JPH03160811A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To prevent contamination of the surface of the device and to improve the performance by adhering an SiO2 film whose refractive index is within a specific range to the surface of the piezoelectric substrate on which electrodes mainly made of aluminum are formed and setting the thickness of the SiO2 film based on the wavelength and the propagation speed of surface acoustic waves generated on the surface. CONSTITUTION:Plural electrodes 3 are formed on the surface of a piezoelectric substrate 2 in a surface acoustic wave device 1 and a passivation film 4 made of SiO2 whose refractive index is nearly 1.5-1.8 is coated on the surface. The surface contamination and adhesion of dust are prevented by coating the film 4 and since the refractive index of the film 4 is large, the effect onto the temperature characteristic is less, but the propagation speed of the surface acoustic wave is constant with respect to a ratio H/lambda (H is thickness and lambda is wavelength) and the oscillating frequency is made stable. Thus, the contamination of the device surface is prevented and the performance is improved.

Description

[Detailed Description of the Invention] [Summary] Regarding the structure of a surface acoustic wave device, particularly a surface acoustic wave device in which an electrode mainly composed of aluminum is formed on a piezoelectric substrate, the present invention provides stabilization of characteristics and stabilization of the surface acoustic wave device. With the aim of eliminating manufacturing problems associated with
, a film is applied, the thickness of the SiO film is H, and the wavelength of the surface acoustic wave generated on the surface is λ. When the propagation velocity of the surface acoustic wave is V, from the relationship between V and H/λ, V is approximately constant H/λ.
The feature is that the thickness H is set higher than λ.

[Industrial application field]

The present invention relates to a surface acoustic wave device, and particularly to a structure of a device in which electrodes mainly made of aluminum are formed on the surface of a piezoelectric substrate.

The surface acoustic wave device is vC○ (Vol.
stage-controlled oscillato
r) and filters in wireless communications, transmission communications, audio, etc. In particular, in the communication field, as the amount of information increases, a frequency exceeding 100 MHz is required, and a frequency exceeding 800 MHz is required for car phones, etc., and VCOs and filters are also required to be miniaturized in order to miniaturize the devices. ing.

(Conventional technology) Conventionally, VCOs and filters in high frequency bands have used vC○, which combines a crystal oscillation (bulk wave) with a frequency lower than the operating frequency and a multiplier circuit, and filters using dielectric resonators. Ta.

In this way, CO using crystal oscillation, which requires a multiplier circuit because the frequency of the oscillator itself is determined by the thickness of the vibrating substrate, and filters using dielectric resonators,
Due to its structure, miniaturization is impossible, and as an alternative, resonators and filters using surface acoustic wave devices in which electrodes are formed on a pressure-sensitive substrate have come to be used.

Resonators and filters using surface acoustic wave devices are easy to manufacture and can be miniaturized to several mm square because the pitch of the formed finger-like electrodes is proportional to the A wavelength of the oscillation frequency. be.

[Problem to be solved by the invention]

However, conventional surface acoustic wave devices in which electrodes are exposed on the surface of a piezoelectric substrate have the disadvantage that their performance is affected by surface contamination and dust adhesion, which makes them difficult to handle during manufacturing. There was a problem.

An object of the present invention is to eliminate the above-mentioned drawbacks and problems and provide a surface acoustic wave device with good performance.

[Means to solve the problem]

According to FIG. 1, which shows an embodiment of the surface acoustic wave device according to the present invention, an electrode 3 mainly made of aluminum has a certain pressure. The surface of the substrate 2 has a light refractive index of approximately 1.5 to
1.8, a SiO2 film 4 having an SiO. The thickness of membrane 4 is H. When the wavelength of the surface acoustic wave generated on the surface is λ, and the propagation velocity of the surface acoustic wave is V, then from the relationship between V and H/λ, V is approximately constant H/λ.
The feature is that the thickness H is set higher than that.

? Effect] The above means uses SiO. By depositing a film (passivation film), the surface is free from contamination and dust adhesion, and the SiO2 film maintains a stable region of surface acoustic wave propagation velocity against film thickness variations. By limiting the optical refractive index to approximately 1.5 to 1.8, performance is stabilized for surface acoustic wave devices with high oscillation frequencies, and handling and manufacturing are facilitated.

〔Example〕

Below, a surface acoustic wave device according to an embodiment of the present invention will be described using the drawings.

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

In FIG. 1, a surface acoustic wave device 1 has a plurality of electrodes (finger-like electrodes) 3 formed on the surface of a piezoelectric substrate 2, and a Stow electrode having an optical refractive index N of approximately 1.5 to 1.8. A passivation film (S i O! II) 4 is deposited.

SiO. The refractive index N of the film is about 1.46,
Surface acoustic wave devices using a SiO film with a refractive index N close to that of quartz as a passivation film have the advantage of good temperature characteristics, but the oscillation frequency changes depending on the film thickness, so the film thickness must be controlled. become.

Therefore, the present inventors investigated SfO films with different refractive indexes N, and found that SiO films with a high refractive index N can stabilize the oscillation frequency, although they have no effect on temperature characteristics.

FIG. 2 is a diagram showing the relationship between the propagation velocity V of surface acoustic waves and the thickness ratio H/λ of the SiCh film.

In Fig. 2, the vertical axis is the propagation velocity v(a+
/s), and the horizontal axis is SiO, the ratio H/λ (%) between the thickness H of the film and the wavelength of the surface acoustic wave.

? A surface acoustic wave device in which an aluminum electrode 3 is formed on a piezoelectric substrate 2 cut out from a 36-degree rotated Y plate of lithium tantalate and whose surface acoustic wave propagation direction is the X direction, and then a SiO film 4 covering the electrode 3 is attached. 1, normal SiO! has an optical refractive index N of 1.467! When the film 4 is deposited, the propagation velocity V becomes equal to the film thickness ratio H/λ, as shown by the broken line in FIG.
decreases as the value increases.

However, when the refractive index N of the SiO film 4 was changed, for example, the SiO film 4 with a refractive index N of 1.5, as shown by the dashed line in FIG. In this case, the propagation velocity V is significantly stabilized compared to that with a refractive index of 1.467, and when the refractive index N is set to 1.6 as shown by the solid line in Fig. 2, the propagation velocity V becomes H/λ by 5% or more. By doing so, it becomes almost constant.

Therefore, from Figure 2, if the refractive index is set to about 1.5 or more, H/
It can be seen that in a region where λ exceeds a certain value, a stabilized propagation velocity V can be obtained regardless of changes in H/λ.

Incidentally, the shape or method of the SiO film 4 is generally c■D (Ch
e+++ical Vapor Deposition
) + methods using sputtering, vapor deposition, M conversion, etc. are known, but as a method with excellent step coverage, the heating temperature of the electrode 3 mainly made of aluminum is 400 ° C or less, CVD is known. is a good idea. C
SiO. Membrane 4 is N. O gas and S
iH. It is known that the refractive index N changes by changing the gas mixture ratio. Figure 3 is a diagram showing the relationship between SiO, the refractive index N of the film, and NtO/SiH4. SiO! The refractive index N of the film, the horizontal axis is the molar ratio N*O/SiHa of NtO gas and SiHa gas, and the curve connecting the plot points in the figure with a solid line is the SiO. The refractive index characteristic of the film, the curve connecting the plot points in the figure with a dashed line is the refractive index characteristic of the Stow film formed under heating at 250'C. When the refractive index exceeds about 1.8 in the refractive index characteristics shown by the solid line and the dashed-dotted line, SiOg
The membrane becomes cloudy and becomes dangerous.

FIG. 4 shows the disconnection rate n of the electrode shown in FIG. 1 and the SiO. A diagram showing the relationship with the refractive index N of the film, FIG. 5 shows the S i shown in FIG. 1.
1 is a diagram showing the main configuration of an Ot membrane permeability test device.

In Fig. 4, the vertical line is the disconnection rate n of the electrode 3, and the horizontal axis is S.
The surface acoustic wave device 1, in which the SiOz film 4 was subjected to the resistance test using the apparatus shown in FIG. 5, had a refractive index N of the SiOg film 4 of 1. When it exceeds 8, the disconnection rate n of the electrode 3 increases rapidly.

The permeability of the SiOt film 4 in relation to the disconnection rate n is determined by immersing the surface acoustic wave device 1 and the test electrode 5 in water 7 in a container 6, and comparing the surface acoustic wave device 1 and the test electrode 5 with the substrate 2 connected to the positive electrode of the device, as shown in FIG. A direct current is passed through the electrode 5 connected to the negative electrode of the device. Then, SiO. While the electrode 3 covered with the film 4 does not show any change, the electrode 3 covered with the Sift film 4, which has poor insulation, corrodes and becomes cut. As explained above, in the present invention, SiO. The refractive index of the film is approximately 1.5 or more as shown in FIG. 2, and approximately 1.5 as shown in FIG.
It is limited to 8 or less. [Effects of the Invention] As explained above, the surface acoustic wave device according to the present invention is characterized in that the surface of the piezoelectric substrate on which electrodes are formed is not contaminated and dust is not attached, and that surface acoustic wave Since the propagation speed does not change, performance is stable and handling and manufacturing are facilitated.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a surface acoustic wave device according to an embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the propagation velocity V of the surface acoustic wave and the thickness ratio H/λ of the SiOg film. is the refractive index N of the SiOg film and NzO/S
FIG. 4 is a diagram showing the relationship between the disconnection rate n of the electrode shown in FIG. 1 and the refractive index N of the SiO2 film. FIG. This is the main configuration diagram of the permeability test device. In the figure, 1 is a surface acoustic wave device, 2 is a piezoelectric substrate, 3 is an electrode, and 4 is SiO, W! , shows, f bullet/l main t-plane: anti-teeth high-chair Figure 5, 02 waist, Manihi CHl person) Kuoki/1i Tao: skin's i-yunae length V and S: (h my 7 maji Lhi H/λ and/) relationship and ho1 figure 2 5 iDz false ER-g￥-N and NZ O/S; 8
Figure 3 shows the connection with 4 iffi Figure 3 Figure 1 Figure 1: Dry 13'+Oz Fake: Tatami → 1 Main area of the examination technique 6 diagrams 5 Figure

Claims (1)

[Claims] The surface of the piezoelectric substrate (2) on which the electrode (3) mainly composed of aluminum is formed has a light refractive index of approximately 1.5 to 1.8.
A SiO_2 film (4) is deposited, and the SiO_2
When the thickness of the film (4) is H, the wavelength of the surface acoustic wave generated on the surface is λ, and the propagation speed of the surface acoustic wave is v, then v is approximately constant from the relationship between v and H/λ. A surface acoustic wave device characterized in that a thickness H is set from H/λ.