The present invention provides a high-accuracy low-noise MEMS accelerometer by using at least two symmetric out-of-plane proof masses for both out-of-plane and in-plane axes. Movement of the proof masses in one or more in-plane sense axes is measured by comb capacitors with mirrored comb electrodes that minimise cross-axis error from in-plane movement of the proof mass out of the sense axis of the capacitor. The two out-of-plane proof masses rotate in opposite directions, thus maintaining their combined centre of mass at the centre of the accelerometer even as they rotate out of plane.

TECHNICAL FIELD

The present invention relates to MEMS (microelectromechanical systems) accelerometers, in particular to an accelerometer designed to reduce noise in the accelerometer output.

BACKGROUND

When used to support autonomous driving, MEMS accelerometers must be highly accurate and have low noise in their outputs. High accuracy accelerometers, such as those used in electronic stability control systems, generally require independence of each of the sense axes—i.e. an individual proof mass for each sense axis—in order to reduce inaccuracies that result from cross-axis interference. However, the use of individual proof masses for the sense axes reduces the maximum size of each individual proof masses since MEMS systems are generally subject to significant size constraints. As a result of the smaller proof masses, the accelerometer is more susceptible to noise. Other MEMS accelerometers, such as those used in smartphones, often use a single proof mass for multiple axes in order to keep the complexity of the system (and therefore cost) down. While this enables a larger proof mass to be used, these accelerometers are highly susceptible to cross-axis error and parasitic modes of movement of the proof mass, which significantly reduce the accuracy of these accelerometers.

SUMMARY OF THE INVENTION

The present invention provides a high-accuracy low-noise MEMS accelerometer by using the at least two symmetric out-of-plane proof masses—preferably see-saw type proof masses—for both the out-of-plane and in-plane axes. Movement of the proof masses in one or more in-plane sense axes is measured by comb capacitors with mirrored comb electrodes that minimise cross-axis error from in-plane movement of the proof mass out of the sense axis of the capacitor. The two out-of-plane proof masses rotate in opposite directions, thus maintaining their combined centre of mass at the centre of the accelerometer even as they rotate out of plane. As a result, external acceleration does not produce a torque on the combined proof masses, reducing parasitic modes of movement and improving accuracy. Furthermore, the total mass of the out-of-plane proof axes can be used to provide highly accurate low-noise detection of acceleration in the in-plane axes. Thus, for a given size of MEMS accelerometer, the out-of-plane proof masses can be larger because space does not have to be sacrificed to provide separate in-plane proof masses. This enables a smaller accelerometer with the same accuracy/noise performance, or a similarly sized accelerometer with improved accuracy/noise performance.

More specifically, the invention provides a MEMS accelerometer, which comprises:a substrate, which defines a substrate plane;at least two proof masses, wherein both proof masses are configured to rotate out of parallel to the substrate plane and move parallel to the substrate plane and wherein the combined centre of mass of the at least two proof masses is at the centre of the accelerometer;first sense circuitry configured to sense movement of the at least two proof masses parallel to the substrate plane; andsecond sense circuitry configured to sense rotation of the at least two proof masses out of parallel to the substrate plane.

The centre of mass of each proof mass is preferably offset from its axis of rotation such that external acceleration in the out-of-plane axis acts upon the asymmetric mass of each proof mass to cause rotation of the proof mass about the axis of rotation and external acceleration in a first axis parallel to the substrate plane acts upon the asymmetric mass and symmetric mass of both proof masses to cause movement of the proof masses parallel to the substrate plane.

The second sense circuitry may comprise one or more moveable electrodes located on each out-of-plane proof mass and stationary electrodes that are in a fixed position relative to the substrate, wherein the moveable electrode and the stationary electrode form a capacitor, the capacitance of which changes as the out-of-plane proof mass rotates about its rotation axis.

The MEMS accelerometer may further comprise at least one in-plane structure configured to move parallel to the substrate plane, wherein the at least two proof masses are rotatably connected to the at least one in-plane structure such that movement of the at least two proof masses parallel to the substrate plane causes movement of the at least one in-plane structure parallel to the substrate plane.

The at least one in-plane structure is preferably anchored to the substrate via springs at one or more anchor points, wherein the springs allow movement of the at least one in-plane proof mass parallel to the substrate plane and resist movement of the at least one in-plane proof mass out of the substrate plane.

The centre of mass of the at least one in plane structure may be advantageously located at the centre of the accelerometer.

The one or more anchor points may be located symmetrically with respect to the combined centre of mass of the at least one in-plane structure and at least two out-of-plane proof masses such that linear acceleration of the MEMS accelerometer produces no overall torque on the combined at least one in-plane structure and at least two proof masses. The one or more anchor points may be advantageously located at or close to the combined centre of mass of the at least one in-plane structure and at least two proof masses.

The at least two proof masses may be connected to the at least one in-plane structure via torsion springs, wherein the torsion springs allow rotation of the at least two proof masses out of parallel to the substrate plane and resist movement of the at least two proof masses parallel to the substrate plane relative to the at least one in-plane structure. The at least two out-of-plane proof masses are preferably located towards the exterior of the MEMS accelerometer relative to the at least one in-plane structure.

The out of plane proof masses may comprise two C-shaped seesaw proof masses which extend around the exterior of the at least one in-plane mass, such that the two C-shaped out-of-plane proof masses are arranged as mirror-images and the proof masses is configured to rotate in opposite direction in response to an external acceleration in the out-of-plane axis. Advantageously, the two C-shaped out-of-plane proof masses may be coupled by at least two springs, wherein at least one spring is disposed at each end of the C shape, such that the springs allow rotation of the two C-shaped out-of-plane proof masses in opposite directions but resist rotation of the two C-shaped out-of-plane proof masses in the same direction.

The first sense circuitry may comprises comb capacitors, wherein one or more stationary electrodes of each comb capacitor are anchored to the substrate and one or more moveable electrodes of each comb capacitor are connected to the at least one in-plane structure, and wherein the MEMS accelerometer is configured to measure movement of the at least two out-of-plane proof masses and at least one in-plane proof mass using differential capacitive measurements.

Each of said comb capacitors may comprise:a first set of moveable comb teeth that extend away from the at least one in-plane structure in a first direction along a capacitor axis which is parallel to the substrate plane;a second set of moveable comb teeth that extend away from the at least one in-plane structure in a second direction, opposite the first direction, along the capacitor axis;a first set of stationary comb teeth opposite to and interdigitated with the first set of moveable comb teeth, wherein the first set of stationary comb teeth extend towards the at least one in-plane structure in the second direction; anda second set of stationary comb teeth opposite to and interdigitated with the second set of moveable comb teeth, wherein the second set of stationary comb teeth extend towards the at least one in-plane structure in the first direction;
wherein movement of the at least one in-plane structure and at least two proof masses in the first direction causes the first set of moveable comb teeth and first set of stationary comb teeth to move closer together and causes the second set of moveable comb teeth and second set of stationary teeth to move further apart, and wherein movement of the at least one in-plane structure and at least two proof masses in the second direction causes the first set of moveable comb teeth and first set of stationary comb teeth to move further apart and the second set of moveable comb teeth and second set of stationary teeth to move closer together.

Movement of the at least one in-plane structure and at least two proof masses in the first direction may advantageously cause the first set of moveable comb teeth and first set of stationary comb teeth to move closer together by a first distance and causes the second set of moveable comb teeth and second set of stationary teeth to move further apart by the first distance, and wherein movement of the at least one in-plane structure and at least two proof masses in the second direction causes the first set of moveable comb teeth and first set of stationary comb teeth to move further apart by a second distance and the second set of moveable comb teeth and second set of stationary teeth to move closer together by the second distance.

The second sense circuitry preferably comprises eight electrodes disposed on the see-saw proof masses and eight electrodes disposed above or below the see-saw proof masses forming eight gap detection capacitors, each capacitor being formed from one of the see-saw proof mass electrodes and one of the electrodes disposed above or below the see-saw proof masses, wherein:four electrodes are located on each proof mass;on each proof mass, a first pair of electrodes is located at a first end of the C-shape of the see-saw proof mass and a second pair of electrodes is located at a second end of the C-shape of the see-saw proof mass; andwithin each pair of electrodes, a first electrode is located towards the middle of the C-shape of the see-saw proof mass from the rotation axis of the see-saw proof mass and a second electrode is located towards the end of the C-shape of the see-saw proof mass from the rotation axis of the see-saw proof mass.

Advantageously, acceleration of the accelerometer in the Z direction is measured from the changes in capacitance of the gap detection capacitors using a double differential measurement.

DETAILED DESCRIPTION

FIG. 1shows a schematic drawing of a 2-axis MEMS accelerometer according to the present invention. The accelerometer includes two out-of-plane proof masses,101and102, which are preferably see-saw type proof masses. The see-saw proof masses101,102are rotatably coupled to an in-plane structure111, which is also referred to as an in-plane proof mass, along rotation axes RA1and RA2.

The in-plane structure111and the see-saw proof masses101,102(when at rest) generally lie in a plane referred to as the substrate plane. As is known in MEMS manufacturing, MEMS devices are largely formed by removing material from a layer of material, e.g. silicon, which is referred to as the substrate, to produce the structures such as those depicted and described herein. The “substrate plane” is a geometric plane that intersects the substrate or some/all of the components that have been formed from the substrate parallel to the upper and lower surfaces of the original substrate layer. The substrate plane is therefore parallel to the plane of the page shown inFIG. 1, and may lie above the upper surface of the remainder of the substrate layer following formation of the MEMS features from the substrate. Where the term “in-plane” is used in this disclosure, it is intended to mean an orientation, axis, direction or movement that is oriented in or parallel to the substrate plane. Similarly, the term “out-of-plane” is intended to mean an orientation, axis, direction or movement that has some component in a direction perpendicular to, i.e. out of, the substrate plane. Another way to define the substrate plane is the plane in which the centres of mass of all of the proof masses lie when the accelerometer is at rest.

The see-saw proof masses101,102are connected to the in-plane structure111via torsion springs, which allow rotation of the see-saw proof masses101,102out of the substrate plane about rotation axes RA1, RA2. The see-saw proof masses101,102are roughly C- or U-shaped and are arranged such that see-saw proof mass101is a mirror image of see-saw proof mass102and the proof masses101,102extend around the outside of and enclose the in-plane structure111. The see-saw proof masses101,102are equally sized.

The rotor rotation axes RA1and RA2are positioned within the substrate plane along with the centres of mass of the see-saw proof masses101,102. However, the centre of mass of each see-saw proof mass101,102is offset from its rotation axis RA1, RA2in the substrate plane. In other words, more of the see-saw proof mass101,102lies on one side of the rotation axis RA1, RA2than on the other side. In this way, the see-saw proof masses101,102each define an asymmetric mass and a symmetric mass. The symmetric mass is the double the mass of the smaller side of the see-saw proof mass101,102(i.e. the mass of the smaller side plus an equal mass from the large side which balances the smaller side). The asymmetric mass is the remaining mass of the see-saw proof mass101,102, i.e. the part of the proof mass that is acted upon by the external acceleration (i.e. the acceleration applied to the accelerometer package which is to be measured) to cause rotation of the see-saw proof mass101,102about the rotation axis RA1, RA2. The combined centre of mass of both see-saw proof masses101,102is located at the centre of the accelerometer, along with the centre of mass of the in-plane structure111.

The see-saw proof masses101,102are coupled together by springs121and122, which are located at the adjacent ends of the see-saw proof masses101,102. Springs121and122permit relative movement of the ends of the see-saw proof masses101,102away from each other parallel to the substrate plane, but resist relative movement of the ends of the see-saw proof masses101,102away from each other perpendicular to the substrate plane. In this way, the see-saw proof masses101,102are generally free to rotate about rotation axes RA1and RA2in opposite directions, i.e. see-saw proof mass101rotates clockwise and see-saw proof mass102rotates anti-clockwise, or vice versa, since this causes relative motion of the ends of the see-saw proof masses101,102away or towards each other parallel to the substrate plane. However, rotation of the see-saw proof masses101,102in the same direction is resisted by the springs121,122, since such rotation causes relative motion of the ends of the see-saw proof masses101,102perpendicular to the substrate plane. This coupling prevents the see-saw proof masses101,102from moving in response to external angular acceleration, in which case the see-saw proof masses101,102would rotate in the same direction, while still allowing the see-saw proof masses101,102to move in response to linear acceleration perpendicular to the substrate plane, which causes the see-saw proof masses101,102to rotate in opposite directions.

The in-plane structure111is a rigid structure connected via springs131,132to anchor points141,142on the substrate. The anchor points and springs are arranged such that the centre of mass of the in-plane structure111and the combined centre of mass of the see-saw proof masses101,102are located at the centre of the accelerometer and the centre of the anchor points. In this way, out-of-plane accelerations do not produce a torque on the in-plane structure111or the combined see-saw proof masses101,102. The springs131,132permit movement of the in-plane structure111along an axis, labelled X, that lies in the substrate plane. Preferably, the springs131,132resist any movement of the in-plane structure111that is not along the X-axis, i.e. perpendicular to the X-axis within the substrate plane, and any movement out of the substrate plane, including rotation. The torsion springs connecting the see-saw proof masses101,102to the in-plane structure11resist motion of the see-saw proof masses101,102relative to the in-plane structure111along any direction in the substrate plane. Thus, the see-saw proof masses101,102can also move along the X-axis along with the in-plane structure in response to acceleration of the accelerometer along the X-axis. Importantly, the combined mass of the symmetric and asymmetric masses of the see-saw proof masses101,102and the in-plane structure are acted upon by external acceleration to cause movement of the see-saw proof masses101,102and in-plane structure111relative to the substrate along the X-axis. The amount of noise in the accelerometer output is inversely proportional to the mass of the proof mass, therefore a higher proof mass for the in-plane X axis reduces the noise in the accelerometer output. Furthermore, for a given package size of MEMS accelerometer, the out-of-plane proof masses can be larger because space does not have to be sacrificed to provide separate in-plane proof masses. This enables a similarly sized accelerometer with improved accuracy/noise performance or a smaller accelerometer with the same accuracy/noise performance.

Movement of the see-saw proof masses101,102and/or the in-plane structure111is measured capacitively. Rotation of the see-saw proof masses101,102can be measured by gap detection capacitors located above or below the see-saw proof masses101,102(not shown inFIG. 1), or can be measured by comb capacitors with moveable electrodes located on the see-saw proof masses101,102and stationary electrodes located on the in-plane structure111(also not shown inFIG. 1). Preferably, gap-detection capacitors are formed between each of the electrodes171-174and181-184, disposed on the see-saw proof masses101,102, and counterpart electrodes formed on the substrate or cap wafer (not shown) above or below the see-saw proof masses101,102. The electrodes171-174are located on the first see-saw proof mass101, with a pair of electrodes171,172located at a first end of the C-shape of the see-saw proof mass101and a second pair of electrodes173,174located at a second end of the C-shape of the see-saw proof mass101. Within each pair171,172and173,174, one electrode171,173is located towards the middle of the C-shape of the see-saw proof mass101from the rotation axis RA1; the other electrode172,174is located towards the end of the C-shape of the see-saw proof mass101from the rotation axis RA1. Similarly, the electrodes181-184are located on the second see-saw proof mass102, with a pair of electrodes181,182located at a first end of the C-shape of the see-saw proof mass102and a second pair of electrodes183,184located at a second end of the C-shape of the see-saw proof mass102. Within each pair181,182and183,184, one electrode182,184is located towards the middle of the C-shape of the see-saw proof mass102from the rotation axis RA2; the other electrode181,183is located towards the end of the C-shape of the see-saw proof mass102from the rotation axis RA2.

In ideal conditions, when the capacitors formed from electrodes171-174and181-184are used to measure movement of the see-saw proof masses101,102out of parallel to the substrate plane (i.e. orthogonal to the substrate plane), in-plane movement of the see-saw proof masses101,102does not affect the out-of-plane capacitance measurement as long as the area of overlap between the electrodes171-174,181-184and the corresponding electrodes above/below the see-saw proof masses101,102does not change, i.e. if the electrodes171-174,181-184or plate electrodes above and/or below the see-saw proof masses101,102cover a large enough area.

In practice, alignment of the electrodes171-174,181-184and the corresponding electrodes above/below the see-saw proof masses101,102may not be perfectly parallel, e.g. due to imperfect alignment of the cap wafer during manufacturing or stress on the device during operation. Under such conditions, in-plane movement of the proof masses101,102may cause changes in the individual capacitances measured by each of the individual capacitors formed from electrodes171-174and181-184. However, taking a single and double differential measurements of the capacitance changes cancels out any such changes.

For example, if the counterpart electrodes are located on the cap wafer and the cap wafer is out of alignment so that the cap wafer electrodes on the left hand side are closer to the see-saw proof mass electrodes171,173than the cap wafer electrodes are to the see-saw proof mass electrode182,184, then movement of the see-saw proof masses in the X direction would result in a change of capacitance as the electrodes moved closer together or further apart due to the mis-alignment. Similarly, if the counterpart electrodes are located on the cap wafer and the cap wafer is out of alignment so that the cap wafer electrodes on the bottom side are closer to the see-saw proof mass electrodes173,174,183,184than the cap wafer electrodes are to the see-saw proof mass electrodes171,172,181,182, then movement of the see-saw proof masses in the Y direction would result in a change of capacitance as the electrodes moved closer together or further apart due to the mis-alignment. These effects would produce an unwanted signal indicating fictitious acceleration in the Z direction.

However, the capacitance can be measured according to the following formula:
(C171−C172)+(C173−C174)+(C182−C181)+(C184−C183)
Where C171indicates the change in capacitance of the capacitor formed from the electrode171and its counterpart electrode above/below the see-saw proof mass101. Within each single differential, e.g. C171−C172, any change in capacitance due to movement of the see-saw proof masses101,102in the Y direction is cancelled out, since the capacitance of the capacitor formed from electrode171is equally affected by the Y direction movement as the capacitor formed from electrode172.

Movement of the see-saw proof masses101,102in the X direction causes equal difference in the change in capacitance between the capacitors formed by each of electrodes171and172and in the change in capacitance between the capacitors formed by each of electrodes181and182. However, each double differential, i.e. (C171−C172)+(C182−C181) can be re-written as (C171−C172)−(C181−C182), thus the error caused by movement in the X direction is also cancelled out.

Thus the arrangement of the electrodes171-174and181-184on either side of the rotation axes RA1and RA2, coupled with the opposite directions of rotation of each of the see-saw proof masses101,102allows for any unwanted change in capacitance of the capacitors used for Z axis sensing to be automatically and efficiently cancelled out.

Movement of the in-plane structure111and the see-saw proof masses101,102along the X-axis is measured by comb capacitors, with moveable electrodes located on the in-plane structure111and stationary electrodes anchored to the substrate (at anchor points151-154) and located interior to the in-plane structure111. While an accelerometer including both the arrangement of proof masses described above and the comb capacitor arrangement described below is particularly advantageous, it will be appreciated that both features can be advantageously used independently.

FIG. 1shows two comb capacitors for measuring movement of the in-plane structure111and see-saw proof masses101,102along the X-axis. Each comb capacitor is made up of two pairs of electrodes161/162and163/164. A first moveable electrode in each pair includes comb teeth that extend away from the in-plane structure111in a first direction parallel to the substrate plane and perpendicular to the X-axis, towards the middle of the accelerometer. The comb teeth of a second moveable electrode extend from the opposite side of the in-plane structure111and extend away from the in-plane structure in a second direction, opposite the first direction, towards the middle of the accelerometer. A first stationary electrode is attached to anchor point151located close to or at the middle of the accelerometer. The first stationary electrode's comb teeth extend towards the in-plane structure111and are interdigitated with the comb teeth of the first moveable electrode. A second stationary electrode is attached to anchor point152, also located close to or at the middle of the accelerometer, and its comb teeth extend towards the in-plane structure111and are interdigitated with the comb teeth of the second moveable electrode. WhileFIG. 1depicts separate anchor points151,152for the two stationary electrodes, it is also possible for the two electrodes to share a common anchor point. The two sets of electrodes161and162form a single capacitor that has mirror symmetry between the stationary electrodes.

This arrangement of electrodes means that movement of the in-plane structure111and see-saw proof masses101,102in the first direction (i.e. down the page, as shown inFIG. 1) causes the first moveable electrode and first stationary electrode to move closer together in the direction parallel to the comb fingers, and causes the second moveable electrode and second stationary electrode to move further apart by the same amount. Thus, the increase in capacitance caused by the increased area of overlap between the first electrodes161is offset by an equal decrease in capacitance caused by the decreased area of overlap between the second electrodes162. Similarly, movement of the in-plane structure111and see-saw proof masses101,102in the second direction (i.e. up the page, as shown inFIG. 1) causes the area of overlap between the first moveable electrode and first stationary moveable electrode to decrease and causes the area of overlap between the second moveable electrode and second stationary electrode to increase by the same amount. The decrease in capacitance caused by the decreased overlap between the first electrodes161is offset by an equal increase in capacitance caused by the increased overlap between the second electrodes162. In this way, the accelerometer shown inFIG. 1passively compensates for cross-axis error, i.e. changes in the capacitance caused by movement perpendicular to the X-axis: the capacitance of the capacitor formed from electrodes161/162increases or decreases as the in-plane structure111and see-saw proof masses101,102moves along the X-axis (i.e. towards the right of the page, as shown inFIG. 1) as the interdigitated comb teeth of the moveable and stationary electrodes get closer together or further apart, but does not significantly change due to movement perpendicular to the X-axis. The second capacitor formed from electrodes163,164and anchor points153,154is formed in the same manner as the first capacitor formed from electrodes161,162and anchor points151,152; however, the second capacitor is a mirror image of the first capacitor about an axis perpendicular to the X-axis, such that as the comb teeth of the first capacitor move closer together due to movement along the X-axis, the comb teeth of the second capacitor move further apart and vice versa. This enables differential capacitive measurements to be used to determine the extent of the movement of the in-plane structure111and see-saw proof masses101,102, and therefore determine the external acceleration.

FIG. 2shows a schematic drawing of a 3-axis accelerometer according to the present invention. See-saw proof masses201and202are positioned and function in the same manner as described above with respect to see-saw masses101,102inFIG. 1. In contrast to the single in-plane structure111of the two-axis accelerometer ofFIG. 1, the accelerometer ofFIG. 2includes a first in-plane structure211and a second in-plane structure212to which the see-saw proof masses201,202are connected. The first211and second in-plane structures212are suspended from the substrate via springs231and233and rigid connecting portion232. The springs231are flexible in the Y-direction but rigid in the X-direction, while springs233are flexible in the X-direction and rigid in the Y-direction. Both springs231and233are rigid in the out-of-plane direction. The two springs231and233are connected by rigid connecting portion232. Spring233is connected at the other end to rigid support structure241, which is in turn anchored to the substrate at or close to the centre of the accelerometer. This same structure of springs231,233and rigid connection portion232is repeated around the accelerometer. In this way, the combined structure of springs231and233and rigid connecting structure232allows the first and second in-plane structures to move relative to the substrate along the X and Y-axes shown inFIG. 2while resisting movement of the in-plane structures211and212out of the substrate plane. Furthermore, since the first and second in-plane structures211,212are both connected to both see-saw proof masses201,202, the combined structure of first and second in-plane structures211,212and see-saw proof masses201,202moves together in the X- and Y-directions.

Anchor points250, to which stationary capacitor electrodes of electrode pairs261-268and the rigid support structures241,242are fixed, are located at or close to the centre of the accelerometer. As with the two-axis accelerometer ofFIG. 1, the anchor points250and rigid support structures241,242are arranged such that the combined centre of mass of the in-plane structures211and212and the combined centre of mass of the see-saw proof masses201,202is located such that out-of-plane accelerations do not produce a torque on the combined in-plane structures211,212or the combined see-saw proof masses201,202. Furthermore, by positioning the anchor points250close together, close to the centre of the accelerometer, the accelerometer is less sensitive to mechanical deformations of the substrate, e.g. caused by temperature changes.

The three-axis accelerometer depicted inFIG. 2has four comb capacitors, each made up of two-pairs of interdigitated comb electrodes as described above with respect toFIG. 1. A first capacitor, which is configured to measure motion in the X direction is made up of electrode pairs261and267. A second capacitor, which is configured to measure motion in Y direction, is made up of electrode pairs262and268. A third capacitor, which is configured to measure motion in the X direction, is made up of electrode pairs263and265. A fourth capacitor, which is configured to measure motion in the Y direction, is made up of electrode pairs264and266. As described above with respect toFIG. 1, each of the arrangement and orientation of these electrodes is such that, for each capacitor, as the moveable electrode and stationary electrode of one pair of electrodes move closer together, the moveable electrode and stationary electrode of the other pair of electrodes moves further apart by the same amount. Motion of the see-saw proof masses201,202and the in-plane structures211,212is shown inFIG. 3.

In the same way as for the accelerometer depicted inFIG. 2, the electrodes271-274and281-284for measuring rotation of the see-saw proof masses101,102are arranged such that movement of the see-saw proof masses101,102in the X-Y plane relative to the substrate either does not cause a change in capacitance of the Z-axis sense capacitors, or if it does, the changes in capacitance is cancelled out in differential or double differential measurement. The description above with respect to electrodes171-174and181-184applies equally to this embodiment.

FIG. 3shows a perspective view of the three-axis accelerometer depicted inFIG. 2, demonstrating the movement of the see-saw proof masses201,202. WhileFIG. 3depicts the in-plane structures211and212corresponding to the accelerometer ofFIG. 2, it will be appreciated that the see-saw proof masses101,102of the accelerometer shown inFIG. 1are configured to move in the same manner as the see-saw proof masses201,202of the accelerometer shown inFIG. 2.