JP2003347880A - Surface acoustic wave device and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave device, capable of preventing strength degradation of bumps due to excessive diffusion of gold, forming bumps in interdigital electrodes, and to provide its manufacturing method. <P>SOLUTION: A base layer 2a for bonding pad electrodes is formed as a part of the interdigital electrodes, on a piezoelectric substrate 1. An aluminum oxide film 8 is formed on the base layer 2a. A raised layer 7 is formed on the aluminum oxide film 8. By subjecting to heat treatment, gold diffuses to some parts of the base layer 2a through parts of the aluminum oxide film 8, and conductivity is thereby assured. Since the aluminum oxide film 8 suppresses diffusion of gold, strength degradation of the bonding pad electrodes 5 is prevented due to excessive generation of gold-aluminum alloy. <P>COPYRIGHT: (C)2004,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a surface acoustic wave device used for a cellular phone, a car phone, a wireless device, and the like. The present invention relates to a small surface acoustic wave device using the same and a method for manufacturing the same.

[0002]

2. Description of the Related Art A surface acoustic wave device, that is, a surface acoustic wave filter or a surface acoustic wave resonator, has been increasingly used as a filter in a high-frequency circuit section of a cellular phone or the like instead of a dielectric filter. This is because the surface acoustic wave filter has a smaller element size than the dielectric filter, and has better electrical characteristics when compared with the same element size.

As shown in FIG. 10, this surface acoustic wave device includes an interdigital electrode 2, that is, a surface acoustic wave converter (IDT) and a reflector on a surface of a piezoelectric substrate 1 such as lithium niobate or lithium tantalate. In general, aluminum or an aluminum alloy such as aluminum-copper is used as the electrode material. Generally, aluminum or an aluminum alloy is formed by sputtering.

Conventionally, an aluminum wire has been used for the electrical connection from the IDT to the outside. However, in order to meet the demand for downsizing of the surface acoustic wave device, a flip-chip method capable of saving an area where a wire is stretched is used. Is being heavily used. In this method, the size of the package 3 for accommodating the piezoelectric substrate 1 may be the same as the size of the piezoelectric substrate 1 cut out by forming the interdigital electrode 2, and can be downsized. As the flip chip method, there is a method of forming a gold bump 4 on a pad electrode base layer 2a formed as a part of an interdigital electrode 2 on a piezoelectric substrate 1. The bonding pad electrode 5 has a chromium layer 6 on the pad electrode base 2a for increasing adhesion strength,
A raised layer 7 made of aluminum is provided, and a gold bump 4 is formed thereon.

In the case of flip chip bonding with the gold bumps 4, the role played by the gold bumps 4 is that in addition to the electrical connection, it is necessary to mechanically hold the surface acoustic wave device chip in the package. Requires a thicker bonding pad electrode 5. For this reason, the diameter of the bonding pad electrode 5 needs to be as large as 60 to 150 μm, and a thicker bonding pad electrode 5 is required. When the thickness of the bonding pad electrode 5 is small, there occurs a problem that the bonding pad electrode 5 and the gold bump 4 cannot be bonded when the gold bump 4 is formed.

[0006]

When the bonding pad electrode 5 having a larger film thickness than the interdigital electrode 2 is formed,
After removing the electrically insulating natural oxide film from the surface of the interdigital electrode 2 made of aluminum or aluminum alloy, the bonding pad electrode 5 is formed, or a vacuum is applied so as not to form the natural oxide film. It was necessary to form the drooping electrode 2 and the bonding pad electrode 5 at the same time. This is because the presence of the natural oxide film of aluminum increases the electric resistance of the bonding pad electrode 5 and thus increases the loss of the surface acoustic wave device.

However, in this conventional surface acoustic wave device, when it is soldered to a printed circuit board by reflow,
The thermal diffusion of gold forming the gold bumps 4 occurs, whereby a mechanically fragile gold-aluminum alloy is formed at the interface between the piezoelectric substrate 1 and the interdigital electrode 2, causing a problem that the bump strength is reduced. This is because the gold element forming the gold bump 4 is excessively diffused by the temperature rise during reflow.

The present invention has been made in view of the above-mentioned problems of the prior art,
An object of the present invention is to provide a surface acoustic wave device capable of preventing a reduction in the strength of a bump due to excessive diffusion of gold forming a bump into an interdigital electrode, and a method for manufacturing the same.

[0009]

(1) In a surface acoustic wave device according to the present invention, an IDT made of aluminum or an aluminum alloy is formed on a piezoelectric substrate, and a bonding pad which is a part of the IDT is formed. A padding layer of aluminum or an aluminum alloy is formed on a base layer of the electrode, a bump layer is formed on the bonding pad electrode, and a gold bump is formed on the bonding pad electrode. The gold bump is used to flip-chip bond the piezoelectric substrate to a package. A surface acoustic wave device, wherein an aluminum oxide film is formed between the underlayer and the raised layer.

As described above, by forming the aluminum oxide film between the base layer and the raising layer of the bonding pad electrode, the diffusion of gold forming the gold bump into the interdigital electrode is suppressed, and the gold-aluminum alloy is formed. It is possible to prevent a decrease in bonding strength of the bump to the piezoelectric substrate due to the formation.

(2) In the surface acoustic wave device according to the present invention, preferably, an aluminum oxide film is formed between the interdigital transducer and the bonding pad electrode, and the gold of the gold bump is formed on the aluminum oxide film. It is configured to pass through and diffuse into the underlayer.

Even when gold is passed through the aluminum oxide film in this manner, the amount of the gold-aluminum alloy layer formed on the IDTs is smaller than in the conventional case without an oxide film, and the piezoelectric properties of the bumps are reduced. A decrease in bonding strength to the substrate can be prevented, and conductivity can be ensured.

(3) In the surface acoustic wave device of the present invention, preferably, a chromium layer is formed between the aluminum oxide film and the raised layer.

According to this structure, when the raised layer is formed by etching aluminum or aluminum alloy as the upper layer, excessive etching can be prevented by stopping the etching when the chromium layer is exposed.

(4) In the method of manufacturing a surface acoustic wave device according to the present invention, aluminum or an aluminum alloy is formed on a piezoelectric substrate, and the aluminum or aluminum alloy film is etched to form interdigital electrodes and a part thereof. An aluminum oxide film is formed on the underlayer by forming an underlayer of the bonding pad electrode, and then left in an oxygen atmosphere or air, and aluminum or aluminum is formed on the underlayer on which the aluminum oxide film is formed. Forming a raising layer made of an alloy, forming a bonding pad electrode by etching, forming a gold bump on the bonding pad electrode, and performing a heat treatment at a temperature range of 200 to 300 ° C. Features.

As described above, by forming the aluminum oxide film on the underlayer by leaving the film, the film can be formed without preparing a special film forming apparatus. In addition, by performing a heat treatment at 200 to 300 ° C. after the formation of the gold bump, appropriate diffusion of gold is caused, and conductivity is obtained, and the bump strength can be secured. If the heat treatment temperature is lower than 200 ° C., gold cannot sufficiently pass through the oxide film, making it difficult to secure conductivity. On the other hand, if the heat treatment exceeds 300 ° C., gold is diffused over the entire surface of the pad electrode, and the bump strength is reduced.

(5) In the method of manufacturing a surface acoustic wave device according to the present invention, preferably, as a raised layer of the bonding pad electrode, a laminated electrode in which a lower layer is a chromium film and an upper layer is an aluminum or aluminum alloy film is formed.

As described above, when the raised layer is formed by etching, excessive etching can be prevented by stopping the etching when the chromium layer is exposed.

[0019]

FIG. 1A is a partial cross-sectional view schematically showing one embodiment of a surface acoustic wave device according to the present invention. In FIG. 1A, reference numeral 1 denotes a piezoelectric substrate made of lithium tantalate, lithium niobate, or the like, and reference numeral 2 denotes an interdigital transducer made of aluminum or an aluminum alloy such as aluminum-copper, as shown in FIG. , Part 2
a is formed as a base layer of the bonding pad electrode 5. FIG. 3 is a partially enlarged cross-sectional view of FIG. 1 of FIG. 2. As shown in FIG. 2, an aluminum oxide film 8 is formed on the surface of the interdigital electrode 2. Reference numeral 9 denotes a terminal electrode for soldering the package 3 to a printed board (not shown).

Reference numeral 6 denotes a chromium layer provided as needed to increase the adhesion strength between the interdigital electrode 2 and the raised layer 7 made of aluminum or an aluminum alloy.
The gold bump 4 is formed on the raised layer 7.

In the surface acoustic wave element main body before being mounted on the package 3, aluminum or an aluminum alloy is formed on the piezoelectric substrate 1, and the aluminum or aluminum alloy film is etched to form the interdigital electrodes 2.
Thereafter, the aluminum oxide film 8 is formed on the IDT 2 by being left in an oxygen atmosphere or air. The aluminum oxide film 8 is formed, and the underlayer 2a is formed.
A chromium layer 6 is formed thereon, aluminum or an aluminum alloy is formed thereon, and a raised layer 7 is formed by etching. Then, the gold bump 4 is formed on the bonding pad electrode 5 thus formed,
Heat treatment is performed in a temperature range of 00 ° C.

As a result, as shown in FIG. 2, gold is diffused through the raised layer 7, the chromium layer 6, and the aluminum oxide film 8, and a gold diffusion region 10 is formed in a part of the base layer 2a. I do. By partially passing gold through the aluminum oxide film 8, the conductivity between the bonding pad electrode 5 and the interdigital electrode 2 is ensured.

The thickness of the interdigital transducer 2 is determined by the specifications of the piezoelectric substrate to be used and the surface acoustic wave filter to be manufactured. The thickness of the center electrode formed on the 36-degree Y-rotation X-propagating lithium tantalate of the present embodiment is determined. In the case of a filter having a frequency of 1842 MHz, the thickness of the IDT 2 is 170 to 2
The thickness of the aluminum oxide film 8 is preferably 1 to 5 nm (more preferably, 2 to 3 nm).
Has a thickness of 50 to 150 nm (more preferably 50 to 12 nm).
0 nm), the thickness of the raising layer 7 is 0.3 to 1.5 μm (more preferably 0.5 to 1.0 μm), and the thickness of the gold bump 4 is 10 to 30 μm (more preferably 15 to 25 μm). Is done.

Example 1 An Al-Cu alloy electrode was formed on a 36 ° Y-rotation X-propagating lithium tantalate substrate, which is a piezoelectric substrate 1, by sputtering, and a resist pattern was formed thereon by photolithography to form a dry pattern. The Al—Cu alloy film was etched by the etching method to obtain a pattern of the IDT 2 including the underlying layer a of the bonding pad electrode.

Thereafter, it was left in dry air for one day. After that, a chromium layer 6 and an aluminum layer 7 are formed,
A resist pattern was formed in the shape of a bonding pad electrode again by photolithography, and a pattern of the aluminum raised layer 7 was formed by dry etching. At this time, the chromium layer of the lower layer 6 acts as an etching stopper at the time of etching the aluminum of the lower layer, and the aluminum layer is not etched.
The etching was stopped when the chromium layer was visible. Next, the chromium is selectively etched by changing an unnecessary dry etching gas type, and the Al-Cu
The alloy film was not affected. Next, the gold bumps 4 were formed on the raised layer 7 by ultrasonically welding gold balls. Thereafter, a heat treatment was performed at 250 ° C. for 2 hours.
Next, after the piezoelectric substrate 1 was divided into chips by dicing, the gold bumps 4 were ultrasonically bonded to the package 3.

[Comparative Example 1] A sample manufactured by a conventional manufacturing method without the step of forming the aluminum oxide film was used as Comparative Example 1.

[Test for disconnection caused by reflow] The surface acoustic wave devices of Example 1 and Comparative Example 1 produced by the above process were subjected to a solder reflow test at 260 ° C., and the number of disconnections caused by bump destruction was examined. As a test method, first, the center frequency of the sample was measured, and then the sample was passed through a solder reflow furnace set at a peak temperature of 260 ° C. The method of measuring the frequency and observing the amount of change in the center frequency was repeated. The sample used in this test was designed as a receiving filter of DCS1800 which is a European mobile phone system, and the center frequency of the filter was around 1842 MHz. The number of samples
Each of the samples of Example 1 and Comparative Example 1 was performed on 15 samples.

FIG. 3 shows the results of Example 1 and Comparative Example 1.
The figure shows the change of the center frequency when each of the five samples is passed through the reflow furnace 20 times. In the case of Example 1,
The change of the center frequency was slight, and it was found that the bump connection portion was not broken. In the case of Comparative Example 1, the change of the center frequency was larger than that of Example 1.
In the case of Comparative Example 1, one out of fifteen out of five solder reflow furnace passages had a very large filter insertion loss, and the center frequency could not be measured. As a result of investigating the cause, it was found that the gold bump was peeled off from the underlayer 2a on the surface acoustic wave element chip. This is because gold excessively diffuses into the underlayer 2a and mechanically brittle Al-Au alloy is excessively formed, which causes a sharp rise in temperature when passing through a reflow furnace,
This is because peeling occurred due to thermal shock due to rapid cooling.

[Effect of Heat Treatment Temperature on Contact Resistance] In the manufacturing method of the first embodiment, the IDT 2
The contact resistance between the metal bumps and the gold bumps 4 was examined by changing the heat treatment temperature. FIG. 4 shows the results. As can be seen from FIG. 4, the contact resistance is 1 Ω or less at 200 ° C. or higher, which does not affect the characteristics of the surface acoustic wave device. This is because when the heat treatment temperature rises to 200 ° C. or more, gold is converted to
To diffuse appropriately into the underlying layer 2a.

Example 2 An Al-Cu alloy electrode was formed by sputtering on a 36 ° Y-rotation X-propagating lithium tantalate substrate, which is a piezoelectric substrate 1, and a resist pattern was formed thereon by photolithography to form a dry pattern. The pattern of the IDT 2 including the underlayer 2a made of the Al-Cu alloy film was obtained by the etching method.

After that, the residual chlorine cleaning solution after dry etching which is an aqueous solution of fluoride (manufactured by Mitsubishi Gas Chemical Company, Ltd.)
ELM-C30). The aluminum oxide film is removed by this cleaning liquid. After removing the aluminum oxide film with the residual chlorine cleaning liquid, the aluminum oxide film 8 was formed by being left for 3 days in an oxygen atmosphere, that is, in dry air.

Thereafter, a chromium layer and an aluminum layer were formed, a resist pattern was formed in the shape of a bonding pad electrode again by photolithography, and a pattern of the aluminum raised layer 7 was formed by a dry etching method. At this time, the etching was stopped at the lower chromium layer 6 so as not to affect the Al-Cu alloy film of the underlayer 2a. Next, the gold bumps 4 were formed by ultrasonically welding gold balls to the raised layers 7. Then 250
Heat treatment was performed at 2 ° C for 2 hours. Next, after the piezoelectric substrate 1 was divided into chips by dicing, the gold bumps 4 were ultrasonically bonded to the package 3.

[Comparative Example 2] For comparison, after eluting and removing the aluminum oxide film 8 with a residual chlorine cleaning solution after dry etching, the aluminum oxide film 8 was housed in a vacuum apparatus so as not to cause re-oxidation. An aluminum layer was formed.
The subsequent steps were performed in the same manner as in Example 2 to prepare a flip chip mounted sample.

[Chip Share Test] A chip share test was performed on the samples of Example 2 and Comparative Example 2 after flip chip mounting. The shear strength test of the flip chip is a destructive test, and is a test for examining the fracture strength of the gold bump due to shearing. The test is performed on 15 samples each of the samples of Example 2 and Comparative Example 2, and the average shear strength and the minimum Shore strength I asked. Table 1 shows the results.

[0035]

[Table 1]

As is clear from Table 1, in the case of Example 2, a much larger shear strength is obtained than in Comparative Example 2, and the value of the minimum shear strength is closer to the average value as compared with Comparative Example 2. It was found that stable strength was obtained.

In the sample of Example 2, the area was a part of the area of the gold bump 4, whereas in the case of Comparative Example 2, the area was the entire area of the gold bump 4. Thus, in Comparative Example 2, it can be seen that excessive diffusion that gold diffusion extends over the entire surface of the gold bump 4 is a cause of the reduction in bump strength.

[Relationship Between Heat Treatment Temperature and Shear Strength] In the process of Example 2, gold bumps were prepared on a sample manufactured under the manufacturing conditions of forming a raised layer 7 after storing in an oxygen atmosphere for 3 days after cleaning after etching. 4 Flip chip strength was evaluated when the heat treatment temperature after formation was changed. Table 2 shows the results. As can be seen from Table 2,
When the temperature exceeds 300 ° C., the shear strength of the heat treatment tends to decrease. This is a reduction in strength due to the gold breaking the barrier of the aluminum oxide film 8 and excessive diffusion of the gold under the entire surface.

As can be seen from Table 2, 200 to 27
In a temperature range of 0 ° C., a shear strength higher than 300 ° C. is obtained, and it is understood that heat treatment in this temperature range is more preferable. Further, as described above, at a heat treatment temperature of less than 200 ° C., it is also difficult to reduce the electric resistance of the electrode to a practical level of 1 Ω or less.
The temperature is preferably 0 ° C. or higher.

[0040]

[Table 2]

[Observation of Cross Section of Bonding Pad Electrode]
In the present invention, the underlayer 2 of the bonding pad electrode is used.
In order to confirm that the gold diffusion in a is partially performed and excessive diffusion is suppressed, the cross section of the pad electrode portion is observed with a transmission electron microscope (TEM), and at the same time, the elements and compounds are analyzed by the selected area electron beam diffraction. Identification was performed.

FIG. 5A is a TEM image showing a TEM image of a portion of the bonding pad electrode where diffusion is determined to be partially occurring (a raised portion of the chrome layer 6). FIG. 5B is a cross-sectional view for clarifying the position of each layer. The gold bumps 4 are not shown in these figures.

In FIG. 5A, portions indicated as P18, P19, etc. are portions of the underlayer 2a formed simultaneously with the interdigital electrodes, and are Al-Cu alloy layers.

The diffusion of gold into this layer is illustrated in FIG.
It became clear from the selected area electron beam diffraction image shown in FIG. That is, it was found that Al ₂ Au was rich (aluminum rich) at the point P19 around the raised portion of the chromium layer 6 and AlAu ₄ was rich (gold rich) at the center point P20. That is, it was found that the center of the raised portion was rich in gold, and the peripheral portion of the raised portion had little gold diffusion. From these facts, it can be determined that the protruding portion has caused the volume expansion due to the formation of the Al-Au alloy in the pad electrode, and it can be understood that the gold element has penetrated the chromium layer 6 and has reached the pad electrode.

FIG. 8A is a photograph showing a TEM image of a portion where diffusion is prevented by the aluminum oxide film, that is, a portion where the chromium layer 6 does not rise. FIG.
(B) is a cross-sectional view additionally shown to clarify the position of each layer. The gold bumps 4 are not shown in these figures.

In FIG. 8A, the area of the underlayer 2a indicated by P8 to P10 was subjected to selected area electron diffraction, but the presence of gold was not confirmed.

The proportions of these elements were analyzed by selected area electron beam diffraction, and the results are summarized in FIG. By comparing FIG. 9 with FIGS. 5 and 8 showing the analysis positions, it can be seen that the gold element forming the gold bump 4 partially diffuses to the underlayer 2a of the pad electrode, thereby obtaining electrical conduction. At the same time, it can be seen that the mechanical strength is maintained.

In the above embodiment, the chromium layer 6 is provided between the underlying layer 2a of the bonding pad electrode and the raised layer 7, but the chromium layer 6 is not necessarily required.

[0049]

According to the present invention, since the aluminum oxide film is formed on the underlying layer of the bonding pad electrode, the diffusion of gold into the underlying layer by heat treatment is partially performed, and the bonding of the gold bump to the pad electrode is performed. It is possible to provide a surface acoustic wave device that has increased strength, secures electrical conduction, and is thermally stable.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view schematically showing one embodiment of a surface acoustic wave device according to the present invention, and FIG. 1B is a plan view showing an example of the shape of an interdigital electrode.

FIG. 2 is an enlarged sectional view of a bonding pad electrode of the surface acoustic wave device according to the present invention.

FIG. 3 is a diagram showing a change in center frequency before and after a reflow furnace passage test in a surface acoustic wave device according to the present invention and a conventional product.

FIG. 4 is a correlation diagram between a heat treatment temperature and a contact resistance of the surface acoustic wave device according to the present invention.

FIG. 5A is a photographic image showing a TEM image of a portion of a bonding pad electrode which is determined to have partially diffused gold in the surface acoustic wave device of the present invention;
(B) is a cross-sectional view additionally shown to clarify the position of each layer.

FIG. 6 is a selected area electron beam diffraction image of part P18 in FIG. 5 (A).

FIG. 7 is a selected area electron beam diffraction image of a portion P19 in FIG. 5 (A).

8A is a photograph showing a TEM image of a portion of the bonding pad electrode where it is determined that gold has not diffused in the surface acoustic wave device of the present invention, and FIG. 8B is a view showing the position of each layer. FIG. 4 is a cross-sectional view additionally shown in order to clarify.

FIG. 9 is a diagram summarizing detection elements and detection intensities at respective portions of a cross section of a bonding pad electrode in the surface acoustic wave device of the present invention.

FIG. 10 is a cross-sectional view schematically showing a cross section of a bonding pad electrode of a conventional surface acoustic wave device.

[Explanation of symbols]

1: piezoelectric substrate, 2: interdigital electrode, 2a: base layer of bonding pad electrode, 3: package, 4: gold bump,
5: bonding pad electrode, 6: chromium layer, 7: raised layer, 8: aluminum oxide film, 9: terminal electrode, 10:
Gold diffusion area

Continuation of front page    F term (reference) 5J097 AA24 BB11 DD24 DD29 FF03                       FF08 GG03 GG04 HA04 JJ09                       KK01 KK10

Claims (5)

[Claims] An interdigital electrode made of aluminum or an aluminum alloy is formed on a piezoelectric substrate, and a bonding pad electrode made of aluminum or an aluminum alloy is formed on an underlying layer of the bonding pad electrode which is a part of the interdigital electrode. Forming a raised layer, forming a gold bump on the bonding pad electrode, and flip-chip bonding the piezoelectric substrate to a package by the gold bump, wherein the underlayer and the raised layer A surface acoustic wave device characterized in that an aluminum oxide film is formed therebetween. 2. An interdigitated electrode made of aluminum or an aluminum alloy is formed on a piezoelectric substrate, and a bonding pad electrode made of aluminum or an aluminum alloy is formed on a base layer of the bonding pad electrode which is a part of the interdigital electrode. Forming a raised layer, forming a gold bump on the bonding electrode, flip-chip bonding the piezoelectric substrate to a package by the gold bump, a surface acoustic wave device, wherein the base layer and the raised layer A surface acoustic wave device, wherein an aluminum oxide film is formed therebetween, and gold of the gold bump diffuses into the aluminum or aluminum alloy layer forming the underlayer through the aluminum oxide film. 3. The surface acoustic wave device according to claim 1, wherein a chromium layer is formed between the aluminum oxide film and the raised layer. 4. An aluminum or aluminum alloy film is formed on a piezoelectric substrate, and the aluminum or aluminum alloy film is etched to form a base layer for the interdigital electrode and a bonding pad electrode as a part thereof, By leaving in an atmosphere or air, an aluminum oxide film is formed on the underlayer, and a raised layer made of aluminum or an aluminum alloy is formed on the underlayer on which the aluminum oxide film is formed,
Forming a bonding pad electrode by etching; forming a gold bump on the bonding pad electrode;
A method for manufacturing a surface acoustic wave device, wherein a heat treatment is performed in a temperature range of 0 to 300 ° C. 5. The method of manufacturing a surface acoustic wave device according to claim 4, wherein a chromium layer is formed on the aluminum oxide film, and an aluminum or aluminum alloy layer is formed thereon as a raising layer. Of manufacturing a surface acoustic wave device.