d4.3 - activity report on cnt based filteroscillator and ... · 3.1.2 microwave device performances...

CARBON BASED SMART SYSTEM FOR WIRELESS APPLICATION

Start Date : 01/09/12 Project n° 318352 Duration : 45 months Topic addressed: Very advanced nanoelectronic components: design, engineering, technology and manufacturability

WORK PACKAGE 4

DELIVERABLE D4.3 Activity report on the CNT filter/oscillator

Due date : T0+40 Submission date : T0+48

Lead contractor for this deliverable : UPMC

Dissemination level : PU – Public

D4.3 Activity report on CNT based filter/oscillator and graphene

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WORK PACKAGE 4: Test activities

PARTNERS ORGANISATION APPROVAL

Name Function Date Signature

Prepared by: Mircea Dragoman Professor 04/10/2016

Martino Aldrigo Postdoc 04/10/2016

Approved by: Afshin Ziaei Research Program Manager 04/10/2016

DISTRIBUTION LIST

QUANTITY ORGANIZATION NAMES

1 ex Thales Research and Technology TRT Afshin ZIAEI

1 ex Chalmers University of Technology CHALMERS Johan LIU

1 ex Foundation for Research & Technology - Hellas FORTH George KONSTANDINIS

1 ex Laboratoire d’Architecture et d’Analyse des Systèmes CNRS-LAAS Patrick PONS

1 ex Université Pierre et Marie Curie UPMC Charlotte TRIPON-CANSELIET

1 ex National Research and Development Institute for Microtechnologies

IMT Mircea DRAGOMAN

1 ex Graphene Industries GI Peter BLAKE

1 ex Thales Systèmes Aéroportés TSA Yves MANCUSO

1 ex SHT Smart High-Tech AB SHT Yifeng FU

1 ex Universita politecnica delle Marche UNIVPM Luca PIERANTONI

1 ex Linköping University LiU Rositsa YAKIMOVA

1 ex Fundacio Privada Institute Catala de Nanorfnologia ICN Clivia SOTOMAYOR

1 ex Tyndall-UCC Tyndall Mircea MODREANU


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REVISION LETTER DATE PAGE NUMBER DESCRIPTION

Template

V1 20/05/2016 22 IMT contribution

V2 04/10/2016 16 Final version


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CONTENTS 1 ABBREVIATION / DEFINITION ......................... ............................................................................5

2 INTRODUCTION ...........................................................................................................................6

3 FABRICATION AND CHARACTERIZATION OF THE CARBON NANO TUBE-BASED TUNABLE MICROWAVE (MW) BAND-PASS FILTER ................... .......................................................................6

3.1.1 Device technology description ..........................................................................................6

3.1.2 Microwave device performances ......................................................................................8

3.1.3 Feedback electromagnetic simulation for device design optimization ............................11

4 GRAPHENE DETECTOR PERFORMANCES .................... .........................................................14

5 CONCLUSIONS AND PERSPECTIVES ...................... ...............................................................16


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1 ABBREVIATION / DEFINITION

CNT Carbon Nano Tubes DC Direct Current LC Inductance/Capacitance RF Radio Frequency IDT MW Micro Wave


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2 INTRODUCTION This deliverable covers WP4 testing activities on CNT based filter designed in WP2 and fabricated in WP3. The filter design started from the device requirements specified in the NANO RF Project, such as:

� Operability in the 8-12 GHz band; � Exploitation of carbon nanotubes (CNTs) for designing a variable capacitor (varactor).

3 FABRICATION AND CHARACTERIZATION OF THE CARBON NANOTUBE-BASED TUNABLE MICROWAVE (MW) BAND-PASS FIL TER

3.1.1 Device technology description The filter structure has been designed in a “T-like” configuration with three meander inductors and three varactors, with physical description and simulated performances reported in D2.3. The circuit was thought to be designed in coplanar waveguide (CPW) technology with external dimensions of 39 mm x 24 mm (Fig.1). The two tapers on the left and right sides of the structure are mandatory for mechanical compatibility with standard CPW measurement probe tips. All the inductors/varactors are characterized by the same value of inductance/capacitance (respectively) for technological simplicity. The selected technological process implies only two masks for meander inductors (one mask is based on optical lithography for L-C realization and the other one is based on e-beam lithography for CNT growth). Thanks to the CPW configuration, the CNT-based varactors and, hence, the whole filter, can be polarized by simply applying positive and a negative potentials on the signal line and ground planes of the CPW line, respectively. The calculated CNTs’ capacitance is 0.251 pF under a DC bias of 0 V, whereas the approximated value of each inductance at 10 GHz is 0.8 nH. The guided wavelength λg corresponding to a frequency of 10 GHz, is about 11 mm, so that the filter is about 2 times (in width) and 4 times (in length) the above mentioned value of λg: which induces conduction losses over the microwave frequency band. From the first fabrication run, a limited CNT growth implying CNT vacancy, low aspect ratio and rigidity has been detected by SEM characterization (Fig. 2a), affecting filter performances. The technological process has been optimized in order to improve the overall CNT density and structure improved with respect to the first run and fulfill the technological specifications (Fig. 2b).


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(a) (b)

Fig. 2: (a) first fabrication run of CNT growth; (b) second fabrication run of CNT growth. The

improvement (w.r.t. the first run) is evident in CNT density and mechanical structure.

Fig. 1: fabricated CNT-based tunable MW filter.


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3.1.2 Microwave device performances S-parameters measurements of the filter prototype from the second technological process have been performed over a 6-12 GHz frequency band with RF input power of -16 dBm. These experimental results in terms of return and transmission losses are presented in figure 3a without any DC bias. Minimum reflection losses value (|S11|) of -21.4 dB is measured at 8.24 GHz, with corresponding transmission losses (|S21|) of -9.31 dB at the same frequency. Figure 3b magnifies bandpass filtering performances in a reduced frequency band of 7-9 GHz around frequency resonance. In comparison with simulations results (Fig. 3c) a small frequency shift of 70 MHz in frequency resonance is observed as well as a transmission loss increase of 4.41 dB, and bandpass filter frequency bandwidth widening, without any radiation effects. In comparison to first technological process (Fig. 4a), transmission losses have been improved of a value of 2.2 dB at the filtering frequency, which means that the transmitted power is 40% higher with CNTs. In opposition, return losses have been affected of 7.2 dB at a frequency of 8.24 GHz (Fig. 4b), These latter results clearly demonstrate the effects and advantages of an optimized growth of the CNTs on the IDTs’ digits, since this has a macroscopic influence on the overall capacitance value of the varactors. Device performances modifications may be attributed to:

1. Gold thickness which is lower than its skin depth at 10 GHz. This entails losses and coupling with the underlying ground plane, since the electromagnetic (EM) field penetrates in the substrate.

2. A capacitive effect when measuring the device directly on the ground plane; 3. The three varactors dot not possess the same number of CNTs and the structure is not

geometrically symmetric. This entails a non-symmetrical device (i.e. S11 ≠ S22); 4. The structure is not perfectly flat.

From these observations, an optimized layout of the proposed CNT-based tunable MW band-pass filter and a new solution for the three inductors are provided, overcoming the limitations of the fabricated device.


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(a)

(b) (c)

Fig. 3: (a) RF measurements of the fabricated CNT-based tunable MW filter (first run) without

DC bias; (b) magnification of the measurement results around the filtering frequency; (c) magnification of the simulation results around the filtering frequency.


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(a)

(b)

Fig. 4: Comparison between the first (V1) and the second fabrication run (V2):

(a) Measured return loss |S11|; (b) Measured transmission |S21|.


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3.1.3 Feedback electromagnetic simulation for devic e design optimization From modelling tools developed in WP2, additional electromagnetic simulations were carried out with CST Microwave Studio (MWS) for physical dimensions reduction and performances enhancement. The new layout is modified at several levels, ie by implementation of:

1. Three wires (diameter: 25 µm, length: 1 mm) in parallel for the inductors on the CPW signal (which is 100 µm wide), or three meander inductors with modified dimensions

2. Three CNT-based varactors (same as first fabricated filter).

Figures 5a-5b-5c report the new filter design with wire-based inductors, and previous filter design with optimized overall dimensions. The two configurations are 4.2-mm wide and 8.4-mm long, i.e. with a surface area reduction of a factor of five. Additional CNT growth process improvements are introduced. CPW dimensions are already in agreement with standard CPW measurement probe tips, so that no tapers on the left and right sides of the structure are required. The technical details of the filters in Figs. 5a and 5c are:

1. Molybdenum thickness of 50 nm is conserved for the local CNT growth. 2. Gold metallization is used and deposited with thickness of 1 µm for CPW signal and ground

accesses, in order to prevent from skin depth effects. 3. the inductance of each wire is about 5.121 nH, so that the inductance of 3 wires in parallel is

about 1.707 nH (it could be possible to use up to 5 wires to tune the inductance value); the inductance of each meander is about 0.92 nH.

4. All the distances among the various filter components (inductors and varactors) are optimized to avoid mutual coupling effects and to provide the best filtering performances. As in the case of the first prototype, filtering frequency tunability is assumed by a direct DC bias applied on CPW accesses. Fig. 6a synthesizes EM simulations results of the filter design with wire inductors (modelled as thin perfect electric conductor – PEC – components) in terms of return loss |S11| and transmission |S21|, whereas Fig. 6b provides the simulation results for the filter with meander inductors. In both case, we considered only the presence of the IDTs without CNTs. Each IDT exhibits an estimated capacitance value of 0.387 pF, so that the presence of the CNTs is expected to modify the overall varactor capacitance (hence, tuning the filtering frequency) according to the applied DC bias voltage applied.


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(a)

(b)

(c)

Fig. 5: CST MWS design (with main dimensions) of the new compact CNT-based tunable MW

band-pass filter with: (a)-(b) wire inductors; (c) meander inductors.


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(a)

(b)

Fig. 6: EM simulation results (performed by means of CST MWS) for the new compact CNT-

based tunable MW band-pass filter with: (a) wire inductors; (b) meander inductors.


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Expected device

performances Wires-based design Meanders-based design

Filtering frequency 9.98 GHz 8.63 GHz |S11|/|S22| -10.73/-30.45 dB -46.78/-11.07 dB

Insertion loss 1.68 dB 1.59 dB Bandwidth (-3 dB) 120 MHz 519 MHz

Q-factor 617 489 Table I: Main performances of the two proposed CNT-based tunable MW band-pass filters.

Table I summarizes and compares the performance of the two solutions. Number of wires implemented in the wires-based design adds an additional degree of freedom. Indeed assuming presence of 5 wires, a new bandpass filtering transfer function appears at a higher frequency (at about 12 GHz). Moreover, the first filtering frequency is down-shifted w.r.t. the case with 3 wires (9.86 GHz instead of 9.98 GHz, i.e. a down-shift of about 120 MHz). From table I, it is apparent how the layout with wires offers the best performance in terms of bandwidth and Q-factor. Again, like in the case of the first prototype, the device is not geometrically symmetrical, so that |S11| ≠ |S22|. Nevertheless, the improvement w.r.t. the filter described in paragraph 1 is evident, especially as regards the insertion loss value (which now is less than 2 dB). Finally, Fig. 7 reports simulations results for the filter with meander inductors performed with a circuit-based simulator (AWR) in order to verify |S11| and |S21| changes by putting in parallel to the each IDT’s scattering matrix a lumped element (capacitor) emulating the CNTs’ effect. This design procedure step is a fast method to test if changing the overall varactor capacitance affects the performance of the filter. In detail, by assuming a capacitance of 0.2 pF for each CNT matrix, we observed a down-shift of the filtering frequency of about 500 MHz.

4 GRAPHENE DETECTOR PERFORMANCES For convenience, this part is reported in D4.5


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(a)

(b)

Fig. 7: (a) Device equivalent schematic of the new compact CNT-based tunable MW band-pass

filter with meander inductors and CNTs’ capacitance of 0.2 pF; (b) simulated return loss and transmission without and with CNTs. The down-shift of the filtering frequency is evident.


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5 CONCLUSIONS AND PERSPECTIVES This report summarizes performances of first prototypes of CNT-based bandpass microwave filter smatching device specifications in terms of frequency band operation, in association with technological process efforts. Ccomplementary modelling and design works predict device performances enhancement. The new masks production and new devices fabrication will be performed at Thales France and IMT Bucharest for wires implementation, in order to verify their contribution on the overall filtering performances.

d4.3 - activity report on cnt based filteroscillator and ... · 3.1.2 microwave device performances...

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