magnetoresistive nanosensors with high spatial resolution for
TRANSCRIPT
Magnetoresistive nanosensors with high spatial resolution fordetecting ultra low magnetic fields
Luıs Manuel Portelinha Gameiro
Dissertacao para a obtencao do Grau de Mestre em
Engenharia Fısica Tecnologica
Juri
Presidente: Doutor Jose Luıs Rodrigues Julio MartinsOrientador: Doutora Susana Isabel Pinheiro Cardoso de FreitasVogal: Doutor Paulo Jorge Peixeiro de Freitas
Novembro 2012
Agradecimentos
Gostaria de agradecer em primeiro lugar a Professora Susana Cardoso, por me ter orientado e ajudadodurante este projecto final de curso, e ao Professor Paulo Freitas pela oportunidade de trabalhar no INESC-MN,onde foi realizado este traballho.
Um grande obrigado a todos os colegas e tecnicos do INESC-MN pela a ajuda sempre que foi necessario.Um especial agradacecimento aos Pos-Docs Diana Leitao e Filipe Cardoso por todo a pacencia e apoio dadonas mais pequenas duvidas.
Obrigado ainda a todos aqueles que fizeram do INESC-MN um bom local de trabalho, relaxado e divertido,dos quais destaco: Ana Silva, Antonio Lopes, Jose Amaral, Miguel Leitao e Ricardo Janeiro.
Agradeco claro minha familia pelo apoio incondicional durante o curso. Um muito obrigado ao meu PaiManuel Gameiro que esteve sempre presente para me puxar as orelhas quando necessario. A minha reles irmaRita Gameiro por ser uma fixe. Um beijo especial de agradecimento para a minha tia Guadalupe Portelinha eprima Ana Margarido por todo o apoio em Lisboa durante todo o periodo que o curso. Um muito obrigado aminha mae Candida Portelinha por tudo, tudo mesmo! especialmente pelo o amor e carinho, fazes muita falta.
Agradecer aos meus amigos de sempre Diogo Calcada, Henrique Neves e Jose Seguro pelo companheirismo.Para acabar mas nao menos importante, um grande obrigado a minha ciganita Violeta Tataru por me porsempre um sorriso na cara.
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Resumo
Em tecnicas Biomedicas de imagiologia, tais como magneto-cardiografia tem a necessidade de sensoresaltamente sensiveis. Sensores magneticos capazes de detectar campos baixo na gama dos sub-picoTesla, a baixasfrequencias baixas. Actualmente, estas tecnicas usao magnetometros supercondutores de interferencia quanticaou hibridos supercondutor/magnetoresistive sao usados para cumprir tais requisitos, com o inconveniente detrabalharem a temperaturas alguns graus acima do zero absoluto tornando uma tecnica muito cara por causada necessidade de um aparelho criogenicos.
Sensores Magnetoresistivos tem a vantagem em custo, tamanho e cosumo de energia. Particularmentejuncoes magnetoresitivas de efeito de tunel (MTJ) pode ser aa alternativa fiavel para estas tecnicsa, sendocapaz de detectar de campo baixo a temperatura ambiente.
O objectivo para este desenvolver e optimizar sensores MgO MTJ, combinada com CoCrPt magnetospermanentes (PM) e guias de fluxo (FG) CoZrNb para detectar campos magneticos ultra-baixos.
MTJ’s com PM integrados obtiveream-se curvas de transporte linear e pouca coercividade antigindosensibilidades de ate 200 %/mT. Combinado com FG, a resposta linear foi mantida exibindo um aumento nasensibilidade ate 2000 %/mT.
Sensores de MgO MTJ capazes de detectar campos magneticos de a pT/Hz0,5 foram demonstradas. Sendoobtidos sensores com alta sensibilidade e ruıdo 1/f reduzindo foi-se capaz de chegar a um detectividade de ate49 pT em 10 Hz e 3,5 pT para altas frequencias.
Palavras-chave: Microtecnologias, Magnetoresistencia, Juncoes de Efeito Tunel de Spin Magneticas,Concentradores de Fluxo Magnetico, Ruıdo a Baixa Frequencia.
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Abstract
Biomedical imaging techniques such as magneto-cardiography demand highly sensitive magnetic sensorscapable to detect fields down to sub-picoTesla range, at low frequencies. Currently, systems composed ofsuperconducting quantum interference devices magnetometers or hybrid superconducting/magnetoresistivefulfill such requirements, with the inconvenient of working at temperatures of few degrees above absolutezero, making a very expensive techniques because of the necessity of a cryogenic apparatus.
Solid state Magnetoresistive sensors have an inherent advantage in size, cost and power. Particularymagnetic tunnel junctions (MTJ) can be a a reliable alternative for this techniques, being able to achievesuch low field detections at room temperatures.
The objective for this to develop and optimize MgO MTJ’s sensors combined with CoCrPt permanentmagnets and CoZrNb flux guides to detect ultra low magnetic fields.
MTJ’s with integrated PM, showed a linear transfer curve and low coercivity with sensitivities up to 200%/mT. Combined with FG, the linear response was maintained displaying an increase in the sensitivity up to2000 %/mT.
Low frequency MgO MTJ sensors targeted at pT/Hz0.5 applications were demonstrated. Sensor with highsensitivities and reduced 1/f noise component were able to reach to a detectivity down to 49 pT at 10 Hz and3.5 pT for high frequencies.
During this thesis other important task was to be responsible for the noise setup at INESC-MN, thisinvolved the maintenance of the setup and to help or perform the noise measurement. Also an improve tothis setup as also done, with the main objective to be able to test the limit detection of a sensor.
Keywords: Microtechnology, Magnetoresistance, Magnetic Tunnel Junction, Magnetic Flux Guides,Low Frequency Noise, High detectivity sensors.
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Contents
Agradecimentos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiResumo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Contents ix
List of Figures xi
List of Abbreviations xiii
1 Introduction 11.1 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Frame Work at INESC-MN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Background Theory 32.1 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Anisotropic Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 Giant Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.3 Tunneling Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Magnetic Tunnel Junctions Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1 Sensor Linearization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2 MTJ Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.3 Bias Voltage Dependance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.4 MTJ Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 Noise in Magnetoresistive Sensors 113.1 General Noise Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1 White Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.2 Frequency Dependent Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Field Detection Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3 Noise Characterization in Magnetoresistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.1 Spin Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.2 Magnetoresistive Tunnel Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4 Noise Measurement Setup at INESC-MN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.4.1 Amplifier Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.4.2 Noise Program of data treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.4.3 Testing the Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 Low Field Detection Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4 Experimental Techniques 234.1 Sputtering Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.1.1 Nordiko 7000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.2 UHV I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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4.1.3 UHV II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.1.4 Alcatel SCM450 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2 Ion Beam Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.3 Optical Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.4 Pattern Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4.1 Ion Beam Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.4.2 Reactive Ion Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.5 Characterization Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.5.1 Profilometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.5.2 Ellipsometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.5.3 Vibrating Sample Magnetometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.5.4 Transport Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.6 Micromachining System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5 Sensor Design 315.1 Stack Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.2 Bottom Contact Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.3 Tunnel Junction Pillar Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.4 Flux Guides Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.5 Permanent Magnets Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.6 Second Contact Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.7 Top Contact Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.8 Final Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6 Results 376.1 First Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.2 Second Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7 Conclusion and Outlook 55
A Amplifier Calibration 57
B Run Sheet 61
C Time Flow 73
D Masks 75
Bibliography 77
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List of Figures
2.1 Density of states in TMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Square and linear transfer curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Simulation of the field created by the permanent magnets . . . . . . . . . . . . . . . . . . . . . . 7
2.4 TMR vs Bias voltage dependance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 Effects of random telegraph noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Example of a MTJ noise spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3 Simulation for MTJ detectivity optimum bias voltage . . . . . . . . . . . . . . . . . . . . . . . . 16
3.4 Example of a MTJ detectivity spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.5 Exempla of a MTJ array noise and detectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.6 Noise measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.7 Noise setup primary shielded box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.8 Noise setup second shielded box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.9 Amplifier calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.10 Noise software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.11 Resistance calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.1 Nordiko7000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.2 UHV I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3 UHV II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Alcatel SCM450 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.5 Nordiko 3600 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.6 SVG track and DWL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.7 Lift-off and Etch steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.8 Etching conditions in Nordiko 3000/3600 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.9 LAM Rainbow 4400 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.10 Profilometer and ellipsometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.11 Vibrating Sample Magnetometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.12 Transport Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.13 Micromachining system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1 Sensor 3D scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.2 Stack deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.3 Bottom electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.4 Junction pillar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.5 Flux Guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.6 CZN hysteresis loop and FG scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.7 Permanent magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.8 CoCrPt hysteresis loop and PM scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.9 Second contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.10 Top contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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6.1 MTJ Stack of TJ39 and TMR/R×A variation for this wafer . . . . . . . . . . . . . . . . . . . . 376.2 TMR vs R×A dispersion for the first process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386.3 Variation of the transfer curve with the MTJ width . . . . . . . . . . . . . . . . . . . . . . . . . 386.4 Variation of the coercivity and sensitivity with the MTJ width . . . . . . . . . . . . . . . . . . . 396.5 Transfer curves with and without PM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406.6 Variation of the coercivity and sensitivity with and without PM . . . . . . . . . . . . . . . . . . 406.7 Influence of Flux Guides in the transfer curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.8 The sensitivity and gain given with different FG tip . . . . . . . . . . . . . . . . . . . . . . . . . 416.9 Second contact influence in the transfer curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426.10 Transfer curve with higher sensitivities in the first process . . . . . . . . . . . . . . . . . . . . . . 426.11 Noise of MTJ’s with the best sensitivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436.12 Detectivity for the sensors with the best detectivities . . . . . . . . . . . . . . . . . . . . . . . . . 436.13 TMR vs R×A dispersion of the second process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.14 Table with the different characteristics of the sensors patterned . . . . . . . . . . . . . . . . . . . 456.15 Transfer curves with circle shape MTJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466.16 Coercivity and sensitivity Vs the area for circle shape MTJ . . . . . . . . . . . . . . . . . . . . . 466.17 Transfer curves with square shape MTJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.18 Coercivity and sensitivity Vs the area for square shape MTJ . . . . . . . . . . . . . . . . . . . . 476.19 Transfer curves for the square shape junctions with FG . . . . . . . . . . . . . . . . . . . . . . . 486.20 Transfer curves with rectangle shape MTJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486.21 Coercivity and sensitivity Vs the width for rectangle shape MTJ . . . . . . . . . . . . . . . . . . 496.22 Transfer curve with and without FG for the rectangle shape junctions . . . . . . . . . . . . . . . 496.23 Sensitivity Vs the width for rectangle shape MTJ with FG . . . . . . . . . . . . . . . . . . . . . 496.24 Noise for sensors with and without FG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506.25 Detectivity for sensors with and without FG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506.26 TMR curve for the sensor with best result in the detectivity . . . . . . . . . . . . . . . . . . . . . 516.27 Noise spectrum of the sensor with best result in the detectivity . . . . . . . . . . . . . . . . . . . 516.28 Detectivity spectrum of the sensor with best result in the detectivity . . . . . . . . . . . . . . . . 516.29 The effects of each sensor component in is performance . . . . . . . . . . . . . . . . . . . . . . . 53
A.1 Noise measurement setup equivalent electrical circuit for the Fetmo amplifier . . . . . . . . . . . 57A.2 The noise sources in the Fetmo amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58A.3 The noise sources in the SRS SIM910 amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
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List of Abbreviations
• AC, Alternating current
• CIPT, Current-In-Plane-Tunneling
• BC, Botom contact
• DC, Direct current
• DWL, Direct write laser
• DUT, Device under test
• FL, Free layer
• fT, femtoTesla
• FG, Flux guides
• IBD, Ion beam deposition
• MCG, Magnetocardiography
• MEG, Magnetoencephalography
• MR, Magnetoresistive
• MTJ, Magnetic tunnel junction
• AMR, Anisotropic Magnetoresistance
• TMR, Tunnel Magnetoresistance
• N, Demagnetizing factor
• PCB, Printed circuit board
• PL, Pinned laye
• PM, Permanet magnets
• pT, PicoTesla
• RF, Radio frequency
• R, Electrical resistance
• SA, Spectrum analyzer
• SQUID, Superconducting quantum interferencedevices
• SV, Spin valve
• SVG, Silicon Valey Group
• T, Temperature
• VSM, Vibrating Sample Magnetometer
• EA, Easy-axis
• HA, Hard-axis
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Chapter 1
Introduction
In this thesis we propose to achieve detection levels of B = 1 picoTesla (pT) at low-frequencies (1-100Hz), using magnetoresistive sensors. The importance of being able to detect such low fields comes fromthe necessity in medical imaging of non-invasive studies of the human body. The organs that have moreinterest for study the magnetic field are the heart with magnetocardiography (MCG) [1] and the brain withMagnetoencephalography (MEG) [2], this hagiology studies are very important for a more accurate diagnostic.
The magnetic fields produced by the human body arise from its normal activity. In the heart the field iscreated by the circulation of ions(potassium) responsible for the contractions of the heart muscles, presentinga field amplitude of 1pT within frequencies of 0.1-100 Hz [3]. In the brain, the magnetic field results fromthe synchronized neural ionic currents with dipolar configuration flowing in the dendrites of neurons duringsynaptic transmission. The brain cells have the particularity of being oriented tangentially to the scalpproducing dipolar current and a measurable orthogonal magnetic field. The brain activity has amplitudedown to few femtoTesla (fT) in a frequency range of 3-30 Hz [3]. The fields that this techniques pretend todetect are extremely small and at low frequencies, so being able to measure them is not an easy task.
1.1 State of the Art
The state of the art, for MCG and MEG nowadays is based on Superconducting Quantum InterferenceDevices (SQUID) [3] that is a very sensitive magnetometer used to measure extremely subtle magnetic fields,based on superconducting loops containing Josephson junctions [4]. There are two types of SQUIDs operatingat two temperatures: 4 K (low-Tc) and 77 K (high-Tc) []. Because the Low-Tc SQUIDs have smaller noiselevel it can reach higher sensitivities (2 fT/Hz1/2), but because it operates at lower temperatures the costincreases more then 30%. A MEG system based on low-Tc SQUID working at 4 K costs 3 M$.
In alternatives to the SQUIDs, there is a mix technology that uses Giant Magnetoresistance (GMR) locatedon top of a superconducting loop, the magnetic field applied gives rise to a supercurrent which flows in theloop to counteract the applied field. Part of the loop is narrowed to increase the local current density, resultingin a local amplification of the applied field by the ratio of the loop size and constriction width. The amplifiedfield can be detected by magnetoresistive sensors or below the constriction. Giant MagnetoResistance devicesare suitable because of their high sensitivity to in-plane. With this technic was able to reach a field detectionof 30 fT/Hz1/2 with the superconductor at 77 K [5].
1
1.2 Frame Work at INESC-MN
INESC-MN was one of to labs to participating in the development of first generation MTJ, having AlOxtunnel junctions with controllable R×A and TMR ∼ 40% - 50% [6][7].
A big improvement in the MTJ technology was the change of the insulator barrier with MgO, increasingthe TMR ratio and maintaining similar R×A values. At INESC-MN the first works with MgO based MTJ’swere deposit at Singulus (Germany) with TMR ∼ 150%. After deposition of MgO MTJ’s was achievedat INESC-MN (TMR ∼ 80%), maintaining a continuous work in the search to improve MgO crystallineorientation, and controlling the CoFe and CoFeB electrode growth orientation. With the acquisition by INLof a Singulus machine let INESC-MN to have state of the art complex MgO stacks with a TMR ∼ 200 %.
The first noise characterization of MTJ’s stacks were made at INESC-MN where made by J. ALmeida [8]and R. Ferreira [9]. By working with large areas (around 1000 µm2) these first sensors were able to detect 1-2nT at 10Hz.
To achieved high detectivities at INESC-MN the designed and fabrication of Flux Guides using amorphousCoZrNb alloys films and been of great importance, being already report sensitivity gain factor up to 100×[10]. Also the linearization bias with incorporation of permanent magnet (CoCrPt) to provide smooth sensorresponse to transverse fields were already done in INESC-MN [11].
In a previous by R. Chaves et al. the he Combination of a reduced 1/f noise sensors with flux guides anda linearization by permanent magnets were being able to reach a detectivity of 97 pT at 10 Hz [12].
1.3 Objectives
This work targets to optimized Magnetic Tunnel Junctions (MTJ), combined with magnetic flux guides,reach the maximum field detectivity possible at room temperature. To achieve this it is necessary thecombination of:
• The design of a linear MTJ sensors with a high TMR;
• Sensors with a minimum 1/f noise contribution with the increasing of the junction area;
• Increase the sensitivity of sensors with the incorporation of flux guides.
MTJ sensors are chosen due to the compact design, the low-cost and the low power. This work waspresented in a poster on the conference INTERMAG 2012.
Other important task during this thesis was to be responsible of the noise setup at INESC-MN. This taskinvolves the maintenance of the noise setup, perform or help to perform the noise measurement. Some of thiswork was publish in IEEE Transactions on Magnetics:.
• Field detection in spin valve sensors using CoFeB/Ru synthetic-antiferromagnetic multilayers as magneticflux concentrators [13].
• Linearization and Field Detectivity in Magnetic Tunnel Junction Sensors Connected in Series Incorpo-rating 16 nm-Thick NiFe Free Layers [14].
2
Chapter 2
Background Theory
2.1 Magnetoresistance
Magnetoresistance is the property of a material or a device to change the value of its electrical resistancewhen an external magnetic field is applied to it. This effect is expressed by measuring the electrical resistance(R), sweeping the magnetic field (H) to the maximum resistance (Rmax) and minimum resistance (Rmin)values. Throughout this thesis the minimum resistance value will be used as the resistance reference andmagnetoresistance will be expressed as a ratio:
MR =Rmax −Rmin
Rmin× 100 [%]. (2.1)
The three main magnetoresistive (MR) mechanisms used in thin film technologies are:
• Anisotropic Magnetoresistance (AMR)
• Giant Magnetoresistance (GMR)
• Tunneling Magnetoresistance (TMR)
Anisotropic Magnetoresistance (AMR)
The AMR Effect was discovered in 1857 by Lord Kelvin. It consists of the change in the resistance withthe magnetic field orientation relative to the direction of the electrical current in the material.
This effect is found in 3d transition metals and their alloys [15] and it results from the conduction electronssuffering scattering in the d orbitals (elliptic shape). The scattering (and resistance) is minimized when theorbitals are perpendicular to the current direction and maximize when they have the same direction, thedirection of the d orbitals are controlled by the external magnetic field.
The largest AMR effect at room temperature is found for Ni1−xCox alloys MR ∼ 6% [15].
Giant Magnetoresistance (GMR)
The GMR phenomenon was first discovered in 1988 by Fert (at low temperatures) [16] and by Grunberg(at room temperatures) [17] when measuring the electrical resistance in multiple metallic layers, which lead tothe attribution of the Nobel Prize in Physics in 2007 [18].
The origin of the GMR effect is the spin dependent electron scattering. The total current flowing throughthe device results from two parallel currents, one due to spin-up and the other due to spin-down electrons.Each current senses different resistance depending on the magnetic configuration of the ferromagnetic layers.
One well known application of the GMR is the Spin Valve (SV) structure introduced in 1991 [19]. Thisstructure consists in four layers, two ferromagnetic layers separated by a spacer of a non magnetic conductive
3
material, and an antiferromagnetic layer. The antiferromagnetic layer will allow the magnetization pinningof one of the ferromagnetic layers, called the pinned layer. The other ferromagnetic layer is free to rotate,called free layer. With the rotation of the free layer the resistance will change, being able to manipulate thestructure to have a square response (memory) or a linear response (sensor) to the external magnetic field.
Standard spin valves can reach a MR value up to ∼ 9 % at room temperature, still with the introductionof an additional nano-oxide-layers next to the ferromagnets the MR can reach ∼ 20 % [20].
Tunneling Magnetoresistance (TMR)
The discovery of the TMR dates back to 1975, when Julliere [21] proposed the first model to explain theeffect. The tunneling effect takes place in a structure very similar to a spin valve, in this case those structuresare called magnetic tunnel junctions (MTJ). But contrary to the spin valves the spacer is an insulator andthe current flow is perpendicular to the plane of the layers. The insulator layer has to be thin enough toallow the tunneling effect to take place.
TMR is a result of spin-dependent tunneling, during the tunneling process the electrons spin is conserved,each electron leaving a ferromagnetic layer and going towards the other one, will fill the states correspondingto their spin in the other ferromagnetic layer, i.e., spin up electrons will fill spin up states and spin downelectrons will fill spin down states, so the tunneling of spin up and spin down electrons are two independentprocesses, so the conductance occurs in two independent spin channels. Therefore, if both ferromagneticlayers have parallel magnetizations, the majority states tunnel to fill the majority states and minority spinstunnel to fill minority states. However, if both ferromagnetic layers have antiparallel magnetizations, we havethe majority spins tunneling to fill minority states and vice versa.
The conductance for a particular spin orientation is proportional to the product of the effective tunnelingdensity of states (DOS) of the two ferromagnetic layers, the conductance (G) of both parallel (P) andantiparallel (AP) configurations (fig. 2.1) could be written as:
GP ∝ D1↑D2↑ +D1↓D2↓) GAP ∝ D1↑D2↓ +D1↓D2↑, (2.2)
where D1 and D2 represent the DOS at the Fermi level of ferromagnetic layer 1 and 2, respectively, sinceonly electrons near the Fermi level contribute for the conduction process.
Figure 2.1: Density of states illustration of spin-dependent tunneling across an insulating barrier.
The TMR ratio can be defined as the conductance difference between parallel and antiparallel, and giventhat the conductance is inversely proportional to the resistance and expression is obtained:
TMR =GAP −GP
GP=Rmax −Rmin
Rmin× 100 [%]. (2.3)
MTJ with the AlOx barrier lead to a maximum TMR of ∼ 70 % [22], with the crystalline MgO barrierswas achieved TMR of ∼ 600% for at room temperature [23].
4
2.2 Magnetic Tunnel Junctions Sensors
2.2.1 Sensor Linearization
The resistance field curve (the called transfer curve) of a MTJ is obtained by applying a voltage dropacross the system FM/I/FM while varying the magnetic field. It can have two types of responses: square orlinear. The linear response is used in application that need magnetic fields detection (sensors), and the squarebehavior is useful in data storage industry (memories).
In this thesis the objective is to use the MTJ’s as sensors for magnetic field detection. Ideal TMR sensorsshould have a linear response free of hysteresis, symmetric (centered at H = 0 T) and with suppressedBarkhausen noise. A coherent and linear rotation of the free layer (FL) is achieved when the starting point ofthe FL magnetization (easy axis) is at 90 in-plane with respect to the easy axis direction of the pinned layer(PL).
In this thesis the stacks used in the microfabrication process had in bulk a parallel anisotropy, this meansthat the FL and PL have the same magnetic orientation. When patterning the sensor it is necessary to assurethe rotation of the FL, to guaranteing that is necessary take in consideration the next energy terms [24]:
• Zeeman term: µ0H ·Mf , where H is the external applied field, and Mf is the magnetization of the FL.
• Crystalline anisotropy term: Ksin2θ, where K = 12µ0HkMf
s, and Mfs is the saturation magnetization of
the FL.
• Demagnetizing field of the free layer: − 12µ0Hf
d ·Mf , where Hfd is the demagnetizing field of the FL.
• Demagnetizing field of the pinned layer: −µ0Hpd ·Mf , where Hp
d is the demagnetizing field of the PL.
• Neel term: −µ0HN ·Mf , where HN is the Neel field.
The energy in the FL (normalized by its volume) can be calculated by the sum of the previous energyterms, obtaining:
Ef
V= −µ0H ·Mf + Ksin2θ − 1
2µ0Hf
d ·Mf − µ0Hpd ·M
f − µ0HN ·Mf [J]. (2.4)
The demagnetizing field is assumed to be Hfd = NMf
scosθi, with N the demagnetizing factor being dependenton the geometrical shape of the magnetic layer. So calculating when the energy is minimum:
∂ Ef
V
∂θ= 0⇒ sinθ[cosθ(Hk −NMf
s) + H−Hpd + HN], (2.5)
there are 3 possible solutions:
1. sinθ = 0⇔ θ = 0 ∨ θ = π;
2. Hk −NMfs = 0 ∧H−Hp
d + HN = 0;
3. cosΘ =H−Hp
d+HN
NMfs−Hk
.
Taking into consideration the second derivative of Eq 2.4, we get the follow:
∂2 Ef
V
∂θ2> 0⇒
H > Hp
d −HN + (NMfs −Hk), for θ = 0
H < Hpd −HN + (NMf
s −Hk), for θ = π(2.6)
If Hk > NMfs, there are only two possible states for the orientation of the FL: θ = 0 and θ = π. Therefore,
we obtain a square magnetic response as showed in fig. 2.2.a). As observed, the component NMfs −Hk defines
the field at which occurs the state change while Hpd −HN shifts the whole curve.
However if Hk < NMfs, this solution gives an incomplete magnetic response curve. To complete the curve
the third solution has to be taken into account, this solution corresponds to a minimum of the energy ifHk < NMf
s and to a maximum otherwise. In the case of Hk < NMfs , all the derivatives vanish and nothing
5
can be concluded. Therefore a linear magnetic response curve is obtained if the demagnetizing field is higherthan the Hk of the material. As for the square magnetic response, the component NMf
s −Hk defines thefield at which the response starts while Hp
d −HN shifts the whole curve. The sensitivity of the sensor will beproportional to the slope given by Slope = 1
NMfs−Hk
as observed in fig. 2.2.b). An increase of the demagnetizing
field will reduce the slope and therefore the sensor sensitivity, but a high demagnetizing field is important tostabilize the free layer and achieve a linear sensor. So, to achieve a linear response is necessary to guaranteethat the sensors have a large and positive aspect ratio N, or using an external magnetic field that ensuresHk < NMf
s.For stacks with crossed anisotropy (weakly pinned), in bulk the FL is already rotated 90 in relation to
the PL. This produces a linear sensor without the necessity for aspect ratio or an external magnetic field [24].
Figure 2.2: a) Square transfer curve. b) Linear transfer curve.
Permanent Magnets (PM)
In a MTJ sensors the linearization can be forced by applying a longitudinal bias (LB) magnetic field,setting the FL perpendicular to the PL, in order to have linear response free of hysteresis. Different approacheshave been used in magnetoresistive sensors to create the LB field, based either on PM [29] or ferromagneticfilms exchange coupled by antiferromagnets [28].
In this thesis integrated PM were used, with the deposition of CoCrPt alloy [25][26]. The direction ofmagnetic field created by the PM is settled by an external magnetic field, without the need to anneal thesample.
The LB field created by the permanent magnet at the FL level depends on the material magnetization
of the PM (−→M), the dimensions (thickness, width and height) and the gap between the PM. The LB field
[−→Bm(−→r )] can be calculated by:
−→Bm(−→r ) =
∫v
∇.−→M(−→r ′) (−→r −−→r ′)
|−→r −−→r ′|3d3−→r ′ + µ0
4π
∫S
n.−→M(−→r ′) (−→r −−→r ′)
|−→r −−→r ′|3d2−→r ′ [T], (2.7)
where n is the normalized vector perpendicular to the surface (S) that delimits the volume (V). Higher LBfields result in better sensing layer stability but also in lower field sensitivities [29]. Assuming constant
magnetization over the volume: ∇.−→M = 0, is possible to calculate the field created by the PM with the chosen
geometry.Figure 2.3 shows the simulation result for the magnetic field along the gap created by the PM, for the
dimension specified. This simulations where made using the a software program called Mathematica.
6
Figure 2.3: Simulation of the magnetic field created by the permanent magnets.
2.2.2 MTJ Resistivity
In a MTJ the resistance depends on the area and thickness of the barrier. An important parameter thatcharacterizes the stacks is the Resistance Area product (R×A). The R×A can be tuned from MΩ.µm2 down tofew Ω.µm2 by decreasing the thickness of the insulator barrier. The barrier minimum thickness is limited bythe surface roughness of the barrier interface and at a critical thickness pinholes are formed in the insulatingbarrier resulting in ballistic electrical conduction instead of tunneling conduction. High roughness also implieshigher Hp
d and therefore higher offsets in the field response.
The spatial resolution required for MCG and MEG applications is up to 1 mm2, therefore the area ofthe sensor can be large, in order to suppress noise at low frequencies as explained in chapter 3. For a largearea sensor > 100 µm, the R×A must be high enough so the final device resistance will be bigger than thecontacts resistance (> 5 Ω) so there is no loss in TMR.
2.2.3 Bias Voltage Dependance
The TMR response of a MTJ depends on the applied voltage, as showed in figure 2.4. The TMR is almostconstant for low bias voltage (V< 20 mV), and then it starts to decrease almost linear with the increase ofvoltage.
Figure 2.4: TMR vs Bias voltage dependance, for a MTJ sensor (4×20µm2) with a MgO barrier ∼ 10 A(stack TJ377).
7
Therefore, the following simple model can be assumed to describe the TMR behavior width the voltage:
TMR(V) = TMR0(1− V
2V1/2) [%], (2.8)
in the equation V1/2 is the voltage at which the TMR is half of the initial TMR0. This parameter is importantto characterize the electrical robustness and it is typically between the 0.3 to 0.5 V for a single MTJ. Thedecrease of the TMR is due to the presence of defects in the barrier which starts to conduct as the voltageincreases [31].
The barrier of the MTJ is a dielectric which can be electrically disrupted if the bias voltage increasesbeyond a certain value, breakdown voltage. In this regime pinholes are formed providing direct low resistancemetallic conduction, resulting in the lost of TMR. To void this limitation, series of MTJs can be used byincreasing the total voltage while the voltage across each junction is reduced [14].
2.2.4 MTJ Sensitivity
The sensitivity of a MTJ sensor is obtained by the slope of the linear part of the transfer curve, thus:
S =∂R
∂H[Ω/T]. (2.9)
Assuming that the sensor is perfectly linear:
S =Rmax − Rmin
∆H=
∆R
∆H[Ω/T], (2.10)
In order to be able to compare the sensitivity of different sensors, the sensitivity is normalized to Rmin
and therefore expressed by:
S =∆R
Rmin∆H=
TMR
∆H[%/T]. (2.11)
Since the TMR depends on the bias voltage (Eq.2.8), the sensitivity will also decrease with the increase ofthe bias voltage:
S(V) = S0(1− V
2×V1/2) [%/T], (2.12)
where S0 is the maximum sensitivity of the MTJ obtained at low bias voltages. In the linear range, theresistance of a MTJ can be written as:
R(H) = R0 + S(V)RminH [Ω], (2.13)
where R0 is the resistance at zero field. Using this sensitivity, the signal variation (4V ) on a MTJ sensordue to an external magnetic field (H) can be written:
4V = (R(H)− R0)I⇔4V = S(V)RminHI [V]. (2.14)
Taking in consideration the objective of a sensor is to detect the external magnetic field, the sensor detectvalue for the external magnetic field can then be taken from variation on the output voltage by:
H =4V
S(V)RminI[T]. (2.15)
8
Increasing the Sensitive with Magnetic Flux Guides (FG)
Magnetic flux guides are used to improve the sensor sensitivity by increasing the magnetic flux throughthe free layer. FG are made of soft ferromagnetic material, presenting a linear magnetization hysteresis loopalong the hard axis, which slope is proportional to the material’s permeability µr. Magnetic materials showinghigh magnetic relative permeability (µr >> 1) tend to attract the magnetic flux lines towards it, if FG followsa certain geometry can guide and concentrate the magnetic field to the sensor region. Considering that themagnetic field (B) inside the ferromagnetic material as a function of its magnetization (M) given by:
B = µ0(H + M), (2.16)
where the µ0 = 4π× 10−7 H/m is the permeability of the free space and H is the external field. The slope of alinear ferromagnetic magnetization hysteresis loop, is defined by the magnetic susceptibility χ, expressed by:
χ =M
H⇒ B = µ0(1 + χ)H, (2.17)
where from the magnetic permeability can be defined as:
µr = 1 + χ. (2.18)
The relative permeability µr (µ = µrµ0), rather that the susceptibility χ, is the common quantity usedto refer to the material’s strength to attract the magnetic field lines. This concentration of field inside themagnetic material occurs due to the abrupt change of permeability (from air to the magnetic material),resulting in a bound surface current distribution that induces an additional magnetic field.
The FG gain observed is due to a geometry effect but also depends on the magnetic properties, in particularthe susceptibility of the pattern element. In this thesis the design was chosen taking in consideration that anhigher susceptibility was obtained for this configuration, after a shape optimization work by R. Chaves [32].
With a correct configuration of the FG, it is possible to focus the magnetic field captured inside it throughthe magnetic sensor. The typical approach is to use two flux concentrators separated by a gap (g), where thesensor is placed.
The gain in sensitivity (G), provided by the FG is defined by the ratio of the magnetic field reaching thesensor region, Hsensor, and the external applied field, Hext:
G =Hsensor
Hext[×]. (2.19)
The gain given by the FG is dependent of the geometry, being dependent of the length (L), the gap betweenconcentrators (g), the area difference of the two lateral sections (A1/A2), and the relative permeability (µr) ofthe material composing the concentrator. Gain in sensitivity given by the FG up to 100 ×, in magnetoresistivesensors, was already reported [10]. The results obtained during this thesis for the gain provided by the FGwere similar to the results calculated by an empirical equation, obtained by A. Guedes with simulation inFeem [30].
9
Chapter 3
Noise in Magnetoresistive Sensors
Noise is a fundamental physical phenomenon, consisting in fluctuations of a physical property such as:energy, temperature, charge, current or voltage.
In an electrical circuit, noise is generated by every component and is manifested by fluctuations in thevoltage and the current. In this thesis the noise will be presented as power spectral density (Sν) with theunits of V2/Hz.
The noise characterization of any sensor is essential to know the field detection limit, ensuring that thefield pretended to detect is within the sensing limit at the operating frequencies. Noise studies can alsoprovide valuable information about the physical properties of the device under study (DUT).
In this chapter we are going to overview the noise sources and noise contribution in different type of MRsensors at low frequencies, and a detailed description of the noise measurement setup.
3.1 General Noise Sources
3.1.1 White Noise
White noise (WN) means that the source is independent of the frequency, therefore the frequency content,the phase and the amplitudes are equally distributed across the entire spectrum. Two types of white noiseare known:
Thermal Noise
The thermal noise was first observed by Johnson [34], after Nyquist [35] demonstrated that a resistor R inthermal equilibrium at a temperature T does have an electromotive force across its terminals. The Thermalnoise results from the random thermal motion of electrons causing a voltage variation according to Eq. 3.1.:
SThermalν = 4kBTR [V2/Hz], (3.1)
where kB represents the Boltzman constant (kB = 1.38× 10−23 J/K) , the T the temperature (K) and Rthe resistance of the devise (Ω). Thermal noise is directly proportional to the temperature, vanishing asthe temperature approaches to absolute zero. It does not depend on the current flowing because the driftvelocities of electrons in conductors are small compared with the electrons thermal velocities.
Shot Noise
Shot noise results from current flowing through discontinuity (insulator) in a electric circuit (eg. MTJ).Elements without discontinuities, such as SV, do not exhibit shot noise. The concept of shot noise was firstintroduced by Schottky who studied fluctuations of current in vacuum tubes.
11
This phenomenon is associated with the discrete nature of the electrical charge caused by thermal effectsand the stochastic noise nature of the electron passing through a discontinuity. The result is a fluctuation inthe current going through the circuit around a long-term average current [37].
Due to the discrete nature of the electrical charges, shot noise is described by a Poisson distribution.However, if the electrical current is high enough, the number of electrons going through the junction will bevery large and shot noise can approximately be described by a normal distribution. In such a case, the shotnoise value is given by:
SShotν = 2eIR2 [V2/Hz], (3.2)
where e represents electron charge (e = 1.60× 10−19 C), I the current (A) and R the resistance of the sensorΩ. The shot noise is dependent on the quality of the barrier [36].
3.1.2 Frequency Dependent Noise
In a the magnetoresistive sensor the magnetic field detection limit is determined by the white noise level.However, there are other noise mechanisms that can limit the detection capability of these devices dependingon the operating frequency. For low frequency operating systems (eg. biological detection) the major concernis the 1/f noise, because is the predominant noise at low frequencies. In some cases random telegraph noise(RTN) may also be present.
Electric 1/f Noise
The 1/f noise occurs in many physical, biological and economic systems, is near ubiquity does not meanthat some features of the explanation should not depend on system [38]. To describe the effects of the 1/fnoise the best known model, and the one that will be used throughout this thesis, is the phenomenologicalmodel proposed by Hooge [39].Here the most universal model of 1/f noise in electronic devices is provided bythe following expressions:
S1/fν = K
IaR2
fb[V2/Hz], (3.3)
where K is a constant for a particular device, a is a constant in the range 0.5 - 2 and b ∼ 1.For magnetoresistive devices, namely SV and MTJ. We consider the transport processes as the source of
noise and Eq. 3.3 is replaced with the values for the constants a = 2 and b = 1.In the case of a SV can be shown that the spectral density of 1/f noise is inversely proportional to the
number of charge carriers Nc [40], and Eq. 3.3 is substituted by:
S1/fνSV
=γHNc
I2R2
f[V2/Hz]. (3.4)
In the case of the MTJ’s the 1/f noise being inversely proportional area (A) of the barrier. The origin ofthe 1/f noise in a MTJ consistent with charge trapping in the barrier. Therefore, the Eq. 3.3 changes to theform [41]:
S1/fνMTJ
=αH
A
I2R2
f[V2/Hz]. (3.5)
Although the 1/f noise spectral density decreases with frequency, in particulary devices it can have animportant contribution well into the MHz range with major consequences in practical applications.
Magnetic 1/f Noise
Magnetic 1/f noise is a crucial characteristic of magnetic-based devices such as SV’s and MTJ’s in thelow frequency regime, reaching values several orders of magnitude above the non-magnetic 1/f noise level.The magnetic noise in a MTJ arises from the magnetization fluctuations originated by thermally activated
12
domain rotation or magnetic domain walls hopping between pinning sites [42]. Usually the magnetic 1/f noiseis extracted from:
αH = αmagnetic + αelectric ⇔ αmagnetic = αH − αelectric [µm−2]. (3.6)
The Hooge parameter (αH), obtained from the well known Eq. 3.3/3.4, can be decomposed into itsmagnetic and electric contributions. αmagnetic obtained by measuring the sensors 1/f noise in saturation,which minimizes the magnetic contribution and enable to take the electrical component of Hooge parameter.There are many propose model that try to describe the magnetic 1/f noise in a MTJ [43], one of the moreacceptable model give the follow equation [44]:
S1/fνmagnetic
=αmagneticV
2
ΩBSatf(∆R
R)2 [V2/Hz], (3.7)
with
αmagnetic =kBT
πMS[χ′′
χ′], (3.8)
where Ω is the free layer’s volume, χ′ and χ′′ stand for the real and imaginary parts of the susceptibilityrespectively , Bsat represents the field required to saturate the sensor, Ms the free layer saturation magnetiza-tion, ∆R is the resistance variation between parallel and antiparallel states, and R is the average resistance.The most important aspect of all this models is the suggestion that the magnetic 1/f noise can be controlledby varying the properties of the sensor free layer, specially that can be reduced by increasing its volume (Ω).
Random Telegraph Noise
The Random Telegraph Noise (RTN) origin depends on the particular system being considered. For a MRsensor the RTN arises from abrupt changes in the resistance. In a MTJ the noise can be caused by: i) thethermal activation of unstable magnetic domains in the pinned or free layer [42], ii) the Charge trapping atthe oxide barrier and/or barrier/metal interfaces [45] iii) the displacement of weakly bonded oxygen ions inthe barrier [46].
When abrupt resistance changes are restricted to its simplest classical appearance with two-level variations,the corresponding noise spectrum [Fig 3.1.b)] shows a Lorentzian type response:
SRTNν =
S0
1 + ( ff0
)2[V2/Hz], (3.9)
where S0 stands for noise power intensity at 0 Hz, and f0 the characteristic roll-off frequency of the Lorentzian.Figure 3.1.a) represents the energy barrier model of RTN, the energy barrier that separates the two
possible stable configurations is called activation energy (Ea). If this energy barrier is of the same order ofthermal energy then the system can spontaneously change from one configuration into the other. Alternatively,the system can simply change its configuration by tunnel effect. Since the two configurations have the sameenergy, we can expect the system to spend 50% of its time in each configuration. If the energy imbalance wasgreater the system would spend a larger portion of time on the lower energy configuration. Although theseconfigurations have a similar energy, it is conceivable that the electrical resistance might be different.
Figure 3.1: a) Two-configuration system model for telegraph noise b) Typical telegraph noise spectrum.
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3.2 Field Detection Limit
The noise level in a sensor can be expressed in units of magnetic field, that corresponds to the sensor lowerfield that a sensor can detect. This result is obtained by [48]:
Dν =
√Sν(V2/Hz)
Ssensor(%/T)IBiasR[T/Hz1/2], (3.10)
where Ssensor is the sensor field sensitivity, IBias stands for the sensor bias current and R the sensor dynamicresistance in the operating point.
3.3 Noise Characterization in Magnetoresistive Sensors
When more than one source of noise is present in an electrical circuit, the circuit output voltage iscalculated using the superposition theorem. This theorem is a direct consequence of the linear nature of theMaxwell equations and states that the output of a circuit with multiple sources is given by the sum of theoutputs obtained in the circuits produced by eliminating all the sources but one.
3.3.1 Spin Valve (SV)
Isolated Spin Valve
In a SV the noise arises from thermal noise, 1/f noise, and in particular cases RTN can also be present.The general equation for describing the noise in a SV is given by:
SSingle SVν = 4kBTR +
γHNc
I2R2
f[V2/Hz], (3.11)
where (Nc) is the number of charge carriers, that can be calculated as follows:
Nc = VC [cm−3], (3.12)
with C the atomic concentration of the metallic spacer (e.g. Cu = 8.45 ×1022 cm−3) and V is the volume ofthe SV.
Series of Spin Valves
In the case of SV array, all the SV have to be consider a independent sources of noise:
SN SVν = S1
ν + S2ν + ...+ SN−1
ν + SNν =
N∑k=1
(4kBTRk +γHNc
I2R2k
f) [V2/Hz]. (3.13)
Considering the SV array as a individual SV with the total of resistance (RT) and the total number ofcharge carries (NcT) given by:
RT =
N∑k=1
(Rk) = N < R > [Ω]; (3.14)
NcT =
N∑k=1
(NCk) = N < NC > [cm−3], (3.15)
where < R > and < NC > are the average resistance and number of carries, respectively. Then we canrewritten Eq. 3.13 as:
14
SN SVν = 4kBTRT +
γHNcT
I2RT2
f⇔ SN SV
ν = 4kBTN < R > +NγH
< NC >
I2< R >2
f[V2/Hz]. (3.16)
If we compare the noise level of an individual SV with a SV array (with the same characteristics), resultsin:
SN SVν = SSingle SV
ν ×√
N [V2/Hz]. (3.17)
As direct consequence of this result, the field detection limits of an array of SV comparing to a single SVis:
DN SVν =
1
N
√SNSVν (V2/Hz)
Ssensor(%/T)IBiasR=√
N
√SSingle SVν
Ssensor(%/T)IBiasR=√
NDSingle SVν [T/Hz1/2]. (3.18)
Therefore having a array of sensors increases the detectivity of the device by a factor of√N , being one of
the possible ways to increase the detectivity of the sensor.
3.3.2 Magnetoresistive Tunnel Junction
Single Magnetic Tunnel Junction
In a MTJ the white noise is composed by shot and thermal noise. Typically the derived quantitativeexpressions are independent and the noise power densities are additive, however, in a tunneling process bothnoises are related and a more rigorous derivation can be obtained [47]:
SWNBν (V2/Hz) = 2eIR2coth(
eV
2kBT) (3.19)
It is noteworthy that with bias voltage (V) tending to 0 the Eq. 3.19 tends to 4kBTR , corresponding tothe thermal noise. And in the hypothetical situation of non thermal agitation, T = 0, the equation will beeRI2, equal to the shot noise. Then the general noise equation for a MTJ can be written as:
SMTJν (V2/Hz) = 2eIR2coth(
eV
2kBT) +
αH
A
I2R2
f(3.20)
Figure 3.2 shows an exemplificative noise measurement for a MTJ:
Figure 3.2: Noise measurement ranging from DC - 1 kHz [a)] and from DC - 100 kHz [b)] for a MTJ with: area of1300 µm2, TMR of 205%, sensitivity of 98 %/mT and Rmin= 5.26 Ω (Stack: TJ377)
15
In the figure is observed that the 1/f noise component is dominant at lower frequencies and at higherfrequencies the spectrum reaches to the white noise baseline, also as expected the noise increases with thebias voltage. The Hooge parameter was calculated making a fit in the part of the spectrum where the 1/fnoise is dominant. The fit equation is:
FitαH(f) =
A
fC+ B. (3.21)
In a converging fit the values for the constants are:
1. A is the 1/f competent: A = αH
Area I2R2;
2. B should have a value similar to the white noise: B = 2eIR2coth( eV2kBT );
3. C should be near the unity: C ' 1.
After being sure that the fit converge with the curve we can extract the Hooge parameter by:
αH =A×Area
I2R2. (3.22)
The detectivity for a MTJ can be extracted knowing the noise spectrum, using the next equation:
DνMTJ=
√2eIR2coth( eV
2kBT ) + αH
AI2R2
f
Ssensor(%/T)IBiasR[T/Hz1/2]. (3.23)
Taking in consideration that the sensitivity decreases with increase of the bias voltage (Eq. 2.12), thedetectivity equation takes now the form of:
DνMTJ=
√2eIR2coth( eV
2kBT ) + αH
AI2R2
f
(S0(1− V2∗V1/2
)(%/T)IBiasR[T/Hz1/2]. (3.24)
This results in an optimum operation bias voltage were a higher detectivity is achieved. This voltage canbe calculate having all the parameters of the sensor [fig. 3.3], in this case is with a voltage bias of 5 mV.
Figure 3.3: Simulation of detectivity for a MTJ to know the optimum bias voltage.
Now is showed an example of a detectivity spectrum as illustrative figure (fig.3.4):
16
Figure 3.4: Detectivity spectrum ranging from DC - 1 kHz [a)] and from DC - 100 kHz [b)] for a MTJ with: area of1300 µm2, TMR of 205%, sensitivity of 98 %/mT and Rmin = 5.26 Ω (Stack: TJ377)
Series of Magnetic Tunnel Junction
For an array of MTJ the general equation that describes the noise is given by:
SMTJν = 2N2eIR2coth(
eV
2kBT) + N
αH
A
I2R2
f[V2/Hz]. (3.25)
This equation comes from similar reasoning then for series of spin valve starting from the Eq. 3.2. Figure3.5 shows an example of a noise measurement and a detectivity limit spectrum for a series of MTJ.
Figure 3.5: Noise [a)] and detectivity spectrum [b)] for a MTJ array with: 82 elements, area of 5×20 µm2, TMR of182%, sensitivity of 11 %/mT and Rmin = 4.7 kΩ
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3.4 Noise Measurement Setup at INESC-MN
The noise setup used during this thesis was developed by R. Ferreira [51] and J. Almeida [33]. Themeasurements consist in applying current to the device and measuring the fluctuation of the voltage. Figure3.6 shows the noise measurement setup installed at INESC-MN, the system can be divided in: Primary box,Secondary box, and the experimental setup.
Figure 3.6: Noise measurement setup with: the spectrum analyzer, the multimeter, the current source and secondshielded box.
The primary box (fig.3.7) corresponds to a chip socket (40 pins) where the on-chip sensor is placed, attachedto a printed circuit board (PCB) enabling to choose which sensor to measure. The sensor is connected with aseries of 9 V batteries (2 or more) and with two potentiometers, one with 100 kΩ and the other with 10 kΩ.In this circuit there are 3 switches: one to turn on the batteries, another to cut off the sensor ofthe circuitand the other to connect an multimeter closed that during measurement.
Figure 3.7: a) Top view of the opened primary shielded box with all its components; b) Primary noise box generalview. In this box is where the conditions of the measurement is controlled.
The second box (fig.3.8) is fully covered with 0.1 mm thick mu-metal foil for magnetic field shielding.Inside is placed the primary box, the amplifier and its corresponding battery supply. The battery supply isconstituted of 2 rechargeable batteries of 15 V each.
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Figure 3.8: Second shielded box.
The sensors noise spectrum is acquired in a Tektronix RSA3308A real time spectrum analyzer (SA) (DC- 8 GHz), which has a noise level of 90 nV/Hz1/2. Since the noise level of the MR sensors are lower thanspectrum analyzer noise, an amplifier is necessary. In this setup there is also a multimeter to read the theparameters in the measurement and a current source to supply the coils.
Two different voltage amplifiers were used during this work. The choice of amplifier was performedaccording to the objective of the study and the properties of the device:
• Femto DPVLA-100-BLN-S (Femto): has a high input resistance (1 MΩ), low noise (0.7 nV/Hz1/2).The amplifier as a first amplification stage with 40 dB that amplifies both the AC and DC componentsof the input signal. Afterwards, there is a optional capacitive coupling to remove of the DC component.The capacitive coupling was always selected to amplify only the fluctuations of the sensors voltageoutput. Afterwards, there is a second stage of amplification with a variable gain of 0, 20, 40 or 60 dB.Finally, there is a programmable low pass-filter with a selectable cut-off frequency of 1 kHz or 100 kHzlimiting the bandwidth DC - 1kHz or 100kHz. This amplifier has the limitation of saturating with avoltage input > 100 mV due to the first amplifier stage, being good to measure devices with a lowresistance.
• Stanford Research Systems (SRS) SIM910: has ahigh noise level (4nV/Hz1/2), high inputresistance (100MΩ) and bandwidth DC - 100 kHz. This amplifier has two different inputs and performdifferential measurement between the two inputs. Before the first, and only, stage of amplification thereis an optional capacitive coupling, enabling (up to ± 5 V) providing that the amplifier output voltage isinside a ±10 V range. The gain can be variable with a maximum of 40 dB. This amplifier can onlybe used for sensors with higher resistance (white noise background of the sensor above the amplifiernoise level) or if the frequency range of interest is in the 1/f dominated regime (with the 1/f noise of thesensor also above the amplifier noise level).
Amplifier Calibration
The noise spectrum obtained with the SA during measurement is not the correct noise spectrum of theDUT. To obtain the real noise spectrum of the device is necessary to remove both the SA and amplifier noisecontributions. The amplifier noise is modeled with two noise sources, a current source (Iai) and a voltagesource (Vai). Characterizing this parameters is necessary:
• The gain of the amplifier, measured with a network analyzer (HP 4195A) at INESC-ID;
19
• The Vai is measured using the spectrum analyzer in an short circuit in the amplifier input for bothamplifiers;
• The Iai is measured using the spectrum analyzer. The measurement for the SRS amplifier is made withan open circuit in the amplifier input and for the Femto amplifier is made with a 10 kΩ film resistanceto avoid the amplifier overload.
Figure 3.9 shows the calibrations measurements of the 3 parameters for both amplifiers.
Figure 3.9: Gain [a)], Vai noise source [b)] and Iai noise source [c)] spectrum from both amplifiers.
In the SRS amplifier the gain remained constant at 40 dB, from DC up to 100 kHz. Although in theamplifier specification state a constant gain from DC to 1 MHz, it was found that the amplifier gain andnoise sources show a non-linear behavior at frequencies above 100 kHz. This may be due to a non optimizedinput impedance causing non-linearities at high frequencies. Since this setup was mainly designed to performmeasurements at low frequency, in this work the amplifier will be used in the range of DC to 100 kHz. Thevoltage source is the larger source of noise, decreasing with the frequency after ∼ 100 kHz the value remainsconstant. The Current source s also higher at low frequencies becoming virtually zero around 2 kHz.
In the Fetmo amplifier gain remains constant (59 dB) until 10 kHz, then it starts to decrease because of islow-pass filter at 100 kHz. The current source is the bigger source of noise for this amplifier. Both sourceshave a slight high value in the low frequencies, after the ∼ 150 kHz there value remains constant.
Software Programs for Noise Data Treatment
Once the amplifier parameters are known for each frequency, we can extract the DUT noise spectrum. Inorder to retrieve the noise measurement of the DUT, two computer programs developed at INESC-MN [51](Fig. ??) were used.
The Calibration Wizard program receives the raw data files gain, the Iai and the Vai , compiling to aunique file that will be used in the data processer program.
20
Figure 3.10: a) Computer program used to create amplifier calibration files. These files contain the information aboutthe amplifier characterization. b) Computer program used to extract the DUT noise spectral density from the rawspectrum analyzer data files using the amplifier calibration file and the resistance of the DUT as inputs.
The Data Processor program receives the raw data file of the DUT noise measurement. As a first step,the program interpolates the data of the gain, the Iai and the Vai file generated by the Calibration Wizardprogram. The program then uses this information together with the DUT resistance to calculate the DUTnoise spectral density by using equations that is described in detail in annex A. The whole process can beused to correct a batch of raw data files at once and it is virtually instantaneous.
Testing the Measurements
To confirm that the amplifiers were well characterized, film resistors with different resistances weremeasured. A film resistor have the propriety to only present thermal noise for low frequencies. So themeasurement should be equal to the thermal noise given by Eq. 3.1. Figure 3.11 shows the result of the testmeasurement, for both amplifiers with the different resistors with the respective theoretical value line.
With the Femto amplifier the measurement with 2 lower resistors (33-600 Ω), the measurement is accurate.However for the larger resistors the measurement were not accurate good because of the low input resistanceof the amplifier.
With the SRS amplifier the measurement of the lower resistance were not accurate since the noise of theamplifier is larger than the thermal noise of the resistance. For the 2 larger resistances (5.6 kΩ and 15.2kΩ) at frequencies above 30 kHz the measurement stops to be accurate due to intrinsic characteristics of theamplifier.
21
Figure 3.11: Noise measurement of different film resistors with the Fetmo amplifier [a)] and with the SRS amplifier[b)]
3.5 Low Field Detection Setup
The obective of this work is to achieve the pT detection. To prove that the sensor can detect the valuesthat detection limit equation states, was built a measurement setup. The measurement consist in applying anexternal AC magnetic field to the sensor and the measuring the potential difference.
A PCB circuit was fabricated to connect the on-chip sensors to the setup. The current bias is applied bya battery , and the sensor voltage is acquired by spectrum analyzer (Tektronix RSA3308A). The magneticfield that the sensor is measuring can be taken by the Eq. 2.15.
The magnetic field is applied by two Helmholtz coils [53]. The field created is given by:
B(I) =4
5
3/2µ0nI
d) [T], (3.26)
where µ0 is the permeability of the free space (1.26×10−6 T.m/A), n is the number of turns in each coil, Ithe current applied to the coils and d is the radius of the coils and the distance between then. The currentsupplying the coils is generator by wave form generator (Agilent 33210A), generating a AC voltage from4 mVrms - 3.54 Vrms in a frequency range of 0 - 10 MHz. the generator voltage is couple to a resistance(100MΩ) and a current amplifier as a buffer. The current amplifier is necessary to guarantee the currentoutput desired.
22
Chapter 4
Experimental Techniques
In this chapter we describe the deposition techniques (sputtering and ion beam), optical lithography,pattern transference techniques and the characterization tools used during this thesis.
Clean Room
In order to fabricate high performance devices it is required a clean room environment to avoid airimpurities which can interfere with the performance of the device. In INESC-MN there are three areas withdifferent classes [49] and these are:
• Class 10000 (Grey area): Sputtering machines, wet bench;
• Class 100 (White area): IBDs, wet bench and characterization tools;
• Class 10 (Yellow room): Optical lithography system, the SVG tracks and the microscope.
The class number translates the number of dust particles larger than 1 µm per cubic feet of air.
4.1 Sputtering Systems
Sputtering [52] is a physical deposition method whereby atoms are ejected from a solid target material dueto transfer of momentum between the highly energetic ions forming the plasma, and the atoms of a target.
The sputtering occurs in a vacuum chamber, which is controllably filled with an inert gas (e.g. Ar) creatinga plasma. The target is held at a negative bias voltage while the shielding surrounding it is grounded. Thisnegative voltage will accelerate ions present in the gas towards the target, which will remove material thatwill be deposited in the sample. This process is valid only for conductive targets, called DC sputtering. Forthe case of non conductive targets a RF voltage source is needed. More information about the sputteringmachines used can be found in [50][51].
4.1.1 Nordiko 7000
The Nordiko 7000 is an automated system designed to operate with 6 inch diameter wafers and installedin the grey area of the clean room, having the user interface inside the clean room (fig. 4.1). This machineis composed by 4 process modules, a dealer and a loadlock. The loadlock pressure is typically ∼ 5× 10−6
Torr and dealer and modules pressures ∼ 5× 10−9 Torr. The function of each module is illustrated briefly asfollows:
• Module 1: Flash annealing;
23
• Module 2: Soft etch. This etch is performed before the electrical contacts deposition to remove thenatural oxidation of the materials, ensuring a better contact between the stack and the material beingdeposited;
• Module 3: Deposition of Ti12.5W50(N37.5). This is a protective layer which prevents physical andchemical damages to the samples, working at the same time, as an anti-reflective layer for opticallithography;
• Module 4: Aluminum deposition. The metal alloy of Al98.5Si1.0Cu0.5 is used for the electrical contacts.
Figure 4.1: a) Nordiko7000 Interface. b) Machine scheme. c) Typical deposition condition.
4.1.2 Ultra High Vacuum I (UHV I)
The magnetic flux guides are composed of Co93.5Zr2.8Nb3.7 deposited in the UHV I (fig. 4.2). The UHV Iis a home made dedicated DC Magnetron Sputtering, with only one chamber with base pressure of ∼ 5× 10−7
Torr. The mosaic target is mounted on a 4 inch magnetron and the substrate holder includes permanentmagnets to create a field of ∼ 12 mT defining the magnetic easy axis of the film.
Figure 4.2: a) UHV I. b) Deposition support with a magnetic field. d) CoZrNb target. d) Typical depositionconditions.
24
4.1.3 Ultra High Vacuum II (UHV II)
The UHV II (fig. 4.3) is a dedicated RF sputtering deposition system used to deposit Al2O3, required toinsulate the two electrodes in the sensors or to work as a protective passivation layer. UHV II is one chambersystem with the base pressure ∼ ×10−7 Torr. The target is a six inch diameter amorphous Al2O3, placed onthe top part of the chamber near a 4 inch diameter magnetron, this allows a confined and uniform deposition.
Figure 4.3: a) UHV II. b) Machine scheme. c) Typical deposition conditions.
4.1.4 Alcatel SCM450
The Alcatel SMC450 system is composed by a single deposition chamber with a base pressure of ∼ 5× 10−7
Torr. The system incorporates four targets mounted on magnetrons (4 inch), four substrate holders (4 inch)with rotation capability up to 4 rpm and three independent shutters. This system was used for Co66Cr16Pt18(permanent magnets) and SiO2 (passivation layer) deposition.
Figure 4.4: a) Alcatel SCM450. b) Machine scheme. c) Typical deposition condition. d) CoCrPt target.
25
4.2 Ion Beam Deposition
The Ion Beam Deposition (IBD) technique uses a highly energetic ions created by a deposition gun, focuson a grounded metallic or dielectric target. The plasma is created in the gun, distant from the targets allowingdeposition pressures one order of magnitude lower than sputtering. Most IBD systems also use a secondaryion source (assist gun), acting directly on the substrate to deliver energetic noble or reactive ions to thedepositing material, consequently improving the quality of the deposited films. The assist gun is also usedfor ion milling etching, using a high energy ions bombardment to remove material with a controlled etchingrate. When insulating materials are deposited, a neutralizer is used to cancel the accumulation of ion chargecreated on top of the insulating target.
At INESC-MN two IBD systems are used: Nordiko 3000 and Nordiko 3600 (fig. 4.5). Both machines areused to deposit thin film multilayers and to perform highly uniform etch. More information can be found in[50][51].
Figure 4.5: a) Nordiko 3600. b) Machine operation scheme. c) Typical deposition conditions of some materials.
4.3 Optical Lithography
Direct write laser (DWL) optical lithography is the core of our microfabrication process, being the techniqueused to apply the design patterns into the sample.
This process starts with the masks design necessary for each fabrication step/layer to complete thefabrication, the design is made using AUTOCAD software [fig. 4.6.a)]. Having the designs concluded, thesample goes through the next steps till reaching a mask with the desired pattern:
1. The sample is submitted to a vapor prime step to improve the surface adhesion of the photo-resit;
2. Then it is coated with a ∼ 1.5 µm thick of positive photo-resist in a Silicon Valey Group (SVG) track[fig. 4.6.b)];
3. The pattern is written into the resist using a DWL 2.0 direct write laser system, fig. 4.6.c), using a 440nm NeAr laser. When exposed to specific wavelength the photo-resist becomes soluble;
4. Finally, the samples passes throught a developer at the SVG track.
The minimum feature size is limited by the size of the laser spot (0.8 µm) and the alignment precision islimited by the stage resolution of 0.1 µm.
26
Figure 4.6: a) Example of an Autocad design. b) SVG track. c) DWL.
4.4 Pattern Transfer
Lift-off
In a lift-off process, the substrate is covered by a mask with the desired design. Afterwards, the materialto be patterned is deposited over the mask, then the sample is submersed in a remover. Therefore, the maskis removed taking with it the material that was on top. The result is a substrate with patterned materialcovering the area previously non-occupied by the mask.
Although this technic is very good to pattern over existing stack, it has the drawback of having roughedge features (rabbit ears) [52].
Etch
The etch technique, starts with a substrate where the material to be patterned is already deposited over it.Following this, the substrate is covered with a mask of the wanted design. After the covering, the materialwhich is not protected by the mask is then removed through a physic or chemical process. Finally, the mask isremoved leaving a substrate with the patterned material covering the area previously occupied by the mask.
This approach usually leads to sharp features but requires a method of implementation and has thedifficulty to control the stopping point. There are several methods used to remove the material [52], duringthis thesis were used: ion milling (IBD’s) and reactive ion etch (LAM).
Figure 4.7: Illustration of a Lift-off and Etch steps.
4.4.1 Ion Beam Milling
The main etching method used in this work was the physical ion milling etch. It was done both in Nordiko3000 and Nordiko 3600 systems. This etch is neither selective or anisotropic. Figure 4.6 summarizes theetching conditions for N3000 and N3600.
27
Figure 4.8: Etching conditions in Nordiko 3000 and Nordiko 3600.
4.4.2 Reactive Ion Etching
Reactive ion etching (RIE) was performed in a LAM Rainbow 4400 system (fig. 4.9). RIE combines aphysical and chemical etching processes using a chemically reactive plasma to remove the material showing agood selectivity and anisotropy. The sample is placed inside a grounded vacuum chamber, where the plasmais created and its ions are directed towards the sample . The choice of the plasma species is chosen accordingto the material to be etched, so that the ions react chemically with the materials on the surface of the samples.For etching SiO2 the sulfur hexafluoride is a common choice. Due to the kinetic energy of ions, they alsoextract material by transfer of moment, as in the sputter etch process. The etching rate is higher in RIE thanin sputter etch or milling.
Figure 4.9: a) LAM Rainbow 4400 system picture. b) Typical etching condition.
4.5 Characterization Methods
4.5.1 Profilometer
The profilometer (Dektak 3030 ST Veeco) [fig. 4.10.a)] was used to measure the thickness of a thin filmby measuring the step height or the trench depth. The measurement is a surface contact technique with apiezoresistive sensor, where a very low force stylus is dragged across the surface. The vertical resolution is5 A, however due to environment noise a thickness higher than 300 Ais required. The lateral resolution islimited by the tip dimensions typically about 20 µm.
4.5.2 Ellipsometer
The ellipsometer (AutoEl) [fig. 4.10.b)] is used to determine the refraction index and dielectric thicknessof the film. A monochromatic and collimated beam incides in the sample with a known angle and wavelength.The transparent film such as an oxide or a nitride must be deposited on an opaque substrate such as Sifor total reflection. The differences between the polarization state of the incident and reflected beams arequantified by a numerical model to obtain the refraction index and thickness.
28
Figure 4.10: a) Profilometer picture. b) Ellipsometer picture.
4.5.3 Vibrating Sample Magnetometer (VSM)
The VSM is a commercial DMS model 880 system (fig. 4.11) used to study the magnetic behaviour of thinfilm samples. This field is generated by two electromagnets and can reach a maximum of 1.3 T (small gap).The VSM has a field resolution of 0.01 mT and a sensitivity down to 10−5 emu/cm3.
Figure 4.11: a) VSM picture. b) Typical VSM curve measurement of a MTJ [stack: Si/Ru 400/Ta 150/Ru 20/ Ta90/NiFe 50/MnIr 90/CoFe 40/Al 10+20’’/CoFe 30/NiFe 40/Ta 250 (A)].
4.5.4 Transport Measurement Setup
In order to characterize the electrical transfer curves of devices a home made setup developed at INESC-MNwas used. The setup allows the measurement of sensors already integrated on a chip carrier, or as processedsamples through the use of micropositioners probes with tungsten needles. The measurement can be doneusing 2 or 4 probes.
The applied magnetic field during the transfer curve measurement is created by two Helmholtz coils,powered by a dc current source Kepco BOP 50-4 D 4-quadrant current supply (±4 A), creating a maximumfield of ±14 mT. The sensor current bias is applied by a Keythley 220, with a 1 pA - 100 mA range limit.The voltmeter is a Keythley 182 with 7 digits resolution and 3 mV minimum scale and an input resistance ofabout 100 . All the setup components are connected to a computer through a GPIB, and a software developedin INESC-MN controls the measurements automatically. Two shunt switches are connected in parallel tothe current source and the voltmeter, preventing any damage to the sensors by charge accumulation at theprobes. Figure 4.12 shows the measurement setup and typical measurement.
29
Figure 4.12: a) Transport Measurement Setup system picture. b) Typical TMR curve measurement of a SV [Stack:glass /Ta 20/NiFe 25/Ru 8/NiFe 15/CoFe 20/Cu 20/CoFe 30/Ru 8/CoFe 20/MnIr 60/Ta 20 (A)].
4.6 Micromachining System
The micromachining system (fig.4.13) is a computer control Micro Mill machine associated to a highfrequency motor TAIG (5 to 50 Krpm). This machine supports drill/mill tools with diameters ranging from0.1 mm to 3 mm . The Micro Mill machine is controlled using a program called Mach2Mill. The desireddesign is created in AutoCad and converted to the machine language by a program called DesKAM2000,where the action to perform (contour, pocketing or drilling) can be chosen together with the depth.
The tools need to be changed manually and the zero height position defined by the user. During the micromachining it is crucial to cool the tool when dealing with polymer plates otherwise they will melt and fuse tothe tool damaging it. This machine was used for manufacturing the Helmotz coils and is supports for the lowfield detection setup, all this pieces were build with Poly(methyl methacrylate) (PMMA).
Figure 4.13: Micro Mill tool installed at INESC-MN inside a plastic box for protection; a) The software Mach2Millcontrols the tool with precision; b) Micro mill tool and x, y stage; c) spindle to control the rotation speed of the motor.
30
Chapter 5
Sensor Design
In this chapter the microfabrication steps employed to achieve the sensor design for pT detection aredescribed. For more details the run sheet used during this process are presented in the Annex B, withrespective optimize time flow in Annex C and in Annex D is showed one of the masks layout. Figure 5.1shows a 3D scheme of the final sensor design.
Figure 5.1: Sensor 3D scheme.
5.1 Stack Deposition
The first step in this fabrication is the deposition of the MTJ structure on Si. This step was performed atINL[55] by sputtering in a Singulus Timaris tool. The stack is composed by (fig. 5.2): Si/ SiO2 /Ta 50 /CuN500 /Ta 30 /CuN 500 /Ta 30 /Ru 50 /IrMn 75 /CoFe30 20 /Ru 8.5 /CoFe40B20 26 /MgO ∼ 10 /CoFe40B20
30 /Ta 2.1 /NiFe 160 /Ta 100 /CuN 300 /Ru 70 (thickness in A). To define the anisotropy axis of the pinnedand the free layer (parallel) a magnetic field of 100 Oe was applied during deposition and afterwards annealingwas made, in vacuum, at 360C for 1 hour under a magnetic field of 8 kOe.
The bulk properties of the sample was characterized at INL, using a CIPT Capres tool measuring theTMR and R×A. The free layer in this structure (CoFe40B20 3 /Ta 0.21 /NiFe 16) was optimized for reducedcoercivity (Hc) and ferromagnetic coupling (Hf), with the values of measured in VSM of Hc= 0.38 mT, Hf =0.4 mT [54].
31
Figure 5.2: Example of a MTJ Stack from INL.
5.2 Bottom Contact Definition
In this step the bottom contact (BC) is defined. First, the photo-resist is patterned according to the firstlayer of the design and then the sample is etched with an angle of 70 by ion milling in N3000 or N3600,removing the entire stack down to the substrate. Finally the sample is submitted to resist-strip (fig. 5.3).
Figure 5.3: a)Bottom contact scheme b)Bottom contact picture
5.3 Tunnel Junction Pillar Definition
The tunnel junction pillars are defined by performing a 2nd lithography, followed by ion milling etch. Thisetch is performed to remove the material down to about half of the IrMn layer, thus ensuring that entirepinned layer has the same geometry of the free layer, important to get the desired output. This step is verycritical because if the etch does not cross the barrier the sensor will be in a short circuit, but if the etch is toexcessive the BC can be over-etched. The sample is etched at 70 until the barrier, and after at 40 to avoidmaterial redeposition, that can cause short circuit.
After the definition of the pillars an insulating layer of ∼ 1000A of Al2O3 is deposited in the UHV II. Thislayer insulates the pillars and the BC, guaranteeing that the electrical current will only flow through thebarrier. Finally the sample is submitted lift-off of the oxide (fig. 5.4).
32
Figure 5.4: a) Pillar junction scheme, b) Pillar junction picture.
5.4 Flux Guides Deposition
The next step is the deposition of the flux guides (FG). It starts with a third lithography step followed bya deposition in the UHV I ∼ 6000A of CZN. To improve the CZN adhesion, the sample is submitted to a 30second etch to clean the surface and a deposition of 10 A of Ta in the IBD, thus avoids undesired peeling ofthe CZN. Afterwards the lift-off is done leaving the designed wanted for the FG.
Figure 5.5: a) Flux guides scheme, b) Flux guides picture.
The CZN in this thesis presents a typical magnetic behavior shown in fig. 5.6.a). In this thesis the FGwere design to achieved the gain in field of ∼ 19− 20×, and was patterned with the dimensions [fig. 5.6.b)]:
• Entrance width of A1 = 500 µm;
• Pole-width(tip) equal to the length of the sensor pillar, A2 = 10 - 60 µm;
• Total length of L = 250 µm;
• Gaps ranging from g = 5 - 60 µm.
Figure 5.6: a) CZN hysteresis loop b) FG schematic.
33
5.5 Permanent Magnets Deposition
This step starts with a fourth lithography and then is deposit ∼ 1500A of CoCrPt in Alcatel. The CoCrPtis going to form the permanent magnets that are very important to force the linearization of the sensor.Afterwards the lift-off of the CoCrPt was performed (fig. 5.7).
Figure 5.7: a) Permanent magnets scheme, b) Permanent magnets picture.
Typical magnetic behavior of the CoCrPt is shown in the fig. 5.8.a). The moment of the PM has to bealigned in the longitudinal direction (at 90 of the measurement direction) for that was used a magnate with1 T. The PM was patterned with dimensions [fig. 5.8.b)]:
• Width of W = ∼ 60 µm;
• Length of L = ∼ 460 µm;
• Gap of G = ∼ 30 µm;
• Thickness of t = ∼ 1200 A.
Figure 5.8: a) Hysteresis loop of a CoCrPt thin film B) Schematic of the PM.
5.6 Second Contact Definition
After the deposition of the flux guides and the permanent magnets, is necessary to deposit the top contacts,but to prevent a short circuit between the bottom and the top contact a passivation layer is deposited. Thepassivation layer is ∼ 3000A SiO2, the reason for such a thick layer is because of the sensor topography. Afterthe SiO2 deposition is necessary to open the contacts, for that the fifth lithography is performed and thenreactive ion etch (LAM) is chosen to ensure that only the SiO2 is removed (fig. 5.9). Afterwards the sampleis submitted to resist strip.
34
Figure 5.9: a) Second contact scheme b) Second contact picture.
5.7 Top Contact Deposition
This step starts with the sixth lithography and after the top contact is deposited in N7000 of ∼ 1500 A ofAl, plus ∼ 150 A of TiW(N) as protective layer. After the lift-off we reach to a working sensor (fig. 5.10).
Figure 5.10: a) Top contact scheme b) Top contact picture
5.8 Final Passivation
The goal of this step is to protect the sensors from oxidation and physical damage, for that an oxide layeris deposited, leaving open pathways to the pads of the sensors electrodes. To achieved that photo-resist ispatterned over the pads and a layer of Al2O3 is deposited with the thickness of ∼ 1000 A. After performingthe lift-off of the Al2O3 the sensor fabrication is conclude.
35
Chapter 6
Results
To achieve the detection of low magnetic fields (∼ pT), the design described in chapter 5 was chosen.During this thesis two distinct process with good results ware obtained.
6.1 First Process
The first fabrication process was made with the stack TJ39. This stack was composed by [fig. 6.1.a)]: Si/SiO2 /Ta 50 /CuN 500 /Ta 30 /CuN 500 /Ta 30 /Ru 50 /IrMn 75 /CoFe30 20 /Ru 8.5 /CoFe40B20 26 /MgO∼ 10 /CoFe40B20 30 /Ta 2.1 /NiFe 160 /Ta 100 /CuN 300 /Ru 70 (thickness in A). CIPT measurementshowed a TMR ∼ 200 % and R×A = 0 - 1400 Ω.µm2 [fig.6.1.b)]. This considerable variation of R×A comesfrom wedge deposition of the MgO.
Figure 6.1: a) Scheme of the MTJ Stack - TJ39, b) Variation of TMR and R × A along the wafer.
In this process the goal was to study the influence of some components of the sensor, and to optimize itto reach the best detectivity. The MTJ sensors were patterned with different designs varying, the junctiondimensions and with or without FG and/or PM and the size of second contact . With all this components isimportant to study:
• The influence of the tunnel junction dimension.
• The linearization achieved by PM.
37
• The increase in sensitivity by FG.
• The influence of the second contact.
After the sensors being patterned, a TMR dispersion versus R×A trend was obtained(fig.6.2). An averagevalue of the TMR ∼ 141% and for the R×A ∼ 112Ω.µm2 were obtain. This R×A value is going to beconsidered as this sample bulk R×A.
Figure 6.2: TMR vs R × A dispersion for the first process with stack TJ39.
Tunnel Junction Dimensions
The tunnel junctions were pattern in a rectangle shape with a constant length (L = 20 µm) and widthranging from h = 2, 3, 4, 5, 6, 10 µm. Figure 6.3 shows the behavior of the transfer curve with differentsensors widths.
Figure 6.3: Variation of the transfer curve for MTJs (stack TJ39) with rectangle shape with length and different withwidth (h).
38
The linearization for this type of stack was achieved by setting the PL and FL magnetization orthogonal toeach other (section 2.2). One way is to guarantee L >> h when patterning, causing a strong shape anisotropy(N= h
L ) induced by the elongated design, forcing the FL magnetization to lie along the length of the sensorand thus minimizing the energy of the system. The demagnetizing field of the FL (Hf
d) layer created whenpatterned, for L >> h, is given by [57]:
Hfd = 4πMf
s
tf
h, (6.1)
where Mfs is the saturation magnetization of the FL and t the thickness of the free layer.
For the sensors with lower width (h = 2 µm) a linear response was obtained with low coercivity of Hc =0.03 mT, meaning a coherent rotation of the free layer. For the sensors with the higher widths, was obtaineda transfer curve with high coercivity Hc = 0.4 mT and with visible jumps. In this case the magnetic responseis governed by mix of linear rotation and domain wall motion, which is irreversible and hence hysteretic[56]. For sensor with this dimensions is visible the importance of a lower width and consequently the highdemagnetizing field to achieve the pretended linear behavior. Figure 6.4.a) shows the coercivity of the sensors,presenting a linear tendency to increasing with the width. This behavior comes from the demagnetizing fielddecreasing with the width, causing the rotation of the FL less coherent and causing a coercivity increase.During this thesis we going to consider as one of parameter necessary for a sensor be linear to present acoercivity lower than 0.2 mT.
The sensitivity of the sensor is defined in the linear zone of the transfer curve by Eq. 2.11 and since for anideal sensor the 4H ' 2×Hsat ' 2[Hk + Hd]. Then the sensor sensitivity can be rewritten as [57]:
S ' TMR
4H' TMR
2[Hk + Hd]∝ TMR× h. (6.2)
As expected by the result given by Eq. 6.2 the sensitivity shows a linear tendency to increase with thewidth of the junction [6.4.b)], for the cases where L >> h (N < 4). In the case when L ≈ h the relation givenby the Eq. 6.2 can not be applied.
Figure 6.4: Variation of the coercivity [a)] and sensitivity [b)] for MTJs (TJ39) with the same length and varying thewidth dimensions.
Influence of the Permanent Magnets in Linearization
The linearization of a sensor can be achieved by applying a perpendicular field to the easy axis of thefree layer. In this work this was accomplished by PM integration. The PM were pattern with the followingdimensions: length of L = 463 µm, width of W = 40 µm, gap of G = 24 µm and thickness of tPM ∼ 1000A. The PM were dimensioned to create a bias field at the center of the magnets of HLB ∼ 1 mT. Figure 6.5shows the transfer curves for the same dimensions with and without PM integration.
39
Figure 6.5: Transfer curves with and without the influence of the PM for MTJs (TJ39) with different width dimension
In the case of sensors with PM Hsat can be describe as Hsat ' 2[Hk + Hd + HLB], where HLB is the externalfield created by the PM. In this case the sensor sensitivity can be rewritten for this case as:
S ' TMR
4H∝ TMR
4πMfstf
h + Hex
. (6.3)
As result of Eq. 6.3 the sensors with integrated PM will have a lower sensitivity [6.6.b)], taking also alinear tendency to increase with the width. For the sensor with a lower width (h = 2 µm), the field applied inthe sensor is only reducing the sensitivity, since the sensor was already linear with only the demagnetizingfield, being for this case counterproductive the PM integration. In the other cases a reduction in coercivity isvisible, meaning that the field applied by the PM is favoring a more coherent rotation. For higher widths (h= 5 µm and h = 6 µm) is possible to see the elimination of the non-coherent jumps (Barkhausen), sowing alinear transfer curves with integrated PM.
With the addition of PM there is a reduction in the coercivity [fig. 6.6.a)], this comes directly from a betterlinearization of the sensors with the field created by the PM. There is a higher reduction of the coercivity forthe higher widths, due to an higher the influence by the PM field since a lower demagnetization field.
Figure 6.6: Variation of the coercivity [a)] and sensitivity [b)] with and without PM, with the width of the tunneljunction.
40
Influence of Flux Guides in the Increase Sensitivity
The FG are used to increase the sensitivity of the MTJ by concentrating the field in the sensitive part ofthe sensor. The FGs were patterned with design and dimensions similar to showed in the section 5.4. Inthis case were patterned with different tips dimensions: A2 = 10 or A2 = 20 µm and the gap between theFG where the width of sensor + 4 µm. This FG was dimensioned to achieve a gain in sensitivity of ∼ 20×.Figure 6.7 shows the effect of the FG in the transfer curves:
Figure 6.7: The influence of Flux Guides in the transfer curves
The FG integration in the sensor produce a more steep transfer curve corresponding to a an increase insensitivity. Figure 6.8.a) display the sensitivity with and without FG for sensor with different dimensions,showing a shapr increase in the sensitivity with FG addition. The sensor with tip A2 = 20 µm (equal to thelength of the FL) displays a higher sensitivity then with the tip A2 = 10 µm.
The gain in the sensitivity resulting from the FG integration decreases with the width of the sensor [fig.6.8.b)], because the gap between FG also increases leading to an accentuated dispersion in field lines. Isnoteworthily that fluctuation in the sensitivity gain provided by the FG was register, because it depends in agood alignment of the FG with the tunnel junction that can have variation along the sample.
Figure 6.8: a) The MTJ’s sensitivity with and without FG, b) The gain in sensitivity with different FG.
Second Contact Influence
The goal of using the second contact is to increase the resistance of the junction keeping a big area. Figure6.9 shows a comparison between transfer curves of sensors with the same characteristics with and withoutsecond contact.
41
Figure 6.9: The second contact influence in the transfer curves, for sensor with the same characteristics: without FG[a)] and with [b)] (TJ39)
The increase of the resistance results from strangulation of the contact and can be tuned by changing thearea of the second contact. Was observed a deterioration of TMR by using this method because of a TMRdilution with the contact resistance.
With this we can conclude that this is not a reliable way to increase the resistance of a MTJ,for the stackswith intrinsically low R×A.
Detectivity Results in the First Process
In this run two sensor with high sensitivities was obtained, namely MTJ A and MTJ B. MTJ A had thecharacteristics of: tunnel junction dimension L = 10 × W = 10 µm2; FG tip A2 = 10 µm, and PM withdimensions described before. And MTJ B had the characteristics of: tunnel junction dimension L = 20 × W= 10 µm2; FG tip A2 = 20 µm, and PM with the same dimensions. This MTJ’s present a magnetic responsedisplayed in the fig. 6.10:
Figure 6.10: The transfer curve of MTJ A (a) and MTJ B (b)
The high sensitivity for this MTJs result from a good linearization by the PM combined with FG. MTJ Afor the bias voltage of 5 mV presents a TMR = 164 % and a maximum sensitivity of S = 784 %/mT. MTJ Bpresents a maximum sensitivity of 735 %/mT with a of TMR = 148 %.
For this MTJs where a good sensitivity was obtained a more detail study was performed by measuring thenoise of each one as can be seen in fig. 6.11.
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Figure 6.11: Noise of MTJ A (a) and MTJ B(b)
Figure 6.11.a) shows the noise spectrum for the MTJ A, presenting a Hooge constant of αH ∼ 2.38×10−9
µm−2. The theoretical value for thermal and shot noise (5 mV) for this MTJ, at H = 0, is SWNB = 4.36×10−20
V2/Hz. The noise spectrum does not reach the white noise baseline because the 1/f noise component is stilldominant at this frequency spectrum. When analyzing the noise spectrum of a device is necessary to becareful when the data lower then the amplifier noise. Is visible that when the noise spectrum is lower thenthe amplifier noise the measurement gets with more interferences.
In the noise spectrum of the MTJ B [fig. 6.11.b)] a Hooge parameters of αH ∼ 4.29 ×10−9 µm−2 wasobtained. The value calculated for the white noise is SWNB ∼ 1.99 ×10−20 V2/Hz, the noise spectrum alsodoes not reach this value because as it was explain before the 1/f noise component is still dominant.
In both curves the predominant source of noise is the 1/f noise contribution, for our application importantto reduce this contribution in order to achieve the lowest noise possible at low frequencies, being necessary toincrease the area of the tunnel junction. The MTJ B has a lower noise then MTJ A resulting mostly fromlargest area implying a lower noise at low frequencies given from 1/f noise equation (Eq. 3.5). The whitenoise is also lower consequence of a lower resistance. The spikes that appears at high frequencies results fromintrinsic problems from the noise setup, not interfering with the measurements.
Is important also to refer that in MTJ B is visible a RTN noise for the voltage bias of 20 mV, presentinga bump in the noise spectrum.
Knowing the noise spectrum is possible to take the detectivity limit spectrum for each sensor, this resultsare showed in the fig. 6.12:
Figure 6.12: Detectivity of MTJ A (a) and MTJ B(b)
43
With MTJ A the best results obtained was with the bias voltage of 20 mV, where the detection limitis ∼ 8 pT/
√Hz at 100 kHz and by interpolation the predict result is ∼ 318 pT/
√Hz at 10 Hz and ∼ 802
pT/√Hz at 1 Hz.
For MTJ B an improved detectivity was obtain due too a lower noise. Reaching detectivities of ∼ 4pT/√Hz for 20 mV bias voltage. For low frequencies, with the bias voltage of 5 mV detectivities of ∼ 160
pT/√Hz and ∼ 352 pT/
√Hz at 10 Hz and at 1 Hz was estimated, respectively.
The detectivity obtained were much higher then the desired (∼ pT), being necessary to improve furtherthe detectivity by optimizing the components of sensor.
Summary
In this run the first conclusion is that the sensors linearization has to be carefully study to achieve a linearrotation of the free layer without losing sensitivity. The combination of aspect ratio and the field created bythe PM has to be optimize to reach higher sensitivities.
The use of the FG to increase the sensitivity is important, to be able to reach the desired detectivities.Since the results are what was expected for the FG, the design do not need to be amended.
The second contact was proved unnecessary and counter-productive, so it was excluded from the nextprocess.
The noise spectrum at low frequencies, were higher then the pretended values and consequently lowerdetectivities were obtained. Principally the contribution of the 1/f noise is essential to reduce, for that isnecessary to increase the area of the sensors and this can only be viable if the R×A of the MTJ stack islarger, since the the sensors obtained in this Run had a to small resistance.
In this process the best result obtain was for the MTJ B, with the characteristics of having a rectangletunnel junction with the area of 200 (10×20) µm2 combined with PM and FG, presenting a sensitivity of 735%/mT and reaching a detectivity at 10 Hz of 160 pT/
√Hz.
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6.2 Second Process
A second generation of sensors were microfabricated with two samples from the same stack (TJ377), thisstack had the same magnetic layers [fig. 6.1.a)] as the previous one, but with a thicker MgO barrier andconsequently having a higher R×A. The sample in bulk was measured by CPIT, exhibiting a TMR ∼ 206%and R×A ∼ 7600Ω.µm2.
Figure 6.13 shows the TMR dispersion versus R×A, obtained for both samples an average of < TMR >∼ 196%. The samples presented a difference in the R×A average, obtaining for the sample 1 a < R×A >∼ 9020 Ω.µm2 and for the sample 2 a < R×A > ∼ 6495 Ω.µm2.
Figure 6.13: TMR vs R × A dispersion for the second run
Pattern
The MTJ’s were patterned in with tree different tunnel junction shapes: circles, squares and rectangles.For each shape sensors with different dimensions were defined. In the table showed in the fig. 6.14 is describedin detail the different tunnel junction dimensions, the PM and FG gap dimensions. The other dimensionsof the FG were similar for the first process, with the width of the tip equal to the length of the sensor andthickness of ∼ 7000 A. The dimensions of the PM were: width 80 µm, length 450 µm, and thickness of ∼ 1200A. This PM magnets were designed to create a higher field then in the previous process, being dimensioned tocreate a field of ∼ 3.5 mT in the center of the PM for the smallest gap, this value decreases with the PM gap.The higher field created by the PM was because an higher magnetization of the material and also a higherthickness.
Figure 6.14: Table with the different characteristics of the sensors patterned.
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Results for the MTJs with a Circle Shape Tunnel Junction
Circle shape MTJs sensors were only linearized by the PM (fig. 6.15). This transfer curves present a linearresponse, with no jumps low coercivity and good sensitivities. The Rmin of the transfer curves ranges from 9 -77 Ω depending the area of the tunnel junction, and they show a shift of the transfer curve center in about ofHf = -0,57 mT.
Figure 6.15: Two exemplificative transfer curves with circle shape tunnel junction.
Figure 6.16 shows the coercivity and sensitivity versus tunnel junctions area. This MTJ sensors present alow coercivity [Hc < 0.2 mT] with no clear tendency for the behavior of the coercivity with the area increase.
In the case of the sensors sensitivity a tendency was registered with the increase of the tunnel junctionarea. In circle shape tunnel junctions the demagnetizing field is null because there is no shape anisotropybeing the PM the only linearization agent, and consequently the sensitivity inversely proportional to thefield created by the PM (Eq. 6.3). For the MTJs with larger areas, the gap between the PM increases thusdecreasing the field created and can be the reason for an increase in sensitivity.
Is noteworthy that sensitivities up to 150 %/mT was observed for the larger areas. This is a very goodsensitivity taking in consideration that such is only achieve from the optimization of FL linearization.
Figure 6.16: Coercivity [a)] and sensitivity [b)] Vs the area of the junction, for circle shape tunnel junction.
Further analysis with this shape were not possible because with the FG addition in the transfer curve wasvisible discontinuities, making the systematic analysis of the sensitivity were not consistent.
46
Results with Square Tunnel Junctions
The MTJs sensors with the square shape presented the transfer curve like the ones showed in fig. 6.17.The majority had a linear response with low coercivity and jumps were not observed. The Rmin ranged from5 - 44 Ω depending the area, the center of the transfer curve were shifted Hf = -0,71 mT, in average.
Figure 6.17: Two examples transfer curves with square shape tunnel junction.
Figure 6.18.a) shows the coercivity for sensors versus the area. Was obtained sensors with low coercivityHc < 0.2 mT and no visible tendency for the coercivity with the tunnel junction area increase.
The sensitivity of the MTJs presented a tendency to increase with the junction area [fig. 6.18.b)]. Thiscan be explain with similar as in the case of circle MTJs, because of the PM gap increase with the area of thejunctions therefore creating a lower field. With this MTJs we were able to reach sensitivity up to 200 %/mTfor the larger areas.
Figure 6.18: Coercivity [a)] and sensitivity [b)] Vs the area of the junction, for square shapes tunnel junction.
As for the circle shape MTJs further studies with this sensors were not possible to perform because thesensors with FG addition presented transfer curves with discontinuities. Figure 6.19 shows transfer curve ofMTJs with the same characteristic with and without FG. Both curve seem to present linear transfer curves.However a more precise transport measurement for sensors with FG, with steps of 0.01 mT and zooming thelinear part [fig. 6.19.b)] is possible to see jumps, and having a non-linear behavior. This jumps can arise froma deficient linearization by the PM combined with coupling domains of the FG with the FL of the sensor.
47
Figure 6.19: a) Square tunnel junction with a without FG, b) Zoom of the linear part of the transfer curve
Results for the MTJs with Rectangle Tunnel Junction
Figure 6.20 show two examples of the transfer curves obtained for the junction with rectangle shape. Thisjunction present a linear magnetic response with no discontinuities and low coercivity. The Rmin of thisMTJ’s range from 5 Ω to 44 Ω according to the tunnel junction area and there is a shift of the transfer curvecenter in average of Hf = -0,66 mT.
Figure 6.20: Two exemplificative transfer curves with rectangle shape.
As for the other shapes this sensors presented low coercivity (Hc < 0.2 mT) and no relation was registerwith the variation of the tunnel junction dimensions [fig. 6.21.a)].
For the sensitivity as can be seen in fig. 6.21.b), the MTJ show a tendency to increase the sensitivity withthe width (h) for the same length (L), this tendency can be explain by the difference in the shape anisotropy.In general the sensitivity also increases with the area, as was observed for the other shapes, because of thePM gap increase. For L = 50 µm a clear difference in sensitivity between samples, was visible mainly due tothe uniformity in the PM deposition.
For this sensors, the sensitivity reached values larger 200 %/mT in the larger areas. This values ofsensitivities are very good taking in consideration arises only from of a good linearization.
For this shape there was no visible discontinuities even with the addition of the FGs. The majority ofsensors presented a linear transfer curve, making possible to continuo the study, in sensitivity increase by theFG addition and also the noise measurement and correspondent detectivity spectrum.
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Figure 6.21: Coercivity [a)] and sensitivity [b)] Vs the width of the junction, for rectangle shapes.
MTJs with Rectangle Shape and Flux Guides
Figure 6.22 shows a transfer curve for two sensor with the same dimensions with and without FG. As canbe seen in the figure there is large increase in the sensitivity of ∼ 19 ×. With FG the sensor present similarresistances than without.
The TMR response of the sensors with FG have more coercivity, it is not clear the origin of this hysteresisand how it can be further reduced, but probably arises from the coupling of the domains of the FG with thesensor causing hindering the FL reversal [58].
Figure 6.22: Exemplificative transfer curve of a MTJ sensor with and without FG.
Figures 6.25.a) shows the increase in the sensitivity provided by the FG, having a gain of 14-26×. Figure6.25.b) shows a detail view of sensors sensitivity with FG. In this case a sensitivity of more then ∼ 2000%/mT was obtained. As for the MTJs without FG, is visible a tendency in the sensitivity increase by thewidth (h) for the same length of the sensor.
Figure 6.23: Sensitivity Vs the width of the junction, for rectangle shapes with FG.
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Noise Studies of the Second Process
The noise measurement were perform the linear sensor. Figure 6.24 shows the noise measurements ofthe MTJs presented in fig. 6.22. Figure 6.24.b) shows the noise spectrum from 400Hz to 100kHz, where ispossible to see the noise reaching the white noise baseline. For senors with FG the 1/f noise corner was at ∼2 kHz and ∼ 9 kHz for the a voltage of 5 mV and 15 mV, respectively.
The Hooge parameter in the linear region was similar for both sensors (αH ∼ 1.6×10−8 µm−2). Measuringthe noise with a magnetic field applied to saturate the sensor, a Hooge constant for the component of 1/felectric was αHeletric ∼ 7, 5 × 10−10 µm−2, and consequently the magnetic component for 1/f magnetic isαHmagnetic
∼ 1, 53× 10−8 µm−2. This allow us to conclude that the magnetic component of the 1/f noise isdominant and thus reducing is contribution is of major importance.
Notice that there was no evidence for this case, that the addition of the FG increased the contribution ofthe magnetic noise.
Figure 6.24: Noise for the sensor presented in fig. 6.22.
Figure 6.25 shows the the detectivity spectrum of the noise measurement. In the figure we can see thedetectivity at 10 Hz for each sensor. The sensor without FG shows a detectivity of ∼ 1.8 nT/
√Hz and with
FG of ∼ 94 pT/√Hz. With extrapolation we can take the value at 1 Hz obtaining for the sensor without FG
of ∼ 5.7 nT/√Hz and for the sensor with FG of ∼ 296 pT/
√Hz.
At high frequencies (> 100 kHz) the detectivity values obtained are: ∼ 50 pT/√Hz, without FG and ∼
3.6 pT/√Hz for the sensor with FG.
Comparing both sensors there is a detectivity increase of ∼ 19 × because of the sensitivity increase fromthe use of FG.
Figure 6.25: Detectivity for the sensor presented in fig. 6.22.
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Result in Detectivity
The best result in the detectivity reached was for a sensor with the the dimensions of: length L = 50 µm,width W = 30 µm, combined with PM and FG. Figure 6.26 shows the magnetic response of this sensor thatpresent a TMR of 206% with Rmin = 5,6 Ω and a maximum sensitivity of 1860 %/mT.
Figure 6.26: TMR curve for the sensor with best result in the detectivity
Measuring the noise spectrum for this device we obtained the graphics presented in fig. 6.27. The Hoogeparameter obtained for this sensor was αH ∼ 3.2 × 10−8 µm−2. Is noteworthily that for the 5 bias voltagethe sensor present a 1/f corner at ∼ 1.2 kHz, this corresponds to a good evolution in the reducing the 1/fnoise contribution in this sensors.
Figure 6.27: Noise spectrum of the sensor with best result in the detectivity
Finally, the detectivity can be calculated as showed in the fig. 6.28. The detectivity spectrum presentthe best results for the Voltage bias of 15 mV obtaining at 10Hz the detectivity of 49 pT/
√Hz and at high
frequencies of 3.5 pT/√Hz, by extrapolation is possible to take the value for 1 Hz that is 144pT/
√Hz.
Figure 6.28: Detectivity spectrum of the sensor with best result in the detectivity
51
Summary
In this process the majority of the MTJ presented a full magnetic response with TMR of ∼ 200 %. Thesensors linearized by the PM had a linear magnetic response with low coercivity and few presented jumps.This sensor reach to values in sensitivities up to 200 %/mT with the biggest areas, being the gap between thePM the key factor for an increase sensitivity of sensors indicating field created by the PM for smaller areaswere too high.
The sensors combined with FG and PM presented a more steep magnetic response. In the sensor withcircular and square tunnel junction when combined with the FG the transfer curve presented a non linearmagnetic response. With the Rectangle shape such discontinuities were piratically absent which enabled us tocontinue the analysis. The FG contribute to a gain in the sensitivity up to 24 ×, and being able to reachsensitivities more than 2000 %/mT.
The best detectivity was accomplished for the sensor with an area of 1500 µm2, combined with PM andFG. Showing a magnetic response with TMR of 206 % and a maximum sensitivity of 1860 %/mT. The Hoogeconstant obtained for this sensor was αH ∼ 3.2 × 10−8 µm−2 being the magnetic noise responsible for thishigh value. The 1/f corner for 5 mV was at 1.2 kHz. The best detectivity was obtained for a bias voltage of15 mV, with the result of 49 pT/
√Hz at 10Hz, and at high frequencies of 3.5 pT/
√Hz.
Discussion and Comparison with Previous Work
In a previous work by Chaves et al. [12], using a MgO based MTJ stack: Ta 30/CuN 300/Ta 50/PtMn200/CoFe 25/Ru 7/CoFeB 30/MgO 12/CoFeB 30/Ta 50/TiWN2 150 (A), with a R×A values of ∼ 150Ω/µm2 and TMR ∼ 150 %. The sensor was patterned with a square shape, with an area of 676 (26×26) µm2
and a contact area of 100 (10×10) µm2. This sensor showed a transfer curve with a TMR = 97 % and Rmin= 1.36 Ω, and a sensitivity of 720 %/mT. The noise spectrum for this sensor presented a αH = 3.2 × 10−9
µm2 (linear region) and αH = 8.2× 10−10 µm2 (parallel state). The corresponding detectivity for this sensorwith a current bias of 10 mA has of 97 pT/
√Hz at 10 Hz and for high frequencies (500kHz) of 2 pT/
√Hz.
The results obtained for the detectivity during first process was for a MTJ with a rectangle shape, withan area of 200 (10×20) µm2, presenting a sensibility of 735 %/mT. The Hooge parameter obtained was αH
∼ 4.29 ×10−9 µm−2 and the 1/f noise component was dominant, over the the spectrum measured. Thedetectivity reached 160 pT/
√Hz at 10Hz. The 1/f noise component was larger than and thus is of major
importance to reduce it to achieve lower field detectivities. This was accomplished by increasing the area ofthe junction in a second process being necessary to increase the R×A of the stack.
In the second process higher detectivities was accomplish for a rectangle shape sensor with an area of1500 µm2, combined with PM and FG. Showing a magnetic response with TMR of 206 % and a maximumsensitivity of 1860 %/mT, having a higher sensor resistance also allow us to obtain a full TMR. The Hoogeconstant obtained in this case was αH ∼ 3.2 × 10−8 µm−2 being the magnetic noise responsible for this highvalue. The 1/f corner for 5 mV was at 1.2 kHz being a good advance comparing to results obtained in theprevious process, but reducing this value by increasing the area is still important. Was obtained with a biasvoltage of 15 mV and a detectivity of 49 pT/
√Hz at 10 Hz, while at high frequencies 3.5 pT/
√Hz. Where
observed an increase of two times more in the detectivity comparing with a previous work.
The big improvement during this work was to obtain sensors with high sensitivities, being able to reachsensitivities up to 200 %/mT combining PM. For sensor with combined PM and FG the sensitivities reachedvalues up to 2000 %/mT. This was accomplished in part with a stack with higher TMR but also a goodlinearization by the PM. Comparing to the previous work this represents an increase in sensitivity of almost 3times more.
In the noise part, there was not such a accentuated difference comparing the Hooge parameter in stackwith similar R×A. In the stack with higher R×A there is an increase in this value caused by the higher barrierthickness. This parameter is very important to be reduced, reducing the noise and consequently reach betterdetectivities. Further studies should be perform to optimize the NiFe layer to reduce the magnetic 1/f noise.The suppression of 1 /f noise by increasing the area was better then in a previous work, being able to reachthe 1/f corner at 1.2 kHz, although further improvement can still be accomplished by increasing the area ofthe MTJ. In the noise case, is necessary to make a compromise between low R×A stack and consequentlylower Hooge parameter and large area MTJ and consequently lower that reduces the 1/f noise component.
52
To finalize, figure 6.29 summarizes the different parameters of a sensor that can influence its performance.From this work point of view that targets sensors with extremely high sensitivities and lower detectivitiesat low frequencies, we pretend to choose a sensor with some aspect ratio (to avoid discontinuities with FGintegration), with a large area and decent resistance (Higher that the contact resistance) and with integrationof PM and FG with dimensions carefully chosen to optimize is sensitivity.
Figure 6.29: The effects of each sensor component in is performance
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Chapter 7
Conclusion and Outlook
Most of the work during this theses was in the optimization of magnetoresistive sensors with an ultimatepurpose in the use for biomedical imaging , such as in magnetocardiography which involves the detection ofmagnetic fields down to picoTesla at low frequencies.
The first sample was fabricated with a stack presenting a TMR ∼ 200% and a R×A ∼ 112Ω.µm2. Thesensors were patterned a with rectangle shapes with different widths and integrating PM and FG. In thisprocess was studied the influence of the aspect ratio, the integration of PM and FG and also the noise wasmeasured. The conclusion was:
• Higher aspect ratio causes transfer curvex with higher coercivity and higher sensitivity.
• With the integration of the PM the sensors were better linearized, presenting transfer curve with lowercoercivity and sensitivity. Although with integration of PM and lower aspect ratio we were able toreach to linear sensors with higher sensitivities.
• Integrating FG to the sensor, there was obtained a visible increase in the sensitivity (up to 26 ×),having better results with the tip equal to the sensor length.
• The strangulation of tunnel junction area was counter-productive reducing the TMR of the sensor.
• Measuring the noise was observed that in the entire spectrum the 1/f noise was dominant increasing thedetectivity reached.
With this result we can conclude the necessity of increase the area of the sensor to reduce further the 1/fcontribution. Since the sensors presented low resistance, causing the sensor to not present the full TMR. Sois important to increase the stack R×A to ensure resistance higher then the contact resistance.
In a second process the sensors were fabricated using a stack with a TMR∼ 206% and a R×A ∼ 7.6kΩ.µm2.The MTJs were patterned with different shapes and with PM and FG. In this samples were study: the shapeinfluence, the linearization by PM, the increase in sensitivity by FG and the noise. The main conclusion was:
• The linearization of the sensors by the PM was successfully being able to reach sensitivities up to 200%/mT, increasing with the gap between PM.
• With the FG integration, sensors with some aspect ratio is necessary since with no aspect ratio sensor(circular and square shape) the FG caused non-linear transfer curves. With the rectangle shapesensitivities up to 2000 %/mT was reached.
• Comparing sensors with and without FG, both present similar noise values and similar Hooge constant.Sensors with FG reached higher detectivities due to the higher sensitivity.
• There was observed a reduction in a 1/f noise component obtaining a 1/f corner at 1.2 kHz for thelargest areas.
55
The maximum detectivity demonstrated in this process was for a rectangle shape sensor with the area of1500 µm2. Presenting a tranfer curve with a TMR of 206 % and a maximum sensitivity of 1860 %/mT. TheHooge constant obtain was of αH ∼ 3.2 × 10−8 µm−2 and the 1/f corner at 1.2 kHz. All this combined let usreach a detectivity of 49 pT/
√Hz at 10 Hz and at high frequencies of 3.5 pT/
√Hz. This result is 2 times
better than previous work [11] value but still one order above of the objective (1 pT/√Hz at 10Hz).
MgO MTJ picoTesla field low cost sensors were demonstrated but further optimizations can be performedto reach better detectivities: increasing the sensitivity and reducing the noise. In this thesis was reachedsensitivities up to 2000 %/mT with the combination of FG and PM. This value can be increase by increasingthe FG dimensions. Is also important to be consider the definition of the flux guides by etch that was provedto have a higher sensitivity gains and also the use of multylayer FG since that they present lower noise byreduced domain coupling between the FG and the FL.
The sensors presented a higher noise then the desired at low frequencies due to an higher contribution ofthe 1/f noise, this arises because of is magnetic component, being important the reduce this parameter bystack engineering (controlling the thickness of the FL). To reduce 1/f noise contribution with this Hoogeconstant is necessary to increase more the junction area to be able to reach better detectivities.
Is also important to test the detectivities reached by this sensors by applying a controlled and low field tothen and see lowest field possible to measured, with the setup created during this thesis.
Furthermore work is still important to be done as example to try to achieve this type of sensitivity withdifferent strategies of linearization, using weakly pinned stacks for example. The modulation techniques basedon the MEMS should also be considered.
56
Appendix A
Amplifier Calibration
To obtain the real noise spectrum of the sensor it is necessary to correct the noise measurement obtainedwith the spectrum analyzer (SA). To obtain that is needed to remove both the SA and amplifier noisecontributions.
Femto DPVLA-100-BLN-S (Fetmo)
Taking in consideration the figure B.1 that represents the equivalent electrical circuit of the noisemeasurement setup with the Femto amplifier.
The noise contribution from the amplifier to the measurement can be represented by two sources: voltageand current noise, VnA and InA respectively. If the amplifier input is shorted, VnA is responsible for theamplifier output. If the amplifier input is open then InA is the responsible one. When a finite resistance isplaced at the amplifier input, both noise sources contribute to the amplifier output. The resistance in theinput of this amplifier is RAI = 1 MΩ, and in the output is RAO = 50 Ω.
Figure A.1: Noise measurement setup equivalent electrical circuit using the Femto amplifier
The voltage (VSA) across the SA input impedance (RSAI = 50 Ω) is related to the amplifier input voltageaccording to equation B.1, since the output is a voltage divider.
VSA =RSAI
RAO +RSAIVOut =
RSAIRAO +RSAI
Gain.VIN (A.1)
57
The amplifier has a capacitive coupling in its input, interrupting the DC power making only the ACcontributions the only are taken into account to the VIN . The contribution of each independent noise sourceto the Femto amplifier input voltage is displayed in figure B.2.
Figure A.2: Contribution from each independent noise source to the amplifier input voltage for the circuit describedin Figure B.1.
VIN1 is the noise contribution of the device(VnSUT ), VIN2 is the voltage contribution of the amplifier(VnA)and VIN3 is the current contribution of the amplifier(InA). VnA and InA is measured with the SA with ashort circuit and open circuit in the input of the amplifier respectively. Using the superposition theorem, VINcan be obtained summing the noise power of the contributions from each one of the independent noise voltagesources:
VIN =√V 2IN1 + V 2
IN2 + V 2IN3 (A.2)
Combining the equation B.1, the equation B.2 and equations presented in Figure B.2, it is possible todetermine the relation between the SUT voltage and the signal measured in the SA when the Femto amplifieris used:
VSUT =RSUT +RAI//RP
RAI//RP
√(VSAGain
RSAI +RAORSAI
)2 − V 2IN2 − V 2
IN3 (A.3)
Stanford Research Systems (SRS) SIM910
Similar calculations can be applied for the SRS amplifier, the difference consists that this amplifier asmore a capacitance (CAI = 35pF ) in parallel with this amplifier input impedance (RAI = 100 MΩ). Theoutput resistance is the same as in the Femto, RAO = 50 Ω. Figure B.3 represents the contribution fromeach independent noise source to the SRS amplifier input voltage
Combining the equation B.1, the equation B.2 and equations presented in Figure B.2, it is possible toexpress the SUT voltage as a function of the signal VSA measured in the SA when the SRS amplifier is used:
58
Figure A.3: Contribution from each independent noise source to the SRS SIM910 amplifier input voltage. VIN1,VIN2 and VIN3 are the contributions from VnSUT, VnA and InA respectively.
VSUT =
√[(1 +
RSUTRP //Rai
)2 + (wRSUTCAI)2][(VSAGain
RSAI +RAORSAI
)2 − V 2IN2 − V 2
IN3] (A.4)
The amplifier Gain(f) parameter is measured using a Network Analyzer, for the frequency range of interest,after correcting the attenuation introduced by the connecting cables. The amplifier output is connectedto the SA while its input is short-circuited, eliminating the current noise source contribution, this way theamplifier input voltage noise source dependence on frequency VnAI(f) is measured. Knowing both Gain(f)and VnAI(f), the noise current noise source InAI(f) is also measured with the spectrum analyzer, but witha finite resistance R = 10kΩ at the amplifier input.
59
61
Appendix B
Run Sheet
MTJ with Large Area, PM and FG Responsible: Project: / Run: MTJ_L_FG_PM 2 Process Start : ___ / ___ / ___ Process Finish : ___ / ___ / ___
STEP 1: Tunnel Junction deposition Small piece from a 8’’ wafer
TJ376 - Etch 120W 180sccm 60s
5 Ta / 50 CuN / 5 Ta / 50 CuN / 5 Ta / 5 Ru /
7.5 IrMn / 2.0 CoFe30 / 0.85 Ru / 2.6 CoFe40B20/
MgO 4x97 3kW 600sccm /
3.0 CoFe40B20/ 0.21 Ta / 16 NiFe / 10 Ta / 30 CuN / 7 Ru
TJ377 - Etch 120W 180sccm 60s
5 Ta / 50 CuN / 5 Ta / 50 CuN / 5 Ta / 5 Ru /
7.5 IrMn / 2.0 CoFe30 / 0.85 Ru / 2.6 CoFe40B20/
MgO 4x80 3kW 600sccm /
3.0 CoFe40B20/ 0.21 Ta / 16 NiFe / 10 Ta / 30 CuN / 7 Ru
Wafer # TMR RxA [.µm]
TJ376 200% 624
TJ377 200% 7631
Comments: Annealing was made to form a magnetic anisotropy axis of the pinned and the free layer.
Defined by a magnetic field of 100Oe during deposition and posterior annealed on-wafer at 360ºC for 1
hour under a magnetic field of 8kOe.
STEP 2: 1st Exposure - Bottom contacts definition
Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
Masks: PMFGBT7K / PMFGBT600 (inverted) Map: AMSION
62
Energy:_____ Focus: ____ Power:_____
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 100s:
Comments:
STEP 3: Ion Beam Milling (IBM) – Etch 1991 Å Date: Operator:
Equipment: Nordiko 3600
Conditions
Batch: junction_etch
wafer #1: etch_junction_stack: etch pan 60 degrees/ end etch junction
Base Pressure (Torr): ______________ Tcryo3(k), stg1:________(~ 85); stg2:_______ (<20)
Etch time: 6*220s(etch)+5*200s(cooling) = 1420s (1308,7s) Total etch thickness:1991,6Å Etch rate: CuN- 1.6 Å/s, the other layers 1.05 Å/s
Pdep ~ ____________ Torr; Target # ____________
Assist Gun Power
[WRF]
V+
[V]
I+
[mA]
V-
[V]
I-
[ma]
Ar Flux
[sccm]
Set Values 170,0 730,0 105 350,0 - 10,0 (Ar)
Read
Neutralizer Voltage
[V]
Current
[ma]
Gas (Ar)
[sccm]
Set Values - 120.0 3.0
Read
Substrate Rotation
[%]
Pan
[deg]
Set Values 30 60
Read
Comments:
e.a.
63
STEP 3: 1st Photoresist stripping
Date: Operator:
Start: Finished:
Equipment: Wet bench in gray area
Total time in Acetone:______________ Ultrasonic Time:_______________
Stripping
ACETONE + Ultrasounds
Rinse with IPA + DI water + dry N2
Cleaning
IPA rinse; DI water rinse; Dry with N2 gun
Comments:
STEP 4: 2nd
Exposure - Junction and Bottom contacts definition Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
Masks: PMFGTJ7K / PMFGTJ600 (inverted) Map: AMSION
Energy:_____ Focus: ____ Power:_____
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 100s:
64
Comments:
STEP 5: Ion Beam Milling (IBM) - Junction and Bottom contacts etch, (2nd
etch) Date: Operator:
Equipment: Nordiko 3600
Conditions
Batch: junction_etch wafer #1: etch_junction_stack: etch pan 30 degrees/ end etch junction
Base Pressure (Torr): ______________ Tcryo3(k), stg1:________(~ 85); stg2:_______ (<20)
Etch time: 3*195s(etch)+2*200s(cooling) = 585s Total etch thickness:754.1Å
Etch rate: CuN- 1.6 Å/s, the other layers 1.05 Å/s
Etch pan 60º (496s) + pan 30º (94s) (Process Steps)
Pdep ~ _______ Torr; Target # ______
Assist Gun Power
[WRF]
V+
[V]
I+
[mA]
V-
[V]
I-
[ma]
Ar Flux
[sccm]
Set Values 170,0 730,0 105 350,0 - 10,0 (Ar)
Read 60
Read 30
Neutralizer Voltage
[V]
Current
[ma]
Gas (Ar)
[sccm]
Set Values - 120.0 3.0
Read 60
Read 30
Substrate Rotation
[%]
Pan
[deg]
Set Values 30 60/30
Read 60
Read 30
Comments:
65
STEP 6: Protective Layer Deposition – Al2O3 1000 Å Deposition Date: Operator:
Equipment: UHV2
RF Sputtering deposition (PVD)
B. P.:_________Torr
Material P [WRF] Pdep [mTorr] Ar [sccm] Thickness R [A/min] T [min]
Al2O3 200 3 45.0 1000 A 13,1 77’
Observations:
Use a test sample of Si
Use Dektak for profile and Measure the refraction index
Comments:
STEP 7: Al2O3 Lift-Off Date: Operator:
Start: Finished:
Equipment: Wet bench in gray area
Total time in Acetone:______________ Ultrasonic Time:_______________
Stripping
ACETONE + Ultrasounds
Rinse with IPA + DI water + dry N2
Cleaning
IPA rinse; DI water rinse; Dry with N2 gun
Comments:
STEP 8: 3rd
Exposure – F.G. definition Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
Masks: PMFGFG7K / PMFGFG600 (non-inverted) Map: AMSION
Energy:_____ Focus: ____ Power:_____
66
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 100s:
Comments:
STEP 9: DC Magnetron Sputtering, CoZrNb 6000 Å Deposition Date: Operator:
Equipment: UVH1 DC Magnetron Sputtering (PVD)
B.P.:
Presputtering target, 30’ – before putting the sample
Material P [W] V[V] Pdep[mbar] Ar [sccm] T [A] R [A/min] Time
CoZrNb 32 400 3 6 6000 1,6 1h15'
Observations:
Use a test sample of Si – measure in VSM the Hk, Hsat and magnetization
Use Dektak for profile and Measure the refraction index
Verify magnetic orientation (E.A. perpendicular to the deposition field direction)
Comments:
STEP 10: CZN Lift-Off Date: Operator:
Start: Finished:
Equipment: Wet bench in gray area
Total time in Acetone:______________ Ultrasonic Time:_______________
Stripping
ACETONE + Ultrasounds
Rinse with IPA + DI water + dry N2
Cleaning
IPA rinse; DI water rinse; Dry with N2 gun;
Observations:
Use Dektak for profile;
Comments:
67
STEP 11: 4th
Exposure – P.M. definition Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
Masks: PMFGPM7K / PMFGPM600 (non-inverted) Map: AMSION
Energy:_____ Focus: ____ Power:_____
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 100s:
Comments:
STEP 12: RF Sputtering, CoCrPt 1000 Å Date: Operator:
Equipment: Alcatel SCM450
RF Sputtering deposition (PVD)
B.P.:
Presputtering target, 30’
Material P [W] V[V] Pdep[mbar] Ar [sccm] T [A] R [A/min] Time
CoCrPt 50 885 20.0 1000 4,2 238’
Observations:
Use a test sample of Si
Use Dektak for profile and Measure the refraction index
Comments:
68
STEP 13: CCP Lift-Off Date: Operator:
Start: Finished:
Equipment: Wet bench in gray area
Total time in Acetone:______________ Ultrasonic Time:_______________
Stripping
ACETONE + Ultrasounds
Rinse with IPA + DI water + dry N2
Cleaning
IPA rinse; DI water rinse; Dry with N2 gun;
Observations:
Use Dektak for profile;
Comments:
STEP 15: Insulating Layer Deposition – SiO2 3000 Å Deposition Date: Operator:
Equipment: Alcatel SCM450
RF Sputtering deposition (PVD)
B.P.:
Presputtering target, 15’
Material P [W] V[V] Pdep[mTorr] Ar [sccm] t [A] R [A/min] Time[s]
SiO2 200 --- 3 20.0 3000 28,6 104’
Observations:
Use a sample of Si
Use Dektak for profile and Measure the refraction index
Comments:
STEP 16: 5th
Exposure – Contact Area Definition Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
69
Masks: PMFG2C (non-inverted)
Map: ______________________ Alig. Recp.: _________
Energy:_____ Focus: ____ Power:_____
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 60s:
Comments:
STEP 17: Reactive etch of SiO2 Lift-Off Date: Operator:
Equipment: LAM
Material P(W) P(mTorr) CF4(sscm) CHF3(sscm) AR(sscm) Rate(Å/s) T (s)
SiO2 200 275 40 40 350 4,5 800
Comments:
STEP 18: 6th
Exposure – Top contacts definition Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
Masks: PMFGTC7K / PMFGTC600 (non-inverted) Map: AMSION
Energy:_____ Focus: ____ Power:_____
70
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 60s
Comments:
STEP 19: Al 3000Å and TiW(N2) 300 Å Deposition Date: Operator:
Equipment: Nordiko 7000
Conditions
B.P.: torr (Mod#3); torr (Mod#4); torr (Dealer)
Run # ______; Seq. _______
Time
[min]
Rate
[A/s]
Power
Gas Flux
[sccm]
Pressure
[mTorr]
Mod2 F9 1’.00’’ - 70W+40W 50(Ar)+25(N2) 3.0
Mod4 1’.20’’ 5,6 2.0k WDC 50 (Ar) 3.0
Mod3 F19 54’’ 37,5 0.5k WDC 50(Ar) + 10(N2) 3.0
Observations:
Al98.5Si1.0Cu0.5
Use a sample of Si
Use Dektak for profile and Measure the refraction index
Comments:
STEP 20: Al Lift-Off Date: Operator:
Start: Finished:
Equipment: Wet bench in gray area
Total time in Acetone:______________ Ultrasonic Time:_______________
Stripping
ACETONE + Ultrasounds
Rinse with IPA + DI water + dry N2
Cleaning
IPA rinse; DI water rinse; Dry with N2 gun
Comments:
71
STEP 21: 7th
Exposure – Insulating Layer Definition (PADS) Date: Operator:
Equipment: SVG Photoresist track, DWL
Vapor prime, 30 min (Recipe – 0)
Purge oven for 25 min at 130ºC in N2 atmosphere at 10Torr;
Bake with surfactant for 5 min at 130ºC and 5 Torr pressure; Surfactant: hexadimethylsilane
Photoresist coating, 1.5 µm PR (Recipe 6/2)
Type: Positive photoresist PFR7790G 2cP JSR Electronics
Sample heating for 60s at 110ºC; Cool down for 30s;
Photoresist spinning for 40s at 2500 rpm (1.5 µm photoresist); Baking for 60s at 100ºC
Direct laser write exposure Start:_______ finish: _______ (Total time ~ ____ h)
Masks: PMFGPD7K / PMFGPD600 (inverted) Map: AMSION
Energy:_____ Focus: ____ Power:_____
Die dimensions: [X: 18000µm Y: 16000 µm]
Origin: X=168.00 ; Y=54.00 µm
Alignment marks :
(168,8054) (3131,8054) (6094, 8054) (12020, 8054) (14983, 8054) (17946, 8054)
(168,54) (3131,54) (6094,54) (12020,54) (14983,54) (17946,54)
Dist: 2963/2963/5926/2963/2963- 17778
Photoresist developing, (Recipe 6/2); (6/2: developing only)
Sample heating for 60s at 110ºC
Cool down for 30s
Developing for 60s:
Comments:
STEP 22: Insulating Layer Deposition – Al2O3 3000 Å Deposition Date: Operator:
Equipment: UHV2
RF Sputtering deposition (PVD)
B. P.:_________Torr
Material P [W] Pdep[mTorr] Ar [sccm] Thickness R [A/min] T [min]
Al2O3 200 3 45.0 3000 13,1 229’
Observations:
Use a sample of Si
Use Dektak for profile and Measure the refraction index
Comments:
72
STEP 23: Al2O3 Lift-Off Date: Operator:
Start: Finished:
Equipment: Wet bench in gray area
Total time in Acetone:______________ Ultrasonic Time:_______________
Stripping
ACETONE + Ultrasounds
Rinse with IPA + DI water + dry N2
Cleaning
IPA rinse; DI water rinse; Dry with N2 gun
Comments:
73
Appendix C
Time Flow
Optimize time flow of the fabrication process is summarized in the next table:
Steps Description Machine
Time scale in days 1 2 3 4 5 6 7 8 9 10 11 12 13
Substrate
preparation Sample from INL
Bottom
contact
definition
1st lithography– pillar definition DWL
Pillar etching IBD 3600
Resist strip Wet bench
Tunnel
Junction
definition
2nd lithography– T.J. definition DWL
2nd etching IBD 3600
Al2O3 deposition UHV 2
Lift-off Wet bench
Flux Guides
definition
3nd lithography– F.G. definition DWL
CoZrNb deposition UHV 2
CoZrNb lift-off Wet bench
Permanet
magnets
definition
4nd lithography– P.M. definition DWL
CoCrPt+SiO2 deposition Alcatel
SCM450
CoCrPt lift-off Wet bench
Contact Area
definition
SiO2 deposition Alcatel
SCM450
5nd lithography– 2nd.C. definition DWL
Reactive etch LAM
Resist strip Wet bench
Top contact
definition
6nd lithography– T.C. definition DWL
Al + TiWN2 depositon N7000
Metal lift-off Wet bench
Vias to TJ
pads
Lithography - Vias DWL
Al2O3 deposition UHV 2
Lift-off Wet bench
Device measurements Measurement
Setup
74
75
Appendix D
Mask A
An example mask used during the thesis (2nd
Run) for picotesla sensors:
Bibliography
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