JP2003221294A - Diamond substrate for surface acoustic wave element, and surface acoustic wave element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diamond substrate for a surface acoustic wave element and the surface acoustic element comprising a diamond thin film that has a large propagation velocity of a surface acoustic wave and a small propagation loss to achieve a surface acoustic wave element having a good frequency response. <P>SOLUTION: A filament CVD process is used to form a diamond thin film 240 having a thickness of 20 μm on a silicon wafer 220. In this process, an adjustment is made so that the ratio I<SB>(220)</SB>/IT is 0.8 or more, wherein I<SB>(220)</SB>refers to the peak intensity for the diamond crystal face (220) and IT refers to the sum of the peak intensities for the diamond crystal faces (111), (220), (311), (400) and (331), when the diamond thin film 240 is measured by X-ray diffraction, and a hydrogen content in the diamond thin film 240 is in the range 1%-5%, by atomic percent. The diamond thin film is smoothed by a mechanical polishing using a grindstone containing diamond abrasive grains so that the arithmetical mean roughness (Ra) of the surface is 20 nm or less. <P>COPYRIGHT: (C)2003,JPO

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 applied to a high frequency filter or the like and a diamond substrate used for such a surface acoustic wave device.

[0002]

2. Description of the Related Art With the increasing frequency in the communication field in recent years, a surface acoustic wave element that can be used in a high frequency (for example, 2.5 GHz) region (as a typical example of the surface acoustic wave element,
Comb-shaped electrodes (IDT: Inter-Digital Transdu
cer) is formed in the surface acoustic wave element, and the device which detects the surface acoustic wave by exciting the surface acoustic wave into the piezoelectric body or a medium other than the piezoelectric body by the action of the comb-shaped electrode and the piezoelectric body. Can be mentioned. ) Development is required. In order to realize a surface acoustic wave device that can be used in a high frequency range, it is necessary to narrow the electrode spacing of the comb-shaped electrodes or increase the propagation velocity of the surface acoustic wave. Since the comb electrodes are usually formed by photolithography, there is a limit to their miniaturization. Therefore, it is necessary to realize a medium in which the propagation velocity of surface acoustic waves is high.

Since diamond has the highest elastic modulus in a substance, the propagation velocity of surface acoustic waves using this as a medium is high. Therefore, conventionally, as a surface acoustic wave element that can be used in a high frequency region, a surface acoustic wave in which a piezoelectric layer is formed on a diamond thin film that is a medium of the surface acoustic wave, and a comb-shaped electrode is further formed on the piezoelectric layer. Wave elements were used.

[0004]

However, the conventional surface acoustic wave device using the diamond thin film has a problem that the insertion loss becomes large because the propagation loss of the surface acoustic wave in the diamond thin film is large. .

Further, in the surface acoustic wave element diamond substrate, in order to enable use in a high frequency range, the surface acoustic wave has a high propagation speed (preferably 9000 m / s).
Above) is desired.

Further, a surface acoustic wave device applied to a high frequency filter or the like is desired to have excellent frequency characteristics, that is, a large Q value (preferably 700 or more). Here, the Q value means Q = f ₀ / Δf; full width at half maximum Δf =
Defined by f ₁ -f ₂ . However, f ₀ denotes the resonance frequency in the surface acoustic wave device, f _1, f _2, respectively, the high frequency side of the f _0, the low-frequency side, ^{2-1 / 2} times the amplitude of the amplitude at f ₀ It means that the frequency becomes (frequency that becomes 1/2 in amplitude intensity).

The present invention has been made in order to solve the above problems, and provides a surface acoustic wave device which is a diamond thin film having a large propagation velocity of surface acoustic waves and a small propagation loss, and having excellent frequency characteristics. An object of the present invention is to provide a surface acoustic wave device and a diamond substrate for a surface acoustic wave device, which is provided with a diamond thin film to be realized.

[0008]

In order to achieve the above object, the diamond substrate of the present invention is a diamond substrate for a surface acoustic wave device, comprising a substrate and a diamond thin film formed on the substrate, The hydrogen content of the diamond thin film is 1% or more and 5% or less in atomic ratio, the arithmetic mean roughness (Ra) of the surface of the diamond thin film is 20 nm or less, and the diamond crystal when the diamond thin film is measured by X-ray diffraction The peak intensity I ₍₂₂₀₎ of the plane ( ₂₂₀₎ and the diamond crystal planes (111), (220), (31
Sum of peak intensities of 1), (400) and (331) I _T
The ratio I ₍₂₂₀₎ / I _T of the is equal to or less than 0.8.

When hydrogen is taken in during the formation of the diamond thin film, the hydrogen intervenes in the bond of carbon and reduces the crystallinity of the diamond thin film. Especially when the hydrogen content is about 30%, the diamond thin film is diamond-like.
It tends to have an amorphous structure like carbon (DLC). When the hydrogen content of the diamond thin film is large, the propagation velocity of the surface acoustic wave becomes small due to the decrease in hardness (reduction in elastic modulus) of the diamond thin film.

On the other hand, when the hydrogen content of the diamond thin film is small, the crystallinity of the diamond thin film is good, so that the scattering of surface acoustic waves at the grain boundaries becomes large and the propagation loss increases.

If the surface of the diamond thin film is rough,
The scattering of surface acoustic waves becomes large and the propagation loss increases.

In order to increase the Q value, it is possible to improve the design of the surface acoustic wave device by increasing the number of pairs of comb electrodes, but in that case, the insertion loss of the surface acoustic wave device increases. There is a point. Therefore, in order to increase the Q value, it is desirable to limit the constituent conditions of the diamond thin film to increase the Q value while maintaining a suitable propagation velocity and propagation loss of the surface acoustic wave.

As a result of earnest studies, the present inventor has found that the hydrogen content of the diamond thin film is 1% or more and 5% or less in atomic ratio, and the arithmetic average roughness (R
It has been found that when a) is 20 nm or less, preferable propagation velocity and propagation loss of surface acoustic waves can be realized at the same time. Furthermore, the peak intensity I ₍₂₂₀₎ of the diamond crystal plane (220) when X-ray diffraction measurement was performed on the diamond thin film,
Diamond crystal faces (111), (220), (31
Sum of peak intensities of 1), (400) and (331) I _T
It has been found that when the ratio I ₍₂₂₀₎ / _IT of 0.8 or more is 0.8 or more, the Q value increases to a suitable level while maintaining a suitable propagation velocity and propagation loss of the surface acoustic wave.

In the diamond substrate for surface acoustic wave device of the present invention, the diamond thin film is
Has a peak in the vicinity of a wave number of 1333 cm ^-1, it is preferable that the half value width of the peak is 10 cm ^-1 or more.

The diamond film has a low hydrogen content,
When the crystallinity of the diamond thin film is good, that is, SP ²
When the proportion of sexual bonds is small, the half-width of the peak in Raman spectroscopy is reduced. In this case, the scattering of surface acoustic waves at the crystal grain boundaries becomes large, and the propagation loss increases.

As a result of earnest research by the present inventor, the diamond thin film was found to have a wave number of 1333 cm in Raman spectroscopic analysis.
^It has been found that a more suitable propagation loss of the surface acoustic wave can be realized when it has a peak near ^-1 and the half width of the peak is 10 cm ^-1 or more.

The diamond substrate for a surface acoustic wave device of the present invention has a Knoop hardness of 6000 kgf / mm ² or more and 8500 kgf /
It is preferably not more than mm ² .

When the hardness of the diamond thin film is low, the propagation velocity of surface acoustic waves becomes low.

On the other hand, when the hardness of the diamond thin film is high, that is, when the crystallinity of the diamond thin film is good, the scattering of surface acoustic waves at the crystal grain boundaries becomes large and the propagation loss increases.

As a result of earnest studies, the present inventor has found that the Knoop hardness of a diamond thin film measured under a load of 500 g is 6000.
When kgf / mm ² or more 8500kgf / mm ² or less, and knowledge to be able to realize further propagation speed and propagation loss of suitable surface acoustic wave at the same time.

In the diamond substrate for surface acoustic wave device of the present invention, it is preferable that the diamond crystal has an average grain size of 1 μm or less.

When the average grain size of diamond crystals is large,
Surface acoustic waves are easily scattered and propagation loss increases.

As a result of earnest research, the present inventor has found that when the average grain size of the diamond crystals in the diamond thin film is 1 μm or less, more suitable surface acoustic wave propagation loss can be realized.

The diamond substrate for a surface acoustic wave device of the present invention has an arithmetic mean roughness (Ra) of the surface of a diamond thin film.
Is preferably 10 nm or less.

As a result of earnest research, the present inventor has found that when the arithmetic average roughness (Ra) of the surface of the diamond thin film is 10 nm or less, a more suitable propagation loss of the surface acoustic wave can be realized.

In order to achieve the above object, in the surface acoustic wave device of the present invention, a piezoelectric thin film is formed on a diamond substrate for a surface acoustic wave device according to any one of the above, and a comb is formed on the piezoelectric thin film. A mold electrode is formed.

In the surface acoustic wave device of the present invention, the comb-shaped electrode is formed on the diamond substrate for surface acoustic wave device according to any one of the above, and the piezoelectric is formed on the diamond substrate on which the comb-shaped electrode is formed. It is preferable that a body thin film is formed.

When the surface acoustic wave device of the present invention is provided with any one of the above-mentioned diamond substrates for surface acoustic wave devices, it is possible to simultaneously realize a suitable surface acoustic wave propagation velocity, propagation loss and Q value.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the surface acoustic wave element diamond substrate of the present invention and the surface acoustic wave element to which the surface acoustic wave element diamond substrate of the present invention is applied will now be described in detail with reference to the accompanying drawings. explain.

FIG. 1 shows a surface acoustic wave device 10 of this embodiment.
It is a schematic perspective view of 0. FIG. 2 is a cross-sectional view of the surface acoustic wave device 100 shown in FIG. 1 taken along the line II-II.

The surface acoustic wave device 100 includes a diamond substrate 200 of this embodiment and a piezoelectric thin film 120 (diamond substrate 200 formed on the diamond substrate 200.
An intermediate layer may be formed between and the piezoelectric thin film 120. ) And a comb-shaped electrode 140 and a comb-shaped electrode 160 formed on the piezoelectric thin film 120. The diamond substrate 200 is composed of a silicon wafer 220 and a diamond thin film 240. The surface acoustic wave device of the present invention may be manufactured by forming a comb-shaped electrode on a diamond substrate and then further forming a piezoelectric thin film on the diamond substrate.

The diamond thin film 240 is formed on the surface of the silicon wafer 220 by the CVD (chemical vapor deposition) method, the hot filament CVD method, the microwave plasma CVD method, the high frequency plasma CVD method, the DC plasma jet method, or the like. At the time of film formation, the hydrogen content of the diamond thin film 240 is 1% or more and 5% or less in atomic ratio, and the peak intensity I ₍₂₂₀₎ of the diamond crystal plane (220) at the time of X-ray diffraction measurement, and the diamond Crystal plane (111), (22
0), (311), it is adjusted to be the (400) and (331 ratio I ₍₂₂₀ of the total I _T of the peak intensity _of)) / I _T is 0.8 or more. Here, FIG. 5 shows an example of an X-ray diffraction chart of the diamond substrate. Note that in this example, I ₍₂₂₀₎
/ I _T is 0.66, different from the scope of the present invention.

The surface of the formed diamond thin film 240 is smoothed by polishing so that the arithmetic mean roughness (Ra) is 20 nm or less (more preferably 10 nm or less). The polishing method can be performed by mechanical polishing using a grindstone containing diamond abrasive grains. Alternatively, instead of smoothing by polishing, the crystal grain size at the time of forming the diamond thin film 240 may be reduced to the nanometer order, and the arithmetic average roughness (Ra) may be 20 nm or less (more preferably 10 nm or less). In order to reduce the surface roughness by polishing, it is preferable that the grindstone has a small distance from the surface of the bond to the flat end surface of the diamond abrasive grain and a large area of the flat end surface of the diamond abrasive grain.

The material of the substrate for forming the diamond thin film 240 is not limited to silicon, but in addition to GaAs, SiC, AlN, InP and GaN, ceramics and metals such as Si ₃ N ₄ and Mo. Can be used. Cemented carbide and cermet can be used depending on the application.

The diamond thin film 240 preferably satisfies the following conditions (1) to (3).

[0036] (1) In the Raman spectroscopic analysis using an Ar laser with a wavelength of 514.5 nm, has a peak near the wave number 1333 cm ^-1, the half-value width of the peak is 10 cm ^-1 or more. (2) Knoop hardness measured at a load of 500g is 6000k
gf / mm ² or more and 8500 kgf / mm ² or less. (3) The average grain size of diamond crystals is 1 μm or less.

Diamond substrate 20 obtained as described above
0, CVD method, microwave plasma CVD method, P
VD (physical vapor deposition) method, sputtering method, ion
By plating method, laser ablation method, etc.
The piezoelectric thin film 120 is formed. As the piezoelectric body, A
1N, ZnO, LiNbO₃, LiTaO₃, KNb
O ₃, PZT (PbZr_xTi_(1-x)O₃) Etc. are used
It

A comb-shaped electrode 140 and a comb-shaped electrode 160 are formed on the surface of the piezoelectric thin film 120. The comb electrode 140 includes an input terminal 142 and an input terminal 144, and the comb electrode 160 includes an output terminal 162 and an output terminal 164.

The diamond substrate 200 of this embodiment is
Since the hydrogen content of the diamond thin film 240 is 5% or less in atomic ratio, the propagation velocity is 9 in the secondary Sezawa.
A high propagation speed of 000 m / s is realized. In addition, the diamond substrate 200 has a hydrogen content of 1% or more in atomic ratio of the diamond thin film 240, so
When applied to a surface acoustic wave device of GHz, the propagation loss becomes 0.022 dB / λ or less, and the (220) intensity ratio (I
₍₂₂₀₎ / _IT ) is 0.8 or more, the Q value is 7
00 or more. The present invention is the first time to realize such a high propagation velocity, a small propagation loss, and a large Q value at the same time, and this diamond substrate realizes a high frequency surface acoustic wave device having low loss and excellent frequency characteristics. it can.

[0040]

The contents of the present invention will be described more specifically below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

(Example 1) Using a filament CVD method, a silicon wafer 220 having a thickness of 0.8 mm and a thickness of 2 was formed.
A 0 μm diamond thin film 240 was formed. W wire was used for the filament. Filament temperature is 2100
The temperature of the silicon wafer 220 is 70
It was set to about 0 to 800 ° C. Hydrogen and methane were used as raw material gases. Hydrogen gas flow rate is 500-5000sc
cm, and the ratio of the methane gas flow rate to the hydrogen gas flow rate was 0.5 to 5.0%. Gas pressure is 5 to 120
It was set to torr.

Diamond substrate 20 obtained as described above
After fixing 0 to the metal holder, the diamond thin film 240 was smoothed by mechanical polishing using a grindstone containing diamond abrasive grains. The average diameter of diamond grains is 20 μm,
A grindstone having a distance from the surface of the bond to the flat end surface of the diamond abrasive grains of 2 μm and a maximum length of the flat end surface of the diamond abrasive grains of 6 μm was used. Grinding condition is 2 stone rotation
The rotation speed was 00 rpm, the holder rotation speed was 200 rpm, and the load was 4 kg. Table 1 shows film forming conditions for the diamond thin film 240.

On the diamond thin film 240 after polishing, R
A ZnO film (piezoelectric thin film 120) having a thickness of 500 nm was formed by the F sputtering method.

An Al film having a thickness of 80 nm was formed on the ZnO film by DC sputtering. A part of the Al film was removed by photolithography to form a comb electrode 140 and a comb electrode 160. The wavelength of the surface acoustic wave is λ =
The electrode width of the comb-shaped electrode 140 and the comb-shaped electrode 160 was 1 μm, the electrode crossing width was 50 times the wavelength λ, and the number of electrode pairs was 40.

In Table 1, the diamond film 240 (22
0) Examples and comparative examples of the intensity ratio, hydrogen content (atomic ratio), arithmetic average roughness (Ra) of the surface, propagation loss and propagation velocity of surface acoustic waves, and Q value are shown. The hydrogen content is based on JAPANESE JOURNALOF APPLIED PHYSICS VOL23,
No.7, can use the methods reported in PP810-814, determined from the absorption peak of CH _n near 3000 cm ^-1 in the infrared absorption analysis. The arithmetic average roughness (Ra) of the surface can be measured by an atomic force microscope (AFM).

[0046]

[Table 1]

In Examples 1-1 to 1-5, the (220) intensity ratio of the diamond thin film 240 was adjusted to 0.80 or more, the hydrogen content was adjusted to 1% to 5% in atomic ratio, and the surface arithmetic was performed. By polishing so that the average roughness (Ra) is 20 nm or less, suitable propagation loss, propagation velocity and Q
Value (propagation loss of surface acoustic wave of 2.5 GHz is 0.022
dB / λ or less and the propagation velocity of the second-order Sezawa wave is 9
000 m / s or more, Q value of 700 or more) was realized.
By applying such a diamond substrate 200, a surface acoustic wave device that can be used in a high frequency range of 2.5 GHz or more, has a small insertion loss, and has excellent frequency characteristics is realized.

In Comparative Examples 1-1 and 1-2, respectively,
(220) Since the strength ratio is as small as 0.75 and 0.71, Q
The problem that the value becomes small occurred. In Comparative Example 1-3, the hydrogen content was as small as 0.3%, and there was a problem that the propagation loss was increased because the surface acoustic waves were largely scattered at the crystal grain boundaries. In Comparative Example 1-4, there was a problem that the arithmetic average roughness (Ra) of the surface was as large as 25 nm and the scattering of the surface acoustic wave was large, so that the propagation loss increased. In Comparative Example 1-5, the hydrogen content was as large as 6.8%, and there was a problem that the propagation speed was lowered due to the decrease in hardness (decrease in elastic modulus) of the diamond thin film 240.

(Embodiment 2) Using a microwave CVD method with a frequency of 2.45 GHz, a diamond thin film 240 with a thickness of 20 μm is formed on a silicon wafer 220 with a thickness of 1.0 mm.
Was formed. The temperature of the silicon wafer 220 is 700 to 8
It was set to about 00 ° C. Hydrogen and methane were used as raw material gases. The hydrogen gas flow rate is 500 to 5000 sccm, and the ratio of the methane gas flow rate to the hydrogen gas flow rate is
It was set to 0.5 to 5.0%. Gas pressure is 5-120 to
rr.

The surface of the diamond thin film 240 was smoothed by polishing in the same manner as in Example 1. In addition, as in Example 1, the surface acoustic wave device 100 was manufactured by applying the diamond substrate 200.

In Table 2, the diamond film 240 (22
0) Strength ratio, hydrogen content, surface arithmetic mean roughness (R
Examples and comparative examples of a), peak half width in Raman spectroscopy, propagation loss and propagation velocity of surface acoustic waves, and Q value are shown. FIG. 3 shows an example of a spectrum in Raman spectroscopic analysis of a diamond thin film. The peak near the wave number of 1333 cm ⁻¹ shown in FIG. 3 is the peak of diamond. The peak has a width due to the Raman effect.

[0052]

[Table 2]

In Examples 2-1 to 2-3, by adjusting the crystallinity of the diamond thin film 240 so that the half-value width of the peak is 10 cm ^-1 or more, suitable propagation loss, propagation velocity and Q value ( The propagation loss of the surface acoustic wave of 2.5 GHz is 0.022 dB / λ or less, the propagation velocity of the secondary Sezawa wave is 9000 m / s or more, and the Q value is 700 or more). Such a diamond substrate 200
By applying, a surface acoustic wave device that can be used even in a high frequency range of 2.5 GHz or higher, has a small insertion loss, and has excellent frequency characteristics is realized.

In Comparative Examples 2-1 and 2-2, respectively,
Half-width of peak is 5 cm^-1, 8 cm ^-1And small, surface bullet
Propagation loss increases due to large scattering of sex waves
Occurred. In Comparative Example 2-3, (220) intensity ratio, hydrogen
Values of content and surface roughness (Ra) are suitable, but half value
9 cm wide^-1And the effect of reducing the propagation loss
On the contrary, it was small.

(Embodiment 3) A silicon wafer 220 having a thickness of 0.8 mm and a thickness of 2 is formed by using the filament CVD method.
A 0 μm diamond thin film 240 was formed. W wire was used for the filament. Filament temperature is 2100
The temperature of the silicon wafer 220 is 70
It was set to about 0 to 800 ° C. Hydrogen and methane were used as raw material gases. Hydrogen gas flow rate is 500-5000sc
cm, and the ratio of the methane gas flow rate to the hydrogen gas flow rate was 0.5 to 5.0%. Gas pressure is 5 to 120
It was set to torr.

The surface of the diamond thin film 240 was smoothed by polishing in the same manner as in Example 1. In addition, as in Example 1, the surface acoustic wave device 100 was manufactured by applying the diamond substrate 200.

In Table 3, the diamond film 240 (22
0) Strength ratio, hydrogen content, surface arithmetic mean roughness (R
Examples and comparative examples of a), hardness (Knoop hardness measured at a load of 500 g), propagation loss and propagation velocity of surface acoustic waves, and Q value are shown.

[0058]

[Table 3]

In Examples 3-1 to 3-3, by adjusting the Knoop hardness of the diamond thin film 240 measured under a load of 500 g to be 6000 kgf / mm ² or more and 8500 kgf / mm ² or less, a suitable propagation loss, Propagation velocity and Q value (propagation loss of surface acoustic wave at 2.5 GHz is 0.022 dB / λ or less, and propagation velocity of secondary Sezawa wave is 9000 m / s or more, Q value is 700 or more) were realized. . By applying such a diamond substrate 200, a surface acoustic wave device that can be used in a high frequency range of 2.5 GHz or more, has a small insertion loss, and has excellent frequency characteristics is realized.

In Comparative Example 3-1, the Knoop hardness measured with a load of 500 g was as small as 5300 kgf / mm ^2, and there was a problem that the propagation velocity was lowered. In Comparative Example 3-2, the Knoop hardness measured with a load of 500 g was 9400 kg.
The f / mm ² is large and the scattering of the surface acoustic waves at the crystal grain boundaries is large, which causes a problem that the propagation loss is increased. In Comparative Example 3-3, the values of (220) strength ratio, hydrogen content and surface roughness (Ra) are suitable, but the Knoop hardness measured at a load of 500 g is as small as 5500 kgf / mm ² ,
The effect of increasing the propagation velocity was relatively small. In Comparative Example 3-4, the values of (220) strength ratio, hydrogen content and surface roughness (Ra) are suitable, but the Knoop hardness measured at a load of 500 g is as large as 8700 kgf / mm ² and the propagation loss is reduced. The effect of making it was relatively small.

(Embodiment 4) Using a filament CVD method, a silicon wafer 220 having a thickness of 1.0 mm is formed with a thickness of 2 mm.
A 0 μm diamond thin film 240 was formed. W wire was used for the filament. Filament temperature is 2100
The temperature of the silicon wafer 220 is 70
It was set to about 0 to 800 ° C. Hydrogen and methane were used as raw material gases. Hydrogen gas flow rate is 500-5000sc
cm, and the ratio of the methane gas flow rate to the hydrogen gas flow rate was 0.5 to 5.0%. Gas pressure is 5 to 120
It was set to torr.

The surface of the diamond thin film 240 was smoothed by polishing in the same manner as in Example 1. In addition, as in Example 1, the surface acoustic wave device 100 was manufactured by applying the diamond substrate 200.

In Table 4, the diamond thin film 240 (22
0) Strength ratio, hydrogen content, surface arithmetic mean roughness (R
Examples and comparative examples of a), average grain size of diamond crystal, propagation loss and propagation velocity of surface acoustic wave, and Q value are shown. The crystal grain size is determined by scanning electron microscope (SE
It can be measured by M). FIG. 4 shows an example of observing the diamond thin film with a scanning electron microscope (SEM).

[0064]

[Table 4]

In Examples 4-1 to 4-3, by adjusting the average grain size of the diamond crystals in the diamond thin film 240 to be 1 μm or less, suitable propagation loss, propagation velocity and Q value (2.5 GHz) were obtained. Has a propagation loss of 0.022 dB / λ or less, a propagation velocity of secondary Sezawa waves of 9000 m / s or more, and a Q value of 70.
0 or more) was realized. Such a diamond substrate 2
By applying 00, a surface acoustic wave device that can be used in a high frequency range of 2.5 GHz or higher, has a small insertion loss, and has excellent frequency characteristics is realized.

In Comparative Example 4-1 and Comparative Example 4-2, the average grain size of the diamond crystals in the diamond thin film 240 was large at 2.0 μm and 3.0 μm, respectively, and the surface acoustic waves were easily scattered, resulting in propagation loss. There was a problem that In Comparative Example 4-3, the values of the (220) strength ratio, the hydrogen content and the surface roughness (Ra) are suitable, but the average grain size of the diamond crystals in the diamond thin film 240 is large at 2.0 μm, and the propagation loss is large. The effect of reducing the was relatively small.

(Embodiment 5) Using a filament CVD method, a silicon wafer 220 having a thickness of 1.2 mm has a thickness of 3 mm.
A 0 μm diamond thin film 240 was formed. W wire was used for the filament. Filament temperature is 2100
The temperature of the silicon wafer 220 is 70
It was set to about 0 to 800 ° C. Hydrogen and methane were used as raw material gases. The hydrogen gas flow rate was 1500 sccm, and the methane gas flow rate was 60 sccm. The gas pressure was 20 torr.

Based on the above, the hydrogen content is 3.2 in atomic ratio.
%, (220) intensity ratio 0.88 diamond substrate 2
00 was obtained. After fixing the diamond substrate 200 to the metal holder, the diamond thin film 2 is mechanically polished by using a grindstone containing diamond abrasive grains having an average diameter of 30 μm.
40 was smoothed. Table 5 shows the distance from the surface of the bond in the grindstone to the flat end surface of the diamond abrasive grain, the maximum length of the flat end surface of the diamond abrasive grain, and the load during polishing.
Shown in. The polishing conditions were a grindstone rotation speed of 200 rpm and a holder rotation speed of 200 rpm.

Similar to the first embodiment, the diamond substrate 20 is used.
0 was applied to produce the surface acoustic wave device 100.

Table 5 shows examples and comparative examples of arithmetic mean roughness (Ra) of the surface of the diamond thin film 240, propagation loss and propagation velocity of surface acoustic waves, and Q value.

[0071]

[Table 5]

In Examples 5-1 to 5-3, the arithmetic average roughness (Ra) of the surface of the diamond thin film 240 was 10 n.
By adjusting so as to be equal to or less than m, the scattering of the surface acoustic wave becomes small, and the suitable propagation loss, propagation velocity and Q value (the propagation loss of the surface acoustic wave of 2.5 GHz is 0.022d
B / λ or less and the propagation velocity of the secondary Sezawa wave is 90
00 m / s or more and a Q value of 700 or more) were realized. By applying such a diamond substrate 200, a surface acoustic wave device that can be used in a high frequency range of 2.5 GHz or more, has a small insertion loss, and has excellent frequency characteristics is realized.

In Comparative Example 5-2, the diamond thin film 24 was used.
0 has a large arithmetic average roughness (Ra) of 26 nm,
Propagation loss increased due to the increased scattering of surface acoustic waves. In Comparative Example 5-1, the arithmetic average roughness (Ra) of the surface of the diamond thin film 240 was as large as 15 nm, and the effect of reducing the propagation loss was relatively small.

[0074]

As described above, according to the present invention, there is provided a diamond thin film having a high propagation velocity of surface acoustic waves and a small propagation loss, which realizes a surface acoustic wave device having excellent frequency characteristics. A diamond substrate for a surface acoustic wave device and a surface acoustic wave device which are provided are realized.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of a surface acoustic wave device 100 according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along the line II-II of the surface acoustic wave device 100 shown in FIG.

FIG. 3 is an example of a spectrum in Raman spectroscopic analysis of a diamond thin film.

FIG. 4 Scanning electron microscope (SE
It is an example observed in M).

FIG. 5 is an example of an X-ray diffraction chart of a diamond substrate.

[Explanation of symbols]

100 ... Surface acoustic wave element, 120 ... Piezoelectric thin film, 14
0, 160 ... Comb type electrodes, 142, 144 ... Input terminals,
162, 164 ... Output terminal, 200 ... Diamond substrate, 220 ... Silicon wafer, 240 ... Diamond thin film.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tomoki Uemura             Sumitomo, 1-1 1-1 Koyokita, Itami City, Hyogo Prefecture             Electric Industry Co., Ltd. Itami Works (72) Inventor Daiki Kawaguchi             Sumitomo, 1-1 1-1 Koyokita, Itami City, Hyogo Prefecture             Electric Industry Co., Ltd. Itami Works (72) Inventor Hideaki Nakahata             Sumitomo, 1-1 1-1 Koyokita, Itami City, Hyogo Prefecture             Electric Industry Co., Ltd. Itami Works F-term (reference) 4G077 AA03 AB01 BA03 DB19 DB21                       EA02 ED06 FG11 HA04 HA11                 5J097 AA23 EE08 FF02 GG06 HA03                       HB08 KK01 KK05

Claims (6)

[Claims] 1. A diamond substrate for a surface acoustic wave device comprising a substrate and a diamond thin film formed on the substrate, wherein the hydrogen content of the diamond thin film is 1% or more and 5% or less in atomic ratio. The arithmetic mean roughness (Ra) of the surface of the diamond thin film is 20 nm or less, and the peak intensity I ₍₂₂₀₎ of the diamond crystal plane (220) at the time of X-ray diffraction measurement of the diamond thin film and the diamond crystal Faces (111), (220), (311),
Ratio of peak intensities of (400) and (331) to total I _T
A diamond substrate for a surface acoustic wave device, which has an I ₍₂₂₀₎ / _IT of 0.8 or more. 2. The diamond thin film has a peak near a wave number of 1333 cm ⁻¹ in Raman spectroscopic analysis,
The diamond substrate for a surface acoustic wave device according to claim 1, wherein the half width of the peak is 10 cm ^-1 or more. 3. The Knoop hardness of the diamond thin film measured with a load of 500 g is 6000 kgf / mm ² or more 85
The diamond substrate for surface acoustic wave device according to claim 1, wherein the diamond substrate has a pressure of not more than 00 kgf / mm ² . 4. The diamond substrate for a surface acoustic wave device according to claim 1, wherein the diamond crystals in the diamond thin film have an average grain size of 1 μm or less. 5. The diamond substrate for surface acoustic wave device according to claim 1, wherein the diamond thin film has an arithmetic mean roughness (Ra) of 10 nm or less. 6. A piezoelectric thin film is formed on the diamond substrate for a surface acoustic wave device according to claim 1, and a comb-shaped electrode is formed on the piezoelectric thin film. A surface acoustic wave device characterized by: