Patent ID: 7573105

Claim:
An isolated CMOS arrangement of transistors formed in a semiconductor substrate, said substrate being doped with P-type impurity and not comprising an epitaxial layer, said CMOS arrangement comprising an isolated CMOS pair, said CMOS pair comprising a PMOS and a NMOS, and an associated isolation structure: said PMOS comprising: an N well having a relatively deep central portion and relatively shallow side portions, said side portions of said N well underlying a field oxide layer, said central portion of said N well underlying a first opening in said field oxide layer; a first gate overlying a channel region of said N well and separated from said substrate by a first gate oxide layer; a P-type source region located at the surface of said substrate on one side of said first gate; and a P-type drain region located at the surface of said substrate on an opposite side of said first gate from said P-type source region; electrical contacts to said first gate, P-type source region, P-type drain region and N well; said NMOS comprising: a P well having a relatively deep central portion and relatively shallow side portions, said relatively shallow side portions of said P well underlying the field oxide layer, said relatively deep central portion of said P well underlying a second opening in said field oxide layer, said P well being electrically isolated from the surrounding P-type substrate by said isolation structure; a second gate overlying a channel region of said P well and separated from said substrate by a second gate oxide layer; an N-type source region located at the surface of said substrate on one side of said second gate; and an N-type drain region located at the surface of said substrate on an opposite side of said second gate from said N-type source region; electrical contacts to said second gate, N-type source region, N-type drain region and P well; said isolation structure comprising: said N well, said N well extending downward from the surface of said substrate and laterally surrounding said P well; and an N-type isolation layer located in said substrate and underlying said P well and at least a portion of said N well, said N-type isolation layer extending laterally beyond said P well a sufficient distance and said second N well being sufficiently deep in said substrate such that said N-type isolation layer overlaps said N well so as to isolate said P well from said P-type substrate; wherein a doping concentration profile taken at a vertical cross-section through said central portion of said N well comprises a series of two or more vertically separated Gaussian profiles differing in depth and concentration and overlapping to form a continuous N-type region.