Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: PHENOL-FREE PHOTORESIST STRIPPER
INVENTOR: JOHN E. VANDER MEY
BACKGROUND OF THE INVENTION
During manufacture of semiconductors and semicon-
ductor micro circuits, it is frequently necessary to coat the
materials from which the semiconductors and micro circuits
are manufactured with a polymeric organic substance, generally
referred to as a photoresist, e.g., a substance which forms
an etch resist upon exposure to light. These photoresists are
used to protect selected areas of the surface of the substrate,
e.g. silicon, SiO2 or aluminum, from the action of the etching
solution, while such etchant selectively attacks the unprotected
area of the substrate. Following completion of the etching -
operation and washing away of the residual etchant, it is
necessary that the resist be removed from the protective
surface to permit essential finishing operations.
A common method used in removing the photoresist from
the substrate is by contacting the substrate with an organic
stripper. Heretofore these organic strippers have been composed
of various components whose purpose it was to llft and remove
;~ the polymeric photoresist from the substrate. However, these
stripping solutions have heretofore contained phenol or phenol
compounds. The use of phenol or phenol compounds results in
a distinct dlsadvantage due to the toxicity of phenol as well
as the pollution problems arising from the disposal of phenol
or phenol compounds such as cresols, phenol sulfonic acid and
the like.
.,
It is an ob~ect of this invention to provide an
effective photoresist stripping solution which is essentially
free of phenol and phenol compounds.
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It is an additional object of this invention to
provide a method for removing photoresist polymeric eoatings
from inorganic substrates with a stripping solution free from
phenol and phenol compounds.
These and other objects will become apparent from the
description which follows.
BRIEF DESCRIPTION OF THE INVENTION
Briefly, in aceordance with this invention, there is
provided a stripping solution for removing polymerie organic
substanees from an inorganic substrate, said solution being
essentially free of phenol compounds and comprising 30 - 80
weight percent of an aryl sulfonic acid having the formula
SO3H
whereln R is hydrogen or an alkyl group containing 1 to 14
earbon atoms, in admixture with 20 - 7U weight pereent of a
solvent whieh is perchloroethylene, dichlerobenzene, dodecyl-
benzene or an isoparaffinie hydroearbon having an averagemolecular weight between 150 and 210 and a boiling point
between 160 and 220C.
Also in aceordanee with this invention, there is
provided a method for removing a photoresist organie polymerie
eoating material from the surfaee of an inorganic substrate
which cornprises contacting said coated substrate for a time
sufficient to loosen said eoating with a stripping composition
containing at least 30 weight pereent of the aryl sulfonic
acid defined above and whieh eomposition is essentially free
of phenol eompounds.
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DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a composition which
is effective in removing photoresist, and a method for removing
photoresist from inorganic substrates, without the use of
phenol or phenol compounds and their corresponding disadvan-
tages. Surprisingly, it has been found that the aryl sulfonic
acid itself acts effectively as a photoresist stripper. This
is unexpected in the sense that the prior art relating to
such stripping solutions employed such compounds in their
compositions as surface active agents and yet felt that phenol
10 or phenol compounds were necessary to effectively remove the
photoresist from the substrate.
The aryl sulfonic acids which have been found to be
effective, per Se, for removing organiC photoresist are
those represented by the formula
R
SO3H
wherein R lS hydrogen or a straight or branched chain alkyl
group containing 1 to 14 carbon atoms. Exemplary of such
aryl sulfonic acids suitable for use in accordance with the
present invention are benzene sulfonic acid, toluene sulfonic
acid, heptylbenzene sulfonic acid, octylbenzene sulfonic acid,
decylbenzene sulfonic acid, dodecylbenzene sulfonic acid,
cumene sulfonic acid, and the like. Blends of these sulfonic
acids may also be employed. The preferred aryl sulfonic acid
for use in accordance with this invention is dodecylbenzene
sulfonic acid, although cumene sulfonic acid has also been
found to be particularly effective. While, as stated above,
~'the aryl sulfonic acids can be used alone in the removal of
' 30 organic photoresists, it has been found that sometimes the
viscosity of the sulfonic acids is falrly high and it is thus
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desirable to lower the viscosity to reduce the quantity of
material that is taken out of the stripping solution upon
removal of the substrate after treatment with the stripping
solution. The viscosity of the aryl sulfonic acid can be
reduced by the addition of a solvent. In fact, in some cases
it is desirable to employ the aryl sulfonic acid in conjunc-
tion with the solvent. The solvent or solvent systems which
are to be used must be materials which do not deter from the
stripping effectiveness of the aryl sulfonic acid. The solvent
or solvent system itself is not critical to the stripping,
i.e., the function of the sulfonic acid, since the function
of the solvent is merely to reduce the viscosity and to render
the sulfonic acid more readily water rinsable. Of course, the
solvent should be miscible with the sulfonic acid and not react
therewith and the most desirable solvents are those which do
not induce corrosion on materials such as aluminum. Within
these guidelines, it has been found that solvents employed
in conjunction with the aryl sulfonic acids in accordance with
this invention may be perchloroethylene, dichlorobenzene,
dodecylbenzene or an isoparaffinic hydrocarbon having an
average molecular weight between 150 and 210 and a boiling
point between 160 and 220C. The isoparaffinic hydrocarbons
suitable for use as solvents in accordance with this invention
are relatively high boiling narrow cut isoparaffinic solvents
and are produced by modern synthesis from selected petroleum-
derived raw materials. They may be straight or branched chained
as long as they fall within the selected parameters. Typical
isoparaffinic hydrocarbons are available under the trade name
Isopar and can be generally characterized as containing about
60 percent of C-ll compounds, 20 percent of C-10 compounds and
20 percent of C-12 compounds. Isopar is a trademark of Exxon
Chemical Company for such mixtures of isoparaffinic hydrocar-
bons. Isopar M is Exxon's trademark for one such product.
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The polymeric organic substances which are to be
removed by the strlpping solutions of this invention are
photoresist which generally comprise polymers selected from
relatively low molecular weight polyisoprenes, polyvinyl
cinnamates and phenol formaldehyde resins. These photoresists
are applied to a substrate, e.g., SiO2, silicon or aluminum
and portions are masked. The masked substrate is then exposed
to light, e.g., a 120 volt 650 watt quartz lamp for 1-15
seconds at a distance of 6-12 inches to harden the exposed
photoresist. The portion of the photoresist which is not
exposed, i.e., masked from the light, is then removed by a
mild solvent which does not dissolve the exposed photoresist
thus leaving a pattern, e.g., a portion of an electrical
circuit pattern, on the exposed substrate. The remaining
- photoreslst is then baked for further hardening and the portionof the substrate which is not covered by the photoresist is
etched or otherwise treated. The hardened photoresist must then
be removed before the substrate can be further processed or
used. In employing the stripping solutions of this invention,
the substrate covered with the baked photoresist is contacted
with the stripping solution at a temperature of from about
; 50 to about 180C, preferably between 90 and 120C. Times
required for stripping the photoresist vary to quite an extent,
depending on the specific polymer used in the photoresist and
photoresist processing conditions. Generally, the time lnvolved
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wlll be between 1 and 10 minutes although some resists, de-
pending upon the bake temperature, may require 15 minutes, 30
mlnutes or even an hour of contact with the stripping solution
before the polymeric photoresist is loosened from the substrate.
After the photoresist has been stripped from the sub-
strate, the substrate is rinsed in any suitable liquid. Examples
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of suitable rinsing liquids are: ethanol, isopropyl alcohol,
trichloroethylene, perchloroethylene, methylene chloride and
water.
When the aryl sulfonic acid is employed in con~unction
with a solvent, it is necessary to use at least 30 percent of
the sulfonic acid in such an admixture. The amount of solvent
employed should be at least about 20 weight percent of the
admixture since amounts lower than 20 percent do not effec-
tively function as a viscosity reducer. Thus, the compositions
of this invention comprise an aryl sulfonic acid having the
formula ~
11 t R
. \~
i SO3H
wherein R ls as defined above, in an amount between 30 and 80
weight percent, in admixture with 20 - 70 weight percent of a
solvent as specified above. The preferred amount of aryl sul-
fonic acid is between 40 - 70 weight percent, and corres-
; pondingly, the preferred amount of solvent is between 30 -
60 weight percent.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A silicon dioxide substrate was coated with various
organic photoresists and baked at various temperatures before
treatrnent in accordance with this invention. The photoresist,
the bake temperature, the stripping time and the stripping
composltion are indicated in the following table.
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As the table indicates, very effective results are
achieved in employing an aryl sulfonic acid in accordance
with this invention either alone, or in conjunction with a
solvent, to remove polymeric photoresist from inorganic
substrates. Although Run No. 8 required more than 45 minutes
for complete removal of the polymer from the substrate, the
polyvinyl cinnamate baked at 220C is particularly difficult
to remove and the use of DDBSA alone compares favorably with
prior art strippers in this regard.
