The present invention relates to substrates for ICs and method for forming the same. The method comprises the steps of: forming a hard mask layer on the bulk silicon material; etching the hard mask layer and the bulk silicon material to form a first part for shallow trench isolation of at least one trench; forming a dielectric film on the sidewall of the at least one trench; further etching the bulk silicon material to deepen the at least one trench so as to form a second part of the at least one trench; completely oxidizing or nitridizing parts of the bulk silicon material which are between the second parts of the trenches, and parts of the bulk silicon material which are between the second parts of the trenches and side surfaces of the bulk silicon substrate; filling dielectric materials in the first and second parts of the at least one trench; and removing the hard mask layer.