Method of forming SGT MOSFETs with improved termination breakdown voltage

A method for preparing a trench power semiconductor device comprising:

• providing a semiconductor substrate comprising a bottom substrate and an epitaxial layer formed atop the bottom substrate;
• performing a first etching of the epitaxial layer to form an active trench with an initial depth in an active area of the semiconductor substrate and a termination trench with a desired depth in a termination area of the semiconductor substrate, wherein the initial depth of the active trench is less than the desired depth of the termination trench;
• applying a mask to cover the termination trench but exposing the active trench;
• performing a second etching to increase the depth of the active trench to a desired depth of the active trench, wherein a depth difference between the desired depth of the active trench and the desired depth of the termination trench is smaller less than a depth difference between the initial depth of the active trench and the desired depth of the termination trench;
• forming an insulating layer lining bottom and sidewalls of the active trench and the termination trench;
• filling a conductive material into the active trench and the termination trench;
• etching back the conductive material in an upper portion of the active trench and the termination trench leaving only the conductive material in the lower portion of the active trench and the termination trench;
• filling the upper portion of the active trench and the termination trench with an insulating material;
• etching back a top portion of the insulating material in the active trench and a portion of the insulting material on sidewall of the termination trench near the active area forming an insulating layer on top of the conductive material in the lower portion of the active trench and the termination trench and a remaining portion of the insulating layer at the sidewall of the termination trench farther from the active area;
• and
• forming another insulating layer lining on exposed sidewall on the upper portion of the active trench and the termination trench and then filling the upper portion of the active trench and the termination trench with a conductive material.