CA1305627C

CA1305627C - Pellicle

Info

Publication number: CA1305627C
Application number: CA000582249A
Authority: CA
Inventors: Hiroaki Nakagawa; Hitomi Matsuzaki
Original assignee: Mitsui Petrochemical Industries Ltd
Current assignee: Mitsui Chemicals Inc
Priority date: 1987-11-05
Filing date: 1988-11-04
Publication date: 1992-07-28
Anticipated expiration: 2009-07-28
Also published as: KR910004848B1; MY103634A; KR890008613A; ATE93978T1

Abstract

PELLICLE

ABSTRACT OF THE DISCLOSURE

A pellicle is composed of a pellicle frame, a pellicle film spread on one side face of the pellicle frame, and a tacky substance layer having a high light transmittance formed on the inner side face of the pellicle film.

Description

9~ 30 ,5: t' ~7 - MPC - 6 9 3 2 PELLICLE

BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a pellicle used as the dust-proof cover during exposure of a photomask or a reticle.

2. Description of the Related Art At the lithographing step, a mask comprising a glass sheet and a circuit pattern of a vacuum deposition film of chromium or the like (hereinafter called mask) formed on the surface of the gl~ss sheet is used, and the circuit pattern is transferred to a resist-coated silicon wafer. At this step, if light exposure is carried out when a foreign substance such as dust is adhered to the circuit pattern on the mask, the foreign substance is transferred to the wafer and a defective product is formed. ~specially, when light exposure is carried out by using a stepper, the risk that all of the chips formed on a wafer will be defective is increasedt and an adhesion of foreign matter to a circuit pattern of a mask or the like becomes a serious problem.
Therefore, a pellicle has recently been developed and used by which this problem i.s eliminated.
In general, the pellicle comprises a trans-parent film of nitrocellulose or the like spread on one side face of a pellicle frame made of aluminum, a double-adhesive tape is applied to the other side face of the pellicle, and the pellicle is attached to a mask.
When this pellicle is used, the intrusion of a foreign substance from the outside can be prevented, and even if a foreign substance adheres to the film, the foreign substance is transferred in the fuzzy state during light exposure and no particular problem arises. Neverthe-less, the foreign substance already attached on the innerside of the film or the pellicle frame can no longer be removed, and when such a pellicle is used, to . ' ~

~ 3~ b~ ~

~ 2 - 70065-14 prevent interference with the exposure because foreign mattar attached to the innerside of the pellicle has fallen onto the photomask or the reticle, a pellicle provided with a tacky film on the inner side face of S the pellicle frame has been proposed (see: J~panese Unexamined Patent Public~tion (Kokai) No. 60-5~84l).
On the other hand, a single layer ~hin film of nitroce~lulo3e has been primarily utilized as the pellicle film, and to improve the stability of the light l~ tran~mittance and ~he like during the exposure step, a pellicle film having a reflection preventive layer comprising a fluorine type polymer or a silicon type polymer and the like on a transparent film of nitro-cellulose has been proposed (see: Japanese Unexamined Patent Publication (Kokai) No. 60-23~450).
Nevertheless, in the pellicle described above (see; Japanese Unexamined Patent Publication (~okai) No. 60-5784l), since a double-adhesive tape is used at the adhered face between the pellicle frame and the mask, a ~oreign substance ~uch as dus~ is genera~ed from the foamed tape which i9 the subs~rate for the double-adhesive tape, and becomes a sourca of contamination.

Th~re is proposed a pellicle which does not use a double-adhesive tape at the adhered surface. But, even in this method, the foreign matter attached on the inner face of the pellicle film may be sometimes caused to fall onto khe mask due to shock during usage, to thereby cause the production of defective LSI's.
3n On the other hand, the pellicle film used as a du~t-proof film must have a high light ray trMnsmlttance, and accordingly, the presence of a foreign substance i9 extremely disadvantageous, and thu4 a material to which the attachment o~ a foreign substance i8 difficult ha~ been used. When ~o.rming a reflection preventive film as disclosed in the above-men~ioned Japanese Unexamined Pa-tent Publication ~Kokai) ~ .

r~

No. 60-237450, however, a material without tackiness on the surface must be used to prevent the attachment of a foreign substance. Nevertheless, although such a material without tackiness on the surface can form a film on which a foreign substance can be attached only with difficulty, any foreign substance attached thereto will readily drop therefrom, and thus the problem arises that such a foreign substance will interfere with the exposure.
SUMMARY OF THE INVENTION
Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantages of the prior art and to provide a pellicle which prevents the dropping of a foreign substance onto a photomask or a reticle by attaching the foreign substance to a tacky substance layer having a high light transmittance and formed on the inner face of the pellicle film.
Other objects and advantages of the present inven-tion will be apparent from the following description~
In accordance with the present invention, there is provided a pellicle comprising a pellicle frame, a pellicle film spread on one side face of the pellicle frame, and a tacky su~stance layer having a hi~h light transmittance formed on the inner side face of the pellicle film.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood from the descripkion set forth below with reference to the accompanying drawing of Fig. 1, wh.ich is a sectional view of a part showing the pellicle of the Example, in which 1 is a pellicle, 2 a pellicle frame, 3 a pellicle film, 4, 6, and 7 are tacky substance layers, 5 a non-tacky layer,.and 8 a material to be adhered.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The pellicle frame usable in the present invention is not particularly limited, but an almite-treated aluminum frame used in the prior art may bP utilized, ~3~ 2~

and other materials can be used. The shape of the pellicle frame may be as desired, for example, cylin-drical, square. Also, preferably a tacky substance layer is formed on the inner side face of the pellicle frame.
In the present invention, the transparent thin film, which becomes the main body of the pellicle film, may be a film having a large average light transmittance at the wavelengths of 350 to 450 nm used for exposure, but preferably is a cellulose derivative thin film such as nitrocellulose, ethylcellulose~ propionic acid cellu-lose, and the like. Among the above, from the aspects of an average light transmittance at 350 to 450 nm and the film strength, nitrocellulose is preferred. Of the nitrocelluloses, those having a nitration degree (N%) of 11 to 12.S~, particularly 11.5 to 12.2%, and an average molecular weight (weight average Mw) of 50,000 to 350,000, particularly, 70,000 to 320,000 are preferred.
Here, the term, average light transmittance, refers to an average value of the transmittance at the same number of peak portions and valley port;ions of interference waves occurring between 350 and 450 nm.
The thicXness o the trans~arent thin film is selected so that transmittance to the desired wavelength between 350 and 450 nm may be higher, but a thickness of 2.85 ~m is generally selected to raise the transmittance to around 436 nm ~ 405 nm + 365 nm of the exposed wave-length currently in use, and a thickness of 0.865 ~m is used for raising the transmittance at 436 nm.
As the tacky substance for forming the tacky substance layer on the inner side face of the trans~
parent thin film which becomes the main body of the pellicle film, there may be employed a substance having a high light transmittance and a tackiness, but pref-erably the substance used can firmly hold a tacky foreign substance attached thereto. The tacky substance preferably has a reEractive index not higher than that ~3~

of the transparent thin film, without decomposition by the light rays at 350 to 450 nm. Also, the tacky substance may have a reflection preventive effect.
As such tacky substances, for example, khere may be included fluorine type polymers and silicone type polymers. As the fluorine type polymer, acrylic fluorine polymers are preferred, for example, poly~
fluoro(meth) acrylates comprising at least one monomer selected from CH2=CHCOOR1 or CH2=C(CH3)COOR2 wherein Rl, R2 are each a fluoroalkyl group which may also contain an ethereal oxygen atom interposed, namely, homopolymers or copolymer~ of fluorine containing acrylates, homo-polymers, copolymers of fluorine containing meth-acrylates, copolymers of fluorine containing acrylates, and fluorine containing methacrylates, and by suitably varying the kind of the fluoroalkyl group of Rl and R2 and the copolymer composition, a polymer with a desired 1uorine content can be used.
Examples of R1 and R2 in the above-mentioned formulae are as follows:
-CH2CF3 , -CH2C2F5 , -cH2c3F7 , -CH2C4F9 , -CH2C5Fll ~ -cH2c7Fl5 , -CH2C3F17, -CH2CgFlg , -CH2clOF2l / ~cH2cH2cF3 , -CH2CH2C2F5 , -CH2CH2C3F7 ~ -cH2cH2c4F9, -CH2CH2C5F
-CH2CH2C7F15, -CH2CH~C8F17, -CH2CH2CgFl g -CH2cH2clOF2~ H2(cF2)2H, -CH2(CF2)4H, -CH2(CF2)6Hr -CH2(CF2)~H, -CH2(CF2)10H, -CH(CF3)2 ~ -CF(CF3)2 / -(CH2)5(~CF(CF3)2 , -(cH2)ll~cF(cF3)2 , -cH2~(cF2)OcF3 , -CH20(CF2)0C2F5 , -CH20(CF2)20C3F7 -CH20(CF2)20C4F9~
CH2cF(cH3)ocF2cF(cF3)ocF2cF2cF3 In the present invention, preerably a poly-1uoro(meth)acrylate having a fluoxine content of 50% by weight is used.
Among the above, a pxeferable tacky substance is a poly1uoroacrylate which is a copolymer of ~rifluoroethyl acrylate and perfluorooctylethyl acrylate. The monomer (A) of this copolymer, tri-fluoroe~hyl acrylate is represented by CH2CHCOOCH2CF3 , and the monomer (B), per1uorooctylethyl acrylate by 5 CH2=CHCOOC2H4C8FI7~ -The above polyfluoro(meth)acrylate also has a reflection preventive property. ~he tacky substance layer is ormed on the inner side face of the trans-parent thin film of the cellulose derivative, with the above tacky substance, and the film thickness at that time is preferably 1/4 n (n is the refractive index) of the wavelength of the target light.
A layer such as a reflection preventive layer or the liXe can be formed on the outer side face of the transparerlt thin film which becomes the main body of the pellicle film, as in the prior art. In this case, the layer such as reflection preventive layer or the like to be formed on the outer side face is preferably a non-tacky substance layer. As t:he non-tacky substance layer, a non-tacky fluorine type polymer or silicone type pol~mer used in the prior art as a re1ection preventive layer may be utilizecl. As the fluorine type polymer mentioned above, there may be included copolymers of tetrafluoroethylene and vinylidene chloride, or vinylidene fluoride, ternary copolymers o~ tetrafluoroethylene, vinylidene chloride, or vinylidene fluorîde, and hexafluoropropylene and the like.
The layers formed on the inner side face and the outer side face of the transparent thin film may be either a single layer or multiple layers. In the case of multiple layers, a tacky substance layer can be used for the innermost layer and a non-tacky substance layer for the outermost layer.
The method for preparing the pellicle film having a tacky substance layer may be practiced as follows, in the case of forming a tacky substance layer comprising a ~3`~

polyfluoroacrylate on a cellulose derivative transparent thin film.
First, a cellulose derivative solution is fed onto a smooth substrate such as glass, and a transparent film of the cellulose derivative is formed by the rotatory film fabrication method. The cellulose derivative is dissolved in a good solvent and the solution is purified by, for example/ filtration, before use if necessary.
As the solvent, there may be employed ketones such as methyl ethyl ketone, methyl isobutyl ketone, acetone and the like; lower fatty acid esters such as butyl acetate, isobutyl acetate and the like; and mixtures of these solvents with alcohols such as isopropyl alcohol and the like. The thickness of the transparent thin film formed can be suitably varied by chanqing the solution vis-cosity or the rotational speed of the substrate.
The cellulose derivative transparent thin film formed on the substrate is dried by, for example, hot air or IR~ray lamp irradiation, to remove the residual solvent.
Subsequently, a solution of a tacky substance such as the above polyfluoroacrylate is applied over the dried cellulose derivative thin film, and a tacky substance layer comprising a fluoxine polymer is formed by the rotatory film fabrication method, as for the cellulose derivative thin film. In this case, the solvent for dissolving polyfluoroacrylate is selected from among m~xylenehexafluoride i ~ - CF3 / benzotrifluoride and pentafluoropropanol, ~3~3~6`~

and amonq these, m-xylenehexafluoride is particularly preferred. The use of such specific solvents enables a polyfluoroacrylate solution with a good rotatory film fabricability to be obtained, and further, adverse influences which dissolve or swell the cellulose deriva-tive thin film, which becomes the base layer, can be prevented during the formation of the polyfluoroacrylate reflection preventive film.
The thickness of the tacky substance layer can be controlled by suitably varying the solution viscosity and the rotational speed of the substrate, e~c., as in the case of the cellulose derivative thin film.
On the other hand, when a pellicle film has a tacky substance layer on one face and a non-tac~y substance layer on the other face, the transparent thin film having the tacky substance layer on one face is peeled from the substrate, plastered on a tentative frame, and on the other face not having the tacky substance layer, is formed a solution of, for example, a tetrafluoro-ethylene/vinylidene chloride or vinylidenefluoride/hexafluoropropylene ternary copolymer (50/29/21 weight ratio) dissolved in a solvent such as perfluoro-~-methyl-l-oxy-3-thiacyclohexane-3,3-dioxide whereby a reflection preventive layer comprising a non-tacky substance layer can be formed.
As the method for continuously preparing a pellicle film having a tacky substance layer and a non-tacky substance layer, a polyfluoroacrylate solution (note, the solvent is not limited to the solvents mentioned above and any solvent which will dissolve the same polymer can be used) is fed onto a smooth substrate such as glass, etc., a polyfluoroacrylate thin film is formed by the rotatory film abrication method, and the film is dried by, for example, hot air or IR-ray lamp irradia-tion, to remove the residual solvent. Then, after atransparent thin film of a cellulose derivative is formed on the thin film by performing the operation as ~L3~

described above, a solution of tetrafluoroethylene/vi-nylidene chloride or vinylidene fluoride/hexa-fluoropropylene ternary copolymer is applied to form a non-tacky substance layer, whereby a three-layer structure pellicle film can be formed.
The pellicle film having the tacky substance layer thus formed on the substrate is peeled from the substrate and used as the pellicle. In this case, -~ for example, a ~ ~ ~ne tape or a frame-shaped implement having an adhesive coated thereon is pushed against the outermost layer of the laminated film formed on the substrate. Namely, the polyfluoroacrylate film is in contact with the outer atmosphere to be adhered thereon, and the film can be directly peeled from the substrate by lifting the cellophane tape or the frame-shaped implement, from one end, by hand or by a mechanical means. In this case, since the interlayer adhesive force between the cellulose derivative layer and the polyfluoroacrylate layer is great, the film can be peeled without separation and the pellicle film thus obtai.ned plastered onto a pellicle frame to form a pellicle.
The pellicle thus prepared is used as dust-proof cover during exposure, by being superposed on a photomask or a reticle as in the prior art. During the preparation of the pellicle, care is taken that no oreign substance such as dust, etc., is adhered thereto, but any foreign substance accidentally brought in is adhered to the tacky substance layer and will not fall on~o the photomask. The foreign substance attached on the outer side face of the pell.icle can be removed by air blowing, but the foreign substance attached on the inner side face will not fall onto a photomask or reticle, and thus will not cause defective LSI's, because substances having a size of about 50 ~m or less will not form images on the transfer face.
~ Jr4d~3- ~csrk EXAMPLES
The present invention will now be further illus-trated by, but is by no means limited to, the following Examples, with reference to the drawing.
Fiyure 1 is a sectional view of a part showing the pellicle of the Example. In the drawing, 1 is a pellicle having a pellicle film 3 spread on one side face of a pellicle -frame 2. On the inner side face of the pellicle film 3 is ormed a tacky substance layer 4, and a non-tacky substance layer 5 is formed on the outer side face. This layer 5 is also utilized as the reflec-tion preventive layer. Also, on the inner side face and the lower side face of a pellicle frame 2 are formed tacky substance layers 6 and 7, and the tacky substance layer 7 is used for adhesion with the material 8 to be adhered, such as a photomask or reticle.
The pellicle 1 having the above constitution is used as the dust-proof cover during exposure, while adhered to the material 8 to be adhered by utilizing the tacky substance layer 7. At this time, the foreign substance attachad to the non-tacky substance layer 5 is removed by air blowing, but the foreign substances attached to the tacky substance layers 6 and 7, will not fall and will not interfere with the exposure.
ExamPle and Comparative Example Nitrocellulose was dissolved in methyl isobutyl ketone to form a 6% by weight solution. Also, a copolymer of trifluoroethyl acrylate and perfluoro-octylethyl acrylate (trifluoroethyl acrylate 67 mol%, perfluorooctylethyl acrylate 33 mol%, fluorine content 52.8% by weight) was dissolved in m-xylenehexafluoride to form a 1.0% by weight solution.
The above fluorine polymer solution was added dropwise by the rotatory coating method and filmed or sheeted by rotation at 500 rpm for 60 seconds to form a tacky substance layer, and then a transparent thin film of nitrocellulose was formed, followed by drying.

~S~2~

Further, a solution of a tetraf:Luoroethylene/vinylidene chloride or vinylidene fluoride/hexafluoropropylene terpolymer (50/29/21 weight ratio) dissolved in perfluoro-2 methyl-1-oxy-3-thiacyclohexane-3,3-dioxide at a concentration of 0.6% by weight was added dropwise to form a non-tacky substance l~yer. The pellicle film was peeled from the substrate and spread on a pellicle frame so that the tacky substance was at the inner side face.
On the innerside of the pellicle were attached 12 oreign substances 5 to 10 ~m in size, and the pellicle then plastered onto a quartz s~lbstrate and permitted to fall three times from a height of 5 cm. The foreign substances were examined, and as a result, it was found that they were in the same position as before the dropping of the pellicle, and had not fallen onto the quartz substrate.
On th~ other hand, when the same experiment was conducted using a pellicle of nitrocellulose alone (8 foreign substances attached), three foreign substances were found to have fallen onto the quartz substrate.
Also, the positions of the remainins 5 foreign substances on the pellicle were found to be changed.
As described above, according to the present invention, a plasterable substance layer is formed on the inner side face of the pellicle film, and therefore, it is possible to prevent interference with the exposure by foreign substances attached on the innerside of the pellicle which have fallen onto the material to be adhered, such as a photomask.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

l. A pellicle comprising:
(a) a pellicle frame;
(b) a transparent pellicle film spread on one side face of the pellicle frame, the pellicle film having a refractive index, and an inner side face and an outer side face; and (c) a tacky substance layer formed on the inner side face of the pellicle film, the tacky substance layer composed of tacky polyfluoro (meth)acrylate having a fluorine content of 50% by weight or more, having a high light transmittance and having a refractive index not higher than that of said pellicle film.
2. A pellicle as claimed in Claim l, wherein a non-tacky substance layer is formed on the outer side face of the pellicle film.
3. A pellicle adapted to be used as a dust-proof cover during exposure to light of a photomask or a reticle in a lithographic step, the said pellicle comprising:
a pellicle frame having an inner side face to be adhered to the photomask or the reticle and an outer side f-ace;
a transparent thin pellicle film spread on the outer side face of the pellicle frame, the pellicle film having inner and outer side faces, having a large average light transmittance of light with a wavelength of 350 to 450 nm and being composed of a cellulose derivative; and a tacky substance layer having a high light transmittance formed on the inner side face of the pellicle film, the said tacky substance having a refractive index not higher than the pellicle film in a wavelength range of 350 to 450 nm and comprising a fluoroalkyl (meth)acrylate.
4. A pellicle as claimed in claim 3, which further comprises:
a non-tacky reflection-preventive substance layer on an outside side face of the pellicle film.