JP2012222458A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize insertion loss in a surface acoustic wave device having an IDT electrode disposed on a piezoelectric substrate in order to use a surface acoustic wave even when a frequency band of the device is a high frequency band of 2 GHz or more.SOLUTION: In an SAW resonator 1, an IDT electrode 2 is configured so that a longitudinal wave type leaky wave propagates at a frequency of 2 GHz or more on a piezoelectric substrate 10. Each of electrode fingers 6 is comprised of a laminate film 23 consisting of an aluminum film 22, and a molybdenum film 21 disposed on a lower layer side of the aluminum film 22 and having a larger elastic constant than the aluminum film 22. A film thickness h1 of the molybdenum film 21 is set at two seventh of a film thickness of the entirety of the laminate film 23.

Description

  The present invention relates to a surface acoustic wave device using SAW (surface acoustic wave).

  As a surface acoustic wave device using SAW (surface acoustic wave), an IDT (interdigital transducer) electrode is arranged on a piezoelectric substrate, and both sides of the IDT electrode in the propagation direction of the acoustic wave (surface acoustic wave). There are known SAW resonators in which reflectors are arranged in a ladder, ladder type filters in which SAW resonators are connected in a ladder type, and the like. In this resonator, the pitch (period length) of the electrode fingers in the IDT electrode is set so as to correspond to the wavelength of the elastic wave propagating on the surface of the piezoelectric substrate. Therefore, as the frequency band of use of such a device (for example, the pass band of a filter) increases, the period length decreases (shortens).

  Here, for example, aluminum (Al) is often used as a main electrode material constituting the SAW resonator. In Patent Document 1, a titanium (Ti) film or the like is formed as a base film on the lower layer side of the aluminum film in order to improve power durability in the duplexer, and an electrode material is constituted by the aluminum film and the base film. The technology is described. Such a base film is often set to a thickness of about 1/5 of the electrode material. Therefore, when providing the base film, the aluminum film is the main material of the electrode material. Further, as disclosed in Patent Document 2, a technique of alternately laminating aluminum films and titanium, titanium boride, or the like, or a technique of mixing copper into a metal film (Document 3 described later) is also known.

On the other hand, when the frequency band described above is a high frequency band of 2 GHz or more, the insertion loss of the surface acoustic wave device becomes too large due to various causes, the Q value is lowered, and it becomes impossible to withstand practical use. . For this reason, in a high frequency band of 2 GHz or more, for example, an FBAR (film bulk acoustic resonator), which is a piezoelectric vibration device, is usually used instead of the surface acoustic wave device.
In Patent Documents 3 to 14, surface acoustic wave devices are described, but the above-described problems are not studied.

JP 2001-94382 A JP-A-9-69748 JP 2008-125131 A International Publication 2006-16544 JP 2002-368568 International Publication 2006-46545 JP 2002-305425 A JP2003-152498A JP 2005-253034 A JP 2003-258593 A JP 2003-209455 A International Publication 2000-74235 JP2003-101372A JP-A-9-135143

  The present invention has been made in view of such circumstances, and an object thereof is a surface acoustic wave device in which an IDT electrode is disposed on a piezoelectric substrate in order to use surface acoustic waves, and the frequency band of this device is 2 GHz or more. An object of the present invention is to provide a surface acoustic wave device that can suppress insertion loss even in a high frequency band.

The elastic surface device of the present invention comprises:
In a surface acoustic wave device using a surface acoustic wave having a frequency of 2 GHz or more,
A piezoelectric substrate;
A pair of bus bars disposed on the piezoelectric substrate so as to extend along the propagation direction of the surface acoustic wave and to be separated from each other in a direction perpendicular to the propagation direction of the surface acoustic wave;
A plurality of electrode fingers disposed on the piezoelectric substrate so as to cross each other from each bus bar toward the opposite bus bar,
Each electrode finger is disposed such that a period length including a width dimension of the electrode finger and a spacing dimension between adjacent electrode fingers corresponds to a frequency of the surface acoustic wave propagating on the piezoelectric substrate,
The electrode fingers are each composed of a laminated film including an aluminum film and a conductive film provided on a lower layer side of the aluminum film and having a larger elastic constant than the aluminum film,
The film thickness of the conductive film is set to ¼ to ３ of the total film thickness of the laminated film.

The conductive film may be at least one of molybdenum and titanium.
When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ, and the film thicknesses of the conductive film and the aluminum film are h1 and h2, respectively,
The wavelength specific thickness h1 / λ of the conductive film is 0.02,
The wavelength specific film thickness h2 / λ of the aluminum film is 0.05,
The piezoelectric substrate is a Y-cut Z-propagating lithium niobate substrate,
Each of the electrode fingers may have a periodic length so that a longitudinal type leaky wave of surface acoustic waves propagates on the piezoelectric substrate.

Another elastic surface device of the present invention is:
In a surface acoustic wave device using a surface acoustic wave having a frequency of 2 GHz or more,
A piezoelectric substrate;
A pair of bus bars disposed on the piezoelectric substrate so as to extend along the propagation direction of the surface acoustic wave and to be separated from each other in a direction perpendicular to the propagation direction of the surface acoustic wave;
A plurality of electrode fingers disposed on the piezoelectric substrate so as to cross each other from each bus bar toward the opposite bus bar,
The periodic length including the width dimension of the electrode fingers and the separation dimension between the electrode fingers adjacent to each other is arranged to correspond to the frequency of the surface acoustic wave propagating on the piezoelectric substrate,
The electrode fingers are each composed of a conductive film containing a boride having a larger elastic constant than an aluminum film. The conductive film may be titanium boride.

When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ and the film thickness of the conductive film is h,
The conductive film has a wavelength specific thickness h / λ of 0.07 to 0.1,
The piezoelectric substrate is a Y-cut Z-propagating lithium niobate substrate,
Each of the electrode fingers may have a periodic length so that a longitudinal type leaky wave of surface acoustic waves propagates on the piezoelectric substrate.

The piezoelectric substrate may be a 39 ° -50 ° Y-cut X-propagating lithium tantalate substrate.
When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ and the film thickness of the conductive film is h,
The wavelength specific thickness h / λ of the conductive film is 0.075 to 0.105,
The piezoelectric substrate is a 48 ° Y-cut X-propagating lithium tantalate substrate,
Each of the electrode fingers may have a periodic length so that a transverse-type leaky wave of surface acoustic waves propagates on the piezoelectric substrate.

When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ and the film thickness of the conductive film is h,
The wavelength specific thickness h / λ of the conductive film is 0.035 to 0.065,
The piezoelectric substrate is a 42 ° Y-cut X-propagating lithium tantalate substrate,
Each of the electrode fingers may have a periodic length so that a transverse-type leaky wave of surface acoustic waves propagates on the piezoelectric substrate.
An auxiliary conductive film having a higher conductivity than that of the conductive film may be stacked on the upper side of the conductive film.

  The present invention relates to a surface acoustic wave device using a surface acoustic wave having a frequency of 2 GHz or more. A pair of bus bars and a plurality of electrode fingers are arranged on a piezoelectric substrate, and each of these electrode fingers is adjacent to the width dimension and each other. The period length including the distance between the electrode fingers is set to correspond to the frequency of the surface acoustic wave propagating on the piezoelectric substrate. Each electrode finger is formed of a laminated film including an aluminum film and a conductive film provided on a lower layer side of the aluminum film and having a larger elastic constant than the aluminum film. The film thickness is set to 1/4 to 1/3 of the total film thickness of the laminated film. Therefore, the lower layer side conductive film bears most of the surface acoustic wave energy propagating on the piezoelectric substrate, while the upper layer side aluminum film secures the conductivity of the electrode fingers, thereby increasing the electrical resistance. While suppressing the energy loss of the surface acoustic wave, the energy loss of the surface acoustic wave due to the viscosity of the electrode finger can be suppressed. Therefore, even if it is a surface acoustic wave device used in a high frequency band of 2 GHz or more, insertion loss can be suppressed. In another invention, each electrode finger is composed of a boride having an elastic constant larger than that of the aluminum film, so that it is caused by the viscosity of the electrode finger while suppressing the energy loss of the surface acoustic wave due to the increase in electric resistance. It is possible to suppress energy loss of surface acoustic waves.

It is a top view which shows an example of the SAW device of this invention. It is a longitudinal cross-sectional view which shows the said device. It is a longitudinal cross-sectional view which shows typically the energy distribution of the elastic wave which propagates in the said device. It is the characteristic view obtained about the said device. It is a longitudinal cross-sectional view which shows the other example of the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is explanatory drawing about the cut angle of the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is the characteristic view obtained about the said device. It is a longitudinal cross-sectional view which shows the other example of the said device. It is a top view which shows the other example of the said device.

[Overview of surface acoustic wave devices]
An example of an embodiment of a surface acoustic wave device according to the present invention will be described with reference to FIGS. First, an overview of the surface acoustic wave device will be described. In this example, the surface acoustic wave device is a ladder type filter in which a plurality of SAW (surface acoustic wave) resonators 1 are combined in a ladder type. Y-cut Z-propagating lithium niobate (a substrate on which surface acoustic waves propagate in the Z-direction of the crystal axis on lithium niobate cut perpendicular to the Y-axis) is formed on the piezoelectric substrate 10. In this example, three SAW resonators 1 are arranged as series arms between the input port 11 and the output port 12 so as to be in series with each other, and one SAW resonator 1 is interposed between the SAW resonators 1 and 1. Are connected in parallel as parallel arms. In FIG. 1, 13 is a ground port, and 4 is a lead electrode for electrically connecting the SAW resonators 1, 1 to each other or the SAW resonator 1 and the ports 11, 12, 13. In FIG. 1, each SAW resonator 1 is schematically depicted in a simplified manner.

  Each SAW resonator 1 includes an IDT electrode 2 and reflectors 3 and 3 formed on both sides of the IDT electrode 2 in the propagation direction of the surface acoustic wave (hereinafter referred to as “elastic wave”). . The IDT electrode 2 extends along the propagation direction of the elastic wave and is spaced apart in a direction perpendicular to the propagation direction of the elastic wave, and between the bus bars 5 and 5. , And a plurality of electrode fingers 6 formed in a comb-teeth shape so as to cross each other. In this example, the IDT electrode 2 includes an electrode finger 6 extending from one bus bar 5 of the pair of bus bars 5 and 5 and an electrode finger 6 extending from the other bus bar 5 adjacent to the electrode finger 6. The electrodes are arranged alternately along the propagation direction of the elastic wave to form a regular electrode. In FIG. 1, 7 is a reflector bus bar, and 8 is a reflector electrode finger.

  Each electrode finger 6 is, as shown in FIG. 2, a periodic length composed of the width dimension of each of the two electrode fingers 6, 6 adjacent to each other and the separation dimension between the electrode fingers 6, 6. Is configured to correspond to the frequency of the elastic wave propagating on the piezoelectric substrate 10. Specifically, the period length has the same dimension as the wavelength λ of the elastic wave at a desired frequency. In this embodiment, the longitudinal wave type leaky wave propagating on the piezoelectric substrate 10 is used so as to utilize the longitudinal type leaky wave among the surface acoustic waves propagating on the piezoelectric substrate 10, and the frequency f of the longitudinal wave type leaky wave propagating on the piezoelectric substrate 10 is 2 GHz or more. In the example, the period length λ is configured to be 3 GHz. That is, on this piezoelectric substrate 10, the propagation speed V of the longitudinal wave type leaky wave propagating through the arrangement region of the electrode finger 6 is, for example, 6100 m / s. Therefore, the period length λ is set so that f = V / λ. Is set. Specifically, the cycle length λ is about 2 μm. The number N of electrode fingers 6 and 6 that intersect each other (the number of pairs of electrode fingers 6) N is set to 150 pairs, for example, and the intersection length W at which the electrode fingers 6 and 6 adjacent to each other intersect is 30λ, for example.

  Here, as shown in FIG. 2, each electrode finger 6 is formed by a laminated film 23 in which, for example, a molybdenum (Mo) film 21 that is a conductive film and an aluminum film 22 are laminated in this order from the lower side. It is configured. Specifically, each resonator 1 is formed by forming a molybdenum film 21 and an aluminum film 22 from the lower side in this order over the entire surface of the piezoelectric substrate 10 by, for example, sputtering, and then using photolithography. It is formed by patterning the laminated film 23. The film thickness h1 of the molybdenum film 21 and the film thickness h2 of the aluminum film 22 are, for example, 0.02λ and 0.05λ, respectively. Therefore, the film thickness h1 of the molybdenum film 21 is 2/7 of the total film thickness (h1 + h2) of the laminated film 23. 2 shows a longitudinal sectional view of the piezoelectric substrate 10 taken along line AA in FIG. 1, and the thickness dimension of the piezoelectric substrate 10 is schematically shown.

  As will be described in detail below, the molybdenum film 21 has a much smaller attenuation than the aluminum film 22 when ultrasonic waves (elastic waves) propagate through the molybdenum film 21. On the other hand, the molybdenum film 21 has a higher electrical resistance than the aluminum film 22, and therefore has a large loss (ohmic cross) due to the electrical resistance. Therefore, in the present invention, as shown in FIG. 3 where the energy distribution of the elastic wave is schematically shown by a thick line, the lower molybdenum film 21 bears most of the energy of the elastic wave propagating on the piezoelectric substrate 10, while the upper layer The above-described two-layer structure is used so that the aluminum film 22 on the side ensures the conductivity of the IDT electrode 2. Hereinafter, the reason why the two-layer structure is adopted will be described.

[Estimation of the cause of energy loss of elastic waves]
When an elastic wave propagates inside a solid material, energy loss occurs. The energy loss of such elastic waves generally increases in the order of nonconductor, semiconductor, and conductor, and thus is so small that it can be ignored by the piezoelectric substrate 10 (nonconductor), while constituting the IDT electrode 2 described above. For example, the aluminum film 22 (conductor) has a size that cannot be ignored. It is known that the magnitude of energy loss of such an elastic wave per wavelength (the magnitude of energy loss when the elastic wave propagates by one wavelength) increases almost in proportion to the frequency, Therefore, the higher the frequency, the greater the influence (loss increases). Therefore, the upper limit frequency of practical SAW devices so far has been less than 2 GHz.

  This loss is due to propagation loss due to bulk wave radiation (loss due to being emitted as a bulk wave) in the case of using ohmic crosses or leaky waves of the electrode (aluminum film 22), and in a direction perpendicular to the main propagation direction. Radiation loss due to acoustic wave leakage was dominant. On the other hand, mechanical loss (due to physical properties) of the electrode itself has not been actively addressed so far. Therefore, in the present invention, the mechanical loss of the electrode was examined as follows.

Reference 1 shows the results of evaluating the attenuation of ultrasonic waves propagating in the metal thin film, each of which was formed by sputtering, and several kinds of metal thin films such as aluminum and copper (Cu) were formed. . With respect to the evaluation result of Document 1, when the amount of attenuation per wavelength of the longitudinal bulk wave is calculated assuming that the amount of attenuation is proportional to the frequency,
Aluminum: 0.1472 dB / λ
Molybdenum: 0.0010 dB / λ
Titanium: 0.0044 dB / λ
It becomes.

It can be seen that molybdenum and titanium have much smaller ultrasonic propagation loss than aluminum. On the other hand, Document 2 describes a longitudinal wave propagation loss as a bulk material (a value in a state in which physical properties do not change due to a shape such as a thin film), and similarly, the amount of attenuation per wavelength is set for each material. For each
Aluminum: 0.048 dB / λ
Copper: 0.129 dB / λ
Gold (Au): 0.065 dB / λ
It becomes.

  Comparing the values of these documents 1 and 2 for the loss in aluminum, it can be seen that the loss of the thin film is about three times that of the bulk material. In addition, copper that may be used as an electrode material for SAW devices has a loss of about 2.7 times that of aluminum in the state of a bulk material. . As described above, even if the conductor is the same, there is a large difference in the propagation loss of the elastic wave depending on the material. Even if the material is the same, the propagation loss is larger in the thin film than in the bulk material. The viscosity loss shown in this document increases in proportion to the frequency. Therefore, in a SAW device in which an elastic wave having a high frequency of 2 GHz or more propagates, it is considered that the mechanical characteristics (physical characteristics) of such an electrode material itself greatly affect the frequency characteristics of the entire device. Therefore, for the SAW resonator using an elastic wave having a frequency of 2 GHz or more, the characteristics of the electrode material constituting the SAW resonator were specifically examined.

  First, on a piezoelectric substrate made of Y-cut Z-propagating lithium niobate, a periodic length λ is about 2 μm so that a longitudinal leaky wave (LLSAW: Longitudinal Leaky SAW) resonates at a frequency of 3 GHz on the piezoelectric substrate. The SAW resonator was formed from an aluminum film. The number N of IDT electrodes, the crossing length W, and the wavelength specific film thickness h / λ (h: film thickness of the IDT electrode) were 150 pairs, 30 and 0.0795, respectively. FIG. 4 shows the admittance and the mechanical loss actually obtained by experiments for this SAW resonator as the characteristics 1 and 2, respectively. This mechanical loss expresses the characteristics of the SAW resonator as a standard lumped constant equivalent circuit (a circuit in which another capacitance component is connected in parallel to the inductance, capacitance component, and resistance component connected in series with each other). The loss obtained in the above-described series arm was evaluated. In this characteristic 2, ohmic cross due to electric resistance is removed as a resistance connected in series with the circuit. This mechanical loss is a quadratic function of frequency.

  Further, the above-described SAW resonator was manufactured by changing the logarithm N and the crossing length W in various ways, and mechanical loss was similarly evaluated. Then, from these evaluation results, the mechanical loss when the logarithm N and the intersection length W are sufficiently large (infinitely) is estimated and shown as characteristic 3 in FIG. That is, assuming that the region in which the elastic wave propagates is infinite, the above estimation is performed assuming that there is no loss that the elastic wave leaks from the region as a surface wave in the horizontal direction. Therefore, the difference between the characteristic 2 and the characteristic 3 represents a loss due to leakage of elastic waves from the region.

  Furthermore, the loss due to bulk radiation was calculated by theoretical calculation based on the finite element method, and is shown as characteristic 4 in FIG. This theoretical calculation was determined on the assumption that the electrode constituting the SAW resonator was a perfect conductor, did not include ohmic loss, and had no mechanical loss of electrode (loss due to electrode viscosity described later). In calculating the characteristic 4, the SAW resonator is uniform in the cross length direction (that is, the cross length W is infinitely long) and has an infinite periodic structure in the elastic wave propagation direction. It was. Therefore, the characteristic 3 and the characteristic 4 both represent the mechanical loss when the logarithm N and the intersection length W are sufficiently large, and can be compared with each other.

  When these characteristics 3 and 4 are compared, the loss of characteristic 3 is larger than the loss of characteristic 4. Here, since the secondary coefficient of the characteristic 3 and the secondary coefficient of the characteristic 4 are equivalent, when the difference between the characteristic 3 and the characteristic 4 is taken and shown as the characteristic 5 in FIG. 4, the value of the characteristic 5 is It was found that the frequency gradually increased as the frequency increased. That is, this characteristic 5 represents the loss excluded (not included) from the calculation in calculating the above-described characteristic 4, more specifically, the ohmic loss and mechanical loss of the electrodes constituting the SAW resonator, For example, at 3 GHz, it was about 0.12 dB / λ. Therefore, from the result of the characteristic 5, it can be seen that at least one of the ohmic resistance and the mechanical loss increases as the frequency increases, as described above. In the characteristic 4 of FIG. 4, the bulk wave radiation is zero at a certain frequency (3 GHz) between the resonance point and the antiresonance point of the SAW resonator.

  Subsequently, based on the attenuation obtained when the frequency is 1 GHz in Document 1, the attenuation in the 3 GHz band is calculated (proportional calculation), and the attenuation is converted into the viscosity of the aluminum electrode. The above-mentioned characteristic 4 was calculated in consideration of the rate (including viscosity). As a result, although not shown, this characteristic shows a quadratic curve having a minimum value at 3 GHz as in the case of the characteristic 4 in FIG. 4, but the minimum value of loss is not zero but 0.074 dB / It was λ. This minimum value corresponded to 60% of the value (about 0.12 dB / λ) at 3 GHz of characteristic 5 (difference between the estimated value from the experimental value and the theoretical value).

  From the above description, the mechanical loss in the SAW resonator includes loss due to elastic waves radiated as bulk waves (characteristic 4), loss due to the finite logarithm N and crossing length W (characteristics 2 and 3). In addition to the ohmic loss and the ohmic loss, it can be said that the loss due to the viscosity of the electrodes constituting the SAW resonator is included. That is, the SAW resonator used in the above-described experiment has a Q value at the resonance point as long as the loss is only ohmic loss and bulk wave loss even when the logarithm N and the crossing length W are finite. However, in simulation, it should be about 600, but it was actually about 170. Therefore, the electrode is softer than the piezoelectric substrate (has a small elastic constant) and has a high viscosity, so it is considered that the energy of the elastic wave is dissipated as heat.

[Reduction of mechanical loss]
Thus, metal materials (conductors) such as aluminum, copper, and gold that have been used as the main electrode materials of SAW devices so far cannot be ignored at frequencies as high as 2 GHz or higher (mechanical loss). It is considered that the loss of the SAW device is increased. Therefore, in order to reduce this viscous loss, it turns out that it is necessary to use a conductor whose viscosity loss is smaller than an aluminum film etc. as an electrode material. However, when using a conductor with low viscosity loss as an electrode material, if the electrical resistance of the electrode material is larger than that of an aluminum film or the like, the film thickness of the electrode is reduced as described above, particularly at a frequency of 2 GHz or more. Loss increases. For example, the aforementioned molybdenum film and titanium film have a smaller mechanical loss than the aluminum film, but the electrical resistance as a bulk material is about twice and 30 times that of aluminum, respectively. Therefore, at a frequency of 2 GHz or more, if a molybdenum film or a titanium film is used as an electrode material instead of an aluminum film, an aluminum film is used by balancing the reduction in mechanical loss and the increase in ohmic loss. It is necessary to design so that the loss of the whole electrode becomes smaller than the case.

  Hereinafter, the results of simulation using the finite element method when a molybdenum film is used instead of the aluminum film as an electrode material will be described. When the aluminum film is used as the electrode material, the wavelength specific film thickness h / λ that minimizes the bulk wave radiation loss is 0.08. However, when the molybdenum film is used as the electrode material, the wavelength specific film The thickness h / λ was 0.043. Therefore, when a molybdenum film is used as an electrode material instead of an aluminum film, the viscosity loss is negligibly small, 1/150, compared to the case of using an aluminum film, but the electrical resistance is high and the film thickness is increased. Since it is necessary to reduce the thickness, the ohmic resistance increases by 2 × 8 ÷ 4.3 = 3.72 times (the electric resistance is twice that of the aluminum film and the film thickness is (4.3 / 8) times). .

  Therefore, in order to minimize the viscosity loss without increasing the ohmic resistance as much as possible, an electrode configuration in which an aluminum film and a molybdenum film are stacked was examined. First, in such a laminated structure, it is necessary to make the aluminum film thicker than the molybdenum film in order not to increase the ohmic resistance as compared with the case where only the aluminum film is used as the electrode material. Therefore, as such a laminated structure, a simulation using the finite element method is similarly applied to a configuration in which an aluminum film and a molybdenum film are laminated in this order from the lower side on a piezoelectric substrate made of Y-cut Z-propagating lithium niobate. went. As a result, the viscosity loss did not become less than half that when only the aluminum film was used. That is, when a part of the aluminum film is replaced with a molybdenum film in the film thickness direction of the electrode, the ohmic resistance increases, so that the viscosity loss is not less than half that when only the aluminum film is used. It is considered that the overall loss including viscosity loss is not improved.

On the other hand, the laminated structure which laminated | stacked the molybdenum film | membrane and the aluminum film | membrane in this order from the lower side on the piezoelectric substrate was examined. When the wavelength specific film thickness h1 / λ of the molybdenum film is set to 0.02 and the wavelength specific film thickness h2 / λ of the aluminum film is set to 0.05, that is, the film thickness of the molybdenum film is set to the entire film thickness of the laminated structure. 2/7 (0.02 ÷ (0.02 + 0.05)), the viscosity loss becomes 1/6 compared to the case of using only the aluminum film, and the ohmic resistance is expressed by the following equation (1). As shown, it was found that an increase of 1.33 times is sufficient.

  That is, since the elastic wave propagates on the piezoelectric substrate, in the electrode formed on the piezoelectric substrate, the closer to the surface of the piezoelectric substrate (the interface between the piezoelectric substrate and the electrode), the propagation and excitation of the elastic wave. It is considered that the proportion contributing to (generation) increases, while the proportion contributing to the propagation and excitation of elastic waves decreases as the distance from the surface of the piezoelectric substrate increases. Therefore, by arranging a molybdenum film under the aluminum film and setting the ratio of the film thickness of the aluminum film and the molybdenum film as described above, the viscosity loss is smaller than that of the aluminum film (the elastic constant is large). ) The molybdenum film bears most of the energy of the elastic wave propagating on the piezoelectric substrate, and an aluminum film having an electric resistance smaller than that of the molybdenum film ensures the conductivity of the electrode.

  According to the above-described embodiment, in the SAW resonator 1 in which the IDT electrode 2 is configured such that a longitudinal wave type leaky wave propagates on the piezoelectric substrate 10 at a frequency of 2 GHz or more, an aluminum film is formed on each electrode finger 6. 22 and a molybdenum film 21 provided on the lower layer side of the aluminum film 22 and having a larger elastic constant than the aluminum film 22, and a film thickness h1 of the molybdenum film 21 is set to the laminated film. The total film thickness of 23 is set to 2/7. Therefore, the molybdenum film 32 bears most of the energy of the surface acoustic wave propagating on the piezoelectric substrate 10, while the aluminum film 22 ensures the conductivity of the electrode fingers 6. The energy loss of the surface acoustic wave due to the viscosity of the electrode finger 6 can be suppressed while suppressing the energy loss. Therefore, even with the SAW resonator 1 used in a high frequency band of 2 GHz or more, insertion loss can be suppressed. Therefore, the application range of the SAW device can be expanded in a frequency band of 2 GHz or higher, which has been difficult to apply in the past.

  When only the molybdenum film 21 is used as the electrode material instead of the aluminum film 22, the molybdenum film 21 has a thickness of 0.043 / 0.08 when the aluminum film 22 is used as described above. Therefore, it is necessary to set the accuracy of the film thickness more strictly (strictly) than when only the aluminum film 22 is used. However, by adopting a laminated structure of the molybdenum film 21 and the aluminum film 22, the film thickness of the laminated structure becomes thicker than when only the molybdenum film 21 is used, and therefore high accuracy is not required for the film thickness of the laminated structure. Therefore, it becomes easy to manufacture the SAW resonator 1.

  In the example described above, the film thickness h1 of the molybdenum film is set to 2/7 of the total film thickness of the laminated film 23. However, in order to reduce the viscosity loss while suppressing the increase in ohmic resistance, the molybdenum film is used. The film thickness h <b> 1 of 21 may be set to ¼ to 膜厚 of the total film thickness of the laminated film 23. Although the molybdenum film 21 is used as a conductive film formed below the aluminum film 22, at least one of a titanium film, ruthenium (Ru), and tungsten (W) is used instead of or together with the molybdenum film 21. A single film may be used.

[Other embodiments]
Next, another embodiment of the SAW device of the present invention will be described with reference to FIG. In this embodiment, each SAW resonator 1 is formed by a titanium boride (TiB 2) film 25 instead of being formed by a laminated structure of a molybdenum film 21 and an aluminum film 22. The reason why the SAW resonator 1 is constituted by the titanium boride film 25 will be described in detail below. In this embodiment, the description of the same configuration as the SAW device described with reference to FIGS. 1 and 2 is omitted.

  As described in Document 2, in general, there is a correlation between the amount of attenuation of the elastic wave when the elastic wave (sound wave) propagates inside the solid material and the electrical conductivity of the solid material. Yes, the attenuation increases in the order of nonconductor (including oxide crystal), semiconductor and conductor. In addition, even with the same conductor, a material having a very small electric resistance tends to have a large attenuation of sound waves. That is, for each material of the conductor, it can be said that the attenuation amount of the sound wave decreases as the electric resistance increases. Therefore, it may be said that a material suitable as the electrode material of the SAW resonator 1 is a material having high electrical resistance among conductors.

  For each material described in the above-described example, the loss of elastic waves (loss due to viscosity) is large for aluminum, copper, gold, or the like having a low electric resistance, but is small for molybdenum, titanium, or the like having a high electric resistance. Such a material such as molybdenum or titanium having a small elastic wave loss has a common characteristic that it has a large elastic constant and an electric resistance of 6 μΩcm or more as compared with a material having a large elastic wave loss.

  Therefore, a material having an electric resistance of, for example, 5 μΩcm or more, an electric resistance of 20 μΩcm or less so that the ohmic resistance is not so large, and an elastic constant as large as possible is considered suitable as an electrode material for the SAW resonator 1. Here, as shown in the following table, borides such as titanium boride (zirconium boride (ZrB2), niobium boride (NbB2), tantalum boride (TaB2), chromium boride (CrB2), molybdenum boride (Mo2B3), tungsten boride (W2B3), and lanthanum boride (LaB6)) have a resistivity of 20 μΩcm or less, which is relatively low compared to other compounds, especially titanium boride (7 μΩcm) and zirconium boride (6 μΩcm). ) Can be sufficiently used as the electrode material of the SAW resonator 1 (Reference 4). Such borides have an extremely large elastic constant than aluminum.

(table)

In this table, “c11” is the elastic constant related to the longitudinal wave, “c44” is the elastic constant related to the transverse wave, and the phase velocity V (longitudinal wave) and V (transverse wave) of the longitudinal wave and the transverse wave are expressed by the following equations, respectively. It is calculated | required by (2) and Formula (3).

次いで、ホウ化チタン膜２５をＳＡＷ共振子１の電極材料として用いた場合に得られる特性について説明する。具体的には、同軸ＹカットＸ伝搬タンタル酸リチウム上を伝搬するＬＳＡＷの代表として、４８°ＹカットＸ伝搬タンタル酸リチウム（結晶軸のＸ軸回りに４８°回転したＹ軸に垂直に切断したタンタル酸リチウム上をＸ方向に弾性表面波が伝搬する基板）からなる圧電基板上に、横波型リーキー波が伝搬するように正規型のＳＡＷ共振子を形成した場合に得られる周波数特性を例に挙げてシミュレーションした。この時、ＳＡＷ共振子におけるＩＤＴ電極のライン占有率（電極指の幅寸法÷（電極指の幅寸法＋互いに隣接する電極指間の距離））は、０．５とした。また、対数Ｎ及び交差長Ｗについては、既述のように各々無限長とした。

Next, characteristics obtained when the titanium boride film 25 is used as the electrode material of the SAW resonator 1 will be described. Specifically, as a representative of LSAW propagating on coaxial Y-cut X-propagating lithium tantalate, 48 ° Y-cut X-propagating lithium tantalate (cut perpendicular to the Y axis rotated 48 ° around the X axis of the crystal axis) An example of frequency characteristics obtained when a regular SAW resonator is formed on a piezoelectric substrate (a substrate on which surface acoustic waves propagate in the X direction on lithium tantalate) so that transverse wave leaky waves propagate. I gave a simulation. At this time, the line occupation ratio of the IDT electrodes in the SAW resonator (electrode finger width dimension ÷ (electrode finger width dimension + distance between adjacent electrode fingers)) was set to 0.5. Further, the logarithm N and the intersection length W are each infinite length as described above.

  Then, the wavelength specific film thickness h / λ was variously changed, and the resonance frequency fr, the antiresonance frequency fa, and the frequency fmin at which the propagation loss due to the bulk radiation was minimized were calculated. At this time, simulation was performed for each of the cases where aluminum (FIG. 6), molybdenum (FIG. 7), and titanium boride (FIG. 8) were used as the electrode material of the SAW resonator. 6 to 8, the horizontal axis indicates the wavelength ratio film thickness h / λ described above, and the vertical axis indicates the frequency f normalized by the fast horizontal wave cut-off frequency (black frequency) fB.

  When the transverse wave type leaky wave is used, the propagation loss due to the bulk radiation is theoretically zero at the frequency fmin. Therefore, in the resonance type device, in order to minimize the average loss amount, the resonance frequency fr and the antiresonance are obtained. It is preferable that the frequency fmin is located at an approximately middle position between the frequency fa. Accordingly, when the wavelength specific film thickness h / λ when the loss is minimized in each of FIGS. 6 to 8 is read, it is 0.10 for aluminum, 0.043 for molybdenum, and 0.085 for titanium boride. In addition, in the simulation of FIGS. 6-8, the viscous loss of an electrode and other loss are excluded from calculation.

  At this time, regarding these three kinds of electrode materials (aluminum, molybdenum and titanium boride), the electrical resistance when the film thickness is adjusted so as to minimize the propagation loss due to bulk radiation as described above will be examined. Assuming that the electrical resistance when aluminum is used is 1, molybdenum has (5.6 / 2.7) / (0.043 / 0.1) = 4.8 times, but with titanium boride ( 7 / 2.7) / (0.085 / 0.1) = 3 times. Therefore, although the electrical resistance of titanium boride is larger than that of molybdenum in the above-mentioned table, the electrical resistance is examined after adjusting the film thickness of the electrode so that the propagation loss due to bulk radiation is minimized. It has been found that titanium halide has a lower electrical resistance than molybdenum. As described above, the electrical resistance in the table is a value for the bulk, and in the case of a thin film, it is considered to be larger (becomes worse) than the value in this table. It is clear that titanium has a lower electrical resistance than molybdenum. Therefore, by forming a SAW resonator (electrode finger) using a titanium boride film, the increase in loss due to electric resistance is minimized as compared with the case where an aluminum film is used. Loss due to the viscosity of the film can be kept small. Therefore, it is possible to obtain a SAW resonator (SAW device) with a smaller loss than when aluminum is used.

  Subsequently, when the film thickness sensitivity (fr / fB) / (h / λ) at the resonance frequency fr is examined, it is −1.08 for aluminum and −2.16 for molybdenum, whereas titanium boride. It was -0.97. Therefore, molybdenum requires about twice as much film thickness accuracy as aluminum, but titanium boride has a slightly lower film thickness sensitivity than aluminum, so it can be said that it is easy to manufacture a SAW resonator. Therefore, from FIG. 8, when the SAW resonator is formed on the above-described piezoelectric substrate using the titanium boride film so that the transverse wave type leaky wave propagates, the preferable range of the wavelength specific film thickness h / λ is as follows. 0.075 to 0.105.

  In the above-described example, the 48 ° Y-cut X-propagating lithium tantalate is described as an example of the coaxial Y-cut X-propagating lithium tantalate. However, as this substrate, 42 ° Y-cut X-propagating lithium tantalate is used. Also good. For the 42 ° Y-cut X-propagating lithium tantalate, the same calculation as in FIGS. 6 to 8 described above was performed for the case where the SAW resonator was configured using an aluminum film and a titanium boride film, As shown in FIG. 9, the preferable range of the wavelength specific film thickness h / λ was ± 0.015 centered on the optimum film thickness 0.05. Specifically, since an ohmic cross is applied at the resonance point fr, fmin is preferably slightly closer to fr. Therefore, the optimum film thickness described above is set to 0.05.

  The above 48 ° Y-cut X-propagating lithium tantalate and 42 ° Y-cut X-propagating lithium tantalate are typical examples of Y-cut X-propagating lithium tantalate, and the wavelength ratio film thickness h / λ is as shown in FIG. In addition, it is known that the transition is almost linear according to the cut angle. Therefore, it can be said that the preferable cut angle of the Y-cut X-propagating lithium tantalate in constructing the SAW resonator using the titanium boride film is 39 ° to 50 °.

  Similarly, characteristics when a regular SAW resonator is arranged on a piezoelectric substrate made of Y-cut Z-propagating lithium niobate so that a longitudinal-wave leaky wave propagates will be described. Also in this example, the line occupation ratio is set to 0.5. Similarly, the results calculated for aluminum, molybdenum, and titanium boride are shown in FIGS. Moreover, about these aluminum, molybdenum, and titanium boride, the propagation loss (alpha) by the bulk radiation in the frequency fmin is shown in FIGS. 14-16, respectively. 14 to 16, the horizontal axis represents the wavelength specific film thickness h / λ, and the vertical axis represents the propagation loss α. As shown in FIGS. 14 to 16, the propagation loss α is a quadratic curve that becomes zero at a certain film thickness. Thus, at the film thickness at which the propagation loss α is zero, the frequency fmin is located in the middle between the resonance frequency fr and the antiresonance frequency fa, so that the optimum propagation loss due to average bulk radiation can be minimized. The film thickness is great. This film thickness (wavelength specific film thickness h / λ) was 0.080 for aluminum, 0.043 for molybdenum, and 0.086 for titanium boride.

  In this case as well, when the electric resistance considering the film thickness is calculated with aluminum as 1, it is (5.6 / 2.7) / (0.043 / 0.080) = 3.9 times for molybdenum. On the other hand, in titanium boride, (7 / 2.7) / (0.086 / 0.080) = 2.4 times, and it can be said that titanium boride has lower electrical resistance than molybdenum. The film thickness sensitivity of the resonance frequency fr is -2.82 for aluminum, -3.27 for molybdenum, and -1.44 for titanium boride, and titanium boride is the easiest among these three types of materials. In other words, it can be said that a SAW resonator can be manufactured (without requiring high accuracy). Further, in each of the quadratic curves of FIGS. 14 to 16, the curve of titanium boride has the gradual slope, and therefore, the titanium boride has a wider range of film thickness with less loss than aluminum or molybdenum. ing. Therefore, when a regular SAW resonator is disposed on a piezoelectric substrate made of Y-cut Z-propagating lithium niobate so that a longitudinal-wave leaky wave propagates, the loss is smaller than when an aluminum film is used. A SAW resonator can be obtained, and a preferable range of the wavelength specific film thickness h / λ is 0.07 to 0.1 in which the frequency fmin is located between the resonance frequency fr and the antiresonance frequency fa. .

In constructing the SAW resonator using the titanium boride film as described above, the bus bar 5, the reflector 3 and the lead-out electrode 4 have higher conductivity than the titanium boride film on the titanium boride film. A material such as an aluminum film may be laminated. In that case, the electrical resistance of the entire SAW resonator can be lowered. Further, as already described in detail, the aluminum film and the titanium boride film are very close to each other when the propagation loss due to bulk radiation is minimized. Even if it is replaced by an aluminum film, the influence on the propagation characteristics of the elastic wave is small. Therefore, as shown in FIG. 17, for example, an aluminum film may be laminated as an auxiliary conductive film on the upper side of the titanium boride film on the piezoelectric substrate to lower the electrical resistance of the entire SAW resonator. Further, instead of or together with titanium boride, the boride described above (zirconium boride, niobium boride, tantalum boride, chromium boride, molybdenum boride, tungsten boride, lanthanum boride). At least one of them may be used.
Each of the electrode fingers 6 of the IDT electrode 2 adopts a structure in which an aluminum film 22 is laminated on the upper layer of the molybdenum film 21 or a structure formed of a titanium boride film 25, and the bus bar 5, the reflector 3 and the routing electrode 4. May be composed of, for example, an aluminum film.

  In addition, as the electrode material constituting the electrode finger 6, the molybdenum film 21, the aluminum film 22, or the titanium boride film 25 is given as an example, but each electrode material is not only a simple substance (containing no impurities), It may be in a state where impurities are contained to such an extent that a compound with other materials is not formed, or an alloy state in which other compositions are mixed to such an extent that the above-described physical properties are not greatly inhibited (several percent).

  Further, in addition to the surface acoustic wave piezoelectric substrate described in each of the above examples, the SAW device of the present invention has any one of a longitudinal wave type leaky wave, a transverse wave type leaky wave and a Rayleigh wave having a frequency of 2 GHz or more. You may use waves. The SAW device of the present invention can be applied to a duplexer in which a ladder type filter is applied to a reception side filter and a transmission side filter, respectively, or an oscillator using the resonator 1, in addition to the ladder type filter and the resonator 1 described above. It may be applied. Further, as the SAW device of the present invention, as shown in FIG. 18, the IDT electrodes 2 described above are respectively arranged as an input side electrode and an output side electrode so as to be separated from each other along the propagation direction of the elastic wave. A filter may be configured. In FIG. 18, 50 is an input port, 51 is an output port, 52 is a damper for absorbing elastic waves, and 53 is a shield electrode. Furthermore, in each of the above examples, the IDT electrode 2 is configured as a regular electrode. However, a reflective electrode (not shown) may be disposed adjacent to the electrode finger 6, or the electrode finger 6 may be weighted. You may do it.

Reference 1: GDMansfeld, SGAlekseev, IMKotelyansky, “ACOUSTIC HBAR SPECTROSCOPY OF METAL (W, Ti, Mo, Al) THIN FILMS”, Proc. 2001 Ultrasonics Symposium, pp.415-418
Reference 2: BAAuld "Acoustic fields and waves in solids-Second Edition", vol. 1, Florida, Krieger Publishing, p. 95, 1990
Reference 3: Kadota et al., “SAW duplexer for small WCDMA full band with good temperature characteristics”, Proc. Symp. Ultrason. Electron., Vol. 27, pp. 39-40, 2006. /M.Kadota et.al., Jpn.J.Appl.Phys., Pp.4714-4717, 2007.
Reference 4: Basics and application of thin film by plasma process (Nikkan Kogyo Shimbun)

DESCRIPTION OF SYMBOLS 1 SAW resonator 2 IDT electrode 6 Electrode finger 10 Piezoelectric substrate 21 Molybdenum film 22 Aluminum film 23 Laminated film 25 Titanium boride film

Claims (10)

In a surface acoustic wave device using a surface acoustic wave having a frequency of 2 GHz or more,
A piezoelectric substrate;
A pair of bus bars disposed on the piezoelectric substrate so as to extend along the propagation direction of the surface acoustic wave and to be separated from each other in a direction perpendicular to the propagation direction of the surface acoustic wave;
A plurality of electrode fingers disposed on the piezoelectric substrate so as to cross each other from each bus bar toward the opposite bus bar,
Each electrode finger is disposed such that a period length including a width dimension of the electrode finger and a spacing dimension between adjacent electrode fingers corresponds to a frequency of the surface acoustic wave propagating on the piezoelectric substrate,
The electrode fingers are each composed of a laminated film including an aluminum film and a conductive film provided on a lower layer side of the aluminum film and having a larger elastic constant than the aluminum film,
The surface acoustic wave device according to claim 1, wherein a thickness of the conductive film is set to ¼ to ３ of a total thickness of the laminated film.   The surface acoustic wave device according to claim 1, wherein the conductive film is at least one of molybdenum and titanium. When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ, and the film thicknesses of the conductive film and the aluminum film are h1 and h2, respectively,
The wavelength specific thickness h1 / λ of the conductive film is 0.02,
The wavelength specific film thickness h2 / λ of the aluminum film is 0.05,
The piezoelectric substrate is a Y-cut Z-propagating lithium niobate substrate,
3. The surface acoustic wave according to claim 1, wherein each of the electrode fingers is configured to have a periodic length so that a longitudinal-type leaky wave among the surface acoustic waves propagates on the piezoelectric substrate. device. In a surface acoustic wave device using a surface acoustic wave having a frequency of 2 GHz or more,
A piezoelectric substrate;
A pair of bus bars disposed on the piezoelectric substrate so as to extend along the propagation direction of the surface acoustic wave and to be separated from each other in a direction perpendicular to the propagation direction of the surface acoustic wave;
A plurality of electrode fingers disposed on the piezoelectric substrate so as to cross each other from each bus bar toward the opposite bus bar,
The periodic length including the width dimension of the electrode fingers and the separation dimension between the electrode fingers adjacent to each other is arranged to correspond to the frequency of the surface acoustic wave propagating on the piezoelectric substrate,
2. The surface acoustic wave device according to claim 1, wherein each of the electrode fingers is composed of a conductive film containing a boride having a larger elastic constant than that of the aluminum film.   The surface acoustic wave device according to claim 4, wherein the conductive film is titanium boride. When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ and the film thickness of the conductive film is h,
The conductive film has a wavelength specific thickness h / λ of 0.07 to 0.1,
The piezoelectric substrate is a Y-cut Z-propagating lithium niobate substrate,
6. The surface acoustic wave according to claim 4, wherein each of the electrode fingers is configured to have a periodic length so that a longitudinal type leaky wave among surface acoustic waves propagates on the piezoelectric substrate. device.   6. The surface acoustic wave device according to claim 4, wherein the piezoelectric substrate is a 39 ° to 50 ° Y-cut X-propagating lithium tantalate substrate. When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ and the film thickness of the conductive film is h,
The wavelength specific thickness h / λ of the conductive film is 0.075 to 0.105,
The piezoelectric substrate is a 48 ° Y-cut X-propagating lithium tantalate substrate,
8. The surface acoustic wave device according to claim 7, wherein each of the electrode fingers is configured to have a periodic length so that a transverse-type leaky wave among the surface acoustic waves propagates on the piezoelectric substrate. When the wavelength of the surface acoustic wave propagating on the piezoelectric substrate is λ and the film thickness of the conductive film is h,
The wavelength specific thickness h / λ of the conductive film is 0.035 to 0.065,
The piezoelectric substrate is a 42 ° Y-cut X-propagating lithium tantalate substrate,
8. The surface acoustic wave device according to claim 7, wherein each of the electrode fingers is configured to have a periodic length so that a transverse-type leaky wave among the surface acoustic waves propagates on the piezoelectric substrate.   10. The surface acoustic wave device according to claim 4, wherein an auxiliary conductive film having a higher conductivity than that of the conductive film is laminated on the upper side of the conductive film. .

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