Method for fabricating semiconductor device

A method for fabricating a semiconductor structure, comprising:

• forming a plurality of mandrels over a substrate, wherein the substrate comprises a semiconductor substrate as a base;
• forming a first dielectric layer to cover on a predetermined mandrel of the mandrels;
• forming a second dielectric layer over the substrate to cover the mandrels;
• removing the mandrels, wherein a remaining portion of the first dielectric layer and the second dielectric layer at a sidewall of the mandrels remains on the substrate;
• performing an anisotropic etching process over the substrate until a top portion of the semiconductor substrate is etched to form a plurality of fins corresponding to the remaining portion of the first dielectric layer and the second dielectric layer, wherein the fins comprises first fins corresponding to the first dielectric layer with the second dielectric layer and second fins corresponding to the second dielectric layer without the first dielectric layer, and the first fins are thicker than the second fins;
• removing a dummy fin of the fins, wherein at least one fin of the first fins subjecting to the first dielectric layer at an area adapting for a separately isolated device remains;
• depositing oxide over the substrate to form an oxide layer covering the fins as remained;
• and
• performing an annealing process over the substrate, wherein the at least one fin of the first fins has more surface oxidation thickness due to the annealing process than the second fins.