Selective atomic layer etching

1. A method for selectively etching a dielectric layer with respect to an epitaxial layer, wherein the epitaxial layer is a germanium containing material, or metal-based hardmask, and improving surface roughness of the etch front, the method comprising:

flowing a thiol containing treatment gas into the etch chamber;

selectively forming a passivation layer from the thiol containing treatment gas on the epitaxial layer or metal-based hardmask with respect to the dielectric layer;

stopping the flow of the thiol containing treatment gas into the etch chamber; and

performing a plurality of cycles in an etch chamber, wherein each cycle comprises:

a deposition phase, comprising:

flowing a deposition gas into the etch chamber, wherein the deposition gas comprises helium and a hydrofluorocarbon or fluorocarbon;

forming the deposition gas into a plasma to effect a fluorinated polymer deposition; and

stopping the flow of the deposition gas into the etch chamber; and

an activation phase, comprising:

flowing an activation gas comprising an ion bombardment gas into the etch chamber;

forming the activation gas into a plasma;

providing an activation bias in the etch chamber to cause ion bombardment of the fluorinated polymer deposition, wherein the ion bombardment activates fluorine from the fluorinated polymer deposition to etch the dielectric layer; and

stopping the flow of the activation gas into the etch chamber.

2. The method, as recited in claim 1 , wherein the activation gas comprises argon.

3. The method, as recited in claim 1 , wherein the deposition gas is argon free.

4. The method, as recited in claim 3 , wherein the activation gas consists essentially of argon.

5. The method, as recited in claim 1 , wherein the fluorocarbon or hydrofluorocarbon comprises at least one of C 4 F 6 or C 4 F 8 .

6. The method, as recited in claim 1 , wherein the dielectric layer is a silicon oxide containing layer.

7. The method, as recited in claim 1 , wherein the dielectric layer is a silicon oxide layer.

8. The method, as recited in claim 1 , wherein the plurality of cycles is at least five cycles.

9. The method, as recited in claim 1 , wherein the deposition gas has a helium to a hydrofluorocarbon or fluorocarbon flow rate ratio of between about 10:1 to 300:1.

10. The method, as recited in claim 1 , wherein the activation bias is less than a bias needed to sputter the dielectric layer.