JP2007333618A - Acceleration sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an acceleration sensor exhibiting high sensitivity, manufacturable at low cost. <P>SOLUTION: The acceleration sensor includes an inertial mass plate varying its position by the applied acceleration, a first electrode and a second electrode disposed on one main surface of the inertial mass plate, and a dielectric body confronted with the first electrode and the second electrode. The acceleration is measured based on electrostatic capacity between the first electrode and the second electrode varying in correspondence with position variation of the inertial mass plate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an acceleration sensor configured using an inertial mass body.

Conventionally, various types of acceleration sensors using inertial force acting when acceleration is applied have been used.
FIG. 6 shows a basic configuration of a conventional example having two electrodes facing each other. In this example, a moving electrode 102 a is provided on the lower surface of the inertial mass plate 102, a fixed electrode 101 a is formed on the substrate 101 so as to face the moving electrode 102 a, and the electrode is moved when the inertial mass plate 102 is displaced with respect to the substrate 101. It takes advantage of the change in capacitance between. In this conventional example, the inertial mass plate 102 is supported by an elastic beam 103 called a beam so as to face the substrate 101. When an acceleration is applied, the inertial mass plate 102 is displaced with respect to the substrate 101, The acceleration is measured based on a change in capacitance caused by the displacement.

Recently, in an acceleration sensor having a fixed electrode formed on a substrate and a moving electrode formed on an inertial mass body, the inertial mass body is configured to oscillate and oscillate in a horizontal plane. The thing which improved this and the thing using a comb-tooth electrode are also proposed.
However, these electrode-facing acceleration sensors have a problem that it is difficult to manufacture at low cost, such as bonding two wafers or requiring deep reactive ion etching.
In addition, since the inertial mass is usually made using a brittle material such as silicon, there is a problem that it is inferior in impact resistance.

  Therefore, in order to realize an acceleration sensor that is relatively simple and can be manufactured at low cost, the present inventors have arranged electrodes on a substrate in place of two opposing electrodes, and a fringe generated between the electrodes. An acceleration sensor using an electric field was previously proposed (Patent Document 1). As shown in FIG. 7, in the acceleration sensor of the prior example proposed earlier, two fixed electrodes 104 and 105 are formed side by side on a substrate 101, and the fringe electric field generated when a potential difference is applied between the electrodes is affected. The inertia mass body 110 made of a dielectric is provided so as to be movable. In this preceding example, the inertial mass body 110 is supported by the elastic beam 103 so as to be movable with respect to the substrate, as in FIG.

  In the preceding example described above, when acceleration is applied to the inertial mass body 110, the distance between the substrate 101 and the inertial mass body 110 changes due to the action of the inertial force, and the capacitance between the fixed electrodes 104 and 105 increases. Change, and acceleration can be measured. The acceleration sensor of the preceding example has an advantage that it can be manufactured at low cost because it can be manufactured by so-called surface micromachining technology without using expensive equipment.

  In addition, in the acceleration sensor of the preceding example, the inertial mass body 110 is configured by using polyparaxylylene called parylene that can be deposited at room temperature, and thus has the following advantages.

That is, since parylene can be vapor-deposited at room temperature, for example, an inertial mass body or the like can be formed after forming an arithmetic circuit composed of a C-MOS circuit, so that an acceleration detection unit and a C-MOS circuit can be formed. Can be integrally formed on the substrate.
Moreover, since parylene has ductility compared to brittle materials such as silicon, it is possible to provide an acceleration sensor that is excellent in impact resistance and the like.
US Patent Publication 2004 / 0239341A1

  However, since the acceleration sensor of the preceding example uses a fringe electric field different from the primary electric field used for the counter electrode, there is a problem that the sensitivity cannot be increased.

  Accordingly, an object of the present invention is to provide an acceleration sensor that can be manufactured at low cost and has high sensitivity.

  In order to achieve the above object, an acceleration sensor according to the present invention includes an inertial mass plate whose position is changed by an applied acceleration, a first electrode and a second electrode provided on one main surface of the inertial mass plate. An electrostatic capacitance between the first electrode and the second electrode that changes in response to a change in the position of the inertial mass plate. Based on the above, the acceleration is measured.

The acceleration sensor according to the present invention configured as described above is provided with a first electrode and a second electrode on an inertial mass plate that is displaced in accordance with acceleration, and is opposed to the first electrode and the second electrode. Since the body is provided, a dielectric having a large relative dielectric constant can be easily formed on the substrate.
Therefore, according to the present invention, an acceleration sensor that can be manufactured at low cost and has high sensitivity can be provided.

Hereinafter, an acceleration sensor according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram schematically showing a basic configuration of an acceleration sensor according to an embodiment of the present invention.
As shown in FIG. 1, the acceleration sensor according to the present embodiment is formed on an inertial mass plate 2 that is supported on a substrate 1 by an elastic beam 3 and whose position is changed by applied acceleration, and on the lower surface of the inertial mass plate 2. A pair of electrodes 2a and 2b, and a dielectric 7 provided to face the lower surface of the inertial mass plate 2.

  That is, in the acceleration sensor according to the present embodiment, the pair of electrodes 2a and 2b are formed on the lower surface of the inertial mass plate 2 whose position is changed by the applied acceleration, and the dielectric 7 is inertially mounted on the substrate 1 on the fixed side. It is provided close to the lower surface of the mass plate 2.

In the acceleration sensor of the embodiment configured as described above, when a potential difference is applied between the two electrodes 2a and 2b, an electric field is generated in the space around the electrodes and in the dielectric 7, and the electric field is schematically shown in FIG. It is represented by electric field lines as shown.
In such a state, if the position of the inertial mass plate 2 changes so that the distance (distance) between the dielectric 7 and the electrodes 2a and 2b changes, the distance (distance) between the dielectric 7 and the electrodes 2a and 2b corresponds. Thus, the electric flux density in the dielectric 7 changes, and the capacitance between the electrodes 2a and 2b changes.

  Therefore, in the acceleration sensor of the present embodiment, the dielectric 7 and the electrodes 2a, 2b are measured by measuring the change in capacitance between the electrodes 2a, 2b when acceleration is applied to the inertial mass plate 2. It is possible to detect how the distance (distance) changes, and the acceleration applied to the inertial mass plate 2 is calculated based on the detected value.

The acceleration sensor of the embodiment configured as described above is characterized in that the electrodes 2a and 2b are formed on the inertial mass plate 2 which is a floating body, and the dielectric 7 is provided on the fixed side. With this type of acceleration sensor, it is possible to select an appropriate material for the dielectric 7, and it is possible to configure an acceleration sensor with high sensitivity and high reliability.
That is, in the acceleration sensor of the present embodiment, by providing the dielectric 7 on the fixed side, the dielectric 7 can be formed by various manufacturing methods, and the dielectric 7 itself is deformed by the applied acceleration. Therefore, it is possible to use a material made of brittle crystals.

Therefore, according to the present invention, for example, a high dielectric such as a ferroelectric ceramic can be used, and a highly sensitive acceleration sensor can be provided.
Further, according to the acceleration sensor of the present invention, since the dielectric is provided not in the movable part but in the fixed part, the dielectric is not deformed, and is configured using, for example, a brittle dielectric such as ceramic. Even in this case, it is possible to ensure high reliability.

Hereinafter, the acceleration sensor of the present embodiment will be described in detail.
First, the dielectric constant and acceleration detection sensitivity of the dielectric 7 used in the acceleration sensor of the embodiment will be described.
Here, based on a model having symmetry with respect to the electrode 2a shown in FIG. 2, a simulation was performed, and the dielectric constant and acceleration detection sensitivity of the dielectric 7 were confirmed.

Here, a simulation was performed on the cross-sectional model having symmetry shown in FIG. 2 using a two-dimensional finite element method.
In the cross-sectional model shown in FIG. 2, the electrode 2a (width w) formed on the lower surface of the inertial mass plate 2 is arranged in the center, and the electrodes 2b are arranged on both sides of the electrode 2a. The interval between the electrode 2a and the electrode 2b was g.

In this simulation, the inertial mass plate 2 has a thickness of 5 μm, a dielectric constant of 3.15 (dielectric constant of polyparaxylylene), and the dielectric 7 has a thickness twice as large as (g + w). The rate was 2600 (dielectric constant of Pb (ZrTi) O ₃ ).
Further, the electric field distribution was calculated in the band-like range shown in FIG. Even if the calculation area is limited to such a range, the area where the electrode is formed (the lower surface of the inertial mass plate 2) is at least three times as long as (g + w) and the lower surface of the inertial mass plate 2 (g + w) In the region separated by more than 2 times, the electric field distribution is substantially zero, and does not affect the calculation of the capacitance evaluated by this simulation.

Further, when the electric field distribution is calculated from the center of one electrode 2b to the center of the other electrode 2b of the two electrodes 2b on both sides of the electrode 2a, for example, an electrode 2a and an electrode 2b, which will be described later, are calculated. The electric field distribution in the comb electrode structure in which are alternately arranged can be represented by repetition of the electric field distribution calculated based on FIG.
Therefore, it is possible to easily evaluate the comb electrode structure capable of further improving the sensitivity based on the simulation result.

The simulation results using the cross-sectional model shown in FIG. 2 are shown in FIGS.
In this simulation, the electrode 2a is 5V, the electrode 2b on both sides is 0V, the electrode width w = 3 μm, the electrode interval g = 7 μm, and the gap h between the inertial mass plate 2 and the dielectric 7 is changed. The change in capacitance between 2a and the electrode 2b was simulated.
In FIG. 3, (a) shows the calculated value when the gap h between the inertial mass plate 2 and the dielectric 7 is in the range of 0.2 μm to 4 μm, and (b) shows the electrostatic value in (a). The gap h between the inertial mass plate 2 and the dielectric 7 where the change in capacitance is remarkable shows the range of 0.2 μm to 1 μm with the horizontal axis extended.

  In FIG. 3, there are two lines shown as relative permittivity εr = 4, one of which is simulated based on the configuration of the preceding example shown in FIG.

As shown in FIG. 3, in the acceleration sensor of the first embodiment, when the dielectric constant of the dielectric 7 is increased, the change in capacitance can be increased with respect to the change in the interval h, and the sensitivity can be improved. It was confirmed that.
In addition, it can be seen from FIG. 3 that in the acceleration sensor of the present embodiment, the gap h between the inertial mass plate 2 and the dielectric 7 is preferably relatively narrow, and more preferably set to 1 μm or less.
The preferable range of the gap h between the inertial mass plate 2 and the dielectric 7 is the same even in the case of the comb electrode structure.

Next, a method for manufacturing the acceleration sensor according to the present embodiment will be described.
In this manufacturing method, first, for example, bulk lead zirconate titanate {Pb (Zr, Ti) O ₃ : hereinafter simply referred to as PZT on a substrate (for example, made of a silicon wafer or a quartz wafer) 1. } Is joined (FIGS. 5A and 5B). Here, the term “bulk” refers to, for example, a form of a substance having a property unique to the substance, unlike a fine particle or a thin film having physical properties different from normal properties. In this specification, the bulk is formed as a thin film. It means not a thing. Here, a PZT plate obtained by cutting bulk PZT so as to have a thickness of 0.2 mm was used as the dielectric plate 7.

  In addition, it is preferable to use a chemically stable bonding material for bonding the substrate 1 and the PZT plate in consideration of organic solvent resistance in a later process, for example, an adhesive such as polydimethylsiloxane (PDMS). Is preferred.

  Next, a sacrificial layer 12 made of, for example, polycrystalline silicon (Si) or amorphous silicon deposited by sputtering or CVD is formed on the PZT plate (dielectric plate 7), and along the outer periphery of the sacrificial layer 12. A slot 12s is formed, and a large number of dimples 12b are formed on the inside (FIGS. 5B (a) and 5 (b)). The slot 12s and the sacrificial layer around the slot 12s are portions for fixing one end of the elastic beam 3 after the sacrificial layer is removed in the final process, and the slot 12s more securely and firmly fixes the elastic beam 3. To be provided. Here, the three slots 12s are formed so that their centers coincide. The dimples 12b are for forming irregularities on the lower surface of the inertial mass body 2, and bumps corresponding to the dimples 12b are formed on the lower surface of the inertial mass body 2. Thus, by forming irregularities (bumps) on the lower surface of the inertial mass body 2, the lower surface of the inertial mass body 2 and the dielectric 7 are in contact with each other only at the upper portion of the bump, so that the contact area can be reduced and the inertial Adsorption of the lower surface of the mass body 2 and the dielectric 7 can be prevented.

Next, a protective film 13 made of, for example, polyparaxylylene is applied to the outer peripheral portion of the sacrificial layer 12 (the outer peripheral portion of the sacrificial layer that remains without being removed when the sacrificial layer is removed) so as to cover the slot 12s. It forms (FIG. 5C (a) (b)). This protective film 13 protects the sacrificial layer (for example, amorphous Si) left immediately below the portion where the electrode pad is to be formed and the outer peripheral portion thereof from the etching species (for example, XeF ₂ gas) in the subsequent process, and the wiring shown in FIG. It is formed to cover the slot where the electrodes 2wa and 2wb are to be formed to eliminate the unevenness. This protective film 13 makes it possible to leave a sacrificial layer that is not damaged under the electrode pad, and facilitates the formation of the wiring electrodes 2wa and 2wb.
As described above, the ends of the elastic beams are fixed, and the fixed portions to be the foundation on which the wiring electrodes 2wa and 2wb for applying a voltage to the electrodes 2a and 2b are formed.

Next, an Al film is formed on the sacrificial layer 12 and the protective film 13 by sputtering, and the Al film is patterned into a predetermined shape by mixed acid etching. Thereby, the electrodes 2a and 2b and the wiring electrodes 2wa and 2wb for applying a voltage to them are formed (FIGS. 5D (a) and (b)).
Next, the protective film is removed by dry etching using O ₂ plasma except for the protective film under the wiring electrodes 2wa and 2wb and the protective film in the outer peripheral portion where the electrode pads are formed.
In this dry etching using O ₂ plasma, etching may be performed using a patterned Al film as a mask. However, a sacrificial layer that needs to be left due to an alignment error of the photomask is etched from the side in a later step and damaged. There is a possibility of receiving. In order to prevent this, it is preferable to form a mask separately from the Al film in order to protect the sacrificial layer that needs to be left in a wide area by adding a margin and to remove other protective films.

Here, the electrode 2a is a comb electrode composed of a plurality of electrode branches 2a2 and a connection electrode 2a1 to which one ends of the electrode branches 2a2 are respectively connected, and a wiring electrode 2wa is connected to one end of the connection electrode 2a1 (FIG. 4). The electrode 2b is a comb electrode composed of a plurality of electrode branches 2b2 and a connection electrode 2b1 to which one ends of the electrode branches 2b2 are respectively connected, and a wiring electrode 2wb is connected to one end of the connection electrode 2b1 (FIG. 4). ). As described above, the electrodes 2a and 2b made of comb electrodes are arranged such that the electrode branches 2a2 and the electrode branches 2b2 are alternately positioned.
As described above, the electrodes 2a and 2b are comb electrodes, and the electrode branches 2a2 and the electrode branches 2b2 are alternately arranged, whereby the capacitance between the electrodes 2a and 2b can be increased and the measurement sensitivity can be improved. become.

Next, a structure film made of, for example, polyparaxylylene is formed so as to cover the electrode 2a, the electrode 2b, and the sacrificial layer 12, and is patterned by etching, for example, with O ₂ ashing. Thereby, the structure part which becomes the inertial mass body 2 and the elastic beam 3 and the fixing portion around the inertial mass body 2 for fixing the elastic beam 3 on the substrate are formed (FIG. 5E (a) ( b) (c)).
At this time, the fixed end of the elastic beam 3 is formed so as to fill the slot 12s of the sacrificial layer 12, and the fixed end of the elastic beam 3 is firmly fixed to the substrate. In addition, since polyparaxylylene can have a conformal film volume, the surface of the fixed end can be made flat.
Moreover, the window of the electrode pad portion is also opened by O ₂ ashing. The opening of the window is necessary for bonding a wire to the electrode in a later process or for detecting a signal by contacting a prober.

Finally, the sacrificial layer 12 made of Si is removed, for example, by etching with XeF ₂ gas, and the structure in which the inertial mass body 2 is held by the elastic beam 3 in a state of being separated from the substrate (dielectric 7). Is formed (FIGS. 5F (a), (b), and (c)).

  As described above, the acceleration sensor of the embodiment in which the inertial mass body 2 having the electrodes 2a and 2b formed on the lower surface is held hollow by the elastic beam 3 so as to be movable on the dielectric body 7 is manufactured. .

  According to the acceleration sensor manufacturing method of the first embodiment described above, the floating mass structure that holds the inertial mass body 2 on the dielectric 7 by the elastic beam 3 is manufactured using the sacrificial layer. The distance between the body 2 and the dielectric 7 can be easily set to 1 μm or less (preferably 0.2 μm to 0.5 μm), and a highly sensitive acceleration sensor can be manufactured.

  Further, according to the method for manufacturing the acceleration sensor of the present embodiment, the inertial mass body 2 can be made of a material suitable for forming a floating body using a micromachining process such as polyparaxylylene. That is, polyparaxylylene is excellent in electrical insulation, dielectric properties, chemical resistance, can be formed into a film with a desired thickness, and is suitable for the above production method. Since it has excellent adhesion to plastic or the like, the highly reliable inertial mass body 2 and elastic beam can be configured.

In addition, since the dielectric 7 is formed by bonding on the substrate, it is possible to form the dielectric 7 using a material having an extremely high dielectric constant, such as a bulk PZT plate, and the manufacturing cost. Can be kept low.
Therefore, according to the acceleration sensor manufacturing method of the present embodiment, a highly sensitive acceleration sensor can be manufactured at low cost.

In the above embodiment, the dielectric 7 is bonded onto the substrate 1. However, the present invention is not limited to such a configuration, and a ferroelectric material is used instead of the substrate 1 and the dielectric 7. A structure in which the substrate 1 and the dielectric 7 are integrated with each other may be used. Thus, if it comprises using the dielectric substrate with which the function of the board | substrate 1 and the dielectric material 7 was integrated, since the process of joining the dielectric material 7 on a board | substrate can be skipped, it can manufacture more cheaply.
Further, when the dielectric substrate in which the functions of the substrate 1 and the dielectric 7 are integrated is used, the dielectric 7 is free from the bonding portion of the substrate 1, so that the bonding portion does not peel off and is higher. A reliable acceleration sensor can be provided.

  In the acceleration sensor according to the embodiment, the four corners of the inertial mass body 2 having a substantially square plane shape are supported by the elastic beams 3. However, the present invention is not limited to this and is substantially limited. The central portion of each side of the square inertia mass body 2 may be supported by an elastic support beam. For example, the inertia mass body 2 having a circular planar shape may be used to support a plurality of peripheral portions thereof by the elastic beam. It may be.

  Further, in the present embodiment, the inertial mass body 2 is supported at four locations, but the present invention is not limited to this, and the inertial mass body 2 is supported at two or three locations, or five or more locations. You may make it support.

  Further, in the present embodiment, the inertial mass body 2 is supported by a so-called both-end supported beam structure. However, in the present invention, an inertial mass body 2 is formed by a cantilever beam using an elastic beam having a relatively high Young's modulus. You may make it support.

The acceleration sensor of the present embodiment may further include a package having a space for housing a sensor structure including the elastic beam 3, the inertial mass plate 2, the electrodes 2a and 2b, and the dielectric 7. Good.
In this case, by setting the space for housing the sensor structure to a vacuum or a reduced pressure state, it is possible to reduce the air resistance that hinders the displacement of the inertial mass body 2 and further improve the measurement sensitivity.

  In the acceleration sensor of the above embodiment, the dielectric 7 is configured by using a bulk PZT plate. However, the present invention is not limited to this, and a so-called thin film forming technique such as sputtering is used, for example, a ferroelectric. A dielectric 7 made of a body may be formed. Even if configured as described above, the dielectric constant is lower than that of bulk ferroelectrics, but dielectrics having a relative dielectric constant of several hundreds or more can be formed. Can be high.

In the present embodiment, an example in which a PZT plate is used as the dielectric 7 has been described. However, the present invention is not limited to this, and other ferroelectrics such as barium titanate (BaTiO ₃ ) are used. Alternatively, a paraelectric material having a relatively high dielectric constant may be used.

  That is, the present invention is characterized in that a first electrode and a second electrode are formed on an inertial mass plate whose position changes according to applied acceleration, and a dielectric is provided on the fixed side. Various modifications are possible within the scope of the idea.

  The acceleration sensor according to the present invention can be used in a wide range of applications such as car navigation systems, conveyors, elevators, and airbag systems.

It is a figure which shows typically the basic composition of the acceleration sensor of embodiment which concerns on this invention. It is a two-dimensional cross-sectional model for evaluating the capacitance between electrodes in the acceleration sensor of the embodiment. It is a graph which shows the electrostatic capacitance with respect to the space | interval between a dielectric material and an inertial mass body in the acceleration sensor of embodiment. It is a top view which shows the pattern structure of an electrode in the acceleration sensor of embodiment. FIG. 5A is a plan view after a dielectric is bonded on a substrate and a cross-sectional view taken along line A-A ′ in the acceleration sensor manufacturing method of the embodiment. FIG. 5A is a plan view after a sacrificial layer is formed on a dielectric in the method for manufacturing an acceleration sensor according to the embodiment, and a cross-sectional view taken along line B-B ′. FIG. 6A is a plan view after forming a protective film so as to cover the periphery of the sacrificial layer and a cross-sectional view taken along the line C-C ′ in the method for manufacturing an acceleration sensor according to the embodiment. FIG. 5A is a plan view after a pair of electrodes are formed at the center of a sacrificial layer and a cross-sectional view taken along the line D-D ′ in the acceleration sensor manufacturing method of the embodiment. In the acceleration sensor manufacturing method according to the embodiment, a plan view after an inertial mass body is formed on a pair of electrodes, a cross-sectional view of the EE ′ line (b), and an E1-E1 ′ line It is sectional drawing (c) about. In the acceleration sensor manufacturing method of the embodiment, a plan view after removing the sacrificial layer (a), a cross-sectional view taken along line FF ′, and a cross-sectional view taken along line F1-F1 ′ (c). It is. It is a figure which shows typically the basic composition of the acceleration sensor of a prior art example. It is a figure which shows typically the basic composition of the acceleration sensor disclosed by patent document 1. FIG.

Explanation of symbols

  1 substrate, 2 inertia mass plate, 2a, 2b electrode, 2wa, 2wb wiring electrode, 2a1, 2b1 connection electrode, 2a2, 2b2 electrode branch, 3 elastic beam, 7 dielectric, 12 sacrificial layer, 12s slot, 12b dimple, 13 Protective film.

Claims (7)

  Inertial mass plate whose position is changed by the applied acceleration, first and second electrodes provided on one main surface of the inertial mass plate, and dielectric provided opposite to the one main surface And an acceleration sensor that measures the acceleration based on a capacitance between the first electrode and the second electrode that changes in response to a change in position of the inertial mass plate. The dielectric comprises a dielectric plate provided on a substrate,
The acceleration sensor according to claim 1, wherein the inertia mass plate is supported on the substrate by an elastic beam.   The acceleration sensor according to claim 2, wherein one end of the elastic beam is connected to the inertia mass plate and the other end is connected to the dielectric plate via a fixing portion. The dielectric comprises a dielectric substrate,
The acceleration sensor according to claim 1, wherein the inertial mass plate is supported on the dielectric substrate by an elastic beam.   The first electrode is a comb electrode having a plurality of first electrode branches, the second electrode is a comb electrode having a plurality of second electrode branches, and the second electrode is disposed between adjacent first electrode branches. The acceleration sensor according to any one of claims 1 to 4, wherein an electrode branch is disposed.   The acceleration sensor according to any one of claims 1 to 5, wherein the inertial mass body is made of polyparaxylylene.   The acceleration sensor according to claim 1, wherein the dielectric is made of a ferroelectric ceramic.

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