JP6750822B2 - Electret element, electromechanical converter, and method for manufacturing electret element - Google Patents

Description

The present invention relates to an electret element, an electromechanical converter, and a method for manufacturing an electret element.

As a conventional method of fixing electric charges to an insulating film such as SiO ₂ , there is a method of using “corona discharge” or “electron beam” to drive charges from the surface of the insulating film (for example, refer to Patent Document 1).
.. However, since it is difficult to charge the narrow gap portion such as the side surface of the comb structure by this method, a method of assembling after the charging process is usually used. Therefore, it is difficult to reduce the gap, and the performance as a power generation device or an actuator is limited.

Therefore, as a method of charging such a narrow gap portion, a method of ionizing air with soft X-rays and implanting ions with a bias voltage has been proposed (see, for example, Patent Document 2).

In addition, a method has been proposed in which ions of an alkali metal such as potassium ions contained in the SiO ₂ layer are moved and immobilized by applying a bias voltage at high temperature (for example, see Patent Document 3).

JP, 9-283373, A Japanese Patent No. 5551914 Japanese Patent No. 5627130

By the way, in the method described in Patent Document 2, it is necessary to maintain a state in which an electric field acts on a portion to be charged during the charging process. For example, in order to charge the comb teeth to the root, it is necessary to keep the comb teeth deeply inserted. However, as the charging progresses, the electrostatic force decreases and the comb tooth insertion amount decreases, so that a special mechanism for pressing the comb tooth is required to maintain the insertion amount. Further, since air is required for the treatment, it is difficult to charge the sealed portion.

Further, since any of the methods is a method of implanting charges from the surface, it is difficult to control the fixed position of the charges (depth from the surface), and it is not possible to uniformly charge the deep portion of the insulator. The electric charges fixed near the surface react with water vapor in the air and are neutralized,
There is a drawback that the life of the electret is shortened.

On the other hand, in the method described in Patent Document 3, alkali metal ions are used, but in general, alkali metal deteriorates the electrical characteristics of the semiconductor element, and is therefore excluded from the manufacturing apparatus.
Therefore, with this method, it is difficult to form the electret in a part of the CMOS device, and the application range is limited. In this method, the alkali metal ions are
Since it is fixed near the surface ₂ , it is necessary to additionally perform a treatment such as a water repellent film in order to prevent the life of the electret from being shortened.

The electret element according to the invention of claim 1 comprises a Si layer, a SiO ₂ layer formed on the surface of the Si layer, and an electret formed in the vicinity of an interface between the SiO ₂ layer and the Si layer. the surface charge constituting the electret is fixed near the interface, the SiO ₂ layers that acts as a protective film of the electret.
An electromechanical converter according to the invention of claim 2 is arranged so as to face the first electrode composed of the electret element according to claim 1, and to be displaced relative to the first electrode. A possible second electrode, the relative displacement of the first and second electrodes providing a conversion between electrical energy and mechanical energy.
According to a third aspect of the present invention, in the electromechanical converter according to the second aspect, the Si layer is composed of a Si substrate, and at least a part of a circuit element for driving the electromechanical converter is the Si substrate. Is formed in.
According to a fourth aspect of the invention, in the electromechanical converter according to the second or third aspect, one of the first and second electrodes moves with respect to the other electrode by the action of an external force to generate power.
According to a fifth aspect of the present invention, in the electromechanical converter according to the second or third aspect, a stationary portion provided with the first electrode, a movable portion provided with the second electrode, and the first electrode are provided. A voltage source that applies a voltage between the second electrode and a control unit that controls the voltage applied by the voltage source to drive the movable unit are provided.
A sixth aspect of the present invention is a method of manufacturing an electret element , wherein the Si layer having the SiO ₂ layer is formed at a first temperature while maintaining the first temperature at which the SiO ₂ layer becomes a semiconductor state. a voltage is applied between the SiO ₂ layer, while applying the voltage, the said Si layer SiO ₂ layer is formed, the SiO ₂ layer from said first temperature to recover the insulation The temperature is changed to a second temperature, the electret of the electret element is formed in the vicinity of the interface with the Si layer in the SiO ₂ layer, and the surface charge forming the electret is fixed near the interface, and the SiO ₂ layer There that acts as a protective film of the electret.

ADVANTAGE OF THE INVENTION According to this invention, the electret element provided with the electret excellent in life performance can be provided.

FIG. 1 is a diagram illustrating an electret element according to the first embodiment. FIG. 2 is a diagram showing the electrical characteristics of the Si/SiO ₂ interface. FIG. 3 is a diagram for explaining the charging principle of the electret element according to the present embodiment. FIG. 4 is a diagram for explaining the charging principle in the electret element of the present embodiment, and shows the state when the applied voltage is zero. FIG. 5 is a diagram illustrating the charging process in detail. FIG. 6 is a diagram illustrating the charging process in detail. FIG. 7 is a diagram illustrating the charging process in detail. FIG. 8 is a schematic diagram showing a schematic configuration of the vibration power generation device. FIG. 9 is a view showing a cross sectional shape taken along line B1-B1 of FIG. FIG. 10 is a diagram showing a cross-sectional shape of B1-B1 after forming an oxide film and performing a charging process. FIG. 11 is a diagram illustrating an application form of the bias voltage V1. FIG. 12 is a schematic diagram showing in detail the electric double layer formed on the comb-teeth electrode. FIG. 13 is a schematic diagram showing in detail the structure of the area surrounded by the broken line C in FIG. FIG. 14 is a diagram showing a state in which the applied voltage is V1. FIG. 15 is a diagram showing a state in which the applied voltage is zero. FIG. 16 is a diagram illustrating the power generation operation of the vibration power generation device. FIG. 17 is a diagram showing a schematic configuration of the MEMS shutter. FIG. 18 is a diagram for explaining the driving operation of the comb-teeth type actuator, and shows the case where the applied voltage V is V=0. FIG. 19 is a diagram for explaining the driving operation of the comb-teeth type actuator, and shows the case where the applied voltage V is set to 0<V<V1. FIG. 20 is a diagram for explaining the driving operation of the comb-teeth type actuator, and shows the case where the applied voltage V is V=V1. FIG. 21 is a diagram for explaining the effect of the electret.

Hereinafter, an embodiment for carrying out the present invention will be described with reference to the drawings.
-First Embodiment-
The electret element according to the first embodiment is one in which a Si layer and a SiO ₂ layer are formed across an interface, and the electret is formed near the interface on the SiO ₂ layer side. The present inventor found the electrical characteristics of the Si/SiO ₂ interface as described below, and formed the electret in the SiO ₂ layer by utilizing the electrical characteristics.

In FIG. 1, a sample 100 has a SiO ₂ layer 102 formed on one surface of a Si layer 101. Au layers 103 and 104 are formed as electrodes on the Si layer 101 and the SiO ₂ layer 102. When Si is heated to a high temperature (about 500 to 700° C.), the electrical resistivity decreases due to an increase in the intrinsic carrier concentration, and it can be regarded as a conductor. Further, SiO ₂ is an excellent insulator at room temperature, but at high temperatures (about 500 to 700° C.), the electrical resistivity can drop to the order of 10^4 Ωm (similar to semiconductors) due to the effect of thermally excited electrons. Are known.

Therefore, the present inventor manufactured a sample 100 having a Si/SiO ₂ interface as shown in FIG. 1 and examined the electrical characteristics of the Si/SiO ₂ interface in a high temperature (about 610° C.) state. FIG. 2 shows the relationship between the applied voltage V1 and the current i, and it was found that the Si/SiO ₂ interface in a high temperature state has a rectifying effect like a Schottky junction.

(Explanation of charging principle)
FIG. 3 is a diagram for explaining the charging principle of the electret element according to the present embodiment. Si
A substrate having a structure in which an O ₂ layer 201 is sandwiched between Si layers 202 and 203 (for example, SOI (Silicon On Insul)
(a substrate) 200 is heated to a high temperature (500 to 700° C.) where SiO ₂ becomes a semiconductor, and a voltage V1 is applied as shown in FIG. 3, an electric double layer is formed with the Si/SiO ₂ interface 204 interposed. It is formed. Incidentally, the Si layer is usually doped with impurities, in which case,
Both p-type and n-type can be used. Further, it may be a Si layer containing no impurities.

As described above, the Si/SiO ₂ interface has a rectifying effect as shown in FIG. 2 in the high temperature state. Therefore, positive charges are accumulated on the Si layer 202 side across the Si/SiO ₂ interface 204 on the upper side, and Si
Negative charges are accumulated on the O ₂ layer 201 side. On the other hand, since a voltage is applied to the lower Si/SiO ₂ interface in the direction of current flow, no electric double layer is formed.

Next, when the temperature of the substrate 200 is returned to room temperature with the voltage applied, that is, the SiO ₂ layer 20
When the insulating property of No. 1 is lowered to the temperature at which the insulating property is restored, the SiO ₂ layer 20 sandwiching the Si/SiO ₂ interface 204 is formed.
The negative charges accumulated on the 1st side cannot be moved while being trapped in the region. After that, as shown in FIG. 4, when the application of the voltage V1 is stopped and the Si layer 202 and the Si layer 203 are connected, part of the positive charges move from the Si layer 202 to the Si layer 203.

On the other hand, the negative charges in the SiO ₂ layer 201, SiO ₂ layer 201 is insulative, the applied voltage V1
Even after being released, it remains trapped in the vicinity of the Si/SiO ₂ interface 204. As a result, an electric field E is formed in the SiO ₂ layer 201 as shown in FIG. This electric field E is an electric field due to the electret, and the potential difference between the Si/SiO ₂ interface 204 and the Si/SiO ₂ interface 205 is V1. That is, the electret of the voltage V1 is formed.

The charge amount and the like will be described in detail with reference to FIGS. FIG. 5 is a schematic diagram of a structure in which a SiO ₂ layer is sandwiched between Si layers. The surface charges Q2 and Q3 are charges forming the electric double layer shown in FIG. Although the distance d between the surface charges Q2 and Q3 in the electric double layer is very small, the distance d is exaggerated and shown large in FIG. 5 for easy understanding, and the surface charge Q2 in the SiO ₂ layer 201 is shown. It shall be fixed in position. In FIG. 5, it is assumed that the Si layer 203 is also charged with the surface charge Q1, and the entire structure is neutral. Therefore, it is assumed that only the electric fields E1 and E2 in the SiO ₂ layer 201 have a non-zero magnitude and the potential difference between the Si layers 202 and 203 due to the electric fields E1 and E2 is V.

First, the distribution of the surface charges Q1 and Q3 will be described. Area where the surface charge Q1 is sandwiched, surface charge Q2
When Gauss's law is applied to each of the region sandwiching and the surface charge Q3, the following formulas (1) to (3) are obtained. In addition, S is the cross-sectional area of the SiO ₂ layer 201, the Si layers 202 and 203,
ε1 is the dielectric constant of the SiO ₂ layer 201.
ε1·E1·S=Q1 (1)
(Ε1・E2-ε1・E1)・S=Q2 (2)
-Ε1, E2, S=Q3 (3)

Further, since the potential difference between the upper and lower Si layers is V, the following equation (4) is established. d is the surface charge Q2
, Q3, and g is the distance between surface charges Q1, Q2.
g·E1+d·E2=−V (4)

By rearranging the expressions (1) to (4), the surface charges Q1 and Q3 are obtained as in the following expressions (5) and (6).
Q1=-d·Q2/(g+d)-ε1·S·V/(g+d) (5)
Q3=-Q2-Q1 (6)

Next, the relationship between the voltage V applied during the charging process and the surface charge Q2 will be described with reference to FIG. The applied voltage V is set to V=V1, and since a current flows through the Si/SiO ₂ interface 205 as can be seen from FIG. 2, the following equations (7) and (8) are established in the state of FIG. Then, equations (7) and (8
) Is applied to equation (5), equation (9) is obtained. This Q2 is the fixed surface charge charged on the SiO ₂ layer 201 and forms an electret. Here, the applied voltage V1 is V1
If >0, then Q2<0. In FIG. 6, the potential change in the stacking direction is shown on the right side of the substrate 200. In the case of FIG. 6, an electric double layer is formed at the Si/SiO ₂ interface 204, and the voltage V1 is concentrated on this electric double layer.
V=V1 (7)
Q1=0 (8)
Q2=-ε1·S·V1/d (9)

Next, the surface charge at room temperature after the charging process shown in FIG. 4 will be described. Note that SiO ₂
Since the insulating property of the layer 201 is restored at room temperature and the charge remains trapped, the surface charge Q2
Holds the value of equation (9). The surface charge Q1 becomes as shown in Expression (10) by substituting Expression (9) into Expression (5).
Q1=-ε1·S·(V1-V)/(g+d) (10)

In the state shown in FIG. 6, there is a potential difference V1 between the Si layer 202 and the Si layer 203, but if the Si layer 202 and the Si layer 203 are connected as shown in FIG.
Positive charges move to the layer 203 and the potential difference decreases. The above-mentioned formula (10) represents the amount of movement of positive charges when the potential difference changes from V1 to V. Finally, as shown in FIG. 7, the potential difference V becomes V=0, so the surface charge Q1 of the Si layer 203 is expressed by the following equation (11).
Q1=-ε1·S·V1/(g+d) (11)

In addition, when the equation (10) and the equation (9) are compared, the relation of the following equation (12) is established. However, |V|<|V1| and d<<g.
|Q1|<<|Q2|... (12)

On the other hand, the surface charge Q3 is the sum of the charge −Q2 induced by the surface charge Q2 and the charge −Q1 due to the outflow of the minute charge Q1, as can be seen from the equation (6). Therefore,
Basically, there is an electric double layer {Q2, -Q2} with high charge density.
It is an image that a small amount of charge Q1 moves between layers.

The advantage of such an electret is that Q1≠0 (that is, electric field E1≠0) at the potential difference V=0 as shown in FIG. As can be seen from the equation (11), the magnitude of the electric field E1 generated at this time is determined by the external bias voltage V when there is no electret (Q2=0).
It has the same magnitude as the electric field generated when 1 is applied. From this, it is expressed that "the electret charging voltage is V1".

In the example shown in FIG. 7, the electric field E1 remains inside the SiO ₂ layer 201 and has a small utility value, but an electric field can be generated in the gap space by performing a charging process on a predetermined structure as described later. The electric field generated in this gap space can be used for electric-mechanical conversion (conversion between electric energy and mechanical energy), and can be used for power generation, sensors, actuators, and the like.

-Second Embodiment-
In the second embodiment, the electret element of the first embodiment is applied to a vibration power generation device having a comb tooth structure, which is an example of a mechanical-electrical converter. FIG. 8 shows a vibration power generation device 3
It is a schematic diagram which shows schematic structure of 00. Similar to the case of the electret element according to the first embodiment, this vibration power generation device 300 has the same semiconductor integrated circuit manufacturing technique as in the case of the general MEMS for the SOI substrate (for example, deep etching by ICP-RIE). It is formed by processing with.

The vibration power generation device 300 includes a fixed comb-teeth electrode 302 and a movable comb-teeth electrode 303 on a pedestal 301 having a rectangular ring shape. The movable comb-tooth electrode 303 is elastically supported on the pedestal 301 by the elastic support portion 305. Each comb tooth of the movable comb-teeth electrode 303 has a fixed comb-teeth electrode 3
It is arranged with a gap between each comb tooth of No. 02. The movable comb-tooth electrode 303 has a weight 304.
Is provided. When external vibration is applied to the vibration power generation device 300, the movable comb-teeth electrode 3
03 vibrates in the direction of arrow R. The load 320 is a fixed comb-teeth electrode 302 and a movable comb-teeth electrode 303.
Connected between and. As will be described later, an electret is formed on the fixed comb-teeth electrode 302, and when an external force is applied to the vibration power generation device 300 to vibrate the movable comb-teeth electrode 303, power is generated.

In this embodiment, after processing into the shape shown an SOI substrate in FIG. 9, as SiO ₂ layer electret is formed, oxide film on the surface of the Si layer (SiO ₂ layer) (thickness t = 0.2 to (About 1 μm) is formed by the thermal oxidation method (see FIG. 10). After that, in the same manner as in the first embodiment, electric charges are fixed to the oxide film to form an electret. Although the oxide film (SiO ₂ layer) formed on the surface of the Si layer is formed by the thermal oxidation method in the present embodiment, the present invention is not limited to this, and the oxide film (SiO ₂ layer) may be formed by various oxide film forming methods. May be. For example, an oxide film (SiO ₂ layer) may be formed by depositing SiO ₂ on the Si layer by CVD.

FIG. 9 is a view showing the B1-B1 cross-sectional shape of FIG. 8 and shows the shape before the oxide film is formed. The pedestal 301 is formed by the handle layer (Si) of the SOI substrate. The fixed comb-teeth electrode 302 is formed by the device layer (Si) of the SOI substrate. A portion indicated by reference numeral 307 is a buried oxide film (SiO ₂ ) called a BOX layer of the SOI substrate. Although illustration is omitted, the movable comb-teeth electrode 303, the elastic support portion 305, and the weight 304 are formed by the device layer of the SOI substrate.

FIG. 10 is a view showing a cross-sectional shape of B1-B1 after the oxide film is formed and the charging process is performed. Si
An oxide film 310 is formed on each of the surfaces of the fixed comb-teeth electrode 302 and the pedestal 301 formed of layers. When the oxide film 310 is charged, as in the case of the first embodiment, a heater or the like is used to heat the oxide film 310, which is a SiO ₂ layer, to a temperature at which it becomes a semiconductor. Then, when the oxide film 310 becomes semiconductor, it is cooled to a temperature at which the semiconductor film oxide film 310 recovers its insulating property with the bias voltage V1 (10 to 200 V) applied. As shown in FIG. 10, since the edge portion of the Si layer has an R shape due to thermal oxidation, the electric field concentration is relaxed when a bias voltage is applied, and the dielectric breakdown strength increases. Therefore, a relatively high bias voltage can be applied despite the small gap size (about 2 μm) between the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303.

(Detailed description of charging process)
When the charging process is performed, the bias voltage V1 is applied between the fixed comb-teeth electrode 302, the movable comb-teeth electrode 303, and the pedestal 301 as shown in FIG. First, the vibration power generation device 300
Is heated to a temperature (500 to 700° C.) at which the oxide film 310 made of SiO ₂ becomes a semiconductor. Then, the bias voltage V1 is applied so that the electric double layer is formed so as to sandwich the Si/SiO ₂ interface 306 of the fixed comb electrode 302 (see FIG. 10).

FIG. 12 schematically shows a cross section (cross section parallel to the paper surface of FIG. 11) of a portion where the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303 overlap each other in a state where the electric double layer is formed. It is a thing. In the following, the oxide film formed on the fixed comb-teeth electrode 302 is denoted by reference numeral 310a, and the oxide film formed on the movable comb-teeth electrode 303 is denoted by reference numeral 310b. Further, the Si layer of the fixed comb-teeth electrode 302 is denoted by reference numeral 311a, and the Si layer of the movable comb-teeth electrode 303 is denoted by reference numeral 311b. When a bias voltage is applied, the potential difference of the electric double layer at the Si/SiO ₂ interface 306 gradually rises and eventually becomes the voltage V1 (several seconds to several minutes).

Since the SiO ₂ layer (the oxide film 310a and the BOX layer 307) is made into a semiconductor and its electric resistivity is lowered, the inside of the SiO ₂ layer has almost the same potential. Therefore, the charge density becomes uniform in the entire Si/SiO ₂ interface 306, and an electric double layer is formed up to the tip of the comb teeth. In addition, Si/SiO ₂ interface 3
When the electric double layer is formed on the entire 06, it is electrostatically shielded by the SiO ₂ layer having a reduced resistivity, and thus no electric field is generated outside the electric double layer. As a result, the electrostatic force between the comb-teeth electrodes becomes zero, and by observing this, it can be used as a guide for the completion of the charging process.

FIG. 13 is a schematic diagram showing in detail the structure of the area surrounded by the broken line C in FIG. 12, and corresponds to FIG. 5 of the first embodiment. Surface charges Q5 and Q6 forming an electric double layer sandwiching the Si/SiO ₂ interface 306 are formed in the oxide film 310a and the Si layer 311a of the fixed comb electrode 302. The surface charge Q4 represents the charge that is charged on the Si layer 311b of the movable comb-tooth electrode 303. E3 is an electric field formed in the oxide film 310b of the movable comb electrode 303. E5 and E6 are electric fields formed in the oxide film 310a of the fixed comb electrode 302. E4 is the comb-teeth electrode 302,
It is an electric field formed in the gap space G between 303.

13, a region including the surface charge Q4, a region including the interface between the oxide film 310b and the gap space G, a region including the interface between the oxide film 310a and the gap space G, a region including the surface charge Q5, and a surface charge Q6. When Gauss's law is applied to each of the regions including, the following equations (13) to (17)
) Is obtained. In addition, S is a cross-sectional area when the area|region C of FIG. 12 is cut out. ε0, ε1
Is the dielectric constant of the gap space G and the oxide film (SiO ₂ ).
ε1, E3, S=Q4 (13)
(Ε0・E4-ε1・E3)・S=0 (14)
(Ε1・E5-ε0・E4)・S=0... (15)
(Ε1・E6−ε1・E5)・S=Q5 (16)
-Ε1·E6·S=Q6 (17)

Since the potential difference between the upper and lower Si layers 311a and 311b is V, the distances d and g shown in FIG.
The following equation (18) holds for 1, g2 and g3.
g1·E3+g2·E4+g3·E5+d·E6=-V (18)

From the expressions (13) to (17), the following expression (19) representing the relationship between the surface charges Q4, Q5, Q6 is obtained.
Q6=-Q5-Q4 (19)

Further, from the equations (13) to (18), the following equation (which represents the surface charge Q5 which is the electret charge)
20) is obtained.
Q5=-[(d+g1+g2(ε1/ε0)+g3)/d]Q4
-Ε1・S・V/d (20)

As shown in FIG. 14, when a bias voltage V1 is applied, a current flows through the Si/SiO ₂ interface 308, so V=V1 and Q4=0 in the bias voltage applied state. When V=V1 and Q4=0 in the equation (20), the surface charge Q5 is represented by the following equation (21). Applied voltage V1
Is V1>0, Q5<0. If Q4=0 in the equation (19), then Q6
=-Q5. Thus, in the case of FIG. 14, an electric double layer is formed at the Si/SiO ₂ interface 306, and the voltage V1 is concentrated on this electric double layer.
Q5=-ε1·S·V1/d (21)

As shown in FIG. 14, when the electric double layer is formed on the Si/SiO ₂ interface 306, that is, when the bias voltage V1 is applied, the temperature at which SiO ₂ regains its insulating property (for example, room temperature)
Then, the surface charge Q5 charged on the oxide film 310a is fixed at the position shown in FIG. After that, as shown in FIG. 15, the Si layer 311 a of the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 3
No. 03 Si layer 311b is connected to the Si layer 311b due to the potential difference (see FIG. 14) between them.
The charge (Q4) moves from 11a to the Si layer 311b, and the potential difference decreases. When the potential difference changes from V1 to V, the amount of charge movement at that time is expressed by the following equation (22). When the potential difference finally becomes zero as shown in FIG. 15, the surface charge Q4 becomes as shown in the following expression (23).
Q4=-ε0·S·(V1-V)/[g′+d·(ε0/ε1)] (22)
However, g′=g2+(g1+g3)·(ε0/ε1)
Q4=-ε0・S・V1/[g'+d・(ε0/ε1)] (23)

Since Q4=ε0·E4·S from equations (13) and (14), the electric field E4 of the gap space G in FIG. 15 is represented by the following equation (24) from this equation and equation (23). This agrees with the electric field formed when the voltage V1 is applied when there is no electret (surface charge Q5).
E4=-V1/[g'+d·(ε0/ε1)] (24)

(Explanation of power generation operation)
Next, the power generation operation of the vibration power generation device 300 will be described. FIG. 16 shows a state in which the movable comb-tooth electrode 303 slides relative to the fixed comb-tooth electrode 302 so that the overlap between the comb teeth becomes zero (c) and a state in which half of the comb teeth overlap ( 3B is a schematic view showing a state (a) in which the entire comb teeth overlap. This corresponds to the case where the load 320 having the low impedance limit is connected, and corresponds to the change in the area S (corresponding to the overlap area) in Expression (22) and the change in the charge amount of the surface charge Q4. ing. In order to simplify the description, one minus sign shown in the surface charge Q5 is regarded as the charge amount −q, and one plus sign shown in the surface charges Q4 and Q6 is regarded as the charge amount +q, so that the change in the charge amount is changed. Will be explained.

The state (a) of FIG. 16 is the same as the state shown in FIG.
The potential of the layer 311a and the potential of the Si layer 311b of the movable comb-tooth electrode 303 are equal. That is, the potential difference V=0. Therefore, no current flows through the load 320. At this time, the amount of surface charge Q6 is +6q, the amount of surface charge Q5 is -8q, and the amount of surface charge Q4 is +2q.
Has become.

In the state (b), the movable comb-tooth electrode 303 moves to the left in the drawing with respect to the fixed comb-tooth electrode 302, and the overlap area of the comb teeth is reduced to half. The amount of surface charge Q4 decreases from +2q to +q as the overlap area decreases, and the amount of surface charge Q6 increases by +6q.
To +7q. As a result, a current I flows from the Si layer 311b of the movable comb-teeth electrode 303 to the Si layer 311a of the fixed comb-teeth electrode 302.

When the overlap area further decreases from the state (b), the amount of surface charge Q4 decreases with the decrease of the overlap area. When the overlap area becomes zero as in the state (c), the charge amount of the surface charge Q4 becomes zero and the charge amount of the surface charge Q6 becomes +8q.

When the movable comb-tooth electrode 303 vibrates with respect to the fixed comb-tooth electrode 302, the states (a) to (c) shown in FIG. 16 are changed to (a)→(b)→(c)→(b). )→(a)→(b)→・・
Is repeated, and an alternating current flows through the load 320. When a load having a high impedance limit is connected as the load 320, the overlap area changes without changing the charge amount of the surface charge Q4, so that the potential difference V changes. Generally, the power taken out is maximized by adjusting the load impedance.

-Third Embodiment-
In the third embodiment, the electret element of the first embodiment is applied to the comb-teeth type actuator of the MEMS shutter. FIG. 17 is a diagram showing a schematic configuration of the MEMS shutter 400 of this embodiment. The same components as those of the vibration power generation device 300 shown in FIG. 8 are designated by the same reference numerals. That is, the MEMS shutter 400 is formed by processing an SOI substrate, and the fixed comb-teeth electrode 30 fixed to the pedestal 301 having a rectangular ring shape.
2. The movable comb-tooth electrode 303 fixed to the pedestal 301 by the elastic support portion 305 is provided. The fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303 form a comb-teeth type actuator. The movable comb electrode 303 is provided with a shutter portion 404 having an opening 404a.

A voltage for driving the actuator is applied between the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303 by the voltage source 401. The control unit 402 controls the applied voltage V of the voltage source 401 to move the movable comb-teeth electrode 303 provided with the shutter unit 404 in the direction of arrow R. The shutter section 404 is arranged on the optical path, and the movement of the movable comb-tooth electrode 303 causes the shutter section 4 to move.
When the opening 404a of 04 is arranged in the optical path, the light ray passes through the shutter unit 404. On the other hand, by arranging the non-opening area (shielding area) of the shutter section 404 in the optical path, light rays are blocked by the shutter section 404.

The configuration and forming method of the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303, and further the method of forming an electret on the fixed comb-teeth electrode 302 are the same as those in the second embodiment described above, and here, The description is omitted.

(Operation explanation)
18 to 20 are views for explaining the driving operation of the comb-teeth type actuator. FIG. 18 shows the case where the applied voltage V of the voltage source 401 is V=0. In FIG. 18, (a) shows the movable comb-teeth electrode 30.
3 shows forces F1 and F2 acting on No. 3 and (b) is a diagram showing a relationship between an applied voltage V and an electric field E4. When the applied voltage V=0, the Si layer 311a and the Si layer 311b have the same potential.
The state is the same as that shown in FIG. In the gap space G between the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303, the electric field E4 represented by the above formula (24) is formed. Due to the electric field E4, a rightward force F1 in the drawing is applied to the movable comb-tooth electrode 303 so as to be drawn between the comb teeth of the fixed comb-tooth electrode 302.

When the movable comb-teeth electrode 303 is moved so as to be drawn into the fixed comb-teeth electrode 302 by the force F1 of the electric field E4, the elastic support portion 305 is deformed as shown in FIG. As a result, due to the elastic force of the elastic support portion 305, a force F2 that pulls back to the left side in the drawing acts on the movable comb tooth electrode 303. The movable comb-teeth electrode 303 stops at a position where the force F1 and the force F2 are in balance.

FIG. 19 shows a case where the applied voltage V is set to 0<V<V1. In this case, the gap space G
The electric field E4 of Eq. (2) obtained by applying Eqs. (13) and (14) to Eq.
It is represented by 5). As can be seen from the equations (24) and (25), the strength of the electric field E4 in FIG. 19 is weaker than when the applied voltage V=0. As a result, the electrostatic force F1 attracted by the movable comb-teeth electrode 303 toward the fixed comb-teeth electrode 302 becomes small, and the movable comb-teeth electrode 303 elastically supports the electrostatic force F1 as shown in FIG. It moves to the left in the drawing to a position where it balances with the elastic force F2 of the portion 305.
E4=-(V1-V)/[g'+d.([epsilon]0/[epsilon]1)] (25)

FIG. 20 is a diagram showing a case where the applied voltage V is V=V1. At this time, the charge amounts of the surface charge Q5 and the surface charge Q6 become equal, and the potential difference in this electric double layer becomes equal to V1. As a result, the electric field E4 of the gap space G becomes zero, and the electrostatic force F1 between the fixed comb electrode 302 and the movable comb electrode 303 also becomes zero. Therefore, as shown in FIG.
The deformation of 5 is also zero.

As described above, in the present embodiment, the voltage V applied by the voltage source 401 is changed to slide and drive the movable comb-tooth electrode 303, so that the shutter section 404 can open and close the shutter. Further, as shown in FIGS. 18 to 20, by incorporating the electret in the comb-teeth electrode, the strength of the electric field E4 in the gap space G is maximized at the applied voltage V=0.

By the way, the electrostatic force acting between the comb teeth in the comb-tooth actuator is proportional to the square of the electric field. Therefore, in the case of a configuration in which the comb-tooth actuator is driven only by the applied voltage V without using the electret, the relationship between the applied voltage V and the electrostatic force F1 becomes a quadratic curve as shown by the line L1 in FIG. .. On the other hand, in the case of the comb-teeth actuator in which the electret is formed as in the present embodiment, the relationship between the applied voltage V and the electrostatic force F1 is as shown by the line L2. The line L2 is a line obtained by moving the line L1 in the positive direction on the horizontal axis by an amount corresponding to the electrification charging voltage V1. Therefore, for the same applied voltage ΔV, the electrostatic force ΔFb with the electret is larger than the electrostatic force ΔFa without the electret. That is, when the electret is formed, a larger electrostatic force can be obtained as compared with the configuration using only the external bias voltage.

As described above, the electret element has the Si layer 202 and the Si layer 2 as shown in FIG.
02, a SiO ₂ layer 201 formed on the surface of the SiO _2, and an electret formed near the interface between the SiO ₂ layer 201 and the Si layer 202. The charges that make up the electret are Si/SiO.
Since it is fixed near the _two interfaces, the SiO ₂ layer 201 functions as a protective film and the life of the electret can be improved.

The electret maintains the Si layer 202 on which the SiO ₂ layer 201 is formed at a first temperature (about 500 to 700° C.) at which the SiO ₂ layer 201 becomes a semiconductor state, while the Si layer 202 and the SiO ₂ layer 20 are maintained.
1 and a voltage is applied between the SiO ₂ layer 201 and the SiO ₂ layer 201.
The i layer 202 is formed by changing from the first temperature to a second temperature (for example, a temperature of about 300° C. or lower) at which the SiO ₂ layer 201 recovers its insulating property.

As described above, since the electret is formed by the method of moving and fixing the electric charges in the SiO ₂ layer, the electret is arranged in a narrow gap part or a closed space like the comb tooth side surface of the comb tooth electrode as shown in FIG. Even with the formed electrode, the electret can be easily formed. Since the electret can be easily formed in the narrow gap portion, the gap size can be designed to be smaller, and the performance as the power generation device or the actuator is improved.

Furthermore, since the charges move without being directly related to the electric field on the device surface, it is possible to charge the charges with a uniform charge density without special measures during the charging process (electret forming process). Further, as shown in FIG. 3, since the electric double layer is formed to perform the charging, the gap between the charges to be charged across the interface is very small, and a large charge density can be obtained even at a small potential.

Further, as in the second embodiment, the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303 are arranged so as to face each other, and the fixed comb-teeth electrode 302 is composed of an electret element. Then, the movable comb-teeth electrode 303 moves, that is, the movable comb-teeth electrode 303 is displaced with respect to the fixed comb-teeth electrode 302, so that electromechanical conversion is performed to convert between electrical energy and mechanical energy. It functions as a container (for example, the vibration power generation device 300).

Although the electret is formed on the fixed comb-teeth electrode 302 side in the above-described embodiment,
An electret may be formed on the movable comb-tooth electrode 303 side. Furthermore, the configuration is not limited to one in which one of the pair of comb-teeth electrodes is movable, and a configuration in which both of the pair of comb-teeth electrodes are movable is also possible.

As the electromechanical converter, in addition to the power generation device, a shutter unit 404 as shown in FIG.
There are an actuator for driving the motor, an electret condenser microphone, and the like. In the case of the electret element of the above-described embodiment, it does not have a configuration including alkali metal ions unlike the electret described in Patent Document 3, so that it can coexist with a CMOS device. For example, the control unit 402 in FIG. It is possible to form a part of the circuit elements in the Si layer (device layer) of the pedestal 301. Such circuit elements include, for example, transistors for drive circuits, FETs and resistors for amplification circuits of microphones and sensors, and rectifying diodes for power generation elements.

In the above-described second embodiment, the electrodes 302 and 303 have the comb-teeth structure, but they may have a parallel plate structure in which the gap distance changes. Thus, the electret element can be used for a parallel plate type vibration power generation device and a condenser microphone.

Further, in the above-described embodiment, the entire device including the fixed comb-teeth electrode 302 and the movable comb-teeth electrode 303 is heated to perform the charging process. However, the region related to the formation of the electret (the SiO ₂ layer to be charged). And the Si layer to which a current is applied) may be locally heated by a laser or the like. As a result, it can be applied to a device such as an electret microphone having a built-in amplifier circuit.

Note that the above description is merely an example, and when interpreting the invention, there is no limitation or restriction on the correspondence between the items described in the above embodiment and the items described in the claims.

101, 202, 203, 311a, 311b... Si layer, 102, 201... SiO ₂ layer, 2
04, 205, 306, 308... Si/SiO ₂ interface, 300... Vibration power generation device, 301... Pedestal, 302... Fixed comb electrode, 303... Movable comb electrode, 304... Weight, 305... Elastic support,
310, 310a, 310b... Oxide film, 320... Load, 400... MEMS shutter, 40
1... Voltage source, 402... Control part, 404... Shutter part, G... Gap space

Claims (6)

A Si layer,
A SiO ₂ layer formed on the surface of the Si layer,
An electret formed in the vicinity of an interface between the SiO ₂ layer and the Si layer ,
The surface charge constituting the electret is fixed near the interface, electret element the SiO ₂ layers that acts as a protective film of the electret. A first electrode and a second electrode that are arranged to face each other and at least one of which is movable,
The first electrode is composed of the electret element according to claim 1,
An electromechanical converter for converting between electrical energy and mechanical energy by moving at least one of the first and second electrodes. The electromechanical converter according to claim 2,
The Si layer is composed of a Si substrate,
An electromechanical converter in which at least a part of a circuit element for driving the electromechanical converter is formed on the Si substrate. The electromechanical converter according to claim 2 or 3,
An electromechanical converter that generates power by moving at least one of the first and second electrodes by the action of an external force. The electromechanical converter according to claim 2 or 3,
A stationary part provided with the first electrode,
A movable part provided with the second electrode,
A voltage source for applying a voltage between the first electrode and the second electrode,
An electromechanical converter comprising: a control unit that controls a voltage applied by the voltage source to drive the movable unit. A method of manufacturing an electret element, comprising:
A voltage is applied between the Si layer and the SiO ₂ layer while maintaining the Si layer on which the SiO ₂ layer is formed at a first temperature at which the SiO ₂ layer becomes a semiconductor state ,
In the state in which the voltage is applied, the Si layer on which the SiO ₂ layer is formed is changed from the first temperature to a _second temperature at which the SiO ₂ layer restores insulating properties ,
An electret of the electret element is formed in the vicinity of an interface between the SiO ₂ layer and the Si layer,
The surface charge constituting the electret is fixed near the interface method of the electret device the SiO ₂ layers that acts as a protective film of the electret.

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