Performance of 320x240 Uncooled Bolometer-typeInfrared Focal Plane ArraysYutaka Tanaka*a, Akio Tanaka c, Kiyoshi Iida b, Tokuhito Sasaki b, Shigeru Tohyama b,Akira Ajisawa b, Akihiro Kawahara b, Seiji Kurasina d Tsutomu Endoh a, Katsuya Kawano a, Kuniyuki Okuyama a, Kazuyuki Egashira b, Hideo Aoki b, Naoki Oda ba 1st Business Development Operations Unit, 1st Custom LSI Division, NEC Electronics1753 Shimonumabe, Nakahara-ku, Kawasaki, Kanagawa211-8666, Japanb Guidance and Electro-Optics Division, NEC Corporation1-10 Nisshin-cho, Fuchu, Tokyo183-8501, Japanc Silicon Systems Research Laboratories, NEC Corporation1120 Shimokuzawa, Sagamihara, Kanagawa 229-1198, Japand 2nd Operations Division, NEC Glass Component7-8 Ebisu-cho, Kanagawa-ku, Yokohama, Kanagawa, JapanABSTRACTThe performance of a 320 x 240 bolometer-type uncooled infrared (IR) Focal Plane Array (FPA) is described.Vanadium oxide thin film is adopted for the bolometer material, having a sheet resistance of ~10 kohms /square. It is patterned such that the bolometer resistance is by a factor of 10 larger than the sheet resistance. On-chip readout integrated circuit(ROIC) is designed to reduce signal drift , extend dynamic range for object temperature and extend ambient temperature range in which operates non-uniformity correction is carried out with about 1/10 fewer frequency than the former ROIC.The 320 x 240 FPA consists of pixels sensitive to IR radiation and optical black (OB) pixels covered with plate which shuts out IR radiation. Drift is reduced by current mirror circuit, using the OB pixels and digital compensation circuit based on voltage change of OB pixels resulting from change in operation temperature. Both the dynamic range and the ambient temperature range are extended by decreasing integration gain and developing low-noise, low-power and large swing operational amplifier (OP-AMP).Since decrease in integration gain degrades noise equivalent temperature difference (NETD), bias voltage forbolometer is increased by factor of 2 and bandwidth is reduced by route half. Finally, IR image was obtained with prototype camera and NETD value was found to be smaller than 0.1K for F/1 optics at 60Hz frame rate and thermal time constant was measured to be 12msec.Keywords: uncooled, microbolometer, infrared, focal plane array, noise, readout1.INTRODUCTIONSince uncooled IR cameras have characteristics of low price and maintenance-free, the demand for the IR cameras has increased. The uncooled IR cameras have been applied to such fields as security, surveillance, thermography, driver’s vision enhancement, intelligent transport systems and so forth. IR markets require several improvements to achieve user * Correspondence: Email : y-tanaka@; Telephone:+81-44-435-1634; Fax:+81-44-435-1911414Infrared Technology and Applications XXIX, Bjørn F. Andresen, Gabor F. Fulop, Editors,Proceedings of SPIE Vol. 5074 (2003) © 2003 SPIE · 0277-786X/03/$15.00friendliness, for example, compactness, light weight, low price and no interruption due to shutter during IR image acquisition. Reduction in pixel size makes optics more compact, lighter and cheaper. However, there is a problem of decrease in output signal due to smaller amount of incident IR radiation on each pixel. Therefore, resposivity has tobe increased and low noise ROIC should be developed. Furthermore, it is a problem for surveillance camera that realtime IR images are often interrupted by shutter operations which are indispensable for non-uniformity correction (NUC). Although, there is a solution to use radiation shield and obstruction-free aperture, optics is more expensive due to increase in number of lenses.This paper describes the following improvements;(1) Low noise, low power and large swing OP-AMPs in ROIC are designed,(2) Responsivity of bolometer array is increased,(3) Low noise and low power circuit for signal processing in camera is developed,(4) Shutter operations for NUC are hardly required to obtain realtime imagery, even in the case of a large amount of background radiation change resulting from variation in ambient temperature (about a couple of tens degrees C) for a wide range of object temperature (a couple of hundreds degrees C), and there improvements make NETD smaller than 0.1K for F/1 optics at 60Hz frame rate.2. VO X MICROBOLOMETER ARRAY2.1 STRUCTURE AND FABRICATIONThermal detectors should be thermally isolated from the heat sink to increase their sensitivities. A thermal isolation structure, i.e., a suspended microbridge structure, achieves the sensitivity increase because it prevents the heat collected by areas sensitive to IR radiation from escaping to the heat sink.Figures 1 (a) and 1 (b) show schematic structure of the bolometer pixel. The pixel is divided into two parts (figure 1 (a)), a silicon (Si) readout integrated circuit in the lower part and a suspended microbridge structure in the upper part. The two parts are separated by a cavity. The microbridge structure is composed of a diaphragm and two beams (figure 1 (b)). The diaphragm is supported by the two beams and is therefore thermally well isolated from the SiROIC heat sink. Figures 2 (a) and 2 (b) show secondary electron micrograph (SEM) pictures. The pixel size is37×37micrometer and the fill factor is 72%. The IR radiation is partly absorbed by the SiN passivation layer in the diaphragm and partly transmitted. The transmitted radiation is perfectly reflected back to the diaphragm by the reflecting layer and is absorbed again by the SiN passivation layer. Thus, about 80% of the incident IR radiation in10micrometer wavelength region is absorbed. The absorbed IR radiation heats the diaphragm and changes the bolometer resistance. Supplying a bias current enables the resistance change to be read out as a voltage change.(b)Figures 1. (a) Schematic cross-sectional view of bolometer pixel along bias current path, and(b) Schematic plan view of bolometer pixel.Proc. of SPIE Vol. 5074 4152.2 CHARACTERISTICSThe properties of a 320×240 bolometer-type uncooled IRFPA were investigated form the viewpoint of uniformity. Figures 3 (a), 3 (b) and 3 (c) are histograms of bolometer resistance (Rb), the temperature coefficient of resistance (TCR) and thermal conductance (Gth), respectively. Data was acquired every sixteen pixels.The distributions of Rb, TCR and Gth have non-uniformity of 10~20% peak-to-peak, ~5% peak-to-peak and ~10% peak-to-peak, respectively, for more than 50 fabrication runs. The distribution of TCR/Gth value, which is a good indicator of voltage responsivity (see the equation (1) mentioned later), has non-uniformity of 10~20% peak-to-peak. The typical value of thermal conductance is 0.1µW/K. It should be mentioned that the mean TCR value of –1.64%/K measured at 40degrees C in the figure corresponds to -1.87%/K at 20degrees C.The voltage responsivity (R V ) is expressed by equation (1), which includes key parameters of the deviceperformance, such as the temperature coefficient of bolometer resistance (TCR), thermal conductance (Gth), IR absorbance (N) and thermal time constant (Ttc).Gth Cth Ttc W V Ttc f πGth N Vb TCR Rv =]/[)×2(+11××=2ޓޓޓޓ (1)Here, Vb, f and Cth are bias voltage, chopping frequency and thermal mass, respectively.The relative responsivity is measured as a function of the chopping frequency (see figure 4). By fitting the equation (1) with the measured curves, the thermal time constant is calculated to be 12 msec. This is an adequate response time for obtaining thermal images at a TV frame rate. From the measured ፧thermal time constant value of 12msec and the measured Gth value of 0.1micro-W/K, the thermal mass of the diaphragm is estimated to be 1.2 nJ/K. The performance characteristics of doped VO X microbolometer array are summarized in Table 1.020406080100120140160180767880828486889092949698100 (a)(b)of resistance (%/K) measured at 40degrees CResistance(Kohms)Figure 3 (a). Histogram for bolometer resistanceFigures 2. (a) SEM picture of 320 x 240 bolometer arrays, and (b) Magnified pictureDiaphragmBeamN u m be r of p i x e l s416 Proc. of SPIE Vol. 5074Figure 3 (c). Histogram for thermal conductance Figure 4. Frequency dependence of responsivityTable1.Summary of design and performance characteristics of doped VO X microbolometer3.FPA ARCHITECTURE AND FPA DESIGNA block diagram of FPA is shown in Figure 5(a) and circuit configuration of readout channel is shown in Figure 5(b) and the chip photograph of the FPA is shown in Figure 6. The FPA has 320 x 240 pixels with 37 x 37 micrometer pitch. The uncooled IR sensor LSI was fabricated with the 0.5-micrometer CMOS process. The on-chip ROIC፧is composed of FPA, band-gap reference circuit(BGR), bias generation circuit(BGC), integration circuits, sample and hold(SH) circuits, multiplexers, output OP-AMP and horizontal and vertical shift registers, the ROIC is driven by only external three clocks and 4bit data. The 4bit data is externally provided to data buses in ROIC and the data consists of sensor bias, resistance of bias cancel circuit, integration capacitor, NUC data and so forth.(see table2) The ROIC can be operated in NTSC mode or in PAL mode and each pixel is pulse-biased. The 183 readout channels are employed for the column of the bolometer array and each readout channel reads out the 2-columns of the array. The readout channel is composed of bolometer bias circuit that supplies the bolometer with bias voltage, bias cancel circuit that subtracts pedestal component from the bias current and integrator that integrates the residual bolometer current. In the final stage of the readout channel, one analog output voltage is selected by two SH circuits. Combination of the integration capacitor with resistance in the bias cancel circuit should be selected to keep NETD value smaller than 0.1K,because the FPA’s operation temperature ranges from -30degrees C to 60degrees C,which changes the resistance of the bolometer from ~20kohms to ~500 kohms.Proc. of SPIE Vol. 5074 417Figure 5(b) Circuit configuration of readout channel 418 Proc. of SPIE Vol. 5074320 x 240 PIXELSBOLOMETER ARRY183 READOUT CHANNELSS/H AND MUXThe bolometer has high temperature coefficient of resistance to realize high responsivity to the incident infrared. This means that the bolometer is extremely sensitive to the chip temperature. In order to reduce the influence of change in the chip temperature (CT), this ROIC has the configuration shown in figure7. This circuit automatically removes change in the pedestal component of the bias current due to change in the chip temperature, so that it mitigates the required stability of the thermoelectric cooler (TEC), which is stringent, especially in turning on the power supply. In this configuration, OB pixels covered with a plate which shuts out IR radiation, act as bolometers whose resistances are changed by the chip temperature. The current that flows in the OB pixels are transformed into voltage by diffusion resister, and the voltage is entered into background radiation correction (BRC) through a low pass filter (LPF). This means that the current in the OB pixels are mirrored to the current generated by bias cancel circuit. In this way, the influence of chip temperature change, which exerts to both OB pixels and bolometer, is canceled. The OP-AMPs in each readout channel, which cancel temperature dependence of V GS in MOSFET, are also effective for reducing drift current. Moreover, the resistance of bias cancel circuit shown in figure 5(b) is, indeed, composed of two resistances with reverse TCRs, which is also effective for reducing drift current. Shutter frequency resulting from the drift is reduced by about 1/10 fewer than the former FPA[1] owing to the temperature compensated configuration of the readout channel.Figure 7 Temperature compensation configurationProc. of SPIE Vol. 5074 419Dynamic range of output signal and noise has to be simultaneously considered to improve performance of IR camera. Requirement for IR camera is as follows; shutter operations for NUC are hardly required to obtain realtime imagery, even in the case of a large amount of background radiation change resulting from variation in ambient temperature (about a couple of tens degrees C) for a wide range of object temperature (from -30degrees C to +150degrees C). This requirement can be achieved without radiation shield nor obstruction-free to lower cost of IR camera.In order to satisfy both dynamic ranges of temperature, it is necessary to decrease an integration gain. However, there is a problem due to increase in input conversion noise. Therefore, bias voltage of bolometer has to be increased to improve responsivity and ROIC with low noise and large voltage swing should be developed.A bias voltage of bolometer is actually increased by a factor of 2, namely 4V. The ROIC mentioned above is realized by the followings;(1) Integration time is doubled by increasing the number of readout channel.(Bandwidth is reduced by route half), (2) OP-AMPs with low noise, low power, small size and large voltage swing is designed, (3) Resistance of bias cancel circuit is selected,(4) Resister of bias cancel circuit is designed to have as large size as possible to decrease the 1/f noise,(5) Integration capacitor of bias cancel circuit is selected to make integration time as long time as possible.The thermal noise of each OP-AMP is reduced by increasing bias current. The 1/f noise of each OP-AMP is reduced by using large size MOSFET. Figure 8 shows the measured waveform for the analog output (Vout) of the OP-AMP. The dynamic range of an output OP-AMP is found to be in the range from +2.5V to +7.5V,using signals of defective pixels. This voltage range corresponds to the object temperature range from -30degrees C to +150degrees C and the ambient temperature range of a couple of tens degrees C.As is shown in figure 9, dynamic range (5V) of Vout contains object signal voltage, background IR radiation signal Figure 8. Output waveform of large voltage swing OP-AMPFigure 10 shows the calculated results for thermal noise and 1/f noise in each part of ROIC and camera. All noise values are converted into the values at the bolometer node. Total noise at the bolometer node is measured to be 20micro voltage rms at 60Hz fame rate, which agrees with the calculation. Although this value is roughly twice as large as the noise value of the former sensor [1], the bias voltage of bolometer is increased by a factor of 2 in this ROIC, and then NETD value doesn’t change.Figure 10. The calculated results for thermal noise and 1/f noise in each part of ROIC and cameraThe ROIC thus developed has such advantage that dynamic ranges of both object and ambient temperatures enlarged without changing NETD value. The design and performance characteristics of FPA and ROIC are summarized in Table 2.Table2.Summary of design and performance characteristics of FPA and ROICProc. of SPIE Vol. 5074 4214. PROTOTYPE CAMERA4.1 DESIGNThe prototype camera is shown in Figure 11.The camera has characteristics, such as compactness, light weight, low cost and low power. The camera utilizes 25mm focal length F/1 Ge optics with anti-reflection coating adapted to the 10micrometer wavelength region. A 320×240 bolometer-type uncooled FPA chip is incorporated into a vacuum sealed package and is placed at focal plane of the IR camera. The performance characteristics of camera are summarized in Table 3.The camera has a power consumption smaller than 6W. The weight is 800g(except for optics).The size of the prototype camera is as small as 80mm(height) x 77mm(width) x 129mm(length).ᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳᱳThe block diagram of the prototype camera is illustrated in figure 12. The camera electronics consist of a power supply board (PSB), analog circuit board (ACB) and two digital signal processing boards(DSPB). One can adjust output level, offset, gain, digital zoom, polarity, NUC and selection of pseudo color or white/black, with user interface on the rear panel of camera. The camera operates at 60Hz (NTSC) or 50Hz (PAL) frame rates .The PSB is designed to use commercial Li-ion battery. Therefore, the input power supply voltage ranges from 5.5V to 12V. The switching power supply is adopted to reduce power consumption. The PSB provides +3.3Vdc (digital), +5Vdc(digital), +15Vdc(analog) and +8Vdc (analog).The ACB consists of the low-noise power supply for ROIC and the temperature stabilization control circuit for thermoelectric cooler(TEC). The analog signal voltage of the ROIC is digitized to 14bit data .The DSPB has a wide variety of functions, including generation of timing clocks for ROIC, correlated double sampling which is used with thermally shorted pixels (see figure 6), NUC, digital noise reduction, defective pixels substitution ,gain and offset adjustment, and look up table for pseudo color and IR radiation correction. DSPB outputs include NTSC or PAL as well as LVDS 16bit parallel digital data. The DSPB accepts RS-232C commands.Two point corrections are made for the digitized data by exposing the FPA to two black bodies with different temperatures. The data is, thus, corrected for non-uniformity of bolometer responsivity.After the power supply is turned on, first, TEC is stabilized and a shutter temperature which is close to an ambient temperature, is measured. The shutter temperature is memorized to memory of DSPB. Next, the background IR radiation correction is performed by digital processing of DSPB which is to search IR radiation correction data with binary search method. If the shutter temperature is changed by a certain amount of temperature, the correctionprocessing is iterated. The certain amount of temperature is changed by the ambient temperature which is measured as the shutter temperature, and then is controlled by CPU of DSPB. The certain amount of temperature is slightly larger than a couple of tens degrees C.Figure 11. The photograph of the prototype camera Table3 Summary of performance characteristics of prototype cameraArray Size 320x240Optics FOCAL LENGTH 25mm F=1 Camera Size 80(H) x 77(W) x 129(L) mmCamera Weight 800gFrame Rate NTSC/PAL Conformity Analog:NTSC/PAL Conformity Outputs Digital:LVDS 8-bit Parallel output portRemote control RS-232C SerialContrast/Brightness AutomaticPower Consumption <6W NETD <100mKOparation Temparature-10~+50 degrees C422 Proc. of SPIE Vol. 5074Figure 12. The block diagram of the prototype camera4.2 TEST RESULTSA thermal IR image is, thus, obtained for F/1 optics and at 60 Hz frame rate (see figure 13). NETD value is measured for the prototype camera, by introducing IR radiation of calibrated black body source to a certain part of FPA. Values of signal and noise are read out, using 16bit digital output data of all pixels, and standard deviation is calculated and is taken as the noise value. The NETD value is found to be smaller than 0.1K.Proc. of SPIE Vol. 5074 4235. CONCLUSIONThe authors design, fabricate and evaluate a 320x240 bolometer-type uncooled IR FPA, on-chip readout circuit and camera. A thermal image with an NETD value smaller than 0.1 K is obtained for F/1 optics and at 60Hz frame rate.The drift characteristics of the ROIC are improved by the temperature compensated configuration and therefore, shutter operation frequency is reduced by 1/10 fewer than the former camera.This performance is good enough for uncooled IRFPA to be used for surveillance camera, radiometric camera and so forth.ACKNOWLEDGMENTSThe authors would like to thank Messrs. S.Matsumoto, H.Murofushi, K.Katoh, M.Miyoshi and M.Hijikawa (NEC Corporation), Messrs. H.Gotoh, Y.Tsuruta, A.Nakazato and F.Nishio (NEC Glass Components) for their collaboration during development and fabrication of detectors and camera.REFERENCES[1] A. Tanaka, K. Chiba, T.Endoh, K.Okuyama, A. Kawahara, K. Iida, N. Tsukamoto, “Low-noise readout circuitfor uncooled infrared FPA”, SPIE, 4130, pp.160-167, 2000.424 Proc. of SPIE Vol. 5074。