The present invention provides straight forward methods for
plasma etching a trench having rounded top corners, or rounded bottom corners, or both in a
silicon substrate. A first method for creating a rounded top corner on the etched
silicon trench comprises
etching both an overlying
silicon oxide layer and an upper portion of the silicon substrate during a "break-through" step which immediately precedes the step in which the silicon trench is etched. The
plasma feed gas for the break-through step comprises carbon and
fluorine. In this method, the
photoresist layer used to pattern the etch stack is preferably not removed prior to the break-through
etching step. Subsequent to the break-through step, a trench is etched to a desired depth in the silicon substrate using a different
plasma feed
gas composition. A second method for creating a rounded top corner on the etched silicon trench comprises formation of a built-up extension on the sidewall of an overlying patterned
silicon nitride hard mask during etch (break-through) of a
silicon oxide adhesion layer which lies between the
hard mask and a
silicone substrate. The built-up extension upon the
silicon nitride sidewall acts as a sacrificial masking material during etch of the silicon trench, delaying etching of the silicon at the outer edges of the top of the trench. This permits completion of trench etching with delayed etching of the top corner of the trench and provides a more gentle
rounding (increased
radius) at the top corners of the trench. During the etching of the silicon trench to its final dimensions, it is desirable to round the bottom corners of the finished silicon trench. We have discovered that a more rounded bottom trench corner is obtained using a two-step silicon etch process where the second step of the process is carried out at a higher process
chamber pressure than the first step.