A. DURAND, J.L TISSOT, P. ROBERT, S. CORTIAL, M. VILAIN, O. LEGRAS

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1 Thermogram µm VGA MICROBOLOMETER SENSORS FOR THERMOGRAPHIC APPLICATIONS A. DURAND, J.L TISSOT, P. ROBERT, S. CORTIAL, M. VILAIN, O. LEGRAS ULIS BP27 F Veurey-Voroize Abstract Since its first development in 2002, ULIS is dedicated to address infrared imaging and thermographic imaging. Ranges of applications and sensibility requirements are widespread among our customers. Ulis endeavored through a selection of product, with pixels size ranging from 35 µm down to 17 µm and FPA size of 160 x 120 up to 1024 x 768, to meet its customer needs whatever the application. This paper details our latest achievement on 640 x 480 / 17µm sensor and uses it as a reference to introduce thermographic system development recommendations. It also gives a general background of a-si bolometer s behaviour that impact thermographic applications. Châlons en Champagne, 8 et 9 décembre 2011 Congrès National de Thermographie THERMOGRAM' 2011

2 1 GENERAL DESCRIPTION OF ULIS BOLOMETER A - Bolometer Structure Bolometer pixel detector consists of an insulated plate above a quarter-wave cavity. This plate has a double function : first to absorb the incoming infrared flux and secondly, as a thermistor, it gives a direct access to the temperature raise through its electrical resistance variation. Figure 1 Schematics of the bolometer structure Absorption in the plate is optimized thanks to a quarter-wave cavity tune to the peak wavelength of interest. This cavity consists of a metallic reflector underneath the absorption plate. Insulation of the plate is paramount to ensure temperature rise of the bolometric material. This insulation is made possible by designing stretched arms for the plate, attached to the CMOS by stud. B - Amorphous Silicon ULIS s base thermometric material is amorphous silicon. As a reference material in micro-electronic industry, amorphous silicon is easily processed hence allowing very homogeneous deposition. The relative homogeneity throughout the focal plane array of key parameters such as resistivity or activation energy turns out to be of great importance in most applications. In particular, when it comes to address TEC-less capability, amorphous silicon devices are coping quite well with temperature variation. The bolometer resistance variation versus temperature is described by Arrhenius law in which activation energy Ea depends on the sensitive material. R = R0.exp( Ea / kt ) [1] As amorphous silicon is not an alloy, every pixel has the same activation energy (standard deviation on mean value < 0.4%) leading to a high spatial uniformity of pixels temperature behavior. Moreover, amorphous silicon activation energy Ea remains essentially constant throughout a large range of operational temperature [1]. C - Circuit The bolometer layer is processed directly on a dedicated imaging ASIC. The readout is operated in rolling shutter mode. Each imaging pixel is addressed through an injection TMOS and is coupled with a skimming bolometer as describe in the figure 2. The bolometer current is first skimmed to remove a large part of the useless offset current going through the bolometer then integrated to ensure high frequency filtering. The analog or numerical output is then multiplexed.

3 VDET Thermogram Pixel Active Bolometer Skimming Current-Voltage Conversion GAIN Figure 2 - Pixel readout architecture GFID CTIA To Sampling VDDA Blind Bolometer VBUS GSK Ground X 480 NEW GENERATION 17 µm A - Design Our new product is a 640 by 480 pixels focal plane array with pixel pitch of 17 µm. Pixel structure stemming from our nominal 25 µm pitch pixel has been partly re-design to ensure high absorption and keep up with the 25 µm pitch sensibility. The readout circuit has been tweak but keep the inheritance of the previous generation. The electric output range stands between 1.4V and 4.4V. The power supply rejection ratio is 30 db. A single analog output is available working up to 50 Hz with a nominal pixel clock at 10 MHz. This readout architecture presents power consumption below 155 mw in analog video mode. Figure 3 - Readout circuit block diagram This new product is packaged in a compact (24 x 24 x 4.1 mm 3 ), low weight (< 6 g) and TEC-less ceramic package. Specific care has been taken to ensure ascendant compatibility with previously developed 384 x 288 / 25 µm sensor. The mechanical and optical interfaces remain the same as ¼ VGA / 25 µm ones and electrical interface is almost compatible with the existing electronics boards. Figure x 480 / 17 µm ceramics package Châlons en Champagne, 8 et 9 décembre 2011 Congrès National de Thermographie THERMOGRAM' 2011

4 Regarding the environmental tests, this package design is compliant with the automotive AEC- Q / Grade 3 standard. Particularly, the detector can withstand high level thermal shocks (-50 C / +125 C cycles) and High Temperature Storage Life Test (HTSL 125 C 1000 h). Moreover, high combined temperature / humidity tests (95 C 95 % RH) have been successfully achieved. Several aging and storage tests (> C and 200 days at 170 C) are demonstrating a very good package vacuum behavior as computed reliability reaches > 93 % after 15 years. The packaging pin-out is full compatible with the previous TEC-less packaging of the ULIS 384x288 / 25 µm pixel pitch product. B - Performances * Electro-optical performances Prior to bringing up temperature dependence of this new device, we hereafter present the measured performances of one detector. All electrical and electro-optical tests are carried out using f/1 optical aperture and a frame rate of 30 Hz. This device can operate up to 50 Hz in analog mode. Responsivity is measured using two blackbodies respectively set at 293 K and 308 K. The main characteristics are presented in this section with an ambient temperature of 303 K. Figure 5 - VGA Output DC level 293K Figure 6 - NEDT histogram The overall output range dynamic is 3 V (between 1.4 V and 4.4 V). In the standard operational mode, the responsivity mean value stands at 12.1 mv/k 4 pf Gain), hence offering a scene dynamic higher than 150 K. Figure shows the output DC level for a standard component. The total distribution span (~ 350 mv) represents only 11.5 % of output dynamic. The resulting mean NEDT 303 K of focal plane temperature, stands at 46 mk (figure 6). Residual Fixed Pattern Noise (RFPN) for this component (in TEC-less operation) reaches 295 µv that is 75 % of the RMS noise. The microbolometer thermal constant has been measured over a range of 8.8 ms to 9.3 ms. Keeping this parameter lower than 10 ms is one of ULIS commitment to ensure good image quality at high video frame rate. If NEDT is required the VGA can be pushed to its limits by working on a sub array (320*240 for instance), hence the NEDT can be lowered down to 38 mk. In this mode, the Figure Of Merit, which combines NEDT and time constant, reached around 350 mk.ms. * TECLESS behaviour Our new product has been specifically design for low cost and low weight application. It does not have any temperature regulation system either heater or thermo-electric cooler. However, specific care have been taken to ensure operation of the FPA in a range of temperature going from -40 C to 85 C without any setting adjustment.

5 Thermogram Figure 1 shows how the mean DC level and signal-spread change with ambient temperature. The limitation due to operational temperature appears when the overall signal-spread kills the available scene dynamic. The VGA design ensures a dynamic over 100 C of scene temperature even at 85 C. Figure 1 - DC level drift with ambient temperature. In the standard recommended settings, NEDT and scene dynamic are changing with temperature as detailed below. Figure 2 - NEDT and scene dynamic evolution with temperature NEDT changes with temperature from 72 mk down to 45 mk at 30 C. Dynamic available on the scene is also shrinking as temperature goes higher. 3 BOLOMETERS IN THERMOGRAPHIC APPLICATIONS Bolometer detectors are perfectly design to ensure accurate temperature measurement. However, accuracy requires to pay a close attention to system design in order to take advantage of bolometer natural properties or to compensate specific bolometer behaviour. A - Advantages Bolometers advantages in thermographic applications are numerous. * Linearity At a given operating temperature the signal output is linear with the incoming flux. Second order deviation can occur for high dynamic range or very high flux. Châlons en Champagne, 8 et 9 décembre 2011 Congrès National de Thermographie THERMOGRAM' 2011

6 Figure 3 - Extraction of DC level output with incoming flux, on a temperature range of 5 C to 80 C and in 8 to 15 µm spectral range. * Uniformity Important parameters responsible for bolometer sensitivity are very uniform throughout an array. Resistance value dispersion < 0.5 %. It ensures a wide range of dynamic and a reduced dispersion of responsivity over the array ; Thermal resistance dispersion : < 2 %. Directly linked with the sensitivity it ensures, it ensures reduced dispersion of responsivity ; Activation energy dispersion < 0.4 %. It ensures a common temperature behaviour of the entire array with ambient temperature. * Predictability As described modelling chapter further on, bolometers follow simple set of behavioural law that can be easily put into equation. This ensures a good understanding and help foreseeing causes of deviation. B - Specific bolometer behaviour As a consequence of uncooled operation of bolometers, there is no IR obscurity due to the absence of cold shielding. Therefore, the output signal is not only linearly sensitive to the incoming flux from the scene; it is also sensitive to ambient temperature and incoming flux outside the scene field of view. On π Figure 4, the scene is focused to the FPA from the optic with a geometric etendue of, N is 4. N the f number of the optic. All the surrounding incoming flux (in orange on the figure), has a etendue of π π. 4. N The ratio of the geometrical etendue of the background and the one of the scene is 4. N that is to say four times if F#=1. If temperature variations induce a deterministic evolution on the output, 1/f noise of the bolometer can on the other hand cause very small variation that can reduce the accuracy of the measurement.

7 Thermogram Figure 4 - Geometrical etendue schematic 4 THERMOGRAPHIC DESIGN RECOMMENDATIONS In light of bolometer specific behaviour in thermographic application quoted in the previous chapter, we can elaborate advices when it comes to design and built thermographic camera. There is obviously many declinations of a thermographic camera design based mainly on different sets of constraints such as cost, accuracy, calibration time, system memory or system CPU capability. Hereafter, we evoke few of the trade-off according to what is most important for the system. A - Thermal mitigation The most important thing to handle in thermographic application is the ambient temperature variation. When developing a thermographic camera, one of the most important parameter is the thermal design. It must be done in order to prevent or to limit any temperature variation close to the detector. The sources of thermal variation are threefold and must be tackled at the same time. Figure 5 - Variation sources of output DC level To compensate for FPA temperature variation recommendations are : a) To tune the parameter settings to reduce the influence of this temperature : on figure 12, we have plotted the DC level variation with ambient temperature for a set of parameter that reduces the overall variation. Hence between -40 c to 50 C the deviation stay below 200mV. b) To reduce rapid temperature variation by insulating the circuit from ambient temperature and/or use a thermal mass to compensate for rapid changes. c) To use a set of calibration table according to ambient temperature based on circuit internal temperature sensor and external ones. And reprocess the signal accordingly. d) Ultimately, it is still possible to regulate the FPA temperature by using an external heater or TEC. This solution, though extremely costly and power consuming, will ensure accuracy and strongly reduce calibration time. Formerly used with older bolometer generation, this technique is more and more abandoned by low cost camera designer. Châlons en Champagne, 8 et 9 décembre 2011 Congrès National de Thermographie THERMOGRAM' 2011

Figure 6 - Example of DC level variation with temperature and bias settings 4.4 3.9 DC level variation with FPA temperature and bias change. DC level in Volt 3.4 2.9 2.4 1.9 1.

For the d one, it entails that optic s temperature has to be regulated. B - Calibration When it comes to measure with accuracy the incoming flux, as any detector, the IRFPA needs to be calibrated.

8 Figure 6 - Example of DC level variation with temperature and bias settings DC level variation with FPA temperature and bias change. DC level in Volt Temperature in C To compensate for off the scene temperature variation (optics, mechanics ), both previous c and d recommendations are applicable. For the d one, it entails that optic s temperature has to be regulated. B - Calibration When it comes to measure with accuracy the incoming flux, as any detector, the IRFPA needs to be calibrated. The level of calibration depends on : a) the range of acceptable temperature variation (if temperature stays still, calibration consists only of a 2 points corrections for linearity estimation) ; b) the accuracy required : the more accuracy is needed the more table covering the temperature span must be registered ; c) the calculation power of the camera (DSP or controller) : temperature variations induce rather smooth and continuous DC level variations ; those variations can be interpolated by polynomials if calculation power allows it. Calibration takes into account DC level variation due to ambient temperature change but also the responsivity change with FPA temperature. The figure below gives an illustration of how the responsivity is changing with temperature. Figure 7 - Responsivity evolution with temperature

Thermogram 2011 137 C Modelling At last, a good knowledge of bolometer physical behaviour can come up handy when accurate thermography is required.

For example, the combination of the resistor versus temperature variation equation and our knowledge of the activation energy homogeneity can help creating a first order forecast of the responsivity

9 Thermogram C Modelling At last, a good knowledge of bolometer physical behaviour can come up handy when accurate thermography is required. Not only does it help making good design choice but it can also help compensating small deviation by implementing adequate algorithm. For example, the combination of the resistor versus temperature variation equation and our knowledge of the activation energy homogeneity can help creating a first order forecast of the responsivity ( Figure 7) variation with temperature hence reducing dramatically calibration time. Essential behavioural equations : R = R0.exp( Ea / kt ) : Resistor variation with temperature. Re V out V R T. C int sponsivity V : Bolometer bias V V = R R s s int Cint : Capacitance of CTIA Tint : integration time. Tint. Vbus Vs : Skimming Bolometer bias Cint Rs : Skimming bolometer resistor Vbus : CTIA reference bias 5 IMAGES : ILLUSTRATION OF OUR NEW 640*480 ARRAY Figure 8 - Thermal skin image Figure 9 Landscape thermal image 6 CONCLUSION In this paper, we have presented the performances of our latest Infrared Focal Plane array featuring a 17 µm pixel pitch for a VGA resolution. Small pixel size and high performances (NEDT < 50 mk) allows its integration in high definition thermographic camera. We have also detailed how to handle infrared camera design for thermographic camera: It is paramount to take care of any thermal parasitic signal by mechanical design, to compensate for FPA variation using adequate settings and to use calibration table in accordance with the required accuracy. By respecting this recommendation, high precision thermographic camera can be built with thermal resolution close to 0.2 C. Châlons en Champagne, 8 et 9 décembre 2011 Congrès National de Thermographie THERMOGRAM' 2011

10 ACKNOWLEDGEMENTS The authors would like to thank the uncooled detectors teams of CEA/LETI LIR and ULIS who develop and produce uncooled infrared micro-bolometer detectors. REFERENCES [1] High performance Uncooled amorphous silicon VGA and XGA IRFPA with 17µm pixel-pitch, J.L Tissot, S. Tinnes, A. Durand, C. Minassian, P. Robert, M. Vilain, Electro-Optical and Infrared Systems: Technology and Applications, proc. of SPIE Vol 7834, (2010). [2] Latest amorphous silicon microbolometer development at LETI/LIR, J.J. Yon, E. Mottin, J.L. Tissot, Infrared technology and Applications XXXIV, proc. of SPIE Vol. 6940, (2008) [3] Uncooled amorphous silicon TEC-less ¼ VGA IRFPA with 25 µm pixel-pitch for high volume applications, C. Minassian, J.L. Tissot, M. Vilain, O. Legras, S. Tinnes, B. Fieque, J.M Chiappa, P. Robert, Infrared Technology and Applications XXXIV, proc. of SPIE Vol.6940 (2008) [4] High-performance uncooled amorphous silicon TEC less XGA IRFPA with 17µm pixel-pitch, C. Trouilleau, B. Fièque, S. Noblet, F. Giner, D. Pochic, A. Durand, P. Robert, S. Cortial, M. Vilain, J.L. Tissot, JJ. Yon, Infrared Technology and Applications XXXV, proc. of SPIE Vol.7298 (2009) [5] VOX based uncooled microbolometric detector: Recent development at SCD, A. Fraenkel, U. Mizrahi, L. Bykov, A. Adin, D. Seter, A. Giladi, E. Malkinson, Y. Gebil, Z. Kopolovich, Infrared Technology and Applications XXXII, proc. of SPIE Vol.6206 (2006)

1711. Analysis on vibrations and infrared absorption of uncooled microbolometer

1711. Analysis on vibrations and infrared absorption of uncooled microbolometer Chao Chen 1, Long Zhang 2, Yun Zhou 3, Xing Zheng 4, Jianghui Dong 5 1, 2, 3, 4 School of Optoelectronic Information, University