PURPOSE: To obtain a semiconductor element substrate of SOI structure, which prevents leakage and breaking of wiring at stepped parts by a simplified process, by causing semiconductor crystal to grow in the form of islands by making use of difference of nucleation density, filling the region between the semiconductor crystals with amorphous insulator including impurity, executing a heat treatment of the substrate, doping the side surface and bottom surface of semiconductor crystals with impurities and then flattening the surface.

CONSTITUTION: PSG film 2 is deposited by the DVD method on a substrate 1. SiNx film is then formed on the PSG film 2 by the CVD method. The SiNx film is patterned to a size of several μm or less by a photolithography technique. The PSG film 2 has a small nucleation density ND and becomes a non-nucleating face (SNDS). Meanwhile, SiN has a large core forming density of Si and becomes the nucleating face (SNDL). Next, the Si single crystal 4 is formed by the thermal DVD method on the SiNx nucleating face (SNDL). Next, after a PSG film 5 is deposited by the CVD method, and reflow is carried out to make the surface of PSG 5 flat. Thereafter, the etching is carried out under a condition that the etching rate ratio between the PSG 5 and Si single crystal 4 becomes almost 1 by RIE and thereby the semiconductor element substrate of SOI structure having a flat surface and no leakage from the side surface and bottom surface can be obtained.