Process for producing a field-effect transistor

A process for manufacturing a field-effect transistor of the vertical-operation DMOS type, from a silicon wafer having a first type of conductivity comprising a principal first face and a second face opposite this first face, the zone of the wafer situated in the vicinity of the second face presenting a conductivity of the same type as, and greater than, the conductivity of the wafer, forming the drain of the transistor, comprising the following steps:

• forming, by thermal growth, a first silicon oxide layer of given thickness on the first face of the wafer,
• forming, on the first silicon oxide layer, a layer of silicon nitride,
• depositing, on the silicon nitride layer, a layer of doped polycrystalline silicon and opening this layer by masking and etching to form the grid electrodes of the transistor,
• forming implantation zones by ionic implantation of a dopant having a type of conductivity opposite that of the wafer by means of the mask formed by the grid electrodes,
• heating so as to develop, from the polycrystalline silicon forming the grid electrodes, a second silicon oxide layer whose thickness is greater than that of the first layer and so as to cause the implanted dopant to diffuse and form in the silicon wafer zones with a type of conductivity opposite that of the wafer,
• selectively etching the silicon nitride layer and etching the silicon oxide over a thickness corresponding to that of the first silicon oxide layer so as to bare the wafer,
• depositing a second layer of doped polycrystalline silicon having the same type of conductivity as that of the zone of the wafer forming the drain of the transistor,
• heating to cause the dopant of the second polycrystalline silicon layer to diffuse into the underlying portions of the wafer so as form the source zones of the transistor.