JPH0263A - Photomask cover - Google Patents

Abstract

PURPOSE:To utilize the intrinsic performance of cellulose ester to obtain the adequate photomask cover by using the thin film of the cellulose ester which is not added with a plasticizer. CONSTITUTION:The processability of the cellulose is improved by addition of the plasticizer thereto but the contamination of the photomask by the absorption of a UV region and the evaporation of the plasticizer is resulted. The thin film having no orientability is formed by a casting system using the cellulose nitrate which is not added with the plasticizer. The cellulose ester dissolves easily in relatively low boiling solvents such as ketone and the finish thickness can be controlled to a prescribed thickness by regulating the soln. concn. and casting thickness. The thin film having 2.8+ or -0.3mum thickness is used for protection of the photomask. The thin film peels naturally from a smooth glass plate if the glass plate is used as a casting substrate and is immersed in water after the solvent removal. The support is obtd. in the uniformly tensed state by placing the thin film in the half-dried state directly on an Al supporting frame and coating a volatile solvent to the contact part of the frame and the film, then drying the coating.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photomask cover made of a thin resin film for protecting a photomask in lithography.

In lithography for manufacturing integrated circuits, photoresists using ultraviolet rays as a light source are characterized by high resolution and high productivity, and are particularly useful for manufacturing highly integrated wafers or chips.

Although this method has high resolution, small dust adhering to the image surface of the photomask reduces the accuracy of the etched image and causes defective products, and the process of removing dust can damage the photomask itself. easily, reducing its lifespan.

As a countermeasure against the above problem, a method is used in which a photomask cover made of a thin resin film is inserted into the optical path on the image side of the photomask to protect the photomask image from adhesion of dust in the air.

In this case, the dust will adhere to the surface of the resin thin film instead of adhering to the image surface of the photomask. At this time, if the thickness of the thin film itself and the distance between the photomask image and the thin film are constant over the entire surface, it is possible to outfocus the effect of foreign matter, that is, dust, on the thin film on the resist surface, and the photomask A pattern faithful to the image can be obtained by exposure.

As a result of intensive research, the inventors have recognized that a photomask cover suitable for protecting a photomask can be obtained by using a cellulose ester thin film to which no plasticizer is added as the thin film material for the photomask cover. , arrived at the present invention.

The physical properties required of the thin film material used in the present invention are as follows.

b. The specific area of light used for exposure (especially in this case UV
area), with as little absorption as possible.

Even though the film is thin, it has sufficient strength and does not creep or shrink after being supported by a support frame.

C. Directionality should not be caused in transmitted light due to crystals or orientation.

2. There should be no substances that would adversely affect the photomask compared to the thin film.

The cellulose ester thin film of the present invention to which no plasticizer is added satisfies these conditions.

That is, in the present invention, cellulose ester without added plasticizer is dissolved in an organic solvent, and a thin film is cast on a smooth glass plate, peeled off in water, supported on a support frame in a wet state, and then dried. This is the photomask cover obtained by this method.

Cellulose ester is suitable as the membrane material used in the present invention, and cellulose ester refers to cellulose nitrate, cellulose acetate, cellulose propionate, cellulose acetate butyrate, and the like. Among these, cellulose nitrate is particularly suitable for the purposes of the present invention.

In manufacturing the resin thin film, a so-called casting method is used to obtain a film without orientation. Cellulose esters are easily dissolved in relatively low boiling point solvents such as ketones and lower fatty acid esters, and by regulating the solution concentration and casting thickness, a thin film with a predetermined finished thickness can be produced.

For example, the thickness of the photomask protective thin film is 2.8±0.3μ.
A material having a certain thickness such as m or 4.5±0.3 μm is used.

When a smooth glass plate is used as the casting substrate during casting, and the thin film is immersed in water after removing the solvent, the thin film is naturally peeled off from the glass plate and can be easily recovered. This thin film is directly mounted on an aluminum support frame, and when it is partially dry, a small amount of volatile solvent is applied along the contact area between the support frame and the membrane and dried. The membrane attempts to contract slightly, providing support under uniform tension.

Cellulose esters such as cellulose nitrate are highly transparent, have little absorption in the UV region, and are optically excellent materials. In addition, although the crystallinity is originally low, the molecular structure is rigid, so it has dimensional stability after molding.

Appropriate casting solvents can be used, and since it has poor adhesion to the glass plate, only a thin film can be collected without applying force.
Thin films have various advantages, such as being swollen very slightly by water, and the present invention takes advantage of these advantages.

-m The processability of cellulose ester can be improved by using a plasticizer, but the use of a plasticizer can cause problems such as absorption in the UV region, adhesion to glass surfaces, creep property, and photomask formation due to volatilization of the plasticizer. Effects that are undesirable for the purposes of the present invention, such as contamination, occur. According to the present invention, by using a cellulose ester thin film to which no plasticizer is added, the above-mentioned inherent performance of cellulose ester is utilized.
The object of the present invention can be achieved.

The following examples further illustrate the invention.

Example 1 Nitrified cotton R3-5 (manufactured by Daicel Chemical Industries, isopropanol blended cotton, solid content 70%) 64 g, 1 methyl ethyl ketone
46 g, butyl acetate 120 g and toluene 120 g
A nitrified cotton dope consisting of the following was applied onto a smooth glass plate using a percoater with a clearance of 50 μm, left to dry at room temperature (20° C.) for 24 hours, and further dried at 60° C. for 1 hour. The dried film-formed nitrified cotton was gently immersed together with the glass plate in clean water. After being left for a while, the nitrified cotton film naturally peeled off from the glass plate, so I scooped it up using a circular aluminum frame with a diameter of about 150 mm without losing its shape, and cut the part inside the frame into an inner diameter of 100 mm.
, was mounted on the upper surface of an aluminum support frame having an outer diameter of 110 mm and a thickness of 1Q mm. The film was then adhered to the support frame by applying a small amount of methyl ethyl ketone along the contact interface of the film with the support frame and air drying. After cutting off the part of the film protruding outside the support frame and drying it at 60°C for 3 hours, a photomask cover made of cellulose nitrate film with a thickness of 3 μm (specific gravity 1.6) supported with uniform tension was created. I got it.

Example 2 Instead of the nitrified cotton dope of Example 1, acetic acid cotton LT105 (
Example 1 Using acetic acid cotton consisting of 60 g (manufactured by Daicel Chemical Industries), 846 g of methylene chloride, and 94 g of methanol,
A cellulose acetate film was prepared according to . However, the casting thickness was 50 μm, and the solvent used for bonding the support frame and the film was a mixed solvent of methylene chloride/methanol=9/1.

Similarly, a photomask cover made of a film with a thickness of 3 μm (specific gravity 1.3) supported with uniform tension was obtained.

Example 3 Nitrified cotton R5-7 (manufactured by Daicel Chemical Industries, Ltd., isopropanol blended cotton, solid content 70% > 20 g, nitrified cotton dope consisting of 50 g of butyl acetate, 50 g of isobutyl acetate, and 90 g of cyclohexanone was prepared.

The viscosity of this dope at 23°C was 480 cp. A film was prepared using this dope by spin coating. That is, a glass plate was set in a spinner, and while rotating at 70 rpm, the above-mentioned dope was dropped onto the center of rotation over a period of 5 seconds. Immediately after the completion of dripping, increase the rotation speed to 105°rpm (required start-up time: 0.
2 seconds), centrifugal casting was performed while maintaining this speed for 15 seconds, and then the rotation was stopped.

Next, the glass plate was removed from the spinner, left to dry indoors (23° C., 60% RH) for 24 hours, and further dried at 60° C. for 1 hour. Thereafter, the glass plate was immersed in water in the same manner as in Example 1, and the resulting film was peeled off from the glass plate, mounted on an aluminum support frame, and supported by adhesive. 60℃ for each support frame
, dried for 3 hours, average thickness 0.29 μm, thickness unevenness 0.
A photomask cover consisting of a cellulose nitrate film supported with a uniform tension of 0.03 μm or less was obtained.

Example 4 Nitrified cotton R3-120 (manufactured by Daicel Chemical Industries, Improper Tool blended cotton, solid content 70%) 10g, butyl acetate 47.
6 g of isobutyl acetate, and 94.8 g of cyclohexanone.

The viscosity of this dope at 23°C was 300 cp.

Using the above dope and rotating the centrifugal casting at a rotation speed of 780
The photomask cover was prepared in the same manner as in Example 3 except that the rotation speed was changed to 0.90 μm in average thickness and a uniform tension of 0.005 μm or less in thickness.

Example 5 Acetate cotton L-20 (manufactured by Daicel Chemical Industries, drying loss rate 5% N
8.5 g, ethylene glycol acetate monomethyl ether 91.2 g, cyclohexanone 45.6 g
, butyl acetate 22.8 g and isobutyl acetate 22,
An acetic acid cotton dope containing 8 g was prepared.

The viscosity of this dope at 23° C. was 550 CI].

Using the above dope, the same procedure as in Example 3 was carried out, except that the rotation speed when dropping onto the glass plate was 20 Orpm, the time required for dropping was 7 seconds, and the rotation speed of centrifugal casting was 850 rpm. .29μm 1 thickness unevenness 0.03μm
A photomask cover consisting of a cellulose acetate film supported with the following uniform tension was obtained.

Claims (1)

[Claims] A photomask cover comprising a non-oriented cellulose ester thin film held in a uniform tension state, characterized in that no plasticizer is added to the cellulose ester.