Proposed is a novel frame-supported pellicle for dust-proof protection of a photomask used in the photolithographic patterning work in the manufacture of semiconductor devices and the like. The frame-supported pellicle of the invention consists of a pellicle membrane made from a specific fluorocarbon-containing polymer which is adhesively bonded in a slack-free fashion to a surface of a rigid pellicle frame by means of a hot-melt adhesive which is a fluorocarbon-containing polymer of the same type as or similar to the fluorocarbon-containing polymer of the membrane so that no problems are involved in the adhesive bonding relative to the compatibility between the adhesive and the membrane which otherwise is poorly susceptible to adhesive bonding.

BACKGROUND OF THE INVENTION 
The present invention relates to a frame-supported pellicle for 
photolithography or, more particularly, to a pellicle consisting of a 
rigid frame and a pellicle membrane supported by the frame in a slack-free 
fashion as being adhesively bonded thereto and used for dust-proof 
protection of a photomask used in the photolithographic patterning work of 
various kinds of electronic devices such as LSIs, VLSIs, liquid-crystal 
display units and the like as well as to a method for the preparation 
thereof. 
As is well known, the so-called photolithographic patterning work is widely 
undertaken in the manufacture of various kinds of electronic devices such 
as LSIs, VLSIs, liquid-crystal display units and the like, in which the 
photoresist layer on the substrate such as semiconductor silicon wafers 
and base plates of liquid-crystal display units is exposed pattern-wise to 
actinic rays such as ultraviolet light through a so-called photomask which 
is a transparency bearing the fine pattern to be formed in the photoresist 
layer. The pattern on the photomask is so fine that any dust particles 
deposited on the photomask greatly affect the quality of pattern 
reproduction resulting in a decrease in the yield of acceptable products. 
It is a usual practice accordingly that the photolithographic patterning 
work is conducted in a clean room freed from dust particles floating in 
the air as completely as possible but perfect cleanness of photomasks can 
hardly be obtained even in a clean room of the highest degree of 
cleanness. Therefore, a conventional means to ensure cleanness of a 
photomask is to protect the photomask by mounting a frame-supported 
pellicle thereon, which consists of a rigid frame to be mounted on and 
fixed to the photomask by means of a pressure-sensitive adhesive and a 
highly transparent pellicle membrane of a plastic resin adhesively bonded 
to the frame in a slack-free fashion so that dust particles are never 
deposited on the surface of the photomask but are deposited on the 
pellicle membrane. The dust particles deposited on the pellicle membrane 
have little adverse influences on the quality of pattern reproduction 
since the light for exposure is focused on the photomask and not on the 
pellicle membrane at a distance from the photomask. 
Needless to say, a primary requirement for the pellicle membranes is that 
transmission of light for the pattern-wise exposure therethrough is as 
high as possible. In this regard, several polymeric materials such as 
nitrocellulose, cellulose acetate and the like are widely used as a 
material of pellicle membranes. In connection with the method for adhesive 
bonding of a pellicle membrane to a frame, Japanese Patent Kokai No. 
58-219023 proposes that a pellicle membrane made from these polymeric 
materials is attached and bonded to the surface of a pellicle frame, which 
is made from aluminum, stainless steel, polyethylene and the like, wet 
with an organic solvent capable of dissolving the polymer of the membrane. 
Alternatively, the pellicle membrane can be adhesively bonded to the 
surface of a pellicle frame by using an acrylic or epoxy resin-based 
adhesive (see, for example, U.S. Pat. No. 4,861,402 and Japanese Patent 
Publication No. 63-27707). Besides the above mentioned cellulose 
derivatives as a material of the pellicle membrane, a proposal has been 
recently made for the use of an amorphous fluorocarbon polymer. The lower 
surface of the pellicle frame, i.e. the surface opposite to the surface to 
which the pellicle membrane is adhesively bonded, is usually coated with a 
pressure-sensitive adhesive based on a polybutene resin, polyvinyl acetate 
resin, acrylic resin and the like in order to facilitate mounting of the 
pellicle on a photomask. Commercial products of frame-supported pellicles 
are usually provided with a release paper sheet to temporarily protect the 
adhesive layer on the lower surface of the frame from inadvertent sticking 
and to ensure easy handling thereof. 
The adhesive, by which the pellicle frame and the pellicle membrane are 
adhesively bonded together, also must satisfy various requirements. In 
addition to a high adhesive bonding strength obtained therewith to ensure 
stability of the supported state of the membrane on the frame, the 
adhesive is required to be highly resistant against irradiation with 
ultraviolet light because the adhesive layer between the pellicle membrane 
and the frame is directly exposed to the ultraviolet light during the 
photolithographic patterning process since otherwise the adhesive causes 
degradation after a relatively short time of service under ultraviolet 
light to become brittle so that the thus embrittled adhesive per se 
produces dust particles if not to mention the decrease in the adhesive 
bonding strength eventually leading to separation of the membrane from the 
frame. Conventional acrylic and epoxy resin-based adhesives are not quite 
satisfactory in this regard. When the pellicle membrane is formed from an 
amorphous fluorocarbon polymer, no satisfactory adhesive bonding strength 
can be obtained between the frame and the membrane because fluorocarbon 
polymers are generally little susceptible to adhesive bonding with a 
conventional adhesive based on an acrylic or epoxy resin. 
SUMMARY OF THE INVENTION 
The present invention accordingly has an object to provide a novel and 
improved frame-supported pellicle for dust-proof protection of a photomask 
in the photolithographic patterning work free from the above described 
problems and disadvantages in the conventional frame-supported pellicles. 
Another object of the present invention is to provide a method for the 
preparation of the above mentioned novel and improved frame-supported 
pellicle. 
Thus, the frame-supported pellicle for dust-proof protection of a photomask 
provided by the present invention is an integral body consisting of: 
(a) a rigid pellicle frame; 
(b) a layer of a hot-melt adhesive; and 
(c) a pellicle membrane made from a first amorphous fluorocarbon-containing 
polymer having a cyclic structure in the molecule, the adhesive layer 
intervening between a surface of the pellicle frame and the pellicle 
membrane so as to support the pellicle membrane by adhesively bonding the 
same to the pellicle frame and the adhesive being made from a second 
amorphous fluorocarbon-containing polymer of a hot-melt type having a 
cyclic structure in the molecule. 
In particular, each of the first and second amorphous 
fluorocarbon-containing polymers having a cyclic structure is a copolymer 
consisting of a tetrafluoroethylene moiety and a cyclic perfluoro ether 
moiety. 
The inventive method for the preparation of the above defined 
frame-supported pellicle comprises the steps off 
(A) forming a film of a first amorphous fluorocarbon-containing polymer 
having a cyclic structure by the casting method of a solution of the 
polymer on the surface of a substrate followed by drying; 
(B) forming a layer of a hot-melt adhesive of a second amorphous 
fluorocarbon-containing polymer having a cyclic structure on one surface 
of a rigid pellicle frame; 
(C) heating the layer of the hot-melt adhesive on the surface of the 
pellicle frame at a temperature higher than the glass transition point of 
the hot-melt adhesive; 
(D) mounting the film of the first amorphous fluorocarbon-containing 
polymer formed on the substrate surface on the surface of the pellicle 
frame coated with the heated hot-melt adhesive followed by cooling to 
effect adhesive bonding by the hot-melt adhesive; and 
(E) removing the substrate from the film of the first amorphous 
fluorocarbon-containing polymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is described above, the inventive frame-supported pellicle is an 
integral body consisting of a rigid frame and a pellicle membrane 
adhesively bonded to the frame in a slack-free fashion by means of a layer 
of a hot-melt adhesive intervening therebetween. Characteristically, each 
of the pellicle membrane and the hot-melt adhesive is made from an 
amorphous fluorocarbon-containing polymer having a cyclic structure in the 
molecule which may be the same one or different but similar ones so that 
the adhesive bonding strength therebetween can be high enough without the 
problem of incompatibility and, since these fluorocarbon-containing 
polymers are highly resistant against ultraviolet light, a 
high-performance frame-supported pellicle with a long serviceable life can 
be obtained. 
The amorphous fluorocarbon-containing polymer having a cyclic structure in 
the molecule for the pellicle membrane is preferably an amorphous 
copolymer consisting of a tetrafluoroethylene moiety and a cyclic 
perfluoro ether moiety. Examples of commercially available products of 
such an amorphous fluorocarbon-containing polymer include Teflon AF (trade 
name by Du Pont Co.) which is a copolymer consisting of the moieties of 
the formula 
##STR1## 
and Sytops (trade name by Asahi Glass Co.) which are each a copolymer 
consisting of the moleties of the formula 
##STR2## 
in which the subscripts n and m are each a positive integer (see Japanese 
Patent Kokai No. 4-104155). 
The membrane of the above mentioned fluorocarbon-containing copolymeric 
resin can be prepared by the method of casting of a solution of the resin 
on a substrate having a flat and smooth surface such as a well polished 
fused quartz glass plate. Suitable organic solvents include 
fluorocarbon-containing compounds such as perfluoro(2-butyl 
tetrahydrofuran) and perfluoro(2-propyl tetrahydropyran). The casting 
solution can be prepared by dissolving the copolymeric resin in these 
solvents in a concentration of 3 to 10% by weight and the solution is 
uniformly spread over the surface of the substrate by using a known 
coating machine such as a spin coater, knife coater and the like. The film 
to serve as the pellicle membrane should have a thickness in the range 
from 0.5 to 10 .mu.m or, preferably, from 0.8 to 5 .mu.m in view of the 
balance between mechanical strength and transmissivity to ultraviolet 
light. It is desirable that the pellicle membrane has at least 95% of the 
transmission of ultraviolet light. 
The thus prepared pellicle membrane is very stable and free from yellowing 
retaining high transparency and occurrence of rupture over a long period 
of service. Needless to say, the pellicle membrane has excellent 
transmission to the light of the wavelength in the range from 210 to 500 
nm including the g-line of 436 nm wavelength, i-line of 365 nm wavelength 
and excimer laser beam or KrF laser beam of 248 nm wavelength frequently 
used in the photolithographic patterning works. 
In the frame-supported pellicle of the invention, as is mentioned before, 
the adhesive, with which the pellicle membrane and the pellicle frame are 
adhesively bonded together, is also a polymeric material which is a 
fluorocarbon-containing hot-melt adhesive compound. The same 
fluorocarbon-containing polymers as used in the preparation of the 
pellicle membrane, such as Teflon AF and Sytops, as such can be used as 
the adhesive so that no problems are involved in the adhesive bonding in 
connection with the compatibility between the pellicle membrane and the 
adhesive by which the membrane is adhesively bonded to the pellicle frame. 
In conducting adhesive bonding of the pellicle membrane and the pellicle 
frame by using the above mentioned hot-melt adhesive, the pellicle frame 
is coated on one surface with a solution of the hot-melt adhesive and, 
after drying the adhesive layer by evaporating the solvent, the dried 
adhesive layer is heated at a temperature higher than the glass transition 
point of the hot-melt adhesive followed by mounting of the pellicle 
membrane formed by the solution casting method on a substrate plate such 
as a fused quartz glass plate and, after cooling of the heated adhesive, 
removal of the substrate plate from the pellicle membrane so that the 
pellicle membrane is supported by the pellicle frame in a slack-free 
fashion. 
Since the adhesive to bond the pellicle frame and the pellicle frame is a 
polymer of the same type as or similar to that of the membrane, excellent 
adhesive bonding can be obtained therebetween without the problem of poor 
compatibility of the adhesive with the membrane. Moreover, the adhesive is 
highly resistant against ultraviolet irradiation so that the 
flame-supported pellicle of the invention has good serviceable life. 
In the following, an example is given to illustrate the frame-supported 
pellicle and the method for the preparation thereof according to the 
invention in more detail. 
EXAMPLE 
A fluorocarbon-containing polymer (Sytop CTXS, a product by Asahi Glass 
Co.), consisting of the tetrafluoroethylene moiety and the perfluorinated 
cyclic ether moiety as shown by the above given general formula (II), in 
which the molar ratio of the tetrafluoroethylene moiety to the 
perfiuorinated cyclic ether moiety is 0.52:1, was dissolved in a 
fluorocarbon-containing solvent (CT Solve 180, a product by the same 
company, supra) in a concentration of 6.0% by weight. This solution was 
uniformly applied to the surface of the well polished surface of a fused 
quartz glass disc of 200 mm diameter and 3 mm thickness by using a spin 
coater followed by drying at 180 .degree. C. for 15 minutes to give a 
dried film of the polymer having a thickness of 0.84 .mu.m. 
Separately, a surface-anodized square aluminum frame having dimensions of 
120 mm a side (outer), 116 mm a side (inner) and thickness of 6 mm was 
coated on one surface with a 9.0% by weight solution of Sytop CTXA (supra) 
in CT Solve 180 (supra) followed by air-drying for 3 hours to form a layer 
of the dried polymer to serve as a hot-melt adhesive. 
The frame having a coating layer of the polymer on one surface was mounted 
on a hot plate kept at 130 .degree. C. with the coating layer of the 
polymer facing upwardly and kept there for 5 minutes to heat the polymer 
at a temperature higher than the glass transition point thereof. 
Thereafter, the polymeric film formed on the fused quartz glass disc as 
the substrate was mounted thereon together with the substrate followed by 
cooling to room temperature so as to adhesively bond the film as the 
pellicle membrane to the aluminum frame. After cooling, the fused quartz 
glass disc as the substrate was removed from the polymeric film which was 
trimmed along the outer periphery of the aluminum frame so as to give a 
frame-supported pellicle. The pellicle was slack-free and free from the 
troubles of separation from the frame or occurrence of rupture and could 
withstand a long time of service in the photolithographic patterning 
works.