Process for forming a buried plate

A method of forming a trench capacitor in a substrate including a single-crystal semiconductor region, comprising:

• etching the single-crystal semiconductor region to pattern a trench elongated in a direction extending downwardly into the single-crystal semiconductor region, the trench having a trench sidewall, the trench sidewall having an upper portion adjacent to a major surface of the substrate and a lower portion disposed below the upper portion at greater depth from the major surface;
• forming a dopant source layer overlying a lower portion of the trench sidewall, the dopant source layer not overlying an upper portion of the trench sidewall;
• forming a covering layer overlying the upper portion of the trench sidewall;
• while the covering layer overlies the single-crystal semiconductor region adjacent to the upper portion, annealing to drive a dopant from the dopant source layer into the single-crystal semiconductor region adjacent to the lower portion to form a buried plate region;
• removing the dopant source layer;
• forming a node dielectric along the lower portion of the trench sidewall;
• forming a node electrode on a side of the node dielectric opposite the buried plate region to form the trench capacitor;
• and
• forming an isolation collar overlying at least a part of the upper portion of the trench sidewall.