Methods for preserving strained semiconductor substrate layers during CMOS processing
Oxidation methods, which avoid consuming undesirably large amounts of surface material in Si/SiGe heterostructure-based wafers, replace various intermediate CMOS thermal oxidation steps. First, by using oxide deposition methods, arbitrarily thick oxides may be formed with little or no consumption of surface silicon. These oxides, such as screening oxide and pad oxide, are formed by deposition onto, rather than reaction with and consumption of the surface layer. Alternatively, oxide deposition is preceded by a thermal oxidation step of short duration, e.g., rapid thermal oxidation. Here, the short thermal oxidation consumes little surface Si, and the Si/oxide interface is of high quality. The oxide may then be thickened to a desired final thickness by deposition. Furthermore, the thin thermal oxide may act as a barrier layer to prevent contamination associated with subsequent oxide deposition.
1. A method for forming a structure, the method comprising:
forming a strained semiconductor layer over a substrate;
growing an oxide layer over at least a portion of a top surface of the strained semiconductor layer;
depositing a pad oxide layer over the oxide layer; and
forming a masking layer over the pad oxide layer; the pad oxide layer substantially inhibiting formation of stress-induced defects in the strained semiconductor layer.
2. The method of claim 1 wherein the masking layer comprises silicon nitride.
3. The method of claim 1 wherein the oxide layer is grown by a rapid thermal oxidation.
4. The method of claim 1 wherein the thickness of the oxide layer ranges from about 5 Å to about 30 Å.
5. The method of claim 1 wherein the substrate comprises at least one of silicon and germanium.
6. The method of claim 1 wherein the strained semiconductor layer is tensilely strained.
7. The method of claim 6 wherein the strained semiconductor layer comprises tensilely strained silicon or tensilely strained silicon-germanium alloy.
8. The method of claim 1 wherein the strained semiconductor layer is compressively strained.
9. The method of claim 8 wherein the strained semiconductor layer comprises compressively strained germanium or compressively strained silicon-germanium alloy.
10. The method of claim 1 wherein the strained layer has a thickness ranging from about 50 Å to about 1000 Å.
11. The method of claim 10 wherein the thickness of the strained layer does not exceed about 300 Å.
12. The method of claim 11 wherein the thickness of the strained layer does not exceed about 200 Å.
13. The method of claim 10 wherein the thickness of the strained semiconductor is substantially unchanged following the deposition of the pad oxide layer.
14. The method of claim 1 wherein the substrate comprises an insulating layer disposed underneath the strained semiconductor layer.
15. The method of claim 1 wherein the substrate comprises a relaxed semiconductor layer disposed underneath the strained semiconductor layer.
16. The method of claim 15 wherein the substrate further comprises a compositionally graded layer disposed underneath the relaxed semiconductor layer.
17. The method of claim 16 wherein the graded layer comprises at least one of a group II, a group III, a group IV, a group V, and a group VI element.
18. The method of claim 17 wherein the graded layer comprises at least one of silicon and germanium.
19. The method of claim 18 wherein the graded layer is graded to a concentration of greater than about 10% germanium.
20. The method of claim 16 wherein the thickness of the graded layer ranges from about 0.5 μm to about 10.0 μm.
21. The method of claim 1 wherein the step of depositing the pad oxide layer comprises chemical vapor deposition.
22. The method of claim 1 wherein the pad oxide layer is selected from the group consisting of: silicon dioxide, silicon oxynitride, silicon germanium oxide, and germanium oxide.
23. The method of claim 1 wherein the pad oxide layer has a thickness ranging from about 50 Å to about 500 Å.
24. A method for forming a structure, the method comprising:
forming a strained semiconductor layer over a substrate;
depositing a pad oxide layer over at least a portion of a top surface of the strained semiconductor layer; and
forming a masking layer over the pad oxide layer; the pad oxide layer substantially inhibiting formation of stress-induced defects in the strained semiconductor layer,
wherein the pad oxide layer is at least one of silicon oxynitride, silicon germanium oxide, or germanium oxide.