KR100611297B1 - Surface acoustic wave element - Google Patents

Abstract

In a surface acoustic wave device in which an interdigital electrode (2) (3) composed mainly of aluminum is formed on a substrate (1) made of lithium tantalate or lithium niobate, the paw electrodes (2) (3) are common. The ratio of the film thickness to the finger pitch of the plurality of electrode fingers connected to the terminal optimizes the propagation loss as a target function and is set within the range of 0.03 to 0.10. As a result, the propagation loss can be lowered than before.

Description

Surface acoustic wave device {SURFACE ACOUSTIC WAVE ELEMENT}

The present invention relates to a surface acoustic wave device using a substrate capable of excitation of a surface acoustic wave in which a longitudinal wave component is superior to a shear wave component, a pseudo surface acoustic wave in which a longitudinal wave component is superior to a shear wave component, or a surface slide volume wave in which a longitudinal wave component is superior to a shear wave component. It is about.

Background Art In recent years, surface acoustic wave devices have been widely used as communication devices such as resonator filters and delay lines for signal processing in communication devices such as automobile telephones. The surface acoustic wave element is, for example, an electrode 2 (3) or a lattice-shaped reflector (not shown) of an interdigital type on the surface of a substrate 1 having piezoelectricity as shown in FIG. And convert the electrical signal and the surface acoustic wave to each other.

Here, the surface acoustic wave is a surface wave that literally propagates the surface of the elastic body and its energy is not radiated inside the substrate. As such a surface acoustic wave, a plurality of excitation modes have been found so far, and for example, Rayleigh waves, three wave waves, love waves, piezoelectric surface slide waves, and the like are known.

In the Rayleigh wave and the three-wave wave, both the components of the longitudinal wave having the displacement in the same direction as the propagation direction and the transverse wave having the displacement in the substrate depth direction are superior. On the other hand, in a love wave and a piezoelectric surface slide wave, the component of the transverse wave which has a displacement parallel to a substrate surface and perpendicular | vertical to a propagation direction predominates. In addition, although three types of volume waves (bulk waves) exist in a piezoelectric substrate, such as "slow transverse wave", "fast transverse wave", and "long wave", surface acoustic waves propagate at a slower phase speed than "slow transverse wave".

Moreover, the acoustic wave which propagates a surface, radiating energy in the depth direction of an elastic body is known, and is called a pseudo surface acoustic wave or a leaky surface acoustic wave. The pseudo surface acoustic waves originally found are predominantly components of a transverse wave parallel to the substrate surface and having a vertical displacement in the propagation direction, and the phase velocity is located between the "slow transverse wave" and the "fast transverse wave".

In addition, recently, pseudo surface acoustic waves with dominant constituent components have been found in succession (JP-A-6-112763, Symposium Lecture Preliminary Proceedings of the 15th Ultrasonic Electronics, 6 Years, 185 ~) See page 186). The phase velocity of the pseudo surface acoustic wave mainly composed of these longitudinal waves is located in the middle of the "fast transverse wave" and the "normal wave".

On the other hand, the volume wave propagating along the surface vicinity of the substrate may be excited by the paw electrode and detected by another paw electrode on the same substrate. Such a volume wave is called a surface slide volume wave. It is conceivable that three kinds of surface slide volume waves exist in correspondence with normal volume waves. However, what is mainly handled at this time is the surface slide volume wave in which the component of the transverse wave which has a displacement parallel to the substrate surface or perpendicular to the propagation direction is dominant.

By the way, the characteristics of the acoustic wave include sound velocity, propagation loss, electromechanical coupling coefficient, and the like, and these characteristics are directly related to the design parameters of the circuit to which the surface acoustic wave element is applied.

Since the period T of the electrode finger of the paw-shaped electrode or the lattice-shaped reflector has a value equal to the wavelength of the acoustic wave, when the frequency is constant, the wavelength is smaller as the sound velocity is lower, and the production of the electrode becomes difficult. Therefore, it is preferable that the speed of sound is high.

In addition, since the resonance sharpness of the surface acoustic wave resonator and the insertion loss of the surface acoustic wave filter directly depend on the propagation loss of the surface acoustic wave, the propagation loss is preferably small.

On the other hand, the electromechanical coupling coefficient represents the conversion rate when the energy of the input electrical signal is converted into the energy of the surface acoustic wave. If the number of electrode fingers of the paw electrode is sufficiently increased, the seismic wave of any energy can be excited even if the electromechanical coupling coefficient is small, but in this case, the capacitance of the paw electrode becomes large, so that impedance matching with an external circuit is difficult. Therefore, a separate matching circuit is required for impedance matching. It is also known that the number of electrode fingers of the foot electrode is approximately inversely proportional to the operating frequency range of the surface acoustic wave element, and increasing the number of electrode fingers limits the feasible characteristics to a narrow band. Therefore, the electromechanical coupling coefficient is preferably large.

Conventionally, an acoustic wave in which two components of a longitudinal wave having a longitudinal wave and a displacement in a depth direction prevail (for example, Rayleigh wave and a ceza wave), or an acoustic wave in which a component of a transverse wave having a displacement perpendicular to the surface or perpendicular to the traveling direction is predominant. For example, for piezoelectric surface slide waves, love waves, transverse wave pseudo surface acoustic waves, and transverse wave surface volumetric waves, the substrate conditions (e.g., the relationship between the crystal axis and the surface acoustic wave propagation direction) to improve the above characteristics And electrode conditions (e.g., electrode finger cycle and film thickness) are known (A-437, Spring 1994, Electronics and Telecommunications Society Spring Conference Preliminary Proceedings, Japanese Journal of Applied Physics, vol. 29 (1990) Supplement 29-1, pp. 119-121, Japanese Journal of Applied Physics, vol. 30 (1991) Supplement 30-1, pp. 143-145, etc.).

By the way, surface acoustic wave (long wave surface acoustic wave) in which longitudinal component is superior to shear wave component, pseudo surface acoustic wave (long wave pseudo surface acoustic wave) in which longitudinal wave component is superior to shear wave component, and surface slide volume wave in which longitudinal wave component is superior to shear wave component (long wave) Type surface slide volume wave), the electrode conditions for improving the respective characteristics are not yet clear.

In particular, the longitudinal pseudo-surface surface acoustic wave has a sound velocity of more than 6000 Hz and an electromechanical coupling coefficient of more than 2%. In this respect, it is advantageous for practical use as a surface acoustic wave device. In this case, the propagation loss is a very large value of 0.5 Hz per wavelength, which is an obstacle to practical use.

An object of the present invention is to clarify the electrode conditions for reducing the propagation loss in a surface acoustic wave device using a substrate capable of excitation of a longitudinal surface acoustic wave, a longitudinal pseudo surface acoustic wave, or a longitudinal surface surface slide volume wave, thereby improving performance. It is to provide a surface acoustic wave device.

<Start of invention>

The surface acoustic wave device according to the present invention is a surface acoustic wave in which a longitudinal wave component is superior to a shear wave component, a pseudo surface acoustic wave in which a longitudinal wave component is superior to a shear wave component, or a surface slide volume wave in which a longitudinal wave component is superior to a shear wave component. It is comprised by forming a foot-shaped electrode which consists of a conductive thin film. Here, the ratio of the film thickness to the finger period of the plurality of electrode fingers connected to the common terminal is optimized for the propagation loss as a target function.

For example, the longitudinal pseudo-surface surface acoustic wave concentrates most of its energy in the range of several wavelengths from the surface. Therefore, when a thin film is formed on the substrate, the characteristics of the acoustic wave are affected by the thin film. In particular, since the conductive thin film serving as the shaped electrode has a lower sound speed than the substrate, radiation of energy in the direction of the substrate depth is suppressed and propagation loss is reduced.

The effect of suppressing this energy radiation increases as the thickness of the conductive thin film increases, and radiation to the substrate is eliminated at a thickness greater than or equal to a certain value, so that the longitudinal wave pseudo surface acoustic wave becomes a longitudinal wave surface acoustic wave. However, when the film thickness of the conductive thin film exceeds a certain value in relation to the electrode finger period, a new surface acoustic wave is excited inside the conductive thin film to generate a higher-order mode, and energy is collected inside the conductive thin film, so that the propagation loss is rather large. .

Therefore, as the thickness of the conductive thin film, there is an optimum value in which the propagation loss is a target function in relation to the electrode finger period.

Therefore, in the surface acoustic wave device of the present invention, the propagation loss is minimized by optimizing the ratio of the film thickness to the electrode finger period of the foot electrode.

In a specific configuration in which the substrate is made of lithium tantalate and the horn electrode is made of a conductive material having aluminum as a main component or a conductive material having a specific gravity equivalent to that of aluminum, the ratio of the film thickness to the electrode finger period of the horn electrode is 0.03. By setting it within the range of 0.1 to 0.10, it is possible to sufficiently reduce the propagation loss than before, and by setting the ratio to a value within the range of 0.05 to 0.09, more preferably 0.08 or substantially 0.08, the propagation loss is minimized. Can be suppressed.

More specifically, the propagation direction of a surface acoustic wave in which the longitudinal component is superior to the shear wave component, a pseudo surface acoustic wave in which the longitudinal wave component is superior to the shear wave component, or a surface slide volume wave in which the longitudinal wave component is superior to the shear wave component is represented by the Euler angle display (40 degrees to 90 degrees). Degrees, 40 degrees to 90 degrees, 0 degrees to 60 degrees) and a range equivalent to this. As a result, a higher sound velocity and a large electromechanical coupling coefficient are obtained.

In a specific configuration in which the substrate is made of lithium niobate and the horn electrode is made of a conductive material having aluminum as a main component or a conductive material having a specific gravity equivalent to that of aluminum, the ratio of the film thickness to the electrode finger period of the horn electrode is 0.03 to By setting it within the range of 0.10, it is possible to sufficiently reduce the propagation loss than before, and by setting the ratio to a value within the range of 0.07 to 0.09, more preferably 0.08 or substantially 0.08, the propagation loss is minimized. It can be suppressed.

More specifically, the propagation direction of a surface acoustic wave in which the longitudinal component is superior to the shear wave component, a pseudo surface acoustic wave in which the longitudinal wave component is superior to the shear wave component, or a surface slide volume wave in which the longitudinal wave component is superior to the shear wave component is represented by the Euler angle display (40 degrees to 90 degrees). Degrees, 40 degrees to 90 degrees, 0 degrees to 70 degrees) and a range equivalent to this. As a result, a higher sound velocity and a large electromechanical coupling coefficient are obtained.

In addition, Euler angle display is a well-known display method which specifies the cut surface and the surface acoustic wave propagation direction by the combination of three angles (phi, (theta), (phi)). That is, as shown in Fig. 6, when the crystal axes are X, Y, and Z, the X axis is rotated by the angle φ toward the Y axis side around the Z axis, and this is A1 axis. Next, the Z axis is rotated counterclockwise by the angle θ around the A1 axis, and this is the A2 axis. Using this A2 axis as a normal line, it cuts to the surface direction containing A1 axis, and sets it as a board | substrate. In the substrate cut in the plane direction, the axis in which the A1 axis is rotated counterclockwise by an angle φ about the A2 axis is referred to as the A3 axis, and the A3 axis is the surface acoustic wave propagation direction. At this time, the cut surface and the surface acoustic wave propagation direction are expressed by Euler angles (?,?,?).

In the surface acoustic wave device according to the present invention, by forming a shaped electrode having an appropriate electrode finger period and film thickness on a surface of a substrate capable of excitation of a longitudinal surface acoustic wave, a longitudinal wave pseudo surface acoustic wave, or a longitudinal surface slide volume wave The propagation loss of the longitudinal surface acoustic wave, the longitudinal pseudo surface acoustic wave, or the longitudinal surface slide volume wave can be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the film thickness ratio and insertion loss with respect to the electrode finger period of a paw electrode in a surface acoustic wave device in which an aluminum paw electrode is formed on a lithium tantalate substrate.

FIG. 2 is a graph illustrating a relationship between a film thickness ratio and a sound velocity with respect to an electrode finger period of a foot electrode in the surface acoustic wave device of FIG. 1.

Fig. 3 is a graph showing the relationship between the film thickness ratio and insertion loss with respect to the electrode finger period of the paw electrode in a surface acoustic wave device in which a paw electrode of aluminum is formed on a lithium niobate substrate.

FIG. 4 is a graph illustrating a relationship between a film thickness ratio and a sound velocity with respect to an electrode finger period of a foot electrode in the surface acoustic wave device of FIG. 3.

Fig. 5 is a plan view of the shaped electrode of the surface acoustic wave filter.

It is a figure explaining an Euler angle display.

First embodiment

The surface acoustic wave device of this embodiment employs lithium tantalate as a material of a substrate capable of excitation of longitudinal wave pseudo surface acoustic wave, and forms a foot-shaped electrode made of aluminum on the substrate.

For the surface acoustic wave device, the input foot electrode 2 made of an aluminum thin film on the substrate 1 made of lithium tantalate as shown in FIG. 5 so as to clarify the electrode conditions for minimizing the propagation loss. ) And a plurality of samples having different film thicknesses and electrode finger periods T in the surface acoustic wave filter in which the output electrode 3 was formed, and their insertion loss and sound velocity were measured by a network analyzer.

In addition, the thickness of the board | substrate 1 is 0.35 mm, the number of electrode fingers of each foot-shaped electrode 2 and 3 is 100, and the electrode finger crossing width W is 600 micrometers.

Further, the propagation direction of the longitudinal wave pseudo surface acoustic wave is in the Euler angle display (40 degrees to 90 degrees, 40 degrees to 90 degrees, 0 degrees to 60 degrees), preferably (80 degrees to 90 degrees, 80 degrees to 90 degrees). , 20 degrees to 40 degrees), more preferably (88 degrees to 90 degrees, 88 degrees to 90 degrees, 30 degrees to 32 degrees), and most preferably (90 degrees, 90 degrees, 31 degrees). . The superiority of these angular ranges has already been reported (see, for example, the Symposium Lecture Preliminary Proceedings of the 15th Ultrasound Electronics, 6 years, 185-186).

Fig. 1 is a graph of the measurement results for a plurality of samples by taking the ratio of the film thickness to the electrode finger period of the foot electrode on the horizontal axis and the insertion loss on the vertical axis.

As is clear from this graph, as the ratio of the film thickness to the electrode finger period increases from zero, the insertion loss gradually decreases from 20 kPa, and the decrease tends to increase sharply at the point of 0.03. And when the ratio exceeds 0.05, insertion loss is less than 15 microseconds, and the ratio is set to about 10 microseconds of the minimum from about 0.08. In addition, the insertion loss slightly increased in the range until the ratio exceeded 0.08 and reached about 0.10, but the value was suppressed to 12 dB or less.

However, when the ratio reaches about 0.11, the insertion loss value rapidly increases to a value exceeding 35 Hz. As the ratio becomes larger than 0.11, the insertion loss gradually decreases, but the value is a large value exceeding 25 Hz.

Therefore, the ratio of the film thickness to the electrode finger period of the paw-shaped electrode is set to a value that is within the range of 0.03 to 0.10, preferably within the range of 0.05 to 0.09, more preferably 0.08 or substantially 0.08. . As a result, the propagation loss is lower than that of the related art and is reduced to practical range.

In addition, as shown in FIG. 1, the change curve of the insertion loss changes abruptly along the way, and the two curves A and B are separated so that the ratio of the film thickness to the electrode finger period exceeds about 0.11. This is because the surface acoustic wave is excited and a higher order mode is generated. The generation of this higher order mode is also apparent from the result of FIG.

Fig. 2 is a graph of the measurement results for a plurality of samples, taking the ratio of the film thickness to the electrode finger period of the foot electrode on the horizontal axis and the sound velocity on the vertical axis. As shown, the change in the speed of sound becomes discontinuous when the ratio of the film thickness to the electrode finger period is about 0.11, and is clearly separated into two change curves A 'and B'. From this result, it can be said that a higher-order mode occurs at about 0.11.

As described above, in the surface acoustic wave device of the present invention, by forming a shaped electrode of aluminum having a specific electrode condition on a lithium tantalate substrate having a specific substrate condition, the longitudinal wave pseudo surface acoustic wave is excited to generate a high sound speed and a large electric machine. It is possible to obtain the coupling coefficient and to significantly reduce the propagation loss than before.

Second embodiment

The surface acoustic wave device of this embodiment employs lithium niobate as a material of a substrate capable of excitation of a longitudinal wave pseudo surface acoustic wave, and forms a foot-shaped electrode made of aluminum on the substrate.

As an object of the surface acoustic wave element, an input foot electrode composed of an aluminum thin film on a substrate made of lithium niobate (1) as in the first embodiment so as to clarify electrode conditions for minimizing the propagation loss ( 2) and the surface acoustic wave filter in which the output paw electrode 3 was formed, a plurality of samples having different film thicknesses and electrode finger periods T were prepared, and their insertion loss and sound velocity were measured by a network analyzer.

In addition, the thickness of the board | substrate 1 is 0.35 mm, the number of pairs of electrode fingers of each foot-shaped electrode 2 and 3 is 100, and the electrode finger crossing width W is 600 micrometers.

The propagation direction of the longitudinal wave pseudo surface acoustic wave is in the Euler angle display (40 degrees to 90 degrees, 40 degrees to 90 degrees, 0 degrees to 70 degrees), preferably (80 degrees to 90 degrees, 80 degrees to 90 degrees). , 20 degrees to 50 degrees), more preferably (88 degrees to 90 degrees, 88 to 90 degrees, 35 degrees to 40 degrees), and most preferably (90 degrees, 90 degrees, 37 degrees). The superiority of these angular ranges has already been reported (see, for example, Symposium Lecture Preliminary Proceedings of the 15th Ultrasound Electronics, 6 years flat, pages 185-186).

Fig. 3 is a graph of the measurement results for the plurality of samples, taking the ratio of the film thickness to the electrode finger period of the foot electrode on the horizontal axis and the insertion loss on the vertical axis.

As is clear from this graph, as the ratio of the film thickness to the electrode finger period increases from 0, the insertion loss gradually decreases from 23 kPa, and the decrease tends to increase rapidly at the point of 0.03 points. And when the ratio exceeds 0.07, insertion loss is less than 15 microseconds, and the ratio is set to about 11.5 microseconds of the minimum from about 0.08. In the range of the ratio exceeding 0.08 and reaching about 0.10, the insertion loss slightly increased, but the value was suppressed to 12 Hz or less.

However, when the ratio reaches about 0.11, the insertion loss value rapidly increases to a value exceeding 27 Hz. As the ratio is larger than 0.11, the insertion loss gradually decreases, but the value is a large value exceeding 21 Hz.

Therefore, the ratio of the film thickness to the electrode finger period of the paw-shaped electrode is set to a value within the range of 0.03 to 0.10, preferably within the range of 0.07 to 0.09, more preferably 0.08 or substantially 0.08. . As a result, the propagation loss is conventionally low and is reduced to the practical range.

In addition, as shown in FIG. 3, the change curve of the insertion loss changes abruptly along the way, and is separated into two curves A and B as in the first embodiment, and the ratio of the film thickness to the electrode finger period is about. This is because higher order mode is generated by exceeding 0.11. The occurrence of this higher order mode is also apparent from the result of FIG.

Fig. 4 is a graph of the measurement results for the plurality of samples, taking the ratio of the film thickness to the electrode finger period of the foot electrode on the horizontal axis and the sound velocity on the vertical axis. As shown, the change in the speed of sound becomes discontinuous when the ratio of the film thickness to the electrode finger period is about 0.11, and is clearly separated into two change curves A 'and B'. From this result, it can be said that the higher-order mode occurs at about 0.11.

As described above, in the surface acoustic wave device of the present embodiment, by forming a shaped electrode of aluminum having a specific electrode condition on a lithium niobate substrate having a specific substrate condition, the longitudinal wave pseudo surface acoustic wave is excited to provide high sound velocity and large electromechanical coupling. It is possible to obtain a coefficient and to significantly reduce propagation loss than before.

In addition, as a substrate capable of excitation of the longitudinal wave pseudo surface acoustic wave, a lithium tetraborate substrate can be employed in addition to the lithium tantalate substrate and the lithium niobate substrate. Similarly, it is possible to reduce the propagation loss by optimizing the ratio of the film thickness to the electrode finger period.

In a surface acoustic wave device in which a thin film made of aluminum is formed on a substrate made of lithium tetraborate, the propagation direction of the longitudinal wave pseudo surface acoustic wave is represented by an Euler angle display (0 degrees to 50 degrees, 15 degrees to 75 degrees, and 40 degrees to 90 degrees). Degrees), preferably (0 degrees to 10 degrees, 40 degrees to 50 degrees, 80 degrees to 90 degrees), more preferably (0 degrees to 2 degrees, 44 degrees to 46 degrees, 88 degrees to 90 degrees), most preferably (0 degrees, 45 degrees, 90 degrees). As a result, a high sound velocity is obtained and a large electromechanical coupling coefficient is obtained.

Description of the said embodiment is for demonstrating this invention, Comprising: It should not be understood to limit the invention as described in a claim, or to reduce the range. In addition, it is a matter of course that the various parts of the present invention are not limited to the above-described embodiments and various modifications can be made within the technical scope described in the claims.

The surface acoustic wave element according to the present invention is suitable for use as a circuit element such as a high frequency filter and a signal processing delay line in a communication device such as a cellular phone.

Claims (6)

It is formed by forming an electrode having a conductive thin film on the surface of a substrate made of lithium tantalate, and the propagation direction of the surface acoustic wave is at the Euler angle display (40 degrees to 90 degrees, 40 degrees to 90 degrees, 0). In the surface acoustic wave element set within a range equivalent to this, The conductive thin film is formed of a conductive material containing aluminum, or a conductive material having a specific gravity equivalent to that of aluminum, The surface electrode according to claim 1, wherein the ratio of the film thickness to the finger period of the plurality of electrode fingers connected to the common terminal is set in the range of 0.03 to 0.10. The surface acoustic wave device as claimed in claim 1, wherein the ratio of the film thickness to the electrode finger period of the foot electrode is set within a range of 0.05 to 0.09. The surface acoustic wave device as claimed in claim 2, wherein the ratio of the film thickness to the electrode finger period of the foot electrode is set to a value regarded as 0.08 or substantially 0.08. Formed on the surface of a substrate made of lithium niobate, a foot-shaped electrode made of a conductive thin film is formed, and the propagation direction of the surface acoustic wave is in the Euler angle display (40 degrees to 90 degrees, 40 degrees to 90 degrees, 0 degrees to 70 degrees). And a surface acoustic wave element set within a range equivalent to The conductive thin film is formed of a conductive material containing aluminum as a main component or a conductive material having a specific gravity equivalent to that of aluminum, The surface electrode according to claim 1, wherein the ratio of the film thickness to the finger period of the plurality of electrode fingers connected to the common terminal is set in the range of 0.03 to 0.10. The surface acoustic wave device as claimed in claim 4, wherein the ratio of the film thickness to the electrode finger period of the paw electrode is set within a range of 0.07 to 0.09. 6. The surface acoustic wave device as claimed in claim 5, wherein the ratio of the film thickness to the electrode finger period of the foot electrode is set to a value regarded as 0.08 or substantially 0.08.