Fully differential accelerometer

Abstract

The present disclosure relates to fully differential accelerometers. Aspects of a multi-mass, multi-axis microelectromechanical system (MEMS) accelerometer sensor device having a fully differential sensing design are disclosed herein that applies a differential drive signal to a movable detection mass and senses a differential motion signal at a sense finger coupled to a substrate. In some embodiments, the capacitive signals from different sense fingers are combined together at a sense signal node disposed on the substrate that supports the detection mass. In some embodiments, split shields can be provided in which a first shield beneath the detection mass is coupled to the same drive signal applied to the detection mass, and a second shield and a first shield provided beneath the sense finger are electrically isolated and use a constant voltage bias to provide shielding for the sense finger.

Description

Technical Field

[0001] The present application relates generally to accelerometers, and in particular to microelectromechanical systems (MEMS) accelerometers. Background Art

[0002] A microelectromechanical system (MEMS) accelerometer can include a movable mass, sometimes referred to as a proof mass. The proof mass is movable in response to the accelerometer's acceleration, and the movement of the proof mass can be sensed and processed by circuitry in the accelerometer to produce an output signal representative of the acceleration. In a linear MEMS accelerometer, the proof mass is movable along one or more linear axes, and the linear movement of the proof mass can be sensed to indicate linear acceleration along the one or more linear axes.

[0003] Some MEMS accelerometers use capacitive sensing to sense the motion of a proof mass that is capacitively coupled to electrodes on a semiconductor substrate. As the proof mass moves in response to the sensed acceleration, the magnitude of the capacitive coupling between the proof mass and the electrodes is electrically measured and processed to indicate the magnitude and direction of the sensed linear acceleration. Summary of the Invention

[0004] Aspects of a multi-mass, multi-axis microelectromechanical system (MEMS) accelerometer sensor device with a fully differential sensing design are disclosed herein. The design applies a differential drive signal to a movable proof mass and senses the differential motion signal at sense fingers coupled to a substrate. In some embodiments, the capacitive signals from the different sense fingers are combined at a sense signal node disposed on the substrate supporting the proof mass. In some embodiments, a split shield can be provided, wherein a first shield below the proof mass couples the same drive signal applied to the proof mass, and a second shield provided below the sense fingers is electrically isolated from the first shield and biased with a constant voltage to provide shielding from the sense fingers.

[0005] According to some embodiments, a differential, multi-mass, multi-axis accelerometer is provided. The accelerometer includes: first and second masses arranged side by side and in a plane in a substrate, movable in the X and Y directions in the plane; and a configuration to have differential drive signals applied thereto, the differential drive signals including a first polarity signal applied to the first mass and a second polarity signal applied to the second mass. The accelerometer also includes: a first plurality of sensing fingers coupled to the substrate, capacitively coupled to the first and second masses, and configured to sense X-direction movement of the first and second masses; and a second plurality of sensing fingers coupled to the substrate, capacitively coupled to the first and second masses, and configured to sense Y-direction movement of the first and second masses. The first plurality of sensing fingers are configured in combination to output a differential X-direction motion signal. The second plurality of sensing fingers are configured in combination to output a differential Y-direction motion signal.

[0006] According to some embodiments, a multi-axis differential accelerometer is provided. The multi-axis differential accelerometer includes: a first movable mass and a second movable mass, disposed side by side in a substrate and each movable in an X-direction and a Y-direction; and a first set of electrodes capacitively coupled to the first movable mass. The multi-axis differential accelerometer also includes: a second set of electrodes capacitively coupled to the second movable mass; and an X-sense signal node disposed on the substrate and coupled to a first subset of the first set of electrodes and a first subset of the second set of electrodes. The X-sense signal node is configured to provide an X-sense signal indicating movement of the accelerometer in the X-direction when the first movable mass and the second movable mass are differentially biased. The multi-axis differential accelerometer also includes: a Y-sense signal node disposed on the substrate and coupled to a second subset of the first set of electrodes and a second subset of the second set of electrodes. The Y-sense signal node is configured to provide an X-sense signal indicating movement of the accelerometer in the Y-direction when the first movable mass and the second movable mass are differentially biased.

[0007] According to some embodiments, a method for measuring multi-axis acceleration using a multi-mass differential accelerometer is provided. The accelerometer includes first and second proof masses disposed side by side in a substrate. The method includes biasing the first and second proof masses using a differential drive signal; combining, at an X sense signal node disposed on the substrate, a first capacitance signal from a first plurality of sense fingers capacitively coupled to the first proof mass with a second capacitance signal from a second plurality of sense fingers capacitively coupled to the second proof mass; combining, at a Y sense signal node disposed on the substrate, a third capacitance signal from a third plurality of sense fingers capacitively coupled to the first proof mass with a fourth capacitance signal from a second plurality of sense fingers capacitively coupled to the second proof mass; outputting a differential X-direction motion signal based on the signal at the X sense signal node; and outputting a differential Y-direction motion signal based on the signal at the Y sense signal node. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Various aspects and embodiments of the present application will be described with reference to the following drawings. It should be understood that the drawings are not necessarily drawn to scale. Items that appear in multiple figures are represented by the same reference numerals in all figures in which they appear. In the drawings:

[0009] Figure 1A is a schematic top view of an accelerometer having a single proof mass;

[0010] Figure 1B Shows the instructions Figure 1A Two timing diagrams of an example of forward charge sensing of an accelerometer shown in;

[0011] Figure 1C Shows the instructions Figure 1A A timing diagram of an example of reverse charge sensing of an accelerometer shown in FIG.

[0012] Figure 2A It can be with Figure 1A and 1B Schematic circuit diagram of a sensing circuit used in conjunction with the forward sensing scheme shown;

[0013] Figure 2B It can be with Figure 1A and 1C A schematic circuit diagram of a sensing circuit used in conjunction with the reverse sensing scheme shown;

[0014] Figure 3 is a schematic top view of a differential, multi-proof accelerometer according to some embodiments;

[0015] Figure 4 Available for operation Figure 3 A schematic circuit diagram of a sensing circuit of a differential multi-mass accelerometer is shown;

[0016] Figure 5 is a schematic top view of a multi-mass accelerometer with X-axis and Y-axis differential sensing according to some embodiments;

[0017] Figure 6A is a schematic side view of a multi-axis differential accelerometer that can provide Z-direction acceleration sensing according to some embodiments;

[0018] Figure 6B is a schematic side view of a multi-axis differential accelerometer with tilt-mode Z-direction acceleration sensing according to some embodiments;

[0019] Figure 7 is a schematic side view of an accelerometer according to some embodiments;

[0020] Figure 8A and 8B is a schematic top view of a multi-axis MEMS accelerometer with fully differential reverse sensing according to some embodiments;

[0021] Figure 9A and 9B is a schematic top view of a multi-axis MEMS accelerometer with fully differential reverse sensing according to some embodiments;

[0022] Figure 10A and 10B is a schematic top view of a multi-axis MEMS accelerometer 1000 that uses a separate proof mass for fully differential inverse sensing in the Z-axis compared to sensing in the X-axis and Y-axis, according to some embodiments;

[0023] Figure 11 is a schematic diagram of an electronic device that houses one or more MEMS accelerometers. DETAILED DESCRIPTION

[0024] Aspects of the present application provide a differential, multi-mass, multi-axis microelectromechanical system (MEMS) accelerometer having a fully differential sensing design that applies differential drive signals to a movable proof mass and senses differential motion signals at sensing fingers coupled to a substrate.

[0025] Some aspects are directed to fully differential sensing of linear acceleration using multiple proof masses. In some embodiments, two proof masses are provided to differentially sense acceleration in the X-direction. Each proof mass is biased by a differential drive signal. A first capacitance signal from a sensing finger capacitively coupled to the first proof mass and a second capacitance signal from a sensing finger capacitively coupled to the second proof mass are combined to output a differential motion signal in the X-direction. The inventors have recognized and appreciated that by combining the signals from the sensing fingers of two adjacent differentially biased movable proof masses to generate a differential motion signal representing acceleration in the X-direction, undesirable common-mode signals can be attenuated or completely eliminated before the output motion signal is processed by sensing circuitry within the accelerometer. This differential sensing design can provide a number of benefits, such as reducing inductive crosstalk within the accelerometer, simplifying the sensing circuit design using an application-specific integrated circuit (ASIC), reducing charging overload in the sensing circuit, and reducing the complexity of overrange detection in the sensing circuit.

[0026] In some embodiments, the first and second capacitance signals from different sensing fingers are combined at a sensing signal node, which is located on the substrate supporting the proof mass. The sensing signal node can be further connected to sensing circuitry, such as a differential amplifier, to further process the combined capacitance signal. By combining the capacitance signals on the substrate, the complexity of the sensing circuit design can be reduced and the system noise performance can be improved.

[0027] In one aspect, the inventors have recognized and appreciated that, due to the relatively large surface area of ​​the proof mass facing the underlying substrate, applying a time-varying drive signal to the proof mass results in substantial capacitive coupling between the proof mass and the substrate, which can undesirably interfere with the proof mass's in-plane motion in response to input acceleration and generate crosstalk in the sensed electronic signal. Furthermore, due to the time-varying nature of the drive signal on the proof mass, the proof mass-substrate interaction cannot be mitigated by a constant voltage shield on the substrate. As a solution, split shields can be provided on the substrate beneath the proof mass and the sensing fingers, allowing separate shielding voltages to be applied to the proof mass and the sensing fingers, respectively. In some embodiments, a first shield beneath the proof mass is coupled to the same drive signal applied to the proof mass to reduce electrostatic interactions between the substrate and the proof mass. A second shield, electrically isolated from the first shield, is disposed beneath the sensing fingers and biased at a constant voltage to provide shielding for the sensing fingers.

[0028] Another aspect relates to differential sensing of multi-axis acceleration along multiple directions. In some embodiments, each of the two proof masses used for differential sensing can move in more than one direction, such as in the X and Y directions, or in three axes including the X, Y, and Z directions. Because the drive signal is applied to the proof masses, a single set of drive signals can be used to sense acceleration in multiple directions, which can simplify clock signal generation and reduce crosstalk between multiple different clock signals.

[0029] Figure 1A is a schematic top view of an accelerometer with a single proof mass. Figure 1B and 1C is a schematic timing diagram showing the Figure 1A Forward and reverse sensing schemes for the motion of the detection mass in.

[0030] Figure 1A An accelerometer 100 is shown having a proof mass 112 and sensing fingers 121, 122 disposed on a substrate 180. The proof mass 112 can move in one or more directions relative to the substrate 180. Figure 1A As shown, the proof mass 112 may be suspended above the substrate 180 and may move along an XY plane parallel to the top surface of the substrate 180 .

[0031] Although not shown, accelerometer 100 includes one or more tethers that mechanically connect proof mass 112 to substrate 180 and allow proof mass 112 to move while providing a restoring force when the proof mass deviates from its equilibrium. One end of the tether is connected to the movable proof mass, while the other end of the tether is connected to the substrate. In addition, the space between sensing finger 121 and movable proof mass 112 can include air or an inert gas to allow free relative movement between the sensing finger and the proof mass. In some embodiments, the accelerometer can include a cavity (not shown) formed on substrate 180, with proof mass 112 disposed within the cavity. Any suitable pressure or gas composition can be provided in the cavity, such as, but not limited to, an inert gas environment. The cavity can be sealed from the surrounding environment so that environmental influences such as moisture, oxidation, pressure changes, etc. are not affected by components within the cavity.

[0032] Proof mass 112 can be shaped as a plate, one or more beams, or have any suitable size or shape. Proof mass 112 can include a conductive material, such as a semiconductor, a metal, a metal alloy, a carbon allotrope, a conductive ceramic, or any suitable material or combination thereof. In some embodiments, proof mass 112 includes one or more connection points, such as conductive traces or pads, that can be coupled to a voltage signal to bias the conductive material within proof mass 112 at any suitable potential.

[0033] like Figure 1A As shown, in some embodiments, proof mass 112 can be rectangular and can have protrusions such as fingers 114 that are shaped and positioned to provide capacitive coupling with adjacent sensing fingers 121, 122. In some embodiments, proof mass 112 is a resonator.

[0034] The X-direction motion of the proof mass 112 can be capacitively sensed based on the change in capacitive coupling between the proof mass 112 and the sensing fingers 121, 122. Figure 1A As shown, the sensing finger 121 forms a variable capacitor C1 with the finger 114 of the detection mass 112, and the sensing finger 122 forms a variable capacitor C2 with the finger 114 of the detection mass 112. When the detection mass 112 moves in the positive X direction, or Figure 1A As the proof mass moves leftward, C1 increases due to the decrease in the horizontal distance between sensing finger 121 and proof mass 112. Sensing finger 121 can be referred to as the p-finger in discussing the sensing finger used to sense X-direction motion. Simultaneously, as the proof mass moves leftward, C2 decreases due to the increase in the horizontal distance between sensing finger 122 and proof mass 112. Sensing finger 122 can be referred to as the n-finger.

[0035] The sensing fingers 121, 122 are disposed on the substrate 180 and may be electrodes formed of any suitable conductive material, such as semiconductors, metals, metal alloys, carbon allotropes, conductive ceramics, or any suitable material or combination thereof. Figure 1A For simplicity, only one pair of sensing fingers is shown. Embodiments of the present application may have multiple sensing fingers for sensing the X-direction motion of the proof mass, wherein a first subset of sensing fingers including sensing finger 121 is p-fingers, and a second subset of sensing fingers including sensing finger 122 is n-fingers. The proof mass 112 may also have multiple fingers 114, which are arranged in an interdigitated or comb-like structure along the X-direction between the p-finger and n-finger pairs. The sensing fingers may have any suitable size or shape and are not limited to the following: Figure 1A In some embodiments, the sensing fingers are fixedly attached to the substrate 180, although fixation of the sensing fingers is not a requirement.

[0036] X-direction motion of proof mass 112 may be capacitively sensed based on changes in the amount of charge Q stored on the capacitor plates of C1 and C2 in response to the motion. Figure 1B Shows the instructions Figure 1A Two timing diagrams of an example of forward charge sensing of an accelerometer are shown in FIG. In forward sensing, the proof mass is at a constant voltage potential and is used to sense the output signal, while the sensing fingers are driven by a time-varying drive signal. Figure 1BAs shown, the p-finger and n-finger are driven by clock signals or differential drive signals V_p 131 and V_n 132 of opposite polarity, respectively, and an output signal is generated at the detection mass 112 which can be fixed at a constant voltage potential. Since the amount of charge stored on the capacitor plate is Q = V·C, where V is the voltage between the two plates in the capacitor and C is the capacitance value of the capacitive coupling, the change in the amount of charge stored on C1 can be expressed as Q = ΔV·ΔC, where ΔV is the voltage difference between the high and low potentials of the applied clock signals 131 and 132, and ΔC is the change in the capacitive coupling during movement. Figure 1A In the example, the output signal at proof mass 112 is the sum of the charge changes in C1 and C2 due to the opposite bias polarity on the p- and n-fingers. The change in charge Q at the capacitor plates can be a capacitance signal representing the motion-induced change in capacitive coupling.

[0037] Figure 1C Shows the instructions Figure 1A 1 and 2. A timing diagram illustrating an example of reverse charge sensing of an accelerometer is shown in FIG. In reverse sensing, a drive signal V_proof_mass 133 is applied to the proof mass 112 and a pair of differential output signals are sensed at the sensing fingers 121 , 122 and differentially combined to indicate a change in charge Q on capacitors C1 and C2 .

[0038] The inventors have recognized and appreciated that for a single proof mass, Figure 1B and 1C Neither the forward sensing nor the reverse sensing in the present invention can eliminate the undesired common-mode signal that is not related to the motion of the proof mass. Techniques such as using compensation circuits can be used to compensate for the common-mode signal, but such techniques increase the complexity of the sensing circuit. Figure 2A and 2B Shown are the combined Figure 1B and 1C Two examples of forward and reverse sensing schemes using compensation circuits are shown.

[0039] Figure 2A It can be with Figure 1A and 1B The schematic circuit diagram of the sensing circuit used in conjunction with the forward sensing scheme shown in FIG. Figure 2A In FIG, the changes caused by the movement of the charges Q from C1 and C2 are sensed from the proof mass 112, which is connected to the first input terminal 204 of the differential amplifier 202. Differential drive signals 131, 132 are applied to the pn fingers 121, 122, respectively. Figure 2AA compensation circuit 220 is shown provided to the second input terminal 206 of the differential amplifier 202. The compensation circuit 220 may include capacitors formed from structures disposed on the substrate 180 in the accelerometer that are driven by the drive signal 232. The compensation circuit 220 may alternatively or additionally include components formed in an ASIC.

[0040] Because the compensation circuit 220 is insensitive to the motion of the proof mass, the differential amplifier can use the signal at the second amplifier input 206 to compensate for the common-mode signal, for example by comparing the output signals at the differential amplifier outputs 208 and 210 to each other. The compensation circuit 220, the amplifier 202, and the feedback capacitor Cfb can be implemented in any suitable manner, for example as part of an ASIC within the accelerometer. The ASIC can be provided on a substrate separate from the MEMS substrate (e.g., substrate 180 having the proof mass and the sensing fingers), and in some embodiments, the ASIC and the MEMS substrate can be combined in a single package to reduce the device footprint and improve electrical performance, for example, by reducing noise.

[0041] Figure 2B It can be with Figure 1A and 1C The schematic circuit diagram of the sensing circuit used in conjunction with the reverse sensing scheme shown in FIG. Figure 2B In the example, proof mass 112 is driven by drive signal 133, and the changes caused by the movement of charge Q from C1 and C2 are sensed by two sensing fingers 121 and 122. P-finger 121 is connected to a first input terminal 204 of differential amplifier 202, while n-finger 122 is connected to a second input terminal 206 of differential amplifier 202. To compensate for common-mode signals, compensation circuit 221 is provided, which can be implemented in any number of suitable ways, such as in an ASIC. In some embodiments, a virtual capacitor is provided in compensation circuit 221. The virtual capacitor can be a virtual beam node in the ASIC.

[0042] Some embodiments of the present application provide two detection masses in a fully differential reverse sensing configuration, i.e., the drive signal is applied to both detection masses, thereby eliminating the need for Figure 2A and 2B The compensation circuit shown in .

[0043] Figure 3 is a schematic top view of a differential multi-mass accelerometer according to some embodiments. Figure 3 Accelerometer 300 is shown including substrate 380 , first and second proof masses 311 , 312 side-by-side and in-plane with each other above substrate 380 , and sensing fingers 321 , 322 , 323 , and 324 coupled to substrate 380 . Figure 3The implementation of the proof mass and sensing fingers in FIG is similar in many respects to that described above. Figure 1A Alternatively and additionally, the accelerometer 300 may include a sealed cavity (not shown) on the substrate 380, with the sensing fingers and the proof mass disposed within the sealed cavity.

[0044] exist Figure 3 , sensing finger 321 forms a capacitive coupling C3 with first detection mass 311, and this capacitive coupling C3 increases as first detection mass 311 moves along the positive X direction. Sensing finger 322 forms a capacitive coupling C4 with first detection mass 311, and this capacitive coupling C4 decreases as detection mass 311 moves along the positive X direction. Sensing finger 323 forms a capacitive coupling C5 with second detection mass 312, and this capacitive coupling C5 decreases as second detection mass 312 moves along the positive X direction. Sensing finger 324 forms a capacitive coupling C6 with second detection mass 312, and this capacitive coupling C6 increases as second detection mass 312 moves along the positive X direction. It should be understood that Figure 3 This is merely a simplified example, and there may be multiple sensing fingers capacitively coupled to the first and second proof masses. Similarly, each of capacitances C3, C4, C5, and C6 may represent the effective or total capacitance of a subset of such multiple sensing fingers capacitively coupled to the corresponding proof mass.

[0045] In accelerometer 300, each proof mass 311, 312 is driven by a pair of differential drive signals 331, 332. Sensing is performed at the sense fingers. Specifically, the signals from sense fingers 321 and 323 are combined to provide a first X-sense signal at signal node 325. Simultaneously, the signals from sense fingers 322 and 324 are combined to provide a second X-sense signal at signal node 326. In some embodiments, proof masses 311, 312 can be mechanically coupled via a suitable linkage but electrically isolated from each other to allow different drive signals to be applied to each proof mass.

[0046] Figure 4 Available for operation Figure 3 Schematic circuit diagram of the sensing circuit of the differential, multi-mass accelerometer shown in . Figure 4Clock signals 331 and 332 of opposite polarity are shown applied to first and second proof masses 311 and 312, respectively. Clock signals 331 and 332 have identical timing and are different representations of the same drive signal. To sense X-direction motion of both proof masses, a first subset of sense fingers 321 forming C3 with the first proof mass and a second subset of sense fingers 323 forming C5 with the second proof mass are connected to a first signal node 325 coupled to first input 204 of differential amplifier 202. Thus, a first capacitance signal from sense finger 321 is combined with a second capacitance signal from sense finger 323 at signal node 325. Differential amplifier 202 amplifies the combined signal at signal node 325 and outputs a first differential X-direction motion signal at first output terminal 208. It should be understood that other suitable circuitry and signal processing may be performed to generate the output motion signal in place of or in addition to the use of amplifier 202.

[0047] As described below, Figure 3 The fully differential inverse sensing configuration shown in [1] is capable of increasing the amplitude of motion-related charge signals while simultaneously canceling non-motion-related noise or background signals. As an illustrative example, when proof masses 311 and 312 move in the same X-direction, capacitive coupling C3 increases while capacitive coupling C5 decreases, resulting in motion-related changes in the capacitances ΔC of the respective sensing fingers 321 and 323 always having opposite polarities. Furthermore, proof masses 311 and 312 are differentially clocked, such that the voltages ΔV1 on sensing finger 321 and ΔV2 on sensing finger 323 always have opposite polarities. Due to this dual negativity in the polarity of ΔC and ΔV, the motion-related charge signals Q = ΔV·ΔC on sensing fingers 321 and 323 have the same polarity and both contribute cumulatively to the amplitude of the first X sense signal at signal node 325. At the same time, non-motion related signals, such as noise or other background signals, have opposite polarities at the sensing fingers 321 and 323 due to the differential biasing of the proof mass and are canceled when the signals are combined at the single signal node 325. As a result, differential sensing can be achieved without the use of virtual capacitors or other compensation circuits, such as Figure 2B The reverse sensing scheme is shown in FIG.

[0048] Return Reference Figure 3, which shows a signal node 325 disposed on substrate 380. In the illustrated embodiment, signal node 325 can be connected to sensing fingers 321, 323 in any suitable manner, such as using conductive structures disposed on substrate 380. The signals from sensing fingers 321 and 323 are combined at signal node 325 before being connected to differential amplifier 202. By combining the capacitance signals from different sensing fingers into a signal node disposed on the substrate, the complexity of the sensing circuit design can be reduced, and system noise performance may be improved.

[0049] Back to Figure 4 , the figure also illustrates further differential sensing of the capacitance signals at the sensing fingers 322, 324, which are combined at a signal node 326 coupled to the second input terminal 206 of the differential amplifier 202. The differential amplifier 202 can output a second differential X-direction motion signal at a second output terminal 210, which is based on the combination of the capacitance signals at the sensing fingers 322 and 326. Optionally, the first differential X-direction motion signal at the output 208 and the second differential X-direction motion signal at the output 210 can be further combined to cancel common-mode signals in the sensing circuit.

[0050] Some aspects are directed to multi-axis differential sensing using two proof masses. Figure 5 is a schematic top view of a multi-proof accelerometer with X-axis and Y-axis differential sensing, according to some embodiments. Figure 5 An accelerometer 500 is shown that includes proof masses 511 and 512, each of which can move in the XY plane in the X direction and the Y direction. Accelerometer 500 is similar in some respects to Figure 3 , wherein like components are labeled with the same reference numerals. Accelerometer 500 has a first set of sensing fingers 321, 322, 324, 323 capacitively coupled to first and second proof masses 511, 512 for sensing motion in the X direction, and a second set of sensing fingers 521, 522, 523, 524 capacitively coupled to the first and second proof masses for sensing motion in the Y direction.

[0051] To sense motion in the X direction, a pair of differential drive signals 331, 332 are applied to the first and second detection masses 511, 512, as shown in FIG. Figure 4 As shown. Figure 5In FIG. 5 , sense finger 321 is part of a first subset of the first set of sense fingers and forms an increasing capacitance with first proof mass 511 when the first proof mass moves in the positive X direction, while sense finger 323 is part of a second subset of the first set of sense fingers and forms a decreasing capacitance with second proof mass 512 when the second proof mass moves in the positive X direction. The capacitance signals at sense finger 321 and sense finger 323 are combined at signal node 325, and the combined signal can be used to generate a differential X-direction motion signal, for example, by using a method similar to that described above with respect to FIG. Figure 4 Description of the differential amplifier.

[0052] Still refer to Figure 5 , the same differential drive signal applied to the proof mass can also sense their Y-direction motion. As shown, sense finger 522 is part of a first subset of the second set of sense fingers that forms an increasing capacitance with the first proof mass 511 when the first proof mass moves in the positive Y-direction, while sense finger 524 is part of a second subset of the second set of sense fingers that forms a decreasing capacitance with the second proof mass 512 when the second proof mass moves in the positive Y-direction. The capacitance signals at sense finger 522 and sense finger 524 are combined at signal node 526, and the combined signal can be used to generate a first differential Y-direction motion signal, for example by using a Figure 4 Furthermore, the capacitance signals at sensing finger 521 and sensing finger 523 can also be combined at signal node 525 and further processed to generate a second differential Y-direction motion signal. The first and second differential Y-direction motion signals can be further combined to cancel common-mode signals.

[0053] The differential, multi-axis, multi-mass accelerometer described herein has several advantages over alternative sensing schemes. For example, embodiments of the present application provide better noise performance. The inventors have recognized and appreciated that noise in the sensed capacitance signal can come from parasitic coupling between the element and the substrate, with the parasitic coupling varying proportionally with the projected area of ​​the element onto the substrate. In some embodiments, the detection mass has a larger projected area on the substrate than the projected area of ​​the sensing fingers. Thus, by driving the detection mass and sensing at the sensing fingers, less noise is picked up due to parasitic coupling. As an added benefit, a detection mass with a large surface area and large mass can be used with little or no effect on the parasitic noise in the sensed signal, which provides additional design flexibility for the accelerometer.

[0054] As another advantage, embodiments that use a fully differential signal path to suppress common-mode effects can provide better offset performance because offset effects such as drift can be compensated for by fully differential sensing. Additionally, because no compensation circuitry is required on the ASIC to implement fully differential sensing, the ASIC circuit design can be simplified.

[0055] Furthermore, since the drive signal is applied to the proof mass moving in both the X and Y directions, a single drive signal, such as a clock signal, can be used, which simplifies the requirements for clock signal generation and reduces the risk of crosstalk between multiple clock signals.

[0056] While the previous examples are directed to embodiments having a proof mass movable in the X and Y directions, it should be understood that aspects of the present application are applicable in directions that are not necessarily orthogonal, as well as directions that are not in plane with the substrate. In some embodiments, the proof mass is additionally movable in the Z direction outside the XY plane and is configured to provide differential inverse sensing of acceleration in the Z direction.

[0057] Figure 6A is a schematic side view of a multi-axis differential accelerometer that can provide Z-direction acceleration sensing according to some embodiments. Figure 6A An accelerometer 600 is shown that includes a substrate 680 , a first proof mass 611 and a second proof mass 612 juxtaposed and in-plane with each other above the substrate 680 , and sense electrodes 621 , 622 , 623 , 624 .

[0058] Proof masses 611 and 612 are each movable in the X, Y, and Z directions, and in some embodiments are each capacitively coupled to a sensing finger (not shown) that can move in a manner similar to that of FIG. Figure 3 and 5 The proof masses 611, 612 are differentially biased by a pair of differential drive signals for X and Y direction sensing at the sense fingers, while the same drive signals also drive capacitive sensing in the Z direction.

[0059] To sense movement in the Z direction, sensing electrode 621 forms a capacitance with first proof mass 611 that increases as first proof mass 611 moves in the positive Z direction. Sensing electrode 622 forms a capacitance with first proof mass 611 that decreases as proof mass 611 moves in the positive Z direction. Sensing electrode 623 forms a capacitance with second proof mass 612 that decreases as second proof mass 612 moves in the positive Z direction. Sensing electrode 624 forms a capacitance with second proof mass 612 that increases as second proof mass 612 moves in the positive Z direction.

[0060] exist Figure 6A 6, sensing electrode 621 and sensing electrode 623 are both coupled to a Z sense signal node 625, and the capacitive signals at sensing electrodes 621, 623 can be combined at signal node 625 to generate a Z sense signal indicating motion of accelerometer 600 in the Z direction. Sense electrode 622 and sensing electrode 624 are both coupled to a Z sense signal node 626. One or both of Z sense signal nodes 625, 626 can be disposed on substrate 680.

[0061] It should be understood that the translational Z sensing configuration between the four sensing electrodes and the two proof masses is Figure 6A , this configuration is for illustrative purposes only, and aspects of the present application are not limited thereto. In some embodiments, each sensing electrode is one of a subset of a plurality of sensing electrodes capacitively coupled to the proof mass to sense motion in the Z direction. The sensing electrodes can be formed of any suitable size or shape of conductive material.

[0062] It should also be understood that Figure 6A This is only a high-level diagram illustrating the capacitive coupling relationship between the sensing electrodes and the detection mass, and the spatial relationship between the components illustrated is not to scale and is not limiting. In the embodiments of the present application, the sensing electrodes 621, 622, 623, 624 for Z-direction sensing can be arranged in any spatial relationship relative to the corresponding detection mass, and the detection mass 611 does not need to be arranged between the sensing electrodes 621, 622, as shown in FIG. Figure 6A Any suitable arrangement of sensing electrodes may be used to provide capacitive coupling to the proof mass that varies positively or negatively with Z-direction motion of the proof mass. For example, sensing electrodes 621, 622 may be disposed on the surface of substrate 680, each facing proof mass 611. In some embodiments, Z sensing may be provided by a proof mass configured using a tilt mode, as described below with respect to Figure 6B As stated.

[0063] Figure 6B is a schematic side view of a multi-axis differential accelerometer with tilt-mode Z-direction acceleration sensing, according to some embodiments. Figure 6B An accelerometer 1600 is shown that includes a substrate 680 , a first proof mass 611 and a second proof mass 612 juxtaposed and in-plane with each other above the substrate 680 , and sense electrodes 1621 , 1622 , 1623 , 1624 .

[0064] exist Figure 6BIn the embodiment of the present invention, first proof mass 611 and second proof mass 612 may also be referred to as pivotable beams, seesaws, or other similar terms. Each of first proof mass 611 and second proof mass 612 is configured to pivot about a respective axis of rotation or pivot point 1661, 1662. Sense electrodes 1621, 1622, 1623, 1624 are disposed on substrate 680 below proof masses 611, 622. In this manner, each proof mass can form a capacitance with the underlying sense electrode. In at least some embodiments, each proof mass is asymmetric in the X direction about its respective pivot point 1661, 1662 to facilitate rotation in response to acceleration in the Z direction. Optionally, an anchor (not shown) can be coupled to proof mass 611 at pivot point 1661, although alternative ways of connecting proof mass 611 to substrate 680 are possible. An example of a proof mass with a tilt-mode Z-axis sensing configuration is described in U.S. Patent No. 10,816,569, Attorney Docket No. G0766.70238US00, entitled “ZAXIS ACCELEROMETER USING VARIABLE VERTICAL GAPS,” published on October 27, 2020. The entire disclosure of that disclosure is incorporated herein by reference.

[0065] Accelerometer 1600 can provide differential sensing of motion in the Z direction in a similar configuration as accelerometer 600. In short, Figure 6B , the proof masses 611, 612 are differentially biased by a pair of differential drive signals. Sense electrode 1621 forms a capacitance C13 with the first proof mass 611 that increases as the first proof mass 611 pivots clockwise about pivot point 1661 in response to the Z motion of accelerometer 1600, while sense electrode 1622 forms a capacitance C14 with the first proof mass 611 that decreases. Simultaneously, sense electrode 1623 forms a capacitance C15 with the second proof mass 612 that decreases as the second proof mass 612 pivots counterclockwise about pivot point 1662 in response to the same Z motion, while sense electrode 1624 forms a capacitance C16 with the second proof mass 612 that increases. In some embodiments, both sense electrode 1621 and sense electrode 1623 are coupled to the Z sense signal node 625, as shown in FIG. Figure 6A As shown, the capacitance signals at the sense electrodes 1621, 1623 can be combined at the signal node 625 to generate a Z sense signal indicating the movement of the accelerometer 1600 in the Z direction. In the same embodiment, the sense electrode 1622 and the sense electrode 1624 are both coupled to the Z sense signal node 626, as shown. Figure 6A shown.

[0066] According to some embodiments, shields 651, 652 are optionally and additionally disposed below respective proof masses 611 and 612 on substrate 680. Each of shields 651, 652 can be biased with the same bias voltage applied to respective proof masses 611 and 612 to electrostatically shield the proof masses from substrate 680. Aspects of shielding are discussed in greater detail below.

[0067] Another aspect relates to a split shield disposed on a substrate. Figure 7 is a schematic side view of an accelerometer according to some embodiments. Figure 7 Two proof masses 711 and 712, a first sensing finger 721, a second sensing finger 722, and finger 714 are shown, arranged side by side in the XY plane above a surface 782 of a substrate 780. Shields 751, 752, and 753 are disposed on surface 782 of substrate 780. Finger 714 is attached to proof mass 711 and moves with proof mass 711. Proof masses 711 and 712 are differentially driven by a pair of oppositely polarized clock signals. X-direction motion of the proof mass is detected based on capacitance signals at sensing fingers 721 and 722 (which form capacitances with finger 714 that change in response to X-direction motion of proof mass 711) and capacitance signals at sensing fingers 723 and 724 (which form capacitances with finger 715 that change in response to X-direction motion of proof mass 712). The capacitive signals from sensing fingers 723, 724 can be used in conjunction with sensing fingers 721, 722 to provide differential sensing of X-direction motion using any of the methods described above.

[0068] Shielding can be provided between the substrate 780 and the proof mass and sense fingers. While a constant voltage can be applied to a shield extending beneath the proof mass and sense fingers, the inventors have recognized and appreciated that such shielding is incompatible with biasing the proof mass and using differential reverse sensing of the sense fingers for transmitting capacitive signals. In particular, due to the large surface area of ​​the proof mass facing the substrate 780, each proof mass forms a large capacitive coupling with the substrate. Because the proof mass applies a time-varying drive signal, even with constant voltage shielding, the time-varying potential difference between the proof mass and the shield can produce crosstalk in the sensed electronic signal and unwanted mechanical motion of the proof mass. Some embodiments provide a split shield to address the different shielding requirements of the biased proof mass and the sense fingers.

[0069] exist Figure 7In an embodiment, shields 751, 752, and 753 are electrically insulated from one another. The shields can be formed from conductive plates of any suitable size, shape, and material, while surface 782 can be an insulating surface of substrate 780. As shown, first shield 751 is disposed below proof mass 711, second shield 752 is disposed below sensing electrodes 721 and 722, and third shield 753 is disposed below proof mass 712. First shield 751 covers an area in the XY plane that substantially includes the projected area of ​​the suspended proof mass on the XY plane. Second shield 752 covers an area in the XY plane that substantially includes the projected area of ​​the suspended sensing fingers.

[0070] To provide shielding for the biased proof mass 711, the first shield 751 can be coupled to the same drive signal applied to the proof mass 711. Similarly, the third shield 753 can be coupled to the same drive signal applied to the proof mass 712 to provide shielding for the proof mass 712. A constant voltage can be applied to the second shield 752. In one non-limiting example, the drive signal is a clock signal that alternates between a high voltage potential and a low voltage potential, and the second shield 752 is maintained at a voltage that is the average of the high and low voltage potentials in the clock signal.

[0071] It should be understood that it is not necessary for all parts of the proof mass 711 to have the first shield 751 underneath. Figure 7 As shown, fingers 714 of proof mass 711 can be outside first shield 751, while second shield 752 is below fingers 714. Because the projected area of ​​fingers 714 is relatively small compared to the rest of proof mass 711, the capacitive coupling between fingers 714 and second shield 752 is small enough not to cause significant interference and electrostatic force issues, while second shield 752 can provide constant voltage shielding for sensing fingers 721, 722, and so on.

[0072] Figure 8A and 8B is a schematic top view of a multi-axis MEMS accelerometer 800 with fully differential reverse sensing according to some embodiments. The MEMS accelerometer 800 may be an accelerometer configured to detect acceleration in the XY directions. Figure 8A In FIG, the first detection mass block 811 and the second detection mass block 812 are clock nodes, each of which is applied with a differential clock signal of opposite polarity. Figure 8BThe same accelerometer 800 is shown, and shows a first set of sense fingers 851 capacitively coupled to a first proof mass 811 to sense XY motion of the accelerometer, and a second set of sense fingers 852 capacitively coupled to a second proof mass 812. Within the first set, a subset of X-direction sense fingers 821 are coupled to a subset of X-direction sense fingers 823 of the second set at an X-sense signal node 825 to provide an X-sense signal. Within the first set, a subset of Y-direction sense fingers 822 are coupled to a subset of Y-direction sense fingers 824 of the second set at a Y-sense signal node 826 to provide a Y-sense signal.

[0073] Figure 9A and 9B is a schematic top view of a multi-axis MEMS accelerometer 900 with fully differential reverse sensing according to some embodiments. The MEMS accelerometer 900 may be an accelerometer configured to detect acceleration in the XY- and Z-directions, i.e., a three-axis accelerometer. Figure 9A In FIG, the first detection mass block 931 and the second detection mass block 932 are clock nodes, each of which is applied with a differential clock signal of opposite polarity. Figure 9B The Z-direction sensing nodes of accelerometer 900 are shown, with the dark areas illustrating a first set of sensing fingers 951 capacitively coupled to a first proof mass 931 to sense Z-direction motion of the accelerometer, and a second set of sensing fingers 952 capacitively coupled to a second proof mass 932 .

[0074] exist Figure 9A and 9B In the multi-axis MEMS accelerometer 900 of FIG. 1 , the first proof mass 931 and the second proof mass 932 can move in each of the three (X, Y, and Z) directions, so that the two proof masses can be used to integrate differential sensing of acceleration in each of the three axes. It should be understood that such integrated multi-axis sensing is not required, and in some embodiments, an additional proof mass can be provided for Z-direction sensing, separate from the first and second proof masses used for XY sensing.

[0075] Figure 10A and 10B is a schematic top view of a multi-axis MEMS accelerometer 1000 that uses a separate proof mass for fully differential inverse sensing in the Z axis compared to sensing in the X and Y axes, according to some embodiments. The MEMS accelerometer 1000 can be an accelerometer configured to detect acceleration in the XY- and Z-directions, i.e., a three-axis accelerometer. Figure 10A As shown, the accelerometer 1000 includes a first detection mass 811 and a second detection mass 812 for XY sensing, as described above with respect to FIG. Figure 8AThe accelerometer 1000 further includes a third detection mass 1031 and a fourth detection mass 1032 for Z-direction sensing. In order to provide full differential sensing, Figure 10A The third and fourth detection masses 1031, 1032 in the CMOS are clock nodes, each having a differential clock signal of opposite polarity applied thereto.

[0076] Figure 10B 1 shows the sensing nodes of the accelerometer 1000. For XY sensing, the accelerometer 1000 includes a first set of sensing fingers 851 and a second set of sensing fingers 852, as described above with respect to FIG. Figure 8B For Z sensing, the accelerometer 1000 includes electrodes 1051 , 1052 capacitively coupled to a third proof mass 1061 , and electrodes 1053 , 1054 capacitively coupled to a fourth proof mass 1062 .

[0077] Figure 11 FIG is a schematic diagram of an electronic device 1100 that houses one or more MEMS accelerometers 1110. According to one aspect of the present application, the MEMS accelerometer 1110 may be disposed in a Figure 11 The wearable electronic device 1100 shown is used in a consumer electronic device to measure acceleration of a sensor device. In one example, the wearable electronic device 1100 is a smartwatch that measures the movement of a user's arm or hand. In another example, the electronic device 1100 may be a linear or rotational accelerometer or a gyroscopic inertial sensor used in a vehicle to track the vehicle's motion based on the MEMS accelerometer 1110. Other uses are possible.

[0078] Various aspects of the present technology can be used alone, in combination, or in various arrangements not specifically discussed in the foregoing embodiments, and thus its application is not limited to the details and arrangements of components set forth in the foregoing description or illustrated in the accompanying drawings. For example, while two proof masses are shown in some examples, it should be understood that an accelerometer may include more than two proof masses. Aspects described in one embodiment may be combined in any manner with aspects described in other embodiments.

[0079] In some embodiments, the terms "about" and "approximately" may be used to mean within ±20% of a target value, in some embodiments within ±10% of a target value, in some embodiments within ±5% of a target value, and in some embodiments within ±2% of a target value. The terms "about" and "approximately" may include the target value.

Claims

1. A differential, multi-mass, multi-axis accelerometer comprising: first and second proof masses, arranged side by side with each other and in a plane in the substrate, movable in the in-plane X and Y directions, and configured to have applied thereto a differential drive signal, the differential drive signal comprising a first polarity signal applied to the first proof mass and a second polarity signal applied to the second proof mass; a first plurality of sensing fingers coupled to the substrate, capacitively coupled to the first and second proof masses, and configured to sense X-direction movement of the first and second proof masses; a second plurality of sensing fingers coupled to the substrate, capacitively coupled to the first and second proof masses, and configured to sense Y-direction movement of the first and second proof masses, a first shielding member disposed on the substrate below the first detection mass block; and a second shield electrically isolated from the first shield and disposed on the substrate below at least some of the first plurality of sensing fingers, wherein the first plurality of sensing fingers are configured in combination to output a differential X-direction motion signal, and The second plurality of sensing fingers are configured in combination to output a differential Y-direction motion signal.

2. The differential, multi-mass, multi-axis accelerometer of claim 1 , wherein the first proof mass and the first shield are electrically coupled to a same bias voltage node.

3. The differential, multi-mass, multi-axis accelerometer of claim 2, wherein the second shield is electrically coupled to a constant voltage node.

4. The differential, multi-mass, multi-axis accelerometer of claim 1 , wherein each of the first and second proof masses is movable in a Z direction, and the accelerometer further comprises: a third plurality of sensing electrodes coupled to the substrate, capacitively coupled to the first and second proof masses, and configured to sense Z-direction movement of the first and second proof masses, The third plurality of sensing electrodes are configured in combination to output a differential Z-direction motion signal.

5. The differential, multi-mass, multi-axis accelerometer of claim 1 , wherein a first subset of the first plurality of sense fingers forms a variable capacitance with the first proof mass that increases as the first proof mass moves in a positive X-direction, a second subset of the first plurality of sense fingers forms a variable capacitance with the second proof mass that decreases as the second proof mass moves in the positive X-direction, and wherein the differential, multi-mass, multi-axis accelerometer further comprises an X sense signal node disposed on the substrate and configured to combine a first X sense signal from the first subset of the first plurality of sense fingers with a second X sense signal from the second subset of the first plurality of sense fingers.

6. The differential, multi-mass, multi-axis accelerometer of claim 5, further comprising: One or more differential amplifiers are coupled to the X sense signal node and configured to amplify the differential X-direction motion signal.

7. The differential, multi-mass, multi-axis accelerometer of claim 1 , wherein: The first plurality of sensing fingers and the second plurality of sensing fingers are fixedly coupled to the substrate.

8. The differential, multi-mass, multi-axis accelerometer of claim 1 , further comprising: a third proof mass and a fourth proof mass disposed side by side in the base plate and each movable in the Z direction; and A third plurality of sense electrodes is coupled to the substrate, capacitively coupled to the third and fourth proof masses, and configured to sense Z-direction movement of the third and fourth proof masses, wherein the third plurality of sense electrodes are configured in combination to output a differential Z-direction motion signal.

9. A multi-axis differential accelerometer comprising: a first movable mass and a second movable mass, disposed side by side in the base plate and each movable in the X direction and the Y direction; a first set of electrodes capacitively coupled to the first movable mass; a second set of electrodes capacitively coupled to the second movable mass; an X sense signal node disposed on the substrate and coupled to a first subset of the first set of electrodes and a first subset of the second set of electrodes, the X sense signal node being configured to provide an X sense signal indicative of motion of the accelerometer in an X direction when the first movable mass and the second movable mass are differentially biased; and a Y sense signal node disposed on the substrate and coupled to a second subset of the first set of electrodes and a second subset of the second set of electrodes, the Y sense signal node configured to provide a Y sense signal indicative of motion of the accelerometer in a Y direction when the first movable mass and the second movable mass are differentially biased; a first shielding member, disposed on the substrate below the first movable mass; and A second shield is electrically isolated from the first shield and disposed on the substrate below at least some of the electrodes in the first set of electrodes. 10 . The multi-axis differential accelerometer of claim 9 , wherein a projected area of ​​the first movable mass on the XY plane is larger than a projected area of ​​the first set of electrodes on the XY plane.

11. The multi-axis differential accelerometer of claim 9, further comprising: One or more differential amplifiers are coupled to the X sense signal node and the Y sense signal node and configured to amplify the X sense signal and the Y sense signal.

12. The multi-axis differential accelerometer of claim 9, wherein the first movable mass and the first shield are electrically coupled to a same bias voltage node.

13. The multi-axis differential accelerometer of claim 12, wherein the second shield is electrically coupled to a constant voltage node.

14. The multi-axis differential accelerometer of claim 9, wherein each of the first and second movable masses is movable in a Z direction, and the accelerometer further comprises: a Z sense signal node disposed on the substrate and coupled to a third subset of the first set of electrodes and a third subset of the second set of electrodes, the Z sense signal node being configured to generate a Z sense signal indicating movement of the accelerometer along a Z direction when the first movable mass and the second movable mass are differentially biased.

15. The multi-axis differential accelerometer of claim 9, wherein a first subset of the first set of electrodes forming a capacitance with the first movable mass that increases as the first movable mass moves in the positive X-direction; and The first subset of the second set of electrodes forms a capacitance with the second movable proof-mass that decreases as the second movable proof-mass moves in the positive X-direction.

16. The multi-axis differential accelerometer of claim 15, wherein the X sense signal node is a first X sense signal node, the X sense signal is a first X sense signal, and the accelerometer further comprises: a second X sense signal node disposed on the substrate and coupled to a fourth subset of the first set of electrodes and a fourth subset of the second set of electrodes, the second X sense signal node being configured to provide a second X sense signal when the first movable mass and the second movable mass are differentially biased; wherein a fourth subset of the first set of electrodes forms a capacitance with the first movable mass, the capacitance decreasing as the first movable mass moves in the positive X-direction, and a fourth subset of the second set of electrodes forms a capacitance with the second movable mass, the capacitance increasing as the second movable mass moves in the positive X-direction; and A differential amplifier has input terminals coupled to the first and second X-sense signal nodes, respectively, and is configured to generate an output signal indicating acceleration of the accelerometer along an X direction based on the first and second X-sense signals.

17. The multi-axis differential accelerometer of claim 9, wherein: The first set of electrodes and the second set of electrodes are fixed to the substrate.

18. The multi-axis differential accelerometer of claim 9, wherein the first movable mass and the second movable mass are mechanically coupled by a linkage mechanism and are electrically isolated from each other by the linkage mechanism.

19. A method for measuring multi-axis acceleration using a multi-mass differential accelerometer, the accelerometer comprising first and second proof masses disposed side-by-side in a substrate, the method comprising: biasing the first and second proof masses using a differential drive signal; combining, at an X sense signal node disposed on the substrate, first capacitance signals from a first plurality of sense fingers capacitively coupled to the first proof mass and second capacitance signals from a second plurality of sense fingers capacitively coupled to the second proof mass; combining, at a Y sense signal node disposed on the substrate, a third capacitance signal from a third plurality of sense fingers capacitively coupled to the first proof mass and a fourth capacitance signal from a second plurality of sense fingers capacitively coupled to the second proof mass; outputting a differential X-direction motion signal based on the signal at the X-sense signal node; outputting a differential Y-direction motion signal based on the signal at the Y sensing signal node; biasing a first shield and the first proof mass using a same drive signal, wherein the first shield is disposed on the substrate below the first proof mass; as well as A second shield is biased using a constant voltage, wherein the second shield is electrically isolated from the first shield and disposed on the substrate beneath at least some of the first and third pluralities of sensing fingers.

20. The method of claim 19, further comprising: combining, at a Z sense signal node disposed on the substrate, a fifth capacitance signal from a fifth plurality of sensing electrodes capacitively coupled to the first proof mass and a sixth capacitance signal from a sixth plurality of sensing electrodes capacitively coupled to the second proof mass; and A differential Z-direction motion signal is output based on the signal at the Z-sense signal node.

21. The method of claim 19, wherein the first capacitance signal increases as the first proof mass moves in the positive X direction, and the second capacitance signal decreases as the second proof mass moves in the positive X direction.

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