Structure of flash memory cell and method for fabricating the same

1. A method for fabricating flash memory, comprising:

providing a substrate;

forming a trench isolation structure, having a base part disposed in the substrate and a protruding part above the substrate protruding from the base part;

forming a dielectric spacer with low dielectric constant (low-K) on a sidewall of the protruding part of the trench isolation structure;

forming a floating gate on the substrate abutting to the dielectric spacer, wherein the dielectric spacer is sandwiched between the floating gate and the protruding part of the trench isolation structure; and

removing the dielectric spacer to have a free space filled with air,

wherein a peripheral bottom flat surface of the floating gate directly covers on a portion of the base part of the trench isolation structure,

the method for fabricating flash memory further comprising performing an oxidation process on the substrate before forming the floating gate and after forming the trench isolation structure to form a tunnel oxide layer between the substrate and the floating gate,

wherein the dielectric spacer and the tunnel oxide layer are separated by a portion of the floating gate on the base part of the trench isolation structure,

wherein a dielectric constant of the dielectric spacer is smaller than a dielectric constant of the trench isolation structure.

2. The method for fabricating flash memory in claim 1 , wherein the tunnel oxide layer is disposed on the substrate between adjacent two of the base part of the trench isolation structure after forming the trench isolation structure.

3. The method for fabricating flash memory in claim 1 , wherein the step of forming the trench isolation structure comprising:

forming a plurality of dielectric islands on the substrate, wherein a gap is between adjacent two of the dielectric islands;

forming a trench in the substrate under the gap;

depositing an isolation material over the substrate to fill the trench and cover over the dielectric islands;

polishing the isolation material to expose the dielectric islands, wherein the protruding part of the trench isolation structure is formed between the dielectric islands; and

removing the dielectric islands.

4. The method for fabricating flash memory in claim 1 , wherein a top surface of the dielectric spacer is coplanar with a top surface of the floating gate.

5. The method for fabricating flash memory in claim 1 , wherein a dielectric constant of the dielectric spacer is smaller than a dielectric constant of silicon oxide.

6. The method for fabricating flash memory in claim 1 , wherein the step of forming the floating gate comprising:

depositing a polysilicon layer over the substrate to at least cover over the protruding part of the trench isolation structure with the dielectric spacer; and

polishing the polysilicon layer to expose the protruding part of the trench isolation structure with the dielectric spacer, wherein a residual part of the polysilicon layer provides the floating gate.