Method of forming implanted regions in a semiconductor device by use of a three layer masking structure

A method of manufacturing a semiconductor device comprising the steps of

• forming at least a first pattern of field insulation at a first major surface of a semiconductor body, said first pattern bounding at least one active region at said first major surface, and said first pattern having a predetermined thickness;
• forming a masking structure on said first major surface, said masking structure having at least one opening above at least a part of said at least one active region;
• carrying out ion implantation to said at least one active region through said at least one opening to dope said at least one active region to a depth at least approximately equal to said predetermined thickness;
• wherein said masking structure is formed on said first major surface by the steps of
• forming a first masking layer on said first major surface, said first masking layer being of a first thickness, and said first masking layer being of a different material from said first pattern of field insulation,
• forming a second masking layer on said first masking layer, said second masking layer being of a second thickness greater than said first thickness, and said second masking layer being of a different material than said first masking layer, and
• forming a third masking layer on said second masking layer, said third masking layer being of a third thickness less than said second thickness, and said third masking layer being of a different material than said second masking layer,
• wherein said at least one opening is formed through said second masking layer and said third masking layer, said first masking layer covering at least parts of said first major surface at said at least one opening through said second and third masking layer.