Recovery of shifted MOS parameters induced by focused ion beam exposure

Abstract

The effects of focused ion beam (FIB) exposure on MOS transistors within a circuit were examined. It was found that FIB exposure does not cause parameter shifts as long as the gate is connected to the drain of other MOS transistors. However, the threshold voltage (V/sub t/) does shift during isolating the gate using a FIB. Further FIB exposure on MOS transistors with a floating gate is shown to cause larger shifts. Thermal annealing was studied to recover shifted V/sub t/. We demonstrated that a 400/spl deg/C-450/spl deg/C anneal could recover shifted V/sub t/ almost completely. Ninety percent recovery can be reached by annealing at 400/spl deg/C-450/spl deg/C for 1-2 hours, and V/sub t/ shifts can be reduced to about 10 mV.