JP2000214140A - Sensor - Google Patents

Abstract

(57) [Problem] To provide a sensor using a surface acoustic wave oscillator, which has both high-speed response and high accuracy and has a simple circuit configuration. SOLUTION: A piezoelectric film 4 is formed on a high-speed propagator 3 having a higher propagation speed of surface acoustic waves than the piezoelectric film 4, and one or more sets of two interdigital transducers 5 are provided on the piezoelectric film 4. 6 and one transducer 5 in each set of interdigital transducers 5, 6 is connected to an amplifier 8.
And the output side 8b of the amplifier 8 is connected to the other transducer 6 to form at least one delay line type surface acoustic wave oscillator 1, and the oscillation frequency of each surface acoustic wave oscillator 1 Is separately provided. More preferably, in at least one surface acoustic wave oscillator 1, a sensitive film 10 sensitive to a predetermined measured quantity is formed between two interdigital transducers 5 and 6 on the piezoelectric film 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of forming two interdigital transducers on a piezoelectric film, one of which is connected to the input of an amplifier, and the output of which is connected to the other. The present invention relates to a sensor for detecting humidity, gas and the like using a delay line type surface acoustic wave oscillator.

[0002]

2. Description of the Related Art In general, as a sensor for detecting humidity, gas, and the like, a semiconductor sensor or the like for detecting a change in electric capacity or electric resistance based on a change in an amount to be measured is known.
Further, as a sensor using the above-mentioned delay line type surface acoustic wave oscillator, a pair of interdigital transducers are formed on a piezoelectric substrate such as quartz or LiNbO ₃ (lithium niobate) to form a delay line type surface acoustic wave. An oscillator is known in which a gas-sensitive film or a humidity-sensitive film is formed on a piezoelectric substrate between a pair of interdigital transducers.

[0003]

However, the semiconductor sensor of the above-mentioned type which detects a change in the electric capacity or electric resistance has a slow response speed, and it takes about three seconds to detect the change. It took about 10 seconds to 1 minute. Furthermore,
Also in a sensor using a surface acoustic wave oscillator fabricated on a piezoelectric substrate, the center frequency of oscillation is as low as about 100 MHz, and the propagation speed of surface acoustic waves of a piezoelectric material such as quartz or LiNbO ₃ is as slow as about 3000 m / s. Therefore, the response speed is slow, and the detection requires a maximum of about 3 seconds. In addition, in the sensor, a higher-accuracy sensor can be obtained as the oscillating frequency is higher. Therefore, it has been desired to increase the oscillating frequency in addition to high-speed response.

As a sensor utilizing surface acoustic waves, the present inventors formed a piezoelectric film on a diamond thin film, formed a pair of interdigital transducers thereon, and placed one on the surface of the surface acoustic wave. A method that detects the effect of a change in the measured quantity on the elastic property of the delay line formed between the two transducers as the transmitting end and the other as the receiving end of the surface acoustic wave as a change in the frequency that appears at the receiving end Has been proposed in an earlier patent application (Japanese Patent Application No. 8-274313). In the sensor of this proposal, since the piezoelectric film is formed on a diamond thin film having a higher propagation speed of the surface acoustic wave than the piezoelectric film, the frequency of the surface acoustic wave is 1 GHz.
Since the above can be used and the propagation speed on the delay line is high, it is possible to provide both high-speed response and high accuracy. However, the sensor proposed above has a configuration of 1 GHz.
Since a high frequency source of z or more is required separately, it is difficult to manufacture an external circuit for making the sensor function effectively.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a sensor using a surface acoustic wave oscillator, which has both high-speed response and high accuracy, and has a simple circuit configuration. The point is to provide a sensor.

[0006]

According to a first feature of a sensor according to the present invention for achieving this object, as described in claim 1 of the claims, the piezoelectric film is formed by a piezoelectric film. Formed on a high-speed propagator in which the propagation speed of surface acoustic waves is higher than that of the conductive film, two interdigital transducers are formed on the piezoelectric film, and one of the transducers is connected to the input side of the amplifier. The output side is connected to the other transducer to constitute a delay line type surface acoustic wave oscillator, and further comprises frequency measuring means for measuring the oscillation frequency of the surface acoustic wave oscillator.

The second characteristic configuration is that, as described in claim 2 of the claims, the piezoelectric film is formed on a high-speed propagator whose propagation speed of surface acoustic waves is higher than that of the piezoelectric film. ,
A plurality of sets of two interdigital transducers are formed on the piezoelectric film, and in each set of the interdigital transducers, one transducer is connected to the input of an amplifier, and the output of the amplifier is connected to the other transducer. Then, a plurality of delay line type surface acoustic wave oscillators are configured, and a frequency measuring means for individually measuring the oscillation frequency of each of the surface acoustic wave oscillators is provided.

[0008] According to a third aspect of the present invention, in addition to the first or second aspect, at least one of the surface acoustic wave oscillators further comprises: The present invention is characterized in that a sensitive film sensitive to a predetermined measured amount is formed between the two interdigital transducers on the piezoelectric film.

The fourth characteristic configuration is, as described in claim 4 of the claims, in addition to the above-mentioned first, second or third characteristic configuration, the high-speed propagating body is provided with a predetermined speed. The point is that it is formed on a substrate.

The fifth feature of the present invention is that the amplifier is formed on the substrate in addition to the fourth feature of the present invention, as described in claim 5 of the claims. is there.

The operation and effects of the above-mentioned features will be described below. According to the first or second characteristic configuration,
The surface acoustic wave propagating through the piezoelectric film propagates at a propagation speed in the high-speed propagating body under the influence of the high-speed propagating body formed below the piezoelectric film. The oscillation frequency of the surface acoustic wave oscillator, which is limited by the propagation speed, can be increased, and a subtle change in the measured amount acting on the piezoelectric film can be detected with high accuracy as a change in the oscillation frequency. Further, since the propagation speed of the surface acoustic wave propagating through the piezoelectric film is increased to the propagation speed of the high-speed propagating body as described above, high-speed response is also achieved. Further, when used as a gas sensor, it is possible to measure the molecular weight of a gas rather than high speed and high frequency. Further, since the surface acoustic wave oscillator is configured and the high-frequency surface acoustic wave is self-excited, a separate high-frequency oscillator for exciting the high-frequency surface acoustic wave is not required, so that the circuit configuration can be simplified. You can do it.

Further, according to the second characteristic configuration, since a plurality of surface acoustic wave oscillators are configured, the object to be measured may be different for each surface acoustic wave oscillator, or the same object may be measured. Also, since the range of the measured quantity can be made different, it is possible to provide a complex high-performance sensor capable of dealing with a plurality of measurement objects in a certain atmosphere and capable of performing a wide range of measurement. Further, since a plurality of sensors can be realized on a single substrate, the sensitive portion of the sensor can be reduced in size.

[0013] According to the third characteristic configuration, it is possible to detect an amount to be measured and a measurement range that cannot be detected by the piezoelectric film alone. In particular, by adding a sensitive film that reacts to a predetermined gas type, a gas sensor of the gas type can be easily manufactured. In particular, the high-performance sensor described above can be easily manufactured by combining it with the second characteristic configuration.

According to the fourth aspect, the high-speed propagator is formed in a thin film on the substrate without using a high-speed propagator such as a diamond substrate. Can be shared, and the manufacturing cost can be reduced. Further, other peripheral circuits and the like can be integrated on the substrate, and the size of the sensor can be reduced.

According to the fifth characteristic configuration, since the entire surface acoustic wave oscillator can be formed on the same substrate, the stray capacitance and the like of the wiring between the amplifier and each transducer can be reduced, and the wiring delay and the signal between them can be reduced. The transition can be sharpened, and the oscillation frequency can be further increased. Also,
This eliminates the need to externally attach an amplifier, thereby reducing the size of the sensor.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. In the first embodiment of the sensor according to the present invention, as shown in FIG. 1, a diamond thin film 3 and a piezoelectric film 4 which are high-speed propagators are formed on a Si substrate 2, and two layers are formed on the piezoelectric film 4. The two interdigital transducers 5 and 6 are provided, and a delay line 7 is provided between the transducers 5 and 6.
And one of the transducers 5 is connected to a high-frequency amplifier 8
The surface acoustic wave oscillator 1 is formed by a feedback loop formed by connecting the input side 8a of the amplifier 8 and the output side 8b of the amplifier 8 to the other transducer 6. The output side 8b of the high-frequency amplifier 8 outputs a high-frequency output signal. The configuration is provided with a frequency measuring means 9 for measuring the oscillation frequency by branching a part, which is a basic configuration of the sensor according to the present invention.

The diamond thin film 3 is formed on the substrate 2 to a thickness of about 10 μm by the CVD method.
The surface is flattened by polishing to the extent. The piezoelectric film 4 is formed on the polished diamond thin film 3 by RF.
A film thickness of 0.5 to 1.0 μm formed by a sputtering method,
More preferably, a ZnO film having a thickness of 0.6 to 0.7 μm is used. The ZnO film normally functions as a piezoelectric film within a thickness range of 0.5 to 1.0 μm. That is, when the thickness of the ZnO film is thinner than 0.5 μm, the piezoelectric conversion efficiency is remarkably reduced, and the intensity of the generated surface acoustic wave is small. a. On the other hand, the thickness of the ZnO film is 1.0 μm.
If the thickness is larger than m, the crystal axis of the ZnO crystal film is broken, the function as a piezoelectric body is impaired, and oscillation becomes impossible.

The interdigital transducers 5, 6
Is a so-called IDT (interdigital transducer) electrode formed by combining two comb-shaped multi-paired electrodes made of Al or an Al alloy, as shown in FIGS. Is formed by a vapor deposition method, a sputtering method, or the like, an unnecessary metal film portion is removed by photolithography by dry etching, lift-off, or the like, thereby forming an electrode pattern. The width w and the interval s of the electrodes of the transducers 5 and 6 are each 3 μm, and the wavelength λ of the surface acoustic wave to be excited is 2 μm.
It is determined by (w + s) to be 12 μm. The comb-shaped multi-pair electrodes are each composed of 25 pairs of electrodes having an intersection width of 300 μm,
The length of each of the transducers 5 and 6 is 300 μm.
The interval between the two transducers 5 and 6 is 500 μm. If this interval becomes longer, the attenuation of the surface acoustic wave in the delay line 7 becomes larger, and a high amplification factor of the high frequency amplifier 8 is required. by the way,
In order to oscillate at a high frequency of 1 GHz or more, the frequency band of the high-frequency amplifier 8 is required to be sufficiently higher than the frequency band. In the present embodiment, the attenuation rate of the delay line 7 is 30 to
Since the gain is about 40 dB, the gain of the high-frequency amplifier 8 is 70 dB. The high-frequency amplifier 8 may be an existing one externally used, or may be a monolithically formed semiconductor integrated circuit on the Si substrate 2.

The frequency measuring means 9 is provided with a frequency counter and a high-frequency spectrum analyzer, is provided with a reference frequency generator as required, and measures a frequency difference from the reference signal. No problem. Further, a correction means for correcting the individual difference of the propagation speed of the surface acoustic wave in the diamond thin film 3 and the relationship between the measured frequency and the measured amount due to the fluctuation of the propagation speed by the sensitive film described later is provided using a microcomputer or the like. It is.

In the above configuration, the amplifier 8
An electric signal output from the output side 8b is input to the transducer 6, and a surface acoustic wave is excited on the surface of the piezoelectric film 4. The surface acoustic wave excited by the transducer 6 directs the delay line 7 toward the transducer 5 and causes the piezoelectric film 4 having a speed higher than the propagation speed of the surface acoustic wave in the piezoelectric film 4 (about 3000 m / s). The propagation velocity v of the surface acoustic wave in the diamond thin film 3 provided on the lower layer side (12000 to 15000 m
/ S), is detected by the transducer 5, converted into an electric signal, input to the input side 8a of the amplifier 8, and oscillates at a predetermined frequency f on the above-mentioned feedback loop. Here, the oscillation frequency f is determined by the relational expression f = v / λ from the wavelength λ of the surface acoustic wave and the propagation velocity v. Incidentally, since the ZnO film serving as the piezoelectric film 4 acts as a humidity sensitive film, the elastic property of the ZnO film changes due to a change in humidity. As a result, the propagation speed of the surface acoustic wave changes and the oscillation frequency f Changes. Phenomenonally, it is considered that the elastic properties of the ZnO film change in response to the molecular weight of water in contact with the surface of the ZnO film.

Next, the measurement results obtained by measuring the characteristics of the sensor of the first embodiment as a relative humidity sensor will be described. The measurement results are shown in FIG. 4 with the horizontal axis representing relative humidity and the vertical axis representing the center frequency of the oscillation frequency of the surface acoustic wave oscillator 1 measured by the frequency measuring means 9. From this, it can be confirmed that the surface acoustic wave oscillator 1 oscillates at an oscillation frequency of 1 GHz or more (1.0372 to 1.0395 GHz), and it can be seen that the accuracy of the sensor is improved. Furthermore, FIG. 4 shows that the oscillation frequency f decreases as the relative humidity increases. In particular, when the relative humidity exceeds 70%, the oscillation frequency f is greatly reduced, so that high-sensitivity measurement can be performed in the range of 70% or more of the relative humidity. In addition, as shown in FIG.
The responsiveness when periodically changing between% and 80% is
The center frequency of the oscillation frequency is about 0.5
Second delay, 1.0395GHz and 1.0372GHz
It can be seen that the response is improved by a factor of 6 or more as compared with the related art.

Next, a second embodiment of the sensor according to the present invention will be described. The sensor according to the second embodiment includes a first sensor.
The sensitive film 1 which is sensitive to a predetermined measured amount between the transducers 5 and 6 on the piezoelectric film 4 of the sensor of the embodiment.
0 is provided. Examples of the sensitive film 10 include cellulose acetate which reacts to a change in humidity, and SnO ₂ which reacts to a predetermined gas shown in Table 1.

The sensitive film 10 is formed with an appropriate thickness by a film forming method corresponding to each material. Generally, by forming the sensitive film 10 on the piezoelectric film 4,
Since the propagation speed of the surface acoustic wave is reduced, the reaction speed and accuracy are slightly reduced. Therefore, a thinner film is preferable from the viewpoints of reaction speed and accuracy, but is unsuitable for detecting a high-concentration gas. Conversely, a thicker film is more suitable for detecting a high-concentration gas. If the sensitive film 10 is conductive, it must be formed so as not to short-circuit the two transducers 5 and 6.

[0024]

[Table 1]

Next, the measurement results of the humidity-sensitive characteristics of a sensor formed by applying a cellulose acetate film as a humidity-sensitive film by a spinner or the like as the sensitive film 10 will be described. This measurement result is shown in FIG. 6 as in the first embodiment, with the horizontal axis representing relative humidity and the vertical axis representing the center frequency of the oscillation frequency of the surface acoustic wave oscillator 1 measured by the frequency measuring means 9. Thus, the surface acoustic wave oscillator 1 has an oscillation frequency of 1 GHz or more (1.0319 to 1.033).
(3 GHz), which indicates that the accuracy of the sensor is improved. Also, it can be seen that the oscillation frequency decreases as the relative humidity increases.
The fluctuation range of the center frequency due to the change of the relative humidity is the first range.
Although slightly shifted to the lower frequency side compared to the case of the embodiment,
This is considered to be due to a decrease in the propagation speed due to the influence of the sensitive film 10 or an individual difference in the propagation speed of the diamond thin film 3 as described above.

In this embodiment, since the ZnO film serving as the piezoelectric film 4 functions as a humidity sensitive film, it is not necessary to provide the cellulose acetate film as the sensitive film 10. In the case where is not sensitive to humidity, the humidity sensor having high sensitivity and high-speed response can be manufactured by providing the cellulose acetate film.

Next, a third embodiment of the sensor according to the present invention will be described. The sensor according to the third embodiment is a composite sensor in which a plurality of the sensors according to the first or second embodiment are formed on the Si substrate 2. Specifically, FIG.
As shown in FIG. 3, the diamond thin film 3 and the piezoelectric film 4 are formed on the Si substrate 2, and a plurality of sets of the two interdigital transducers 5 and 6 are provided on the piezoelectric film 4. For each set of transducers 5, 6, one transducer 5 is connected to the input 8a of the high-frequency amplifier 8 and the output 8b of the amplifier 8 is connected to the other transducer 6, and the surface acoustic wave oscillator is connected. 1, a part of the high-frequency output signal is branched from the output side 8b of the high-frequency amplifier 8 and input to the frequency measuring means 9, and the number of the plurality of surface acoustic wave oscillators 1 The surface acoustic wave oscillator 1 includes the sensitive film 10 between the two transducers 5 and 6 on the piezoelectric film 4. As the sensitive film 10, for example, by selecting and using a sensitive film that reacts to a different gas type for each of the surface acoustic wave oscillators 1, it is possible to improve the gas discrimination ability as a gas sensor. Therefore, according to the present embodiment, it is possible to configure a multifunctional sensor in which a humidity sensor and a gas sensor capable of distinguishing various kinds of gases are formed on a single substrate. Further, the sensor according to the first embodiment also functions as a vibration sensor or a strain sensor because the piezoelectric film 4 also responds to vibration or strain. Note that the sensitive film 10 is not necessarily provided.

The plurality of surface acoustic wave oscillators 1 are formed after the piezoelectric film 4 is formed so that adjacent ones along the traveling direction of the surface acoustic wave do not interfere with each other. In order to separate the wave oscillator 1 in the same direction, the piezoelectric film 4 or the piezoelectric film 4 and the diamond thin film 3 corresponding to the boundary portion of the region where the transducers 5 and 6 are formed are removed by plasma etching or the like, A buffer band 11 for surface acoustic waves is formed in this portion. Further, the plurality of surface acoustic wave oscillators 1 may be completely separated from each other.

Hereinafter, other embodiments of the present invention will be listed. <1> In the above-described embodiment, the diamond thin film 3 which is finally formed on the Si substrate 2 to a thickness of about 5 μm is used as the high-speed propagator of the surface acoustic wave oscillator 1. The thickness is not limited to about 5 μm and may be about 2 μm or more. In addition to the configuration in which the diamond thin film 3 is formed on the Si substrate 2, a single crystal diamond substrate may be used as the high-speed propagation body, and the piezoelectric film 4 may be formed thereon. . Further, the substrate 2 is not limited to the Si substrate, but may be another substrate.

<2> Further, in each of the above-described embodiments, diamond is used as the high-speed propagating member such as the diamond thin film 3 and the diamond substrate 9, but the propagation of surface acoustic waves is larger than that of the piezoelectric film 4. It is sufficient if the speed is sufficiently high. For example, a thin film or substrate such as sapphire or CNx may be used.

<3> In each of the above embodiments, ZnO is used as the material of the piezoelectric film 4.
Other materials having piezoelectricity such as bO3 and ZnS may be used.

<4> In each of the above embodiments, the number of pairs of electrodes of the IDTs 5 and 6 and the dimensions of each part are not limited to those illustrated above, but can be changed as appropriate.

[Brief description of the drawings]

FIG. 1 is a perspective view of a first embodiment of a sensor according to the present invention.

FIG. 2 is a plan view of an interdigital transducer of a surface acoustic wave oscillator.

FIG. 3 is a plan view of a main part of the interdigital transducer shown in FIG. 2;

FIG. 4 is a characteristic diagram showing a humidity response characteristic of the sensor according to the first embodiment.

FIG. 5 is a transient response characteristic diagram showing the response of the sensor according to the first embodiment.

FIG. 6 is a characteristic diagram showing a humidity response characteristic of the sensor according to the second embodiment.

FIG. 7 is a perspective view of a third embodiment of the sensor according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Surface acoustic wave oscillator 2 Substrate 3 High-speed propagation body (diamond thin film) 4 Piezoelectric film 5, 6 IDT 7 Delay line 8 High frequency amplifier 8a Input side 8b Output side 9 Frequency measuring means 10 Sensitive film 11 Buffer band

Claims (5)

[Claims] 1. A piezoelectric film is formed on a high-speed propagator whose surface acoustic wave propagates faster than the piezoelectric film, and two interdigital transducers are formed on the piezoelectric film. A delay line type surface acoustic wave oscillator is formed by connecting the input side of the amplifier to the input side of the amplifier, and the output side of the amplifier is connected to the other transducer, and frequency measuring means for measuring the oscillation frequency of the surface acoustic wave oscillator is provided. Sensor. 2. A piezoelectric film is formed on a high-speed propagator whose surface acoustic wave propagates faster than the piezoelectric film, and a plurality of sets of two interdigital transducers are formed on the piezoelectric film. Connecting one transducer to the input of an amplifier and the output of the amplifier to the other transducer in a set of interdigital transducers;
A sensor comprising a plurality of delay line type surface acoustic wave oscillators and comprising frequency measuring means for individually measuring the oscillation frequency of each of the surface acoustic wave oscillators. 3. The at least one surface acoustic wave oscillator, wherein a sensitive film sensitive to a predetermined measured quantity is formed between the two interdigital transducers on the piezoelectric film. The sensor as described. 4. The sensor according to claim 1, wherein said high-speed propagation body is formed on a predetermined substrate. 5. The sensor according to claim 4, wherein said amplifier is formed on said substrate.