JP5487493B2 - Electret micro power generator - Google Patents

Description

Detailed Description of the Invention

The present invention relates to a power generation device that converts vibration energy into electric energy, and more particularly to a micro power generation device manufactured by a technique represented by MEMS (Micro Electromechanical Systems) technology.

The history of technology for generating electricity from vibrations in the environment is old, and a self-winding power generation watch is a notable example (see Non-Patent Document 1, for example). This is a vibration power generator with a scale of about 1 centimeter, and the voltage and current handled are about 1 V and 1 microampere, respectively. In such a power scale, charging of the capacitor becomes practical by half-wave rectification using a known diode.

Based on this, a method of driving a timepiece circuit by forming a booster circuit by connecting a plurality of capacitors using a semiconductor switch has been put to practical use (for example, see Non-Patent Document 2). As for the rectifier circuit itself, a low-loss rectifier circuit using a FET (Feed Effect Transistor) instead of a diode for the purpose of generating power for medical implant devices or distributed sensor nodes is well known (for example, patents). Reference 1). On the other hand, as research on ultra-low power devices progresses (see, for example, Non-Patent Document 3), there is a potential demand for low-power but ultra-small vibration power generators, particularly in electret mechanical electrical energy conversion elements. Research is progressing along with research on devices using other piezoelectric materials (see, for example, Non-Patent Document 4).

Journal of the Japan Watch Association, No. 120, 1987, pages 40-48 Journal of the Japan Watch Association, No. 126, 1988, pages 28-40 Nature, Vol., 445, pp. 745, 2007, pages 745-748. Proceedings of the 23rd Sensor Symposium, pp. 521-524, 2006

US Pat. No. 5,999,849

In recent years, the importance of developing micro-energy power generation technology that generates necessary power on the spot has been recognized in recent years in preparation for the wider use of microin-vivo implant medical technology in the future or the arrival of a ubiquitous sensor network society. In general, micro energy power generation technology generates electricity from artificial or natural light, vibration, chemistry, thermal energy, etc. in the environment. Of these, in vibration power generation that generates power from vibration energy and sound field energy in the environment, since the energy density is generally relatively small, a very small amount of output power is efficiently rectified and stored as energy, mainly intermittently. The technology used for proper use is critical.

Nevertheless, no such technology currently exists at power scales on the order of nanowatts or less. In order to minimize the forward voltage drop, for example, a circuit using a Schottky diode element is well known, but those schemes are insufficient to be applied to nanowatt scale power. In addition, for a constant amplitude vibration, the theoretical maximum output of the vibration generator is proportional to the fourth power of the vibration frequency, so artificial energy supply requires rectification and power storage technology that operates at higher frequencies However, this becomes more difficult as the power becomes lower. Thus, there is a lack of technology to meet the potentially large-scale demand of supplying power to ultra-low power sensor nodes on the spot.

In order to respond to this situation, it is important not to take out and examine only the rectifier circuit that handles a minute output, but to design it comprehensively including the mechanical and electrical energy conversion elements. Accordingly, an object of the present invention is to obtain only a small output current and voltage, and when they have a frequency that is not so low, this is efficiently rectified and energy is stored with low loss, and a low power circuit is obtained. An object of the present invention is to provide such a device for use by

To briefly explain the summary of typical inventions among the inventions disclosed by the present application in order to achieve the above object, it is as follows.

Under the situation where a very small primary power generation voltage in the present invention is handled, a voltage drop in all active electronic circuit elements including a diode greatly affects. Therefore, as shown in FIG. 2, for example, diodes D1 and D2 are used for rectification, and energy is stored in the capacitor C1, half-wave rectification is fundamental. However, this is not necessarily the case when a primary power generation voltage sufficient for full-wave rectification can be obtained.

Specifically, an electret vibration power generation element having a micro vibrator with a side of 1 cm or less manufactured using, for example, a MEMS technology as a mechanical electric energy conversion element can be considered. FIG. 3 is a schematic diagram of an electret type mechanical electric energy conversion element. In electret mechanical-electrical energy conversion, for example, by electrifying the electret film ELF1 applied to the bottom surface of the micro vibrator OSL1, an opposite charge is induced in the metal electrodes CE1 and CE2 facing the electret film ELF1. If the micro vibrator is now moved to the left and right, the opposite charge also moves in accordance with that, so that a current flowing through the load L is generated. Corresponding to the load characteristics, a voltage corresponding to the current is generated. Here, in general, the load characteristic is not ohmic.

This voltage induced on the electrode facing the electret film (hereinafter referred to as the counter electrode) gives the micro vibrator a force that resists vibration. However, in practice, the more important factor is the high impedance of the input of the rectifier circuit. Long relaxation time, i.e., time constant, due to the combination of the input high resistance Ri due to the presence and the stray capacitance Cs due to the stray capacitance between the counter electrodes CE1 and CE2 or the stray capacitance between CE1 or CE2 and the electric ground. There is a generation of Tr = Ri × Cs . If this exceeds the vibration period of the micro vibrator of about 100 Hz to 100 kHz, for example, the energy recovery rate is lowered. Further, a value obtained by dividing the amount of generated charge by the stray capacitance gives a generated voltage, which must be at least about 0.2 V or more at which a normal semiconductor device operates. This gives another reason why the stray capacitance must be small.

On the other hand, turning to the rectifier circuit side, when diodes D1 and D2 are used as the rectifier circuit elements as shown in FIG. 2, if an element having a large reverse leakage current is used, a larger current is always generated in the machine electric circuit. If it is not generated from the energy conversion element, there is a problem that charges are not accumulated in the capacitor C1. In particular, electric charge accumulated in a stationary state of zero bias Day impedance is not generating power and no more than 10 megohms leaks out. That is, the problem is further increased in the case of intermittent power use where the present invention is focused. Therefore, it is desirable to use a diode having a small reverse leakage current. However, in general, such a device has a problem that the current rise is slow with respect to the forward voltage because the current scale is generally small. In other words, only a small current flows in the forward low voltage region, that is, the input impedance is very large. In the case of an actual electret-type electromechanical energy conversion element of millimeter scale, the appropriate load impedance is typically about 100 megaohms, but for example, a diode having a minute leakage current of about picoamperes is more than gigaohm in a voltage range of about millivolts. Input impedance. As a result of experimental investigation, the inventors of the present invention have found that it is extremely important to match these impedances.

The stray capacitance associated with the counter electrodes CE1 and CE2 can be reduced to some extent by taking away the ground electrode or by taking measures such as reducing the circuit wiring to about 5 mm or less, but is a geometric limit. There is an interelectrode parasitic capacitance C. That is, assuming that the width of the counter electrode is W, the gap distance between adjacent counter electrodes is G, the length of the electret film in the depth direction is L, and the dielectric constant of the substrate is ε, the area of the electrode in the substrate portion where the electrode is located The occupation ratio is H = W / (W + G) . The area of the substrate portion having electrodes without the electrode plate is referred to as an interelectrode gap area. Now, the capacity C is (Equation 1) C = eL {K (sin (pH / 2)) / K (cos (pH / 2))}
It is. Here, K is a first-type complete elliptic integral, and the dimensionless numerical factor including K in the above equation is about 1.5 to 2 in most regions, but becomes about 3 when H is about 0.97. As H increases, it rapidly increases toward infinity. Of course, when H is too small, the bond with the electret film is deteriorated.

One method for solving the above problems is that an electret film and a pair of two electrodes opposed to the electret film are arranged as shown in FIG. 1 while the mechanical micro vibrator of the electret vibration power generation element is kept single. A plurality of sets of counter electrodes are connected in series and three or more sets of counter electrodes are connected in series to add synchronized voltages. As a result, the output voltage can be increased to a value sufficient for the operation of the rectifier circuit, and an appropriate rectification operation can be performed. The plurality of electret films have a stripe shape, and the width of the stripe is 3 mm or less because it is a small electret vibration power generation element.

In the case of a micro device that operates at a relatively high frequency of resonance frequency of 100 Hz or more, undesirable parasitic capacitance is obtained by minimizing the electret film, the counter electrode, the wiring to the rectifying device, and the rectifying circuit to, for example, 5 mm or less by fine processing. It is preferable that the voltage drop can be suppressed low by minimizing the charge and suppressing the charge flowing into it.

In addition to this, in order to avoid an unnecessary increase in the geometric part of the interelectrode parasitic capacitance, the total electrode area of the interelectrode gap area, that is, the sum of the interelectrode gap area and the electrode area, A configuration in which the ratio to is 3% or more is preferable. That is, when expressed using the factor H representing the area occupancy of the electrode, a configuration in which 1-H is 3% = 0.03 or more is preferable.

The electret vibration power generator according to claim 2 minimizes the stray capacitance by integrating the counter electrode and the rectifying diode within an area of 5 mm square of the silicon substrate, and particularly reduces leakage current at zero bias. It is characterized by using a diode with a micro-junction cross-sectional area to minimize. As the diode, a pn junction type may be used, or a Schottky type may be used in order to minimize the forward voltage effect. The rectifier circuit may be an efficient full-wave rectifier circuit using four diodes, or a half-wave rectifier circuit using two diodes and having a low operation threshold voltage.

The electret vibration power generator according to claim 3 is characterized in that stripe electrets are formed on at least two surfaces of the micro vibrator in order to maximize the power generation amount per unit volume of the power generator. And For example, a plurality of stripe electret films are formed on both the upper surface and the lower surface of the micro vibrator, and a counter electrode group facing each of these surfaces is formed on two different substrates. The micro vibrator may be sandwiched.

The electret vibration power generator according to claim 4 is made of a silicon oxide film or a polymer electret material as the electret film. The silicon oxide film may be a silicon oxide film formed by thermally oxidizing a silicon wafer, or a silicon oxide film formed by thin film deposition. As the polymer electret material, CYTOP may be used, or parylene may be used.

The electret vibration power generator according to claim 5 is characterized in that mechanical friction and elastic energy loss are minimized by levitation of the micro vibrator by a combination of a diamagnetic material and a permanent magnet. To do. The micro vibrator may be made of a diamagnetic material and there may be a permanent magnet for floating it around. Conversely, the micro vibrator is made of a permanent magnet and there is a diamagnetic material for floating it around. Also good. As the diamagnetic material, graphite, bismuth, or a high-temperature superconductor may be used.

In the case of rectification using a diode, the rectification operation is performed on the entire current that contributes to the generation of power flowing in the diode. However, the information necessary for rectification is essentially only one bit that indicates where the current flows. Therefore, the electret vibration power generation device according to claim 6 is more efficient by separately acquiring information on the direction of movement of the micro vibrator from the mechanical-electrical energy conversion element and using it for controlling the rectifier circuit. A rectifying operation is performed.

Specifically, when the FET element is used for rectification, it is sufficient to drive the gate electrode with only a minute charge, so the control voltage signal is obtained separately from the mechanical and electrical energy conversion element to increase the efficiency of the rectification operation. it can. In this case, since the amount of charge handled is very small, it is particularly important to minimize the parasitic capacitance. For this reason, the electret vibration power generator according to claim 6 is characterized in that the length of the wiring connecting the mechanical-electrical energy conversion element and the gate electrode of the FET is 5 mm or less. As a result, the gate electrode of the FET can be controlled with a sufficient voltage amplitude by a small amount of generated charge.

Here, in particular, in a MOS (Metal Oxide Semiconductor) type FET element, a gate leakage current is small, and therefore it is essential that there is almost no power loss. In general, a small FET element has a sufficiently small gate input capacitance, In other words, it is important that only a small amount of charge is required. Therefore, the electret vibration power generator according to claim 6 is characterized in that the gate leakage current of the FET element is 10 pA or less. In order to separately generate a small amount of electric charge, it is sufficient to provide a small part of the total effective area of the electromechanical energy conversion element. Therefore, by appropriately designing the electret film and counter electrode area for generating the charge for controlling the FET, even if a large voltage is generated on the counter electrode side, the reaction to the movement of the micro vibrator does not become a problem. Elements can be designed.

The best mode for carrying out the present invention will be described below with reference to the drawings. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description is an example of the best mode of the present invention, and does not limit the scope of the claims.

FIG. 1 is a conceptual diagram of an exemplary embodiment of the present invention. A plurality of electret mechanical electrical energy converters EC1, EC2, EC3 are arranged, and the output voltage is increased by serial connection. The output rectified by the rectifier circuit elements RD1 and RD2 is accumulated in the energy storage device ES. Depending on the implementation status, information on the vibration direction is obtained separately from the mechanical system via the wiring SL, and the rectifier circuit elements RD1 and RD2 are thereby controlled.

FIG. 4 is a perspective view of a vibration power generation element constructed by a semiconductor microfabrication process according to an embodiment of the present invention. In order to minimize the parasitic capacitance generated in the vicinity of the counter electrode of the mechanical-electrical energy conversion element, the entire power generation apparatus is miniaturized. In order to minimize the parasitic capacitance, it is necessary to miniaturize the wiring including the wiring to the rectifying diode elements D3 and D4. Therefore, these rectifying elements are also integrated with the counter electrode group CE3 on the lower silicon substrate SS2. Yes. The micro vibrator OSL2 is formed on the upper silicon substrate SS1 using the MEMS technology. Here, the spring that supports the micro vibrator is, for example, a silicon beam B formed by an ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching) method or the like. An electret film ELF2 made of CYTOP is patterned on the lower side of the OSL2, and further charged by a corona discharge method or the like. The two substrates are bonded to each other while leaving a minute gap distance by a spacer PS made of, for example, polyimide formed on SS1. D3 and D4 have a minute reverse leakage current, but a certain amount of voltage is required to obtain a sufficient forward current. In order to achieve such a voltage, the electret film ELF2 and the counter electrode CE3 are pluralized and integrated in series. The energy storage device is, for example, a capacitor. However, in order to ensure a relatively large capacity, it is preferable that the energy storage device is externally attached to the silicon device via the electrodes EL1 and EL2. That is, one end of the counter electrode group CE3 which is a series electrode circuit, that is, the left end of CE3 in the case of FIG. 4, is connected to one end of an external capacitor via EL2. In addition, the conducting wire starting from the other end of the counter electrode group CE3 minimizes the stray capacitance to the input terminal of the rectifier circuit composed of D3 and D4, that is, the point where the conducting wire from one side of D3 and the other side of D4 is directly connected. In order to do so, it is connected at a distance of 5 mm or less. The output terminal of the rectifier circuit represented by the electrode EL1 is connected to the other end of the capacitor.

FIG. 5 is a configuration diagram of a system in which the rectifying FET is controlled by a separated signal line. In this system, a highly efficient rectification operation is realized by controlling the FET element using a signal representing the direction of current directly obtained separately from the mechanical / electrical energy conversion element. In this configuration, the electret film patterned on the bottom surface of the micro vibrator OSL3 is divided into three sections. The electret films ELF3 and ELF4 each form one section and are installed for the purpose of generating voltage signals for controlling the FET1 and FET2 on the counter electrode.

On the other hand, the electret films ELF5 and ELF6 are intended to generate electric power, and the only purpose of being divided is to obtain a sufficient voltage as in the previous embodiment. Signal lines SL2 and SL3 connected to the gate electrodes of the two FETs are grounded through high resistances R1 and R2. The resistance values of R1 and R2 are selected so as to discharge with a time constant much longer than the reciprocal of the natural frequency of the micro- vibrator OSL3. Therefore, SL2 and SL3 are floated from the viewpoint of power generation operation. However, at the same time, these high resistances prevent the two gate potentials from acting uncontrollably and in the long run serve to prevent the potential from deviating significantly from zero.

The two FETs are N-channel type enhancement mode MOS type devices. Therefore, the source electrode of these FETs is usually connected to a P-type silicon substrate electrode, and when the source electrode is at a higher potential than the drain electrode, a PN junction is passed from the P-type substrate to the N-type region immediately below the drain electrode. Current flows. In this circuit, this current is treated as a forward current. In the case of a diode, there is a problem that the reverse leakage current becomes large if the device is designed so that the forward current becomes large at a low voltage. However, in the case of FET, leakage current can be minimized by applying an appropriate gate voltage when biased in the reverse direction, which is exactly what the circuit of FIG. 5 has achieved. That is, for both FET1 and FET2, when the drain electrode is at a higher potential than the source electrode, it is necessary to stop the drain-source leakage current, but the signal from the counter electrode divided into three sections is Since all are synchronized, the FET voltage that needs to stop the drain-source current is configured so that the gate voltage is greatly negative and the N-type channel is closed. The power energy from the ELF5 and ELF6 thus rectified is stored in the capacitor C2.

In the above configuration, in the power generation apparatus mainly used for intermittent use as the main point of the present invention, the drain voltage between the drain and the source of the FET has a gate voltage in order to avoid energy loss when power generation is not performed. Even if it is zero, it needs to be small enough. In this case, when the forward current should flow, the gate voltage swings positively and the N-type channel is greatly opened, so that the current flowing through the channel from the source to the drain flows into the pn junction forward current from the substrate to the drain. It is suitable if it is designed to greatly improve the overall conductance. In this case as well, the basic design shown in FIG.

In the field of ID tags, the present invention may be used in an extremely large amount for power supply using sound waves or ultrasonic waves instead of power supply from an antenna in the current RFID. In that case, power can be supplied by a much smaller device of 1 cm or less as compared with the rf antenna which is currently about 10 cm. In addition, it can be widely applied to, for example, an in-vivo implant device for collecting data for medical purposes. When using an extremely small implant device without replacing the battery for a long period of time, it is necessary to supply energy from the outside. However, energy injection using sound waves or ultrasonic waves is energy transfer using RF electromagnetic waves that are currently used. Compared to the above, it is advantageous from the viewpoint of minimization of the in-vivo implant device because it does not use an electromagnetic coil that tends to take a place.

The block diagram of the Example of this invention. Half-wave rectifier circuit using diodes. The schematic diagram of an electret type mechanical electrical energy conversion element. The schematic diagram of the vibration electric power generation element comprised by the semiconductor fine processing process. The block diagram of the rectification | straightening system which controls rectification FET by the separated signal line.

EC1, EC2, EC3 electret mechanical electrical energy converter RD1, RD2 Rectifier circuit element SL1, SL2, SL3 Signal line ES Energy storage device D1, D2, D3, D4 Diode element C1, C2 Capacitor ELF1, ELF2, ELF3, ELF4, ELF5 Electret film CE1, CE2, CE3 Counter electrode OSL1, OSL2, OSL3 Micro vibrator L Load SS1, SS2 Silicon substrate B Silicon microfabricated beam PS Polymer spacer EL2, EL2 Electrodes R1, R2 High resistance FET1, FET1 N channel type Enhanced mode MOSFET device

Claims (6)

A micro vibrator having a plurality of electret film stripes of 3 mm or less in width and having a side of 1 cm or less and a resonance frequency of 100 Hz or more;
An electrode pair facing the stripe;
A series electrode circuit in which three or more electrode pairs are connected in series in order to generate a voltage of 0.2 V or more necessary for the operation of the semiconductor element of the rectifier circuit;
Connecting one end of the series electrode circuit to one end of the output capacitor;
Time stray capacitance and thus to supply current at 100Hz above 100kHz frequencies below the rectifier circuit to generate a 0.2V or more voltage required for the operation of the semiconductor device of the rectifier circuit, and high above 10 megohm Zerobaia Sui impedance In order to minimize the constant, the other end of the series electrode circuit is connected to the input terminal of the rectifier circuit at a distance of 5 mm or less,
Connecting the output terminal of the rectifier circuit to the other end of the output capacitor;
The electret vibration power generator according to claim 1, wherein a ratio of the inter-electrode gap area to the sum of the inter-electrode gap area and the electrode area is 3% or more. In the electret vibration power generator according to claim 1 ,
The electrode pair created on the silicon substrate;
The rectifier circuit includes two or four diodes of a pn junction type or a Schottky type having a small junction cross-sectional area with a small leakage current formed within an area of 5 mm square on the silicon substrate. Electret vibration power generator. 3. The electret vibration power generator according to claim 1 , wherein the electret film stripe has the micro vibrator formed on two or more surfaces of the surface of the micro vibrator. 4. An electret vibration power generator. The electret vibration power generator according to any one of claims 1 to 3 , wherein the electret film stripe is made of a silicon oxide film or a polymer electret material. The electret vibration power generator according to any one of claims 1 to 4 , wherein the micro vibrator is diamagnetically levitated by a combination of a diamagnetic material and a permanent magnet . One or more FET-controlled electret film stripes formed on the micro-vibrator according to any one of claims 1 to 5 ,
FET control electrode pairs facing each of the FET control electret film stripes, one of the FET control electrode pairs is grounded, the other is connected to the gate terminal of the FET at a distance of 5 mm or less,
As a result, the FET element functions as a rectifying element having a leakage current of 10 pA or less in the rectifying circuit.

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