Microelectromechanical device

Abstract

The present disclosure provides a microelectromechanical device comprising: a fixed stator and a movable rotor in a device plane; a fixed wall defining a wall plane adjacent to the device plane; and a motion limiter configured to prevent the rotor from being in direct physical contact with the fixed wall. The motion limiter comprises a damper extending from the rotor to the stator and a fixed stop structure protruding from the fixed wall towards the damper.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to microelectromechanical (MEMS) devices, and in particular to motion limiters that prevent undesired contact between movable device components and stationary device components. BACKGROUND

[0002] Microelectromechanical (MEMS) devices such as accelerometers and gyroscopes typically include a mass element suspended from a stationary anchor with a flexible suspension structure that allows the mass element to move relative to an adjacent stationary structure. The movable mass element can be referred to as a rotor, and the stationary device component in which the anchor resides can be referred to as a stator. The stationary structure adjacent to the rotor typically also includes a wall that forms an enclosure around the rotor and other active components of the MEMS device.

[0003] Direct physical contact between the rotor and the stationary structure is generally undesirable, as it can interfere with the operation of the device. Although the rotor and its suspension structure can be sized such that direct contact does not occur in normal operation, an exceptional external impact can still displace the rotor into direct contact with the stationary structure, causing structural damage, stiction, electrical shorts, or other malfunctions.

[0004] A motion limiter can be implemented in a MEMS device to reduce or prevent these detrimental consequences. The motion limiter can for example include a bump attached to the rotor and extending from the rotor toward the adjacent stationary structure. A gap between the motion limiter bump and the stationary structure can narrow in the direction of expected motion, such that the bump will be the first part to contact the stationary structure in the event of an external impact. Damage can be reduced, for example, by positioning the motion limiter bump as far away from the most sensitive areas of the rotor as possible.

[0005] However, the space available for a motion limiter is often limited by cost and design considerations. Furthermore, a common problem with rigidly fixing the motion limiter bump to the rotor is that the bump and the rotor always move together. If the impact between the bump and the stationary structure is severe, particles can be released from the stationary structure or the bump itself. These particles can move toward more sensitive areas of the device, limit the dynamic operating range, and cause shorts or other damage. SUMMARY

[0006] It is an object of the present disclosure to provide a device that overcomes the above-mentioned problems.

[0007] The object of the present disclosure is achieved by a device characterized by what is stated in the independent claim. Preferred embodiments of the present disclosure are disclosed in the dependent claims.

[0008] The present disclosure is based on the idea of establishing a motion limiter on a flexible spring extending from a rotor to a stator. The advantage of this arrangement is that impacts between the motion limiter and the adjacent fixed structure become soft. BRIEF DESCRIPTION OF DRAWINGS

[0009] In the following, the present disclosure will be described in more detail by means of preferred embodiments with reference to the attached drawings, in which:

[0010] Figures la to Id A motion limiter and its operating principle are illustrated.

[0011] Figures 2a to 2b A shock absorber is illustrated.

[0012] Figure 3 A shock absorber is also illustrated. DETAILED DESCRIPTION

[0013] A rotor in a MEMS device is typically formed in a device layer, e.g. by etching the device layer. The device layer can be a silicon wafer, for example. Alternatively, the device layer can be a silicon layer deposited on a substrate. When manufacturing the device, the rotor is partly released from the surrounding fixed structure. The rotor can be suspended from a fixed anchor point by a flexible suspension, for example. The suspension can be formed in the same etching process as the rotor, and the area where the anchor point of the fixed structure is located can be an area of the device layer adjacent to the rotor.

[0014] A stator adjacent to the rotor in a MEMS device can be any part of the device layer that (unlike the rotor) remains fixed to a given position relative to the surrounding fixed structure regardless of the motion the device experiences. The stator can serve as a fixed reference point in a measurement that tracks the motion of the rotor, e.g. in a capacitive measurement device where a set of electrodes is prepared on the stator and a set of adjacent counter electrodes on the rotor. A piezoelectric measurement device can alternatively be prepared on a flexible suspension extending from the stator to the rotor. However, the fixed stator for the purpose of building the motion limiter described in the present disclosure does not have to have the same structure as the structure used for measuring the motion of the rotor. Rather, two separate stator structures can be used for these two purposes.

[0015] The device layer defines a device plane, which is illustrated as the xy-plane in the present disclosure. The x-direction can be referred to as the lateral direction, and the y-direction can be referred to as the transverse direction. The device plane can also be referred to as the horizontal plane. In the present disclosure, the direction perpendicular to the device plane is illustrated with the z-axis, and it can be referred to as the vertical direction or the out-of-plane direction. In the present disclosure, the words "horizontal" and "vertical" refer only to the plane and the direction perpendicular to the plane. They do not imply how the device should be oriented relative to the Earth's gravitational field when manufactured or used. The same applies to terms related to "vertical", such as "above" and "below", or "up" and "down".

[0016] In some technical applications, the rotor can be designed to undergo linear out-of-plane motion, in which the entire rotor is moved out of the device plane. In other applications, the rotor can be designed to undergo rotational out-of-plane motion, in which the rotor is rotated about an axis that lies in the device plane. The present disclosure provides a motion limiter that is intended to limit any kind of out-of-plane motion.

[0017] The present disclosure describes a microelectromechanical device that includes a movable rotor and a fixed stator. The rotor lies in a horizontal device plane in its rest position. The rotor is located in proximity to the stator such that an edge of the rotor is separated from an edge of the stator by a rotor-stator gap.

[0018] The device further includes a fixed wall. The fixed wall defines a wall plane that is adjacent to the horizontal device plane and separated from the horizontal device plane in a vertical direction by a rotor-wall gap. The vertical direction is perpendicular to the horizontal device plane.

[0019] The microelectromechanical device further includes a motion limiter that is configured to prevent the rotor from directly physically contacting the fixed wall across the rotor-wall gap. The motion limiter includes a bumper that extends from the rotor across the rotor-stator gap to the stator. The bumper is flexible in the vertical direction. The motion limiter further includes a fixed stop structure that protrudes in the vertical direction from the fixed wall toward the bumper such that the stop structure is vertically aligned with the bumper in an impact region. The bumper is separated from the stop structure in the vertical direction by a stop gap.

[0020] The stop structure is dimensioned such that, when the movable rotor undergoes motion in the vertical direction toward the fixed wall, the bumper contacts the stop structure in the impact region before the rotor contacts the fixed wall across the rotor-wall gap.

[0021] In some applications, the movable rotor can be referred to as a mass element, a detection mass, or a Coriolis mass. The rotor is in its rest position when it is not undergoing motion relative to a fixed structure. The rotor can be moved away from its rest position by a force sensor and, for example, set into an oscillatory motion. In addition to (or instead of) such intentional actuation, the rotor can be moved away from its rest position by externally applied motion. The motion limiter described in the present disclosure can be configured to limit any kind of out-of-plane motion.

[0022] Figures la to Id The operation of the motion limiter in a microelectromechanical device is schematically illustrated. Figures la to Id The shapes and relative sizes of the elements and gaps shown in the figures are chosen only to illustrate the working principle of the motion limiter. Alternatively, many other shapes can be used, and the relative sizes of the elements and gaps can differ from those illustrated in the figures.

[0023] The device comprises a movable rotor 11 and a fixed stator 12. Figure la The device is illustrated when the rotor is in its rest position and the rotor is in the device plane. The stator 12 is a fixed structure that remains fixed in the device plane even when the rotor 11 undergoes motion. The device comprises a damper 13 that extends from the rotor 11 to the stator 12 in a motion limiter region in the device plane. The gap 181 between the edge of the rotor 11 and the edge of the stator 12 can be referred to as the rotor-stator gap.

[0024] Figure lb The device is illustrated when the rotor is in its rest position and the rotor is in the device plane. The stator 12 is a fixed structure that remains fixed in the device plane even when the rotor 11 undergoes motion. The device comprises a damper 13 that extends from the rotor 11 to the stator 12 in a motion limiter region in the device plane. The gap 181 between the edge of the rotor 11 and the edge of the stator 12 can be referred to as the rotor-stator gap. Figure la The A-A cross section is illustrated. The device comprises a motion limiter region 171 that can coextend with the damper 13 in the device plane when the rotor is in its rest position.

[0025] Figure la The A-A cross section is illustrated. The device comprises a motion limiter region 171 that can coextend with the damper 13 in the device plane when the rotor is in its rest position. Figure lb A fixed wall 14 and a rotor-wall gap 182 are also illustrated. A fixed stop structure 15 is attached to the fixed wall 14. The stop structure can have a top surface 151 and a bottom surface 152 that are attached to the fixed wall. The impact region 171 is the region in which the stop structure 15 and the damper 13 will come into contact with each other when the rotor is moved far enough upwards. In Figures la to Id The impact region 171 is the region in which the vertical protrusion of the bottom surface 152 of the stop structure 15 overlaps with the damper 13.

[0026] The wall plane corresponds in Figures la to Id to the plane in which the illustrated wall 14 lies. If the fixed wall has a more irregular shape, the wall plane can correspond to a horizontal plane that traverses the points at which the stop structure 15 is attached to the fixed wall.

[0027] The fixed wall 14 can for example be a surface on a support wafer that is adjacent to the device layer and provides mechanical support for the device layer on one or more edges (not shown) of the device. Alternatively, the fixed wall 14 can be a surface on a cap wafer that has been placed adjacent to the device layer, or an inner surface of any similar packaging structure. The fixed structure that is adjacent to the device layer typically forms an enclosure around the rotor.

[0028] The damper 13 is separated from the fixed stop structure 15 by a stop gap 183. In order to prevent direct contact between the rotor 11 and the fixed wall 14 that is vertically adjacent to it when the out-of-plane displacement of the rotor 11 approaches a given threshold, the motion limiter is designed so that the damper 13 comes into contact with the stop structure 15 across the stop gap 183 before the rotor 11 can come into contact with the fixed wall 14.

[0029] This is illustrated in Figure lcand 1d is shown in Figure lc and Figure Id The device is shown when the rotor 11 has moved away from its rest position. Figure Id The same cross section is shown in Figure lb The damper 13 is flexible in the vertical direction. Thus, at least some parts of the damper 13 fold when the rotor 11 moves in the out-of-plane direction. The stop gap 183 has been dimensioned such that, if the rotor 11 moves far enough in the out-of-plane direction, the damper 13 comes into contact with the stop structure 15 before the rotor 11 reaches the fixed wall 14. In other words, as Figure lc and 1d The illustrated, when the damper 13 comes into contact with the stop structure 15, there is a non-zero threshold gap 184 between the rotor 11 and the fixed wall 14.

[0030] Due to the placement of the damper 13 and its vertical flexibility, the impact between the damper 13 and the stop structure 15 will be softer than would occur if the rotor 11 (or a protuberance located on the rotor) hit the fixed wall 14.

[0031] The vertical impact velocity V1 of the damper 13 hitting the stop structure 15 is less than the velocity V2 of the rotor 11 moving towards the wall 14 at the moment of impact. The difference between V1 and V2 will depend on the vertical flexibility of the damper 13. If the rotor experiences a rotational out-of-plane motion (rather than a linear out-of-plane translation as shown in Figure lc and 1d The distance between the point where the damper is attached to the rotor and the edge of the rotor closest to the fixed wall will also affect the difference between V1 and V2.

[0032] In the figures of the present disclosure, the impact region is generally placed somewhere near the middle of the rotor-stator gap. However, the impact region can alternatively be closer to the rotor than to the stator, or closer to the stator than to the rotor. The threshold at which the impact occurs (the point at which the rotor moves far enough upwards so that the gap between the rotor and the wall is equal to the threshold gap 184) is determined by the geometry of the damper 13 in the xy-plane and its vertical flexibility and by the height of the stop structure 15.

[0033] Even after the damper 13 comes into contact with the stop structure 15 in Figure Id The vertical flexibility of the damper 13 can allow the rotor 11 to move further upwards even after the damper 13 comes into contact with the stop structure 15 in

[0034] The thickness of the damper 13 in the vertical z-direction can be less than the vertical thickness of the rotor, asFigure lb The shock absorber can be flexible in the vertical direction. However, the vertical flexibility can also be achieved in other ways.

[0035] The shock absorber can for example comprise an impact part aligned with the stop structure in the vertical direction, one or more first shock absorber springs extending from the rotor to the impact part, and one or more second shock absorber springs extending from the impact part to the stator. The one or more first shock absorber springs and the one or more second shock absorber springs can be flexible in the vertical direction.

[0036] In other words, the shock absorber can comprise an impact part designed to be in contact with the stop structure. The impact part can be a substantially rigid plate, which does not have a significant vertical flexibility. The area of the impact part in the xy-plane can be substantially equal to or slightly larger than the area of the bottom surface of the stop structure. In addition to the impact part, the shock absorber can comprise springs providing the shock absorber with vertical flexibility. The impact part and the springs can have the same vertical thickness as the rotor.

[0037] The vertically flexible springs on both sides of the impact part allow the force of the impact (between the stop structure and the impact part) to be transformed into a breaking force, which slows down the movement of the rotor and eventually softly stops the rotor.

[0038] Figures 2a to 2b A microelectromechanical device is illustrated, in Figure 2a In the figure, reference numerals 21, 22, 23 and 281 correspond to reference numerals 11, 12, 13 and 181, respectively.

[0039] The one or more first shock absorber springs can comprise one first shock absorber spring, and the one or more second shock absorber springs can comprise one second shock absorber spring. In Figure 2a In the figure, the shock absorber 23 comprises an impact part 239, first shock absorber springs (231+235+232) and second shock absorber springs (233+236+234).

[0040] Each shock absorber spring comprises two attachment sections and a connecting rod extending between the attachment sections. The first shock absorber spring comprises a first attachment section 231 having torsional flexibility, which is attached to the edge of the rotor 21. The first attachment section 231 extends in the lateral direction towards the stator 22. The first shock absorber spring further comprises a second attachment section 232 having torsional flexibility, which is attached to the impact part 239 and extends in the lateral direction towards the rotor 21. The first shock absorber spring further comprises a first connecting rod 235, which extends from the first attachment section 231 to the second attachment section 232.

[0041] The second shock absorber spring comprises a third attachment section 233 having torsional flexibility, which is attached to the impact part 239 and extends in the transverse direction towards the stator 22. The second shock absorber spring further comprises a fourth attachment section 234 having torsional flexibility, which is attached to the stator 22 and extends in the transverse direction towards the rotor 21. The second shock absorber spring further comprises a second connecting rod 236, which extends from the third attachment section 233 to the fourth attachment section 234.

[0042] The torsional flexibility of the attachment sections 231-234 provides the shock absorber 23 with vertical flexibility. In other words, when the rotor moves in the vertical direction, the attachment sections can twist around their longitudinal axis, e.g. axis 291 for attachment section 231 or axis 292 for attachment section 234. This allows the connecting rods 235 and 236 to turn out of the device plane. The connecting rods 235-236 can also optionally be bent in the vertical direction to provide further vertical flexibility to the shock absorber spring.

[0043] As Figure 2a illustrated in the present disclosure, the attachment sections having torsional flexibility can be straight rods having a narrow width in the x-direction, for example. Such rods can be referred to as torsion rods. The first shock absorber spring can comprise a first torsion rod attached to the rotor, a second torsion rod attached to the impact part, and a first connecting rod extending between the first and second torsion rods, and the second shock absorber spring can comprise a third torsion rod attached to the impact part, a fourth torsion rod attached to the stator, and a second connecting rod extending between the third and fourth torsion rods.

[0044] In any of the embodiments discussed in the present disclosure, the size of each first shock absorber spring can be substantially equal to the size of each second shock absorber spring.

[0045] The number of first and second shock absorber springs can be increased as needed. One or more first shock absorber springs can for example comprise two first shock absorber springs attached to opposite end portions of the impact part, and one or more second shock absorber springs can also comprise two second shock absorber springs attached to opposite end portions of the impact part. This is illustrated in Figure 2b , where each reference number refers to the same elements as in Figure 2a , but there are two first shock absorber springs and two second shock absorber springs. Each first attachment section 231 can be aligned with an opposite fourth attachment section 234 on a transverse axis (axis 291 or axis 292 in Figure 2b ). Correspondingly, each second attachment section 232 can be aligned with an opposite third attachment section 233 on a transverse axis (axis 293 or axis 294 in Figure 2b ).

[0046] As mentioned before, the attachment sections with torsional flexibility can be torsion bars. Thus, Figure 2b Each of the two first damper springs in the device of Fig. 1 can comprise a first torsion bar attached to the rotor, a second torsion bar attached to the impact member and a first connecting rod extending between the first and second torsion bars. Each of the two second damper springs in the device of Fig. 1 can comprise a third torsion bar attached to the impact member, a fourth torsion bar attached to the stator and a second connecting rod extending between the third and fourth torsion bars. The size of each first damper spring and each second damper spring can be substantially equal.

[0047] The attachment sections or the entire damper springs can alternatively be any other type of vertically (and possibly torsionally) flexible structure that allows the rotor and the impact member to move out of the device plane. The one or more first damper springs may, for example, comprise one first damper spring having a meandering shape in the device plane. Correspondingly, the one or more second damper springs can comprise one second damper spring having a meandering shape in the device plane. This is illustrated in Figure 3 Fig. 1 1, where reference numerals 31, 32, 33, 381 and 339 correspond to reference numerals 21, 22, 23, 281 and 239 in Fig. 1, respectively. The first damper spring 331 and the second damper spring 332 have a meandering shape with flexibility in the vertical direction. Figure 2a

[0048] Alternatively, the two first damper springs can extend from the rotor to the impact member and both the two first damper springs can have a meandering shape. The two second damper springs can also extend from the stator to the impact member and both the two second damper springs can have a meandering shape.​

Claims

1. A microelectromechanical device comprising a movable rotor and a fixed stator, wherein, The rotor lies in a horizontal device plane in a rest position of the rotor, and the rotor lies adjacent to the stator such that an edge of the rotor is separated from an edge of the stator by a rotor-stator gap, and the device further comprises a stationary wall, and the stationary wall defines a wall plane adjacent to the horizontal device plane and separated from the horizontal device plane in a vertical direction by a rotor-wall gap, wherein the vertical direction is perpendicular to the horizontal device plane, and the micro-electromechanical device further comprises a motion limiter configured to prevent the rotor from directly physically contacting the stationary wall across the rotor-wall gap, characterized in that the motion limiter comprises: a shock absorber extending from the rotor to the stator across the rotor-stator gap, wherein the shock absorber is flexible in the vertical direction, and a stationary stopper structure protruding from the stationary wall towards the shock absorber in the vertical direction such that the stopper structure is vertically aligned with the shock absorber in an impact region and the shock absorber is separated from the stopper structure in the vertical direction by a stopper gap, and the stopper structure is dimensioned such that, when the movable rotor experiences a motion in the vertical direction towards the stationary wall, the shock absorber contacts the stopper structure in the impact region before the rotor contacts the stationary wall across the rotor-wall gap, and the shock absorber comprises an impact component aligned with the stopper structure in the vertical direction, one or more first shock absorber springs extending from the rotor to the impact component, and one or more second shock absorber springs extending from the impact component to the stator, wherein the one or more first shock absorber springs and the one or more second shock absorber springs are flexible in the vertical direction.

2. The microelectromechanical device of claim 1, wherein, The one or more first shock absorber springs comprise one first shock absorber spring, and the one or more second shock absorber springs comprise one second shock absorber spring.

3. The microelectromechanical device of claim 2, wherein, The one first shock absorber spring comprises a first torsion bar attached to the rotor, a second torsion bar attached to the impact component, and a first connecting rod extending between the first torsion bar and the second torsion bar, and the one second shock absorber spring comprises a third torsion bar attached to the impact component, a fourth torsion bar attached to the stator, and a second connecting rod extending between the third torsion bar and the fourth torsion bar.

4. The microelectromechanical device of claim 2, wherein, The one first shock absorber spring has a meander shape in the device plane, and the one second shock absorber spring also has a meander shape in the device plane.

5. The microelectromechanical device according to any one of claims 1 to 4, wherein The size of each first shock absorber spring is essentially equal to the size of each second shock absorber spring.

6. The microelectromechanical device of claim 1, wherein, The one or more first shock absorber springs comprise two first shock absorber springs attached to opposite ends of the impact component, and the one or more second shock absorber springs comprise two second shock absorber springs attached to opposite ends of the impact component.

7. The microelectromechanical device of claim 6, wherein, Each of the two first shock absorber springs includes a first torsion bar attached to the rotor, a second torsion bar attached to the impact component, and a first connecting rod extending between the first and second torsion bars, and each of the two second shock absorber springs includes a third torsion bar attached to the impact component, a fourth torsion bar attached to the stator, and a second connecting rod extending between the third and fourth torsion bars.

8. The microelectromechanical device of claim 7, wherein, Each first shock absorber spring and each second shock absorber spring are substantially equal in size.

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