U.S. patent application number 09/761291 was filed with the patent office on 2002-07-18 for suspended micromachined structure.
Invention is credited to Knowles, Gary R..
Application Number | 20020093067 09/761291 |
Document ID | / |
Family ID | 25061792 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020093067 |
Kind Code |
A1 |
Knowles, Gary R. |
July 18, 2002 |
SUSPENDED MICROMACHINED STRUCTURE
Abstract
A suspended micromachined structure including a proof mass and
multiple support arms configured to suspend the mass above a
substrate. At least one support arm may include two spring
elements, each attached to the substrate as well as to a rigid
lateral element. Thus, there may be three points of attachment
along each lateral element. These points of attachment create three
effective flexure points along each rigid lateral element that
allow the proof mass to move with a great deal of freedom axially,
parallel to the substrate. The linearity of the spring constant
that acts on the proof mass may be improved.
Inventors: |
Knowles, Gary R.; (Ham Lake,
MN) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Family ID: |
25061792 |
Appl. No.: |
09/761291 |
Filed: |
January 16, 2001 |
Current U.S.
Class: |
257/419 |
Current CPC
Class: |
B81B 3/0051 20130101;
B81B 2203/053 20130101; B81B 2203/051 20130101; B81B 2203/0109
20130101; G01C 19/5719 20130101; B81B 2201/0242 20130101 |
Class at
Publication: |
257/419 |
International
Class: |
H01L 021/00 |
Claims
The embodiments of the invention in which an exclusive property or
right is claimed are defined as follows:
1. A micromachined device comprising: a movable mass; a plurality
of support arms attached to the movable mass; each of the support
arms flexibly coupling the movable mass to a substrate; at least
one of the support arms including a first end coupled to the
substrate and a second end coupled to the substrate; wherein the
movable mass is connected to the at least one support arm at a
point between the first end and the second end of the support
arm.
2. The micromachined device of claim 1, wherein the at least one
support arm comprises: a first spring element attached to the
substrate; a second spring element attached to the substrate; a
rigid lateral element having a first end and a second end, the
first end of the rigid lateral element connected to the first
spring element and the second end of the rigid lateral element
connected to the second spring element; wherein the movable mass is
connected to the rigid lateral element of the at least one support
arm between the first and second ends of the rigid lateral
element.
3. The micromachined device of claim 2, wherein the movable mass is
connected to the middle of the rigid lateral element.
4. The micromachined device of claim 2 wherein the connection of
the first spring element to the rigid lateral element creates a
first end flexure point, the connection of the second spring
element to the rigid lateral element creates a second end flexure
point, and the connection of the movable mass to the rigid lateral
element creates a middle flexure point.
5. The micromachined device of claim 4, wherein the first end
flexure point, the second end flexure point, and the middle flexure
point form a substantially straight line.
6. The micromachined device of claim 5, wherein the substantially
straight line comprises an axis of alignment.
7. The micromachined device of claim 6, wherein the axis of
alignment is parallel to an axis of oscillation of the movable
mass.
8. The micromachined device of claim 7, wherein when the movable
mass moves along the axis of oscillation, the middle flexure point
moves along the axis of alignment.
9. The micromachined device of claim 1, further including a second
support arm having a first end coupled to the substrate and a
second end coupled to the substrate; wherein the movable mass is
connected to the second support arm at a point between the first
end and the second end of the support arm.
10. The micromachined device of claim 9, wherein the at least one
support arm and the second support arm are positioned on the same
side of the movable mass.
11. The micromachined device of claim 9, wherein the at least on
support arm and the second support arm are positioned on opposite
sides of the movable mass.
12. The micromachined device of claim 6, wherein the first and
second spring elements are symmetrical about the axis of
alignment.
13. A micromachined device comprising: a movable mass having first
and second sides; a first support arm extending from the first side
of the movable mass; a second support arm extending from the second
side of the movable mass; each of the support arms flexibly
coupling the movable mass to a substrate; each of the support arms
includes a first end coupled to the substrate and a second end
coupled to the substrate; wherein the movable mass is connected to
each of the support arms at a point between the first end and the
second end of the support arm.
14. The micromachined device of claim 13, wherein each of the
support arms comprise: a first spring element attached to the
substrate; a second spring element attached to the substrate; a
rigid lateral element having a first end and a second end, the
first end of the rigid lateral element connected to the first
spring element and the second end of the rigid lateral element
connected to a second spring element; wherein the movable mass is
connected to the rigid lateral element of each support arm between
the first and second ends of the rigid lateral elements.
15. The micromachined device of claim 14, wherein the movable mass
is connected to the middle of each of the rigid lateral
elements.
16. The micromachined device of claim 14, wherein for each support
arm the connection of the first spring element to the rigid lateral
element creates a first end flexure point, the connection of the
second spring element to the rigid lateral element creates a second
end flexure point, and the connection of the movable mass to the
rigid lateral element creates a third flexure point.
17. The micromachined device of claim 16, wherein the first end
flexure point, the second end flexure point, and the middle flexure
point for each support arm forms a substantially straight line.
18. The micromachined device of claim 17, wherein the substantially
straight line for each support arm comprises an axis of
alignment.
19. The micromachined device of claim 18, wherein the axis of
alignment for each support arm is parallel to every other support
arm's axis of alignment.
20. The micromachined device of claim 19, wherein when the movable
mass moves along the axis of oscillation, the middle flexure point
of each support arm moves along its respective axis of
alignment.
21. A micromachined device comprising: a movable mass having first
and second sides; four support arms extending from the sides of the
movable mass flexibly coupling the movable mass to a substrate;
each of the four support arms including a first end connected to
the substrate, a second end connected to the substrate, and a rigid
lateral element; each rigid lateral element including a first end
and a second end, the first end of the rigid lateral element
connected to a first spring element and the second end of the rigid
lateral element connected to a second spring element; each
connecting arm of the movable mass being connected to each rigid
lateral element between the first and second ends of each rigid
lateral element; wherein the connection of the first spring element
to each rigid lateral element creates a first end flexure point,
the connection of the second spring element to each rigid lateral
element creates a second end flexure point, and the connection of
each connecting arm to each rigid lateral element creates a middle
flexure point.
22. The device of claim 21, wherein the three flexure points of
each support arm are aligned in a substantially straight line that
comprises an axis of alignment.
23. The device of claim 22, wherein each axis of alignment of the
four support arms are parallel to each other and to an axis of
oscillation of the movable mass.
24. The device of claim 22, wherein the middle flexure point of
each support arm moves along its respective axis of alignment when
the movable mass moves along the axis of oscillation.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to micromachined devices and,
more particularly, to a suspension system for micromachined
devices.
[0003] 2. Description of Related Art
[0004] Recent advances in micromachining have enabled the
manufacture of various microelectromechanical systems (MEMS) that
offer potential performance and cost improvements over existing
non-micromachined devices. MEMS devices may be manufactured on a
large scale using photolithographic techniques to etch silicon
wafers, in much the same way that traditional microelectronic
integrated circuits are produced in the electronics industry. In
silicon-based MEMS devices fabricated using conventional integrated
circuit techniques, three-dimensional structures can be integrated
with electronic circuitry on the same chip, offering great
potential for improvements of sensors, actuators, and other
devices. Initially, MEMS devices were strictly silicon-based, like
microelectronic devices, but today the term represents complete
miniature devices that may or may not be silicon-based, and that
can be produced using methods other than photolithographic
techniques.
[0005] One MEMS device is a micro-electromechanical system
gyroscope (MEMS gyro). The MEMS gyro consists of one or more
oscillating proof masses that may be suspended above a substrate by
spring elements mounted to the substrate. The proof mass is made to
oscillate at a precise frequency axially and parallel to the
substrate by an electronic drive mechanism. As used herein, the
term "proof mass" is defined broadly to include any mass suitable
for use in a MEMS system. The MEMS gyro functions by sensing the
coriolis acceleration that acts on the oscillating proof mass when
the gyro is rotated. Further, the substrate typically has a recess
below the proof mass that allows the gyro to oscillate freely above
the substrate. The recess may be formed in the substrate by
deposition of a photoresist mask that allows the substrate to be
selectively etched.
[0006] When spring elements are used to suspend a proof mass above
a substrate, at least one end of the spring element is typically
mounted to the substrate, and the other end is typically attached
to the proof mass. Because one end is fixed, and also because
micro-machined structures do not have pin joints, a spring element
must typically stretch as well as bend when the proof mass
oscillates axially. Adding spring elasticity to each of the spring
elements used to suspend a proof mass can accommodate this
stretching.
[0007] When proof masses are mounted so that their spring elements
must stretch to allow movement, however, the resulting spring
constants in the direction of oscillation are non-linear.
Non-linear spring constants can introduce frequency shifts if the
amplitude of the mass' oscillation varies. Such frequency shifts
are undesirable, as they can affect the accuracy of a MEMS
gyroscope. Moreover, the performance of any micromachined device
that employs a movable mass may be adversely affected by a
non-linear spring constant in the suspension system. Thus, a
suspension system with a more linear spring constant could provide
improved performance in micromachined devices.
[0008] In addition, suspending a proof mass with a spring element
that is configured to stretch and also to bend as the proof mass
oscillates allows some freedom of motion in directions other than
the direction in which the proof mass was designed to oscillate.
Such freedom of motion is undesirable, can adversely affect
measurements made by the gyro and, if it is great enough, may even
damage or destroy the gyro if a portion of the proof mass collides
with a stationary element of the gyro. Thus, a suspension system
that allows great freedom of motion along one axis, while
significantly restricting motion in any other direction in the
plane of the substrate may provide improved reliability and
performance in micromachined devices.
SUMMARY OF THE INVENTION
[0009] A suspended micromachined structure is disclosed. The
structure may include a movable proof mass, and multiple support
arms configured to suspend the proof mass above a substrate. Each
support arm may include one or more spring elements and at least
one rigid lateral element.
[0010] Preferably, the proof mass is integrally connected to a
rigid lateral element of each of four support arms suspending the
proof mass above the substrate. Each support arm preferably
includes a rigid lateral element having spring elements extending
therefrom. The spring elements may in turn be attached to the
substrate at two points, and each rigid lateral element of the
support arms may be attached to the proof mass at one point.
Preferably, the spring elements are attached to opposite ends of
each rigid lateral element. In addition, the proof mass is
preferably attached to the center of the rigid lateral element.
Thus, there are preferably three points of attachment on the rigid
lateral element. These points of attachment create three effective
flexure points or flexure points for each support arm along each
rigid lateral element. Thus, there is a flexure point at the
intersection of the proof mass and the rigid lateral element, as
well as at the intersection of the rigid lateral elements with the
respective spring elements.
[0011] The effective flexure points of the support arms may be
configured to allow the proof mass to move with a great deal of
freedom axially, parallel to the substrate, while allowing
substantially less freedom of movement in any other direction
within a plane parallel to the substrate. Further, the support arms
may be configured so that the spring constants that act on the
proof mass result from bending of the spring elements, greatly
improving the linearity of the net spring constant of the
suspension system. In such a system, the flexure point associated
with the point of attachment between the proof mass and the rigid
lateral element moves in a linear direction parallel to the axial
direction of motion of the proof mass, allowing for improved
performance of the gyro.
[0012] These as well as other aspects and advantages of the present
invention will become apparent to those of ordinary skill in the
art by reading the following detailed description, with appropriate
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Exemplary embodiments of the present invention are described
herein with reference to the drawing, in which:
[0014] FIG. 1 is a plan view of a proof mass and suspension system
of the present invention, with drive and sense elements omitted for
clarity;
[0015] FIG. 2 is a detailed view of one support arm of the present
invention suspending the proof mass above the substrate with the
proof mass in its undisplaced (i.e., centered) position.
[0016] FIG. 3 is a detailed view of one support arm of the present
invention showing in exaggerated fashion the positional
relationship between the proof mass and a support arm as the proof
mass is displaced from its center position.
DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT
[0017] Referring to the drawings, FIG. 1 is a plan view of the
present invention. Micromachined proof mass 12 may be suspended
above substrate 10 by four support arms 14. The invention, however,
would also function even if fewer (or more) than four support arms
were used, and even if fewer or more than all of the support arms
were configured as shown in FIG. 1. Proof mass 12 and support arms
14 may be integrally formed using any suitable micromachining
technique. Proof mass 12 is connected to each support arm 14 by a
connecting arm 22 that intersects each support arm 14 at a point
along rigid lateral element 18. Each support arm 14 is connected to
substrate 10 at two attachment points 16 via spring elements 20
extending from respective ends of each rigid lateral element 18.
During operation, proof mass 12 oscillates along the axis of
oscillation shown. In the preferred embodiment, the MEMS gyroscope
of the present invention may be fabricated from a single, unitary
silicon substrate, but this is neither required nor a limitation of
the invention as it is contemplated; a MEMS gyroscope may be
fabricated from various materials known to be suitable for
micromachining, such as silicon, polycrystalline silicon, and other
crystalline or amorphous materials. Also, although only one proof
mass is shown, the invention is equally applicable to MEMS gyros
having two or more proof masses that oscillate in opposition to
each other for the purpose of canceling undesirable vibrations in
the substrate and providing greater sensitivity, since the
displacement of each proof mass may be measured by the system.
[0018] FIG. 2 shows one of the support arms 14 in greater detail,
and also illustrates the principle of operation of the invention.
Each support arm 14 includes two spring elements 20 and a rigid
lateral element 18, where the spring elements 20 are attached to
substrate 10 at two attachment points 16. Rigid lateral element 18
is relatively wider and thus more rigid than spring elements 20.
Further, connecting arm 22 of proof mass 12 is designed to be
narrow where it attaches to rigid lateral element 18. Due to this
design, there are three effective flexure points associated with
each rigid lateral element 18: two end flexure points 30 where each
spring element 16 meets an end of the rigid lateral element 18, and
a middle flexure point 32, where connecting arm 22 meets the center
of the rigid lateral element 18. Effective flexure points act as
pivoting joints, or pin joints, would in a larger mechanical
system.
[0019] The effective flexure points 30 and 32 can be determined by
a finite element modeling program, such as ANSYS. The symmetrical
design of the suspension system creates an axis of alignment 34. To
minimize the overall flexure of the suspension system, the three
flexure points 30 and 32 should lie on a straight line that is
co-linear with axis of alignment 34.
[0020] If the three flexure points of each rigid lateral element 18
lie along axis of alignment 34, the overall flexure of the
suspension system will be minimized. Moreover, since each support
arm 14 is symmetrical about axis of alignment 34, each middle
flexure point 32 will have substantially more freedom of motion
along its axis of alignment 34 than in any other direction in the
plane parallel to substrate 10. This principle will be described in
greater detail below. Further, even if each support arm 14 is not
symmetrical but the design has the overall configuration shown, it
is still possible to achieve a highly linear spring rate and a
greater degree of freedom of motion of middle flexure point 32
along an axis of alignment than in any other direction in the plane
parallel to substrate 10. In other words, perfect symmetry is not
required for the proper functioning of the invention.
[0021] Preferably, the axes of alignment 34 of all four support
arms 14 will be parallel. Thus, the greater freedom of motion that
each middle flexure point 32 has along its axis of alignment 34
will tend to confine the proof mass to an axis of oscillation that
is parallel to the axes of alignment 34, since the proof mass 12 is
preferably connected to each support arm 14 at a middle flexure
point 32.
[0022] When a driving force is exerted on proof mass 12 in the
general direction of the axis of oscillation by an electrical drive
system (not shown), it will be displaced to the left or right of
the center position shown in FIG. 2. As proof mass 12 is displaced,
for example, to the right (as shown in FIG. 3 in exaggerated form),
the right spring element 20 will also move to the right. As right
spring element 20 moves to the right, its flexure point 30 will
travel along radius of travel 36. At the same time, the left spring
element 20 will also move to the right, and its flexure point 40
will travel along its radius of travel 38.
[0023] Since end flexure points 30 and 40 move along their
respective radii of travel 36 and 38 (and not along axis of
alignment 34), rigid lateral element 18 will rotate in the
clockwise direction as proof mass 12 is displaced in either
direction away from the center position of the system, and rigid
lateral element 18 will rotate in the counter-clockwise direction
as proof mass 12 moves toward the centered position.
[0024] Importantly, though, as the proof mass moves along the axis
of oscillation, the middle flexure point 32 will also remain on and
travel along axis of alignment 34, since support arm 14 is
symmetrical about middle flexure point 32. Because support arm 14
is symmetrical, each end of rigid lateral element 18 is displaced
the same distance away from axis of alignment 34 as the other end
(in the opposite directions) with the result that middle flexure
point 32 travels along axis of alignment 34.
[0025] Although it would not be optimal for keeping flexure at a
minimum, middle flexure point 32 will still travel in a straight
line along the axis of alignment 34 as long as support arms 14 are
symmetrical (that is, even if the three flexure points lie on a
straight line that is not parallel to the axis of alignment 34
associated with a particular support arm 14).
[0026] Restricting proof mass 12 to travel along the axis of
oscillation is desirable because non-linear motion could cause
proof mass 12 to either crash into stationary structures (not
shown) attached to substrate 10, or to adversely affect the
accuracy of the sensor due to the undesirable change in the
positional relationship between the proof mass and the sensing
elements of the device (not shown). The restriction on the motion
of proof mass 12 is limited to motion in a plane co-planar to
substrate 10. In other words, proof mass 12 cannot easily rotate
(for example) within a plane parallel to substrate 10, but it has
(and must have) some freedom to move in either direction
perpendicular to substrate 10, since such out-of-plane motion
results from the torques and accelerations that are ultimately
measured by the MEMS gyro.
[0027] With the configuration depicted in Figures, rigid lateral
element 18 is free to rotate about its middle flexure point 32 due
to the geometry of the system, as described above, elongation of
spring elements 20 is limited or eliminated when allowing proof
mass 12 to move along the axis of oscillation. Instead, spring
elements 20 flex at their effective flexure points 30 and 40 to
allow proof mass 12 to move as desired. This flexure with limited
or no elongation of spring elements 20 results in a net spring
constant acting on proof mass 12 that is much more linear than it
would be if it were necessary for spring elements 20 to elongate as
well as bend in order to allow proof mass 12 to oscillate. In the
Figures shown, four support arms are shown having the described
geometry. It should be understood that a greater or fewer number of
support arms of this configuration could be used. While not
optimal, even using one support arm of this type of configuration
will improve the linearity of the net spring constant of the system
and tend to improve the intended path of oscillation of the proof
mass.
[0028] A linear spring constant is desirable for any sensor
employing an oscillating proof mass because a linear spring
constant tends to keep the proof mass oscillating at a more precise
frequency even if the amplitude of oscillation changes. Precise
frequency control improves sensor accuracy and sensitivity and also
simplifies the electronic drive system used for oscillating mass
sensors.
[0029] Exemplary embodiments of the present invention have been
illustrated and described. It will be understood, however, that
changes and modifications may be made to the invention without
deviating from the spirit and scope of the invention, as defined by
the following claims.
* * * * *