Hiroaki Utsumi

Technologies to improve photo-sensitivity and reduce VOD shutter voltage for CCD image sensors

New technologies to increase the photo-sensitivity and reduce the shutter voltage of the vertical over-flow-drain (VOD) have been developed for CCD image sensors. The photo-sensitivity was increased 40% by forming an anti-reflection film over the photodiode and reducing the thickness of the p/sup +/-layer formed at the photodiode surface. The VOD shutter voltage was reduced from 31 to 18 V by using an epitaxially grown substrate with double impurity concentration layers.

Optical limitations to cell size reduction in IT-CCD image sensors

We have determined the practical limits of cell size reduction in interline-transfer CCD image sensors, limits resulting from diffraction occurring at the aperture above the photodiode. We have found that image cell size cannot be reduced to a level for which aperture width would fall below about 0.2 /spl mu/m. We have also found, however, that image cells with greater than 0.2 /spl mu/m aperture size are sensitive over the entire wavelength range of visible light, and that sensitivity can be increased by thinning the photoshield film.

A 1/2-in 1.3 M-pixel progressive-scan IT-CCD for digital still camera applications

A 1/2-in 1.3 M-pixel progressive-scan interline-transfer charge-coupled-device (IT-CCD) image sensor has been developed for small, low-power mega-pixel digital still cameras (DSCs). The pixel size as small as 5 /spl mu/m square makes small-size progressive-scan IT-CCD (8.3/spl times/7.1 mm/sup 2/) for the SXGA format. A two-phase-drive horizontal-CCD with phosphorus-implanted storage regions helps reduce the driving voltage to 2.5 V, resulting in the power consumption of the device being as low as 146 mW. A new source-follower amplifier with separate p-well driver transistors achieves 12% higher gain than that obtained using a conventional amplifier. An overflow drain with a self-adjusting potential barrier can instantly remove superfluous charges in vertical-CCDs just before an exposure period, which enables DSCs to perform such functions as quick auto-focusing and dark-current removal. New dual operation modes for still and motion pictures can provide not only high-resolution color signals in a 15-frame/s 1050-line progressive mode but also wide-dynamic-range color signals in a 30-frame/s 525-line progressive mode. The latter mode employs a pixel-exchange-and-mix readout operation that helps halve the number of scanning lines with no loss in sensitivity and color information.

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.

A 30 frames/s 2/3-in 1.3 M-pixel progressive scan IT-CCD image sensor

A 30 frames/s 2/3-in 1.3 M-pixel progressive scan interline-transfer charge-coupled device (IT-CCD) image sensor has been developed for video and digital still-camera applications. To obtain high frame-rate images, a 49-MHz driving horizontal CCD (H-CCD) was developed. An 8-phase drive for vertical CCDs (V-CCDs) makes it possible to operate in a variety of modes, such as 1050 line progressive scan mode and 1049 line wide dynamic range interlaced scan mode. For digital still camera use, removing residual charges stored in the V-CCDs before exposure is essential, therefore new narrow-channel barrier over-flow drain (NCB-OFD) attached under the H-CCD was developed. The NCB-OFD automatically drains out extra charges and has the advantages of requiring neither an over-flow control gate nor any additional masks.

A 30 frame/s 2/3 inch 1.3M pixel progressive scan IT-CCD image sensor

For multimedia applications, 30Frame/s 2/3 inch 1.3M pixel progressive scan interline-transfer CCD (IT-CCD) image sensor employs an optimized well for the horizontal CCD (H-CCD) and a wide bandwidth amplifier for 49 MHz operation. An 8-phase drive for vertical CCDs (V-CCDs) makes it possible to carry out a variety of operations such as 1050 line progressive mode and 1049 line wide dynamic range interlaced mode. For still camera use, removing residual charges stored in the V-CCDs before exposure is essential. A narrow-channel barrier overflow drain attached under the H-CCD automatically drains extra charge and requires neither overflow control gate nor additional masks.

A 1/2 inch 1.3 M-pixel progressive-scan IT-CCD for still and motion picture applications

For still pictures, the sensor provides high-resolution color signals in a 15 frame/s 1050-line progressive mode. For motion pictures, it provides wide-dynamic range color signals in a 30 frame/s 525-line progressive mode. This latter mode employs a pixel-exchange-and-mix readout function that helps halve the number of scanning lines. A 5/spl times/5 /spl mu/m/sup 2/ pixel makes the 8.3/spl times/7.1 mm/sup 2/ IT-CCD the smallest yet applicable to the SXGA format. A two-phase drive horizontal CCD with phosphorus-implanted storage regions (PST-HCCD) with optimized wells so that they do not generate potential pockets under inter-electrode gaps helps reduce the driving voltage to 2.5 V. In still camera use, an overflow drain with self-adjusting potential barrier (SA-OFD) is attached to the HCCD to remove superfluous charges in the vertical CCD (VCCD) just before an exposure. A source-follower amplifier with separate p-well driver transistors with suppressed back-gate effects achieves a gain 11% higher than that obtained using a conventional amplifier. Green-sensitivity of the device is 190 mV (F8, 706 nt, 1/30 s), and the saturation signal is 400 mV (1050-line progressive mode).

Utilizing an optical cavity to increase the saturation level in a Schottky-barrier IR image sensor

A precise evaluation of the photodiode storage capacitance in a platinum silicide Schottky- barrier IR image sensor with an optical cavity and in which an aluminum reflector is electrically grounded has revealed that the cavity capacitance between the aluminum reflector and the platinum silicide film, which is one of four components in the photodiode storage capacitance, is dominant. While this area of the photodiode has previously been investigated for its optical characteristics, the present study represents the first reported investigation on its electrical characteristics. In order to increase the saturation level of an image sensor, it is essential to increase the storage capacitance in its photodiode. The storage capacitance in the photodiode was shown to be increased not only by newly using but also by increasing the cavity capacitance. When the SiO2 film between the aluminum reflector and the platinum silicide film was replaced with SiN, total storage capacitance in the photodiode was successfully increased by a factor of 1.6, while the optical characteristics of the photodiode remained the same.

New technologies of photosensitivity improvement and VOD shutter voltage reduction for CCD image sensors

New technologies to increase the photo-sensitivity and reduce the shutter voltage of the vertical over-flow-drain (VOD) have been developed for CCD image sensors. A 40 percent photo-sensitivity increase was obtained by forming an anti-reflection film over the photodiode, in addition to reducing the thickness of the P+ layer formed at the photodiode surface. A VOD shutter voltage reduction from 31 V to 18 V was successfully obtained by using an epitaxially grown substrate with double impurity concentration layers. We found that a stacked film structure of Si3N4 on SiO2 film was suitable for the anti-reflection to obtain the maximum increase in sensitivity. A suitable film thickness was estimated by using an optical multiple- reflection analysis simulator, resulting in a 10 nm SiO2 and a 50 nm Si3N4 films. As a result, a 30 percent higher photo-sensitivity than that of the conventional structure was obtained. Additionally, by reducing the depth of the P+ junction formed at the photodiode surface, a 10 percent photo-sensitivity increase was obtained for a 15 percent depth reduction. The VOD shutter voltage reduction was performed by preventing the photodiode depletion layer depth from spreading deeply into the substrate. An epitaxially grown substrate with double impurity concentration layers, whose impurity concentration of the bottom layer is 10 times higher than that of the top layer, was adopted.

A high fill factor and progressive scan PtSi Schottky-barrier IR-CCD image sensor using new wiring technology

A back surface illuminated 130/spl times/130 pixel PtSi Schottky-barrier (SB) IR-CCD image sensor has been developed by using new wiring technology, referred to as CLOSE Wiring, CLOSE Wiring, designed to effectively utilize the space over the SB photodiodes, brings about flexibility in clock line designing, high fill factor, and large charge handling capability in a vertical CCD (VCCD). This image sensor uses a progressive scanned interline-scheme, and has a 64.4% fill factor in a 30 /spl mu/m/spl times/30 /spl mu/m pixel, a 3.9 mm/spl times/3.9 mm image area, and a 5.5 mm/spl times/5.5 mm chip size. The charge handling capability for the 3.3 /spl mu/m wide VCCD achieves 9.8/spl times/10/sup 5/ electrons, The noise equivalent temperature difference obtained was 0.099 K for operation at 120 frames/sec with a 50 mm f/1.3 lens. >