A method of making a semiconductor device or integrated circuit produces trench isolation structures having rounded corners in order to reduce gate oxide weakness and to avoid the so-called "double hump" curve produced by the corner effect. A LOCOS oxide is removed from the surface of a silicon substrate to produce a depression in the surface. A silicon dioxide or silicon nitride layer is deposited over the surface including the silicon nitride used in the LOCOS process. The surface in anisotropically etched to form sidewall spacers either side of the area where the trench is to be formed. The substrate is then etched to form a trench. In an alternative method, nitride sidewalls are formed prior to removing the LOCOS oxide. The resulting structure is anisotropically etched to leave sidewalls on either side of the trench location, the sidewalls being made of the silicon nitride sidewall and the remainder of the LOCOS oxide. The substrate would then be etched to form the trench.