Method of forming a uniform collar oxide layer over an upper portion of a sidewall of a trench extending into a semiconductor substrate

A method of fabricating a deep trench capacitor, comprising:

• providing a single-crystal silicon substrate;
• forming a trench in the single-crystal silicon substrate with a width of w;
• forming a conformal oxide layer and a conformal mask layer;
• forming a photoresist layer on the mask layer;
• removing a part of the photoresist layer to make the top surface of the photoresist layer lower than the top surface of the single-crystal silicon substrate with a distance d;
• removing the mask layer and the oxide layer to expose an upper portion of the sidewall of the trench;
• removing the photoresist layer;
• performing an ion implantation process to reduce oxidation rate on (110) orientation with a dopant to make the oxidation rates on (110) and (100) orientations existing in the sidewall of the trench equal to each other;
• performing a local oxidation to form a collar oxide layer with a uniform thickness on the upper portion of the sidewall of the trench;
• removing the mask layer and the oxide layer until the surface of the single-crystal silicon substrate is exposed;
• forming a lower electrode in the trench in the single-crystal silicon substrate;
• and
• forming a dielectric layer on the lower electrode.