JPS63249115A - Production of device using lithium tetraborate crystal - Google Patents

Abstract

PURPOSE:To prevent a device from damages, to improve the characteristics of the device, and to miniaturize the device, by forming metallic film or dielectric film with a photoresist splitable at a principal chain, using also an org. solvent for all treating liquid. CONSTITUTION:A high molecular compd. splitable at a principal chain which changes its dissolution velocity into an org. solvent when its principal chain is split is used as photoresist, and an org. solvent alone is used for the treating liquid at the same time. Suitable photoresist is polymethyl isopropenyl ketone. An org. solvent consisting primarily of methyl ethyl ketone, acetone, and xylene, is used as developing liquid. Xylene is used as rinsing liquid,and acetone is used as releasing agent for the resist. By this constitution, a metallic film pattern or dielectric film can be formed without causing damage of lithium tetraborate crystal. Thus, devices such as surface acoustic wave element, etc. can be made to have high performance, and suitability for high frequency. Miniaturization of the device is also possible.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing devices using lithium tetraborate crystal as a substrate. The present invention relates to effective techniques that can be used to form elements.

[Prior art] SAW elements that use surface acoustic waves (SAW) and so-called waveguide type A1 that utilizes the interaction between surface acoustic waves and guided light
Instead of conventional crystal, lithium niobate (L i N b Oi) crystal, lithium tantalate (L x Ta Os) crystal, etc., as the piezoelectric substrate constituting the zero element,
A technique using lithium tetraborate (L x x B 40 t ) crystal has been proposed (Japanese Patent Application Laid-Open No. 60-21430).
No. 7, Japanese Patent Publication No. 60-259012).

Lithium tetraborate crystal has a lower delay time temperature coefficient (
It has a small TCD), a large electromechanical coupling coefficient (K2) compared to quartz, LiTaO, and other crystals, superior SAW reflectance characteristics compared to other crystals, and low reflection at the connection surface with the optical fiber. Because of its low loss, it is expected to be used as a crystal substrate for SAW devices and waveguide type A10 devices.

However, lithium tetraborate crystals have the property of being easily dissolved by acids. Therefore, in the process of SAW devices, etc., which are constructed by forming comb-shaped transmitting electrodes and receiving electrodes made of aluminum layers on a lithium tetraborate crystal substrate, conventionally the aluminum electrodes are formed using a phosphoric acid-based etching solution. In the electrode formation process,
This damages the crystal substrate, deteriorating the device characteristics, and also makes it difficult to finely process the electrodes.

Therefore, a technique has been proposed in which the aluminum electrode is formed by a lift-off method to prevent damage to the substrate.

In the lift-off method, a resist film is applied before the metal film is deposited, exposed and developed, and a reverse pattern is formed by removing the resist in the areas where the metal is desired to remain as an electrode.After that, the metal is deposited on the entire surface, and then the resist is removed. This method involves melting and removing the unnecessary metal film on it, leaving the desired electrode pattern.

A method of manufacturing a surface acoustic wave device using this lift-off method will be described with reference to FIG.

First, a photoresist 2 is placed on a mirror-polished piezoelectric substrate 1.
a) After applying the photomask 3 having the desired pattern 3a onto the photoresist 2 or irradiating it with visible light or ultraviolet light with a narrow gap, b) developing the substrate with a developer, The development is stopped with a rinsing solution, and a resist pattern 2' opposite to the electrode pattern is formed (C).

After that, a metal film 4 is deposited on the substrate having the resist pattern 2'.
' is removed together with the unnecessary metal film thereon to form a desired metal film pattern 4' (e).

Finally, the substrate 1 is cut into a predetermined size, separated into individual devices, and placed in a container to complete the surface acoustic wave device.

By the way, for the photoresist used in the above-mentioned foot-off method, a positive type photoresist using a naphthoquinone diazide group as a photodegrading agent is often used due to resolution, exposure time, etc. This naphthoquinone diazide group is It becomes soluble in alkaline aqueous solution by irradiation with visible light or ultraviolet light. That is, a resist pattern is formed by using an alkaline aqueous solution as a developer. Further, rinsing with pure water or the like is usually performed to stop development and remove the developer.

[Problems to be Solved by the Invention] However, as is clear from Japanese Patent Application No. 61-305409 previously filed by the present applicant, lithium tetraborate crystals are susceptible to H1 ions in hydrochloric acid, acetic acid, pure water, etc. It has the property of dissolving in proportion to the concentration. FIG. 2 shows the dissolution rate of lithium tetraborate crystals with respect to pH.

As can be seen from this graph, lithium tetraborate crystals can be dissolved to some extent in alkaline and aqueous solutions, although the amount is significantly less than in acids. For example, when rinsing with pure water for about 1 to 3 minutes after development with an alkaline aqueous solution, lithium tetraborate crystals
Approximately 00 to 600 people will be etched.

As a result, the characteristics of devices such as SAW elements using lithium tetraborate crystals as substrates as described above deteriorate, and microfabrication of electrodes and the like becomes difficult.

The purpose of this invention is to prevent the crystal substrate from being damaged by various processing solutions used during the process, such as developing solution, rinsing solution, and stripping agent, in a device process using lithium tetraborate crystals. The object of the present invention is to provide a manufacturing technique that improves the characteristics of a device constructed by forming a pattern of a metal film or dielectric film on a substrate, and allows finer processing.

[Means for Solving the Problem] This invention focuses on the property that lithium tetraborate crystals do not dissolve in organic solvents such as acetone, and provides a substitute for a type of photoresist whose dissolution rate in an alkaline aqueous solution changes depending on exposure. In addition, photoresists with main chain cleavage, in which the molecular weight decreases when the main chain of the polymer is cleaved and the dissolution rate in organic solvents changes, are used to form metal films and dielectric films, and they are also used in developing solutions and Organic solvents are used as rinsing liquids, resist stripping agents, and all other processing liquids used in the process.

[Function] According to the above means, in the manufacturing process of a device using lithium tetraborate crystal as a substrate, it is no longer necessary to use water and acid or alkaline aqueous solution in which the lithium tetraborate crystal is dissolved.

This makes it possible to prevent damage to the surface of the crystal substrate caused by a photoresist developer, rinse solution, etc., to improve the characteristics of the device, and to achieve the above-mentioned purpose of enabling finer processing.

[Example] Hereinafter, an example will be described in which the present invention is applied to a process of forming an aluminum electrode on a lithium tetraborate crystal substrate to obtain a SAW element.

Note that the process itself using the lift-off method in this embodiment is the same as the conventional process shown in FIG. 1, and therefore will be described below with reference to FIG. 1.

First, a mirror-polished lithium tetraborate crystal substrate is prepared, and a main chain-cleaved photoresist such as polymethyl isopropenyl ketone is spin-coded on the surface of the crystal substrate 1 to a thickness of 1.0 to 1. Resist film 2 of about 5μm
is formed over the entire surface (Fig. 1(a)). Thereafter, this substrate is prebaked at a temperature of about 100° C. for about 20 to 30 minutes to dry the photoresist. Although polymethyl isopropenyl ketone has an average molecular weight of about 90,000, its dissolution rate in organic solvents changes to a very slow property due to drying by the above-mentioned prebaking.

Next, a photomask 3 having a pattern 3a corresponding to a desired electrode is brought into close contact with the resist film 2, and
irradiate with deep ultraviolet rays (Fig. 1(b)).

By this, polymethyl isopropenyl ketone becomes 3
In the part irradiated with light with a wavelength shorter than 40 nm, -CH2-C-CH, -C-
The main chain of is cleaved into molecules with an average molecular weight of about 20,000.

Shifi, U member, Shi Mg
Shitt.

Therefore, after the substrate is developed with a developer made of an organic solvent such as methyl ethyl ketone, acetone, or xylene, the development is stopped by washing with a rinse solution made of xylene or the like, and the developer is removed. Then, the dissolution rate in the organic solvent increases in the part whose main chain is cut and changed into a low molecular weight substance by the exposure, so the photoresist in the part irradiated with light is dissolved by the developer, and the first As shown in Figure (Q), a resist pattern 2' opposite to the desired electrode pattern remains.

Next, a metal film 4 such as aluminum is formed entirely on the substrate 1 by vapor deposition or sputtering (the first
After cleaning, the remaining resist pattern 2 on the substrate 1 is washed with an organic solvent such as acetone.
' is peeled off, and the unnecessary metal film 4 thereon is removed. As a result, a desired metal film pattern 4' is formed on the substrate 1 (FIG. 1(e)).

Thereafter, the substrate is cut to a desired size and then sealed in a container to complete a device such as a surface acoustic wave element.

According to the above example, no solution such as acid, water, or alkaline aqueous solution that dissolves lithium tetraborate crystals even slightly is used in the process, and only an organic solvent that does not dissolve lithium tetraborate crystals at all is used to form an electrode. can be formed.

Therefore, the substrate surface is not damaged by the developer, rinse solution, and resist stripping agent used in the electrode formation process using the lift-off method.

As a result, although accurate device design has not been possible due to damage to the substrate surface, it is now possible to manufacture devices that are accurate to the designed values, and the characteristics of the device are improved. Further, since variations in damage to the substrate surface are eliminated, variations in device manufacturing are reduced. Furthermore, since there is no change in electrode shape due to substrate melting, it becomes possible to form finer electrode patterns.

The inventor investigated the dissolution of lithium tetraborate crystals in organic solvents other than methyl ethyl ketone, acetone, and xylene used in this example, and found that not only methyl ethyl ketone, acetone, and xylene, but also monochlorobenzene, toluene, It was confirmed that organic solvents such as cyclohexanone do not dissolve lithium tetraborate crystals. It is also known that trichlene does not dissolve lithium tetraborate crystals.

Therefore, in addition to the organic solvents used in the examples, organic solvents such as monochlorobenzene, toluene, cyclohexanone, and trichlene may be used as a developer, a rinse agent, and a resist stripping agent.

Further, in this example, polymethyl isopropenyl ketone was used as the photoresist, but the photoresist is not limited thereto, and other main chain cleavage type photoresists such as polymethyl methacrylate may be used, thereby making it similar to the above example. Development, rinsing, resist stripping, etc. can be performed using only organic solvents.

Further, in this embodiment, aluminum is used as the electrode, but Au, Cr, Ti, and Cu may also be used.

It is clear that similar effects can be obtained using metals that can be deposited by vapor deposition or sputtering, such as Ni or alloys thereof. Incidentally, in this embodiment, exposure was performed using a photomask, but similar effects can be obtained even in the case of direct writing using an electron beam without using a photomask.

Furthermore, instead of metal electrodes, Sin, 5in2, Si,
Since a dielectric film such as N, etc. may be used, the present invention can also be applied to the case where such a dielectric film is formed in a predetermined pattern.

In addition to surface acoustic wave devices, the present invention also applies to optical devices such as optical waveguides and waveguide type A10 devices, bulk vibrators, sensors, etc.
It can be used generally to form devices using lithium tetraborate crystals.

For example, there is a method for manufacturing optical waveguides in which a thin metal film is deposited on a crystal substrate to form a predetermined pattern, and then heated to diffuse the metal into the substrate. The above embodiment can be applied to the formation of a metal pattern in a process.

Furthermore, the present invention can be applied to the case where a device such as a SAW element is formed by a process other than the lift-off method.

[Effects of the Invention] As explained above, the present invention uses a main chain cleavage type resist as a photoresist in the process of a device using lithium tetraborate crystals, and also uses a developer,
Since only organic solvents are used as processing liquids such as rinsing liquid and resist stripping agent, it is possible to manufacture devices using lithium tetraborate crystal as a substrate without using any water or acid or alkaline aqueous solutions. . This makes it possible to form metal film patterns or dielectric film patterns without damaging the lithium tetraborate crystal, making it possible to improve the performance and frequency of devices such as surface acoustic wave elements, and to improve the performance and frequency of devices such as surface acoustic wave elements. Miniaturization becomes possible.

[Brief explanation of drawings]

FIGS. 1(a) to (e) are cross-sectional views showing an example of the electrode formation process on a lithium tetraborate crystal substrate to which the present invention is applied; FIG. 2 is a diagram showing the amount of lithium tetraborate crystal dissolved and the treatment solution. It is a graph showing the relationship between PH and PH. 1...Lithium tetraborate crystal substrate, 2...Photoresist film, 2'...Resist pattern, 3...3
．． Photomask, 4...metal film, 4'...electrode pattern. Figure 1

Claims (3)

[Claims] (1) In the manufacturing process of a device using lithium tetraborate crystal as a substrate, a polymer compound with a main chain cleavage type is used as a photoresist whose dissolution rate in an organic solvent changes when the main chain is cleaved. A method for manufacturing a device using lithium tetraborate crystal, characterized in that only an organic solvent is used as a processing liquid. (2) After applying a photoresist on the lithium tetraborate crystal and exposing the photoresist in a predetermined pattern, the photoresist is developed using a developer, and then rinsed using a rinsing solution. , a device manufacturing method in which a metal film pattern or a dielectric film pattern is formed by depositing a metal film or a dielectric film on a photoresist and removing unnecessary resist using a resist stripping agent, the method comprising: The lithium tetraborate crystal according to claim 1, wherein a main chain-cleaved polymer compound is used as the lithium tetraborate crystal, and an organic solvent is used as the developer, rinse agent, and resist stripping agent. A device manufacturing method using (3) Polymethyl isopropenyl ketone is used as the photoresist, an organic solvent containing methyl ethyl ketone, acetone, and xylene as the main components is used as the developer, xylene is used as the rinsing liquid, and acetone is used as the resist stripping agent. A method for manufacturing a device using a lithium tetraborate crystal according to claim 2, characterized in that the method uses a lithium tetraborate crystal.