Methods for manufacturing isolation layers in stacked transistor structures

1. A method, comprising:

forming a first transistor on a substrate, wherein the first transistor comprises first source/drain (S/D) epitaxial structures;

depositing an oxygen-free dielectric on the first S/D epitaxial structures;

oxidizing a top portion of the oxygen-free dielectric;

etching the oxidized top portion of the oxygen-free dielectric to form a first layer;

depositing an oxygen-containing dielectric on the first layer;

oxidizing a top portion of the oxygen-containing dielectric;

etching the oxidized top portion of the oxygen-containing dielectric to form a second layer; and

forming a second transistor on the first transistor, wherein:

the second transistor comprises second S/D epitaxial structures formed on the first S/D epitaxial structures; and

the first and second S/D epitaxial structures are separated by the first and second layers.

2. The method of claim 1 , wherein oxidizing the top portions of the oxygen-free and oxygen-containing dielectrics comprises treating the oxygen-free and oxygen-containing dielectrics with oxygen radicals generated by an inductively coupled plasma or a microwave plasma.

3. The method of claim 1 , wherein etching the oxidized top portions of the oxygen-free and oxygen-containing dielectrics comprises exposing the oxidized top portions to a wet etching process that includes diluted hydrofluoric acid (HF) with a water to HF ratio between about 100:1 and about 500:1.

4. The method of claim 1 , wherein etching the oxidized top portions of the oxygen-containing dielectric comprises forming a concave top surface.

5. The method of claim 1 , wherein depositing the oxygen-containing dielectric comprises depositing a dielectric material with a different dielectric constant from the oxygen-free dielectric.

6. The method of claim 1 , wherein etching the oxidized top portions of the oxygen-free and oxygen-containing dielectrics comprises forming the first layer thinner than the second layer.

7. The method of claim 1 , wherein depositing the oxygen-containing dielectric comprises silicon oxide or silicon oxy-carbon nitride.

8. A method, comprising:

depositing an oxygen-free dielectric material in an opening over a first epitaxial structure, wherein the oxygen-free dielectric material covers top surfaces of the first epitaxial structure and sidewall surfaces of the opening;

exposing the oxygen-free dielectric material to an oxidizing process to oxidize the oxygen-free dielectric material, wherein the oxidizing process does not oxidize a portion of the oxygen-free dielectric material on the first epitaxial structure;

etching the oxidized oxygen-free dielectric material; and

forming a second epitaxial layer on the oxygen-free dielectric material not removed by the etching to substantially fill the opening.

9. The method of claim 8 , wherein depositing the oxygen-free dielectric material comprises depositing a dielectric layer with a dielectric constant less than about 3.9 at a thickness between about 52 nm and about 104 nm.

10. The method of claim 8 , wherein depositing the oxygen-free dielectric material comprises depositing silicon carbon nitride, silicon carbon boron nitride, or boron nitride.

11. The method of claim 8 , wherein exposing the oxygen-free dielectric material to the oxidizing process comprises exposing the oxygen-free dielectric material to oxygen radicals generated by an inductively coupled plasma or a microwave plasma.

12. The method of claim 8 , wherein etching the oxidized oxygen-free dielectric material comprises exposing the oxidized oxygen-free dielectric material to a wet etching chemistry comprising diluted sulfuric acid with hydrogen peroxide (H 2 SO 4 :H 2 O 2 ), to a solution of ammonium hydroxide, oxygen peroxide, and water (NH 4 OH:H 2 O 2 :H 2 O), or to diluted hydrofluoric acid (DHF).

13. A method, comprising:

forming, on a substrate, a first transistor having first source/drain (S/D) epitaxial structures;

forming, on the first transistor, a second transistor comprising second S/D epitaxial structures aligned with the first S/D epitaxial structures;

forming an isolation structure between the first and second S/D epitaxial structures, the isolation structure having a concave top surface, wherein forming the isolation structure comprises forming a first layer with a planar top surface topography and a second layer; and

capping sidewalls of the first and second S/D epitaxial structures.

14. The method of claim 13 , further comprising capping the isolation structure.

15. The method of claim 13 , wherein forming the isolation structure comprises forming a first layer comprising an oxygen-free dielectric.

16. The method of claim 15 , wherein forming the isolation structure further comprises forming, on the first layer, a second layer comprising an oxygen-containing dielectric.

17. The method of claim 16 , wherein forming the second layer comprises forming the second layer having a lower dielectric constant than that of the first layer.

18. The method of claim 15 , wherein forming the isolation structure further comprises oxidizing a portion of the oxygen-free dielectric.

19. The method of claim 13 , wherein forming the second layer comprises forming the concave top surface of the isolation structure, an edge thickness of the second layer between about 3% and about 10% thicker than a mid-point thickness of the second layer.

20. The method of claim 13 , wherein a thickness of the first layer is between about 2 nm and about 4 nm.