Negative-working photosensitive composition comprising a diphenylamine-melamine condensate and an azide compound

The negative-working photosensitive composition of the invention, which is suitable as a photoresist material in the photolithographic processing of semiconductor devices, comprises (a) a condensation product of a hydroxy-substituted diphenylamine compound such as 4-hydroxy diphenylamine and a methylol melamine or alkoxylated methylol melamine by the reaction in a medium of phosphoric or sulfuric acid and (b) an azide compound capable of strongly absorbing UV or far UV light. The composition gives a photoresist layer having high resistance against heat in the post-baking and the attack of gas plasma encountered in the dry etching for semiconductor processing.

BACKGROUND OF THE INVENTION 
The present invention relates to a negative-working photosensitive 
composition or, more particularly, to a negative-working photosensitive 
composition sensitive to ultraviolet and far ultraviolet light and 
suitable for forming a finely patterned photoresist layer having excellent 
heat resistance against heat and dry etching. 
As is well known, the process of fine patterning in the manufacture of 
semiconductor devices such as integrated circuits involves the techniques 
of photolithography, in which a semiconductor silicon wafer having a 
surface film of an oxide or nitride is first coated with a solution of a 
photoresist composition followed by drying to form a photoresist film 
thereon. Thereafter, a photomask bearing a desired pattern is laid on the 
photoresist film which is exposed to actinic rays through the photomask 
followed by a treatment of developing to form a finely patterned 
photoresist layer. The surface film of oxide or nitride in the thus 
uncovered areas is then subjected to etching to have the underlying 
surface of silicon exposed bare which is doped with a dopant by diffusion 
after removal of the photoresist layer. Semiconductor devices are finished 
by repeating the above described procedures to form a desired electric 
circuit with adjunction of necessary parts such as electrodes. 
The photoresist materials used in the above described photolithographic 
process are classified into positive-working ones, of which the solubility 
in a developer solution is increased by exposure to actinic rays so that 
the photoresist layer is dissolved away by the developer solution in the 
exposed areas, and negative-working ones, of which the solubility in a 
developer solution is decreased by exposure to actinic rays so that the 
photoresist layer is dissolved away by the developer solution in the 
unexposed areas. A typical positive-working photoresist material is a 
photosensitive composition comprising a novolac resin and an o-quinone 
diazide compound and typical negative-working photoresist materials 
include cyclized rubber-based photosensitive compositions comprising a 
cyclized rubber as the polymeric base ingredient admixed with a 
photosensitive bisazide compound and poly(vinyl cinnamate)-based 
photosensitive compositions prepared from polyvinyl alcohol and cinnamyl 
chloride. 
Turning now to the procedure of etching in the processing of semiconductor 
devices, it is a trend in recent years that the conventional wet-etching 
process using an etching solution is increasingly being replaced with the 
dry-etching process utilizing gas plasma in view of several advantages 
over the wet process. A problem in the dry-etching process, however, is 
the poor resistance of the negative-working photoresist materials against 
the attack of the gas plasma in the etching in comparison with the 
positive-working ones and none of the conventional negative-working 
photoresist materials can give quite satisfactory results. Although many 
of the negative-working photoresist materials are superior to 
positive-working ones in respect of the heat resistance, the heat 
resistance of conventionally available negative-working photoresist 
materials is still insufficient to comply with the processing conditions 
in the modern manufacturing process of semiconductor devices. 
SUMMARY OF THE INVENTION 
The present invention completed as a result of the investigations 
undertaken in view of the above described prior art problems accordingly 
has an object to provide a photosensitive composition suitable as a 
negative-working photoresist material usable for forming a finely 
patterned photoresist layer capable of exhibiting high heat resistance to 
withstand the thermal conditions in the post-baking treatment and 
excellent resistance against the attack of a plasma gas in the dry 
etching. 
The photosensitive composition of the present invention to achieve the 
above mentioned object comprises: 
(a) a condensation product of a hydroxy-substituted diphenylamine compound 
of the formula 
EQU C.sub.6 H.sub.5 --NH--C.sub.6 H.sub.5-n (OH).sub.n, (I) 
in which n is 1 or 2, and a methylol melamine or alkoxylated methylol 
melamine compound; and 
(b) an azide compound having strong absorption of light in the wave length 
region of ultraviolet or far ultraviolet. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is understood from the above given summary of the invention, the 
polymeric base ingredient in the inventive photosensitive composition is 
the component (a) which is a condensation product of a hydroxy-substituted 
diphenylamine compound and an optionally alkoxylated methylol melamine 
compound. Such a condensation product can readily be prepared by the 
condensation reaction of the reactants in the presence of an acid 
catalyst. 
The hydroxy-substituted diphenylamine compound in conformity with the above 
given formula (I) is exemplified by 2-, 3- and 4-hydroxy diphenylamines, 
2,4- and 3,5-dihydroxy diphenylamines and the like. These diphenylamine 
compounds can be used either singly or as a combination of two kinds or 
more according to need. 
The other reactant to be condensed with the above mentioned 
hydroxy-substituted diphenylamine compound is a methylol melamine or 
alkoxylated methylol melamine compound. The methylol melamine compound can 
be prepared according to a known method by the methylolation reaction of 
melamine with formaldehyde and the alkoxylated methylol melamine compound 
can be obtained by the alkoxylation of a methylol melamine with an alcohol 
having 1 to 4 carbon atoms in a molecule. Accordingly, the optionally 
alkoxylated methylol melamine compound is represented by the general 
formula 
##STR1## 
in which each of the groups denoted by R is, independently from the 
others, a hydrogen atom, a methylol group or an alkoxy-substituted methyl 
group having 1 to 4 carbon atoms in the alkoxy group, at least one of the 
groups denoted by R in a molecule being a methylol group. The methylol 
melamine compound of the general formula (II) can be in the monomeric form 
or in a dimeric or trimeric form. Mixtures of these monomeric and 
oligomeric forms can of course be used. 
The optionally alkoxylated methylol melamine compound in conformity with 
the general formula (II) is exemplified by penta(butoxymethyl) 
monomethylol melamine, tri(methoxymethyl) monomethylol melamine, 
di(methoxymethyl) monomethylol melamine, trimethylol melamine, 
hexamethylol melamine and the like. These methylol melamine compounds can 
be used either singly or as a combination of two kinds or more according 
to need. It is of course optional that the oligomeric form of the methylol 
melamine compound is a mixed condensation product of different kinds of 
these methylol melamine compounds. 
The condensation reaction of the hydroxy-substituted diphenylamine compound 
and the methylol melamine compound is performed in the presence of an acid 
catalyst such as phosphoric acid, sulfuric acid and the like inorganic 
acids or, rather, in these acids as a reaction medium. For example, the 
starting reactants are added to and dissolved in phosphoric or sulfuric 
acid and the reaction mixture is heated at 30.degree. to 100.degree. C. 
for 2 to 100 hours and then poured into a large volume of water to 
precipitate the polymeric material which is collected by filtration, 
washed with water and dried to give the desired condensation product. The 
amounts of the starting reactants used in this condensation reaction is 
usually such that the weight proportion of the melamine compound to the 
hydroxy-containing diphenylamine compound should be in the range from 1:99 
to 60:40 or, preferably, from 25:75 to 55:45. When the amount of the 
melamine compound is too large, the reaction mixture is sometimes gelled 
as the condensation reaction proceeds or, if not gelled, the solubility of 
the resultant condensation product may be somewhat decreased to cause 
disadvantages. When the amount of the melamine compound is too small, on 
the other hand, the condensation product would have an unduly increased 
solubility also to cause some disadvantages although the reaction mixture 
is free from gelation. 
The other essential ingredient in the inventive photosensitive composition 
as the component (b) besides the above described condensation product as 
the component (a) is an azide compound having strong absorption of light 
in the wave length region of ultraviolet or far ultraviolet. Exemplary of 
such an azide compound are 4,4'-diazidodiphenyl ether, 
4,4'-diazidodiphenyl methane, 3,3'-dichloro-4,4'-diazidodiphenyl methane, 
4,4'-diazidodiphenyl sulfide, 4,4'- and 3,3'-diazidodiphenyl sulfones, 
4,4'- and 2,2'-diazidostilbenes, 4,4'-diazidochalcone, 4,4'-diazidobenzal 
acetone, 4,4'-diazidobenzylidene acetone, 2,6-di(4'-azidobenzal) 
cyclohexanone, 2,6-di(4'-azidobenzal)-4-methyl cyclohexanone, 
2,6-di(4'-azidocinnamylidene) cyclohexanone, 4,4'-diazidobiphenyl, 
3,3'-dimethyl-4,4'-diazidobiphenyl, 1-azidonaphthalene, 1-azidoanthracene, 
1-azidophenanthrene, 1-azidopyrene, 1,8-diazidonaphthalene and the like, 
of which 4,4'-diazidodiphenyl ether, 4,4'-diazidodiphenyl methane, 
4,4'-diazidodiphenyl sulfide and 4,4'- and 3,3'-diazidodiphenyl sulfones 
are particularly preferred. The amount of the azide compound as the 
component (b) in the inventive photosensitive composition should usually 
be in the range from 0.5 to 50% by weight based on the condensation 
product as the component (a). 
The photosensitive composition of the invention may be further admixed with 
various kinds of known additives conventionally added to photosensitive 
compositions such as photosensitizers, coloring agents, stabilizers and 
the like according to need. 
The photosensitive composition of the invention is prepared usually and 
preferably in the form of a solution by dissolving the above described 
components (a) and (b) in a suitable organic solvent exemplified by polar 
solvents such as N-methyl-2-pyrrolidone, N-acetyl-2-pyrrolidone, dimethyl 
sulfoxide, dimethyl acetamide and dimethyl formamide as well as mixtures 
thereof with optional admixture of a small amount of another solvent which 
is miscible with the above mentioned polar solvent though without 
dissolving power for the condensation product. 
To describe a practical way of using the inventive photosensitive 
composition in the processing of semiconductor devices, the inventive 
photosensitive composition in the form of a solution is first applied 
uniformly to the surface of a substrate such as a silicon wafer by use of 
a spinner followed by drying to form a photoresist layer which is then 
exposed patternwise to ultraviolet or far ultraviolet light through a 
photomask bearing a desired pattern and developed with a developer 
solution so that the photoresist layer in the unexposed areas is dissolved 
away to leave a negatively patterned photoresist layer with high fidelity 
of the pattern to the pattern on the photomask. 
The developer solution used in the development above mentioned should be an 
aqueous solution containing an inorganic or organic alkaline compound or a 
certain organic solution. The inorganic alkaline compound is exemplified 
by sodium hydroxide, sodium metasilicate, sodium phosphate and the like 
and the organic alkaline compound is exemplified by tetramethyl ammonium 
hydroxide, trimethyl hydroxyethyl ammonium hydroxide and the like. These 
alkaline compounds can be used either singly or as a combination of two 
kinds or more according to need. When the developer solution is organic, 
the organic solvent used to dissolve the alkaline compound may be a polar 
organic solvent such as N-methyl-2-pyrrolidone, N-acetyl-2-pyrrolidone, 
dimethyl sulfoxide, dimethyl acetamide, dimethyl formamide and the like as 
well as solvent mixtures of these polar organic solvents with other 
organic solvents having miscibility therewith. The organic solvents 
miscible with the above mentioned polar organic solvents include acetone, 
methyl ethyl ketone, methyl alcohol, ethyl alcohol, isopropyl alcohol, 
ethyleneglycol monomethyl ether, ethyleneglycol monoethyl ether, 
ethyleneglycol monomethyl ether acetate, ethyleneglycol monoethyl ether 
acetate and the like. The addition of these solvents to the polar organic 
solvent is sometimes preferable since such a mixed solvent is effective in 
adequately controlling the dissolution of the photoresist layer not to be 
dissolved away in the areas exposed to light. 
The above described negative-working photosensitive composition is suitable 
for use as a photoresist material capable of forming a very finely 
patterned photoresist layer having markedly improved resistance against 
heat encountered in the post-baking treatment and the attack of gas plasma 
in the process of dry etching in comparison with conventional 
negative-working photosensitive compositions. 
In the following, the negative-working photosensitive compositions of the 
invention are described in more detail by way of examples.

EXAMPLE 1 
A condensation reaction was performed by admixing a solution of 585 g of 
4-hydroxy diphenylamine in 2540 g of 85% phosphoric acid with an aqueous 
solution of 390 g of trimethylol melamine dissolved in 390 g of water 
followed by heating the reaction mixture at 50.degree. C. for 72 hours. 
After completion of the reaction, the reaction mixture was poured into 15 
liters of water and the precipitates were collected by filtration, 
thoroughly washed with water and dried to give the condensation product in 
a powdery from. 
A photosensitive composition was prepared by dissolving 30 parts by weight 
of the thus obtained condensation product and 6 parts by weight of 
4,4'-diazidodiphenyl sulfide in 100 parts by weight of N,N-dimethyl 
acetamide. 
The photosensitive composition in the form of a solution was applied to the 
surface of a semiconductor silicon wafer of 3 inch diameter on a spinner 
at 1500 rpm for 40 seconds and then the silicon wafer was prebaked on a 
hot plate at 110.degree. C. for 5 minutes to give a uniform photoresist 
layer having a thickness of 1.06 .mu.m. The photoresist layer on the 
silicon wafer was exposed to ultraviolet by hard-contacting of a photomask 
and developed with a developer solution, which was a 2.38% by weight 
aqueous solution of tetramethyl ammonium hydroxide, at 23.degree. C. for 1 
minute followed by rinse in pure water for 30 seconds to give a negatively 
patterned photoresist layer. 
The silicon wafer thus provided with the patterned photoresist layer was 
subjected to post-baking at 200.degree. C. for 5 minutes to find that the 
photoresist layer was stable without showing noticeable deformation in the 
cross sectional form of the patterned lines. 
The silicon wafer thus provided with the patterned photoresist layer after 
the post-baking treatment was then subjected to dry etching with plasma of 
a 94:6 by volume mixture of carbon tetrafluoride and oxygen gases under a 
pressure of 0.4 Torr for 1 minute with an electric output of 200 watts 
keeping the table at 100.degree. C. to find that the decrease in the 
thickness of the photoresist layer was only about 30 nm. 
COMATIVE EXAMPLE 1 
A silicon wafer was coated on a spinner with a commercial product of a 
cyclized rubber-based photoresist composition (OMR-83, a product by Tokyo 
Ohka Kogyo Co.) and prebaked for 90 seconds on a hot plate kept at 
110.degree. C. to form a photoresist layer having a thickness of 1.0 
.mu.m. Exposure of the photoresist layer to ultraviolet and subsequent 
development were undertaken in substantially the same manner as in Example 
1 except that the developer solution and the rinse liquid were each a 
commercial product recommended by the manufacturer of the photoresist 
composition to give a negatively patterned photoresist layer. 
The silicon wafer provided with the patterned photoresist layer was 
post-baked for 5 minutes on a hot plate kept at 170.degree. C. to find 
some dullness in the shoulder portions in the cross section of the 
patterned photoresist lines. 
COMATIVE EXAMPLE 2 
A silicon wafer was coated with a commercial product of a positive-working 
photoresist composition (OFPR-800, a product by Tokyo Ohka Kogyo Co.) on a 
spinner and pre-baked for 90 seconds on a hot plate kept at 110.degree. C. 
to form a photoresist layer having a thickness of 1.3 .mu.m. The 
photoresist layer on the silicon wafer was subjected to dry etching in the 
same manner as in Example 1 to find that the thickness of the photoresist 
layer was decreased by about 100 nm. 
EXAMPLE 2 
A condensation reaction was performed by admixing a solution of 160 g of 
4-hydroxy diphenylamine and 20 g of 3-hydroxy diphenylamine in 730 g of 
85% phosphoric acid with 100 g of trimethylol melamine and heating the 
mixture at 50.degree. C. for 6 hours. After completion of the reaction, 
the reaction mixture was poured into 4 liters of water and the 
precipitates were collected by filtration, thoroughly washed with water 
and dried to give the condensation product in a powdery form. 
A photosensitive composition was prepared by dissolving 30 parts by weight 
of the thus obtained condensation product and 5 parts by weight of 
4,4'-diazidodiphenyl sulfone in 100 parts by weight of N,N-dimethyl 
acetamide. 
A semiconductor silicon wafer of 3 inch diameter was coated with this 
photosensitive composition to form a photoresist layer having a thickness 
of 1.0 .mu.m followed by a pre-baking treatment, exposure to light and 
development under the same conditions as in Example 1 excepting extension 
of the development time to 75 seconds to give a negatively patterned 
photoresist layer. 
A post-baking treatment of this silicon wafer provided with the patterned 
photoresist layer indicated substantially no deformation in the cross 
section of the patterned lines of the photoresist layer after 5 minutes on 
a hot plate kept at 200.degree. C. Further, the silicon wafer was 
subjected to dry etching in the same manner as in Example 1 to find that 
the decrease in the thickness of the photoresist layer was about 40 nm. 
EXAMPLE 3 
A condensation reaction was performed by admixing a solution of 200 g of 
4-hydroxy diphenylamine and 50 g of 3,5-dihydroxy diphenylamine in 1000 g 
of 98% sulfuric acid with 140 g of di(methoxymethyl) monomethylol melamine 
and heating the mixture at 60.degree. C. for 60 hours. After completion of 
the reaction, the reaction mixture was poured into 5 liters of water and 
the precipitates were collected by filtration, thoroughly washed with 
water and dried to give the condensation product in a powdery form. 
A photosensitive composition was prepared by dissolving 30 parts by weight 
of the thus obtained condensation product and 10 parts by weight of 
4,4'-diazidodiphenyl sulfide in 100 parts by weight of N,N-dimethyl 
acetamide. 
A semiconductor silicon wafer of 3 inch diameter was coated with this 
photosensitive composition to form a photoresist layer having a thickness 
of 1.1 .mu.m followed by a pre-baking treatment, exposure to light and 
development under the same conditions as in Example 1 except that the 
development was performed for 45 seconds with a 1% by weight aqueous 
solution of sodium hydroxide as the developer solution to give a 
negatively patterned photoresist layer. 
A post-baking treatment of this silicon wafer provided with the patterned 
photoresist layer indicated substantially no deformation in the cross 
section of the patterned lines of the photoresist layer after 5 minutes on 
a hot plate kept at 200.degree. C. Further, the silicon wafer was 
subjected to dry etching in the same manner as in Example 1 to find that 
the decrease in the thickness of the photoresist layer was about 40 nm. 
EXAMPLE 4 
A condensation reaction was performed by admixing a solution of 50 g of 
2-hydroxy diphenylamine and 150 g of 2,4-dihydroxy diphenylamine in 860 g 
of 85% phosphoric acid with 140 g of trimethylol melamine and heating the 
mixture at 50.degree. C. for 6 hours. After completion of the reaction, 
the reaction mixture was poured into 4 liters of water and the 
precipitates were collected by filtration, thoroughly washed with water 
and dried to give the condensation product in a powdery form. 
A photosensitive composition was prepared by dissolving 30 parts by weight 
of the thus obtained condensation product and 5 parts by weight of 
4,4'-diazidodiphenyl sulfide in 100 parts by weight of N,N-dimethyl 
acetamide. 
A semiconductor silicon wafer of 3 inch diameter was coated with this 
photosensitive composition to form a photoresist layer having a thickness 
of 1.0 .mu.m followed by a pre-baking treatment, exposure to light and 
development under the same conditions as in Example 1 to give a negatively 
patterned photoresist layer. 
A post-baking treatment of this silicon wafer provided with the patterned 
photoresist layer indicated substantially no deformation in the cross 
section of the patterned lines of the photoresist layer after 5 minutes on 
a hot plate kept at 200.degree. C. Further, the silicon wafer was 
subjected to dry etching in the same manner as in Example 1 to find that 
the decrease in the thickness of the photoresist layer was about 40 nm.