Akihiro Kawahara

Design and performance of 256x256 bolometer-type uncooled infrared detector

This paper discusses the design and performance of a 256 X 256 bolometer-type uncooled infrared detector. First, model calculations are carried out to clarify the relations of the noise equivalent temperature difference (NETD) to the electrical properties of the bolometer material. The properties are mainly resistivity, the temperature coefficient of resistance (TCR) and 1/f noise. To obtain real-time images with NETD values smaller than 0.15 K for F/1 optics, vanadium oxide thin film was developed as the bolometer material, having a sheet resistance range of 5 - 50 k(Omega) /square and a TCR value of -2%/K. This material did not exhibit thermochroism like VO2(A), because it was identified as VO2(B). The bolometer-array was statistically evaluated and put into the infrared camera. Finally, a thermal image with a NETD of 0.15 K was obtained.

New thermally isolated pixel structure for high-resolution (640×480) uncooled infrared focal plane arrays

A new pixel structure with twice-bent beams and eaves structure, suitable for high-resolution uncooled infrared (IR) focal plane arrays (FPAs), is proposed. In comparison with previous results (FPA of 37-µm pixel pitch), the thermal conductance of the test device with the proposed pixel structure of 23.5-µm pitch is reduced about 2.5 times. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3. A 640×480 bolometer-type uncooled IRFPA is demonstrated by utilizing the new pixel structure, with supplementary modification to improve thermal conductance and thermal time constant. It shows a noise equivalent temperature difference (NETD) of 50 mK for F/1.0 optics at 30 frames/sec, a thermal conductance of 0.03 µW/K, and a thermal time constant of 16 msec.

Fabrication process for 256 x 256 bolometer-type uncooled infrared detector

A process for fabricating a monolithic 256 X 256 bolometer-type uncooled IR detector array is presented that utilizes surface micromachining technology. Each pixel of the device is composed of two parts, a silicon readout integrated circuit in the lower part and suspended microbridge structures in the upper part. The device is based on vanadium oxide bolometer films, which typically exhibit a temperature coefficient of resistance of -2 percent K. The vanadium oxide film is subject to damage, especially during wet etching of the sacrificial layer. Hence, the material and deposition process of the passivation layer for vanadium oxide film were investigated toward attaining a damage-free and flat microbridge structure. This was achieved by adjusting both the thickness of the passivation layer and the stresses in the electrode and passivation layers. The stiction problem of the microbridge structure was solved, by investigating drying conditions after etching of the sacrificial layer. Since each pixel has a cavity structure of (lambda) /4 to absorb IR radiation of the wavelength (lambda) , the spectral response of the pixel was measured in the wavelength range of 2 to 12 micrometers . The interference characteristics can clearly be seen. From responsivity measurements both in vacuum and at one atmosphere, the thermal time constant, thermal mass, and thermal conductance were estimated.

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.

Development of MBE-grown HgCdTe 64 x 64 FPA for long-wavelength IR detection

The HgCdTe (MCT) 64 X 64 focal plane array (FPA) for long wavelength infrared (LWIR) detection was developed, using MCT epilayers grown by molecular beam epitaxy (MBE). The n-on-p photodiode array has a cutoff wavelength of 10.7 micrometers . The readout circuit, with off focal plane integration capacitance, was designed for 77 K operation. These components were fabricated independently and were hybridized. The 97.8% operability was obtained. Photodiode characteristics for each pixel were measured directly. Mean R

Hybrid 256 x 256 LWIR FPA using MBE-grown HgCdTe on GaAs

o)A value of 1.9 (Omega) (DOT) cm2 and quantum efficiency of 0.3 were obtained. Using an infrared camera system with nonuniformity correction function, the infrared image was successfully demonstrated. An NETD (noise equivalent temperature difference) value of 0.117 K was attained with an F/2.5 optical lens under the 300 K background condition.

Infrared focal plane array detector and method of producing the same

A hybrid HgCdTe 256 by 256 FPA for LWIR detection was fabricated and an infrared image was demonstrated. MCT epilayers were grown on GaAs substrates by MBE and annealed to p- type. The n+ on p photodiodes were formed by boron ion implantation. The mean value of zero bias differential resistance for the diode array was measured to be 8.0 M(Omega) with a cutoff wavelength of 9.5 micrometer. The effective quantum efficiency was estimated to be 0.55, and the optical cross talk was estimated to be 8.2%. A multiline parallel integration readout circuit designed especially for this 256 by 256 LWIR FPA, had 8.3 X 107 electron capacity, a 190 microsecond integration time, and a single output. This work shows that the MBE growth method on GaAs substrates, pn junction formation process, the MLPI circuit design, and the hybridization technique are useful technologies.