Radiation-hardened power semiconductor devices and methods of forming them

A method of forming a power MOSFET semiconductor device, comprising:

• providing a semiconductor substrate comprising a drain region in ohmic contact with a drain electrode arranged on the semiconductor substrate;
• forming an epitaxial layer on the semiconductor substrate, the epitaxial layer comprising a body region, a source region, and a drift region, the body region, the source region, and the drift region extending to a main surface of the epitaxial layer;
• forming a dielectric layer on the main surface of the epitaxial layer, wherein the dielectric layer has a thick part and a thin part, wherein the thick part of the dielectric layer directly adjoins the drift region at the first surface and directly adjoins a part of the body region that is laterally adjacent to the drift region at the first surface, and wherein the thin part of the dielectric layer is laterally disposed between the thick part of the dielectric layer and the source, and wherein the dielectric layer is formed at a temperature less than 950° C.;
• and
• forming a gate electrode on the dielectric layer at least above the body region,
• wherein forming the epitaxial layer comprises:
• forming a superjunction structure in the epitaxial layer, wherein
• (a) an upper superjunction zone of the superjunction structure below the body region is formed with a higher first-type doping than a lower superjunction zone of the superjunction structure below the upper superjunction zone, or
• (b) an upper drift zone of the drift region laterally adjacent to the upper superjunction zone is formed with a higher second-type doping than a lower drift zone below the upper drift zone, or
• (c) the upper superjunction zone is formed with a higher first-type doping than the lower superjunction zone and the upper drift zone is formed with a higher second-type doping than the lower drift zone;
• wherein the difference between a net dose of the higher first-type net doping dose of the upper superjunction zone and a net dose of the higher second-type net doping dose of the upper drift zone is larger than the difference between a net dose of the higher first-type net doping dose of the lower superjunction zone and a net dose of the second-type net doping dose of the lower drift zone, and
• wherein forming the epitaxial layer comprises:
• forming a second-type doped uppermost drift zone above the upper drift zone and laterally adjacent to the body region, wherein the second-type doping of the uppermost drift zone is higher than the second-type doping of the at least one lower drift zone.