TECHNICAL FIELD OF THE DISCLOSURE
The technical field of the examples to be disclosed in the following sections is related generally to the art of operation of microstructures, and, more particularly, to operation of MEMS devices comprising MEMS magnetic sensing structures.
BACKGROUND OF THE DISCLOSURE
Microstructures, such as microelectromechanical (hereafter MEMS) devices (e.g. accelerometers, DC relay and RF switches, optical cross connects and optical switches, microlenses, reflectors and beam splitters, filters, oscillators and antenna system components, variable capacitors and inductors, switched banks of filters, resonant comb-drives and resonant beams, and micromirror arrays for direct view and projection displays) have many applications in basic signal transduction. For example, a MEMS gyroscope measures angular rate.
A gyroscope (hereafter “gyro” or “gyroscope”) is based on the Coriolis effect as diagrammatically illustrated in FIG. 1. Proof-mass 100 is moving with velocity Vd. Under external angular velocity Ω, the Coriolis effect causes movement of the poof-mass (100) with velocity Vs. With fixed Vd, the external angular velocity can be measured from Vd. A typical example based on the theory shown in FIG. 1 is capacitive MEMS gyroscope, as diagrammatically illustrated in FIG. 2.
The MEMS gyro is a typical capacitive MEMS gyro, which has been widely studied. Regardless of various structural variations, the capacitive MEMS gyro in FIG. 2 includes the very basic theory based on which all other variations are built. In this typical structure, capacitive MEMS gyro 102 is comprised of proof-mass 100, driving mode 104, and sensing mode 102. The driving mode (104) causes the proof-mass (100) to move in a predefined direction, and such movement is often in a form of resonance vibration. Under external angular rotation, the proof-mass (100) also moves along the Vs direction with velocity Vs. Such movement of Vs is detected by the capacitor structure of the sensing mode (102). Both of the driving and sensing modes use capacitive structures, whereas the capacitive structure of the driving mode changes the overlaps of the capacitors, and the capacitive structure of the sensing mode changes the gaps of the capacitors.
Current capacitive MEMS gyros, however, are hard to achieve submicro-g/rtHz because the capacitance between sensing electrodes decreases with the miniaturization of the movable structure of the sensing element and the impact of the stray and parasitic capacitance increase at the same time, even with large and high aspect ratio proof-masses.
Therefore, what is desired is a MEMS device capable of sensing angular velocities and methods of operating the same.
SUMMARY OF THE DISCLOSURE
In view of the foregoing, a MEMS gyroscope is disclosed herein, wherein the gyroscope comprises: a first substrate, comprising: a movable portion that is movable in response to an external angular velocity; a driving mechanism capable of moving the movable portion; a magnetic source for generating magnetic field; a second substrate, comprising: a magnetic sensor of a spintronic device for detecting the magnetic field from said magnetic source; and wherein the first and second substrates are bonded into a substrate assembly; and a package, comprising: the substrate assembly; and a magnet disposed at a side of the spintronic device for providing a bias magnetic field for the spintronic device along a hard axis of the spintronic device.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 diagrammatically illustrates the Coriolis effect in a MEMS structure;
FIG. 2 is a top view of a typical existing capacitive MEMS gyroscope having a driving mode and a sensing mode, wherein both of the driving and sensing mode utilize capacitance structures;
FIG. 3 illustrates an exemplary MEMS gyroscope having a magnetic sensing mechanism;
FIG. 4
a to FIG. 4c illustrates a top view of a portion of an exemplary MEMS gyroscope having a magnetic driving mechanism;
FIG. 5 illustrates a perspective view of a portion of another exemplary implementation of the MEMS gyroscope illustrated in FIG. 3, wherein the MEMS gyroscope illustrated in FIG. 5 having a magnetic driving mechanism for the driving mode and a magnetic sensing mechanism for the sensing mode
FIG. 6 illustrates an exemplary magnetic driving mechanism of the MEMS gyroscope in FIG. 5;
FIG. 7 illustrates an exemplary magnetic source of the MEMS gyroscope illustrated in FIG. 3;
FIG. 8 illustrates an exemplary magnetic sensing mechanism that can be used in the MEMS gyroscope illustrated in FIG. 3;
FIG. 9 shows an exemplary thin-film stack that can be configured into a CIP or CPP structure for use in the magnetic sensing mechanism illustrated in FIG. 8;
FIG. 10 illustrates an exemplary MEMS gyroscope that comprises multiple magnetic sensing structures;
FIG. 11
a and 11b illustrates an exemplary electrical diagram of a magnetic sensor, wherein electric wires connecting the magnetic sensor is covered by a magnetic insulation layer; and
FIG. 12
a and FIG. 12b illustrate an exemplary package having a MEMS gyroscope, wherein the package further comprises magnets for improving the performance of the MEMS gyroscope.
DETAILED DESCRIPTION OF SELECTED EXAMPLES
Disclosed herein is a MEMS gyroscope for sensing an angular velocity, wherein the MEMS gyroscope utilizes a magnetic sensing mechanism. It will be appreciated by those skilled in the art that the following discussion is for demonstration purposes, and should not be interpreted as a limitation. Many other variations within the scope of the following disclosure are also applicable. For example, the MEMS gyroscope and the method disclosed in the following are applicable for use in accelerometers.
Referring to FIG. 3, an exemplary MEMS gyroscope is illustrated herein. In this example, MEMS gyroscope 106 comprises magnetic sensing mechanism 114 for sensing the target angular velocity through the measurement of proof-mass 112. Specifically, MEMS gyroscope 106 comprises mass-substrate 108 and sensor substrate 110. Mass-substrate 108 comprises proof-mass 112 that is capable of responding to an angular velocity. The two substrates (108 and 110) are spaced apart, for example, by a pillar (not shown herein for simplicity) such that at least the proof-mass (112) is movable in response to an angular velocity' under the Coriolis effect. The movement of the proof-mass (112) and thus the target angular velocity can be measured by magnetic sensing mechanism 114.
The magnetic sensing mechanism (114) in this example comprises a magnetic source 116 and magnetic sensor 118. The magnetic source (116) generates a magnetic field, and the magnetic sensor (118) detects the magnetic field and/or the magnetic field variations that is generated by the magnetic source (116). In the example illustrated herein in FIG. 3, the magnetic source is placed on/in the proof-mass (112) and moves with the proof-mass (112). The magnetic sensor (118) is placed on/in the sensor substrate (120) and non-movable relative to the moving proof-mass (112) and the magnetic source (116). With this configuration, the movement of the proof-mass (112) can be measured from the measurement of the magnetic field from the magnetic source (116).
Other than placing the magnetic source on/in the movable proof-mass (112), the magnetic source (116) can be placed on/in the sensor substrate (120); and the magnetic sensor (118) can be placed on/in the proof-mass (112).
It is also noted that the MEMS gyroscope illustrated in FIG. 3 can also be used as an accelerometer.
The MEMS gyroscope as discussed above with reference to FIG. 3 can be implemented in many ways, one of which is illustrated in FIG. 4a to FIG. 4c. Referring to 4a, proof-mass 122 is attached thereto movable wire loop 123. Static loop 121 is affixed to anchor 125 and is electrically coupled to movable loop 121. By driving current in the coupled loops in selected directions, the loops generate attractive and repel forces, as illustrated in FIG. 4b and FIG. 4c.
Referring to FIG. 4b, the direction of the current through movable loop 123 can be fixed, for example counter-clockwise. When the current in static loop 121 has a clockwise direction, the loops 121 and 123 generate attractive force. Because the static loop (121) affixed to anchor 125 and static, the movable loop (123) is moves towards the static loop (121) under the attractive force, and so does the proof-mass (122).
When both of the current in the static loop (121) and movable loop (123) have counter-clockwise direction, as illustrated in FIG. 4c, the force between the two loops is repellent. The movable loop (123), so does the proof-mass (122), moves away from the static loop (121) under the repellent force.
As can be seen from FIG. 4a to FIG. 4c, by changing the direction of the current in the static loop while keeping the current direction in the movable loop unchanged, the proof-mass (122) can be moved towards and away from the static loop (121). In other examples, the current direction of the static loop (121) can be unchanged during operation, while the direction of the current in the movable loop (123) is varied. In another example, both of the directions of the current in the static and movable loops can be varied during operation to driving the movable loop, as well as the proof-mass, away and towards the anchor (125). In any examples, the frequency of changing the current direction can be equal or close to the resonate frequency of the proof-mass in the driving direction. It is noted that multiple static and movable loop pairs can be provided for a proof-mass to increase the driving efficiency, even though FIG. 4a shows two loop pairs.
Another exemplary magnetic driving mechanism is illustrated in FIG. 5. Referring to FIG. 5, the mass substrate (108) comprises a movable proof-mass (126) that is supported by flexible structures such as flexures 128, 129, and 130. The layout of the flexures enables the proof-mass to move in a plane substantially parallel to the major planes of mass substrate 108. In particular, the flexures enables the proof-mass to move along the length and the width directions, wherein the length direction can be the driving mode direction and the width direction can be the sensing mode direction of the MEMS gyro device. The proof-mass (126) is connected to frame 132 through flexures (128, 129, and 130). The frame (132) is anchored by non-movable structures such as pillar 134. The mass-substrate (108) and sensing substrate 110 are spaced apart by the pillar (134). The proof-mass (112) in this example is driving by a magnetic driving mechanism (136). Specifically, the proof-mass (126) can move (e.g. vibrate) in the driving mode under magnetic force applied by magnetic driving mechanism 136, which is better illustrated in FIG. 6.
Referring to FIG. 6, the magnetic driving mechanism 136 comprise a magnet core 138 surrounded by coil 140. By applying an alternating current through coil 140, an alternating magnetic field can be generated from the coil 140. The alternating magnetic field applies magnetic force to the magnet core 140 so as to move the magnet core. The magnet core thus moves the proof-mass.
With the magnetic driving mechanisms as discussed above with reference to FIG. 4a to FIG. 4c and FIG. 5, the proof-mass (112) can be composed of any suitable materials, not necessarily conductive materials that are required by existing capacitive gyroscopes. In particular, the proof-mass can be composed of a material with superior thermo-mechanical property so as to reduce the noise signal caused by thermo-mechanical coupling of the proof-mass and associated features, such as beams and frames for supporting the proof-mass. In one example, the proof-mass can be composed of ceramic material, such as Sitall glass, which is also known as Sitall CO-115M or Astrositall.
The magnetic source (114) of the MEMS gyroscope (106) illustrated in FIG. 3 can be implemented in many ways, one of which is illustrated in FIG. 7. Referring to FIG. 7, conductive wire 142 is displaced on/in proof-mass 112. In one example, conductive wire 142 can be placed on the lower surface of the proof-mass (112), wherein the lower surface is facing the magnetic sensors (118 in FIG. 3) on the sensor substrate (110, in FIG. 3). Alternatively, the conductive wire (142) can be placed on the top surface of the proof-mass (112), i.e. on the opposite side of the proof-mass (112) in view of the magnetic sensor (118). In another example, the conductive wire (142) can be placed inside the proof-mass, e.g. laminated or embedded inside the proof-mass (112), which will not be detailed herein as those examples are obvious to those skilled in the art of the related technical field.
The conductive wire (142) can be implemented in many suitable ways, one of which is illustrated in FIG. 7. In this example, the conductive wire (142) comprises a center conductive segment 146 and tapered contacts 144 and 148 that extend the central conductive segment to terminals, through the terminals of which current can be driven through the central segment. The conductive wire (142) may have other configurations. For example, the contact tapered contacts (144 and 148) and the central segment (146) maybe U-shaped such that the tapered contacts may be substantially parallel but are substantially perpendicular to the central segment, which is not shown for its obviousness.
The magnetic sensor (118) illustrated in FIG. 3 can be implemented to comprise a reference sensor (150) and a signal sensor (152) as illustrated in FIG. 8. Referring to FIG. 8, magnetic senor 118 on/in sensor substrate 120 comprises reference sensor 150 and signal sensor 152. The reference sensor (150) can be designated for dynamically measuring the magnetic signal background in which the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7) co-exists. The signal sensor (152) can be designated for dynamically measuring the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7). In other examples, the signal sensor (152) can be designated for dynamically measuring the magnetic signal background in which the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7) co-exists, while the signal sensor (150) can be designated for dynamically measuring the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7).
The reference sensor (150) and the signal sensor (152) preferably comprise magneto-resistors, such as AMRs, giant-magneto-resistors (such as spin-valves, hereafter SV), or tunneling-magneto-resistors (TMR). For demonstration purpose, FIG. 9 illustrates a magneto-resistor structure, which can be configured into CIP (current-in-plane, such as a spin-valve) or a CPP (current-perpendicular-to-plane, such as TMR structure). As illustrated in FIG. 9, the magneto-resistor stack comprises top pin-layer 154, free-layer 156, spacer 158, reference layer 160, bottom pin layer 162, and substrate 120. Top pin layer 154 is provided for magnetically pinning free layer 156. The top pin layer can be comprised of IrMn, PtMn or other suitable magnetic materials. The free layer (156) can be comprised of a ferromagnetic material, such as NiFe, CoFe, CoFeB, or other suitable materials or the combinations thereof. The spacer (158) can be comprised of a non-magnetic conductive material, such as Cu, or an oxide material, such as Al2O3 or MgO or other suitable materials. The reference layer (160) can be comprised of a ferromagnetic magnetic material, such as NiFe, CoFe, CoFeB, or other materials or the combinations thereof. The bottom pin layer (162) is provided for magnetic pinning the reference layer (160), which can be comprised of a IrMn, PtMn or other suitable materials or the combinations thereof. The substrate (120) can be comprised of any suitable materials, such as glass, silicon, or other materials or the combinations thereof.
In examples wherein the spacer (158) is comprised of a non-magnetic conductive layer, such as Cu, the magneto-resistor (118) stack can be configured into a CIP structure (i.e. spin-valve, SV), wherein the current is driven in the plane of the stack layers. When the spacer (158) is comprised of an oxide such as Al2O3, MgO or the like, the magneto-resistor stack (118) can be configured into a CPP structure (i.e. TMR), wherein the current is driven perpendicularly to the stack layers.
In the example as illustrated in FIG. 9, the free layer (156) is magnetically pinned by the top pin layer (154), and the reference layer (160) is also magnetically pinned by bottom pin layer 162. The top pin layer (154) and the bottom pin layer (162) preferably having different blocking temperatures. In this specification, a blocking temperature is referred to as the temperature, above which the magnetic pin layer is magnetically decoupled with the associated pinned magnetic layer. For example, the top pin layer (154) is magnetically decoupled with the free layer (156) above the blocking temperature TB of the top pin layer (154) such that the free layer (156) is “freed” from the magnetic pinning of top pin layer (154). Equal to or below the blocking temperature TB of the top pin layer (154), the free layer (156) is magnetically pinned by the top pin layer (154) such that the magnetic orientation of the free layer (156) is substantially not affected by the external magnetic field. Similarly, the bottom pin layer (162) is magnetically decoupled with the reference layer (160) above the blocking temperature TB of the bottom pin layer (162) such that the reference layer (160) is “freed” from the magnetic pinning of bottom pin layer (162). Equal to or below the blocking temperature TB of the bottom pin layer (162), the reference layer (160) is magnetically pinned by the bottom pin layer (162) such that the magnetic orientation of the reference layer (162) is substantially not affected by the external magnetic field.
The top and bottom pin layers (154 and 162, respectively) preferably have different blocking temperatures. When the free layer (156) is “freed” from being pinned by the top pin layer (154), the reference layer (160) preferably remains being pinned by the bottom pin layer (162). Alternatively, when the free layer (156) is still pinned by the top pin layer (154), the reference layer (160) can be “freed” from being pinned by the bottom pin layer (162). In the later example, the reference layer (160) can be used as a “sensing layer” for responding to the external magnetic field such as the target magnetic field, while the free layer (156) is used as a reference layer to provide a reference magnetic orientation.
The different blocking temperatures can be accomplished by using different magnetic materials for the top pin layer (154) and bottom pin layer (162). In one example, the top pin layer (154) can be comprised of IrMn, while the bottom pin layer (162) can be comprised of PtMn, vice versa. In another example, both of the top and bottom pin layers (154 and 162) may be comprised of the same material, such as IrMn or PtMn, but with different thicknesses such that they have different blocking temperatures.
It is noted by those skilled in the art that the magneto-resistor stack (118) is configured into sensors for sensing magnetic signals. As such, the magnetic orientations of the free layer (156) and the reference layer (160) are substantially perpendicular at the initial state. Other layers, such as protective layer Ta, seed layers for growing the stack layers on substrate 120 can be provided. It is further noted that the magnetic stack layers (118) illustrated in FIG. 9 are what is often referred to as “bottom pin” configuration in the field of art. In other examples, the stack can be configured into what is often referred as “top pinned” configuration in the field of art, which will not be detailed herein.
In some applications, multiple magnetic sensing mechanisms can be provided, an example of which is illustrated in FIG. 10. Referring to FIG. 10, magnetic sensing mechanisms 116 and 164 are provided for detecting the movements of proof-mass 112. The multiple magnetic sensing mechanisms can be used for detecting the movements of proof-mass 112 in driving mode and sensing mode respectively. Alternatively, the multiple magnetic sensing mechanisms 116 and 164 can be provided for detecting the same modes (e.g. the driving mode and/or the sensing mode).
The magnetic field from the magnetic sources is measured by magnetic sensors so as to extract the movements of the proof-mass. For measuring the magnetic field, current is driven through the magnetic sensor so as to obtain the resistance of the magnetic sensor. The current through the magnetic sensor, however, also generates magnetic field around the magnetic sensor; and such magnetic field can be mixed with the magnetic field from the magnetic source. Obviously, such mixture causes error. There are multiple ways to eliminate such error. One of the methods is to magnetically insulate the wires connecting the magnetic sensor, as illustrated in FIG. 11a and FIG. 11b.
Referring to FIG. 11a, magnetic sensor 208 is formed on substrate 112. The magnetic sensor (208) is connected to terminals 200 and 202 through wires 204 and 206 such that current can flow through the magnetic sensor through terminals 200 and 202 and wires 204 and 206. To eliminate mixture of magnetic field generated by the wires 204 and 206 to magnetic field generated by the magnetic source, wires 204 and 206 are covered by a magnetic insulating layer, which is better illustrated in a side view illustrated in FIG. 11b.
Referring to FIG. 11b, magnetic sensor 208 is connected to wires 210 and 214 through which current can be delivered to magnetic sensor 208. Wires 210 and 214 are covered by magnetic insulating layers 208 and 212 for eliminating magnetic field generated by wires 210 and 214 from being mixed with magnetic field generated by magnetic sources. The magnetic insulating layers 208 and 204 can be composed of soft magnetic materials such as nickel-iron metal alloys.
In examples wherein the magnetic sensors are spintronic devices such as spin-valves or MTJs (magnetic-tunneling-junction), it is often preferred that a bias magnetic field is applied to the magnetic sensors along the hard axis. One approach is to fabricate integrated magnets at the sides of the magnetic sensors on the sensor substrate. Another approach is to integrate magnets on the packages of the MEMS gyroscope, an example of which is illustrated in FIG. 12a and FIG. 12b.
Referring to FIG. 12a, MEMS gyroscope 218 having a spintronic device as magnetic sensor is placed in a cavity of package 224, wherein the package may have lid that is removed from the figure for simplicity. Magnets 220 and 222 are disposed near at the sides of the MEMS gyroscope to provide bias magnetic field for the spintronic device of the MEMS gyroscope. FIG. 12b better illustrates the arrangement.
Referring to FIG. 12b, the spintronic device of magnetic sensor 218 has a hard axis along the width direction. The magnets 220 and 222 are disposed at the sides of the MEMS gyroscope 218 such that the bias magnetic field generated by magnets 220 and 222 are along the hard axis of the spintronic device of magnetic sensor 218.
It will be appreciated by those of skilled in the art that a new and useful MEMS gyroscope has been described herein. In view of the many possible embodiments, however, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of what is claimed. Those of skill in the art will recognize that the illustrated embodiments can be modified in arrangement and detail. Therefore, the devices and methods as described herein contemplate all such embodiments as may come within the scope of the following claims and equivalents thereof. In the claims, only elements denoted by the words “means for” are intended to be interpreted as means plus function claims under 35 U.S.C. §112, the sixth paragraph.
Patent Application
20150033856
