Shallow trench isolation

1. A method for forming an isolation structure on a semiconductor substrate, the method comprising:

forming a pad oxide layer over the substrate;

forming a nitride layer over the pad oxide;

opening a portion of the nitride layer, the nitride layer opening defining a first edge;

opening a portion of a pad oxide layer exposed by the nitride opening step using a process gas including an etchant gas and a polymer-forming gas to form a polymer residue at a second edge of the opened portion of the pad oxide;

etching a portion of the substrate exposed by the opening step to form a trench having a first slope and a second slope, the first slope being greater than the second slope;

forming a first liner oxide layer over the trench;

etching the nitride layer using an isotropic etch to cause the first edge of the nitride layer to recede outwardly from the trench, the first liner oxide protecting the substrate during the isotropic nitride etch; and

filling the trench to form the isolation structure.

2. The method of claim 1 , wherein the isolation structure is a shallow trench isolation (STI) structure.

3. The method of claim 1 , wherein the pad oxide opening step has an etch rate of less than 3 angstroms per second.

4. The method of claim 1 , wherein the first slope is greater than 45 degrees and the second slope is less than 45 degrees.

5. The method of claim 1 , wherein the pad oxide has a thickness of less than 200 angstroms and the pad oxide opening step is performed in an environment having pressure of 50 mT or greater.

6. The method of claim 5 , wherein the etchant gas is CF 4 and the polymer forming gas is CHF 3 .

7. The method of claim 1 , wherein the nitride layer formed over the pad oxide is provided with an excess thickness of at least 150 angstroms that is to be removed during the subsequent isotropic nitride etching step.

8. The method of claim 1 , further comprising:

forming a second liner oxide layer over the trench after performing the isotropic. nitride etching.

9. The method of claim 8 , further comprising:

forming a thick gate oxide layer over the substrate; and

forming a thin gate oxide layer over the substrate.

10. The method of claim 8 , wherein the isolation structure is a shallow trench isolation structure having a body and a shoulder, wherein the body defines the first slope greater than 45 degrees and the shoulder defines the second slope less than 45 degrees.

11. The method of claim 1 , wherein the isolation structure is a shallow trench isolation structure having a body and a shoulder, wherein the body defines the first slope greater than 45 degrees and the shoulder defines the second slope less than 45 degrees.