VIBRATION DAMPING MOUNT

Abstract

A MEMS sensor package includes a MEMS sensor fixed to a vibration damping mount. The mount includes a silicon substrate defining an outer frame; a moveable support to which the MEMS sensor is fixed; and a vibration damping structure connected between the outer frame and the moveable support to damp movement of the support. The MEMS sensor and vibration damping mount are enclosed by a casing that is backfilled with gas.

Claims

1. A microelectromechanical systems (MEMS) sensor package comprising a MEMS sensor fixed to a vibration damping mount, the mount comprising a silicon substrate defining: an outer frame; a moveable support to which the MEMS sensor is fixed; and a vibration damping structure connected between the outer frame and the moveable support to damp movement of the support; wherein the MEMS sensor and vibration damping mount are enclosed by a casing that is backfilled with gas.

2. A MEMS sensor package according to claim 1, comprising a further vibration damping structure arranged to damp movement of the support out of the plane of the outer frame.

3. A MEMS sensor package according to claim 1, wherein the further vibration damping structure comprises a plurality of apertures extending through the moveable support in a direction out of its plane.

4. A MEMS sensor package according to claim 3, wherein the number and/or size of the apertures is chosen to provide critical damping.

5. A MEMS sensor package according to claim 1, wherein at least one of the outer frame and moveable support defined by the silicon substrate has a depth d, and the vibration damping structure comprises a support arrangement having a second depth that is less than the depth d.

6. A MEMS sensor package according to claim 5, wherein the support arrangement comprises one or more compliant legs extending between the outer frame and the moveable support to damp movement of the support in the plane of the outer frame and/or out of the plane of the outer frame.

7. A MEMS sensor package according to claim 6, wherein the compliant legs comprise a plurality of serpentine legs extending between the outer frame and the moveable support.

8. A MEMS sensor package according to claim 5, wherein the support arrangement provides a resonant frequency f.sub.z for movement of the support out of the plane of the outer frame.

9. A MEMS sensor package according to claim 8, wherein f.sub.z is about 1 kHz.

10. A MEMS sensor package according to claim 1, wherein the vibration damping structure comprises one or more sets of interdigitated fingers arranged to damp movement of the support in the plane of the outer frame.

11. A MEMS sensor package or a vibration damping mount according to claim 10, wherein the number or spacing of the one or more sets of interdigitated fingers is chosen to provide critical damping.

12. A MEMS sensor package according to claim 10, wherein the one or more sets of interdigitated fingers provide a resonant frequency f.sub.xy for movement of the support in the plane of the outer frame and the support arrangement is configured to provide a resonant frequency f.sub.z for movement of the support out of the plane of the outer frame that substantially matches f.sub.xy.

13. A vibration damping mount for a MEMS sensor, the mount comprising a silicon substrate of depth d, the substrate defining: an outer frame; a moveable support for supporting a MEMS sensor; and a vibration damping structure connected between the outer frame and the moveable support to damp movement of the support; wherein the vibration damping structure comprises a support arrangement having a second depth that is less than the depth d.

14. A vibration damping mount according to claim 13, wherein the silicon substrate is anodically bonded to an underlying glass substrate.

15. A vibration damping mount according to claim 13, wherein the support arrangement comprises one or more compliant legs extending between the outer frame and the moveable support to damp movement of the support in the plane of the outer frame and/or out of the plane of the outer frame.

16. A vibration damping mount according to claim 15, wherein the compliant legs comprise a plurality of serpentine legs extending between the outer frame and the moveable support.

17. A vibration damping mount according to claim 13, wherein the support arrangement provides a resonant frequency f.sub.z for movement of the support out of the plane of the outer frame.

18. A vibration damping mount according to claim 17, wherein f.sub.z is about 1 kHz.

19. A vibration damping mount according to claim 13, wherein the vibration damping structure comprises one or more sets of interdigitated fingers arranged to damp movement of the support in the plane of the outer frame.

20. A vibration damping mount according to claim 19, wherein the one or more sets of interdigitated fingers provide a resonant frequency f.sub.xy for movement of the support in the plane of the outer frame and the support arrangement is configured to provide a resonant frequency f.sub.z for movement of the support out of the plane of the outer frame that substantially matches f.sub.xy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] An example of the present disclosure is described hereinbelow with reference to the accompanying drawings in which:

[0034] FIG. 1 is a plan view of a MEMS sensor package comprising a vibration damping mount for a MEMS sensor in accordance with the present disclosure;

[0035] FIG. 2 is a side sectional view of the MEMS sensor package of FIG. 1; and

[0036] FIG. 3 is a close-up view of a compliant leg in the vibration damping structure of the vibration damping mount of FIG. 1.

DETAILED DESCRIPTION

[0037] FIG. 1 is a plan view of a MEMS sensor package comprising a vibration damping mount 2 for a MEMS sensor 4 in accordance with the present disclosure. In this figure, the MEMS sensor 4 is shown as being mounted on the vibration damping mount 2; However, it should be appreciated that the MEMS sensor 4 does not form part of the vibration damping mount 2 itself. The x- and y-axes that will be referred to below are shown in the upper left corner of FIG. 1.

[0038] The vibration damping mount 2 comprises a movable support 6 onto which the MEMS sensor 4 is mounted and an outer frame 8, which are both formed from the same silicon substrate e.g. by deep reactive-ion etching (DRIE) or other known semiconductor fabrication techniques. The outer frame 8 is mounted to a base substrate as will be described with reference to FIG. 2 further below. The vibration damping mount 2 and the MEMS sensor 4 are enclosed by a casing 10 to form a hermetic package that is backfilled with air or a different gas such as argon or neon. As the vibration damping mount 2 and the MEMS sensor 4 are sealed within the same package casing 10, the MEMS sensor 4 can be openi.e. the MEMS sensor 4 does not need to be sealed within its own package before mounting it on the moveable support 6, e.g. an inductive gyroscope. Otherwise, the MEMS sensor 4 may be sealed within its own hermetic package (e.g. a capacitive gyroscope in a vacuum or an accelerometer in a gas environment for squeeze film damping purposes), where this hermetic package is then fixed to the mount 2. The casing 10 of the overall MEMS sensor package therefore defines a sensor chip having its own internal vibration mounting.

[0039] The vibration damping mount 2 is arranged to isolate the MEMS sensor 4 from external vibrations in the x-, y-, and z-directions. In order to provide damping in the x- and y-directions, the vibration damping mount 2 comprises a first damping structure in the form of orthogonal sets of interdigitated fingers 12, 14 that provide squeeze film damping in the x- and y-directions. The outer frame 8 is provided with four arrays of fingers 12 that protrude from the frame 8 in the x- and y-directions towards the moveable support 6. As these fingers 12 protrude from the frame 8 which is fixed to the substrate (described with reference to FIG. 2), these fingers 12 are also fixed in position and do not move. However, the moveable support 6 is also provided with an array of fingers 14 that protrude from the moveable support 6 in the direction of the frame 8. The set of fingers 12 that protrude from the frame 8 and the set of fingers 14 that protrude from the moveable support 6 interdigitate with one another so as to provide a comb-like structure, preferably with an equal gap both sides to give as much squeeze film damping as possible. The set of fingers 14 that protrude from the moveable support 6 are able to move relative to the fixed fingers 12 when the vibration damping mount 2 undergoes acceleration, e.g. due to vibration. The interdigitated fingers 12, 14 are conveniently formed from the same silicon substrate as the outer frame 8 and moveable support 6.

[0040] The moveable support 6 is connected to the outer frame 8 by a support arrangement comprising a number of compliant legs 16, that forms part of the first vibration damping structure. These compliant legs 16 act like springs and allow the moveable support 6 to move in the x-, y- and z-directions when the vibration damping mount 2 undergoes an acceleration, e.g. vibration. These legs 16 may be serpentine or meandering in structure as described further below with reference to FIG. 3. However, when the vibration damping mount 2 undergoes an acceleration in the x- or y-direction, the interdigitated fingers 12, 14 provide squeeze film damping that reduces the magnitude of motion of the moveable support 6. The squeeze film damping effect arises due to gas (e.g. air, argon, neon, etc.) between the fingers 12 attached to the frame 8 and the fingers 14 connected to the moveable support 6 being compressed as the gap between adjacent fingers 12, 14 is reduced when the moveable support 6 moves.

[0041] The vibration damping mount 2 is also arranged to isolate the MEMS sensor 4 from vibrations in the z-direction using a second vibration damping structure. This is described in further detail with reference to FIG. 2, however it should be noted that the moveable support 6 comprises an array of apertures 18 that extend through the support 6 in the z-direction and contribute to the z-direction damping characteristics of the vibration damping mount 2 that are more clearly visible in the plan view of FIG. 1 than the side view of FIG. 2.

[0042] FIG. 2 is a side sectional view of the vibration damping mount 2 of FIG. 1. It can be seen in FIG. 2 that the silicon structure comprising the moveable support 6, the outer frame 8, the interdigitated fingers 12, 14, and the compliant legs 16 is anodically bonded to a glass substrate 20. A spacing layer 22 separates the frame 8 (and thus the moveable support 6 which is connected to the frame 8 via the compliant legs 16) from the glass substrate 20 thus providing a cavity 24 between the glass substrate 20 and the moveable support 6, the frame 8, and the compliant legs 16. This cavity 24 provides a bump stop that sets a limit on how far the moveable support 6 can move when the vibration damping mount 2 undergoes an acceleration in the z-direction, where the size of the bump stop can be set by choosing an appropriate size of the spacing layer 22, e.g. 10-20 microns. In addition, one or more bump stop protrusions may be located in the cavity 24. The x- and z-axes are shown in the upper left corner of FIG. 2.

[0043] The frame 8 is fixed to the sides of the hermetic package casing 10 such that vibrations applied to the casing 10 are absorbed by the vibration damping mount 2. The compliant legs 16 effectively decouple the moveable support 6 from the frame 8 and, by extension, the hermetic package casing 10. Thus unwanted vibrations applied to the MEMS sensor package have minimal impact on the moveable support 6 and hence the MEMS sensor 4 mounted on the moveable support 6.

[0044] As can be seen schematically in FIG. 2, the compliant legs 16 are thinned in the z-direction compared to the frame 8 and the support 6. While the interdigitated fingers 12, 14 are the same thickness d as the frame 8 and the moveable support 6 respectively, the compliant legs 16 are substantially thinner than the frame 8 and the moveable support 6. For example, the frame 8, the moveable support 6, and the interdigitated fingers 12, 14 may have a thickness of around 100 microns while the compliant legs 16 may have a thickness of only 10-20 microns. The thickness of the compliant legs 16 sets the resonant frequency f.sub.z of the support 6 in the z-direction in conjunction with the carried mass of the support 6 and the MEMS sensor 4. Ideally, the resonant frequency f.sub.z in the z-direction is the same as the resonant frequency f.sub.xy in the x- and y-directions. The ideal value of these resonant frequencies f.sub.xy and f.sub.z will depend on the requirements of the MEMS sensor 4, however a typical MEMS sensor such as a vibrating structure gyroscope or accelerometer may have an operational input vibration spectrum of 20 Hz to 2 kHz (g rms) with an operational bandwidth of 100 Hz, and typically 200 Hz. A suitable value for the resonant frequencies f.sub.xy and f.sub.z may therefore be, by way of example only, 1 kHz as this frequency may provide an acceptable compromise so as to provide sufficient stiffness without absorbing too much ambient vibration and giving fidelity of the low frequency vibration signals. Accordingly, low frequency vibrations, for example up to 200 Hz, will not be filtered out by the compliant legs 16 and thus will be passed to the MEMS sensor 4. This is important as it allows accelerations and vibrations within the bandwidth of the MEMS sensor 4 to be detected and recorded correctly.

[0045] Ideally, the damping ratios .sub.x, .sub.y and .sub.z in the x-, y-, and z-directions respectively are the same, i.e. the vibration damping mount 2 provides isotropic damping such that .sub.x=.sub.y=.sub.z.

[0046] The undamped resonant frequencies can be determined using the relationship

[00001]$f_0=\frac{1}{2.Math.}.Math.\sqrt{\frac{k}{m}}$

where f.sub.0 is the undamped resonant frequency, k is the stiffness of the damping structure, and m is the carried mass. The resonant frequencies are typically set during design using finite element modelling methods, known in the art per se, taking account of the carried mass and the elastic properties of the silicon. Thus, it is important to account for the mass of the MEMS sensor 4 when designing the vibration damping mount 2.

[0047] The geometry of the vibration damping mount 2 is preferably selected such that the moving parts (i.e. the moveable support 6 and the corresponding fingers 14) do not touchdown under typical ambient vibration levels. For example, if the vibration damping mount 2 is to be used in aerospace applications, the geometry of the vibration damping mount 2 may need to accommodate accelerations of around 8 g rms without the moveable support 6 touching the glass substrate 20 or the moveable fingers 14 touching the fixed fingers 12.

[0048] The MEMS sensor 4 is typically fixed to the moveable support 6 using, by way of example only, a thin epoxy layer (e.g. 5 microns thick) or using Si-Si fusion bonding. Alternatively, the MEMS sensor 4 may be fixed to the moveable support 6 using eutectic bonding (e.g. using a AgSn alloy), glass frit bonding (sometimes referred to as glass soldering) or any other suitable bonding method known in the art per se. Although not seen in FIG. 2, the support 6 also provides electrical interconnects for the MEMS sensor 4.

[0049] While the damping ratios in the x- and y-directions can be set by choosing an appropriate gap between adjacent interdigitated fingers 12, 14, it can be more difficult to control the damping ratio in the z-direction. The apertures 18 referred to previously with reference to FIG. 1 provide a mechanism for controlling the magnitude of the squeeze film damping in the z-direction and thus the damping ratio .sub.z in the z-direction. Ideally, the number, placement, size, and density of these apertures 18 are selected so as to provide critical damping in the z-direction. Similarly, the size of the gaps between adjacent fingers 12, 14 is also selected in order to provide critical damping in the x- and y-directions (i.e. .sub.x=.sub.y=1). While the apertures 18 shown in FIG. 1 are circular holes, it will be appreciated that other shapes may be used for the apertures 18, for example ellipsoidal or rectangular holes or slots may be used instead, or a variety of different shapes including irregular shapes may be used as appropriate.

[0050] FIG. 3 is a close up view of a compliant support leg 16 in the support arrangement of the vibration damping mount 2 of FIG. 1. As it can be seen from FIG. 3, the compliant support legs 16 are not simply a straight beam connecting the outer frame 8 to the moveable support 6, but instead have a serpentine form that allows them to act as a spring connecting the moveable support 6 to the frame 8. Over a certain distance 26, the support leg 16 folds back and forth on itself to form a coil-like structure. Such serpentine legs are described in further detail in WO 2013/050752, the contents of which are hereby incorporated by reference. This serpentine form allows the leg 16 to be extended or compressed like a spring and thus permits motion of the moveable support 6 relative to the frame 8 in the x-, y- and z-directions when the vibration damping mount 2 experiences an acceleration in these directions (e.g. due to vibration). This serpentine form also allows the legs 16 to isolate the mount 2 and hence the MEMS sensor 4 from mechanical stresses due to thermal expansion rate mismatches. The legs 16 may carry metal tracking to provide an electrical connection to the moveable support 6 and therefore to the MEMS sensor 4 mounted thereon. The stiffness of the support arrangement is set by the number of these compliant legs 16, and a typical vibration damping mount 2 may have ten to twelve of these legs 16, for example with three legs 16 per quarter of the mount 2. Each leg 16 may carry a conductive track to convey electrical signals as required and the design can be adjusted so as to accommodate the number of tracks required for a particular application.

[0051] Although a single MEMS sensor 4 is shown, it will be appreciated that more than one sensor may be fixed to the mount 6, e.g. a gyroscope and one or two accelerometers (such as a pair of x- and a y-direction accelerometers and a gyroscope arranged to measure angular rate in the x-y plane) or two accelerometers (e.g. to measure accelerations in the x- and y-directions). For example, the Gemini system manufactured by Silicon Sensing provides a dual-axis MEMS accelerometer that may be fixed to the mount 6. Where multiple sensors are fixed to the mount 6, each sensor may be sealed in its own hermetic package or may be open, and a mix of open and sealed sensors may be mounted within the same MEMS sensor package.

[0052] Thus it will be seen that a vibration damping mount for a MEMS sensor that provides critical damping in the x-, y-, and z-directions has been described herein. Although particular examples have been described in detail, it will be appreciated by those skilled in the art that many variations and modifications are possible using the principles of the disclosure set out herein.