the masking layer 26 is also selected based at least somewhat in part upon its use in the formation of the material of the isolation structure as described in more detail below the masking layer 26 is etched to reveal the precursor material layer 24 at the bottom 22 of the trench 18 in the preferred embodiment the trench 18 is too narrow to effectively mask the masking layer 26 with a photoresist and wet etch the masking layer 26 to expose the precursor material layer 24 further a wet etch would tend to remove the masking layer 26 from the sidewalls 20 of the trench 18 as well as from the bottom 22 of the trench 18 therefore in the preferred embodiment the amorphous silicon masking layer 26 is etched with a highly direction accelerated ion etch to substantially expose the nitridized silicon oxide precursor material layer 24 at the bottom 22 of the trench 18 while allowing a substantial portion of the masking layer 26 to remain over the precursor material layer 24 at the sidewalls 20 of the trench 18 this type of etch may be accomplished in a 9400se tcp reactor manufactured by lam research of fremont calif the etch is preferably accomplished at a pressure of between about zero torr and about 0 1 torr and most preferably about.