JPS6035733A - Manufacture of dustproof cover for photomask - Google Patents

Abstract

PURPOSE:To obtain easily the titled cover of a uniform thickness with high reproducibility by adhering a frame to a thin nitrocellulose film formed on a smooth substrate and by immersing the film in water to strip the substrate. CONSTITUTION:A smooth substrate such as a silicon substrate having >=lambda/4 smoothness is coated with a soln. contg. 3-15wt% nitrocellulose having 10.7- 12.2% nitrogen content by means of a spin coater under conditions of 25-30 deg.C casting temp., 5-10cc extent of casting and 500-3,000r.p.m. number of revolutions of the spinner to form a thin nitrocellulose film of 0.5-3mum thickness. A frame is adhered to the film, and the film is immersed in water for >=15min to strip the substrate. The film is then taken out of the water and dried. The thin nitrocellulose film can be stretched tight on the frame.

Description

DETAILED DESCRIPTION OF THE INVENTION In recent years, the density of large-scale integrated circuits has increased, and the lines in the circuits have become extremely thin, as small as 2 to 3 microns.

As the image line becomes thinner due to higher density, dust on the photomask becomes a problem. In other words, even if invisible dust of several microns exists on the photomask,
When the circuit image on the photomask is projected and exposed onto the resist on the wafer 1 using a projection or stepper, dust is transferred along with the image, resulting in short circuits, disconnections, etc. of the circuit. Even if we proceed with strict dust prevention measures such as performing all exposure operations in a clean room to prevent the removal of chemicals, it is impossible to eliminate dust generated from workers, equipment, etc. It cannot be considered perfect.

Therefore, a method has recently been adopted to prevent fleas from directly adhering to the circuit image portion on the photomask by placing a seal on the photomask.

This method involves placing a transparent thin film at a position a few feet away from the photomask. Since the chemical adheres to this thin film, when the circuit image of the photomask is imaged onto the resist on the wafer, The principle is that dust is not imaged. According to this method, even in an environment with slightly poor cleanliness, chemicals will only adhere to this thin film, greatly reducing defects due to short circuits, disconnections, etc., and making it easier to manufacture large-scale integrated circuits. It will be very useful.

Since this thin film is installed at a position several distances away from the photomask, it is normally used by attaching it to an aluminum frame and adhering it to the photomask. Dustproof power for photomasks
I'll call it Gu.

This dustproof power? - The thin film has a light transmittance; this is important, and when used for projection, the wavelength of the light of the exposure source, i.e. 92 nm to 450 nm, is important.
% or higher, and when using stainless steel, a transmittance of 98 coefficient or higher at 436 nm is required.

A film with such high light transmittance cannot be achieved unless it is made of a material that does not absorb at this wavelength, and the thickness of the film must be 5 microns or less. Further, since variations in the thickness within the film cause fluctuations in light transmittance as well as disturbances in the image, the accuracy of the film thickness must also be within ±2 factors.

In addition, although dust adhering to the top of the dustproof cover has little effect on short circuits or disconnections during exposure, large dust can become a problem, making it difficult for dust to adhere to it, and removing adhering dust by air. It is desirable that the thin film be non-static so that it can be easily removed by blowing or the like.

In order to meet these required characteristics, a nitrocellulose thin film is used for this thin film.

The method for producing a dust-proof cutter using nitrocellulose is generally to cast a nitrocellulose solution onto a fluid surface such as a water surface or a mercury surface, and then stretch the formed thin film onto an aluminum frame. Thin membranes are difficult to handle, wrinkle easily, and are extremely difficult to install in a taut condition without wrinkles on the frame. Also, when forming a thin film on a fluid surface by casting, it is necessary to keep the fluid surface stationary, but slight vibrations and wind may cause the wavelength of the fluid surface to change, causing wrinkles and thin film thickness. The uniformity of the film is impaired, making it impossible to obtain the desired thin film.

Therefore, the inventor of the present invention investigated a method of applying a thin film of nitrocellulose to a frame without wrinkles, and after studying a method of easily creating a thin film of uniform thickness, the present invention was achieved.

The present invention involves forming a nitrocellulose thin film with a thickness of 5 microns or less on a smooth substrate using a solution casting method, bonding this thin film to an aluminum frame with an adhesive, and then immersing the substrate in water. This is a method for manufacturing a dustproof cover for a photomask by separating the material from water, then taking it out of water and drying it. According to the present invention, excellent dustproofing power for a photomask, S- can be easily obtained. Moreover, it can be manufactured with good reproducibility.

The present invention will be explained in more detail below.

As mentioned in the casting method on the fluid surface, the thin film of nitrocellulose is difficult to handle alone, so the thin film is formed on the substrate, the thin film and the frame are bonded, and the substrate is removed to apply the thin film to the frame. USP 437
8953. According to this method, the substrate can be peeled off as it is, or by applying a mold release agent on the substrate in advance, peeling can be facilitated. However, if you try to separate the substrate and thin film as they are, then... If + [ is broken h or b - elongation γ , then 1 - → cannot be obtained. In addition, in the method of pre-coating a mold release agent on the substrate, the flatness of the surface when applying the mold release agent may affect the thickness of the nitrocellulose thin film applied thereon. of film thickness)
As the racking increases, part of the release agent may be transferred to the thin film, which has a negative effect on performance.

Therefore, as a result of studying various methods for peeling off the substrate, the present inventor found that if a frame is bonded in advance to the nitrocellulose thin film formed on the substrate, then immersed in water and the substrate is peeled off underwater, the substrate can be easily removed. It may peel off or the thin film may be torn.
I found that it does not stretch. Also, since nitrocellulose absorbs about 1% of moisture, after peeling off the substrate, take it out of the water and dry it to make a thin #r nitrocellulose glued to the frame! , Yield L, can be hung on the frame without sagging. This is something that only appears when a nitrocellulose thin film is immersed in water, and in addition to making it easier to peel off the substrate, it is also extremely useful in manufacturing dust-proofing and coating for photomasks. .

It should be noted that succinic acid in water only needs to be in substantially the same condition as in water, for example, after being immersed in water and then taken out of the water. Even in the case of separation, if there is water on the surface to be separated, succinic acid can be produced satisfactorily, but if the water is wiped off, succinic acid will be insufficient.

The water used in the present invention may be water alone, or additives such as surfactants that do not eliminate the effects of the present invention.
In addition, nitrocellulose and adhesives can be added as long as they do not cause deterioration.

Such additives do not form water droplets on the surface of the nitrocellulose thin film after peeling and uniformly wet the thin film with water, resulting in the effect that no traces of water remain after drying. Also, if the time of soaking in water is too short, the water will not penetrate sufficiently and succinic acid will not be formed easily, so it is necessary to soak for 15 minutes or more.

On the other hand, as for the substrate, it can be used as long as it has a smooth surface and is not attacked by the nitrocellulose solution, but it is limited in practical use because the flatness of the substrate affects the accuracy of the film thickness. In other words, in order to maintain the internal thickness accuracy of a thin film within ±22 degrees, the flatness of the substrate must be A/4 or higher, and those with such a smooth surface include silicon substrates, glass plates, and Although there are substrates on which metal is vapor-deposited, a silicon substrate is the most excellent substrate because of the ease with which succinic acid forms with the nitrocellulose thin film.

Of the solution casting methods that form a thin film of nitrocellulose on a smooth substrate, the methods that can produce a dust-proof thin film for photomasks with high precision are those that use a spin coater, and those that use a substrate with a weir around the substrate. Methods include keeping the surface level and casting the solution onto it, and using a knife coater.

When a thin film of nitrocellulose is formed on a substrate using a spin coater, the thickness of the thin film depends on the concentration of nitrocellulose, the type of solvent, the casting temperature, the rotational speed of the spin coater, the softness of the cast, and the like. For this reason, in order to create a thin film with a desired thickness, it is necessary to consider various conditions, and although it cannot be generally defined, an example would be a film thickness of Mil.

For 5-3 microns, use n-butyl acetate as a solvent, nitrocellulose concentration 3-15% by weight, casting temperature 25°C-30°C, cast amount 5-10cc, spinner rotation grade soo-3000 r, p. , rn, it can be formed on a substrate of J, 125 m.

On the other hand, in a method in which a weir is provided around a smooth substrate and a nitrocellulose solution is cast, the thickness of the film is determined by the nitrocellulose concentration and the amount of cast. The most important point in this method is the horizontality of the substrate surface, and in order to keep the thickness accuracy within the film within ±2%, the horizontality of the size of the film to be manufactured must be within 0.5 microns. This is achieved by In addition, if the concentration of the nitrohirulose bath solution is too high, it will not be uniformly cast on the surface of the substrate within the film, and if it is too low, it will take a long time to evaporate the solvent, so it is preferably 0.5 to 2% by weight. Delicious.

In addition, in the method using a knife coater, the accuracy of the film thickness is determined by the distance between the substrate and the knife, so it is important to set this distance precisely. By setting it within .5 microns, the film thickness within the film can be within ±2% error.

Furthermore, if the concentration of nitrocellulose is too high, it will not be possible to apply it uniformly, and if it is too low, the solution will flow after coating and the thickness will change, so it should be 10 to 200 to 20% by weight.

Note that the nitrocellulose used in the present invention has a nitrogen content of 1
It refers to those in the 0.7 to 12.2 section.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

The thickness of the film was calculated from the interference wave of the film transmittance measured with a spectrophotometer. In other words, the peak wavelengths of the measured peaks of adjacent interference waves are each expressed as 1.

22 microns (λl>λ2), and if the number of interferences at λ2 is N, then the number of interferences at λ2 is N+1, so if the refractive index of nitrocellulose is 1.51, the film thickness d
Micron is given by the following formula.

a = (λt
0 nitrogen content (11.5-12.2%) was dissolved in n-butyl acetate to prepare a 5% by weight solution.

On the other hand, after degreasing and cleaning a 5-inch piece of silicone Ueno with MEK and air drying, it was set in a spin coater, the prepared 2) cellulose fat solution was added dropwise, and the rotor was rotated at a low speed of 500 r.
10 seconds at p, m, high speed rotation 1050 r, p, m.

It was applied for 15 seconds and air-dried to form a thin film of nitrocellulose on Uni-No.

Next, an epoxy adhesive (Araldite Rapid, manufactured by Showa Kobunshi Co., Ltd.) was applied to the end face of the aluminum frame (inner diameter 105 mm), and was adhered to the nitrocellulose thin film, followed by curing for 1 hour.

Is this one made by film draining agent (Fuji Photo Film @)? By soaking the aluminum frame in water containing 1 part of nitrocellulose, leaving it for 30 minutes, gently removing it, removing it from the water, and letting it air dry, a thin nitrocellulose film is applied to the aluminum frame without wrinkles, making it waterproof. I got what I didn't have.

The thickness of the thin film of the obtained dustproof cover was 0.867 microns, and the variation in film thickness was a factor of ±0.6, indicating extremely good uniformity. Further, the transmittance of light at 436 nm was 99.2%.

Example 2 The same nitrocellulose as in Example 1 was dissolved in n-butyl acetate to create a 1% by weight solution.

On the other hand, around the 5-inch silicon wafer, a height of 3 meters was placed.
A stainless steel flamer with an inner diameter of 100m+n was installed so that the horizontality was within 0.5 microns per 100mm, and a nitrocellulose solution of 3.63 mm was placed on top of this.
was added dropwise and dried to form a nitrocellulose thin film on the wafer.

Next, an epoxy adhesive (Araldide Rapid P manufactured by Showa Kobunshi@) was applied to the end face of an aluminum frame (inner diameter 80 mm) and adhered to the nitrocellulose thin film, and the frame was cured for 1 hour.

After that, the wafer was immersed in water, left for 30 minutes, peeled off, taken out of the water, and air-dried.
An aluminum frame with a nitrocellulose thin film stretched without wrinkles was obtained.

The thickness of the obtained dustproof film was 2.87 microns, the thickness accuracy was ±1.3, and it was highly uniform. In addition, the transmittance of light in the range of 350 to 450 nm is 9
It was Section 2.1.

Example 3 The same nitrocellulose as in Example 1 was dissolved in n-butyl acetate to prepare a 1O wt % solution.

On the other hand, a knife coater was installed on a 6 inch 2 silicon substrate, and the distance between the knife and the substrate was set to 47 microns.
The prepared nitrocellulose solution was cast and air-dried to form a thin film of nitrocellulose on Ueno.

Next, an epoxy adhesive (Araldite Rapid, manufactured by Showa Kobunshi Co., Ltd.) was applied to the end face of the aluminum frame (inner diameter: 60 mm), and the adhesive was bonded to the nitrocellulose thin film, followed by curing for 1 hour.

This product was immersed in water, left for 30 minutes, peeled off, removed from the water, and air-dried to obtain a nitrocellulose thin film on an aluminum frame without wrinkles.

The thickness of the obtained dust-proof thin film was 2.89 microns, the thickness accuracy was ±1.7%, and it was highly uniform. Also, the transmittance of light in the range of 350 to 450 nm is 9
I was in charge of 2.0.

Comparative Example 1 When peeling off a nitrocellulose thin film on a silicon wafer prepared in the same manner as in Example 1, it was difficult to peel off the wafer when the wafer was peeled off without being immersed in water. , the nitrocellulose thin film stretched and it was not possible to obtain a frame that was stretched without wrinkles.

Comparative Example 2 A dustproof cover was produced in exactly the same manner as in Example 3, except that a glass plate with a surface area of 2/2 was used instead of the silicon substrate.

The thickness of the thin film for the dust-proof cutter was 2.85 microns, and the thickness accuracy was ±3.2, and the uniformity of the thickness was insufficient for a thin film for a dust-proof cutter.

Patent applicant: Asahi Kasei Industries, Ltd.

Claims (2)

[Claims] (1) For photomasks, which are characterized in that a nitrocellulose thin film is formed on a smooth substrate by a solution casting method, a frame is adhered to the thin film, the substrate is peeled off by immersion in water, and then dried. How to manufacture dustproof cover (2) A method for producing a dustproof power for a photomask according to claim 1, wherein the smooth substrate is a silicon substrate.