JP2021013133A - Oscillator and sensor element - Google Patents

Abstract

To improve frequency-temperature characteristics over a wider temperature range without causing attenuation of piezoelectric vibrational energy.SOLUTION: The oscillator includes a first plate-shaped layer 101 made of piezoelectric material whose resonant frequency has a positive temperature coefficient, and a second plate-shaped layer 102 formed on top of the first layer 101 and made of piezoelectric material whose resonant frequency has a negative temperature coefficient. The oscillator further includes a first electrode 103 formed in contact with the bottom of the first layer 101, and a second electrode 104 formed in contact with the top of the second layer 102. The oscillator can be used as a sensor element, and an external force applied to at least one of the first and second electrodes 103 and 104 can be measured as a change in resonance frequency.SELECTED DRAWING: Figure 1

Description

The present invention relates to an oscillator and a sensor element made of a piezoelectric material.

A crystal oscillator is an oscillator whose frequency changes relatively small with respect to changes in environmental temperature, and is currently used in various electronic products as a clock source for generating a timing signal. Quartz can obtain characteristics suitable for various uses by selecting the cutting angle of the crystal. For example, AT cut has the best temperature stability among various piezoelectric vibrators because the frequency temperature characteristic has a cubic curve, but it exceeds this AT cut as it is almost 90 years since its discovery. No new material with temperature characteristics has been found. For this reason, for electronic devices that require high-precision frequency stability, the structure is such that the oscillator is held in a constant temperature bath kept at a constant temperature so that it is not affected by changes in the environmental temperature. Measures have been taken such as adding a circuit that performs temperature compensation based on temperature information such as a temperature sensor.

On the other hand, the crystal unit is also used as a film thickness sensor in the thin film forming process. When a film adheres to one main surface of the crystal unit, the resonance frequency changes due to the mass load effect. This frequency change is related to the film thickness change, and is used as a film thickness sensor in a film forming apparatus or the like. However, as with crystal oscillators for timing devices, temperature changes in frequency have become an issue. That is, there is a problem that the frequency change due to the temperature is added to the frequency change due to the film thickness, and the measurement accuracy is deteriorated. In particular, in the case of BT cut, which is generally used for sensor applications, the frequency temperature characteristic becomes a quadratic curve, and there is a problem in temperature stability.

Although the Langasite-type single crystal has excellent characteristics such as a large electromechanical coupling coefficient and a small crystal impedance as compared with quartz, the frequency and temperature characteristics cannot exceed the characteristics of quartz AT cut. Even if various element substitutions are performed, it is not easy to improve the temperature characteristics of the material alone.

Further, a piezoelectric thin film oscillator using a piezoelectric thin film is known. Improving the temperature characteristics is also an issue for these piezoelectric thin film transducers, and it is known that the temperature characteristics can be improved by adding a film having a delay time temperature coefficient code different from that of the piezoelectric thin film. However, since the film to be added does not have piezoelectricity, if the film to be added is made thicker in order to improve the temperature characteristics, the effect of attenuating the vibration energy of the piezoelectric thin film becomes remarkable, so that there is a limit to the improvement of the temperature characteristics. is there.

As described above, although quartz has attractive characteristics for sensor applications, it is not always sufficient in terms of frequency and temperature characteristics, and has been a barrier to practical use. Piezoelectric devices using AT cut can obtain sufficient frequency temperature stability in the operating temperature range of 20 to 80 ° C for consumer products, but in the operating temperature range of 40 to 120 ° C for in-vehicle products. Not always enough. In particular, for mobile phone base stations and measuring instrument applications that require highly accurate frequency stability, the above-mentioned conventional techniques are insufficient in temperature stability.

For this reason, in general, it is necessary to add an extra circuit, such as by combining it with an electronic circuit for temperature compensation, which has a demerit such that the labor and cost of circuit design are increased. Furthermore, when quartz is used for sensor applications such as film thickness measurement at a high temperature of 200 ° C or higher, AT-cut quartz cannot handle it, and different cuts such as BT-cut are used, and its frequency-temperature characteristics are not always sufficient. It wasn't good. Further, in the technique for improving the temperature characteristics using the piezoelectric thin film vibration, there is a problem that the increase of the added film causes the attenuation of the piezoelectric vibration energy.

The present invention has been made to solve the above problems, and an object of the present invention is to improve frequency temperature characteristics over a wider temperature range without causing attenuation of piezoelectric vibration energy.

The vibrator according to the present invention has a plate-shaped first layer made of a piezoelectric material, a plate-shaped second layer formed on one surface of the first layer, and made of a piezoelectric material, and a first layer. It includes a first electrode formed in contact with another surface of the layer and a second electrode formed in contact with the second layer, and the first layer and the second layer have temperature characteristics of resonance frequency. Is different.

In one configuration example of the above oscillator, the first layer is composed of a piezoelectric material having a positive resonance frequency and a second layer is composed of a piezoelectric material having a negative resonance frequency. ..

In one configuration example of the above vibrator, each of the temperature coefficient of the thickness of the first layer and the resonance frequency and the temperature coefficient of the thickness of the second layer and the temperature coefficient of the resonance frequency are adjusted to the temperature characteristics of the set resonance frequency of the element. Is set.

In one configuration example of the above oscillator, a third layer arranged between the first layer and the second layer and made of a piezoelectric material is further provided.

The sensor element according to the present invention is a sensor element using the above-mentioned oscillator, and measures an external force applied to at least one of the first electrode and the second electrode as a change in resonance frequency.

As described above, according to the present invention, the frequency temperature characteristic is improved over a wider temperature range without causing the attenuation of the piezoelectric vibration energy.

FIG. 1 is a cross-sectional view showing the configuration of an oscillator according to an embodiment of the present invention. FIG. 2 is a characteristic diagram showing the temperature dependence of the transverse wave sound velocity in two different propagation directions (the first layer is in the 122 ° Y-axis direction and the second layer is in the 171 ° Y-axis direction) with respect to the CTGS single crystal. FIG. 3 shows the temperature characteristics of the linear expansion coefficient with respect to the crystal orientation in two different propagation directions (the first layer is in the 122 ° Y-axis direction and the second layer is in the 171 ° Y-axis direction) with respect to the CTGS single crystal. It is a figure. FIG. 4 shows the resonance frequency of an oscillator having two different propagation directions (the first layer is the 122 ° Y-axis direction and the second layer is the 171 ° Y-axis direction) with respect to the CTGS single crystal in the plane direction of the layer. It is a characteristic diagram which shows the temperature dependence of. In FIG. 5, the first layer and the second layer in two different propagation directions (the first layer is in the 122 ° Y-axis direction and the second layer is in the 171 ° Y-axis direction) with respect to the CTGS single crystal are bonded together to form an element. It is a characteristic diagram which shows the resonance frequency temperature characteristic when the thickness ratio x of two layers is changed. FIG. 6 shows the temperature characteristic measurement result (solid line) of the resonance frequency when x = 0.2448 in FIG. 5 and the resonance frequency of the oscillator having the main surface which is the minimum temperature characteristic in the case of the CTGS single layer. It is a characteristic diagram which shows the temperature characteristic measurement result (broken line). FIG. 7A is a characteristic diagram showing a first-order temperature coefficient when a quadratic curve approximation is performed with respect to the resonance frequency temperature characteristic of the vibrator for each direction of the CTGS single crystal. FIG. 7B is a characteristic diagram showing a quadratic temperature coefficient when a quadratic curve approximation is performed with respect to the resonance frequency temperature characteristic of the vibrator for each direction of the CTGS single crystal. FIG. 8 is a characteristic diagram showing the electromechanical coupling coefficient of the transverse wave for each direction of the CTGS single crystal.

Hereinafter, the oscillator according to the embodiment of the present invention will be described with reference to FIG. This vibrator includes a plate-shaped first layer 101 made of a piezoelectric material and a plate-shaped second layer 102 formed on one surface of the first layer 101 and made of a piezoelectric material. Further, the temperature characteristics of the resonance frequency are different between the first layer 101 and the second layer 102. For example, the first layer 101 may be composed of a piezoelectric material having a positive resonance frequency and a temperature coefficient, and the second layer 102 may be composed of a piezoelectric material having a negative resonance frequency. The first layer 101 and the second layer 102 can be composed of, for example, a Ca ₃ TaGa ₃ Si ₂ O ₁₄ (hereinafter referred to as CTGS) single crystal. Further, the first layer 101 and the second layer 102 can be made of, for example, a piezoelectric material having a thickness slip in the vibration mode.

Further, this oscillator includes a first electrode 103 formed in contact with the lower surface (on the other surface) of the first layer 101, and a second electrode 104 formed in contact with the second layer 102. The first electrode 103 and the second electrode 104 are arranged so as to face each other, for example.

Further, it is also possible to sandwich a layer of non-piezoelectric material between the first layer 101 and the second layer 102 for the purpose of stress relaxation and the like as long as the function of the present invention described later is not impaired.

This oscillator can be used as a sensor element. This sensor element measures an external force applied to at least one of the first electrode 103 and the second electrode 104 as a change in resonance frequency. For example, when a mass change due to the deposition of a predetermined material is applied to the first electrode 103 as an external force, it can be measured as a change in the resonance frequency of the element and can be used as a film thickness sensor. Further, when a pressure change is applied to the first electrode 103 as an external force, it can be measured as a change in the resonance frequency of the element and can be used as a pressure sensor. Further, when strain is applied to the first electrode 103 as an external force, it can be measured as a change in the resonance frequency of the element and can be used as a strain sensor.

For example, a thick film sensor is used in a film forming process of a semiconductor or the like. It is used for applications such as installing a thick film sensor near the wafer on which film is formed in the film forming apparatus and precisely controlling the film thickness while monitoring the increase in film thickness. This film thickness sensor captures the change in film thickness as a change in frequency, but as the film formation process progresses, the temperature change in the film forming apparatus cannot be avoided, so in addition to the frequency change due to the change in film thickness, Frequency change due to temperature change is added. Therefore, in this thick film sensor, it is important to make the amount of frequency change due to temperature change as close to zero as possible.

Each of the temperature coefficient of the thickness and the resonance frequency of the first layer 101 and the temperature coefficient of the thickness and the resonance frequency of the second layer 102 are set according to the temperature characteristics of the resonance frequency of the set element. The temperature characteristics of the resonance frequencies of the first layer 101 and the second layer 102 are different from each other, and the temperature dependence of the resonance frequency of the entire element is canceled by appropriately selecting the ratio of the thicknesses of the first layer 101 and the second layer 102. , The temperature coefficient of the resonance frequency of the entire element can be brought close to zero. For example, the temperature coefficient of the resonance frequency of the first layer 101 and the temperature coefficient of the resonance frequency of the second layer 102 are selected to be positive and negative, respectively, and the ratio x of the thickness of each layer is adjusted. This makes it possible to cancel the temperature dependence of the resonance frequency of the entire element and bring the temperature coefficient of the resonance frequency of the entire element close to zero.

For example, a first plate member of a piezoelectric material having a positive temperature coefficient of resonance frequency and a second plate member of a piezoelectric material having a negative temperature coefficient of resonance frequency are prepared. Next, the bonded surface of the first plate member and the bonded surface of the second plate member are bonded to each other by a known joining technique to form a bonded substrate. Next, a predetermined metal material is deposited on the electrode forming surface of the first plate member of the bonded substrate to form the first metal film. Further, a predetermined metal material is deposited on the electrode forming surface of the second plate member of the bonded substrate to form the second metal film. Next, the vibrator according to the embodiment is obtained by cutting out the bonded substrate to a predetermined size. When the obtained vibrator is used as a sensor element, a film thickness measuring device or a pressure measuring device can be formed by connecting a measuring device having a resonance frequency measuring unit or the like to each of the first electrode 103 and the second electrode 104. Can be configured. These can be used by being incorporated into a film forming apparatus or other apparatus.

Next, it will be described in more detail using the results of the experiment. In the following, a Ca ₃ TaGa ₃ Si ₂ O ₁₄ (hereinafter referred to as CTGS) single crystal will be described as a piezoelectric material. FIG. 2 shows the temperature dependence of the transverse wave sound velocity in two different propagation directions (the first layer is in the 122 ° Y-axis direction and the second layer is in the 171 ° Y-axis direction) with respect to the CTGS single crystal. As shown in FIG. 2A, the result of the first layer is that the first-order temperature coefficient is positive and the second-order temperature coefficient is negative (the quadratic curve is convex upward). On the other hand, as shown in FIG. 2B, the result of the second layer is conversely that the temperature coefficient of the first order is negative and the temperature coefficient of the second order is positive (the shape of the quadratic curve is convex downward). ing.

FIG. 3 shows the temperature characteristics of the coefficient of linear expansion for the two types of crystal orientations taken up in FIG. FIG. 3A shows the temperature characteristic of the coefficient of linear expansion of the first layer, and FIG. 3B shows the temperature characteristic of the coefficient of linear expansion of the second layer. Although the two have slightly different gradients, they both have a positive first-order temperature coefficient.

FIG. 4 shows the temperature dependence of the resonance frequency of the oscillator having the two types of crystal orientations taken up in FIGS. 2 and 3 in the plane direction of the layer. As shown in FIG. 4A, the result of the first layer is that the primary temperature coefficient is positive and the secondary temperature coefficient is negative. On the other hand, as shown in FIG. 4B, the result of the second layer is conversely that the primary temperature coefficient is negative and the secondary temperature coefficient is positive.

FIG. 5 shows the resonance frequency temperature characteristics when the thickness ratio x of the two layers is changed by laminating the first layer and the second layer of the two types of crystal orientations shown in FIG. 4 to form an element. There is. The thickness of the first layer is hx and the thickness of the second layer is h (1-x) with respect to the thickness h of the entire element. In the case of x = 0.2448, the frequency fluctuation of the device was the minimum of 19 ppm in the range of −50 to 100 ° C.

FIG. 6 shows the result (broken line) of the oscillator having the main surface having the minimum temperature characteristic in the case of the CTGS single layer in comparison with the result (solid line) in the case of x = 0.2448 in FIG. There is. In this temperature range, as shown in FIG. 6 (b), the single layer could only reduce the frequency fluctuation to 120 ppm. On the other hand, as shown in FIG. 6A, by adopting the structure of the element according to the embodiment, the frequency fluctuation can be reduced to about 1/6 of 19 ppm.

FIG. 7A shows the first-order temperature coefficient when the quadratic curve approximation is performed with respect to the resonance frequency temperature characteristic of the oscillator for each direction of the CTGS single crystal. Further, FIG. 7B shows the quadratic temperature coefficient when the quadratic curve approximation is performed with respect to the resonance frequency temperature characteristic of the oscillator for each direction of the CTGS single crystal. As shown in FIGS. 7A and 7B, it can be seen that the primary and secondary temperature coefficients change in a complicated manner depending on the crystal orientation. For this reason, it is difficult to combine the characteristics of both the primary and secondary temperature coefficients so that they can be offset, and as the result of the solid line in FIG. 6 shows, the temperature characteristics are completely flat even when the laminated structure is used. It turns out that things are difficult. It is also possible to develop oscillators with perfectly flat temperature characteristics by searching for combinations of different materials that can offset both the primary and secondary temperature coefficient characteristics.

FIG. 8 shows the electromechanical coupling coefficient of the transverse wave for each direction of the CTGS single crystal. In order to produce a more energy-efficient transducer, it is important to use a piezoelectric material with a crystal orientation that has a larger electromechanical coupling coefficient, and the crystal orientations of the main surfaces of the two types of layers selected above are The crystal orientations are 1.67% and 2.79%, respectively, which is a combination of piezoelectric materials having a relatively large electromechanical coupling coefficient.

By the way, a third layer made of a piezoelectric material may be further provided between the first layer 101 and the second layer 102. For example, if the first layer 101 is divided into two layers in the thickness direction, the above-mentioned three-layer structure is obtained. For example, a piezoelectric material selected so that the second layer 102 is composed of a CTGS 171 ° Y plate and the two layers obtained by dividing the first layer 101 have a temperature characteristic of a resonance frequency equivalent to that of the 122 ° Y plate. It can be configured from. In this case, the two layers obtained by dividing the first layer 101 can have a configuration in which a 121 ° Y plate and a 123 ° Y plate are bonded together at a thickness ratio of 0.5.

As described above, the same performance as the result shown in FIG. 6A was obtained for the vibrator in which the first layer 101 was composed of two layers and the whole was made into three layers. Further, a plurality of layers, each of which is made of a piezoelectric material, may be further provided between the first layer 101 and the second layer 102. As described above, even if the number of layers is four or more, if a substrate to be bonded is selected by the same method as described above, the oscillator can obtain the same performance.

As described above, according to the present invention, from a piezoelectric material having a negative temperature coefficient of resonance frequency on a plate-shaped first layer composed of a piezoelectric material having a positive temperature coefficient of resonance frequency. Since the formed plate-shaped second layer is brought into contact with each other, the frequency temperature characteristic can be improved over a wider temperature range without causing the attenuation of the piezoelectric vibration energy.

The present invention is not limited to the embodiments described above, and within the technical idea of the present invention, a person having ordinary knowledge in the art may combine many materials or dissimilar materials. It is clear that the combination in is feasible.

For example, in the above description, the case of CTGS of a Langasite-based single crystal has been illustrated, but the present invention is not limited to this, and for example, a crystal or a ferroelectric single crystal such as LiTaO ₃ and LiNbO ₃ can be used. Further, the first layer and the second layer are not limited to a combination of the same kind of materials, but may be a combination of different materials. Further, the temperature characteristics of the materials to be combined are not limited to the combination in which one is positive and the other is negative, and even if they have the same positive or negative temperature coefficient, the slope of the temperature coefficient and the intercept are different. You can also do it. Further, the degree (Nth-order curve: a straight line when N = 1 and a quadratic curve when N = 2) when the frequency-temperature characteristics are polynomial-approximated may be different. Further, even if the above-mentioned order is the same, it is possible to use a combination of materials having different extreme temperatures, or a combination of materials having different shapes of frequency temperature characteristics such as different amounts of frequency temperature changes.

Further, the film thicknesses of the materials to be bonded are not limited to the same, and each can be in a different state. Further, a different material such as an ultrathin metal or an oxide may be sandwiched between the joints of the plate members of the piezoelectric material, and different thin films such as oxides may be formed on the front and back surfaces of the bonded plate members. .. Further, the shape of the element does not have to be a rectangular body in a plan view, for example, and the end portion may be surface-removed (tapered or rounded). Further, the shape of the element may be a convex lens shape = so-called convex processing or concave lens processing. It should be noted that only one main surface may have a convex shape.

Further, the crystal orientation in the in-plane direction of the substrates to be bonded can be appropriately adjusted in the same direction or different directions. For example, when laminating rotating Y plates having different crystal orientations in the in-plane direction, the method of selecting the laminating surface of each rotating Y plate is such that the directions of vibration displacement (particle displacement direction) with respect to the electric field are the same. The orientations of the rotating Y plates can be matched (in the above embodiment, the + 122 ° Y direction and the + 171 ° Y direction are matched), or the combinations can be non-matched.

The reason for this is that the temperature characteristics do not change, only the frequency band that resonates efficiently changes between the basic mode and the higher-order mode, and there is a degree of freedom in selection according to the design. Further, the X-axis directions of the respective rotating Y plates can be matched in the in-plane direction of the bonding plane, or the combinations can be made not to match. In the case of the above-described embodiment, when the X-axis directions of the crystals are matched, the linear expansion coefficients in the X-axis direction are completely the same because of the same material, so that the influence of strain due to thermal expansion in this direction is eliminated. effective. That is, the relative relationship between the upper and lower surfaces of each substrate and the in-plane orientation of the bonded surface can be appropriately changed according to the desired resonance frequency temperature dependence.

Furthermore, it is clear to the person concerned that the present invention is not limited to sensor applications and can be applied to oscillators for timing devices and the like that use the same piezoelectric effect.

The oscillator according to the present invention and the sensor element using this oscillator can be composed only of a piezoelectric material and electrodes. It is significantly different from the structure in which a film such as a SiO ₂ film, which is a non-piezoelectric film, is added to the surface of a conventional crystal oscillator or piezoelectric thin film oscillator for the purpose of improving temperature characteristics. In this conventional structure, since the non-piezoelectric film is used as the temperature compensation film, vibration energy is lost and the resonance characteristic, which is the main characteristic, is deteriorated. On the other hand, the vibrator according to the present invention has a two-layer structure made of a piezoelectric material and performs piezoelectric vibration by an electric field generated by an AC voltage applied by an electrode, so that almost no loss of vibration energy occurs. In particular, in a desirable configuration, a piezoelectric single crystal is used as the piezoelectric material, and it is also advantageous that the selection range of crystal orientation is wide.

101 ... 1st layer, 102 ... 2nd layer, 103 ... 1st electrode, 104 ... 2nd electrode.

Claims (5)

A plate-shaped first layer made of a piezoelectric material and
A plate-shaped second layer formed on one surface of the first layer and made of a piezoelectric material,
With the first electrode formed in contact with the other surface of the first layer,
A second electrode formed in contact with the second layer is provided.
An oscillator characterized in that the first layer and the second layer have different temperature characteristics of resonance frequencies. In the oscillator according to claim 1,
The first layer is composed of a piezoelectric material whose resonance frequency has a positive temperature coefficient.
The second layer is a vibrator characterized in that the resonance frequency is made of a piezoelectric material having a negative temperature coefficient. In the oscillator according to claim 1 or 2.
The temperature coefficient of the thickness of the first layer and the resonance frequency, and the temperature coefficient of the thickness of the second layer and the temperature coefficient of the resonance frequency are each set according to the temperature characteristics of the resonance frequency of the set element. Characterized oscillator. In the vibrator according to any one of claims 1 to 3,
An oscillator which is arranged between the first layer and the second layer and further includes a third layer made of a piezoelectric material. A sensor element using the vibrator according to any one of claims 1 to 4.
A sensor element characterized in that an external force applied to at least one of the first electrode and the second electrode is measured as a change in resonance frequency.

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