MANUFACTURE OF SEMICONDUCTOR DEVICE

PURPOSE:To avoid the bird's beak formation by a method wherein the bottom surfaces of the first opening parts are covered to form the second thermal resistant insulating oxide films and then the second opening parts are formed in the first opening parts. CONSTITUTION:The second Si3N4 13 in film thickness of 100-300Angstrom thinner than that of the first Si3N4 film 11 is formed by CVD process. Successively, resist patterns are formed and the second Si3N4 film 13 is selectively etched away by CF4 gas using the resist patterns as masks to form the second opening parts 14 in the width corresponding to 0.7-0.9 of the width of the first opening parts 12 i.e. 2.1-2.7mum in the first opening parts 12. Next, boron ion (B) is implanted in an Si substrate 10 through the intermediary of the second Si3N4 film 13 and the second opening parts 14. Next, the Si substrate 10 is oxidized by leading oxygen wetted by passing through hot water into a heated heat treatment furnace. At this time, the end of bird's beak still remains inside the sidewalls of the first opening parts 12. The oxidation is limited within the range of the first opening parts 12 while ths oxide films in sufficient thickness are formed. Through these procedures, the bird's beak formation can be avoided.