Isolation trench with a rounded top edge using an etch buffer layer

Abstract

The present invention provides a method of manufacturing a trench having rounded top corners 28 in a substrate. The rounded top edges allow the formation of a gate oxide with a uniform thickness around the trench thereby reducing parasitic field FET problems. The method begins by forming a pad layer 14 over a semiconductor substrate 10. Next, an insulating layer 18 composed of silicon nitride is formed over the pad layer 14. A first opening 19 is formed in the insulating layer 18 and the pad layer 14 exposing the surface of the substrate. The first opening is defined by sidewalls of the pad layer 14 and of the insulating layer 18. An etch buffer layer 20 composed of polysilicon is formed over the resultant surface. In one etch step, the etch buffer layer 20 is anisotropically etched forming spacers 22 on the sidewalls of the pad layer 14 and of the insulating layer 18. The same etch step continues by etching the spacers 22 and the exposed substrate in the first opening 19 thereby forming a trench 26 in the substrate 10. Because the etch has to etch through the spacers before it reached the substrate, the trench 26 has rounded top edges 28 near the pad layer 14. Lastly, the pad layer 14 and the first insulating layer 18 are removed thereby forming the trench 26 with rounded top edges 28.