Semiconductor memory device and a manufacturing method thereof

A manufacturing method of a semiconductor memory device, comprising:

• providing a semiconductor substrate, wherein a shallow trench isolation is formed in the semiconductor substrate and defines a plurality of active regions;
• performing a first etching process for forming a bit line contact opening in the semiconductor substrate, wherein the bit line contact opening is formed corresponding and exposes one of the active regions, and an edge of the bit line contact opening has an under-cut structure;
• forming a bit line structure on the semiconductor substrate, wherein the bit line structure is partially disposed in the bit line contact opening and in contact with one of the active regions corresponding to the bit line contact opening, and a bottom width of the bit line contact opening is greater than a top width of the bit line contact opening;
• and
• forming an isolation structure in the under-cut structure of the bit line contact opening,
• wherein a bottom of the isolation structure is wider than a top of the isolation structure,
• wherein a manufacturing method of the isolation structure comprises:
• forming an insulating layer on the semiconductor substrate and in the bit line contact opening, wherein the under-cut structure of the bit line contact opening is filled with the insulating layer;
• and
• performing a second etching process to the insulating layer for removing the insulating layer outside the bit line contact opening and a portion of the insulating layer in the bit line contact opening, so as to form the isolation structure in the under-cut structure.