Surface doping using ion implantation for optimum guard layer design in COS/MOS structures

Theory and experimental results using COS/MOS devices are presented for using ion-implanted surface guard layers to replace guard bands in LSI COS/MOS circuits. The function of a guardband is to reduce leakage between neighboring devices; surface guard layers accomplish the same end with the addition of a considerable reduction in the space required for a given COS/MOS memory cell. The price paid for the elimination of the guardbands is a tradeoff between the onset of leakage and the onset of avalanche breakdown between drain and substrate. A systematic method is presented for optimizing these two limiting voltages for a given value of field oxide thickness and for defined limits of the Q
s
oxide charge.