Negative photoresist composition

Abstract: A negative photoresist composition suitable for use in far ultraviolet lithographies and particularly KrF excimer laser lithography, excellent in properties such as resolution, sensitivity, film thickness retention, profile, heat resistance, etc., and small in standing wave, which comprises a resin obtainable through a copolymerization of p-hydroxystyrene and styrene, a crosslinking agent, which is a melamine derivative, represented by the general formula (II): wherein Z represents -NRR' and the like and R, R' and R₁ to R₄ each represents -CH₂OR a and the like in which R a is lower alkyl group, and a photo-induced acid precursor represented by the general formula (III): wherein Y₃ to Y₅ each represents mono-, di- or tri-halogen-substituted alkyl group or the like.

Description: The present invention relates to a resist composition usable as a pattern-forming material in the production of semiconductors, and more particularly to a negative photoresist composition suitable for use in lithographies using far ultraviolet rays including excimer laser. Recently, with a rise in the integration level of integrated circuits, formation of patterns of sub-micron order is required. Particularly, excimer laser lithography using excimer laser as a light source makes it possible to produce 64 and 256 MDRAMs. As a result of such an alteration of light source, the following properties are demanded of the today's resists in addition to the hitherto required properties such as heat resistance, or film thickness retention: (1) a high sensitivity to the light sources mentioned above, (2) a high resolution, (3) a good profile, and (4) a small standing wave. Prior negative photoresist compositions cannot be said to be satisfactory from the above-mentioned viewpoints. For example, JP-A-1-293339 (1989) discloses a photoresist composition comprising a polymer containing a group represented by the following formula (I): wherein R 1 represents a hydrogen atom or an alkyl group having from 1 to 5 carbons atoms, a crosslinking agent containing a group represented by the following formula: wherein R 2 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and a photo-induced acid precursor containing a sulfonic acid ester group. If a pattern is formed from this composition, however, the profile is not good or the film thickness retention is low and a pattern of 0.4 µm or less cannot sufficiently be resolved. Thus, it is the object of the present invention to solve the problems mentioned above and to provide a negative photoresist composition suitable for use in far ultraviolet lithographies and particularly KrF excimer laser lithography, excellent in properties such as resolution, sensitivity, heat resistance, profile or film thickness retention, and small in standing wave. This object has been achieved by a negative photoresist composition comprising 20 to 90 % by weight of a copolymer of p-hydroxystyrene and styrene having a molar ratio of p-hydroxystyrene to styrene of from 50/50 to 95/5, 5 to 50% by weight of a crosslinking agent represented by the following general formula (II): wherein Z represents -NRR' or an optionally substituted aryl group, and R, R' and R 1 to R 4 independently of one another each represents a hydrogen atom, a -CH 2 OH group or a -CH 2 OR a group in which R a is a lower alkyl group, and 0.1 to 20% by weight of a photo-induced acid precursor represented by the following general formula (III): wherein Y 3 , Y 4 , and Y 5 independently of one another each represents an optionally substituted alkyl, alkenyl, aralkyl or piperidino group, a -N(Y 6 )(Y 7 ) group, an -OY 8 group or a -SY 9 group in which Y 6 to Y 9 independently of one another each represents a hydrogen atom or an optionally substituted alkyi or aryl group, provided that at least one of Y 3 , Y 4 and Y 5 represents mono-, di- or tri-halogen-substituted alkyl group, or 2-phenyl-4,6-bis (trichloromethyl)-s-triazine, 2-(p-acethylphenyl)-4,6-bis(trichloromethyl-s-triazine or 2-(p-chlorophenyl)4,6-bis(trichloromethyl)-s-triazine. The above-mentioned resin can easily be produced by a radical copolymerization or ion copolymerization of p-hydroxystyrene and styrene. The molar ratio of p-hydroxystyrene and styrene in the copolymer is from about 50/50 to about 95/5, and preferably from about 70/30 to about 85/15. If the proportion of p-hydroxystyrene is smaller than the above, the copolymer is difficult to dissolve in solvent at the time of preparing a resist solution or difficult to dissolve in aqueous alkali solution at the time of development. On the other hand, if the proportion of p-hydroxystyrene is greater than the above, the copolymer is too readily soluble in aqueous alkali solution, so that no fine pattern can be formed therefrom. A weight average molecular weight of the resin is usually from about 1,000 to about 5,000, preferably from about 2,500 to about 4,000. In the crosslinking agent represented by the general formula (II), preferable examples of the lower alkyl group represented by R a include straight chain and branched chain alkyl groups having 1 to 7 carbon atoms among which methyl, ethyl, n-propyl and n-butyl are preferable and methyl and ethyl are more preferable, and examples of the optionally substituted aryl group include optionally substituted phenyl or naphtyl. Among the crosslinking agents represented by the general formula (II), melamine and benzoguanamine are readily available commercially, and their methylol derivatives are obtainable through a condensation reaction of melamine or benzoguanamine with formalin. Their methylol ether derivatives can be obtained by reacting a methylol derivative with an alcohol according to known methods. In the general formula (III), examples of the alkyl group represented by Y 3 to Y 5 include straight chain and branched chain alkyl groups having 1 to 7 carbon atoms, among which methyl, ethyl, n-propyl, i-propyl, n-butyl and t-butyl are preferable. As the substituent present on the alkyl group, halogen atoms can be referred to, for example. Preferable examples of the alkenyl group represented by Y 3 to Y 5 include straight chain and branched chain alkenyl groups having 2 to 7 carbon atoms. As the substituent present on the alkenyl group, aryl groups such as phenyl or naphthyl and heterocyclic groups such as benzoxazolyl or benzothiazolyl can be referred to, for example. These aryl groups and heterocyclic groups may be additionally substituted by halogen atoms, alkoxy groups having 1 to 7 carbon atoms, an acetyl group or a nitro group. Preferable examples of the aralkyl group represented by Y 3 to Y 5 include phenylalkyl groups having 7 to 10 carbon atoms. As preferable substituents present on the phenyl moiety of these aralkyl groups, lower alkyl groups, lower alkoxy groups, halogen atom, nitro group, cyano group, dimethylamino group and diethylamino group can be referred to. Preferable examples of the optionally substituted alkyl group represented by Y 6 to Y 9 include straight chain and branched chain alkyl groups having 1 to 6 carbon atoms, among which methyl, ethyl, n-propyl, and n-butyl are more preferable. Preferable examples of the optionally substituted aryl group represented by Y 6 to Y 9 include a phenyl or naphthyl group among which the phenyl group is more preferable. The photo-induced acid precursor represented by the formula (III) can be produced by, for example, reacting the corresponding nitrile compound with aluminum bromide in the presence of hydrogen chloride or by using the corresponding imidate compound, according to Bull. Chem. Soc. Japan, 42 , 2924 (1969); US-A- 3,987,037; or F. C. Schaefer et. al, J. Org. Chem. 29 , 1527 (1964). Examples of the photo-induced acid precursor represented by the formula (III) include the compound disclosed in the papers mentioned above and the compound disclosed in GB-B- 1,388,492 and JP-A-53-133428 (1978). Among these compounds, preferable are 2,4,6-tris(trichloromethyl)-s-triazine, 2-phenyl-4,6-bis(trichloromethyl)-s-triazine, 2,4,6-tris(dibromopropyl)-s-triazine, 2-(p-acetylphenyl)-4,6-bis(trichloromethyl)-s-triazine, 2-(p-chlorophenyl)-4,6-bis(trichloromethyl)-s-triazine and 2-(p-methoxyphenyl)-4,6-bis(trichloromethyl)-s-triazine. These photo-induced acid precursors may be used either singly or in the form of a mixture of two or more members. In the negative photoresist composition of the present invention, the above-mentioned resin is used usually in an amount of from 20 to 90% by weight (preferably in an amount of from 40 to 80% by weight), the crosslinking agent is used usually in an amount of from 5 to 50% by weight (preferably in an amount of from 5 to 20% by weight) and the photo-induced acid precursor is used usually in an amount of from 0.1 to 20% by weight (preferably in an amount of from 1 to 20% by weight), respectively. If necessary, the negative photoresist composition of the present invention may contain a sensitizer, and a dye. Generally speaking, a negative resist solution is prepared by mixing the above-mentioned resin, a crosslinking agent, and a photo-induced acid precursor into a solvent. These ingredients are used in an amount of from 1 to 50% by weight in the resulting negative resist solution. The solvent is preferably one that evaporates at an appropriate drying rate to give a uniform and smooth coating film. Examples of such solvent include methyl cellosolve acetate, ethyl cellosolve acetate, methyl cellosolve, ethyl cellosolve, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, butyl acetate, methyl isobutyl ketone, xylene, ethyl lactate, ethyl pyruvate and 2-heptanone. These solvents are used either singly or in the form of a mixture of two or more members. Next, the present invention will be explained by the following examples. In examples, "parts" are by weight. Example 1 50 Parts of diethylene glycol dimethyl ether were mixed with 13.33 parts of a resin [LYNCUR CST-70, manufactured by Maruzen Sekiyukagaku K. K., weight average molecular weight 2,900, p-hydroxystyrene/styrene molar ratio in copolymer 70/30], 1 part of hexamethylolmelamine hexamethyl ether as a crosslinking agent and 3 parts of tris(2,3-dibromopropyl)-1,3,5-triazine (a reagent manufactured by Tokyo Kasei Kogyo K. K.] as a photo-induced acid precursor. The resulting mixture was filtered through a tetrafluoroethylene filter having a pore size of 0.2 µm to obtain a resist solution. Using a spinner, the resist solution was coated on a silicon wafer which had been rinsed in a usual manner to form a resist film of 0.7 µm in thickness. Subsequently, the silicon wafer was pre-baked in an oven at 100°C for one minute, and the coated film on the wafer was exposed to light by means of KrF Excimer Laser Stepper (manufactured by Nikon Co., NSR1755EX NA=0.45) having an exposure wavelength of 248 nm. After the exposure, the wafer was heated on a hot plate at 100°C for one minute to make progress a crosslinking reaction of the exposed area. Then, the wafer was developed with a 2.0% (by weight) aqueous solution of tetramethylammonium hydroxide to obtain a negative pattern. An observation by means of an electron microscope revealed that a 0.25 µm fine pattern was resolved with a good profile, provided that exposure dose was 60 mJ/cm 2 (effective sensitivity). Example 2 A negative pattern was obtained by repeating Example 1, except that the resin was altered to LYNCUR CST-50 (manufactured by Maruzen Sekiyukagaku K. K., weight average molecular weight 3,600, p-hydroxystyrene/styrene ratio in copolymer 50/50). A 0.25 µm fine pattern was resolved with a good profile, provided that exposure dose was 80 mJ/cm 2 (effective sensitivity). Comparative Example 1 The procedure of Example 1 was repeated, except that the resin was altered to 13.33 parts of poly(p-hydroxystyrene) (MARUKA LYNCUR M, manufactured by Maruzen Sekiyukagaku K. K., weight average molecular weight 4,100). As a result, a sensitivity of 35 mJ/cm 2 was obtained. However, a 0.25 µm pattern could not be resolved.

Claims: (Amended) A negative photoresist composition comprising 20 to 90% by weight of a copolymer of p-hydroxystyrene and styrene having a molar ratio of p-hydroxystyrene to styrene of from 50/50 to 95/5, 5 to 50% by weight of a crosslinking agent represented by the formula (II): wherein Z represents -NRR' or an optionally substituted aryl group, and R, R', R 1 , R 2 , R 3 and R 4 independently of one another each represents a hydrogen atom, a -CH 2 OH group or a -CH 2 OR a group in which R a is a lower alkyl group, and 0.1 to 20% by weight of a photo-induced acid precursor represented by the formula (III): wherein Y 3 , Y 4 and Y 5 independently of one another each represents an optionally substituted alkyl, alkenyl, aralkyl or piperidino group, a -N(Y 6 )(Y 7 ) group , an -OY 8 group or a -SY 9 group in which Y 6 , Y 7 , Y 8 and Y 9 independently of one another each represents a hydrogen atom or an optionally substituted alkyl or aryl group, provided that at least one of Y 3 , Y 4 and Y 5 represents a mono-, di- or tri-halogen-substituted alkyl group , or 2-phenyl-4,6-bis(trichloromethyl)-s-triazine, 2-(p-acetylphenyl)-4,6 -bis(trichloromethyl)-s-triazine or 2-(p-chlorophenyl)4,6-bis(trichloromethyl)-s-triazine . The negative photoresist composition according to claim 1, wherein the molar ratio of p-hydroxystyrene to styrene in the copolymer is from 70/30 to 85/15. The negative photoresist composition according to claim 1, wherein the weight average molecular weight of the copolymer is from 1,000 to 5,000. The negative photoresist composition according to claim 1, wherein the weight average molecular weight of the copolymer is from 2,500 to 4,000. The negative photoresist composition according to claim 1, wherein the lower alkyl group represented by R a is selected from the group consisting of methyl, ethyl, n-propyl and n-butyl. The negative photoresist composition according to claim 1, wherein the crosslinking agent is melamine or benzoguanamine. The negative photoresist composition according to claim 1, wherein the alkyl group represented by Y 3 , Y 4 and Y 5 is a straight or branched chain alkyl group having 1 to 7 carbon atoms that may be substituted with a halogen atom. The negative photoresist composition according to claim 1, wherein the alkyl group represented by Y 3 , Y 4 and Y 5 is selected from the group consisting of methyl, ethyl, n-propyl, i-propyl, n-butyl and t-butyl groups that may be substituted with a halogen atom. The negative photoresist composition according to claim 1, wherein the alkenyl group represented by Y 3 , Y 4 and Y 5 is a straight or branched chain alkenyl group having 2 to 7 carbon-atoms that may be substituted with an aryl group or heterocyclic group. The negative photoresist composition according to claim 1, wherein the aralkyl group represented by Y 3 , Y 4 and Y 5 is a phenylalkyl group having 7 to 10 carbon atoms. The negative photoresist composition according to claim 1, wherein the optionally substituted alkyl group represented by Y 6 to Y 9 is a straight or branched chain alkyl group having 1 to 6 carbon atoms. The negative photoresist composition according to claim 11, wherein the optionally substituted alkyl group represented by Y 6 to Y 9 is selected from the group consisting of methyl, ethyl, n-propyl and n-butyl groups. The negative photoresist composition according to claim 1, wherein the optionally substituted aryl group represented by Y 6 to Y 9 is a phenyl or naphthyl group. The negative photoresist composition according to claim 1, wherein the composition comprises 40 to 80% by weight of the copolymer, 5 to 20% by weight of the crosslinking agent and 1 to 20% by weight of the photo-induced acid precursor.

Inventors: KUSUMOTO TAKEHIRO, NAKANO YUKO, TAKEYAMA NAOKI, UEDA YUJI, UEKI HIROMI, KUSUMOTO, TAKEHIRO, NAKANO, YUKO, TAKEYAMA, NAOKI, UEDA, YUJI, UEKI, HIROMI
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY LIMITED
Application year: 1993
Main classification: G03F7
IPC codes: G03F7, H01L21