Curing photo resist for improving etching selectivity

1. A method comprising:

forming a wafer comprising:

forming a hard mask over a low-k dielectric layer, wherein the hard mask comprises a trench therein;

forming a bottom layer over the hard mask, wherein the bottom layer is a photo-resist containing layer, and the bottom layer is filled into the trench;

forming a middle layer over the bottom layer; and

forming a patterned photo resist over the middle layer, wherein the patterned photo resist has an opening;

after the patterned photo resist is formed over the middle layer, performing a treatment on the patterned photo resist to increase cross-linking in the patterned photo resist, wherein the treatment comprises using a process gas, wherein the process gas is free of an etchant gas suitable for etching the middle layer;

after performing the treatment, etching the middle layer using an etching gas and the patterned photo resist as an etching mask;

etching the bottom layer; and

etching the low-k dielectric layer, wherein a pattern of the opening in the patterned photo resist is transferred into the low-k dielectric layer.

2. The method of claim 1 , wherein the treatment comprises generating secondary electrons, wherein the treatment comprises bombarding the patterned photo resist with the secondary electrons.

3. The method of claim 2 , wherein generating the secondary electrons comprising:

using a Radio-Frequency (RF) power source to generate plasma from the process gas; and

applying a negative DC voltage on an electrode under the wafer to generate the secondary electrons.

4. The method of claim 1 , wherein the treatment is performed in an etcher.

5. The method of claim 1 , wherein at a time after the treatment is performed and before the middle layer is etched, the patterned photo resist has a thickness greater than about 250 Å.

6. The method of claim 1 , wherein the patterned photo resist comprises a negative photo resist.

7. The method of claim 1 , wherein during the treatment, the process gas is free of hydrogen gas.

8. The method of claim 1 , wherein during the treatment, the process gas is free of fluorine-containing gas.

9. The method of claim 1 , wherein the treatment has a duration shorter than about 10 seconds.

10. A method comprising:

forming a hard mask;

forming a bottom layer over the hard mask, wherein the bottom layer comprises a photo resist material;

forming a middle layer over the bottom layer;

forming a photo resist over and contacting the middle layer;

forming an opening in the photo resist;

generating secondary electrons;

performing a treatment on the photo resist using the secondary electrons to form a treated photo resist, the treated photo resist comprising hydrocarbonate; and

after the treatment, etching the middle layer using the treated photo resist as an etching mask.

11. The method of claim 10 further comprising transferring a pattern of the photo resist into a low-k dielectric layer underlying the hard mask to form a via opening.

12. The method of claim ii further comprising:

forming a trench pattern in the hard mask; and

transferring the trench pattern into the low-k dielectric layer to form a trench, wherein the pattern of the photo resist is transferred into the via opening in the low-k dielectric layer, with the via opening underlying the trench.

13. The method of claim 10 , wherein the photo resist is a negative photo resist.

14. The method of claim 10 , wherein the treatment is performed using a process gas comprising hydrogen.

15. The method of claim 14 , wherein the process gas further comprises argon, and during the treatment, a ratio of a first flow rate of the hydrogen to a second flow rate of the argon is between ⅛ and ¼ .

16. A method comprising:

forming a wafer comprising:

a hard mask over a low-k dielectric layer;

a bottom layer over the hard mask, wherein the bottom layer comprises a photo resist;

a middle layer over the bottom layer; and

a patterned photo resist over the middle layer, the patterned photo resist having an opening;

after forming the wafer having the patterned photo resist, treating the patterned photo resist using a process gas mixture comprising a hydrogen (H 2 ) gas and an argon gas, wherein treating the patterned photoresist converts at least a portion of the patterned photo resist to hydrocarbonate;

after the patterned photo resist is treated, etching the middle layer using the patterned photo resist as a first etching mask; and

patterning the bottom layer and the low-k dielectric layer using the etched middle layer as a second etching mask.

17. The method of claim 16 , wherein the treating results in the patterned photo resist to have increased cross-linking.

18. The method of claim 16 , wherein the treating results in the patterned photo resist to have a reduced etching rate when the middle layer is etched.

19. The method of claim 16 further comprising filling a conductive material into the patterned low-k dielectric layer.

20. The method of claim 16 , wherein the patterned photo resist is treated using secondary electrons generated from the hydrogen gas and the argon gas.