JPH10126207A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To protect an excitation electrode and to obtain superior characteristics and reliability by laminating an excitation electrode, which generates a surface acoustic wave and a semiconductor layer satisfying a specific relational expression and has a specific resistance value in order on a piezoelectric substrate. SOLUTION: On a piezoelectric substrate 3 of lithium tantalate single crystal, etc., an IDT electrode 4 as the excitation electrode formed of aluminum, etc., and reflectors 5 at both its ends are arranged respectively. On those IDT electrode 4 and reflectors 5, a semiconductor layer 6 as a protective film is laminated in sequentially, by using a semiconductor material such as a compound material containing conductive materials of silicon, etc. Consequently, a foreign body is prevented from sticking on the excitation electrode. Further, an electrode such as the excitation electrode or its semiconductor layer is prevented from being destroyed, owing to discharge caused by the pyroelectricity of the piezoelectric substrate. In this case, the specific resistance value of the semiconductor layer is 10<2> to 10<7> Ω.cm and 0.07<hs<he<0.15 is made to hold. Here, hs is the thickness of the semiconductor layer, and he is the thickness of the excitation electrode.

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 (SAW) filter formed by providing an excitation electrode on a piezoelectric substrate such as a lithium tantalate single crystal, a lithium niobate single crystal, or a lithium tetraborate single crystal. The present invention relates to a surface acoustic wave device.

[0002]

2. Description of the Related Art At present, a single crystal such as lithium tantalate (LiTaO ₃ ) has attracted a great deal of attention as a material having a large electromechanical coupling coefficient generally used for evaluating the performance of piezoelectric materials. Promising as a material used for various piezoelectric devices such as wave devices and bulk wave devices.

As an interdigital (hereinafter abbreviated as IDT) electrode material, aluminum (Al) or an alloy mainly composed of aluminum (for example, an aluminum-copper (Al-Cu) alloy or the like) is used. Since the basic characteristics of the surface acoustic wave device change depending on the thickness of the IDT electrode, it is necessary to search for and study the optimum thickness.
Research in this area is being actively conducted.

For example, a 36 ° Y-cut X-propagation lithium tantalate single crystal is used as a substrate, and aluminum is used for ID.
When used as a T electrode material, it is said that the normalized thickness H / λ of the IDT electrode with respect to the wavelength of the surface acoustic wave to be propagated is optimally set to 0.1 or less (for example, Japanese Patent Application Laid-Open No. 6-188873). Reference).

However, for example, cellular phones, PH
To configure a front-end SAW filter (analog: 900 MHz band, digital: 15 GHz band) used for a mobile communication phone such as S (Personal Handy-phone System), for example, an AMPS (Advanced Mobile Phone)
Service) method, even if a large out-of-band attenuation of 20 dB or more is required at least, even if an attempt is made to apply a lithium tantalate single crystal excellent in characteristics to a substrate material, Nothing has an out-of-band attenuation and a small insertion loss.

The following is considered as one of the causes. That is, it is considered that a conductive foreign matter adheres to the substrate and the excitation electrode during the manufacturing process or the like, which causes deterioration of characteristics. In particular, in the surface acoustic wave device using the frequency in the high-frequency band as described above, since the distance between the electrode fingers of the excitation electrode is very narrow and fine, such adhesion of foreign matter may cause variations in the characteristics of the finished product and the like. This has caused a problem of inducing the occurrence of characteristic failure. For example,
In the case of a surface acoustic wave filter, the pass band width, which is a filter characteristic, may be reduced or the insertion loss may be increased. For this reason, a method has been proposed in which an insulating film such as SiO ₂ is coated on the excitation electrode to protect the excitation electrode from foreign substances (see, for example, JP-A-59-210708). When an insulating film is formed on a pyroelectric piezoelectric substrate, dielectric breakdown easily occurs, which is a serious problem.

Therefore, while protecting the excitation electrode,
It is an object of the present invention to provide a surface acoustic wave device which is improved in characteristics without deterioration in characteristics as in the prior art, and which has extremely excellent reliability.

[0008]

In order to achieve the above object, a surface acoustic wave device according to the present invention comprises: an excitation electrode for generating a surface acoustic wave on a piezoelectric substrate; Is formed by sequentially stacking semiconductor layers of 10 ² to 10 ⁷ Ω · cm.

0.07 <hS / he <0.15 (hs: thickness of semiconductor layer, he: thickness of excitation electrode) Further, the piezoelectric substrate is a single crystal of lithium tantalate, and the semiconductor layer is silicon. It is characterized by being.

Here, the semiconductor layer refers to a composite material or the like in which a conductive material such as carbon or a metal material is added to resin or glass in addition to silicon.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. A surface acoustic wave device S of the present invention is composed of a plurality of surface acoustic wave resonators 1 connected in series and a plurality of surface acoustic wave resonators 2 connected in parallel, as shown in FIG. A surface acoustic wave filter such as a filter is referred to, but is not particularly limited to this.

In the surface acoustic wave device S shown in FIG. 1, a low-pass filter is constituted by a plurality of surface acoustic wave resonators 1 connected in series, and a high-pass filter is constituted by a surface acoustic wave resonator 2 connected in parallel. Thus, desired characteristics are obtained.

As shown in FIG. 2A, the surface acoustic wave resonator 1 or 2 is made of a piezoelectric material such as a lithium tantalate single crystal, a lithium niobate single crystal, or a lithium tetraborate single crystal. Aluminum or an alloy containing aluminum as a main component (Al-Si based, Al-C
IDT which is an excitation electrode composed of u-based, Al-Ti-based, etc.
The electrode 4 is arranged, and IDTs are provided at both ends of the IDT electrode 4.
A reflector 5 made of the same material as that of the T electrode 4 is disposed, and further, as shown in FIG. 2B, silicon or the like, carbon or resin or glass on the IDT electrode 4 and the reflector 5. A semiconductor material such as a composite material containing a conductive material such as a metal material (having a specific resistance in the range of 10 ² to 10 ⁷ Ω · cm), and a semiconductor layer 6 serving as a protective film is sequentially laminated. Then, the surface acoustic wave having the wavelength λ is propagated in the X direction to the substrate 3 configured as described above. In FIG. 2A, the semiconductor layer 6 is shown for simplicity.
Is omitted and a part of the surface acoustic wave resonator is illustrated.

In such a surface acoustic wave device, foreign matter is prevented from adhering to the excitation electrode, so that characteristic deterioration due to foreign matter adhesion is prevented as much as possible. In addition, the discharge caused by the pyroelectricity of the piezoelectric substrate prevents the electrodes such as the excitation electrodes and the semiconductor layers thereof from being broken as much as possible. Such an effect is obtained when the specific resistance of the semiconductor layer is in the range of 10 ² to 10 ⁷ Ω · cm and 0.07 <hS / he <0.15 (where hS is the thickness of the semiconductor layer, he : Thickness of excitation electrode) must be satisfied. That is, if the specific resistance value is smaller than this value, the characteristics are degraded, and if it is larger, the dielectric breakdown is likely to occur. Hs / he is 0.07
This is because, when it is smaller than 0.15 or larger than 0.15, not only the change amount of the pass band width changes to the negative side, but also the change amount of the insertion loss increases.

The surface acoustic wave device is not limited to the above-described embodiment, but includes an electrode such as an excitation electrode disposed on a piezoelectric substrate and a semiconductor layer coated on at least the excitation electrode. As long as the piezoelectric substrate, the excitation electrode,
The material of the semiconductor layer and the like is not limited to those described above,
The present invention can be appropriately changed and implemented without departing from the scope.

[0016]

Next, specific embodiments will be described. As shown in FIG. 1, four or less series and parallel surface acoustic wave resonators were prepared. Further, a 36 ° rotation Y-cut lithium tantalate single crystal is used as a material of the substrate 3, and a metal film made of aluminum is formed as the IDT electrode 4 and the reflector 5, as shown in FIG. A non-single-crystal silicon film 6 (having a specific resistance of about 4) is formed as a semiconductor layer on the IDT electrode 4 as an excitation electrode and the reflector 5.
× 10 ⁵ Ω · cm) with a predetermined thickness. Here, after forming the metal film, the resist was patterned, then a silicon film was deposited, and finally, a region other than the extraction electrode was covered with the silicon film 6 by a lift-off method. Then, a filter was measured for the obtained surface acoustic wave device. In addition, those without the silicon film 6 and those on which SiO ₂ was laminated instead of silicon were also prepared, and compared with those with the silicon film 6 deposited. In order to examine the dielectric breakdown, the sample was heated at 150 ° C. for about 2 hours, and then cooled to room temperature while flowing ionized air.

Here, the logarithm N of the IDT electrode 4 of the surface acoustic wave resonator 1 is 40 to 120, and the intersection width W is 10 (λ) to 4
0 (λ) and the IDT logarithm N of the surface acoustic wave resonator 2 were the same as those of the IDT electrode of the surface acoustic wave resonator 1. Here, λ is the wavelength of the surface acoustic wave.

Next, the film thickness h of the IDT electrode 4 and the reflector 5
e is 150 to 200 nm, and the thickness hs of the silicon film 6 formed on the IDT electrode 4 and the reflector 5 is 10 to 40 nm.
A description will be given of the results of producing thirty surface acoustic wave devices S under the conditions described above, measuring the filter characteristics, and evaluating from the average value. The center frequency is 1900M
The filter characteristics were measured at Hz ± 250 MHz.

First, regarding the relationship between the thickness of the silicon film and the variation of the pass band width, the variation was compared with the case where the silicon film was not stacked, and as shown in FIG.
Under the above conditions, the thickness ratio of the silicon film is 0.07 <hS /
he <0.15 (hs: thickness of the semiconductor layer, h
e: thickness of the IDT electrode), the change amount of the pass band width is 0
It was found to increase (becomes broadband) in the range of ７7 MHz, and to decrease at other film thicknesses.

Further, the relationship between the thickness of the silicon film and the amount of change in the insertion loss was examined in comparison with the case where the silicon film was not laminated. As shown in FIG. The film thickness ratio hs / he is 0.08 to
At 0.14, it was found that the amount of change in insertion loss was negative, and it was found to increase at other film thicknesses.

Further, when the occurrence rate of dielectric breakdown due to electric discharge was examined using a large number of samples, it was found that when a silicon oxide (SiO ₂ ) film as an insulating material was laminated, it was 4 times larger than when a silicon film was laminated. The incidence was more than doubled. That is, in the present embodiment, when the silicon film was laminated, the incidence rate was 0.1% or less,
When the O ₂ film was laminated, the incidence was 0.4%.

Further, in order to examine the change in characteristics due to the metallic foreign matter, silver powder was deposited on the substrate and the excitation electrode, and the characteristics before and after the deposition were compared. As shown in FIG. The wave device did not change its characteristics even after the silver powder was deposited, whereas the surface acoustic wave device without the silicon film had an increased insertion loss after the silver powder was deposited, as shown in FIG. Significant property changes occurred.

From the above results, the silicon film thickness ratio hs
By setting / he to be 0.07 to 0.15, the amount of change in the pass band width increases to the positive side, and it has been found that a suitable surface acoustic wave device having excellent characteristics can be provided. Further, it has been found that by setting the thickness ratio hS / he of the silicon film to 0.08 to 0.14, it is possible to provide a surface acoustic wave device having a smaller insertion loss and more excellent characteristics. Furthermore, according to the above configuration, there is no occurrence due to dielectric breakdown.

[0024]

As described above, according to the surface acoustic wave device of the present invention, it is possible to prevent foreign substances from adhering to electrodes such as the excitation electrode as much as possible. You. In addition, the discharge caused by the pyroelectricity of the piezoelectric substrate prevents breakage of the electrodes such as the excitation electrodes and the semiconductor layer as much as possible, so that a highly reliable surface acoustic wave device can be provided.

In particular, when the piezoelectric substrate is a single crystal of lithium tantalate and the semiconductor layer is a silicon film, the thickness of the semiconductor layer is set to 0.07 to 0.15 to change the pass band width. And a suitable surface acoustic wave device having excellent characteristics can be provided. Further, the thickness ratio of the silicon film is set to 0.0
By setting it to 8 to 0.14, the insertion loss is very small,
A surface acoustic wave device having more excellent characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view showing a surface acoustic wave device according to one embodiment of the present invention.

FIG. 2A is a plan view showing a surface acoustic wave resonator constituting a surface acoustic wave device according to the present invention, and FIG.
Line sectional view.

FIG. 3 is a graph showing the relationship between the thickness of a silicon semiconductor layer and the variation of the passband, as compared to the case where no silicon semiconductor layer is stacked.

FIG. 4 is a graph showing the relationship between the thickness of a silicon semiconductor layer and the amount of change in insertion loss as compared to the case where no silicon semiconductor layer is stacked.

FIG. 5 is a graph showing the relationship between the frequency and the amount of attenuation before and after silver powder adhesion of the present invention.

FIG. 6 is a graph showing the relationship between frequency and attenuation before and after silver powder adhesion in a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave resonator (for series) 2 ... Surface acoustic wave resonator (for parallel) 3 ... Substrate 4 ... IDT electrode (excitation electrode) 5 ... Reflector 6 ...・ Semiconductor layer S ・ ・ ・ Surface acoustic wave device

Claims (2)

[Claims] 1. An excitation electrode for generating a surface acoustic wave on a piezoelectric substrate, and a specific resistance value satisfying the following expression and having a specific resistance value of 10 ² or more:
A surface acoustic wave device formed by sequentially laminating semiconductor layers of 10 ⁷ Ω · cm. 0.07 <hS / he <0.15 (hs: thickness of semiconductor layer, he: thickness of excitation electrode) 2. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is a single crystal of lithium tantalate, and the semiconductor layer is silicon.