Transcript
A stitched 24×24 field-effect transistor detector array and low-noise readout electronics for real-time imaging at 590 GHz Justinas Zdanevičius1, Sebastian Boppel2, Maris Bauer2, Alvydas Lisauskas1,2, Vilius Palenskis1, Viktor Krozer2, and Hartmut G. Roskos2 1
Radiophysics Department, Vilnius University, Vilnius, LT, 10222 Lithuania Physikalisches Institut, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, DE, 60438 Germany
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Abstract─We report on developments of the camera for 590 GHz radiation which has been stitched from four 12×12 antenna-coupled field-effect transistor detector (TeraFET) arrays and supplemented with parallelized low noise read-out electronics. In the current state, the read-out speed of 740 frames-per-second (fps) for a 12×12 element segment has been achieved. It is anticipated that in the final state the full 24×24 detector camera will be able to deliver images with a maximum of 1.3 kfps rate and will be able to resolve <1 nW at 30 Hz repetition rate. I. INTRODUCTION The steadily growing interest in terahertz (THz) imaging technologies resulted to recent emergence of commercial or soon-to-be-commercialized THz imagers [1,2,3] which operate at room-temperature. These cameras employ bolometric detection approach and are based on welldeveloped infrared imager technologies after required optimization of sensors for THz frequencies. Alternative commercialized technologies are also coming from microwave detection approach involving scalable electronic rectifier arrays [4,5]. It has been recently reported on realization of THz detector arrays using field-effect transistor detectors implemented in standard CMOS technologies [6,7]. To the moment, the sensitivities of both cameras are reported to be only of about 10 nW with 25 fps rates contrasting to the typical noiseequivalent power values of tens of pW/√Hz achievable with single detectors without limitations posed by read-out electronics [8,9]. This strongly indicates for a need in development of customized low-noise read-out schemes. In this contribution a special attention is given for the minimization of read-out noise. II. CHARACTERISTICS OF THE READ-OUT ELECTRONICS FOR THE DETECTOR ARRAY
As a basic sensor element we have used a die comprising 12×12 detector pixels with both horizontal and vertical pitch of 200 µm [7]. Two corner pixels have been omitted for technological manufacturing reasons. In order to increas the area of imaging array, we have stitched 4 dies together resulting to the toltal of 24×24 pixel focal plane array (FPA). The toltal number of active pixels is 568. Every single detector is comprised of a 590 GHz resonant patch antenna coupled to the pair of field-effect transistors. Electrical tests did not show even a single bad-pixel over the tested dies. The channel resistances of all detectors in the array have been found to be statistically distributed around 14 kΩ at 0.7 V gate bias with the root-mean-square deviation of about 1.5 kΩ.
Each pixel in the FPA besides TeraFET detector has two additional FET switches which enables the pixel to be addressed via row-select lines. Input at each row-select line switches 12 pixels and signals at column-output lines can be read simultaneously. FET switches and TeraFETs are ESDprotected with additional diodes. The schematic of a single pixel is shown in Fig. 1. The gate bias of the detectors (VG) is set precisely by the adjustable low-noise voltage source between 0 V and 2.048 V in 8 mV steps. VDD is fixed to 1.8 V and is set by another voltage source. Row-select lines are driven by the digital signal demultiplexer which sets 1.2 V at the output when the line is switched on. Detected signal from the pixel is fed to the low-noise preamplifier at each column-output. Further parallel read-out is done simultaneously and no analog multiplexing is employed to commute signals. Analog multiplexing can be the major source of noise where low level signals occur and presents a crosstalk between analog and digital signals. In order to achieve high speed and low-noise read-out, any analog multiplexing after output of FPA was omitted from design. Column out
VG
VDD
Row select
Fig. 1. Schematic of the single pixel in a CMOS detector array and the control lines.
Total of 48 buffered 16-bit successive approximation register (SAR)-based analog-to-digital converters (ADCs) is used to convert signals simultaneously. Each ADC accepts symmetrical input and has an internal buffered reference source. Converted signals are sent over the serial-peripheralinterface (SPI) to the microcontroller which also controls a digital demultiplexer and voltage sources. Processed data and control of the device is available and can be sent to PC over high-speed universal serial bus (USB). The block diagram of read-out electronics is presented in Fig. 2.
Row-select inputs
1×48 digital DEMUX
1.3 kfps rate and will be able to resolve <1 nW at 30 Hz repetition rate. The imaging experim ments are ongoing. FPA: four 12×12 detector arrays (24×24)
Column-outputs
Loow noise 0÷2.0048 V source
µC (322-bit ARM Corteex – M4F)
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24×2 low-noise preamplifiers
1.8 V voltage source
USB
24×2 16-bit ADCs
PC
Fig. 2. Block diagram of the read-out electtronics.
The low noise preamplifiers were developed on the base of pre-selected OPA827 operational amplifiers with junction field-effect transistors at the input stage. T The advantage of such OPs is that their own noise level doees not depend on signal source resistance for a very wide rangge of resistances. The equivalent input noise resistances of thee preamplifiers do not exceed 1 kΩ, and are about 14 times smaller than the thermal noise of the channel resistance of the TeraFET detector in the bias optimized for operationn. The schematics and the noise characteristics of an OPA827-based preamplifier is shown in Fig. 3. The preamplifier has gain of 1000 and a flat frequency band from DC up to 30 kHz. F Finally, the photo of assembled THz camera is shown in Fig. 4. 10
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Rin=14 kΩ
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Fig. 3. Schematic of the single OPA827-based preampllifier and the noise at the output of four pre-selected preamplifiers at zeero input resistance (shown for estimation of amplifier noise) and equuivalent resistance of TeraFET detector biased to the lowest NEP P conditions.
III. SUMMARY In the current state, developed readd-out algorithms achieve 740 fps read-out for a 12×12 elemeent segment. It is anticipated that in the final state the fulll 24×24 detector camera will be able to deliver images withh a maximum of
Fig. 4. The photo of the designed and d assembled THz camera.
ACKNOWLEDGE EMENT Vilnius group acknowledges fundin ng by the European Social Fund under the Global Grant measure. m Frankfurt team acknowledges funding from LOEW WE initiative by state of Hesse “Sensors towards Terahertz”. ES REFERENCE [1]. N. Oda, “Uncooled bolometer-type Terahertz T focal plane array and camera for real-time imaging,” Comptes Reendus Physique, vol. 11, no. 7–8, pp. 496–509, Aug. 2010. [2]. D.-T. Nguyen, F. Simoens, J.-L. Ouvrrier-Buffet, J. Meilhan, and J.-L. Coutaz, “Broadband THz Uncooled Antennaa-Coupled Microbolometer Array: Electromagnetic Design, Simulations and Meeasurements,” IEEE Transactions on Terahertz Science and Technology, vol. 2,, no. 3, pp. 299–305, May 2012. [3]. M. Bolduc, M. Terroux, B. Tremblay y, L. Marchese, E. Savard, M. Doucet, H. Oulachgar, C. Alain, H. Jerom minek, and A. Bergeron, “Noiseequivalent power characterization of an unco ooled microbolometer-based THz imaging camera,”Proc. SPIE8023, 2011, p. 80230C–80230C–10. 8 [4]. http://www.traycer.com/products-2/ [5]. http://terasense.com/products/ hao, W. Forster, H. M. Keller, A. [6]. R. Al Hadi, H. Sherry, J. Grzyb, Y. Zh Cathelin, A. Kaiser, and U. R. Pfeiffer, “A 1 k-Pixel Video Camera for 0.7-1.1 Terahertz Imaging Applications in 65-nm CMOS,” C IEEE Journal of SolidState Circuits, vol. 47, no. 12, pp. 2999–3012 2, Dec. 2012. [7]. A. Lisauskas, S. Boppel, M. Saphar, V. Krozer, L. Minkevičius, R. Venckevičius, D. Seliuta, I. Kašalynas, V. Taamošiūnas, G. Valušis, and H. G. Roskos, “Detectors for terahertz multii-pixel coherent imaging and demonstration of real-time imaging with a 12x12-pixel 1 CMOS array,” Proc. SPIE 84960, p. 84960J–84960J–9. 2012 [8].S. Boppel, A. Lisauskas, M. Mundt, D. Seliuta, L. Minkevicius, I. Kasalynas, G. Valusis, M. Mittendorff, S. Winnerl, V. Krozer, and H. G. Roskos, “CMOS Integrated Antenna-Coupleed Field-Effect Transistors for the Detection of Radiation From 0.2 to 4.3 THz,” IEEE Transactions on Microwave Theory and Techniques, vol. 60, no. 12, pp. 3834–3843, Dec. 2012. B Y. Zhao, E. Ojefors, A. [9]. R. Al Hadi, H. Sherry, J. Grzyb, N. Baktash, Kaiser, A. Cathelin, and U. Pfeiffer, “A broadband b 0.6 to 1 THz CMOS imaging detector with an integrated lens,” in i Microwave Symposium Digest (MTT), 2011 IEEE MTT-S International, 201 11, pp. 1–4.
