MEMS MOTION SENSOR AND METHOD OF MANUFACTURING
A MEMS motion sensor and its manufacturing method are provided. The sensor includes a MEMS wafer including a proof mass and flexible springs suspending the proof mass and enabling the proof mass to move relative to an outer frame along mutually orthogonal x, y and z axes. The sensor includes top and bottom cap wafers including top and bottom cap electrodes forming capacitors with the proof mass, the electrodes being configured to detect a motion of the proof mass. Electrical contacts are provided on the top cap wafer, some of which are connected to the respective top cap electrodes, while others are connected to the respective bottom cap electrodes by way of insulated conducting pathways, extending along the z axis from one of the respective bottom cap electrodes and upward successively through the bottom cap wafer, the outer frame of the MEMS wafer and the top cap wafer.
This patent application is a continuation-in-part of international application no. PCT/CA2014/050730 filed on Aug. 1, 2014, which claims priority from U.S. application No. 61/861,786 filed on Aug. 2, 2013 and from U.S. application No. 61/861,821 filed on Aug. 2, 2013. The disclosures of each of these applications are incorporated herein by reference in their entirety.
TECHNICAL FIELD OF THE INVENTIONThis invention relates to MicroElectroMechanical Systems (MEMS) motion sensors enabling electrical measurements from top and/or bottom caps. The invention also relates to a method for manufacturing MEMS motion sensors.
BACKGROUNDMEMS inertial sensors, which include accelerometers and angular rate sensors or gyroscopes, are used in a growing number of applications which have been increasing steadily over the past decade.
Presently, most MEMS gyroscopes use polysilicon as their mechanical material. However, due to the build-up of stresses in films deposited during the fabrication of these devices, processes for physical and chemical deposition are limited to only a few micrometers of material. Consequently polysilicon devices tend to have small masses. Small sensing masses provide low measurement sensitivity and higher vulnerability to thermal noise. Additionally, since springs and comb electrodes are patterned in the same material as the mass, the spring and electrode widths are limited to only a few microns, leading to small sense capacitances and weak springs. Furthermore, the dimensions of the capacitors, springs, and proof mass are all determined by the mechanical polysilicon film thickness. Some MEMS gyroscope manufacturers have tried to address sensitivity and noise issues by using a thicker MEMS layer made out of a single crystal silicon layer. However, as with the polysilicon devices, the spring width cannot be decoupled from the mass thickness. If the mass thickness is increased to increase sensitivity or decrease noise, the spring stiffness will increase, counteracting the effects of the mass increase.
MEMS gyroscopes are generally two-dimensional architectures using comb drives and detectors. The directions parallel to the plane of the device (typically denoted x and y) are similar (in mass distribution, symmetry, etc.), but the direction perpendicular to the plane (z) is different from the other two. Consequently, different angular rate transduction methods must be used for each, resulting in two classes of gyroscopes: 2 axis x/y gyroscopes and 1 axis z gyroscopes. Devices marketed as three axis gyroscopes typically consist of three gyroscopes integrated onto the same chip with as many as four to six proof masses.
Numerous subsequent improvements in MEMS inertial measurement unit (IMU) packaging have been made to simplify the package and reduce cost. Most of these approaches take advantage of the 2D planar nature of silicon microelectronics fabrication. Most MEMS devices are fabricated by successively depositing thin films, using a photolithographic process to form the desired 2D shape of the film, such as the MEMS inertial sensor proof mass, and etching the pattern into the film. In some cases the photolithographic process produces a form into which the film is plated or deposited to form the desired pattern. This process sequence is repeated over and over to form the final device. As a result, most MEMS devices are planar or two-dimensional since they consist of a stack of very thin films, each typically on the order of micrometer thick or less.
In all these cases a cap (e.g. silicon or glass) is placed over the MEMS to protect it and electrical contact is made to the top of the MEMS and/or CMOS. Most of these integration approaches are based on the 2D nature of the sensors with detection and signal transduction in the plane of the device. For example, almost all accelerometers and gyroscopes use comb capacitors for drive and detection in the plane of the device. Consequently the electrical leads have to be brought out on the MEMS wafer under the cap, so IMU packaging still requires wire bonding and packaging.
Efforts have been made to overcome the sensitivity limitations due to the small mass by using bulk silicon micromachining to fabricate a larger proof mass from the full thickness of the silicon wafer. Most of these efforts have been directed towards the development of accelerometers; little work has been done on large proof mass gyroscopes.
What is needed is a MEMS motion sensor which allows transmitting electrical signals from within the sensor to at least one cap, while enclosing the proof mass. It would also be desirable for the motion sensor to allow measurement of acceleration along three axes, and also the measurement of angular rate. Current pendulous accelerometer designs have not been successfully adapted to angular velocity measurements.
Additionally, what is needed is a wafer-scale fabrication method in which the proof mass is sealed in an enclosure which provides electrodes above and also below the proof mass, to drive and sense the motion.
SUMMARY OF THE INVENTIONA MEMS motion sensor is provided. The MEMS wafer has first and second opposed sides and includes an outer frame, a proof mass and flexible springs suspending the proof mass relative to the outer frame and enabling the proof mass to move relative to the outer frame along mutually orthogonal x, y and z axes. The sensor also includes top and bottom cap wafers respectively bonded to the first and second sides of the MEMS wafer. The top cap wafer, the bottom cap wafer and the outer frame of the MEMS wafer define a cavity for housing the proof mass. The MEMS wafer, the top cap wafer and the bottom cap wafer are electrically conductive, and are preferably made of silicon-based semiconductor. Top and bottom cap electrodes are respectively provided in the top and bottom cap wafers and form capacitors with the proof mass, the top and bottom cap electrodes are configurable to detect a motion of the proof mass. Electrical contacts are provided on the top cap wafer and form first and second sets of electrical contacts. The electrical contact of the first set are connected to the respective top cap electrodes, and the electrical contacts of the second set are connected to the respective bottom cap electrodes by way of respective insulated conducting pathways, each extending along the z axis from one of the respective bottom cap electrodes and upward successively through the bottom cap wafer, the outer frame of the MEMS wafer and the top cap wafer.
In some embodiments, the proof mass and flexible springs form a resonant structure having resonant frequencies fx, fy and fz for motion along the x, y and z axes, respectively.
In some embodiments, the MEMS motion sensor comprises electrode assemblies (or sets of electrodes), each including at least one pair of the top and/or bottom cap electrodes. Preferably, the motion sensor includes a first set of electrodes configurable to detect a rocking motion of the proof mass about the y axis, indicative of an acceleration of the proof mass along the x axis; a second set of electrodes configurable to detect a rocking motion of the proof mass about the x axis, indicative of an acceleration of the proof mass along the y axis; and a third set of electrodes configured to detect a translational motion of the proof mass along the z axis, indicative of an acceleration of the proof mass along the z axis.
In some embodiments, one set of electrode is configured to vibrate the proof mass at a drive frequency along the z axis, and two other sets of electrodes are configured to detect Coriolis-induced oscillations of the proof mass along the x and y axes, indicative of an angular motion of the proof mass about the y and x axes, respectively.
The drive frequency preferably corresponds to the resonant frequency fz. In some embodiments, the resonant frequency fz is substantially identical to each of the respective resonant frequencies fx, fy, in order to provide matched resonance conditions. Preferably, a relative difference between any two of the resonant frequencies fz, fx, fy is no more than 10%. It is also possible that the resonant structure be shaped, sized and configured with each of the resonant frequencies fx, fy and fz being substantially different, for example with mutually non-overlapping 3 dB-bandwidths, in order to provide non-matched resonance conditions.
In some embodiments, the drive frequency is lower than at least one of the respective resonant frequencies fx and fy, such as 10-40% lower.
In some embodiments, one set of electrodes is configured to vibrate the proof mass at a drive frequency along a corresponding one of the x and y axes, respectively, and another set of electrodes is configured to detect Coriolis-induced oscillations of the proof mass along the other one of the x and y axes, indicative of an angular motion of the proof mass about the z axis.
In some embodiments, the resonant structure is shaped, sized and configured such that each of the resonant frequencies fx, fy and fz is substantially higher than sensing frequencies at which the electrode assemblies are configured to detect the motion of the proof mass in response to accelerations of the proof mass along to the x, y and z axes, respectively.
In some embodiments, the top and bottom cap electrodes may comprise a pair of said top and bottom electrodes aligned with the z axis, which is centered relative to the proof mass. The top and bottom cap electrodes may also comprise two pairs of said top and bottom electrodes disposed along the x axis on each side of the y axis, and also possibly two pairs of said top and bottom electrodes disposed along the y axis on each side of the x axis.
In some embodiments, the proof mass can be shaped as a convex polygonal prism, which is preferably regular, such as an octagonal prism. Typically, the motion sensor includes four flexible springs.
The top and bottom electrodes typically extend through the entire thicknesses of the top and bottom cap wafers, respectively, and are preferably delimited by insulated channels. Preferably, the MEMS wafer is a silicon on insulator (SOI) wafer with an insulating layer separating a device layer from a handle layer, and the proof mass can be patterned in both the device and handle layers.
In some embodiments, the motion sensor comprise an additional insulated conducting pathway extending through the bottom cap wafer, through the frame of the MEMS wafer, and though the top cap wafer, between one of the electrical contacts of the top cap wafer to the electrical contact of the bottom cap wafer, thereby forming a conductive feedthrough.
A method for manufacturing the MEMS motion sensor is also provided. The method comprises the steps of:
- a) providing the top and bottom cap wafers and forming insulated conducting cap wafer channels; patterning trenches and filling the trenches to form electrodes on the inner sides of the cap wafers, some of the insulated conducting cap wafer channels being electrically connected to the respective electrodes;
- b) providing a MEMS wafer and patterning portions of the proof mass, of the flexible springs and of the outer frame with insulated conducting MEMS wafer channels in one of the first and second sides;
- c) bonding the side of the MEMS wafer patterned in step b) to the inner side of the top or bottom cap wafer by aligning the insulated conducting cap wafer channels with the corresponding portions of the insulated conducting MEMS channels, and by aligning the electrodes relative to the proof mass and the springs;
- d) patterning the remaining portions of the proof mass, of the flexible springs and of the outer frame (164) with the insulated conducting MEMS wafer channels on the other side of the MEMS wafer;
- e) bonding the side of the MEMS wafer patterned in step d) to the inner side of the other top or bottom cap wafer, by aligning the electrodes of the top cap wafer with the electrodes of the bottom cap wafer and by aligning the insulated conducting cap wafer channels of the other cap wafer with the remaining portions of the insulated conducting MEMS channels, creating insulated conducting pathways, some of which extend from the electrodes of the bottom cap wafer, through the outer frame of the MEMS wafer and through the top cap wafer, and enclosing the proof mass suspended relative to the outer frame by the flexible springs within a cavity formed by the top and bottom cap wafers and by the outer frame (164) of the MEMS wafer (16); and
- f) removing a portion of the top and bottom cap wafers to expose and isolate the insulated conducting pathways and the electrodes in the top and bottom cap wafers.
The method can also include a step of forming electrical contacts on the outer side of the top cap wafer connected with the insulated conducting pathways, allowing routing of electrical signals from the electrodes of the bottom cap wafer to these electrical contacts. The method can also include a step of forming electrical contacts on the bottom cap wafer, connected to some of the insulated conducting pathways, allowing routing of electrical signals to the electrical contacts on the bottom cap wafer.
Of course, other processing steps may be performed prior, during or after the above described steps. The order of the steps may also differ, and some of the steps may be combined.
It should be noted that the appended drawings illustrate only exemplary embodiments of the invention and should therefore not be considered limiting of its scope, as the invention may admit to other equally effective embodiments.
In the following description, similar features of the drawings have been given similar reference numerals. To preserve the clarity of the drawings, some reference numerals have been omitted when they were already identified in a preceding figure.
The present invention provides a MEMS motion sensor formed by a top cap wafer, a central MEMS wafer and a bottom cap wafer, the wafers being made of an electrically conducting material, such as silicon. Both the top and bottom cap wafers are provided with electrodes on both sides of a pendulous proof mass. The MEMS motion sensor also includes insulated conducting pathways, at least some of which extend from electrodes in the bottom cap wafer, through the MEMS wafer and to the top cap wafer, allowing routing or transmitting electrical signals sensed by the electrodes of the bottom cap through the MEMS wafer, and more specifically through the lateral frame of the sensor, from the bottom cap wafer to the top cap wafer. This architecture of the MEMS motion sensor enables the placement of electrodes and electrical leads above, below, and/or around a pendulous proof mass, for measuring acceleration and/or angular velocity. This architecture of the MEMS motion sensor thus not only allows encapsulating the proof mass, it also makes efficient use of the protective caps by including electrodes in the caps, and by providing insulated conducted pathways which allow routing signals from the bottom side of the sensor to the top side, allowing the placement of the electrical contacts on a single side of the sensor. Of course, if needed, electrical contacts can also be placed on the bottom side of the sensor. Yet another advantage of the present MEMS motion sensor resides in the patterning of a bulk, pendulous proof mass (having for example a thickness varying from 400 to 700 um), which is suspended by flexible springs patterned such that they are much thinner than the proof mass. Further details regarding devices and methods of operating motion sensors are described in international application number PCT/CA2014/050635 entitled “MEMS Device and Method of Manufacturing” filed on Jul. 4, 2014, and the corresponding U.S. Application No. filed on Feb. 13, 2015, the entire contents of these applications being incorporated herein by reference.
Referring to
The motion sensor 10 includes top and bottom cap electrodes 13, 15 respectively provided in the top and bottom cap wafers 12, 14, and forming capacitors with the proof mass 17. The electrodes are configured to detect a motion of the proof mass 17, such as a translation along the z axis, or a rocking along the x or y axis. Electrical contacts 42 are provided on the top cap wafer 12. The contacts 42 form first and second sets of electrical contacts: the electrical contact of the first set are connected to the top cap electrodes 13, and the electrical contacts of the second set are connected to the bottom cap electrodes 15 by way of respective insulated conducting pathways, such as pathway 33ii. The pathways connected to the bottom cap electrodes extend upward along the z axis, successively through the bottom cap wafer 14, the outer frame 164 of the MEMS wafer 16 and the top cap wafer 12. Of course, other electrical contacts can be provided on the top cap wafer, such as for connecting feedthroughs extending from the bottom to the top cap for example, and other insulated conducting pathways, such as pathway 33i, can be provided for connecting electrodes of the top cap wafer, and also possibly of the proof mass.
In the present description, the terms “top” and “bottom” relate to the position of the wafers as shown in the figures. Unless otherwise indicated, positional descriptions such as “top”, “bottom” and the like should be taken in the context of the figures and should not be considered as being limitative. The top cap wafer can also be referred as a first cap wafer, and the bottom cap wafer can be referred as a second cap wafer. The terms “top” and “bottom” are used to facilitate reading of the description, and persons skilled in the art of MEMS know that, when in use, MEMS devices can be placed in different orientations such that the “top cap wafer” and the “bottom cap wafer” are positioned upside down. In this particular embodiment, the “top” refers to the direction of the device layer.
In this specific embodiment, the proof mass 17 is suspended by four flexible springs (27i, 27ii, 27iii and 27iv—identified in
The proof mass 17 and flexible springs 27 form together a resonant structure having resonant frequencies fx, fy and fz for motion along the x, y and z axes, respectively. The resonant frequencies can be set by adjusting the width and thickness of the springs and/or the size and shape of the proof mass.
Referring to
In
As shown in
Finally, the motion sensor includes a pair of top and bottom electrodes 13v and 15v. Electrode 15v is similar to electrode 13v, but hidden underneath proof mass 17. The electrodes 13v and 15v are aligned with the z axis, which is centered relative to the proof mass.
Of course, the electrode assemblies can be grouped and/or positioned differently, and include more or less electrodes, as long as they are able to detect motion of the proof mass in all three directions x, y and z.
In addition to detecting accelerations of the proof mass, the MEMS motion sensor can also be configured to detect angular rate or angular velocity (deg/sec). Typically, MEMS gyroscopes use vibrating mechanical elements to sense angular rotation via the Coriolis Effect. The Coriolis Effect arises when a mass M is moving at velocity {right arrow over (ν)} in a reference frame rotating with angular rate {right arrow over (Ω)}. An observer sitting in the rotating frame perceives the mass to be deflected from its straight-line trajectory by the Coriolis Force, given by {right arrow over (F)}Coriolis=2M{right arrow over (ν)}×{right arrow over (Ω)}, where × denotes the vector cross-product.
In order to detect angular motion of the suspended proof mass, a periodic force is applied to the proof mass along one direction. When the sensor, and by extension the proof mass, is subjected to an angular rotation, a periodic Coriolis force proportional to the rate of rotation at the same frequency as the drive, but out of phase by 90 degrees, is induced along a direction perpendicular to both the drive signal and the axis of rotation. The magnitude of this motion can measured using capacitive sensing techniques.
The MEMS motion sensor can sense motion over 5 degrees of freedom (5 DOF), that is, accelerations along x, y and z axes, and angular velocity along the x and y axes. In this case, an electrode assembly is configured to vibrate the proof mass 17 at a drive frequency along the z axis, and two other electrode assemblies are configured to detect Coriolis-induced oscillations of the proof mass along the x and y axes, indicative of an angular motion of the proof mass with respect to the y and x axes, respectively.
The resonant structure formed by the proof mass 17 and flexible springs 27 can be sized, shaped and configured to provide either matched or unmatched resonance conditions, depending on the objective sought. Referring to
Referring now to
The ratios of the frequencies can be adjusted by modifying the ratios of the rocking moment of inertia to the total mass. The ratios of the rocking frequencies fx, fy to the vertical resonant frequency fz depend chiefly on the ratio of the rocking moment of inertia to the mass,
where Kz is the z spring constant, J is the moment of inertia along one of the rocking axes, M is the mass, and Krot is the rotational spring constant, which for a four spring architecture is roughly
with S being the width of the proof mass. So the frequency ratio reduces to
is the definition of the radius of gyration, the distance from the axis of rotation of an extended object at which its mass, if concentrated into a point mass, would have the same moment of inertia as the extended object, i.e. appear as a simple pendulum. In other words, =MrG2, so
Thus, to operate non-resonantly and ensure that the rocking frequency fr (fx, fy) is higher than the resonant frequency fz, the proof mass can be designed such that
For proof masses with large lobes, J is large (i.e. large radius of gyration), so the rocking frequency fx or fy is lower than the z frequency fz. Low moment of inertia is obtained when most of the mass is concentrated beneath the axis. This occurs more naturally for proof masses with simple or “regular” cross sections. Similarly, to have the y rocking frequency fy higher than the x rocking frequency fx, the y axis moment of inertial must be smaller than the x axis moment. This can be accomplished by reducing the proof mass width along the y axis relative to the x axis.
In another embodiment, it is possible to measure angular motion about the z axis as well. In this case the MEMS motion sensor detects motion over 6 degrees of freedom (6 DOF). The x and y angular velocities are measured separately from the z angular velocity. Existing surface micromachined MEMS gyroscopes having small proof masses and sense electrodes require the gyroscope to be operated in a resonant sense mode. Advantageously, the MEMS motion sensor of the present invention can be operated in either a resonant or a non-resonant mode, due to the relatively large proof mass and sense electrodes. For higher sensitivity, the MEMS motion sensor is preferably designed with matched resonant frequencies fx, fy and fz. Alternatively, to reduce the impact temperature, fabrication, and phase sensitivities which are exacerbated by working near the peak of the sense frequency response curve, the MEMS motion sensor can be designed with non-matched resonant frequencies.
The angular velocity around the 6th or z axis is measured in a different way since the drive axis must be along an orthogonal axis. In this case, one of the first and second electrode assemblies is configured to vibrate the proof mass at a drive frequency along a corresponding one of the x and y axes, respectively, the first electrode assembly being configured to detect Coriolis-induced oscillations of the proof mass along the other one of the x and y axes, indicative of an angular motion of the proof mass about the z axis. Preferably, the drive frequency along the corresponding one of the x and y axes corresponds to a respective one of the resonant frequencies fx and fy.
The proof mass is driven along one of the lateral axes, e.g. the x-axis, at the rocking frequency, such as shown in
For a symmetric proof mass, the x and y rocking modes occur at the same frequency, so a matched-mode angular rate measurement is more natural, such as shown in
It will be appreciated that in either one of the matched or unmatched resonant modes, the resonant structure is shaped, sized and configured such that each of the resonant frequencies fx, fy and fz is substantially higher than sensing frequencies at which the electrode assemblies are configured to detect the motion of the proof mass in response to accelerations of the proof mass along to the x, y and z axes, respectively.
Depending of the application of the MEMS motion sensor (3 DOF accelerometer and/or 5 DOF or 6 DOF gyroscope) some of the top and/or bottom electrodes are connectable to driving means, and other ones of the top and/or bottom electrodes are connectable to sensing means. The top and bottom electrodes can also be reconfigurably connectable to driving and sensing means, for switching between drive and sense modes. The terms “driving means” and “sensing means” refer to any electronic circuitry configured to transmit and/or read electric signals.
The proof mass can take different shapes, such as a cross-shape as shown in
As shown in any one of
Referring now to
Referring to
Referring to
Referring to
Referring to
The motion sensor is a multi-wafer stack consisting of top and bottom cap wafers containing sense electrodes and the center MEMS wafer containing the proof mass and springs. As described previously, the stack is combined with insulated conducting pathways, which can also be referred to as electrically isolated “3 dimensional through-chip vias” (3DTCVs) to route signals from electrodes on the bottom cap and MEMS wafer through the MEMS wafer to and through the top cap wafer to bond pads on the surface, thus providing a means of monitoring the position of the proof mass in three-dimensional space.
The method for manufacturing the MEMS device will be described in connection with a preferred embodiment. However, it will be understood that there is no intent to limit the invention to the embodiment described.
Referring to
Referring to
Referring to
Referring to
Referring to
In this example, the MEMS wafer channel 163 will eventually form part of a device feedthrough, located in the periphery of the handle layer 22. Trenches 28 are etched around the conductive silicon wafer plug 26 to isolate it from the rest of the layer 22. The SOI conducting shunt 34 in the device and insulating layers 20, 24 provides electrical conductivity within the channel 163. If there were no shunt 34, the silicon plug would merely be a mechanical support.
Referring to
Similar to the bonding of the other cap wafer, the bond is a conductive bond, which can be performed using various bonding method such as fusion bonding or bonding with a conducting material, such as gold thermocompression bonding or gold-silicon eutectic bonding for example. The bond is used to provide electrical contact between the channels in the MEMS wafer and the channels in the cap wafer 14, some of which are connected electrically to the bottom electrodes 15. In this manner, a conductive pathway 33i is provided from a bottom electrode 15 through the bottom cap silicon pad, handle feedthrough, SOI conducting shunt, and SOI device layer pad to the top cap wafer pad. At this point the MEMS wafer 16 is hermetically sealed between the cap wafers 12, 14. The proof mass is aligned with electrodes of the top cap and/or bottom cap and/or any handle side electrodes. Because the insulating channels do not yet fully penetrate the caps, the electrodes (such those illustrated—13i, 13ii, 13v and 15i 15ii and 15v) on each cap are shorted together through the remaining silicon.
Referring to
However, manufacturing the MEMS motion sensor 10 typically comprises the step of forming electrical contacts on at least the outer side 122 of the top cap wafer 12. The electrical contacts on the top cap are connected with the insulated conducting pathway 33i and feedthrough 25, and allow to route electrical signals from the bottom cap wafer 14 to the electrical contacts on the top cap wafer 12. Preferably, the method further comprises forming electrical contacts on the outer side 142 of the bottom cap wafer 14 as well. These electrical contacts 43, being connected to some of the insulated conducting pathway 33i, allow the routing of electrical signals from the conducting pathway 33i to the electrical contacts on the bottom cap wafer 14.
This step of forming electrical contacts on the outer sides of the top and/or bottom cap wafers can be accomplished as follows. The procedure is illustrated for one side of the MEMS device only, but of course the same steps can be performed on the other side as well.
Referring to
Referring to
Referring to
A possible embodiment of a completed IMU is shown in 25C. At this point in the process the MEMS IMU wafer is still in wafer form. For wafer scale system packaging, the I/O bond pads of the MEMS IMU and feedthroughs are designed to match the I/O pads of the sense electronics IC. The sense electronics IC wafer 44 can then be flip chip bonded directly to the top of the MEMS IMU wafer using an underfill and solder-bump wafer bonding process. These wafer bonding processes are known in the semiconductor industry and any can be used by implementing the appropriate bond pad and solder metallurgies. The bonded wafers can be diced into chips, or “MEMS IMU cubes”. The diced and hermetically sealed IMU cubes can be treated as packaged chips ready to be solder-attached to other chips, multi-chip package, or PC (printed circuit) board.
The benefits of this approach are:
- 1) The MEMS motion sensor and IC can be matched in size with the bond pad layout so that at singulation, no IC bond pads extend outward beyond the extent of the MEMS chip and the MEMS chip does not have to include any wasted area that is cut away to expose the bond pads. Both the MEMS sensor and IC wafers can be used more efficiently. This enables true MEMS/IC wafer scale packaging since dicing results in usable packaged devices.
- 2) Bond wires are eliminated between the MEMS and the IC and between the IMU system and the processing electronics. This eliminates stray inductance and capacitance that can affect measurements, as well as the additional cost of wire bonding.
- 3) No Through Silicon Vias (TSVs) are required in the IC wafer. This reduces IC costs by eliminating the additional processes required at the IC foundry to produce TSVs, eliminates the IC space required for the TSVs, and opens up sourcing for the IC wafers since many IC foundries do not have TSV capabilities.
- 4) The 3DTCV architecture enables through-MEMS-chip IC Input/Output without adding any additional TSV processes beyond those already used to fabricate the MEMS IMU itself. The only additional process steps are the contact etch and bond pad metallization required for the bottom cap.
The figures illustrate only an exemplary embodiment of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective or equivalent embodiments. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
Claims
1. A MEMS motion sensor comprising:
- an electrically conductive MEMS wafer having a first side and a second side[s] and including an outer frame, a proof mass and flexible springs suspending the proof mass relative to the outer frame and enabling the proof mass to move relative to the outer frame along mutually orthogonal x, y and z axes;
- an electrically conductive top cap wafer and an electrically conductive bottom cap wafer respectively bonded to the first side and the second side of the MEMS wafer such that the top cap wafer, the bottom cap wafer and the outer frame of the MEMS wafer define a cavity for housing the proof mass;
- a plurality of top cap wafer electrodes and a plurality of bottom cap wafer electrodes that are respectively positioned with the top cap wafer and the bottom cap wafer, the electrodes forming capacitors with the proof mass that are operative to detect a motion of the proof mass; and
- a first set of electrical contacts connected to the plurality of top cap wafer electrodes, and a second set of electrical contacts being conductively connected to the bottom cap wafer electrodes with insulated conducting pathways that extend upwardly through the bottom cap wafer, the outer frame of the MEMS wafer and the top cap wafer.
2. The MEMS motion sensor according to claim 1, wherein the proof mass and flexible springs form a resonant structure having resonant frequencies fx, fy and fz for motion along the x, y and z axes, respectively.
3. The MEMS motion sensor according to claim 2, comprising electrode assemblies, each including at least one pair of said top cap electrodes, or at least one pair of said bottom cap electrodes or a combination of said top cap electrodes and bottom cap electrodes, said electrode assemblies comprising:
- a first electrode assembly configured to detect a rocking motion of the proof mass about the y axis, indicative of an acceleration of the proof mass along the x axis;
- a second electrode assembly configured to detect a rocking motion of the proof mass about the x axis, indicative of an acceleration of the proof mass along the y axis; and
- a third electrode assembly configured to detect a translational motion of the proof mass along the z axis, indicative of an acceleration of the proof mass along the z axis.
4. The MEMS motion sensor according to claim 3, wherein one of the electrode assemblies is connectable to a drive circuit configured to vibrate the proof mass at a drive frequency along the z axis, and two other of the electrode assemblies are configured to detect Coriolis-induced oscillations of the proof mass along the x and y axes, indicative of an angular motion of the proof mass about the y and x axes, respectively.
5. The MEMS motion sensor according to claim 4, wherein the drive frequency corresponds to the resonant frequency fz.
6. The MEMS motion sensor according to claim 4, wherein the resonant frequency fz is substantially identical to each of the respective resonant frequencies fx, fy, in order to provide matched resonance conditions.
7. The MEMS motion sensor according to claim 4, wherein a relative difference between any two of the resonant frequencies fz, fx, fy is no more than 10%.
8. The MEMS motion sensor according to claim 4, wherein the drive frequency is lower than at least one of the respective resonant frequencies fx and fy.
9. The MEMS motion sensor according to claim 4, wherein the drive frequency is 10-40% lower than each of the respective resonant frequencies fx and fy.
10. The MEMS motion sensor according to claim 3, wherein one of the electrode assemblies is configured to vibrate the proof mass at a drive frequency along a corresponding one of the x and y axes, respectively, and another one of the electrode assemblies is configured to detect Coriolis-induced oscillations of the proof mass along the other one of the x and y axes, indicative of an angular motion of the proof mass about the z axis.
11. The MEMS motion sensor according to claim 10, wherein the drive frequency along the corresponding one of the x and y axes corresponds to a respective one of the resonant frequencies fx and fy.
12. The MEMS motion sensor according to claim 3, wherein the resonant structure is shaped, sized and configured such that each of the resonant frequencies fx, fy and fz is substantially higher than sensing frequencies at which the electrode assemblies are configurable with a sensing circuit to detect the motion of the proof mass in response to accelerations of the proof mass along to the x, y and z axes, respectively.
13. The MEMS motion sensor according to claim 1, wherein the resonant structure is shaped, sized and configured with each of the resonant frequencies fx, fy and fz being substantially different.
14. The MEMS motion sensor according to claim 13, wherein the resonant structure is shaped, sized and configured with the resonant frequencies fx, fy and fz having mutually non-overlapping 3 dB-bandwidths.
15. The MEMS motion sensor according to claim 1, wherein said top and bottom cap electrodes comprise a pair of said top and bottom electrodes, aligned with the z axis, the electrodes being centered relative to the proof mass.
16. The MEMS motion sensor according to claim 1, wherein said top and bottom cap electrodes comprise two pairs of said top and bottom electrodes disposed along the x axis on each side of the y axis.
17. The MEMS motion sensor according to claim 1, wherein said top and bottom cap electrodes comprise two pairs of said top and bottom electrodes disposed along the y axis on each side of the x axis.
18. The MEMS motion sensor according to claim 1, wherein the proof mass is shaped as a convex polygonal prism.
19. The MEMS motion sensor according to claim 1, wherein the proof mass is shaped as a regular convex polygonal prism.
20. The MEMS motion sensor according to claim 1, wherein the proof mass is shaped as an octagonal prism.
21. The MEMS motion sensor according to claim 1, wherein the flexible springs comprise four flexible springs.
22. The MEMS motion sensor according to claim 1, wherein the top and bottom cap wafers have respective thicknesses, the top and bottom electrodes extending through the entire thicknesses of the top and bottom cap wafers, respectively.
23. The MEMS motion sensor according to claim 1, wherein the top cap wafer, the MEMS wafer and the bottom cap wafer comprise a silicon semiconductor.
24. The MEMS motion sensor according to claim 1, wherein the MEMS wafer is a silicon on-insulator (SOI) wafer with an insulating layer separating a device layer from a handle layer.
25. The MEMS motion sensor according to claim 24, wherein the proof mass is patterned in both the device and handle layers.
26. The MEMS motion sensor according to 1, wherein top and bottom electrodes are delimited by insulated channels.
27. The MEMS motion sensor according to claim 1, wherein the bottom cap wafer is provided with an electrical contact, the MEMS motion sensor comprising an additional insulated conducting pathway extending through the bottom cap wafer, through the frame of the MEMS wafer, and through the top cap wafer along the z axis and optionally in an x-y plane, between one of the electrical contacts of the top cap wafer to one of the electrical contacts of the bottom cap wafer, thereby forming a conductive feedthrough.
28. A method for manufacturing a MEMS motion sensor, the method comprising the steps of:
- a) providing top and bottom cap wafers having respective inner and outer sides; forming insulated conducting cap wafer channels; patterning trenches and filling the trenches to form electrodes on the inner sides of said cap wafers, some of the insulated conducting cap wafer channels being electrically connected to the respective electrodes;
- b) providing a MEMS wafer having first and second sides, and patterning portions of a proof mass, of flexible springs, of an outer frame with insulated conducting MEMS wafer channels, in one of the first and second sides;
- c) bonding the side of the MEMS wafer patterned in step b) to the inner side of the top or bottom cap wafer by aligning the insulated conducting cap wafer channels with the corresponding portions of the insulated conducting MEMS channels, and by aligning the electrodes relative to the proof mass and the springs;
- d) patterning the remaining portions of the proof mass, the flexible springs, the outer frame with the insulated conducting MEMS wafer channels on the other side of the MEMS wafer;
- e) bonding the side of the MEMS wafer patterned in step d) to the inner side of the other top or bottom cap wafer, by aligning the electrodes of the top cap wafer with the electrodes of the bottom cap wafer and by aligning the insulated conducting cap wafer channels of said other cap wafer with the remaining portions of the insulated conducting MEMS channels, creating insulated conducting pathways, with some of said insulated conducting pathways extending from the electrodes of the bottom cap wafer, through the outer frame of the MEMS wafer and through the top cap wafer, and enclosing the proof mass suspended relative to the outer frame by the flexible springs within a cavity formed by the top and bottom cap wafers and by the outer frame of the MEMS wafer; and
- f) removing a portion of the outer sides of the top and bottom cap wafers to expose and isolate the insulated conducting pathways and the electrodes in the top and bottom cap wafers.
29. A method for operating a motion sensor comprising:
- operating a motion sensor including a proof mass in a cavity, the cavity defined by a conductive top cap wafer having an inner top cap side bonded to a first side of a conductive MEMS wafer that includes the proof mass suspended from flexible springs and an outer frame, the cavity being further defined by a conductive bottom cap wafer having an inner bottom cap side bonded to a second side of the MEMS wafer, the top cap wafer having a first set of top cap electrical contacts and a second set of top cap electrical contacts, the first set of electrical contacts being connected to top cap wafer electrodes and the second set of electrical contacts being connected to bottom cap wafer electrodes; and
- conducting electrical signals between the bottom cap wafer through the outer frame to electrical contacts on the top cap wafer with insulated conducting pathways that extend from the bottom cap wafer through the outer frame and the top cap wafer, the insulated conducting pathways being in conductive contact with corresponding electrical contacts on the top cap wafer.
30. The method of claim 29 further comprising driving the proof mass at a drive frequency with a drive circuit and sensing motion of the proof mass with a sensing circuit.
Type: Application
Filed: Feb 13, 2015
Publication Date: Sep 28, 2023
Inventors: Robert Mark Boysel (Delaware, OH), Louis Ross (Tokyo)
Application Number: 14/622,548