Stress-Isolated MEMS Device and Method Therefor

• US 20080290430A1
• Filed: 05/25/2007
• Published: 11/27/2008
• Est. Priority Date: 05/25/2007
• Status: Abandoned Application

• Current Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Koury, Daniel N. JR., Mahadevan, Dave S.
• Application Number:
  US11/753,851
• Publication Number:
  US 20080290430A1
• Time in Patent Office: Days
• US Class Current: 257/418
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 7/0048 between the MEMS die and the substrate

Abstract

A stress-isolated MEMS device (14) includes a platform (26) suspended over a substrate wafer (24). In one embodiment, the platform (26) is suspended by springs (38), but other suspension techniques may also be used. A transducer (28) is formed over the platform (26). The transducer (28) includes immovable portions (50) and movable portions (52). The transducer (28) and platform (26) are sealed within a cavity (62) formed within a cap support (30) between a cap wafer (32) and the substrate wafer (24). A leadframe (22) is affixed to the substrate wafer (24). The cap wafer (32) and other portions of the device (14) become embedded in a package material (20) so that a substantially solid boundary forms between the cap wafer (32) and the package material (20).

Claims

1. A MEMS device comprising:

a substrate wafer;

a sacrificial layer overlying a first portion of said substrate wafer, said sacrificial layer not overlying a second portion of said substrate wafer, and said sacrificial layer being formed of a different material than said substrate wafer;

a platform movably suspended over said second portion of said substrate wafer; and

a transducer having an immovable portion positioned over and rigidly affixed to said platform and having a movable portion movably suspended over said platform.

2. A MEMS device as claimed in claim 1 additionally comprising a cap support surrounding said transducer and said platform.

3. A MEMS device as claimed in claim 1 additionally comprising a cap wafer overlying said transducer and bonded to said substrate wafer so that said transducer and said platform are enclosed together within a cavity formed between said substrate wafer and said cap wafer.

4. A MEMS device as claimed in claim 3 wherein said cap wafer is embedded within a package material so as to provide a substantially solid boundary between said cap wafer and said package material.

5. A MEMS device as claimed in claim 3 wherein said transducer and said platform are hermetically sealed together within a cavity formed inside said cap support and between said substrate wafer and said cap wafer.

6. A MEMS device as claimed in claim 1 wherein said immovable portion of said transducer overlies a first footprint area, said platform overlies a second footprint area greater than or equal to said first footprint area, and said substrate wafer overlies a third footprint area greater than said second footprint area.

7. A MEMS device as claimed in claim 1 additionally comprising a leadframe rigidly affixed to said substrate wafer.

8. A MEMS device as claimed in claim 1 wherein:

said MEMS device additionally comprises a perimeter wall anchored to said substrate wafer and surrounding said platform; and

said platform is suspended on springs within said perimeter wall.

9. A MEMS device as claimed in claim 1 wherein:

said MEMS device additionally comprises a platform anchor rigidly coupled to said substrate wafer; and

said platform has an edge at which said platform is hinged to said platform anchor and from which said platform extends over said substrate wafer.

10. A MEMS device as claimed in claim 9 additionally comprising a movable support affixed to said platform, positioned between said substrate wafer and said platform, and configured to accommodate movement of said substrate wafer relative to said platform.

11. A MEMS device as claimed in claim 1 wherein said platform and said movable portion of said transducer are formed of substantially identical materials.

12. A method of isolating a MEMS device from temperature-induced stress, said method comprising:

applying a sacrificial layer overlying a substrate wafer, said sacrificial layer being formed from a material different than said substrate wafer;

suspending a platform over said substrate wafer by removing a portion of said sacrificial layer so as to allow movement of said platform relative to said substrate wafer; and

forming a transducer on said platform, said transducer having an immovable portion fixedly attached over said platform and a moveable portion suspended over said platform so as to allow movement of said moveable portion relative to said immovable portion and said platform.

13. A method as claimed in claim 12 wherein said suspending activity and said forming activity form said platform and said movable portion of said transducer from substantially identical materials.

14. A method as claimed in claim 12 additionally comprising:

bonding a cap wafer to said substrate wafer so that said transducer and said platform are enclosed together within a cavity formed between said substrate wafer and said cap wafer.

15. A method as claimed in claim 14 additionally comprising embedding said cap wafer within a package material so as to provide a substantially solid boundary between said cap wafer and said package material.

16. A method as claimed in claim 12 wherein:

said forming activity configures said movable portion of said transducer to exhibit a first resonant frequency; and

said suspending activity configures said platform and transducer to collectively exhibit a second resonant frequency different from said first resonant frequency.

17. An apparatus about which physical effects are transduced with electrical signals comprising:

a base positioned to experience said physical effects;

a MEMS sensor device mounted on said base, said MEMS sensor device comprising;

a leadframe;

a substrate affixed to said leadframe;

a platform movably suspended over said substrate; and

a transducer formed over said platform, said transducer having an immovable portion fixed over said platform and a movable portion movably coupled to said platform and configured to respond to one of said physical effects and said electrical signals.

18. An apparatus as claimed in claim 17 additionally comprising a sacrificial layer overlying a first portion of said substrate wafer and underlying a layer from which said platform is formed, said sacrificial layer not overlying a second portion of said substrate wafer, said sacrificial layer not underlying said platform, and said sacrificial layer being formed of a different material than said substrate wafer.

19. An apparatus as claimed in claim 18 wherein said platform and said movable portion of said transducer are formed of substantially identical materials.

20. An apparatus as claimed in claim 19 additionally comprising:

a cap wafer overlying said MEMS device and bonded to said substrate so that said MEMS device and said platform are enclosed within a cavity formed between said substrate and said cap wafer; and

a package material within which said cap wafer is embedded to form a substantially solid boundary between said cap wafer and said package material.