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EP-4379396-B1 - DESIGN OPTIMISATION OF ACCELEROMETER SUPPORTS

EP4379396B1EP 4379396 B1EP4379396 B1EP 4379396B1EP-4379396-B1

Inventors

  • FELL, CHRISTOPHER PAUL
  • BAXTER, JASON
  • TOWNSEND, KEVIN
  • MILWARD, Thomas Oliver

Dates

Publication Date
20260506
Application Date
20221201

Claims (14)

  1. A sensing structure (100) for a capacitive accelerometer, the sensing structure (100) comprising: a resonator structure (101, 107a, 107b) comprising a proof mass (101) and one or more flexible supports (107a, 107b) for mounting the resonator structure to a substrate, the proof mass (101) being moveable along a sensing axis (102) in response to an applied acceleration; and first and second fixed capacitive electrodes (103a, 103b) arranged either side of the proof mass (101) along the sensing axis (102); wherein a gap (d small ) is defined along the sensing axis (102) between each of the first and second fixed capacitive electrodes (103a, 103b) and the proof mass (101) under zero applied acceleration; and wherein the one or more flexible supports (107a, 107b) has a width (d width ) along the sensing axis (102) that is substantially equal to the size of the gap (d small ) defined between at least one of the first and second fixed capacitive electrodes (103a, 103b) and the proof mass (101) under zero applied acceleration; characterised in that : a second gap (d 2 ) is defined along the sensing axis (102) between at least one of the one or more flexible supports (107a, 107b) and at least one of the fixed capacitive electrodes (103a, 103b), said second gap (d 2 ) having the same size as the gap (d small ) defined between at least one of the first and second fixed capacitive electrodes (103a, 103b) and the proof mass (101) under zero applied acceleration.
  2. The sensing structure (100) of claim 1, further comprising an external frame (105) surrounding the resonator structure (101, 107a, 107b) and the fixed electrodes (103a, 103b), wherein a third gap (d 1 ) is defined along the sensing axis (102) between the one or more flexible supports (107a, 107b) and the external frame (105), said third gap (d 1 ) having the same size as the gap (d small ) defined between at least one of the first and second fixed capacitive electrodes (103a, 103b) and the proof mass (101) under zero applied acceleration.
  3. The sensing structure of claim 1 or 2, wherein the proof mass (101) comprises a plurality of moveable capacitive electrode fingers (113, 115) extending perpendicular to the sensing axis (102) and spaced apart in the direction of the sensing axis (102).
  4. The sensing structure of claim 3, wherein the first and/or second fixed capacitive electrode (103a, 103b) comprises a plurality of fixed capacitive electrode fingers (117, 119) extending substantially perpendicular to the sensing axis (102) and spaced apart in the direction of the sensing axis (102).
  5. The sensing structure (100) of claim 4, wherein the fixed capacitive electrode fingers (117, 119) are arranged to interdigitate with the moveable capacitive electrode fingers (113, 115) of the proof mass (101).
  6. The sensing structure (100) of claim 5, wherein a first set of fixed capacitive electrode fingers (117) is arranged to interdigitate with the moveable electrode fingers (113, 115) of the proof mass (101) with a first offset in one direction from a median line therebetween, and a second set of fixed capacitive electrode fingers (119) is arranged to interdigitate with the moveable electrode fingers (113, 115) of the proof mass (101) with a second offset in the opposite direction from a median line therebetween.
  7. The sensing structure (100) of any preceding claim, wherein the resonator structure (101, 107a, 107b) forms an outer frame surrounding the first and second fixed capacitive electrodes (103a, 103b).
  8. The sensing structure (100) of claim 7, wherein the proof mass comprises a first rigid section (101a) and second rigid section (101b), connected by a rigid beam (104) extending between the first rigid section (101a) and the second rigid section (101b) perpendicular to the sensing axis (102).
  9. The sensing structure of claim 8, wherein the first fixed electrode (103a) is positioned above the rigid beam (104) along the sensing axis (102) and the second fixed electrode (103b) is positioned below the rigid beam (104) along the sensing axis (102).
  10. The sensing structure (100) of any preceding claim, wherein the resonator structure (101, 107a, 107b) comprises a pair of flexible supports (107a, 107b) located at opposite sides of the proof mass (101), each flexible support (107a, 107b) being arranged to mount the resonator structure to the substrate.
  11. The sensing structure (100) of any preceding claim, wherein the one or more flexible supports (107a, 107b) extends perpendicular to the sensing axis (102) inwardly from the proof mass (101) to an anchor (109a, 109b) for mounting to the substrate, the anchor (109a, 109b) having a position along the sensing axis (102) that is centred with respect to the proof mass (101).
  12. The sensing structure (100) of claim 11, wherein the one or more flexible supports (107a, 107b) extends linearly between the proof mass (101) and the anchor (109a, 109b), and/or wherein the one or more flexible supports (107a, 107b) has a substantially constant width along the sensing axis (102).
  13. The sensing structure (100) of any preceding claim, wherein the proof mass (101) is substantially planar, and wherein the first and second fixed capacitive electrodes (103a, 103b) are in the same plane as the proof mass (101).
  14. The sensing structure (100) of any preceding claim, wherein the proof mass (101), the flexible supports (107a, 107b) and first and second fixed capacitive electrodes (103a, 103b) are formed from a common material layer.

Description

Technical Field The present disclosure relates to a sensing structure for a capacitive accelerometer with high sensitivity. Background Accelerometers are electromechanical devices that are widely used to measure acceleration forces due to motion and/or vibration. Capacitive accelerometers are typically implemented as micro-electromechanical systems (MEMS) and may be manufactured from a semiconductor material such as silicon. A typical MEMS for a capacitive accelerometer includes a sensing structure, comprising a resonator structure in the form of a proof mass moveably mounted to one or more flexible spring supports, with a set of moveable electrode fingers extending from the proof mass. The electrode fingers of the sensing structure are interdigitated with one or more sets of fixed capacitive electrode fingers so as to form a differential capacitor. The sensing structure is connected, via a plurality of electrodes, to suitable drive and pickoff electronics to drive and sense the in-plane motion of the proof mass relative to the fixed capacitive electrode fingers. Such accelerometers may be operated in an "open loop" configuration, in which the electronics are arranged to drive the fixed capacitive electrode fingers with a suitable waveform, such as a sine or square wave signal, such that when the proof mass moves under an applied acceleration a pickoff voltage signal appears on the output. Alternatively, accelerometers may be operated in a "closed loop" configuration, in which the proof mass is maintained in a fixed position at all times by applying electrostatic forces to the fixed capacitive electrode fingers to achieve force balancing. The force required to maintain the proof mass in the fixed position while under acceleration can then be used to determine the acceleration applied to the accelerometer. In order to provide an accelerometer with high sensitivity, it is desirable for the flexible supports of the resonator structure to have low stiffness, which allows for greater displacement of the proof mass in response to an applied acceleration. In closed loop operation, high voltages are required to control the motion of the proof mass during high-g operation. However, these voltages can cause the proof mass to come into contact with the fixed capacitive electrode fingers as a result of the electrostatic attraction between the two components overcoming the mechanical restoring force of the flexible supports. If this occurs, the proof mass collides with the fixed electrodes, and will remain fixed at the electrode surface. To prevent this, the minimum stiffness of the flexible supports is typically limited, effectively limiting the maximum sensitivity of the accelerometer that may be achieved. The magnitude of the mechanical restoring force is sensitive to variations in the stiffness of the flexible supports and the magnitude of the electrostatic attraction is sensitive to the width of the gaps between the fixed and movable capacitive electrodes. Both of these parameters may change due to variations in the fabrication process. This results in significant variations in the total stiffness even for devices where the mechanical restoring force is greater than the electrostatic attraction. This introduces a variation in the accelerometer performance that is undesirable and which also contributes to yield loss during device manufacture. It is an object of the present disclosure to address one or more of the disadvantages outlined above, in order to provide a capacitive accelerometer with high sensitivity. Majid Taghavi et al: "A Closed-Loop MOEMS Accelerometer", Arxiv.Org, Cornell University Library, 201 Olin Library Cornell University Ithaca, NY 14853, 7 February 2022, XP091158138 discloses a closed-loop micro-opto-electro-mechanical system (MOEMS) accelerometer based on the Fabry-Perot (FP) interferometer. US 2017/010300 A1 discloses a physical quantity sensor including a movable electrode side fixed section, a first fixed electrode side fixed section which has a first fixed electrode section and a second fixed electrode side fixed section which has a second fixed electrode section, a movable mass section which has a first movable electrode section that has a portion facing the first fixed electrode section and a second movable electrode section that has a portion facing the second fixed electrode section and which is formed in a shape that encloses the movable electrode side fixed section, the first fixed electrode side fixed section, and the second fixed electrode side fixed section in planar view, and an elastic section which connects the movable electrode side fixed section and the movable mass section. Summary From a first aspect of the disclosure, there is provided a sensing structure for a capacitive accelerometer according to claim 1. As will be described further below, by employing one or more flexible supports having a width along the sensing axis that is substantially equal to the size of a gap de