THICKNESS DEPENDENCE OF BORON PENETRATION THROUGH O-2-GROWN AND N2O-GROWN GATE OXIDES AND ITS IMPACT ON THRESHOLD VOLTAGE VARIATION

We report an a quantitative study of boron penetration from p(+) polys ilicon through 5- to 8-nm gate dielectrics prepared by rapid thermal o xidation in O-2 Of N2O. Using MOS capacitor measurements, we show that boron penetration exponentially increases with decreasing oxide thick ness. We successfully describe this behavior with a simple physical mo del, and then use the model to predict the magnitude of boron penetrat ion, N-B, for thicknesses other than those measured. We find that the minimum t(ox) required to inhibit boron penetration is always 2-4 nm l ess when N2O-grown gate oxides are used in place of O-2-grown oxides. We also employ the boron penetration model to explore the conditions u nder which borun-induced threshold voltage variation can become signif icant in ULSI technologies. Because of the strong dependence of boron penetration on t(ox), incremental variations in oxide thickness result in a large variation in N-B, leading to increased threshold voltage s preading and degraded process control. While the sensitivity of thresh old voltage to oxide thickness variation is normally determined by cha nnel doping and the resultant depletion charge, we find that for a nom inal thickness of 6 mn, threshold voltage control is further degraded by penetrated boron densities as low as 10(11) cm(-2).