MEMS accelerometer with proof masses moving in anti-phase direction normal to the plane of the substrate

This application claims benefit under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/920,246, filed on Dec. 23, 2013, entitled “MEMS ACCELEROMETER WITH PROOF MASSES MOVING IN ANTI-PHASE DIRECTION NORMAL TO THE PLANE OF THE SUBSTRATE,” which is incorporated herein by reference in its entirety.

The present invention relates generally to inertial sensors and more particularly to MEMS accelerometers.

Inertial sensors such as accelerometers are widely used for motion sensing applications. Conventionally, an accelerometer consists of a suspended proof mass and a means of measuring the proof mass displacement with respect to the reference frame. There is always a need to provide improvements in the performance and reliability of an accelerometer for commercial applications. Typically, performance is determined by the mechanical and electrical sensitivities of the sensor and reliability is determined, among many other parameters, by the required breakout force to unstuck moving parts of the structure in case of accidental contact with other fixed or moving parts.

What is needed therefore is a system and method that provides such inertial sensors. The method and system should be easily implemented, cost effective and adaptable to existing environments. The present invention addresses the above-identified issues.

A sensor is disclosed. In a first aspect, the sensor includes a substrate and at least two proof masses. The sensor also includes a flexible coupling between the at least two proof masses and the substrate. The at least two coupling proof masses move in an anti-phase direction normal to a plane of the substrate in response to acceleration.

In a second aspect, a combined accelerometer and a magnetometer includes a substrate and at least two proof masses. The combined accelerometer and the magnetometer also includes a flexible coupling between the two proof masses and the substrate. The proof masses move in an anti-phase direction normal to a plane of the substrate in response to acceleration. The combined accelerometer and the magnetometer also includes a magnetic material that causes the combined accelerometer and magnetometer to rotate around a first axis in response to a magnetic field. The acceleration and magnetic field can be sensed separately because they have two different sensing modes, respectively the anti-phase movement of the proof masses and the tilting around the first axis of the proof masses. For instance outputs of the combined accelerometer and magnetometer can be measured by altering the configuration of transducer to sense separate the outputs given by the acceleration and magnetic field.

FIG. 1 shows an accelerometer that responds to a linear acceleration in a Z direction.

FIG. 2 shows a single anchor accelerometer in accordance with an embodiment.

FIG. 3 shows a single lever and single anchor accelerometer in accordance with an embodiment.

FIG. 4 shows an alternative single lever accelerometer in accordance with an embodiment.

FIG. 5 a shows the configuration of the sensor to be a capacitive wheatstone bridge in accordance with an embodiment.

FIG. 5 b shows a configuration for sensing acceleration in a Wheatstone bridge configuration as shown in accordance with an embodiment.

FIG. 5 c shows a configuration for sensing magnetic field in a Wheatstone bridge configuration as shown in accordance with an embodiment.

FIG. 6 shows an accelerometer where a capacitive readout is performed on a rotational lever instead on proof masses in accordance with an embodiment.

FIG. 7 shows an accelerometer including the tilt cancellation paddles in accordance with an embodiment.

FIG. 8 shows a combined accelerometer and magnetometer sensor in accordance with an embodiment.

FIGS. 9-12 show a various embodiments of sensors that includes a cap as part thereof.

The present invention relates generally to inertial sensors and more particularly to MEMS accelerometers. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein.

In the described embodiments micro-electro-mechanical systems (MEMS) refers to a class of structures or devices fabricated using semiconductor-like processes and exhibiting mechanical characteristics such as the ability to move or deform. MEMS devices often, but not always, interact with electrical signals. MEMS devices include but are not limited to gyroscopes, accelerometers, magnetometers, pressure sensors, microphones, and radio-frequency components. Silicon wafers containing MEMS structures are referred to as MEMS wafers.

In the described embodiments, the MEMS device may refer to a semiconductor device implemented as a micro-electro-mechanical system. The MEMS structure may refer to any feature that may be part of a larger MEMS device. The semiconductor layer with the mechanically active MEMS structure is referred to as the device layer. An engineered silicon-on-insulator (ESOI) wafer may refer to a SOI wafer with cavities beneath the silicon device layer or substrate. A handle wafer typically refers to a thicker substrate used as a carrier for the thinner silicon device substrate in a silicon-on-insulator wafer. A handle substrate and a handle wafer can be interchanged.

In the described embodiments, a cavity may refer to an opening or recession in a substrate wafer and an enclosure may refer to a fully enclosed space. A post may be a vertical structure in the cavity of the MEMS device for mechanical support. A standoff is a vertical structure providing electrical contact.

In the described embodiments, a rigid structure within a MEMS device that moves when subject to force may be referred to as a plate. Although rigid plates are preferred for the described embodiments, semi rigid plates or deformable membranes could replace rigid plates. Plates may comprise of silicon, silicon containing materials (e.g. poly-silicon, silicon oxide, silicon nitride), metals and materials that are used in semiconductor processes (e.g. aluminum nitride, germanium). A back plate may be a solid or perforated plate comprising at least one electrode. The electrode can be comprised of semiconductor process compatible conductive materials (e.g. poly-silicon, silicon, aluminum, copper, nickel, titanium, chromium, gold). The electrodes may have insulating films on one or more surfaces.

FIG. 1 shows top and side views of an accelerometer 100 that responds to a linear acceleration in a Z direction. The accelerometer 100 comprises two proof masses PM1 102 B and PM2 102 A that respond to a linear acceleration in the Z direction by moving in anti-phase direction normal to a plane of a substrate 106 . The anti-phase movement is constrained by a flexible coupling between the two proof masses PM1 102 B and PM2 102 A and the substrate 106 . The flexible coupling comprises two separated anchors A1 110 A and A2 110 B, two central torsional springs B1 114 A and B2 114 B, two rotational levers L1 116 A and L2 116 B and four external torsional springs B11 118 A, B21 118 B, B12 118 C and B22 118 D. The motion of the accelerometer 100 is measured by an out-of-plane transducer on the proof masses, for instance a set of capacitive differential electrode C1 and C2 120 A- 120 D.

The springs B1 114 A and B2 114 B connect the anchors A1 110 A and A2 110 B to the levers L1 116 A and L2 116 B. The four external torsional springs B11 118 A, B21 118 B, B12 118 C and B22 118 D connect the end of one lever to the end of the other lever on the opposite side trough the two proof masses PM1 102 B and PM2 102 A. In particular spring B11 118 A connects the top of the left lever L1 116 A to internal proof mass PM1 102 B that connects the bottom of the right lever L2 116 B trough the spring B22 118 D. In the same way the bottom of the left lever L1 116 A is coupled to the top of the right lever L2 116 B with the springs B12 118 C and B22 118 D.

For simplicity, suppose that the proof masses have the center of gravity on the axis of the central springs (B1 114 A and B2 114 B) and that the external springs (B12 118 C, B21 118 B, B11 118 A and B22 118 D) are coupled to the proof masses with the same distance from the center of gravity orthogonal to this axis. A more general case is described in the following.

A linear acceleration a in the Z direction will create a force in Z for each proof mass:

F PM 1 =m 1 a

F PM 2 =m 2 a

Where m 1 and m 2 are the masses of PM1 102 B and PM2 102 A respectively. On each proof mass half of this force acts on each one of the external springs, B11 118 A and B22 118 D for PM1 102 B and B12 118 C and B21 118 B for PM2 102 A. Finally this force is transferred on the extreme of the lever so in the center of the lever there is a torque that is the difference of this force times the lever of PM1 102 B and PM2 102 A:

M = m 1 ⁢ a 2 ⁢  PM ⁢ ⁢ 1 ⁢ - m 2 ⁢ a 2  PM ⁢ ⁢ 2

where the lever length of PM1 l PM1 is the distance from the springs B11 118 A to B1 114 A and B22 118 D to B2 114 B and l PM2 is the distance from the springs B12 118 C to B1 114 A and B21 118 B to B2 114 B. The torque M causes the central springs and the two levers to rotate in anti-phase and so one proof mass moves towards the substrate and the other moves in the opposite direction.

In order to cause the anti-phase movement there must be an unbalancing torque M. This unbalanced torque M can be given by a difference in the mass (m 1 ≠m 2 ), by difference in the lever (l PM1 ≠l PM2 ), or by a difference in the mass lever product (m 1 l PM1 ≠m 2 l PM2 ).

In a more general example, where the center of gravity of the mass is not lying on the spring axis or the external springs are not coupled to proof masses with the same distance orthogonal to this axis, the acceleration cause a torque in addition to the force in the Z direction. In this case the structure of sensor also rotates. The sensor also includes a transducer to measure the motion of the sensor. For instance capacitive sensing can be performed by means of electrodes on the substrate measuring the capacitance change due to the mass motion.

Single Anchor Accelerometer

FIG. 2 shows a single anchor accelerometer 200 in accordance with an embodiment. The single anchor comprises an anchor A 210 B, two central springs B1 214 A and B2 214 B, torsional springs B11 218 A, B21 218 B, B12 218 C and B22 218 D, rotational levers 216 A and 216 B, DOWN electrodes C11 220 B and C11 220 C, and UP electrodes C21 220 A and C21 220 D. In this embodiment as is seen there is only one anchor A 210 B that connects the two central springs B1 214 A and B2 214 B.

Single Lever Design

FIG. 3 shows a single lever and single anchor accelerometer 300 in accordance with an embodiment.

The spring B 314 B connects the anchor A 310 B to the lever L 316 B. The two external torsional springs B21 318 B and B22 318 D connect the end of the lever to the two proof masses PM1 302 B and PM2 302 A. The two proof masses are coupled to the substrate with two translating out-of-plane springs.

The two translating springs B11 318 A and B12 318 C must be stiff for rotation around X to have the two proof masses moving in anti-phase direction normal to the plane of the substrate. If those springs are compliant for rotation around the X axis then the sensor 300 rotates around X axis.

A linear acceleration cause a force on each of the two proof masses PM1 302 A ad PM2 302 B that is transferred to the lever L 316 B and provides a torque on the central spring B 314 B. Moreover, if there is an unbalancing in the mass and lever product then the central spring B 314 B and the lever L 316 B rotate and one of the proof masses moves towards the substrate and the other of the proof masses PM1 302 A and PM2 302 B moves in the opposite direction.

Alternative Single Lever Accelerometer

FIG. 4 shows an alternative single lever accelerometer 400 in accordance with an embodiment. The single lever comprises proof masses 402 A and 402 B, anchor 410 B, rotational lever 416 B, translating springs B11 418 A and B12 418 C, torsional springs B21 4188 , B 4148 and B22 418 D, DOWN electrodes C1 420 A, and UP electrodes C2 420 B. As explained in FIG. 3 , the translating springs B11 418 A and B12 418 C must stiff for rotation around the X axis to have the two proof masses moving in anti-phase direction normal to the plane of the substrate.

Two MEMS Accelerometers

FIG. 5 a shows the configuration of the sensor to be a capacitive wheatstone bridge in accordance with an embodiment. FIG. 5 b shows a configuration for sensing acceleration in a Wheatstone bridge configuration as shown in accordance with an embodiment. FIG. 5 c shows a configuration for sensing magnetic field in a Wheatstone bridge configuration as shown in accordance with an embodiment. FIG. 5 a shows the configuration of the sensor to be a capacitive wheatstone bridge in accordance with an embodiment. The sensor 500 a comprises proof masses 502 A and 502 B, anchors 510 A and 510 B, torsional springs 514 A, 514 B, 518 A, 518 B, 518 C and 518 D, rotational levers 516 A and 516 B, UP electrodes C21 ( FIGS. 5 b and 5 c ) 520 A, C21 ( FIGS. 5 b and 5 c ) 520 D, C22 ( FIGS. 5 b and 5 c ) 520 A and C22 ( FIGS. 5 b and 5 c ) 520 D, and DOWN electrodes C11 ( FIGS. 5 b and 5 c ) 520 B, C11 ( FIGS. 5 b and 5 c ) 520 C, C12 ( FIGS. 5 b and 5 c ) 520 B and C12 ( FIGS. 5 b and 5 c ) 520 C. In response to acceleration the proof masses 502 A and 502 B moves in an anti-phase direction normal to the substrate increasing the capacitance of capacitors C11 and C12 and decreasing the capacitance of the capacitors C21 and C22. For sensing acceleration, the capacitors C11, C21, C12, and C22 can be connected in a Wheatstone bridge configuration as shown in FIGS. 5 b and 5 c.

MEMS Accelerometer with Capacitive Readout on Rotational Lever

FIG. 6 shows an accelerometer 600 where a capacitive readout is performed on electrodes on levers instead on proof masses in accordance with an embodiment. The sensor comprises proof masses 602 A and 602 B, rotational levers 616 A and 616 B, anchors 610 A and 610 B, torsional springs 614 A, 614 B, 618 A, 618 B, 618 C, 618 D, UP electrodes C21 620 A and C21 620 D, and DOWN electrodes C11 620 B and 620 C. As is seen, the electrodes 620 A- 620 D are utilized to detect movement of the rotational levers 616 A and 616 B.

MEMS Accelerometer with Tilt Cancellation Paddles

FIG. 7 shows an accelerometer 700 including the tilt cancellation paddles in accordance with an embodiment. The sensor comprises proof masses 702 A and 702 B, torsional springs 714 A, 714 B, 718 A, 7188 , 718 C and 718 D, rotational levers 716 A and 7168 , UP electrodes C21 720 A and C21 720 D, DOWN electrodes C11 720 B and C11 720 C, and tilt cancellation paddles 722 A, 722 B, 722 C and 722 D. The proof masses 702 A and 702 B move in anti-phase direction normal to the plane of the substrate with the tilt cancellation paddles 722 A and 722 B. The output induced by a tilt between proof masses and the substrate is canceled by measuring the differential capacitance between tilt cancellation electrodes and UP and DOWN electrodes.

Accelerometer and Magnetometer Sensor with Differential Capacitive Sensing

FIG. 8 shows a combined accelerometer and magnetometer sensor in accordance with an embodiment. The combined accelerometer and magnetometer sensor 800 comprises proof masses 802 A and 802 B, anchors 810 A and 810 B, torsional springs 814 A, 814 B, 818 A, 818 B, 818 C and 818 D, magnetic materials 816 A and 816 B, UP electrodes C21 820 A and C22 820 D, and DOWN electrodes C11 820 C and C12 820 B. The magnetic material induces a torque in response to a magnetic field. This torque will cause the sensor 800 to rotate around x axis. Moreover, acceleration causes the proof masses 802 A and 802 B to move in anti-phase direction normal to the plane of the substrate.

The acceleration and magnetic field can be sensed separately because they cause two different sensing modes, respectively the anti-phase movement of the proof masses and the tilting around the first axis. For instance the combined accelerometer and magnetometer sensor 800 measures by altering the configuration of a capacitive readout on a circuit shown in FIG. 5 b and FIG. 5 c . In particular the signals are given by:

where A out is the output given by the acceleration and M out is the output given by the magnetic field.

Placement of Sensing Electrodes when Utilizing a Cap

A cap or a cover provides mechanical protection to a structural layer and optionally forms a portion of an enclosure for the sensor. When utilizing a cap on top of the substrate the sensing electrodes can be located in different areas on the sensor. To describe this feature in more detail refer now to the following discussion in conjunction with the accompanying Figures.

FIG. 9 illustrates a sensor 900 that includes a substrate 906 with a cap 950 coupled thereover with a sensor structure 901 coupled to the substrate by anchor 914 within a cavity 908 of the sensor. In this embodiment, the UP and DOWN electrodes 904 A and 904 B are coupled to the substrate 906 .

FIG. 10 illustrates a sensor 900 ′ similar to that of FIG. 9 except that the UP and DOWN electrodes 904 A and 904 B are coupled to the cap. FIG. 11 illustrates a sensor 900 ″ that is also similar to that of FIG. 9 except that an UP electrode 904 A and a DOWN electrode 904 B are on the substrate and there is a DOWN electrode 904 C and an UP electrode 904 D on the cap 950 . FIG. 12 illustrates a sensor 900 ′″ similar to FIG. 9 except that anchor 914 couples the sensor structure 901 to the cap 950 rather that the substrate 906 . As is readily recognized by one of ordinary skill in the art, for example other embodiments could include on the sensor 900 ′″ electrodes 904 on both the cap 950 and the substrate 906 or any combination thereof or one up electrode on the cap and one electrode on the substrate or any combination thereof.

The advantages provided by the anti-phase movement of the proof masses normal to the plane are both a high mechanical sensitivity and a high electrical sensitivity. The high mechanical sensitivity is provided because any part of each proof mass is moving by the same amount and so maximizing the inertia that transduces the acceleration in the force causing the proof masses movement. The high electrical sensitivity is provided also by the translating movement because the gap between the proof masses and the electrodes will reduce or increase everywhere by the same amount maximizing the capacitance change that is the output signal of the transducer. Moreover the movement and so the capacitive readout are differential since the proof masses move in anti-phase.

The high mechanical and electrical sensitivity combined provides a variety of other advantages such as high breakout force, high full scale, high signal to noise ratio, or allow to reduce the size of the sensor by keeping similar performances.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the present invention.