Flash memory device and method of forming the same with improved gate breakdown and endurance
View Patent ↗The present invention provides a flash memory device and method for making the same having a floating gate structure with a semiconductor substrate and shallow trench isolation (STI) structure formed in the substrate. A first polysilicon layer is formed over the substrate and the STI structure. The recess formed within the first polysilicon layer is over the STI structure and extends through the first polysilicon layer to the STI structure. An oxide fill is provided within the recess and is etched back. ONO (oxide-nitride-oxide) layer conformally covers the oxide fill and the first polysilicon layer. The second polysilicon layer covers the ONO layer. The oxide fill within the recess provides a minimum spacing between the second polysilicon layer and the corner of the STI regions, thereby avoiding the creation of a weak spot and reducing the risk of gate breakdown, gate leakage, and improving device reliability.
1. A flash memory device having a floating gate structure, comprising:
a semiconductor substrate;
a shallow trench isolation (STI) structure formed in the substrate;
a first polysilicon layer over the substrate and the STI structure;
a recess formed within the first polysilicon layer over the STI structure and extending through the first polysilicon layer to the STI structure;
oxide fill only within the recess;
an oxide-nitride-oxide (ONO) layer conformally covering the oxide fill and the first polysilicon layer, a portion of the ONO layer being formed within the recess, wherein the ONO layer is in direct physical contact with the STI structure; and
a second polysilicon layer covering the ONO layer, a portion of the second polysilicon layer being formed within the recess.
2. The device of claim 1 , wherein the first polysilicon layer is between about 400 Å to about 1200 Å thick.
3. The device of claim 2 , wherein the oxide fill is at least 100 Å thick.
4. The device according to claim 1 , wherein the first polysilicon layer is misaligned with respect to an upper surface of the STI structure.
5. The device according to claim 1 , wherein the recess extends to the substrate.
6. The device according to claim 1 , further comprising spaced apart source/drain regions in the substrate adjoining the STI structure.
7. The device according to claim 6 , wherein the second polysilicon layer is separated from a source/drain region at a corner of the STI structure by an amount equal to a thickness of the oxide fill.
8. The device according to claim 1 , wherein the oxide fill occupies at least 10% of the recess.
9. The device according to claim 1 , wherein the oxide fill is in the form of spacers.
10. The device of claim 1 , wherein a width of the recess is equal to or less than a width of the STI structure.