JP2018056866A - Piezoelectric composite substrate for surface acoustic wave element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric composite substrate for a surface acoustic wave element, from which a surface acoustic wave (SAW) filter responding to higher frequencies can be inexpensively fabricated.SOLUTION: A substrate having a Vickers hardness scale of 1200 or less is prepared and disposed in a vacuum chamber, to which hydrocarbon is introduced to form a diamond-like carbon (DLC) film having a film thickness of 0.1 μm or more and 0.3 μm or less by an ion chemical vapor deposition method on the substrate surface. The substrate is mounted on a DC magnetron sputtering device, in which a metal Zn target is used together with a mixture gas of oxygen and argon as a reaction gas introduced so as to form a zinc oxide thin film having a film thickness of 0.1 μm or more and 10 μm or less on the DLC film while heating the substrate to 160°C or higher and 250°C or lower.SELECTED DRAWING: None

Description

The present invention relates to a piezoelectric composite substrate for a surface acoustic wave element and a method for manufacturing the same.

A surface acoustic wave (SAW) is a wave propagating on the surface of a substance, and a SAW filter using this characteristic is known. This SAW SAW filter is an important component for high-speed communication, which is used for selecting an arbitrary frequency used in a mobile phone and a smart phone, utilizing the characteristics of small size and light weight.

In addition, it is applied to multifunctional applications such as resonators, signal processing delay elements, pressure sensors, and temperature sensors.

For example, as a surface acoustic wave element used as a SAW filter, a piezoelectric layer as a surface acoustic wave propagation medium on a substrate and a pair of comb-like electrodes (Inter) made of aluminum formed on the piezoelectric layer. Digital Transducer (IDT). (For example, see Patent Document 1)

In the surface acoustic wave element, when an electric signal (alternating current power) is supplied to the input IDT, a distortion occurs in the piezoelectric layer due to an electric field generated by the electric signal. Here, since the electrode has a comb shape, a difference in density occurs in the piezoelectric layer, and a surface acoustic wave is generated. The surface acoustic wave propagates to the output IDT, and the energy of the surface acoustic wave is converted into electrical energy and output by the output IDT.

Further, in Patent Document 2, the surface acoustic wave element has an acoustic wave whose propagation speed exceeds 10,000 m / s using diamond as a hard layer and zinc oxide (ZnO) as a piezoelectric layer for the purpose of speeding up. An element is described, which corresponds to the higher frequency required for a surface acoustic wave element recently.

In general, the center frequency f of the SAW filter is expressed by V / λ from the relationship between the propagation speed V of the surface acoustic wave and the electrode pitch (λ / 4: λ is the wavelength of the surface acoustic wave). It has been known. That is, in order to increase the frequency of the surface acoustic wave element, a method of reducing the wavelength of the surface acoustic wave and a method of increasing the propagation speed of the surface acoustic wave are conceivable. However, the actual situation is that the surface acoustic wave wavelength is limited by the electrode pitch of the IDT and is limited in manufacturing technology. At the current mass production level, it is about 0.4 μm, and 2400 MHz can be said to be the limit at the propagation speed of 3800 m / s of the lithium tantalate substrate (LT) which is often used for SAW filters recently.

Therefore, a device using an ALN vibrator called FBAR (Film Bulk Acoustic Resonator) has been studied for high frequency applications. However, since the manufacturing process is complicated and expensive, it is actually used only for some devices.

Japanese Patent Laid-Open No. 9-51248 JP 2008-258780 A

  The present inventor has eagerly studied a method for increasing the propagation speed of surface acoustic waves in order to cope with higher frequencies, and in order to efficiently increase the propagation speed of surface acoustic waves, a substrate having low hardness. This can be achieved by arranging a DLC (diamond-like carbon) film on the substrate so as to have a high hardness as a supporting substrate, and further arranging a sputtered thin film made of zinc oxide on the DLC film as a highly oriented piezoelectric film. As a result, the present invention has been completed.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems and to provide a piezoelectric composite substrate for a surface acoustic wave element and a method of manufacturing the same capable of inexpensively producing a SAW filter corresponding to a higher frequency. And

According to the present invention, a DLC (diamond-like carbon) film having a film thickness of 0.1 μm or more and 0.3 μm or less is disposed on a substrate having a Vickers hardness of 1200 or less, and the film thickness is 0 on the DLC film. A piezoelectric composite substrate for a surface acoustic wave device is provided, in which a zinc oxide film having a thickness of 1 μm to 10 μm is disposed.

Moreover, according to the present invention,
A first step of preparing a substrate having a Vickers hardness of 1200 or less as a substrate;
Next, a second step of placing the substrate in a vacuum vessel, introducing hydrocarbons, and forming a DLC film having a film thickness of 0.1 μm or more and 0.3 μm or less on the substrate surface by ionized vapor deposition, ,
Thereafter, the obtained substrate is mounted on a DC magnetron sputtering apparatus, a mixed gas of oxygen and argon gas is introduced as a reaction gas using a metal Zn target, and the substrate temperature is heated to 160 ° C. or higher and 250 ° C. or lower. And a third step of forming a zinc oxide film having a thickness of 0.1 μm or more and 10 μm or less on the DLC film, and a method for manufacturing a piezoelectric composite substrate for a surface acoustic wave element.

According to the present invention, by using an inexpensive sapphire substrate and using a zinc oxide thin film as a piezoelectric material, a SAW filter applicable to the 3 GHz band, which greatly exceeds the propagation speed of a SAW filter using a conventional LT substrate, is efficiently used. Can be manufactured.

Comb electrode formed on the piezoelectric film

Hereinafter, a piezoelectric composite substrate for a surface acoustic wave element according to the present invention and a manufacturing method thereof will be described.

(1) Substrate for surface acoustic wave element It is known that the propagation speed transmitted through a substrate used for a surface acoustic wave element is higher as the hardness of the substrate is higher.

For example, the highest level of propagation speed exceeding 10,000 m / s is obtained in a substrate in which a piezoelectric substrate of aluminum nitride or zinc oxide is formed on a diamond substrate having the highest hardness. However, since the price of the substrate is too expensive, it cannot be used practically.

An inexpensive and general-purpose soda glass substrate as a support substrate has a Vickers hardness of 500 to 600, a silicon substrate of about 1040, and a sapphire substrate of 2300.

Glass substrates and silicon substrates are inexpensive and are produced in large quantities and are inexpensive, so they are desired to be used as substrates. However, propagation speeds corresponding to higher frequencies cannot be obtained.

Therefore, a general-purpose glass substrate or silicon substrate with low hardness is used, and a DLC (diamond-like carbon) film having a high hardness is formed thinly on this substrate, thereby increasing the hardness of the substrate and on the DLC film. It was thought that the composite substrate with the piezoelectric film disposed on the surface may have a higher propagation speed than the lithium tantalate (LT) piezoelectric substrate used as the surface acoustic wave device substrate corresponding to higher frequency. .

(2) DLC (diamond-like carbon) film In the present invention, a diamond-like carbon film formed on a general-purpose glass substrate or silicon substrate having low hardness is a carbon of both diamond and graphite (graphite). It is a thin film made of a carbon-based material with carbon bonds. A structure having both diamond and graphite bonds is an amorphous structure (amorphous structure), and the diamond bond is called SP3 and the graphite bond is called SP2.

Diamond is made of only SP3 bonds, and graphite is made of only SP2. However, the DLC film is made of a complex combination of SP3 bonds and SP2 bonds.

In general, amorphous carbon has physical properties similar to diamond when the SP3 ratio is large, and physical properties similar to graphite when the SP2 ratio is large. By adjusting the ratio, amorphous carbon having various characteristics can be obtained. be able to. Therefore, it was studied that a DLC film having a Vickers hardness of 3000 or more is formed on a glass substrate or a silicon substrate to increase the hardness as a support substrate to obtain a large propagation speed.

The thickness of the DLC film is preferably 0.1 μm or more and 0.3 μm or less. A film thickness of less than 0.1 μm is not preferable because it is too thin and the strength to maintain the film structure is insufficient. It is not preferable because it does not improve and the cost increases. The DCL film is more preferably about 0.2 μm.

Since the DLC film has an amorphous structure and does not have a crystal grain boundary, the DLC film has a smooth surface. Accordingly, since the surface roughness Ra of the DLC film is as extremely smooth as the 0.5 nm level, the piezoelectric film disposed thereon can be formed smoothly.

(3) Piezoelectric film Next, it was considered that a zinc oxide film was the most suitable material for the piezoelectric film disposed on the DLC film. This is because the glass substrate or silicon substrate of the support substrate on which the DLC film is formed does not have piezoelectricity, so it is necessary to crystallize the piezoelectric film to provide piezoelectricity, and zinc oxide is easy to obtain orientation A membrane was selected. The film thickness of the piezoelectric film made of zinc oxide is preferably in the range of 0.1 μm to 10 μm.

If the thickness is less than 0.1 μm, the characteristics as a stable piezoelectric film may not be obtained. Even if the thickness exceeds 10 μm, the propagation speed of the surface acoustic wave is not improved, and stable crystallinity is obtained. Can be difficult.

(4) Method for Manufacturing Piezoelectric Composite Substrate Next, a method for manufacturing the piezoelectric composite substrate of the present invention will be described. First, a glass substrate or a silicon substrate having a Vickers hardness of 1200 or less is prepared as a substrate.

Next, a DLC film having a thickness of 0.1 μm to 0.3 μm is formed on the prepared substrate. This DLC film can be obtained by film formation by ionization vapor deposition which is a plasma process in high vacuum. When benzene (C ₆ H ₆ ) gas or other hydrocarbon gas is introduced into the vacuum chamber, hydrocarbon ions and excited radicals are generated in the DC arc discharge plasma, and the hydrocarbon ions are biased to a DC negative voltage. The resulting substrate collides with energy corresponding to the bias voltage to solidify and form a film. Since non-equilibrium plasma is used, the substrate temperature during film formation is usually 200 ° C. or lower.

Next, a zinc oxide thin film is formed as a piezoelectric film. In general, a nitride film such as aluminum nitride or an oxide film such as barium titanate, lithium tantalate, lithium niobate, zinc oxide, or the like is used as the piezoelectric film. From the viewpoint of producing a thin film having excellent orientation at low cost, it was considered preferable to form zinc oxide by sputtering.

The influence of oxygen in the vacuum apparatus is large in the nitride film, and it is difficult to form a film that stably exhibits piezoelectricity because the metal and oxygen first react to inhibit the growth of the nitride film. In order to solve this, the problem can be solved by using a load-lock type apparatus that can continuously sputter without forming a vacuum in forming a nitride film, but this apparatus has a complicated structure and is extremely expensive. It is. Therefore, an oxide was selected as the piezoelectric body, and zinc oxide was selected as the most suitable material from various oxide piezoelectric materials for the above reasons.

Further, in terms of cost, an inexpensive metal Zn target is used instead of an expensive sintered body target such as ZnO. As a sputtering reaction gas, a mixed gas of oxygen and argon gas is introduced, and a zinc oxide film having a thickness of 0.1 μm or more and 10 μm or less is formed by reactive sputtering using a DC magnetron sputtering apparatus.

As specific film forming conditions for sputtering, the substrate heating temperature is preferably in the range of 160 ° C. to 250 ° C., and more preferably in the range of 170 ° C. to 200 ° C. When the substrate heating temperature is lower than 160 ° C., the crystallinity and orientation of the zinc oxide film are suddenly lowered and become amorphous, and when it exceeds 250 ° C., it takes time to raise the temperature from room temperature and return to room temperature, and the film formation time becomes longer. Therefore, it is not preferable.

The DC output of DC magnetron sputtering is preferably performed in the range of 80 W to 250 W. More preferably, it is 150W or more and 200W or less. If the DC output is lower than 80 W, it is difficult to obtain stable sputtering, and if it exceeds 250 W, unevenness and grain growth occur on the surface of the zinc oxide film, and the smoothness is impaired.

As a reaction gas at the time of sputtering, a mixed gas of oxygen and argon is used to form a zinc oxide film. As the reactive gas, an argon gas flow rate of 10 sccm to 15 sccm and an oxygen gas flow rate of 8 sccm to 10 sccm can be efficiently formed.

  EXAMPLES Hereinafter, although this invention is demonstrated in more detail using an Example, this invention is not limited to an Example at all.

(Production and evaluation of piezoelectric composite substrate)
Example 1
First, soda glass (substrate size: 30 × 40 × 2 mm) was prepared as a substrate. The prepared substrate was cleaned with acetone.

Next, a DLC film having a thickness of 0.2 μm was formed on the washed soda glass substrate under the following conditions. The specific conditions for forming the DLC film are shown below.
-Pretreatment: 140 ° C x 60 minutes baking. Ar Bombard (20 minutes)
・ Gas and flow rate: benzene (C ₆ H ₆ ) 3 ml / min ・ Vapor deposition method: DC ion plating ・ Substrate voltage: 2 KV, 50 mA
・ Degree of vacuum: 4.5 × 10 ⁻³ Pa
-Substrate temperature: 160 ° C
・ Deposition time: 30 minutes

Next, the soda glass substrate on which the DLC film was formed was set in a DC magnetron sputtering apparatus (manufactured by Shibaura Seisakusho, model CFS-4ES). Also, a Zn metal target having a diameter of 3 inches is mounted as a sputtering target, and a mixed gas of argon gas and oxygen is supplied as a reactive gas while heating the substrate temperature to 170 ° C. (the flow rate of argon gas is 15 sccm, the flow rate of oxygen 10 sccm), reactive sputtering was performed to form a 0.8 μm-thick zinc oxide film as the piezoelectric film.

The ultimate vacuum during sputtering was 6.5 × 10 ⁻³ Pa, the sputtering output was 200 W, and the film formation time was 40 minutes.

  When the half width of the crystallinity of the obtained zinc oxide thin film was measured by X-ray diffraction (XRD), (001) orientation was observed at 14.4 °.

(Example 2)
A piezoelectric composite substrate was fabricated in the same manner as in Example 1 except that a 2-inch diameter silicon substrate was used as the substrate. The thickness of the DLC film was 0.2 μm, and the thickness of the zinc oxide film was 0.8 μm.

  When the half width of the crystallinity of the obtained zinc oxide thin film was measured by X-ray diffraction (XRD), (001) orientation was observed at 13.8 °.

(Comparative Example 1)
The same substrate as in Example 1 was used, and a treatment was performed in the same manner as in Example 1 except that no DCL film was formed, so that a piezoelectric composite substrate was produced. The thickness of the zinc oxide film was 1.3 μm.
When the half width of the crystallinity of the obtained zinc oxide thin film was measured by X-ray diffraction (XRD), (001) orientation was observed at 14.4 °.

(Production and evaluation of SAW filter)
A Cu comb-shaped electrode shown in FIG. 1 was formed on the zinc oxide thin film of the piezoelectric composite substrate obtained in Example 1 to produce a SAW filter. This comb-shaped electrode was an electrode having an electrode width of 1 μm, an electrode interval of 1 μm, and a copper film thickness of 0.5 μm. The SAW filter had a propagation velocity of 5100 m / s, a frequency temperature characteristic of −12.6 ppm / ° C., and an electromechanical coupling coefficient of 10%.

It can also be seen that a SAW filter of 3200 MHz can be obtained by setting the surface acoustic wave wavelength λ to 0.4 μm.

A Cu comb-shaped electrode was formed on the zinc oxide thin film of the piezoelectric composite substrate obtained in Example 2 in the same manner as in the piezoelectric composite substrate of Example 1 to produce a SAW filter. The SAW filter had a propagation speed of 5200 m / s, a frequency temperature characteristic of −10.8 ppm / ° C., and an electromechanical coupling coefficient of 9.8%.

It can also be seen that a 3250 MHz SAW filter can be obtained by setting the surface acoustic wave wavelength λ to 0.4 μm.

Next, the comb comb-shaped electrode shown in FIG. 1 was formed on the zinc oxide thin film of the piezoelectric composite substrate obtained in Comparative Example 1 to produce a SAW filter. The SAW filter had a propagation speed of 2400 m / s, a frequency temperature characteristic of −17.6 ppm / ° C., and an electromechanical coupling coefficient of 8.5%.

Further, using a LT substrate having a thickness of 200 μm, a Cu comb-shaped electrode shown in FIG. 1 was formed on the surface of the LT substrate in the same manner as the piezoelectric composite substrate, thereby producing a SAW filter. This comb-shaped electrode was an electrode having an electrode width of 1 μm, an electrode interval of 1 μm, and a copper film thickness of 0.5 μm. The SAW filter had a propagation speed of 3900 m / s and a frequency temperature characteristic of -35.0 ppm / ° C.

From these evaluation results, it can be seen that the piezoelectric composite substrate of the present invention has a propagation velocity and frequency temperature characteristics higher than those of the conventional LT piezoelectric substrate.

Claims (6)

A DLC (diamond-like carbon) film having a thickness of 0.1 μm or more and 0.3 μm or less is disposed on a substrate having a Vickers hardness of 1200 or less, and the film thickness is 0.1 μm or more and 10 μm or less on the DLC film. A piezoelectric composite substrate for a surface acoustic wave device, characterized in that a zinc oxide film is disposed   The piezoelectric composite substrate for a surface acoustic wave device according to claim 2, wherein the substrate is a glass substrate or a silicon substrate. A first step of preparing a substrate having a Vickers hardness of 1200 or less as a substrate;
Next, a second step of placing the substrate in a vacuum vessel, introducing hydrocarbons, and forming a DLC film having a film thickness of 0.1 μm or more and 0.3 μm or less on the substrate surface by ionized vapor deposition, ,
Thereafter, the obtained substrate is mounted on a DC magnetron sputtering apparatus, a mixed gas of oxygen and argon gas is introduced into the reaction gas using a metal Zn target, and the substrate temperature is heated to 160 to 250 ° C., And a third step of forming a zinc oxide film having a film thickness of 0.1 μm or more and 10 μm or less on the DLC film, and a method for manufacturing a piezoelectric composite substrate for a surface acoustic wave element.   4. The method of manufacturing a piezoelectric composite substrate for a surface acoustic wave element according to claim 3, wherein the hydrocarbon is benzene. 4. The method of manufacturing a piezoelectric composite substrate for a surface acoustic wave element according to claim 3, wherein the DC magnetron sputtering apparatus has a DC output in a range of 80 W to 250 W. 4. The surface acoustic wave according to claim 3, wherein a flow rate of argon gas and oxygen of the reaction gas is 10 sccm to 15 sccm, and a flow rate of oxygen gas is 8 sccm to 10 sccm. A method for manufacturing a piezoelectric composite substrate for an element.


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