> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)

The International Conference Centre Birmingham

20-23 August 20112, Birmingham, United Kingdom

Antenna-Coupled Nanowire Thermocouples for

Infrared Detection

Gergo P_ Szakmany, Peter M. Krenz, Member IEEE, Alexei O. Orlov, Gary H. Bernstein, Fellow, IEEE, Wolfgang Porod, Fellow, IEEE

Abstract - Unbiased, uncooled, and frequency-selective

antenna-coupled nanothermocouples have been fabricated and

characterized for infrared detection. The relative Seebeck

coefficient of the nanothermocouples was measured by the 2(0 technique, which uses a test setup co-located on the same chip as

the detectors. The hot junction of the nanothermocouple is less

than 80 nm x 80 nm. The measured normalized detectivity (D*)

is 1.94x105 cm HZI/2/W.

Index Terms - Antennas, Infrared detectors, Nanowire

thermocouples, Thin-film devices

I. INTRODUCTION

INFRARED detectors for the long-wave infrared (L WIR)

range are of special interest due to a window in the

atmospheric transmission [1]. These detectors have many

applications, including thermal imaging, energy harvesting,

and target tracking [2, 3]. Our work is directed at fabricating a

thermal L WIR image sensor that operates without biasing, at

room temperature without the need of cooling, and has a small

"footprint."

We built antenna-coupled thermocouples from a dipole

antenna for receiving the infrared radiation coupled to a

nanowire thermocouple whose hot junction is located at the

feed point of the antenna. The operating principle of these IR

detectors is based on the heating of the hot junction by

radiation-induced antenna currents, and the subsequent

detection of this heating by measuring the open-circuit voltage

of the nanowire thermocouple.

Due to their small size, thin film metal thermocouples have

different thermoelectric powers compared to their bulk

counterparts. It has been found that the relative Seebeck

coefficient for thin films decreases as the thickness is reduced

below 200 nrn [4 , 5]. Characterizing thermocouples that were

fabricated under nominally identical conditions and located on

the same chip as the detectors allows us to compare different

fabrication processes and to study their impact on the Seebeck

coefficients and the infrared response.

Manuscript received June 1 2012. G. P. Szakmany, P. M. Krenz, A. O. Orlov, G. H. Bernstein, W. Porod are

with the Center for Nano Science and Technology, Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556 USA (e-mail: gszakman@nd.edu).

The project was funded by the ONR-MURI program.

II. DESIGN AND F ABRICA TION

A. Design 1) Characterization platform: This platform consists of a

Joule heater, a resistive thermometer, and a thermocouple as

shown in Fig. 1 (b). The thermometer and the thermocouple

are symmetric around the heater, to ensure uniform heat flow

and uniform temperature distribution around the heater.

Applying current to the heater locally increases the

temperature of the hot junction of the thermocouple, but not of

the cold junction, which is located approximately 4 0 11m away

from the heater.

2) Antenna-coupled thermocouple: The dipole-antenna

serves as a receiving element that collects the incident

electromagnetic wave and heats up the hot junction of the

thermocouple by dissipation of the radiation induced antenna

currents.

The directivity of the antenna was increased by separating it

from a ground plane by a quarter-wave thick Si02 standoff

layer. The radiation reflected by the ground plane adds in

phase with the incident radiation at the antenna, causing an

increase of the antenna currents and therefore a larger

response.

50r-r-r-r-����'-�-r-r-r-r-r������

45

40

S2' -S 35

Q) <J) ctI 30 !!! o .S: 25

!!! .a 20 � Q) � 15

� 10

5

Dipole Length (llm) Fig. 1. COMSOL Multiphysics® simulation to determine the resonant dipole antenna length at 10.6 llm. A dipole antenna length of 2.2 llm causes the maximum temperature increase of the hot junction of the thermocouple due to incident irradiation of 1.42 W/cm2

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The optimum length of the dipole antenna on a Si02

substrate with a ground plane was found with COMSOL

Multiphysics simulations. The optical properties of the

materials at 28. 3 THz were found in [6] and their thermal

properties were taken from [7]. The simulated temperature

increase of the hot junction of the thermocouple due to

illumination with 1.42 W/cm2

is shown in Fig. 1 as a function

of the antenna length. The maximum temperature increase is

achieved for an antenna length of2.2 f.!m.

B. Fabrication A silicon substrate was coated with a 100 nm thick

aluminum ground plane. A 1. 3 f.!m thick Si02 standoff layer

was deposited onto the ground plane using plasma enhance

chemical vapor deposition. The bonding pads, which are used

to measure the open-circuit voltage of the thermocouples and

the resistance change of the thermometers were patterned on

top of the oxide using optical lithography and metallized with

200 nm of gold with a 10 nrn thick titanium adhesion layer.

Fabrication of the characterization platform and the

antenna-coupled thermocouples involves two-step electron

beam lithography. In the first step, the thermometer, the

heater, and one part of the thermocouple or dipole antenna

were patterned into a conventional MMAIPMMA resist layer.

Palladium was deposited by electron beam evaporation and

lift-off was performed with NMP. In the second step, the

remaining parts of the devices were patterned and metalized

with one of four different metals: AI, Au, Cr, and Ni. The hot

junction of the thermocouples is located where the two metal

layers overlap. A fabricated antenna-coupled thermocouple

and characterization platform are shown in Fig. 2.

Fig. 2. (a) Scanning electron micrograph of an antenna-coupled thermocouple, and (b) the characterization platform to measure the Seebeck coefficient of the nanowire thermocouple. The hot junction is located where the two different metals overlap and the cold junction where the leads are connected to the bonding pads.

III. MEASUREMENTS

A. Electrical Measurement The palladium thermometers were first calibrated in a N2

cryostat, and the temperature coefficient of resistance (TCR)

was determined with a four-terminal resistance measurement.

The thermocouples were calibrated by applying an AC current

of frequency (0 to the heater and by measuring the resistance

change of the thermometer and the open-circuit voltage of the

thermocouple at 2(0. Details of the measurement can be found

in [8]. The measured Seebeck coefficient is 15 f.!V/K for a

palladium-chromium thermocouple and 3 f.! V /K for a

palladium-gold thermocouple.

B. Infrared Measurement The antenna-coupled nanowire thermocouples were

illuminated with 1.42 W Icm2

using a linearly polarized CO2

laser operating at 10. 6 f.!m. The schematic of the beam path is

shown in Fig. 3.

Visible Half-wave ate

alignment lase�

•

C02 laser Polarizer Chopper

Fig. 3. Experimental setup for infrared testing. The 10.6 /lm infrared radiation is provided by the CO2 laser. The beam is linearly polarized and chopped to add a square wave modulation of the incident beam on the device under test (OUT).

IV. RESULTS

Device responses were measured when the dipole axis was

parallel to the linear polarization of the beam. The open­

circuit voltage of the devices was recorded with a lock-in

amplifier. Fig. 4 shows the relative Seebeck coefficient vs.

open-circuit voltage for different devices.

220

200

180

:> 160 c: 'Q; 140 Ol .!!! 120 (5 > - 100 '5 � 80 U c 60 Q) a. 0 40

20

0 0 5

• Pd-Ni • Pd-AI ... Pd-Au ... Pd-Cr

10 15 20 25

Relative Seebeck Coefficient (flV/K)

1

30 35

Fig. 4. Device response vs. relative Seebeck coefficient. Devices with larger thermoelectric power result in a larger open-circuit voltage under identical illumination conditions.

As expected, the larger response is achieved by devices

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with larger Seebeck coefficients under nominally identical

illumination conditions. The Seebeck coefficient

measurement of Pd-Ni and Pd-AI are in progress, and

therefore the corresponding bulk material properties were used

in Fig. 4 .

Polarization dependence of the devices was measured by

rotating the linear polarization of the laser beam with a half­

wave plate. In accordance with antenna theory [9], the

maximum signal response occurs when the linearly polarized

incident electric field is parallel to the axis of the antenna (0°,

180°). The minimum response is detected when the electric

field is polarized perpendicular to the antenna axis (90°, 270°).

In agreement with antenna theory, these devices follow the

cosine-square dependence shown in Fig. 5, proving that the

detected signal is due to the radiation-induced currents on the

antenna.

160

140

120 > .s Ql 100 0> .l!! (5 80 >

'5 � 60 <3 c Ql

40 a. 0

20

0 0

Parallel polarization

Perpendicular polarization

30 60 90 120 150 180 210 240 270 300 330 360

Polarization (degree) Fig. 5. Polarization dependent response of an antenna-coupled nanothennocouple for 10.6 J.Im radiation at nonnal incidence.

Signal-to-noise ratio of a palladium-chromium device was

measured with a signal analyzer and found to be 28 dB. The

normalized detectivity, which is independent of device area, of

this sensor was calculated to be 1.94 x 105 cm.JHz / W .

V. CONCLUSION

Relative Seebeck coefficient and antenna IR response were

measured on the same chip. The infrared response of the

antenna-coupled thermocouples is linearly proportional to the

magnitude of the relative Seebeck coefficient of the

thermocouple.

The normalized detectivity was measured at 10. 6 /-lm

illumination, and found to be 1.94 x 105 cm.JHz / W .

ACKNOWLEDGMENT

The authors would like to thank Dr. Badri Tiwari for his

comments and ideas during this project.

REFERENCES

[I] S. Lord, "A new software tool for computing Earth's atmospheric transmission of near- and far-infrared radiation," NASA Technical Memorandum, vol. 103957, 1992.

[2] A. Zare,J. Bolton,P. Gader, and M. Schatten, "Vegetation mapping for landmine detection using long-wave hyperspectral imagery,", iEEE Trans. Geosci. Remote Sens., vo1.46, no. I , pp.I72-178, Jan. 2008.

[3] P. Krenz, B. Tiwari, G. Szakmany, A. Orlov, F. Gonzalez, G. Boreman, and W. Porod, "Response increase of IR antenna-coupled thermocouple using impedance matching," iEEE J. Quantum Electron., vol. 48, no. 5, May 2012.

[4] M. C. Salvadori, A. R. Vaz, F. S. Teixeira, T. G. Brown, and M. Cattani, "Thermoelectric effect in very thin film PtiAu thermocouples," App.

Phys. Lett., vol. 88,2006. [5] D. Chu, W. Wong, K. E. Goodson, R. Fabian, and W. Pease, "Transient

temperature measurement of resist heating using nanothermocouples," J. Vac. Sci. Technol. B, vol. 21, n. 6, pp. 2985-2989, 2003.

[6] E. Palik, Handbook ojOptical Constants ojSo/ids, Elsevier Inc. 1997. [7] D. Lide, CRC Handbook oj Chemistry and Physics, 90th ed., Boca

Raton, FL: CRC Press, 2009. [8] G. Szakmany, P. Krenz, A. Orlov, and W. Porod, "Nanowire

thermocouple characterization platfonn," in preparation. [9] C. Balanis Antenna Theory, Wiley-Interscience 3'd edition, pp. 46, 2005.

Gergo P. Szakmany received his diploma in electrical and computer engineering from the Pazmany Peter Catholic University, Budapest, Hungary in 2007. He received his MS degree in electrical engineering from the University of Notre Dame, Notre Dame, IN in 2011. He is working toward his PhD degree in electrical engineering at the University of Notre Dame, IN on antenna­coupled infrared detectors. His research interests focuses on submicron device fabrication and characterization.

Peter M. Krenz (M'04) received his BS degree in electrical engineering from the Oklahoma State University, Stillwater, OK in 2003 and his MS degree in optics from the University of Central Florida, Orlando, FL in 2008. He completed his PhD in optics at the College of Optics and Photonics (CREOL) at the University of Central Florida, Orlando, FL in 2010.

Currently, he is a Postdoctoral Researcher with the Center for Nano Science and Technology at the University of Notre Dame, Notre Dame, IN. His

research interests include the simulation, fabrication, and characterization of uncooled antenna-coupled infrared detectors and infrared transmission lines.

Alexei O. Orlov currently is a Research Professor at the University of Notre Dame. He received his M.S. degree in Physics from the Moscow State University in 1983. From 1983 to 1993 he worked at the Institute of Radio Engineering and Electronics of the Russian Academy of Sciences, Moscow. During this time he conducted research on mesoscopic and quantum ballistic effects in electron transport of GaAs field-effect transistors.

He received his Ph.D. from the same Institute in 1990. He has been a visiting fellow at the University

of Exeter, UK in 1993, and joined the Department of Electrical Engineering at the University of Notre Dame, IN, in 1994. His topics of research include experimental studies of mesoscopic, single-electron and molecular electronic devices and sensors, nanomagnetics and quantum-dot cellular automata. Alexei Orlov has authored or co-authored more than 70 journal pUblications.

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Gary H. Bernstein received his BSEE from the University of Connecticut, Storrs, with honors, in 1979 and MSEE from Purdue University, W. Lafayette, Indiana in 1981. During the summers of 1979 and '80, he was a graduate assistant at Los Alamos National Laboratory, and in the summer of 1983 interned at the Motorola Semiconductor Research and Development Laboratory, Phoenix, Arizona. He received his Ph.D. in Electrical Engineering from Arizona State University, Tempe, in 1987, after which he spent a year there as a

postdoctoral fellow. He joined the Department of Electrical Engineering at the University of Notre Dame, Notre Dame, Indiana, in 1988 as an assistant professor, and was the founding Director of the Notre Dame Nanoelectronics Facility (NDNF) from 1989 to 1998. Dr. Bernstein received an NSF White House Presidential Faculty Fellowship in 1992, was promoted to rank of Professor in 1998, and served as the Associate Chairman of his Department from 1999 to 2006. Bernstein was named a Frank M. Freimann Professor of Electrical Engineering in 2010. He has authored or co-authored more than 200 publications in the areas of electron beam lithography, nanomagnetics, quantum electronics, high-speed integrated circuits, electromigration, MEMS, and electronics packaging.

Wolfgang Porod currently is Frank M. Freimann Professor of Electrical Engineering at the University of Notre Dame. He received his Diplom (M.S.) and Ph.D. degrees from the University of Graz, Austria, in 1979 and 1981, respectively. After appointments as a postdoctoral fellow at Colorado State University and as a senior research analyst at Arizona State University, he joined the University of Notre Dame in 1986 as an Associate Professor. He now also serves as the Director of Notre Dame's Center for Nano Science and Technology. His research interests

are in the area of nanoelectronics, with an emphasis on new circuit concepts for novel devices. He has authored some 300 publications and presentations. He has served as the Vice President for Publications for the IEEE Nanotechnology Council (2002-2003), and he was appointed an Associate Editor for the IEEE Transactions on Nanotechnology (2001-2005). He has been active on several committees, in organizing special sessions and tutorials, and as a speaker in IEEE Distinguished Lecturer Programs. He is a Fellow of the IEEE.

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