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).
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.
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 Poisson's equation are used to determine the charge capacity as a function of pixel dimension.