Pattern forming method

Abstract: A photosensitive resin composition for far-ultraviolet exposure of from 170 to 220 nm, which comprises an N-hydroxymaleinimide-type sulfonate photo acid generator having a specific structure and a resin having a substituent capable of increasing the solubility to an alkali developer by an acid, and a pattern forming method using the photosensitive resin composition. The photosensitive resin composition has a high transparency to light having a wavelength of from 170 to 220 nm and is excellent in the photocomposition property and the acid generating efficiency.

Description: The present invention relates to a pattern-forming method employing a photosensitive resin composition suitable for exposure to light of a very short wavelength of from 170 to 220 nm. In the field of producing various kinds of electronic devices requiring fine working such as a semiconductor element, the demand for increasing the density and the integration of devices has been increased more and more. With the demand, the required performance for the photographic techniques for realizing fining of patterns becomes extremely severe. The technique of playing a part of the fining techniques is the increase of the high resolving power of a photoresist and also is shortening the wavelength of the exposure light. In general, the resolution (Res) in an optical system can be expressed by the Rayleigh's equation, that is Res = k•λ/NA (wherein k is a process factor, is a wavelength of the exposure light source, and NA is a numerical aperture of a lens). From the equation, it can be seen that for more fining the reproduced line width and resolving fine patterns (that is, for obtaining a high resolving power), the wavelength at exposure may be shortened. Certainly, with the reduction of the smallest reproduced line width, the exposure wavelength is shifted to the g ray (436 nm) and the i ray (365 nm) of a high-pressure mercury lamp, and further, the production of devices using a KrF excimer laser (248 nm) has been investigated. Also, for finer working, the utilization of shorter excimer laser, in particular, ArF excimer laser (193 nm) has been promising. On the other hand, when a photoresist which is exposed with the light of a short wavelength is watched, the increase of the high integration by not a single layer resist conventionally produced but by double layer or more multilayer resist system utilizing a surface lithography has been investigated. However, there is yet a problem about the complication of process which has hitherto hindered the practical use of a multilayer resist. Also, in the case of an excimer laser such as a KrF excimer laser, because the life of the gas is short and the exposure apparatus itself is expensive, it is generally considered to be necessary to increase the cost performance of laser. In response to the necessity, there is a so-called chemical amplification-type resist which becomes the main current from the use for the KrF excimer laser exposure. The chemical amplification resist has a mechanism that an acid is generated from a photo acid generator existing in the system in a catalytic amount by a light exposure, by the catalytic amount of the acid, the protective group of an alkali-soluble group of a binder or a low molecular compound is released as in catalytic reaction and the discrimination of the solubility to an alkali developer is secured. Because the chemical amplification-type resist catalytic reactively utilizes the acid generated by a photoreaction, the increase of the sensitivity can be expected. The greater part of the photoresists for the excimer laser exposure under development at present are the chemical amplification-type resists and this is also applied to the application for an ArF excimer laser having an exposure wavelength of 220 nm or shorter. Now, the largest theme in the photoresist system corresponding to a stepper having exposure wavelengths of from 170 to 220 nm aimed at by the present invention is securing of the transparency of the resist film to the wavelength of the exposed light. The quinonediazido series compound which is used in the case of the g ray (436 nm) and the i ray (365 nm) of a high-pressure mercury lamp as the exposure light has a property of reducing the absorption to the wavelength of the exposed light by the light exposure, that is, has a light bleaching property, and thus the absorption by the film before the light exposure does not give a problem as at present. On the other hand, in the case of the chemical amplification-type resist, because the resist does not have such a light bleaching property, when the light absorption before light exposure is high, a large difference of the light intensity occurs between the surface of the film and the bottom portion of the resist and as the result thereof, a high resolving property is not obtained. Thus, in the case of the chemical amplification-type resist, the transparency of the resist film before light exposure is regarded as important. In the case of planning a transparent material to a light having a wavelength of 220 nm or shorter, first, it is said that the use of aromatic compounds conventionally used as materials for positive resists is difficult. For example, even novolac resins and further polyhydroxystyrene which are conventionally frequently used as binders have a high absorption to the light of the wavelength of 220 nm or lower. Accordingly, for a lithography utilizing the exposing light wavelength of 220 nm or shorter, these materials cannot be used. The above thing is also applied to the other material, the photo acid generator. For example, in SPIE Proceedings, pages 422-431, Vol.2438 (1995), it is reported that the added amount of the triphenylsulfonium salt-type light acid generating agent, which is frequently used in the resists for the KrF excimer laser, is limited because of a large light absorption in a vacuum ultraviolet region. On the other hand, it is reported that among the aromatic series compounds, naphthalene has a very small light absorption to the ArF excimer laser (exposure wavelength: 193 nm), e.g., in J. Photopolym. Sci. Technol., pages 423-432, Vol. 7 (1994), and SPIE Proceedings, pages 445-454, Vol. 2438 (1995). That is, in the aromatic series materials which are generally considered to be unusable in the lithography utilizing the light for exposure having a wavelength of 220 nm or shorter, it can be said that usable materials exist. Also, in regard to the photo acid generators not having an aromatic substituent in the molecule and the small absorbance to the light having a wavelength of 220 nm or shorter, for example, the photo acid generator represented by following formula is reported in JP-A-7-199467, JP-A-7-252324, and JP-A-8-12626 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application"); However, the photo acid generator is unsatisfactory on the photodecomposition property, i.e., the 'acid generating efficiency, which is another important property for the photo acid generator. As described above, in the lithography utilizing the light for exposure having a wavelength of 220 nm or shorter, such as the ArF excimer laser, systematic investigations of the photo acid generator have not yet been made and thus it is very difficult to anticipate the relation of the structure and the light absorption. Furthermore, it can be said that there was no knowledge about the transparency and photodecomposition property or the acid generating efficiency to exposed light. That is, the means of obtaining a photo acid generator having a high transparency to an exposure light, satisfying the photodecomposition property or the acid generating efficiency to the exposure light, and being usable in the lithography utilizing the exposure light having a wavelength of from 170 to 220 nm, which is aimed in the present invention, have hitherto been entirely not known. Accordingly, an object of the present invention is to provide a pattern forming method employing a photosensitive resin composition for far-ultraviolet exposure having a high transparency to an exposure light within a wavelength range of from 170 to 220 nm, satisfactory photodecomposition property and acid generating efficiency to the exposure light, and being usable in lithography utilizing the exposure light having a wavelength of from 170 to 220 nm. As the result of various investigations taking into consideration of the above-described various characteristics, the inventors have discovered that the above objects can be achieved by a pattern forming method which comprises: forming a thin film of a photosensitive resin composition for far-ultraviolet exposure of from 170 to 220 nm on a substrate, exposing the thin film with light from an ArF excimer laser having a wavelength within the range of from 170 to 220 nm, and developing the exposed thin film, said photosensitive resin composition comprising an N-hydroxymaleinimide-type sulfonate photo acid generator represented by following formula (I) and a resin having a substituent capable of increasing the solubility in an alkali developer by an acid; wherein R 1 and R 2 , which may be the same or 'different, each represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, or a cycloalkyl group having not more than 6 carbon atoms; R 1 and R 2 may combine via an alkylene chain to form a ring; and R 3 represents an alkyl group-, a perfluoroalkyl group, a cyclic alkyl group, or a camphor group, N-(trifluoromethylsulfonyloxy)-bicycle[2.2.1]-heptene-2,3-dicarboxyimide being excluded. The present invention is described in detail below. The groups represented by R 1 and R 2 in formula (I) are described below. Examples of the alkyl group having from 1 to 6 carbon atoms include methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, and n-hexyl. In these alkyl groups, methyl, ethyl, and propyl are preferred and methyl and ethyl are more preferred. Examples of the cycloalkyl group having not more than 6 carbon atoms include cyclopropyl, cyclopentyl and cyclohexyl. In these groups, cyclopentyl and cyclohexyl are preferred. As the case of forming a ring by combining R 1 and R 2 via an alkylene chain, there are the cases of forming, for example, a cyclohexyl group, a norbornyl group, and a tricyclodecanyl group. The group represented by R 3 is described below. Examples of the alkyl group include a straight chain alkyl group having from 1 to 20 carbon atoms such as methyl, ethyl and propyl, and a branched alkyl group having from 1 to 20 carbon atoms such as isopropyl, isobutyl, tert-butyl and neopentyl. In these alkyl groups, straight chain or branched alkyl groups having from 1 to 16 carbon atoms are preferred and straight chain or branched alkyl groups having from 4 to 15 carbon atoms are more preferred. Examples of the perfluoroalkyl group includes a straight chain perfluoroalkyl group having from 1 to 20 carbon atoms such as trifluoromethyl and pentafluoroethyl, and a branched perfluoroalkyl group having from 1 to 20 carbon atoms such as heptafluoroisopropyl and nonafluoro tert-butyl. In these groups, straight chain or branched perfluoroalkyl groups having from 1 to 16 carbon atoms are preferred. Examples of the cyclic alkyl group includes a single cyclic alkyl group such as cyclopentyl and cyclohexyl and a plural cyclic alkyl group such as decalyl, norbornyl and tricyclodecanyl. The camphor group as R 3 means the group as shown in Compound I-9 exemplified later. Examples of the compound represented by formula (I) are shown below but the compounds of formula (I) are not limited to these compounds. The compound represented by formula (I) described above can be synthesized by reacting the N-hydroxyimide compound synthesized according to the method of G.F. Jaubert, Ber., 28, 360(1895), the method of D.E. Ames et al., J. Chem. Soc., 3518 (1955), or the method of M.A. Stolberg, et al., J. Am. Chem. Soc., 79 , 2615(1957) and sulfonic acid chloride under a basic condition, for example, according to the method of L. Bauer, et al., J. Org. Chem. , 24 , 1294, (1959). The content of the compound represented by formula (I) thus synthesized in the photosensitive resin composition is preferably from 0.1 to 20% by weight, more preferably from 0.5 to 15% by weight, and most preferably from 1 to 10% by weight based on the total solid components. The resin having a substituent capable of increasing the solubility in an alkali developer by an acid is explained below. As the fundamental properties of the resin, the following properties are required. (1) Transparency to the wavelength of the exposed light. (2) Dry etching resistance. (3) Solubility to a coating solvent. (4) Appropriate glass transition temperature. As a binder resin for a resist, a novolac resin and a styrene series resin such as parahydroxtstyrene, have hitherto been used. The reason is because the aromatic ring contributes to a high dry etching resistance. However, since such resins have a high absorption to the light for exposure to be used in the present invention, which is originated in the aromatic ring, these materials can not be used. That is, the dry etching resistance must be ensured using non-aromatic series materials. The above-described properties (1) and (2) can be attained by skillfully using a compound having an alicyclic alkyl group at the side chains. The resins having a substituent capable of increasing the solubility to an alkali developer by an acid are shown below. The ratio of each monomer unit is by mole. wherein, R 11 , R 13 , R 15 , and R 16 , which may be the same or different, each represents a hydrogen atom or a methyl group; R 12 represents R 14 represents R 17 represents COOCH 3 , COOC 2 H 5 , or CN; v represents from 0.3 to 0.7; x represents from 0.2 to 0.6; y represents from 0.05 to 0.3; z represents from 0 to 0.2; and v + x + y + z = 1. v, x, y and z indicate the molar ratio of each monomer unit. Also, the resins each having introduced into the linking portions of the side chains an alicyclic compound as described below can be used as the resin having a substituent capable of increasing the solubility to an alkali developer by an acid in the present invention. wherein, R 18 , R 20 , and R 21 , which may be the same or different, each represents a hydrogen atom or a methyl group; R 19 represents and A 1 represents wherein R 22 , R 24 , R 25 , and R 27 , which may be the same or different, each represents a hydrogen atom or a methyl group; R 23 and R 26 , which may be the same or different, each represents a tert-butoxycarbonyl group, a tetrahydrofuran-2-yl group, a 4-methoxytetrahydropyranyl group, a 3-oxocyclohexyl group, a 1-methoxyethyl group, a 1-ethoxyethyl group, a 1-isopropoxyethyl group, a 1-(n-butoxy)ethyl group, or a 1-(tert-butoxy)ethyl group; and A 2 represents Furthermore, the resins each having introduced into the main chain portion an alicyclic compound can be used as the resin having a substituent capable of increasing the solubility to an alkali developer by an acid in the present invention, For example, there are resins having the following monomer unit The above-described resins can be prepared by subjecting suitable monomers to radical polymerization in the presence of a radical polymerization initiator such as peroxides and azo type compounds. It is preferred that the resins have a weight average molecular weight of 2,000 to 200,000. If the weight average molecular weight is less than 2,000, the resistance to dry etching and heat may be deteriorated. On the other hand, if it exceeds 200,000, the developability or film-forming properties may be deteriorated. The deterioration of film-forming properties- appears to be caused by the high viscosity. In the present invention, the content of the resin having a substituent capable of increasing the solubility to an alkali developer by an acid in the composition is preferably from 40% by weight to 98% by weight and more preferably from 50% by weight to 95% by weight based on the total solid components. To the photosensitive resin composition employed in the method of the present invention may be added an appropriate alkali-soluble low-molecular compound for the purpose of improving the alkali solubility of the system and for the purposes of controlling the glass transition temperature of the system and preventing the film from becoming brittle and the heat resistance from being deteriorated. The alkali-soluble low-molecular compound includes the compounds having an acid group in the molecule, such as a dialkylsulfonamide compound, a dialkylsulfonylimide (-SO 2 -NH-CO-) compound, and a dialkyldisulfonylimide (-SO 2 -NH-SO 2 -) compound. The content of the alkali-soluble low-molecular compound is preferably 40% by weight or less, more preferably 30% by weight or less, and still more preferably 25% by weight or less to the resin having a substituent capable of increasing the solubility to an alkali developer by an acid described above. As the solvent, any organic solvents can be used in the present invention, as long as the components constituting the composition such as the resin and photo acid generator are sufficiently dissolved therein and the resulting solution can form a uniform coated film by a spin coating method etc. The organic solvents may be used singly or as a mixture thereof. Examples of the solvent include n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, tert-butyl alcohol, methylcellosolve acetate, ethylcellosolve acetate, propylene glycol monoethyl ether acetate, methyl lactate, ethyl lactate, 2-methoxybutyl acetate, 2-ethoxyethyl acetate, methyl pyruvate, ethyl pyruvate, methyl 3-methoxypropionate, ethyl 3-methoxypropionate, N-methyl-2-pyrrolidinone, cyclohexanone, cyclopentanone, cyclohexanol, methyl ethyl ketone, 1,4-dioxane, ethylene glycol monomethyl ether, ethylene glycol monomehyl ether acetate, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, and diethylene glycol dimethyl ether. As a matter of course, the organic solvents which can be used in the present invention are not limited to these. Although the fundamental constituting components of the photosensitive resin composition employed in the present invention are the N-hydroxyimidosulfonate compound, the resin, and the solvent (if necessary, the alkali-soluble low-molecular compound) described above but if necessary, other components such as a surface active agent, a dye, a stabilizer, a coatability improving agent and a dyestuff, may be added to the composition. Examples of the developer which can be used in forming the fine patterns using the photosensitive resin composition of the present invention include an appropriate organic solvent, a mixture of such organic solvents, an alkali solution having an appropriate concentration, an aqueous solution, a mixture thereof, and a mixture of an alkali solution, an appropriate organic compound, and/or an organic solvent, which can be selected according to the solubility of the high-molecular compound used. Suitable examples of the organic solvent and organic compound include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone, 2,6-dimethylcyclohexanone, and 3-methyl-2-cyclopentanone; alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, cyclopentanol, cyclohexanol, 2-methylcyclohexanol, 3-methylyclohexanol, 3,5-dimethylcyclohexanol, 1,4-cyclohexanedimethanol, and 1,4-cyclohexanediol; and other organic solvents such as tetrahydrofuran, dioxane, ethyl acetate, butyl acetate, isoamyl acetate, benzene, toluene, xylene, phenol, acetonitrile, and dimethylformamide. Suitable examples of the alkali solution include an aqueous solution, an organic solvent, and a mixture thereof containing an inorganic alkali (e.g., sodium hydroxide, potassium hydroxide, sodium silicate, and ammonia); an organic amine (e.g., ethylamine, propylamine, diethylamine, dipropylamine, trimethylamine, and triethylamine; or an organic ammonium salt (e.g., tetramethylammonium hydroxide, tetraethylammonium hydroxide, trimethylhydroxymethylammonium hydroxide, triethylhydroxymethylammonium hydroxide, and trimethylhydroxyethylammonium hydroxide). The photosensitive resin composition employed in the present invention is dissolved in an appropriate solvent as described above and coated on a substrate to form a thin film. The thickness of the thin film is preferably from 0.4 to 1.5 µm. EXAMPLES The present invention is explained practically by the following example but the present invention is not limited to the example. (1) Synthesis of Compound (I-1) In a four-neck flask equipped with a reflux condenser, a thermometer, a dropping apparatus, and a stirrer were placed 28.4 g of N-hydroxyamine (hydrochloride) and 100 ml of distilled water and an aqueous solution prepared by dissolving 15.8 g of sodium hydroxide in 50 ml of distilled water was added dropwise to the mixture over a period of 20 minutes. Thereafter, 50 g of methyl-substituted maleic anhydride was added to the mixture and after stirring the resultant mixture for 3 hours at room temperature, the mixture was stirred under heating for further 3 hours under a reflux condition. After the reaction was over, an aqueous solution of hydrochloric acid was added to the reaction mixture and after further saturating with sodium chloride, the product was extracted with ethyl acetate. The ethyl acetate solution obtained was concentrated to provide the crude crystals of the N-hydroxyimide product of methyl-substituted maleic anhydride. The crude crystals were recrystallized from toluene to provide 32 g of the crystals of the N-hydroxyimide product of methyl-substituted maleic anhydride. Then, in the same apparatus as above were placed 5 g of the N-hydroxyimide product of methyl-substituted maleic anhydride obtained above, 6 g of trifluoromethanesulfonic acid sulfonylchloride, and 100 ml of acetone followed by stirring, 3,76 g of triethylamine was added dropwise to the mixture over a period of 30 minutes, and the resultant mixture was stirred for one hour at room temperature. The reaction mixture obtained was crystallized in distilled water and the powders thus deposited were collected by filtration to provide 8.8 g of the desired compound (I-1). In addition, by changing the acid anhydride as the raw material, Compounds (I-2) and (I-3) could be obtained by the same procedure as above. (2) Synthesis of Compound (I-9) By reacting the N-hydroxyimide product of methyl-substituted maleic anhydride and camphor sulfonylchloride in place of trifluoromethanesulfonic acid sulfonyl chloride according to the above-described manners, the desired compound (I-9) was obtained. (3) Measurement of Optical Absorption In 4.5 g of propylene glycol monomethyl ether acetate were dissolved 0.2 mmol of each of the photo acid generator obtained in the above-described synthetic examples, N-hydroxysuccinimidotrifluoromethane sulfonate, or N-hydroxyphthalimidotrifluoromethane sulfonate and 1 g of a methacrylic acid-methyl methacrylate copolymer, each solution was filtered with a filter of 0.2 µm, each solution obtained was uniformly coated on a quartz glass plate, and dried for 90 seconds at 100°C to form each film. The absorption of light having a wavelength of 193 nm by the film formed was measured by a ultraviolet absorption measurement apparatus to obtain the optical density of the film. The results obtained are shown in Table 1 below. Photo Acid Generator Optical Density (µm -1 ) (I-1) 0.210 (I-2) 0.222 (I-3) 0.230 (I-9) 0.215 N-Hydroxysuccinimidotrifluoromethane sulfonate 0.254 N-Hydroxyphthalimidotrifluoromethane sulfonate 0.850 From the results, it can be seen that N-hydroxyphthalimidotrifluoromethane sulfonate had a particularly high optical absorption and was hard to be used for the lithography using the exposure light having a wavelength of 193 nm. Also, it can be seen that in regard to the photo acid generator of the present invention and N-hydroxysuccinimidotrifluoromethane sulfonate, the transparency to the exposed light was high. (4) Comparison of Acid Generating Efficiency 0.41 g of the photo acid generator (I-1) of this invention or 0.45 g of N-hydroxysuccinimidotrifluoromethane sulfonate with 10 g of a t-butyl methacrylate-methyl methacrylate-acrolein copolymer (copolymerization ratio: 30/40/30) and 40 g of propylene glycol monomethyl ether acetate were mixed, followed by dissolving, and each solution was filtered by a filter of 0.2 µm to provide each resist liquid. The resist liquid was coated on a silicon wafer using a spin coater and dried for 90 seconds at 120°C to form each resist film. The resist film was exposed with a light of 172 nm, immediately after the exposure, the resist film was heated at 110°C for 60 seconds, the silicon wafer having the resist film was immediately immersed in an aqueous solution of 2.38% tetramethylammonium hydroxide for 60 seconds, rinsed with water for 30 seconds, and dried. From the ratio of residual film of the pattern on the silicon wafer thus obtained, the samples were compared in terms of sensitivity. Because the same acid was generated from the comparative photo acid generator, the sensitivity of the resist becomes an indicator of the acid generating efficiency of the photo acid generator. The sensitivity of the resist containing the comparative photo acid generator, N-hydroxysuccinimidotrifluoromethane sulfonate was taken as 100, and the relative sensitivity (exposure amount) was employed. If the relative sensitivity is less than 100, it has higher sensitivity and higher acid generating ratio. The results are shown in Table 2 below. Photo Acid Generator Relative Sensitivity (I-1) 70 N-Hydroxysuccinimidotrifluoromethane sulfonate 100 As shown in the above results, it can be seen that the photo acid generator of the present invention had a higher acid generating efficiency. As described above, the photosensitive resin composition of the present invention for far-ultraviolet exposure of the present invention containing the specific photo acid generator has a high transparency to the light having wavelengths of from 170 to 220 nm and shows a high photo acid generating efficiency when exposed by the light having the foregoing wavelengths. Accordingly, the photosensitive resin composition of the present invention is suitably used as the material for the lithography using light having a wavelength of from 170 to 220 nm for exposure.

Claims: A pattern forming method which comprises: forming a thin film of a photosensitive resin composition for far-ultraviolet exposure of from 170 to 220 nm on a substrate, exposing the thin film with light from an ArF excimer laser having a wavelength within the range of from 170 to 220 nm, and developing the exposed thin film, said photosensitive resin composition comprising an N-hydroxymaleinimide-type sulfonate photo acid generator represented by following formula (I) and a resin having a substituent capable of increasing the solubility in an alkali developer by an acid; wherein R 1 and R 2 , which may be the same or different, each represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, or a cycloalkyl group having not more than 6 carbon atoms; R 1 and R 2 may combine via an alkylene chain to form a ring; and R 3 represents an alkyl group, a perfluoroalkyl group, a cyclic alkyl group, or a camphor group, N-(trifluoromethylsulfonyloxy)-bicyclo[2.2.1]-heptene-2,3-dicarboxyimide being excluded. The pattern forming method of claim 1, wherein the composition contains the photo acid generator represented by formula (I) in an amount of from 0.1 to 20% by weight based on the total solid components of the composition. The pattern forming method of claim 1 or 2, wherein the composition contains the resin having a substituent capable of increasing the solubility to an alkali developer by an acid in an amount of from 40 to 98% by weight based on the total solid components of the composition.

Inventors: AOAI TOSHIAKI, KODAMA KUNIHIKO, SATO KENICHIRO, UENISHI KAZUYA, AOAI, TOSHIAKI, KODAMA, KUNIHIKO, SATO, KENICHIRO, UENISHI, KAZUYA
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1997
Main classification: G03F7
IPC codes: G03F7, H01L21