JP2018093329A - Elastic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable further suppression of the fluctuation in frequency band owing to the change in temperature in a small-size elastic wave element.SOLUTION: An elastic wave element comprises: a support substrate; a propagation substrate made of piezoelectric monocrystalline; and an electrode provided on an upper face of the propagation substrate. The upper face of the propagation substrate has an area of 20 mmor less. The piezoelectric monocrystalline includes a lithium niobate, lithium tantalate or a lithium niobate-lithium tantalate solid solution. The linear thermal expansion coefficient of the material of the support substrate in an elastic wave propagation direction is a negative value.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an acoustic wave device.

  Surface acoustic wave devices that can function as filter elements and oscillators used in cellular phones, etc., and acoustic wave devices such as Lamb wave elements using piezoelectric thin films and thin film resonators (FBARs) Are known. As such an acoustic wave device, a device in which a supporting substrate and a piezoelectric substrate that propagates a surface acoustic wave are bonded together and a comb-shaped electrode capable of exciting the surface acoustic wave is provided on the surface of the piezoelectric substrate is known.

  However, the surface acoustic wave element has a problem that the pass band moves due to a temperature change. In particular, lithium niobate and lithium tantalate, which are widely used at present, have a large electromechanical coupling coefficient, and are advantageous for realizing broadband filter characteristics. However, lithium niobate and lithium tantalate are inferior in temperature stability (Patent Document 1).

  For example, the temperature coefficient of frequency change of lithium tantalate is −35 ppm / K, and the frequency variation is large in the temperature range of −30 to + 85 ° C. For this reason, it is necessary to reduce the temperature coefficient of the frequency change.

  Patent Document 2 describes a device in which a SAW propagation substrate and a support substrate are bonded by an organic thin film layer. The propagation substrate is a lithium tantalate substrate having a thickness of 30 μm, for example, and is bonded to a glass substrate with an organic adhesive having a thickness of 1 μm or less.

  A so-called FAB (Fast Atom Beam) type direct bonding method is known (Patent Document 3). In this method, a neutralized atom beam is irradiated to each bonding surface at normal temperature to activate and bond directly.

JP2008-301066 Patent 4956569 JP2014-086400

  However, recently, the requirement for the stability of the frequency band has become stricter. On the other hand, surface acoustic wave devices have environmental changes, especially temperature changes during transportation and movement, so it is necessary to prevent damage such as peeling and cracks from occurring even if sudden temperature changes occur. is there.

  An object of the present invention is to provide a support substrate, a propagation substrate made of a piezoelectric single crystal, and an acoustic wave device including an electrode provided on the upper surface of the propagation substrate, and further suppress fluctuations in the frequency band due to temperature changes, It is to prevent the device from peeling or cracking even after applying the thermal shock cycle.

The present invention
Support substrate,
An acoustic wave device comprising a propagation substrate made of a piezoelectric single crystal, and an electrode provided on the upper surface of the propagation substrate,
The area of the upper surface of the propagation substrate is 20 mm ² or less,
The piezoelectric single crystal is made of lithium niobate, lithium tantalate or lithium niobate-lithium tantalate solid solution, and the linear thermal expansion coefficient of the material of the support substrate in the propagation direction of the elastic wave is a negative numerical value. It is characterized by.

According to the present invention, a propagation substrate made of a piezoelectric single crystal is joined to a propagation substrate made of a material having a negative linear thermal expansion coefficient of the material of the support substrate in the propagation direction. By making the area of the upper surface of 20 mm ² or less, fluctuations in the frequency band of the element with respect to temperature changes can be significantly reduced, and peeling and cracking of the element after applying a thermal shock cycle can be suppressed.

(A) shows the state where the main surface of the support substrate 1 is activated by the neutralizing beam A, and (b) shows the state where the main surface of the piezoelectric single crystal substrate 6 is activated by the neutralizing beam A. Indicates. (A) shows the joined body 3 in which the support substrate 1 and the piezoelectric single crystal substrate 6 are directly joined, (b) shows a state in which the piezoelectric single crystal substrate is thinned by processing, and (c) shows 1 shows an acoustic wave element 5 in which an electrode 10 is provided on a propagation substrate 6A.

Hereinafter, the present invention will be described in detail with appropriate reference to the drawings.
1 and 2 relate to an embodiment in which a support substrate is directly bonded to the surface of a piezoelectric single crystal substrate.

  As shown in FIG. 1A, the surface of the support substrate is irradiated with a neutralization beam as indicated by an arrow A to activate the surface to be an activated surface 1a. On the other hand, as shown in FIG. 1B, the surface of the piezoelectric single crystal substrate 6 is activated by irradiating the surface with a neutralized beam, thereby forming an activated surface 6a. Then, as shown in FIG. 2A, the joined body 3 is obtained by directly joining the activation surface 6 a of the piezoelectric single crystal substrate 6 and the activation surface 1 a of the support substrate 1.

In a preferred embodiment, the surface 6b of the piezoelectric single crystal substrate of the bonded body 3 is further polished to reduce the thickness of the piezoelectric single crystal substrate as shown in FIG. obtain. 6c is a polished surface.
In FIG. 2C, the acoustic wave element 5 is produced by forming a predetermined electrode 10 on the polishing surface 6c of the propagation substrate 6A.

  Known acoustic wave elements include surface acoustic wave devices, Lamb wave elements, thin film resonators (FBARs), and the like. For example, a surface acoustic wave device has an IDT (Interdigital Transducer) electrode (also referred to as a comb-shaped electrode or a comb-shaped electrode) for exciting surface acoustic waves on the surface of a piezoelectric material substrate and an output side for receiving surface acoustic waves. IDT electrodes are provided. When a high frequency signal is applied to the IDT electrode on the input side, an electric field is generated between the electrodes, and a surface acoustic wave is excited and propagates on the piezoelectric substrate. Then, the propagated surface acoustic wave can be taken out as an electric signal from the IDT electrode on the output side provided in the propagation direction.

Here, the area of the upper surface 6c of the propagation substrate 6A is 20 mm ² or less. In the case of such a small acoustic wave element, the fluctuation of the frequency band with respect to the temperature change is particularly large. From this viewpoint, the area of the upper surface of the propagation substrate is more preferably 6 mm ² or less. The lower limit of the area of the upper surface 1b of the propagation substrate is not particularly but can be a 0.01 mm ² or more, and particularly preferably 3 mm ² or more.

  The piezoelectric single crystal constituting the propagation substrate is made of lithium niobate, lithium tantalate, or lithium niobate-lithium tantalate solid solution. Since these materials have a high propagation speed of surface acoustic waves and a large electromechanical coupling coefficient, they are suitable as acoustic wave devices for high frequencies and wideband frequencies. Further, the normal direction of the upper surface of the propagation substrate is not particularly limited. For example, when the propagation substrate is made of lithium tantalate, 36 to 36 from the Y axis to the Z axis centering on the X axis that is the propagation direction of the surface acoustic wave. It is preferable to use the one rotated by 47 ° (for example, 42 °) because the propagation loss is small. When the propagation substrate is made of lithium niobate, it is a propagation loss to use the one rotated by 60 to 68 ° (for example, 64 °) from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave. Is preferable because it is small.

  A metal film may be provided on the bonding surface 6a of the propagation substrate. The metal film plays a role of increasing the electromechanical coupling coefficient in the vicinity of the back surface of the piezoelectric substrate when a Lamb wave element is manufactured as an elastic wave device. In this case, the Lamb wave element has a structure in which comb electrodes are formed on the upper surface 6c of the propagation substrate, and the metal film of the piezoelectric substrate is exposed by the cavity provided in the support substrate. Examples of the material of such a metal film include aluminum, an aluminum alloy, copper, and gold. When a Lamb wave device is manufactured, a joined body including a propagation substrate that does not have a metal film on the bottom surface may be used.

  Moreover, you may have a metal film and an insulating film in the joint surface 6a of a propagation board | substrate. The metal film serves as an electrode when a thin film resonator is manufactured as an acoustic wave device. In this case, the thin film resonator has a structure in which electrodes are formed on both the upper surface and the bonding surface of the piezoelectric substrate, and the metal film of the piezoelectric substrate is exposed by using the insulating film as a cavity. Examples of the material for such a metal film include molybdenum, ruthenium, tungsten, chromium, and aluminum. Examples of the material for the insulating film include silicon dioxide, phosphorous silica glass, and boron phosphorous silica glass.

  In the present invention, the linear thermal expansion coefficient of the material of the support substrate in the elastic wave propagation direction B is a negative numerical value. Here, the main line thermal expansion coefficient is measured by a linear expansion coefficient measuring apparatus. Specifically, a sample having a diameter of 3.5 mm and a length of 15 mm and a standard sample of quartz glass are prepared and measured from the difference in thermal expansion when the temperature is increased at a constant speed.

  Examples of the material of the support substrate include low expansion crystallized glass and ceramic material. Specific examples of the ceramic material include trademark CERSAT manufactured by Nippon Electric Glass Co., Ltd.

  Also, depending on the combination of the piezoelectric substrate and the support substrate to be used, when lithium tantalate is used for the piezoelectric substrate and CERSAT is used for the support substrate, the thickness (TP) of the propagation substrate and the thickness of the support substrate The linear thermal expansion of the entire element in the propagation direction of the elastic wave by setting the ratio (TP / TS) to the thickness (TS) of 0.02 to 0.3 (preferably 0.1 to 0.2) The coefficient can be made closer to zero.

  The support substrate and the propagation substrate are preferably directly bonded, but there are combinations that are difficult to directly bond, and there may be a bonding layer between the two. Such a bonding layer is preferably a resin, more preferably a thermosetting resin or an ultraviolet curable resin, and particularly preferably an acrylic resin or an epoxy resin.

  The thickness of the bonding layer is preferably 1.0 μm or less, and more preferably 0.1 μm or less, from the viewpoint of reducing the influence of changes in linear expansion due to resin or the like. In addition to the resin, for example, when alumina, tantalum pentoxide, or the like is formed as a bonding layer on the support substrate side, bonding through the bonding layer may be possible without using a resin. The thinner the alumina or tantalum pentoxide film used as the bonding layer, the smaller the contribution to the linear expansion change, preferably 1.0 μm or less, and more preferably 0.1 μm or less.

In a preferred bonding method, the surfaces of the piezoelectric single crystal substrate and the support substrate are flattened to obtain a flat surface. Here, methods for flattening the surface include lap polishing and chemical mechanical polishing (CMP). The flat surface is preferably Ra ≦ 1 nm, but more preferably 0.3 nm or less.
However, depending on the substrate to be handled, smoothing may be difficult. In this case, bonding using a resin may be performed, or alumina or tantalum pentoxide may be formed to smooth the film. When the alumina and tantalum pentoxide films are smoothed, an extra film thickness of 0.2 μm to 0.5 μm, which is necessary for smoothing, is formed in advance.

  Next, in a preferred embodiment, the flat surface is activated by irradiating the surface of the piezoelectric single crystal substrate and the surface of the support substrate with a neutral beam.

When performing surface activation with a neutralized beam, it is preferable to generate and irradiate the neutralized beam using an apparatus as described in Patent Document 3. That is, a saddle field type fast atomic beam source is used as the beam source. Then, an inert gas is introduced into the chamber, and a high voltage is applied to the electrodes from a DC power source. As a result, the saddle field type electric field generated between the electrode (positive electrode) and the casing (negative electrode) moves the electrons e, thereby generating atomic and ion beams by the inert gas. Of the beams that reach the grid, the ion beam is neutralized by the grid, so that a beam of neutral atoms is emitted from the fast atom beam source. The atomic species constituting the beam is preferably an inert gas (argon, nitrogen, etc.).
The voltage upon activation by beam irradiation is preferably 0.5 to 2.0 kV, and the current is preferably 50 to 200 mA.

  Next, the activated surfaces are brought into contact with each other and bonded in a vacuum atmosphere. The temperature at this time is room temperature, but specifically, it is preferably 40 ° C. or lower, more preferably 30 ° C. or lower. The temperature at the time of joining is particularly preferably 20 ° C. or higher and 25 ° C. or lower. The pressure at the time of joining is preferably 100 to 20000N.

(Experiment 1)
An acoustic wave device was manufactured according to the method described with reference to FIGS.
Specifically, a lithium tantalate substrate (LT substrate) having an orientation flat portion (OF portion), a diameter of 4 inches, and a thickness of 250 μm was used as the piezoelectric single crystal substrate 6. However, a 46 ° Y-cut X-propagating lithium tantalate substrate in which the propagation direction of the surface acoustic wave (SAW) is X and the cutting angle is a rotating Y-cut plate was used. The surface 6a of the piezoelectric single crystal substrate 6 was mirror-polished so that the arithmetic average roughness Ra was 0.2 nm.

  Further, as the support substrate 1, a support substrate 1 having a diameter of 30 mm and a thickness of 230 μm was prepared. The material of the support substrate is a tantalum pentoxide film having a thickness of 0.4 μm formed on a ceramic substrate having a negative linear expansion coefficient. The arithmetic average roughness Ra of the surface of the support substrate is 1 nm. Arithmetic mean roughness was evaluated with an atomic force microscope (AFM) in a square field of 10 μm length × 10 μm width.

Here, the linear thermal expansion coefficient of the material of the support substrate in the propagation direction B of the elastic wave is −8.4 ppm / K, and the linear thermal expansion coefficient of the material of the propagation substrate is 16.1 ppm / K.
The ceramic substrate used as the support substrate 1 is difficult to smooth the surface, and after forming 0.4 μm of tantalum pentoxide film, the surface was smoothed to reduce Ra.

Next, the surface of the support substrate and the surface of the piezoelectric single crystal substrate were washed, removed, and introduced into a vacuum chamber. After evacuating to the 10 ⁻⁶ Pa level, the surface of each substrate was irradiated with a high-speed atomic beam (acceleration voltage 1 kV, Ar flow rate 27 sccm) for 120 seconds. Next, the beam irradiation surface (activation surface) 1a of the support substrate 1 and the activation surface 6a of the piezoelectric single crystal substrate 6 were brought into contact with each other, and then pressed at 10000 N for 2 minutes to bond both substrates.

Next, the surface 6b of the piezoelectric single crystal substrate 6 was ground and polished so that the thickness was changed from the initial 250 μm to 75 μm (see FIG. 2B). During the grinding and polishing process, no peeling of the joint could be confirmed. Moreover, it was 1.4 J / m < ² > when the joining strength was evaluated by the crack opening method. Here, the area of the upper surface of the propagation substrate is 900 mm ² . As a result of measuring the linear thermal expansion coefficient in the propagation direction of the entire joined body in this state, it was found to be +3.5 ppm / K, approaching zero.

  Next, the propagation substrate was further polished to a thickness of 40 μm. As a result of measuring the linear thermal expansion coefficient in the entire propagation direction of the obtained joined body, it was +0.2 ppm / K. As described above, the linear thermal expansion coefficient of the entire element can be reduced to almost zero.

  Next, a thermal shock cycle test was performed in order to examine the environmental resistance of the joined body thinned to the thickness of 40 μm. As test conditions for the thermal shock cycle, the low temperature side was −40 ° C., the high temperature side was + 85 ° C., and the temperature could be switched within 2 seconds. The storage temperature of each temperature was 30 minutes, and the test was repeated 1000 times. .

  After the test, as a result of observing the bonded portion of the bonded body, peeling was observed at the bonding interface, and cracks that progressed inward from the substrate edge were also observed. In particular, it is presumed that peeling or cracking occurred due to the large amount of strain at the edge of the substrate due to the difference in linear expansion.

Next, the propagation substrate was cut into small pieces by dicing, and the thermal shock cycle test was performed. As a result, it was found that separation at the joint portion does not occur when the area of the upper surface of the propagation substrate is 20 mm ² or less.

Experiment 1 shows that if the linear expansion coefficient approaches zero due to the bonded body and the substrate size is not less than 20 mm ² , peeling occurs at the bonded portion. It was designed to be 20 mm ² . Specifically, taking into account the width (0.1 mm) of the dicing grindstone used for cutting, the propagation direction is set to 5.1 mm pitch, and the vertical direction is set to 4.1 mm pitch. Designed as follows.

  The IDT electrode for generating the surface acoustic wave was formed through a photolithography process. After forming the electrode, it was cut into small pieces by dicing, and an element having a propagation direction of 5 mm and a vertical direction of 4 mm was obtained. In addition, a reference substrate of the same size for measuring the linear expansion coefficient was prepared without forming the IDT electrode.

First, as a result of evaluating the linear expansion coefficient with the reference substrate, the linear expansion coefficient was +0.03 ppm / K. Although it increased slightly before cutting, it was confirmed that the linear expansion coefficient was close to zero.
When the temperature characteristic of the frequency was measured in the range of 25 to 80 ° C. with the element formed with the IDT electrode, it was −5 ppm / K, and the temperature change was smaller than that of the piezoelectric element formed only on the piezoelectric single crystal substrate. I was able to confirm.
Next, in the above experiment, elements having an area of the upper surface of the propagation substrate of 10 mm ² and 3.0 mm ² were designed. Other elements were obtained in the same manner as described above. As a result, the linear expansion coefficients were +0.03 ppm / K and +0.04 ppm / K, respectively, and the temperature characteristics of the frequency in the range of 25 to 80 ° C. were −6 ppm / K and −0.07 ppm / K, respectively.

(Experiment 2)
A surface acoustic wave device was produced in the same manner as in Experiment 1.
However, a lithium niobate substrate (LN substrate) having an orientation flat portion (OF portion), a diameter of 4 inches, and a thickness of 250 μm was used as the piezoelectric single crystal substrate 6. However, a 46 ° Y-cut X-propagating lithium niobate substrate in which the propagation direction of the surface acoustic wave (SAW) is X and the cutting angle is a rotating Y-cut plate was used. The surface 6a of the propagation substrate 6 was mirror-polished so that the arithmetic average roughness Ra was 0.2 nm.
As the support substrate 1, the same one as in Experiment 1 was used.

  Here, the linear thermal expansion coefficient of the material of the support substrate in the propagation direction B of the elastic wave is −8.4 ppm / K, and the linear thermal expansion coefficient of the material of the propagation substrate is 15.1 ppm / K.

  Next, the surface of the support substrate and the surface of the piezoelectric single crystal substrate were bonded in the same manner as in Experiment 1 to obtain a bonded body.

Next, the surface 6b of the piezoelectric single crystal substrate 6 was ground and polished so that the thickness was changed from the initial 250 μm to 75 μm (see FIG. 2B). During the grinding and polishing process, no peeling of the joint could be confirmed. Moreover, it was 1.4 J / m < ² > when the joining strength was evaluated by the crack opening method.
As a result of measuring the linear thermal expansion coefficient in the propagation direction of the entire joined body in this state, it was found to be +2.5 ppm / K, approaching zero.

  Next, the propagation substrate was further polished to a thickness of 50 μm. As a result of measuring the linear thermal expansion coefficient in the entire propagation direction of the obtained joined body, it was +0.1 ppm / K. As described above, the linear thermal expansion coefficient of the entire element can be reduced to almost zero.

  Subsequently, the same thermal shock cycle test as that in Experiment 1 was performed in order to examine the environmental resistance of the joined body thinned to the above-described thickness of 50 μm. After the test, as a result of observing the bonded portion of the bonded body, peeling was observed at the bonding interface, and cracks that progressed inward from the substrate edge were also observed. In particular, it is presumed that peeling or cracking occurred due to the large amount of strain at the edge of the substrate due to the difference in linear expansion.

Next, the propagation substrate was cut into small pieces by dicing, and the thermal shock cycle test was performed. As a result, it was found that separation at the joint portion does not occur when the area of the upper surface of the propagation substrate is 20 mm ² or less.

Experiment 2 shows that even with a lithium niobate substrate, if the substrate size is not less than 20 mm ² , peeling occurs at the joint, so that the area of the upper surface of the propagation substrate is 20 mm ² as in Experiment 1. Designed. The device size was the same as in Experiment 1. Similarly, a reference substrate on which no IDT electrode is formed was also prepared.

As a result of measuring the linear expansion coefficient with the reference substrate, the linear expansion coefficient was +0.04 ppm / K. Although it increased slightly before cutting, it was confirmed that the linear expansion coefficient was close to zero.
A surface acoustic wave electrode was provided on the joined body to produce an element, and the temperature characteristic of the frequency was measured in the range of 25 to 80 ° C., and it was −15 ppm / K.
Next, in the above experiment, elements having an area of the upper surface of the propagation substrate of 10 mm ² and 3.0 mm ² were designed. Other elements were obtained in the same manner as described above. As a result, the linear expansion coefficients were +0.05 ppm / K and +0.06 ppm / K, respectively, and the temperature characteristics of the frequency in the range of 25 to 80 ° C. were −16 ppm / K and −18 ppm / K, respectively.

(Experiment 3)
A joined body was produced in the same manner as in Experiment 1.
However, the material of the support substrate was silicon (Si). The linear thermal expansion coefficient in the propagation direction B of silicon is 2.35 ppm / K. Silicon with high flatness can be polished, and an arithmetic average roughness Ra of 0.2 nm was prepared and bonded to the propagation substrate without forming tantalum pentoxide.

  Next, the propagation substrate was further polished to a thickness of 75 μm. As a result of measuring the linear thermal expansion coefficient in the entire propagation direction of the obtained joined body, it was +7.5 ppm / K.

Thereafter, the propagation substrate was further polished to a thickness of 0.001 μm. However, as a result of measuring the linear thermal expansion coefficient in the entire propagation direction of the obtained joined body, it was +3.5 ppm / K.
A surface acoustic wave electrode was provided on the joined body to manufacture an element, and the temperature characteristic of the frequency was measured in the range of 25 to 80 ° C. and found to be −30 ppm / K.

Claims (3)

Support substrate,
An acoustic wave device comprising a propagation substrate made of a piezoelectric single crystal, and an electrode provided on the upper surface of the propagation substrate,
An area of the upper surface of the propagation substrate is 20 mm ² or less;
The piezoelectric single crystal is made of lithium niobate, lithium tantalate or lithium niobate-lithium tantalate solid solution, and the linear thermal expansion coefficient of the material of the support substrate in the propagation direction of the elastic wave is a negative numerical value. An elastic wave device characterized by the above.
The element according to claim 1, wherein the material of the support substrate is ceramics.
The element according to claim 1, wherein the support substrate and the propagation substrate are directly bonded to each other.

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