Etch profile control of via opening

1. A method comprising:

forming a source/drain contact over a source/drain region;

forming an etch stop layer over the source/drain contact and an interlayer dielectric (ILD) layer over the etch stop layer;

performing a first etching process to form a via opening extending through the ILD layer and a recess in a nitride material of the etch stop layer;

oxidizing a sidewall of the recess in the etch stop layer, wherein after oxidizing the sidewall of the recess in the etch stop layer, the etch stop layer has an un-oxidized region under the recess;

after oxidizing the sidewall of the recess in the etch stop layer, performing a second etching process to extend the via opening down to the source/drain contact; and

after performing the second etching process, forming a source/drain via in the via opening.

2. The method of claim 1 , wherein the sidewall of the recess in the etch stop layer is oxidized using an oxygen plasma.

3. The method of claim 2 , wherein the oxygen plasma is generated from an O 2 gas.

4. The method of claim 2 , wherein the oxygen plasma is generated from a gaseous mixture of an O 2 gas and one or more of an Ar gas, a He gas, a Ne gas, a Kr gas, a N 2 gas, a CO gas, a CO 2 gas, a C x H y F z gas, an NF 3 gas, a Carbonyl sulfide (COS) gas, and an SO 2 gas, wherein x, y and z are greater than zero.

5. The method of claim 1 , wherein the second etching process uses a different etchant than that used in the first etching process.

6. The method of claim 1 , wherein the first etching process is a plasma etching process using a plasma generated from a hydrogen-free gaseous mixture.

7. The method of claim 1 , wherein the second etching process is a plasma etching process using a plasma generated from a hydrogen-containing gaseous mixture.

8. The method of claim 7 , wherein the hydrogen-containing gaseous mixture is a mixture of a fluorine-containing gas and a hydrogen gas.

9. The method of claim 8 , wherein the fluorine-containing gas is a CHF 3 gas, a CF 4 gas, or a combination thereof.

10. The method of claim 1 , wherein the second etching process results in less lateral etching on the oxidized sidewall of the etch stop layer than on the un-oxidized region of the etch stop layer.

11. The method of claim 1 , wherein after oxidizing the sidewall of the recess in the etch stop layer, the etch stop layer has an oxygen-containing region having an oxygen concentration gradient.

12. The method of claim 1 , wherein the source/drain region is over a semiconductor fin.

13. A method, comprising:

forming a source/drain contact over an epitaxy structure;

depositing in sequence an etch stop layer and an interlayer dielectric (ILD) layer over the source/drain contact;

performing a first etching process on the ILD layer until the etch stop layer has a recess;

oxidizing a surface of the recess in the etch stop layer;

after oxidizing the surface of the recess in the etch stop layer, performing a second etching process to break through the etch stop layer, wherein the etch stop layer has an un-oxidized region under the oxidized surface of the recess; and

after performing the second etching process, forming a source/drain via extending through the etch stop layer.

14. The method of claim 13 , wherein the surface of the recess in the etch stop layer is oxidized using an oxygen plasma.

15. The method of claim 13 , wherein after oxidizing the surface of the recess in the etch stop layer, the etch stop layer has an oxygen-containing region having an oxygen concentration gradient.

16. The method of claim 15 , wherein the oxygen-containing region has an oxygen atomic percentage decreasing as a distance from the oxidized surface of the recess increases.

17. The method of claim 13 , wherein the second etching process uses a gas mixture with a hydrogen gas, and the first etching process is free of the hydrogen gas.

18. A method, comprising:

forming a source/drain contact over a source/drain region;

forming an etch stop layer over the source/drain contact and an interlayer dielectric (ILD) layer over the etch stop layer;

performing a first etching step on the ILD layer until the etch stop layer has a recess;

oxidizing a first region of the etch stop layer exposed in the recess, wherein the etch stop layer has a second region remaining un-oxidized after oxidizing the first region;

performing a second etching step on the oxidized first region of the etch stop layer until the source/drain contact is exposed; and

forming a source/drain via on the exposed source/drain contact.

19. The method of claim 18 , wherein the second etching step etches the oxidized first region of the etch stop layer at a slower etch rate than etching the un-oxidized second region.

20. The method of claim 18 , wherein the first region of the etch stop layer is oxidized using an oxygen plasma.