Method of making a trench MOSFET device with improved on-resistance

A method of forming a trench MOSFET device comprising:

• providing a substrate of a first conductivity type;
• depositing an epitaxial layer of said first conductivity type over said substrate, said epitaxial layer having a lower majority carrier concentration than said substrate;
• forming a body region of a second conductivity type within an upper portion of said epitaxial layer;
• etching a trench extending into said epitaxial layer from an upper surface of said epitaxial layer, said trench having side portions and a bottom portion and extending to a greater depth from said upper surface of said epitaxial layer than does said body region;
• forming a single doped region of said first conductivity type between said bottom portion of said trench and said substrate, said single doped region having a majority carrier concentration that is lower than that of said substrate and higher than that of said epitaxial layer,
• wherein said single doped region is diffused and spans 100% of the distance from said trench bottom portion to said substrate, and
• wherein the steps of etching said trench and forming said doped region comprise:
• (a) forming a trench mask directly on said epitaxial layer;
• (b) etching said trench through said trench mask;
• (c) implanting a dopant of said first conductivity type through said trench mask;
• and (d) diffusing said dopant of said first conductivity type at elevated temperature;
• forming an insulating layer lining at least a portion of said trench;
• forming a conductive region within said trench adjacent said insulating layer;
• and
• forming a source region of said first conductivity type within an upper portion of said body region and adjacent said trench subsequent to said steps of etching said trench and forming said doped region.