Fluoride gas etching of silicon with improved selectivity

• US 6,290,864 B1
• Filed: 10/26/1999
• Issued: 09/18/2001
• Est. Priority Date: 10/26/1999
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Reflectivity, Inc. (Texas Instruments, Inc.)
• Inventors: MacDonald, Douglas B., Patel, Satyadev R., Schaadt, Gregory P.
• Primary Examiner(s): Stinson, Frankie L.
• Assistant Examiner(s): OLSEN, ALLAN W
• Application Number:
  US09/427,841
• Time in Patent Office: 693 Days
• Field of Search: 252/79.1, 216/58, 216/73, 216/79
• US Class Current: 216/79
• CPC Class Codes: B81C 1/00595 Control etch selectivity
• CPC Class Codes: H01L 21/3065 Plasma etching; Reactive-ion etching
• CPC Class Codes: H01L 21/3081 characterised by their composition, e.g. multilayer masks, materials
• CPC Class Codes: H01L 21/32135 by vapour etching only

Abstract

The etching of a sacrificial silicon portion in a microstructure such as a microelectromechanical structure by the use of etchant gases that are noble gas fluorides or halogen fluorides is performed with greater selectivity toward the silicon portion relative to other portions of the microstructure by the addition of non-etchant gaseous additives to the etchant gas. An additional discovery is that non-etchant gaseous additives that have a molar averaged formula weight that is below that of molecular nitrogen offer significant advantages over gaseous additives of higher formula weights by causing completion of the etch in a shorter period of time while still achieving the same improvement in selectivity.

Claims

1. In a method for selectively etching a silicon portion relative to a non-silicon portion of a sample containing both silicon and non-silicon portions, said non-silicon portion consisting of a member selected from the group consisting of a non-silicon metal, a compound of a non-silicon metal, and a silicon-containing compound in which silicon is bonded to a non-silicon element, by exposing both said silicon portion and said non-silicon portion to an etchant gas selected from the group consisting of noble gas fluorides and halogen fluorides, the improvement in which said etchant gas is utilized in the form of a gas mixture in which said etchant gas is mixed with a non-etchant gaseous additive, the partial pressure of said etchant gas in said gas mixture being at least about 0.1 mbar, and the molar ratio of said non-etchant gaseous additive to said etchant gas being from about 1:

1 to about 500;

1, such that said gas mixture achieves greater etching selectivity toward said silicon portion than would be achieved with said etchant gas alone.

2. A method in accordance with claim 1 in which said non-etchant gaseous additive has a molar-averaged formula weight of less than about 25.

3. A method in accordance with claim 1 in which said non-etchant gaseous additive has a molar-averaged formula weight of from about 4 to about 25.

4. A method in accordance with claim 1 in which said non-etchant gaseous additive has a molar-averaged formula weight of from about 4 to about 20.

5. A method in accordance with claim 1 in which said non-etchant gaseous additive has a molar-averaged formula weight of from about 4 to about 10.

6. A method in accordance with claim 1 in which said non-etchant gaseous additive has a molar-averaged thermal conductivity at 300 K and atmospheric pressure of from about 10 mW/(m K) to about 200 mW/(m K).

7. A method in accordance with claim 1 in which said non-etchant gaseous additive has a molar-averaged thermal conductivity at 300 K and atmospheric pressure of from about 140 mW/(m K) to about 190 mW/(m K).

8. A method in accordance with claim 1 in which said molar ratio is from about 10:

1 to about 200;

1.

9. A method in accordance with claim 1 in which said molar ratio is from about 20:

1 to about 100;

1.

10. A method in accordance with claim 1 in which said non-etchant gaseous additive is a member selected from the group consisting of nitrogen, argon, helium, neon, and mixtures thereof.

11. A method in accordance with claim 1 in which said non-etchant gaseous additive is a member selected from the group consisting of helium, neon, mixtures of helium and neon, and mixtures of one or both of helium and neon with one or both of nitrogen and argon.

12. A method in accordance with claim 1 in which said non-etchant gaseous additive is a member selected from the group consisting of helium, a mixture of helium and nitrogen, and a mixture of helium and argon.

13. A method in accordance with claim 1 in which said non-etchant gaseous additive is a member selected from the group consisting of helium and a mixture of helium and nitrogen.

14. A method in accordance with claim 1 in which said non-etchant gaseous additive is helium.

15. A method in accordance with claim 1 in which said etchant gas is a noble gas fluoride.

16. A method in accordance with claim 15 in which said noble gas fluoride is a member selected from the group consisting of krypton difluoride and the xenon fluorides.

17. A method in accordance with claim 15 in which said noble gas fluoride is a member selected from the group consisting of xenon difluoride, xenon tetrafluoride, and xenon hexafluoride.

18. A method in accordance with claim 15 in which said noble gas fluoride is xenon difluoride.

19. A method in accordance with claim 15 in which said noble gas fluoride is xenon difluoride and said non-etchant gaseous additive is a member selected from the group consisting of helium, neon, and mixtures one or more or helium and neon with one or more of nitrogen and argon.

20. A method in accordance with claim 15 in which said noble gas fluoride is xenon difluoride and said non-etchant gaseous additive is a member selected from the group consisting of helium and a mixture of nitrogen and helium.

21. A method in accordance with claim 1 in which said etchant gas is a halogen fluoride.

22. A method in accordance with claim 21 in which said halogen fluoride is a member selected from the group consisting of chlorine trifluoride, bromine trifluoride, and iodine pentafluoride.

23. A method in accordance with claim 21 in which said halogen fluoride is a member selected from the group consisting of chlorine trifluoride and bromine trifluoride.

24. A method in accordance with claim 21 in which said halogen fluoride is bromine trifluoride.

25. A method in accordance with claim 1 in which the partial pressure of said etchant gas is from about 0.3 mbar to about 30 mbar.

26. A method in accordance with claim 1 in which the partial pressure of said etchant gas is from about 1 mbar to about 15 mbar.

27. A method in accordance with claim 1 in which the partial pressure of said etchant gas is from about 1 mbar to about 15 mbar, and the mole ratio of said non-etchant gaseous additive to said etchant gas is from about 10:

1 to about 200;

1.

28. A method in accordance with claim 1 in which the partial pressure of said etchant gas is from about 1 mbar to about 15 mbar, and the mole ratio of said non-etchant gaseous additive to said etchant gas is from about 20:

1 to about 100;

1.

29. A method in accordance with claim 1 in which said non-silicon portion is a member selected form the group consisting of non-silicon metals and compounds of non-silicon metals.

30. A method in accordance with claim 29 in which said non-silicon portion is a member selected from the group consisting of titanium, gold, tungsten, and compounds thereof.

31. A method in accordance with claim 29 in which said non-silicon portion is gold.

32. A method in accordance with claim 1 in which said silicon portion is a silicon layer deposited over a substrate and said non-silicon portion is a layer of a member selected from the group consisting of silicon nitride, silicon carbide, and silicon oxide deposited over said silicon layer, said non-silicon layer being patterned to leave vias therein for access of said gas mixture to said silicon layer, the exposure to said gas mixture being of sufficient duration to laterally etch away substantially all of said silicon layer by access through said vias.

33. A method in accordance with claim 1 in which said silicon layer is a polysilicon layer deposited over a substrate and said non-silicon portion is a layer of silicon nitride, said silicon nitride layer being patterned to leave vias therein for access of said gas mixture to said polysilicon layer, said exposure to said gas mixture being of sufficient duration to laterally etch away substantially all of said polysilicon layer by access through said vias.

34. A method in accordance with claim 33 in which said polysilicon layer is from about 200 nm to about 5000 nm in thickness.

35. A method in accordance with claim 33 in which said polysilicon layer is from about 250 nm to about 3000 nm in thickness.

36. A method in accordance with claim 33 in which said polysilicon layer is from about 300 nm to about 1000 nm in thickness.

37. A method in accordance with claim 33 in which said silicon nitride layer is from about 10 nm to about 500 nm in thickness.

38. A method in accordance with claim 33 in which said silicon nitride layer is from about 20 nm to about 200 nm in thickness.