Photoresist compositions based on hydroxystyrene copolymers

Negative photoresist compositions are made from copolymers of 4-hydroxystyrene and dialkyl muconates, alkyl sorbates, alkadiene monomers or allyl esters of ethylenically unsaturated acids plus a photosensitizer. Such compositions are useful in photolithographic and photomasking operations in fabricating microelectronic devices, printed circuits, semiconductors, printing plates, dies and the like.

BACKGROUND OF THE INVENTION 
The field of art to which this invention pertains is photoreactive 
compositions for use in photoresist coatings. 
The physical properties of photoresist coatings change when the coatings 
are exposed to light. The change usually is one of solubility and results 
in solvent discrimination between exposed and unexposed areas. Exposure of 
coatings to light through a pattern results in solubility changes and, 
therefore, image boundaries that can be used to form resist images. Images 
can be produced by solvent development in either negative-or 
positive-working modes. 
The negative-working mode depends on a light induced decrease in solubility 
of polymer molecules. The positive-working mode is just the opposite, 
i.e., exposure to light makes the polymer more soluble in the solvent 
(developer). 
Photoresist polymer compositions must fulfill a number of physical property 
dependent requirements under practical working conditions. The polymers 
must form homogeneous solutions in ecologically acceptable solvents; form 
smooth, flaw-free coatings with good adhesion in all process steps; be 
characterized by good solution stability for storage; form tack-free 
coatings; provide good image discrimination; resist flow or creep during 
processing at high temperatures; form thermally stable images that do no 
flow; and be removable or strippable after the required fabrication steps. 
Negative acting photoresists based on polydienes cross-linked with 
bisazides are described in The Journal of Photographical Science, Volume 
12, p 181 (1964). Photoresist systems based on poly(cis-isoprene) and 
bis(4-azidobenzal)cyclohexanone have been studied using infrared and 
ultraviolet spectroscopy, solvent extraction and thin layer chromatography 
as described in The Journal of the Electrochemical Society, Volume 124, p 
1394 (1977). 
Photoresist compositions based on poly(vinylphenols) are described in U.S. 
Pat. No. 3,869,292 and U.S. Pat. No. 4,439,516. Such compositions are made 
from poly(vinylphenols) and sensitizers, such as aromatic esters or amides 
of ortho-naphthoquinone diazide sulfonic or carboxylic acid. 
Positive acting photoresists are described in U.S. Pat. No. 4,409,317. Such 
compositions contain alkali soluble resins such as poly(vinylphenol). 
Photosensitive compositions made from polymers of the reaction product of 
p-vinylphenol and cinnamic acid chloride are described in U.S. Pat. No. 
3,882,084. 
Positive or negative acting photoresist compositions are described in U.S. 
Pat. No. 4,491,628. Such compositions are made from polymers which contain 
acid labile groups, such as tertiary butyl carbonates of 
poly(vinylphenols). 
In U.S. Pat. No. 4,469,778, photosensitive compositions in which the 
photosensitizers are bisazide compounds are described. 
SUMMARY OF THE INVENTION 
This invention pertains to negative-acting photoresist compositions based 
on 4-hydroxystyrene copolymers. 
The negative-acting photoresist compositions of this invention are made 
from (A) copolymers of 4-hydroxystyrene and other monomers selected from 
the group consisting of dialkyl muconates, alkyl sorbates, alkadiene 
monomers and allyl esters of ethylenically unsaturated acids wherein the 
copolymers contain about 10 to about 90 weight percent 4-hydroxystyrene 
and about 90 to about 10 weight percent other monomer and (B) an azide 
photosensitizer. 
By varying the particular azide photosensitizer in the composition, 
negative-acting photoresist compositions can be made that are 
photosensitive to radiation in the deep ultraviolet up to visible light. 
DESCRIPTION OF THE INVENTION 
The 4-hydroxystyrene copolymers useful in this invention are prepared by 
copolymerizing 4-acetoxystyrene with the other monomers, followed by 
selective hydrolysis of the acetoxy groups to phenol groups using the 
procedures set forth in copending applications, Ser. No.: 099,429, filed 
Sept. 11, 1987, and Ser. No.:099,430, filed Sept. 11, 1987. 
The 4-acetoxystyrene monomers can be made by the process described in 
Corson et. al., Journal of Organic Chemistry 23, 544-549 (1958). As 
described in this article, phenol is acetylated to 4-hydroxyacetophenone 
which is then acetylated to 4-acetoxyacetophenone. This compound is then 
hydrogenated to 4-acetoxyphenylmethylcarbinol, which is then dehydrated to 
4-hydroxystyrene. 
The dialkyl muconates which are used in preparing copolymers utilized in 
this invention are diester derivatives of muconic acid. Muconic acid is a 
diolefinically unsaturated adipic acid derivative and can be made by the 
microbiological oxidation of various hydrocarbon substrates, as described 
in U.S. Pat. Nos. 4,480,034 and 4,534,059. The dialkyl muconates used in 
this invention contain 1 to 4 carbon atoms in each alkyl group and are 
made by the direct esterification of muconic acid with methanol, ethanol, 
the propanols, or the butanols. The preferred dialkyl muconates useful in 
this invention are dimethyl muconate and diethyl muconate. 
The alkyl sorbates which are used in preparing copolymers reacted in this 
invention are ester derivatives of sorbic acid. Sorbic acid, or 
hexadienoic acid, is a diolefinically unsaturated acid which can be made 
by the ring opening of the beta-lactone made from crotonaldehyde and 
ketene. The alkyl sorbates used in this invention contain 1 to 4 carbon 
atoms in each alkyl group and can be made by the direct esterification of 
sorbic acid with methanol, ethanol, the propanols, or the butanols. The 
preferred alkyl sorbates useful in this invention are methyl sorbate and 
ethyl sorbate. 
The alkadiene monomers useful in this invention are hydrocarbon monomers 
which contain two conjugated ethylenically unsaturated groups and which 
polymerize through 1,2 or 1,4 addition leaving one unsaturated group 
unpolymerized. Examples of such monomers are 1,3-butadiene, 
1,3-pentadiene, 1,3-hexadiene, 2,4-hexadiene, isoprene and chloroprene. 
The allyl ester monomers useful in this invention are mono which contain 
one polymerizable ethylenically unsaturated group. Examples of such esters 
include allyl acrylate, allyl methacrylate, allyl ethacrylate, diallyl 
maleate, alkyl allyl maleate, diallyl fumarate, alkyl allyl fumarate, 
diallyl itaconate, alkyl allyl itaconate, allyl sorbate, diallyl muconate 
and alkyl allyl muconate. 
In preparing copolymers with the alkadiene monomers, the sorbate monomers 
and the muconate monomers, polymerization takes place either 1,2 or 1,4 
leaving an unsaturated group which is polymerizable by radiation. The 
allyl ester monomers polymerize through the ethylenic unsaturation in the 
acid portion of the monomer leaving the allyl unsaturation intact. 
Polymers formed from the diene monomers and from the allyl monomers can be 
crosslinked by radiation. 
The copolymers are prepared by the free radical polymerization of 
4-hydroxystyrene and the other monomers in solution, emulsion, or 
suspension using well known polymerization techniques. The copolymers 
contain about 10 to about 90 weight percent 4-hydroxystyrene to about 90 
to about 10 weight percent other monomers. Preferred copolymers contain 
about 40 to about 80 weight percent 4-hydroxystyrene to about 60 to about 
20 weight percent other monomers. 
The 4-acetoxystyrene copolymers are converted to 4-hydroxystyrene 
copolymers by acid or base hydrolysis using the procedures described in 
copending applications, Ser. No.: 099,429, filed Sept. 11, 1987, and Ser. 
No.: 099,430, filed Sept. 11, 1987, which applications are hereby 
incorporated by reference. 
The photosensitizers useful in this invention are aromatic azides and 
bisazides. These photosensitizers are selected based on their 
compatibility with the hydroxystyrene copolymers, their solubility in the 
solvents used in the formulations, and their sensitivity to ultraviolet 
(UV) radiation, i.e., the wavelength range of UV light which can be varied 
from near UV to mid UV to deep UV radiation. Near UV refers to ultraviolet 
light having a wavelength ranging from about 310 to about 465 nanometers, 
mid UV refers to ultraviolet light having a wavelength range of from about 
280 to about 310 nanometers and deep UV refers to ultraviolet light having 
a wavelength ranging from about 230 to about 280 nanometers. Such 
photosensitizers are described in U.S. Pat. No. 3,669,669, U.S. Pat. No. 
3,917,794, U.S. Pat. No. 4,329,419 and U.S. Pat. No. 4,469,778, which 
patents are hereby incorporated by reference. Publications which are 
incorporated by reference are "Photoresist Materials and Processes," W. S. 
DeForest, McGraw-Hill Book Company, 1975, Chapter 2, Pages 38-40 and T. 
Iwayanagi, et al., J. Electrochem. Soc., Vol. 127, Page 2759 (1980). 
Examples of photosensitizers useful in this invention are 
2,6-bis(4-azidyl-benzylidene)-cyclohexanone, 
2,6-bis(4-azidylbenzylidene)-4-methylcyclohexanone, 4,4'-diazidodiphenyl 
ether, 4,4'-diazidophenyl sulfide, 4,4'-diazidodiphenyl sulfone, 
3,3'-diazidodiphenyl sufone, 4,4'-diazidodiphenylmethane, 
3,3'-dichloro-4,4'-diazidodiphenylmethane, 4,4'-diazidodiphenyl disulfide, 
4,4'-diazidobibenzyl and 4,4'diazido-stilbene-2,2'-disodium sulfonate. 
The azide photosensitizers are used with the 4-hydroxystyrene copolymers in 
the amount of about 1 to about 5 weight percent based on the weight of 
photosensitizer and copolymer. The preferred range is about 2 to about 3 
weight percent. 
In utilizing the compositions of this invention as photoresist materials, 
the 4-hydroxystyrene copolymers and the photosensitizer are dissolved in a 
solvent, are spin coated on a suitable substrate and are heated to remove 
the solvent. The coating is then exposed to radiation in an image-wise 
configuration, e.g., exposed to U.V. radiation through a photomask, and is 
again heated . The portions of the coating on which the radiation falls 
are crosslinked. When the coating is placed in a suitable solvent, the 
unexposed portions of the coating are dissolved, leaving a negative image 
on the substrate. 
Any solvents which are not photoreactive and which dissolve the 
4-hydroxystyrene copolymers and the photosensitizers can be used in this 
invention. Examples of such solvents are esters, e.g., ethyl acetate, 
butyl acetate and amyl acetate, ketones, e.g., acetone, methylethyl 
ketone, methylisobutyl ketone, alcohols, e.g., 2-ethoxyethanol, 
2-methoxyethanol, 2-methoxypropanol-l and the like. Aqueous solvents such 
as aqueous solutions of quaternary ammonium hydroxide, e.g., tetralkyl 
ammonium hydroxide, can also be used provided the bisazides that are 
soluble in these solvents are used as photosensitizers. Mixtures of such 
solvents can also be used. 
Generally the copolymers and photosensitizers used in this invention are 
dissolved in the solvents at a concentration of about 5 weight percent up 
to about 50 weight percent, preferably, up to about 25 weight percent. The 
so formed solutions are then filtered through filters having pore sizes of 
1 micrometer or less. The solutions are then spun coated on glass, quartz 
or silicon wafers to a coating thickness of about 0.5 to about 3 
micrometers. The coatings are then heated at a temperature of about 
50.degree. to about 100.degree. C. for a time sufficient to remove the 
solvent, generally about 10 minutes to about 1 hour. A photomask is then 
fitted over the wafer and the wafer is exposed to U.V. radiation. The 
coated wafers can then be heated again before developing them in a 
suitable solvent. Preferred solvents are the solvents which are used to 
form solutions of the copolymers and photosensitizers as described 
hereinbefore. Other solvents which will dissolve the unexposed coating but 
not the exposed and crosslinked coating can also be used. The resulting 
negative images can be used as fine line photoresists in the manufacture 
of semiconductor chips, printed circuit boards, microelectronics, and 
other electronic photographic applications, e.g., photolithography and 
graphic arts, printing plates and dies. 
The following examples describe the invention in more detail. Parts and 
percentages, unless otherwise designated, are parts and percentages by 
weight.

EXAMPLE 1 
One part of a copolymer of 4-hydroxystyrene and dimethylmuconate in a molar 
weight ratio of 5:3, 0.03 part of 
2,6-bis-(4-azidyl-benzylidene)-4-methylcyclohexanone, are dissolved in 10 
parts by volume of butyl acetate. The solution is filtered through a 
Millipore 1 micrometerffilter and is spun coated on 2 inch by 2 inch glass 
plates at 1500 rpm for 60 seconds. The coated plates are heated at 
80.degree. C. for 30 minutes in a forced air oven. The coatings are then 
contacted with a chrome photomask, are exposed to ultraviolet radiation 
--37 mw/cm.sup.2 --for 2 seconds and are then dipped in butyl acetate to 
dissolve the unexposed coating. Fine line negative images are formed. 
EXAMPLE 2 
Using the same procedure described in Example 1, coatings are made on glass 
plates using 4-hydroxystyrene (HSM) copolymerized with dimethyl muconate 
(MMu), diethyl muconate (EMu) or methyl sorbate (MSb) in admixture with 
2,6-bis(4-azidyl-benzylidene)-4-methylcyclohexanone (bisazide). After 
photomasking, and radiating with ultraviolet, negative images comparable 
to those described in Example 1 are obtained. The copolymers and amounts 
of components are listed in the Table. 
TABLE 
______________________________________ 
Copolymer Bisazide 
Molar Part by 
Parts by 
Example Monomers Ratio Weight 
Weight 
______________________________________ 
2A HSM:EMu 5:1 1 0.03 
2B HSM:MMu 5:1 1 0.03 
2C HSM:EMu 10:1 1 0.03 
2D HSM:MSb 5:1 1 0.03 
______________________________________ 
EXAMPLE 3 
Using the same procedure described in Example 1, coatings on glass plates 
are made using a copolymer of 4-hydroxystyrene and diallyl maleate in a 
weight ratio of 8 to 2 and 3 weight percent 
2,6-bis-(4-azidyl-benzylidene)-cyclohexanone dissolved in 
2-methoxypropanol-l. After photomasking and radiating with UV, negative 
images comparable to those described in Example 1 are obtained. 
EXAMPLE 4 
Using the same procedure described in Example 1, coatings on glass plates 
are made from a solution in 2-methoxy-propanol-1 of a copolymer of 
4-hydroxystyrene and 2,5-dimethyl-2,4-hexadiene in a weight ratio of 7 to 
3 and 3 weight percent 2,6-bis-(4-azidylbenzylidene)-cyclohexanone. After 
photomasking and radiating with UV, negative images comparable to those 
described in Example 1 are obtained. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrating rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.