12 are then removed preferably sequentially using the photoresist pattern 16 a as a mask to form a thermal oxidation mask which comprises a silicon nitride film pattern 14 a and a pad oxide film pattern 12 a afterwards the exposed silicon substrate 10 is removed to a desired depth using the thermal oxidation mask which comprises the photoresist pattern 16 a to form a channel 18 the photoresist pattern 16 a is removed and a silicon epitaxial layer 30 is produced on the exposed surface of the channel 18 as shown in fig2 c preferably the silicon epitaxial layer thickness is from about 50 to about 100 u00e5 thereafter a silicon oxide film 32 is produced e g by a chemical vapor deposition cvd method on the surface of the structure see fig2 d preferably the thickness of the silicon oxide film 32 is from about 50 to about 100 u00e5 the silicon oxide film 32 is then removed e g by etching such that a spacer 32 a is produced on the side surfaces of the silicon nitride film pattern 14 a and the pad oxide film pattern 12 a as shown in fig2 e thereafter the u201c corner portions u201d of the silicon.