Use of dilute steam ambient for improvement of flash devices
The present invention provides a flash memory integrated circuit and a method for fabricating the same. The method includes etching a gate stack that includes an initial oxide layer directly in contact with a silicon layer, defining an oxide-silicon interface therebetween. By exposing the etched gate stack to elevated temperatures and a dilute steam ambient, additional oxide material is formed substantially uniformly along the oxide-silicon interface. Polysilicon grain boundaries at the interface are thereby passivated after etching. In the preferred embodiment, the interface is formed between a tunnel oxide and a floating gate, and passivating the grain boundaries reduces erase variability due to enhanced charge transfer along grain boundaries. At the same time, oxide in an upper storage dielectric layer (oxide -nitride-oxide or ONO) is enhanced in the dilute steam oxidation. Thermal budget can be radically conserved by growing thin oxide layers on either side of a nitride layer prior to etching, and enhancing the oxide layers by dilute steam oxidation through the exposed sidewall after etching. The thin oxide layers, like the initial tunnel oxide, serve as diffusion paths to enhance uniform distribution of OH species across the buried interfaces being oxidized.
1. A method of using an integrated circuit, the method comprising:
providing an integrated circuit comprising a transistor comprising a floating gate layer directly in contact with a tunnel dielectric layer and having an electrode-dielectric interface therebetween, the floating gate layer having grain boundaries, the grain boundaries having oxidized portions across the entire electrode-dielectric interface, wherein the tunnel dielectric layer has a minimum thickness and a maximum thickness, the maximum thickness being no more than approximately 8 Angstroms larger than the minimum thickness; and
storing charge on the floating gate layer by transferring electrons through the tunnel dielectric layer to the floating gate layer.
2. The method of claim 1 , further comprising removing charge from the floating gate layer.
3. The method of claim 2 , wherein charge is removed from the floating gate layer by Fowler-Nordheim tunneling.
4. The method of claim 1 , wherein the tunnel dielectric layer is approximately 100 Angstroms thick.
5. The method of claim 1 , wherein the floating gate layer comprises polysilicon, the tunnel dielectric layer comprises an oxide layer, and the oxidized portions of the grain boundaries comprise bonds between silicon and OH.
6. The method claim 5 , wherein the oxide layer has a substantially uniform thickness across the entire electrode-dielectric interface.
7. The method of claim 5 , wherein the grain boundaries are passivated by the oxide bonds.
8. The method of claim 5 , wherein the maximum thickness is no more than approximately 4 Angstroms larger than the minimum thickness.
9. The method of claim 5 , wherein the polysilicon of the floating gate layer comprises a dopant.
10. The method of claim 5 , wherein the oxide layer comprises silicon oxide.
11. The method of claim 5 , wherein the oxide layer comprises tantalum pentoxide.
12. The method of claim 5 , wherein the oxide layer forms part of a storage dielectric layer, the storage dielectric layer overlying the floating gate layer.
13. The method of claim 12 , wherein the storage dielectric layer an oxide-nitride-oxide construction.
14. The method of claim 12 , wherein the storage dielectric layer comprises a lower oxide layer at an upper surface of the floating gate, a nitride layer on top of the lower oxide layer, and an upper oxide layer on top of the nitride layer, a control gate layer comprising polysilicon serving as the polysilicon layer, and the upper oxide layer serving as the oxide layer in direct contact with the polysilicon layer.
15. The apparatus of claim 14 , wherein the thickness of the lower oxide layer is approximately 40 Angstroms.
16. The apparatus of claim 14 , wherein the thickness of the nitride layer is approximately 30 Angstroms and the thickness of the upper oxide layer is approximately 40 Angstroms.
17. A method of using an integrated circuit, the method comprising:
providing an integrated circuit comprising a transistor comprising a polysilicon layer directly in contact with an oxide layer and having an oxide-polysilicon interface therebetween, the polysilicon layer having grain boundaries, the grain boundaries having OH-passivated portions across the entire oxide-polysilicon interface; and
storing charge on the floating gate layer by transferring electrons through the tunnel dielectric layer to the floating gate layer.
18. The method of claim 17 , further comprising removing charge from the polysilicon layer.
19. The method of claim 18 , wherein charge is removed from the polysilicon layer by Fowler-Nordheim tunneling.
20. The method of claim 17 , wherein the oxide layer has a minimum thickness and a maximum thickness, the maximum thickness being no more than approximately 8 Angstroms larger than the minimum thickness.