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

• US 7,291,513 B2
• Filed: 12/07/2004
• Issued: 11/06/2007
• Est. Priority Date: 12/15/2003
• Status: Active Grant

• Current Assignee: Teledyne Digital Imaging, Inc. (Teledyne Technologies Incorporated)
• Original Assignee: Dalsa Semiconductor Inc (Teledyne Technologies Incorporated)
• Inventors: Ouellet, Luc, Turcotte, Karine
• Primary Examiner(s): Fourson; George R.
• Assistant Examiner(s): Parker; John M.
• Application Number:
  US11/004,973
• Publication Number:
  US 20050142685A1
• Time in Patent Office: 1,064 Days
• Field of Search: 438/51, 438/118, 438/119, 438/455, 216/33, 148/DIG.12
• US Class Current: 438/64
• 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;

wherein said first and second bonding structures comprise respective anti-oxidation layers that are bonded together by an intervening solder layer between said respective anti-oxidation layers;

wherein said anti-oxidation layers comprise a noble metal selected from the group consisting of platinum, rhodium and ruthenium;

wherein said first bonding structure comprises a first seed layer on a MEMS device, a first structural underlayer over said first seed layer, and a first said anti-oxidation layer over said first structural underlayer; and

wherein said second bonding structure comprises a second seed layer, a second structural underlayer over said second seed layer, a second said anti-oxidation over said second structural underlayer, and

wherein said solder layer is provided on one of said anti-oxidation layers.

2. A method as claimed in claim 1, wherein each said seed layer is selected from the group consisting of:

a doped silicon layer and a metal layer with a bulk resistivity lower than 100 mohm.cm.

3. A method as claimed in claim 1, 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.

4. A method as claimed in claim 1, wherein

wherein each said structural under-layer is deposited using a substantially alkali-free and chlorine-free electroless plating chemistry employing electroless nickel, and

wherein said electroless nickel is obtained according to the equation:

NiSO₄+Ni(H₂PO₂)₂+2H₂O→

Ni⁰+H₂SO₄+Ni(H₂PO₃)₂+H₂↑

5. A method as claimed in claim 1,

wherein each said structural under-layer is deposited using a substantially alkali-free and chlorine-free electroless plating chemistry employing electroless nickel, and;

wherein said electroless nickel is obtained according to the equation;

Ni(H₂PO₂)₂+Ni(H₂PO₂)₂+2H₂O→

Ni⁰+2H₃PO₂+Ni(H₂PO₃)₂+H₂↑

10. A method as claimed in claim 1, 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.

11. A method as claimed in claim 10, wherein the lead-free metal solder is gallium.

12. A method as claimed in claim 10, wherein the lead-free solder is indium.

13. A method as claimed in claim 12, wherein said indium is electroless plated using a source of indium selected from the group consisting of:

indium sulfate, indium nitride, and indium methanesulfonate.

14. A method as claimed in claim 12, 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.

15. A method as claimed in claim 12, wherein said indium is electroless plated using thiourea.

16. A method as claimed in claim 10, wherein the lead-free metal solder is tin.

17. A method as claimed in claim 16, wherein said tin is electroless plated using stannous sulfate as a source of the tin.

18. A method as claimed in claim 16, 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.

19. A method as claimed in claim 16, 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.

20. A method as claimed in claim 10, 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.

21. A method as claimed in claim 20, wherein said lead-free metal solder is electroless plated.

6. 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;

wherein said first and second bonding structures comprise respective anti-oxidation layers that are bonded together by an intervening solder layer;

wherein said anti-oxidation layers comprise a noble metal selected from the group consisting of platinum, rhodium and ruthenium; and

wherein the noble metal layer is deposited using a process selected from the group consisting of;

immersion and electroless plating.

7. A method as claimed in claim 6, wherein a sulfate of the noble metal is used as a source thereof.

8. A method as claimed in claim 6, wherein a plating solution is employed that contains a component selected from the group consisting of:

ethylenediaminetetraacetic acid (LDTA) and dimethylamine borane.

9. A method as claimed in claim 6, wherein a plating solution is employed that contains ammonium hydroxide as a buffer to control the pH of the solution.

22. 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;

wherein each of said first and second bonding structures includes a structural under-layer; and

wherein each said structural under-layer is deposited using a substantially alkali-free and chlorine-free electroless plating chemistry employing electroless nickel, and

wherein said electroless nickel is obtained according to an equation selected from the group consisting of;

NiSO₄+Ni(H₂PO₂)₂+2H₂O→

Ni⁰

+H₂SO₄+Ni(H₂PO₃)₂+H₂↑

;

and

Ni(H₂PO₂)₂+Ni(H₂PO₂)₂+2H₂

O→

Ni⁰+2H₃PO₂+Ni(H₂PO₃)₂+H₂↑