JPH04196803A - Piezoelectric substrate for surface acoustic wave device, method for manufacturing the same, and surface acoustic wave device using the same - 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 provides a piezoelectric substrate in which electrodes formed on the surface of the piezoelectric substrate for excitation of surface acoustic waves, etc., are made difficult to corrode by high-precision processing; The present invention relates to a manufacturing method thereof and a surface acoustic wave device using the same.

[Prior Art] Surface acoustic wave devices are widely used in signal processing as elements such as filters and delay lines. In particular, in the field of mobile vehicle compensation, surface acoustic wave devices are being used as suitable elements because small size and light weight are required at high frequencies. Its basic structure consists of interdigital electrodes and grating reflectors formed of a thin film of aluminum or aluminum alloy on a piezoelectric substrate.

The period of forming the interdigital electrodes and grating reflectors is determined by the speed of the surface acoustic wave propagating through the piezoelectric substrate and the frequency of the electrical signal to be responded to, and becomes smaller as the frequency becomes higher. The interdigital electrodes of high-frequency surface acoustic wave devices are solid-type electrodes from the viewpoint of processing accuracy.
4 width electrodes are often used. For example, 900
In a MHz band surface acoustic wave device, the width of the λ/4 electrode is approximately 1
It is μm.

When forming such fine electrodes as described above, processing accuracy is particularly important. This is because when an electrode made of a metal thin film is formed on the surface of a piezoelectric substrate, the speed of the propagating surface acoustic wave differs between the part with the electrode and the part without it due to the mass addition effect and the short-circuit effect of the electric field. This is because variations in frequency lead to variations in frequency characteristics, resulting in a decrease in yield during manufacturing. In particular, at high frequencies of I GHz or higher, the width of the electrode is 1 micron or submicron, so high precision is required for electrode processing.

Similar to LSI, photolithography is used as a processing technology for the above electrodes. Conventionally, when processing aluminum thin films, an etching solution containing nitric acid and phosphoric acid as the main ingredients was used to chemically etch the electrodes. was forming. However, in the above conventional technique, isotropic etching is performed.

There is a problem with precision in forming minute electrodes as described above. As a high-precision etching technology, there is a dry etching technology that has superior anisotropy compared to conventional chemical etching.As one of the dry etching technologies used for processing aluminum or aluminum alloy, it has a high etching speed and is anisotropic. There is a reactive ion etching technology that is excellent for reactive etching. This etching technique uses a chlorine-based gas, turns the gas into a plasma state through electrical discharge, and causes the chlorine to react with the aluminum thin film that is the material to be etched, producing volatile chloride to proceed with etching. Therefore, the lower the boiling point of the chloride produced by the reaction, the more convenient it is for etching. Conversely, if the chloride produced has a higher boiling point, it will remain on the substrate surface, which is disadvantageous. If such a residue exists, when the substrate after etching is taken out into the atmosphere, the remaining chloride and moisture in the atmosphere will generate acid, which will corrode the aluminum electrode.

Chlorides that cause the corrosion mentioned above are conventionally known as
Aluminum chloride was considered, but it was discovered that chlorides of alkali metal elements or alkaline earth metal elements, which have even higher boiling points, may be produced. When we analyzed the reactants that had adhered to the walls of the reaction chamber of the etching equipment after etching a large number of substrates, we found that along with aluminum and iron, we found alkali metal elements and alkaline earth metal elements that should not exist in the first place. was detected. These elements were attached to the substrate in extremely small amounts, and it was assumed that they were deposited in the reaction tank due to the etching process of a large number of substrates. The detected elements include sodium, calcium, and magnesium, and these elements react with chlorine to easily generate chloride, which is very difficult to volatilize. For example, comparing the boiling points at atmospheric pressure, aluminum trichloride, which is produced when etching aluminum, has a boiling point of 182.7°C, while sodium chloride has a boiling point of 1413°C.

Calcium chloride has a very high boiling point of 1,600°C, and magnesium chloride has a very high boiling point of 1,412°C, and as mentioned above, it is understood that they tend to remain on the substrate surface.

Therefore, when performing electrode processing using a chlorine-based gas such as 1-reactive ion etching, it is important to remove impurities of alkali metal elements or alkaline earth metal elements from the substrate surface.

As a method for manufacturing a piezoelectric substrate for a surface acoustic wave device, for example, Japanese Patent Publication No. 56-33872 describes a method of chemically etching the back side of the substrate in order to correct warping of the substrate.
Japanese Patent Publication No. 33873 describes a method of mirror-polishing both sides of a substrate and then chemically etching only one side to make the surface rough, eliminating the influence of bulk waves and reducing warpage.
The publication also describes a method of etching to improve the warpage of the substrate.

[Problems to be Solved by the Invention] All of the above-mentioned conventional techniques aim to prevent warping of the substrate and prevent deterioration of yield due to reduction in pattern dimensional accuracy in the photolithography process. No consideration is given to impurities of alkali metal elements or alkaline earth metal elements on the surface, and as mentioned above, when reactive ion etching is performed, chloride residues tend to remain.

There were problems such as the occurrence of defects due to electrode corrosion and the inability to obtain long-term reliability.

In the case of chemical etching, there is no problem with the production or residual chloride that causes electrode corrosion, as mentioned above, but as mentioned above, with high-frequency surface acoustic wave devices, the width of the electrode fingers becomes narrower and the processing accuracy becomes smaller. This makes it difficult to put it into practical use due to problems such as reduced yield and impossibility of electrode processing, and reactive ion etching, which allows anisotropic etching and has high processing accuracy, is essential.

The present invention reduces the residual chloride after reactive ion etching to a non-problematic level, solves the above-mentioned problems that were not considered in the conventional technology, and thereby realizes high-precision electrode processing and steps. The purpose of this invention is to provide a highly reliable surface acoustic wave device with improved retention.

[Means for Solving the Problems] In order to achieve the above object, in the present invention, impurities of alkali metal elements or alkaline earth metal elements on the surface of the piezoelectric substrate are removed in advance to below a predetermined level value. I did it like that. Furthermore, it was decided to use polishing or etching as a method for removing impurities.

In particular, substances to be targeted for impurity removal are impurity elements detected by analyzing deposits in the reaction tank of a reactive ion etching device. That is, sodium, calcium, magnesium, etc. All of these elements produce chlorine and chlorides with high boiling points, which cause electrode corrosion. Furthermore, impurity elements other than those mentioned above, such as alkali metal elements or alkaline earth metal elements, must be removed because they tend to combine with chlorine and are expected to cause electrode corrosion.

[Function] In a surface acoustic wave device, interdigital electrodes or grating reflectors made of a thin film of aluminum or aluminum alloy are formed on the surface of a single-crystal piezoelectric substrate such as LiNbO3 or LiTaO3, and this is housed in a package. Alternatively, it is configured by applying wiring such as aluminum wire and sealing.

When using reactive ion etching to form interdigital electrodes, grating reflectors, etc., if impurities of alkali metal elements or alkaline earth metal elements are present on the surface of the piezoelectric substrate as described above, chlorides that are difficult to volatilize may be generated. is,
Electrode corrosion occurs due to the action of moisture in the atmosphere. but,
According to the present invention, since impurities on the surface of the piezoelectric substrate are removed in advance, chlorides that are difficult to volatilize are not generated even when reactive ion etching is performed, and electrode corrosion does not occur.

Piezoelectric substrates are made by slicing a single crystal ingot into wafers.
It is manufactured through processes such as beveling, wrapping, and polishing. The surface of such a piezoelectric substrate is processed into a mirror-like surface to allow surface acoustic waves to propagate without loss, but when viewed microscopically, a process-altered layer with microcracks as shown in Figure 2 appears. exist. Further, since the substrate itself has a piezoelectric effect and a pyroelectric effect, it becomes electrically charged due to mechanical polishing and heat generation during manufacturing, and easily attracts dust.

It is thought that due to the above-mentioned condition of the substrate, alkali metal elements or alkaline earth metal elements are adsorbed onto the substrate surface during the manufacturing process.

Even if the surface of the piezoelectric substrate is mechanically cleaned using a brush in advance during the cleaning process when forming the interdigital electrode, the occurrence of electrode corrosion after reactive ion etching is the same as when no cleaning is done. It is thought that the impurity elements involved in the generation of are adsorbed within the aforementioned microcrank.

Therefore, in order to remove impurity elements, it is necessary to remove at least the process-affected layer on the surface of the piezoelectric substrate. Also.

Care must be taken to prevent impurities from re-attaching to the substrate surface after removal.

In the present invention, a polishing method or an etching method is used as a method for removing impurities.

By these methods, impurities adsorbed on the surface of the piezoelectric substrate are removed, and electrode corrosion that conventionally occurs when forming interdigital electrodes is prevented.

[Example] An embodiment of the present invention will be described below with reference to the drawings.

FIGS. 1(,) to 1(e) schematically show cross sections of a piezoelectric substrate at each step in manufacturing the piezoelectric substrate.

Single crystal ingots such as LiNbO2 and LiTaO3 are
First, it is sliced into wafers as shown in Figure 1(a), then the end faces are beveled as shown in Figure 1(b), and the surface is polished by lapping as shown in Figure 1(C). Polished. In this state, there are many polishing damages such as microcracks on the substrate surface due to the influence of mechanical polishing. Conventionally, the piezoelectric substrate 1 was manufactured by polishing and roughing the back surface in this state. Further, active removal of impurities was not performed.

As shown in FIG. 2, the conventional piezoelectric substrate described above has a polishing damage layer A on the substrate surface and has a piezoelectric effect and a pyroelectric effect, so it is charged by thermal and mechanical stimulation during the manufacturing process. This makes it easier to adsorb impurities.

When such a substrate is used to form interdigital electrodes for a surface acoustic wave device by reactive ion etching technology, as shown in FIG. As shown in Figure 2), the impurities react with the chlorine of the etching gas, and chloride 4 is produced. In particular, when the impurity is an alkali metal or an alkaline earth metal, chloride that is extremely difficult to volatilize is generated, and this will remain on the substrate surface and the formed electrode portion. Therefore, when the substrate is taken out into the atmosphere, moisture in the atmosphere acts on the chloride to generate acid, which corrodes the electrode.

In the present invention, we focused on impurities of alkali metal elements and alkaline earth metal elements, which are a particular problem, and by removing impurities from the substrate after lapping and polishing, we developed a surface acoustic wave device using reactive ion etching technology. This provides a piezoelectric substrate suitable for manufacturing.

The degree to which impurities should be removed was estimated from the amount of chlorine remaining after reactive ion etching.

Here, the amount of residual chlorine was analyzed by ion chromatography.

When the amount of residual chlorine was 0.05 μg/cm 2 , corrosion of the electrode was observed, but when the amount of residual chlorine was 0.03 μg/cm 2 , no corrosion occurred. Therefore, the amount of residual chlorine is at least 0.0
It has been found that corrosion can be prevented by removing impurities from the substrate so that the concentration is 3 μg/cm 2 or less.

In this case, for example, if chlorine remains as sodium chloride, the remaining amount of sodium is 0.02 μg.
/cm2 or less. Regarding other impurities such as calcium or magnesium, the remaining amount is approximately the same, and from now on, the total amount of residual impurities of alkali metal elements or alkaline earth metal elements remaining on the substrate should be 0.02 μg/Cm2 or less. It turns out that this is necessary to prevent corrosion. In addition, the above sodium,
Calcium, magnesium, etc. are detected in the deposits in the vacuum chamber of the reactive ion etching equipment after etching a large number of substrates. It is thought that this was done. In addition to the above-mentioned impurities, alkali metal elements and alkaline earth metal elements tend to react with chlorine, so the amount remaining on the substrate must be small as well.

In this embodiment, surface etching using a hydrofluoric acid-based etching solution was used as a method for removing impurities from the substrate.

FIG. 4 is a diagram showing an example of a surface acoustic wave device using a piezoelectric substrate of this embodiment, in which 1 is a piezoelectric substrate, 5 is an interdigital electrode, and 6 is a surface acoustic wave device using a piezoelectric substrate.
is a wiring, and 7 is a bonding pad. In particular, 900
At high frequencies of MHz or higher, the electrode finger width is 1 μm or less, so high processing accuracy is required, and electrode processing is performed by reactive ion etching. Figure 5 shows the electrode processing process. First, a thin film 2 of aluminum or aluminum alloy is formed on a piezoelectric substrate 1 to a thickness of 1100 nm by vapor deposition or sputtering, and a photoresist pattern is formed by coating, exposing and developing a photoresist 3. Next, using the photoresist pattern as a mask material, reactive ion etching is performed, and finally the photoresist is peeled off to form electrodes.

When a conventional piezoelectric substrate is used, as described above, when the substrate is taken out into the atmosphere after reactive ion etching, there is a risk that electrode corrosion may occur due to the action of residual chlorine in the substrate and moisture in the atmosphere. Therefore, when using the conventional technology, it is inevitable that nonvolatile chloride remains on the substrate.
The electrode processing process required special treatment to remove residual chlorine. For example, after etching, the surface of the substrate is sputtered with a gas different from the etching gas, or immediately after the substrate is taken out, it is washed with water to remove chloride from the surface. However, even with Kowara's method, it is difficult to completely prevent corrosion, and there are problems such as the manufacturing process becomes complicated and long-term reliability cannot be obtained. According to the present invention, the amount of residual chlorine can be reduced without any special treatment in the electrode processing step, and electrode corrosion can be prevented.

In the above embodiment, impurities were removed by etching, but it is also possible to remove impurities by ordinary mechanical polishing.

When a surface acoustic wave device is manufactured by reactive ion etching using a piezoelectric substrate from which impurities have been removed according to the present invention, defects caused by electrode corrosion that conventionally occur can be eliminated by almost 100%.
A preventive effect was obtained.

[Effects of the Invention] As explained above, according to the present invention, a highly reliable surface acoustic wave device can be obtained relatively easily.

That is, it is possible to simplify processes such as sputter cleaning and washing of the substrate after etching, which were conventionally performed to prevent electrode corrosion. Furthermore, in the present invention, the electrodes can be formed using reactive dry etching technology with high processing accuracy.

In particular, great practical effects can be obtained in the manufacture of high-frequency surface acoustic wave devices. Furthermore, since the cause of electrode corrosion in surface acoustic wave devices is fundamentally eliminated, the long-term reliability of the product is significantly improved.

[Brief explanation of the drawing]

FIGS. 1(a) to (e) are cross-sectional views of a piezoelectric substrate according to an embodiment of the present invention and its manufacturing process; FIG. 2 is a cross-sectional view showing polishing damage on the surface of a conventional piezoelectric substrate; FIG. is a diagram showing the adhesion of chloride to the substrate surface when reactive ion etching is performed, FIG. 4 is a plan view of the surface acoustic wave device according to the present invention, and FIGS. FIG. 3 is a diagram showing an electrode processing process of a surface acoustic wave device using ion etching. 1... Piezoelectric substrate, 2... Aluminum thin film or aluminum alloy thin film, 3... Photoresist, 4
...Chloride, 5.. Interdigital electrode, 6.. Wiring, 7.. Ponding pad. ::1〒; B [door? ] (-e) (C) (CI) 岑 2 Figure 3 Figure 4

Claims (5)

[Claims] 1. The total amount of impurities of either alkali metal elements or alkaline earth metal elements present on the surface of the piezoelectric substrate is reduced to 0.
．． A piezoelectric substrate for a surface acoustic wave device, characterized in that the density is 0.02 μg/cm^2 or less. 2. 2. The piezoelectric substrate for a surface acoustic wave device according to claim 1, wherein the target impurity contains one or more of sodium, calcium, and magnesium. 3. A method for manufacturing a piezoelectric substrate for a surface acoustic wave device, comprising a step of removing alkali metal or alkaline earth metal impurities present on the surface of the piezoelectric substrate to 0.02 μg/cm^2 or less. A method for manufacturing a piezoelectric substrate for a wave device. 4. 4. The method of manufacturing a piezoelectric substrate for a surface acoustic wave device according to claim 3, wherein alkali metal or alkaline earth metal impurities present on the surface of the piezoelectric substrate are removed by a polishing step or an etching step. 5. A surface acoustic wave device comprising the piezoelectric substrate for a surface acoustic wave device according to claim 1 or 2.