JPH0154883B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a surface acoustic wave (hereinafter simply referred to as surface wave) interdigital electrode.

A surface wave interdigital electrode is made by intersecting comb-shaped electrodes on the surface of a piezoelectric substrate, and is well known as a transducer that can efficiently transmit and receive surface waves. Surface wave devices using this technology are well known as telecommunication circuit devices with excellent performance that perform various signal processing functions, including filters and delay lines, in a wide frequency range from megahertz (MHz) to gigahertz (GHz) bands. It is being

The interdigital electrode is usually obtained by depositing an electrode material on the surface of a substrate and then processing it into a desired shape using photoetching technology. The fine structure of such interdigital electrodes often forms a step with a height equal to the electrode film thickness on a flat substrate surface. When surface waves propagate through interdigital electrodes having such steps, undesirable phenomena generally occur. One of these is that the surface waves are reflected unnecessarily by the difference in level, and the other is that the surface wave is converted into an unnecessary bulk wave by the difference in level. The former is undesirable because it induces surface wave echoes that interfere with the main signal, often resulting in unnecessary ripples in the response of the surface wave element. Moreover, the latter is also a cause of surface wave propagation loss, which is also undesirable. In particular, these problems become serious as the frequency of surface wave elements becomes higher. For this reason, a structure is conventionally known in which interdigital electrodes are embedded in the substrate surface to eliminate the level difference on the surface wave propagation surface.

The following methods are well known as methods for manufacturing such embedded interdigital electrodes. Part 1
One is to form a photoresist pattern on the substrate surface in advance, and using this as a mask, use ion beam etching technology to form grooves in the same shape as the interdigital electrodes on the substrate surface. Thereafter, an electrode material film is deposited by vapor deposition to fill the groove with the electrode material. Finally, unnecessary electrodes on the photoresist pattern are removed together with the photoresist mask pattern using a lift-off technique. The disadvantage of this method is that in the lift-off step,
The problem lies in the difficulty of separating unnecessary electrodes on the photoresist pattern from the electrodes buried in the grooves. For this reason, this manufacturing method has the drawback of generally lacking in practicality.

Another method, as shown in FIG. 1, is to fill the gap between the stepped electrodes with another insulator to flatten the surface of the substrate. That is, in FIG. 1A, first, an interdigital electrode 12 is formed on the substrate surface 11 using ordinary photo etching and chemical etching. Next, an insulating film 13 is deposited on the entire surface by vacuum deposition or sputtering. At this time, the step caused by the electrode 12 is directly transferred onto the surface of the insulating film 13. Thereafter, a photoresist film 14 is applied by spin coating. In this state, the level difference before application has been slightly eliminated.
Here, the entire surface of the substrate is etched by ion beam etching from diagonally above the resist film 14. At this time, by appropriately selecting the angle θ formed by the ion beam 15 and the film 14, even if there is a slight step at the beginning of etching, the surface will be flat when the ion beam etching is completed, that is, when the electrode 12 is exposed. The gap has been eliminated. That is, the first
As shown in FIG. 2B, an insulating film 16 is embedded in the gap between the electrodes 12 on the surface of the substrate 11, eliminating the level difference on the surface. In this method, it is generally possible to strengthen the adhesion of each film, and the surface is also good, so it can be said to be practical for multilayer wiring in integrated circuits and magnetic bubble devices. . However, when this method is applied to planarizing the electrode portion of a surface wave device, an important drawback occurs. That is,
The insulating film that fills the space between the electrodes is generally amorphous or polycrystalline, which increases the propagation loss of surface waves. It is well known that the propagation loss of surface waves is much smaller in single crystal materials than in amorphous or polycrystal materials. Therefore, in surface wave devices using single-crystal substrates that are often put into practical use, a planar structure in which electrodes are buried in the substrate is better than a planar structure in which an insulator is buried between the electrodes for the reasons mentioned above. It can be said that it is superior.

To summarize the shortcomings of the conventional examples mentioned above, the method of burying electrodes in the substrate uses a lift-off method, which has drawbacks in reliability and yield, and the method of burying an insulator between the electrodes has problems with surface waves. The disadvantage is that it increases the propagation loss.

An object of the present invention is to provide a new method for embedding interdigital electrodes that is highly practical and eliminates the drawbacks of the conventional method.

After forming a concavo-convex structure with grooves forming a desired interdigital electrode pattern, and then uniformly depositing an electrode material film over the entire surface of the concavo-convex structure, etching is performed on the electrode material film having the concavo-convex structure at an etching rate of the electrode material. When a material smaller than that of is applied, and the thickness of the electrode film on the convex part is Te, the thickness of the coating film is Ta, and the thickness of the coating film on the concave part is Tb, the relationship Ta + Te > Tb > Ta is satisfied. After that, the entire surface of the substrate is irradiated with an ion beam from a direction inclined at a predetermined angle with respect to the surface normal direction to etch away the film and unnecessary electrode film. As a result, a method for manufacturing an interdigital electrode characterized by forming an embedded interdigital electrode is obtained.

Next, details of the present invention will be explained using the drawings.

FIG. 2 is a process schematic diagram in one embodiment of the present invention. In the same figure a, on the surface of the substrate 21,
A photoresist pattern 22 is formed, and then, using this as a mask, grooves 23 are formed by ion beam etching. Resist pattern 22 here
is an inverted pattern of interdigital electrodes. The groove 23 thus has a shape equivalent to the desired interdigital electrode. Next, as shown in Figure b, after removing the resist film 22 shown in the previous figure, an electrode material film 24 of aluminum (Al) having a uniform thickness is deposited on the entire surface of the uneven substrate by vapor deposition. Let the Al film thickness at this time be Te. Next, a photoresist film 25 is deposited again by a spin coating method. At this time, the thickness of the photoresist film at the convex portion and concave portion is Ta, respectively.
Let it be Tb. Here, each film thickness is controlled so that Ta+Te>Tb>Ta. Thereafter, the resist film 25 and unnecessary electrode portion 28 are etched by irradiating the substrate surface with an ion beam 26 from an oblique direction of about 45 degrees. At this time, after the etching is completed for a predetermined time, the electrode 27 is embedded in the surface of the substrate 21, as shown in FIG.

The reason for this will be explained with reference to FIG. 2b. The electrode film 24 is Al, and the resist film 25 is AZ-1350J.
(Trademark: Shipley, Inc., USA), the etching rate of the electrode film is higher than that of the resist film. According to actual measurements, when the angle θ between the ion beam 26 and the normal direction of the substrate is 0<θ<80°,
The relative etching rate R of the electrode film to the resist film is 1R2. Next, a resist film 25 is applied onto the electrode film 24 having the steps by a spin coating method, and the resist film thickness is controlled so that the thickness of the resist film is larger at the bottom of the groove than at the convex part. At this time, the level difference is somewhat alleviated, but still remains. When etching is started in this state, the step remains unchanged at the initial stage because both the concave and convex portions sandwiching the step are the same film. However, when the electrode film appears on the convex portion, etching progresses faster on the convex portion due to the difference in etching speed mentioned above. In other words, the total film thickness of resist and Al (Ta +
Even if there is a relationship between Ta+Te>Tb as mentioned above between the resist thickness Tb at the concave part and the resist thickness Tb at the concave part, if the incident angle θ of the ion beam and the etching time are appropriately selected, the electrode film at the convex part can be removed. At the time when all 28 are etched, it is possible to eliminate the step.

The above-mentioned predetermined etching time is determined by the thickness of the resist film 25 and the electrode film 28.
As a trial, when the final amount of the electrode film 27 buried in the substrate is 1000 Å, the depth of the groove 23 and the thickness of the electrode film 24 are set to be about 1000 Å, and the thickness of the resist film 25 is set in the range of about 2000 Å to 5000 Å. If selected, by selecting the etching time to a desired value of 3 to 10 minutes, the object of the present invention, flattening the surface, can be achieved.

Next, the advantages of the present invention will be described in comparison with the conventional example.
Among the conventional examples described above, the first one is to deposit an electrode material in the state shown in FIG. The method was to do so. However, in this case, as described above, unless the side walls of the resist pattern 22 are steep, it is difficult to strip off the unnecessary electrode film. In order to increase the reproducibility and reliability of the stripping process, it is desirable that the side walls of the resist pattern 22 have an inverted trapezoidal shape, as is well known. However, if the strip is made into an inverted trapezoid, there will be a gap between the side wall of the buried electrode obtained in exchange for improving the reliability of the stripping process and the side wall of the substrate groove.
A gap is created. Such a gap is undesirable for the propagation of surface waves because it causes reflection and scattering. On the other hand, according to the method of the present invention, the side walls of the embedded electrode and the side walls of the substrate groove can be closely attached, so that surface waves can be reflected by the gap at the end of the electrode as described above. No scattering occurs. Next, the present invention will be compared with the second conventional example described above. The flattening means of the present invention is not a method of burying an insulator in the groove between the electrodes, but a method of embedding the electrodes themselves in the substrate. Therefore, when considering the reduction of surface wave propagation loss as described above, it is clear that the present invention is superior to the conventional example. Therefore, to summarize the advantages of the present invention, the present invention embeds an electrode within a substrate, and also closely adheres the electrode and the substrate so that grooves do not occur along the edges of the embedded electrode. Since this is a manufacturing method for flattening electrodes with various structures, reflection, scattering, and
It can be said that this reduces attenuation.

In addition, in the description of the embodiments of the present invention using the drawings,
Before the step of attaching the electrode material 24 in FIG. 2b, the resist film 22 in FIG. 2a was removed. However, this step is not an essential requirement of the present invention. That is, if the final thickness of the electrode 27 is selected to be large, the groove 23 shown in FIG. I have to choose. In such a case, the difference in level caused by both the groove 23 and the remaining resist film 22 is large, so
In consideration of reducing the level difference in the subsequent process, the resist film 22
It is preferable to remove it. However, when the thickness of the electrode 27 and the depth of the groove 23 are selected to be small, the resist film 22 can also be made thinner, so it is not removed.
A post-process may be performed. Even in this case, the remaining resist is removed in the final oblique ion etching step.

Further, in the embodiment of the present invention described above, ion beam etching technology was used to form the groove 23 shown in FIG. 2a. However, it goes without saying that this process is not limited to the ion etching method, and other etching methods including reactive sputter etching and chemical etching are also permissible.

Although the details of the present invention have been explained above, the rights of the present invention are not limited to the above-mentioned embodiments, but extend to all interdigital electrode manufacturing methods shown in the above-mentioned claims.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a process in a conventional example, and FIG. 2 is a schematic diagram of a process in an embodiment of the present invention. In the figure, 11 and 21 are the substrates, 12,
27 is an interdigital electrode, 14, 22, and 25 are photoresist films, and 16 is an insulating film.

Claims (1)

[Claims] 1. A concavo-convex structure with grooves forming a desired interdigital electrode pattern is formed on the surface of a piezoelectric substrate, and then an electrode material film is uniformly deposited over the concave-convex surface, and then the concavo-convex structure is formed. A material having an etching rate lower than that of the electrode material is coated on the electrode material film having an etched surface, and the thickness of the electrode film at the convex portion is Te, and the thickness of the coating film is
When Ta and the thickness of the coating film in the recess are Tb, a structure is formed in which the thickness of each film is controlled to satisfy the relationship Ta + Te > Tb > Ta, and then at a predetermined angle with respect to the surface normal direction. A method of manufacturing a surface acoustic wave interdigital electrode, characterized in that a buried interdigital electrode is formed by irradiating the entire surface of the substrate with an ion beam from an inclined direction and etching the coating and unnecessary electrode film.