mehdi fardmanesh supercondutor electronics research laboratory (serl)

DETECTIVITY OPTIMIZATION OF HTS BOLOMETERS

Mehdi FardmaneshSupercondutor Electronics Research Laboratory (SERL) Electrical Engineering DepartmentSharif university of technology

Superconductor Electronics Research Laboratory(SERL) 1

Out linesPart I : Theoretical optimizationOperation principleTheoretical LimitDetectivity parameters

Substrate thicknessModulation frequencyThermal conductivity

Part II :Experimental ResultsSubstrate thinningAbsorber effectDetectivity calculationConclusion and Summary


Part ITheoretical Detectivity

Optimization


Bolometer - Principles

4

A device which transforms a change in the incident input power into a change of electrical resistance.

A bolometer in general consists of: 1- Absorber 2- Thermometer 3- A thermal link to the temperature reservoir

Superconductor Electronics Research Laboratory(SERL)

5

Operation Mechanism

Transition curve and bolometric response Comparison of different types of infrared

detectors [SERL]

Cut off frequency for semiconductor detectors:

cc

hcU


Detectivity

A criterion to determine device performance

Different noise sources

1/2*

n

V

A cmHzDNEP W

V WNEPr Hz

1/22 2 2 2 2 2 2. . 1/ . .nTot n Phot Phon R f Amp ExtV V V V V V V V


Theoretical limit

Considering only background noise

It can be enhanced by reducing field of view of detector

1/22

1/2* 11

2

8

2.56 10 cm Hz /W8

v B BPhoton

photon

B Bphoton

r Ak TV

DK ATNEP


Important terms in total NEP

Phonon NEP

Johnson NEP

1/f NEP

4 bPhonon B

TNEP k G

4 B b bJohnson

v

k T RNEP

r

1b bH

fv

I RNEPNAdf r


Responsivity

Optical Response

Maximum bias current

9

22 2b

vI dRr

dTG fC

2

max1bI dR GIdRG dT

dT

Maximum Response & Total NEPMaximum Optical Response

Considering linear transition

Total NEP

,max 22 2

v

GdR dRdTr

dTG fC

2222 222 2

248 42 2

b B bB B Htotal B

T G fCT k Tk AT T WNEP k G HzG NAdf


2bR T

dRdT

Detectivity vs. thermal conductance & Normal resistance(1mm thickness)

Almost independent of device resistanceStrong variation versus thermal conductanceDecreases at low and high value of thermal conductance


Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 1mm at 80Hz modulation frequency.

Detectivity vs. thermal conductance & Normal resistance(10µm thickness)

Lower values of optimum thermal conductanceHigher detectivity Decreases at low and high value of thermal conductance


Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 10µm at 80Hz modulation frequency.

Detectivity Vs. thermal conductance & Normal resistance(10µm thickness)

Ignoring 1/f noiseDetectivity as high as 2×1010

Very strong dependence on thermal conductanceDecreases at low and high value of thermal conductance


Detectivity versus thermal conductance and normal resistance of a bolometer over SrTiO3 substrate with thickness of 10µm at 2Hz modulation frequency.

Substrate thickness effectsThinning the substrate would increase device detectivityThickness independent at high thermal conductanceThere is an optimum value for thermal conductance

0.0E+00

1.0E+09

2.0E+09

3.0E+09

4.0E+09

0 0.04 0.08 0.12 0.16 0.2

Det

ectiv

ity(C

m.H

z^0.

5/W

)

Thermal conductance(W/K)

1000µm

200µm

100µm

50µm

20µm

Detectivity versus thermal conductance of a bolometer over SrTiO3 substrate with device area of 9 mm2, surface absorption of 20% and modulation frequency of 30Hz.Superconductor Electronics Research Laboratory(SERL) 14

Modulation frequency effectVery low detectivity at low value of modulation frequency Frequency independent at high value of thermal conductanceIncreasing frequency decreases the detectivityThere is an optimum value of thermal conductance

0.0E+00

1.0E+09

2.0E+09

3.0E+09

0 0.04 0.08 0.12 0.16 0.2

Det

ectiv

ity(C

mH

z^0.

5/W

)

Thermal conductance(W/K)

2Hz

12Hz

30Hz

90Hz

200Hz

Detectivity versus thermal conductance of a bolometer over SrTiO3 substrate with device area of 9 mm2, surface absorption of 20% and substrate thickness of 100µm.Superconductor Electronics Research Laboratory(SERL) 15

Analytical optimizationOptimum thermal conductance

Optimum NEP

16

2 2

2

(2 ) 10 02

22

total s

optimal s

NEP T fCT

G GTG fCT

222

22

4 2 22

2 22

162

Boptimum s s

s

Boptimum s

K T T TNEP T fC fCT TfC

T

K T TNEP fC T


Optimum thermal conductanceIncreases by increasing modulation frequencyDecreases by decreasing substrate thickness

Optimum thermal conductance versus modulation frequency and substrate thickness


Maximum detectivity Decrease by increasing modulation frequencyincreases by decreasing substrate thickness

Maximum Detectivity versus modulation frequency and substrate thickness


Optimum detectivity

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0

5E+09

1E+10

1.5E+10

2E+10

2.5E+10

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Max

imum

Det

ectiv

ity(C

m.H

z^0.

5/W

)

Substrate Thickness(µm)

12Hz

30Hz

90Hz

3Hz

Maximum detectivity versus substrate thickness of a bolometer over SrTiO3 substrate with device area of 9 mm2 and surface absorption of 20%.


For detectivity in the range of 1010, substrate thickness should be as low as 30µm whereas modulation frequency should be as low as possible .

Surface Conductance Limitation“Effect of Substrate Thickness on Responsivity of Free-Membrane Bolometric Detectors,” To appear in IEEE Journal of Sensors�

Part IIExperimental Optimization


Device fabrication 200nm thin YBCO film deposited on 1mm thick SrTiO3 substrate using PLD

YBCO film was patterned using standard photolithography process

Au coated pads

Silver paste for contacts


Sample ‘A’

Sample ‘B’

New design of cold headReducing thermal conductance is essential in order to achieve high detectivity

Reducing thermal contact between substrate and cold headFree standing bolometer


Thinning the substrateThinning the substrate using our customized grinding machine Initial thickness 1000µmFinal thickness 200µm


Thermal diffusion length Depends on modulation frequency and substrate materialsDecreases at high value of modulation frequencyDetermines the knee frequency of device


Thermal diffusion length of common substrate materials versus modulation frequency

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

The

rmal

Diff

usio

n le

ngth

(µm

)

Frequency(Hz)

Lf(LaAlO3)(µm)Lf(SrTiO3)(µm)Lf(MgO)(µm)

Thinning effect on the responsivity and noise

No major effect on the device noise at high frequenciesIncreasing device response in the frequency range lower than the knee frequencySlight increase of 1/f noiseSlight destructive effect on the film quality


Measured values of optical response and voltage noise of sample A before and after thinning the substrate.

Knee frequencies

f -1 slope

f -0.5 slope

Absorber effect

Backside absorber no destructive effect on film quality Wider range of absorber materialsOnly enhances responsivity in the frequency range in which Lf>Ls


Measured values of optical response and voltage noise of sample “A_200” before and after applying absorber layer on the back side of device.

Increasing surface absorption of device increases device response Applying thin layer of black paint as an absorber on the backside of device

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Detectivity calculationAbout one order of magnitude gain in device detectivity by thinning and using absorber

Calculated detectivity versus modulation frequency of sample “B”


Thinner substrate thickness Higher device detectivitySrTiO3 is brittle at low thicknesses

Using silicon substrate as a support

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Results conclusion Sample

codeA

mm2Ls

µmR

T=300K(kΩ)

Ronset

(kΩ)ΔT(K)

GmW/K

dR/dTmax

kΩ/K

D*max

cmHz^1/2/W

A_1000 8.97 1000

7 1.4 1.2 14.076 1.4 2×108

B_1000 1.257 1000

5 1.2 0.8 9.16 1.8 4.3×108

A_200 8.97 200 7 1.4 1.4 5.2 1.2 4.35×108

B_200 1.257 200 5 1.2 0.87 2.22 1.75 9.5×108

A_200_Absorber

8.97 200 7 1.4 1.4 5.2 1.2 7.73×108

B_200_Absorber

1.257 200 5 1.2 0.87 2.22 1.75 3.6×109

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New device structure

Structural optimization of bolometer

Silicon Substrate

Buffer Layer

YBCO thin film

Gold Layer

Film Substrate

Absorber Layer

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Extrapolation of Responsivity

20 times increase in device responsivityExpected value of detectivity is

7.2×1010


Extrapolated value of responsivity for 10µm substrate thickness

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Planned response 10µm thickness

SummaryFormulation of Detectivity Optimization

Thinning substrateReducing thermal conductanceUsing absorber layer

One order of magnitude gain in detectivity by model based device engineeringA bolometer with detectivity of 3.6×109 Obtained Successful use of Absorber on the backside

No destructive effect on film qualityPossibility of wider range of absorber materials

Target: Wide Band D* over 1010

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Thanks for you attention

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mehdi fardmanesh supercondutor electronics research laboratory (serl)

Documents

thermal conductancedecreases

thermal link

high thermal conductancethere

srtio3 substrate

hz modulation frequency

optimum value

device area

semiconductor detectors