RESONANT ACCELEROMETER WITH LOW SENSITIVITY TO PACKAGE STRESS

• US 20090255339A1
• Filed: 04/14/2008
• Published: 10/15/2009
• Est. Priority Date: 04/14/2008
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Lin, Yizhen, McNeil, Andrew C.
• Granted Patent: US 8,468,887 B2
• Time in Patent Office: Days
• US Class Current: 73/514.150
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 2203/0307 Anchors
• CPC Class Codes: B81B 3/0072 For controlling internal stress or strain in moving or flexible elements, e.g. stress compensating layers
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 2015/084 the mass being suspended at more than one of its sides, e.g. membrane-type suspension, so as to permit multi-axis movement of the mass
• CPC Class Codes: Y10T 29/49009 Dynamoelectric machine

Abstract

A resonant accelerometer (24) includes a single anchor (28) fixed to a substrate (32). A proof mass (34) is positioned above a surface (30) of the substrate (32) and is positioned symmetrically about the anchor (28). The proof mass (34) has a central opening (38). Each of a number of suspension beams (42, 44, 46, 48) resides in the central opening (38) and has one end (50) affixed to the anchor (28) and another end (52) attached to an inner peripheral wall (40) of the proof mass (34). A resonant frequency of the beams (42, 44) in a direction (64) aligned with a common axis (58) of the beams (42, 44) changes according to acceleration in the direction (64). A resonant frequency of the beams (46, 48) in a direction (66) aligned with a common axis (62) of the beams (46, 48) changes according to acceleration in the direction (66).

Claims

1. A resonant accelerometer comprising:

a substrate;

an anchor formed on a surface of said substrate;

a proof mass positioned in parallel spaced apart relationship above said surface of said substrate, said proof mass having a central opening defined by an inner peripheral wall, said central opening extending through said proof mass; and

suspension beams residing in said central opening, each of said suspension beams having one end affixed to said anchor and having another end attached to said proof mass at said inner peripheral wall.

2. A resonant accelerometer as claimed in claim 1 wherein said resonant accelerometer comprises a single one of said anchor.

3. A resonant accelerometer as claimed in claim 1 wherein said proof mass is positioned generally symmetrically about said anchor.

4. A resonant accelerometer as claimed in claim 1 wherein said proof mass is a rectangular-shaped structure and said opening is correspondingly rectangular-shaped.

5. A resonant accelerometer as claimed in claim 1 wherein said proof mass includes a plurality of vent holes extending through said proof mass.

6. A resonant accelerometer as claimed in claim 1 wherein said suspension beams include first and second beams affixed at first opposing sides of said anchor such that respective longitudinal axes of said first and second beams form a common axis through said proof mass.

7. A resonant accelerometer as claimed in claim 6 wherein said suspension beams include third and fourth beams affixed at second opposing sides of said anchor such that respective second longitudinal axes of said third and fourth beams form a second common axis through said proof mass, said second common axis being perpendicular to said common axis.

8. A resonant accelerometer as claimed in claim 1 wherein each of said suspension beams is substantially straight.

9. A resonant accelerometer as claimed in claim 1 wherein said suspension beams comprise:

a first pair of beams exhibiting a first dimensional design, said first pair of beams having respective first longitudinal axes that form a first common axis through said proof mass; and

a second pair of beams exhibiting a second dimensional design that differs from said first design, said second pair of beams being arranged perpendicular to said first pair of beams, and said second pair of beams having respective second longitudinal axes that form a second common axis through said proof mass.

10. A resonant accelerometer as claimed in claim 1 wherein said suspension beams comprise:

a first pair of beams, said first pair of beams having respective first longitudinal axes that form a first common axis through said proof mass, wherein a resonant frequency of each of said first pair of beams in a first direction aligned with said first common axis changes in accordance with acceleration in said first direction; and

a second pair of beams arranged perpendicular to said first pair of beams, said second pair of beams having respective second longitudinal axes that form a second common axis through said proof mass, said second common axis being perpendicular to said first common axis, wherein said resonant frequency of each of said second pair of beams in a second direction aligned with said second common axis changes in accordance with said acceleration in said second direction.

11. A resonant accelerometer as claimed in claim 1 wherein said resonant accelerometer is a microelectromechanical systems (MEMS) sensor.

12. A method of fabricating a microelectromechanical (MEMS) resonant accelerometer comprising:

providing a substrate;

forming on said substrate a proof mass having a central opening extending through said proof mass, an anchor located at a center of said central opening so that said proof mass is positioned generally symmetrically about said anchor, and suspension beams residing in said central opening, each of said suspension beams having one end affixed to said anchor and having another end attached to said proof mass at an inner peripheral wall of said opening; and

performing a timed etch process to release said suspension beams and said proof mass from said substrate.

13. A method as claimed in claim 12 wherein said forming operation comprises forming a plurality of vent holes extending through said proof mass.

14. A method as claimed in claim 13 wherein said vent holes are positioned in said proof mass such that an etchant used in said timed etch process passes through said vent holes to remove a sacrificial material underlying said proof mass to release said proof mass and to leave remaining said sacrificial material underlying said anchor.

15. A method as claimed in claim 12 wherein said forming operation comprises:

forming a first pair of said suspension beams affixed at first opposing sides of said anchor, said first pair of suspension beams having respective first longitudinal axes that form a first common axis through said proof mass; and

forming a second pair of said suspension beams arranged perpendicular to said first pair of beams and affixed at second opposing sides of said anchor, said first pair of suspension beams having respective second longitudinal axes that form a second common axis through said proof mass, said second common axis being perpendicular to said first common axis.

16. A method as claimed in claim 12 wherein said forming operation comprises forming each of said suspension beams to be substantially straight.

20. A resonant accelerometer as claimed in claim 16 wherein said resonant accelerometer is a microelectromechanical systems (MEMS) sensor.

17. A resonant accelerometer comprising:

a substrate;

an anchor formed on a surface of said substrate;

a proof mass positioned in parallel spaced apart relationship above said surface of said substrate, said proof mass having a central opening extending through said proof mass, said proof mass being positioned generally symmetrically about said anchor;

suspension beams residing in said central opening, each of said suspension beams having one end affixed to said anchor and having another end attached to said proof mass at an inner peripheral wall of said central opening, said suspension beams including;

first and second beams having respective first longitudinal axes that form a first common axis through said proof mass; and

third and fourth beams having respective second longitudinal axes that form a second common axis through said proof mass, said second common axis being perpendicular to said first common axis.

18. A resonant accelerometer as claimed in claim 17 wherein a resonant frequency of each of said first and second beams in a first direction aligned with said first common axis changes in accordance with acceleration in said first direction, and said resonant frequency of said third and fourth beams in a second direction aligned with said second common axis changes in accordance with said acceleration in said second direction.

19. A resonant accelerometer as claimed in claim 17 wherein said proof mass includes a plurality of vent holes extending through said proof mass.