JP2020145530A - Elastic wave device - Google Patents

Abstract

To provide an elastic wave device that is excellent in electrical characteristics.SOLUTION: An elastic wave device 1 comprises: an IDT electrode 9 that includes a plurality of electrode fingers; a piezoelectric film 7 that includes a first surface 7a and a second surface 7b, has the IDT electrode 9 located on the first surface 7a, has a thickness less than a wavelength λ defined by twice the repeating interval of the plurality of electrode fingers, and is formed of piezoelectric crystal containing a dopant Dp with a density of 10 ppm or more and 10000 ppm or less; and an insulating unit 5 that is formed of an insulating material directly or indirectly joined to the second surface 7b of the piezoelectric film 7.SELECTED DRAWING: Figure 1

Description

The present invention relates to an elastic wave device.

Conventionally, a low-sound velocity film, which is laminated on a piezoelectric film and a propagating bulk wave whose sound velocity is lower than the sound velocity of a bulk wave propagating through the piezoelectric film, and a low-sound velocity film on the opposite side of the piezoelectric film. An elastic wave device including a high sound velocity substrate which is laminated on a surface and whose sound velocity of the propagating bulk wave is higher than the sound velocity of the bulk wave propagating through the piezoelectric film has been proposed. (See Patent Document 1). According to such an elastic wave device, the Q value can be increased.

Japanese Unexamined Patent Publication No. 2015-73331

Further, an elastic wave device having excellent electrical characteristics is required.

The present disclosure has been made in view of such a situation, and an object of the present disclosure is to provide an elastic wave device having excellent electrical characteristics.

The surface acoustic wave device of the present disclosure includes an IDT electrode having a plurality of electrode fingers, a first surface and a second surface, the IDT electrode is located on the first surface, and the plurality of electrode fingers are repeated. A piezoelectric film having a thickness less than the surface acoustic wave wavelength λ defined by twice the interval and composed of a piezoelectric crystal containing a dopant having a concentration of 10 ppm or more and 10000 ppm or less, and directly or indirectly on the second surface of the piezoelectric film. It is provided with an insulating portion made of an insulating material to be joined to.

According to the above configuration, it is possible to provide an elastic wave element having excellent electrical characteristics.

It is a schematic cross-sectional view of the elastic wave device which concerns on this disclosure. It is a top view of the elastic wave device of FIG. It is a diagram which shows the frequency characteristic of the elastic wave apparatus. FIG. 4A is a diagram showing an AFM analysis result, and FIG. 4B is a diagram showing an analysis result by a piezoelectric response microscope. 5 (a) and 5 (b) are cross-sectional views showing modified examples of the elastic wave device according to FIG. 1, respectively. It is sectional drawing which shows the modification of the elastic wave apparatus which concerns on FIG.

Hereinafter, the present invention will be clarified by explaining specific embodiments of the present disclosure with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of an elastic wave device (hereinafter referred to as a SAW device) 1 according to the embodiment of the present disclosure.

The SAW device 1 includes a support substrate 3, an insulating portion 5, a piezoelectric film 7, and an IDT electrode 9. The support substrate 3, the insulating portion 5, and the piezoelectric film 7 are laminated in this order.

In this example, the support substrate 3 supports the insulating portion 5 and the piezoelectric film 7 laminated on the support substrate 3, and is not particularly limited as long as it has a certain strength. For example, when the material is made of a material having a coefficient of linear expansion smaller than that of the piezoelectric film 7, it is possible to reduce the characteristic change due to the temperature change by reducing the deformation of the piezoelectric film 7 due to the temperature change. Further, when the material is selected so that the transverse wave sound velocity of the elastic wave propagating in the support substrate 3 is faster than the transverse wave sound velocity of the elastic wave propagating in the piezoelectric film 7, the elastic wave is confined in the piezoelectric film 7. It is possible to provide the SAW device 1 having excellent frequency characteristics.

Examples of such a material include a sapphire substrate and a Si substrate. In this embodiment, a case where a Si substrate is used as the support substrate 3 will be described as an example.

The material of the support substrate 3 may be selected so as to reduce the warp of the laminate of the support substrate 3, the insulating portion 5, and the piezoelectric film 7. For example, when the coefficient of linear expansion of the insulating portion 5 is smaller than the coefficient of linear expansion of the piezoelectric film 7, the support substrate 3 may be made of a material having a coefficient of linear expansion larger than that of the insulating portion 5.

The insulating portion 5 is made of an insulating material such as silicon oxide, silicon nitrogen nitrogen, or aluminum oxide, and its crystallinity is not particularly limited. By providing the insulating portion 5, it is possible to reduce the formation of unnecessary potentials and unnecessary capacitances, so that the electrical characteristics of the SAW device 1 can be improved. In this example, silicon oxide is used as the insulating portion 5.

In particular, when a Si substrate, which is a semiconductor material, is used as the support substrate 3 as in this example, the influence of the support substrate 3 is reduced by providing an insulating portion 5 between the piezoelectric film 7 and the support substrate 3. Can be done. In order to ensure the insulating property and to utilize the characteristics of the high sound velocity material of the support substrate 3, the thickness of the insulating portion 3 is 0.01p or more and 2p with respect to the pitch p defined by the IDT electrode 9 described later. It may be as follows. In particular, when the value is 0.1p to 0.4p, the influence of the conductivity of the support substrate 3 (Si in this case) can be avoided. Further, when silicon oxide is used as the insulating portion 5, the transverse wave sound velocity of the elastic wave propagating in the insulating portion 5 is slower than the transverse wave sound velocity of the elastic wave propagating in the piezoelectric film 7. In this case, when the piezoelectric film 7 becomes thinner than the specified value and the resonance frequency of the SAW device 1 becomes high, the distribution amount of elastic waves in the insulating portion 5 increases and works to lower the frequency, so that the piezoelectric film 7 is specified. When it becomes thicker and the resonance frequency of the SAW device 1 becomes lower, the distribution amount of elastic waves in the insulating portion 5 decreases, and it works to increase the frequency. Therefore, even if the thickness of the piezoelectric film 7 fluctuates in the SAW device 1, the change in frequency characteristics can be reduced, so that the robustness can be improved. Furthermore, the effect of improving the temperature characteristics can be expected.

The piezoelectric film 7 includes a first surface 7a and a second surface 7b facing the first surface 7a. For convenience, the direction from the second surface 7b to the first surface 7a may be referred to as upward. The insulating portion 5 is joined to the second surface 7b of the piezoelectric film 7.

The piezoelectric film 7 is, for example, a single crystal substrate having piezoelectricity made of lithium tantalate (LiTaO ₃ ; hereinafter referred to as LT) crystal or a single having piezoelectricity made of lithium niobate (LiNbO ₃ ; hereinafter referred to as LN) crystal. A crystalline substrate can be used.

The piezoelectric film 7 has a thickness of 2 p or less, that is, an elastic wave wavelength λ or less. By setting the thickness to be equal to or less than the wavelength λ of the elastic wave, the elastic wave can be confined in the piezoelectric film 7, and the Q value of the SAW device 1 can be increased. Further, the resonance frequency can be increased by selecting the Euler angles of the piezoelectric film 7.

Here, the piezoelectric film 7 contains a dopant Dp. That is, when an LT crystal is used, it contains an impurity element in addition to Li, Ta, and O. The dopant Dp is composed of, for example, a metal element, and its concentration is 10 ppm or more and 10000 ppm or less.

The IDT electrode 9 is located on the first surface 7a of the piezoelectric film 7. The IDT electrode 9 is formed by using a conductive material, and in this example, it is formed of an Al—Cu alloy in which Cu is added to Al. Various conductive materials such as Al, Cu, Pt, Mo, and Au can be used for the IDT electrode 9, and a plurality of layers thereof may be laminated. Also. When it is composed of a laminated body of a plurality of layers, a base layer may be interposed at the laminated interface.

FIG. 2 shows the shape of the IDT electrode 9. FIG. 2 is a top view of the SAW device 1. As shown in FIG. 2, in the IDT electrode 9, a plurality of two bus bars 91 and a plurality of elongated electrode fingers 92 connected to any of the bus bars 91 are arranged in one direction. The electrode fingers 92 connected to one bus bar 91 and the electrode fingers 92 connected to the other bus bar 91 are alternately arranged. Further, a dummy electrode 93 facing the tip of the electrode finger 92 connected to one bus bar 91 and connected to the other bus bar 91 is provided. In the figure, one is shaded in order to distinguish between the configuration connected to one bus bar 91 and the configuration connected to the other bus bar 91.

When a high-frequency signal is applied to such an IDT electrode 9, a standing wave having a half-wavelength between the centers of the electrode fingers 92 is excited.

Reflectors 11 are located on both sides of the IDT electrode 9 in the arrangement direction of the electrode fingers 92. As a result, the IDT electrode 9 and the reflector 11 function as a 1-port type resonator. The SAW element 1 of the present disclosure may include such an IDT electrode 9, and the number, arrangement, and the like thereof are not particularly limited. For example, a ladder type filter including a plurality of such resonators, a vertically coupled type filter, and the like can be configured.

According to the SAW device 1 of the present disclosure, by providing the above-mentioned configuration, the electrical characteristics are excellent. The mechanics will be described in detail below.

The inventor has discovered a phenomenon in which the frequency characteristics deteriorate as the thickness of the piezoelectric film 7 is reduced. Specifically, it was discovered that ripples may occur in the vicinity of the resonance frequency and the antiresonance frequency.

FIG. 3 shows the frequency characteristics in the vicinity of the pass band of the SAW device 1 when the filter is configured by the IDT electrode 9. The horizontal axis is the frequency, and the vertical axis is the transmission characteristic. It can be seen that the line L1 has a ripple as shown by an arrow in the figure in the pass band as compared with the line L2, and the electrical characteristics are deteriorated.

Generally, in order to improve the pyroelectricity of the piezoelectric film 7, it is known to use an LT substrate or an LN substrate which has been subjected to a reduction treatment in advance as the piezoelectric film. By using the piezoelectric film subjected to such a reduction treatment, ripple did not occur when the piezoelectric film was thick. Further, even if the piezoelectric film becomes thin, ripple does not occur in a configuration in which the piezoelectric film and the Si substrate are directly bonded, for example. On the other hand, the inventor has found that when the piezoelectric film is thin and the insulating material is located directly under the piezoelectric film, ripple may occur even if a reduction-treated piezoelectric film is used. It was.

As a result of diligent studies by the inventor on this phenomenon, it has been found that the ripple of the line L1 is caused by the polarization reversal that occurs in a part of the piezoelectric film 7 due to heat. Here, "heat" includes heat history in manufacturing the SAW device 1 added after the formation of the IDT electrode 7 and heat generation due to application of a high frequency signal.

Since the piezoelectric film 7 is thin, oxidation proceeds even if the reduction treatment is performed, and the pyroelectric effect is likely to be exhibited. In addition, the surface potential is further unstable due to the contact with the insulating material. Therefore, the pyroelectric effect is likely to appear. Under such circumstances, it is presumed that the pyroelectric effect is generated by heat, a voltage is generated at the IDT electrode 9, and polarization reversal occurs.

In order to suppress this, in the present disclosure, the piezoelectric film 7 contains a dopant Dp. As a result, the conductivity can be improved and the pyroelectricity can be improved. Therefore, the dopant Dp is preferably a metal element. However, since the mixture of the dopant Dp also leads to the improvement of the conductivity of the piezoelectric film 7, it is necessary to make it 10,000 ppm or less in order to function as the SAW device 1. Similarly, it is necessary to include 10 ppm or more in order to exhibit the effect of the dopant Dp. If the amount of dopant Dp is too small, the effect of making it difficult to invert the crystal domain described below becomes small, and if the amount of dopant Dp is too large, crystal growth becomes difficult or the crystal becomes brittle. Therefore, the range of dopant Dp is 100 ppm. It may be ~ 500 ppm.

In addition, the dopant Dp not only improves the conductivity, but also has the following effects. That is, by including the dopant Dp, distortion and defects are generated in the crystal lattice of the piezoelectric crystal constituting the piezoelectric film 7, so that the crystal domain is hard to be inverted and the movement of the inverted domain is reduced. Can be done. From this point of view, an element having a somewhat large atomic number may be selected as the dopant Dp. For example, it may be selected from the elements of the transition metal, and Fe can be exemplified as an example.

It was confirmed that by using such a piezoelectric film 7, a waveform like the line L2 can be obtained without generating a ripple like the line L1.

The piezoelectric film 7 may be subjected to a reduction treatment after containing the dopant Dp.

The SAW device 1 as described above was actually manufactured. Specifically, the film thickness of the piezoelectric film 7 was 0.8 μm (0.7p), and the film thickness of the insulating portion 5 was 0.32 μm (0.3p). Ripple occurs when a high-frequency signal having a frequency close to the resonance frequency is applied to such a SAW device 1 to perform a power withstand test, and when a SAW device having no thermal history is heated. I confirmed not to. On the other hand, as a comparative example, when a similar test was performed even when a piezoelectric crystal containing only the reduction treatment as the piezoelectric film 7 and not containing the dopant Dp was used, it was confirmed that the ripple of the line L1 could be reproduced. did.

Further, with respect to the SAW apparatus according to such a comparative example, the IDT electrode 9 was removed by wet etching to expose the piezoelectric film 7, and the polarization state was measured by a piezoelectric response force microscope (PFM). The result is shown in FIG. FIG. 4A is an AFM image near the location where the dummy electrode 93 on the surface of the piezoelectric film 7 was located. Although the IDT electrode 9 itself has been removed, unevenness on the surface of the piezoelectric film 7 formed in the manufacturing process has been detected, and traces of the IDT electrode 9 can be confirmed. In FIG. 4A, there is a bus bar 91 at the left end, from which a dummy electrode 93 extends to the right of the paper surface and faces the electrode finger 92 extending from the left side of the paper surface with a gap. FIG. 4B is a PFM image at the same location. From FIG. 4B, the voltage V is negative along the dummy electrode and the counter electrode, centering on the vicinity of the gap between the dummy electrode 93 and the counter electrode, and the direction of polarization is reversed from the other parts. Was confirmed. In the SAW device 1, no positive / negative inverted portion was confirmed.

In this way, in the dummy electrode 93, a voltage is generated due to the pyroelectricity during heating and cooling, and at the same time, the direction of the electric field is in the direction along the polarization direction (the direction of the dummy electrode and its electrode finger), so that the polarization is reversed. Is presumed to occur. Therefore, when the dummy electrode 93 is provided and the IDT electrode 9 to which a high voltage may be applied is included, the SAW device 1 including the piezoelectric film 7 is particularly preferable. Similarly, the polarization determination is likely to occur in the electrode portion that has a floating potential.

(Other examples)
In the above example, the configuration includes the support substrate 3, but the configuration is not limited to that configuration. For example, as shown in FIG. 5A, the insulating portion 5 is thick and the support substrate 3 is not provided, or as shown in FIG. 5B, the supporting substrate 3 does not exist directly under the insulating portion 5. There may be.

As an example shown in FIG. 5A, for example, a case where a sapphire substrate is used as the insulating portion 5 can be exemplified.

Further, as shown in FIG. 6, a plurality of layers may be interposed between the support substrate 3 and the insulating portion 5. In FIG. 6, the acoustic multilayer film 15 may be included between the support substrate 3 and the piezoelectric film 7. As the piezoelectric film 7, if the Euler angles of the LT substrate are set to around (0.25.0), the sound velocity of the elastic wave generated becomes high. An acoustic multilayer film 15 is required to confine this high sound velocity elastic wave on the side of the piezoelectric film 7.

The acoustic multilayer film 15 is formed by alternately stacking a plurality of high acoustic impedance layers 15a and low acoustic impedance layers 15b. Examples of the high acoustic impedance layer 15a include tantalum oxide and hafnium oxide. Silicon oxide can be exemplified as the low acoustic impedance layer 15b. The low acoustic impedance layer 15b functions as the insulating portion 5.

In the case of such a configuration, since the high sound velocity elastic wave is not reflected to the side of the piezoelectric film 7 and leaked to the side of the support substrate 3, a low-loss SAW device can be obtained, and the polarization is reversed. Can be suppressed, so that high electrical characteristics can be realized.

In the above example, the case where the piezoelectric film 7 and the insulating portion 5 are directly bonded is described as an example, but a conductive layer having a thickness and conductivity within a range that does not affect the electrical characteristics is provided between the two. You may be. For example, it may be a conductive layer having a thickness of 5 nm or less. In this case, the potential can be stabilized and the effect of pyroelectricity can be reduced. Similarly, a metal element such as Fe, Ni, or Cu may be dispersed between the piezoelectric film 7 and the insulating portion 5. By interspersing the elements having conductivity, the potential can be stabilized on the second surface 7b of the piezoelectric film 7. The dispersion concentration may be less than the number of atoms of the piezoelectric film 7 exposed at the bonding interface, for example, 1/10 or less, more preferably 1/1000 or less. The specific concentration may be, for example, 10 ¹⁴ atoms / cm ² or less.

1: SAW device 3: Support substrate 5: Insulation part 7: Piezoelectric film 9: IDT electrode

Claims (9)

IDT electrodes with multiple electrode fingers and
It has a first surface and a second surface, and the IDT electrode is located on the first surface, and has a thickness less than the surface acoustic wave wavelength λ defined by twice the repetition interval of the plurality of electrode fingers. A piezoelectric film made of a piezoelectric crystal containing a dopant having a concentration of 10 ppm or more and 10000 ppm or less,
An elastic wave device including an insulating portion made of an insulating material that is directly or indirectly bonded to the second surface of the piezoelectric film. The elastic wave device according to claim 1, wherein the dopant is a transition metal element. The elastic wave device according to claim 1 or 2, wherein the dopant is Fe. The insulating portion is made of a material having a slower transverse sound velocity than the piezoelectric film.
The elastic wave device according to any one of claims 1 to 3, further comprising a support substrate directly or indirectly bonded to a surface of the insulating portion opposite to the side on which the piezoelectric film is located. The elastic wave device according to claim 4, wherein the support substrate is made of a material having a transverse sound velocity faster than that of the piezoelectric film. The elastic wave device according to any one of claims 1 to 5, wherein the insulating portion is silicon oxide. Claims 1 to 3 wherein an acoustic multilayer film formed by alternately laminating high acoustic impedance layers and low acoustic impedance layers is bonded to the lower surface of the piezoelectric film, and the low acoustic impedance layer is composed of the insulating portion. , 6. The elastic wave device according to any one of 6. The elastic wave device according to any one of claims 1 to 7, wherein the IDT electrode includes a dummy electrode. The elastic wave device according to any one of claims 1 to 6, further comprising a conductive layer between the piezoelectric film and the insulating portion.