3-Dimensional comb structure and actuator and inertia detection sensor both using the comb structure

A 3-dimensional comb structure using an electrical force, and an inertial detection sensor and an actuator both using the 3-dimensional comb structure are provided. In the 3-dimensional comb structure, a suspension structure, which is an inertia body, is separated a predetermined height from a substrate, maintaining the predetermined height from the substrate. A movable comb, which has at least one movable comb finger, protrudes perpendicularly from the suspension structure. A fixed comb, which has at least one fixed comb finger, protrudes perpendicularly from the substrate, in mesh with the movable comb. The 3-dimensional comb structure is driven by a voltage provided from a power supply unit which is connected to the movable comb and the fixed comb.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a 3-dimensional comb structure using an
 electrostatic force, and an inertia detection sensor and an actuator which
 uses the 3-dimensional comb structure.
 2. Description of the Related Art
 A 3-dimensional comb structure using an electrostatic force protrudes
 perpendicularly with respect to a flat plane surface, and has a structure
 such that an electrostatic force, which is generated between a pair of
 interlocked combs by applying a voltage thereto, is constant with respect
 to the relative motion between the combs.
 In many cases, electrostatic actuators are used to move micro structures.
 An electrostatic comb drive (U.S. Pat. No. 5,025,346) is well known as an
 electrostatic actuator. The basic principle of the electrostatic comb
 drive will now be described with reference to FIG. 1.
 A pair of combs 1 and 2 mesh with each other with a gap s between fingers
 thereof. When a power supply 3 is connected to each of the combs 1 and 2
 via conductive wires 4 and 5, respectively, a horizontal electrostatic
 force (F.sub.s) 6 acting upon a finger of the comb 2 is expressed by the
 following Equation 1:
 ##EQU1##
 wherein .epsilon..sub.0, t, s and V denote the permittivity of free space,
 the thickness of a finger in a direction perpendicular to the surface, the
 interval between a finger of the comb 1 and an adjacent finger of the comb
 2, and a voltage 3 applied to a bridge between fingers, respectively. The
 electrostatic comb drive can be manufactured by a CMOS process such as a
 process for manufacturing a semiconductor RAM, and has a constant force
 with respect to the motion of a comb, as shown in Equation 1.
 FIG. 2 disclosed in U.S. Pat. No. 5,025,346 can be taken as an example of
 an actuator using the principle of FIG. 1, which is the principle of an
 existing electrostatic comb drive. This electrostatic actuator 20 includes
 a mass body 22 having a plurality of movable comb fingers 27, at least one
 elastic member 23 connected to the mass body 22, and a plurality of fixed
 combs 25 which are meshed with the movable comb fingers 27, facing the
 movable comb fingers 27. Here, the plurality of movable comb fingers 27
 are supported by a substrate 21 via supporters 24, and the fixed comb
 fingers 25 are supported by the substrate 21 via fixed comb supporters 26.
 When a voltage is applied to the fixed comb fingers 25 and the movable
 comb fingers 27 via an appropriate means (not shown), the mass body 22 is
 moved linearly in a horizontal direction with respect to the substrate 21
 by the electrostatic force generated by Equation 1. The electrostatic
 force generated in this structure is constant with respect to the distance
 of motion, as shown in Equation 1. However, according to this structure,
 the movable comb fingers 27 and the fixed comb fingers 25 are parallel to
 the substrate 21. Also, since the movable comb fingers 27 and the fixed
 comb fingers 25 are installed on both ends of the flat mass body which is
 parallel to the substrate 21, the number of combs can increase in
 proportion to the length of each end of the mass body. Thus, an
 electrostatic force is small due to the limit in the number of combs.
 Furthermore, the mass body must move largely to be used in acceleration
 sensors or gyro sensors. However, in this conventional comb structure, a
 small amount of electrostatic force makes it difficult to directly drive
 the mass body, so that the mass body can only be driven at the resonance
 point.
 SUMMARY OF THE INVENTION
 To solve the above problems, an objective of the present invention is to
 provide a 3-dimensional comb structure which is strong enough to drive a
 large-sized structure and having fingers arranged perpendicularly over a
 flat plane to make it easy to control the position of the structure, and
 an actuator and an inertia detection sensor both using the 3-dimensional
 comb structure.
 To achieve the above objective of the invention, there is provided a
 3-dimensional comb structure including: a substrate; a suspension
 structure separated a predetermined height from the substrate, maintaining
 the predetermined height therefrom, such that the suspension structure can
 vibrate over the substrate to detect an inertial movement; at least one
 elastic member connected to the suspension structure, for supporting the
 suspension structure so that the suspension structure makes an inertial
 movement; a movable comb having at least one comb finger, the movable comb
 protruding from the suspension structure; and a fixed comb having at least
 one comb finger, the fixed comb fingers protruding from the substrate
 opposite to and in mesh with the movable comb fingers.
 In the present invention, preferably, an electrostatic force is generated
 perpendicular to the direction of protrusion of the movable comb fingers
 from the suspension structure, when a voltage is applied to the movable
 comb and the fixed comb, so that the 3-dimensional comb structure is
 excited parallel to the substrate. It is preferable that the movable comb
 is meshed with the fixed comb having a predetermined gap between a finger
 of the movable comb and a finger of the fixed comb.
 It is also preferable that the movable comb and the fixed comb face each
 other, and are arranged on the suspension structure and the substrate,
 respectively, in a circular symmetrical manner, such that the suspension
 structure is horizontal to the substrate and rotates around the center of
 the circular symmetry.
 To achieve the above objective of the invention, there is provided an
 inertia detection sensor adopting a 3-dimensional comb structure
 including: a substrate; a suspension structure separated a predetermined
 height from the substrate, maintaining the predetermined height therefrom,
 such that the suspension structure can vibrate over the substrate to
 detect an inertial movement; at least one elastic member connected to the
 suspension structure, for supporting the suspension structure so that the
 suspension structure makes an inertial movement; a movable comb having at
 least one comb finger, the movable comb protruding from the suspension
 structure; a fixed comb having at least one comb finger, the fixed comb
 fingers protruding from the substrate opposite to and in mesh with the
 movable comb fingers; and a sensing unit for detecting an acceleration by
 sensing a capacity change between the movable comb and the fixed comb.
 Preferably, an electrostatic force is generated perpendicular to the
 direction of protrusion of the movable comb fingers from the suspension
 structure, when a voltage is applied to the movable comb and the fixed
 comb, so that the 3-dimensional comb structure is excited parallel to the
 substrate. It is preferable that the movable comb is meshed with the fixed
 comb having a predetermined gap between a finger of the movable comb and a
 finger of the fixed comb.
 It is also preferable that the movable comb and the fixed comb face each
 other, and are arranged on the suspension structure and the substrate,
 respectively, in a circular symmetrical manner, such that the suspension
 structure is horizontal to the substrate and rotates around the center of
 the circular symmetry.
 To achieve the above objective of the invention, there is provided an
 actuator adopting a 3-dimensional comb structure comprising: a substrate;
 a suspension structure separated a predetermined height from the
 substrate, maintaining the predetermined height therefrom, such that the
 suspension structure can vibrate over the substrate to detect an inertial
 movement; at least one elastic member connected to the suspension
 structure, for supporting the suspension structure so that the suspension
 structure makes an inertial movement; a movable comb having at least one
 comb finger, the movable comb protruding from the suspension structure; a
 fixed comb having at least one comb finger, the fixed comb fingers
 protruding from the substrate opposite to and in mesh with the movable
 comb fingers; and a power supply for providing a voltage between the
 movable comb and the fixed comb to excite the suspension structure.
 Preferably, an electrostatic force is generated perpendicular to the
 direction of protrusion of the movable comb fingers from the suspension
 structure, when a voltage is applied to the movable comb and the fixed
 comb, so that the 3-dimensional comb structure is excited perpendicular to
 the direction of protrusion of the fixed comb. It is preferable that the
 movable comb is meshed with the fixed comb having a predetermined gap
 between a finger of the movable comb and a finger of the fixed comb.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 3, a 3-dimensional comb structure using an electrostatic
 force according to the present invention includes at least one fixed comb
 finger 32 installed perpendicularly on a substrate 31, and at least one
 movable comb finger 35 which is meshed with the fixed comb finger 32,
 opposite to the fixed comb finger 32. The fixed comb finger 32 and the
 movable comb finger 35 are spaced apart by a gap s, and overlap with each
 other by a distance h in a perpendicular direction with respect to the
 substrate 31. A pair of comb fingers 32 and 35 are electrically connected
 to each other, as shown in FIG. 3, such that they are driven by an
 electrical field which is generated between them by a voltage provided
 from a power supply means 36.
 As shown in FIG. 3, when a voltage is applied from the power supply means
 36, a capacitor is formed between the fixed comb finger 32 and the movable
 comb finger 35, so that energies are stored by charge. The capacitance C
 of the capacitor formed at this time is expressed by the following
 Equation 2:
 ##EQU2##
 wherein .epsilon..sub.0, s, h, and x denote the permittivity of a gap, the
 gap between comb fingers, the length by which a pair of comb fingers
 overlap each other perpendicularly with respect to a substrate, and the
 length by which a pair of comb fingers overlap each other in parallel to
 the substrate, respectively. Since the gap s exists between two comb
 fingers, as shown in FIG. 3, an energy U stored in the capacitor is
 expressed by the following Equation 3 using the capacitance given by
 Equation 2:
 ##EQU3##
 A force F which is applied to the movable comb finger 32 and the fixed comb
 finger 35 parallel to the substrate 31, that is, in a direction indicated
 by arrow 39 in FIG. 3, is given by the following Equation 4:
 ##EQU4##
 Equation 4 calculates the electrostatic force between the movable comb
 finger 32 and the fixed comb finger 35 with reference to the basic
 conceptual diagram showing the operational principle of a 3-dimensional
 comb structure according to the present invention shown in FIG. 3. It can
 be seen from Equation 4 that the electrostatic force F acts in a
 perpendicular direction to the movable comb finger 32 and the fixed comb
 finger 35, that is, in the direction 39 parallel to the substrate 31.
 An embodiment of the 3-dimensional comb structure using an electrostatic
 force having such a configuration will now be described with reference to
 FIG. 4. Referring to FIG. 4, which shows an embodiment of a
 straight-vibrating driver using the 3-dimensional comb structure of FIG.
 3, a suspension structure 42 has a plurality of movable comb fingers 43
 which protrude perpendicularly thereto, and supported by a substrate via a
 plurality of support springs 44 and a plurality of supporters 45. A
 plurality of fixed comb fingers 46 are arranged opposite to and in mesh
 with the movable comb fingers 43, and supported by the substrate 41. This
 3-dimensional comb structure is driven by an electrostatic force, as
 described with reference to FIG. 3. Here, the vertical width of the
 support springs 44 is made larger than the horizontal width thereof with
 respect to the substrate, such that each of the support springs 44 moves
 with a flexible elasticity in a horizontal direction indicated by arrow
 47, and is fixed without flexibility in the vertical direction. That is,
 when the vertical width of the support spring in FIG. 4 is h, the
 horizontal width thereof is b, the length thereof is L, and the rigidity
 thereof is K, the rigidity in an excitation direction and that in a
 measuring direction are expressed by the following Equation 5:
 ##EQU5##
 It can be seen from Equation 5 that the horizontal rigidity and the
 vertical rigidity of the support spring 44 are proportional to the cube of
 the horizontal width and the cube of the vertical width, respectively.
 When a voltage is applied from a power supply means (not shown) to the
 movable comb fingers 43 and the fixed comb fingers 46, a capacitance as
 given by Equation 2 is formed between the movable comb fingers 43 and the
 fixed comb fingers 46. Also, the force given by Equation 4 is generated in
 a direction which is parallel to the substrate, thus moving the suspension
 structure 42 to the right. Here, when the voltage from the power supply
 means (not shown) is an alternating current voltage, the support spring 44
 allows the suspension structure 42 to reciprocate in the directions
 indicated by double-headed arrow 47 according to the applied alternating
 current voltage while supporting the suspension structure 42. In order to
 achieve this operation, it is preferable that the plurality of supporters
 45 are electrically insulated from the substrate 41.
 In an existing comb structure, combs are installed on both ends of a mass
 body, such that the number of comb fingers can be increased in proportion
 to the length of the mass body. On the other hand, in the 3-dimensional
 comb structure according to the present invention, comb fingers protrude
 perpendicularly with respect to the mass body, such that the number of
 comb fingers can be increased in proportion to the area of the mass body.
 Therefore, the 3-dimensional comb structure according to the present
 invention can increase the number of comb fingers per unit area as
 compared to the existing comb structure shown in FIG. 2, so that it
 becomes much stronger. This structure is used in various actuators and
 inertial detection sensors.
 An acceleration detection sensor can be taken as an example of an inertia
 detection sensor which adopts the 3-dimensional comb structure using an
 electrostatic force. The suspension structure 42 in the comb structure of
 FIG. 4 is spaced from the substrate 41 by a given distance, such that it
 can move in the horizontal direction 47 with respect to the substrate 41.
 At this time, when an acceleration in a direction indicated by arrow x is
 applied, the suspension structure 42 moves in the direction indicated by
 arrow x. This motion is sensed as a capacitance change caused by the
 movable comb fingers 43 and the fixed comb fingers 46, thereby detecting
 the variation in acceleration.
 A gyro sensor can be taken as another example of an inertia detection
 sensor which adopts the 3-dimensional comb structure using an
 electrostatic force. While the suspension structure 42 in FIG. 4 moves in
 the horizontal direction 47 with respect to the substrate 41, a Coriolis
 force in a direction indicated by arrow z is generated when an
 acceleration is applied in a direction indicated by arrow y, thus
 vibrating the suspension structure 42 in the direction indicated by arrow
 z. This vibration is sensed by an appropriate sensor (not shown in FIG.
 4), thereby detecting the applied acceleration.
 The 3-dimensional comb structure using an electrostatic force according to
 the present invention can also be used in a variety of actuators for
 moving a suspension structure which is an inertia body, and in various
 inertial sensors and magnetic flux detection sensors.
 FIG. 5 is a schematic view illustrating a rotary driving structure 50 taken
 as an example of still another embodiment of the 3-dimensional comb
 structure according to the present invention. In this embodiment which is
 a structure which can apply a force to a circular structure, a rotary
 suspension structure 52 has a plurality of movable comb fingers 53 which
 protrude perpendicularly with respect to the rotatory suspension structure
 52, and is supported by a substrate 51 while maintaining a predetermined
 distance from the substrate 51 via a plurality of support springs 55 and a
 plurality of supporters 56. At least one fixed comb finger 54, which is
 installed in opposite to and mesh with the movable comb fingers 53, stands
 perpendicularly on the substrate 51. The operational principle of this
 comb structure is similar to that of the aforementioned comb structure.
 When a voltage is applied from a power supply means (not shown) to the
 movable comb fingers 53 and at least one fixed comb finger 54, a
 capacitance as given by Equation 2 is formed between the movable comb
 fingers and the fixed comb finger, and a force given by Equation 4 is
 generated parallel to the substrate, thus rotating the rotary suspension
 structure 52. Here, when the voltage applied from the power supply means
 is an alternating current voltage, a support spring 55 allows the rotary
 suspension structure 52 to reciprocate a circular arc section having a
 predetermined angle according to the applied alternating current voltage,
 while supporting the rotary suspension structure 52. In order to achieve
 this operation, it is preferable that the supporters 56 are electrically
 insulated from the substrate 51 to prevent flow of current. The
 operational principle of FIG. 6, in which the supporter is positioned at
 the center of a rotary mass body, is the same as that of the structure
 shown in FIG. 5. The structures shown in FIGS. 5 and 6 can be used in
 various actuators and sensors according to the principle described in the
 latter part of the description of the structure shown in FIG. 4.
 As described above, in a 3-dimensional comb structure according to the
 present invention and an inertial detection sensor and an actuator both
 using the 3-dimensional comb structure, a suspension structure, which is
 an inertia body, is separated a predetermined height from a substrate,
 maintaining the predetermined height from the substrate. A movable comb,
 which has at least one movable comb finger, protrudes perpendicularly from
 the suspension structure. A fixed comb, which has at least one fixed comb
 finger, protrudes perpendicularly from the substrate, in mesh with the
 movable comb. The 3-dimensional comb structure is driven by a voltage
 provided from a power supply unit which is connected to the movable comb
 and the fixed comb, so that the actuator and the inertial detection sensor
 both using the 3-dimensional comb structure can obtain the following
 effects.
 Firstly, since comb fingers are manufactured perpendicular to the
 suspension structure and the substrate, the present invention can greatly
 increase the number of comb fingers per unit area as compared to an
 existing driver. Thus, the present invention can reinforce an
 electrostatic force greater than an existing comb driver.
 Secondly, the direction of an electrostatic force in the existing comb
 structure is the same as that of the protrusion of comb fingers, whereas
 the direction of an electrostatic force in the 3-dimensional comb driver
 according to the present invention is perpendicular to that of protrusion
 of comb fingers.
 Thirdly, the 3-dimensional comb driver according to the present invention
 is stronger than the existing comb driver, so that it can increase the
 driving displacement by which a suspension structure is driven.
 Fourthly, since the comb structure according to the present invention is
 strong, the suspension structure can be driven at an arbitrary frequency
 without the need to drive the suspension structure at the resonance point
 of the comb structure to move the comb structure. Therefore, when a sensor
 using the resonance such as microgyro is manufactured using the comb
 structure according to the present invention, a resonance frequency in the
 excitation direction of the structure does not need to be consistent with
 a resonance frequency in the sensing direction thereof.
 Fifthly, the comb driver according to the present invention can generate a
 sufficiently large force at a small amount of driving voltage, so that the
 comb structure according to the present invention can be driven with a
 lower voltage than the existing comb driver.