Method of fabricating high yield wafer level packages integrating MMIC and MEMS components

• US 7,067,397 B1
• Filed: 06/23/2005
• Issued: 06/27/2006
• Est. Priority Date: 06/23/2005
• Status: Active Grant

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Chang-Chien, Patty P., Tomquist, Kelly J., Geiger, Craig B., Kong, Alvin M.
• Primary Examiner(s): Coleman, W. David
• Assistant Examiner(s): STARK, JARRETT J
• Application Number:
  US11/159,473
• Time in Patent Office: 369 Days
• Field of Search: 438/48, 438/459, 438/106, 438/121
• US Class Current: 438/459
• CPC Class Codes: B81B 7/0064 for protecting against electromagnetic or electrostatic interferences
• 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
• CPC Class Codes: H01L 2223/6683 for monolithic microwave integrated circuit [MMIC]
• CPC Class Codes: H01L 23/66 High-frequency adaptations
• CPC Class Codes: H01L 25/16 the devices being of types provided for in two or more different main groups of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. forming hybrid circuits
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Abstract

Monolithic microwave integrated circuit (MMIC) components and micro electromechanical systems (MEMS) components are integrated onto a single substrate at a wafer scale, by first performing MMIC and MEMS fabrication on a front face of a thick substrate wafer, bonding the substrate wafer to a cover wafer, thinning the back face of the substrate wafer and, finally, completing MMIC and MEMS fabrication on the back face of the thinned substrate wafer. The fabrication process is facilitated by use of a guard ring between the wafers to provide additional mechanical support to the substrate wafer and to protect the devices while the MMIC/MEMS fabrication is completed, and by a low temperature bonding process to join the substrate wafer and the cover wafer at multiple device cavity seal rings.

Claims

1. A method for fabricating wafer-level packages containing MMIC (monolithic microwave integrated circuit) components and MEMS (micro electromechanical systems) components, the method comprising the steps of:

providing a substrate wafer having a front face and a back face, and providing a cover wafer of comparable size to the substrate wafer;

fabricating multiple MMIC and MEMS components on the front face of the substrate wafer;

forming a first set of seal rings around selected device areas on the front face of the substrate wafer;

forming a second set of seal rings on a surface of the cover wafer, in positions corresponding the first set of seal rings;

bonding the first and second sets of seal rings to provide a double-wafer structure that includes multiple device cavities formed between the substrate wafer and the cover wafer;

thinning the substrate wafer by removing material from the back face; and

completing MMIC and MEMS fabrication on the back face of the substrate wafer;

whereby integration of MMIC and MEMS components provides enhanced circuit performance for each fabricated device.

2. A method as defined in claim 1, wherein the thinning step comprises lapping and polishing.

3. A method as defined in claim 1, and further comprising:

mounting the double-wafer structure by adhering the cover wafer to a dummy carrier prior to performing the thinning step; and

removing the double-wafer structure from the dummy carrier after completing MMIC and MEMS fabrication.

4. A method as defined in claim 1, and further comprising:

dicing the double-wafer structure into multiple device chips.

5. A method as defined in claim 1, and further comprising:

forming a continuous guard ring surrounding the multiple device cavities, wherein the step of forming a continuous guard ring comprises forming one component of the guard ring on the cover wafer and another component on the front face of the substrate wafer, and bonding the guard ring components together, simultaneously with the step of bonding the first and second sets of seal rings together;

wherein the continuous guard ring provides mechanical support to the substrate wafer during the thinning step and protects the MMIC and MEMS components during the step of completing MMIC and MEMS fabrication.

6. A method as defined in claim 5, wherein the continuous guard ring is circular in shape and extends circumferentially about the double-wafer structure.

7. A method as defined in claim 5, wherein the continuous guard ring is rectangular in shape and encompasses the multiple device cavities.

8. A method as defined in claim 5, wherein the continuous guard ring is irregular in shape and encompasses the multiple device cavities.

9. A method as defined in claim 1, wherein:

the step of forming a first set of seal rings comprises forming a first cavity spacer layer in selected regions of the front face of the substrate wafer and forming a gold layer over the first cavity spacer layer;

the step of for a second set of seal rings comprises a second cavity spacer layer in the selected regions of the face of the cover wafer forming a gold layer over the second cavity spacer layer;

one of the steps for forming first and second sets of seal rings Her comprises forming a layer of indium over the gold layer;

the bonding step comprises heat treating the first and second sets of seal rings to begin melting the indium layer, and forcing the first and second sets of seal rings together to intermix the indium layer with the gold layers and form a bonded structure that seals each of the multiple device cavities.

10. A method as defined in claim 9, wherein the bonding step further comprises;

applying and releasing compressive force multiple times to effect intermixing of indium and gold and to form an indium/gold alloy at each seal ring; and

subsequently cooling the double-wafer structure.

17. A method as defined in claim 5, wherein, the continuous guard ring fills the entire space between the substrate wafer and the cover wafer except for spaces occupied by the multiple device cavities.

18. A method as defined in claim 5, wherein the continuous guard ring is located near a periphery of the substrate wafer to provide physical strength in addition to the first and second sets of seal rings;

wherein the continuous guard ring surrounds the multiple MMIC and MEMS components and the first and second sets of seal rings;

wherein the continuous guard ring serves to prevent exposure of the first and second sets of seal rings to chemical substances used in wet processing during the step of completing MMIC and MEMS fabrication.

19. A method as defined in claim 1, wherein the substrate wafer is thinned during the thinning step after the substrate wafer and the cover wafer have been bonded together during the bonding step.

11. A method for bonding first and second wafers together using relatively low processing temperatures, the method comprising:

forming a first set of seal rings around selected areas on a face of the first wafer, wherein the step of forming the first set of seal rings comprises forming a first cavity spacer layer in the selected areas, and forming a gold layer over the first cavity spacer layer;

forming a second set of seal rings on a face of the second wafer, in positions corresponding the first set of seal rings, wherein the step of forming the second set of seal rings comprises forming a second cavity spacer layer in the selected areas, and forming a gold layer over the second cavity spacer layer;

forming an indium layer over one of the gold layers formed in the first and second sets of seal rings;

heat treating the first and second sets of seal rings to begin melting the indium layer; and

forcing the first and second sets of seal together to intermix the indium layer with the gold layers and to form a bonded structure that seals each of the areas defined by the sets of seal rings.

12. A method as defined in claim 11, wherein the forcing step comprises alternately applying and releasing a compressive force multiple times.

13. A method as defined in claim 12, and further comprising subsequently cooling the first and second sets of seal rings.

14. A method as defined in claim 11, wherein each of the first and second cavity spacer layers comprises multiple metal sublayers at least one of which acts as a barrier against intermixing of indium into the cavity spacer layers.

15. A method as defined in claim 14, wherein each of the first and second cavity spacer layers comprises sublayers of titanium, platinum, and gold.

16. A method as defined in claim 11, where the heat treating and bonding steps are performed at a selected temperature in the range of approximately 175°

C. to 240°

C.

20. A method as defined in claim 11, wherein each of the first and second cavity spacer layers comprises a titanium sublayer, a platinum sublayer, and a gold sublayer;

wherein the titanium and platinum sublayers serve to prevent intermixing interesting of the indium layer with the gold sublayer of the first and second cavity spacer layers.