JPH05243895A - Surface acoustic wave element and manufacture of the element - Google Patents

Abstract

PURPOSE:To provide the surface acoustic wave (SAW) element, which can be driven at a high frequency, at low cost by forming layers according to an ARE method or a PVD method while using aluminium nitride at high SAW velocity. CONSTITUTION:The aluminium nitride layer is formed by introducing nitrogen gas, adjusting nitrogen pressure division, opening a shutter 15 when the gas pressure is stablized, and impressing a high-frequency bias to a sapphire substrate 13 after the inside of a vacuum container is set at prescribed pressure, a hollow cathode 6 is discharged and aluminium 9 is heated and evaporated by beams. Next, nitrogen gas is exhausted, and the aluminium layer to be an interdigital electrode is formed with prescribed thickness on the aluminium nitride and worked into the interdigital shape. Then, the SAW element is obtained by executing wire bonding. Thus, a piezoelectric layer can be formed at the low temperature, the SAW element to be driven at the high frequency can be obtained at low cost on the substrate of a semiconductor or glass, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter element used in communication equipment and the like, a surface acoustic wave element such as a surface acoustic wave resonator, and a method for producing the same.

[0002]

2. Description of the Related Art Interdigital electrodes have been formed on the surface of crystal or lithium tantalate crystal.

[0003]

The velocity of the surface acoustic wave is v, and the interdigital electrodes (IDT, Inte) of the surface acoustic wave element are used.
Let d be the electrode pitch of the digital-transducer) and f be the resonance frequency of the surface acoustic wave element.
= Fd. A surface acoustic wave device using a crystal or a lithium tantalate crystal has a propagation velocity of 2800 to 3700 m.
Since it is as small as / sec, the electrode pitch must be designed small in order to drive at a high frequency.

The interdigital electrode is generally formed by processing a metal thin film formed by vapor deposition or sputtering by photolithography and etching. As the electrode pitch becomes smaller, finer processing is required, so in the photolithography process, a precise aligner with high resolution,
A dustproof room with high cleanliness is required. Further, in the etching process, a sophisticated technique such as a reactive ion beam etching method or an ECR plasma etching method is required. Therefore, the equipment becomes expensive, and as a result, the manufacturing cost of the surface acoustic wave element becomes high.

On the other hand, the surface acoustic wave of aluminum nitride has a large propagation velocity of about 6000 m / sec, and the electrode pitch can be designed large by using aluminum nitride for the piezoelectric body. Although it has been reported that an aluminum nitride film can be synthesized by the CVD method (chemical vapor deposition method), aluminum chloride, trimethylaluminum, and ammonia used as the material gas in the CVD method are toxic and should be handled with great care. In addition to this, it is expensive and increases the manufacturing cost. Further, since the CVD method requires a high temperature of 800 ° C. or higher for the synthesis reaction, the surface acoustic wave device cannot be formed on a heat-sensitive semiconductor substrate or glass substrate.

[0006]

By using aluminum nitride having a high surface acoustic wave velocity as a piezoelectric material, a surface acoustic wave element that can be driven at a high frequency even if the electrode pitch is relatively large can be manufactured. it can. As a method of forming an aluminum nitride layer, CVD utilizing a reaction by heat
Activated plasma deposition method (AR)
PV method called E method) or ion plating method
By adopting the method (physical vapor deposition method), it becomes possible to form the piezoelectric layer while keeping the substrate temperature low.

[0007]

The surface acoustic wave element using the aluminum nitride film has a large surface acoustic wave propagation speed of about 6000 m / s, so that the pitch of the interdigital transducer can be increased.
The activated reactive vapor deposition method is a method in which a solid metal is heated and vaporized by an electron beam, and at the same time, plasma is generated by direct current or high frequency discharge to activate vaporized atoms and reaction gas to obtain a compound film. Since plasma energy is used instead of thermal energy, a compound film can be obtained even at a relatively low substrate temperature. By applying this thin film forming method and using aluminum as the solid metal and nitrogen as the reaction gas, an aluminum nitride film can be obtained.

In the activated reactive vapor deposition method, it is a common practice to apply a DC bias voltage to the substrate in order to improve the adhesion and crystallinity of the film formed. However, if the substrate or the thin film formed is an insulator,
DC bias has no effect. By applying a high frequency bias voltage, the adhesion and crystallinity can be improved even if the substrate or the thin film is an insulator.

When the piezoelectric layer is an aluminum nitride thin film, it is said that a surface acoustic wave device with good performance can be realized by using sapphire as a substrate. However, in order to downsize and integrate the device, a semiconductor substrate is required. It is desirable to be able to form the piezoelectric layer directly on top. Depending on the material of the substrate, the velocity of the surface acoustic wave may slow down or the energy conversion efficiency between the electric signal and the surface acoustic wave may decrease, but this can be prevented by providing the aluminum oxide layer as the intermediate layer.

[0010]

EXAMPLE 1 FIG. 1 is a sectional view showing the structure of a surface acoustic wave device according to an example. The material of the substrate 1 is sapphire, the piezoelectric layer 2 is aluminum nitride, and the interdigital transducer 3 is aluminum.

FIG. 3 is a sectional view showing the structure of a thin film forming apparatus used for manufacturing the surface acoustic wave element of the embodiment. An electron beam 7 provided with a hollow cathode 6 and produced by a hollow cathode discharge
By this, the aluminum 9 in the hearth 8 can be heated and evaporated. A high frequency power source 12 is connected to the substrate holder 10 via a matching unit 11, and a high frequency bias can be applied to the substrate 13.

An embodiment in which the surface acoustic wave element of FIG. 1 is manufactured by using the thin film forming apparatus of FIG. 2 will be described. First, the substrate 13 is attached to the substrate holder 10, and the inside of the vacuum container 14 is evacuated to a pressure of 0.01 mTorr or less. Argon gas is caused to flow through the hollow cathode 6 with the shutter 15 kept closed to start hollow cathode discharge, and aluminum 9 is heated by the electron beam 7 and evaporated. The discharge current of the hollow cathode discharge was 200 A and the argon gas flow rate was 18 CCM.

Nitrogen gas is introduced into the vacuum vessel 14 to adjust the nitrogen partial pressure. In the example, the partial pressure of nitrogen was 2 mTorr. When the hollow cathode discharge and the nitrogen gas pressure have stabilized, the shutter 15 is opened and an aluminum nitride layer is formed on the sapphire substrate 13. A high frequency bias was applied to the substrate 13 during the formation of the aluminum nitride layer. The high frequency output was 20W. In the example, the temperature of the thermocouple installed in the vacuum container 13 was about 200 ° C. In the example, the thickness of the aluminum nitride layer was 5 μm.

After forming the aluminum nitride layer, an aluminum layer to be a comb-shaped electrode is continuously formed. The shutter 15 is once closed and the introduction of nitrogen gas is stopped. After confirming that the gas pressure is stable and the nitrogen gas is exhausted, the shutter 15 is opened and an aluminum layer is formed on the aluminum nitride. In the example, the thickness of the aluminum layer was about 0.2 μm.

After cooling, the substrate 13 is taken out, and the aluminum layer is processed by the following method to form a comb-shaped electrode.
Apply photoresist evenly on the aluminum layer,
Pre-bake at ℃ for 30 minutes. The photoresist used was a positive type. The resist is exposed using an aligner, and the pattern is transferred from the mask. The electrode pitch was 34 μm. After developing the resist, aluminum was wet-etched with an etching solution containing a mixture of phosphoric acid, acetic acid and nitric acid.

The resist was stripped using a dedicated stripping solution, the substrate was placed in a package, and wire-bonding was performed on the interdigital electrode processed by the above method to complete a surface acoustic wave device. When the S parameter of this surface acoustic wave element was measured with a network analyzer, the resonance frequency was about 174 MHz. Since the surface acoustic wave device of the quartz substrate made with the same electrode pitch has a frequency of about 90 MHz, the surface acoustic wave device of the example has a surface acoustic wave device of about 2 with the same electrode pitch.
It will be possible to drive at nearly double the frequency.

(Embodiment 2) FIG. 2 is a sectional view showing the structure of a surface acoustic wave device according to an embodiment. The material of the substrate 4 is a single crystal silicon wafer, the intermediate layer 5 is aluminum oxide, the piezoelectric layer 2 is aluminum nitride, and the electrode 3 is aluminum. An example of manufacturing the surface acoustic wave device of FIG. 2 using the thin film forming apparatus of FIG. 3 will be described.

First, the substrate 13 is attached to the substrate holder 10, and the inside of the vacuum container 14 is evacuated to a pressure of 0.01 mTorr or less. Argon gas is caused to flow through the hollow cathode 6 while the shutter 15 is closed to start hollow cathode discharge,
Aluminum 9 is heated by the electron beam 7 and evaporated. The discharge current of the hollow cathode discharge was 200 A and the argon gas flow rate was 18 CCM.

Oxygen gas is introduced into the vacuum vessel 14 to adjust the nitrogen partial pressure. In the example, the partial pressure of nitrogen was 3 mTorr. When the hollow cathode discharge and the nitrogen gas pressure have stabilized, the shutter 15 is opened and an aluminum oxide layer is formed on the substrate 13. Substrate 1 during formation of aluminum oxide layer
A high frequency bias was applied to No. 3. The high frequency output was 20W. In the example, the temperature of the thermocouple installed in the vacuum vessel was about 200 ° C. In the example, the thickness of the aluminum oxide layer was 5 μm.

After forming the aluminum oxide layer, the inside of the vacuum container 14 was evacuated, the atmosphere was switched to nitrogen, and an aluminum nitride layer was formed in the same manner as in Example 1 to form an aluminum layer. After the above steps, the substrate was taken out and the interdigital electrodes were processed by the same method as in Example 1 to fabricate a surface acoustic wave device.

When the characteristics of this element were measured, the same characteristics as in Example 1 were obtained.

[0022]

The surface acoustic wave device that can be driven at a high frequency can be manufactured at low cost. Since the piezoelectric layer can be formed at a relatively low temperature, the surface acoustic wave device can be formed on a substrate such as a semiconductor or glass which is weak against high temperature. If a surface acoustic wave device can be fabricated on a semiconductor substrate,
It is possible to manufacture a small-sized composite element in which a semiconductor element and a surface acoustic wave element are integrated.

Since each of the layers of aluminum nitride, aluminum oxide and aluminum can be formed by one apparatus, the equipment investment can be reduced. By forming each layer continuously, the number of steps can be reduced, the time required for the process can be shortened, and the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a surface acoustic wave device of Example 1.

FIG. 2 is a sectional view showing a structure of a surface acoustic wave device according to a second embodiment.

FIG. 3 is a schematic view showing a structure of a thin film forming apparatus used in an example.

[Explanation of symbols]

 1 Substrate 2 Piezoelectric layer 3 Interdigital electrode 4 Substrate 5 Intermediate layer 6 Hollow cathode 7 Electron beam 8 Hearth 9 Aluminum 10 Substrate holder 11 Matching device 12 High frequency power supply 13 Substrate 14 Vacuum container 15 Shutter 16 Argon gas introduction tube 17 Hollow cathode Power supply 18 Nitrogen gas introduction pipe 19 Heater

Claims (7)

[Claims] 1. A surface acoustic wave device comprising a piezoelectric layer made of aluminum nitride formed on a substrate made of sapphire, and interdigital electrodes made of aluminum formed on the surface of the piezoelectric layer. 2. An intermediate layer made of aluminum oxide formed on a substrate such as a single crystal silicon wafer, a piezoelectric layer made of aluminum nitride formed on the surface of the intermediate layer, and further formed on the surface of the piezoelectric layer. A surface acoustic wave device having a comb-shaped electrode made of aluminum. 3. A method of forming a piezoelectric layer, wherein a substrate is placed in a vacuum container of a thin film forming apparatus equipped with an electron beam heating evaporation source, nitrogen gas is introduced into the vacuum container, and the substrate is placed in a nitrogen atmosphere. 3. The surface acoustic wave device according to claim 1, wherein the aluminum nitride layer is formed on the substrate by simultaneously heating and evaporating the metallic aluminum with an electron beam and generating plasma. Manufacturing method. 4. An oxygen gas is introduced into a vacuum container, and an aluminum oxide layer is formed on a substrate by heating and evaporating aluminum metal with an electron beam in an oxygen atmosphere, and then the atmosphere is switched to nitrogen gas, The method for manufacturing a surface acoustic wave device according to claim 2, wherein an aluminum nitride layer is formed. 5. The method of manufacturing a surface acoustic wave device according to claim 3, wherein the electron beam source of the electron beam heating evaporation source is a hollow cathode discharge. 6. The method of manufacturing a surface acoustic wave device according to claim 3, wherein a high frequency bias voltage is applied to the substrate when the piezoelectric layer is formed. 7. The aluminum layer serving as an electrode is formed by evacuating the vacuum vessel immediately after forming the piezoelectric layer and subsequently evaporating aluminum in a vacuum to form an aluminum layer serving as an electrode. A method for manufacturing the surface acoustic wave device according to claim 1.