Tsutomu Endoh

Uncooled infrared focal plane array having 128 x 128 thermopile detector elements

A 128 X 128 element thermopile infrared image sensor has been developed. This device has a monolithically integrated structure to increase fill factor. The CCD for signal charge accumulation and signal charge read-out is fabricated on the silicon surface. Over the CCD, silicon dioxide diaphragms for thermal isolation are made by using micromachining technology. On each diaphragm, 32 pairs of p-type polysilicon and n-type polysilicon thermopile are formed. The noise equivalent temperature difference obtained by the device is 0.5 degree(s)C with an f/1 lens. Since the materials used are the same as those for silicon ICs, and since the whole fabrication process is carried out at the silicon IC plant, it can be said that a low cost uncooled infrared image sensor is realized by this technology.

Silicon IC Process Compatible Bolometer Infrared Focal Plane Array

A 128 x 128 element bolometer infrared image sensor using thin film titanium is proposed. The device is a monolithically integrated structure having a titanium bolometer detector and a CMOS circuit underneath it for reading out the bolometers signals. By employing a metallic material like titanium and refining the CMOS readout circuit, l/f noise can be minimized. It was demonstrated that by using small l/f noise material, a bias current is increased and S/N ratio can be subsequently improved. Since the fabrication process is silicon-process compatible, low-cost inkared image senson can be realized by using this technology.

Influence of bias heating on a titanium bolometer infrared sensor

A 128 by 128 pixel bolometer infrared focal plane array using thin film titanium has been developed. The device is a monolithically integrated structure with a titanium bolometer detector located over a CMOS circuit that reads out the bolometers signals. Since the thermal conductance of the bolometer detector is minimized, the temperature of the detector itself is increased by applying the bias current. Under the present operating conditions of the titanium bolometer, this temperature increase becomes about 30 degrees Celsius. The influence of this bias heating on device destruction and degradation was experimentally investigated and is discussed. The noise equivalent temperature difference obtained with the device is 0.07 degrees Celsius. Since the fabrication process is silicon-process compatible, costs can be kept low.

Performance of 320x240 uncooled bolometer-type infrared focal plane arrays

The 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 approximately 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 for bolometer 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 12 msec.

Low-noise readout circuit for uncooled infrared FPA

A low-noise architecture for uncooled microbolometer focal plane arrays is described. The on-chip readout circuit contains an integration circuit in which the bolometer current is directly injected into a capacitor, and exhibits extremely low noise with no decrease in signal. The simple configuration of the integration circuit makes it possible to operate more circuits in parallel, and increases the integration time and number of pixels. The bias circuit for the integration circuit is formed on the chip to reduce the effect of changes in the substrate temperature. The equivalent input noise, in which all readout noise is converted into that at the bolometer node, was 6.2(mu) V rms. A noise at this level is so low that can loosen the required TCR in the bolometer material. A 37-micrometers -pitch 320 x 240 ROIC was fabricated, and its expected NETD was 67-34 mK at a TCR of 1-2%/K. This architecture makes it possible to produce low-cost miniature cameras.

Uncooled infrared detectors toward smaller pixel pitch with newly proposed pixel structure

Abstract. An uncooled infrared (IR) focal plane array (FPA) with 23.5 μm pixel pitch has been successfully demonstrated and has found wide commercial applications in the areas of thermography, security cameras, and other applications. One of the key issues for uncooled IRFPA technology is to shrink the pixel pitch because the size of the pixel pitch determines the overall size of the FPA, which, in turn, determines the cost of the IR camera products. This paper proposes an innovative pixel structure with a diaphragm and beams placed in different levels to realize an uncooled IRFPA with smaller pixel pitch (≦17  μm). The upper level consists of a diaphragm with VOx bolometer and IR absorber layers, while the lower level consists of the two beams, which are designed to be placed on the adjacent pixels. The test devices of this pixel design with 12, 15, and 17 μm pitch have been fabricated on the Si read-out integrated circuit (ROIC) of quarter video graphics array (QVGA) (320×240) with 23.5 μm pitch. Their performances are nearly equal to those of the IRFPA with 23.5 μm pitch. For example, a noise equivalent temperature difference of 12 μm pixel is 63.1 mK for F/1 optics with the thermal time constant of 14.5 ms. Then, the proposed structure is shown to be effective for the existing IRFPA with 23.5 μm pitch because of the improvements in IR sensitivity. Furthermore, the advanced pixel structure that has the beams composed of two levels are demonstrated to be realizable.

Uncooled infrared detector with 12μm pixel pitch video graphics array

Uncooled infrared detectors with 12μm pixel pitch video graphics array (VGA) have been developed. To improve the signal to noise ratio (SNR) for 12μm pixel pitch, a highly sensitive bolometer material, an advanced pixel structure for thermal isolation and a newly designed read-out IC (ROIC) have been also developed. The bolometer material has been improved by using vanadium niobate. Over a wide range of temperature, temperature coefficient of resistance (TCR) is achieved higher level than -3.6%/K, which is 2 times higher than that for the conventional bolometer material. For thermal isolation, thermal conductance (Gth) value for the new pixel structure, fabricated by using triple level sacrificial layer process, is estimated to be 5nW/K, which is 1/5 times lower than that for the conventional pixel structure. On the other hand, since the imaging area is reduced by the pixel pitch, the uniformity of pixel can be improved. This enables to remove the non-uniformity correction (NUC) circuit in the ROIC. Removal of this circuit is effective for low power and low noise. This 12μm pixel pitch VGA detector is packaged in a compact (24 × 24 × 6.5 mm) and lightweight (11g) ceramic package. In addition, it has been incorporated in a newly developed prototype miniature imager. The miniature imager has dimension of 25(H) ×25(W) ×28(L) mm and weight of 30g. This imager is compact and small enough to fit in your hand. Hereafter, this imager is greatly expected to be applied to mobile systems.

Estimating temperature and emissivity for infrared measurements using a PtSi Schottky-barrier infrared CCD image sensor

A new analytical method to estimate temperature and emissivity for infrared measurements is described. There are four steps in the method. First, in calculating the output voltage, the dependence of temperature and emissivity of the object was evaluated. The result was the output voltage increased in proportion to the second power of the object temperature and the dependence of the emissivity was linear for the 250 K to 400 K temperature range. Second, in the fitting of these polynomial equations, the orders of six coefficients were also evaluated. Third, in measuring the output voltage of the standard imaging area, the unit transfer coefficient from digital unit (LSB) to voltage (V) was computed. Finally, an inversion problem for estimating temperature and emissivity of the object was proposed. We have developed a new kind of 3 approximately 5 micrometers band Schottky-barrier infrared CCD image sensor, which we call SCAT648, to verify the validity of the estimating method. The SCAT648 image sensor is composed of the different types of pixels. These pixels have different spectral responsivities and capabilities of measuring target temperature and target emissivity. Four standard temperature-controlled samples were imaged with the newly developed SCAT648 camera system. We estimate the error of the temperature and emissivity measurements to be a low +/- 0.5 K and +/- 5%, respectively.