JPH02290316A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To attain highly efficient operation by forming a zinc oxide film having a tilt less than a specified value with respect to a silicon layer face with a specific film thickness onto a silicon layer of a face cut off on nearly equivalent to the (110) plane and using a Sezawa wave as a surface acoustic wave element. CONSTITUTION:A dielectric film 7 made of a silicon dioxide or the like is formed on a silicon substrate 5 cut at a face equivalent to the (110) plane. A zinc oxide film 6 is formed on the substrate 5 and the film 7 so that its film thickness (h) is in a range of 0.9<2pih/lambda<3.0 with respect to a wavelength lambdaof a surface acoustic wave and a piezoelectric axis is tilted less than 10 deg. from the direction perpendicular to the face of the substrate 5. When a Sezawa wave is excited as a surface acoustic wave in a direction equivalent to the direction of the (011) axis of the substrate 5 to the input electrode 8, the surface acoustic wave propagates on the surface of the film 8 and reaches an output electrode 9. Thus, the electromechanical coefficient is increased and the element is operated with a high efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device that operates with high efficiency and has a structure that has a small temperature coefficient. Recently, various surface acoustic wave devices that utilize surface acoustic waves propagating along the surface of an elastic body have been actively developed. Lithium niobate (
Piezoelectric single crystals such as LiNb○, ), piezoelectric ceramics such as zirconium titanate buttons (PZT), and piezoelectric thin films such as zinc oxide (ZnO) provided on non-piezoelectric substrates are known. Among these, lithium niobate has a large electromechanical coupling coefficient K and small surface wave propagation loss, but has the disadvantage of a large temperature coefficient. Due to the nature of the body, the higher the frequency, the greater the surface wave propagation loss. Furthermore, since the piezoelectric single crystals and piezoelectric ceramics described above have only a single function, their uses are limited, and it is difficult to manufacture devices with new functions by combining them with ICs.

In this regard, as shown in FIG. 1, the piezoelectric thin film described above is constructed by providing a non-piezoelectric substrate such as a silicon substrate 1, a zinc oxide film 2, etc. on the surface thereof, and electrodes 3 and 4 being provided on the surface of the non-piezoelectric substrate. Therefore, by forming other semiconductor elements on the silicon substrate 1, it becomes possible to realize a device with new functions.

However, the surface acoustic wave device using the piezoelectric thin pancreas has a drawback that it does not operate efficiently because the electromechanical coupling coefficient K is smaller than that of the piezoelectric single crystal and piezoelectric ceramic. Furthermore, since the temperature coefficient is relatively large, it is difficult to apply it to devices where signal delay time is a problem.

The present invention has been made in response to the above problems, and uses a silicon substrate cut in a plane approximately equivalent to the (1 0 0) plane as a substrate material, forms a dielectric film on this silicon substrate, and at least The present invention provides a surface acoustic wave element having a structure in which a zinc oxide film is formed on the surface of this dielectric film, and electrodes are formed in contact with this zinc oxide film.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a cross-sectional view showing a surface acoustic wave device according to an embodiment of the present invention, in which 5 is a silicon substrate cut in a plane equivalent to the (110) plane, and 7 is the silicon substrate 5. A dielectric film partially formed on the top is made of silicon dioxide, for example, and 6 is an oxide film formed on the silicon substrate 5 and the dielectric film 7 so that the piezoelectric axis is perpendicular to the surface of the silicon substrate 5. Zinc films 8 and 9 are comb-shaped electrodes formed on the surface of the zinc oxide film 6.

The zinc oxide film 6 and dielectric film 7 are formed using the well-known sputtering method. The comb-shaped electrodes 8 and 9 are formed by a CVD method or the like.
A metal such as aluminum is formed by a well-known vapor deposition method.

A Sezawa wave is excited in the input electrode 8 of the surface acoustic wave device having the above structure in a direction equivalent to the [001F axis direction of the silicon 5 by surface acoustic wave binding. As a result, the surface acoustic waves propagate on the surface of the zinc oxide film 6 and reach the output electrode 9.

FIG. 3 shows the characteristic curve obtained by the above embodiment of the present invention, where the horizontal axis represents the normalized thickness h of the zinc oxide film 6, which is 2πh/λ (here, λ is the surface elasticity wave wavelength)
The vertical axis indicates the square value K2 of the electromechanical coupling coefficient K in percentage. In the structure of the embodiment of the present invention shown in FIG. 2, when the conductivity near the boundary between the silicon substrate 5 and the zinc oxide film 6 is high, the square value K2 of the electromechanical coupling coefficient K is curved A in the characteristic shown in FIG. change like this. Note that this curve A shows a curve for the above-mentioned Sezawa wave among surface acoustic waves.

In addition, straight line B is lithium diobate (L iN b O
:l) Indicates the maximum value of K2 in the substrate, approximately 5
．． The value is 5%.

As is clear from the characteristics shown in Figure 3, [
001] When a surface acoustic wave is propagated in a direction equivalent to the axial direction, the thickness h of the zinc oxide film 6 is 0.9<2πh/λ<3
．． By selecting a value in the range of 0, it is possible to obtain a large electromechanical coupling coefficient that allows high efficiency operation.

Note that the Sezawa wave is a second-order higher-order mode among a plurality of modes of surface acoustic waves that exist in the structure shown in FIG. 2, for example. Incidentally, the first-order fundamental mode is called a Rayleigh wave. For example r 1977 Ultra
sonic symposium proceedings
Fig from gsIEE J pages 814 to 818,
4, the 1st wave is a Rayleigh wave and the 2nd wave is a Sezawa wave.

The characteristics of the electromechanical coupling coefficient K2 when using Sezawa waves are shown in Fig. 3, but when using Rayleigh waves, for example, rNational TechnicalRe
port, December 1976J, pages 905 to 923, shown in FIG.
2 is large. This coefficient is a very important parameter for surface acoustic wave devices, and has a great effect on the efficiency, bandwidth, etc. of the surface acoustic wave transducer, so the practical effects of using Sezawa waves are great.

An example of K2, which is currently widely used as a piezoelectric material, is shown below.
3 is other material 128'Y LiNb○,K''=5.5%YZ L
iNbO. K2=4.8%IjTaO,
K"=Q.8% quartz has K2 which is one order of magnitude larger than K2α0.2%, but in the ZnO/Si structure device using Sezawa waves of the present invention, the maximum value of K2 is L x N bO 3, and even when the ZnO film thickness h is selected outside of 0.9<2πh/λ<3.o, it is larger than other widely used piezoelectric materials other than LiNbO3. Since K2 is obtained, even if K2 changes depending on the thickness of the ZnO film as shown in Figure 3,
You can get big advantages. FIG. 4 is a cross-sectional view showing a surface acoustic wave device according to another embodiment of the invention, in which 5' is a silicon substrate cut in a plane equivalent to the (1 0 0) plane; The same parts are indicated by the same numbers.

Regarding the input electrode 8 of the surface acoustic wave device having the above structure,
A Sezawa wave is excited as a surface acoustic wave in a direction equivalent to the [0111 axis direction of silicon 5'.

FIG. 5 shows the characteristic curve obtained by the above embodiment. In the above structure, when the conductivity near the boundary between the silicon substrate 5' and the zinc oxide film 6 is high, the electromechanical coupling coefficient K is The square value K2 changes like curve A. Note that this curve A shows a curve for the above-mentioned Séza wave among surface acoustic waves.

Straight line B indicates the maximum value of K2 in the lithium niobate (LiNbO) substrate, which is approximately 5.5%. As is clear from the characteristics shown in FIG. 5, when a surface acoustic wave is propagated in a direction equivalent to the [011F axis direction of the silicon 5' surface, the film thickness h of the zinc oxide film 6 is 0.9<2πh/λ <
By selecting a value in the range of 3.5, it is possible to obtain a large electromechanical coupling coefficient that enables highly efficient operation.

As mentioned above, the fact that the conductivity near the boundary between the silicon substrate 5' and the zinc oxide film 6 is high means that the electric conductivity at the boundary between the silicon substrate 5' and the zinc oxide film 6 is high as shown in FIG.
This means that the same effect can be obtained with a structure in which the metal film 10 is formed in the portion corresponding to 8.9.

Further, even if the silicon substrate 5' has an epitaxial growth layer, the same effect as the structure shown in FIG. 2 can be obtained because the bulk resistance can be lowered.

FIG. 7 shows another embodiment of the present invention, in which a dielectric film 11 made of silicon dioxide or the like having a thickness sufficiently smaller than the wavelength of the surface acoustic wave is uniformly formed on the entire surface of silicon 5'. This structure also provides the same effect as the structures shown in FIGS. 2 and 4.

With the above configuration, the silicon dioxide constituting the dielectric film 11 acts to cancel out the temperature coefficient of the element determined by the silicon substrate 5' and the zinc oxide film 6, so that the overall element It can have a small temperature coefficient. Furthermore, as another embodiment of the present invention, the comb-shaped electrodes can be provided on a silicon substrate or a dielectric film. Alternatively, a structure may be adopted in which a metal film is attached on the zinc oxide film facing the interdigitated electrode. In the examples in this text, a case is shown in which the piezoelectric axis of the zinc oxide film 6 is formed perpendicular to the silicon substrate 5', but a piezoelectric film whose inclination from the direction perpendicular to the silicon substrate 5' is approximately 10 degrees or less Almost the same characteristics can be obtained for the shaft. In addition, the cut surface of the silicon substrate 5' and the propagation axis for exciting surface acoustic waves are shifted by several degrees from the (1 1 0) plane, (100) plane, [001] axis direction, and [0 1 11 axis direction, respectively. It was found that almost the same characteristics can be obtained even when As is clear from the above description, according to the present invention, a silicon substrate cut along a predetermined crystal plane is used as a substrate material, a dielectric film is formed on this silicon substrate, and at least the surface of this dielectric film is Forms a zinc oxide film,
Since the structure is such that electrodes are formed on the surface of this zinc oxide film, the electromechanical coupling coefficient can be made flexible and can be set to an arbitrary value. Also, by providing a dielectric film, the temperature coefficient can be reduced. Since the electromechanical coupling coefficient can be increased in this manner, the impedance of the surface acoustic wave transducer can be reduced and matching can be easily achieved, so that a surface acoustic wave element that can operate with high efficiency can be realized.

Additionally, since the number of electrode pairs in the surface acoustic wave transducer can be reduced, the device can be made smaller and costs can be reduced. Furthermore, since the temperature coefficient is reduced, the surface acoustic wave element can operate stably.

When a Sezawa wave is used as a surface acoustic wave as in the present invention, it is particularly advantageous to realize a high frequency device by utilizing its property of having a large phase velocity.

In particular, by using a common silicon substrate as an IC substrate, the present invention can be effectively applied to a wide range of applications, since it is possible to obtain a smaller, highly integrated device with new functions.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a conventional example, FIGS. 2, 4, 6 and 7 are sectional views showing an embodiment of the present invention, and FIGS. 3 and 5 are sectional views showing an embodiment of the present invention. This is a characteristic diagram showing the results obtained.

Claims (3)

[Claims] (1) A silicon substrate cut in a plane approximately equivalent to the (110) plane, a dielectric film partially formed on the surface of this silicon substrate, and a layer on the silicon substrate so as to cover this dielectric film. and an electrode formed on the zinc oxide film, and propagates a Sezawa wave as a surface acoustic wave in a direction approximately equivalent to the [001] axis direction of the silicon substrate. The film thickness h of the zinc film is selected in the range of 0.9<2πh/λ<3.0, where the wavelength of the surface acoustic wave is λ, and the piezoelectric axis of the zinc oxide film is perpendicular or perpendicular to the silicon substrate. A surface acoustic wave element having an inclination of 10 degrees or less with respect to a direction. (2) A silicon substrate cut in a plane approximately equivalent to the (110) plane, a dielectric film partially formed on the surface of this silicon substrate, and a layer on the silicon substrate so as to cover this dielectric film. a zinc oxide film formed on the silicon oxide film, an electrode formed on the zinc oxide film, and a metal film provided in a corresponding portion of the electric field between the silicon substrate and the zinc oxide film to the electrode; A Sezawa wave is propagated as a surface acoustic wave in a direction substantially equivalent to the [001] axis direction of the substrate, and the film thickness h of the zinc oxide film is set to 0.0.
9<2πh/λ<3.0, and the piezoelectric axis of the zinc oxide film is perpendicular to the silicon substrate surface or has an inclination of 10 degrees or less with respect to the perpendicular direction. Acoustic wave element. (3) The surface acoustic wave device according to claim 1 or 2, wherein the dielectric film is made of silicon dioxide.