Field inversion control for n-channel device integrated circuits

Abstract

The field inversion properties of integrated circuits incorporating N-channel MOS devices are improved by using a silicon substrate whose bulk dopant concentration is low, but whose local dopant concentration is high at the field surfaces under the field oxide separating the active surface areas where the individual N-channel MOS devices are formed. The differential doping between surface areas under the field oxide and the active surface areas of the substrate is done by nonselectively ion-implanting boron into the substrate to form a uniform low resistivity layer, removing selected portions of the low resistivity layer to expose the unimplanted, high resistivity substrate and forming the active devices at the unimplanted substrate portions. As an option, the unimplanted surface portion can be doped to an intermediate dopant concentration to improve performance. The remaining pattern of the low resistivity layer is covered with field oxide. The invention allows the use of relatively inexpensive, low dopant concentration substrates to conveniently manufacture high performance N-channel MOS integrated circuits.