JP2012085108A - Surface acoustic wave sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave sensor in which measurement and monitoring of a desired medium are achieved based on the propagation characteristics of a surface acoustic wave on a piezoelectric substrate, performance is improved without increasing the cost sharply, and variation in characteristics is avoided stably and reliably.SOLUTION: The surface acoustic wave sensor which is formed on a piezoelectric substrate, converts an electric signal into an elastic wave and converts the elastic wave into an electric signal comprises two wave transmission electrodes which are arranged on the piezoelectric substrate and convert an electric signal into an elastic wave, two wave reception electrodes which are formed on the piezoelectric substrate to face the two wave transmission electrodes individually and convert individual elastic waves arrived via the piezoelectric substrate, respectively, into electric signals, and an reflective electrode which is formed in a region on the piezoelectric substrate sandwiched by one of two wave transmission electrodes and one wave reception electrode formed to face the one wave transmission electrode, and reflects the surface acoustic wave out of the elastic waves in the direction other than the two wave reception electrodes.

Description

  The present invention relates to a surface acoustic wave sensor that realizes measurement and monitoring of a desired medium based on propagation characteristics of surface acoustic waves on a piezoelectric substrate.

  Since surface acoustic wave devices such as surface acoustic wave sensors have the following desirable characteristics, various devices are being developed that can be easily reduced in size, reduced in weight, and made non-adjustable by utilizing these characteristics. .

(1) The wavelength of the surface acoustic wave obtained by internally converting the input electrical signal is significantly short, about one millionth of the wavelength of the electromagnetic wave.
(2) It is composed of a piezoelectric substrate serving as a surface acoustic wave propagation path, and a film, a pattern, and the like that can be finely formed on the piezoelectric substrate.
(3) For polishing the surface of the piezoelectric substrate, only one surface on which the propagation path of the surface acoustic wave is formed is sufficient.

(4) Since the main energy of the surface acoustic wave is concentrated and propagates on the surface of the piezoelectric substrate, the acoustic coupling between the electrode formed on such a surface or the medium in contact with the surface is tight. Or achieved at will.
(5) The process applied to the surface acoustic wave under the above-described coupling or the change that occurs can be realized stably and accurately not only in the linear region but also in the nonlinear region.

As a conventional surface acoustic wave sensor, for example, there is a surface acoustic wave device configured as follows as disclosed in Patent Document 1 described later.
(1) Two propagation paths having the same propagation characteristics are formed on the piezoelectric substrate.

(2) On one of these propagation paths, the propagation of the surface acoustic wave is blocked by the blocking means (resin applied on the propagation path or the groove formed) on the propagation path, A bulk wave generated in the other propagation path is predicted.
(3) The predicted bulk wave is subtracted from the elastic wave (including the surface acoustic wave and bulk wave) propagated through the other propagation path, thereby suppressing the bulk wave component.

  In the surface acoustic wave sensor having such a configuration, deterioration in accuracy and performance due to bulk waves is reduced.

  In addition, as a prior art relevant to this invention, there exists patent document 2 mentioned later other than patent document 1 already stated. The outline of these prior arts is listed below.

(1) “On the piezoelectric substrate, an input electrode for exciting a surface acoustic wave and an output electrode for converting the surface acoustic wave from the input electrode into an electric signal and outputting it are obtained to obtain a desired frequency characteristic. Therefore, in the surface acoustic wave device weighted to the electrode finger crossing width, the input electrodes are arranged to face each other in a direction substantially orthogonal to the propagation direction of the surface acoustic wave, and are excited together with the surface acoustic wave from each. First and second input electrodes configured so that bulk waves traveling parallel to the surface of the piezoelectric substrate are in opposite phases to each other, and from each of the first and second input electrodes By providing a blocking means to block any one of the surface acoustic waves propagating to the output electrode ”,“ outside of the band by suppressing unwanted bulk waves that travel mainly parallel to the piezoelectric substrate, regardless of the substrate characteristics. To improve the degree of suppression Characteristic surface acoustic wave device ... Patent Document 1

(2) By “inclining the back surface of the surface acoustic wave element to an angle satisfying a predetermined condition”, “incident at different positions of the output electrode or the surface acoustic wave waveguide reflected on the back surface and formed on the substrate surface It is characterized in that the signal due to the bulk wave is canceled and the output signal can be extracted with a high SN ratio.

Japanese Patent No. 2821263 JP-A-5-129886

  By the way, in the conventional surface acoustic wave sensor described above, the propagation and suppression of the surface acoustic wave are realized by a resin applied to the propagation path of the surface acoustic wave or a groove formed.

  In addition, since the application of the resin and the formation of the groove are realized as a process different from the formation of the electrode on the surface of the elastic substrate, not only the cost is increased but also the propagation of the surface acoustic wave is prevented. There was a high possibility that this would cause variations in characteristics.

  However, since the variation in the above characteristics is a factor that hinders further improvement in sensitivity and performance required for the surface acoustic wave sensor, there is a strong demand for a technique that can be eliminated or greatly reduced.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave sensor that can improve performance and stably avoid variations in characteristics without significantly increasing costs.

  The invention according to claim 1 is a surface acoustic wave sensor which is formed on a piezoelectric substrate and converts an electric signal into an elastic wave, and converts an elastic wave propagated through the piezoelectric substrate into an electric signal. A wave electrode is disposed on the piezoelectric substrate and converts the electrical signal into the elastic wave. The two receiving electrodes are formed on the piezoelectric substrate so as to face the two transmitting electrodes, respectively, and each elastic wave that has arrived through the piezoelectric substrate is converted into an electric signal. The reflective electrode is formed in a region on the piezoelectric substrate sandwiched between one of the two transmitting electrodes and one receiving electrode formed opposite to the one transmitting electrode, Among them, the surface acoustic wave is reflected in a direction other than the two receiving electrodes.

  That is, one of the two receiving electrodes receives both surface acoustic waves and bulk waves from one opposing transmitting electrode, and the other receives only bulk waves from the other transmitting electrode. Therefore, the characteristics of two propagation paths formed in parallel between the two transmitting electrodes and the two receiving electrodes described above are the same, or the difference between these characteristics is known. Then, in conjunction with the cancellation of the bulk wave, it is possible to extract the surface acoustic wave that has arrived at one of the receiving electrodes via the piezoelectric substrate with a high SN ratio.

  Further, in the present invention, the reflection electrode has a narrow occupation band of the electric signal input to the surface acoustic wave sensor and can be set in a fine shape, arrangement, and dimensions. Similarly, it can be formed as a pattern, and reflection of surface acoustic waves with high efficiency is realized.

According to a second aspect of the present invention, in the surface acoustic wave sensor according to the first aspect, the two receiving electrodes convert elastic waves that individually arrive via the piezoelectric substrate into electrical signals having opposite phases to each other. To do.
In other words, bulk wave components arriving at each of the two receiving electrodes via the piezoelectric substrate are obtained in opposite phases.

According to a third aspect of the present invention, in the surface acoustic wave sensor according to the first aspect, the two transmission electrodes convert the electric signals into elastic waves having opposite phases to each other.
That is, the components of the bulk waves that have arrived at each via the piezoelectric substrate can be obtained in opposite phases even when the structures of these receiving electrodes are the same.

According to a fourth aspect of the present invention, in the surface acoustic wave sensor according to any one of the first to third aspects, the two receiving electrodes are connected to the two receiving electrodes via the piezoelectric substrate. The sum of the electric signals obtained by the conversion of the elastic wave that arrived at is generated.
That is, in the surface acoustic wave sensor according to the present invention, even if an active circuit or an active element is not provided, the above-described cancellation of the bulk wave and the improvement of the S / N ratio due to the cancellation of the bulk wave are realized.

  According to a fifth aspect of the present invention, in the surface acoustic wave sensor according to any one of the first to fourth aspects, the propagation separator is formed on the piezoelectric substrate with the one transmission electrode and the one. Formed on the boundary between the first region sandwiched between the other receiving electrode and the second region defined by the other transmitting electrode and the other receiving electrode, and the elastic wave or the surface acoustic wave This prevents or alleviates the coupling of the propagation paths.

  That is, acoustic rough coupling or isolation is achieved between the propagation paths of the elastic waves formed between the two transmitting electrodes and the two receiving electrodes, respectively.

  In the present invention, the reflective electrode can be formed as a pattern in a desired shape, arrangement and dimensions in the same manner as the two transmitting electrodes and the two receiving electrodes, and the reflective area of such a reflective electrode is elastic. By adapting to the occupied band of the surface wave, a high S / N ratio is stably achieved as compared with the conventional example.

  Further, according to the present invention, it is possible to cancel the bulk wave component by adding or multiplying the electric signals output from the two receiving electrodes.

Furthermore, in the present invention, compared with the case where rough coupling and isolation between the propagation paths respectively formed between the two transmitting electrodes and the two receiving electrodes are not achieved, the bulk wave component is reduced. Offset is achieved with high accuracy.
Therefore, according to the present invention, the sensitivity and performance of the surface acoustic wave sensor can be improved at a low cost.

It is a figure which shows one Embodiment of this invention. It is a figure (1) which shows operation | movement of this embodiment. It is a figure (2) which shows operation | movement of this embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing an embodiment of the present invention.
In the figure, a rectangular propagation path 12 similar to one side is secured at the center of one side of the piezoelectric substrate 11 and a film of protein or the like is formed. In the vicinity of such a specific side of the propagation path 12, comb-shaped electrodes (IDT: interdigital) that have the same configuration and are electrically connected in parallel as a pattern on the piezoelectric substrate 11.
transducers 13S-a and 13S-b are formed, and comb electrodes 13R-a and 13R-b are connected in parallel in the vicinity of the other side facing the specific side, and the comb electrodes 13S-a , 13S-b, respectively. The configuration of the comb-shaped electrode 13R-a is the same as the configuration of the above-described comb-shaped electrode 13S-a, but the comb-shaped electrode 13R-b is configured symmetrically with the pattern forming the comb-shaped electrode 13R-a.

  The output of the signal source 20 is connected to one terminal of the comb-shaped electrode 13S-a, and the other terminal of the comb-shaped electrode 13S-a and one terminal of the comb-shaped electrode 13S-b are formed on the piezoelectric substrate 11. The pattern is electrically connected directly to the ground and grounded. The other terminal of the comb-shaped electrode 13S-b is also connected to the output of the signal source 20.

  One terminal of the comb-shaped electrode 13R-a is connected to the input of the circuit 21, and the other terminal of the comb-shaped electrode 13R-a and one terminal of the comb-shaped electrode 13R-b are patterns formed on the piezoelectric substrate 11. Are electrically connected directly to each other and grounded. The other terminal of the comb-shaped electrode 13R-b is also connected to the input of the circuit 21.

  In the propagation path 12, a predetermined area of a region sandwiched between the comb electrodes 13S-b and 13R-b (hereinafter referred to as “surface propagation blocking region”) has a comb electrode 13S-b and 13R-b. A reflective electrode 14 is formed that crosses in the direction in which the surface acoustic wave propagates between them and has a reflection region in the occupation band of the surface acoustic wave on the frequency axis. Such a reflective electrode 14 may be configured as a grating reflector, for example.

  The remaining area adjacent to the “surface propagation blocking area” in the area of the propagation path 12 is hereinafter referred to as a “surface propagation area”.

  Among the sides of the propagation path 12, other than the sides closest to the comb electrodes 13 </ b> S-b and 13 </ b> R-b and outside the side opposite to the above-mentioned “surface propagation region” A body 15 is arranged.

The operation of this embodiment will be described below with reference to FIG.
The signal source 20 drops an AC signal having a predetermined frequency suitable for an item to be measured with respect to a medium which is dropped or contacted with a reaction field corresponding to the center of the propagation path 12, for example, the comb-shaped electrode 13S-. a, given in parallel with 13S-b.

  The comb-shaped electrode 13S-a is excited by such an AC signal, and as indicated by a thick solid arrow in FIG. 1, a surface acoustic wave (hereinafter referred to as “surface acoustic wave a”) is generated in the propagation path 12 (surface propagation region). .) Is sent out. Some of these surface acoustic waves are caused by, for example, a dotted line in FIG. 1 due to the discontinuity of the propagation speed between the portion of the metal film forming the comb-shaped electrode 13S-a and the portion other than the metal film. As shown by the arrow of FIG. 6, the piezoelectric substrate 11 is converted into a bulk wave (hereinafter referred to as “bulk wave a”) that propagates inside rather than the surface.

  The comb-shaped electrode 13S-b is excited when the AC signal is applied in a phase opposite to that of the comb-shaped electrode 13S-a, and as indicated by a thin solid line arrow in FIG. 2 transmits a surface acoustic wave having a phase opposite to that of the aforementioned “surface acoustic wave a” (hereinafter referred to as “surface acoustic wave b”). Some of these surface acoustic waves are caused by, for example, a dotted line in FIG. 1 due to the discontinuity of the propagation speed between the portion of the metal film forming the comb-shaped electrode 13S-b and the portion other than the metal film. As shown by the arrow of FIG. 6, the piezoelectric substrate 11 is converted into a bulk wave (hereinafter referred to as “bulk wave b”) that propagates inside rather than the surface.

  The surface acoustic wave a propagates through the reaction field where the medium is dropped or contacted (and the medium) to reach the comb electrode 13R-a, and the electric signal Ea (FIG. a)).

Further, the bulk wave a propagates through the inside (inner layer) of the piezoelectric substrate 11 and reaches the comb electrode 13R-a, and is converted into an electric signal EBa (FIG. 2B) by the comb electrode 13R-a.
On the other hand, the surface acoustic wave b propagates through the reaction field (and its medium), but most of the (major) components of the surface acoustic wave b reach the comb electrode 13R-b before reaching the comb-shaped electrode 13R-b. As indicated by a broken line in FIG. 1, the light is reflected by the reflective electrode 14 in the direction of the absorber 15 and is absorbed by the absorber 15.

Further, the bulk wave b propagates through the inside (inner layer) of the piezoelectric substrate 11 and reaches the comb electrode 13R-b, and is converted into an electric signal EBb (= −EBa) by the comb electrode 13R-b.
That is, in the electric signal E inputted to the circuit 21, the components of the bulk waves a and b are canceled out in opposite phases as shown in the following equation and FIG. It is hardly included.
E = Ea + EBa + EBb
= Ea + EBa-EBa
= Ea

  Note that the S / N ratio obtained under such cancellation is, for example, 40 decibels or more as shown in FIGS.

  Further, since the bulk wave components are canceled by subtracting the instantaneous values of the bulk wave a and the bulk wave b on the time axis, these bulk waves are obtained as shown in FIGS. Regardless of the frequency spectrum of the wave and the surface acoustic wave, it can be realized accurately and stably.

  Furthermore, in this embodiment, the reflective electrode 14 is different from “resin” coated on the propagation path (reaction field) and “scratches” formed on the propagation path (reaction field) as in the conventional example. Unlike the comb-shaped electrodes 13S-a, 13S-b, 13R-a, and 13R-b, they can be generally formed as a pattern on the piezoelectric substrate 11, and the arrangement, shape, dimensions, etc. are fine. Can be set.

  Therefore, according to the present embodiment, deterioration in accuracy and performance due to the bulk wave can be stably avoided without significantly increasing the cost.

In the present embodiment, the comb-shaped electrode 13S-a and the comb-shaped electrode 13S-b are excited in opposite phases by the signal source 20.
However, the present invention is not limited to such a configuration. For example, when the comb electrodes 13S-a and 13S-b and the comb electrodes 13R-a and 13R-b are interchanged, the bulk wave a and the bulk wave b May be realized as well.

  In this embodiment, the surface propagation blocking area and the surface propagation area are formed adjacent to the propagation path 12 in parallel. The surface propagation blocking area and the surface propagation area are different from each other in the propagation characteristics. May be formed in the following form, for example.

(1) It is formed in two physically separated areas on the common piezoelectric substrate 11. (2) Although formed on the common piezoelectric substrate 11, a separator that roughly sets the coupling of the propagation path of the surface acoustic wave and the bulk wave between them is formed together.
(3) Formed individually on different piezoelectric substrates.

Furthermore, in this embodiment, the absorber 15 is provided.
However, the absorber 15 has an elastic surface reflected by the reflective electrode 14 on any one of the surface propagation blocking region, the surface propagation region, and the comb electrodes 13S-a, 13S-b, 13R-a, and 13R-b. If deterioration of accuracy and performance due to the arrival of the wave b without being absorbed is allowed, it may not be provided.

  Further, in the present embodiment, the surface acoustic wave b is reflected by the reflective electrode 14 in such a manner that the surface acoustic wave b, the bulk wave a and the bulk wave b, and the surface acoustic wave b are acoustic on the piezoelectric substrate 11. As long as accuracy and performance due to interference with the above and a decrease in the stability of these are allowed, it may be in any direction.

  Furthermore, in this embodiment, the cancellation of the bulk wave a and the bulk wave b is performed by connecting the outputs of the comb electrodes 13R-a and 13R-b, which are configured symmetrically as described above, to the circuit 21 in parallel. It is realized by.

However, such cancellation may be realized in any of the following forms, for example.
(1) When the number of terminals of the surface acoustic wave sensor according to the present embodiment is allowed to be “4”, the output is performed after the outputs of the comb electrodes 13R-a and 13R-b are individually delivered to the outside. "Addition or subtraction of these outputs"
(2) “Addition or subtraction of outputs of comb electrodes 13R-a and 13R-b” performed by a built-in operational amplification circuit or signal processing equivalent thereto

  In any of these forms (1) and (2), what is the configuration identity and complementarity of the comb electrodes 13S-a, 13S-b, 13R-a, and 13R-b? It may be.

  In the present embodiment, the connection point between the comb electrode 13S-a and the comb electrode 13S-b and the connection point between the comb electrode 13R-a and the comb electrode 13R-b are both grounded.

  However, these connection points may not be grounded if the propagation characteristics of the surface acoustic wave in the surface propagation region and the surface propagation blocking region are the same with the desired accuracy.

  Furthermore, in the present embodiment, for example, due to the type, amount, distribution, and the like of the medium (that may exist) on the reaction field, the surface acoustic wave propagation characteristics in the surface propagation region and the surface propagation inhibition region are allowed. If a deviation or change occurs to the extent that it is not performed, the addition or subtraction that realizes the above-described cancellation is realized as a product-sum operation based on weights Wa and Wb given by a circuit or device composed of the following elements: Also good.

(1) First means for identifying the type, amount, distribution, etc. of the above medium, or enabling manual designation
(2) Second means for obtaining weights Wa and Wb adapted to the type, amount, distribution, etc. of the identified or designated medium

  Such a circuit or device may not be incorporated in the surface acoustic wave sensor according to the present embodiment, and may be provided separately.

  Moreover, when said 1st means identifies the kind of medium, quantity, distribution, etc., it cooperates with what kind of sensor for the identification, or such a sensor may be provided.

  Further, the second means may calculate the weights Wa and Wb by any of arithmetic operations based on the type, amount, distribution, etc. of the medium and a table lookup according to a combination of these types, amount, distribution, etc. You may ask for it.

  Further, the present invention is not limited to the surface acoustic wave sensor as in the present embodiment. For example, the surface acoustic wave filter, the surface acoustic wave oscillator, the surface acoustic wave resonator, the surface acoustic wave waveguide, the surface acoustic wave delay device, The present invention can be similarly applied to various surface acoustic wave devices such as a surface acoustic wave correlator.

  Further, the present invention is not limited to the above-described embodiments, and various configurations of the embodiments are possible within the scope of the present invention, and any improvements may be made to all or some of the components.

  Hereinafter, among the inventions disclosed in the present application, the inventions not described in “Claims” are listed in a format according to the descriptions in the “Claims” and “Means for Solving the Problems” column. To do.

[Claim 6] In the surface acoustic wave sensor according to any one of claims 1 to 5,
The direction is
A surface acoustic wave sensor, wherein the surface acoustic wave sensor is in a direction other than a region sandwiched between the two transmitting electrodes and the two receiving electrodes on the piezoelectric substrate.

  In the surface acoustic wave sensor having such a configuration, in the surface acoustic wave sensor according to any one of claims 1 to 5, the direction is the two transmitting electrodes and the two on the piezoelectric substrate. The direction is other than the region sandwiched between the two receiving electrodes.

  That is, even if the surface acoustic wave reflected by the reflecting electrode is reflected, it reaches the region sandwiched between the two transmitting electrodes and the two receiving electrodes directly via some path.

  Therefore, a decrease in the S / N ratio and sensitivity due to the presence of the region in the direction in which the surface acoustic wave is reflected by the reflective electrode is avoided or alleviated.

[Claim 7] In the surface acoustic wave sensor according to any one of claims 1 to 5,
An absorber that is disposed in a region sandwiched between the two transmitting electrodes and the two receiving electrodes on the piezoelectric substrate and that absorbs the surface acoustic wave;
The direction is
A surface acoustic wave sensor, wherein the absorber is in a direction in which the absorber is located.

  In the surface acoustic wave sensor having such a configuration, in the surface acoustic wave sensor according to any one of claims 1 to 5, an absorber is formed on the piezoelectric substrate on the piezoelectric substrate. It is disposed in a region sandwiched between the wave electrode and the two receiving electrodes, and absorbs the surface acoustic wave. The direction is a direction in which the absorber is located.

That is, the surface acoustic wave reflected by the reflective electrode is absorbed by the absorber.
Therefore, compared with the case where the surface acoustic wave reflected by the reflective electrode is not actively absorbed, the sensitivity and the SN ratio are increased and maintained stably.

[Claim 8] In the surface acoustic wave sensor according to any one of claims 1 to 7,
The reflective electrode is
A surface acoustic wave sensor comprising a reflection region in an occupation band of the elastic wave on a frequency axis.
In the surface acoustic wave sensor having such a configuration, in the surface acoustic wave sensor according to any one of claims 1 to 7, the reflective electrode has a reflection region in a band occupied by the acoustic wave on a frequency axis. Have

  That is, the surface acoustic wave is efficiently reflected by the reflective electrode between the other transmitting electrode and the other receiving electrode.

  Therefore, the higher the efficiency with which the surface acoustic wave is reflected by the reflective electrode, the higher the sensitivity and SN ratio, and the more stable it is.

11 Piezoelectric substrate 12 Propagation path 13S, 13R Comb electrode 14 Reflective electrode 15 Absorber 20 Signal source 21 Circuit

Claims (5)

A surface acoustic wave sensor formed on a piezoelectric substrate for converting an electric signal into an elastic wave, and converting an elastic wave propagated through the piezoelectric substrate into an electric signal,
Two transmitting electrodes disposed on the piezoelectric substrate and converting the electrical signal into the elastic wave;
Two receiving electrodes that are individually formed on the piezoelectric substrate so as to oppose the two transmitting electrodes and that convert individual elastic waves that have arrived through the piezoelectric substrate into electrical signals;
Formed in a region on the piezoelectric substrate sandwiched between one of the two transmitting electrodes and one receiving electrode formed opposite to the one transmitting electrode; A surface acoustic wave sensor comprising: a reflection electrode that reflects a wave in a direction other than the two reception electrodes. The surface acoustic wave sensor according to claim 1,
The two receiving electrodes are:
A surface acoustic wave sensor, wherein the acoustic waves that individually arrive via the piezoelectric substrate are converted into electrical signals having opposite phases. The surface acoustic wave sensor according to claim 1,
The two transmitting electrodes are:
A surface acoustic wave sensor, wherein the electrical signal is converted into elastic waves having opposite phases to each other. The surface acoustic wave sensor according to any one of claims 1 to 3,
The two receiving electrodes are:
A surface acoustic wave sensor characterized by generating a sum of electrical signals obtained by converting acoustic waves arriving at the two receiving electrodes via the piezoelectric substrate. The surface acoustic wave sensor according to any one of claims 1 to 4,
On the piezoelectric substrate, a first region sandwiched between the one transmitting electrode and the one receiving electrode, and a second region sandwiched between the other transmitting electrode and the other receiving electrode. A surface acoustic wave sensor, comprising: a propagation separator that prevents or relaxes coupling of propagation paths of the acoustic wave or the surface acoustic wave.