Edward T. Nelson

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.

A model for charge transfer in buried-channel charge-coupled devices at low temperature

Charge transfer in buried-channel charge-coupled devices (CCDs) is explored with a one-dimensional numerical model which describes the capture and emission of electrons from a shallow donor level in silicon through the use of the Shockley-Read-Hall generation-recombination theory. Incorporated in the model are the three-dimensional Poole-Frenkel barrier lowering theory of A. K. Jonscher (1967) and J. L. Hartke (1968) and the low-temperature form of Poissons equation. Reasonable agreement of the model with experimental data taken from the buried-channel CCDs of a PtSi Schottky barrier infrared image sensor is found. Moreover, the value for the capture cross section of electrons to the shallow phosphorus level in silicon inferred from the model follows the cascade theory for capture by M. Lax (1959) and agrees roughly with determinations made by other experimenters. >

Latch-up and image crosstalk suppression by internal gettering

Internal gettering can be used to reduce crosstalk in imagers and latch-up susceptibility in CMOS circuits. The internal gettering process forms defects in the bulk of the silicon wafers that are effective recombination sites for minority carriers in the substrate. Experimental and theoretical results are presented for the crosstalk reduction obtained in an area imager. Also, the current gain β of the parasitic lateral n-p-n transistors formed in the substrate in CMOS circuits was considerably lower for the internally gettered wafers. The trigger current needed to initiate latch-up in the n-p-n-p structures increased as 1/β, in accordance with the theory. A Monte Carlo method was developed to calculate the expected lateral transistor current gain. The calculated βs are in excellent agreement with the measured values.

Monte Carlo simulation of the photoelectron crosstalk in silicon imaging devices

The Monte Carlo method is used to evaluate the extent of the crosstalk in solid-state imagers. The calculations are performed in three dimensions and are in excellent agreement with experiment. The Monte Carlo method is used because it handles adjacent regions that either collect or reflect minority carriers.

A 1/3" format image sensor with refractory metal light shield for color video applications

The authors report results obtained on a full-color interline transfer CCD (charged-coupled device) image sensor with pixel dimensions of 8.6 mu m(H)*6.8 mu m(V) using 1.2- mu m design rules and a two-phase, single-polysilicon-per-phase technology. In order to reduce image smear and to provide suitable topography for integral color filters, a refractory light shield with a flowed glass overlayer was incorporated. The basic sensor and pixel architecture is shown. Image smear as a percent of full well, measured with 10% vertical illumination at saturated intensity, is shown as a function of wavelength. Smear is lowest at short wavelengths but is at an acceptable level for applications with controlled illumination.<<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.

Reduction of lateral diffusion of photoelectrons in silicon photodiode imager arrays by internal gettering

The lateral diffusion of photoelectrons to adjacent picture elements in a silicon linear photodiode array is reduced in substrates with a high density of oxygen precipitates formed by internal gettering. The signal due to diffusion in adjacent pixels, normalized to the illuminated pixel signal, was reduced by a factor of 1.6 for pixels with centers 48 µm apart and by a factor of 10 for pixels farther apart; there was no significant decrease in sensor quantum efficiency. These results are interpreted with a numerical model that solves the three-dimensional diffusion equation for a substrate with different lifetimes in the surface and internal regions.

Photography with an 11-megapixel 35-mm format CCD

The new Kodak KAI-11000CM image sensor-a 35-mm format, 11-Megapixel interline CCD-has been characterized to evaluate its performance in photography applications. Traditional sensor performance parameters, including quantum efficiency, charge capacity, dark current, and read noise are summarized. The impact of these performance parameters on image quality is discussed. A photographic evaluation of the sensor, including measurements of signal-to-noise and color fidelity, is described.

Latch-up and image crosstalk suppression by internal gettering [in CMOS]

Internal gettering can be used to reduce crosstalk in imagers and latch-up susceptibility in CMOS circuits. The internal gettering process forms defects in the bulk of the silicon wafers that are effective recombination sites for minority carriers in the substrate. Experimental and theoretical results are presented for the crosstalk reduction obtained in an area imager. Also, the current gain β of the parasitic lateral n-p-n transistors formed in the substrate in CMOS circuits was considerably lower for the internally gettered wafers. The trigger current needed to initiate latch-up in the n-p-n-p structures increased as 1/β, in accordance with the theory. A Monte Carlo method was developed to calculate the expected lateral transistor current gain. The calculated βs are in excellent agreement with the measured values.

Design and performance of a 486 x 640 pixel platinum silicide IR imaging system

An infrared imaging system based on a high resolution platinum silicide detector has been developed. The detector is a 486 X 640 Schottky Barrier photodiode array with a CCD readout multiplexer. The imaging system includes signal processing electronics to correct the fixed pattern noise and implements a histogram projection algorithm for automatic gain and offset adjustment and dynamic range compression. The performance of the pattern noise correction has been demonstrated to reduce the residual pattern noise below the image shot noise over a scene temperature range of more than 50 degree(s)C. The effect of optical cross talk in the imager has been examined. The magnitude of the side lobes has been found to be a factor of about 7 X 104 smaller than the central spot.