Timothy J. Tredwell

The pinned photodiode for an interline-transfer CCD image sensor

A pinned photodiode has been developed for use in an interline-transfer CCD. This photoelement has excellent blue response and high charge capacity. Both modeling and experimental results will be presented, including process considerations necessary to avoid unwanted barriers at the diode/transfer-gate edge.

Steady-state photocarrier collection in silicon imaging devices

Solid-state imagers lose resolution when photocarriers generated in one imaging site diffuse to a nearby site where they are collected. These processes are modeled by solving the steady-state diffusion equation for minority carriers. A source term represents the absorption of photons and the generation of photocarriers, and a linear term represents the loss of photocarriers by recombination. This is equivalent to studying the Helmholtz equation with an inhomogeneous term. The problem is simplified when the light source has symmetry. A line source or a cylindrically symmetric source leads to a two-dimensional problem. The approach of Seib, Crowell, and Labuda allows a solution by quadrature if the further assumption of a smooth top boundary is made. We calculate the integrated normal flux over each imaging site to see how many carriers diffuse from under the illuminated site to another site. We compare our predicted line- and point-spread functions to those measured on imagers and find reasonable agreement. This allows us to extract minority-carrier diffusion lengths. Further calculations show how the diffusion of carriers depends on the photon wavelength and the pixel size. We generalize Seibs approach and apply it to a solid-state imager covered with color filters. This allows us to see the extent of color mixing due to carrier diffusion. We also discuss a finite-difference solution of the diffusion equation that employs the method of conjugate gradients. This approach is useful for problems where the top boundary is not smooth.

Threshold voltage in short-channel MOS devices

The threshold voltage in short-channel MOS transistors was investigated by use of a two-dimensional numerical solution of Poissons equation and experimental measurements on devices of 5.15-, 3.15-, and 2.15-µm channel length. The assumption of constant equipotential surface in the oxide implicit in the charge-sharing technique is not valid in devices of shorter Channel lengths and at larger operating voltages. The numerical determination of the threshold voltage from the two-dimensional analysis agrees with experimental results. Unlike previous work, the charge-sharing model was investigated from an electric-field point of view. The inadequacies of the charge-sharing model are elucidated qualitatively and quantitatively.

A 1-Megapixel, progressive-scan image sensor with antiblooming control and lag-free operation

A 1024-pixel*1024-pixel interline charge-coupled device (IL CCD) image sensor has been developed. It incorporates antiblooming and electronic exposure control while eliminating lag and obtaining a high responsivity. The novel features of this device include a noninterlaced, or progressive-scan, architecture and dual-horizontal registers that can be used to clock out the image area by one or two lines at a time. These features make it well suited for applications demanding high-resolution-image capture from a single, high-speed scan. The progressive-scan architecture of this device covers the same resolution in an electronic-camera application as that of a 2-million-element, interlaced device. >

A digital video signal processor for color image sensors

The authors describe signal processor (DSP) for CCD (charge-coupled-device) cameras using a specified color filter array pattern. A block diagram of the DSP chip is shown. The chip has been designed and fabricated in a 2- mu m single-poly double-metal CMOS process. Eight scan-test registers were used at selected points in the processing chain to enable the entire chip to be tested, including ROMs and line delays, with 16 k vectors. The chip is functional at a maximum clock rate of 14.3 MHz. An image processed by the device is shown. The data path is designed with simple ripple-carry adders and dynamic registers. The on-chip programmable delay lines and 14.3-MHz clock-rate allow the chip to accommodate sensors for up to 768 active pixels, making it suitable for NTSC, CCIR 601 and PAL video standards.<<ETX>>

Wide-field-of-view PtSi infrared focal plane array

A 640 x 486 pixel monolithic focal plane array detector using PtSi Schottky barrier photodiodes was developed. This detector uses 1.2-micron design rules to achieve a 54-percent fill factor with 25-micron square pixels. The detector array used an interline CCD configuration with a progressive scan (noninterlaced) readout of the field, and two-phase clocking of both the vertical and horizontal registers.

A 1.4 million element, full frame CCD image sensor with vertical overflow drain for anti-blooming and low color crosstalk

Blooming and color crosstalk must be greatly suppressed in solid-state image sensors for nearly all imaging applications. A vertical overflow drain has been developed for a 1.4 megapixel image sensor for blooming suppression and low color crosstalk. The overflow drain is formed using a uniform flat p-well. This paper describes the modeling, fabrication, and experimental data associated with implementing vertical overflow in this device.

Channel potential and channel width in narrow buried-channel MOSFET's

A new method is described for determining the effective width over which incremental charge spreads in a narrow buried-channel transistor. The method is based on the transconductance in the buried-channel mode. Experimental results for effective widths and channel potential shifts are presented for MOSFETs with effective channel widths from 2 to 10 µm. Two-dimensional numerical calculations verify the experimental results.

A 360,000 pixel color image sensor for imaging photographic negatives

We describe a740(H) \times 242(V) × 2 charge-coupled color image sensor for imaging photographic negatives. The sensor achieves charge capacity of1 \times 10^{6}electrons per pixel, random noise of 300 rms electrons per pixel, and dynamic range of 70 dB. Sensor design, spectral sensitivity, charge capacity, and noise are discussed.

Segmented Amorphous Silicon n-i-p Photodiodes on Stainless-Steel Foils for Flexible Imaging Arrays

This paper reports the first successful attempt to fabricate amorphous silicon (a-Si:H) n-i-p photodiodes on a thin stainless-steel foil substrate for medical X-ray imaging applications. Two architectures of the n-i-p-photosensor, where the top electrode is based on amorphous or polycrystalline ITO, have been developed and characterized. The impact of critical fabrication steps including the deposition of semiconductor layers, dry etch of the NIP stack, diode passivation and encapsulation, as well as a contact formation on the device performance is presented and discussed. The test structures comprising segmented photodiodes with an active area ranged from 0.126 × 0.126 to 1 × 1 mm 2 have been fabricated on stainless-steel foils and on glass substrates for the purposes of process characterization. The fabricated samples are evaluated in terms of current-voltage, capacitance-voltage, and spectral response characteristics.