BACKGROUND OF THE INVENTION In recent years, the spread of IC cards has been promoted, and further reduction of the thickness thereof is now demanded. Accordingly, it is now required that the thickness of semiconductor chips, which has been about 350 um, be reduced to 50-100 ptm or less.

As a technology for attaining the above reduction of chip thickness, there can be mentioned the process for producing semiconductor chips disclosed in Japanese Patent

Laid-open Publication No. 5 (1993) -335411 in which grooves of given depth are formed from the wafer surface side and thereafter the wafer back side is ground. This process is also known as the"predicing process". At the time of wafer back grinding, a surface protective sheet is stuck onto the grooved wafer surface so that the circuits on wafer surface can be protected and so that the wafer (chips) is fixed.

The semiconductor chips produced by this process, with maintaining the wafer configuration and in the state of being fixed by a surface protective sheet, are subjected to recognition of the wafer (chip) position and direction prior to the subsequent step. The wafer direction is recognized through direction of grooves provided between chips.

Although the width of grooves provided between chips originally depends on the blade width employed at the stage of forming grooves, when there is an internal strain in the surface protective sheet employed, the internal strain is released at the time of division of the wafer into chips with the result that the chip interval is reduced to thereby disenable recognition of the wafer direction. The internal strain of the surface protective sheet is generated from residual strain attributed to tension imparted at wafer sticking or sheeting of base material, and it is practically impossible to reduce the internal strain to zero.

When the internal strain of the surface protective sheet is extremely large, the interval between neighboring chips (calf interval) becomes nearly nil. Thus, there has been the danger of, at the stage of chip pickup, mutual contacting of neighboring chips leading to damaging of chips.

Therefore, it has been proposed to use a film of high stress relaxation as the base material of surface protective sheet (see Japanese Patent Laid-open Publication No.

2000-212530). Using the above base material of high stress relaxation enables rapid decrease of the residual stress, so that the above problem attributed to the internal strain can be resolved. However, the film of high stress relaxation is generally soft, so that it may be delicately deformed by the pressure from a grinder at the time of wafer back grinding or various loads suffered at later steps (for example, transfer onto a pressure sensitive adhesive sheet for pickup) with the result that there occurs changing of the calf interval between neighboring chips. When changing of the calf interval occurs, there is the danger of recognition failure or malfunctioning of pickup unit used at the stage of pickup operation.

For keeping the calf interval constant, it is being proposed to use a film of high rigidity such as polyethylene terephthalate as the base material of surface protective sheet.

However, using a rigid film such as polyethylene terephthalate as the base material poses the following problem.

For detaching the surface protective sheet in the predicing process, it is only proposed to bond a heat seal tape as a stripping tape to an end of the surface protective sheet and thereafter detach the surface protective sheet together with the heat seal tape (see Japanese Patent Laid-open Publication No. 2000-68293). However, the polyethylene terephthalate film has a high melting point, so that the heat sealing cannot be effected. This is disadvantageous in realizing an automatic process.

OBJECT OF THE INVENTION The present invention has been made in view of the above state of the prior art, and it is an object of the present invention to provide a surface protective sheet which, in the so-called predicing process, not only enables maintaining a given calf interval during and after wafer back grinding but also facilitates fitting of a stripping tape. It is another object of the present invention to provide a process for producing semiconductor chips with the use of the surface protective sheet.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a surface protective sheet for semiconductor wafer, used in semiconductor wafer back grinding during a process comprising providing a semiconductor wafer of predetermined thickness having a surface furnished with circuits and a back, forming grooves of a cut depth smaller than the thickness of the semiconductor wafer from the semiconductor wafer surface and grinding the back of the semiconductor wafer so that the thickness of the semiconductor wafer is reduced and so that the semiconductor wafer is finally divided into individual semiconductor chips, which surface protective sheet comprises a base material and, superimposed thereon, a pressure sensitive adhesive layer, the base material comprising two or more constituent layers which include: a first constituent layer of 10 to 300 gm thickness exhibiting a Young's modulus of 3000 to 30,000 N/m2, and a second constituent layer exhibiting a glass transition temperature, measured by DSC (differential scanning calorimetry), of 70°C or below, the second constituent layer being an outermost layer.

In the present invention, it is preferred that the second constituent layer have an endotherm of 0.1 J/g or

more exhibited at the DSC measuring. Also, it is preferred that the second constituent layer be constituted of a low density polyethylene.

Preferably, the first constituent layer is constituted of polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, a polyamide, a polyacetal, a polycarbonate, a modified polyphenylene oxide, polyphenylene sulfide, polysulfone, a wholly aromatic polyester, a polyether ketone or a polyimide.

According to another aspect of the present invention, there is provided a process for producing semiconductor chips, comprising the steps of: providing a semiconductor wafer of predetermined thickness having a surface furnished with circuits and a back; forming grooves of a cut depth smaller than the thickness of the semiconductor wafer from the semiconductor wafer surface; sticking a surface protective sheet to the semiconductor wafer surface, which surface protective sheet is as defined above (namely, comprising a base material and, superimposed thereon, a pressure sensitive adhesive layer, the base material comprising two or more constituent layers which include a first constituent layer of 10 to 300 pm thickness

exhibiting a Young's modulus of 3000 to 30,000 N/m2, and a second constituent layer exhibiting a glass transition temperature, measured by DSC (differential scanning calorimetry), of 70°C or below, the second constituent layer provided as an outermost layer); grinding the back of the semiconductor wafer so that the thickness of the semiconductor wafer is reduced and so that the semiconductor wafer is finally divided into individual semiconductor chips ; sticking a pressure sensitive adhesive sheet for pickup to the back of the semiconductor chips ; bonding a stripping tape to the second constituent layer by heat seal ; and peeling the surface protective sheet with the stripping tape as a starting point to thereby enable transfer of the semiconductor chips onto the pressure sensitive adhesive sheet for pickup.

The above surface protective sheet of the present invention, in the so-called predicing process, not only enables maintaining a given calf interval during and after the wafer back grinding but also facilitates fitting of the stripping tape.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a sectional view of one form of surface protective sheet according to the present invention ; Fig. 2-6 are a view for explaining a process for producing thin semiconductor chips with the use of the surface protective sheet of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail below with reference to the appended drawings.

Referring to Fig. 1, the surface protective sheet 10 of the present invention comprises base material 11 and, superimposed thereon, pressure sensitive adhesive layer 12, the base material 11 comprising two or more constituent layers.

The base material 11 comprises at least two constituent layers. The constituent layer arranged on the pressure sensitive adhesive side of the base material is referred to as the first constituent layer 1 while the constituent layer arranged on the opposite side is referred to as the second constituent layer 2. The particulars thereof are explained below.

The thickness of the first constituent layer 1 is in the range of 10 to 300 llm, preferably 25 to 250 pm, and still preferably 50 to 200 pm.

The Young's modulus (measured in accordance with Japanese Industrial Standard (JIS) K-7127) of the first constituent layer 1 is in the range of 3000 to 30,000 N/m2, preferably 5000 to 15,000 N/m2.

When both the Young's modulus and thickness of the first constituent layer 1 fall within the above ranges, not only is the machine adaptability at the stage of sticking to the wafer excellent but also the stress applied to the wafer (chips) at the stage of peeling can be reduced.

The first constituent layer 1 is not particularly limited as long as the above properties can be satisfied, and various thin layer items can be used. From the viewpoint of, for example, water resistance, thermal stability and rigidity, it is preferred to use synthetic resin films.

The first constituent layer 1 is constituted of, for example, any of polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, a polyamide, a polyacetal, a polycarbonate, a modified polyphenylene oxide, polyphenylene sulfide, polysulfone, a wholly aromatic polyester, a polyether ketone and a polyimide. The first constituent layer 1 may be in the form of a monolayer film or a film laminate constituted of these various polymers.

Among the above polymers, it is preferred to use those which do not exert adverse effects, such as ion contamination, on the wafer in the first constituent layer 1. For example,

it is especially preferred to use polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate or a polyamide in the first constituent layer 1.

In the base material 11 for use in the present invention, the second constituent layer 2 is laminated onto one side of the first constituent layer 1, and is provided as an outermost layer. The second constituent layer 2 is provided as an outermost layer because it is used to bond the stripping tape described later by heat seal.

The second constituent layer 2 is constituted of a resin whose glass transition temperature, measured by DSC (differential scanning calorimetry), is 70°C or below, preferably in the range of 10 to 70°C, still preferably 20 to 60°C, and optimally 30 to 50°C.

In the present invention, the second constituent layer 2 is preferably constituted of a resin whose endotherm exhibited at the DSC measuring is preferably 0.1 J/g or more, still preferably in the range of 0.1 to 2.0 J/g, and optimally 0.2 to 1.0 J/g.

The base material 11 can be obtained by bonding a resin film for forming the first constituent layer 1 and a resin film for forming the second constituent layer 2 to each other with the use of an adhesive so as to form a laminate, or by coating the first constituent layer 1 with a resin for forming the second constituent layer 2 and forming a film

therefrom. The resin for forming the second constituent layer 2 may be in the form of a solution or a dispersion, or may be in the melting state which is extruded by means ofanextruder. The second constituent layer 2 is preferably constituted of polyethylene or an ethylene copolymer, especially a low-density polyethylene. The density of low density polyethylene is preferably in the range of about 0.910 to 0.940 g/cm3, still preferably about 0.915 to 0.930 g/cm3. When the density of low density polyethylene falls within the above ranges, the heat seal strength with the stripping tape is strong so as to ensure peeling of the surface protective sheet 10.

When the first constituent layer 1 and the second constituent layer 2 are bonded to each other with the use of an adhesive, use may be made of an adhesive for dry laminating, or a pressure sensitive adhesive.

When the second constituent layer 2 is applied by coating, the second constituent layer 2 is preferably constituted of a composition comprising a thermoplastic elastomer as a principal component and a hardening resin.

As the thermoplastic elastomer, there can be employed, for example, a styrene/isoprene/styrene copolymer, a styrene/butadiene/styrene copolymer, a styrene/ethylene/propylene/styrene copolymer, natural rubber, an isoprene rubber, an acrylic rubber or an

ethylene/propylene rubber. The hardening resin may be selected from among common thermosetting resins, or photohardening (photocurable) resins which are hardened (cured) by irradiation with light, for example, ultraviolet light. As the photohardening resin, there can be preferably employed a low-molecular-weight compound having at least two photopolymerizable carbon to carbon double bonds in each molecule thereof. The low-molecular-weight compounds include trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol monohydroxypentaacrylate, dipentaerythritol hexaacrylate, 1,4-butylene glycol diacrylate, 1, 6-hexanediol diacrylate, polyethylene glycol diacrylate, an oligoester acrylate or a urethane acrylate.

The above resin composition may comprise a photopolymerization initiator, a solvent, etc. according to necessity.

In the preparation of the above hardening resin composition, generally, 10 to 50 parts by weight of hardening resin is blended with 100 parts by weight of thermoplastic elastomer. When the photohardening (photocurable) resin is used as the hardening resin, further, 0.1 to 2 parts by weight of photopolymerization initiator is added to the resin composition. A solvent is preferably also added in an appropriate amount in order to regulate coatability of the

resin for forming the constituent layer 2. As the solvent, use can be made of, for example, toluene, ethyl acetate, isopropyl alcohol or methyl ethyl ketone.

Moreover, in order to regulate the coefficient of static friction of layer surface, the resin composition for forming the second constituent layer 2 may comprise about 10 to 50 parts by weight of silica, calcium carbonate, zinc oxide, etc. per 100 parts by weight of thermoplastic elastomer.

The base material 11 of the surface protective sheet 10 according to the present invention may consist of the first constituent layer 1 and the second constituent layer 2 as mentioned above. Alternatively, the base material 11 may have a three-layer structure, for example, consisting of second constituent layer 2/first constituent layer 1 /second constituent layer 2. This three-layer structure would suppress the curling tendency of base material to thereby enable avoiding a workability lowering in chip production.

The total thickness of base material 11 is preferably in the range of 30 to 1000 pm, still preferably 50 to 800 pm, and optimally 80 to 500 pm.

The surface of base material 11 on which the pressure sensitive adhesive layer 12 is formed may be subjected to corona treatment or may be provided with another layer such

as a primer in order to increase the adherence to the pressure sensitive adhesive layer 12.

The surface protective sheet 10 of the present invention can be produced by superimposing the pressure sensitive adhesive layer 12 on the above base material 11. When the pressure sensitive adhesive layer 12 is constituted of an ultraviolet curable pressure sensitive adhesive, use is made of the base material 11 whose constituent layers are all transparent.

In the present invention, the elastic modulus at 23°C of the pressure sensitive adhesive for constituting the pressure sensitive adhesive layer 12 is preferably in the range of 5.0 X 104 to 1.0 X 108 Pa, still preferably 6.0 X 104 to 8.0 X 107 Pa, and optimally 7.0 X 104 to 5.0 X 107 Pa. When the elastic modulus of the pressure sensitive adhesive falls within these ranges, the fixing of wafer on the surface protective sheet 10 can be securely effected, thereby enhancing the wafer adaptability to grinding. When the pressure sensitive adhesive layer 12 is constituted of an energy radiation curable pressure sensitive adhesive as described later, the above elastic modulus means that before the curing by exposure to energy radiation.

The pressure sensitive adhesive layer 12 can be formed from various conventional pressure sensitive adhesives. As useful pressure sensitive adhesives, there can be mentioned,

for example, rubber, acrylic resin, silicone and polyvinyl ether based pressure sensitive adhesives, although these are not limitative. Further, use can be made of energy radiation curable and hydrophilic pressure sensitive adhesives. Energy radiation curable pressure sensitive adhesives are especially preferably used in the present invention.

The energy radiation curable pressure sensitive adhesives generally contains an acrylic pressure sensitive adhesive and an energy radiation polymerizable compound as main components.

For example, low-molecular-weight compounds having in the molecule thereof at least two photopolymerizable carbon to carbon double bonds that can be converted into a three-dimensional network structure by light irradiation as disclosed in Japanese Patent Laid-open Publication Nos.

60 (1985) -196956 and 60 (1985) -223139 are widely used as the energy radiation polymerizable compounds to be incorporated in the energy radiation curable pressure sensitive adhesives.

For example, use can be made of trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol monohydroxypentaacrylate, dipentaerythritol hexaacrylate, 1, 4-butyleneglycoldiacrylate, 1, 6-hexanedioldiacrylate,

polyethylene glycol diacrylate, an oligoester acrylate or a urethane acrylate oligomer.

With respect to the blending ratio of acrylic pressure sensitive adhesive and energy radiation polymerizable compound in the energy radiation curable pressure sensitive adhesive, it is preferred that 50 to 200 parts by weight, especially 50 to 150 parts by weight, and still especially 70 to 120 parts by weight of energy radiation polymerizable compound be blended with 100 parts by weight of acrylic pressure sensitive adhesive. When the blending ratio is as above, the obtained surface protective sheet exhibits a large initial bonding strength and, upon exposure to energy radiation, realizes a sharp drop of adhesive strength.

Therefore, the interfacial peeling of the energy radiation curable pressure sensitive adhesive layer from the wafer (chips) after the completion of wafer back grinding can be facilitated.

The energy radiation curable pressure sensitive adhesive may be prepared from an energy radiation curable copolymer having an energy radiation polymerizable group at its side chain. This energy radiation curable copolymer has such properties as ensure both pressure sensitive adherence and energy radiation curability. Detailed description of the energy radiation curable copolymer having an energy radiation polymerizable group at its side chain

is found in, for example, Japanese Patent Laid-open Publication Nos. 5 (1993) -32946 and 8 (1996)-27239.

The above acrylic energy radiation curable pressure sensitive adhesive exhibits satisfactory adhesive strength to the wafer before the exposure to energy radiation, which adhesive strength however markedly drops after the exposure to energy radiation. That is, the surface protective sheet 10 is stuck to the wafer with satisfactory adhesive strength before the exposure to energy radiation, thereby enabling surface protection. On the other hand, after the exposure to energy radiation, the surface protective sheet 10 can be easily peeled from the ground wafer (chips).

Moreover, as the hydrophilic pressure sensitive adhesive, use can be made of, for example, pressure sensitive adhesives described in Japanese Patent Laid-open Publication No. 10 (1998) -226776. The composition for providing such pressure sensitive adhesives comprises a copolymer, which is prepared from a carboxylated monomer (monomer containing a carboxyl group) and another monomer copolymerizable with the carboxylated monomer, a neutralizer and a crosslinking agent.

The carboxylated monomer is, for example, selected from among acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid and fumaric acid. As the other monomer copolymerizable with the carboxylated monomer, use

can be made of (meth) acrylic acid ester monomers containing alkyl group, such as 2-methoxyethyl (meth) acrylate, 2-ethoxyethyl (meth) acrylate, 3-methoxybutyl (meth) acrylate and 2-butoxyethyl (meth) acrylate, and also (meth) acrylic acid esters each having an alkyl group of 1 to 18 carbon atoms.

The neutralizer is used for neutralizing some or all of the carboxyl groups of the above copolymer so that hydrophilicity is imparted to the pressure sensitive adhesive composition. As this neutralizer, use can be made of organic amino compounds, such as monoethylamine, monoethanolamine, diethylamine, diethanolamine, triethylamine, triethanolamine and N, N, N'-trimethylethylenediamine.

The crosslinking agent is used for partially crosslinking the above copolymer. Examples of suitable crosslinking agents include epoxy crosslinking agents, isocyanate crosslinking agents, methylol crosslinking agents, chelate crosslinking agents and aziridine crosslinking agents.

With respect to the above hydrophilic pressure sensitive adhesive, not only is the amount of adhesive residue at the stage of peeling the surface protective sheet extremely small but also hydrophilicity is imparted to the adherent polymer per se, so that the water washability

thereof is excellent and, even when the pressure sensitive adhesive remains sticking to the wafer, it can be washed away with the use of pure water only.

The. surface protective sheet 10of the present invention can be obtained by coating the base material 11 with the above pressure sensitive adhesive at an appropriate thickness by customary means such as a knife coater, a roll coater, a gravure coater, a die coater or a reverse coater, and drying the coating to thereby form the pressure sensitive adhesive layer 12, followed by, if desired, sticking of a release sheet onto the pressure sensitive adhesive layer.

The thickness of the pressure sensitive adhesive layer 12, although depending on the characteristic of material of pressure sensitive adhesive layer per se and the shape of surface of the wafer as an adherend, is generally in the range of about 10 to 500 m, preferably about 10 to 300 pm.

With respect to chips of large surface height differences (high bump chips), the use of which is increasing in recent years, the requirement for smooth back grinding is now met by burying the surface height differences into the pressure sensitive adhesive layer. That is, the surface height differences between concave portions and convex portions are leveled by burying the pressure sensitive adhesive into concave portions to thereby enable smooth back grinding. Therefore, in the processing of such high bump

chips, it is preferred that the thickness of the pressure sensitive adhesive layer 12 be in the range of about 1.0 to 5.0 times, especially about 1.1 to 3.0 times the bump height difference. In place of increasing the thickness of the pressure sensitive adhesive layer 12, it is also appropriate to interpose a buffer layer between the base material 11 and the pressure sensitive adhesive layer 12.

When the buffer layer is interposed, the thickness of the pressure sensitive adhesive layer 12 is preferably in the range of about 5 to 30 pm.

The elastic modulus at 23°C of the buffer layer is preferably in the range of 5.0 X 104 to 1.0 X 107 Pa. The tan 8 value thereof is preferably 0.3 or greater. It is preferred that the thickness of the buffer layer be in the range of about 1.0 to 5.0 times, especially about 1.1 to 3.0 times the bump height.

As long as the buffer layer has the above thickness and characteristics, the material of the buffer layer is not limited. For example, the buffer layer may be composed of components used as pressure sensitive adhesives.

Although the buffer layer may be composed of the same components as those of the above pressure sensitive adhesive layer 12, it is appropriate to form general-purpose pressure sensitive adhesives into the buffer layer because the peelability is not needed.

The tan 8 is called the loss tangent and defined as the ratio of loss elastic modulus to storage elastic modulus.

In particular, the tan 8 is measured by the use of a dynamic viscoelasticity measuring instrument on the basis of response to a stress, such as tension or torsion, applied to an object.

The surface protective sheet 10 of the present invention is used for the protection of wafer surface and used as means for temporarily fixing the wafer in the wafer back grinding process comprising forming grooves of a cut depth smaller than the thickness of the wafer from the wafer surface furnished with circuits and grinding the back of the wafer so that the thickness of the wafer is reduced and so that the wafer is finally divided into individual chips.

More specifically, the surface protective sheet 10 is used in the wafer back grinding process comprising the following steps.

First step: forming grooves 4 of given depth along the wafer cutting sites partitioning a plurality of circuits from the surface of wafer 3 (see Fig. 2).

Second step: sticking the surface protective sheet 10 of the present invention so that the entire surface of the wafer 3 is covered by the surface protective sheet 10 (see Fig. 3).

Third step: grinding the back of the wafer until the bottom portion of the grooves 4 is removed and until the thickness of the wafer reaches given value, thereby dividing the wafer into individual chips 5 (see Fig. 4).

Thereafter, when the pressure sensitive adhesive layer 12 is constituted of the energy radiation curable pressure sensitive adhesive, the pressure sensitive adhesive layer 12 is exposed to energy radiation, thereby lowering the adhesive strength thereof. Upon recognition of the chip position and direction, pressure sensitive adhesive tape for pickup 20 (for example, mounting tape described in Japanese Patent Laid-open Publication Nos. 5 (1993)-335411) is stuck to the ground side of chips. The chip position and direction are adjusted so as to enable pickup, and the pressure sensitive adhesive tape for pickup 20 is secured onto a ring frame.

Subsequently, stripping tape 21 is bonded by heat sealing to the base material 11 of the surface protective sheet 10 (see Fig. 5). The stripping tape 21 is generally a heat salable tape comprising a base material of, for example, polyethylene terephthalate and, superimposed thereon, an adhesive layer composed mainly of, for example, a styrene/butadiene copolymer resin or an ethylene/olefin copolymer resin. This stripping tape 21 is heat sealed to the second constituent layer 2 being the outermost layer

of the base material 11. In the present invention, the second constituent layer 2 is constituted of the aforementioned resin with specified properties, so that the heat sealing of the stripping tape 21 can be easily accomplished.

Although the heat sealing conditions are appropriately selected depending on the softening points of resins constituting the stripping tape 21 and the second constituent layer 2, the heat sealing is generally performed at about 150 to 250°C under a pressure of about 0.1 to 1.0 MPa.

Thereafter, the surface protective sheet 10 is peeled with the stripping tape 21 as a starting point, thereby accomplishing transfer of the semiconductor chips 5 onto the pressure sensitive adhesive sheet for pickup 20 (see Fig. 6). Then, according to customary processing, the semiconductor chips 5 are picked up from the pressure sensitive adhesive sheet for pickup 20, and mounted on a given substrate.

The described production process is free from changing of the calf interval during the grinding step through the pickup step and enables enhancing the capability of recognition of semiconductor chips, thereby enabling efficiently producing semiconductor chips.

EFFECT OF THE INVENTION

The use of the surface protective sheet of the present invention, in the so-called predicing process, not only enables maintaining a given calf interval during and after the wafer back grinding but also facilitates fitting of the stripping tape.

EXAMPLE The present invention will further be illustrated below with reference to the following Examples which in no way limit the scope of the invention.

In the following Examples and Comparative Examples, <BR> the"Young's modulus", "elastic modulus","tan 8","ratio of reduction of calf interval","alignment","recognition of chip", "machine adaptability"and"condition after polishing"were evaluated in the following manner.

"Young's modulus" The Young's modulus was measured at a test speed of 200 mm/min in accordance with Japanese Industrial Standard (JIS) K-7127.

"Elastic modulus", G' (torsion shear method) With the use of viscoelasticity measuring instrument (trade name: Dynamic Analyzer RDA II, manufactured by <BR> Rheometric Scientific F. E. Ltd. ), the elastic modulus G' was measured with respect to a 8 mm diameter X 3 mm sample

according to the torsion shear method at 1 Hz in 23°C atmosphere.

"tan 8" The tan 8 was measured at a 11 Hz tensile stress by means of a dynamic viscoelasticity measuring instrument.

Specifically, each resin, for example, pressure sensitive adhesive was sampled into a given size, and the tan 8 thereof at 23°C was measured at a frequency of 11 Hz with the use of Rheovibron DDV-11-EP manufactured by Orientec Corporation.

"Ratio of reduction of calf interval" Dicingtape (trade name : Adwill D-628 producedbyLintec Corporation) was stuck to a given dummy wafer. Grooves were formed in the dummy wafer by means of half-cut dicing machine (DFD-651, manufactured by Disco Corporation) using a 35 ptm thick blade under conditions such that the cut depth was 70 jim and the chip sizes were 8 mm square and 12 mm square.

Subsequently, each of the surface protective sheets produced in the Examples and Comparative Examples was stuck to the grooved surface of the wafer by means of tape mounter (trade name: Adwill RAD-3500F/8 manufactured by Lintec Corporation). The dicing tape was peeled, and the back of the wafer was polished by means of grinding machine (DFG 850, manufactured by Disco Corporation) until the wafer thickness became 50 pn. The resultant wafer, with the

polished side up, was placed on a suction stage of an optical microscope, and the calf interval was measured. In this measurement, the calf intervals perpendicular to the sheet sticking direction were measured at 25 points from the wafer front side, rear side, right side, left side and center each in the directions parallel to and perpendicular to orientation flat, and an average thereof was calculated.

The average was defined as the calf interval. The ratio of reduction of calf interval was calculated by the formula: { (35 m-average of measurements)/35 lem} X 100.

In Examples 1, 3 and 4 and Comparative Examples 1 and 2, as the dummy wafer, use was made of wafers of 6 inch diameter, 650 to 700 pm thickness (thickness of portion where no dot printing was effected), 500 to 600 pm dot diameter, 105 urn dot height and 2.0 mm dot pitch (no printing was effected up to 20 mm from the wafer circumference).

In Example 2, as the dummy wafer, use was made of a wafer of 6 inch diameter, 650 to 700 pm thickness (thickness of portion where no dot printing was effected), 100 to 200 pm dot diameter, 25 pu dot height and 2. 0 mm dot pitch (no printing was effected up to 20 mm from the wafer circumference).

"Alignment" The thus produced chipped wafer was visually inspected, and evaluated as being"poor"when the calf intervals were

extremely different at varied points of the wafer or when chip dislocation occurred while evaluated as being"good" when uniform calf interval was ascertained by visual inspection.

"Recognition of chip" With the use of wafer transfer apparatus (LTD-2500f/8 manufactured by Lintec Corporation), the thus produced chipped wafer was aligned through recognition of grooves formed between chips, and transferred from the surface protective sheet to a pressure sensitive adhesive tape for pickup.

At the transfer, the stripping tape consisting of a heat salable tape comprising a polyethylene terephthalate film and, superimposed thereon, a styrene/butadiene heat salable adhesive was fitted to the base material of the surface protective sheet at 180 °C, and the surface protective sheet was peeled with the stripping tape as a starting point.

The recognition of chip was evaluated as being"good" when the transfer was achieved without any problem while evaluated as being"poor"when errors occurred.

"Machine adaptability" In the above sequence of predicing process, the machine adaptability was evaluated as being"good"when the steps till the peeling of surface protective sheet were completed without any problem while evaluated as being"poor"when

failure occurred during the process. The contents of failure were indicated.

"Condition after polishing" The chip back after polishing was inspected, and evaluated as being"good"when there was no crack and no dimple of 2 pu or greater while evaluated as being"poor" when there were cracks and/or dimples of 2 pm or greater.

Example 1 Polyethylene terephthalate (PET) film (thickness: 25 pm and Young's modulus: 5.4 kN/m2) as the first constituent layer and low density polyethylene (LDPE) film (thickness: 25 pu, density: 0.923 g/cm3, Tg: 47. 5°C and DSC endotherm: 0.515 J/g) as the second constituent layer were bonded to each other with the use of a urethane dry laminate adhesive to prepare a base material.

This base material on the PET film side was coated with a pressure sensitive adhesive composition consisting of a mixture of 100 parts by weight of acrylic pressure sensitive adhesive (copolymer of 90 parts by weight of n-butyl acrylate and 10 parts by weight of acrylic acid), 50 parts by weight of an oligomeric urethane resin of 8000 molecular weight, 2 parts by weight of a toluylene diisocyanate adduct of trimethylolpropane (hereinafter simply referred to as "TM-TDI") as a curing agent and a diluent solvent (toluene),

and dried, so that a 200 pm thick buffer layer (elastic modulus: 6.0 X 105 Pa and tan8 : 0.58) was formed on the base material. The buffer layer on the upper surface was coated with a pressure sensitive adhesive composition consisting of a mixture of 100 parts by weight of acrylic pressure sensitive adhesive (copolymer of 84 parts by weight of n-butyl acrylate, 8 parts by weight of methyl methacrylate, 5 parts by weight of 2-hydroxyethyl acrylate and 3 parts by weight of acrylic acid), 4 parts by weight of curing agent (TM-TDI) and a diluent solvent, and dried, so that a 10 pm thick pressure sensitive adhesive layer for fixing the wafer was formed. Thus, a surface protective sheet was obtained.

Evaluation results with respect to the"ratio of <BR> <BR> reduction of calf interval", "alignment","recognition of<BR> chip", "machine adaptability"and"condition after polishing"are listed in Table 1.

Example 2 PET film (thickness: 50 pm and Young's modulus: 5.4 kN/m2), as the first constituent layer, on its one side was coated with a coating liquid consisting of a mixture of 100 parts by weight of styrene/isoprene/styrene block copolymer as a thermoplastic elastomer, 30 parts by weight of dipentaerythritol hexaacrylate as an ultraviolet curable

resin, 0.9 part by weight of 1-hydroxycyclohexyl phenyl ketone as a photopolymerization initiator, 5 parts by weight of silica of 1. 5 to 2 jum average particle diameter and a diluent solvent (toluene), dried and exposed to ultraviolet light. Thus, the second constituent layer (thickness: 2 jim, Tg: 37. 2°C and DSC endotherm: 0.242 J/g) was formed on the first constituent layer. As a result, a base material was obtained.

This base material on the PET film side was coated with a pressure sensitive adhesive composition consisting of a mixture of 100 parts by weight of acrylic pressure sensitive adhesive (copolymer of 90 parts by weight of n-butyl acrylate and 10 parts by weight of acrylic acid) and 2 parts by weight of curing agent (TM-TDI), and dried, so that a 40 pm thick buffer layer (elastic modulus : 1. 4 X 105 Pa and tan 6 : 0. 41) was formed on the base material. The buffer layer on the upper surface was coated with a pressure sensitive adhesive composition consisting of a mixture of 100 parts by weight of acrylic pressure sensitive adhesive (copolymer of 84 parts by weight of n-butyl acrylate, 8 parts by weight of methyl methacrylate, 5 parts by weight of 2-hydroxyethyl acrylate and 3 parts by weight of acrylic acid) and 7 parts by weight of curing agent (TM-TDI), and dried, so that a 10 pm thick pressure sensitive adhesive layer for holding the wafer was formed. Thus, a surface protective sheet was obtained.

Evaluation results are listed in Table 1.

Example 3 Surface protective sheet was produced in the same manner as in Example 1 except that the structure of base material was changed to LDPE (25 pm)/PET (25 pm)/LDPE (25, um) [second constituent layer (glass transition temperature: 47. 5°C and DSC endotherm: 0.515 J/g) /first constituent layer (Young's modulus: 5. 4 kN/m2)/second constituent layer mentioned above].

Evaluation results are listed in Table 1.

Example 4 Surface protective sheet was produced in the same manner as in Example 1 except that the buffer layer was formed from pressure sensitive adhesive composition (elastic modulus: 7. 9 X 105 Pa and tan 8 : 0.44) consisting of a mixture of 100 parts by weight of acrylic pressure sensitive adhesive (ethyl acetate solution of copolymer of 70 parts by weight of 2-ethylhexyl acrylate, 28 parts by weight of vinyl acetate and 2 parts by weight of acrylic acid) and 0.5 part by weight of chelate curing agent (aluminum trisacetylacetonate) and except that the thickness of the buffer layer was 200 fJLm.

Evaluation results are listed in Table 1.

Comparative Example 1 Surface protective sheet was produced in the same manner as in Example 1 except that use was made of a base material consisting of a single layer of LDPE (thickness: 25 pm and Young's modulus: 2.0 kN/m2).

Evaluation results are listed in Table 1.

Comparative Example 2 Surface protective sheet was produced in the same manner as in Example 1 except that use was made of a base material consisting of a single layer of PET (thickness: 25 go, glass transition temperature: 77°C and DSC endotherm: less than 0.01 J/g).

Evaluation results are listed in Table 1.

Table 1 Calf interval (pm) Calf interval parallel to Perpendicular reduction orientation to orientation ratio flat flat Example 1 30 35 7. 1 Example 2 30 35 7. 1 Example 3 30 35 7. 1 Example 4 30 35 7. 1 Comp. Ex. 1 10 20 57 Comp. Ex. 2 30 35 7. 1 Table 1 (Continued) Recogni-Alignment Machine Condition tion of adapta-after chip bility polishing Example 1 good good good good Example 2 good good good good Example 3 good good good good Example 4 good good good good Comp. Ex. 1 poor poor poorl) poor Comp. Ex. 2 good good poor2) poor Poorl) : Poor recognition occurred by shrinkage of calf interval at the stage of sticking of pressure sensitive adhesive sheet for pickup.

Poor2) : Surface protective sheet could not be peeled.