JP2021162486A - Sensor element and gas sensor - Google Patents

Abstract

To provide a sensor element which offers stable oscillation characteristics and improved sensitivity, and a gas sensor.SOLUTION: A sensor element is provided, comprising a quartz oscillator, a first electrode, a second electrode, and a sensitive film. The quartz oscillator has a first surface with a surface roughness of 0.19-0.36 μm, inclusive, and a second surface. The first electrode is provided on the first surface. The second electrode is provided on the second surface. The sensitive film is provided on the first electrode.SELECTED DRAWING: Figure 1

Description

The present invention relates to a sensor element using a crystal oscillator and a gas sensor.

A crystal oscillator is an element that oscillates at a constant frequency by utilizing the piezoelectric effect of quartz, and is used in various electronic devices such as automobiles, smartphones, and personal computers. A crystal oscillator is configured by processing a crystal wafer into a crystal element piece having a predetermined shape and dimensions, and then forming an electrode film on the surface thereof. The frequency and stability of the crystal unit are largely determined by the thickness and surface condition of the crystal element piece. In recent years, with the demand for higher frequency and higher stability of the crystal unit, it has become necessary to process the crystal element piece thinner and smoother.

Patent Document 1 describes a QCM sensor that detects chemical contaminants. A crystal oscillator is used for the QCM sensor, and a silicon layer having irregularities is formed on an electrode provided on the surface of the crystal oscillator. The sensor can detect chemical pollutants by utilizing the fact that the frequency change corresponding to the weight change due to the adsorption of the substance on the silicon layer is output.
Patent Document 2 discloses a QCM device in which an organic polymer film is formed as an odor adsorption film. When irregularities are formed on an adsorption film made of an organic polymer film such as the QCM device of Patent Document 2 by using, for example, a laser, the adsorption film is broken, which causes a problem in adsorption characteristics.

Japanese Unexamined Patent Publication No. 2012-20395 Japanese Unexamined Patent Publication No. 2010-07716

A sensor element using a crystal oscillator is required to have stable oscillation characteristics and improve detection sensitivity.
In view of the above circumstances, an object of the present invention is to provide a sensor element and a gas sensor having stable oscillation characteristics and improved sensitivity.

In order to achieve the above object, the sensor element according to one embodiment of the present invention includes a crystal oscillator, a first electrode, a second electrode, and a sensitive film.
The crystal unit has a first surface having a surface roughness of 0.19 μm or more and 0.36 μm or less, and a second surface.
The first electrode is provided on the first surface.
The second electrode is provided on the second surface.
The sensitive film is provided on the first electrode.

According to such a configuration of the present invention, it is possible to obtain a sensor element capable of improving sensitivity to components such as odor and gas while maintaining stable oscillation characteristics.

The gas sensor according to one embodiment of the present invention includes a sensor element, an oscillation circuit, and a detection circuit.
The sensor element includes a crystal oscillator having a first surface having a surface roughness of 0.19 μm or more and 0.36 μm or less, a second surface, and a first electrode provided on the first surface. A second electrode provided on the second surface and a sensitive film provided on the first electrode are provided.
The oscillation circuit vibrates the sensor element.
The detection circuit detects a change in the resonance frequency of the sensor element.

As described above, according to the present invention, it is possible to improve the detection sensitivity for components such as odor and gas while having stable oscillation characteristics.

It is a front view of the sensor element which concerns on embodiment of this invention. It is a schematic cross-sectional view cut along the line II-II of FIG. It is a schematic diagram of the gas sensor provided with the said sensor element. It is a figure which shows the relationship between the surface roughness and the crystal impedance in a crystal oscillator. It is a figure explaining the difference of the gas detection function of the sensor element which the surface roughness is different from each other.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Sensor element configuration]
FIG. 1 is a front view showing the sensor element 1. FIG. 2 is a cross-sectional view taken along the line II-II of FIG.
As shown in FIGS. 1 and 2, the sensor element 1 includes a crystal oscillator 13, a first excitation electrode 11A (hereinafter referred to as a first electrode 11A), and a second excitation electrode 11B (hereinafter, a second). The electrode 11B), the sensitive film 12, the lead lands 16A and 16B, the leads 14A and 14B, the pin terminals 19A and 19B, and the holder 18.
The sensor element 1 specifies the type and amount of odor and gas components. In the sensor element 1, when a detection object such as gas is adsorbed on the sensitive film 12 in a state of being vibrated at a predetermined frequency, the resonance frequency of the crystal oscillator 13 of the sensor element 1 changes. From this change in resonance frequency, the type and amount of introduced gas can be specified.
The sensor element 1 has a width (W) axis direction, a length (L) axis direction, and a thickness (t) axis direction orthogonal to each other, and the t-axis direction coincides with the thickness direction of the crystal oscillator 13.

As the crystal oscillator 13, a crystal oscillator having a fundamental frequency of 32 MHz at 25 ° C. is used from the viewpoint of ease of manufacture and good sensitivity. The fundamental frequency of the crystal oscillator 13 is not limited to this. In general, a crystal unit having a relatively low frequency as a fundamental frequency is easy to manufacture, but its sensitivity is lowered. A crystal unit having a relatively high frequency as a fundamental frequency is difficult to manufacture, but its sensitivity is improved.

This embodiment will be described with reference to FIG. The crystal oscillator 13 has a dimension of 1.32 mm in the W-axis direction, a dimension of 2.00 mm in the L-axis direction, a dimension of 0.0506 mm in the t-axis direction, and has a plate shape having a rectangular WL plane shape. The dimensions are not limited to this.

As the crystal unit 13 which is a piezoelectric diaphragm, an AT-cut type crystal unit having relatively excellent temperature characteristics is typically used. The AT-cut type crystal oscillator is perpendicular to the Y-axis when viewed from the positive direction of the X-axis with respect to the X-axis (electrical axis), Y-axis (mechanical axis), and Z-axis defined by the right crystal of JIS standard JIS C6704. The Y-plate crystal is cut out by rotating it counterclockwise at an angle θx = 35 ° 15'± 1 °. A pair of first electrodes 11A and second electrodes 11B having different polarities are arranged to face each other on a plane perpendicular to the new axis after rotation of the crystal oscillator 13 (corresponding to the xy plane). The rotation direction of the AT-cut crystal oscillator is the direction in the right crystal, and in the case of the left crystal, the angle θx = 35 ° clockwise of the Y-plate crystal that is perpendicular to the Y-axis when viewed from the plus direction of the X-axis. It is cut out by rotating 15'± 1 °.
Further, when the frequency temperature characteristic changes by applying the sensitive film 12 on the crystal oscillator 13, the angle to be cut out may be set to θx = 35 ° 15'± 2 ° in order to adjust the frequency temperature characteristic.
Further, as the vibration form of the crystal oscillator, a tuning fork type, a bending mode type, a twisting mode type, a length longitudinal mode type, a thickness sliding mode type and the like can be used.

A crystal piece is cut from a crystal to obtain a crystal oscillator 13, but the demand for the cutting angle is extremely strict, and it is necessary to set the initial position accurately.
A so-called r-plane (01-11) plane is naturally formed around the Z-axis of the crystal. The r-plane is a counterclockwise Y-plate crystal that is perpendicular to the Y-axis when viewed from the positive direction of the X-axis with respect to the X-axis (electrical axis), Y-axis (mechanical axis), and Z-axis defined by the right crystal of JIS C6704. It is a surface rotated by an angle of 38 ° 13'around. The rotation of the r-plane is the rotation direction of the right crystal, and in the case of the left crystal, the Y-plate crystal perpendicular to the Y-axis when viewed from the plus direction of the X-axis is rotated clockwise at an angle of 38 ° 13'. It becomes a face.
The difference between the r-plane and the typical angle of 35 ° 15'cut out by the AT-cut crystal oscillator is 2 ° 58', which is a value close to parallel to the angle at which the AT-cut crystal is cut out. Further, when the r-plane is irradiated with X-rays, the X-rays diffracted on this plane have a strong peak intensity and a narrow half-value width. Therefore, the angle of the r-plane can be accurately known by X-ray diffraction, and the initial position can be accurately determined in seconds with respect to the r-plane using an X-ray angle measuring device using X-ray diffraction.
For the film formation of the first electrode 11A and the second electrode 11B, for example, a physical vapor deposition method (sputtering or the like) is used.
An electrode is formed by covering the crystal oscillator 13 with a mask having holes in the electrode shape and forming a film. Alternatively, the electrode film may be formed on the entire surface and then processed into a predetermined shape by using, for example, a photographic technique to form an electrode.
Metals such as Au and Ag are used as the material of the electrode. When Au is used, since the adhesion between Au and the crystal is weak, the Au film can be formed after forming the Cr film on the crystal oscillator for the purpose of improving the adhesion strength.

The crystal oscillator 13 has a first surface 13A and a second surface 13B that face each other. The first surface 13A is roughened to a surface roughness Ra of 0.19 μm or more and 0.36 μm or less. In the present embodiment, an example of roughening the first surface 13A, which is the surface on which the sensitive film 12 is formed, is given, but in addition to this, the second surface 13B is also roughened. You can do it. The roughening process will be described later.

The size of the crystal oscillator is 0.6 mm to 2.4 mm in the W-axis direction, 0.6 mm to 2.4 mm in the L-axis direction, and 0.015 mm to the dimension in the t-axis direction, that is, the thickness direction. A range of 0.067 mm is good. In this case, the surface roughness Ra of the first surface 13A of the crystal oscillator 13 is preferably 0.36 μm or less from the viewpoint of oscillation stability.
If the surface roughness Ra of the crystal oscillator 13 is larger than 0.36 μm, the thickness of the crystal oscillator 13 is unevenly distributed in the plane, which affects the vibration mode at the time of resonance of the crystal oscillator 13. Therefore, the resonance frequency at the time of resonance does not become a constant value. Therefore, the oscillation becomes unstable and it becomes difficult to control the resonance characteristics. On the other hand, in the present embodiment, destabilization can be suppressed by setting the surface roughness Ra of the crystal oscillator 13 to 0.36 μm or less.
The stability of the crystal oscillator can be determined from the oscillation margin. The oscillation margin represents the margin from the oscillating state to the oscillation stop, and can be calculated by the following formula using the negative resistance and the equivalent series resistance standard value of the crystal oscillator. can. In the formula, | −R | represents a negative resistance, and R1sp represents a transmission series resistance standard value of a crystal unit.
Oscillation margin [times] = | -R | / R1sp
The negative resistance value is measured by adding a pure resistance in series with the crystal oscillator and checking how long it continues to oscillate with an oscilloscope or the like. The negative resistance value is the value obtained by adding the effective resistance value of the crystal unit used for the measurement to the value of the pure resistance just before the oscillation stops completely.
The sensor element 1 of this embodiment is preferably used for a gas sensor. In the gas sensor, the oscillation of the crystal oscillator may become unstable depending on the environment in which it is used, so it is preferable to set it so that the oscillation margin is large. This makes it possible to obtain a sensor element having stable sensitivity characteristics under a wide range of environmental conditions. Here, when the oscillation margin is 7 or more (the crystal impedance value (hereinafter referred to as CI value) is 100Ω or less), it is judged to be stable, and when it is less than 7 (CI value is larger than 100Ω), it is slightly. Judged as unstable or unstable.

The surface roughness Ra of the crystal oscillator 13 is 0.19 μm or more, more preferably 0.26 μm or more from the viewpoint of improving the sensitivity. The first electrode 11A and the sensitive film 12 are formed on the crystal oscillator 13 in this order, and the first electrode 11A and the sensitive film 12 are formed along the unevenness of the roughened first surface 13A. A film is formed. As a result, unevenness can be easily formed on the surface of the sensitive film 12. By providing the surface of the sensitive film 12 with irregularities, the surface area of the sensitive film can be increased, and the detection sensitivity to odors and gas components can be improved.

In this way, by roughening the first surface 13A of the crystal oscillator 13 so that the surface roughness Ra is 0.19 μm or more and 0.36 μm or less, stable oscillation characteristics are obtained and sensitivity is easily achieved. The surface of the film 12 can be roughened to obtain the sensor element 1 capable of improving the sensitivity to odors and gas components.

A first electrode 11A and a second electrode 11B formed by patterning a metal thin film into a predetermined shape are formed on each of the first surface 13A and the second surface 13B. In the present embodiment, a laminated film of Cr and Au is used as the electrode material, but the present invention is not limited to this. The first electrode 11A and the second electrode 11B can be formed into a film by using sputtering or the like.

The first electrode 11A and the second electrode 11B have a rectangular shape, for example, the dimension in the W-axis direction is 0.8 mm, the dimension in the L-axis direction is 1.0 mm, and the dimension in the t-axis direction, that is, the film thickness. Is 0.1 μm. If the film thickness is around 0.1 μm, for example, 0.1 μm ± 0.02 μm, the surface roughness Ra value is almost unchanged. The shapes of the first electrode 11A and the second electrode 11B may be circular or rhombic, and are not limited to a rectangle.
The first electrode 11A is formed along the unevenness of the first surface 13A of the crystal unit 13 that has been roughened, and the surface of the first electrode 11A has the unevenness. The surface roughness of the first electrode 11A and the surface roughness of the first surface of the crystal unit are substantially the same. The surface roughness Ra of the first electrode 11A is 0.19 μm or more and 0.36 μm or less.

The sensitive film 12 is formed on the first electrode 11A and adsorbs a specific gas. The surface of the sensitive film 12 has irregularities because the surface of the sensitive film 12 is formed on the first electrode 11A having irregularities. The surface roughness of the sensitive film 12 and the surface roughness of the first surface 13A of the crystal oscillator 13 are substantially the same.
The sensitive film 12 can be formed by using various materials depending on the type of gas to be detected.
For example, cellulose acetylbutyrate can be used as a sensitive film that mainly selectively adsorbs toluene or 2,3-dimethylpentane. Specifically, an acetone solution in which cellulose acetylbutyrate is dissolved in acetone is prepared, and this solution is applied on the first electrode 11A to a predetermined thickness, here, 0.5 μm, by spray coating. The solvent is volatilized in the drying furnace to form the sensitive film 12. The film forming method is not limited to spray coating, and may be spin coating, vapor deposition, or the like. The surface roughness Ra of the sensitive film 12 is 0.19 μm or more and 0.36 μm or less.

The lead land 16A is integrally formed with the first electrode 11A, and the lead land 16B is integrally formed with the second electrode 11B.
The leads 14A and 14B are made of a metal spring material and are arranged parallel to each other.
The lead 14A is configured such that one end is electrically connected to the first electrode 11A via the lead land 16A and the other end is connected to the pin terminal 19A. The lead 14B is configured such that one end is electrically connected to the second electrode 11B via the lead land 16B and the other end is connected to the pin terminal 19B.

The holder 18 is made of an insulating member and has a through hole through which the pin terminals 19A and 19B penetrate. By holding the crystal oscillator 13 so that the pin terminals 19A and 19B penetrate through the through hole of the holder 18, the crystal oscillator 13 is oscillatedly supported by the holder 18.

The pin terminals 19A and 19B of the sensor element 1 are connected to the oscillation circuit 4, and a driving voltage is applied to the gas sensor element 1. When a driving voltage is applied to the gas sensor element 1, the crystal oscillator 13 vibrates at a unique frequency (32 MHz).
Then, the mass of the sensitive film 12 changes due to the adsorption of the gas, and the resonance frequency of the crystal oscillator 13 decreases according to the amount of the adsorption.

[Gas sensor configuration]
As shown in FIG. 3, the gas sensor 2 has a gas sensor unit 3 and a controller 10. The controller 10 is composed of a computer having a CPU (Central Processing Unit), a memory, and the like, and has an oscillation circuit 4 and a detection circuit 5.
The gas sensor unit 3 includes a chamber 31 and a sensor element 1 which is a gas sensor element housed in the chamber 31. The chamber 31 houses the sensor element 1. The gas to be detected can be introduced into the chamber 31. In the present embodiment, an example including one sensor element has been given, but a plurality of sensor elements having the same or different types of sensitive films may be provided.
The oscillation circuit 4 vibrates the crystal oscillator 13 of the sensor element 1 at a predetermined frequency (resonance frequency: 32 MHz).
The detection circuit 5 detects the resonance frequency of the sensor element or its change.

When a detection object such as gas is adsorbed on the sensitive film 12 in a state where the sensor element 1 is vibrated at the predetermined frequency by the oscillation circuit 4, the resonance frequency of the crystal oscillator 13 of the sensor element 1 changes. The change in the resonance frequency is detected by the detection circuit 5. The type and amount of gas introduced can be specified from the type of sensitive film used and the change in the detected resonance frequency.

[Roughening treatment of crystal unit]
As described above, the first surface 13A of the crystal unit has been roughened. A known method can be used as the roughening treatment method, but a roughening treatment by wrapping is typically used. Lapping polishing is free abrasive grain processing in which fine abrasive grains are poured on the bottom plate for processing.

In the present embodiment, in the roughening treatment of the crystal oscillator, the surface roughness Ra of the first surface 13A is set to 0 by using a wrapping machine using free abrasive grains, for example, according to the following procedure. The surface roughening treatment can be performed so that the range is 19 μm or more and 0.36 μm or less.
That is, the lapping process is performed in the order of rough polishing, intermediate polishing, and finish polishing.
In rough polishing, the crystal unit is roughly polished to a thickness slightly lower than the desired fundamental frequency.
In the intermediate polishing, specifically, for example, wrapping processing is performed using abrasive grains having a grain size (count) of # 2000 (Japanese Industrial Standards (JIS) R6001-2: 2017) or more and # 4000 or less. I do. The film-forming surface is preferably close to a mirror surface from the viewpoint of the electrical characteristics of the crystal unit. Polishing for mirror finishing may be performed after intermediate polishing.
In finish polishing, in order to roughen the surface, at least one surface is wrapped with coarse-grained abrasive grains. Specifically, for example, the wrapping process is performed using abrasive grains having a particle size of # 800 or more and # 1500 or less.
If necessary, chemical etching may be used in combination. Typically, the quartz plate can be processed by wet etching with at least hydrofluoric acid or ammonium fluoride. Since the chemical etching does not apply processing stress, it is possible to remove the processing strain after the lapping process and the polishing process, and it is possible to prevent the secondary cracking. Further, chemical etching may be performed to compensate for the lack of accuracy of the lapping process and the polishing process, or to prepare a thin crystal plate.

[Measurement method of surface roughness Ra]
A laser microscope VK-9700 manufactured by KEYENCE Co., Ltd. was used for measuring the surface roughness Ra. The measurement was performed using a 150x (WDO2) lens in the range of 70 μm × 94 μm at the center of the gold electrode.
The surface roughness Ra of the crystal oscillator 13 in the sensor element 1 is measured by measuring the surface roughness Ra of the exposed first electrode 11A after dissolving the sensitive film 12 with a solvent, for example, acetone in the present embodiment. Can be done. Alternatively, when the second surface 13B side is roughened, the surface roughness Ra of the crystal oscillator 13 is measured by measuring the surface roughness Ra of the second electrode 11B on which the sensitive film is not formed. You may. That is, as described above, the surface roughness Ra of the crystal oscillator 13 and the surface roughness Ra of the first electrode 11A and the second electrode 11B formed on the crystal oscillator 13 are substantially the same. Therefore, the surface roughness Ra of the first electrode 11A or the second electrode 11B can be measured, and the measurement result can be used as the surface roughness Ra of the crystal oscillator 13.

[Example]
Eight sensor elements equipped with crystal units having different surface roughness Ra were prepared as samples, and the stability and sensitivity were judged and evaluated. The results are shown in Table 1.

The sensor elements of sample numbers 1 to 8 have the same configuration except that the surface roughness of the first surface of the crystal oscillator is different. With reference to FIG. 1, the dimension of the crystal oscillator 13 in the W-axis direction was 1.32 mm, the dimension in the L-axis direction was 2.00 mm, and the thickness (dimension in the t-axis direction) was 0.0506 mm. The dimensions of the first electrode 11A and the second electrode 11B in the W-axis direction were 0.8 mm, the dimensions in the L-axis direction were 1.0 mm, and the thickness (dimensions in the t-axis direction) was 0.105 μm. The electrode is a laminated film having a Cr film of 5 nm and an Au film of 100 nm. The dimension of the sensitive film 12 in the W-axis direction is 0.8 mm, the dimension in the L-axis direction is 1.0 mm, the thickness (dimension in the t-axis direction) is 0.5 μm, and cellulose acetylbutyrate is used as the material of the sensitive film 12. board. In a plan view, the first electrode 11A and the second electrode 11B and the sensitive film 12 overlap each other. In addition, in FIGS. 1 and 2, the sensitive film 12 is located in the first electrode 11A and the second electrode 11B in a plan view, and the sensitive film 12 is formed from the first electrode 11A and the second electrode 11B. Also illustrates the form of a plane shape with a small area.

The crystal oscillators of the sensor elements of sample numbers 1 to 8 have different surface roughness Ras by changing the counts of the abrasive grains used in the lapping process. The sensor elements of sample numbers 2 to 5 correspond to the sensor elements according to the present embodiment, and the sensor elements of sample numbers 1 and 6 to 8 correspond to comparative examples.

Each item shown in Table 1 will be described.
The surface roughness Ra is the surface roughness Ra on the surface of the first electrode measured in a state where the first electrode is formed on the crystal unit.
CI indicates the crystal impedance of the crystal unit. FIG. 4 shows the crystal impedance of the crystal transducers of the sensor elements of sample numbers 1 to 8. In FIG. 4, the numbers on the horizontal axis indicate the surface roughness Ra [μm], and the vertical axis indicates the crystal impedance (CI [Ω]).
The oscillation margin is obtained using the above equation.
As described above, the stability is determined to be stable when the oscillation margin is 7 or more, and is indicated by ◯ in Table 1. When the oscillation margin is 5 or more and less than 7, it is determined that the oscillation margin is slightly unstable, and is indicated by Δ in the table. When the oscillation margin is less than 5, it is judged to be unstable and is indicated by x.
The columns for toluene and 2,3-dimethylpentane are detected when the sensor elements of sample numbers 1 to 8 are installed in the gas sensor 2 described above and toluene and 2,3-dimethylpentane are used as detection gases, respectively. The value of the resonance frequency change amount (Δf [Hz]) is shown. FIG. 5 is a bar graph of these detected data. In FIG. 5, the number on the horizontal axis indicates the sample number, and the vertical axis indicates the amount of change in resonance frequency (Δf [Hz]).
The sensitivity is determined to be good when the amount of change in resonance frequency is 1000 Hz or more, and is indicated by ◯ in Table 1. When the amount of change in the resonance frequency is 800 Hz or more and less than 1000 Hz, it is determined that the sensitivity is slightly poor, and it is indicated by Δ in the table. When the amount of change in resonance frequency is less than 800 Hz, it is determined that the sensitivity is poor. Here, data that causes poor sensitivity has not been acquired, but in the case of poor sensitivity, it can be indicated by x in the table.
In the evaluation, when both stability and sensitivity are judged as ◯, it is judged that it is very suitable for the sensor element used for the gas sensor, and it is indicated by ⊚ in the table. If one of the stability and sensitivity is 〇 but the other is judged to be Δ, it is determined that the sensor element used for the gas sensor is suitable or slightly unsuitable. It is indicated by Δ. Further, when at least one of the stability and the sensitivity is judged to be ×, it is determined that the sensor element used for the gas sensor is not suitable, and is indicated by × in the table.

As shown in Table 1 and FIG. 5, by roughening the first surface so that the surface roughness of the crystal oscillator 13 is 0.19 μm or more and 0.36 μm or less, stable oscillation characteristics are obtained. The sensor element 1 can improve the sensitivity to odors and gas components.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made.

1 ... Sensor element 2 ... Gas sensor 4 ... Oscillation circuit 5 ... Detection circuit 11A ... First electrode 11B ... Second electrode 12 ... Sensitive film 13 ... Crystal oscillator 13A ... First surface 13B ... Second surface

Claims (2)

A crystal oscillator having a first surface having a surface roughness of 0.19 μm or more and 0.36 μm or less and a second surface.
With the first electrode provided on the first surface,
With the second electrode provided on the second surface,
A sensor element including a sensitive film provided on the first electrode. A crystal oscillator having a first surface having a surface roughness of 0.19 μm or more and 0.36 μm or less and a second surface.
With the first electrode provided on the first surface,
With the second electrode provided on the second surface,
A sensor element having a sensitive film provided on the first electrode and
An oscillator circuit that vibrates the sensor element and
A gas sensor including a detection circuit for detecting a change in the resonance frequency of the sensor element.

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