Flash memory device and method for forming the same

A method for forming a flash memory device, comprising:

• forming a semiconductor stack structure and isolation features on the substrate, wherein the isolation features protrude from the substrate, so that the semiconductor stack structure is sandwiched between the isolation features, wherein the semiconductor stack structure comprises:
• a first floating gate;
• and
• a second floating gate disposed on the first floating gate;
• wherein the formation of each of the isolation features comprises:
• forming a first dielectric filling layer in the substrate adjacent to the semiconductor stack structure, wherein a first top surface of the first dielectric filling layer is recessed from a second top surface of the semiconductor stack structure;
• conformally forming a second dielectric liner material layer on the semiconductor stack structure and the first top surface of the first dielectric filling layer;
• forming a third dielectric filling material layer on the second dielectric liner material layer;
• and
• removing a portion of the second dielectric liner material layer and a portion of the third dielectric filling material layer to expose second side surfaces of the second floating gate, thereby forming a second dielectric liner and a third dielectric filling layer of each of the isolation features, wherein a first top surface of each of the isolation features includes a top surface of the third dielectric filling layer and a top surface of the second dielectric liner,
• wherein the first dielectric filling layer and the third dielectric filling material layer are formed of a first material, the second dielectric liner material layer is formed of a second material, and the second material is different from the first material.