JP2000341073A - Surface acoustic wave waveguide structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the loss as a SAW filter by arranging short-circuit type gratings outside both an open type grating and a grating opening on a piezoelectric substrate, having a recessed surface as the opposite speed surface in the main propagation direction of a surface acoustic wave, in the propagation direction of the surface wave. SOLUTION: On the piezoelectric substrate of 360 deg. LiTaO3, an area A inside a waveguide is formed by arranging an open type inside grating 2 along the propagation direction of the surface wave. At right angles to the propagation direction of the surface wave, outside gratings (electrode finger array) 3a and 3b are arranged on both the sides of the inside grating (electrode finger array) 2 in parallel to and at the same pitch with the inside grating 2. The outsides of the outside gratings 3a and 3b are connected by bus bars 4a and 4b to form an area B for a short-circuit type grating. The open type inside grating 2 is always faster in phase speed than the short-circuit type outside gratings 3a and 3b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave waveguide structure such as a surface acoustic wave converter or a reflector, and more particularly, to a surface acoustic wave waveguide structure with more stable waveguide conditions and a device thereof.

[0002]

2. Description of the Related Art In recent years, surface acoustic waves (hereinafter referred to as SAW) have been developed.
Devices are widely used in the communications field,
Because of its excellent features such as mold and mass production,
It is often used for talking. FIG. 3 shows a conventional SAW device.
Electrode 20 used as a surface acoustic wave waveguide
FIG. 3 is a plan view showing a configuration of
You. As is well known, the IDT electrode 20 has a surface on a piezoelectric substrate.
Width L along the direction of wave propagation_TThe electrode finger of the gap S_TAnd pick
Chi P₁(P₁ = L_T + S_T ), Arranged alternately positive and negative bus
This is a structure that is electrically connected to the bars 22a and 22b. Immediately
The electrode period L of the IDT electrode 20 is the pitch P ₁2 times L = 2 P₁
Becomes On the other hand, the grating electrode 21 is an IDT electrode.
Width L same as 20_RThe electrode finger of the gap S_RPitch P_Two(P
_Two= L_R + S_R ), And generally have a bus at both ends.
This is a structure that is electrically connected to the bars 22c and 22d.

A surface wave propagating through the IDT electrode 20 and the grating electrode 21 shown in FIG. 3 will be considered from the viewpoint of waveguide. The surface wave excited by the IDT electrode 20 is
At the excited position, the plane wave is close to a plane wave having a phase velocity in the direction perpendicular to the electrode array, but diffracts and spreads as it propagates, and the wavefront of the plane wave gradually disturbs. Where IDT
Since the electrode finger of the electrode 20 has the effect of electric field short-circuiting, reflection and speed change due to the mass load effect similarly to the electrode finger of the grating electrode 21, when describing the grating effect, both the IDT electrode 20 and the grating electrode 21 use the grating. I will say. The configuration of the IDT electrode 20 and the grating electrode 21 shown in FIG. 3 is continuous for a certain length or more, or a grating electrode (not shown) similar to the grating electrode 21 is provided.
Are arranged symmetrically with respect to the center, so that if the excited surface wave reciprocates between the grating electrodes many times, the IDT electrode 20 and the grating electrode 21
Function as a waveguide, and a waveguide mode may be formed.

This is because the phase velocity of the surface wave in the grating area A shown in FIG.
It can be considered that the difference between the phase velocities of the surface waves in the region B of b, c, and 22d is due to the total reflection of the surface wave at the boundary between both A and B. At this time, the displacement distribution of the lowest-order symmetric mode in the direction perpendicular to the waveguide is roughly expressed as
As shown by the broken line α in FIG. 3, the vibration energy of the surface wave is confined in the grating region A inside the waveguide.

In order to form the above-mentioned waveguide state, that is, a state in which the vibration energy of the surface wave is confined,
Several conditions need to be met. As shown in Hashimoto et al.'S "High-speed analysis method of surface acoustic waves obliquely propagating in a finite-thickness metal grating" (IEICE Technical Report, US95-47 (1995-09)), a waveguide state is formed. In order for the ID
In the range where the angle between the vertical direction (main propagation direction) of the electrode finger of the T electrode 20 and the propagation direction of the surface wave is small, the reverse velocity surface of the surface wave (reverse velocity is the reciprocal of the phase velocity of the surface wave, ie, Slowne).
ss) is convex near the main propagation direction as shown in FIG. 4A, the velocity of the surface wave in the inner grating area A shown in FIG. Also, as shown in FIG. 4B, when the reverse velocity surface is concave near the main propagation direction, it is shown that the velocity of the area B must be lower than that of the area A. ing.

For example, when the above condition is not satisfied, the energy of the excited surface wave is
, And its displacement distribution becomes as shown by a solid line β in FIG. 3, the energy of the surface wave cannot be confined in the region A, and no guided mode is formed. The loss due to this radiation increases as the length of the waveguide increases. If the resonator is a resonator, the Q value decreases, and if the filter is a filter, the insertion loss increases.

The difference between the propagation speeds in the regions A and B shown in FIG. 3 will be described in more detail. As described above, Hashimoto et al. Described in the above document that if the reverse velocity surface is convex in the traveling direction, the speed of the surface wave in the outer region B must be faster than that in the region A in order to form a guided state. It is good to say. For example, when the bus bar region B is formed of a metallized film having the same thickness as the electrode fingers of the IDT electrode 20 and the grating electrode 21, the bus bar region B has a larger electrode area density than the grating region A, and thus has a short-circuit effect. And the mass loading effect is large, the propagation speed in region B should be slower than that in region A. However, if the thickness of the electrode pattern is equal to or greater than a certain thickness, even if the region B is the same metallized surface as the grating region A, the region due to the electrical and mechanical energy storage effect at the electrode finger end of the grating region A Since the propagation speed of the surface wave in A is lower than that in the region B, it is known that the surface wave is guided even with the electrode configuration of FIG. Actually, when an aluminum (Al) electrode is used for the ST-cut quartz substrate (the reverse speed surface is convex), 0.3 to 0.4
It has been found that the propagation velocity in the grating region A is slower than that in the region B when the film thickness is as extremely small as% L (L = 2P ₁ ) or more. Here, L is the electrode period of the IDT electrode 20, and is substantially equal to the wavelength of the surface wave at the center frequency of the device.

On the other hand, when a substrate material having a surface wave reverse velocity surface concave in the traveling direction of the surface wave is used as shown in FIG. Must be slower than grating area A. For example, as shown in the above-mentioned Hashimoto document, a typical piezoelectric substrate having a concave reverse velocity surface in the main propagation direction has 36
There is ゜ Y-cut X-propagation LiTaO ₃ (or 42） Y-cut), and this substrate is used in many cases for RF filters of mobile phones and the like. As described above, when the electrode film thickness is increased, the phase velocities A and B of the regions A and B become A <B, and when the reverse velocity surface is concave, it becomes impossible to form a waveguide state. As a result, a problem occurs that the insertion loss of the filter increases. In order to solve this problem, the present inventor proposed a waveguide structure as shown in FIG. 5 in a prior patent application (Japanese Patent Application No. 10-116018). That is, on a piezoelectric substrate (not shown), I
The DT electrode 32 and the grating electrode 33 are arranged and I
Grating electrodes 34a on both sides of the opening of the DT electrode 32;
34b is juxtaposed with the IDT electrode 32 and the grating electrode 33. The IDT electrode 32 is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and the grating electrodes 33 are arranged with electrode fingers at substantially equal pitch.

The distance between the centers of the adjacent electrode fingers of the IDT electrode 32, the grating electrode 33, and the grating electrodes 34a and 34b, that is, the electrode finger pitch is made substantially equal. I
Electrode finger width of DT electrode 32 and grating electrode 33
L _T and L _R are almost equal, but the grating electrodes 34a,
The electrode finger width L _G of 34b the electrode finger width L _T, is formed wider than L _R. IDT electrode 32, grating 33 and grating
The common bus bars 35a and 35b of the respective 34a and 34b are provided at the boundary between the grating area A and the area B. With such a configuration, it is possible to make the propagation speed of the surface wave in the grating region A including the IDT electrode 32 and the grating electrode 33 and the propagation speed of the surface wave in the region B including the grating electrodes 34a and 34b different. As a result, even in the case of a piezoelectric substrate in which the reverse velocity surface of the surface wave is concave, it is possible to conduct the wave as shown by the displacement distribution curve α shown at the left end in the figure.

FIG. 6 shows a case in which a 36 ° Y-cut X-propagation LiTaO ₃ is used as a substrate, an electrode film thickness h = 0.07L (L is an electrode period), and a line occupancy η of the IDT electrode 32 and the grating electrode 33 is η = 0. 5 and the grating electrode 34
The speed dispersion curve when the line occupancy rate of the lines a and b is η = 0.6. The speed of the line occupancy rate η = 0.6 is lower than the speed of the occupancy rate η = 0.5 at any frequency. That is, FIG.
If the line occupancy of the region B is larger than that of the grating region A, the phase velocity of the surface wave in the region B can be made slower than that of the region A, so that the surface wave can be guided. Is shown.

[0011]

However, in order to form a waveguide state using a substrate material in which the reverse velocity surface of the surface wave is concave in the traveling direction of the surface wave, the electrode thickness is set to h = 0.07L. In the example shown in FIG. 5 where the line occupancy η = 0.5 in the region A and the line occupancy η = 0.6 in the region B, the asymptote of the speed in the region A with the line occupancy η = 0.5 as shown in FIG. The speed difference between the stop band and the asymptote of the speed in the region B with the line occupancy η = 0.6 (shown by a solid line) is ΔV / V _f = 0.0007. small. Even if only the line occupancy of the region B is set to η = 0.7, there is a problem that the velocity difference ΔV / V _f = 0.0036, which is less than 0.01, and a stable waveguide state cannot be created. The present invention has been made to solve the above problems, and has as its object to provide a grating in which the speed difference between regions A and B is increased to stabilize the waveguide state, and a device using the same. .

[0012]

In order to achieve the above object, a surface acoustic wave waveguide structure according to the present invention and a device using the same have a reverse velocity with respect to the main propagation direction of the surface acoustic wave. The surface acoustic wave waveguide structure is characterized in that an open grating and short-circuit gratings are arranged on both sides of an opening of the grating along a propagation direction of a surface wave on a piezoelectric substrate having a concave surface. According to a second aspect of the present invention, an IDT electrode is provided on a piezoelectric substrate.
In a surface acoustic wave waveguide provided with a grating reflector arranged close to an electrode, a region corresponding to the opening of the IDT electrode was set as an open grating as the grating reflector, and a short-circuited grating was arranged outside the region. It is a surface acoustic wave waveguide structure characterized by the above.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1A is a plan view showing the configuration of a grating 1 according to the present invention. An inner grating 2 is formed on a piezoelectric substrate (not shown) along a propagation direction of a surface wave, and a direction perpendicular to the propagation direction of the surface wave. Outer gratings 3 on both sides of the grating 2
The gratings 3a and 3b are arranged in parallel with the grating 2, and the gratings 3a and 3b are connected to bus bars 4a and 4b respectively arranged outside these gratings. Region A, grating 3a, bus bar 4a (short-circuit grating), grating 3b, bus bar 4
The region composed of b (short-circuit type grating) is referred to as a region B. FIG. 1B shows another embodiment according to the present invention.
The grating C shown in (a) and the IDT shown in FIG.
The electrode D is arranged in close proximity.

FIG. 1 shows a 36 ° Y-X LiTaO ₃ piezoelectric substrate.
As shown in (a), a region A inside the waveguide is constituted by an open grating 2, and a region B outside the waveguide is short-circuited gratings 3a, 4a having the same pitch as the open grating 2, respectively. Consider a waveguide reflector in which 3b and 4b are arranged. FIG. 2 shows the open grating 2 and the short-circuit gratings 3a, 4a, 3
The simulation result of the phase velocity in the main propagation direction in the stop band respectively formed by b and 4b is shown. FIG. 2A is a diagram showing a state near the stop band when the electrode film thickness h is small. The stop band of the short-circuit grating indicated by β is small, and the stop band of the open grating indicated by α is short-circuited. It is formed on the high frequency side in contact with it at frequency P2. The asymptote (V / V _f ) (indicated by the arrow) of the phase velocity in the B and A regions is 0.98, respectively.
17 and 0.9959, and the speed difference ΔV / V _f is 0.0142, which is larger than the conventional one. As described above, in the configuration according to the present invention, the phase velocity is always larger in the open grating 2 than in the short-circuited gratings 3a, 4a, 3b, and 4b, and the phase velocity in the 36 ° Y-X LiTaO ₃ having a concave reverse velocity surface is reduced. The wave condition will be satisfied.

On the other hand, when the electrode thickness h is large, FIG.
As shown in the figure, the short-circuit gratings 3a, 4a, 3b,
4b increases, the stop band α of the open grating 2 is included in that of the short-circuit gratings 3a, 4a, 3b, and 4b, and is formed at a frequency P3 whose upper ends match each other. The asymptotes (V / V _f ) of the phase velocities in the B and A regions are 0.9665 and 0.98, respectively.
It becomes 36. Therefore, it can be seen that the speed difference ΔV / V _f is 0.0171, which is larger than the conventional one.

The present invention has realized that the speed difference between the inside and outside of the waveguide structure is increased by utilizing the speed difference between the short-circuit grating and the open grating due to the piezoelectric reaction as described above. In the above description, 36 ° Y-X LiTaO ₃ was used, but it goes without saying that the present invention can be applied to other piezoelectric substrates having a concave reverse surface.

The present invention utilizes a difference in speed due to a difference in electrical connection between a short circuit and an open grating. By combining this, as shown in FIG. If it is larger than the above, the speed difference between the inside and outside of the waveguide can be further increased.

[0018]

According to the present invention, as described above, in a surface acoustic wave piezoelectric substrate in which the reverse velocity surface in the main propagation direction is concave, the waveguide can be made more reliable than conventional means. If used for a SAW filter, a filter with lower loss can be realized. Therefore, when the SAW device according to the present invention is used for a mobile communication device or the like, an excellent effect is produced in manufacturing a communication device having excellent call quality.

[Brief description of the drawings]

FIG. 1A is a plan view showing a structure of a grating according to the present invention, and FIG. 1B is another embodiment of the present invention.

FIGS. 2A and 2B are diagrams showing a relationship between a normalized frequency and a normalized phase velocity in the vicinity of a stop band formed by the grating having the structure of FIG. 1; FIG.
Is thick.

FIG. 3 is a diagram illustrating a configuration of a conventional IDT electrode and a grating reflector, and a diagram illustrating a displacement distribution.

FIG. 4A shows a case where the reverse speed surface is a convex surface, and FIG. 4B shows a case where the reverse speed surface is a concave surface.

FIG. 5 is a diagram showing a configuration of an IDT electrode and a reflector for forming a waveguide structure when the reverse velocity surface is a concave surface.

6 is a diagram illustrating a relationship between a normalized frequency and a normalized phase velocity near a stop band formed by the grating having the structure of FIG. 5;

[Explanation of symbols]

1 Grating 2 Electrode array 3a, 3b Electrode array 4a, 4b Bus bar f Surface wave frequency f ₀ Free surface velocity V _f Frequency divided by IDT electrode cycle V V・ Surface wave velocity V _f・ ・ Free surface velocity

Claims (2)

[Claims] 1. An open-type grating and short-circuited gratings disposed on both outer sides of an opening of the grating along a surface-wave propagation direction on a piezoelectric substrate having a concave surface opposite to a main surface of the surface acoustic wave in a direction opposite to the main propagation direction. A surface acoustic wave waveguide structure characterized in that: 2. A surface acoustic wave waveguide having an IDT electrode and a grating reflector disposed close to the IDT electrode on a piezoelectric substrate, wherein a region corresponding to an opening of the IDT electrode is used as the grating reflector. A surface acoustic wave waveguide structure comprising an open grating and a short-circuit grating disposed outside the open grating.