JPH043510A - Ultra-thin, anti-migration electrode - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to electrodes of highly reliable surface acoustic wave elements, and particularly relates to surface acoustic wave devices that transmit large amounts of power or surface acoustic wave waves with large amplitude and high frequency. The present invention relates to a thin film electrode suitable for a surface acoustic wave device in which

[Conventional technology]

Regarding surface acoustic wave devices using fine interdigitated comb-shaped electrode fingers, see Transactions of the Institute of Electronics and Communication Engineers, Vol. 9, J.

As described in No. 67C, pp. 278-285 (March 1984), protrusions (protrusions) caused by electromigration that occur on aluminum wiring electrodes of semiconductor integrated circuits.
In surface acoustic wave resonators, defects such as hillocks and voids occur, causing failures such as resonance frequency shifts, and in surface acoustic wave filters that transmit large amounts of power, failures such as short circuits and disconnections due to the growth of hillocks. This occurred frequently and lacked long-term reliability.

In the above literature, the mechanism by which such defects occur is that the strain on the substrate surface caused by surface acoustic waves generates internal stress in the AQ thin film electrode formed on the substrate surface, and the area where this stress exceeds a threshold value occurs. It is said that grain boundary movement of AQ occurs, resulting in voids and hillocks.

Grain boundary movement due to internal stress is caused by I.E.
E.E., Transactions, Parts.

Hybrids, and Packaging, P.H.B. Vol., No. 5 (1971), pp. 134 to 138 (IEEE Trans, Parts Hybrids
andPackagingPHP-7N(15(19
71) It is argued that the same mechanism is the case in temperature cycling of integrated circuits as shown in PP134-138). In addition, the above-mentioned document describes a method of adding a trace amount (1 to 4%) of Cu to the AQ of wiring materials used in semiconductor integrated circuits as a countermeasure against defects caused by such migration, and discusses its effectiveness in suppressing migration. It shows.

However, the conventional technique of adding a small amount of Cu to AQ according to the above-mentioned document has the drawback that the hardness of the film increases, making it difficult to bond it to an external circuit board such as by wire bonding. In high-frequency surface acoustic wave devices, when dry etching is used to form microelectrodes with high precision, corrosion of the electrodes is more likely to occur than with pure AQ electrodes, and device manufacturing The disadvantage is that the yield is significantly lower. In addition, in surface acoustic wave devices used in high frequency bands, even if the transmitted power or amplitude remains unchanged and is the same as the conventional technology, the distortion of the surface waves increases as the frequency increases, so the above literature Cu-doped AQ electrodes made by resistance heating or electron beam heating evaporation, such as those seen here, have the disadvantage that a sufficient lifespan cannot be guaranteed when operating in a high frequency band where large power (large amplitude) or large strain is excited.

[Problem to be solved by the invention]

The present invention is an electrode for a surface acoustic wave device that handles high-frequency surface acoustic waves that excite high power, large amplitude, and large strain as described above, and solves the problems and drawbacks that have conventionally occurred. Another object of the present invention is to provide a highly reliable electrode for a surface acoustic wave device that can withstand long-term use even under high frequency conditions.

[Means to solve the problem]

In order to achieve the above object, the present invention uses a comb-shaped interdigital electrode that excites, propagates, or reflects surface acoustic waves arranged on a piezoelectric substrate for a surface acoustic wave device, and is made of a pure AQ thin film or a thin film with impurities at a low concentration. It is formed of a multilayer film with a hyperperiodic structure in which a first layer made of a doped AQ thin film and a second layer made of a pure metal thin film or a metal thin film doped with impurities at a low concentration are superimposed. Furthermore, in order to improve the migration resistance of the electrode, the multilayer film with a super periodic structure constituting the electrode is a multilayer film in which there is no interdiffusion between the layers. Furthermore, in order to create an electrode with good migration resistance and a small change in resistivity over time, the multilayer film constituting the electrode has a crystal grain size distribution width of 10 nm.
m or less, and its specific resistance value is 6 μΩ·■ or less.

[Effect]

A super-periodic structure in which a first electrode layer made of a pure AQ thin film or an AQ thin film doped with impurities at a low concentration is overlapped with a second electrode layer made of a pure metal thin film or a metal thin film doped with impurities at a low concentration. The multilayer electrode structure is such that the periodically arranged second layer acts to suppress the grain boundary diffusion of AQ atoms in the periodically arranged first layer, so that the AQ Alternatively, the migration resistance of the first inner layer made of AQ containing a low concentration of impurities is improved. Thereby, the power durability and long-term reliability of the surface acoustic wave device are significantly improved.

In a multilayer electrode with a hyperperiodic structure, creating a multilayer electrode without interdiffusion between the first layer and the second layer is as follows:
It acts to reduce the resistance value of the thin film electrode. This lower resistance makes it possible to reduce the temperature rise due to heat generation of the electrode during surface acoustic wave excitation at high power or high frequency bands, suppressing AQ and grain boundary diffusion of atoms, and significantly improving migration resistance. do.

Reducing the crystal grain size distribution width of the multilayer film with a hyperperiodic structure constituting the electrode to 10 nm or less acts to suppress the occurrence of migration resulting from the nonuniformity of the grain size distribution, that is, the crystal grains. . As a result, changes over time in the resistivity of the electrodes are significantly reduced, and, for example, fluctuations in the center frequency of the surface acoustic wave device are suppressed, and the life of the device can be significantly improved.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG.

FIG. 1(a) is a plan view of a comb-shaped interdigital electrode pattern of a surface acoustic wave element, and FIG. 1(b) is a sectional view of a portion indicated by AA' in the same figure.

The electrode pattern 1 in the figure is the piezoelectric substrate LST cut crystal 2
It is formed by photo-etching on top of the . What is the minimum line width and gap width of the pattern?

Each is 0.4 μm.

The first layer 3 is a pure aluminum film with a thickness of 50 ml, and the second layer 4 is a Cu film with a thickness of 10 ml, for a total thickness of about 300 ml.

These electrodes are made by cooling the substrate below the liquid nitrogen temperature (77K) and sequentially depositing the first and second layers to ensure that mutual diffusion between the layers is achieved between the piezoelectric substrate, the first layer, and the second layer. It is designed so that it does not occur.

Such interdiffusion at the interface between the substrate and the first layer of the electrode, and between the first layer and the second layer of the electrode can be achieved by forming the electrode at a low temperature.

FIG. 2 is a cross-sectional view of an electrode showing another embodiment in the same electrode configuration as FIG. 1. In (a) of the figure, the first layer 31 is an aluminum film with a thickness of 70 mm, and the second layer 41 is an A Q /Cu (Cu: 2 wt %) film with a thickness of 5 mm.

In (b) of the same figure, the first layer 32 is made of AQ/Cu with a film thickness of 50 mm.
(Cu: 2wt%) film, and the first layer 42 is composed of a magnesium (Mg) film with a thickness of 5 mm.

In (c) of the same figure, the first layer 33 is made of AR/Cu with a film thickness of 40 mm.
(Cu: 2wt%) film, the second layer 43 has a film thickness of 10
It is composed of a human AQ/Ti (Ti: 1wt%) film.

Using the comb-shaped interdigital electrodes of this example as a basic pattern, a surface acoustic wave resonator was formed with a wave transmitting/receiving electrode 2, a reflector electrode, and a bonding pad, and the results of an accelerated deterioration test were presented in the third section. As shown in the figure.

The figure also shows, as a comparative example, the test results of a resonator whose electrodes were formed by vapor deposition of AQ added with several percent of Cu, which has been used in the past. In the figure, the horizontal axis is AQ-
In the case of Cu (conventional example) film, the average concentration of the additive element Cu in the Afl film is shown in wt%, and the Afl/Cu multilayer film (Example, AQ: 50 people).

In the case of Cu: 5 people), the number of laminated layers of AQ/Cu is shown, and the vertical axis shows the deterioration time TF as the time when the resonance frequency changes.

Accelerated deterioration test conditions are temperature 120℃, hexagonal power 100m
It is W.

The broken line 5 in the figure represents the test results of a comparative example in which the electrode was formed with an AQ film added with Cu, and the solid line 6 represents the test results of the example of the present invention. In the example, the deterioration time was approximately three times longer than in the comparative example in the case of five layers, and the withstand power was significantly improved.Moreover, the resistivity of the film hardly increased, and the frequency characteristics as a resonator were not deteriorated. was not recognized at all.

As shown in the figure, in this example, the effect of the multilayer film electrode tends to be such that the deterioration time is saturated when the number of laminated layers is approximately 9.

The above example is a case of a two-opening surface acoustic wave resonator using a reflector made of multilayer metal film strips (electrodes).
The present invention is not limited to - Sekiguchi surface acoustic wave resonators, surface acoustic wave filters that send large power from input side electrodes to output side electrodes, and surface acoustic wave transducers that resonate. Even if it is used as a vessel, its effect remains the same. The piezoelectric substrate that excites surface acoustic waves is not limited to LST-cut crystal, but LiN
b0. , LiTaO3, etc., and cut plane orientations are also effective.

In addition, in the above embodiment, the −th layer was a pure aluminum film, but
As shown in FIGS. 2(b) and 2(c), it is more effective in terms of life if Cu is added to the negative layer within a range that does not increase the resistivity.

FIG. 4(a) is a diagram showing another embodiment of the present invention, in which the crystal grain size distribution width of the electrode is This figure shows an electrode pattern of a surface acoustic wave resonator that is made small to 10 nm or less by low-temperature formation. Further, FIG. 2B shows the rate of change in resonance frequency over time when this element was operated at a temperature of 65° C. and an excitation power of 2 mW.

As for the design conditions of the surface acoustic wave resonator, an X-cut LiTa○ was used as the piezoelectric substrate 7, and the propagation direction of the surface acoustic wave was set to a direction inclined by 112 degrees from the Y axis. The interdigital electrode 8, which is formed by combining a pair of comb-shaped electrodes 8a and 8b, is composed of 11 pairs of electrode fingers, and the grating reflectors 9 and 10 for reflecting surface acoustic waves each have 20 pairs of interdigitated electrode fingers.
It consisted of 0 strips.

In order to compare with the surface acoustic wave resonator of the present invention, interdigital electrodes and grating reflectors were deposited at a conventional substrate temperature of 200°C, and the crystal grain size distribution width was 1100°C.
A resonator made of a pure aluminum film with a thickness of n to 500 nm was similarly fabricated.

In FIG. 4(b), curve 11 shows the case where the interdigital electrodes and grating reflector are formed of the pure aluminum film (conventional example) with a single layer (-layer) crystal grain size distribution width of 100 to 500 nm. It shows the characteristics of the resonator and also curve 12
A Q / interdigital electrode and grating reflector
Cu-AQ/Ti multilayer film (Example, AQ/Ti multilayer film
4 shows the characteristics of a resonator of the present invention formed using Cu: 40 people, AQ/Ti: 5 people.

In a resonator in which the electrodes and reflector are made of a pure aluminum film with a wide crystal grain size distribution, the resonance frequency significantly decreases over time. In this case, fluctuations in the resonant frequency are significantly reduced. In other words, after 1000 hours, the rate of change in the resonant frequency when using a pure aluminum film is -
In contrast, in the present invention, in which the crystal grain size distribution is reduced using a multilayer electrode without interfacial interdiffusion, -0.0%.
It is 06%. In this way, the reduction in the resonant frequency is reduced by approximately 1/
It can be suppressed to 8.

As described above, by reducing the crystal grain size distribution width of the multilayer film with a hyperperiodic structure constituting the electrode and reflector to 10 nm or less, the occurrence of migration resulting from the nonuniformity of the grain size distribution, that is, the crystal grains, can be reduced. As a result, changes in the resistivity of electrodes, etc. over time are significantly reduced, and fluctuations in the center frequency of the surface acoustic wave device are suppressed, and the life of the device can be greatly improved.

〔Effect of the invention〕

According to the present invention, in a multilayer thin film electrode with a superperiodic structure,
Since the periodically arranged second electrode layer acts to suppress grain boundary diffusion of AQ atoms in the periodically arranged first electrode layer, the durability of the first electrode layer is increased. Migration properties are improved and the life of the electrode is significantly extended.

Furthermore, as shown in FIG. 5, by forming the electrode layer at an extremely low temperature, mutual diffusion at the interface between AQ and the substrate (L i T a O,) is suppressed, and the constituent element T in the substrate is suppressed.
Since a and ○ (oxygen) are not diffused into the electrode layer, the resistivity of the thin film electrode can be reduced. By this extremely low temperature formation, the first and second layers of the multilayer thin film electrode are
Since it is also possible to form a multilayer electrode without interdiffusion between the layers, the resistivity of the multilayer thin film electrode can be significantly reduced.

By reducing the resistance of the electrode in this manner, the temperature rise due to heat generation of the electrode is reduced, and grain boundary diffusion of AQ atoms can be suppressed, so that the power durability of the electrode layer can be significantly improved.

Furthermore, by reducing the crystal grain size distribution width of the electrode layer to 10 nm or less, it is possible to suppress the occurrence of migration caused by non-uniformity of crystal grains, so the change in resistivity of the electrode over time is significantly reduced. This has the effect of suppressing fluctuations in the center frequency of the surface acoustic wave element and significantly improving the long-term reliability of the element.

[Brief explanation of drawings]

FIG. 1 is a plan view of a comb-shaped interdigital electrode pattern according to an embodiment of the present invention, and a cross-sectional view of a portion indicated by A-A' in the figure, and FIG. 2 is a plan view of an electrode pattern showing another embodiment of the present invention. 3 is a graph showing the accelerated deterioration test results of a surface acoustic wave resonator according to an embodiment of the present invention, and FIG. 4 is a plan view of an electrode pattern of a surface acoustic wave resonator according to an embodiment of the present invention, and FIG. FIG. 5 is a graph showing the change over time in the resonant frequency change rate, and is a composition distribution map obtained by Auger analysis showing a cross-sectional view of the electrode/substrate interface in the electrode layer and the diffusion state. 11. Electrode pattern, 2... Piezoelectric substrate (LST cut crystal), 3... First layer of electrode constituent material (31, 32° 33), 4... Second layer of electrode constituent material (41, 42° 4)
3), 5... Test results of Cu-added AQ electrode (comparative example)
, 6... Test results of the multilayer thin film electrode of the present invention (Example)
, 7... Piezoelectric substrate (L x Ta Oa),
8... Interdigital electrodes, 8a and 8b... Comb-shaped electrodes, 9 and 10... Grating reflectors, 11...
Characteristics of a resonator formed with a single layer aluminum electrode (conventional example), 1
2...Kura 1 Figure (ku) Fu 2 Figure Zume) (shi) (C-) Takugo R 饗翳迳觳觳 (￥34 圀(θ、n10θ lθθθ time 藺 (hhi)

Claims (5)

[Claims] 1. A surface acoustic wave device comprising a piezoelectric substrate and a number of periodically arranged metal strips provided on the piezoelectric substrate for converting an input electric signal into a surface acoustic wave propagating on the piezoelectric substrate. A surface acoustic wave element comprising a transducer and a reflector formed by periodically arranging a large number of metal strips for reflecting the surface acoustic wave, which is provided on the piezoelectric substrate to face the transducer. The metal strip constituting the transducer and the reflector is made of a first layer made of a pure aluminum thin film or a lightly doped aluminum thin film, and a pure metal thin film or a lightly doped metal thin film. An ultra-thin, anti-migration electrode characterized by being composed of a super-periodic structure film overlaid with a second layer. 2. 2. The ultra-thin anti-migration electrode according to claim 1, wherein the second layer is thinner than the first layer. 3. The ultra-periodic structure film in which the first layer and the second layer are stacked is an ultra-thin and durable film according to claims 1 and 2, characterized in that there is no interdiffusion between each layer. Migration electrode. 4. The ultra-periodic structure film constituting the metal strip has a specific resistance value of 6 μΩ・with a thickness in the range of 10 to 100 nm.
4. The ultra-thin, anti-migration electrode according to any one of claims 1 to 3, characterized in that the electrode has a thickness of less than cm. 5. The ultra-thin migration-resistant electrode according to claims 1 to 4, wherein the crystal grain size of the thin film constituting the first and second layers has a grain size distribution of 10 nm or less. .