JPH08148966A - Surface acoustic wave element electrode - Google Patents

Abstract

PURPOSE: To provide a durable surface acoustic wave element electrode for which the power resistance of an electrode film is high, internal loss is less and characteristics are improved. CONSTITUTION: In the fine structure of the electrode film, the ratio of (the standard deviation of a crystal grain size/the average grain size of the crystal grain size) is defined as being equal to or less than 0.3, the ratio of (an average crystal grain size/a film thickness) is defined as being equal to or more than 0.2 and equal to or less than 1.0 and AlxMy (where M is at least one of Ti, Pd, Nb, Sc, Ni, Mg, Ge, Si, Co, Zn, Li, Ta, Au, Ag, Pt, Cf, Hf, Zr, Cd, W, V and Cu and (x) and (y) are 0<(y)<=20 and (x)+(y)=100 in wt.% display) is defined as electrode film material composition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode of a surface acoustic wave device (that is, a surface acoustic wave device electrode) formed by forming a polycrystalline electrode made of a metal thin film on a piezoelectric substrate. The present invention relates to a surface acoustic wave element electrode having power resistance that is not damaged even with high input power and capable of withstanding long-term use by reducing internal loss.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave devices have expanded the range of applications as compact high-performance bandpass filters and resonators, and have been required to have high operating power as well as operating frequencies of several hundred MHz to several GHz. Is becoming more common. Further, in order to achieve higher output, a new structure with reduced internal loss and high power resistance has been required. In order to increase the frequency, it is necessary to narrow the pitch of the comb-shaped electrodes used and at the same time to narrow the electrode width. When the center frequency is 1 GHz, the electrode width is about 1 μm.
m. The problem in terms of reliability of the surface acoustic wave device using such a fine electrode is that the distortion of the substrate surface caused by the surface acoustic wave during operation causes internal stress in the electrode film formed on the surface, As the stress acts on the electrode film, the electrode material atoms move with the passage of the crystal grain boundaries along the passage of time, causing voids (voids) and protrusions (hillocks) in the electrode, resulting in deterioration of characteristics and electrode destruction. There is a point to do. To address this issue, traditionally, for example,
As described in Japanese Patent Publication No. 61-47010, it has been practiced to add a small amount of Cu to Al as an electrode material to be used and cure the metal thin film of the electrode. The means for curing the electrode film is not only Cu but also Ti,
A method of adding Ni, Mg, Pd, etc. is also performed.

However, the conventional surface acoustic wave device electrode is
When a small amount of Cu, Ti, Ni, Mg, Pd or the like is added to the 1 film to cure the electrode film, the curing strength increases and the power resistance increases as the addition amount increases. However, there is a problem that the internal loss is increased due to the increase in the specific resistance, the element to be added and the amount to be added are significantly limited, and the fine structure of the electrode film has not been optimized.

[0004]

SUMMARY OF THE INVENTION The above prior art is to increase the hardening strength of the electrode film to improve the electric power resistance.
It has not been considered to reduce the specific resistance of the electrode film to reduce the internal loss during operation, and to realize both at the same time.

An object of the present invention is to provide a fine structure of an electrode of a surface acoustic wave device and an electrode material having high power resistance and low internal loss during operation.

[0006]

In order to achieve the above object, in the present invention, an electrode of a surface acoustic wave device in which a polycrystalline electrode made of a metal thin film is formed on a piezoelectric substrate is used. When a transmission electron microscope observation was performed on a circular electrode, the ratio of the standard deviation of the crystal grain size of the electrode to the average grain size of the crystal grain size of the electrode, that is, (standard deviation of the crystal grain size / The value of (average particle size) was set to 0.3 or less.

Further, in an electrode of a surface acoustic wave device in which a polycrystalline electrode made of a metal thin film is formed on a piezoelectric substrate, when the film-shaped electrode is observed by a transmission electron microscope, the electrode is The ratio of the average crystal grain size to the film thickness of the electrode, that is, the value of (average crystal grain size / film thickness) is 0.2 or more and 1.0 or less.

Further, under the above conditions, the average composition in the film thickness direction of the metal thin film forming the surface acoustic wave element electrode is Alx.
The alloy is called My (alloy containing x% by weight of Al and y% by weight of metal M). However, the metal M is Ti, P
d, Nb, Sc, Ni, Mg, Ge, Si, Co, Z
n, Li, Ta, Au, Ag, Pt, Cr, Hf, Z
represents at least one metal selected from r, Cd, W, V and Cu, and 0 <y ≦ 20, x + y = 100
The following conditions shall be satisfied.

[0009]

As a method of forming an electrode film on a piezoelectric substrate, a sputtering method and a vacuum vapor deposition method are used, but the sputtering method is mainly used from the viewpoint of the denseness of the film and the composition stability of the alloy film. In most cases, the electrode film formed by the sputtering method is a polycrystalline thin film composed of many crystal grains. Therefore, even if the film composition is the same, the fine structure of the film, for example, the average crystal grain size, The film characteristics vary widely depending on the standard deviation of crystal grain size and orientation.

The present inventor has paid attention to the crystal grain size distribution consisting of the average crystal grain size of the electrode thin film and the standard deviation of the crystal grain size. As a result, the inventors have found that the optimum crystal grain size is from the viewpoint of power resistance and specific resistance. It was found that a range of distribution exists. That is, after creating a histogram of the crystal grain size distribution by observing the electrode film with a transmission electron microscope, the average crystal grain size and standard deviation of the crystal grain size are derived, and ((standard deviation of crystal grain size / average grain size of crystal grain size As a result of examining the relationship between the value of (diameter) and the value of (average crystal grain size / film thickness) and the power resistance, it was confirmed that the lower the value, the higher the power resistance.

As a result of the experiment, in order to obtain good power resistance, the value of (standard deviation of crystal grain size / average grain size of crystal grain size) is preferably 0.3 or less, and 0.2 or less. It has been clarified that it is more preferable. The value of (average crystal grain size / film thickness) is
Affects power resistance and specific resistance. The smaller the average crystal grain size, the more the power resistance tends to improve, while the specific resistance tends to increase and the internal loss tends to increase. Therefore, the average grain size has an optimum range. As a result of the experiment, when the value of (average crystal grain size / film thickness) is 1.0 or less, it is effective in improving the power resistance, but when it is less than 0.2, the specific resistance increases and the internal loss increases. Clarified that it is not preferable to. Therefore, the value of (average crystal grain size / film thickness) is 0.2 or more and 1.
It must be within the range of 0 or less.

The value of (standard deviation of crystal grain size / average grain size of crystal grain size) and the value of (average crystal grain size / film thickness) are set in the above numerical ranges, and the metal constituting the electrode is set. By making the average composition in the film thickness direction of the thin film an alloy of AlxMy (alloy containing x% by weight of Al and y% by weight of metal M), it is possible to realize a great improvement in power resistance and reduction of internal loss. Metal M is Ti, Pd, Nb, Sc, Ni, M
g, Ge, Si, Co, Zn, Li, Ta, Au, A
represents at least one metal selected from g, Pt, Cr, Hf, Zr, Cd, W, V and Cu, and 0 <
It is assumed that the conditions of y ≦ 20 and x + y = 100 are satisfied. Even if the metal M is contained in an amount of more than 20% by weight, it has almost no effect on improving the electric power resistance, and rather increases the specific resistance of the electrode film and causes an increase in internal loss.
It should be 0% by weight or less.

[0013]

The present invention will be described in more detail below with reference to the drawings.

Example 1 In FIG. 1, the electrode film material is Al--
The value of (standard deviation of crystal grain size / average grain size of crystal grain size) in the case of 0.6 wt% Ti (alloy in which titanium is 0.6 wt% and aluminum is the remaining 99.4 wt%) It is a characteristic showing the relationship with the life. The value of (standard deviation of crystal grain size / average grain size of crystal grain size) can be obtained by changing the sputtering film forming conditions of the electrode film. The accelerated deterioration test conditions are an ambient temperature of 120 ° C. and an output of 1W. It is indicated that the lower the ratio is, the longer the life (power resistance) is. In this accelerated deterioration test, it is practically necessary to maintain a life of 10 hours or more. Therefore, the value of (standard deviation of crystal grain size / average grain size of crystal grain size) is preferably 0.3 or less,
0.25 or less is next preferable, and 0.2 or less is further preferable.

FIG. 2A is a plan view showing an element structure used in the accelerated deterioration test for obtaining the characteristics of FIG. 1, and FIG. 2B is taken along the line AA 'in FIG. 2A. FIG. The piezoelectric substrate 1 propagates SH surface pseudo surface waves at 36 °
It is composed of rotating Y-axis cutting and X-axis propagating LiTaO ₃ .

In the electrode structure, the input electrodes 2 and the output electrodes 3 are alternately arranged, and the number of input / output electrodes is the same as that of the input electrodes 2
Has a multi-electrode structure with two and three output electrodes 3. Each of the input electrode 2 and the output electrode 3 is composed of a comb-shaped electrode finger 4, and as shown in the sectional view of FIG.
The electrode width of the comb-shaped electrode fingers 4 is equal to the width of the portion (space portion) where the comb-shaped electrode fingers 4 are not present. A ground electrode pattern 5 is formed between the input / output electrodes 2 and 3. Further, the surface of the piezoelectric substrate 1 is covered with a floating electrode pattern 6 which is electrically insulated from the input / output electrodes 2 and 3 and the ground electrode pattern 5.

The center frequency of this multi-electrode type surface acoustic wave device is 880 MHz, the electrode width and space width of the comb-shaped electrode fingers of the input / output electrodes 2 and 3 are both 1.2 μm, and the width of the grounding electrode pattern 5. Is 5 μm. The electrode is formed by the DC magnetron sputtering method, and the film thickness is about 100n.
m. It was created by patterning by photolithography technology. The life is zero loss at the center frequency.
The time was increased by 5 dB.

Example 2 FIG. 3 shows Al-0.6 wt%.
It is a characteristic diagram showing that the life of the device electrode in the Ti electrode film is greatly influenced by the value of (average crystal grain size / film thickness). That is, FIG. 3 shows the relationship between the ratio of the average particle size of the electrode film to the electrode film thickness and the life of the device electrode by the curve of ●. The accelerated deterioration test conditions and the device are the same as those in Example 1.
It is similar to that shown in.

The ratio of the average crystal grain size of the electrode film to the electrode film thickness can be obtained by changing the sputtering film forming conditions of the electrode film. The value of (standard deviation of crystal grain size / average grain size of crystal grain size) is between 0.24 and 0.3. The life is improved as the ratio becomes lower, and it is practically necessary to maintain the life of 10 hours or more in this accelerated deterioration test. Therefore, the ratio needs to be 1.0 or less.

On the other hand, when the ratio is lower than 0.2, the specific resistance of the electrode film increases as shown by the characteristics of the black square marks. An increase in specific resistance causes an increase in internal loss of the device, which is not preferable. Therefore, from the above two conditions, the preferable ratio is 0.2 or more and 1.0 or less.

(Example 3) (standard deviation of crystal grain size / average grain size of crystal grain size) ratio is 0.3 or less, and (average crystal grain size / film thickness) ratio is 0.2 to 1 Table 1 shows the relationship between the additive element to the Al film and the life, with the film forming conditions set so as to be 0.0. The life was evaluated by the life magnification for the Al film. The accelerated deterioration test conditions and device structure are the same as those described above.

[0022]

[Table 1]

To improve the life, Ti, Pd, Nb, S
c, Ni, Mg, Ge, Co, Zn, Li, Ta, A
u, Ag, Pt, Cr, Hf, Zr, Cd, W, V, C
It can be seen from the data of sample numbers 1-27 in Table 1 that it is effective to add at least one element of u to Al.

For reference, sample numbers 28 to 30 in Table 1 show that Sb, In and Sn are not effective as additive elements. If the added amount of the additional element is more than 20 wt%, the specific resistance becomes high, the internal loss as an element is large and it is not suitable for practical use, and therefore it was necessary to set it to 20 wt% or less.

As described above, in the present embodiment, the electrode made of a single layer film is shown, but a multilayer film having a plurality of layers is also effective. Further, although the thickness of the electrode film is 100 nm in this embodiment,
It does not matter whether it is thicker or thinner. The piezoelectric substrate is
The material is not limited to LiTaO ₃ in this embodiment, but may be quartz, LiNbO ₃ , Li ₂ B ₄ O ₇ , ZnO, AlN, or the like. Further, the element structure does not have to be limited to the multi-electrode structure of this embodiment, and may be a resonator structure or the like.

[0026]

According to the present invention, in the surface acoustic wave device electrode, the power resistance can be greatly improved and the specific resistance of the electrode film can be reduced, so that the internal loss of the surface acoustic wave device can be reduced. Become. Therefore, the surface acoustic wave device electrode according to the present invention is suitable for a SAW filter transmitting relatively high power.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the relationship between (standard deviation of crystal grain size / average grain size of crystal grain size) ratio and life when the electrode film material is Al-0.6 wt% Ti.

FIG. 2 is an explanatory diagram of a surface acoustic wave device according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing a relationship between (average crystal grain size / film thickness) and life and normalized specific resistance when the electrode film material is Al-0.6 wt% Ti.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate, 2 ... Input electrode, 3 ... Output electrode, 4 ... Comb-shaped electrode finger, 5 ... Grounding electrode pattern, 6 ... Floating electrode pattern.

Claims (3)

[Claims] 1. A surface acoustic wave device electrode comprising a piezoelectric substrate on which a polycrystal electrode made of a metal thin film is formed, and the crystal of the electrode when observed by a transmission electron microscope. A surface acoustic wave device electrode, wherein the ratio of the average grain size of the crystal grain size of the electrode to the standard deviation of grain size is 0.3 or less. 2. A surface acoustic wave device electrode comprising a piezoelectric substrate on which a polycrystal electrode made of a metal thin film is formed, and the average of the electrodes when observed by a transmission electron microscope. The ratio of the film thickness of the electrode to the crystal grain size is 0.
2. A surface acoustic wave device electrode, which is 2 or more and 1.0 or less. 3. The elasticity according to claim 1, wherein an average composition in the film thickness direction of the metal thin film forming the electrode is represented by an alloy of AlxMy (alloy containing x% by weight of Al and y% by weight of metal M). Surface wave element electrode. (However, the metal M is Ti, P
d, Nb, Sc, Ni, Mg, Ge, Si, Co, Z
n, Li, Ta, Au, Ag, Pt, Cr, Hf, Z
represents at least one metal selected from r, Cd, W, V and Cu, and 0 <y ≦ 20, x + y = 100
The following conditions must be met)