Note: Descriptions are shown in the official language in which they were submitted.
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
. ' ~
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~ 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~
-- 4 --
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
-- 6 --
~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
CF3
i ~ - CF3 / benzotrifluoride
CF3
and pentafluoropropanol,
~3~3~6`~
-- 8
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
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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~
-- 11 --
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.
