Method of reducing the aspect ratio of a trench

A method of reducing an aspect ratio of a trench, comprising the steps of:

• forming a trench in a substrate;
• using HDP-CVD to form a conformal Si-rich oxide layer on a surface of the trench;
• using HDP-CVD to form a conformal first oxide layer on the Si-rich oxide layer;
• using LP-CVD to form a conformal second oxide layer on the first oxide layer;
• anisotropically etching the Si-rich oxide layer, the first oxide layer and the second oxide layer to form a remaining Si-rich oxide layer on a sidewall of the trench, a remaining first oxide layer on the remaining Si-rich layer and a remaining second oxide layer on the remaining first oxide layer;
• and
• performing an etching procedure with an etchant to remove the remaining second oxide layer at a third etching rate, part of the remaining first oxide layer at a second etching rate and part of the remaining Si-rich oxide layer at a first etching rate;
• wherein the third etching rate is greater than the second etching rate and the second etching rate is greater than the first etching rate.

A method of reducing an aspect ratio of a trench, comprising the steps of:

• forming a trench in a Si substrate;
• using HDP-CVD, forming a conformal Si-rich oxide layer on a surface of the trench, wherein a thickness of the Si-rich layer is 50˜100 Å;
• using HDP-CVD, forming a conformal first silicon oxide layer on the Si-rich oxide layer, wherein a thickness of the first oxide layer is 100˜120 Å;
• using LP-CVD, forming a conformal second silicon oxide layer on the first silicon oxide layer, wherein a thickness of the second oxide layer is 100˜150 Å;
• anisotropically etching the Si-rich oxide layer, the first silicon oxide layer and the second silicon oxide layer to form a remaining Si-rich oxide layer on a sidewall of the trench, a remaining first silicon oxide layer on the remaining Si-rich layer and a remaining second silicon oxide layer on the remaining first silicon oxide layer;
• and
• performing an etching procedure with a BOE solution to remove the remaining second silicon oxide layer at a third etching rate, part of the remaining first silicon oxide layer at a second etching rate and part of the remaining Si-rich oxide layer at a first etching rate;
• wherein the third etching rate is greater than the second etching rate and the second etching rate is greater than the first etching rate.