Patent Application
20210328569
The present invention relates to a bonded body of a piezoelectric material substrate and supporting substrate and an acoustic wave device.
It is known that, according to a surface acoustic wave filter obtained by adhering lithium tantalate and sapphire through a silicon oxide layer, bulk wave is generated at the bonding interface so that unnecessary response is generated in transmission band and high frequency band. For preventing this, it is proposed that roughened surface in introduced at the bonding interface to scatter the bulk wave and to suppress the unnecessary response (Patent documents 1 and 2).
According to patent document 1, in the case that the bonding surface is made a roughened surface, as to the geometrical specification of the roughened surface, the ratio of an average length RSm of a factor in a cross sectional profile of convex-concave structure forming the roughened surface and a wavelength λ of a surface acoustic wave is made 0.2 or larger and 7.0 or smaller and the arithmetic average roughness Ra of the cross sectional profile of the convex-concave structure is made 100 nm or larger. Further, it is defined a difference in height of a roughened surface in the patent document 2.
In prior arts, it was measured the convex-concave morphology (for example, RSm or Ra) of a bonding surface of a supporting substrate or piezoelectric material substrate and the values were adjusted to be slightly large for suppressing the spurious wave. However, even in the case that RSm's of the bonding surfaces are the same, there may be cases that the spurious wave could not be suppressed. It is thus proved that the spurious wave cannot be suppressed, by simply adjusting the convex-concave morphology of the bonding surface.
An object of the present invention is to provide a novel structure for suppressing the spurious wave which cannot be suppressed by adjusting the surface morphology of a bonding surface of a piezoelectric material substrate or supporting substrate of a bonded body.
The present invention provides a bonded body comprising:
a supporting substrate;
a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and
a bonding layer bonding the supporting substrate and the piezoelectric material substrate, said bonding layer contacting a main surface of said piezoelectric material substrate,
provided that at least one of a bonding surface of the supporting substrate and a bonding surface of the piezoelectric material substrate is measured by X-ray reflectivity method and that 1 is assigned to a signal intensity in the case of total reflection, a relative intensity I of a reflected light from the bonding surface is approximated by the following formula (1) in a range of 1.0×10−4 or larger and 1.0×101 or smaller.
I=α(2θ)−b (1)
(In the formula (1),
θ represents an incident angle of an X-ray with respect to said bonding surface,
a is 1.0×10−5 or larger and 2.0×10−3 or smaller, and
The inventors have tried to make the bonding surface of the supporting substrate or piezoelectric material substrate a mirror surface and then to form a roughened surface by mechanical processing, and the microstructure was observed and analyzed in detail. As a result, it is found that minute defects or denaturing of the film, which cannot be evaluated based on the surface convex-concave morphology, are generated on the bonding surface after the mechanical processing. Based on such measurement results, it is found that the effect of suppressing the spurious wave is needed to be controlled based not on the surface convex-concave morphology and based on the effective crystallographic and geometric characteristics in surface regions of the piezoelectric material substrate and bonding layer.
Based on such findings, the present inventors have researched various kinds of processing and measuring methods of the bonding surface. During the research, the inventors noticed X-ray reflectivity method (XRR method, X-ray Reflection).
According to the X-ray reflectivity method, X-ray is made incident onto a surface of a sample at an extremely low angle, and it is measured the X-ray intensity profile of reflected light reflected in the mirror direction of the incident angle. According to the method, the profile obtained by the measurement is compared with the simulation results and the simulation parameters are optimized so that the film thickness and density of the sample are determined. That is, the X-ray reflectivity method is originally the method of measuring the film thickness and density of a thin film utilizing X-ray reflection from the thin film. It means that the reflected light reflected from the thin film carries information of the convex-concave of the surface of the thin film as well as information in the direction of depth of the thin film (film thickness and density).
The inventors tried to apply the X-ray reflectivity method onto the bonding surface after the roughening process of the piezoelectric material substrate or supporting substrate and to obtain information of denaturing or change of density in a region near the surface of the bonding surface, and researched the relationship of the information and the effect of suppressing the spurious wave.
That is, the bonding surface of the piezoelectric material substrate or supporting substrate is subjected to roughening process. Thereafter, the information of crystallographic state in the surface region of the bonding surface is obtained by the X-ray reflectivity method. Specifically, X-ray is made incident onto the bonding surface at a very low angle and the reflected light is then measured. Here, provided that θ is assigned to the incident angle of the X-ray, and θ is gradually changed from 0° to record the relative intensity I of the reflected light. At this time, the relative intensity I is defined as a relative intensity provided that 1 is assigned to the signal intensity at the total reflection. It is known that the relative intensity I is damped in proportion with 1×10−4 in the case that the bonding surface is ideally flat. However, it is proved that the spurious wave is generated when the bonding surface is a flat mirror surface and that the spurious wave still cannot be suppressed even when the bonding surface is roughened as far as the relative intensity has a proportional coefficient near 1×10−4.
Thus, the inventors performed the measurement of the bonding surface after various kinds of roughening processes by the X-ray reflectivity method, respectively, and measured the degree of suppressing the spurious wave. As a result, it is found that the relative density I of the reflected light from the bonding surface can be approximated with the formula (1), in a range of 1.0×10−4 or larger and 1.0×10−1 or smaller.
That is, as schematically shown in
Here, the index b is 5.0 or larger and coefficient a can be approximated as 1.0×10−5 to 2.0×10−3. It means that the relative intensity I is decreased more rapidly as the increase of the incident angle θ than the case that the bonding surface is a mirror surface. Further, it means that the crystals are denatured in the surface region of the bonding surface or defects are generated other than the minute convex-concave morphology on the surface. It is found that the spurious wave can be suppressed under such condition.
Further, as the index b exceeds 9.0, the spurious wave is actually increased. It is thus necessary to make b 9.0 or smaller.
Embodiments of the present invention will be described in detail below, appropriately referring to the drawings.
First, as shown in
Further, as shown in
The activated bonding surface 2b of the bonding layer 2 on the supporting substrate and the activated bonding surface 3b of the piezoelectric material substrate 3 are contacted with each other and subjected to direct bonding, to obtain a bonded body 5 shown in
At this stage, electrodes may be provided on the piezoelectric material substrate 3. However, preferably, as shown in
Further, an intermediate layer may be provided between the bonding layer 2 and piezoelectric material substrate 3.
According to the present example, as shown in
Further, as shown in
Then, the activated surface 2b of the bonding layer 2 on the supporting substrate and the activated bonding surface 12a of the intermediate layer 12 on the piezoelectric material substrate 3 are contacted with and directly bonded with each other, to obtain a bonded body 15 shown in
At this stage, electrodes may be provided on the piezoelectric material substrate 3. However, preferably, as shown in
Alternatively, after the bonding layer 2 is film-formed, the intermediate layer 12 may be subsequently film-formed on the bonding layer 2. In this case, CMP processing is performed on the surface of the intermediate layer 12 to obtain a bonding surface (mirror surface). Plasma is irradiated onto the thus obtained bonding surface to activate the surface. The surface of the supporting substrate is then subjected to plasma activation and directly bonded with the bonding surface of the intermediate layer.
According to the present invention, b is made 5.0 or larger. Further, b is 9.0 or smaller and may preferably be 7.0 or smaller. Further, according to the present invention, a is made 1.0×10−5 or larger, and may preferably be 1.0×10−4 or larger. Further, a is 2.0×10−3 or smaller and may preferably be 1.0×10−3 or smaller.
According to a preferred embodiment, a and b satisfy the following formula (2).
−0.713·ln(a)+0.5≤b≤−0.713·ln(a)+0.7 (2)
According to a still preferred embodiment, a and b satisfy the following formula (3).
b=−0.713·ln(a)+0.6 (3)
The measurement condition for the X-ray reflectivity method is as follows.
method for controlling the measurement results by the X-ray reflectivity method of the bonding surfaces of the piezoelectric material substrate and supporting substrate as described above.
As to the method for roughening the surfaces, it is listed a mechanical processing method such as grinding process by grinding stones or blast processing by micro media such as alumina or silicon nitride, and ion beam processing of colliding ions at a high speed.
Respective constituents of the present invention will be described further in detail below.
Although the material of the supporting substrate 1 is not particularly limited, the material is preferably selected from the group consisting of silicon, quartz, sialon, mullite, sapphire and translucent alumina. It is thus possible to further improve the temperature characteristics on frequency of the acoustic wave device 6 or 16.
Although the film-forming method of the bonding layer or intermediate layer is not particularly limited, sputtering method, chemical vapor deposition (CVD) method and vapor deposition method may be listed.
Although the material of the bonding layer 2 is not particularly limited as far as the surface activation process is possible, a metal oxide film is preferred, and the material may preferably be selected from the group consisting of silicon oxide, silicon nitride, aluminum nitride, alumina, tantalum pentoxide, mullite, niobium pentoxide and titanium oxide. Further, as the surface activation method, an appropriate method may be selected depending on the material used for the bonding layer. Such surface activation method includes plasma activation and FAB (Ar atomic beam).
Although the material of the intermediate layer 12 is not particularly limited as far as the surface activation processing is possible, a metal oxide film is preferred, and it is particularly preferred a material selected from the group consisting of silicon oxide, silicon nitride, aluminum nitride, alumina, tantalum pentoxide, mullite, niobium pentoxide and titanium oxide. Further, it is preferred to select the material of the intermediate layer different from that of the bonding layer.
The thickness of the bonding layer 2 may preferably be 0.05 μm or larger, more preferably be 0.1 μm or larger and most preferably be 0.2 μm or larger on the viewpoint of the present invention. Further, the thickness of the bonding layer 2 may preferably be 3 μm or smaller, more preferably be 2 μm or smaller and further preferably be 1 μm or smaller.
The piezoelectric material substrate 3 used in the present invention is composed of lithium tantalate (LT) single crystal, lithium niobate (LN) single crystal, or lithium niobate-lithium tantalate solid solution. As the materials have high propagation speed of an acoustic wave and large electromechanical coupling factor, they are suitable for a surface acoustic wave device of a high frequency and wide-band frequency application.
Further, although the normal line direction of the main surface 3a of the piezoelectric material substrate 3 is not particularly limited, for example, in the case that the piezoelectric material substrate 3 is made of LT, it is preferred to use the substrate rotated from Y-axis toward Z-axis by 32 to 55°, (180°, 58 to 35°, 180°) on Euler angle representation, around X-axis, which is a direction of propagation of a surface acoustic wave, because of a low propagation loss. In the case that the piezoelectric material substrate 3 is made of LN, (i) it is preferred to use the substrate rotated from Z axis toward −Y-axis by 37.8°, (0°, 37.8°, 0°) on Euler angle representation, around X-axis, which is the direction of propagation of the surface acoustic wave, because of a larger electromechanical coupling factor. Alternatively, (ii) it is preferred to use the substrate rotated from Y axis toward Z axis by 40 to 65°, (180°, 50 to 25°, 180°) on Euler angle representation, around X-axis, which is the direction of propagation of the surface acoustic wave, because of a high sound velocity. Further, although the size of the piezoelectric material substrate 3 is not particularly limited, for example, the diameter may be 100 to 200 mm and thickness may be 0.15 to 1 μm.
Plasma is then irradiated onto the bonding surfaces of the bonding layer 2 on the supporting substrate 1, piezoelectric material substrate 3 and intermediate layer 12 on the piezoelectric material substrate 3 at a temperature of 150° C. or lower, to activate the bonding surfaces. On the viewpoint of the present invention, although it is preferred to irradiate nitrogen plasma, it is possible to obtain the inventive bonded body even in the case that oxygen plasma is irradiated.
The pressure during the surface activation may preferably be 100 Pa or lower and more preferably be 80 Pa or lower. Further, the atmosphere may be composed of nitrogen only, oxygen only or mixture of nitrogen and oxygen.
The temperature during plasma irradiation is made 150° C. or lower. It is thus possible to obtain a bonded body having a high bonding strength and without deterioration of crystallinity. On the viewpoint, the temperature during the plasma irradiation is made 150° C. or lower, and may preferably be made 100° C. or lower.
Further, the energy of the plasma irradiation may preferably be 30 to 150 W. Further, the product of the energy and irradiation time duration of the irradiated plasma may preferably be 0.12 to 1.0 Wh.
The plasma-treated bonding surfaces of the piezoelectric material substrate and bonding layer are contacted with each other at room temperature. The treatment may be performed in vacuum and the contact may preferably be performed in atmosphere.
When the activation of the surfaces is performed using the argon atomic beam, it is preferred to use a system described in Japanese patent publication No. 2014-086400A to generate the argon atomic beam, which is irradiated. That is, it is used a high-speed atomic beam source of saddle field type as the beam source. Then, inert gas is introduced into the chamber and a high voltage is applied onto electrodes from a direct current electric source. By this, electric field of saddle field type generated between the electrode (positive electrode) and a housing (negative electrode) causes motion of electrons, e, so that argon atomic and ion beams are generated. Among the beams reached at a grid, the ion beam is neutralized at the grid, and the beam of argon atoms is emitted from the high-speed atomic beam source. In the activation step by beam irradiation, the voltage may preferably be made 0.5 to 2.0 kV, and the current may preferably be made 50 to 200 mA.
According to a preferred embodiment, before the surface activation treatment, the bonding surfaces of the bonding layer on the supporting substrate, of the piezoelectric material substrate and of the intermediate layer on the piezoelectric material substrate are subjected to flattening process. The method of flattening the respective surfaces includes lapping, chemical mechanical polishing (CMP) and the like. Further, the flattened surfaces may preferably have Ra of 1 nm or lower and more preferably be 0.3 nm or lower.
Then, the bonding surface of the bonding layer on the supporting substrate and the bonding surface of the piezoelectric material substrate 3 or bonding surface of the intermediate layer are contacted and bonded with each other. Thereafter, it is preferred to improve the bonding strength by performing annealing treatment. The temperature for the annealing treatment may preferably be 100° C. or higher and 300° C. or lower.
The bonded bodies 5, 5A, 15 and 15A of the present invention can be appropriately used for the acoustic wave devices 6 and 16. That is, it is provided an acoustic wave device having the bonded body of the present invention and an electrode provided on the piezoelectric material substrate.
Specifically, as the acoustic wave devices 6 and 16, a surface acoustic wave device, Lamb wave-type device, thin film resonator (FBAR) and the like are known. For example, the surface acoustic wave device is produced by providing input side IDT (Interdigital transducer) electrodes (also referred to as comb electrodes or interdigitated electrodes) for oscillating surface acoustic wave and IDT electrodes on the output side for receiving the surface acoustic wave on the surface of the piezoelectric single crystal substrate. By applying high frequency signal on the IDT electrodes on the input side, electric field is generated between the electrodes, so that the surface acoustic wave is oscillated and propagated on the piezoelectric substrate. Then, the propagated surface acoustic wave is drawn as an electrical signal from the IDT electrodes on the output side provided in the direction of the propagation.
The material forming the electrodes 10 on the piezoelectric material substrate 3A may preferably be aluminum, aluminum alloy, copper or gold and more preferably be aluminum or aluminum alloy. The aluminum alloy used may preferably be Al doped with 0.3 to 5 weight percent of Cu. In this case, Ti, Mg, Ni, Mo, or Ta may be used instead of Cu.
It was produced an acoustic wave device 6 shown in
Specifically, one main surface 3c of a 42Y-cut X-propagation LiTaO3 substrate (piezoelectric material substrate) 3 having a thickness of 250 μm was polished into mirror surface and the other main surface 3a was subjected to lapping with GC #1000. Further, it was prepared a Si(100) substrate (supporting substrate) 1 having a thickness of 0.23 mm and high resistance (>2 kΩ·cm). The sizes of the respective substrates were 150 mm.
The bonding surface of the supporting substrate was then subjected to processing into a roughened surface. According to the present example, it was performed grinding process using grinding stones of number of #6000. The amount of the processing was made about 3 μm.
It was obtained the spectrum by X-ray reflectivity method of the bonding surface of the supporting substrate and the relative signal intensity was approximated based on the formula (1), to obtain values of a of 9.2×10−4 and b of 5.55.
A silicon oxide film 2 was the film-formed in 0.7 μm on the bonding surface 1a of the supporting substrate 1, and the surface was polished by CMP (chemical mechanical polishing) in about 0.2 um for the flattening. Then, the bonding surface 3b of the piezoelectric material substrate 3 and bonding surface of the silicon oxide film 2 were activated by N2 plasma, respectively, and then bonded with each other in atmosphere. Specifically, as the surface roughness of the bonding layer after the polishing was measured by AFM (Atomic force Microscope), it was confirmed that a mirror surface having Ra of 0.4 nm could be obtained sufficient for the bonding.
Then, the bonding surface 3b of the piezoelectric material substrate 3 and bonding surface 2b of the bonding layer 2 were subjected to cleaning and surface activation, respectively. Specifically, ultrasonic cleaning by pure water was performed and the substrate surface was dried by spin drying. Then, the supporting substrate after the cleaning was introduced into a plasma activation chamber and the bonding surface of the bonding layer was activated at 30° C. by nitrogen plasma. The piezoelectric material substrate 3 was similarly introduced in the plasma activation chamber and subjected to surface activation by nitrogen plasma at 30° C. The time period of the surface activation was made 40 seconds, and the energy was made 100 W. The ultrasonic cleaning and spin drying were performed again as described above, for removing particles adhered during the surface activation.
The positions of the respective substrates were then adjusted so that the bonding surfaces of the respective activated substrates were contacted with each other at room temperature. The piezoelectric material substrate 3 was positioned on the upper side when the substrates were contacted. As a result, it was observed the state (so-called bonding wave) that the adhesion of the substrates was spreading to prove that good preliminary bonding was accomplished. The bonded body was then charged into an oven filled with nitrogen atmosphere and held at 130° C. for 40 hours, for improving the bonding strength.
The surface 3c of the piezoelectric material substrate 3 of the bonded body after the heating was subjected to grinding, lapping and CMP processing so that the thickness of the piezoelectric material substrate 3A reached 7 μm.
Then, for confirming the effects of the present invention, a comb-shaped electrode made of aluminum metal was formed on the piezoelectric material substrate of the bonded body, to produce a resonator of a surface acoustic wave device. The specification was shown as follows.
As the reflection characteristics of the resonator was measured by a network analyzer, spurious was hardly observed in a range higher than an anti-resonance frequency, as shown in
The results were shown in table 1.
It was produced the resonator of the surface acoustic wave device according to the same procedure as that in the inventive example 1, and the reflection characteristics of the resonator was measured by the network analyzer. However, the processing of the bonding surface of the supporting substrate was performed by grinding the bonding surface by grinding stones of #8000.
As spectrum of the bonding surface of the supporting substrate was obtained by X-ray reflectivity method and the relative signal intensity was approximated by the formula (1), it was obtained a of 7.1×10−4 and b of 5.80. The magnitude of the spurious wave was proved to be 3.2 dB.
It was produced the resonator of the surface acoustic wave device according to the same procedure as the inventive example 1, and the reflection characteristics of the resonator was measured by the network analyzer. However, the processing of the bonding layer of the supporting substrate was performed by blasting of the whole surface of the substrate by silicon nitride particles. The amount of the processing was evaluated and proved to be as small as 10 nm.
As spectrum of the bonding surface of the supporting substrate was obtained by X-ray reflectivity method and the relative signal intensity was approximated by the formula (1), it was obtained a of 2.2×10−5 and b of 8.84. The magnitude of the spurious wave was proved to be 4.8 dB.
The resonator of the surface acoustic wave device was produced according to the same procedure as that of the inventive example 1, and the reflection characteristics of the resonator was measured by the network analyzer. However, the processing of the bonding surface of the supporting substrate was performed by charging the supporting substrate in an ion processing machine and by colliding Ar ions accelerated at 0.5 keV.
As spectrum of the bonding surface of the supporting substrate was obtained by X-ray reflectivity method and the relative signal intensity was approximated by the formula (1), it was obtained a of 5.6×10−5 and b of 7.63. The magnitude of the spurious wave was proved to be 3.3 dB.
The resonator of the surface acoustic wave device was produced according to the same procedure as that of the inventive example 1, and the reflection characteristics of the resonator was measured by the network analyzer. However, the processing of the bonding surface was performed by charging the supporting substrate in an ion processing machine and by colliding Ar ions accelerated at 1.0 keV.
As spectrum of the bonding surface of the supporting substrate was obtained by X-ray reflectivity method and the relative signal intensity was approximated by the formula (1), it was obtained a of 1.8×10−3 and b of 5.12. The magnitude of the spurious wave was proved to be 3.5 dB.
The resonator of the surface acoustic wave device was produced according to the same procedure as that of the inventive example 1, and the reflection characteristics of the resonator was measured by the network analyzer. However, as the bonding surface of the supporting substrate was made a mirror surface, Ra was 0.02 nm and the approximation by the formula (1) could not be made. Spurious was observed in the reflection characteristics, as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-216520 | Nov 2019 | JP | national |
This is a continuation of PCT/JP2020/042099, filed Nov. 11, 2020, which claims priority to Japanese Application No. 2019-216520 filed on Nov. 29, 2019, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2020/042099 | Nov 2020 | US |
| Child | 17358121 | US |
