JPH0523084B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a novel surface acoustic wave device. More specifically, the present invention relates to a surface acoustic wave element in which a piezoelectric thin film and an electrode are laminated on at least one surface of a plate-shaped substrate made of a new specific aluminum nitride sintered body.

Surface acoustic wave elements are widely used as part of electronic machines and equipment. For example, a surface acoustic wave element in which a thin film of zinc oxide is formed on a glass substrate and electrodes are provided on the surface thereof is used as a radio wave filter material, such as a filter for video intermediate frequencies of televisions and a filter for FM intermediate frequencies. However, conventional surface acoustic wave elements have a defect that prevents them from fully exhibiting their functions when used for high frequencies. Therefore, in recent years, there has been a demand for the development of surface acoustic wave elements that can be used as high-frequency filters.

The present inventor has been earnestly involved in the development of surface acoustic wave devices, and discovered that a new surface acoustic wave device using a new aluminum nitride substrate exhibits significantly superior characteristics, and continued research. As a result, the present invention has been completed and proposed.

That is, the present invention provides a surface acoustic wave element comprising a piezoelectric thin film and an electrode laminated on at least one surface of a plate-like substrate made of an aluminum nitride sintered body, in which the aluminum nitride sintered body has a mechanically fractured surface. It is formed by a tightly packed state of fine grains that are distinguished from each other by clear contours, and the clear contours at the fracture surface of the fine grains are polygonal, and the fine grains have a polygonal shape. The average particle diameter of the crystal at the fracture surface defined by the clear contour is D(μ
A surface acoustic wave device characterized in that the number of crystal grains having a particle size in the range of 0.3D to 1.8D accounts for at least 70% when defined in (m). .

This is a new aluminum nitride sintered body, and the following are used.

That is, it is an aluminum nitride sintered body whose mechanically fractured fracture surface is formed by a tightly packed state of fine crystal grains that are distinguished from each other by clear contours, and the fine crystal grains are The clear contour on the fracture surface is polygonal, and the fine crystals have an average particle size of 0.3D to 0.3D (μm) when the average particle diameter on the fracture surface defined by the clear contour is defined by D (μm). The aluminum nitride sintered body is composed of at least 70% of crystal grains having a particle diameter of 1.8D, preferably 0.5 to 1.5D. The aluminum nitride sintered body preferably has a density of 3.20 g/cm ³ or more, preferably 3.23 g/cm ³ or more. Furthermore, the content of cationic impurities is 0.3% by weight or less, preferably 0.1% by weight.
An aluminum nitride sintered body having the above-mentioned properties of less than % by weight exhibits particularly excellent transparency and is preferably employed.

The aluminum nitride sintered body is new, and such a sintered body may be manufactured, for example, as follows. That is, it contains particles with an average particle size of 2 μm or less and 3 μm or less in a ratio of 70% by volume or more, an oxygen content of 3% by weight or less, preferably 1.5% by weight or less, and an aluminum nitride purity of 95% or more, preferably 97%. % or more, preferably 0.3% by weight or less, preferably 0.1% by weight or less of cationic impurities, and at least one metal oxide selected from Group A metals and Group A metals of the Periodic Table. Or, in the presence or absence of a metal compound that can become the metal during sintering, at normal pressure or under a pressure of 30 to 500 kg/ ^cm2 ,
Obtained by heating and sintering at a temperature of 2100°C. Note that the aluminum nitride mentioned above is a 1:1 compound of aluminum and nitrogen, and anything other than this is treated as an impurity in principle. However, since the surface of the aluminum nitride powder is inevitably oxidized in the air and the AL-N bond is replaced by the AL-O bond, the aluminum bonded to the AL-O bond is not considered to be a cationic impurity. Also, components that serve as sintering aids are not considered cationic impurities. Furthermore, the average particle diameter in the following description refers to the volume-based median particle diameter measured by a light transmission type particle size distribution analyzer.

The aluminum nitride powder having the above characteristics is also new, and for example, one obtained as follows can be suitably used. That is, (1) Fine aluminum oxide particles with an average particle size of 2 μm or less and fine carbon powder with an ash content of 0.2% by weight or less and an average particle size of 1 μm or less are mixed in a liquid dispersion medium such as alcohol, hydrocarbons, or water. The weight ratio of the aluminum oxide fine powder to the carbon fine powder is 1:
0.36 to 1:1; (2) The intimate mixture obtained is optionally dried and calcined at a temperature of 1400 to 1700°C under an atmosphere of nitrogen or ammonia; (3) The fine powder obtained is then exposed to oxygen. The unreacted carbon is removed by heating at a temperature of 600-900℃ in an atmosphere containing
3.0% by weight, preferably 1.5% by weight, and can be produced by producing aluminum oxide powder with an average particle size of 2 μm or less, containing a maximum of 0.3% by weight of metal compounds as impurities. .

In addition, impurities mixed in with raw materials such as alumina and carbon, or mixed in during the manufacturing process of aluminum nitride powder should be minimized.
For example, it is preferable to use a metal compound whose content as an impurity is 0.3% by weight or less, preferably 0.2% by weight or less, more preferably 0.1% by weight or less. Especially among the metals of the above impurity components,
When the total content of iron, chromium, nickel, cobalt, copper, zinc, or titanium components is 0.1% by weight or less, the aluminum nitride powder imparts translucency to the aluminum nitride sintered body. It is suitable because it gives

As described above, the surface acoustic wave device of the present invention requires a piezoelectric thin film to be laminated on at least one surface of a plate-like substrate made of a sintered aluminum nitride body.
Any known material can be used for the piezoelectric thin film as long as it does not hinder its use as a surface acoustic wave element. For example, it is generally preferable to select materials such as zinc oxide and aluminum nitride.
Further, the method of laminating the piezoelectric thin films is not particularly limited, and any known means and apparatus may be used. Generally, for example, a piezoelectric thin film can be formed using a material such as a zinc oxide sintered body, an aluminum nitride sintered body, etc., and a method such as a high-frequency magnetron sputtering method. Furthermore, as a method for laminating aluminum nitride thin films, a method may be adopted in which metal aluminum is deposited as a reactive thin film by high-frequency magnetron sputtering in an atmosphere containing nitrogen.

Furthermore, the method of providing the electrodes is not particularly limited, and any known method can be employed. Generally, a comb-shaped electrode made of gold or aluminum is laminated on the surface of the plate-like substrate made of the aluminum nitride sintered body, or after a piezoelectric thin film is laminated on the surface of the plate-shaped substrate made of the aluminum nitride sintered body. A method of providing this can be suitably adopted.

The surface acoustic wave device of the present invention exhibits characteristics of having a high electromechanical coupling coefficient and a low temperature coefficient, as will be clear from the examples described below. Furthermore, since the aluminum nitride sintered body is used as a substrate, the propagation speed is several times higher than that of a conventional substrate, for example, a glass substrate, so it is extremely excellent as a high frequency surface acoustic wave device.

EXAMPLES In order to explain the present invention more specifically, the present invention will be described below with reference to Examples, but the present invention is not limited to these Examples.

Example 1 The purity is 99.99% (impurity analysis values are shown in Table 1), the average particle diameter is 0.52 μm, and the proportion of particles of 3 μm or less is
100 parts by weight of 95 vol% alumina and 50 parts by weight of carbon black having an ash content of 0.08 wt% and an average particle size of 0.45 μm were uniformly mixed in a ball mill using a nylon pot and a nylon-coated ball with ethanol as a dispersion medium. After drying the resulting mixture, it was placed in a flat plate made of high-purity graphite and heated at 1600°C while continuously supplying nitrogen gas at 3/min in an electric furnace.
The mixture was heated at a temperature of 6 hours. The resulting reaction mixture was heated in air at a temperature of 750° C. for 4 hours to oxidize and remove unreacted carbon. As a result of X-ray diffraction analysis, the obtained white powder was found to be single phase AlN, Al ₂ O ₃
There were no diffraction peaks. In addition, the average particle diameter of the powder was measured using a particle size distribution analyzer (CAPA-500 manufactured by Horiba, Ltd.).
As a result of measurement, it was 1.31 μm, and 90% by volume was 3 μm or less. When observed using a scanning electron microscope, this powder was found to be uniform particles with an average size of about 0.7 μm. Further, the measured value of the specific surface area was 4.0 m ² /g. The analytical values of this powder are shown in Table 2.

Table 1 Al ₂ O ₃ powder analysis values Al ₂ O ₃ content 99.99% Element Content (ppm) Mg <5 Cr <10 Si 30 Zn <5 Fe 22 Cu <5 Ca <20 Ni 15 Ti <5 Table 2 AlN Powder analysis AlN content 97.8% Element Content Mg <5 (ppm) Cr 21 (ppm) Si 125 (ppm) Zn 9 (ppm) Fe 20 (ppm) Cu <5 (ppm) Mn 5 (ppm) Ni 27 (ppm) Ti <5 (ppm) Co <5 (ppm) Al 64.8 (wt%) N 33.4 (wt%) O 1.1 (wt%) C 0.11 (wt%) Aluminum nitride powder thus obtained Calcium nitrate and Ca(NO ₈ ) ₂ ·4H ₂ O were added to the solution to give a concentration of 0.5% by weight in terms of CaO, and the mixture was uniformly mixed using ethanol as a dispersion medium. After mixing, the mixture was dried by gradually removing ethanol while stirring. This mixed powder was filled into a graphite mold with an inner diameter of 40 mm, and hot press sintered at 1 atm of nitrogen gas at a pressure of 200 Kg/cm ² at 1900° C. for 1 hour. The resulting translucent sintered body having a thickness of about 2 mm had a density of 3.26 Kg/cm ³ . After cutting and polishing this sintered body, the final surface roughness is approximately 500 Å, the size is 13 mm x 13 mm, and the thickness is 0.6 mm.
A mm board was created. On this substrate, the period is 100μm.
25 pairs of Au comb-shaped electrodes with an opening of 4.4 mm and a center distance of 5.5 mm were provided by photoetching. Furthermore, a lithium-containing ZnO target was deposited on this substrate to a thickness of 5.33~ by high-frequency magnetron sputtering at a pressure of 1 × 10 ^-2 using a mixed gas of equal amounts of oxygen and argon.
A C-axis oriented thin film of 42.1 μm was formed. Furthermore, ZnO
In order to short-circuit the surface, Al was evaporated onto the ZnO thin film in the same area as the comb-shaped electrode, creating a device with an upper electrode. Electromechanical coupling coefficient (K ² ) of this element
When measured using the first mode for the ratio h/λ of film thickness (h) and wavelength (λ), K ² = 6.6% when h/λ = 0.12, and K when h/λ = 0.19. ² = 5.5%, h/λ
= 0.33 and K ² = 2.2%. Also, the relationship between propagation velocity (Vp) and h/λ is h/λ=0.12, 0.19,
0.33, Vp=6550, 5540, and 5020 m/sec, respectively. Further, when h/λ=0.19, the temperature dependence of the center frequency was -40 ppm/°C.

In addition, a scanning electron micrograph of the mechanically fractured surface of the aluminum nitride sintered body was taken. This photograph is 2000x magnification and corresponds to 1cm5μm, and is attached as Figure 1. This photo (7cm x 10
cm) to measure the particle shape and particle size constituting the sintered body. The particles are formed from polygonal fine crystal grains with clear outlines, and the average particle diameter D (average value of major axis and minor axis) is 6.9 μm, and 0.3D.
The proportion of crystal grains having a particle diameter in the range of ~1.8D, that is, 2.1 μm to 12.4 μm, accounted for 97%.

Example 2 Metal aluminum was used as a target instead of the ZnO target in Example 1, and a mixed gas of equal amounts of nitrogen and argon was used at a pressure of 1×10 ^-2 Torr and a substrate temperature.
A C-axis oriented AlN thin film was formed by high-frequency magnetron sputtering at 200°C, and an element with a comb-shaped electrode at the interface between the AlN substrate and the AlN thin film was created. When the electromechanical coefficient was measured in the same manner as in Example 1, h/λ = 0.18.
K ² = 6.0% and Vp = 7220 m/sec.

[Brief explanation of the drawing]

FIG. 1 of the accompanying drawings is a photograph of a mechanically fractured aluminum nitride sintered body obtained in Example 1, and a portion thereof taken with a microscope.

Claims (1)

[Scope of Claims] 1. A surface acoustic wave element comprising a piezoelectric thin film and an electrode laminated on at least one surface of a plate-like substrate made of an aluminum nitride sintered body, in which the aluminum nitride sintered body is mechanically broken. the cross section is formed by a close packing of fine grains that are distinguished from each other by clear contours, and the distinct contour of the fracture surface of the fine grains is polygonal; The fine crystals have at least 70 crystal grains having a particle size in the range of 0.3D to 1.8D when the average particle size at the fracture surface defined by the clear contour is defined as D (μm). %. 2. The surface acoustic wave device according to claim 1, wherein the aluminum nitride sintered body has a density of 3.20 g/cm ³ or more. 3 Aluminum nitride sintered body removes cation impurities
The surface acoustic wave device according to claim 1, which contains 0.3% by weight or less. 4. The surface acoustic wave device according to claim 1, wherein the piezoelectric thin film is a thin film of zinc oxide or aluminum nitride. 5. The surface acoustic wave device according to claim 1, wherein the electrode is a comb-shaped electrode.