SURFACE ACOUSTIC WAVE DEVICE
A surface acoustic wave device includes a piezoelectric substrate made of LiTaO3 or LiNbO3 having an electromechanical coefficient of about 15% or more, at least one electrode which is disposed on the piezoelectric substrate and which is a laminate film having a metal layer defining a primary metal layer primarily composed of a metal having a density higher than that of Al or an alloy of the metal and a metal layer which is laminated on the primary metal layer and which is composed of another metal, and a first SiO2 layer which is disposed in a remaining area other than that at which the at least one electrode is located and which has a thickness approximately equivalent to that of the electrode. In the surface acoustic wave device described above, the density of the electrode is at least about 1.5 times that of the first SiO2 layer. In addition, a second SiO2 layer disposed so as to cover the electrode and the first SiO2 layer and a silicon nitride compound layer disposed on the second SiO2 layer are further provided.
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1. Field of the Invention
The present invention relates to a surface acoustic wave device used, for example, for a resonator and a band pass filter, and more particularly, relates to a surface acoustic wave device having a structure in which an insulating layer is arranged so as to cover electrodes.
2. Description of the Related Art
A DPX and an RF filter used for mobile communication systems have been required to satisfy broadband properties and superior temperature properties. Heretofore, in a surface acoustic wave device used for a DPX and an RF filter, a piezoelectric substrate of 36° to 50° rotated Y-plate X-propagation LiTaO3 has been used. This piezoelectric substrate has a frequency-temperature coefficient of approximately −40 ppm/° C. to −30 ppm/° C. Hence, in order to improve the temperature properties, a method has been used in which a SiO2 film having a positive frequency-temperature coefficient is formed on the piezoelectric substrate so as to cover an IDT electrode. In
As shown in
On the other hand, for a purpose different from that for improving frequency-temperature properties described above, a manufacturing method of a surface acoustic wave device is disclosed in Japanese Unexamined Patent Application Publication No. 11-186866 (Patent Document 1) in which an insulating or an semi-conductive protective film is formed so as to cover an IDT electrode of the surface acoustic wave device.
In addition, in Japanese Unexamined Patent Application Publication No. 61-136312 (Patent Document 2), a one-port type surface acoustic wave resonator is disclosed in which after electrodes made of a metal such as aluminum or gold are formed on a piezoelectric substrate made of quartz or lithium niobate, and a SiO2 film is further formed, planarization of the SiO2 film is performed. In this case, due to the planarization, superior resonant properties are obtained.
As shown in
On the other hand, in the surface acoustic wave device described in Patent Document 1, after the inter-electrode-finger film 64 is formed between electrode fingers of the IDT 63, the protective film 65 is formed. Accordingly, the surface of the protective film 65 can be planarized.
However, according to the structure described in Patent Document 1, the IDT electrode 63 is formed of Al or an alloy primarily composed of Al. Although the inter-electrode-finger film 64 is formed so as to be in contact with this IDT electrode 63, a sufficient reflection coefficient cannot be obtained at the IDT electrode 63. As a result, for example, there has been a problem in that ripples are generated in the resonant properties.
Furthermore, in the manufacturing method described in Patent Document 1, prior to the formation of the protective film 65, the resist formed on the inter-electrode-finger film 64 must be removed using a resist stripping solution. However, in this step, the IDT electrode 63 may be corroded by a resist stripping solution. Thus, as a metal forming the IDT electrode, a metal that is susceptible to corrosion cannot be used. That is, the type of metal forming the IDT electrode is limited.
On the other hand, in the one-port type surface acoustic wave resonator described in Patent Document 2, it has been disclosed that quartz or lithium niobate is used as the piezoelectric substrate and the electrode is formed of aluminum or gold. However, the situation in which an electrode made of Al is formed on a quartz substrate is only described as an example. That is, a surface acoustic wave device using another substrate material and/or another metal material has not been specifically described.
SUMMARY OF THE INVENTIONTo overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which an insulating layer is formed on an electrode and in which the properties are not degraded by ripples which appear, for example, in resonant properties. That is, a surface acoustic wave device is provided which has superior resonant properties and filter properties.
As described above, in Patent Document 2, it has been disclosed that superior resonant properties can be obtained by planarization of the upper surface of the SiO2 film. Hence, the inventors of the present invention formed a one-port type surface acoustic wave resonator having a structure that is equivalent to that described in Patent Document 2 except that a LiTaO3 substrate having a high electromechanical coefficient was used as the piezoelectric substrate, and subsequently, the properties of the resonator were determined. That is, after an electrode of Al was formed on the LiTaO3 substrate, a SiO2 film was formed, and the surface of the SiO2 film was then planarized. However, after the SiO2 film was formed, the properties were severely degraded, and it was determined that the resonator cannot be practically used.
When a LiTaO3 substrate or a LiNbO3 substrate is used which has a high electromechanical coefficient as compared to that of quartz, the fractional band width is significantly increased. However, according to detailed investigation performed by the inventors of the present invention, as shown in
As shown in
In addition, heretofore, it has been known that the reflection coefficient is increased as the electrode thickness is increased. However, as shown in
On the other hand, as shown in
According to a preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate made of LiTaO3 or LiNbO3, which has an electromechanical coefficient of at least about 15%, at least one electrode disposed on the piezoelectric substrate and which is a laminate having a metal layer defining a primary metal layer primarily composed of a metal having a density higher than that of Al or an alloy primarily composed of the metal and a metal layer which is laminated on the primary metal layer and which is composed of another metal, and a first SiO2 layer which is disposed in a remaining area other than that at which the at least one electrode is disposed and which has a thickness approximately equal to that of the at least one electrode. In the surface acoustic wave device described above, the density of the electrode is at least about 1.5 times that of the first SiO2 layer. In addition, a second SiO2 layer arranged so as to cover the electrode and the first SiO2 layer and a silicon nitride compound layer disposed on the second SiO2 layer are further provided.
In another preferred embodiment of the surface acoustic wave device according to the present invention, when the thickness of the second SiO2 film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0.08≦h/λ≦0.5 is preferably satisfied.
In another preferred embodiment of the surface acoustic wave device according to the present invention, when the silicon nitride compound layer is composed of an SiN layer, the thickness of the SiN layer is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0<h/λ≦0.1 is preferably satisfied.
In this case, the silicon nitride compound layer may be Si3N4 or other suitable silicon nitride compound other than SiN.
In another preferred embodiment of the surface acoustic wave device according to the present invention, a diffusion inhibition film is further provided which is made of SiN and which is disposed between the second SiO2 layer and the electrode, and when the thickness of the diffusion inhibition film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0.005≦h/λ≦0.05 is preferably satisfied.
In another preferred embodiment of the surface acoustic wave device according to the present invention, the electrode is composed of one of Cu or a Cu alloy, or a laminate film having a metal layer primarily composed of Cu or a Cu alloy.
In another preferred embodiment of the surface acoustic wave device according to the present invention, when the piezoelectric substrate is formed of rotated-Y plate X propagating LiTaO3 or LiNbO3, the thickness of the second SiO2 layer is represented by h1, the thickness of the silicon nitride compound layer formed on the second SiO2 layer is represented by h2, the wavelength of a surface acoustic wave is represented by λ, and the following equations are satisfied: coefficient A1=−190.48, coefficient A2=76.19, coefficient A3=−120.00, coefficient A4=−47.30, coefficient A5=55.25, H1=h1/λ, H2=h2/λ, and θ=(A1H12+A2H1+A3) H2+A4H1+A5, a Y-X cut angle of the rotated Y-plate X-propagation piezoelectric substrate is in the range of θ±5°.
In the surface acoustic wave device according to preferred embodiments of the present invention, on the piezoelectric substrate, the electrode is preferably composed of a metal having a density higher than that of Al, an alloy primarily composed of the metal, or a laminate having a metal layer composed of a metal having a density higher than that of Al or an alloy primarily composed of the metal, the first SiO2 layer having a thickness approximately equal to that of the electrode is disposed in a remaining area other than that at which the electrode is provided, and the second SiO2 layer is disposed so as to cover the electrode and the first SiO2 layer. In the surface acoustic wave device described above, the density of the electrode is preferably set to at least about 1.5 times that of the first SiO2 layer. Thus, when the upper surface of the second SiO2 layer is planarized, ripples which appear in resonant properties and/or filter properties are moved out of the band, and in addition, the ripples are prevented and minimized. In addition, superior frequency-temperature properties are achieved.
In addition, since the silicon nitride compound layer is disposed on the second SiO2 layer, the properties can be adjusted and improved. Furthermore, by performing dry etching of the SiN, the frequency can be adjusted without changing TCF and the fractional band width.
In addition, in the case in which 0.08≦h/λ≦0.5 is satisfied, where the thickness of the second SiO2 film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, as will be apparent from experimental results described below, the frequency-temperature properties TCF is improved. That is, the absolute value of TCF is decreased.
In addition, in the case in which 0<h/λ≦0.1 is satisfied, where the silicon nitride compound layer is composed of SiN, the thickness of the SiN is represented by h, and the wavelength of a surface acoustic wave is represented by λ, the absolute value of the frequency-temperature properties TCF is further decreased, and a surface acoustic wave device having a smaller change in frequency-temperature properties TCF is provided.
Furthermore, in the case in which 0.005≦h/λ≦0.05 is satisfied, where the diffusion inhibition film made of SiN is disposed between the second SiO2 layer and the electrode, the thickness of the diffusion inhibition film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, the amount of change in frequency-temperature properties TCF is decreased, and in addition, resistance properties are improved when a direct current voltage is applied. In addition, when the diffusion inhibition film is provided, diffusion of an electrode material to the second SiO2 layer is prevented, and thus, the surface of the second SiO2 layer can be further planarized. Since the diffusion of the metal forming the electrode to the SiO2 layer is prevented, degradation defects of properties in a high temperature load test are not likely to occur. Thus, the resistance properties are improved when a direct current is applied.
When an electrode of Cu or an alloy primarily composed of Cu or an electrode made of a laminate having an electrode layer of Cu or an alloy primarily composed of Cu is used, an electrode can be provided which is inexpensive and which has superior conductivity.
Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.
Hereinafter, with reference to the figures, particular preferred embodiments of the present invention will be described so as to facilitate the understanding of the present invention.
With reference to
As shown in
A first SiO2 layer 2 is formed over the entire surface of the LiTaO3 substrate 1. In this reference example, the first SiO2 layer 2 is made of a SiO2 film.
A method for forming the first SiO2 layer 2 may be performed by an optional method, such as printing, deposition, or sputtering. In addition, the thickness of the first SiO2 layer 2 is set to be equal to that of an IDT electrode which is to be formed in a subsequent step.
Next, as shown in
Next, with a reactive ion etching method (RIE) performed by ion irradiation as shown by arrows in
When SiO2 is etched by RIE using a fluorinated gas, residues may be generated by a polymerization reaction. In this case, after RIE, treatment may be performed using BHF (buffered hydrofluoric acid) or other suitable treatments.
Subsequently, a Cu film and a Ti film are formed so as to have a thickness equivalent to that of the first SiO2 layer 2. As shown in
Next, using a resist stripping solution, the resist pattern 3 is removed. As described above, as shown in
Then, as shown in
As described above, a one-port type surface acoustic wave resonator 11 having the electrode structure shown in
In
In the above reference example, since the one-port type surface acoustic wave resonator 11 is formed, the IDT electrode 4A is formed on the LiTaO3 substrate 1. However, depending on the application of the surface acoustic wave device, a plurality of IDT electrodes may be formed, and in addition, reflectors may be formed in the same step as the IDTs as described above or may not be provided at all.
For comparison purposes, in accordance with the conventional manufacturing method of a surface acoustic wave device having a SiO2 film, shown in
In addition, in
That is, when the IDT electrode and the SiO2 film are formed in accordance with the conventional method shown in
On the other hand, according to the manufacturing method of this reference example, even when the thickness of the SiO2 film is increased, the properties are not substantially degraded as shown in
In addition, in
When the results in
In addition, from the results shown in
That is, according to the manufacturing method of the reference example, even when the thickness of the SiO2 film is increased as described above, the impedance ratio is not likely to substantially decrease, and the degradation in properties is prevented and minimized.
In addition,
In
As shown in
Thus, it is understood that when the manufacturing method of the above reference example is used, a surface acoustic wave resonator is provided in which the properties are not likely to be degraded and in which the temperature properties are effectively improved.
In addition, in the manufacturing method of the reference example, the IDT electrode is formed of Cu which has a density greater than that of Al. Thus, the IDT electrode 4A has a sufficient reflection coefficient, and undesired ripples which appear in the resonant properties are prevented and minimized. The reasons for this will be described below.
A surface acoustic wave resonator according to a second comparative example was formed in a similar manner to that of the above reference example except that an Al film was used instead of Cu. However, the normalized thickness Hs/λ of the SiO2 film was set to about 0.08. That is, the normalized thickness of the thickness of the first SiO2 film was set to about 0.08. The impedance and the phase properties of the surface acoustic wave resonator are shown by solid lines in
In addition, the impedance and the phase properties of a surface acoustic wave resonator, which was formed in a manner similar to that of the second comparative example except that the SiO2 film was not formed, are shown by dotted lines in
As apparent from the solid lines shown in
Hence, it is understood that although it is intended to improve the frequency-temperature properties by the formation of the SiO2 film, when the IDT electrode is formed of Al, the ripple A appears, and as a result, the properties are degraded. Through investigation further performed on the above point by the inventors of the present invention, it was determined that when a metal having a density greater than that of Al is used for the IDT electrode, the reflection coefficient thereof can be increased, and that the ripple A is thereby eliminated.
That is, in accordance with a manufacturing method similar to that of the above reference example, surface acoustic wave resonators are formed in a similar manner to that of the above reference example, while the density of a metal forming the IDT electrode 4 was variously changed. The impedance properties of the surface acoustic wave resonators formed as described above are shown in
As shown in
Thus, from the results shown in
In
Therefore, in a surface acoustic wave resonator in which the IDT electrode is covered with a SiO2 film, it is understood that when the density of the IDT electrode or the average density of the laminate of the IDT electrode and the protective metal film is greater than the density of the first SiO2 layer located at the side of the IDT electrode, the reflection coefficient of the IDT electrode is increased and that, as a result, the degradation in properties shown between the resonant point and the antiresonant point is prevented and minimized.
In addition, as the metal or the alloy, which has a density greater than that of Al, instead of Cu, for example, Ag, Au, or an alloy primarily formed therefrom may be used.
In addition, when the protective metal film is laminated on the IDT electrode as in the above reference example, as shown in the manufacturing method of
Furthermore, instead of SiO2, the first and the second SiO2 layers may be formed of an insulating material, such as SiOxNy, having a temperature-property improving effect. In addition, the first and the second SiO2 layers may be formed of different insulating materials or may be formed of the same material as described above.
The normalized thickness of the electrode at which the electromechanical coefficient is greater than that of Al was investigated for each metal from the data obtained from
In
Thus, as apparent from the description of particular examples using various different electrode materials, which will be described later, in the structure in which the electrode is formed on a piezoelectric substrate of a 14°˜50° rotated Y-plate X-propagation (Euler angles: (0°, 104°˜140°, 0°) LiTaO3, and in which the SiO2 film is further formed so as to have a normalized thickness H/λ in the range of about 0.03 to about 0.45, when the normalized thickness H/λ of the electrode satisfies the following equation:
0.005≦H/λ≦0.00025xρ2−0.01056xρ+0.16473 Equation (1),
as apparent from the results shown in
In addition, the electrode is formed of the afore-mentioned metal having a density greater than that of aluminum. In this case, the electrode may be formed of a metal having a density greater than that of aluminum or may be formed of an alloy primarily composed of aluminum. In addition, the electrode may be formed of a laminate having a major metal film made of aluminum or an alloy primarily composed of aluminum and a minor metal film made of a metal different from that used for the above-described major metal film. When the electrode is formed of a laminate film, the average density of the electrode preferably satisfies the equation ρ0x0.7≦ρ≦ρx1.3, where ρ0 indicates the metal density of the major electrode layer.
In addition, in the present invention, although the surface of the second SiO2 layer is planarized as described above, the planarization may be performed such that the irregularities are about 30% or less of the thickness of the electrode. When the irregularities are more than about 30%, the advantageous effects of the planarization may not be sufficiently obtained.
Furthermore, as described above, the planarization of the second SiO2 layer may be performed using various methods. For example, a planarization method performed by an etch-back step, a planarization method using an oblique incidence effect by a reverse sputtering effect, a method for polishing an insulating layer surface, or a method for polishing an electrode may be used. Alternatively, at least two of these methods may be used in combination.
First Preferred EmbodimentA surface acoustic wave device 21 of this preferred embodiment is formed in a similar manner to that of the above surface acoustic wave device 11 except that a SiN layer 22 is provided at the topmost portion.
That is, as shown in
With reference to
In addition, the thickness of the first SiO2 layer 2 is set to be approximately equal to that of the electrode. Thus, the upper surface of the structure including the electrode and the first SiO2 layer 2 is planarized, as in the case of the above-described reference example. In other words, the upper surface of the electrode and the upper surface of the first SiO2 layer 2 are disposed at approximately the same height.
The second SiO2 layer 6 is disposed so as to cover the electrode and the first SiO2 layer 2.
When the second SiO2 layer 6 is formed using a thin-film forming method such as sputtering, the upper surface of the second SiO2 layer 6 is formed so as to be flat. That is, since the upper surface of the first SiO2 layer 2 and that of the electrodes are disposed at approximately the same height, when the second SiO2 layer 6 is formed by a thin-film forming method, the upper surface of the second SiO2 layer 6 has an approximately flat surface, and as a result, the generation of undesired ripples is effectively eliminated.
In addition, with the various planarization methods described above, the upper surface of the second SiO2 layer 6 may also be planarized. The meaning of the planarization is the same as that described above.
In addition, as described with reference to
As described above, the surface acoustic wave device 21 is substantially the same as the surface acoustic wave device 11 described in the reference example, except for the SiN layer 22. That is, in the surface acoustic wave device 21, the electrode is formed of (1) a metal having a density greater than that of Al or an alloy primarily formed of the metal, or is formed of (2) a laminate film including a metal layer as a major metal layer, which is composed of a metal having a density greater than that of Al or an alloy primarily formed of the metal, and a metal layer made of another metal and provided on the major metal layer. In addition, the density of the electrode is set to be at least about 1.5 times the density of the first SiO2 layer 2. Thus, in the surface acoustic wave device 21 of this preferred embodiment, the same function and advantage as that obtained by the surface acoustic wave device 11 of the above reference example are obtained.
In the surface acoustic wave device 21 of this preferred embodiment, the SiN layer 22 is disposed so as to cover the second SiO2 layer 6. In this preferred embodiment, the SiN layer 22 is preferably defined by a SiN film. The SiN layer 22 is made of a material having an acoustic velocity different from that of the second SiO2 layer 6. In addition to the above-described function and advantage obtained by the surface acoustic wave device 11 in which the surface of the second SiO2 layer 6 is planarized, the fractional band width (fa−fc)/fc (%) is increased in the surface acoustic wave device 21 according to this preferred embodiment. In the fractional band width, fc indicates a resonant frequency, and fa indicates an antiresonant frequency. In addition, since the resistance at the antiresonant frequency fa, that is, antiresonant resistance Ra, is increased, Q at the antiresonant frequency fa is increased, and for example, in a band pass filter including a plurality of the surface acoustic wave devices 21, the attenuation amount in the attenuation region at a high frequency side of the pass band is increased, such that the steepness of the filter properties is increased.
The above-described functions and advantages of the surface acoustic wave device 21 of this preferred embodiment will be described with reference to more particular experimental examples.
A SiO2 film having a thickness h/λ of about 0.04 and the first SiO2 layer 2 were formed over the entire surface of a 36° rotated Y-plate X-propagation LiTaO3 substrate used as the piezoelectric substrate 1. The formation of the SiO2 film may be performed by an optional method such as printing, deposition, or sputtering. However, in this preferred embodiment, the formation was performed by sputtering.
Next, by using a photolithographic technique, the SiO2 film was patterned. Patterning was performed so as to remove the SiO2 film provided in the region at which the electrode was formed.
Next, a Ti film having a thickness h/λ of about 0.0025 and a Cu film having a thickness h/λ of about 0.0325 were sequentially formed. The total thickness h/λ of the Ti film and the Cu film was about 0.035.
Subsequently, the Ti film and the Cu film disposed on the resist pattern provided on the SiO2 film were removed. As described above, the first SiO2 layer 2 and the electrode were formed.
Next, by sputtering, a SiO2 film was formed over the entire surface, such that the second SiO2 layer 6 was formed. Finally, a SiN film was formed by sputtering, such that the SiN layer 22 was formed. In the surface acoustic wave device 21 obtained as described above, after the frequency properties are measured, frequency adjustment is performed by machining the SiN layer 22. This frequency adjustment will be described with reference to
In general, in the surface acoustic wave device 21, when the thickness of the laminate film made of the SiO2 film and the SiN film is increased, the insertion loss is increased, and the frequency properties are significantly degraded. However, in this preferred embodiment, since the upper surface of the second SiO2 layer 6 is planarized as described above, the degradation in properties caused by the increase in thickness of the laminate film is eliminated and minimized.
As shown in
The frequency adjustment performed by the thickness adjustment of the SiN layer 22 as described above can be easily performed at a mother wafer stage to obtain the surface acoustic wave device 21. In addition, when the frequency adjustment is performed so as to decrease the thickness of the SiN layer 22 using, for example, reactive ion etching or physical etching by irradiation of inert ions such as Ar or N2, the frequency adjustment can be performed when the surface acoustic wave device 21 is mounted on a package.
The surface acoustic wave device 11 of the reference example had the same structure as that of this preferred embodiment, except that the SiN layer 22 was not formed on the second SiO2 layer 6. In the surface acoustic wave device 11 of the reference example, by adjusting the thickness of the second SiO2 layer 6, the frequency is adjusted. However, since the SiN layer 22 was not provided, when the frequency was adjusted by adjusting the thickness of the second SiO2 layer 6, the frequency-temperature coefficient TCF (ppm/° C.) and the fractional band width were substantially changed. On the other hand, in this preferred embodiment, the frequency adjustment is performed by adjusting the thickness of the SiN layer 22, and in this case, the change in frequency-temperature coefficient TCF and that in fractional band width are suppressed. These effects will be described with reference to
As shown in
Thus, as described above, since the SiN layer 22 is provided in this preferred embodiment, the frequency adjustment is performed without causing considerable changes in fractional band width and frequency-temperature TCF. In particular, as shown in
In this preferred embodiment, since the layer 22 is formed of SiN, reactive ion etching thereof can be performed with a gas similar to that used for SiO2 forming the second SiO2 layer 6. Thus, in addition to the thickness adjustment of the SiN layer 22 which is easily performed by reactive ion etching, a step of removing an insulating film provided on electrode pads, which are necessarily exposed for electrical connection of the electrode with the exterior, is easily performed.
Next, in the surface acoustic wave device 21 of this preferred embodiment, the change in frequency properties with the change in thickness of the SiN layer 22 is shown in
As shown in
In addition, as shown in
In particular, when the thickness of the SiN film is in the range of about 100 nm to about 200 nm, that is, when the normalized thickness h/λ is in the range of about 0.05 to about 0.1, the antiresonant resistance Ra is at least about 57.5 dB, and when the thickness is about 150 nm, that is, when the normalized thickness h/λ is about 0.075, the antiresonant resistance Ra is maximally increased to about 60 db.
In this preferred embodiment, the surface of the second SiO2 layer 6 is planarized, and the SiN layer 22 is formed on the upper surface of the second SiO2 layer 6, such that the properties are improved as described above. The effect obtained by the formation of the SiN layer 22 is based on the planarization of the surface of the second SiO2 layer 6 as described above. The above effect will be described with reference to
As in the above-described preferred embodiment, the SiN film was formed so as to have various thicknesses, and the fractional band width and the antiresonant resistance Ra were measured. The results are shown in
In addition, as in the above-described preferred embodiment, various surface acoustic wave devices 21 were formed while the thickness of the second SiO2 layer 6, the thickness of the SiN film 22, and the cut angle of the piezoelectric substrate are varied, and the change in antiresonant Q value with the change in cut angle of the piezoelectric substrate was measured. One example of the change in antiresonant Q value with the cut angle is shown in
As shown in
As shown in
In the above equation, coefficients A1 to A5 are as follows.
coefficient A1=−190.48, coefficient A2=76.19, coefficient A3=−120.00, coefficient A4=−47.30, coefficient A5=55.25, H1=h1/λ, and H2=h2/λ.
In addition, h2 indicates the thickness of the SiN layer, and h1 indicates the thickness of the second SiO2 layer 6.
Second Preferred EmbodimentIn addition, the electrode includes a Ti film, a Cu film, and a Ti film having thicknesses of about 5 nm, about 65 nm, and about 10 nm, respectively. Then, a first SiO2 layer 32 is provided so as to have a thickness of about 80 nm.
In this preferred embodiment, a diffusion inhibition film 35 is disposed so as to cover the electrode and the first SiO2 layer 32. The diffusion inhibition film 35 is made of a SiN film in this preferred embodiment.
In addition, on the diffusion inhibition film 35, a second SiO2 layer 36 is provided.
In this preferred embodiment, since the diffusion inhibition film 35 is made of SiN, diffusion of metal particles from the electrode to the second SiO2 layer 36 is effectively eliminated and minimized.
When the surface acoustic wave device 31 of this preferred embodiment is manufactured, after the diffusion inhibition film 35 is formed, the second SiO2 layer 36 is provided as a temperature property-improvement film by forming a SiO2 film so as to improve the temperature properties, and subsequently, an insulating material on electrode pads of the electrode is removed by reactive ion etching to expose the electrode pads.
When the diffusion inhibition film 35 is not formed, while the second SiO2 layer 36 is formed, an electrode material, Cu in this preferred embodiment, diffuses. As a result, as shown in
On the other hand, as shown in
Hereinafter, the above-described effects will be described with reference to particular experimental examples.
The following table 1 shows variations in properties of individual surface acoustic wave devices including a surface acoustic wave wafer having a diameter of about 10.16 cm, which were obtained for the surface acoustic wave device of the second preferred embodiment in which the diffusion inhibition film 35 is provided, and when a surface acoustic wave device is formed in a manner equivalent to that described above except that the diffusion inhibition film 35 is not provided. In this case, the thickness of the SiN film used as the diffusion inhibition film 35 was set to about 30 nm (h/λ=0.015).
As shown in Table 1, with the insertion of the diffusion inhibition film 35, the variation in properties, such as variation in frequency, is effectively prevented and minimized.
In addition, in this preferred embodiment, since the diffusion inhibition film 35 is made of SiN, reactive ion etching can be performed using a gas similar to that used for forming the first SiO2 layer 32 and the second SiO2 layer 36. Thus, the step of removing an insulating film on the electrode pads for exposure thereof is simplified.
In the second preferred embodiment, the surface acoustic wave device 31 is received in a package and is then wire-bonded and sealed, such that a surface acoustic wave device product is obtained. For the surface acoustic wave device product thus obtained, a high temperature load test was performed in which the product was maintained for approximately 600 hours under the following conditions.
High Temperature Load Test . . . While a direct current voltage of 6V was applied to a surface acoustic wave device product, the product was placed in a high temperature bath, and the insulating resistance was measured with time.
For comparison purposes, in a manner similar to the above-described experimental example, a high temperature load test was performed for a surface acoustic wave device product formed in a manner similar to that described above except that the diffusion inhibition film 35 was not provided. The results are shown in
As shown in
The thickness h of the diffusion inhibition film 35 made of SiN is preferably set so as to satisfy the equation 0.005≦h/λ≦0.05, where λ indicates the wavelength of a surface acoustic wave. This will be described with reference to
That is, in the second preferred embodiment, various surface acoustic wave filter devices were formed by variously changing the thickness of the SiN film defining the diffusion inhibition film 35, and the properties were measured.
In addition, in the surface acoustic wave filter device of the second preferred embodiment described above, after individual surface acoustic wave devices 21 were formed in a manner similar to that described above except that the thickness of the SiN film was changed to about 5 nm, about 10 nm, and about 30 nm, and furthermore, the surface acoustic wave device 31 having no diffusion inhibition film was formed for comparison purpose, a high temperature load test was performed. The results are shown in Table 2 below.
In Table 2, ◯ indicates no malfunctions (an insulating resistance of at least about 106Ω), Δ indicates that although a suppression effect is observed, some are out of order, and x indicates that all show inferior resistance properties and are out of order.
As shown in
In the second preferred embodiment, SiN is preferably used as the diffusion inhibition film 35. However, another nitride film may also be used. For example, AlN, TiN, TaN, or WN may be used. Furthermore, the diffusion inhibition film may be made of an oxide film, such as, for example, Ta2O5.
In addition, in
In addition, in the second preferred embodiment, the diffusion inhibition film 35 is provided between the electrode and the second SiO2 layer 36, and the SiN layer 22 shown in the first preferred embodiment may also be formed on the second SiO2 layer 36. The configuration described above is preferable since the effects of both of the first and the second preferred embodiments are obtained.
In the first and the second preferred embodiments, as the piezoelectric substrate, a 36° rotated Y-plate X-propagation LiTaO3 substrate is used. However, a LiTaO3 substrate having another cut angle may also be used, or another piezoelectric substrate, such as a LiNbO3 substrate, may also be used.
In addition, in the above first and the second preferred embodiments, a one-port type surface acoustic wave resonator is described. However, the present invention can be applied to various general surface acoustic wave devices, including surface acoustic wave filters, such as a ladder type filter including a plurality of one-port type surface acoustic wave resonators as described above. In addition, instead of the one-port type surface acoustic wave resonator described above, the electrode may be formed so as to have various filter and resonator structures.
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
Claims
1. A method of manufacturing a surface acoustic wave device comprising the step of:
- providing a piezoelectric substrate of made one of LiTaO3 or LiNbO3 and having an electromechanical coefficient of at least about 15%;
- forming at least one electrode having a density greater than that of Al on the piezoelectric substrate;
- forming a first SiO2 layer on a remaining area other than that at which said at least one electrode is formed, the first SiO2 layer having a thickness approximately equal to that of the at least one electrode;
- forming a second SiO2 layer so as to cover the at least one electrode and the first SiO2 layer; and
- forming a silicon nitride compound layer on the second SiO2 layer; wherein
- the density of the at least one electrode is at least about 1.5 times that of the first SiO2 layer.
2. The method of manufacturing a surface acoustic wave device according to claim 1, wherein when the thickness of the second SiO2 film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0.08≦h/λ≦0.5 is satisfied.
3. The method of manufacturing a surface acoustic wave device, according to claim 1, wherein the silicon nitride compound layer is formed of an SiN layer, and when the thickness of the SiN layer is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0<h/λ≦0.1 is maintained.
4. The method of manufacturing a surface acoustic wave device, according to claim 1, further comprising the step of:
- forming a diffusion inhibition film made of SiN between the second SiO2 layer and the at least one electrode; wherein
- when the thickness of the diffusion inhibition film is represented by h, and the wavelength of a surface acoustic wave is represented by λ, 0.005≦h/λ≦0.05 is satisfied.
5. The method of manufacturing a surface acoustic wave device, according to claim 1, wherein the at least one electrode is formed of one of Cu, a Cu alloy, and a laminate film comprising a metal layer primarily composed of Cu or a Cu alloy.
6. The method of manufacturing a surface acoustic wave device, according to claim 1, wherein the piezoelectric substrate is made of one of rotated Y-plate X-propagation LiTaO3 or LiNbO3, and when the thickness of the second SiO2 layer is represented by h1, the thickness of the silicon nitride compound layer formed on the second SiO2 layer is represented by h2, the wavelength of a surface acoustic wave is represented by λ, and the following equations are satisfied: a Y-X cut angle of the rotated Y-plate X-propagation piezoelectric substrate is in the range of θ±5°.
- coefficient A1=−190.48;
- coefficient A2=76.19;
- coefficient A3=−120.00;
- coefficient A4=−47.30;
- coefficient A5=55.25;
- H1=h1/λ,H2=h2/λ, and θ=(A1H12+A2H1+A3)H2+A4H1+A5,
7. The method of manufacturing a surface acoustic wave device according to claim 1, wherein the density of the at least one electrode is at least about 2.5 times that of the first SiO2 layer.
8. The method of manufacturing a surface acoustic wave device according to claim 1, wherein the step of forming the at least one electrode includes the steps of:
- laminating a metal layer as a primary metal layer made of a metal having a density greater than that of Al or an alloy primarily composed of the metal; and
- laminating a metal layer on the primary metal layer and which is made of another metal.
9. The method of manufacturing a surface acoustic wave device according to claim 1, further comprising the step of:
- planarizing an upper surface of each of the at least one electrode and the first SiO2 layer before the step of forming the second SiO2 layer.
10. The method of manufacturing a surface acoustic wave device according to claim 1, further comprising the step of:
- planarizing an upper surface of the second SiO2 layer before the step of forming the silicon nitride compound layer.
Type: Application
Filed: Dec 19, 2007
Publication Date: Jul 3, 2008
Applicant: MURATA MANUFACTURING CO., LTD. (Nagaokakyo-shi)
Inventors: Kenji NISHIYAMA (Yasu-shi), Eiichi TAKATA (Nagaokakyo-shi), Takeshi NAKAO (Omihachiman-shi), Michio KADOTA (Kyoto-shi)
Application Number: 11/960,074
International Classification: H01L 41/24 (20060101); B05D 5/12 (20060101);