U.S. patent application number 14/050201 was filed with the patent office on 2014-09-18 for elastic bump stops for mems devices.
This patent application is currently assigned to Invensense, Inc.. The applicant listed for this patent is Invensense, Inc.. Invention is credited to Jin Qiu, Joseph Seeger.
Application Number | 20140260613 14/050201 |
Document ID | / |
Family ID | 51521264 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140260613 |
Kind Code |
A1 |
Qiu; Jin ; et al. |
September 18, 2014 |
ELASTIC BUMP STOPS FOR MEMS DEVICES
Abstract
A MEMS device includes at least one proof mass, the at least one
proof mass is capable of moving to contact at least one target
structure. The MEMS device further includes at least one elastic
bump stop coupled to the proof mass and situated at a first
distance from the target structure. The MEMS device additionally
includes at least one secondary bump stop situated at a second
distance from the target structure, wherein the second distance is
greater than the first distance, and further wherein the at least
one elastic bump stop moves to reduce the first distance when a
shock is applied.
Inventors: |
Qiu; Jin; (Sunnyvale,
CA) ; Seeger; Joseph; (Menlo Park, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Invensense, Inc. |
San Jose |
CA |
US |
|
|
Assignee: |
Invensense, Inc.
San Jose
CA
|
Family ID: |
51521264 |
Appl. No.: |
14/050201 |
Filed: |
October 9, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61790300 |
Mar 15, 2013 |
|
|
|
Current U.S.
Class: |
73/504.15 ;
73/504.12 |
Current CPC
Class: |
G01P 2015/0871 20130101;
G01C 19/5733 20130101; G01P 2015/0874 20130101 |
Class at
Publication: |
73/504.15 ;
73/504.12 |
International
Class: |
G01C 19/56 20060101
G01C019/56 |
Claims
1. A MEMS device comprising: at least one proof mass capable of
moving relative to a substrate; a first contact surface coupled to
the at least one proof mass; a second contact surface coupled to
the at least one proof mass; a first target surface facing the
first contact surface and separated from the first contact surface
by a first distance; a second target surface facing the second
contact surface and separated from the second contact surface by a
second distance; and the first contact surface is coupled to the
proof mass by a flexible element or the first target surface is
coupled to the flexible element, wherein the at least one proof
mass is coupled to the substrate by a spring, wherein when the at
least one proof mass moves, the first contact surface moves to
reduce the first distance until the first contact surface contacts
the first target surface, wherein when the first contact surface
and the first target surface are in contact and the at least one
proof mass moves, at least one flexible element flexes and the
second contact surface moves to reduce the second distance until
the second contact surface contacts the second target surface.
2. The MEMS device of claim 1, wherein the at least one proof mass
moves at a plane substantially parallel to the substrate.
3. The MEMS device of claim 1, wherein the at least one proof mass
moves out of a plane parallel to the substrate.
4. The MEMS device of claim 1, wherein the flexible element is a
torsional spring.
5. The MEMS device of claim 1, wherein the first target surface is
stationary with respect to the substrate.
6. The MEMS device of claim 5, wherein the flexible element is
coupled to the at least one proof mass.
7. The MEMS device of claim 1, wherein the flexible element is a
clamped-clamped beam.
8. The MEMS device of claim 7, further comprising a first structure
coupled to the first target surface, thereby extending a portion of
the first target surface towards the at least one proof mass.
9. The MEMS device of claim 7, further comprising a first structure
coupled to the first contact surface thereby extending a portion of
the first contact surface towards the at least one proof mass
10. The MEMS device of claim 9, further comprising a second
structure coupled to the second target surface or the second
contact surface.
11. The MEMS device of claim 10, wherein the second structure is a
secondary bump stop.
12. The MEMS device of claim 10, wherein the second structure is
less flexible than the at least one flexible element.
13. The MEMS device of claim 1, wherein the second target surface
is stationary with respect to the substrate.
14. The MEMS device of claim 1, wherein the first contact surface
is smaller than the second contact surface.
15. The MEMS device of claim 1, where in the MEMS device is a
gyroscope or an accelerometer.
16. The MEMS device of claim 1, wherein the at least one proof mass
moves translationally.
17. The MEMS device of claim 1, wherein the at least one proof mass
moves rotationally.
18. The MEMS device of claim 1, further comprising a structure
coupled to the at least one proof mass by a second flexible
element, wherein the first contact surface is formed on a the first
surface of the structure, wherein the second target surface is
formed on a second surface of the structure, wherein the second
contact surface is formed on the at least one proof mass.
19. The MEMS device of claim 18, wherein the second flexible
element is a cantilever beam.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 61/790,300 filed on Mar. 15, 2013, by Qiu et al.,
and entitled "Elastic Bump Stops for MEMS Devices".
BACKGROUND
[0002] Various embodiments of the invention relate generally to
bump stops and particularly to elastic bump stops used in
microelectromechanical systems (MEMS) device. A known problem with
MEMS devices is upon experiencing a shock, the MEMS device
undergoes chipping at corners when a moving structure contacts a
target. This clearly leads to undesirable effects not to mention a
short lifetime of the MEMS device as well as increased costs and
other foreseen deterioration. Bump stops have been historically
employed to aid in reducing the impact experienced by MEMS devices.
However, such measures have yielded little positive results
particularly in light of their stiffness.
[0003] What is needed is a MEMS device with reduced impact force
upon encountering a shock.
SUMMARY
[0004] Briefly, an embodiment of the invention includes a MEMS
device having at least one proof mass, the at least one proof mass
is capable of moving to contact at least one target structure. The
MEMS device further includes at least one elastic bump stop coupled
to the proof mass and situated at a first distance from the target
structure. The MEMS device additionally includes at least one
secondary bump stop situated at a second distance from the target
structure, wherein the second distance is greater than the first
distance, and further wherein the at least one elastic bump stop
moves to reduce the first distance when a shock is applied.
[0005] A further understanding of the nature and the advantages of
particular embodiments disclosed herein may be realized by
reference of the remaining portions of the specification and the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIGS. 1(a) through 2(b) each show a conceptual diagram of a
MEMS device, in accordance embodiments of the invention.
[0007] FIG. 3 shows a portion of a gyroscope with elastic bump
stops, in accordance with an embodiment of the invention.
[0008] FIGS. 4(a) through (d) each show various views of
out-of-plane elastic bump stops, in accordance with other
embodiments of the invention.
[0009] FIGS. 5(a) through (f) show various views of in-plane
elastic bump stops, in accordance with yet another embodiment of
the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0010] The following specification describes a MEMS device with
multiple bump stops (also referred to herein as "elastic bump" or
"bump") to reduce the affect of a shock to the MEMS device.
[0011] As used herein "stiction" is an undesirable situation which
arises when surface adhesion forces are higher than the mechanical
restoring force of a MEMS structure or MEMS device. Stiction is
recognized to often occur in situations where two surfaces with
areas in close proximity come in contact. The greater the contact
area at both macroscopic and microscopic roughness levels, the
greater the risk of stiction. At the microscopic level, soft
materials can deform, effectively increasing contact area. Surfaces
can be unintentionally brought into contact by external
environmental forces including vibration, shock and surface tension
forces that are present during aqueous sacrificial release steps
often used in micro-fabrication processes. Adherence of the two
surfaces may occur causing the undesirable stiction.
[0012] Particular embodiments and methods of the invention disclose
a MEMS device having at least one proof mass. The proof mass is
capable of moving to contact at least one target structure. The
MEMS device further includes at least one primary bump stop that is
coupled to the proof mass and situated at a first distance from the
target structure. The primary bump stops maybe an elastic bump
stop. The MEMS device additionally has at least one secondary bump
stop situated at a second distance from the target structure. The
second distance is greater than the first distance. The primary
bump stop moves to reduce the first distance when a shock is
applied to the MEMS device. Flexible element coupled to the proof
mas reduces the impact of the force experienced by the proof mas to
prevent chipping. In various embodiments of the invention, reduced
impact force permits use of smaller bump stops for less stiction.
Additionally, the increased flexible element restores force for a
secondary bump stop. In some embodiments, the target structure is
positioned at a tilt relative to the elastic bump stops and the
elastic bump stops reduce chipping caused by impact at the corners
of the target structure, such as the MEMS device, because the bump
stop can conform to the tilt. Stated differently, elastic bump
stops are utilized to help prevent chipping by compressing to help
break out a secondary bump stop contact stiction.
[0013] In the described embodiments, MEMS device may include one or
more proof masses, one or more primary bump stops and one or more
secondary bump stops. The primary bump stop may be more flexible
than the secondary bump stop. The primary bump stop may be coupled
to a target structure or connected to the proof mass. Similarly,
the secondary bump stop may be coupled to the target structure or
the proof mass. The target structure may be a stationary or
moveable.
[0014] The MEMS device may include a first contact surface and a
second contact surface coupled to the proof mass. In some
embodiments, the first contact surface may be on the primary bump
stop or the proof mass or the target structure; the second contact
surface may be on the secondary bump stop or the proof mass or the
target structure or the primary bump stop. In some embodiments, the
first target surface may be on the primary bump stop or the proof
mass or the target structure; the second target surface may be on
the secondary bump stop or the proof mass or the target structure.
The second target structure and the second contact are farther
apart than the first target structure and the first contact. The
first contact surface may be coupled to the proof mass via a
flexible element or coupled directly to the proof mass.
[0015] In the described embodiments, elastic member, flexible
element, spring, and flexible structure maybe used interchangingly.
FIGS. 1(a) through 2(b) each show a conceptual diagram of a MEMS
device, in accordance with various embodiments of the invention. In
FIG. 1(a) the first contact is identified as 126 whereas the second
contact is identified as 124. The first and second target contacts
are 110.
[0016] Referring now to FIG. 1(a), a conceptual diagram of MEMS
device 100 with proof mass 102 flexibly extending from a fixed
structure 101, represented by the spring 104 having associated
therewith a spring constant `k1`. The mass `m` moves down in
relation to fixed structure 101 with the spring 104 extending (or
flexing) downward. Along with proof mass 102, two secondary bump
stops 106 and 108 are shown connected to proof mass 102 and move
downwardly with proof mass 102. Secondary bump stops 106 and 108
help to protect proof mass 102 from making contact with another
structure 110 especially upon the proof mass experiencing a great
shock, as shown in FIG. 1(c). In an embodiment structure 110 may be
stationary, in another embodiment, structure 110 may be
moveable.
[0017] Further shown in FIG. 1(a) is primary contact 114 that
flexibly extends downward via and along with a spring 116 that has
a spring constant 12'. Structure 114 makes contact with structure
110 at contact 118, as shown in FIG. 1(c), when structure 114 moves
downwardly toward the structure 110.
[0018] The structure 114 is shown, in FIG. 1(a), to be at a
distance 120, `g1`, from structure 110. Secondary bump stops 106
and 108 are shown to be at a distance, `g1+g2` away from structure
110 prior to proof mass 102 experiencing a shock or moving down
toward the structure 110. The elastic bump stop operation, shown in
FIGS. 1(a)-(c) has at least two advantages. One such advantage is
to reduce the impact force between secondary bump stops 106 and 108
and structure 110, another is to have extra restoring force for
structure 110.
[0019] In FIG. 1(a), in an embodiment of the invention, MEMS device
100 is a gyroscope or an accelerometer. While the MEMS device is at
rest position, secondary bump stop stops 106 and 108 are not
compressed. When proof mass 102 starts to move with a velocity v0
and when the velocity is small, as shown in FIG. 1(b), primary bump
stop 114 contacts structure 110 to reduce g1 to almost zero and
secondary bump stops 106 and 108 are at distance 113 away from
structure 110. In the embodiment of FIG. 1(b), the force upon proof
mass 102 is less than the force proof mass 102 of the embodiment of
FIG. 1(c) experiences. In an embodiment and method of the
invention, the primary bump stop 114 moves translationally toward
the structure 110.
[0020] In FIG. 1(a), secondary bump stops 106 and 108 are shown at
a distance that is the sum of g1 and g2 at rest and primary bump
stop 114 is shown at the gap 120 or the distance of g1 at rest.
Whereas, in FIG. 1(b), the distance g1 is nearly zero and the
distance 113 is smaller than the distance g1+g2 (of FIG. 1(a)). In
FIG. 1(a)-(c), the distances g1 and g2, i.e. gaps 120 and 122,
respectively, are zero. `g1` and `g2` each represent the gaps at
rest.
[0021] Stated differently, FIG. 1(a) shows primary bump stop 114,
with secondary contacts 106 and 108, between proof mass 102 and the
structure 110 at rest, which limits the primary bump stop 114
deflection. Obviously, primary bump stop 114 comes in contact with
structure 110 before the secondary bumps 106 and 108 come in
contact with structure 110. The contact force between the primary
bump stop 114 and structure 110 are indicated with the
following:
f.sub.bounceMax=v.sub.0 {square root over
(mk.sub.2)},f.sub.Max=k.sub.2g.sub.2. Eq. (1)
[0022] If v0 is low and the secondary bump stops 106 and 108 are
not engaged during impact, f_bounceMax is the maximum force
experienced by primary bump stop 114. In the embodiment of FIG.
1(b), v0 is smaller than it is in the embodiment of FIG. 1(c). If
v0 is high as in shown in FIG. 1(c), secondary bump stops 106 and
108 move to establish contact with the structure 110, and f_Max is
the maximum force determined by the stiffness and deflection of
primary bump stop 114. As Eq. (1) indicates, either f_bounceMax or
f_Max are lowered by low elastic bump stop stiffness k2. If the
secondary bump stops 106 and 108 move to contact structure 110, the
elastic member (spring 116 with stiffness k2) applies an extra
force f_Max to restore secondary bump stops 106 and 108 and target
structure 110. Secondary bump stops 106 and 108 are compressed to
help break out the secondary bump stop contact stiction.
[0023] FIGS. 2(a) and 2(b) show that the bump stops can conform to
the contact target tilt at rest position, as shown in FIG. 2(a). At
contact state (shown in FIG. 2(b)), primary bump stop 114 can
conform to the tilted contact target, or structure 110. In this
embodiment, structure 110 is not stationary. FIGS. 2(a) and 2(b)
show another advantage of an elastic bump stop and that is, it
conforms to the contact target structure with its rotational
compliance. By conforming to two contact surfaces, the contact area
increases thus the contact stress, i.e., contact force divided by
contact area, decreases. This reduces the risk of breakage at
contact surfaces.
[0024] As shown in FIG. 2, secondary bump stops 106 and 108 can
also be implemented between two moving structures such as proof
mass 102 and structure 110. Elastic bump stops have the following
advantages. One is flexible contacts that reduce the impact force
to proof mass 102 to prevent chipping, thereby permitting use of
smaller bump stops with less stiction. Another is increasing the
flexible contact to restore the force of the secondary bump stop
124 and reduce the elastic bump stops thereby reducing chipping
caused by impact at the corners of proof mass 102. This is because
the bump stop can conform to the tilt (shown in FIGS. 2(a) and
2(b)).
[0025] FIG. 3 shows a portion of a gyroscope with elastic bump
stops, in accordance with an embodiment of the invention. In FIG.
3, only a quarter of the gyroscope is shown, with the lower left
corner pointing as the symmetric center. The gyroscope has anchor
at 311, a system of springs, proof masses, and linking structures.
The proof mass 313 is the main sensing element; its motion during
shock is constrained by bump stops 316 (and other elastic bump
stops in circles) against linking structures 312, 314, and 315.
Elastic bump stops engage in either the Y-directional shock, or the
Z-directional shock. During a Z-directional shock, proof mass 313
initially only has Z-motion, but due to the constraint of the
springs, the Z-motion energy is partially transferred to RX
(rotation around X-axis) motion. Following this proof mass RX
motion, the elastic bump stops also rotate in RX direction, and
engage to close the contacts. Each primary bump stop 362 is
supported by a flexible beam 361 to the proof mass 364. The
structure 363 is at the opposing structure. There are three gaps
365, 366, 367 in the elastic bump stop structure. In an embodiment,
the flexible beam 361 (also referred to herein as "flexible
element") is a cantilever beam. In an embodiment of the invention,
the flexible beam 361 is a clamped-clamped beam.
[0026] When a shock is applied, first the gap 367 closes, due to
the flexibility of the flexible beam 361, the contact force between
362 and the structure 363 are reduced. If the shock is large
enough, the gaps 365 and 366 also close, to prevent further
relative motion between the proof mass 364 and the structure
363.
[0027] FIGS. 4(a) through (d) each show various views of elastic
bump stops in an accelerometer where the motion of the proof mass
is out-of-plane, in accordance with other embodiments of the
invention. FIG. 4(a) shows an accelerometer to sense linear
acceleration along the Z-axis. FIG. 4(b) shows a portion of the
accelerometer enlarged. In FIGS. 4(a)-4(d), motion along the Z axis
is initiated by asymmetric proof mass couple 423 and 424, supported
by pivot springs 422 to anchor 421 with the proof mass 423 being
heavier than the proof mass 424. Proof masses 424 and 423 are
anchored by anchor 421 to substrate 420. Bump stops are situated on
each side of the masses 423 and 424. Either bump stop can contact
the substrate 420 depending on the direction of the Z directional
shock. The side view of operations, shown in FIGS. 4(c) and 4(d),
shows the bumps on the heavier mass 423. Primary bump stop 426 is
shown coupled to the proof mass 423 by flexure beam 425. A portion
of proof mass 423 that extends beyond flexure beam 425 functions as
a secondary bump stop. During low intensity shock along the Z-axis,
tip 427 of the primary bump stop 426 comes in contact with the
substrate 420. Due to the flexibility of primary bump stop 426, the
contact force is reduced. FIG. 4(d) shows the accelerometer when a
higher intensity shock along the Z-axis, proof mass 423 comes in
contact with substrate 420 at 428. The restoration force from
flexure beam 425 creates a strong restoring force for secondary
bump 428 in addition to the weaker restoration force from pivot
springs 422.
[0028] FIG. 5(a) shows a z-axis accelerometer for sensing linear
acceleration along the Z-axis where the bump stops are in the plane
of the substrate. 502 identifies a bump stop similar to that of
FIG. 5(d).
[0029] FIGS. 5(b) through 5(f) show implementations of the five
embodiments of the bump stop 520 and elastic member 510. In one
embodiment, the elastic member may be a clamp-clamp beam. FIG. 5(c)
shows elastic member 510 being coupled to proof mass 530. Bump 520
is coupled to fixed structure 540. Target structure 540 may be a
fixed structure. When the MEMS device experiences acceleration
along the Z-axis, elastic member 510 contacts bump stop 520 before
secondary bump stops 512 contact the target structure 540. FIG.
5(d) shows bump stop 520 coupled to proof mass 530 by elastic
member 510. When proof mass 530 moves, bump stop 520 contacts
target structure 540 before secondary bump stops 512 contacts the
target structure 540. FIG. 5(e) shows bump stop 520 coupled to
target structure 540 by elastic member 510. When proof mass 530
moves, bump stop 520 contacts proof mass 530 before the secondary
bump stops 512 contact the proof mass 530. In FIG. 5(f), bump stop
520 is coupled to proof mass 530. In FIG. 5 (g) is another
embodiment of FIG. 5 (d) wherein secondary bump stops 524 are
coupled to the target structure 540. When proof mass moves towards
target structure, bump stop 520 contacts target structure 540
before secondary bump stops 524 contact the proof mass 530. In an
embodiment, secondary bump stops 512 may by a flat structure
whereas secondary bump stop 524 may be extensions on the fixed
structure.
[0030] Thus, in accordance with the various embodiments and methods
of the invention, an elastic bump stop design with a second contact
that limits the elastic bump stop deflection so as to prevent the
bump stop function is disclosed. The elastic bump stop design with
the second contact helps restore the second contact and conforms to
the contact surface by the rotational compliance of the elastic
member. The elastic bump stop design with the first contact member
either flattens out, or not, against the contact target. Both
behaviors can be implemented by specific elastic bump stop
designs.
[0031] The embodiments and methods disclosed herein can also apply
to a compass, in addition to a gyroscope, with the compass
comprising of moveable elements. Although the description has been
described with respect to particular embodiments thereof, these
particular embodiments are merely illustrative, and not
restrictive.
[0032] As used in the description herein and throughout the claims
that follow, "a", "an", and "the" includes plural references unless
the context clearly dictates otherwise. Also, as used in the
description herein and throughout the claims that follow, the
meaning of "in" includes "in" and "on" unless the context clearly
dictates otherwise.
[0033] Thus, while particular embodiments have been described
herein, latitudes of modification, various changes, and
substitutions are intended in the foregoing disclosures, and it
will be appreciated that in some instances some features of
particular embodiments will be employed without a corresponding use
of other features without departing from the scope and spirit as
set forth. Therefore, many modifications may be made to adapt a
particular situation or material to the essential scope and
spirit.
* * * * *