Shigeru Tohyama

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.

Influence of Fermi‐level pinning on barrier height inhomogeneity in PtSi/p‐Si Schottky contacts

The barrier height inhomogeneity in PtSi/p‐Si and IrSi/p‐Si was observed by internal photoemission. New Fowler equations were introduced, to analyze the observed properties. Two regions with different barrier heights were assumed to coexist, and the individual barrier heights were evaluated. One of two barrier heights was consistent with the generally obtained value in individual contacts. The other was 0.39 eV in both contacts. The origin of two regions was explained in terms of Fermi‐level pinning.

A standard-television compatible 648*487 pixel Schottky-barrier infrared CCD image sensor

Describes a 648*487 pixel PtSi Schottky-barrier infrared CCD image sensor. Due to the development of the modified inverted-LOCOS process, which can minimize dead regions, and the two-dopant concentration structure, which achieves both a large charge capability and high transfer efficiency, a 40% fill factor in a 21- mu m*21- mu m pixel and a 0.1-K noise equivalent temperature difference were 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.

A new concept silicon homojunction infrared sensor

A novel silicon photovoltaic infrared sensor is proposed. It consists of three regions with a homojunction structure that has a flexibility designed barrier height corresponding to the cutoff wavelength. The operation of the sensor is based on infrared absorption and internal photoemission by free-electrons in a conduction band in a degenerated n/sup ++/-type silicon. Results obtained with fabricated sensors have verified the basic operating principle and have shown that the detectable wavelength range extends past 12 mu m.<<ETX>>

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.

New thermally isolated pixel structure for high-resolution uncooled infrared FPAs

This paper proposes a new thermally isolated pixel structure, having a twice-bent beam structure and eaves structure, suitable for high-resolution uncooled infrared (IR) focal-plane arrays (FPAs). It also describes the properties of test devices, fabricated to verify the effect of the new pixel structure. Although the pixel size of the test devices is 23.5 μm × 23.5 μm, which represents a smaller area by a factor of about 2.5 than the 37 μm × 37 μm pixel size for the 320 × 240 bolometer-type uncooled IRFPA, previously developed by the authors, the test devices have beams with almost the same length as in the previous IRFPA by utilizing the new beam structure. In addition, the cross-sectional area of the beam is reduced. Accordingly, the thermal conductance of the test devices can be reduced by a factor of about 2.5. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3, which is consistent with our calculations. By utilizing the new thermally isolated pixel structure, the test devices with 23.5 μm pixels enable us to achieve thermal sensitivity equivalent to the previous 37 μm pixels.

324*487 Schottky-barrier infrared imager

A standard TV-compatible PtSi Schottky-barrier infrared imager is described. The imager is a 324*487 element area array and has an electronic shutter function. Although the pixel is 42*21 mu m, a large fill factor of 42% is obtained, using a 1.5- mu m minimum design rule and a two-level polysilicon layer, and two-level aluminum layer structure. Using face-down bonding technology, it was possible to reduce the package size to 60% of the conventional ceramic package size. Due to optimization of the Schottky-barrier diode process and the diode structure, the noise equivalent temperature difference is as small as 0.1 K at f/1. >

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.

An infrared-bi-color Schottky-barrier CCD image sensor for precise thermal images

An infrared-bi-color image sensor was developed with a barrier height controlled Schottky-barrier photo diode array for precise temperature images. Low and high barrier height diode pixels are arranged vertically next to one another using a selective area ion implantation technique. Conventional monochrome infrared image sensors frequently give wrong temperature images due to an unreasonable emissivity assumption. The infrared-bi-color image sensor can obtain the temperature image precisely with regard to the emissivity of the object.