JP4433476B2 - Drive element - Google Patents

Description

  The present invention relates to a driving element such as a vibrator or a piezoelectric actuator.

  The hydrogen sensor is a sensor necessary for ensuring the safety of a fuel cell system using hydrogen as a fuel source, and is expected to spread in the future. The hydrogen sensor is used to reliably detect hydrogen leakage at a low concentration stage, and to stop or warn the system. The hydrogen sensor employs various detection methods such as a catalytic combustion method, a semiconductor method, a thermoelectric method, and an optical fiber method, but all have problems in gas type selectivity, cost, and durability.

  In the contact combustion type hydrogen sensor, hydrogen is burned on a catalyst such as platinum, and the change in resistance value of the resistor due to the heat and heat is used. In this method, not only hydrogen but also hydrocarbons are combusted at the same time, so the selectivity of the gas species is poor. Moreover, it is necessary to use expensive materials such as platinum.

  In a semiconductor-type hydrogen sensor, a metal oxide semiconductor sintered body such as tin oxide is heated, and when it comes into contact with hydrogen, the electrical resistance of the semiconductor changes. The hydrogen concentration is calculated using this resistance value change. Even in this method, the same reaction occurs with hydrocarbons and carbon monoxide, so the selectivity of the gas species is poor. In addition, heating of the semiconductor with a heater is essential, and the cost is high. Furthermore, the life of the tin oxide is short because it deteriorates in the usage environment.

In addition, a hydrogen sensor using a crystal resonator with a palladium film has been proposed (Patent Document 1). Palladium has the property that hydrogen is occluded in the gaps between atoms. Since it is impossible for hydrocarbons and carbon monoxide to enter the gaps between the atoms, this hydrogen sensor has high gas species selectivity.
Japanese Patent Publication No. 3-40817

  In the hydrogen sensor described in Patent Document 1, electrodes are provided on both sides of a quartz diaphragm, and a palladium film is formed so as to cover the electrodes. And an alternating voltage is supplied to an electrode and a diaphragm is vibrated. The vibration frequency at this time is determined by the natural resonance frequency of the diaphragm and the electrode. When hydrogen is adsorbed on the palladium film, the frequency of the diaphragm changes slightly, so the hydrogen concentration is calculated from this frequency change.

  However, in the hydrogen sensor described in Patent Document 1, the frequency change based on a very small amount of attached hydrogen is very small, and it is difficult to accurately detect the hydrogen concentration from this frequency change. Since hydrogen gas explodes in the atmosphere at a concentration of about 4%, it is necessary to detect it at the lowest possible concentration, and at least 0.1 to 1%. It is desirable that detection is possible at a concentration of about 100 ppm if possible.

  In the hydrogen sensor as described in Patent Document 1, in order to improve the hydrogen detection sensitivity, it is necessary to increase the amount of hydrogen adsorption per unit area. For this purpose, it is necessary to increase the thickness of the palladium film. is there. However, when the film thickness of the palladium film is increased, the amount of expensive palladium increases and the cost increases. In addition, since the time required for hydrogen to penetrate into the membrane becomes longer, the responsiveness to changes in hydrogen concentration becomes worse.

For this reason, in the patent document 2, the present applicant separates the drive means and the detection means in the hydrogen sensor that measures the hydrogen concentration using the vibration state change caused by the hydrogen adsorption to the adsorption film on the vibrator, It has been disclosed that a fundamental vibration is excited in a sensor by a driving means, and a change in vibration state due to hydrogen adsorption is detected by a detection means. By separating the drive means and the detection means in this way, it becomes easy to accurately detect a desired vibration that reflects the vibration state, and it is possible to provide the drive means and the detection means in the most effective parts. Become. As a result, it has become possible to improve the detection sensitivity of the hydrogen concentration in the sensor.
Japanese Patent Application No. 2004-310379

  However, it has been found that new problems arise in the process of actually manufacturing such a vibrator. That is, in order to manufacture a hydrogen sensor, it is necessary to form a palladium film on the surface of the vibrator. The palladium film also functions as an electrode film. However, since palladium is expensive, it is expensive to form the entire electrode film on the vibrator with palladium, which is not practical. Accordingly, the electrode film for driving the vibrator is mainly formed of a noble metal such as gold.

  However, the vibrator is preferably made of a piezoelectric single crystal such as quartz. Here, since a piezoelectric single crystal such as quartz and a metal film such as a gold film have poor adhesion, a base film such as a chromium film is formed between the metal film and the crystal resonator. Thus, after forming the base film and the electrode film on the surface of the vibrator, an attempt was made to form a palladium film or a palladium alloy film on the electrode film. It turned out to be bad and easy to peel off. In order to prevent this, it was also considered to provide a base film such as a chromium film between the palladium film or palladium alloy film and the electrode film, but could it improve the adhesion to the electrode film such as a palladium film? It was not possible to prevent peeling.

  An object of the present invention is to prevent a peeling of a functional film from an electrode film in a driving element including a substrate that can be driven by voltage application, an electrode film, and a functional film made of palladium or a palladium alloy. It is to improve.

The drive element according to the present invention is:
A substrate that can be driven by applying voltage,
An electrode film formed on the substrate to apply a voltage to the substrate and made of gold, silver, lead or platinum ;
Functional film made of palladium or palladium alloy formed so as not to overlap the electrode film on the substrate,
An electrode film-side base film that is provided between the electrode film and the substrate and is made of a chromium film, a titanium film, or a laminated film of a chromium film and a titanium film; and
A functional film-side base film that is provided between the functional film and the substrate and includes a titanium film, a chromium film, or a laminated film of a titanium film and a chromium film is provided.

  According to the present invention, the functional film made of palladium or a palladium alloy is not formed on the electrode film, but is formed on the substrate so as not to overlap the electrode film when viewed in plan. In other words, when the electrode film and the functional film are projected onto the substrate surface, both projection images are prevented from overlapping. Alternatively, the functional film is prevented from being laminated directly on the electrode film or through another film such as a base film. As a result, peeling of the palladium film or the palladium alloy film from the electrode film can be prevented, and the yield of the drive element is improved.

  The use of the drive element of the present invention is not particularly limited. The drive element can function as a vibrator. The vibrator means that the substrate vibrates in various vibration modes. In other embodiments, the drive element functions as a piezoelectric actuator. In this case, the drive element does not need to vibrate, and includes a case of operating in one direction in response to a predetermined signal. An example of such an operation is an opening / closing operation.

The material of the substrate is not particularly limited. In one example, a piezoelectric single crystal or a piezoelectric ceramic can be used. Examples of the piezoelectric single crystal include quartz crystal, lithium niobate, lithium tantalate, lithium niobate-lithium tantalate solid solution (Li (Nb, Ta) O ₃ ) single crystal, lithium borate single crystal, and langasite single crystal. . Quartz production technology has been established, which can reduce the manufacturing cost of hydrogen sensors. Lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate solid solutions have a high Curie point and can be used up to high temperatures. Examples of piezoelectric ceramics include lead zirconate titanate, lead titanate and so-called PZT-based piezoelectric ceramics with additives added thereto, barium titanate, strontium titanate, bismuth layered compounds, and the like.

  Further, a substrate made of AlN can be used. AlN can operate up to about 1000 ° C. in a reducing atmosphere. Furthermore, a substrate formed by silicon micromachining can also be used. In addition, a substrate made of Langasite can be used. For example, the Langasite vibrator has a high melting point of 1470 ° C. and can operate up to about 1000 ° C.

The material of the electrode film is a gold film, a silver film, a lead film, or a platinum film. A base film is provided between the electrode film and the substrate. Such undercoating film, a chromium film, a titanium layer, Ru multilayer film der the chromium film and a titanium film. When operating at a high temperature, the electrode surface is preferably covered with platinum.

  Impurities may be contained in palladium constituting the functional film. Examples of the metal constituting the palladium alloy constituting the functional film include silver, platinum, gold, ruthenium, iron, molybdenum, cobalt, tin, copper, nickel, yttrium, rhodium, zinc, and vanadium.

  The thickness of the functional film is not particularly limited, but is preferably 90 angstroms or more, and more preferably 500 angstroms or more from the viewpoint of further improving the detection sensitivity of the hydrogen sensor. If the adsorption film becomes too thick, the responsiveness of the sensor to changes in hydrogen concentration tends to decrease. From the viewpoint of responsiveness, the thickness of the adsorption film is preferably 5 microns or less, preferably 1 micron or less. More preferably it is.

A base film is provided between the functional film and the substrate. Such a base film is a titanium film, a chromium film, or a laminated film of a titanium film and a chromium film.

  The formation method of the electrode film, the base film, and the functional film is not particularly limited, and a physical deposition method (PVD method) or a chemical deposition method (CVD method) is used for forming a thin film. A sputtering method can be exemplified.

  In one embodiment, the electrode film and the functional film are separated from each other in plan view. In this case, a detection electrode can be formed between the electrode film and the functional film. The functional film can also be used as an electrode. In this embodiment, since the functional film and the electrode film are separated from each other, the film formation of both is relatively easy.

  In one embodiment, the electrode film and the functional film are continuously formed in a plan view. In this case, the functional film also functions as an electrode together with the electrode film. In this embodiment, the area of the functional film can be reduced, thereby reducing the amount of expensive palladium or palladium alloy constituting the functional film.

  The function of the functional film is not particularly limited. In a preferred embodiment, the functional film functions as a hydrogen adsorption film. The functional membrane can also function as a catalyst.

  In the embodiment of the hydrogen sensor, a detection means for detecting a physical quantity accompanying the vibration of the substrate is provided, and the hydrogen concentration is measured based on the detected value of the physical quantity. The type of physical quantity obtained from the detection means is not limited, but vibration displacement is particularly preferable from the viewpoint of sensitivity. Examples of other physical quantities include electrical resistance, stress, and acceleration.

  The vibration displacement detection means is preferably the detection electrode as described above, but is not limited thereto. For example, the displacement of the central axis of the vibrator and the vicinity thereof can be measured with a laser displacement meter.

  The hydrogen absorption amount of palladium varies depending on the environmental temperature. Such a change in the amount of absorbed hydrogen has little effect in applications for detecting leakage of hydrogen gas, but it is necessary to control the application where it is necessary to know the absolute value of the hydrogen concentration or the measured value of the hydrogen concentration. In use, it causes measurement errors. Therefore, in such a case, the change in the adsorption amount due to the temperature change can be corrected by the following method.

(1) The temperature of the vibrator is kept constant by adding a heater to the hydrogen sensor.
(2) Install a temperature sensor on or around the vibrator and measure the temperature. A calibration curve indicating the relationship between the temperature and the detected value of the physical quantity is stored in the control device. The physical value detected value is corrected by collating the measured value from the temperature sensor with the calibration curve.
(3) The resonance frequency of the vibrator changes according to the temperature change. Therefore, a calibration curve indicating the relationship between the resonance frequency of the vibrator and the detected value of the physical quantity is stored in the control device. The detected value of the physical quantity is corrected by collating the measured value of the resonance frequency obtained from the vibrator with the calibration curve. According to this method, since the temperature of the vibrator can be directly known, an error between the temperature of the temperature sensor and the temperature of the vibrator hardly occurs, and more accurate temperature correction is possible.

  Preferably, the hydrogen concentration is measured based on the difference between the detected value of the physical quantity from the detecting means at the time of non-measurement and the detected value of the physical quantity from the detecting means at the time of measurement. In particular, by measuring vibration displacement, the measurement system of hydrogen can be remarkably improved.

  In a preferred embodiment, in the fundamental vibration, the vibration displacement is substantially symmetric with respect to the central axis of the vibrator. In a preferred embodiment, the detection value from the detection means is set to approximately 0 when not measuring. In this case, since the displacement from about 0 is detected, the measurement sensitivity is further improved and the influence of the environmental change can be reduced.

  In a preferred embodiment, the fundamental vibration is a torsional vibration mode in the thickness direction of the vibrator. By using a thickness torsional vibrator, hydrogen detection can be realized in principle at a hydrogen concentration equivalent to 1 ppm.

  1 to 6 relate to a hydrogen sensor using a thickness torsional vibration mode. 1 is a plan view schematically showing the sensor 1, FIG. 2 is a perspective view schematically showing the sensor 1, and FIGS. 3A and 4 are front sectional views of the sensor 1. It is. FIG. 3B is a cross-sectional view of a sensor outside the present invention. FIG. 5A is a plan view for explaining the thickness torsional vibration mode, FIG. 5B is a perspective view for explaining the thickness torsional vibration mode, and FIG. 6 shows a circuit example.

  As shown in FIGS. 1 and 2, the vibrator 2 of the sensor 1 has, for example, a square plate shape. Drive electrodes 3A, 3B and detection electrodes 4 are formed on the surface 2a of the vibrator 2, and drive electrodes 3A, 3B and detection electrodes 4 are formed on the surface 2b. Further, a hydrogen adsorption film 5 is provided on the vibrator 2. The film 5 is surrounded by the drive electrode film 3A. 15 is a lead, and 16 is a lead terminal.

  More specifically, as shown in FIGS. 3A and 4, the drive electrodes 3 </ b> A, 3 </ b> B, and 4 are formed on the base film 13. The hydrogen adsorption film 5 is also formed on the base film 33. The base films 13 and 33 are continuously formed in a plan view. The hydrogen adsorption film 5 is surrounded by the drive electrode film 3A, and the adsorption film 5 and the electrode film 3A are continuously formed.

  By using the drive power supply 8 of the drive circuit unit 14 (see FIG. 6) and applying AC voltages of opposite phases between the drive electrodes 3A and between the drive electrodes 3B, respectively, FIG. As shown by arrows A and B shown in b), thickness shear vibration is generated. D1 and D2 are AC voltage application terminals, and D1G and D2G are ground terminals. The drive vibrations A and B are substantially line symmetric with respect to the central axis D of the vibrator.

  When the vibrator 2 is displaced between the pair of detection electrodes 4, a voltage is generated between the terminal P and the ground terminal PG. This voltage difference is detected by the detection amplifier 9 of the signal processing unit 6, and phase detection is performed by the phase detection circuit 10 by driving vibration. Then, the vibration having the same phase as the drive vibration is passed through the low-pass filter 11 and output.

  Here, the detection signal at the center detection electrode 4 is made to be substantially zero at the time of non-measurement. This is because the displacements A and B of the drive vibration are substantially line symmetric with respect to the central axis D of the vibrator 2, so that the vibration displacement of the vibrator in the region between the detection electrodes 4 is almost zero. Because.

  If hydrogen adheres to the adsorption film 5 at the time of measurement, the mass of the adsorption film 5 increases and the balance of the masses on the left and right of the central axis D of the vibrator 2 is lost. As a result, the line symmetry of the drive vibrations A and B with respect to the central axis D is lost, and a signal voltage in phase with the drive vibration is generated between the pair of detection electrodes 4. The mass is calculated based on this signal voltage.

  Here, in the sensor described in Patent Document 2, for example, a drive electrode film 3A is formed on the base film 13 as in the sensor 16 shown in FIG. 5 was formed. However, in this method, it has been found that the adhesion between the hydrogen adsorption film 5 and the drive electrode film 3A is low, the adsorption film 5 is easy to peel off, and the yield is low. According to the present invention, for example, as shown in FIG. 3A, the drive electrode film 3A and the adsorption film 5 are provided directly on the substrate 2 or via the base films 13 and 33. At this time, the adsorption film 5 does not overlap with the electrode film 3A in plan view. In this example, the electrode film 3A and the adsorption film 5 are particularly formed substantially continuously. In such a drive element, the adhesion of the adsorption film 5 to the substrate 2 is high, and peeling does not easily occur. Therefore, the yield of the drive element can be improved.

  7 and 8, the drive electrode film and the adsorption film are separated from each other, and the detection electrode film is provided between them. That is, the vibrator 2 of the sensor 21 has a square plate shape, for example. A drive electrode 17, a detection electrode 4, and an adsorption film 18 are formed on the surface 2a of the vibrator 2, and a drive electrode 17 and a detection electrode 4 are formed on the surface 2b. 15 is a lead, and 16 is a lead terminal. More specifically, as shown in FIG. 8, each drive electrode 17, detection electrode 4, and adsorption film 18 are formed on the base films 13 and 33, respectively.

  Driving vibration is excited in the vibrator 2 in the same manner as the sensor of FIGS. At this time, by applying an alternating voltage of opposite phase to the adsorption film 18 made of palladium or palladium alloy and the drive electrode film 17 made of other metal, respectively, FIG. 5A and FIG. As shown by arrows A and B shown in FIG.

  When the vibrator 2 is displaced between the pair of detection electrodes 4, a voltage is generated between the terminal P and the ground terminal PG. This voltage difference is detected by the detection amplifier 9 of the signal processing unit 6, and phase detection is performed by the phase detection circuit 10 by driving vibration. Then, the vibration having the same phase as the drive vibration is passed through the low-pass filter 11 and output.

Example 1
(Adsorption film deposition process)
A hydrogen sensor 1 shown in FIGS. 1 to 6 was manufactured. Specifically, the process shown in FIGS. 9 to 11 was followed. That is, as shown in FIG. 9A, the quartz substrate 2 having a length of 2 inches, a width of 2 inches, and a thickness of 0.16 mm is etched and washed with dilute hydrofluoric acid, and then dried by a spin dryer, and the quartz substrate 2 Both surfaces 2a, 2b were cleaned. Using a sputtering apparatus, a 0.02 μm thick Cr film was formed on the quartz substrate 2 as an underlayer 33, and a 0.1 μm thick Pd film 5 was formed thereon.

  Photoresist was applied on the surface of the Pd film 5 to form a resist film 24 having a thickness of 1 μm as shown in FIG. 9B. In order to remove the solvent component of the resist, pre-baking was performed using an oven.

  Next, using a photomask on which a Pd film formation pattern is drawn, the photomask pattern is transferred onto the resist film surface by an exposure apparatus by photolithography to form a resist pattern 24A as shown in FIG. The exposed portion 24 was removed (FIG. 9 (d)). Next, using this resist pattern 24A as a mask, the Pd film 5 formed on the quartz substrate 2 is etched away with a mixed solution of nitric acid and hydrochloric acid, and the Cr film 33 is etched away with ceric ammonium nitrate. As shown in FIG. 10A, an etching mask pattern composed of the Pd film 5 and the Cr film 33 was formed. The remaining photoresist was dissolved and removed with acetone to form a Pd film 5 (FIG. 10B).

(Gold film deposition process)
Photoresist was again applied to each surface 2a, 2b of the quartz substrate 2 by a spin coater to form resist films 25, 25B having a thickness of 1 μm (FIG. 10C). In order to remove the solvent component of the resist, pre-baking was performed using an oven.

  Using an exposure apparatus capable of high-precision alignment of each surface and a photomask on which only an electrode etching pattern was drawn, the photomask pattern was transferred onto both the front and back surfaces of the quartz substrate 2 by photolithography to form a resist pattern 25A. (FIG. 10 (d)). Reference numeral 25 denotes an unexposed portion. This unexposed portion 25 was removed (FIG. 11A).

  Using a vapor deposition device on each surface, a Cr film having a thickness of 0.02 μm was formed as a base layer 13 on the quartz substrate 2, and an Au film 3 for electrode having a thickness of 0.1 μm was formed thereon (FIG. 11 (b)). The remaining photoresists 25A and 25B were dissolved and removed with acetone to form Au electrode films 3A, 3B, and 4 (FIG. 11C).

  A sensor shown in FIGS. 1 to 6 was constructed using the substrate thus obtained, and a hydrogen adsorption experiment was performed. As a result, hydrogen having a concentration of 0.01% could be detected.

(Example 2)
Sensors as shown in FIGS. 1 to 6 were produced. However, the process up to the formation of the adsorption film 5 is the same as that of the first embodiment (steps from FIG. 9A to FIG. 10B). However, the next drive and detection electrode film forming steps are different from those in the first embodiment.

  That is, using a sputtering apparatus on each surface 2a, 2b of the quartz substrate 2, a 0.02 μm thick Cr film is formed as an underlayer 13, and an 0.1 μm thick Au film 3 for an electrode is formed thereon. A film was formed (FIG. 12A). Photoresist was applied on the film-forming surface of the Au film 3 to form resist films 24 and 24B having a thickness of 1 μm (FIG. 12B). In order to remove the solvent component of the resist, pre-baking was performed using an oven.

  Using an exposure apparatus capable of high-precision alignment of each surface and a photomask on which only an electrode etching pattern was drawn, the photomask pattern was transferred onto both the front and back surfaces of the quartz substrate 2 by photolithography to form a resist pattern 24A. (FIG. 12 (c)). Reference numeral 24 denotes an unexposed portion. This unexposed portion was removed (FIG. 13 (a)).

  Using this resist pattern 24A as a mask, the Au and Cr multilayer films 13 and 3 formed on the quartz substrate were etched away with a mixed solution of iodine and potassium iodide to form an etching mask pattern (FIG. 13). (B)). The remaining photoresist 24A was dissolved and removed with acetone to form Au electrode films 3A, 3B, and 4 (FIG. 13C).

  A sensor shown in FIGS. 1 to 6 was constructed using the substrate thus obtained, and a hydrogen adsorption experiment was performed. As a result, hydrogen having a concentration of 0.01% could be detected.

It is a top view showing typically sensor 1 of an embodiment of the present invention. 1 is a perspective view schematically showing a sensor 1. FIG. (A) is a cross-sectional view schematically showing the sensor 1, and (b) is a cross-sectional view showing a sensor 16 outside the present invention. It is a cross-sectional view which shows the wiring of a sensor typically. (A) is a top view for demonstrating the thickness shear vibration mode of the vibrator | oscillator 2, (b) is a schematic diagram which shows thickness shear vibration mode. It is a figure which shows the example of a circuit. It is a front view which shows typically the sensor 21 which concerns on other embodiment of this invention. It is a cross-sectional view schematically showing the sensor 21 of FIG. (A), (b), (c), (d) is a cross-sectional view which shows typically the manufacturing process of the sensor in Example 1 of this invention. (A), (b), (c), (d) is a cross-sectional view which shows typically the manufacturing process of the sensor in Example 1 of this invention. (A), (b), (c) is a cross-sectional view which shows typically the manufacturing process of the sensor in Example 1 of this invention. (A), (b), (c) is a cross-sectional view which shows typically the manufacturing process of the sensor in Example 2 of this invention. (A), (b), (c) is a cross-sectional view which shows typically the manufacturing process of the sensor in Example 2 of this invention.

Explanation of symbols

  1, 21 Sensor 2 Substrate 3A, 3B, 17 Drive electrode made of metal other than palladium and palladium alloy 4 Detection electrode 5, 18 Adsorption film made of palladium and palladium alloy 13, 33 Undercoat film A, B Thickness sliding vibration D Central axis E Basic vibration

Claims (11)

A substrate that can be driven by applying voltage,
An electrode film formed on the substrate for applying a voltage to the substrate, and made of gold, silver, lead or platinum ,
A functional film made of palladium or a palladium alloy formed so as not to overlap the electrode film on the substrate;
An electrode film-side base film that is provided between the electrode film and the substrate and includes a chromium film, a titanium film, or a laminated film of a chromium film and a titanium film; and
A functional film-side base film which is provided between the functional film and the substrate and is formed of a titanium film, a chromium film, or a laminated film of a titanium film and a chromium film. Driving element.   The drive element according to claim 1, wherein the substrate functions as a vibrator.   The drive element according to claim 1, wherein the drive element functions as a piezoelectric actuator. Wherein characterized in that the electrode film and the functional film is separated, the driving device according to any one of claims 1-3. The drive element according to any one of claims 1 to 3 , wherein the electrode film and the functional film are formed continuously. Characterized in that the functional film functions as a hydrogen adsorption layer, the driving device according to any one of claims 1-5. The driving element according to claim 6 , further comprising a detecting unit that detects a physical quantity associated with vibration of the substrate, and measuring a hydrogen concentration based on a detected value of the physical quantity. The hydrogen concentration is measured based on a difference between a detection value of the physical quantity from the detection means at the time of non-measurement and a detection value of the physical quantity from the detection means at the time of measurement. 8. The driving element according to 7 . Characterized in that said physical quantity is vibration displacement, claim 7 or 8, wherein the drive element. Wherein said detecting means is sandwiched between the electrode layer and the functional film when the surface of the substrate is viewed in a plan view, any one of claims 7-9 The drive element as described in. It said vibration, characterized in that it is a thickness torsional vibration mode of the vibrator, the driving device according to any one of claims 7-10.

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