JPS58156215A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To improve the reproducibility of piezoelectric characteristics and to reduce the transmission loss of a high frequency region, by using an elastic matter structure in which an aluminum nitride film is formed on an elastic substrate which has a positive delay time temperature coefficient to a surface acoustic wave. CONSTITUTION:An aluminum nitride film 2 is formed on a surface equivalent to the crystalline face (0001) of a sapphire substrate 1 having a positive delay time temperature coefficient to a surface acoustic wave. In this case, the piezoelectric axis of the film 2 is set vertical or horizontal to the substrate 1. Then a surface acoustic wave generating electrode 3 and a surface acoustic wave detecting electrode 4 are formed on the surface of the film 2. Thus a surface acoustic wave is excited in the direction equivalent to the direction of an axis 1-100 and on the surface (0001) of the substrate 1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave element with a new structure and excellent characteristics.

8surface Acoustic
Conventionally, the structure (substrate) constituting an elastic surface liquid element for handling various electrical signals has been: 1. A structure consisting only of a piezoelectric substrate (piezoelectric single crystal substrate, piezoelectric ceramic substrate) etc.), 2. A structure in which a piezoelectric film is formed on a non-piezoelectric substrate, and 3. A structure in which a piezoelectric film is formed on a semiconductor substrate.

By the way, as the above-mentioned multilayer structure 2, a structure in which a zinc oxide film (ZnO) is formed on a sapphire substrate or a glass substrate by sputtering method etc. is currently known, but this ZnO film has the following structure. There are problems because there are flaws.

1. Because it is difficult to form a high-quality film, it is not possible to obtain a piezoelectric part with sufficient reproducibility.

2. There is a lot of surface acoustic wave propagation loss in the g-frequency region.

3. Dispersion of surface acoustic wave propagation characteristics is large.

4. Temperature change rate of surface acoustic wave slow decay time τ (1/τ)・
It is difficult to control (aτ/aT). (T: ambient temperature 1) The present invention has been made to address these problems, and is an elastic body in which an aluminum nitride film is formed on an elastic body substrate that has a positive slow-decay time temperature coefficient for surface acoustic waves. structure(
The fundamental feature is that a sapphire substrate is used, and in particular, the purpose is to provide a surface acoustic wave element using a sapphire substrate. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional view showing a surface acoustic wave device according to an embodiment of the present invention, where 1 is a sapphire substrate, and II (0 plane) is equivalent to the (0001) crystal plane, or II (0 plane) is equivalent to the (0112) crystal plane (R plane). ), 2 is an aluminum nitride group formed on this substrate 7, and its piezoelectric axis (C axis or (0001) axis) is perpendicular or parallel to the surface of the sapphire substrate 1. It is formed as follows.

3.4 is a comb-shaped surface acoustic wave generating electrode and a detection electrode formed on the surface of the aluminum nitride film 2; H is the thickness of the aluminum nitride film 2;

A plane (0
For a surface acoustic wave element having the structure shown in Fig. II1 in which the K aluminum nitride film 2 is formed so that its C axis is almost perpendicular, When a surface acoustic wave is excited (propagated) on the (00o1) plane of the sapphire substrate 1 in a direction (Y-axis) equivalent to the (11oo) axis direction, the velocity dispersion of the surface acoustic wave as shown in FIG. Characteristics were obtained. In the figure, the horizontal axis represents the standardized thickness H of the aluminum nitride film 2, which is 2π
It is expressed as H/λ (here, λ is the wavelength of the surface acoustic wave), and the vertical axis indicates the phase velocity Vp of the elasticity 5m1t. As is clear from the sameness, the dispersion of the phase velocity ■2 is small, and a very large value of the phase velocity ■p can be obtained.

FIG. 3 shows a surface acoustic wave element having the above structure with a [1100
This shows the characteristic curve of the electromechanical coupling coefficient when a surface acoustic wave is propagated in the direction equivalent to the axial direction (Y-axis). The horizontal axis is 2πH/λ, and the vertical axis is the electromechanical coupling coefficient. K squared
is expressed as a percentage. As is clear from the figure,
In particular, when the normalized film thickness 2πH/λ is in the range of 2.0 to 6.0, K of 0.22% to 0.27% is obtained, which is usually a desirable value for generating and detecting surface acoustic waves. This shows that it has excellent piezoelectricity.

Figure 4 shows the temperature coefficient of delay time (TCD) for surface acoustic waves when waves are excited on the (0001'') plane of the sapphire substrate l in the same manner as above for the surface acoustic wave element having the above structure.
), the horizontal axis is 2πH/λ,
The vertical axis is the delay time tf) of the surface acoustic wave and the temperature t'R rate (
"/r) ・(at/3T) is expressed in ppm/C percentage. Here, the sapphire substrate l has a positive delay time temperature coefficient, while the aluminum nitride film 2 has a positive delay time temperature coefficient. Since it has a negative delay time temperature coefficient, as is clear from the figure, the overall characteristic is a value in which both of them compensate for each other, and it changes according to changes in the film thickness H of the aluminum nitride film 2. .Especially when the above film thickness H is 3.0<2fH/λ<5
．． By selecting a value in the range of 0, the rate of temperature change during the delay time can be brought close to zero.

As in the above example, K, t equivalent to the (0001) crystal plane
The C-axis of the aluminum nitride film 2 is formed almost vertically on a sapphire substrate cut at h (0 plane), and the sapphire substrate (0003) is formed in a direction perpendicular to the C-axis direction of the aluminum nitride film 2 From K, propagating a surface acoustic wave in a direction (Y axis) equivalent to the (iioo) axis direction on a surface,
A surface acoustic wave element having excellent velocity dispersion characteristics and uniformity as well as delay time temperature change characteristics can be obtained.

Other embodiments of the invention will be described below.

A plane (0
For the surface acoustic wave element having the structure shown in FIG. On the (0001) plane of the above sapphire substrate l (
1120) Direction equivalent to the axial direction CX axis) K elasticity table #ar
When propagating through the IL element, velocity dispersion characteristics of surface acoustic waves as shown in No. 5 #tJK were expected.

As is clear from the figure, the dispersion of the phase velocity 1LVp is small, and a very large value of the phase velocity ■ can be obtained.

Figure 816 shows the surface acoustic wave element having the above structure, with the (3120
) Direction equivalent to the axial direction (X-axis)) c Characteristics of electromechanical coupling coefficient when surface acoustic waves are propagated -- (K41t).

As is clear from the figure, the standard film thickness 2πH/λ is 2
．． K is 0.2% to 0628% in the range of 0 to 8.0
This value is usually desirable for generating and detecting elastic surface waves, and indicates excellent piezoelectricity.

FIG. 7 shows a surface acoustic wave element having the above structure, with the sapphire substrate 5 (0001)-[1120]
Temperature coefficient of delay time (TC
D′) is shown.

As is clear from the rrIU diagram, aluminum nitride is used on the back.
I20 thickness H is 3.0 (2πH/λ<6.0 range 11K
j! The rate of temperature change during the delay time can be brought close to zero by increasing the temperature.

As in the above embodiment, the C-axis of the aluminum nitride film 2 is formed almost perpendicularly on a sapphire substrate cut in a plane (Cff1'') equivalent to the (0001) crystal plane. By propagating a K-elastic wave in a direction (X-axis) equivalent to the [1120]-axis direction on the (0001) II of the sapphire substrate 1 in a direction perpendicular to the C-axis direction, velocity dispersion characteristics and 41 properties can be obtained. In addition, a surface acoustic wave element having excellent opening time degree change characteristics during Kjl grinding can be obtained.

Other embodiments of the invention will be described below.

A plane (R
-), the aluminum nitride film 2 is formed on the surface of the surface acoustic wave element having the structure shown in FIG. (0112) plane (R11) of the sapphire substrate 1 in a direction parallel to the piezoelectric axis (C-axis) direction
111) When an elastic am wave was propagated in a direction equivalent to the axial direction, a moderate dispersion characteristic of the elastic table IOm as shown in FIG. 8 was obtained.

As is clear from the figure, the dispersion of the phase velocity ■ is small;
Moreover, a very large value of the phase velocity ■p can be obtained.

No. ** is for the surface acoustic wave element having the above structure, in the same manner as above, on (0112)
) Characteristics of the electromechanical coupling coefficient of a clamp in which surface acoustic waves are propagated in a direction equivalent to the axial direction -! I (Kq!i property).

As is clear from the figure, especially the standard film thickness'; ltH/λ
In the range of 1.0- & 0, 2 is 0.75%~
A value of 0.8% was obtained, which is usually a desirable value for all-generating and detecting elastic surfaces and indicates excellent piezoelectricity.

Figure 1θ shows the surface acoustic wave device having the above structure, with the (0111)
When a surface acoustic wave is propagated in a direction equivalent to the axial direction, the elastic surface fILK';
) shows the characteristic curve of

As is clear from the figure, the temperature change rate of the delay time can be brought close to zero by selecting the film thickness H of the aluminum nitride group 2 in the range 11 of 2.0 (2fH/λ<5.0). I can do it.

As in the above example, aluminum nitride was placed on a sapphire substrate cut in a plane equivalent to the (0112) crystal direction.
12 is formed parallel to the (0111) axis direction of the sapphire substrate 5, and the C axis of this aluminum nitride jII2 is
(000
1) By propagating surface acoustic waves on a surface in a direction equivalent to the [1120] axis direction, it is possible to obtain a surface acoustic wave element that has moderate dispersion characteristics and uniformity as well as excellent temperature change characteristics of delay time. The aluminum nitride film used in each example has a large band gap of about 6.2 eV, and a resistivity 6 of 10 Ω or more can be easily obtained, so it exhibits good insulation. Although this aluminum nitride film is preferably single crystal, a single crystal epitaxial group can be easily formed by using the well-known MO-CVD technique.

In addition, compared to the conventional zinc oxide film formed by sputtering or the like, an aluminum nitride film has excellent reproducibility and a uniform film quality, so that the propagation loss at a high circumference a can be suppressed to II#.

41. Since the aluminum nitride film has a negative temperature coefficient of delay time for surface acoustic waves, a substrate such as a sapphire substrate that has a positive temperature coefficient of delay time may be used. If formed above, the delay time temperature coefficients are compensated for each other, so that stable characteristics against temperature changes can be obtained. 4IK The stability of an element against temperature changes is the most important performance in narrowband signal processing elements such as resonators and oscillators, but according to the structures of each of the above embodiments, stable operation can be achieved against temperature changes. I can do it. Furthermore, higher frequencies and lower losses can be achieved at the same time.

As the substrate on which the aluminum nitride film is formed, any material other than sapphire can be selected as long as it has a negative temperature coefficient of delay time.

As is clear from the above description, according to the present invention, an elastic structure is used which has a temperature coefficient of K such that an aluminum nitride film is formed on an elastic substrate with a positive delay time temperature coefficient for surface acoustic waves. A surface acoustic wave device with excellent performance can be obtained.

According to the present invention explained above, the following effects can be obtained.

! 6. Since the surface acoustic wave velocity is high, the wavelength at high frequencies becomes large, making it easier to manufacture sheet-shaped electrodes, etc.

2. Since the rate of frequency fluctuation due to film thickness fluctuation is small, it is easy to manufacture elements that match the designed operating frequency of 1kK.
It is possible to reduce costs by improving yield.

3. The delay time of the surface acoustic wave element can be brought close to zero.

4. An aluminum nitride film with high insulating properties can be easily obtained, and its single crystal epitaxial film can also be easily formed using MO-CVD technology.

It should be noted that the crystal orientation of the substrate and the aluminum nitride film formed on the substrate and the crystal orientation for exciting (propagating) surface acoustic waves shown in the examples of the main text may be appropriately selected other than those shown in the examples to obtain the same effect. It is possible to obtain

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing an embodiment of the present invention, @Figure 2 to Figure 1
All figures are characteristic diagrams showing the results obtained by the present invention. DESCRIPTION OF SYMBOLS 1... Sapphire substrate, 2... Fluminilumium nitride film, 3... Electrode for surface acoustic wave generation, 4... Electrode for surface acoustic wave detection. Patent applicant Noriyoshi Mikoshiba Kazuo Utsubo, agent Patent attorney Nagai 1) Engraving of Takesanbu drawing (no changes to the original) Fig. 1 3 4 Fig. 2 2'JCH/?-- Fig. 3 and RuH To - Procedural amendment (method/own ship Showa! S7 April 9, 1, Indication of the incident 1986 @l'Fllll Segs No. 3, Relationship with the arresting person incident) , Agent〒105

Claims (1)

[Scope of Claims] 1. An elastic substrate having a positive temperature coefficient of delay time with respect to surface acoustic waves, an aluminum nitride film formed on the elastic substrate and having a piezoelectric axis oriented; A surface acoustic wave device comprising: an electrode formed thereon. 2. The surface acoustic wave device according to claim 8111, wherein the elastic substrate is made of sapphire. 3. The sapphire substrate has a plane equivalent to the (0001) crystal direction, and the piezoelectric axis of the aluminum nitride film is formed perpendicular or horizontal to the sapphire substrate K[i, and is formed on the (0001) ml of the sapphire substrate. So (11
00) The surface acoustic wave element according to claim 8, wherein the surface acoustic wave element propagates in a direction equivalent to the axis. 4. The film thickness H of the nitride Fluminilum film is 214Vλ.
6 (where λ indicates the wavelength of a surface acoustic wave); 5. The sapphire substrate has a plane equivalent to the (0001) crystal plane, and is formed so that the piezoelectric axis of the aluminum nitride film is perpendicular or horizontal to the above-mentioned fire substrate Kli, and on the (0001) plane of the sapphire substrate. (112G
) A surface acoustic wave element according to claim 2, characterized in that the surface acoustic wave is propagated in a direction equivalent to the axis. 6. The film thickness H of the above-mentioned Fluminiram nitride film is 2×2gH/
A patent claim characterized by belonging to the range of λ〈8〉
Elastic table m-wave element according to item 115. 7. The sapphire substrate is formed with a pattern equivalent to the (0112) crystal orientation, and the piezoelectric axis of the nitrided Fluminilum film is perpendicular or horizontal to the fire substrate,
(0111) on the (0112) plane of the above sapphire substrate
The surface acoustic wave element according to claim 5, characterized in that the surface acoustic wave element propagates an elastic wave in a direction equivalent to the axis. 8. The thickness H of the aluminum nitride film is l<2xH/
Range of λ〈8! ! 41111. The surface acoustic wave device according to claim 181, item 7. 9. The surface acoustic wave device according to any one of claims 111 to 8, wherein the aluminum nitride film is made of a single crystal epitaxial film.