JPH06276049A - Surface acoustic wave element and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the surface acoustic wave element which is fast in the propagation velocity of a surface acoustic wave and high in its propagation speed and has excellent characteristics by forming a carbon film containing diamond of less than specific size as a surface wave propagation film. CONSTITUTION:The surface acoustic wave element 11 has the surface surface propagation film 14 and a piezoelectric film 15 on a substrate 12 in this order and also has an electrode 16 for applying an electric field to the piezoelectric film 15; and the surface wave propagation film 14 is the carbon film containing diamond of <=100nm mean crystal grain size. The film which is small in crystal grain size like this behaviors as a medium which is isotropic to a surface acoustic wave with short wavelength, so the phase of the surface acoustic wave is hardly disordered and excellent element characteristics are obtained. Further, the surface acoustic wave is not scattered by crystal grain borders since the extremely small crystal grain size, so the propagation loss becomes small. The effect can not be obtained when the mean crystal grain size of the diamond exceeds the range. The lower limit of the mean crystal grain size of the diamond is not specially determined, but the sound velocity tends to decreases remarkably when the mean crystal grain size is <=5nm.

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 device and a method for manufacturing the same.

[0002]

2. Description of the Related Art A diamond thin film is a surface acoustic wave (SA
W) Has attracted attention as a surface wave propagation film for devices. The surface acoustic wave element has a structure in which a surface wave propagating film and a piezoelectric film are laminated, and a surface wave is excited in the surface wave propagating film by applying an electric field to the piezoelectric film by a comb-shaped electrode to vibrate the film. To do. The surface acoustic wave element is widely used in the field of communication including filters, but since it has good characteristics with a simple structure, it is expected to be applied to fields such as mobile communication. In mobile communication, since the UHF band is usually used, the operating frequency becomes extremely high. In order to increase the operating frequency of the surface acoustic wave element, for example, the period of the comb-shaped electrode is shortened or a surface acoustic wave propagation film having a high sound velocity is used. Since the comb-shaped electrode is usually formed by photolithography, there is a limit in reducing the electrode period. On the other hand, since the sound velocity in diamond is extremely high, using a diamond thin film as the surface wave propagation film may lead to the realization of a surface acoustic wave device capable of supporting the gigahertz (GHz) band.

The CVD method is preferably used for forming a diamond thin film because a crystal structure close to that of bulk diamond can be obtained. However, the thickness required for the surface wave propagation film is about 10 μm or more, and when a diamond film having such a thickness is formed by a normal CVD method, it becomes a polycrystalline film having a crystal grain size of about 1 μm. I will end up. Since diamond has anisotropy in sound velocity, when such a polycrystalline film is used, the phase of a sound wave propagating through the film varies, and good characteristics cannot be obtained. Further, when the crystal grain size is equal to or larger than the wavelength of the surface acoustic wave, there is a problem that the propagation loss becomes large due to scattering by the crystal grain boundaries. In addition, since such a polycrystalline diamond thin film has severe surface irregularities as compared with the wavelength of the surface acoustic wave, the surface wave is scattered and a propagation loss occurs. The depth at which the surface wave penetrates when propagating is extremely shallow and is about the wavelength thereof. Therefore, the shorter the wavelength used, the larger the proportion of the propagation loss due to the unevenness of the film surface. In order to prevent this, the surface must be polished after the film is formed, and it is difficult to reduce the cost.

On the other hand, an amorphous carbon film having no crystal grains (usually called a diamond-like carbon film) may be used as a surface wave propagation film. The sound velocity of the amorphous carbon film has no anisotropy, but the sound velocity is slower than that of the diamond film. Therefore, the electrode period must be made smaller, and it is difficult to cope with high frequencies.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface acoustic wave device having a high propagation speed of surface acoustic waves, a high propagation efficiency thereof, and excellent characteristics.

[0006]

These objects are achieved by the present invention described in (1) to (8) below. (1) A surface acoustic wave element having a surface wave propagating film and a piezoelectric film in this order on a substrate, and an electrode for applying an electric field to the piezoelectric film, wherein the surface wave propagating film has an average crystal grain size of 100.
A surface acoustic wave device characterized by being a carbon film containing diamond of not more than nm. (2) The surface acoustic wave device according to the above (1), wherein the diamond has an average crystal grain size of 5 nm or more. (3) The surface acoustic wave device according to (1) or (2), wherein the speed of sound in the surface wave propagation film is 5700 m / s or more. (4) A method for manufacturing the surface acoustic wave device according to any one of (1) to (3) above, which comprises 1 × 10 ^{−5 to} 5 × 10 ^−3.
A method of manufacturing a surface acoustic wave device, characterized in that the surface acoustic wave propagation film is formed by a plasma CVD method under a pressure of Torr. (5) Manufacture of the surface acoustic wave device according to (4), wherein the raw material neutral particle gas is caused to collide with a heating body to increase the particle velocity of the gas, and then the molecules of the gas are ionized and dissociated to perform plasma CVD. Method. (6) The method for manufacturing a surface acoustic wave element according to (5), wherein the heating body is a heating tube, and the gas collides with the inner wall of the heating tube. (7) The method for manufacturing a surface acoustic wave device according to the above (5) or (6), wherein the heating body is heated to a temperature of 700 K or higher. (8) The method for manufacturing a surface acoustic wave device according to any one of (5) to (7) above, wherein the molecules are ionized and dissociated by microwave plasma with magnetic field.

[0007]

In the surface acoustic wave device of the present invention, a carbon film containing diamond having an average crystal grain size of 100 nm or less is used as the surface wave propagation film. Since a film having a small crystal grain size behaves as an isotropic medium even for surface acoustic waves of short wavelength, there is almost no disturbance of the phase of the surface acoustic waves.
Good device characteristics can be obtained. Moreover, since the crystal grain size is extremely small, the surface acoustic waves are hardly scattered by the crystal grain boundaries, so that the propagation loss is extremely small. Therefore,
In the present invention, a surface acoustic wave having an extremely short wavelength can be used, and since the surface wave propagating film has a higher sound velocity than the amorphous carbon film, a surface acoustic wave device corresponding to the GHz band can be easily realized. To do.

Further, in the present invention, since the unevenness on the surface of the surface propagation film becomes extremely small, it is not necessary to polish the surface of the film when forming the device.

[0009]

Specific Structure The specific structure of the present invention will be described in detail below.

An example of the structure of the surface acoustic wave device of the present invention is shown in FIG.
Shown in. In the figure, the surface acoustic wave element 11 is a substrate 1
2 has a surface wave propagation film 14 and a piezoelectric film 15 in this order, and a comb-shaped electrode 16 for applying an electric field to the piezoelectric film.
Are provided on the surface wave propagation film 14.

The surface wave propagation film is a carbon film containing diamond having an average crystal grain size of 100 nm or less. If the average crystal grain size of the diamond exceeds the above range, the effects of the present invention described above cannot be obtained. There is no particular lower limit to the average crystal grain size of the diamond, but when the average crystal grain size is less than 5 nm, the tendency for the sound velocity to decrease becomes remarkable. The more preferable average crystal grain size of diamond is 20 to 100 nm. The fact that polycrystalline diamond is contained in the film is X
It can be confirmed by line diffraction, and the average crystal grain size of diamond can be measured by a transmission electron microscope or the like. Further, it can be confirmed by a scanning electron microscope or the like that the surface of the surface wave propagating film is flat and no free-form surface appears.

The preferable thickness of the surface wave propagating film varies depending on various conditions such as an operating frequency band, but is usually 10 to 10.
It is about 20 μm.

The speed of sound in the surface propagation film varies depending on the structure of the piezoelectric film, but is usually 5700 m / s or more, especially 8000 m.
It is faster than / s.

The surface wave propagation film is preferably formed by the CVD method. As the CVD method, various plasma C
It is preferable to select from the VD method and various thermal CVD methods, and it is particularly preferable to use the plasma CVD method.

In the present invention, preferably 1 × 10 ^{-5 to} 5 ×
10 ⁻³ Torr, more preferably 1 × 10 ^{−4 to} 5 × 10 ⁻³ To
A surface wave propagation film is formed by a plasma CVD method under a pressure of rr. If the atmospheric pressure during film formation exceeds the above range, it is difficult to obtain a film having a small crystal grain size. Further, if the atmospheric pressure is less than the above range, film formation becomes difficult.

As a plasma generating source in the plasma CVD method, for example, a plasma generating means utilizing electron cyclotron resonance (ECR) as disclosed in Japanese Patent Laid-Open No. 64-65843 or high frequency induction heating is used. Examples include plasma generating means.
Among these, the one using ECR is preferable because high-density electrons are obtained and the processing speed is high.

In the ECR type plasma CVD apparatus, electrons are resonantly accelerated by the interaction between an electric field and a magnetic field, and the collision of these electrons turns the gas into plasma, so that the ions and radicals in the plasma are deposited on the substrate. By doing so, film formation is performed. Also, electrons are extracted from this plasma, and the extracted electrons are collided with the raw material gas to form plasma,
The generated ions, radicals, etc. can be deposited on the substrate.

A plasma CVD apparatus suitable for forming the surface wave propagation film in the present invention is shown in FIGS. 2 and 3.

The plasma processing apparatus 1 shown in FIGS. 2 and 3 has a plasma generating chamber 7 in a vacuum system 8, and a source gas introducing pipe 2 is provided in communication with the plasma generating chamber 7. A waveguide 9 connected to a microwave power source (not shown) is provided in the plasma generation chamber 7 through a microwave introduction window, and a Helmholtz type magnet as a magnet is provided around the plasma generation chamber 7. The electromagnet 4 is provided, and these constitute ECR type magnetic field microwave plasma generation means. Further, a vacuum exhaust port is provided in the vacuum system 8 so that a predetermined operating pressure can be achieved, and a substrate holder 6 is provided so as to be movable so that the plasma can be generated while the substrate is heated to a predetermined temperature. It can be placed at a predetermined position in the generation chamber 7.

A heating tube 3 as a heating element is connected to the tip end opening of the raw material gas introduction tube 2 so that the introduced raw material neutral particle gas collides with the inner wall of the heating tube 3. The inner wall of the heating pipe 3 is 700K or more, especially 1600 to
The temperature is set to 2000 K, and the gas particles substantially elastically collide with the inner wall to increase the particle velocity (kinetic energy). There is no particular limitation on the structure of the heating tube as the accelerating means and how to collide the gas, and it is possible to spirally heat the heating element around the tube of Ta, Mo, W, etc. or use a honeycomb heater. However, in order to improve the acceleration efficiency, it is preferable to use the heating tube 3 as shown in FIG. In the illustrated example, for example, the inner diameter of W, Ta or the like is 0.7 to
A pair of electrodes 35 are connected to both ends of a tubular body 31 having a thickness of 1.4 mm, a thickness of 0.1 to 0.2 mm, and a length of 15 to 40 mm, and direct current heating is performed while changing an applied voltage to a predetermined temperature. It is said to be a structure. In addition, the heating means may be resistance heating, induction heating, or the like, and the material of the tubular body 31 is not particularly limited.

Raw material neutral particle gas is introduced into the plasma generation chamber 7 from the raw material gas introduction pipe 2 through the heating pipe 3 to increase the particle velocity. At this time, the raw material neutral particle gas is
Usually about 0.025 eV kinetic energy and about 630 m / se
It is introduced with a particle velocity of c, but the raw material neutral particle gas is 0.06 eV or more, especially 0.11 to 0.2 due to the heating tube.
Kinetic energy of 1 eV and 1300 to 1780 m / sec (1
Particle velocities of 300-2400 K) are provided.

In the plasma generation chamber 7, microwaves are introduced through the microwave introduction window, and at the same time, a magnetic field preferably satisfying the ECR condition is applied to the inside of the plasma generation chamber by the electromagnet 4. Therefore, the electrons in the plasma generation chamber 7 are accelerated by the magnetic field and the electric field of the microwave and collide with the accelerated gas particles to generate plasma. Then, plasma CVD can be performed at a low operating pressure as described above, and processing in a large area of 3 inches square or more, particularly 4 to 6 inches square is possible. Also, the substrate temperature can be lowered than usual.

The heating body such as the heating tube 3 is preferably provided vertically above the substrate 5, and the accelerated particles are directed almost vertically to the substrate surface, during which ionization / dissociation may be performed in the plasma. preferable. This improves the processing efficiency.

In the above, an example in which the processing is performed in the plasma generation chamber 7 has been described. However, the plasma processing is performed in a system communicating with the plasma generation chamber such as the ECR cavity, or an ion extraction electrode or the like is used. Then, it is also possible to perform plasma processing outside the generation chamber 7.

In the present invention, such plasma C
In addition to the VD apparatus, a plasma CVD apparatus disclosed in Japanese Patent Application No. 4-355784, that is, a plasma chamber provided with an ECR type plasma generating means and a target object holder, and formed by the plasma generating means A plasma CVD apparatus having a configuration in which a shielding member is provided between the processed plasma high-density region and the object holder, or a plasma chamber in which a plasma generation chamber and a processing chamber are provided. A plasma generating unit is provided, a processing object holder is provided in the processing chamber, and an electron extraction electrode for extracting electrons from the plasma generated in the plasma generation chamber to the processing chamber is provided between the plasma generation chamber and the processing chamber. A plasma CVD apparatus having a structure in which a shielding member is provided between the electron extraction electrode and the object holder can also be preferably used.

CH ₄ is used as a source gas in the CVD method.
And various carbon compounds such as C ₂ H ₄ may be used. If CO ₂ is used in addition to these, the crystallinity of the film is improved. It is preferable to use H ₂ gas as the carrier gas. The H ₂ gas has a function of eliminating impurities in the film. For the total of the source gas flow rate and the carrier gas flow rate,
The carrier gas flow rate is preferably 80% or more, and more preferably 85 to 99%. By setting the carrier gas flow rate within such a range, a film with extremely high purity can be formed.

The substrate temperature during film formation is 400 to 700.
The temperature is preferably set to ° C.

The material of the substrate 12 is not particularly limited and may be appropriately selected from various metals, alloys and ceramics.
For example, Si, W, Mo, Cu, Ta, Al and Ti
A metal or alloy containing one or more elements selected from the above, ceramics such as Al ₂ O ₃ , ZrO ₂ and MgO can be preferably used. Moreover, the size of the substrate is not particularly limited.

The material of the piezoelectric film 15 is LiNbO ₃ or Zn.
It may be selected from various piezoelectric materials such as O and AlN.
The film is formed by a sputtering method or the like. The thickness of the piezoelectric film is usually about 0.1 to 4 μm. Generally, the thicker the piezoelectric film, the higher the conversion efficiency into surface acoustic waves, but the slower the speed of sound.

The electrode 16 is usually formed by a vacuum vapor deposition method or the like, and formed into a comb shape by various etchings.
The electrode period may be appropriately set according to the operating frequency, the speed of sound of the surface wave propagation film, the speed of sound of the piezoelectric film, the thickness thereof, and the like. In the configuration example shown in FIG. 1, the comb-shaped electrode 16 is formed on the surface wave propagation film 14, and the piezoelectric film 15 is formed on the surface wave propagation film 14. However, the present invention is not limited to this mode. Any electrode configuration may be used as long as it can be used as a surface acoustic wave element, such as forming an electrode on a film.

Between the substrate 12 and the surface wave propagation film 14,
A base film may be provided if necessary. The base film is usually
It is provided in order to improve the adhesiveness of the surface wave propagation film, and in that case, it is preferable to use a composition containing a substrate constituent element and carbon, such as SiC or WC.

[0032]

EXAMPLES The present invention will be described in more detail with reference to specific examples.

Plasma CV having the configuration shown in FIG.
Place the Si (100) substrate on the substrate holder of the D device,
The system was evacuated to 1 × 10 ^-5 Torr and the substrate temperature was 600 ° C.
And exhausted.

Next, the heating tube 3 which is an acceleration source of neutral particles
Was electrically heated. When the temperature was monitored with a radiation thermometer, it was 2000K. The heating tube 3 has an inner diameter of 0.
The thickness was 7 mm, the wall thickness was 0.1 mm, and the length was 40 mm. Average speed _{Vave = (8kT / πmCH 4)} 1/2 of CH ₄ is estimated to be 1625m / sec.

After that, a source gas was introduced. The raw material gas was set to have a flow rate ratio of CH ₄ : CO ₂ : H ₂ = 5: 10: 85. Then the microwave and electromagnet were turned on to generate plasma. It was confirmed that the plasma uniformly spreads in the plasma generation chamber 7 of the cavity. Further, a microwave of 2.45 GHz and 500 W was applied to form a surface wave propagation film having a thickness of 15 μm under an applied magnetic field of 875 G, a substrate temperature of 600 ° C. and an operating pressure of 1 × 10 ⁻³ Torr. At this time, the heating tube 3 was placed at a position 50 nm vertically above the substrate 5 with an opening on the substrate surface.

A surface acoustic wave propagation film of a comparative example was also formed in the same manner as above except that the operating pressure was 0.5 Torr.

After the completion of film formation, the substrate was cooled and taken out, and X-ray diffraction was performed on these surface wave propagation films. As a result, it was confirmed that diamond was contained. When observed with a scanning electron microscope at 50000 times, a self-shaped surface was observed on the film surface in the comparative example, but no self-shaped surface was observed in the example of the present invention, and it was a flat surface. As a result of measuring the average crystal grain size of diamond, it was 2 μm in the comparative example, but it was extremely small as 95 nm in the example of the present invention. The crystal grain size was measured by a scanning electron microscope in the comparative example and by a transmission electron microscope in the example of the present invention.

Next, an Al film was formed on the surface wave propagating film by electron beam vapor deposition and etched by photolithography to form a comb-shaped electrode. The electrode period was 8 μm. In the comparative example, the surface of the surface wave propagation film was buffed and smoothed before the Al film was deposited.

Further, a ZnO film having a thickness of 2 μm was formed thereon by a sputtering method to form a piezoelectric film.

The transmission characteristics were measured using the surface acoustic wave devices of the present invention and the comparative example thus manufactured. The results of the device of the present invention are shown in FIG. 5, and the results of the device of the comparative example are shown in FIG. The element of the present invention has a larger amplitude at the resonance frequency than the element of the comparative example, that is, the insertion loss is smaller.
From this, it is understood that the device of the present invention has a smaller propagation loss than the device of the comparative example. Further, the element of the present invention has a smaller amplitude at the zero point than the element of the comparative example, and it can be seen that the phase variation of the surface acoustic wave is small.

In addition to the above-mentioned device of the present invention, devices having different average crystal grain sizes of the surface wave propagation film were prepared. The pressure at the time of forming the surface wave propagation film was set lower than that at the time of the element of the present invention. Surface acoustic wave propagation velocities were measured for these. The average crystal grain size and the sound velocity in the surface wave propagation film of each element were as follows.

[0042]

From the above results, the effect of the present invention is clear.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view showing a configuration example of a surface acoustic wave device of the present invention.

FIG. 2 is a configuration diagram of a plasma CVD apparatus suitable for forming a surface wave propagation film.

FIG. 3 is a configuration diagram of a plasma CVD apparatus suitable for forming a surface wave propagation film.

FIG. 4 is a perspective view of a heating tube.

FIG. 5 is a graph showing the transmission characteristics of the surface acoustic wave device of the present invention.

FIG. 6 is a graph showing the transmission characteristics of a conventional surface acoustic wave device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Plasma processing apparatus 2 Raw material gas introduction pipe 3 Heating pipe 31 Tubular body 35 Electrode 4 Electromagnet 5 Substrate 6 Substrate holder 7 Plasma generation chamber 8 Vacuum system 9 Microwave waveguide 11 Surface acoustic wave element 12 Substrate 14 Surface wave propagation film 15 Piezoelectric Membrane 16 electrodes

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuto Nagano 1-13-1 Nihonbashi, Chuo-ku, Tokyo Inside TDC Corporation

Claims (8)

[Claims] 1. A surface acoustic wave device having a surface wave propagation film and a piezoelectric film in this order on a substrate, and an electrode for applying an electric field to the piezoelectric film, wherein the surface wave propagation film is an average crystal grain. A surface acoustic wave device comprising a carbon film containing diamond having a diameter of 100 nm or less. 2. The average crystal grain size of the diamond is 5 nm
The surface acoustic wave device according to claim 1, which is as described above. 3. The speed of sound in the surface wave propagation film is 5700 m /
The surface acoustic wave device according to claim 1 or 2, which is s or more. 4. A method of manufacturing a surface acoustic wave device according to claim 1, wherein the plasma CVD is performed at a pressure of 1 × 10 ^{−5 to} 5 × 10 ⁻³ Torr.
A method of manufacturing a surface acoustic wave element, characterized in that the surface wave propagation film is formed by a method. 5. The surface acoustic wave device according to claim 4, wherein the raw material neutral particle gas is collided with a heating body to increase the particle velocity of the gas, and then the molecules of the gas are ionized and dissociated to perform plasma CVD. Production method. 6. The method of manufacturing a surface acoustic wave device according to claim 5, wherein the heating body is a heating tube, and the gas collides with the inner wall of the heating tube. 7. The method of manufacturing a surface acoustic wave device according to claim 5, wherein the heating element is heated to a temperature of 700 K or higher. 8. The method of manufacturing a surface acoustic wave device according to claim 5, wherein the ionization / dissociation of the molecules is performed by microwave plasma with magnetic field.