Process for manufacture of low voltage power MOSFET device
View Patent ↗A trench type power MOSFET has a thin vertical gate oxide along its side walls and a thickened oxide with a rounded bottom at the bottom of the trench to provide a low R DSON and increased V DSMAX and V GSMAX and a reduced Miller capacitance. The walls of the trench are first lined with nitride to permit the growth of the thick bottom oxide to, for example 1000 Å to 1400 Å and the nitride is subsequently removed and a thin oxide, for example 320 Å is regrown on the side walls. In another embodiment, the trench bottom in amorphized and the trench walls are left as single crystal silicon so that oxide can be grown much faster and thicker on the trench bottom than on the trench walls during an oxide growth step. A reduced channel length of about 0.7 microns is used. The source diffusion is made deeper than the implant damage depth so that the full 0.7 micron channel is along undamaged silicon. A very lightly doped diffusion of 1000 Å to 2000 Å in depth could also be formed around the bottom of the trench and is depleted at all times by the inherent junction voltage to further reduce Miller capacitance and switching loss.
1. The process for forming a trench type MOSgated device in a semiconductor body comprising a highly doped substrate of a first conductivity and a lightly doped semiconductor layer of said first conductivity formed over said substrate; said process comprising the steps of etching a trench having spaced side walls and a flat bottom surface in said semiconductor layer, said trench terminating in said semiconductor layer and said flat bottom surface and said sidewalls being generally normal to one another thereby forming a sharp edge at the point of their cross-section; amorphizing the bottom surface of the trench but not its side walls; and thereafter growing a silicon dioxide layer on the side walls and bottom of the trench at the same time, wherein the silicon dioxide layer is substantially thicker at the bottom of said trench than on the side walls and said edges are round.
2. The process of claim 1 wherein the said silicon dioxide coating on said walls have a thickness of greater than 1000 Å.
3. The process of claim 1 , wherein the step of amorphizing the silicon at the bottom of said trench is carried out by an ion implant of a neutral species into the bottom of said trench.
4. The process of claim 3 , wherein said ion implant is at a heavy dose of greater than about 1E16 atoms/cm 2 .
5. The process of claim 4 , wherein the silicon oxide coating on said walls have a thickness of greater than about 1000 Å.