Energy beam curable hydrophilic pressure sensitive adhesive composition and use thereof

Abstract: An energy beam curable hydrophilic pressure sensitive adhesive composition comprising a hydrophilic pressure sensitive adhesive (A) and an energy beam polymerizable compound (B), optionally together with a photopolymerization initiator (C) added according to necessity. The pressure sensitive adhesive composition is suitable for use in a wafer surface protective sheet, enabling easy removal of any residual pressure sensitive adhesive by washing with water, even if pressure sensitive adhesive remains on the wafer surface after peeling of the pressure sensitive adhesive sheet.

Description: The present invention relates to an energy beam curable hydrophilic pressure sensitive adhesive composition and a use thereof. More particularly, the present invention relates to a pressure sensitive adhesive composition suitable for use in a pressure sensitive adhesive sheet for protecting a wafer surface which.is used for protecting circuit patterns on a wafer surface from, for example, cutting dust during polishing of the back of the wafer. A circuit pattern is formed on a surface of a wafer of a semiconductor such as silicon or gallium arsenide by, for example, etching or the liftoff method. Subsequently, the wafer, having a pattern formed thereon, has its back polished by means of, for example, a grinder; during this the wafer's patterned surface is generally covered with a pressure sensitive adhesive sheet. The first object of polishing the back of the patterned wafer is to remove any oxide coating which may occur on the back of the wafer in the etching step. The second object of polishing the wafer back is to regulate the thickness of the wafer having a pattern formed thereon. The polishing of the back of the wafer having a pattern formed on its surface is conducted while washing the wafer back with purified water in order to remove dust produced by polishing and heat generated during the polishing. Before the polishing of the wafer back is conducted, the pressure sensitive adhesive sheet (surface protective sheet) is adhered to the wafer surface so that the pattern formed on the wafer surface is protected from polishing dust and cutting water. The pressure sensitive sheet may be: a hydrophilic one such as the pressure sensitive adhesive sheet of Japanese Patent Laid-open Publication No.62(1987)-101678 which involves a pressure sensitive adhesive containing a nonionic surfactant or pressure sensitive adhesive sheet of Japanese Patent Publication No. 5(1993)-77284 (Patent family member of EP-A-0252739) using a water swellable pressure sensitive adhesive, which, after the completion of the polishing, is directly peeled so that the stain on wafer surface caused by pressure sensitive adhesive components can easily be removed by washing with purified water; or an energy beam curable one such as pressure sensitive adhesive sheet disclosed in Japanese Patent Laid-open Publication No. 8(1996)-27239, which, after the completion of back polishing, is irradiated with energy beams to thereby extremely reduce the adhesive strength to the wafer so that traces of pressure sensitive adhesive would not remain even upon peeling of the pressure sensitive adhesive sheet. Alternatively, this type of pressure sensitive adhesive sheet may include such adhesive, for example, as disclosed in GB-A-2184741, EP-A-0745654 and US-A-5302629. GB-A-2184741, for example, discloses an adhesive composition comprising copolymers of acrylic esters (e.g. ethyl methacrylate), methacrylic acid, a radiation polymerisable compound and, a hardner. Similarly, EP-A-0745654 discloses an adhesive composition comprising a UV polymerisable compound and a methacrylate based polymer and US-A-5302629 discloses a composition comprising a C 4-10 acrylic ester, a hydroxyl containing monomer, optionally a multifunctional monomer, and a photoinitiator. Although the above latter two types of pressure sensitive adhesive sheets for wafer back polishing have appropriately been employed in view of the respective characteristics and in conformity with encountered conditions, an energy beam curable pressure sensitive adhesive sheet which exhibits low peeling resistance at the time of peeling of the pressure sensitive adhesive sheet and is free from breakage the wafer will increasingly be demanded in the future in accordance with increase of the diameter of wafers and reduction of the thickness of wafers. However, this energy beam curable pressure sensitive adhesive does not possess water washability. Thus, the problem would be encountered such that, when residual pressure sensitive adhesive remains on a wafer surface for some reason (e.g., shortage in quantity of energy beams applied during irradiation therewith, abnormally high temperature caused by excess irradiation, energy beam irradiation deficiency attributed to adhesion of cutting dust raised at the time of wafer back polishing onto the surface of the sheet), the residual pressure sensitive adhesive cannot be removed by water washing. Thus, when conventional energy beam curable pressure sensitive adhesive remains on a wafer surface, it is common practice to conduct washing with an organic solvent. This, however, is disadvantageous from the viewpoint of, for example, environmental conservation. It is an object of the present invention to provide a pressure sensitive adhesive composition suitable for use in a wafer surface protective sheet which, at the time of polishing of a wafer back, is used to protect the circuit pattern formed on the wafer surface from, for example, cutting dust. It is another object of the present invention to provide a pressure sensitive adhesive composition suitable for use in a wafer surface protective sheet in which, even if pressure sensitive adhesive remains on a wafer surface upon peeling of the pressure sensitive adhesive sheet, the residual pressure sensitive adhesive can easily be removed by washing with water. The energy beam curable hydrophilic pressure sensitive adhesive composition of the present invention comprises a hydrophilic pressure sensitive adhesive (A) and an energy beam polymerizable compound (B) wherein said hydrophilic pressure sensitive adhesive (A) is composed of a pressure sensitive adhesive composition formed from: a carboxyl group containing copolymer of a polymerizable monomer (A1) and another monomer (A2) that is copolymerizable with the carboxyl group containing polymerizable monomer by solution polymerization, a neutralizer (A3), and a crosslinking agent (A4) wherein the other monomer (A2) is a compound represented by the formula: wherein each of X 1 , X 2 and X 3 independently represents hydrogen or a methyl group; R 1 represents a divalent hydrocarbon group having 2 to 12 carbon atoms; R 2 represents a hydrocarbon group having 1 to 10 carbon atoms; and n is 1 to 10. The energy beam polymerizable compound (B) is preferably a compound that contains at least one energy beam polymerizable carbon to carbon double bond in its molecule and has a weight average molecular weight of 100 to 50,000. It is preferred that the energy beam curable hydrophilic pressure sensitive adhesive composition of the present invention comprises a photopolymerization initiator (C) in addition to the above components. A pressure sensitive adhesive sheet comprising a substrate coated with an energy beam curable hydrophilic pressure sensitive adhesive composition as defined hereinbefore is also provided. In particular, this pressure sensitive adhesive sheet is preferably used in wafer processing. The pressure sensitive adhesive may also be used in protecting a wafer surface. The above pressure sensitive adhesive sheet is preferably used in a method of polishing a wafer back comprising the steps of: adhering the above pressure sensitive adhesive sheet to a patterned wafer surface, and polishing the back of the wafer while feeding water to the wafer back. The pressure sensitive adhesive composition of the present invention possesses not only energy beam curability but also hydrophilicity. Therefore, the adhesive strength is sharply reduced by irradiation with energy beams to thereby enable the easily peeling of the pressure sensitive adhesive sheet from an adherend. Further, even when the pressure sensitive adhesive remains on an adherend surface for some reason, the residual pressure sensitive adhesive can easily be removed by washing with water. The energy beam curable hydrophilic pressure sensitive adhesive composition and use thereof according to the present invention will be described in detail below. The energy beam curable hydrophilic pressure sensitive adhesive composition of the present invention comprises a hydrophilic pressure sensitive adhesive (A) and an energy beam polymerizable compound (B). The individual components will be described below with reference to the examples thereof. Hydrophilic pressure sensitive adhesive (A) The hydrophilic pressure sensitive adhesive (A) is preferably a pressure sensitive adhesive which is dissolved or swells upon being brought into contact with water. The hydrophilic pressure sensitive adhesive (A) for use in the present invention is formed from a carboxyl group containing copolymer of a polymerizable monomer (A1) and another monomer (A2) that is copolymerizable with the carboxyl group containing polymerizable monomer by solution polymerization; a neutralizer (A3); and a crosslinking agent (A4) wherein the other monomer (A2) is a compound represented by the formula: wherein each of X 1 , X 2 and X 3 independently represents hydrogen or a methyl group; R 1 represents a divalent hydrocarbon group having 2 to 12 carbon atoms; R 2 represents a hydrocarbon group having 1 to 10 carbon atoms; and n is 1 to 10. The carboxyl group containing copolymer is obtainable by polymerizing a carboxyl group containing polymerizable monomer (A1) and another monomer (A2) as hereinbefore defined that is copolymerizable with the carboxyl group containing polymerizable monomer by solution polymerization. The carboxyl group containing polymerizable monomer (A1) is a monomer which contains both a polymerizable carbon to carbon double bond and a carboxyl group. For example, acrylic acid, methacrylic acid, crotonic acid, itacbnic acid, maleic acid, fumaric acid, monoalkylitaconic acid, monoalkylmaleic acid or monoalkylfumaric acid is used as the carboxyl group containing polymerizable monomer (A1). Of these, acrylic acid and methacrylic acid are preferably used in the present invention. These carboxyl group containing polymerizable monomers can be used either individually or in combination. The other monomer (A2) that is copolymerizable with the above carboxyl group containing polymerisable monomer is represented by the formula: In the formula, each of X 1 , X 2 and X 3 independently represent hydrogen or a methyl group, preferably, hydrogen; n is 1 to 10, preferably, 1 to 4; R 1 represents a divalent hydrocarbon group having 2 to 12 carbon atoms, preferably, an alkylene group having 2 to 5 carbon atoms such as ethylene, n-propylene, isopropylene, n-butylene, iso-butylene, tert-butylene, sec-butylene or n-pentylene. Of these, ethylene, n-propylene, n-butylene and tert-butylene are preferably used as the group R 1 ; and R 2 represents a hydrocarbon group having 1 to 10 carbon atoms, preferably, an alkyl group having 1 to 6 carbon atoms such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, sec-butyl, n-pentyl, iso-pentyl, neopentyl or n-hexyl. Of these, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl and tert-butyl are preferably used as the group R 2 . Accordingly, a preferred example of the monomer (A2) for use in the present invention is an alkoxy group containing (meth)acrylic acid ester such as 2-methoxyethyl (meth) acrylate, 2-ethoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, 2-butoxyethyl (meth)acrylate, methoxydiethylene glycol (meth)acrylate or ethoxydiethylene glycol (meth)acrylate. These alkoxy group containing (meth)acrylic acid esters can be used either individually or in combination. In addition to the above alkoxy group containing (meth)acrylic acid esters the following monomers may be used: (meth)acrylic acid esters of alkyl group having 1 to 18 carbon atoms, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth) acrylate, N, N-dimethylaminoethyl (meth) acrylate, N,N-dimethylacrylamide, N,N-diethylacrylamide, N, N-dimethylaminopropylacrylamide, N, N-diethylaminopropylacrylamide, N, N-di-n-butoxymethylacrylamide, acryloylmorpholine, vinyl acetate, styrene, acrylonitrile, glycidyl (meth)acrylate, methylolacrylamide, vinylpyrrolidone, vinyl methyl ether, maleic anhydride, ethylene oxide and glycosyloxyethyl (meth) acrylate. These are used in combination with the alkoxy group containing (meth)acrylic acid ester. In this instance, the ratio (weight ratio) of alkoxy group containing (meth)acrylic acid ester to other polymerizable monomer preferably ranges from 1/20 to 99/1. The ratio of carboxyl group containing polymerizable monomer (A1) to other monomer (A2) that is copolymerizable with monomer (A1) (weight ratio of monomer (A1) /monomer (A2)) in the carboxyl group containing copolymer for use in the present invention preferably ranges from 1/500 to 1/4, still preferably, from 1/100 to 1/10. The weight average molecular weight of the carboxyl group containing copolymer preferably ranges from 50 thousand to 1.50 million, still preferably from 100 thousand to 1 million. The carboxyl group containing copolymer for use in the present invention can be obtained by copolymerizing the above carboxyl group containing polymerizable monomer (A1) and other monomer (A2) as defined hereinbefore that is copolymerizable with the monomer (A1) in given proportion. This copolymerization is conducted by solution polymerization. The solution polymerization is conveniently carried out by adding appropriate amounts of a solvent such as ethyl acetate and a polymerization initiator, such as azobisisobutyronitrile, to a mixture of the above monomers (A1) and (A2), agitating the mixture under a stream of nitrogen at room temperature for 30 min and performing a reaction at 40 to 100°C for 4 to 10 hr. As compared with emulsion polymerization, this solution polymerization has the advantages that neither emulsifier nor thickener is needed to thereby enable the decrease of the amount of impurity ions and enable the easy performance of control of polymerization conditions and change of obtained polymer composition. Moreover, a surprising effect is:realized such that the polymer produced by the solution polymerization is strong in shearing direction action as compared with the polymer produced by the emulsion polymerization. The above carboxyl group containing copolymers can be used either individually or in combination. The neutralizer (A3) is used to neutralize part or all of the carboxyl groups of the above carboxyl group containing copolymer so as to impart hydrophilicity or water solubility to the carboxyl group containing copolymer. Taking into account ionic impurities which are detrimental to the wafer, it is preferred that a compound not containing any of metals and halogens in its molecule be used as the neutralizer (A3). Although various basic compounds can be employed, unsubstituted or substituted amines are preferably used in the present invention. Of them, water soluble amine compounds are especially preferred. Examples of the above amine compounds include ammonia, ammonium salts exhibiting alkalinity and organic amino compounds exhibiting alkalinity, primary amines such as monoethylamine and monoethanolamine; secondary amines such as diethylamine and diethanolamine; tertiary amines such as triethylamine, triethanolamine, N,N-dimethylformamide diethylacetal, N,N,N'-trimethylethylenediamine, N-methyldiethanolamine and N,N-diethylhydroxylamine; amino compounds having a plurality of Ns in the molecule, such as diamine and polyethyleneimine; and cyclic amino compounds such as pyridine. Triethanolamine and N,N-dimethylformamide diethylacetal are preferably used in the present invention. The above neutralizers can be used either individually or in combination. The neutralizer (A3) is preferably used in an amount of 0.0001 to 1 mol, still preferably, 0.01 to 0.5 mol per mol of carboxyl group of the above carboxyl group containing copolymer. The crosslinking agent (A4) is used to partially crosslink the above carboxyl group containing copolymer. As the crosslinking agent (A4), use can be made of, for example, compounds having a glycidyl group in the molecule, e.g., epoxy crosslinking agents such as neopentyl glycol diglycidyl ether, polyethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, diglycidyl phthalate, dimer acid diglycidyl ether, triglycidyl isocyanurate, diglycerol triglycidyl ether, sorbitol tetraglycidyl ether, N,N,N',N'-tetraglycidyl-m-xylenediamine, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane and N,N,N',N'-tetraglycidyldiaminodiphenylmethane; compounds having an isocyanate group in the molecule, e.g., isocyanate crosslinking agents such as tolylene diisocyanate and diphenylmethane diisocyanate; methylol crosslinking agents such as melamine and phenol; chelating crosslinking agents; and aziridine crosslinking agents. Effecting a partial crosslinking of the carboxyl group containing copolymer with the crosslinking agent (A4) enables the regulation of the peeling strength of the obtained pressure sensitive adhesive to an appropriate value and enables the production of a pressure sensitive adhesive which swells in water without being dissolved therein. The crosslinking agent (A4) is preferably used in an amount of 0.001 to 1.0 mol, still preferably, 0.01 to 0.75 mol per mol of carboxyl group of the above carboxyl group containing copolymer. The pressure sensitive adhesive composition (1) for use in the present invention comprises the carboxyl group containing copolymer and the above components (A3) and (A4) as principal components and can be obtained by appropriately mixing these components together. Preferably, the carboxyl group containing copolymer is first mixed with the neutralizer (A3) and, thereafter, the crosslinking agent (A4) is added to the mixture to thereby crosslink the carboxyl group containing copolymer. The mixing of the carboxyl group containing copolymer with the neutralizer (A3) is conducted, for example, by adding the neutralizer (A3) directly or after dilution with a solvent (e.g., alcohol or acetone) to the carboxyl group containing copolymer at room temperature and, thereafter, agitating the mixture at 5 to 40°C for at least 25 min. Further, the crosslinking agent (A4) is added directly or after dilution with a solvent (e.g., toluene, ethyl acetate or isopropyl alcohol) to the above obtained mixture at room temperature and agitated at 5 to 40°C for at least 5 min, thereby obtaining the pressure sensitive adhesive composition (1) of the present invention. The thus obtained pressure sensitive adhesive composition (1) is a pressure sensitive adhesive composition which is of solvent type and soluble in water (swellable in water) without the use of any trace of surfactant. This pressure sensitive adhesive composition has excellent water washability and exhibits extremely small value of peeling electrification voltage at the time of peeling of pressure sensitive adhesive sheet because hydrophilicity is imparted to the pressure sensitive adhesive polymer per se. Moreover, water solubility (water swellability) is imparted to the pressure sensitive adhesive composition (1) without conducting a polymer blend of water soluble polymer and (meth)acrylic polymer, so that the amount of glue residue which may occur at the time of peeling of pressure sensitive adhesive sheet is extremely small. Energy beam polymerizable compound (B) The energy beam polymerizable compound (B) for use in the present invention is a compound having at least one energy beam polymerizable carbon to carbon double bond in its molecule, which compound preferably has a weight average molecular weight of 100 to 50,000, still preferably, 500 to 30,000 and, optimally, 1000 to 20,000 and can be polymerized by irradiation with energy beams. For example, energy beam polymerizable compounds disclosed in Japanese Patent Laid-open Publication No. 60(1985)-196,956 (patent family member of EP-A-0157508) and Japanese Patent Laid-open Publication No. 60(1985)-223,139 are widely used as the above energy beam polymerizable compound (B). Examples of energy beam polymerizable compounds (B) include trimethylolpropane triacrylate, tetramethylolmethane tetraacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol monohydroxypentaacrylate, dipentaerythritol hexaacrylate, 1,4-butylene glycol diacrylate, 1,6-hexanediol diacrylate, polyethylene glycol diacrylate and commercially available oligoester acrylates. In addition to the above acrylate compounds, urethane acrylate oligomers can be used as the energy beam polymerizable compound. Urethane acrylate oligomers can be obtained by reacting a polyester or polyether type polyol compound with a polyisocyanate compound such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate or diphenylmethane-4,4'-diisocyanate to thereby obtain an isocyanate-terminal urethane prepolymer and by reacting this isocyanate-terminal urethane prepolymer with a hydroxyl group containing acrylate or methacrylate such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropylmethacrylate, polyethylene glycol acrylate or polyethylene glycol methacrylate. Of these urethane acrylates, a hydrophilic urethane acrylate compound is preferably used in the present invention. For example, preferred use is made of a product of reaction of a (meth)acrylic acid ester containing at least one hydroxyl group in its molecule, an organic polyisocyanate and a polyethylene glycol monoalkyl ether having one hydroxyl group. The above polyethylene glycol monoalkyl ether is a compound represented by the formula: H-(OCH 2 CH 2 ) n -OR wherein R represents a lower.alkyl group, and n is an integer of 3 to 40. The details of these preferred urethane acrylate compounds are described in, for example, Japanese Patent Laid-open Publication No. 6(1994)-287260. In the pressure sensitive adhesive composition of the present invention, the energy beam polymerizable compound (B) is preferably used in an amount of 10 to 200 parts by weight, still preferably,'50 to 150 parts by weight and, optimally, 70 to 120 parts by weight per 100 parts by weight of hydrophilic pressure sensitive adhesive (A). In these amounts, the initial adhesive strength of obtained pressure sensitive adhesive is large, and the adhesive strength is sharply decreased upon irradiation with energy beams to thereby facilitate peeling from an adherend and avoid the remaining of pressure sensitive adhesive on the surface of the adherend. When the amount of energy beam polymerizable compound (B) is less than 10 parts by weight, the decrease of the adhesive strength upon irradiation with energy beams is not satisfactory. On the other hand, when the amount exceeds 200 parts by weight, there is unfavorably the possibility that it is difficult to remove the pressure sensitive adhesive with water. Photopolymerization initiator (C) When curing is conducted by irradiation with light, the energy beam curable hydrophilic pressure sensitive adhesive composition of the present invention can be doped with a photopolymerization initiator (C). The period of polymerization/curing by irradiation with light and the light dosage can be reduced by the doping with the photopolymerization initiator (C). Examples of suitable photopolymerization initiators (C) include 1-hydroxycyclohexyl phenyl ketone, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzyl diphenyl sulfide, tetramethylthiuram monosulfide, azobisisobutyronitrile, dibenzyl, diacetyl and β-chloroanthraquinone. It is preferred that the photopolymerization initiator (C) be, used in an amount ranging from 0.05 to 15 parts by weight, especially, from 0.1 to 10 parts by weight and, still especially, from 0.5 to 5 parts by weight per 100 parts by weight of energy beam polymerizable compound (B). Other components Moreover, for regulating the adhesive strength, cohesive strength, tack, molecular weight, molecular weight distribution, elasticity, glass transition temperature, hydrophilicity, water resistance of the pressure sensitive adhesive, the energy beam curable hydrophilic pressure sensitive adhesive composition of the present invention can be doped with a hydrophilic plasticizer such as a water soluble polyhydric alcohol, a tackifier resin, a pigment, a dye, an antifoaming agent, an antiseptic according to necessity in an amount not detrimental to the object of the present invention. These other components are preferably added in an amount of 0.01 to 20 parts by weight per 100 parts by weight of component (A), although depending on the particular purpose of the addition of individual component. Energy beam curable hydrophilic pressure sensitive adhesive composition The energy beam curable hydrophilic pressure sensitive adhesive composition of the present invention can be obtained by blending together the above hydrophilic pressure sensitive adhesive (A) and energy beam polymerizable compound (B), optionally together with the above photopolymerization initiator (C) and other components, by the customary method. This energy beam curable hydrophilic pressure sensitive adhesive composition can be cured by irradiation with energy beams to such a degree that the amount of pressure sensitive adhesive remaining on an adherend after peeling can extremely be reduced. For example, ultraviolet rays and electron beams are used as the energy beams. The energy beam dosage is varied depending on the formulation of the pressure sensitive adhesive and the type of applied energy beams. For example, when use is made of ultraviolet rays, the dosage thereof is preferred to range from 40 to 200 W/cm. When use is made of electron beams, the dosage thereof is preferred to range from 10 to 1000 krad. This energy beam irradiation realizes a marked reduction of the adhesive strength of the composition of the present invention. For example, whereas the adhesive strength to a mirror surface of semiconductor wafer ranges from 100 to 1000 g/25 mm before the energy beam irradiation, control can be effected so that the adhesive strength after the energy beam irradiation is 0.5 to 50% of that before the energy beam irradiation. Moreover, the pressure sensitive adhesive composition of the present invention, even after having been cured by energy beams, maintains hydrophilicity due to the hydrophilic pressure sensitive adhesive (A). For example, when the above pressure sensitive adhesive composition (1) is used as the hydrophilic pressure sensitive adhesive (A), the hydrophilic pressure sensitive adhesive polymer finitely swells in a crosslinked state. The pressure sensitive adhesive polymer swells even after irradiation with energy beams, so that, even if the pressure sensitive adhesive remains on the wafer surface, washing can be performed. The hydrophilicity of the pressure sensitive adhesive is evaluated as follows. That is, only the pressure sensitive adhesive is stuck to an adherend (mirror wafer) and dipped in 23°C water. The time required for the pressure sensitive adhesive to be dissolved or swell in water to thereby be completely removed from the adherend is measured. There is generally a step of washing a polished wafer with water, for example, in order to remove polishing dust therefrom. From the viewpoint of step adaptability, it is preferred that the pressure sensitive adhesive composition of the present invention be removed from the adherend within 30 min, especially, within 10 min. When more than 30 min is taken, the productivity may be unfavorably lowered. As apparent from the above, the pressure sensitive adhesive composition of the present invention possesses hydrophilicity even after having been cured by energy beams, so that, even if the pressure sensitive adhesive remains on the adherend surface after the curing, residual pressure sensitive adhesive can be removed by simple water washing. Pressure sensitive adhesive sheet The pressure sensitive adhesive sheet of the present invention comprises a substrate and, superimposed thereon, a pressure sensitive adhesive layer composed of the above pressure sensitive adhesive composition. The above substrate may be selected from among, for example, films of polyethylene, polypropylene, polybutene, polybutadiene, polymethylpentene, polyvinyl chloride, vinyl chloride copolymer, polyethylene terephthalate, polybutylene terephthalate, polyurethane, ethylene/vinyl acetate copolymer, ionomer resin, ethylene/(meth)acrylic acid copolymer, ethylene/(meth)acrylic acid ester copolymer, polystyrene and polycarbonate, films produced by crosslinking these and films composed of laminates thereof. Furthermore, according to necessity, use can be made of films produced by, for example, coloring the above films, fluororesin films. The thickness of the substrate generally ranges from 50 to 500 µm, preferably, from 80 to 300 µm. When the thickness of the substrate is smaller than these, there is the danger that the surface protecting capability of the pressure sensitive adhesive sheet may be deteriorated. The pressure sensitive adhesive sheet of the present invention can be obtained by coating a variable substrate in an appropriate thickness with the above pressure sensitive adhesive composition according to the customary technique employing a conuiia coater, gravure coater, die coater, reverse coater and drying the composition to thereby form a pressure sensitive adhesive layer on the substrate. When considered necessary, a release sheet may be applied onto the pressure sensitive adhesive layer. Although varied depending on the use, the thickness of the pressure sensitive adhesive layer generally ranges from 10 to 50 µm, preferably, from 20 to 40 µm. When the thickness of the pressure sensitive adhesive layer is smaller than these, there is the danger that the surface protecting capability of the pressure sensitive adhesive sheet may be deteriorated. The configuration of the pressure sensitive adhesive sheet of the present invention may have the form of, for example, a tape or a label. The above pressure sensitive adhesive sheet of the present invention can suitably be used for protecting the surface of a wafer. That is, the pressure sensitive adhesive sheet comprising the above constituents prevents the penetration of polishing dust and cutting dust into the interface between the pressure sensitive adhesive layer and the adherend surface during the step of wafer processing to thereby enable satisfactory protection of the wafer surface. Moreover, not only can the pressure sensitive adhesive sheet be easily peeled from the wafer after the completion of required processing but also, even if the pressure sensitive adhesive sticks to and remains on the wafer surface, the residual pressure sensitive adhesive can easily be washed away with water. Method of use The above pressure sensitive adhesive sheet of the present invention can suitably be used as a surface protective sheet, especially, during wafer processing. A method of polishing a wafer back with the use of the pressure sensitive adhesive sheet of the present invention will be described below. When a release sheet is disposed on an.upper surface of the pressure sensitive adhesive sheet, the release sheet is removed. Subsequently, a wafer whose back is to be polished is adhered to the pressure sensitive adhesive layer of the pressure sensitive adhesive sheet. This adhesion is so conducted that a patterned surface of the wafer contacts the pressure sensitive adhesive layer. In this form, the wafer back is polished by means of, for example, a grinder to thereby not only remove any oxide layer which has occurred on the wafer back but also regulate the thickness of the wafer to that desired. During this step, purified water is sprayed on to the wafer so that polishing dust is washed away from the wafer and heat generated during the polishing is absorbed. Upon completion of the polishing, the pressure sensitive adhesive sheet is peeled from the wafer. In the pressure sensitive adhesive sheet of the present invention, as is apparent from the foregoing, the adhesive strength can be reduced to an extremely low level by irradiating with an energy beam, so that the pressure sensitive adhesive sheet can easily be peeled from the wafer surface. Furthermore, even if the pressure sensitive adhesive remains on the wafer surface after the peeling of the pressure sensitive adhesive sheet, the residual pressure sensitive adhesive can easily be washed away with purified water. In summing up, the pressure sensitive adhesive layer of the pressure sensitive adhesive sheet of the present invention is composed of the pressure sensitive adhesive composition containing specific components, so that the adhesive strength is extremely reduced by irradiation with energy beams to thereby enable easy peeling. Moreover, even if the pressure sensitive adhesive remains on the wafer surface, the residual pressure sensitive adhesive can easily be washed away with purified water without the need to use an organic solvent such as trichlene. Therefore, there is no danger of having a bad influence on human health or causing environmental pollution. Although the prior art has required two steps of washing the wafer surface having the pressure sensitive adhesive adhering thereto with an organic solvent such as trichlene and, thereafter, washing with water, only washing of the wafer surface having the pressure sensitive adhesive adhering thereto with purified water, for example, in an ultrasonic washer is satisfactory in the present invention with the result that the washing can be accomplished by one step in the present invention. Furthermore, the pressure sensitive adhesive sheet of the present invention is adhered to the wafer with satisfactory adhesive strength during the polishing of the wafer back, so that penetration of wafer polishing dust into the interface between the wafer surface and the pressure sensitive adhesive sheet to thereby break the pattern formed on the wafer surface can be avoided. The pressure sensitive adhesive composition of the present invention possesses both energy beam curability and hydrophilicity. Therefore, the adhesive strength is extremely reduced by irradiation with energy beams to thereby enable easy peeling of the pressure sensitive adhesive sheet from the adherend. Further, even if the pressure sensitive adhesive remains on the adherend surface for some reason, the residual pressure sensitive adhesive can easily be removed by washing with water. EXAMPLE The present invention will further be illustrated below with reference to the following Examples. In the following Examples and Comparative Examples, the "residual particle", "water washability" and "adhesive strength" were evaluated in the following manner. Measurement of residual particle The pressure sensitive adhesive sheet obtained in each of the following Examples and Comparative Examples was adhered onto the mirror surface of a 4-inch silicon wafer in 23°C/65% RH atmosphere by reciprocating a 2 kg rubber roller thereon, and allowed to stand still for 20 min. From the substrate film side, the pressure sensitive adhesive sheet was irradiated at a line speed of 5 m/min with ultraviolet rays emitted from a high pressure mercury lamp (80 W/cm) disposed at a distance of 10 cm from the pressure sensitive adhesive sheet. Thereafter, the pressure sensitive adhesive sheet was peeled. The number of particles with a particle size of at least 0.27 µm (diameter) remaining on the wafer surface was measured by the use of a laser surface inspecting instrument (LS 5000 manufactured by Hitachi Electronic Engineering Co., Ltd.). Water washability The pressure sensitive adhesive only produced in each of the following Examples and Comparative Examples was applied and dried to obtain a coating with a thickness of 50 µm, cut into 20 mm squares, adhered onto a mirror wafer and allowed to stand still for 20 min. The pressure sensitive adhesive layer was irradiated at a line speed of 5 m/min with ultraviolet rays emitted from a high pressure mercury lamp (80 W/cm) disposed at a distance of 10 cm from the layer. Thereafter, the wafer with pressure sensitive adhesive coating was immersed in pure water at ordinary temperature, and the time required for the pressure sensitive adhesive to be removed was measured. Judgments indicated in the table were made based on the following criteria: good: < 10 min, fair: 10 to 30 min, and failure: > 30 min. Adhesive strength The pressure sensitive adhesive sheet obtained in each of the following Examples and Comparative Examples was adhered onto the back (polished by #2000) of a semiconductor wafer in 23°C/65% RH atmosphere by reciprocating a 2 kg rubber roller thereon, allowed to stand still for 20 min and peeled by means of a universal tensile tester (TENSILON®/UTM-4-100 manufactured by Orientec Corporation) at a peeling speed of 300 mm/min to thereby determine a 180° peeling adhesive strength (g/25 mm). Also, the pressure sensitive adhesive sheet was adhered and allowed to stand still under the same conditions, and, from the substrate film side, the pressure sensitive adhesive sheet was irradiated at a line speed of 5 m/min with ultraviolet rays emitted from a high pressure mercury lamp (80 W/cm) disposed at a distance of 10 cm from the pressure sensitive adhesive sheet. Thereafter, the 180° peeling adhesive strength was measured in the same manner. Employed abbreviations stand for the following matter. [energy beam polymerizable compound] B1: reaction product (80% ethyl acetate solution) (weight average molecular weight: 2500) from 343.5 parts by weight of hexamethylene diisocyanate trimer (trade name: Coronate® HX, produced by Nippon Polyurethane Co. Ltd.), 166.3 parts by weight of hydroxypropyl acrylate and 190.3 parts by weight of polyethylene glycol monomethyl ether. B2: reaction product (80% ethyl acetate solution) (weight average molecular weight: 2000) from 52.8 parts by weight of hexamethylene diisocyanate, 10.7 parts by weight of pentaerythritol and 36.5 parts by weight of 2-hydroxyethyl acrylate. B3: reaction product (40% toluene solution) (weight average molecular weight: 15,000) from 51.8 parts by weight of hydrogenated tolylene diisocyanate, 623.0 parts by weight of poly(butylene oxide)diol and 25.2 parts by weight of 2-hydroxyethyl acrylate. [photopolymerization initiator] C: 1 -hydroxycyclohexyl phenyl ketone (trade name: Irgacure 184® produced by Ciba-Geigy). Example 1 [Preparation of pressure sensitive adhesive composition] (1) Production of hydrophilic pressure sensitive adhesive: 3 parts by weight of acrylic acid, 90 parts by weight of 2-methoxyethyl acrylate, 7 parts by weight of 2-hydroxyethyl acrylate, 200 parts by weight of ethyl acetate and 0.5 part by weight of azobisisobutyronitrile were placed in a four-necked flask equipped with a reflux condenser, an agitator, a thermometer, an inert gas introducing tube and a dropping funnel. The mixture was agitated in a stream of nitrogen for 30 min and reacted at 60°C for about 8 hr, thereby obtaining a carboxyl group containing copolymer.Triethanolamine as a neutralizer was added in an amount of 1.25 parts by weight (0.2 mol per mol of carboxyl group) per 100 parts by weight, in terms of solid contents, of the above obtained carboxyl group containing copolymer and satisfactorily agitated.Thereafter, epoxy crosslinking agent (trade name: Tetrad® C, produced by MITSUBISHI GAS CHEMICAL CO., INC.) as a crosslinking agent was added in an amount of 0.5 part by weight (0.03 mol per mol of carboxyl group) and satisfactorily agitated, thereby obtaining a hydrophilic pressure sensitive adhesive. (2) Production of energy beam curable hydrophilic pressure sensitive adhesive composition: 120 parts by weight of energy beam polymerizable compound B1 and 4.8 parts by weight of photopolymerization initiator C were added to 101.75 parts by weight of the above obtained hydrophilic pressure sensitive adhesive and satisfactorily blended, thereby obtaining an energy beam curable hydrophilic pressure sensitive adhesive composition. [Production of pressure sensitive adhesive sheet] A corona treated surface of a 110 µm thick polyethylene film was coated with the above obtained pressure sensitive adhesive composition so that the thickness of the coating having been dried was 20 µm. Thus, a pressure sensitive adhesive sheet with a total thickness of 130 µm was obtained. The "residual particle", "water washability" and "adhesive strength" of the pressure sensitive adhesive sheet were evaluated by the above methods. The results are given in Table 2. Example 2 [Preparation of pressure sensitive adhesive composition] (1) Production of hydrophilic pressure sensitive adhesive: A hydrophilic pressure sensitive adhesive was obtained in the same manner as in Example 1 except that the amount of added neutralizer triethanolamine was changed to 1.75 parts by weight (0.3 mol per mol of carboxyl group). (2) Production of energy beam curable hydrophilic pressure sensitive adhesive composition: 120 parts by weight of energy beam polymerizable compound B2 and 4.8 parts by weight of photopolymerization initiator C were added per 102.25 parts by weight, in terms of solid contents, of the above obtained hydrophilic pressure sensitive adhesive and satisfactorily blended, thereby obtaining an energy beam curable hydrophilic pressure sensitive adhesive composition. [Production of pressure sensitive adhesive sheet] A pressure sensitive adhesive sheet was obtained in the same manner as in Example 1. The results are given in Table 2. Example 3 [Preparation of pressure sensitive adhesive composition] (1) Production of hydrophilic pressure sensitive adhesive: A hydrophilic pressure sensitive adhesive was obtained in the same manner as in Example 1. (2) Production of energy beam curable hydrophilic pressure sensitive adhesive composition: 120 parts by weight of energy beam polymerizable compound B3 and 4.8 parts by weight of photopolymerization initiator C were added per 102.25 parts by weight of the above obtained hydrophilic pressure sensitive adhesive and satisfactorily blended, thereby obtaining an energy beam curable hydrophilic pressure sensitive adhesive composition. [Production of pressure sensitive adhesive sheet] A pressure sensitive adhesive sheet was obtained in the same manner as in Example 1. The results are given in Table 2. Comparative Example 1 [Preparation of pressure sensitive adhesive composition] (1) Production of pressure sensitive adhesive: 5 parts by weight of 2-hydroxyethyl acrylate and 95 parts by weight of butyl acrylate were polymerized. 0.2 part by weight of isocyanate crosslinking agent (trade name: Caronate® L, produced by Nippon Polyurethane Co. Ltd.) as a crosslinking agent was added thereto and satisfactorily agitated. Thus, a pressure sensitive adhesive was obtained. (2) Production of energy beam curable pressure sensitive adhesive composition: 100 parts by weight of energy beam polymerizable compound B1 and 4 parts by weight of photopolymerization initiator C were added per 100.2 parts by weight, in terms of solid contents, of the above obtained pressure sensitive adhesive and satisfactorily blended, thereby obtaining an energy beam curable pressure sensitive adhesive composition. [Production of pressure sensitive adhesive sheet] A pressure sensitive adhesive sheet was obtained in the same manner as in Example 1. The results are given in Table 2. Comparative Example 2 The same procedure as in Example 1 was repeated except that the neutralizer triethanolamine was not used and that the amount of added energy beam polymerizable compound B1 was changed to 100 parts by weight. The results are given in Table 2. Pressure sensitive adhesive (pts.wt) Energy beam polymerizable compd. (pts.wt) Photopolymn initiator (pts.wt) B1 B2 B3 Ex. 1 101.75 120 - - 4.8 Ex. 2 102.25 - 120 - 4.8 Ex. 3 102.25 - - 120 4.8 Comp. Ex. 1 100. 2 100 - - 4 Comp. Ex. 2 100. 2 100 - - 4 The pressure sensitive adhesives of the Examples are hydrophilic while those of the Comparative Examples are hydrophobic. No. of residual particles Water washability Adhesive strength before UV irradiation (g/25 mm) Adhesive strength after UV irradiation (g/25 mm) (no. / ( 4") wafer) > 0.27 µm time required for removal judgment Ex. 1 18 2 min good 350 15 Ex. 2 12 9 min good 400 5 Ex. 3 30 12 min fair 450 25 Comp. Ex. 1 27 >2 hrs fail 320 10 Comp. Ex. 2 17 >2 hrs fail 340 15

Claims: An energy beam curable hydrophilic pressure sensitive adhesive composition comprising a hydrophilic pressure sensitive adhesive (A) and an energy beam polymerizable compound (B) wherein said hydrophilic pressure sensitive adhesive (A) is composed of a pressure sensitive adhesive composition obtainable from: a carboxyl group containing copolymer of a polymerizable monomer (A1) and another monomer (A2) that is copolymerizable with the carboxyl group containing polymerizable monomer by solution polymerization, a neutralizer (A3), and a crosslinking agent (A4) wherein the other monomer (A2) is a compound represented by the formula: wherein each of X 1 , X 2 and X 3 independently represents hydrogen or a methyl group; R 1 represents a divalent hydrocarbon group having 2 to 12 carbon atoms; R 2 represents a hydrocarbon group having 1 to 10 carbon atoms; and n is 1 to 10. An energy beam curable hydrophilic pressure sensitive adhesive composition as claimed in claim 1, wherein said energy beam polymerizable compound (B) is a compound that contains at least one energy beam polymerizable carbon to carbon double bond in its molecule and has a weight average molecular weight of 100 to 50,000. An energy beam curable hydrophilic pressure sensitive adhesive composition as claimed in claim 1 or claim 2, which further comprises a photopolymerization initiator (C) A pressure sensitive adhesive sheet comprising a substrate coated with an energy beam curable hydrophilic pressure sensitive adhesive composition as claimed in any of claims 1 to 3. The use of pressure sensitive adhesive sheet as claimed in claim 4, in wafer processing. The use of pressure sensitive adhesive sheet as claimed in claim 4, in protecting a wafer surface.

Inventors: KATO KIICHIRO, KONDOH TAKESHI, TAGUCHI KATSUHISA, TAKAHASHI KAZUHIRO, KATO, KIICHIRO, KONDOH, TAKESHI, TAGUCHI, KATSUHISA, TAKAHASHI, KAZUHIRO
Applicants: LINTEC CORP, LINTEC CORPORATION
Application year: 1998
Main classification: C09J4
IPC codes: C08F290, C08F299, C09J155, C09J133, H01L21, C09J175, C09J7, C09J201, C08F220, C09J4