JPH0578027B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photomask cover for protecting a photomask in lithography.

In lithography for the manufacture of integrated circuits, photoresists using ultraviolet light as the light source are characterized by high resolution and high productivity, and are extremely useful, particularly in the manufacture of highly integrated wafers or chips. In this case, Dave UV light is generally used because the shorter the wavelength of the light source, the higher the resolution.

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 photomask itself is easily damaged during the dust removal process. reduce 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 effects of foreign matter, that is, dust, on the thin film on the resist surface. A pattern faithful to the photomask image can be obtained by exposure.

Although such a photomask cover for protecting a photomask is extremely useful in the semiconductor industry, a manufacturing method thereof has not yet been established.

As a result of intensive research, the inventors have recognized that a photomask cover suitable for protecting a photomask can be obtained by collecting a transparent resin thin film in water, fixing it to an aluminum support frame in a partially dry state, and then drying it. , arrived at the present invention.

That is, in the present invention, an unoriented cellulose ester thin film made by casting from an organic solvent solution of cellulose ester to which no plasticizer is added is
This relates to a photomask cover that is held under uniform tension by an aluminum support frame.

The functions required of the support frame used in the present invention are as follows:
It is as follows.

B. The thin film can be adhered to the support frame by mounting the thin film in a partially dry state on the support frame, applying a small amount of volatile solvent, and drying it.

(b) The thin film must be able to withstand shrinkage when it dries.

(c) The thin film can be maintained in a uniform tension state.

(d) The thickness of the support frame is such that the effect of the presence of foreign matter, ie, dust, on the thin film can be out-focused on the resist surface.

(e) The material of the support frame shall not contain any substances that may have an adverse effect on the thin film or photomask.

The aluminum support frame satisfies the functions required of the support frame described above.

The aluminum used in the aluminum support frame may contain small amounts (10% or less) of other metals to increase its strength.

Organic polymers are also considered as materials for the support frame, but they have drawbacks in terms of adhesiveness, solvent resistance, strength, etc.

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

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

(b) Even if the film is thin, it has sufficient strength and does not cleave or shrink after being supported by a support frame.

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

In addition, it is necessary to satisfy the following conditions for manufacturing a thin film.

D. The film should be thin and uniform in thickness.

(e) The film must have a specified thickness.

(f) Maintaining uniform tension when supported on the frame.

G. Do not orient the membrane during the process of film formation and support.

The resin thin film supported on the aluminum support frame of the present invention satisfies these conditions.

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 photomask protective thin film has a thickness of 2.8±0.3μm or 4.5±
A material with a certain thickness such as 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 mounted directly on an aluminum support frame, and when it is in a partially dry state, a small amount of a volatile solvent is applied along the contact area between the support frame and the membrane, and when it dries, the membrane contacts the support frame and The membrane attempts to contract very slightly, providing support under uniform tension.

Cellulose esters, including cellulose nitrate, are highly transparent and have low absorption in the UV region, making them optically excellent materials. In addition, although the crystallinity is low, the molecular structure is rigid, so it has dimensional stability after molding. A suitable casting solvent can be used, and because it has poor adhesion to the glass plate, only the thin film can be collected without applying force, and the thin film swells only slightly with water.

Generally, the processability of cellulose esters is improved by using plasticizers, but the use of plasticizers has undesirable effects for the purpose of the present invention, such as absorption in the UV region, adhesion to glass surfaces, and cleaving properties. Therefore, it is preferable not to use plasticizers.

The following examples further illustrate the invention.

Example 1 Nitrified cotton RS-5 (manufactured by Daicel Chemical Industries, isopropanol wet cotton, solid content 70%) 64 g, methyl ethyl ketone 146 g, butyl acetate 120 g, and toluene 120 g
Nitrified cotton dope consisting of
Coat it on a smooth glass plate using a Percoater, leave it to dry at room temperature (20℃) for 24 hours, and then
It was dried at 60°C for 1 hour. The dried film of nitric acid 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, taking care not to lose its shape.
mm, and mounted on the top surface of an amminium support frame with a thickness of 10 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. Cut off the part of the film that extends outside the support frame and dry it at 60°C for 3 hours to obtain a film with a thickness of 30 μm (specific gravity
A photomask cover made of cellulose nitrate film as described in 1.6) was obtained.

Note that the thickness of the aluminum support frame acts as a spacer that forms a constant distance in the optical path between the photomask image plane and the resin thin film.

Example 2 In place of the nitrified cotton dope in Example 1, acetic acid cotton LT
-105 (Daicel Chemical Industries) 60g, methylene chloride
A cellulose acetate film was produced according to Example 1 using acetic acid cotton consisting of 846 g and methanol 94 g. However, the casting thickness was 50 μm, and the solvent used for adhering the support frame to the film was a mixed solvent of methylene chloride/methanol=9/1. Thickness 3μm (specific gravity 1.3) supported with uniform tension in the same way
A photomask cover was obtained consisting of a film of.

Example 3 Nitrified cotton RS-7 (manufactured by Daicel Chemical Industries, isopropanol wet cotton, solid content 70%) 20 g, butyl acetate 50
g, isobutyl acetate 50 g and cyclohexanone 90 g
A nitrified cotton dope consisting of g.

The viscosity of this dope at 23°C was 480 cp. A film was prepared using this dope by a spin coating method. That is, a glass plate was set in a spinner, and while rotating at 70 rpm, the above 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 1050 rpm (startup time required 0.2 seconds), and at this speed
After centrifugal casting for 2 seconds, the rotation was stopped.

Next, remove the glass plate from the spinner, leave it to dry indoors (23℃, 60%RH) for 24 hours, and then
It was 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 produced film was peeled off from the glass plate, mounted on an aluminum support frame, and supported by adhesive. The supporting frame was dried at 60° C. for 3 hours to obtain a photomask cover made of cellulose nitrate film supported with uniform tension with an average thickness of 0.29 μm and thickness unevenness of 0.03 μm or less.

Example 4 Nitrified cotton RS-120 (manufactured by Daicel Chemical Industries, isopropanol wet cotton, solid content 70%) 10g, butyl acetate
A nitrified cotton dope was prepared containing 47.6 g of isobutyl 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 setting the rotational speed of centrifugal casting to 780 rpm, the same operation as in Example 3 was carried out,
A photomask cover made of a cellulose nitrate film supported with uniform tension with an average thickness of 0.09 μm and thickness unevenness of 0.005 μm or less was obtained.

Example 5 Acetate cotton L-20 (manufactured by Daicel Chemical Industries, drying loss 5%) 18.5 g, ethylene glycol acetate monomethyl ether 91.2 g, cyclohexanone 45.6
g, butyl acetate 22.8g and isobutyl acetate 22.8g
An acetic acid cotton dope consisting of

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

Using the above dope, the rotation speed when dropping onto the glass plate was 200 rpm, and the time required for dropping was 7 seconds.
A photomask cover consisting of a cellulose acetate film supported under uniform tension with an average thickness of 2.90 μm and thickness unevenness of 0.03 μm or less was prepared in the same manner as in Example 3, except that the rotational speed of centrifugal casting was 850 rpm. Obtained.

Claims (1)

[Claims] 1. A photomask cover in which a non-oriented cellulose ester thin film made by casting from an organic solvent solution of cellulose ester to which no plasticizer is added is held in a uniformly tensioned state on an aluminum support frame.