Methods of Manufacturing Semiconductor devices Having Buried Bit Lines

A method of forming a semiconductor device, comprising:

• forming a plurality of bit lines spaced from one another and extending in a first direction in an upper portion of a semiconductor substrate having a first conductivity type, by selectively doping an upper portion of the substrate with an impurity of a second conductivity type that is opposite to first conductivity type;
• forming a mask layer on a surface of the semiconductor substrate above the upper portion of the substrate;
• forming a plurality of spaced apart recess regions extending through the mask layer and into the substrate between adjacent bit lines of the plurality of bit lines, each of the recess regions having a pair of opposing sidewalls oriented in parallel to a pair of adjacent bit lines and a pair of opposing sidewalls extending between a pair of adjacent bit lines;
• depositing a first filling layer in the recess region;
• removing the mask layer to expose the surface of the semiconductor substrate;
• partially removing the first filling layer from the semiconductor substrate, to form a second filling layer exposing sidewalls of the recess regions;
• forming an ONO layer on the surface of the semiconductor substrate including the sidewalls of the recess regions and the second filling layer;
• and
• forming a gate electrode layer on the ONO layer over the sidewalls of the recess regions extending between adjacent pairs of bit lines.