Method for forming buried doped region

A method for forming a buried doped region, comprising:

• providing a substrate;
• forming a first silicon oxide layer on the substrate;
• patterning the first silicon oxide layer to form an opening that exposes a portion of the substrate, wherein the opening extends in a first direction;
• forming a buried doped region in the substrate exposed by the opening;
• forming a second silicon oxide layer over the substrate to fill the opening;
• planarizing the second silicon oxide layer so that the second silicon oxide layer and the first silicon oxide layer together form a third silicon oxide layer;
• patterning the third silicon oxide layer to form an isolation layer that exposes a portion of the substrate and the buried doped region, wherein the isolation layer extends in a second direction and the second direction and the first direction cross over each other;
• and
• forming a semiconductor layer over the substrate, wherein the semiconductor layer exposes at least the isolation layer.