The present application claims priority from Japanese Patent Application JP 2008-302639 filed on Nov. 27, 2008, the content of which is hereby incorporated by reference into this application.
The present invention relates to a semiconductor physical quantity sensor and a control device using the same. More particularly, the invention relates to a semiconductor physical quantity sensor which is formed using fine fabrication technology for semiconductors (i.e. MEMS process) and which measures a physical quantity, for example, acceleration or an angular rate by detecting a physical quantity associated with an inertial force generated in a vibrating object and a control device using such a semiconductor physical quantity sensor.
There have been known semiconductor physical quantity sensors each of which includes a movable microelectrode formed by removing a sacrifice layer on a silicon substrate and a fixed electrode facing the movable electrode and forming electrostatic capacitance between itself and the movable electrode and which detects a change in a physical quantity based on a change in the electrostatic capacitance between the electrodes.
According to a semiconductor physical quantity sensor and a method for manufacturing the same disclosed in Japanese Patent Application Laid-Open publication No. 2006-84326, a capacitance type angular rate sensor includes a movable electrode and a fixed electrode located opposite to the movable electrode, which are formed on a support substrate of silicon. In the semiconductor physical quantity sensor, a compressive stress layer is formed on the surface of a beam suspending the movable electrode thereby causing the movable electrode to be cambered away from the support substrate. The movable electrode cambered away from the support substrate faces the fixed electrode in a position more shifted, than the fixed electrode, from the support substrate.
When a physical quantity is applied to the sensor in the thickness direction of the support substrate causing the movable electrode to be displaced away from the support substrate, the area of each of the mutually facing surface portions of the movable electrode and the fixed electrode for physical quantity detection decreases causing the capacitance between the electrodes to decrease.
When, on the other hand, a physical quantity is applied to the sensor in the thickness direction of the support substrate causing the movable electrode to be displaced toward the support substrate, the area of each of the mutually facing surface portions of the movable electrode and the fixed electrode for physical quantity detection increases causing the capacitance between the electrodes to increase.
Thus, detecting the direction and magnitude of a capacitance change between the movable electrode and the fixed electrode makes it possible to appropriately detect the direction and magnitude of displacement of the movable electrode in the thickness direction of the support substrate, i.e. the direction and magnitude of a physical quantity applied to the sensor. According to Japanese Patent Application Laid-Open Publication No. 2006-84326, the compressive stress layer is formed of thermally-oxidized film, polysilicon film, or silicon nitride film.
As described above, in the semiconductor physical quantity sensor according to Japanese Patent Application laid-Open Publication No. 2006-84326, a compressive stress layer is formed on the surface of a beam causing the movable electrode to be cambered away from the support substrate. This makes it possible to appropriately detect the direction and magnitude of a change in displacement, in the thickness direction of the support substrate, of the movable electrode, i.e. the direction and magnitude of a physical quantity applied to the sensor.
There are, however, the following problems with the existing technology disclosed in Japanese Patent Application Laid-Open Publication No. 2006-84326.
The present invention has been made in view of the above problems with the existing technology, and it is an object of the invention to provide a low-cost physical quantity sensor with high sensitivity and high reliability and a control device using the physical quantity sensor.
The above and other objects and novel characteristics of the present invention will be apparent from the description of this specification and the accompanying drawings.
A typical structure of the present invention is as follows. The semiconductor physical quantity sensor includes a movable electrode which is displaced when a physical quantity is applied and a fixed electrode which faces the movable electrode and forms electrostatic capacitance. The semiconductor physical quantity sensor detects, when a physical quantity is applied thereto, the physical quantity according to an electrostatic capacitance change caused between the movable electrode and the fixed electrode. In the semiconductor physical quantity sensor: the movable electrode and the fixed electrode are formed on a same conductive layer having a substantially uniform height on a substrate; and the movable electrode and the fixed electrode are placed, using an electrostatic force, in an initial offset state where the movable electrode and the fixed electrode have different distances to the substrate.
According to the present invention, a highly reliable semiconductor physical quantity sensor whose performance does not change much over time and a control device using the physical quantity sensor are provided.
The physical quantity sensor according to a typical embodiment of the present invention includes a movable electrode which is displaced when a physical quantity is applied and a fixed electrode which faces the movable electrode and forms electrostatic capacitance. The semiconductor physical quantity sensor detects, when a physical quantity is applied thereto, the physical quantity according to an electrostatic capacitance change caused between the movable electrode and the fixed electrode. In the semiconductor physical quantity sensor, the movable electrode and the fixed electrode are formed on a same layer, and they are shifted using an electrostatic force such that they have different distances to the substrate.
According to the invention, when an electrostatic force is applied to the movable electrode in the substrate thickness direction (in the direction toward outside the sensor surface), the distance between the movable electrode and the support substrate changes, causing the movable electrode and the fixed electrode fixed to the support substrate to be shifted from each other in the substrate thickness direction.
Based on the positional relationship between the movable electrode and the fixed electrode being shifted from each other, the magnitude and direction of a physical quantity applied in the same direction can be appropriately detected.
The physical quantity sensor of the present invention can be fabricated by preparing a multi-layer substrate including a support substrate on which a conductive layer (active layer) is formed via, in the substrate thickness direction, an interlayer insulation layer; movably connecting a movable electrode formed in the active layer to the support substrate via the interlayer insulation layer; and fixing a fixed electrode formed also in the active layer to the support substrate. The multi-layer substrate may be a silicon-on-insulator substrate (i.e. an SOI wafer) including the support substrate and the active layer both formed of silicon and the insulation layer formed of silicon oxide film.
In the SOI wafer, when a bias voltage is applied to between the active layer on which the movable electrode is formed and the support substrate, the movable electrode suspended by the support substrate via the interlayer insulation layer can be pulled toward the support substrate to be displaced in the substrate thickness direction. Such a sensor structure can be realized without requiring a complicated fabrication process.
The physical quantity sensor according to another embodiment of the present invention includes a multi-layer substrate having a glass or silicon cap disposed thereon and, in the physical quantity sensor, the movable electrode is displaced away from the support substrate by applying a bias voltage to between the cap and the movable electrode. According to the embodiment, the movable electrode is displaced away from the support substrate, so that the displacement is not restricted by the thickness of the interlayer insulation layer or by any pull-in phenomenon. It is therefore possible to reduce the initial capacitance C0 between the movable electrode and the fixed electrode while maintaining the capacitance change (ΔC) caused by a physical quantity applied to the sensor. This makes it possible to detect the applied physical quantity with high sensitivity and high accuracy.
The physical quantity sensor according to still another embodiment of the present invention includes a bias voltage adjusting unit which can actively adjust the amount of shifting between the fixed electrode and the movable electrode. This makes it possible, for example, to adjust the shifting between the electrodes according to a range of physical quantity measurement or to adjust, relative to another physical quantity sensor, the sensor sensitivity or initial output (sensor output with an initial capacitance C0) by adjusting the shifting between the electrodes.
As described above, the invention is suitable for application mainly to acceleration sensors or angular rate sensors with a detection axis extending in the support substrate thickness direction (the direction toward outside the sensor surface).
In the physical quantity sensor according the present invention, the fixed electrode and the movable electrode are shifted from each other using an electrostatic force. The effects of the arrangement are summarized in the following.
The present invention can be applied to a semiconductor physical quantity sensor such as an acceleration sensor or an angular rate sensor. Such sensor can applicable as a speed sensor or an inclination angle sensor. A control device using a semiconductor physical quantity sensor according to the present invention can be used in a large variety of products including an automobile, portable appliances, amusement apparatus, and home information appliances. In the field of the automobile, for example, a device or a system to which the control device can be applied include a travel speed control device, an air-bag system, an attitude control system for stabilizing automobile attitude during a turn, or a navigation system.
Embodiments of the present invention will be described below with reference to drawings. In the following, the description of the invention will be divided into two or more sections or will range over two or more embodiments as required for the sake of convenience. Unless otherwise expressed, such sections and embodiments are not mutually irrelevant. For example, among such sections and embodiments, one is a partial or total modification of another, or one elaborates or supplements another.
Also, numbers referred to in the following description of embodiments (for example, numbers representing counts, amounts, ranges, or other numeric values) are not defined values, that is, they may be smaller or larger unless otherwise expressed or except when they are apparently defined in principle.
Furthermore, the constituent elements (including element steps) of the following embodiments are not necessarily indispensable unless otherwise expressed or except when they are apparently indispensable in principle.
Similarly, the shapes of and positional relationships between constituent elements referred to in the following description are inclusive of those substantially close to or similar to them unless otherwise expressed or except when such shapes and positional relationships are apparently considered strictly defined in principle. This also applies to the numeric values and ranges.
Note that, in the drawings referred to in describing the following embodiments, identical members are denoted, as a rule, by identical reference numerals, and duplicate descriptions of identical members are omitted. Also note that the drawings referred to in the following may include plan views hatched to make them clearer.
A semiconductor physical quantity sensor according to a first embodiment of the present invention will be described with reference to
First, the structure of the acceleration sensor 1A of the first embodiment will be described. The acceleration sensor 1A includes, for example, a silicon-on-insulator (SOI) substrate 2 (See
The thickness of the stack structure of the SOI substrate 2, i.e. the total thickness of the support substrate 2a and the interlayer insulation layer 2b, may range, for example, from several tens of micrometers to several hundreds micrometers. The conductive layer 2c ranges, for example, from several micrometers to several tens of micrometers in thickness. Even though, in the first embodiment, the SOI substrate 2 is used as a semiconductor substrate, the semiconductor substrate need not necessarily be an SOI substrate. The semiconductor substrate may be formed of, for example, conductive polysilicon obtained using surface MEMS (microelectromechanical system) technology. Also, the stack structure may include a conductive layer of, for example, plated nickel.
As shown in
The mass body 4 has movable electrodes 6 which formed to be displaced together with the mass body 4. Namely, the movable electrodes 6 formed integrally with the mass body 4 are held by the fixed part 3 via the beam 5 that is displaced responding to acceleration applied outwardly of the sensor surface. The active layer 2c also includes fixed electrodes 7 formed such that capacitance is formed between the movable electrodes 6 and the fixed electrodes 7. Namely, the movable electrodes 6 and the fixed electrodes 7 are formed in the conductive layer 2c to be substantially at the same height over, via the interlayer insulation layer 2b, the support substrate. Since the fixed electrodes 7 are fixed to the support substrate 2a via the interlayer insulation layer 2b, they are not displaced even when subjected to acceleration. The movable electrodes 6 and the fixed electrodes 7 are positioned like pairs of mutually facing combs whose teeth appear engaged with opposing ones without mutually touching so as to increase the electrostatic capacitance formed between their mutually facing surface portions.
The fixed electrodes 7 are surrounded by a dummy pattern 8 whose potential is fixed to the reference voltage (DC voltage) of the mass body 4 so as to shield against external electromagnetic noise and reduce processing fluctuations during DRIE (deep reactive ion etching).
The fixed part 3 is used also as an electrode for providing the movable electrodes 6 with electrical signals. Electrical signals received from outside are applied to the movable electrodes 6 via a through-electrode 9 connected to the fixed part 3 as being described later with reference to
To form the through-electrode 9, first a through hole is formed in the support substrate 2a; a thermally oxidized insulation film 10 is formed over the support substrate 2a thereby insulating the through hole; and the insulated through hole is filled with a polysilicon film 11 to diffuse impurities and thereby lower the insulation resistivity of the through-electrode 9. The polysilicon film 11 is attached with a metal electrode pad 12 of, for example, aluminum which is used to exchange, via a bonded wire, signals with an external signal processing unit (e.g. an LSI).
Like the movable electrode 6, each of the fixed electrodes 7 is also provided with a through-electrode 13 and a pad 12. The fixed electrode 7 exchanges signals with the outside via the through-electrode 13.
The dummy pattern 8 is also provided with a through-electrode 14 and a pad 12, so that the dummy pattern 8 can be held at a predetermined potential.
The connecting conductive layer 2d is formed to electrically connect the through-electrode 9, the fixed part 3, the beam 5, and the movable electrodes 6. The connecting conductive layer 2d is also formed to electrically connect the through-electrodes 13, the fixed electrodes 7, and the through-electrode 14 to the peripheral dummy pattern 8. The fixed part 3 is formed by patterning the active layer 2c and the connecting conductive layer 2d. Subsequently, a gap 17 is formed by removing a sacrifice layer (a part of the interlayer insulation layer 2b).
A substrate electrode 15 is formed on the support substrate 2a. The substrate electrode 15 is formed, for example, by processing the thermally oxidized film 10 by photolithography, then forming an aluminum film by sputtering, and patterning the aluminum film. The substrate electrode 15 makes up a part of a bias voltage applying unit. The bias voltage applying unit applies a bias voltage to the substrate electrode 15 and places, using an electrostatic force, the movable electrodes 6 and the fixed electrodes 7 in an initial offset state where the distance between the movable electrodes 6 and the support substrate 2a differs from the distance between the fixed electrodes 7 and the support substrate 2a.
Principal constituent elements of the acceleration sensor 1A such as the fixed part 3, the mass body 4, the beam 5, the movable electrodes 6, the fixed electrodes 7, and the peripheral dummy pattern 8 need not necessarily be shaped and arranged as shown in
Next, an arrangement, which is a characteristic of the present invention, where the movable electrode 6 and the fixed electrode 7 in an initial offset state have different distance to the support substrate 2a will be described with reference to
When, in the initial offset state, acceleration is applied to the semiconductor physical quantity sensor causing the movable electrode 6 and the mass body 4 to be displaced in the +z direction, the areas of mutually facing surface portions of the movable electrode 6 and the fixed electrode 7 increase to cause the electrostatic capacitance between the movable electrode 6 and the fixed electrode 7 to increase. Namely, the electrostatic capacitance between the movable electrode 6 and the fixed electrode 7 increases from the initial electrostatic capacitance C0 to C0+ΔC.
When, on the other hand, acceleration is applied to the semiconductor physical quantity sensor causing the movable electrode 6 and the mass body 4 to be displaced in the −z direction, the areas of mutually facing surface portions of the movable electrode 6 and the fixed electrode 7 decrease to cause the electrostatic capacitance between the movable electrode 6 and the fixed electrode 7 to decrease. Namely, the electrostatic capacitance between the movable electrode 6 and the fixed electrode 7 decreases from the initial electrostatic capacitance C0 to C0−ΔC.
As described above, shifting the moving electrode 6 and the fixed electrode 7 formed in the same layer beforehand in the direction of displacement detection makes it possible to appropriately detect the direction and magnitude of acceleration applied to the semiconductor physical quantity sensor.
The semiconductor physical quantity sensor is preferably protected with a protective cover.
Generally, in cases where a movable body is displaced using an electrostatic force, a pull-in phenomenon occurs when the displacement of the movable body reaches two thirds the distance before displacement between the movable body and the fixed electrode facing the movable body to generate an electrostatic force. Therefore, the maximum displacement dmax of the movable electrode 6 is limited to two thirds the distance between the movable electrode 6 and the support substrate 2a (i.e. two thirds the thickness of the interlayer insulation layer 2b). In the first embodiment, the thickness of the interlayer insulation layer 2b of the acceleration sensor 1A is 3 micrometers, so that the value of dmax is 2 micrometers. The initially required displacement value d0 will be explained in detail later in explaining the operating principle of the acceleration sensor. In most cases, the d0 value in the range of 50 nm to 1 micrometer will be appropriate. The interlayer insulation layer 2b ranges in thickness from 100 nm up to 4 micrometers with the sensor fabrication cost and productivity taken into consideration.
Subsequently, the terminals of the signal processing IC 70, the acceleration sensor 1A, and the ceramic package 60 are connected by bonding conductive wires 90. The process is completed by sealing the acceleration sensor 1A with a lid 100.
The operating principle of the acceleration sensor 1A of the first embodiment will be explained below. In the acceleration sensor 1A, when acceleration A is applied from outside to the mass body 4, the mass body 4 is subjected to an inertial force F=mA. The inertial force is converted into a restoring force F=kz of a beam 5 (k) supporting the mass body 4; where z represents the displacement of the mass body 4.
What is described above is represented by the equation (1) shown below, where displacement z is a function of natural frequency f0 determined by mass m, which includes the movable electrodes 6 and the mass body 4, and spring constant k of the beam 5. The scales of the movable electrodes 6 and the fixed electrodes 7 are determined by displacement z, the range of acceleration to be measured, measurement resolution, and the processing capacity of the signal processing IC 70.
The acceleration sensor 1A of the first embodiment, shown in
where:
z=displacement of movable electrodes 6 (=mass body 4)
A=acceleration applied
f0=natural frequency of acceleration sensor 1A
m=mass of movable part (movable electrodes 6+mass body 4)
k=spring constant of beam 5 suspending the movable part
When an acceleration of 1 G is applied to the acceleration sensor 1A whose natural frequency f0 is 3 kHz, the movable electrodes 6 are displaced by 27.58 nm. That is, a minimum value of displacement d required to detect an acceleration of 1 G is 27.58 nm. In the first embodiment, the mass m of the movable part of the acceleration sensor 1A is 120 micrograms and the spring constant k is 43 N/m. The surface portion facing the support substrate 2a of the movable part has an area of 2.4 mm2. The bias voltage required to displace the movable part by 30 nm is 0.8 V.
The electrostatic capacitance in an initial state (initial capacitance value) C0 can be adjusted by adjusting the value of bias voltage VB. In the first embodiment, a bias voltage VB of 3 V is applied to the acceleration sensor 1A in advance to set an initial displacement d0 to about 500 nm. The displacement corresponds to an acceleration of about 16G. The present invention, however, does not require the displacement to be a specific value. The value may be adjusted appropriately, for example, according to a measurement range requirement or the purpose of operation, for example, correcting sensor sensitivity variation.
The equation (2) below is a relational expression for capacitance change ΔC caused by an analog output of the signal processing IC 70 and acceleration applied to the sensor.
where:
Vo=output voltage
C0=initial capacitance between movable electrodes 6 and fixed electrodes 7
ΔC=capacitance change caused by acceleration application
Cf=reference capacitance
Vi=carrier voltage
When, for the above equation (2), the reference capacitance Cf is set to C0 (i.e. C0/Cf=1) and then the known carrier voltage Vi is subtracted, the output voltage Vo of the equation (2) becomes equal to output voltage Vo′ of the following equation (3).
where Vo′=output voltage after signal processing
The above equation (3) indicates that the output voltage varies with the magnitude of and the sign attached to ΔC. Hence, the direction and magnitude of acceleration applied can be determined based on the output voltage.
The above equation (3) is based on the assumption that the initial capacitance C0 between the movable electrodes and the fixed electrodes equals the reference capacitance Cf. If a difference occurs between them, the difference directly affects sensor characteristics (sensitivity and initial displacement). In a real sensor fabrication process, the gap between the movable electrodes 6 and the fixed electrodes 7 or the areas of their mutually facing surface portions often vary from sensor to sensor, so that characteristics of such acceleration sensors also vary.
Characteristic differences between acceleration sensors can be corrected by adjusting the bias voltage between the movable part and the support substrate 2a and thereby equalizing the initial capacitance C0 and the reference capacitance Cf. Namely, in the method in which an electrostatic attractive force is made use of, it is possible, while maintaining the relationship between the physical quantity to be detected and the capacitance change (ΔC) caused between the fixed electrodes and the movable electrodes, to adjust the initial capacitance value (C0) by adjusting the bias voltage.
Even though, for the foregoing equation (3), it is assumed, to make explanation easier to understand, that the initial capacitance C0 between the movable electrodes and the fixed electrodes equals the reference capacitance Cf, there may be a difference between the initial capacitance C0 and the reference capacitance Cf as long as the initial capacitance C0 is adjustable and the sensitivity or the output in an initial state (i.e. the output with an initial displacement d0 with no acceleration applied) can be adjusted by adjusting the initial capacitance C0. As is clear from
For the acceleration sensor of the first embodiment, the initial capacitance C0 is adjusted by adjusting the bias voltage based on the relationships shown in
Assume, for example, that: the initial capacitance C0 between the movable electrodes 6 and the fixed electrodes 7 is 2 pF; the reference capacitance Cf is 1 pF; and an additional capacitance ΔC of 200 fF is generated when an acceleration of 1 G is applied. Based on the equation (2), when the input carrier voltage is 2 V, the output voltage Vo is 4.4 V. When, in this case, Vcc (saturation voltage of operational amplifier) is 4.5 V, and an acceleration of 2 G is applied, the output voltage Vo becomes 4.8 V to be outside the measurable range.
When the initial capacitance C0 is adjusted to 1 pF by adjusting the bias voltage, the output voltage Vo is 2.2 V for an acceleration of 1 G and 3 V for an acceleration of 5 G. The capacitance change ΔC caused by an application of acceleration is 200 fF for an application of 1 G and 1 pF for an application of 5 G regardless of the initial capacitance C0.
From the foregoing, it is known that the measurement range of the acceleration sensor can be changed by adjusting the bias voltage without saturating the C-V conversion section 72.
When, in the physical quantity sensor of the present embodiment, an electrostatic force is applied to the movable electrodes in the thickness direction of the substrate (in the direction toward outside the sensor surface), the distance between the movable electrodes and the support substrate changes causing the movable electrode to be displaced in the substrate thickness direction with respect to the fixed electrodes fixed to the substrate.
Measuring the displacement makes it possible to appropriately determine the magnitude and direction of the physical quantity applied to the sensor in the displacement direction.
The physical quantity sensor of the present embodiment can be fabricated by preparing a multi-layer substrate including a support substrate on which a conductive layer (active layer) is formed via, in the substrate thickness direction, an interlayer insulation layer; movably connecting movable electrodes formed in the active layer to the support substrate via the interlayer insulation layer; and fixing fixed electrodes formed also in the active layer to the support substrate. The multi-layer substrate may be a silicon-on-insulator substrate (i.e. an SOI wafer) including the support substrate and the active layer both formed of silicon and the insulation layer formed of silicon oxide film.
In the SOI wafer, a bias voltage is applied to between the active layer on which the movable electrodes are formed and the support substrate. Therefore, a sensor structure in which the movable electrodes suspended by the support substrate via the interlayer insulation layer can be pulled toward the support substrate to be displaced in the substrate thickness direction can be realized without requiring a complicated fabrication process. The SOI wafer can be fabricated without involving a high-temperature process, so that it offers high flexibility as to circuit mixing and process selection.
The initial displacement of the movable electrodes is a function of the voltage applied, the areas of mutually facing surface portions of the movable and fixed electrodes, and the spring constant in the z direction of the movable electrodes, and is not related with the thickness of the active layer. Characteristics of the physical quantity sensor can therefore be set arbitrarily and actively.
In the physical quantity sensor of the present embodiment, the movable electrodes are displaced toward the support substrate beforehand thereby reducing the gap between the movable electrodes and the support substrate. This generates a damping effect against vibrations of the movable electrodes and improves the vibration resistance of the sensor when subjected to external disturbance.
Next, a semiconductor physical quantity sensor according to a second embodiment of the present invention will be described. The second embodiment is modified in the part of electrodes of the first embodiment. According to the second embodiment of the present invention, the electrodes (the movable electrodes 6 and the fixed electrodes 7) are formed on the cap 50 (See
When the movable electrodes 6 are displaced away from the support substrate 2a by applying a bias voltage to between the cap 50 and the movable electrodes 6, the displacement is not restricted by the thickness of the interlayer insulation layer 2b. It is therefore possible to appropriately adjust the initial displacement d0 and the displacement adjusting value d1 as required by taking into account the magnitude of the applicable bias voltage.
According to the second embodiment, the movable electrodes are displaced away from the support substrate, so that the displacement is not restricted by the thickness of the interlayer insulation layer or by any pull-in phenomenon. It is therefore possible to reduce the initial capacitance C0 between the movable electrodes and the fixed electrodes while maintaining the capacitance change (ΔC) caused by a physical quantity applied to the sensor. This makes it possible to detect the applied physical quantity with high sensitivity and high accuracy.
The semiconductor physical quantity sensor according to a third embodiment of the present invention is an angular rate sensor for detecting an angular rate as a physical quantity. The structure and the principle of operation of an angular rate sensor 1B will be described in detail below with reference to drawings. In the following, description already provided regarding the first embodiment will be omitted to avoid duplication.
The angular rate sensor 1B of the third embodiment includes, broadly classified, two driven elements 25 and two Coriolis elements 31. The driven elements 25 each include driving electrodes 21 and 22 which drive the sensor and monitoring electrodes 23 and 24 which monitor the driving amplitude of the sensor. The Coriolis elements 31 are displaced by angular rate application and respectively include detection electrodes 33 and 34 which detect capacitance changes caused by the displacement.
The driven elements 25 that drive the sensor and generate a driving amplitude will be described below. On the SOI substrate, fixed parts 26 are, as shown in
The driven elements 25 are supported by the supporting beams 27 in a state of being suspended off the support substrate 2a with the interlayer insulation layer 2b removed.
The driven elements 25 each include movable electrodes which face the driving electrodes 21 and 22 and the monitoring electrodes 23 and 24 to form capacitance between such electrodes and themselves. The driving electrodes 21 and 22 have mutually reverse-phased driving signals inputted to them and drive the driven elements 25. The two driven elements 25 are linked to each other via a link beam 29. The two driven elements 25 are therefore vibrated in mutually reversed mode. The frequency of the driving signals is aligned with the reversed mode frequency (the second natural frequency in the case of the angular rate sensor of the present embodiment) of the driven elements 25 so as to obtain a large amplitude using a small driving energy.
The monitoring electrodes 23 and 24 are for measuring the vibration amplitudes of the driven elements 25. They exchange electrical signals with the external signal processing IC 70 via through-electrodes.
The Coriolis elements 31 are each connected to one of the driven elements 25 with four detection beams 32 which are rigid in the y direction perpendicular to each of the excitation direction x and the detection direction z and flexible in the z direction, i.e. the detection direction. Each of the Coriolis elements 31, therefore, vibrates, following the vibration in the x direction of the corresponding driven element 25, in the excitation direction in the same phase as the driven element 25. The amplitudes in the excitation direction x of the Coriolis elements 31 can be made the same as the amplitudes of the driven elements 25 by increasing the rigidity in the x direction of the detection beams 32, or they can be made larger or smaller than the amplitudes of the driven elements 25 by adjusting the rigidity proportion among the detection beams 32, supporting beams 27, and the link beam 29. Namely, regarding the x direction, a mass-spring system including a driven element 25 and a Coriolis element 31 as masses and the supporting beams 27, the link beam 29, and the detection beams 32 as springs may be arranged as a single-degree-of-freedom system or as a two-degree-of-freedom system for use in first-order mode.
The Coriolis elements 31 are suspended by the driven elements 25 and are driven in mutually reversed phases. Therefore, the Coriolis force generated when an angular rate about the y axis is applied to the angular rate sensor 1B causes the Coriolis elements 31 to vibrate in mutually reversed phases.
Detection electrodes 33 and 34 face the Coriolis elements 31, respectively, and electrostatic capacitance is formed between them. Like in the case of the monitoring electrodes 23 and 24, reverse-phased carrier waves are applied to the detection electrodes 33 and 34 via through-electrodes 35 and 36, respectively. Therefore, when, with an angular rate applied to the sensor, the Coriolis elements 31 are vibrated in mutually reversed phases, capacitance changes associated with the detection electrodes 33 and 34 can be differentially detected. The circuit to detect the capacitance changes is similar to that, shown in
In cases where the detection electrodes 33 and 34 and the Coriolis elements 31 are positioned without being shifted, however, applying an angular rate to the sensor causing the Coriolis elements 31 to be displaced in mutually opposite directions causes the areas of mutually facing surfaces portions of the movable electrodes and the fixed electrodes to decrease for both of the detection electrodes 33 and 34, so that the capacitance reduces by AC (i.e. a change of −ΔC) both between the detection electrode 33 and one of the Coriolis elements 31 and between the detection electrode 34 and the other of the Coriolis elements 31 as shown in
When, in a state where the Coriolis elements 31 are displaced toward the support substrate 2a with respect to the fixed detection electrodes 33 and 34 (as illustrated in solid line in
The left-half portions (a) of
The driven elements 25, the Coriolis elements 31, and other elements of the sensor are surrounded, like in the case of the acceleration sensor 1A of the first embodiment, by a dummy pattern 37 kept at a predetermined potential via a through-electrode 38.
In the angular rate sensor of the third embodiment, a voltage corresponding to the capacitance change +ΔC or −ΔC caused by an angular rate applied to the sensor is applied, as shown in the right-half portions (b) of
As in the second embodiment, it is possible, though not illustrated, to displace the Coriolis elements 31 away from the support substrate 2a by installing the movable and fixed electrodes on a cap 50 and applying a bias voltage to between the movable parts (the driven elements 25 and the Coriolis elements 31) and the cap 50. The effects of displacing the Coriolis elements 31 away from the support substrate 2a are the same as those of displacing the
Coriolis elements 31, as described above, toward the support substrate 2a.
The present embodiment can produce effects similar to those produced by the first and the second embodiment.
The invention made by the present inventors has been described in detail based on embodiments. The present invention, however, is not limited to the above embodiments, and it can be modified in various ways without departing from the scope and spirit of the invention.
Number | Date | Country | Kind |
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2008-302639 | Nov 2008 | JP | national |