Hermetic wafer-level packaging for MEMS devices with low-temperature metallurgy

• US 20050142685A1
• Filed: 12/07/2004
• Published: 06/30/2005
• Est. Priority Date: 12/15/2003
• Status: Active Grant

• Current Assignee: Teledyne Digital Imaging, Inc. (Teledyne Technologies Incorporated)
• Original Assignee: Dalsa Incorporated (Teledyne Technologies Incorporated)
• Inventors: Ouellet, Luc, Turcotte, Karine
• Granted Patent: US 7,291,513 B2
• Time in Patent Office: Days
• US Class Current: 438/51
• CPC Class Codes: B81C 1/00269 Bonding of solid lids or wafers to the substrate
• CPC Class Codes: B81C 2203/0118 Bonding a wafer on the substrate, i.e. where the cap consists of another wafer
• CPC Class Codes: B81C 2203/019 characterised by the material or arrangement of seals between parts

Abstract

A method is disclosed for making a wafer-level package for a plurality of MEMS devices. The method involves preparing a MEMS wafer and a lid wafer, each having respective bonding structures. The lid and MEMS wafers are then bonded together through the bonding structures. The wafers are substantially free of alkali metals and/or chlorine. IN a preferred embodiment, each wafer has a seed layer, a structural underlayer and an anti-oxidation layer. A solder layer, normally formed on the lid wafer, bonds the two wafers together.

Claims

1. A method of making a wafer-level package for a plurality of MEMS devices, wherein a sealed and hermetic micro-cavity is formed over each MEMS device, comprising:

preparing a MEMS wafer with a first bonding structure;

preparing a lid wafer with a second bonding structure; and

bonding said lid wafer to said MEMS wafer through said first and second bonding structures to create said wafer level package, and

wherein said lid wafer and said MEMS wafer are substantially free of at least one component selected from the group consisting of alkali metals and chlorine.

2. A method as claimed in claim 1, wherein said first bonding structure of said MEMS wafer is formed by depositing a first seed layer on a MEMS device, depositing a first structural underlayer over said first seed layer, and depositing a first anti-oxidation layer over said first structural underlayer, said anti-oxidation layer being bondable to said second bonding structure.

3. A method as claimed in claim 2, wherein said second bonding structure of said lid wafer comprises a second seed layer, a second structural underlayer over said second seed layer, a second anti-oxidation over said second structural underlayer, and a solder layer capable of bonding to said first anti-oxidation layer.

4. A method as claimed in claim 3, wherein each said seed layer is selected from the group consisting of:

a doped silicon layer, a substrate with a bulk resistivity lower than 100 ohm.cm, a metal layer with a bulk resistivity lower than 100 mohm.cm.

5. A method as claimed in claim 2, wherein each said seed layer is a metal layer selected from the group consisting of:

titanium and compounds thereof, tungsten and compounds thereof, copper, aluminum and an aluminum alloy.

6. A method as claimed in claim 2, wherein each said seed layer is a substrate with a bulk resistivity lower than 100 mohm.cm.

7. A method as claimed in claim 2, wherein each said structural under-layer is deposited using electroless plating.

8. A method as claimed in claim 7, wherein said electroless plating uses a substantially alkali-free and chlorine-free electroless plating chemistry.

9. A method as claimed in claim 7, wherein said electroless plating uses electroless nickel.

10. A method as claimed in claim 9, wherein said electroless nickel is obtained according to the equation:

NiSO₄+Ni(H₂PO₂)₂+2H₂O→

Ni⁰+H₂SO₄+Ni(H₂PO₃)₂+H₂↑

11. A method as claimed in claim 9, wherein said electroless nickel is obtained according to the equation:

Ni(H₂PO₂)₂+Ni(H₂PO₂)₂+2H₂O→

Ni⁰+2H₃PO₂+Ni(H₂PO₃)₂H₂↑

12. A method as claimed in claim 3, wherein each said anti-oxidation layer is a noble metal layer other than gold.

13. A method as claimed in claim 12, wherein said noble metal layer is selected from the group consisting of platinum, rhodium and ruthenium.

14. A method as claimed in claim 13 where the noble metal layer is deposited using a process selected from the group consisting of:

immersion and electroless plating.

15. A method as claimed in claim 14, wherein a sulfate of the noble metal is used as a source thereof.

16. A method as claimed in claim 14, wherein a plating solution is employed that contains a component selected from the group consisting of:

ethylenediaminetetraacetic acid (EDTA) and dimethylamine borane.

17. A method as claimed in claim 14, wherein a plating solution is employed that contains ammonium hydroxide as a buffer to control the pH of the solution.

18. A method as claimed in claim 3, wherein said solder layer is a lead-free metal solder deposited by electroless plating in an alkali-free, hydrazine-free, dimethylamine borane-free, and chlorine-free electroless plating solution.

19. A method as claimed in claim 18, wherein the lead-free metal solder is gallium.

20. A method as claimed in claim 18, wherein the lead-free solder is indium.

21. A method as claimed in claim 20, wherein said indium is electroless plated using a source of indium selected from the group consisting of:

indium sulfate, indium nitride, and indium methanesulfonate.

22. A method as claimed in claim 20, wherein said indium is electroless plated using an agent selected from the group consisting of:

sulfuric acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, tartaric acid, and citric acid to stabilize.

23. A method as claimed in claim 20, wherein said indium is electroless plated using thiourea.

24. A method as claimed in claim 18, wherein the lead-free metal solder is tin.

25. A method as claimed in claim 24, wherein said tin is electroless plated using stannous sulfate as a source of the tin.

26. A method as claimed in claim 24, wherein said tin is electroless plated using an agent selected from the group consisting of:

sulfuric acid, fluoroboric acid, phosphoric acid, an organic sulfonic acid, methane sulfonic acid, to stabilize the pH.

27. A method as claimed in claim 24, wherein said tin is electroless plated using an operator-safe complexing agent selected from the group consisting of:

tartaric acid, citric acid, ethylenediaminetetraacetic acid, thiourea, and triethanol amine.

28. A method as claimed in claim 18, wherein the lead-free metal solder is selected from the group consisting of:

gallium-indium binary metal solder, a gallium-tin binary metal solder, gallium-bismuth binary metal solder, gallium-zinc binary metal solder, an indium-gallium binary metal solder, an indium-tin binary metal solder, an indium-bismuth binary metal solder, an indium-zinc binary metal solder, an indium-silver binary metal solder, a tin-gallium binary metal solder, a tin-indium binary metal solder, a tin-bismuth binary metal solder, a tin-zinc binary metal solder, a tin-magnesium binary metal solder, a tin-silver binary metal solder, and a tin-copper binary metal solder.

29. A method as claimed in claim 28, wherein said lead-free metal solder is electroless plated.

30. A MEMS package comprising:

a MEMS wafer having a first bonding structure;

a lid wafer having a second bonding structure;

said lid wafer being bonded to said MEMS wafer through said first and second bonding structures to form a sealed and hermetic micro-cavity; and

wherein said lid wafer and said MEMS wafer are substantially free of at least one component selected from the group consisting of alkali metals and chlorine.

31. A MEMS package as claimed in claim 30, wherein said first bonding structure of comprises a first seed layer, a first structural underlayer over said first seed layer, and a first anti-oxidation layer over said first structural underlayer, said anti-oxidation layer being bondable to said second bonding structure.

32. A MEMS package as claimed in claim 31, wherein said second bonding structure of said lid wafer comprises a second seed layer, a second structural underlayer over said second seed layer, a second anti-oxidation over said second structural underlayer, and a solder layer capable of bonding to said first anti-oxidation layer.

33. A MEMS package as claimed in claim 32, wherein each said structural underlayer is electrolessly plated nickel.

34. A MEMS package as claimed in claim 33, wherein each said anti-oxidation layer is a noble metal layer other than gold.

35. A MEMS package as claimed in claim 34, wherein said noble metal layer is selected from the group consisting of platinum, rhodium and ruthenium.

36. A MEMS package as claimed in claim 32, wherein said solder layer comprises a lead-free metal solder.

37. A MEMS package as claimed in claim 36, wherein the lead-free metal solder is gallium.

38. A MEMS package as claimed in claim 36, wherein the lead-free solder is indium.

39. A MEMS package as claimed in claim 36, wherein the lead-free metal solder is selected from the group consisting of:

gallium-indium binary metal solder, a gallium-tin binary metal solder, gallium-bismuth binary metal solder, gallium-zinc binary metal solder, an indium-gallium binary metal solder, an indium-tin binary metal solder, an indium-bismuth binary metal solder, an indium-zinc binary metal solder, an indium-silver binary metal solder, a tin-gallium binary metal solder, a tin-indium binary metal solder, a tin-bismuth binary metal solder, a tin-zinc binary metal solder, a tin-magnesium binary metal solder, a tin-silver binary metal solder, and a tin-copper binary metal solder.