Edmund K. Banghart

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. >

Extension of the open-circuit voltage decay technique to include plasma-induced bandgap narrowing

An expression for the open-circuit voltage decay is derived to include the effects of plasma-induced bandgap narrowing, as well as the effects of Fermi-Dirac statistics. As a result, the range of validity of the open-circuit voltage decay technique is extended to highly injected carrier densities in silicon in excess of 10/sup 17/ cm/sup -3/. The theory, when applied to data from the point-contact concentrator cell, in which highly injected carrier densities have been reported, provides a result consistent with recently determined values for the Auger coefficient and the intrinsic carrier concentration. In the course of the modeling, useful analytical expressions for both experimental and theoretical determinations of the bandgap narrowing have also been found. >

The effect of potential obstacles on charge transfer in image sensors

Numerical methods are presented to investigate charge transfer in charge-coupled devices (CCDs) when potential barriers or wells occur. A Monte Carlo simulation of electron thermal diffusion and field-aided drift is used to determine the time scale for charge transfer. The Monte Carlo approach is useful for exploring new problems, but it requires considerable amounts of computer time. A quicker technique, that of the mean first passage time, is introduced. This method reduces the solution of the carrier continuity equation for charge transfer to the evaluation of a double integral that yields the characteristic time /spl tau/ for e/sup -t//spl tau//. This provides the leading or dominant time dependence of the carrier continuity equations solution. Numerical examples are presented to show how /spl tau/ varies with the size and location of the potential obstacle. The mean first passage time approach permits rapid estimates of the effects of potential obstacles on charge transfer in CCDs. These estimates are in excellent agreement with the results of the Monte Carlo simulations.

An LOD with improved breakdown voltage in full-frame CCD devices

In full-frame image sensors, lateral overflow drain (LOD) structures are typically formed along the vertical CCD shift registers to provide a means for preventing charge blooming in the imager pixels. In a conventional LOD structure, the n-type LOD implant is made through the thin gate dielectric stack in the device active area and adjacent to the thick field oxidation that isolates the vertical CCD columns of the imager. In this paper, a novel LOD structure is described in which the n-type LOD impurities are placed directly under the field oxidation and are, therefore, electrically isolated from the gate electrodes. By reducing the electrical fields that cause breakdown at the silicon surface, this new structure permits a larger amount of n-type impurities to be implanted for the purpose of increasing the LOD conductivity. As a consequence of the improved conductance, the LOD width can be significantly reduced, enabling the design of higher resolution imaging arrays without sacrificing charge capacity in the pixels. Numerical simulations with MEDICI of the LOD leakage current are presented that identify the breakdown mechanism, while three-dimensional solutions to Poissons equation are used to determine the charge capacity as a function of pixel dimension.

31 Mp and 39 Mp full-frame CCD image sensors with improved charge capacity and angle response

This paper describes the design and performance of two new high-resolution full-frame architecture CCD imaging devices for use in professional color, digital still-imaging applications. These devices are made using 6.8 μm pixels and contain a dual-split HCCD register with two outputs to increase frame rate. The KODAK KAF-31600 Image Sensor (31 Mp) is designed with microlenses to maximize sensitivity, whereas the KODAK KAF-39000 Image Sensor (39 Mp) is designed without microlenses to maximize incident light-angle response. Of particular interest is the implementation of an under-the-field oxide (UFOX) lateral overflow drain (LOD) and thin light shield process technologies. The new UFOX LOD structure forms the LOD under the thick-field oxide that eliminates a breakdown condition, allowing much higher LOD doping levels to be used. The net result is that the LOD may be scaled to smaller dimensions, thereby enabling larger charge capacities without compromising blooming control. The thin light shield process utilizes only the TiW portion of the TiW/Al metal bilayer to form the pixel aperture. This reduces the overall stack height that helps improve angle response (for pixels using microlenses) or critical crosstalk angles (for pixels without microlenses).

Advanced process and device modeling of full-frame CCD imagers (Invited Paper)

Using Synopsys TCAD tools, several examples of advanced process and device modeling are presented for full-frame CCD image sensors. The topics covered in these examples include channel potential, charge capacity, charge transport, and charge blooming. The simulations provide in depth analysis of the basic principles of operation of CCDs and cover some aspects of antiblooming protection.