JP3296097B2 - Surface acoustic wave device - Google Patents

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 device having an electrode film made of a metal thin film, and more particularly to an electrode of a surface acoustic wave device having high power durability, good characteristics and endurable for a long time.

[0002]

2. Description of the Related Art In recent years, the surface acoustic wave element has been applied to a small and high-performance band-pass filter and a resonator, and the operating frequency has been increased from several hundred MHz to several GHz, and at the same time, high output is required. It is becoming. Further, in order to achieve high output, a new structure having both a reduction in internal loss and high power durability has been required. In order to increase the frequency, it is necessary to narrow the pitch of the interdigital transducers to be 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. As a problem in terms of reliability of a surface acoustic wave element using such a microelectrode, during operation, distortion of the substrate surface caused by surface acoustic waves generates internal stress in an electrode film formed on the surface, At the portion where the stress exceeds the critical shear stress of the electrode film, the electrode material atoms move through the crystal grain boundaries as passages, creating voids (voids) and protrusions (hillocks) in the electrode, causing deterioration of characteristics and electrode breakdown. Point.

In order to cope with the above problem, as described in, for example, Japanese Patent Publication No. 61-47010, a small amount of Cu is added to Al as an electrode material to be used to cure a metal thin film of the electrode. Is being done. A proposal for adding Ti to Al is disclosed in Japanese Patent Application Laid-Open No. 62-1634.
Japanese Patent Application Laid-Open No. 08-08,082 discloses a proposal for adding Pd to Al.
No. 274008. Besides, N to Al
It has been proposed to add i, Mg and the like.

[0004] As an electrode film material, Cu, T
When the electrode film is hardened by adding a small amount of i, Pd, Ni, Mg, etc., the hardening strength increases and the power durability increases as the addition amount increases, but the specific resistance of the electrode film increases. And, because the curing strength is too high, the internal loss increases,
In addition, there is a problem that wire bonding becomes difficult, and there is a great limitation on elements to be added and amounts to be added to satisfy both of them, and optimization on a fine structure of an electrode film has not been performed.

[0005]

The above prior art aims to improve the power durability by increasing the curing strength of the electrode film.
No consideration has been given to simultaneously reducing the internal resistance during operation by reducing the specific resistance of the electrode film and enabling wire bonding.

SUMMARY OF THE INVENTION The present invention solves the conventional problems, and provides an electrode material of a surface acoustic wave element electrode and a fine structure of an electrode thin film having high power durability, low internal loss during operation, and easy wire bonding. The purpose is to:

[0007]

According to the present invention, at least one transmitting and receiving electrode is provided on a piezoelectric substrate, and a surface acoustic wave is propagated including the transmitting and receiving electrode. Or at least part of the reflecting electrode, (1) 0.3 wt% to 2.0 wt% of Cu and Pd
To form a three-element Al alloy thin film containing 0.1 wt% to 3.0 wt% of Cu; (2) 0.3 wt% to 2.0 w of Cu
3 containing 0.1% to 3.0% by weight of Ti and 0.1% by weight
(3) Cu 0.3 wt%
% To 2.0 wt%, and a four-element Al alloy thin film containing 0.1 wt% to 3.0 wt% of the total of both Pd and Ti. (4) The average particle size of the Al alloy thin film is The width is set to 1/50 to 1/3 of the electrode width, and the above four means are adopted.

[0008]

[Function] As a method of forming an electrode film on a piezoelectric substrate,
Although a sputtering method and a vacuum evaporation method are used, the sputtering method is mainly used in view of the denseness of the film and the composition stability of the alloy film. In order to improve the power durability of the electrode film, it is effective to use an Al alloy thin film to which Cu is added. However, the disadvantage of the Al-Cu alloy film is that it is highly corrosive due to the battery effect, and when dry etching is performed using a chlorine-based gas plasma that forms a fine electrode with high precision, the vapor pressure of the chloride of Cu is reduced. Is low, etching is difficult, and electrode chloride is liable to be generated due to residual Cu chloride.

As a result of studying the effect of the additive material to Al and the effect of the electrode film structure on the electrode deterioration of the surface acoustic wave device, the inventors have found the following. The effect of adding Cu is suppression of the self-diffusion of Al atoms in the Al-Cu alloy film, and the self-diffusion rate of Al can be reduced by one digit or more as compared with other added materials. The Al-Cu alloy film has almost the same crystal grain size as the Al film formed under the same conditions, and has no effect of reducing the crystal grain size. On the other hand, for the electrode film structure, miniaturization of the crystal grain size is effective for preventing electrode deterioration. As a result of studying the additive material as a means for reducing the crystal grain size, Pd, T
i, Bi, V, Zr, Sn and Pb were effective. However, Bi, V, Zr, Sn, and Pb are not preferred in terms of resistance increase, oxidizing property, corrosiveness, phase separation and toxicity due to addition to Al, and Pd and Ti are more preferred.

Therefore, when Pd and Ti are added to the Al-Cu alloy film, the crystal grain size is reduced, and the power durability of the electrode is improved by more than three times as compared with the case where the Al-Cu alloy film is used. Make sure you do. The addition amount of Cu is required to be 0.1 wt% or more in order to exert its effect.
0.3 w
t% or more is more preferable. Further, the content needs to be 2.0 wt% or less in order to prevent the resistance increase due to the addition of Cu and the easiness of corrosion due to the battery effect. The addition amount of Pd and Ti is required to be 0.1 wt% or more in order to exert the effect, but it is 3.0 wt% because wire addition becomes impossible due to increase in resistance due to addition of Pd and Ti and increase in film hardness.
It is limited to the following.

The average particle size of the electrode film affects power durability and specific resistance. The smaller the average particle size, the higher the power durability tends to be. On the other hand, the specific resistance tends to increase and the internal loss tends to increase. Therefore, the average particle size has an optimum range. As a result of the experiment, when the average particle size is 1/3 or less of the electrode width,
Although it is effective for improving the power durability, it has been clarified that if it is less than 1/50, the specific resistance increases and the internal loss increases, which is not preferable.

Therefore, the average particle size of the electrode film is one of the electrode width.
It is necessary to be within the range of / 50 or more and 1/3 or less.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the drawings.

Embodiment 1 FIG. 1 (a) is a plan view of a surface acoustic wave two-aperture resonator, and FIG. 1 (b) is an A-line resonator shown in FIG. 1 (a).
FIG. 3 is a sectional view taken along line A ′. An ST-cut quartz substrate is used as the piezoelectric substrate 1a, and a pair of transmitting and receiving electrodes 2 and 2 'are formed on the substrate.
Are provided so as to transmit and receive surface acoustic waves to and from each other with an opening of 1000 μm and 28 pairs.
3 '. The bonding pads 3, 3 'are bonding wires made of an Al wire or an Au wire having a diameter of 25 μm.
4 'is electrically connected. Further, reflectors 5 and 5 'made of 750 metal splits are provided on both sides of the pair of transmitting and receiving electrodes 2 and 2' to constitute a two-aperture surface acoustic wave resonator. The thickness of the transmitting and receiving electrodes 2, 2 'and the electrodes of the reflectors 5, 5' is about 0.1 μm, the resonance frequency is 697 MHz, Q ≒ 4000, and a single layer structure of Al-0.5 wt% Cu-0.3wt% Ti electrode, Al
A -0.5 wt% Cu-0.3 wt% Pd electrode and an Al-0.5 wt% Cu electrode as a comparative example were produced. still,
The substrate 1a on which the transmitting and receiving electrodes 2, 2 'and the reflectors 5, 5' are formed is adhered to the TO-5 can package stem 7 with a conductive adhesive 6. FIG. 2 shows the results of the accelerated deterioration test. The conditions of the accelerated aging test were performed when the input power was changed at an ambient temperature of 120 ° C., and the life was shown by the time when the resonance frequency changed by ± 50 kHz from the start of the test. T
When an Al-Cu electrode to which i, Pd is added is used, A
It can be confirmed that the life is longer than when the l-Cu electrode is used.

Embodiment 2 FIG. 3 shows an element structure used in an accelerated deterioration test of this embodiment. FIG. 3A is a plan view of the element of this embodiment, and FIG. 3B is a cross-sectional view taken along the line AA ′ of FIG. The piezoelectric substrate 1b is made of LiTaO ₃ that propagates a SH-mode pseudo surface wave and is 36 ° rotated Y-axis cut and X-axis propagated. The electrode configuration is such that input electrodes 8 and output electrodes 9 are alternately arranged, and the number of input / output electrodes is 2
The output electrode 9 has three multi-electrode structures. The input electrode 8 and the output electrode 9 are
As shown in the cross-sectional view of FIG. 3B, the electrode width of the comb-shaped electrode finger 10 is equal to the width of the portion without the comb-shaped electrode finger 10 (space portion). A ground electrode pattern 11 is formed between the input / output electrodes 8 and 9. Further, the surface of the piezoelectric substrate 1b is covered with a floating electrode pattern 12 which is electrically insulated from the input / output electrodes 8, 9 and the ground electrode pattern 11. still,
The center frequency of this multi-electrode surface acoustic wave device is 880 MHz.
In z, the electrode width and space width of the comb-shaped electrode fingers of the input / output electrodes 8 and 9 are both 1.2 μm, and the width of the ground electrode pattern 11 is 5 μm. The electrode is a single layer Al-1.0 wt
% Cu-0.5 wt% Ti electrode, Al-1.0 wt% C
u-0.5 wt% Pd electrode, Al-1.0 as a comparative example
A product of a wt% Cu electrode was manufactured, and the film thickness was about 0.1 μm. FIG. 4 shows the results of the accelerated deterioration test. The conditions of the accelerated aging test were performed at an ambient temperature of 120 ° C. while changing the input power, and the life was defined as the time at which the loss at the center frequency increased by 0.5 dB. It can be confirmed that the life is longer when the Al-Cu electrode to which Ti and Pd are added is used than when the Al-Cu electrode is used.

Example 3 The same multi-electrode surface acoustic wave device structure as in Example 2 was used, and the electrodes were of a single-layer structure of Al-1.
A 0 wt% Cu-0.3 wt% Ti-0.3 wt% Pd electrode and an Al-1.5 wt% Cu electrode as a comparative example were prepared and subjected to an accelerated deterioration test. The thickness is about 0.1 μm. FIG. 5 shows the results of the accelerated deterioration test. The conditions of the accelerated aging test were performed at an ambient temperature of 120 ° C. while changing the input power, and the life was defined as the time at which the loss at the center frequency increased by 0.5 dB. It can be confirmed that when the Al-Cu electrode to which Ti and Pd are added is used, the life is longer than when the Al-Cu electrode is used.

Embodiment 4 This embodiment shows the range of the amount of Cu added to Al. FIG. 6 shows, in a surface acoustic wave device using an Al—Cu electrode film, the life with respect to the added amount of Cu normalized by the life when the Al electrode film is used. The surface acoustic wave device used for life evaluation is
It is a two-aperture resonator using an ST crystal substrate, and the conditions for the accelerated deterioration test are an ambient temperature of 120 ° C. and an input power of 1 W.
The life was defined as the time when the resonance frequency changed by ± 50 kHz from the start of the test. The structure and the like of the surface acoustic wave element are described in Example 1.
Is the same as When the amount of Cu added to Al is increased, A
The life multiplier for 1 increases. Although the effect of improving the life appears by adding 0.1 wt% Cu, at least Al
0.3 w to ensure a life twice as long as
It is necessary to add t% Cu or more. However, 2.0wt
If more than% Cu is added, the electrode frequently corrodes during the surface acoustic wave device fabrication process, which is not practical. Therefore, as the addition amount of Cu, 0.1.
Addition of 1 wt% or more is preferable, and addition of 0.3 wt% or more is more preferable, but it is necessary to be 2.0 wt% or less.

Embodiment 5: This embodiment relates to the case where Ti, P
It shows the lower limit of the amount of d added. FIG. 7 shows the average crystal grain size of the alloy film with respect to the amounts of Ti and Pd added to Al. The thickness of each alloy film was evaluated as being constant at 0.13 μm. Both Ti and Pd have the effect of reducing the crystal grain size of the alloy film, and it has been confirmed that Pd has a greater effect at the same addition amount. Although this effect appears even in a small amount of addition, at least 0.1 wt% or more is more desirable.

Embodiment 6: This embodiment relates to the case where Ti, P
It shows the upper limit of the amount of d added. FIG.
It shows the specific resistance of the alloy film with respect to the amount of Ti and Pd added to 0.5 wt% Cu. The thickness of each alloy film is 0.
The evaluation was made constant at 1 μm. Although the specific resistance increases with the addition amount of Ti and Pd, the ratio of the increase is smaller with the addition of Pd. If more than 3.0 wt% is added, the hardness of the alloy film increases and wire bonding becomes impossible, so it is necessary to make the addition amount 3.0 wt% or less.

Considering the results of Example 5, Ti, Pd
Is required to be 0.1 wt% or more and 3.0 wt% or less.

Embodiment 7 FIG. 9 is a view showing that the life of a surface acoustic wave device electrode is greatly affected by (average crystal grain size / electrode width) of an electrode film. That is, the relationship between the ratio of the average particle diameter of the electrode film to the electrode width and the life of the device electrode is indicated by the characteristic line indicated by a black circle. The accelerated deterioration test conditions and elements are the same as those described in Example 2. However, the composition of the electrode film is Al-0.5 wt% Cu-0.4
wt% Pd. The ratio of the average particle size of the electrode film to the electrode width can be obtained by changing the conditions for forming the electrode film. As the ratio decreases, the service life increases, and in the present accelerated test, it is practically necessary to maintain a service life of 10 hours or more. Therefore, it is understood that the ratio needs to be 1/3 or less. On the other hand, if the ratio is lower than 1/50, as shown by the characteristic line indicated by the black square, the specific resistance of the electrode film increases, which is not preferable because the internal loss as an element increases. Therefore, the preferable ratio is 1/3 to 1/50 based on the above two conditions.

Example 8: FIG. 10 shows that Al-0.5 wt%
It shows the average crystal grain size of the alloy film with respect to the addition amount of Ti and Pd to Cu. The thickness of each alloy film is 0.13μ
The evaluation was made constant at m. Both Ti and Pd have the effect of reducing the crystal grain size of the alloy film, and it has been confirmed that Pd has a greater effect at the same addition amount. Although this effect appears even in a small amount of addition, at least 0.1 wt% or more is more desirable.

According to the results of the accelerated deterioration test shown in FIG. 2 of Example 1, the life was longer when Ti was added than when Pd was added.
The result is different from that of the present example in which d is more effective in reducing the average crystal grain size. This fact means that the self-diffusion of Al is smaller in the alloy to which Ti is added than in the alloy to which Pd is added.

Example 9: FIG. 11 shows Al-1.0 wt%
It shows the average crystal grain size of the alloy film with respect to the addition amount of Ti and Pd to Cu. The thickness of each alloy film is 0.13μ
The evaluation was made constant at m. Both Ti and Pd have the effect of reducing the crystal grain size of the alloy film, and it has been confirmed that Pd has a greater effect at the same addition amount. Although this effect appears even in a small amount of addition, at least 0.1 wt% or more is more desirable.

According to the results of the accelerated deterioration test shown in FIG. 4 of Example 2, the life was longer when Ti was added than when Pd was added.
The result is different from that of the present example in which d is more effective in reducing the average crystal grain size. This fact means that the self-diffusion of Al is smaller in the alloy to which Ti is added than in the alloy to which Pd is added.

As described above, in the present embodiment, an electrode composed of a single-layer film has been described. However, it is effective to apply the present invention to at least one layer of a multilayer film composed of two or more layers. The thickness of the electrode film was 0.1 μm in the present embodiment,
It can be thicker or thinner. The piezoelectric substrate is
The present invention is not limited to the quartz crystal and LiTaO ₃ of this embodiment, and any piezoelectric substrate made of LiNbO ₃ , Li ₂ B ₄ O ₇ , ZnO or the like is included in the present invention. Also, the element structure does not need to be limited to the resonator type structure and the multi-electrode type structure of this embodiment.

[0027]

As described above, according to the present invention, in the surface acoustic wave device electrode, the power durability can be greatly improved, the specific resistance of the electrode film can be reduced, and wire bonding can be performed. Can do things.

[Brief description of the drawings]

FIG. 1A is a plan view of a surface acoustic wave device according to one embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along line AA in FIG.

FIG. 2 is a diagram illustrating an Al-0.5 wt% Cu electrode film and an Al-0.5 wt%
% Cu-0.3wt% Ti electrode film, Al-0.5wt%
FIG. 9 is a characteristic diagram showing an accelerated deterioration test result when a Cu-0.3 wt% Pd electrode film is used.

3A is a plan view of a surface acoustic wave device according to one embodiment of the present invention, and FIG. 3B is a cross-sectional view taken along line AA in FIG.

FIG. 4 shows a multi-electrode surface acoustic wave device according to a second embodiment, in which an Al-1.0 wt% Cu electrode film and an Al-1.0 wt% are used.
% Cu-0.5wt% Ti electrode film, Al-1.0wt%
FIG. 9 is a characteristic diagram showing an accelerated deterioration test result when a Cu-0.5 wt% Pd electrode film is used.

FIG. 5 shows a multi-electrode surface acoustic wave device according to a third embodiment, in which an Al-1.5 wt% Cu electrode film and an Al-1.0 wt% are used.
FIG. 9 is a characteristic diagram showing an accelerated deterioration test result when a% Cu-0.3 wt% Ti-0.3 wt% Pd electrode film is used.

FIG. 6 is a characteristic diagram showing the relationship between the amount of Cu added and the life of an Al—Cu electrode film.

FIG. 7 is a characteristic diagram showing the relationship between the amounts of Ti and Pd added to an Al—Ti film and an Al—Pd film and the average crystal grain size.

FIG. 8 shows Ti and Pd on an Al-0.5 wt% Cu electrode film.
It is a characteristic view which shows the relationship between the addition amount and specific resistance.

FIG. 9 is a characteristic diagram showing a relationship between an average crystal grain size / film thickness in an electrode film, life, and standardized specific resistance.

FIG. 10 is a characteristic diagram showing the relationship between the amounts of Ti and Pd added to Al-0.5 wt% Cu and the average crystal grain size.

FIG. 11 is a characteristic diagram showing the relationship between the amounts of Ti and Pd added to Al-1.0 wt% Cu and the average crystal grain size.

[Explanation of symbols]

 1a, 1b: piezoelectric substrate, 2, 2 ': transmitting / receiving electrode, 3, 3': bonding pad, 4, 4 ': input / output pin, 5, 5': reflector, 6: conductive adhesive, 7 ... Can package stem, 8 ... input electrode, 9 ... output electrode, 10 ... comb electrode, 11 ... grounding electrode pattern, 12 ... floating electrode pattern.

Claims (3)

(57) [Claims] An electrode having at least one transmitting / receiving electrode on a piezoelectric substrate, and including at least one of the transmitting / receiving electrodes, at least a part of an electrode for propagating or reflecting a surface acoustic wave is disposed in a thickness direction of the electrode The average composition of Cu is 0.3 wt% to 2.0 w
3 containing 0.1% to 3.0% by weight of Ti and 0.1% by weight
A surface acoustic wave device comprising an element Al alloy. 2. A piezoelectric substrate comprising at least one transmitting / receiving electrode on a piezoelectric substrate, and at least a part of an electrode for transmitting or reflecting a surface acoustic wave, including the transmitting / receiving electrode, is disposed in a thickness direction of the electrode. The average composition of Cu is 0.3 wt% to 2.0 w
t%, and the total of both Pd and Ti is 0.1 wt% or more.
A surface acoustic wave device comprising a four-element Al alloy added with 3.0 wt%. 3. An average particle diameter of at least one electrode for transmitting and receiving a surface acoustic wave, including at least one transmitting / receiving electrode on a piezoelectric substrate, including the transmitting / receiving electrode. 3. The surface acoustic wave device according to claim 1, wherein the width of the surface acoustic wave is 1/50 to 1/3 of the electrode width.