Silicon oxide silicon nitride stack stair step etch

1. A method for forming a stair-step structure in a stack on a substrate in a plasma processing chamber, wherein the stack comprises a plurality of silicon oxide and silicon nitride bilayers under a mask, comprising at least one stair step cycle, wherein each stair step cycle comprises:

trimming the mask; and

etching the stack in a plurality of cycles, wherein each cycle comprises:

etching a SiO 2 layer, comprising:

flowing a SiO 2 etching gas into the plasma processing chamber, wherein the SiO 2 etching gas comprises a hydrofluorocarbon, an inert bombardment gas, and at least one of SF 6 and NF 3 ;

generating a plasma from the SiO 2 etching gas;

providing a bias; and

stopping the SiO 2 layer etch; and

etching a SiN layer, wherein the SiN layer is selectively etched with respect to the SiO 2 layer and the mask, comprising:

flowing a SiN etching gas into the plasma processing chamber, wherein the SiN etching gas comprises a hydrofluorocarbon and oxygen;

generating a plasma from the SiN etching gas;

providing a bias; and

stopping the SiN layer etch.

2. The method, as recited in claim 1 , wherein the SiO 2 etching gas is oxygen free.

3. The method, as recited in claim 2 , wherein the SiO 2 etching gas comprises SF 6 and NF 3 .

4. The method, as recited in claim 2 , wherein the SiO 2 etching gas comprises SF 6 .

5. The method, as recited in claim 1 , wherein the method comprises at least twenty stair step cycles.

6. The method, as recited in claim 1 , wherein the inert bombardment gas is He.

7. The method, as recited in claim 1 , wherein the hydrofluorocarbon in the SiN etching gas is at least one of CH 2 F 2 , CH 3 F, and CHF 3 .

8. The method, as recited in claim 1 , wherein the bias during the etching of the SiN layer has a magnitude that is greater than or equal to a magnitude of the bias during the etching of the SiO 2 layer.

9. The method, as recited in claim 1 , wherein the bias during the etching of the SiN layer has a magnitude that is between 150 to 400 volts inclusive and the bias during the etching of the SiO 2 layer has a magnitude that is less than 150 volts.

10. The method, as recited in claim 1 , further comprising providing a pressure of greater than 30 mTorr during the etching of the SiN layer and providing a pressure of less than 20 mTorr during the etching of the SiO 2 layer.

11. The method, as recited in claim 1 , wherein the etching the SiN layer is less than 10 seconds for each cycle and the etching the SiO 2 layer is less than 10 seconds for each cycle.

12. The method, as recited in claim 1 , wherein the etching the stack comprises three to ten cycles.

13. The method, as recited in claim 1 , wherein the stack comprises more than 60 bilayers.

14. The method, as recited in claim 1 , wherein the flowing a SiN etching gas into the plasma processing chamber flows the hydrofluorocarbon through a center feed and a side feed, wherein the center feed includes one or more nozzles and the side feed includes one or more nozzles that are closer to sides of the substrate than a center of the substrate.

15. The method, as recited in claim 14 , wherein the one or more nozzles of the side feed flow hydrofluorocarbon in a direction from the sides of the substrate towards the center of the substrate.

16. The method, as recited in claim 1 , wherein the etching of the SiN layer has a SiN to SiO 2 etch selectivity in the range of 2:1 to 4:1.

17. The method, as recited in claim 16 , wherein the SiO 2 etch does not selectively etch the SiO 2 layer with respect to the SiN layer.

18. The method, as recited in claim 1 , wherein the SiO 2 etching gas comprises SF 6 and NF 3 .

19. The method, as recited in claim 1 , wherein the SiO 2 etching gas comprises SF 6 .