Method of fabricating a silicon nitride stack

1. A method for fabricating a silicon nitride stack on a semiconductor substrate, comprising:

(a) depositing a base layer comprising silicon nitride on the semiconductor substrate using a first set of process conditions that selectively control a stress of the base layer; and

(b) depositing an upper layer comprising silicon nitride using a second set of process conditions that selectively control at least one of an oxidation resistance and a refractive index of the upper layer.

2. The method of claim 1 , wherein the first set of process conditions that selectively control the stress of the base layer includes at least one of a silicon source gas flow, a total gas flow, and a chamber pressure.

3. The method of claim 1 , wherein the second set of process conditions that selectively control at least one of an oxidation resistance and a refractive index of the upper layer includes a nitrogen source gas to silicon source gas flow ratio.

4. The method of claim 1 , wherein the silicon nitride is deposited using disilane and ammonia.

5. The method of claim 1 , wherein step (a) further comprises minimizing the stress of the base layer.

6. The method of claim 1 , wherein step (b) further comprises maximizing the oxidation resistance of the upper layer.

7. The method of claim 1 , wherein step (b) further comprises optimizing the refractive index of the upper layer.

8. The method of claim 1 , further comprising:

depositing one or more intervening layers comprising silicon nitride between the base and upper layers of the stack.

9. A method of forming a shallow trench isolation structure in a substrate, comprising:

(a) depositing a pad oxide layer on the substrate;

(b) depositing a silicon nitride stack having a base layer and an upper layer, wherein a stress of the base layer is selectively controlled by a first set of deposition conditions and wherein at least one of an oxidation resistance and a refractive index of the upper layer are selectively controlled by a second set of deposition conditions;

(c) depositing and patterning a photoresist layer on the substrate;

(d) etching a trench into the substrate through the pad oxide layer and silicon nitride stack;

(e) oxidizing the trench to form a liner;

(f) filling the linered trench with silicon oxide; and

(g) removing excess silicon oxide.

10. The method of claim 9 , wherein the first set of process conditions that selectively control the stress of the base layer includes at least one of a silicon source gas flow, a total gas flow, and a chamber pressure.

11. The method of claim 9 , wherein the second set of process conditions that selectively control at least one of an oxidation resistance and a refractive index of the upper layer includes a nitrogen source gas to silicon source gas flow ratio.

12. The method of claim 9 , wherein the silicon nitride is deposited using disilane and ammonia.

13. The method of claim 9 , wherein step (b) further comprises minimizing the stress of the base layer.

14. The method of claim 9 , wherein step (b) further comprises maximizing the oxidation resistance of the upper layer.

15. The method of claim 9 , wherein step (b) further comprises optimizing the refractive index of the upper layer.

16. The method of claim 9 , further comprising:

depositing one or more intervening layers comprising silicon nitride between the base and upper layers of the stack.

17. The method of claim 9 , wherein the base layer is about 500-2,500 Angstroms thick.

18. The method of claim 9 , wherein the upper layer is about 100-2,000 Angstroms thick.

19. The method of claim 9 , wherein the silicon nitride stack has a total thickness of about 500-2,500 Angstroms.

20. The method of claim 9 , wherein the silicon nitride stack has a total thickness of about 1,550 Angstroms.