JPH0798869B2 - Dustproof film for photomask protection - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a dustproof film for preventing a photomask or a reticle from adhering dust in a semiconductor manufacturing process.

[Conventional technology]

In the manufacturing of semiconductors such as LSI, it is necessary to prevent dust from adhering to the photomask or reticle during the lithography process that transfers the mask pattern of the photomask to the photoresist. Protected by.

As such a dustproof film for protecting a photomask, a conventional thin film of nitrocellulose, cellulose acetate butyrate, alkyl cellulose or the like is used.

[Problems to be solved by the invention]

The performance required for such a photomask protective dustproof film is that it has a high light-transmitting property and good light resistance, but in the above conventional one, it is possible to use light having a wavelength of 365 nm for photolithography. On the other hand, it exhibits some absorption, and has poor light resistance to light of wavelengths in the vicinity.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a dustproof film for protecting a photomask, which has a high light-transmitting property, an excellent light resistance, and a long life.

[Means for solving problems]

The above problems can be solved by forming a dustproof film for protecting a photomask with cellulose propionate having a weight average molecular weight of 60,000 to 400,000 according to polystyrene standards.

[Action]

The dustproof film for protecting a photomask of the present invention has a high light transmittance of 99% or more for light having a wavelength of 365 nm.

The dustproof film of the present invention can be produced from a solution of cellulose propionate in an organic solvent by a rotary film forming method.

As the organic solvent for cellulose propionate, ketone-based solvents such as methyl isobutyl ketone (MIBK), acetone, methyl ethyl ketone, diethyl ketone, diisobutyl ketone, cyclohexanone, and diacetone alcohol are particularly suitable. Select the most efficient solvent to obtain homogeneity.

Of course, the organic solvent is not limited to those described above, for example, alcohols such as methanol and ethanol, tetrahydrofuran, etals such as 1,2-dimethoxyethane and 1,2-diethoxyethane, ethyl acetate, ethyl lactate, formic acid. It should be understood that esters such as butyl and butyl acetate can also be used alone or in combination of two or more.

The weight ratio of cellulose propionate to a solvent such as methyl isobutyl ketone is 30: 1 to 10: 1, preferably 20: 1 to 1
It should be in the range of 0: 1.

The cellulose propionate solution may be used as it is to form a film by a rotary film forming method, but in order to remove foreign matters, it is better to form a thin film after filtering. The filtration is performed by a pressure filtration method using a 0.2 μm filter, for example.

Further, when a high molecular weight substance is removed in advance from the cellulose propionate solution by the fractional precipitation method, the distribution of the film thickness of the resulting thin film becomes uniform, and the optical characteristics are remarkably improved.

Furthermore, a second organic solvent that is miscible with the organic solvent but is a poor solvent with respect to cellulose propionate is added to the organic solvent solution of the cellulose propionate, and high molecular weight cellulose in the cellulose propionate is added. ,
More preferably, a gelled substance, a low-solubility impurity and the like are precipitated together with the high molecular weight component, the high molecular weight cellulose propionate is separated from the solution, and the separated cellulose propionate solution may be formed into a film by a rotary film forming method.

In this way, by separating and precipitating a high molecular weight substance or by using a poor solvent, the stability and filterability of the raw material solution for film formation, and further the film forming performance can be improved. It is possible to make the thickness distribution of the film uniform and to improve the optical characteristics remarkably.

Examples of the poor solvent for cellulose propionate include hexane, heptane, methylcyclohexane, cyclohexane, toluene, octane, solvent naphtha, cyclonaphtha, xylene, etc., but this poor solvent is required to be desolvated by heating later, methylisobutyl. Those having a boiling point lower than that of the solvent of the ketone, particularly aliphatic hydrocarbons or alicyclic hydrocarbons such as hexane, heptane, methylcyclohexane, cyclohexane and the like, having 5 to 10 carbon atoms are advantageous. Hexane is advantageous in terms of availability and cost.

The amount of the poor solvent added is preferably 0.8 to 1.5 in terms of volume ratio of poor solvent / solvent, and 1.0 to 1.20 is suitable for practical use. If the amount of the poor solvent is less than the amount of the solvent such as methyl isobutyl ketone, the precipitation action of the high molecular weight cellulose is not performed as expected. By investigating how the average molecular weight of cellulose propionate changes, for example, by changing the volume ratio of poor solvent (hexane) and methyl isobutyl ketone, the average amount of cellulose propionate increases as the amount of poor solvent added increases. It can be seen that the molecular weight is reduced, ie the high molecular weight fraction is deposited. However,
If the amount of the poor solvent added is too large, the yield of the purified solution of cellulose propionate to be obtained will be low and the productivity will be poor.

After adding a poor solvent, a solution of cellulose propionate is obtained by removing only high molecular weight cellulose by means such as decantation or centrifugation, and further, the remaining poor solvent is desolvated, and a purified solution of cellulose propionate is obtained. To get
The solvent is removed by heating the solvent below the boiling point of methyl isobutyl ketone to above the boiling point of the poor solvent to evaporate the poor solvent. At that time, the cellulose propionate solution may be placed under reduced pressure.

The thickness of the thin film may vary, but it is generally in the range 0.5 to 6 microns, especially 0.8 to 3 microns. A dust-proof film with a thickness of 2.85 μm is used.

Here, an example of an apparatus used for the rotary film forming method will be described. As shown in FIG. 1, this apparatus has a substrate 2 placed on a turntable 1 which is rotationally driven. The substrate 2 is a rotating body, and the upper surface of the substrate 2 is kept horizontal to form a horizontal plane 3. It is a thing. Then, when the cellulose propionate solution is supplied from the nozzle 4 to the horizontal surface 3 and the turntable is rotated, the substrate 2 also rotates, and the cellulose ester solution spreads thinly along the horizontal surface 3 due to the centrifugal force, and a cellulose ester thin film is formed. To be done.

The rotation speed of the substrate 2 is usually 400 to 4000 rpm, preferably 500.
~ 3000rpm is good, but not only constant speed rotation, but low speed rise (200-1000rpm) and high speed (400-4000rpm) from the middle.

Further, if the release agent is applied to the horizontal surface 3 of the substrate 2 in advance, the thin film can be easily peeled off from the horizontal surface.

The dustproof film of the present invention thus obtained must have a weight average molecular weight of 60,000 to 400,000 according to polystyrene standards. If the weight average molecular weight is too low, the strength of the film cannot be withstood to use, and if it is too high, the solubility in a solvent is deteriorated and a uniform film cannot be produced.

〔Example〕

Hereinafter, examples of the present invention will be described in comparison with comparative examples.

Cellulose propionate (manufactured by Aldrich, high molecular weight grade) 70 g, dissolved in 930 g of methyl isobutyl ketone to form a cellulose propionate solution, hexane 60 in this cellulose propionate solution.
5 g was added dropwise (at this time, the amount of methyl isobutyl ketone was 1170 ml, the amount of hexane was 920 ml, and the relationship between the two was 1.27: 1 in terms of volume ratio), and left for 5 hours for decantation The precipitated high molecular weight cellulose propionate was removed, and the supernatant was placed under reduced pressure of 15 to 20 mmHg at 40 ° C. to desolvate hexane to obtain a purified solution of cellulose propionate.

This purified cellulose propionate solution has a pore size of 0.2 μm.
Then, when circulation filtration was performed under a pressure of 0.5 kg / cm ² · G using a filter having a diameter of 142 mm, the filter was not clogged even after 40 hours and could be filtered. Further, a thin film was produced from the filtered cellulose propionate solution by a rotary film forming method.

The obtained thin film had a weight average molecular weight of 470,000, and was a cellulose propionate thin film having good optical characteristics without color unevenness and streaks, and had a light transmittance of 99% or more.

〔The invention's effect〕

According to the dust mask for protecting a photomask of the present invention, since the light transmittance is extremely good and the light resistance is excellent, the performance as a dustproof film can be sufficiently exhibited, and the life is 2 times that of the nitrocellulose dustproof film. It has an excellent effect of more than double.

[Brief description of drawings]

FIG. 1 is a perspective view of an apparatus used in the method for producing a cellulose ester thin film of the present invention. 1 ... Turntable, 2 ... Board, 3 ... Horizontal surface.

Claims (2)

[Claims] 1. A polystyrene standard weight average molecular weight
Dustproof film for photomask protection consisting of 60,000 to 400,000 cellulose propionate. 2. The invention according to claim 1, wherein the film thickness is 2.85 μm.
The dustproof film for protecting the photomask according to the item.