a method for element isolation utilizing insulating materials in a semiconductor substrate is proposed . in this method an oxidizable material layer of polycrystalline silicon or the like is formed and then the oxidizable material layer is selectively oxidized , using an oxidation - proof mask thereby forming a thick oxide layer . thereafter , the oxidation - proof mask is removed and unoxidized oxidizable material below the mask is perpendicularly etched off , leaving part of the oxidizable material which is then oxidized to form together with the thick oxide layer an element isolation . this invention further proposes a semi - conductor device having element isolation layer whose bird &# 39 ; s beak is very small in length .