Patent Description:
An accelerometer is a type of microelectromechanical systems (MEMS) device which measures acceleration. Typically, the MEMS accelerometer includes, among other component parts, a movable element, also referred to as a proof mass coupled to a compliant spring structure to allow movement of the proof mass in a linear direction. When the MEMS accelerometer experiences an acceleration, the proof mass also experiences the acceleration and moves as a result of the acceleration. The motion of the proof mass may then be converted into an electrical signal having a parameter magnitude (e.g., voltage, current, frequency, etc.) that is proportional to the movement of the proof mass such as the acceleration of the proof mass.

In some instances, the MEMS accelerometer experiences harsh accelerations or a high force which causes the proof mass to move beyond a desired distance or exhibit unstable behavior. Such movement or behavior can potentially damage the MEMS accelerometer. To reduce such damage, many MEMS accelerometers include one or more distance limiters, typically referred to as over travel stops anchored to a substrate. These over-travel stops allow the proof mass to travel a certain distance referred to as a stop gap before movement of the proof mass is limited by the over-travel stops so that the proof mass does not move beyond the desired distance. <CIT> discloses a micromechanical sensor device comprising: at least one movable detection element; and a stop device for the detection element, the stop device being mechanically and electrically connected to the detection element, the stop device being designed such that in the event of movement of the detection element a stop area of the stop device is moved in the opposite direction to the detection element.

According to the invention there is provided microelectromechanical systems (MEMS) accelerometer and a method of operating a MEMS accelerometer, as defined by the appended claims.

The drawings are for the purpose of illustrating example embodiments, but it is understood that the embodiments are not limited to the arrangements and instrumentality shown in the drawings.

A stop gap is a distance that a proof mass is allowed to travel, or move, until an overtravel stop which is anchored to a substrate stops the proof mass from moving. The stop gap is typically a minimum etch size of a semiconductor process associated with fabricating the MEMS accelerometer and the proof mass is allowed to travel no more than the distance of the stop gap defined by the minimum etch size before the over travel stop stops the proof mass from moving.

If a stop gap is too large due to process limitations, then the proof mass might accelerate to a velocity such that contact with the overtravel stop results in chipping of the proof mass, chipping of the overtravel stop, or chipping of both the proof mass and the overtravel stop. Embodiments disclosed herein are directed to forming a stop gap which is smaller than or equal to and defined by the minimum etch size of the semiconductor process. In an example, the stop gap is controlled without changing a pitch of the semiconductor process. The proof mass is coupled to a compliant spring structure in the MEMS accelerometer which allows movement of the proof mass in a certain direction (e.g., linear direction) during an acceleration of the MEMS accelerometer. The compliant spring structure has a tension or stiffness k which causes the proof mass to move back to an initial position when the MEMS accelerometer is no longer subject to an acceleration. An over travel stop in the form of an extension to the compliant spring structure moves in the opposite direction in response to the motion of the proof mass when subject to an acceleration. The extension has a length which is sizeable to control a size of the stop gap and when the extension contacts the proof mass to stop the motion of the proof mass. The extension is anchored to a substrate via the complaint spring structure so that the extension stops the motion of the proof mass when the extension contacts the proof mass. The movement of the proof mass in one direction from a rest position and responsive movement of extension in the opposite direction to the movement of the proof mass has an absolute total displacement equal to a minimum etch size but the displacement of the proof mass is less than the minimum etch size such that the stop gap is less than the minimum etch size. The smaller stop gap results in a smaller velocity of the proof mass before the proof mass is stopped and reduces chances of proof mass, the extension, or both the proof mass and the extension being chipped or damaged as a result of contact compared to the proof mass traveling over a stop gap with a distance of the minimum etch size.

<FIG> are top view and side view respectively of an example microelectromechanical systems (MEMS) accelerometer <NUM> in accordance with an exemplary embodiment of the invention. Accelerometer <NUM> represents a single axis accelerometer which measures an acceleration in a linear direction. The top view of <FIG> may show the accelerometer <NUM> in an xy plane with a z axis being orthogonal to the page. Further, in an example, the accelerometer <NUM> may have a proof mass <NUM>, sense fingers <NUM>-<NUM> to <NUM>-<NUM>, and a compliant spring structure <NUM> to measure the acceleration.

The proof mass <NUM> may be a known quantity of mass coupled to the compliant spring structure <NUM> which allow linear movement of the proof mass <NUM>. In an example, the proof mass <NUM> and compliant spring structure <NUM> may made of a silicon material such as a single crystal silicon or polysilicon. In an example, a compliant mechanism such as the compliant spring structure is a flexible mechanism that achieves force and motion transmission through elastic body deformation. The compliant spring structure <NUM> may enable movement of proof mass <NUM> in a linear direction as a result of acceleration of the MEMS accelerometer <NUM> and resulting acceleration of the proof mass <NUM>. Examples below describe the direction to be in a +y or -y direction for ease of illustration, but the proof mass <NUM> may move in other linear or non-linear directions. The compliant spring structure <NUM> may be composed of compliant members. In an example, the compliant members may include a flexible beam <NUM>, a flexible beam <NUM>, and rigid structure <NUM>. Further, in an example, the flexible beam <NUM>, flexible beam <NUM>, and rigid structure <NUM> may be fabricated by semiconductor patterning and etching process of the silicon material and formed as a single unitary structure that provides a desired flexibility, stiffness, or rigidity. Further, the dimension and number of the flexible beam <NUM>, the flexible beam <NUM>, and the rigid structure <NUM> may be tuned to form the compliant spring structure <NUM> per a design preference. In an example, thickness of the beams may be <NUM> with beams widths of <NUM> and lengths of <NUM> to <NUM>.

The beam <NUM> may have respective ends <NUM>, <NUM> along a longitudinal direction of the beam <NUM>. The end <NUM> of the beam <NUM> may be coupled to a spring anchor <NUM> while the end <NUM> may be coupled to the rigid structure <NUM>. The spring anchor <NUM> may be coupled to a substrate <NUM> as shown in the side view of <FIG> along cross section 1B of <FIG>. The substrate <NUM> may be stationary with respect to the motion of the proof mass <NUM> and fix the end <NUM> of the beam <NUM> from moving relative to the movement of the proof mass <NUM>. In an example, the substrate <NUM> may be an underlying surface or support to the compliant spring structure <NUM> which in an example does not contact the proof mass <NUM>. Further, the beam <NUM> may have respective ends <NUM>, <NUM> along a longitudinal direction of the beam <NUM>. The end <NUM> of the beam <NUM> may be coupled to the rigid structure <NUM> and the end <NUM> of the beam <NUM> may be coupled to the proof mass <NUM>. The coupling of the ends <NUM>, <NUM>, <NUM>, <NUM> may be a fixed joint which fixes the end to the spring anchor <NUM>, proof mass <NUM>, or rigid structure <NUM> without any rotational movement at the joint.

The sense fingers <NUM>-<NUM> to <NUM>-<NUM> may determine the acceleration of the proof mass <NUM>. Further, the MEMS accelerometer <NUM> may have fewer or more sense fingers than what is illustrated.

For ease of explanation, structure and operation of sense finger <NUM>-<NUM> will be now described. The sense fingers <NUM>-<NUM> to <NUM>-<NUM> which have a similar structure and operate similarly may not be described for conciseness. The sense finger <NUM>-<NUM> may have an electrode <NUM> constructed from a silicon material. The electrode <NUM> may be anchored to the substrate <NUM> at an electrode anchor <NUM>. By being anchored to the substrate <NUM>, the sense finger <NUM>-<NUM> may be stationary relative to movement of the proof mass <NUM> so as to measure the movement of the proof mass <NUM> in response to acceleration. A sense gap <NUM> may separate the sense finger <NUM>-<NUM> from the proof mass <NUM> and be an insulating material such as air. The sense finger <NUM>-<NUM>, sense gap <NUM>, and proof mass <NUM> may form a capacitor shown as capacitance C. Further, movement of the proof mass <NUM> may alter a size of the sense gap <NUM> and resulting capacitance such that a differential capacitance as a function of time is indicative of movement such as acceleration of the proof mass <NUM>. The differential capacitance may then be converted into an electrical signal having a parameter magnitude (e.g., voltage, current, frequency, etc.) that is proportional to the acceleration.

Sense fingers <NUM>-<NUM> to <NUM>-<NUM> may be positioned at various positions and orientations with respect to the proof mass <NUM> to measure acceleration of the proof mass <NUM>. Capacitance between a sense finger and proof mass <NUM> may increase when the proof mass <NUM> travels in one direction and decrease when the proof mass <NUM> travels in another direction. For example, when the proof mass moves in a +y direction, then capacitance between the sense finger <NUM>-<NUM> and proof mass <NUM> may increase while a capacitance between the sense finger <NUM>-<NUM> and proof mass <NUM> may decrease. Alternatively, when the proof mass <NUM> moves in a -y direction, then capacitance of the sense finger <NUM>-<NUM> may decrease while a capacitance of the sense finger <NUM>-<NUM> may increase.

An over-travel stop allows a proof mass to travel a certain distance before the over travel stop stops the proof mass so that the proof mass does not move beyond the certain distance. A stop gap defines a distance that the proof mass is allowed to move until the proof mass is stopped. The stop gap typically has a size defined by an etch size of a semiconductor process. The etch size is a size of a gap produced by an etching process of the semiconductor process and is typically a minimum etch size defined by a minimum pitch size or minimum feature size of the semiconductor process. The over travel stop allows the proof mass to typically travel no more than the size of the stop gap defined by the minimum etch size before the over travel stop stops the proof mass from moving.

A small stop gap may be preferable to a larger stop gap so that the proof mass does not accelerate to velocities that cause chipping or damage of the proof mass, chipping or damage of the overtravel stop, or chipping or damage of both the proof mass and the overtravel stop when the over travel stop contacts the proof mass to stop the proof mass. But because the minimum etch size typically defines the size of the stop gap, the stop gap is limited by the semiconductor process and cannot be made any smaller than the minimum etch size without changing the pitch of the semiconductor process.

Embodiments disclosed herein are directed to the compliant spring structure <NUM> comprising an extension <NUM> which allows for forming the stop gap to be smaller than or equal to a minimum etch size and/or in some examples without changing the semiconductor process. In an example, the minimum etch size may define the stop gap. In an example, the extension <NUM> of the compliant spring structure <NUM> is arranged to travel responsively in a direction opposite to motion of the proof mass <NUM> to stop the proof mass <NUM> from moving as a result of acceleration. The opposite direction may be a direction other than direction of travel of the proof mass <NUM> such as substantially <NUM> degrees different from the motion of the proof mass <NUM>. For example, as the proof mass <NUM> moves in the +y direction, the extension <NUM> moves in the -y direction. The movement of the end <NUM> causes the extension <NUM> to move in an opposite y direction resulting in the extension <NUM> contacting the proof mass <NUM> and stopping the motion of the proof mass <NUM> as a result of acceleration of the proof mass <NUM>. A total displacement of the proof mass <NUM> and the extension <NUM> resulting from respective movement may define size of the stop gap. <FIG> shows a rest position of the compliant spring structure <NUM> and proof mass <NUM> when not subject to an acceleration. In an example, the proof mass <NUM> may move from this rest position as a result of acceleration in the +y direction and the extension <NUM> may responsively move in a -y direction such that a total absolute displacement of the proof mass <NUM> and the extension <NUM> is substantially the minimum etch size and the proof mass <NUM> may move less than the minimum etch size. In an example, the motion of the proof mass <NUM> and opposite motion of the extension <NUM> may reduce a size of the stop gap to less than the minimum etch size.

A separation <NUM> is a distance between the extension <NUM> and the proof mass when the proof mass <NUM> is not subject to an acceleration force. The separation <NUM> may be fabricated based on a semiconductor patterning and etching process in a silicon material to be substantially a minimum etch size to allow a total displacement of the proof mass <NUM> and extension <NUM> to be less than or equal to the minimum etch size. The separation <NUM> may then vary when the proof mass <NUM> is subject to an acceleration. Further, in an example, the extension <NUM> may be fabricated based on a semiconductor patterning and etching process in a silicon material to form a single unitary structure with dimension that provides a desired stiffness or rigidity. In an example, the extension <NUM> may have a <NUM> thickness and a width on the order of order of <NUM> to <NUM> wide, but could be made as small as the minimum etch size, typically <NUM>.

The compliant spring structure <NUM> also has an extension <NUM> that operates in a manner similar to that of extension <NUM>. Unlike the extension <NUM>, the extension <NUM> may contact the proof mass <NUM> to stop the motion of the proof mass <NUM> if the proof mass <NUM> moves in a -y direction as a result of acceleration. The extension <NUM> may operate to stop the proof mass <NUM> in the stop gap less than the minimum etch size similar to that described with respect to the extension <NUM>.

A size of the sense gap <NUM> associated with the sense finger <NUM>-<NUM> to <NUM>-<NUM> may also depend on the size of the stop gap. The size of the sense gap <NUM> may vary in proportion to the movement of the proof mass <NUM>. In an example, the size of the sense gap <NUM> may be greater than the stop gap so that the motion of the proof mass <NUM> equal to the size of the stop gap does not result in the sense finger <NUM>-<NUM> to <NUM>-<NUM> contacting the proof mass <NUM> and producing an electrostatic latch up or short between the sense finger <NUM>-<NUM> to <NUM>-<NUM> and proof mass <NUM>. Further, a resolution of acceleration measured by the sense finger <NUM> may depend on the size of the sense gap <NUM>. A larger size of the stop gap may result in maintaining a larger sense gap <NUM> and less resolution of the acceleration measured by the sense finger <NUM>-<NUM> to <NUM>-<NUM>. Conversely, a smaller size of the stop gap may result in maintaining a smaller sense gap <NUM> and higher resolution of the acceleration measured by the sense finger <NUM>-<NUM> to <NUM>-<NUM>.

The smaller stop gap also results in higher restoring forces applied by the compliant spring structure <NUM> to mitigate stiction of the proof mass <NUM>. Stiction is an undesirable situation which arises when surface adhesion forces are higher than the mechanical restoring force of the MEMS accelerometer <NUM>. Stiction is recognized to often occur in situations where two surfaces with areas in close proximity come in contact. For example, the greater the contact area at both macroscopic and microscopic roughness levels, the greater the risk of stiction.

<FIG> illustrates operation of the MEMS accelerometer <NUM> in accordance with an exemplary embodiment of the invention. The operation may include illustrating motion of the proof mass <NUM> in one direction causing the extension <NUM> of the compliant spring structure <NUM> to move in an opposite direction to contact the proof mass <NUM> in accordance with an exemplary embodiment of the invention. Further, the motion may cause one or more of the beam <NUM> and beam <NUM> to deform because the spring anchor <NUM> is coupled to the substrate <NUM>. The proof mass <NUM> may move in a +y direction as a result of acceleration which causes the extension <NUM> to move in a -y direction and contact the proof mass <NUM> at surface <NUM> to cause the proof mass <NUM> to stop moving because the extension <NUM> is coupled to the spring anchor <NUM> via the compliant spring structure <NUM>. In an example, an end of the compliant spring structure <NUM> that is attached to the proof mass <NUM> may be given <NUM> of displacement in the +y direction. The extension <NUM> responsively deflects <NUM> in the -y direction to make contact with the proof mass <NUM> at point <NUM>. The proof mass <NUM> and extension <NUM> may collectively move <NUM> which is the minimum etch size but the extension <NUM> may contact the proof mass <NUM> and stop the motion after ~<NUM> of travel rather than a <NUM> minimum etch size. The stop gap may be reduced by ~<NUM>% and allow a smaller sense gap. The relationship between the displacement of the proof mass <NUM> and extension <NUM> and the separation <NUM> may be represented by the following equation:<MAT> where Δyproof mass displacement is a displacement of the proof mass, Δyextension displacement is a displacement of the extension in response to the proof mass displacement, and Abs is an absolute value function. With a non-zero displacement of the extension, the proof mass may have a displacement less than or equal to the minimum etch size. In an example, the minimum etch size may define the stop gap. In an example, an absolute value of the maximum displacement of the proof mass plus an absolute value of the maximum displacement of the extension which results from the maximum displacement of the proof mass may be less than or equal to the minimum etch size.

The compliant spring structure <NUM> also has an extension <NUM> that operates in a manner similar to that of extension <NUM>. The extension <NUM> may contact the proof mass <NUM> to stop the motion of the proof mass <NUM> if the proof mass <NUM> moves in a -y direction as a result of acceleration. The extension <NUM> may contact the proof mass on a surface <NUM> of the proof mass <NUM> and operate to stop the proof mass <NUM> in the stop gap less than the minimum etch size similar to that described with respect to the extension <NUM>.

<FIG> is an example of a schematic view <NUM> of principles of operation of the compliant spring structure <NUM> according to an exemplary embodiment of the invention. The principles of operation is based on the motion of the proof mass <NUM> in response to an acceleration moving in one direction and rigid structure <NUM> of the compliant spring structure <NUM> rotating which causes the extension <NUM> to move in an opposite direction to define the stop gap for the proof mass <NUM> rather than the stop gap being the minimum etch size of the semiconductor process which the proof mass <NUM> moves.

The schematic view <NUM> models the compliant spring structure <NUM> as a cantilever spring structure with a flexible beam <NUM>, a rigid center mass <NUM>, and a flexible beam <NUM> with stiffness k defined by the beam <NUM>, rigid structure <NUM>, and beam <NUM> respectively in the accelerometer <NUM>. End <NUM> of the beam <NUM> may be fixed to a reference such as to the spring anchor <NUM> while the other end <NUM> of the beam <NUM> may be coupled to the rigid center mass <NUM>. Further, end <NUM> of the beam <NUM> may be coupled to a moving mass such as the proof mass <NUM> while end <NUM> of the beam <NUM> may be coupled to the rigid center mass <NUM>. The beam <NUM> may flex based on movement of the proof mass <NUM> mass in a y direction in response to an acceleration. The flexing may cause the rigid center mass <NUM> to rotate in a z axis which is orthogonal to the x and y directions. The flexing and rotation may be example deformations of the compliant spring structure <NUM>. The rigid center mass <NUM> may have an extension <NUM> which converts the rotation of the rigid center mass <NUM> to a y axis motion in an opposite direction to the motion of the proof mass <NUM> in response to the motion of the proof mass <NUM>. Based on the motion of the proof mass <NUM>, the extension <NUM> may move and eventually contact the proof mass <NUM> and stop travel of the proof mass <NUM>. The motion of the proof mass <NUM> in the y direction and the motion of the extension <NUM> in the opposite y direction result in a total displacement of the proof mass <NUM> and the extension <NUM> to be substantially the minimum etch size but the stop gap may be less than the minimum etch size.

In an example, an absolute displacement of the extension <NUM> and proof mass <NUM> may be no greater than the minimum etch size of the semiconductor process. A length of the extension <NUM> determines a distance when the extension <NUM> contacts the proof mass <NUM> and stops the proof mass <NUM>. If the length of the extension <NUM> is long, the proof mass <NUM> travels a shorter distance before being stopped by the extension <NUM> compared to the displacement of the extension <NUM>. If the length of the extension <NUM> is short, the proof mass <NUM> travels a longer distance before being stopped by the extension <NUM> compared to the displacement of the extension <NUM>.

<FIG> is an example of a schematic view <NUM> of principles of operation of a motion amplifier in accordance with an exemplary embodiment of the invention. The schematic view <NUM> shows the rigid center mass <NUM> coupled to a beam <NUM>. One end <NUM> of the beam <NUM> may be coupled to a fixed reference such as the anchor <NUM> and another end <NUM> of the beam <NUM> may be coupled to the rigid center mass <NUM>. The rigid center mass <NUM> may have a displacement <NUM> in the y direction resulting from motion of the proof mass <NUM> such as an acceleration but this displacement may increase as a distance from the rigid center mass <NUM> increases as shown by displacement <NUM>. The increase in displacement is a motion amplification of the displacement <NUM>. A length of the extension <NUM> may be adjusted based on a desired amount of the motion amplification to achieve a desired stop gap size. A longer extension <NUM> may reduce a stop gap size while a shorter extension <NUM> may increase a stop gap size.

<FIG> is an alternative example of the compliant spring structure <NUM> in accordance with an exemplary embodiment not within the scope of the invention. The compliant spring structure <NUM> which is arranged in the proof mass <NUM> produces a softer spring constant compared to the compliant spring structure <NUM>. The compliant spring structure <NUM> may have a beam <NUM> and extensions <NUM>, <NUM>. One end of the beam <NUM> may be coupled to the anchor <NUM> while another end of the beam <NUM> may be coupled to the extensions <NUM>, <NUM> at a coupler <NUM>. One end of the extensions <NUM>, <NUM> may be coupled to the proof mass <NUM> and another end of the extension <NUM>, <NUM> may be coupled to the beam <NUM> at the coupler <NUM>. The extensions <NUM>, <NUM> may move in a direction opposite to motion of the proof mass <NUM> as a result of acceleration to contact the proof mass <NUM>. A separation <NUM> may determine a distance between the proof mass <NUM> and the extension <NUM>. Similarly, a separation <NUM> may determine a distance between the proof mass <NUM> and the extension <NUM>. The compliant spring structure <NUM> and proof mass <NUM> is shown not subject to an acceleration such that the separation <NUM>, <NUM> in an example is equal to the minimum etch size but will vary based on acceleration and result in the extensions <NUM>, <NUM> contacting the proof mass <NUM> at point <NUM> or point <NUM> respectively on the proof mass <NUM> depending on a direction of movement of the proof mass <NUM>. Further, a length of the extension <NUM>, <NUM> may determine a size of a stop gap which is smaller than a minimum etch size associated with a fabrication process for the MEMS accelerometer <NUM>. In an example, a <NUM> movement of the proof mass <NUM> coupled to the compliant spring structure <NUM> in the +y direction may cause the extension <NUM>, <NUM> to moves -<NUM> in the -y direction. If the minimum etch size is <NUM>, the extension <NUM>, <NUM> may stop the proof mass <NUM> from moving after <NUM> of travel. This design has reduced the stop gap by ~<NUM>%.

<FIG> is a flow chart of functions <NUM> associated with operation of the MEMS accelerometer <NUM> in accordance with an exemplary embodiment of the invention. At <NUM>, a proof mass <NUM> of the MEMS accelerometer <NUM> moves in a first direction in response to an acceleration. The first direction may be a linear direction which corresponds to a +y or -y direction. At <NUM>, an extension <NUM>, <NUM> of the compliant spring structure <NUM> of the MEMS accelerometer <NUM> moves in a second direction which is opposite to the first direction in response to the proof mass <NUM> moving in the first direction. In an example, the compliant spring structure <NUM> may be coupled to the proof mass <NUM>. Further, in an example, the motion of the proof mass <NUM> causes the beam <NUM> to flex, which causes a rotation of a rigid structure <NUM> of the complaint spring structure <NUM> which causes the extension <NUM> to move in the opposite direction to the proof mass <NUM>. At <NUM>, the extension <NUM> contacts the proof mass <NUM> when a sum of an absolute displacement of the proof mass <NUM> and an absolute displacement of the extension <NUM> is substantially the minimum etch size to stop the proof mass <NUM>. The minimum etch size may be based on a fabrication process of the MEMS accelerometer <NUM>. The extension <NUM> may be coupled via an anchor <NUM> to a substrate <NUM> which stops the proof mass <NUM> from moving. In an example, the size of the stop gap may be less than the minimum etch size and a sum of a maximum absolute displacement of the proof mass <NUM> and a maximum absolute displacement of the extension <NUM> may be less than or equal to the minimum etch size.

A few implementations have been described in detail above, and various modifications are possible. The disclosed subject matter, including the functional operations described in this specification, can be implemented in electronic circuitry, computer hardware, firmware, software, or in combinations of them, such as the structural means disclosed in this specification and structural equivalents thereof: including potentially a program operable to cause one or more data processing apparatus such as a processor to perform the operations described (such as program code encoded in a non-transitory computer-readable medium, which can be a memory device, a storage device, a machine-readable storage substrate, or other physical, machine readable medium, or a combination of one or more of them).

While this specification contains many specifics, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular implementations. The present invention is defined by the independent claims and advantageous embodiments are described in the dependent claims.

Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations.

Use of the phrase "at least one of" preceding a list with the conjunction "and" should not be treated as an exclusive list and should not be construed as a list of categories with one item from each category, unless specifically stated otherwise. A clause that recites "at least one of A, B, and C" can be infringed with only one of the listed items, multiple of the listed items, and one or more of the items in the list and another item not listed.

Claim 1:
A microelectromechanical systems, MEMS, accelerometer (<NUM>) comprising:
a compliant spring structure (<NUM>) comprising a first beam (<NUM>);, a second beam (<NUM>), and a rigid structure (<NUM>), wherein one end of the first beam (<NUM>); and one end of the second beam (<NUM>) are coupled to the rigid structure (<NUM>);
a proof mass (<NUM>) coupled to another end of the second beam (<NUM>);
a spring anchor (<NUM>) coupled to another end of the first beam (<NUM>); and characterized by
first and second extensions (<NUM>, <NUM>) coupled to the rigid structure (<NUM>) and extending across respective first and second opposing sides of the spring anchor (<NUM>),
wherein in response to the proof mass (<NUM>) moving, each of the first and second extensions (<NUM>, <NUM>) is configured to move in an opposite direction to the motion of the proof mass (<NUM>) to cause one of the first and second extensions (<NUM>, <NUM>) to contact the proof mass (<NUM>) and stop the movement of the proof mass (<NUM>).