Laff, Whitesel, Conte & Saret, Ltd.
Bonded IC substrate with a high breakdown voltage and large current capabilities
 A semiconductor device includes a groove formed in a surface of a first semiconductor substrate of one conductivity type in order to partition and isolate first and second device regions. A first insulating film on the first semiconductor substrate of the first device region also contacts the groove. A second insulating film covers an inner wall of the groove. The first insulating film is thicker than the second film in order to increase the breakdown voltage and facilitate carrying a higher current. This thickness relationship also aids manufacturing. 
NEC Corporation
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Hiroaki Tomohiro
Kikuchi Hamajima