Wafer level packaging technique for microdevices

• US 20040067604A1
• Filed: 12/06/2002
• Published: 04/08/2004
• Est. Priority Date: 10/04/2002
• Status: Active Grant

• Current Assignee: Teledyne Digital Imaging, Inc. (Teledyne Technologies Incorporated)
• Original Assignee: Dalsa Semiconductor Inc (Teledyne Technologies Incorporated)
• Inventors: Poisson, Jules J., Ouellet, Luc
• Granted Patent: US 7,138,293 B2
• Time in Patent Office: Days
• US Class Current: 438/108
• CPC Class Codes: B81C 1/00269 Bonding of solid lids or wafers to the substrate
• CPC Class Codes: B81C 2203/019 characterised by the material or arrangement of seals between parts
• CPC Class Codes: H01L 2224/48463 the connecting portion on the bonding area of the semiconductor or solid-state body being a ball bond
• CPC Class Codes: H01L 2924/16235 Connecting to a semiconductor or solid-state bodies, i.e. cap-to-chip

Abstract

A method is disclosed for fabricating a integrated device, such as a MEMS device. A first wafer is provided on an exposed surface with a layer of gold, gold alloy or gold compound. A second wafer is provided on its exposed surface with under-layer of gold, gold alloy or gold compound; and an over- of bismuth, bismuth alloy, a compound of bismuth, cadmium, cadmium alloy, a compound of cadmium compound, tin, tin alloy, or a compound of tin. The wafers are then brought into contact and bonded at their surfaces through the deposited layers.

Claims

1. A method of fabricating an integrated microdevice, comprising:

providing a first wafer having on a surface thereof a layer of material selected from the group consisting of;

gold, gold alloy or gold compound;

providing a second wafer with having on a surface thereof an under-layer of material selected from the group consisting of gold, gold alloy or gold compound; and

a solder over-layer selected from the group consisting of bismuth, bismuth alloy, a compound of bismuth, cadmium, cadmium alloy, a compound of cadmium, tin, tin alloy, and a compound of tin; and

bonding said wafers together at said surfaces thereof.

2. The method as claimed in claim 1, wherein said first wafer is a base wafer and said second wafer is a cap wafer.

3. The method as claimed in claim 2, wherein said over-layer is selected from the group consisting of:

bismuth, bismuth alloy, a compound of bismuth.

4. The method as claimed in claim 2, wherein said over-layer is selected from the group consisting of:

tin-silver-copper, tin-copper, tin-silver, tin-bismuth, tin-gold, tin.

5. The method as claimed in claim 2, wherein said over-layer is deposited by electroless plating.

6. The method as claimed in claim 2, wherein said over-layer is deposited by a technique selected from the group consisting of:

auto-catalytic plating, electrolytic plating, electroplating, evaporation, electron beam deposition, a lift-off technique, sputtering, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and metallorganic chemical vapor deposition (MOCVD).

7. The method as claimed in claim 5, wherein said under-layer is deposited by immersion plating.

8. The method as claimed in claim 6, wherein said under-layer is deposited by a technique selected from the group consisting of:

auto-catalytic plating, electrolytic plating, electroplating method, evaporation, electron beam deposition, a lift-off technique, sputtering, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and metallorganic chemical vapor deposition (MOCVD).

9. The method as claimed in claim 1, wherein a barrier layer is deposited under said layer provided on said first wafer.

10. The method as claimed in claim 9, wherein said barrier layer is made of a material selected from the group consisting of:

titanium, Ti, tantalum, Ta, platinum, Pt, rhodium, Rh, palladium, Pd, ruthenium, Ru, titanium tungsten, TiW, titanium tungsten nitride TiW(N), titanium nitride, TiN, titanium carbide, TiC, titanium boride, TiB, tungsten nitride, WN, tungsten carbide, WC, tungsten boride, WB, tantalum nitride, TaN, tantalum carbide, TaC, tantalum boride, TaB, halfmium nitride, HfN, halfmium carbide, HfC, halfmium boride, HfB, or tungsten silicon nitride, WsiN.

11. The method as claimed in claim 9, wherein a spacer layer is deposited under said barrier layer.

12. The method as claimed in claim 11, wherein said spacer layer is made of a material selected from the group consisting of:

gold, Au, nickel, Ni, nickel-phosphorus, Ni(P), nickel-palladium, Ni(Pd), chromium, Cr, copper, Cu, bismuth, Bi, cadmium, Cd or palladium-cobalt, PdCo.

13. The method as claimed in claim 12, wherein said spacer layer is deposited by electroless plating.

14. The method as claimed in claim 12, wherein said spacer layer is deposited by a technique selected from the group consisting of:

auto-catalytic plating, electrolytic plating, electroplating method, evaporation, electron beam deposition, a lift-off technique, sputtering, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD) or metallorganic chemical vapor deposition (MOCVD).

15. The method as claimed in claim 1, wherein if said over-layer is bismuth, a bismuth alloy or a bismuth compound, said second wafer is heated to a maximum temperature above 271°

C. and not more than 450°

C. prior to bonding said wafers;

if said over-layer is cadmium, a cadmium alloy or a cadmium compound, said second wafer is heated to a maximum temperature above 321°

C. and not more than 450°

C. prior to bonding said wafers; and

if said over-layer is tin, a tin alloy or a tin compound, said second wafer is heated to a maximum temperature above 321°

C. and not more than 450°

C. prior to bonding said wafers.

16. The method as claim 15, wherein said maximum temperature is sufficiently high to permit the material of said over-layer to interact with the under-layer of gold, gold alloy or gold compound and form a liquid solution.

17. The method as claimed in claim 16, wherein the amount of material in said over-layer and the amount of gold, gold alloy or gold compound in said the under-layer are such that the liquid solution remains liquid without precipitation of solids at said maximum temperature even if the complete consumption of said under-layer of gold, gold alloy or gold compound occurs due to interaction with said over-layer.

18. The method as claimed in claim 17, wherein the amount of material in the over-layer and the amount of gold, gold alloy or gold compound in the-under-layer are such that the liquid solution at said maximum temperature can dissolve an additional amount of extra gold, gold alloy or gold compound equivalent to at least 10% of the amount of gold, gold alloy or gold compound already dissolved in the liquid solution at said maximum temperature.

19. The method as claimed in claim 18, wherein the amount of extra gold, gold alloy or gold compound that can be dissolved at said maximum temperature is determined from the combined system consisting of the unsaturated liquid solution of the second wafer and the gold, gold alloy or gold compound layer of the first wafer.

20. The method as claimed in claim 19, wherein the temperature of the first wafer is higher than the temperature of the second wafer prior to said bonding step.

21. The method as claimed in claim 20, wherein the higher temperature of the first wafer is maintained prior to contact with said second wafer for a sufficient duration to allow adequate degassing of critical components of said first wafer.

22. The method as claimed in claim 21, wherein said adequate outgassing is determined by a substantially reduced generation of water vapor and hydrogen at said higher temperature.

23. The method as claimed in claim 21, wherein components of said base wafer are located in a region forming a cavity after the bonding of first wafer and said second wafer.

24. The method as claimed in claim 23, wherein prior to bonding of said first and second wafers, said region is exposed, at the moment the first wafer contacts the second wafer, to an environment selected from the group consisting of:

a vacuum, a controlled ambient pressure of;

an inert gas;

an insulating gas, a forming gas;

an oxide reduction gas; and

a non-inert gas selected to perform a specific function in the cavity to be sealed as to obtain this environment inside said cavity.

25. The method as claimed in claim 24, wherein said inert gas is selected from the group consisting of:

nitrogen, helium, neon, argon, krypton, xenon or radon.

26. The method as claimed in claim 24, wherein said insulating gas is selected from the group consisting of:

sulfur hexafluoride (SF₆) and freons.

27. The method as claimed in claim 24, wherein said forming gas consists of hydrogen.

28. The method as claimed in claim 24, wherein the temperature of said first wafer is higher than the temperature of said second wafer at the moment of contact to ensure adequate degassing of the components located in the formed cavity.

29. The method as claimed in claim 28, wherein at the moment of contact between the first wafer and second base wafer, the liquid solution of the surface of the cap wafer is permitted to fill microscopic defects, microscopic cracks, microscopic dimples, or microscopic depressions induced by the gold surface roughness of the base wafer.

30. The method as claimed in claim 29, wherein during said bonding step, intimate contact between the liquid solution of the first wafer and the higher temperature gold, gold alloy or gold compound layer of the base wafer causes at least partial dissolution of the gold, gold alloy or gold compound layer of the first wafer into the liquid solution at the interface, thus increasing the proportion of gold in the liquid solution while reducing the amount of gold, gold alloy or gold compound of the first wafer.

31. The method as claimed in claim 30, wherein the temperature of the first and second wafers is maintained while they are in contact with each other for a certain period of time so as to allow even more dissolution of the gold, gold alloy or gold compound layer of the base wafer into the liquid solution filling the micro-defects of the interface between the first wafer and second wafer.

32. The method as claimed in claim 31, wherein the temperature of the contacted first and second wafers is maintained for a sufficient duration to permit additional dissolution of the gold, gold alloy or gold compound layer of the first wafer into the liquid solution filling the micro-defects of the interface between the two wafers.

33. The method as claimed in claim 32, wherein the dissolution of the gold, gold alloy or gold compound layer of the first wafer causes the complete consumption of the under-layer of gold, gold alloy or gold compound in the second wafer while maintaining the gold-enriched solution in a liquid state, the initial amount of gold, gold alloy or gold compound underlying the over-layer of the cap wafer, the initial amount of gold, gold alloy or gold compound of the base wafer and the temperature of the contacted first wafer and second wafer defining the actual composition of the liquid solution present at the vanishing interface and micro-defects.

34. The method as claimed in claim 33, wherein the temperature of the contacted first wafer and second wafer is such that, for the actual composition of the liquid solution defining the actual liquidus temperature of the gold-bismuth gold-cadmium or gold-tin phase diagram, it approaches the liquidus temperature below which the first gold crystals or gold-bismuth intermetallic, gold-cadmium or gold-tin phase crystals will begin to precipitate.

35. The method as claimed in claim 34, wherein during the bonding step the temperature of the first wafer and second wafer is reduced after contact to cause the crystallization of crystals having a richer content of the material of said overlayer.

36. The method as claimed in claim 35, wherein said material of said overlayer is bismuth, bismuth alloy or bismuth compound and temperature is reduced to the lowest temperature liquidus of the gold-bismuth phase diagram, namely 241°

C., where liquid phase has completely disappeared with the formation of crystals of 13.2% Au/86.8% Bi composition.

37. The method as claimed in claim 35, wherein said material of said overlayer is cadmium, cadmium alloy or cadmium compound and temperature is reduced to the lowest temperature liquidus of the gold-cadmium phase diagram, namely 309°

C., where liquid phase has completely disappeared with the formation of crystals of 6% Au/94% Cd composition.

38. The method as claimed in claim 35, wherein said material of said overlayer is tin, tin alloy or tin compound and temperature is reduced to the lowest temperature liquidus of the gold-tin phase diagram, namely 282°

C. (for 80% Au/20% Sn) or 217°

C. (for 7% Au/93% Sn), where liquid phase has completely disappeared with the formation of crystals of 80% Au/20% Sn or of 7% Au/93% Sn composition.

39. The method as claimed in claim 35, wherein the temperature of the bonded wafers is reduced to room temperature and the cap wafer is micromachined without damaging the underlying base wafer as to remove the portion of the cap wafer outside a sealed cavity within said wafers and to expose a region of the first wafer outside the cavity without damaging a seal thereof.

40. The method as claimed in claim 39, wherein the micro-machining involves the use of one or more processes selected from the group consisting of:

grinding, polishing, a wet etch in TMAH, KOH, EDP, a plasma etch, a reactive ion etch, RIE, a deep reactive ion etch, DRIE, a mechanical blasting technique using silicon carbide, SiC other micro-beeds to remove the portion of the cap wafer outside the sealed cavity.

41. The method as claimed in claim 40, wherein the obtained micro-machined bonded wafers result in a wafer-level packaged wafer.

42. The method as claimed in claim 41, wherein the wafer-level packaged wafer incorporates one or more components selected from the group consisting of:

a silicon semiconductor device, a germanium semiconductor device, a silicon-germanium semiconductor device, a II-V compound semiconductor, optical device, a II-VI compound semiconductor, a silica photonic device, a plastic photonic device, a CMOS, DMOS, BCDMOS, Bipolar, BiCMOS or other high-voltage driver or device, a micro-electro-mechanical system, a sensor, an actuator, an intelligent microsystem, a biochip, a laboratory on a chip, LOAC.

43. The method as claimed in claim 42, wherein the wafer-level package allows at least one bonding pad located outside the sealed cavity to electrically thermally connect at least one component located the inside the sealed cavity.

44. The method as claimed in claim 42, wherein the wafer-level package the back side of the base wafer is processed by a technique selected from the group consisting of:

polishing and grinding to reduce the overall thickness thereof.

45. The method as claimed in claim 44, wherein the wafer-level package is diced by standard or slightly modified standard dicing techniques.

46. The method as claimed in claim 45, wherein the individual dies are packaged in standard molded plastic packages and/or other standard packages.

47. A precursor assembly for making an integrated microdevice comprising:

a base wafer comprising;

a zone to be sealed;

a bonding pad located outside said zone;

a component located inside said zone;

a metal interconnect located at the perimeter of said zone;

an anti-oxidation metal layer selectively deposited said metal barrier layer; and

a cap wafer comprising;

one or more of metal-based interconnects located at said zone;

a metal layer selectively deposited over said metal-based interconnect;

a gold layer selectively deposited over said metal-based interconnect;

a solder layer selectively deposited over the gold layer;

an alignment structure; and

a recessed structure allowing the removal of the portion of the cap wafer outside said zone.

48. The integrated device as claimed in claim 47, wherein said solder layer is selected from the group consisting of bismuth, bismuth alloy, a compound of bismuth, cadmium, cadmium alloy, a compound of cadmium compound, tin, tin alloy, or a compound of tin.

49. The integrated device as claimed in claim 47, further comprising a barrier layer selectively deposited over said metal interconnect.

50. The integrated device as claimed in claim 47, wherein said barrier layer is made of nickel.

51. The integrated device as claimed in claim 47, wherein said anti-oxidation layer is gold.

52. A method of fabricating an integrated microdevice, comprising the steps of:

fabricating a base wafer including a bonding pad located outside a zone to be sealed, a component inside said zone, a metal interconnect located at the perimeter of said zone, an anti-oxidation layer selectively deposited using immersion plating through a temporary mask, and an alignment structure;

fabricating a cap wafer including a metal-based interconnect located at the perimeter of said zone, a gold layer selectively deposited over said metal-based interconnect using electrolytic plating through a temporary mask, a solder layer selectively deposited over the gold layer using electrolytic plating, an alignment structure and a recessed structure allowing the removal of the portion of this second wafer outside said zone; and

bonding said cap wafer and said base wafer together using said solder layer.

53. The method as claimed in claim 52, wherein said solder layer is selected from the group consisting of bismuth, bismuth alloy, a compound of bismuth.

54. The method as claimed in claim 52, wherein said solder layer is selected from the group consisting of:

cadmium, cadmium alloy, a compound of cadmium compound, tin, tin alloy, or a compound of tin.

55. The method as claimed in claim 54, wherein a barrier layer is selectively deposited over said metal interconnect on said cap wafer using electroless plating through a temporary mask and said gold layer is deposited over said barrier layer.

56. The method as claimed in claim 55, wherein said gold layer is selectively deposited over said nickel barrier layer using immersion plating through a temporary mask.

57. The method as claimed in claim 52, wherein said bonding step occurs at a temperature of less than 400°

C. and under vacuum or under a specific pressure of dry nitrogen, dry forming gas or dry sulfur hexafluoride ambient.