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

Abstract

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.

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

“…When the implant dose is increased, the resistance of the LOD decreases, and antiblooming protection is improved. Other important factors include the charge capacity (N sat ), exposure time (∆t) and the number of pixels in the vertical direction (M) whose general relationship is shown in [1] below. As the pixel is scaled to smaller dimensions to increase the resolution, including reductions in the LOD width, it becomes necessary to decrease the resistance of the LOD in order to maintain the same antiblooming performance.…”