Poly(para-t-butoxycarbonyl-oxystyrene) and method of making it

Abstract: Provided is a poly(para-t-butoxycarbonyloxystyrene) having recurring units of the formula: which is prepared by anionic polymerization of para-t-butoxycarbonyloxystyrene. The polymer is of a controlled molecular weight and a narrow molecular weight distribution and has a high degree of resolution and development, meeting the requirements of resist materials. The method allows for simple preparation of such polymers.

Description: This invention relates to a poly(para-t-butoxycarbonyloxystyrene) suitable as LSI resist and methods for preparing and using them. In the prior art, functional polymers are often used as high-resolution lithographic materials in the manufacture of LSI (Large Scale Integration). In accordance with the recent advance of LSI technology toward higher density, the resist material is also required to have a high degree of resolution and development ability. To meet this requirement with novolak resin, a prior art resist material, a technique of fractionation is utilized to control the molecular weight of the novolak resin and thereby increase the degree of resolution and development ability (see JP-A-1217542/1987). The fractionation technique is, however, complicated in operation and time consuming. One recently proposed replacement for prior art resist materials is a class of chemically amplified resist materials. Among such resist materials, those compounds having a functional group which is readily eliminatable with acid and showing a substantial difference in solubility before and after the elimination are preferred, largely because of ease of processing. Styrene derivatives having good plasma resistance are known to be desirable from this point of view, but are not successful in meeting the increasingly rigorous recent demands. Poly (p-tert-butoxycarbonyloxystyrene) is known: see Polymer, 1983 vol 24, pp 995-1000 where it is disclosed as a useful new intermediate for poly(p-hydroxystyrene) when polymerised by free radical or cationic polymerisation. EP-A-102450 and EP-A-251241 describe its use as resist material for making semiconductor circuit devices. See also our own EP-A-498630, in Article 54(3) EPC relation to the present case, which describes a convenient way of making p-t-butoxycarbonyloxystyrene. Therefore, an object of the present invention is to provide a novel polystyrene which has reduced the above - mentioned problems of the prior art and is a useful resist material. The inventors have found that by starting with para-t-butoxycarbonyloxystyrene of the following formula (2) as a monomer and utilizing anionic polymerization, especially living anionic polymerization, a poly(para-t-butoxycarbonyloxystyrene) having recurring units of the following formula (1) and a narrow molecular weight dispersion, of from 1.03 to 1.3, is obtainable. It is possible to control the molecular weight of the polymer to a desired value. This narrow dispersion p-t-butoxycarbonyloxystyrene polymer is quite advantageous as a resist material for use in the manufacture of LSI. As opposed to conventional polymers used in well-known resists as a major component which are obtained by conventional radical polymerization or polycondensation of corresponding monomers without paying attention to the control of molecular weight and its distribution, anionic polymerization, especially living anionic polymerization of p-t-butoxycarbonyloxystyrene can result in a poly(para-t-butoxycarbonyloxystyrene) having a molecular weight dispersion as narrow as 1.03 to 1.3. A resist based on such a polymer having a narrow molecular weight distribution can provide a high degree of resolution and developability, meeting the requirements of an advanced LSI resist material. Accordingly, the present invention in one aspect provides such a poly(p-t-butoxycarbonyloxystyrene), and in a second aspect a resist material based on it. In another aspect, the present invention provides a method for preparing such a poly(p-t-butoxycarbonyloxystyrene), by anionic polymerization of p-t-butoxycarbonyloxystyrene of formula (2). A further aspect is its use as a resist material in making an IC or semiconductor chip. FIG. 1 is a proton-NMR chart of poly(p-t-butoxycarbonyloxystyrene) obtained in Example 1. FIG. 2 is a GPC elution curve of the polymer prepared in Example 1. FIG. 3 is a GPC elution curve of another polymer prepared in Example 2. The present invention provides a specific styrene derivative polymer that is poly(p-t-butoxycarbonyloxystyrene) having recurring units of the following formula (1). This styrene derivative polymer should have a narrow molecular weight dispersion of from 1.03 to 1.3. The molecular weight dispersion used herein is defined as the weight average molecular weight divided by the number average molecular weight, i.e., Mw/Mn. The weight average molecular weight (Mw) of a polymer can be readily calculated, in the case of living polymerization, from the weight of monomer used and the moles (or molecule number) of a polymerization initiator, or measured by an optical scattering technique. The number average molecular weight (Mn) is determined through measurement by a membrane osmometer. The molecular weight distribution is determined by gel permeation chromatography (GPC). The molecular structure is readily identifiable by infrared (IR) spectroscopy and 1 H-NMR. The narrowly dispersed polystyrene derivative of the present invention should preferably have an average molecular weight of about 2,000 to 100,000 for use as resist material although the molecular weight is not limited to this range. The poly(p-t-butoxycarbonyloxystyrene) of the present invention is prepared by carrying out anionic polymerization, preferably living anionic polymerization of para-t-butoxycarbonyloxystyrene of the following formula (2). Accordingly, the poly(p-t-butoxycarbonyloxystyrene) of the invention is obtained as a living polymer. The monomer of formula (2) may be prepared by any prior art well-known method. As disclosed in EP-A-102 450, for example, the monomer is prepared by converting p-hydroxy-acetophenone with potassium t-butoxide into a corresponding potassium salt, reacting the salt with di-t-butyl dicarbonate to form p-t-butoxycarbonyloxybenzaldehyde, and reacting it with a Wittig reagent such as methylenetriphenylphosphorane as shown by reaction scheme A below. Alternatively, the monomer is prepared by converting p-hydroxystyrene with potassium t-butoxide into a corresponding potassium salt and reacting the salt with di-t-butyl dicarbonate as shown by reaction scheme B below. (Bu-t or t-Bu is a tertiary butyl group, and Ph is a phenyl group.) Also, the process that we propose in EP-A-498630 is recommended because p-t-butoxycarbonyloxystyrene can be prepared from a starting material which is readily available and easy to handle in a commercially advantageous manner. The process involves the steps of reacting p-acetoxystyrene with an alkali metal alkoxide to form an alkali metal phenolate, and tertiary carbonyloxylating the alkali metal phenolate, thereby synthesizing p-tert-butoxycarbonyloxystyrene as shown below. Often initiators are used in polymerizing p-tert-butoxycarbonyloxystyrene of formula (2). Although the type of polymerization initiator is not critical, organometallic compounds are preferred initiators. Exemplary are organic alkali metals including n-butyllithium, sec-butyllithium, tert-butyllithium, sodium naphthalene, potassium naphthalene, sodium anthracene, disodium α-methylstyrene tetramer, cumylpotassium, and cumylcaesium. Polymerization is generally carried out in an organic solvent. Examples of the organic solvent include aromatic hydrocarbons such as benzene and toluene, cyclic ethers such as tetrahydrofuran, dioxane and tetrahydropyran, and aliphatic or cycloaliphatic hydrocarbons such as dimethoxyethane, n-hexane and cyclohexane, alone or in admixture of two or more. Most preferred is tetrahydrofuran. In general, a solution of the monomer in an organic solvent preferably in concentrations of about 1 to 30% by weight is subject to polymerization with stirring in high vacuum or in an inert gas atmosphere such as argon and nitrogen. The reaction temperature may range from about -78°C to the boiling point of the solvent used. For example, a reaction temperature of from -78°C to 0°C is selected for tetrahydrofuran system and room temperature is selected for benzene system. Polymerization is continued for about 10 minutes to about 5 hours until a desired molecular weight is reached. Then a suitable stopper such as methanol, water and methylbromide is added to the reaction system for terminating the reaction. During living polymerization, a color characteristic of anionic living appears at active terminals. After the completion of reaction, the polymer is separated, for example, by adding methanol to cause the polymer to precipitate, followed by drying. There is obtained a poly(p-t-butoxycarbonyloxystyrene) having recurring units of formula (1) and a desired molecular weight. EXAMPLE Examples of the present process are given below by way of illustration and not by way of limitation. Example 1 Synthesis of p-t-butoxycarbonyloxystyrene monomer With ice cooling, 310 ml of a 1.0-M solution of potassium tertiary butoxide in tetrahydrofuran was stirred until a temperature of 5°C was reached. To the solution, 50 grams (0.31 mol) of p-acetoxystyrene was added dropwise over 30 minutes. During this addition, the reaction solution was maintained at a temperature below 20°C. At the end of addition, the solution was allowed to stand under the conditions for 30 minutes, obtaining a yellowish orange colored tetrahydrofuran solution of potassium p-vinylphenolate. With stirring and ice water cooling, a solution of 67.28 grams (0.31 mol) of di-t-butyl dicarbonate in 60 ml of tetrahydrofuran was added dropwise to the reaction solution, which was maintained at a temperature below 20°C. At the end of addition, the reaction mixture was stirred for one hour at room temperature. It was combined with 30 ml of saturated saline water and shaken for subsequent decantation. The organic layer was dried over anhydrous sodium sulfate and the solvent distilled off. The oily residue was subjected to vacuum distillation, yielding 65 grams of a colorless oily product as a fraction of 90 to 92°C /0.2 mmHg. It was p-t-butoxycarbonyloxystyrene (GC 98.5%, yield 95.7%). To remove water and other impurities from the p-tert-butoxycarbonyloxystyrene monomer, the monomer was subjected to distillation along with CaH 2 and refined with sodium benzophenone before polymerization. Preparation of poly(p-t-butoxycarbonyloxystyrene) A 1-liter flask was charged with 700 ml of tetrahydrofuran solvent and 2 × 10 -3 mol of potassium naphthalene as an initiator. To the mixture was added 50 grams of the p-t-butoxycarbonyloxystyrene monomer diluted with 50 ml of tetrahydrofuran at -78°C. The reaction solution turned red after one hour of polymerization. Reaction was continued until a desired molecular weight (15,000) was reached. Polymerization was terminated by adding methanol to the reaction solution. The reaction solution was poured into methanol whereby the polymer was precipitated and thereafter, separated and dried. There was obtained 29.5 grams of a white polymer. FIG. 1 is a diagram of 1 H-NMR analysis of the polymer and FIG. 2 is the GPC elution curve of the polymer. Based on IR and 1 H-NMR analysis, the polymer was identified to be a poly(p-t-butoxycarbonyloxystyrene) in which active terminals remained unreacted with p-t-butoxycarbonyloxy groups, that is, styrenic vinyl groups terminated. By membrane osmometry, the number average molecular weight (Mn) of the polymer was measured to be 1.3 × 10 4 g/mol. The molecular weight dispersion (Mw/Mn) was as narrow as 1.15. The GPC elution curve shows that the polymer has a narrow molecular weight distribution and a high degree of mono-dispersion (or narrow dispersion). Example 2 Polymerization was carried out using the refined p-t-butoxycarbonyloxystyrene monomer obtained in Example 1. A 2-liter flask was charged with 1 liter of tetrahydrofuran solvent and 9 × 10 -4 mol of n-butyllithium as an initiator. To the mixture was added 50 grams of the p-t-butoxycarbonyloxystyrene monomer diluted with 100 ml of tetrahydrofuran at -78°C. The reaction solution turned red after 30 minutes of polymerization. Reaction was continued until a desired molecular weight (55,000) was reached. Polymerization was terminated by adding methanol to the reaction solution. The reaction solution was poured into methanol whereby the polymer was precipitated and thereafter, separated and dried. There was obtained 24.5 grams of a white polymer. The 1 H-NMR analysis of the polymer provided a characteristic absorption diagram as in Example 1. The molecular weight dispersion (Mw/Mn) was 1.20. The GPC elution curve of FIG. 3 shows that the polymer has a high degree of mono-dispersion. There have been described p-t-butoxycarbonyloxystyrene polymers having a controlled molecular weight and a narrow molecular weight distribution. The polymers have a high degree of resolution and development as photosensitive polymers, meeting the requirements of the advanced resist materials. The method of the present invention allows for simple preparation of such valuable polymers. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: A poly(para-t-butoxycarbonyloxystyrene) having recurring units of the following formula (1): and a molecular weight dispersion Mw/Mn in the range from 1.03 to 1.3. A polystyrene according to claim 1 whose average molecular weight is in the range from 2000 to 100000. A resist material based on a polystyrene as defined in claim 1 or claim 2. A method for preparing a poly(para-t-butoxycarbonyloxystyrene) according to claim 1 or claim 2, comprising the step of carrying out anionic polymerisation of para-t-butoxycarbonyloxystyrene of the following formula (2): A method of claim 4 wherein the anionic polymerization is living anionic polymerization. A method according to claim 4 or 5 wherein polymerization is carried out in an organic solvent. A method according to any of claims 4 to 6 wherein an initiator is present during at least part of the polymerization step. A method according to any of claims 4 to 7 which comprises adding a stopper to terminate the polymerization, and isolating the polymer. Use of a resist material according to claim 3 in the manufacture of an integrated circuit or semi-conductor chip.

Inventors: FURIHATA TOMOYOSHI, NAKANISHI TOHRU, WATANABE OSAMU, YAGIHASHI FUJIO, YAMADA MOTOYUKI, FURIHATA, TOMOYOSHI, NAKANISHI, TOHRU, WATANABE, OSAMU, YAGIHASHI, FUJIO, YAMADA, MOTOYUKI, YAGIHASHI, FUJIO, NO. 101, AOBADAI APT.
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1992
Main classification: C08F112
IPC codes: C08F12