The present invention relates generally to microelectromechanical systems (MEMS) devices. More specifically, the present invention relates to a Z-axis MEMS inertial sensor with enhanced offset stability over temperature performance and enhanced mechanical robustness.
Microelectromechanical Systems (MEMS) sensors are widely used in applications such as automotive, inertial guidance systems, household appliances, protection systems for a variety of devices, and many other industrial, scientific, and engineering systems. Such MEMS sensors are used to sense a physical condition such as acceleration, pressure, angular rotation, or temperature, and to provide an electrical signal representative of the sensed physical condition.
Capacitive-sensing MEMS designs are highly desirable for operation in both acceleration and angular rotation environments and in miniaturized devices, due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a two layer capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under Z-axis acceleration above a substrate. The accelerometer structure can measure two distinct capacitances to determine differential or relative capacitance.
Aspects of the disclosure are defined in the accompanying claims.
In a first aspect, there is provided an inertial sensor comprising a movable mass spaced apart from a surface of a substrate; a torsion element coupled to the movable mass and configured to enable motion of the movable mass about an axis of rotation in response to a force imposed upon the movable mass in a direction that is perpendicular to the surface of the substrate; and a suspension system configured to suspend the movable mass apart from the surface of the substrate. The suspension system comprises a first anchor attached to the substrate; a first folded spring having first and second spring ends, the first spring end being coupled to the first anchor; a second anchor attached to the substrate, each of the first and second anchors being displaced away from the axis of rotation; a second folded spring having third and fourth spring ends, the third spring end being coupled to the second anchor; and a beam connected to the movable mass via the torsion element, the beam having first and second beam ends, the first beam end being coupled to the second spring end of the first folded spring, and the second beam end being coupled to the fourth spring end of the second folded spring.
In a second aspect, there is provided an inertial sensor comprising a movable mass spaced apart from a surface of a substrate; a torsion element having first and second ends, the first end being coupled to the movable mass, the torsion element being configured to enable motion of the movable mass about an axis of rotation in response to a force imposed upon the movable mass in a direction that is perpendicular to the surface of the substrate; and a suspension system configured to suspend the movable mass apart from the surface of the substrate. The suspension system comprises a first anchor attached to the substrate; a first folded spring having first and second spring ends, the first spring end being coupled to the first anchor; a second anchor attached to the substrate, each of the first and second anchors being displaced away from the axis of rotation; a second folded spring having third and fourth spring ends, the third spring end being coupled to the second anchor; and a beam connected to the movable mass via the torsion element, the beam having first and second beam ends, the first beam end being coupled to the second spring end of the first folded spring, the second beam end being coupled to the fourth spring end of the second folded spring, the second end of the torsion element being attached to the beam at a midpoint of the beam between the first and second beam ends, and a longitudinal dimension of the beam extends on opposing sides of the axis of rotation, the longitudinal dimension being oriented perpendicular to the axis of rotation.
In a third aspect, there is provided an inertial sensor comprising a movable mass spaced apart from a surface of a substrate; first and second torsion elements coupled to the movable mass and configured to enable motion of the movable mass about an axis of rotation in response to a force imposed upon the movable mass in a direction that is perpendicular to the surface of the substrate; and a suspension system configured to suspend the movable mass apart from the surface of the substrate. The suspension system comprises first, second, third, and fourth anchors attached to the substrate, each of the first, second, third, and fourth anchors being displaced away from the axis of rotation; a first folded spring having first and second spring ends, the first spring end being coupled to the first anchor; a second folded spring having third and fourth spring ends, the third spring end being coupled to the second anchor; a third folded spring having fifth and sixth spring ends, the fifth spring end being coupled to the third anchor; a fourth folded spring having seventh and eighth spring ends, the seventh spring end being coupled to the fourth anchor; a first beam connected to the movable mass via the first torsion element, the first beam having first and second beam ends, the first beam end being coupled to the second spring end of the first folded spring, the second beam end being coupled to the fourth spring end of the second folded spring; and a second beam connected to the movable mass via the second torsion element, the second beam having third and fourth beam ends, the third beam end being coupled to the sixth spring end of the third folded spring, and the fourth beam end being coupled to the eighth spring end of the fourth folded spring, wherein a longitudinal dimension of each of the first and second beams is oriented perpendicular to the axis of rotation.
The accompanying figures in which like reference numerals refer to identical or functionally similar elements throughout the separate views, the figures are not necessarily drawn to scale, and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
In overview, the present disclosure concerns a microelectromechanical systems (MEMS) inertial sensor with enhanced offset stability over temperature performance and enhanced mechanical robustness under high-g shock environments. More particularly, the inertial sensor has a movable mass that rotates under Z-axis acceleration above a substrate. The inertial sensor includes anchors distributed on both sides of an axis of rotation and flexure compliance between the distributed anchors. The distributed anchor location and the flexure compliance between the anchors may enable the direction and magnitude of tilt of the movable mass due to warpage of the underlying substrate or offset caused by other process variations to be averaged out. Further, the distributed anchor locations and the flexure compliance between the anchors may effectively reduce the maximum principle stress on the movable mass in response to high-g shock environments (e.g., 30,000 g), relative to prior art central anchor designs. As such, the inertial sensor may have enhanced mechanical robustness under high-g shock environments. Still further, the distributed anchor locations and the flexure compliance between the anchors do not impact the torsional stiffness of the torsion elements that enable motion of the movable mass about an axis of rotation, and therefore do not adversely affect the sensitivity of Z-axis sensing of the inertial sensor.
The instant disclosure is provided to further explain in an enabling fashion at least one embodiment in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit in any manner the invention. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
It should be understood that the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Furthermore, some of the figures may be illustrated using various shading and/or hatching to distinguish the different elements produced within the various structural layers. These different elements within the structural layers may be produced utilizing current and upcoming microfabrication techniques of depositing, patterning, etching, and so forth. Accordingly, although different shading and/or hatching may be utilized in the illustrations, the different elements within the structural layers may be formed out of the same material.
Referring to
Proof mass 36 is adapted for rotational motion in response to acceleration 22 along an axis perpendicular to surface 26 of substrate 24, thus changing its position relative to the underlying first and second sense elements 28, 30. This rotational motion occurs about an axis of rotation 42 positioned between a first end 44 and a second end 46 of proof mass 36. In order to operate as a teeter-totter type accelerometer, a first section 48 of proof mass 36 on one side of axis of rotation 42 is formed with relatively greater mass than a second section 50 on the other side of axis of rotation 42. The greater mass of first section 48 may be created by offsetting axis of rotation 42 such that a first length 52 of first section 48 between axis of rotation 42 and first end 44 of proof mass 36 is greater than a second length 54 of second section 50 between axis of rotation 42 and second end 46 of proof mass 36.
First and second sense elements 28, 30 are symmetrically arranged relative to axis of rotation 42. That is, first and second sense elements 28, 30 are positioned equidistant from axis of rotation 42. The area of first section 48 beyond first sense element 28 to first end 44 is referred to herein as an asymmetric portion 56 of proof mass 34. The presence of asymmetric portion 56 results in the greater mass of first section 48 relative to second section 50. For simplicity, first section 48 of proof mass 36 having the greater mass may alternatively be referred to hereinafter has heavy-end 48 and second section 50 may thus be referred to hereinafter as light-end 50.
Due to the asymmetric configuration of first and second sections 48, 50, proof mass 36 can pivot about axis of rotation 42 in response to Z-axis acceleration 22. Inertial sensor 20 may detect or otherwise measure two distinct capacitances: SNS+ between first section 48 and first sense element 28 and SNS− between second section 50 and second sense element 30. The two capacitances, SNS+ and SNS−, can be used to determine differential or relative capacitance.
In this prior art configuration, suspension anchors 32, 34 and first and second torsion elements 38, 40 are symmetrically arranged relative to axis of rotation 42. More specifically, suspension anchors 32, 34 and first and second torsion elements 38, 40 are aligned with and positioned at axis of rotation 42. However, the greater first length 52 of heavy end 48 relative to second length 54 of light end 50 causes the center of gravity of proof mass 36 to be shifted away from the pivot axis (i.e., axis of rotation 42). As particularly observed in
Embodiments described below entail a structural configuration of suspension anchors in which the suspension anchors are displaced away from the axis of rotation, referred to herein as a split anchor configuration. In the split anchor configuration, the direction and magnitude of tilt due to substrate warpage and/or process variations is averaged out or otherwise significantly reduced to yield enhanced offset over temperature performance and a mechanically robust design.
Referring now to
In the illustrated configuration of
First folded spring 90 has first and second spring ends 108, 110, in which first spring end 108 is coupled to first anchor 82 and second spring end 110 is coupled to a first beam end 112 of first beam 98. Second folded spring 92 has third and fourth spring ends 114, 116, in which third spring end 114 is coupled to second anchor 84 and fourth spring end 116 is coupled to a second beam end 118 of first beam 98. Similarly, third folded spring 94 has fifth and sixth spring ends 120, 122, in which fifth spring end 120 is coupled to third anchor 86 and sixth spring end 122 is coupled to a third beam end 124 of second beam 100. And fourth folded spring 96 has seventh and eighth spring ends 126, 128, in which seventh spring end 126 is coupled to fourth anchor 88 and eighth spring end 128 is coupled to a fourth beam end 130 of second beam 100. As such, each of first and second beams 98, 100 extends on opposing sides of axis of rotation to suitably interconnect with respective first, second, third, and fourth folded springs 90, 92, 94, 96.
Coupler 102 is positioned at and aligned with axis of rotation 80. Coupler 102 includes first and second coupler ends 132, 134. A first midpoint 136 of first beam 98 between first and second beam ends 112, 118 is connected to first coupler end 132 and a second midpoint 138 of second beam 100 between third and fourth beam ends 124, 130 is connected to second coupler end 134. Additionally, first torsion element 76 has a first end 140 attached to proof mass 72 and a second end 142 attached to first coupler end 132 of coupler 102. Likewise, second torsion element 78 has a third end 144 coupled to proof mass 72 and a fourth end 146 attached to second coupler end 134 of coupler 102. Accordingly, first and second torsion elements 76, 78 interconnect proof mass 72 with suspension system 74 so that proof mass 72 is suspended above substrate 64.
Proof mass 72 is adapted for rotational motion in response to acceleration 22 (
As mentioned previously, first and third anchors 82, 86 are positioned at first side 104 of axis of rotation and second and fourth anchors 84, 88 are positioned at second side 106 of axis of rotation. In some embodiments, first and third anchors 82, 86 are displaced away from axis of rotation 80 by a first distance 156 and second and fourth anchors 84, 88 are displaced away from axis of rotation 80 by a second distance 158 that is substantially equivalent to first distance 156. Additionally, proof mass 72 is defined by a midline 160 oriented perpendicular to axis of rotation 80. In some embodiments, first and second anchors 82, 84 are positioned at a third side 162 of midline 160 and are displaced away from midline 160 by a third distance 164. Third and fourth anchors 86, 88 are positioned at a fourth side 166 of midline 160 opposing third side 162 and are displaced away from midline 160 by a fourth distance 168 that is substantially equivalent to third distance 164. Thus, in some embodiments, first, second, third, and fourth anchors 82, 84, 86, 88 are symmetrically arranged relative to axis of rotation 80 and midline 160.
In the illustrated example of
As shown, first beam 98 extends on opposing sides of axis of rotation 80, and a longitudinal dimension 170 of first beam 98 is oriented perpendicular to axis of rotation 80. First and second anchors 82, 84 reside on opposing sides of axis of rotation, and may thus be considered a split anchor design in lieu of the on-axis, central anchor design of
Referring concurrently to
The direction and magnitude of tilt of proof mass 72 is generally averaged out since the distributed anchors (e.g., first, second, third, and fourth anchors 82, 84, 86, 88) are positioned over a relatively large area and the flexures (e.g., first, second, third, and fourth folded springs 90, 92, 94, 96) between the anchors are compliant. By averaging out, or otherwise reducing the direction and magnitude of tilt of proof mass 72, better offset over temperature performance may be achieved.
Referring to
Offset can be caused by other process variations even if there is no substrate warpage or in addition to substrate warpage. In this example, the misalignment of connecting material 172 and the polysilicon material layer 174 could result in asymmetric deformation caused by CTE mismatch between connecting material 172 and polysilicon material layer 174 of second anchor 84. With the distributed anchor configuration of first, second, third, and fourth anchors 82, 84, 86, 88 (
Suspension system 194 includes first, second, third, and fourth anchors 202, 204, 206, 208, first, second, third, and fourth folded springs 210, 212, 214, 216, first and second beams 218, 220, and a coupler 222. First, second, third, and fourth anchors 202, 204, 206, 208 are attached to substrate 184 and each of first, second, third, and fourth anchors 202, 204, 206, 208 is displaced away from axis of rotation 200. More particularly, first and third anchors 202, 206 are positioned at a first side 224 of axis of rotation 200 and second and fourth anchors 204, 208 are positioned at a second side 226 of axis of rotation 200 opposing first side 224.
First folded spring 210 has first and second spring ends 228, 230, in which first spring end 228 is coupled to first anchor 202 and second spring end 230 is coupled to a first beam end 232 of first beam 218. Second folded spring 212 has third and fourth spring ends 234, 236, in which third spring end 234 is coupled to second anchor 204 and fourth spring end 236 is coupled to a second beam end 238 of first beam 218. Similarly, third folded spring 214 has fifth and sixth spring ends 240, 242, in which fifth spring end 240 is coupled to third anchor 206 and sixth spring end 242 is coupled to a third beam end 244 of second beam 220. And fourth folded spring 216 has seventh and eighth spring ends 246, 248, in which seventh spring end 246 is coupled to fourth anchor 208 and eighth spring end 248 is coupled to a fourth beam end 250 of second beam 220. As such, each of first and second beams 218, 220 extends on opposing sides of axis of rotation 200 to suitably interconnect with respective first, second, third, and fourth folded springs 210, 212, 214, 216.
In the illustrated embodiment, the design of first, second, third, and fourth folded springs 210, 212, 214, 216 of inertial sensor 180 differs from first, second, third, and fourth folded springs 90, 92, 94, 96 (
Coupler 222 is positioned at and aligned with axis of rotation 200. Coupler 222 includes first and second coupler ends 252, 254. A first midpoint 256 of first beam 218 between first and second beam ends 232, 238 is connected to first coupler end 252 and a second midpoint 258 of second beam 220 between third and fourth beam ends 244, 250 is connected to second coupler end 254.
In the illustrated embodiment, each of first and second torsion elements 196, 198 is a “T-shaped” structure having a beam 262 oriented perpendicular to axis of rotation 200 and a beam 264 coupled to a midpoint of beam 262 and oriented parallel to axis of rotation 200. Opposing ends 266, 268 of beam 262 of first torsion element 196 are attached to proof mass 192 and an end 270 of beam 264 of first torsion element 196 is attached to first coupler end 252 of coupler 222. Likewise, opposing ends 266, 268 of beam 262 of second torsion element 198 are attached to proof mass 192 and end 270 of beam 264 of second torsion element 198 is attached to second coupler end 254 of coupler 222. Accordingly, first and second torsion elements 196, 198 interconnect proof mass 192 with suspension system 194 so that proof mass 192 is suspended above substrate 184.
In the illustrated embodiment, first beam 218 has an elongate opening 272 aligned with a longitudinal dimension 274 of first beam 218 and centered between first and second beam ends 232, 238. Likewise, second beam 220 has an elongate opening 276 aligned with longitudinal dimension 274 of second beam 220 and centered between third and fourth beam ends 244, 250. Thus, on opposing longitudinal edges of elongate openings 272, 276, the remaining material portions of each of first and second beams 218, 220 is relatively thinner, thus more compliant then the remainder of first and second beams 218, 220. The combination of the “T-shaped” structure of each of first and second torsion elements 196, 198 and the compliancy gained in first and second beams 218, 220 by the presence of elongate openings 272, 276 may provide additional compliance in the vertical direction (e.g., parallel to the Z-axis) to provide some stress relief during a high-g shock event. Thus, such a structural configuration may provide inertial sensor 180 with enhanced protection from damage during a high-g shock event.
The configurations of the suspension systems of inertial sensors 60 (
Embodiments described herein entail microelectromechanical systems (MEMS) inertial sensors with enhanced offset stability over temperature performance and enhanced mechanical robustness under high-g shock environments. More particularly, an inertial sensor has a movable mass that rotates under Z-axis acceleration above a substrate. The inertial sensor includes anchors distributed on both sides of an axis of rotation and flexure compliance between the distributed anchors. The distributed anchor location and the flexure compliance between the anchors may enable the direction and magnitude of tilt of the movable mass due to warpage of the underlying substrate or offset caused by other process variations to be averaged out. Further, the distributed anchor locations and the flexure compliance between the anchors may effectively reduce the maximum principle stress on the movable mass in response to high-g shock environments (e.g., 30,000 g), relative to prior art central anchor designs. As such, the inertial sensor may have enhanced mechanical robustness under high-g shock environments. Still further, the distributed anchor locations and the flexure compliance between the anchors do not impact the torsional stiffness of the torsion elements that enable motion of the movable mass about an axis of rotation, and therefore do not adversely affect the sensitivity of Z-axis sensing of the inertial sensor.
This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.