Naoki Yutani

Low-Temperature Characteristics of Buried-Channel Charge-Coupled Devices

The characteristics of buried-channel CCDs in the temperature range between 40 K and 300 K were studied. It was found that the transfer loss had a peak at about 130 K and increased rapidly below 90 K. In these specific temperature regions, the transfer loss depended on the clock period, the data cycle time and the clock fall time. The characteristics were analyzed based on free-charge transfer mechanisms and the Shockley-Read-Hall theory. The peak at 130 K was caused by a trap at the energy level 0.22 eV below the conduction band, and the increase below 90 K by the carrier freeze-out. Furthermore, the reduction of impurity concentration in the buried channel turned out to be an effective method of improving the transfer efficiency below 90 K.

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>>

High-performance 1040- x 1040-element PtSi Schottky-barrier image sensor

We have developed a monolithic 1040 X 1040 element PtSi Schottky-barrier infrared image sensor. This device uses the Charge Sweep Device readout architecture with four parallel outputs. The pixel size is 17 X 17 micrometers 2, which is 56% of that of our 512 X 512 element PtSi image sensor. In order to keep sufficient sensitivity with such a small pixel, we have developed a 1.5 micrometers Schottky-barrier process technology and improved the fill factor. The fill factor of this device is 53%. As a result of this improvement, a high differential temperature response of 9.6 X 103 electrons/K and a low noise equivalent temperature difference of 0.1 K have been achieved with f/1.2 optics. We have also improved the saturation characteristics of the device by optimizing the impurity concentrations of the isolation region and guard ring. The saturation level is 1.6 X 106 electrons at a detector reset voltage of 4 V.

Realization of low on-resistance 4H-SiC power MOSFETs by using retrograde profile in P-body

Inversion-type 4H-SiC power MOSFETs using p-body implanted with retrograde profiles have been fabricated. The Al concentration at the p-body surface (Nas) is varied in the range from 5×1015 to 2×1018 cm-3. The MOSFETs show normally-off characteristics. While the Ron is 3 cm2 at Eox = (Vg-Vth)/dox ≅ 3 MV/cm for the MOSFET with the Nas of 2×1018 cm-3, the Ron is reduced by a decrease in the Nas and 26 mcm2 is attained for the device with the Nas of 5×1015 cm-3. The inversion channel mobility and threshold voltage are improved with a decrease in the Nas. By modifying the structural parameter of the MOSFET, a still smaller Ron of 7 mcm2 is achieved with a blocking voltage of 1.3 kV.

IrSi Schottky-barrier infrared image sensor

An iridium silicide Schottky-barrier (IrSi SB) infrared image sensor with 512×512 pixels has been developed. The Charge Sweep Device architecture is applied to the device. The detector is an IrSi/p-Si SB diode, which has a cutoff wavelength of 7.3µm. The device is cooled to 62K for reducing thermally generated dark current. The device operates at the NTSC frame rate (30 frames/s). The examples of thermal imaging with this device are demonstrated.

256 X 256 Element Platinum Silicide Schottky-Barrier Infrared Charge-Coupled Device Image Sensor

A 256 x 256 element PtSi Schottky-barrier 1R-CCD image sensor has been developed using a minimum design rule of 2 µm and a three-level polysilicon structure. The pixel size and chip size are 37 x 31 µm2 and 10 x 10 mm2, respectively. In spite of the small pixel size, a large fill factor of 25% has been obtained. The responsivity has been improved by use of a thin metal film and an optical cavity structure. The barrier height and quantum efficiency coefficient obtained from the array performance measurement are 0.23 eV and 0.15 eV -1, respectively. The noise equivalent temperature difference of about 0.1 K is obtained with f/1.4 optics and a 16.7 ms stare time. The noise in this case is limited by the shot noise of the detector. An infrared camera was also developed using the 256 x 256 element IR-CCD image sensor.

1040 X 1040 infrared charge sweep device imager with PtSi Schottky-barrier detectors

The authors have developed a 1-Mpixel infrared charge sweep device (IRCSD) imager for thermal imaging in the 3- to 5-[mu]m band. The device of this imager is a 1040 x 1040 monolithic PtSi Schottky-barrier (SB) array using the charge sweep device (CSD) readout architecture. The pixel size is 17 x 17 [mu]m[sup 2] and the fill factor of this device is 44%. In this imager system, four video signals are read out from four independent channels on the device. The processing of these four outputs, such as sample and hold (S/H), and offset control and image correction, is performed in parallel, after which these outputs are combined to produce high-definition TV (HDTV; 1,125 lines, 30 Hz) format thermal image in real time. The noise-equivalent temperature difference (NETD) with f/1.2 optics at 27 C background is 0.13 C at the HDTV output stage.

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.

A wide spectral band photodetector with PtSi/p-Si Schottky-Barrier

The photoresponse of a front-illuminated PtSi Schottky-barrier detector is measured in the wavelength range between 0.4 and 5.2μm. In the wavelength range longer than 1.1μm, the detection mechanism is the internal photoemission. On the other hand, the intrinsic mechanism becomes dominant in the wavelength range shorter than 1.1μm. The measured data are in good agreement with values calculated from these two detection mechanisms. The photoresponse depends on the PtSi thickness in both wavelength ranges. For getting a high responsivity, it is important to make a thin uniform metal film. The visible and the thermal image with a PtSi Schottky-barrier wide spectral band imager are also demonstrated.

High Density Schottky-Barrier Infrared Image Sensor

In recent years silicide Schottky-barrier focal plane technology has received great interest because it is the most promising technology for high density infrared image sensors. However, improvement in not only the detector performance but also the readout architecture is indispensable for realizing high density infrared image sensors. This paper presents the advances in silicide Schottky-barrier focal plane technology and the design considerations for high resolution infrared image sensors with video quality. A 512x512-element platinum silicide Schottky-barrier infrared image sensor with a novel readout architecture is also described.