Lining and corner rounding method for shallow trench isolation

Abstract

A method for forming shallow trench isolation on a silicon wafer is described wherein a trench is formed using a silicon nitride/pad oxide hardmask having a silicon oxynitride ARC layer over the nitride. After a trench is formed by dry etching, the hardmask is recessed by first selectively recessing the silicon nitride and then exposing the upper corners of the silicon trench by wet etching the pad oxide thereby exposed. A first sacrificial oxidation converts a portion of the silicon oxynitride ARC layer to oxide and rounds off the sharp upper silicon corners of the trench. The sacrificial oxide is removed and a trench lining oxide is grown to a prescribed thickness by a second oxidation which converts the remaining silicon oxynitride into silicon oxide while further rounding the upper silicon trench corners. By converting the entire oxynitride ARC layer to oxide, it becomes possible to planarized the filler oxide into the silicon nitride layer with a CMP process having a high oxide-to-nitride selectivity. This avoids dishing of large field areas which occurs when a low oxide-to-nitride selectivity planarization process must be used with residual oxynitride is present. The rounding of the silicon corners by the dual oxidation reduces oxide stress and thereby results in improved performance of devices formed adjacent to the trench isolation.