Hirofumi Yagi

1040*1040 element PtSi Schottky-barrier IR image sensor

A high-density infrared image sensor has been developed for thermal imaging in the 3-5 mu m infrared band. The array size is 1040*1040. The detector is a platinum silicide (PtSi) Schottky-barrier diode. The charge sweep device (CSD) architecture is used for the device in order to realize a large fill factor and a high saturation level. The device is fabricated with a 1.5 mu m minimum feature size and has a large fill factor of 53% in spite of the small pixel size (17*17 mu m/sup 2/). The saturation level is improved by increasing the detector storage capacity. A high saturation level of 1.6*10/sup 6/ electrons is obtained. The noise equivalent temperature difference with f/1.2 optics is estimated as 0.10 K at 300 K.<<ETX>>

Performance of 320 x 240 uncooled IRFPA with SOI diode detectors

We reported a 320 x 240 uncooled IRFPA with 40 micrometers pitch having diode detectors fabricated on an SOI wafer. Since the fabrication process of the SOI diode detector is compatible with the silicon IC process, only a silicon IC fab is necessary for manufacture of the FPAs. This enables mass production of low cost uncooled FPAs. This paper focuses on the performance of the FPA. In the previous paper, we proposed a novel infrared absorbing structure which offers a very high fill factor. Although this structure exhibited a high infrared absorption because of interference absorbing components incorporated in the structure, large thermal capacitance was an issue. Thus we have improved the infrared absorbing structure in the newly developed FPA. The improved absorbing structure has been devised making use of reflection of metal interconnections including diode metal straps. A thermal time constant of 17 msec has been achieved without degrading the responsivity compared with the conventional absorbing structure.

Pixel scaling for SOI-diode uncooled infrared focal plane arrays

Pixel scaling for SOI diode uncooled infrared focal plane arrays (IRFPAs) was investigated in order to achieve the realization of small size and low cost IRFPAs. Since the SOI diode pixel has two different layers -- one for the temperature sensor and the thermal isolation structure, and the other for the infrared absorption structure -- each layer can be independently designed. Hence, a high fill factor can be maintained when reducing pixel size without changing the basic structure of the pixel, which is advantageous in reducing the pixel size. In order to verify this, the authors have developed an SOI diode IRFPA with the pixel size of 28 μm x 28 μm which is 49% of the previous pixel size (40 μm x 40 μm) and achieved a noise equivalent temperature difference (NETD) of 87 mK. In order to further reduce the pixel size and to improve device sensitivity, we propose a new pixel structure. In this structure, a reflector is fabricated between the infrared absorption structure and support legs. Therefore, the infrared rays which are incident on the support legs, which do not sufficiently function as a reflector, can be used effectively. A new pixel structure with a pixel size of 25 μm x 25 μm was fabricated and realized the thermal conductance of 1.0 x 10-8 W/K and the infrared absorption structure was then verified for its effectiveness.

Monolithic Schottky-barrier infrared image sensor with 71% fill factor

An improved 512 x 512-element PtSi Schottky-barrier IR image sensor (512 x 512 IRCSD) has been developed using the charge sweep device (CSD) readout architecture and 1.2-[mu]m minimum design rules. Finer pattern process technology enhances the advantage of the CSD readout architecture, enlarging the fill factor without sacrificing the saturation signal level. A large fill factor of 71% is achieved in spite of a small pixel size of 26 x 20 [mu]m[sup 2]. At the Schottky-barrier detector reset voltage of 4 V, the differential temperature response with f/1.2 optics at 300 K and saturation signal level were 3.2 [times] 10[sup 4] electrons/K and 2.9 [times] 10[sup 6] electrons, respectively. The noise equivalent temperature difference was estimated as 0.033 K with f/1.2 optics at 300 K. The improved 512 x 512 IRCSD was designed to be operated either in the field or frame integration interlace modes for versatility.

PtSi FPA with improved CSD operation

Over the past ten years we have been developing PtSi focal plane arrays (FPAs) using the charge sweep device (CSD). FPAs are going to high resolution and the power of the FPAs are on an upward trend. Now we have developed a low-power CMOS CSD scanner (LOCCS) for a high resolution FPA. The conventional CSD scanner operates at the same frequency as that of the horizontal CCD to prevent fixed pattern noise (FPN), and generates a frequency pulse higher than the minimum requirement. The LOCCS is a kind of CMOS dynamic shift resistor, which generates clock pulses for vertical signal transfer without the low frequency input pulses that cause FPN. Because the LOCCS generates multi-phase clock pulses, the power consumption can be reduced. We have fabricated test devices to evaluate the improved CSD operation by the LOCCS, and confirmed that the devices operate normally and the reduction of power consumption is in good agreement with the theory. We also applied the LOCCS to a 256 by 256 PtSi FPA and obtained thermal images.

Silicon infrared focal plane arrays

Using Si VLSI technology, we can fabricate various kinds of infrared focal plane arrays (FPAs) which cover spectral bands from short wavelength infrared to long wavelength infrared. The Si-based technology offers many attractive features, such as monolithic integration, high uniformity, low noise, low cost, and high productivity. We have been developing Si-based infrared FPAs for more than 20 years and have verified their usefulness.

Portable high-performance camera with 801 x 512 PtSi-SB IRCSD

We have developed an 801 by 512 element PtSi Schottky-barrier FPA with low-power multiphase CSD readout for a compact high performance infrared camera. The power consumption of this detector is about a half of the conventional IRCSD and the number of pixels is 1.5 times of that. The cryocooler is a high efficiency and compact size Stirling-cycle cooler that has also been developed for this imager at the same time. The cooling capacity is just match for the new IRCSD. This paper describes the basic imager design and features.

High-fill-factor monolithic infrared image sensor

A 256 X 256 element platinum silicide monolithic image sensor with a large fill factor has been developed as a high sensitivity infrared image sensor. It is essential to increase the maximum signal charge of the staring infrared image sensor to obtain higher sensitivity. We used the Charge Sweep Device readout architecture and improved operations of the floating diffusion amplifier to increase the maximum signal charge. A 52 X 40 micrometers 2 pixel using a minimum features size of 2 micrometers has a fill factor of 66%. Evaluating the performance of the device, we confirmed the effectiveness of the improved technologies. The measured saturation level is 2.8 X 106 electrons which is determined by the storage capacity of the detector. We estimated from a measurement point at low background temperature that the noise equivalent temperature difference with a f/1.2 optics is 0.036 K at 300 K background.