JP2024096264A - Pressure-sensitive adhesive composition - Google Patents

Abstract

To provide an adhesive sheet that has sufficient adhesive strength, prevents adhesive leftover where an adhesive layer is transferred onto the adherend from which the adhesive sheet has been removed, and effectively absorbs surface irregularities.SOLUTION: An adhesive sheet includes a sheet-form base material and an adhesive layer formed on one surface of the base material. The adhesive layer comprises a cured product of an adhesive composition. The gel fraction is 50-65 mass%. The adhesive composition includes a polyurethane (A), a (meth)acrylic monomer (B) that comprises a compound having a (meth)acryloyloxy group, a chain transfer agent (C), and a photopolymerization initiator (D). The polyurethane (A) includes a polyurethane (a1) with a skeleton including a structure derived from a polyoxyalkylene polyol and a structure derived from a polyisocyanate, featuring (meth)acryloyl groups at two or more ends.SELECTED DRAWING: None

Description

The present invention relates to a pressure-sensitive adhesive sheet and a pressure-sensitive adhesive composition.
This application claims priority based on Japanese Patent Application No. 2019-138812, filed on July 29, 2019, the contents of which are incorporated herein by reference.

In response to the demand for thinner semiconductor devices, a backgrinding process is carried out on semiconductor wafers during the semiconductor device manufacturing process. In the backgrinding process, the front side of the semiconductor wafer is protected with an adhesive sheet, and the back side is ground to thin the semiconductor wafer.

Conventionally, various types of adhesive sheets have been proposed for use in protecting the surface of semiconductor wafers. In recent years, there has been a demand for adhesive sheets that have sufficient unevenness absorption properties for semiconductor wafers with uneven surfaces, such as semiconductor wafers with bumps (electrodes) made of solder or the like formed on the surface.

For example, Patent Document 1 discloses a backgrind sheet having an unevenness absorbing layer on a substrate, the unevenness absorbing layer being formed from a film-forming composition containing (A) a urethane (meth)acrylate and (B) a polymerizable monomer other than the component (A), and satisfying specific ranges of loss tangent, relaxation rate, and storage modulus.

International Publication No. 2014/046096

When performing the backgrinding process on a semiconductor wafer having bumps on its surface, the adhesive sheet used to protect the surface is required to have sufficient adhesive strength. However, if a highly adhesive adhesive sheet is attached to a semiconductor wafer having bumps on its surface and the backgrinding process is performed, after the backgrinding process, the adhesive layer of the adhesive sheet is transferred to the semiconductor wafer and glue residue is left around the bumps when the adhesive sheet is peeled off.

In addition, when performing a backgrinding process on a semiconductor wafer having bumps on its surface, if the adhesive sheet protecting the surface does not adequately absorb the unevenness of the semiconductor wafer, gaps will form around the bumps. If water used in the backgrinding process seeps into these gaps, the semiconductor wafer may become contaminated.

Thus, adhesive sheets that are to be attached to an adherend having an uneven surface and then peeled off are required to have sufficient adhesive strength, are unlikely to leave any adhesive residue, and have excellent unevenness absorbency.
However, conventional pressure-sensitive adhesive sheets do not have sufficient adhesive strength, are unlikely to leave adhesive residue, and do not have excellent irregularity absorption properties. Therefore, when using conventional pressure-sensitive adhesive sheets, in order to obtain sufficient adhesive strength and irregularity absorption properties for semiconductor wafers, it is necessary to provide an irregularity absorption layer in addition to the adhesive layer of the pressure-sensitive adhesive sheet, or to heat and soften the adhesive layer.

The present invention has been made in consideration of the above circumstances, and has an object to provide an adhesive sheet that has sufficient adhesive strength, is unlikely to leave adhesive residue on an adherend where the adhesive layer is transferred to the adherend after the adhesive sheet is peeled off, and has excellent unevenness absorbency.
Another object of the present invention is to provide a pressure-sensitive adhesive composition which has sufficient adhesive strength, is less likely to leave adhesive residue, and gives a cured product with excellent irregularity absorbency, and which is suitable as a material for the pressure-sensitive adhesive layer of a pressure-sensitive adhesive sheet.

A first aspect of the present invention is a pressure-sensitive adhesive sheet as follows.
[1] A pressure-sensitive adhesive sheet having at least a sheet-like substrate and a pressure-sensitive adhesive layer formed on one side of the substrate,
the pressure-sensitive adhesive layer is made of a cured product of a pressure-sensitive adhesive composition and has a gel fraction of 50 to 65% by mass;
The pressure-sensitive adhesive composition includes a polyurethane (A), a (meth)acrylic monomer (B) composed of a compound having a (meth)acryloyloxy group, a chain transfer agent (C), and a photopolymerization initiator (D),
The pressure-sensitive adhesive sheet, characterized in that the polyurethane (A) comprises a polyurethane (a1) having a skeleton including a structure derived from a polyoxyalkylene polyol and a structure derived from a polyisocyanate and having (meth)acryloyl groups at two or more ends.

The pressure-sensitive adhesive sheet according to the first aspect of the present invention preferably includes the following features [2] to [3]. Two or more of the following features may be combined with each other.
[2] The pressure-sensitive adhesive layer has a storage modulus of 1.0 × 10 ⁴ to 1.0 × 10 ⁵ at 25°C measured at a frequency of 1 Hz;
The pressure-sensitive adhesive sheet according to [1], wherein the loss tangent at 25° C. measured at a frequency of 1 Hz is 0.25 to 0.55.
[3] The pressure-sensitive adhesive sheet according to [1] or [2], wherein the pressure-sensitive adhesive layer has a thickness of 50 to 500 μm.

A second aspect of the present invention is the following adhesive composition.
[4] A polyurethane (A), a (meth)acrylic monomer (B) composed of a compound having a (meth)acryloyloxy group, a chain transfer agent (C), and a photopolymerization initiator (D),
the polyurethane (A) includes a polyurethane (a1) having a skeleton including a structure derived from a polyoxyalkylene polyol and a structure derived from a polyisocyanate and having (meth)acryloyl groups at two or more ends;
A pressure-sensitive adhesive composition, characterized in that the gel fraction of a cured product photocured at an irradiation dose of 1000 mJ/ ^cm2 is 50 to 65 mass%.

The adhesive composition according to the second aspect of the present invention preferably includes the following features [5] to [10]. The following features may be combined in any two or more.
[5] The cured product has a storage modulus of 1.0×10 ⁴ to 1.0×10 ⁵ at 25° C., as measured at a frequency of 1 Hz;
The pressure-sensitive adhesive composition according to [4], wherein the loss tangent at 25° C. measured at a frequency of 1 Hz is 0.25 to 0.55.

[6] The pressure-sensitive adhesive composition according to [4] or [5], wherein the (meth)acrylic monomer (B) contains a monofunctional (meth)acrylate and a polyfunctional (meth)acrylate.
[7] The pressure-sensitive adhesive composition according to [6], containing 85 to 99 mol % of the monofunctional (meth)acrylate and 1 to 15 mol % of the polyfunctional (meth)acrylate, when the total amount of the (meth)acrylic monomers (B) is 100 mol %.

[8] The pressure-sensitive adhesive composition according to any one of [4] to [7], wherein the chain transfer agent (C) is a polyfunctional thiol.
[9] 20 to 50% by mass of the polyurethane (A),
49 to 79% by mass of the (meth)acrylic monomer (B),
0.5 to 5% by mass of the chain transfer agent (C),
The pressure-sensitive adhesive composition according to any one of [4] to [8], comprising 0.01 to 5 mass% of the photopolymerization initiator (D).
[10] The pressure-sensitive adhesive composition according to any one of [4] to [9], further comprising a fatty acid ester (E).

The adhesive sheet of the present invention has an adhesive layer on one side of a sheet-like substrate, which is made of a cured product of a specific adhesive composition and has a gel fraction of 50 to 65% by mass. Therefore, the adhesive sheet of the present invention has sufficient adhesive strength, is unlikely to leave adhesive residue when the adhesive layer is transferred to an adherend after the adhesive sheet is peeled off, and has excellent irregularity absorption properties. Therefore, when the adhesive sheet of the present invention is used for applications in which it is attached to an adherend having an irregular surface and then peeled off, there is no need to provide an irregularity absorption layer separate from the adhesive layer or to heat and soften the adhesive layer, as in the case of using conventional adhesive sheets. Therefore, the adhesive sheet of the present invention is suitable for applications in which it is attached to an adherend having an irregular surface and then peeled off.

The adhesive composition of the present invention has a specific composition and gel fraction. Therefore, by curing the adhesive composition of the present invention, a cured product is obtained that has sufficient adhesive strength, is less likely to leave adhesive residue, and has excellent unevenness absorption properties. Therefore, the adhesive composition of the present invention is suitable as a material for the adhesive layer in an adhesive sheet that is applied to an adherend having uneven surfaces and then peeled off.

The pressure-sensitive adhesive sheet and pressure-sensitive adhesive composition of the present invention will be described in detail below. Note that the present invention is not limited to the following embodiments. Within the scope of the present invention, it is possible to omit, change, replace, and/or add types, amounts, compositions, ratios, numbers, values, positions, sizes, and the like, as necessary.
<Adhesive sheet>
The pressure-sensitive adhesive sheet of the present embodiment has a sheet-like substrate and a pressure-sensitive adhesive layer formed on one surface of the substrate. The pressure-sensitive adhesive layer is made of a cured product of a pressure-sensitive adhesive composition described below.

The material of the substrate can be appropriately selected, and examples thereof include resin materials. Examples of resin materials include polyolefins such as polyethylene (PE) and polypropylene (PP); polyester sheets such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate; polyvinyl chloride (PVC); polyimide (PI); polyphenylene sulfide (PPS); ethylene vinyl acetate (EVA); and polytetrafluoroethylene (PTFE). Among these resin materials, PE, PP, and PET are preferably used because they provide a sheet with appropriate flexibility. The resin materials may be used alone or in combination of two or more types.

When a resin sheet made of a resin material is used as the substrate, the resin sheet may be a single layer or a multi-layer structure of two or more layers (e.g., a three-layer structure). In a resin sheet having a multi-layer structure, the resin material constituting each layer may be a resin material containing only one type alone or a resin material containing two or more types.

The substrate may be one that has been subjected to an antistatic treatment. The antistatic treatment applied to the substrate is not particularly limited, but may be a method of providing an antistatic layer on at least one surface of the substrate, a method of kneading an antistatic agent into the substrate, or the like.
Furthermore, the surface of the substrate on which the pressure-sensitive adhesive layer is formed may be subjected to an adhesion-improving treatment such as acid treatment, alkali treatment, primer treatment, corona treatment, plasma treatment, ultraviolet treatment, or ozone treatment, if necessary.

The thickness of the substrate can be appropriately selected depending on the material of the substrate. When the adhesive sheet is used to protect a semiconductor wafer having an uneven surface during processing of the semiconductor wafer, and a resin sheet is used as the substrate, the thickness of the substrate is preferably, for example, 10 to 1000 μm, and more preferably 50 to 300 μm. When the thickness of the substrate is 10 μm or more, the rigidity (stiffness) of the adhesive sheet is high. Therefore, when the adhesive sheet is attached to or removed from the semiconductor wafer as the adherend, the adhesive sheet tends to be less likely to wrinkle or float. In addition, when the thickness of the substrate is 1000 μm or less, the adhesive sheet attached to the semiconductor wafer can be easily peeled off from the semiconductor wafer, improving workability (handling).

The thickness of the adhesive layer is preferably 50 to 500 μm, more preferably 60 to 400 μm, and even more preferably 70 to 300 μm. If the thickness of the adhesive layer is 50 μm or more, the adhesive sheet will have better unevenness absorption properties. Furthermore, if the thickness of the adhesive layer is 500 μm or less, it will be easier to control the thickness of the adhesive layer.

When the adhesive sheet of this embodiment is an adhesive sheet to be attached to an adherend having an uneven surface, the thickness of the adhesive layer depends largely on the height of the uneven surface. The thickness of the adhesive layer is preferably equal to or greater than the height of the uneven surface so as to obtain sufficient unevenness absorption. Therefore, for example, when the uneven surface is a bump formed on a semiconductor wafer, the thickness of the adhesive layer is preferably at least twice the height of the bump. The height of the bump is usually 30 to 200 μm. For example, when the bump height is 100 μm, the thickness of the adhesive layer is preferably at least 200 μm, and when the bump height is 200 μm, the thickness of the adhesive layer is preferably at least 400 μm.

The adhesive layer of the adhesive sheet of this embodiment has a gel fraction in the range of 50 to 65% by mass. Therefore, the adhesive sheet of this embodiment has sufficient adhesive strength, is less likely to leave adhesive residue, and has excellent unevenness absorbency. The gel fraction of the adhesive layer is preferably 52% by mass or more. In addition, the gel fraction of the adhesive layer is preferably 63% by mass or less.
On the other hand, if the gel fraction is less than 50% by mass, adhesive residue is likely to occur when the adhesive sheet is peeled off after being applied to the adherend. Also, if the gel fraction is more than 65% by mass, the fluidity tends to be insufficient when used as the adhesive layer of the adhesive sheet. Therefore, when the adhesive sheet is sent to an adherend having an uneven surface, gaps are likely to occur between the uneven surface of the adherend and the adhesive sheet.

(Measurement of Gel Fraction of Pressure-Sensitive Adhesive Layer)
The gel fraction of the pressure-sensitive adhesive layer can be measured, for example, by the method described below.
A square sheet measuring 8 cm in length and 8 cm in width is cut out from the pressure-sensitive adhesive sheet, and the substrate is peeled off from the pressure-sensitive adhesive layer of the square sheet obtained. The pressure-sensitive adhesive layer peeled off from the square sheet is used as a measurement sample. The obtained measurement sample can be measured using a method similar to the method for measuring the gel fraction of a measurement sample made of a cured product of a pressure-sensitive adhesive composition described below.

The adhesive layer of the adhesive sheet of this embodiment preferably has a storage modulus of 1.0×10 ⁴ to 1.0×10 ⁵ at 25° C. measured at a frequency of 1 Hz, and a loss tangent of 0.25 to 0.55 at 25° C. measured at a frequency of 1 Hz. An adhesive layer having a storage modulus and loss tangent in the above ranges has good flexibility and flowability. Therefore, when the adhesive sheet is attached to a work (adherend) such as a semiconductor wafer having uneven parts such as bumps, it is possible to prevent the occurrence of voids between the uneven parts and the adhesive sheet.

The storage modulus of the pressure-sensitive adhesive layer is more preferably 2.0×10 ⁴ or more, and even more preferably 3.0×10 ⁴ or more. Moreover, the storage modulus is more preferably 9.0×10 ⁴ or less, and even more preferably 8.0×10 ⁴ or less.
The adhesive layer having a storage modulus of 1.0×10 ⁴ or more is not too soft. Therefore, an adhesive sheet having an adhesive layer having a storage modulus of 1.0×10 ⁴ or more is more unlikely to leave adhesive residue when peeled off after being applied to an adherend, which is preferable. In addition, an adhesive layer having a storage modulus of 1.0×10 ⁵ or less has good flexibility. Therefore, when an adhesive sheet having an adhesive layer having a storage modulus of 1.0×10 ⁵ or less is applied to an adherend having an uneven surface, it is more unlikely to generate a gap between the adherend and the uneven portion, which is preferable.

The loss tangent of the pressure-sensitive adhesive layer is more preferably 0.30 or more. Also, the loss tangent is more preferably 0.50 or less.
The pressure-sensitive adhesive layer having a loss tangent of 0.25 or more has good fluidity. Therefore, when a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer having a loss tangent of 0.25 or more is attached to an adherend having an uneven surface, a gap is unlikely to occur between the adhesive layer and the uneven surface of the adherend, which is preferable. In addition, the pressure-sensitive adhesive layer having a loss tangent of 0.55 or less does not have too high fluidity. Therefore, a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer having a loss tangent of 0.55 or less is unlikely to leave adhesive residue even when peeled off after being attached to an adherend, which is preferable.

(Measurement of storage modulus and loss tangent of adhesive layer)
The storage modulus and loss tangent of the pressure-sensitive adhesive layer can be measured, for example, by the following method. First, the substrate is peeled off from the pressure-sensitive adhesive sheet. Then, the pressure-sensitive adhesive layer from which the substrate has been peeled off is laminated to obtain a laminated sheet having a thickness in the range of 1 to 2 mm. A square sheet having a length of 8 mm and a width of 8 mm is cut out from the obtained laminated sheet to obtain a measurement sample. The obtained measurement sample can be measured using the same method as the measurement method for the storage modulus and loss tangent of a measurement sample made of a cured product of a pressure-sensitive adhesive composition described below.

The adhesive sheet may be provided with a transparent separator on the surface of the adhesive layer opposite the substrate in order to protect the adhesive layer. The separator is preferably laminated on the surface of the adhesive layer. For example, paper, plastic film, etc. can be used as the material for the separator, and it is preferable to use a plastic film in terms of excellent surface smoothness. The plastic film used as the separator is not particularly limited as long as it can protect the above-mentioned adhesive layer, and examples include polyethylene, polypropylene, polyethylene terephthalate, polybutene, etc.

<Adhesive Composition>
The adhesive layer of the adhesive sheet of the present embodiment is made of a cured product of an adhesive composition. The adhesive composition used as a material for the adhesive layer of the adhesive sheet of the present embodiment will be described in detail below.
The pressure-sensitive adhesive composition of the present embodiment contains a polyurethane (A), a (meth)acrylic monomer (B), a chain transfer agent (C), and a photopolymerization initiator (D).

(Polyurethane (A))
The polyurethane (A) contains polyurethane (a1). The polyurethane (A) may contain not only polyurethane (a1) but also polyurethane (a2) described below for the purpose of adjusting the cohesive strength in the cured product of the pressure-sensitive adhesive composition. It is preferable that the polyurethane (A) does not contain any components other than polyurethane (a1) and polyurethane (a2) contained as necessary.

[Polyurethane (a1)]
The polyurethane (a1) has a skeleton including a structure derived from a polyoxyalkylene polyol and a structure derived from a polyisocyanate. The polyurethane (a1) has two or more (meth)acryloyl groups at its terminals, i.e., at a plurality of terminals. The (meth)acryloyl groups at the terminals of the polyurethane (a1) are preferably part of a (meth)acryloyloxy group.

In the present invention, the "multiple ends" of a polyurethane refer to two ends when the polyurethane is a linear polymer, and refer to two or more ends out of the same number of ends as the number of branched chains when the polyurethane is a branched polymer.
In the present invention, the (meth)acryloyl group means one or more groups selected from the functional group represented by the chemical formula CH ₂ ═CH—CO— and the functional group represented by the chemical formula CH ₂ ═C(CH ₃ )—CO—.

[Polyurethane (a2)]
The polyurethane (a2) has a skeleton containing a structure derived from a polyoxyalkylene polyol and a structure derived from a polyisocyanate, similar to the polyurethane (a1). Unlike the polyurethane (a1), the polyurethane (a2) has a (meth)acryloyl group at only one end. The (meth)acryloyl group at the end of the polyurethane (a2) is preferably a part of a (meth)acryloyloxy group. The end of the polyurethane (a2) that does not have a (meth)acryloyl group preferably has any one selected from an isocyanato group, a structure derived from an alkyl alcohol, and a structure derived from an alkyl isocyanate, and more preferably has a structure derived from an alkyl alcohol.

"Polyoxyalkylene polyol-derived structure"
The polyoxyalkylene polyol having a structure derived from the polyoxyalkylene polyol contained in the skeleton of the polyurethane (a1) and the polyurethane (a2) preferably has an alkylene chain having 2 to 4 carbon atoms. Specific examples include polyoxyethylene polyol, polyoxypropylene polyol, polyoxybutylene polyol, etc.

The polyoxyalkylene polyol having a structure derived from the polyoxyalkylene polyol may contain one type of alkylene chain, or may contain two or more types of alkylene chains.
The polyoxyalkylene polyol having a structure derived from polyoxyalkylene polyol is preferably one having two or three hydroxyl groups at the terminals (diol-type or triol-type polyoxyalkylene polyol), more preferably a polyoxyalkylene glycol (diol-type), and particularly preferably a polypropylene glycol having an alkylene chain with three carbon atoms.

For example, when the polyoxyalkylene polyol is polypropylene glycol, the hydroxyl value is preferably 20 to 120 mgKOH/g, more preferably 30 to 100 mgKOH/g, and even more preferably 40 to 80 mgKOH/g. Specific examples of polypropylene glycol include polypropylene glycol having a hydroxyl group (hydroxy group) at the end and a hydroxyl value of 56 mgKOH/g (Actocol D-2000; manufactured by Mitsui Chemicals, number average molecular weight 2000, diol type).

Here, the hydroxyl value of polyoxyalkylene polyol is the hydroxyl value of polyoxyalkylene polyol measured according to JIS K0070. In other words, it means the number of milligrams of potassium hydroxide required to neutralize the free acetic acid when 1 g of polyoxyalkylene polyol is acetylated. Specifically, it can be determined by acetylating the hydroxyl groups in a sample (polyoxyalkylene polyol) with acetic anhydride and titrating the free acetic acid generated during this process with a potassium hydroxide solution.

The number average molecular weight of the polyoxyalkylene polyol is preferably 500 to 5,000, more preferably 800 to 4,000, and even more preferably 1,000 to 3,000. When the number average molecular weight of the polyoxyalkylene polyol is 500 or more, the adhesive sheet having an adhesive layer made of a cured product of an adhesive composition containing polyurethane (A) synthesized using the polyoxyalkylene polyol has high peel strength. When the number average molecular weight of the polyoxyalkylene polyol is 5,000 or less, the polyurethane (A) synthesized using the polyoxyalkylene polyol contains a sufficient amount of urethane bonds. Therefore, the cured product obtained by curing the adhesive composition containing polyurethane (A) has good cohesion.

The structure derived from the polyoxyalkylene polyol contained in the skeleton of each of the polyurethanes (a1) and (a2) may be of only one type, or may be of two or more types.
The polyurethane (a1) and the polyurethane (a2) may have a structure in which structures derived from two or more different polyoxyalkylene polyols are bonded via a structure derived from a polyisocyanate.
The structure derived from a polyoxyalkylene polyol contained in the skeleton of the polyurethane (a1) and the structure derived from a polyoxyalkylene polyol contained in the skeleton of the polyurethane (a2) may be the same as or different from each other.

"Polyisocyanate-derived structure"
As the polyisocyanate having the polyisocyanate-derived structure contained in the skeleton of the polyurethane (a1) and polyurethane (a2), a compound having a plurality of isocyanate groups is used, and it is preferable to use a diisocyanate.
Examples of diisocyanates include tolylene diisocyanate and hydrogenated products thereof, xylylene diisocyanate and hydrogenated products thereof, diphenylmethane diisocyanate and hydrogenated products thereof, 1,5-naphthylene diisocyanate and hydrogenated products thereof, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, tetramethylxylylene diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexyl diisocyanate, 1,3-bis(isocyanatemethyl)cyclohexane, and norbornane diisocyanate.

Among these polyisocyanates, it is preferable to use isophorone diisocyanate or a hydrogenated product of diphenylmethane diisocyanate from the viewpoint of controlling the light resistance of the polyurethane (A) synthesized using the polyisocyanate and the reactivity with polyoxyalkylene polyol. It is more preferable to use a hydrogenated product of diphenylmethane diisocyanate from the viewpoint of reactivity with polyoxyalkylene polyol.

Specific examples of polyisocyanates with a polyisocyanate-derived structure include hydrogenated diphenylmethane diisocyanate (Desmodur W, manufactured by Sumika Covestro Urethanes) and isophorone diisocyanate (Desmodur I, manufactured by Sumika Covestro Urethanes).

The polyisocyanate-derived structure contained in the skeleton of each of the polyurethane (a1) and the polyurethane (a2) may be of only one type, or may be of two or more types.
Furthermore, the structure derived from a polyisocyanate contained in the skeleton of the polyurethane (a1) and the structure derived from a polyisocyanate contained in the skeleton of the polyurethane (a2) may be the same as or different from each other.

When the polyisocyanate-derived structure and the polyoxyalkylene polyol-derived structure contained in the skeletons of polyurethane (a1) and polyurethane (a2) are the same, polyurethane (a1) and polyurethane (a2) can be synthesized simultaneously, and polyurethane (A) can be produced efficiently, which is preferable.

The proportion of polyurethane (a1) contained in polyurethane (A) is preferably 80 to 100%, more preferably 90 to 100%, and even more preferably 100%, of polyurethane (A) in terms of the number of molecules.
The proportion of polyurethane (a2) contained in polyurethane (A) is preferably 0 to 20%, more preferably 0 to 10%, and even more preferably 0%, of polyurethane (A) in terms of the number of molecules.
When the proportion of polyurethane (a1) contained in polyurethane (A) is 80% or more, the cured product of the pressure-sensitive adhesive composition containing polyurethane (A) has sufficiently high cohesive strength, which is preferable.

Of the total number of terminals contained in polyurethane (A) (the total number of terminals of polyurethane (a1) and polyurethane (a2) contained as necessary), it is preferable that 90 to 100% of the terminals are (meth)acryloyl groups are introduced on a molecular number basis, more preferably 95 to 100%, and even more preferably 100%. If the amount of (meth)acryloyl groups introduced is 90% or more of the total number of terminals contained in polyurethane (A) on a molecular number basis, the cohesive strength of the cured product obtained by curing the pressure-sensitive adhesive composition containing polyurethane (A) is sufficiently high.

The ratio of the number of terminals into which a (meth)acryloyl group has been introduced, based on the number of molecules, to the total number of terminals contained in polyurethane (A) can be calculated using the results of analyzing polyurethane (A) using infrared absorption spectroscopy (IR) or nuclear magnetic resonance spectroscopy (NMR), etc.

The ratio of the contents of polyurethane (a1) and polyurethane (a2) contained in polyurethane (A), i.e., the ratio of the number of terminals into which a (meth)acryloyl group has been introduced on a molecular basis to the total number of terminals contained in polyurethane (A), can be adjusted by the manufacturing method of polyurethane (A) described below.

The mass average molecular weight of polyurethane (A) is preferably 30,000 to 200,000, more preferably 50,000 to 150,000, and even more preferably 60,000 to 100,000. When the mass average molecular weight of polyurethane (A) is 30,000 or more, the cured product obtained by curing the adhesive composition containing polyurethane (A) has good flexibility. When the mass average molecular weight of polyurethane (A) is 200,000 or less, the adhesive composition containing polyurethane (A) is easy to handle and has good workability.

(Method for measuring weight average molecular weight of polyurethane (A))
The weight average molecular weight of the polyurethane (A) is a value calculated in terms of polystyrene measured by gel permeation chromatography (GPC-101; Shodex (registered trademark) manufactured by Showa Denko K.K.) (hereinafter referred to as GPC). The measurement conditions for GPC are as follows.
Column: LF-804 (Showa Denko K.K.)
Column temperature: 40°C
Sample: 0.2% by mass solution of polyurethane (A) in tetrahydrofuran Flow rate: 1 ml/min Eluent: tetrahydrofuran Detector: RI detector (differential refractive index detector)

The content of polyurethane (A) in the adhesive composition of this embodiment is preferably 20 to 50% by mass, more preferably 25 to 45% by mass, and even more preferably 30 to 40% by mass. When the content of polyurethane (A) is 20% by mass or more, the cured product obtained by curing the adhesive composition has sufficient cohesive strength and excellent adhesive strength. In addition, an adhesive sheet using this cured product as an adhesive layer has an appropriate range of softness of the adhesive layer, and air bubbles are unlikely to be trapped between the adhesive layer and the adherend. In addition, when the content of polyurethane (A) is 50% by mass or less, the cured product obtained by curing the adhesive composition has sufficient flexibility. Therefore, an adhesive sheet using this cured product as an adhesive layer has good wettability to the adherend.

((Meth)acrylic monomer (B))
The (meth)acrylic monomer (B) is not particularly limited as long as it is a compound having a (meth)acryloyloxy group. As the (meth)acrylic monomer (B), a compound having a (meth)acryloyloxy group may be used alone or in combination of two or more. As the (meth)acrylic monomer (B), a monofunctional (meth)acrylate may be used, a polyfunctional (meth)acrylate may be used, or both a monofunctional (meth)acrylate and a polyfunctional (meth)acrylate may be used.

In the present invention, the term "monofunctional" in the monofunctional (meth)acrylate means a (meth)acrylate having only one (meth)acryloyloxy group.
In the present invention, the term "polyfunctional" in the polyfunctional (meth)acrylate means a (meth)acrylate having two or more (meth)acryloyloxy groups.

As the (meth)acrylic monomer (B), from the viewpoint of the cohesive strength of the cured product obtained by curing the adhesive composition and the curability of the adhesive composition, it is preferable to use a combination of a monofunctional (meth)acrylate and a polyfunctional (meth)acrylate, more preferably a monofunctional (meth)acrylate and a trifunctional or higher (meth)acrylate, and most preferably a monofunctional (meth)acrylate and a trifunctional (meth)acrylate having three (meth)acryloyloxy groups.

Examples of monofunctional (meth)acrylates include alkyl (meth)acrylates, cyclic alkyl (meth)acrylates such as isobornyl (meth)acrylate, alkoxyalkyl (meth)acrylates, alkoxy(poly)alkylene glycol (meth)acrylates, hydroxyl group-containing (meth)acrylates, carboxyl group-containing (meth)acrylates, fluorinated alkyl (meth)acrylates, dialkylaminoalkyl (meth)acrylates, (meth)acrylamides, epoxy group-containing (meth)acrylates, and (meth)acryloylmorpholine.

Among these monofunctional (meth)acrylates, it is preferable to contain an alkyl (meth)acrylate, because the adhesive strength (peel strength), storage modulus, loss tangent, and gel fraction of the cured product obtained by curing the pressure-sensitive adhesive composition are more likely to fall within appropriate ranges when the cured product is used as an adhesive layer of a pressure-sensitive adhesive sheet.
As the alkyl (meth)acrylate, it is more preferable to use an alkyl (meth)acrylate having an alkyl group having 4 to 10 carbon atoms. Specific examples of the alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, etc. Among these, it is particularly preferable to use 2-ethylhexyl (meth)acrylate and/or n-butyl (meth)acrylate.

The polyfunctional (meth)acrylate is a compound having a plurality of (meth)acryloyloxy groups other than the polyurethane (A). As the polyfunctional (meth)acrylate, it is preferable to use a poly(meth)acrylate of a polyol compound.
Specific examples of the polyfunctional (meth)acrylate include polyethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, hydroxypivalic acid ester neopentyl glycol di(meth)acrylate, 1,3-bis(hydroxyethyl)-5,5-dimethylhydantoin di(meth)acrylate, α,ω-di(meth)acryl bisdiethylene glycol phthalate, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, diacryloxyethyl phosphate, and pentaerythritol tetra(meth)acrylate. Among these, it is preferable to use trimethylolpropane tri(meth)acrylate as the polyfunctional (meth)acrylate from the viewpoint of the curability of the pressure-sensitive adhesive composition.

When (meth)acrylic monomer (B) contains both monofunctional (meth)acrylate and polyfunctional (meth)acrylate, it is preferable that the monofunctional (meth)acrylate is contained in an amount of 85 to 99 mol % and the polyfunctional (meth)acrylate is contained in an amount of 1 to 15 mol % when the total amount of (meth)acrylic monomer (B) is taken as 100 mol %. In this case, the content of the monofunctional (meth)acrylate is more preferably 90 to 99 mol %, and even more preferably 95 to 98 mol %. The content of the polyfunctional (meth)acrylate is more preferably 1 to 10 mol %, and even more preferably 2 to 5 mol %.

When the adhesive composition contains a monofunctional (meth)acrylate and a polyfunctional (meth)acrylate, if the content of the monofunctional (meth)acrylate is 85 mol% or more, the fluidity of the cured product obtained by curing the adhesive composition is in a preferred range when the cured product is used as an adhesive layer of an adhesive sheet. Therefore, an adhesive sheet using this cured product as an adhesive layer has sufficient unevenness absorption properties, and when applied to an adherend having uneven parts on its surface, it is less likely to generate gaps between the uneven parts of the adherend, which is preferable. In addition, if the content of the monofunctional (meth)acrylate is 99 mol% or less, an adhesive sheet using the cured product of the adhesive composition as an adhesive layer is less likely to leave adhesive residue when peeled off from the adherend, which is preferable.

When the adhesive composition contains a monofunctional (meth)acrylate and a polyfunctional (meth)acrylate, the content of the polyfunctional (meth)acrylate is 1 mol% or more, and the fluidity of the cured product obtained by curing the adhesive composition is not too high, which is preferable. Also, when the content of the polyfunctional (meth)acrylate is 15 mol% or less, the fluidity of the cured product obtained by curing the adhesive composition is in a preferable range when the cured product is used as an adhesive layer of an adhesive sheet. Therefore, an adhesive sheet using the cured product as an adhesive layer has sufficient unevenness absorption properties, and when the adhesive sheet is attached to an adherend having uneven parts on the surface, voids are unlikely to occur between the adhesive sheet and the uneven parts of the adherend, which is preferable.

The content of (meth)acrylic monomer (B) in the adhesive composition of this embodiment is preferably 49 to 79% by mass, more preferably 53 to 73% by mass, and even more preferably 56 to 66% by mass. When the content of (meth)acrylic monomer (B) is 49% by mass or more, the viscosity of the adhesive composition does not become too high, and excellent coatability is achieved, which is preferable. Furthermore, when the content of (meth)acrylic monomer (B) is 79% by mass or less, the viscosity of the adhesive composition does not become too low, and it is easy to control the thickness of the coating film made of the adhesive composition, which is preferable.

(Chain Transfer Agent (C))
The chain transfer agent (C) is contained in the pressure-sensitive adhesive composition for the purpose of controlling the storage modulus, loss tangent, and gel fraction of the cured product obtained by curing the pressure-sensitive adhesive composition.
As the chain transfer agent (C), for example, a polyfunctional thiol can be preferably used. The polyfunctional thiol is a compound having two or more mercapto groups in the molecule.

The polyfunctional thiol is not particularly limited, but examples thereof include 1,2-ethanedithiol, 1,4-bis(3-mercaptobutyryloxy)butane, tetraethylene glycol bis(3-mercaptopropionate), trimethylolethane tris(3-mercaptobutyrate), trimethylolpropane tris(3-mercaptobutyrate), trimethylolpropane tris(3-mercaptopropionate), pentaerythritol, Examples of the chain transfer agent (C) include pentaerythritol tetrakis(3-mercaptobutyrate), pentaerythritol tetrakis(3-mercaptopropionate), 1,3,5-tris(3-mercaptobutyloxyethyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, tris-[(3-mercaptopropionyloxy)-ethyl]-isocyanurate, and dipentaerythritol hexakis(3-mercaptopropionate). Among the above, it is preferable to use pentaerythritol tetrakis(3-mercaptobutyrate) as the chain transfer agent (C) from the viewpoint of the reactivity of the adhesive composition.

The content of the chain transfer agent (C) in the adhesive composition of this embodiment is preferably 0.5 to 5 mass%, more preferably 1 to 5 mass%, and even more preferably 3 to 4.5 mass%. When the content of the chain transfer agent (C) is 0.5 mass% or more, the fluidity of the cured product obtained by curing the adhesive composition is in a preferred range when the cured product is used as an adhesive layer of an adhesive sheet. Therefore, an adhesive sheet using the cured product as an adhesive layer has sufficient unevenness absorption properties and is less likely to generate gaps between the cured product and the uneven parts of the adherend, which is preferable. When the content is 5 mass% or less, an adhesive sheet using the cured product of the adhesive composition as an adhesive layer is less likely to leave adhesive residue when peeled off from the adherend, which is preferable.

(Photopolymerization Initiator (D))
The photopolymerization initiator (D) is not particularly limited, but a photoradical polymerization initiator is preferred. As the photopolymerization initiator (D), for example, a carbonyl-based photopolymerization initiator, a sulfide-based photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator, a quinone-based photopolymerization initiator, a sulfochloride-based photopolymerization initiator, a thioxanthone-based photopolymerization initiator, etc. can be used. Among these photopolymerization initiators (D), from the viewpoint of the transparency of the cured product obtained by photocuring the pressure-sensitive adhesive composition, it is preferable to use an acylphosphine oxide-based photopolymerization initiator, and specifically, it is preferable to use 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

The content of the photopolymerization initiator (D) in the adhesive composition of this embodiment is preferably 0.01 to 5 mass%, more preferably 0.05 to 3 mass%, and even more preferably 0.1 to 2 mass%. When the content of the photopolymerization initiator (D) is 0.01 mass% or more, the photocuring of the adhesive composition proceeds sufficiently. Furthermore, when the content of the photopolymerization initiator (D) is 5 mass% or less, the amount of low molecular weight components does not become too large during photocuring of the adhesive composition. Therefore, when an adhesive sheet using a cured product of the adhesive composition as an adhesive layer is peeled off from an adherend, adhesive residue is less likely to be left, which is preferable.

(Fatty Acid Ester (E))
The pressure-sensitive adhesive composition of the present embodiment contains a polyurethane (A), a (meth)acrylic monomer (B), a chain transfer agent (C), and a photopolymerization initiator (D), and may further contain a fatty acid ester (E) as necessary.
The fatty acid ester (E) is contained for the purposes of controlling the adhesive strength of an adhesive sheet using a cured product of the adhesive composition as an adhesive layer, and improving the lamination properties (wettability) and bubble release properties (ease of release of air bubbles trapped when the adhesive sheet is attached to an adherend) of the adhesive layer.

As the fatty acid ester (E), an ester of a fatty acid and an alkyl alcohol can be used, and from the viewpoint of compatibility with other components, it is preferable to use a fatty acid ester selected from an ester of a fatty acid having 8 to 18 carbon atoms and a monofunctional alcohol having a branched hydrocarbon group having 3 to 18 carbon atoms, and an ester of an unsaturated fatty acid having 14 to 18 carbon atoms and a di- to tetra-functional alcohol.

Examples of esters of fatty acids having 8 to 18 carbon atoms and monofunctional alcohols having a branched hydrocarbon group having 3 to 18 carbon atoms include isostearyl laurate, isopropyl myristate, isocetyl myristate, octyldodecyl myristate, isopropyl palmitate, isostearyl palmitate, isocetyl stearate, 2-ethylhexyl stearate, octyldodecyl oleate, diisostearyl adipate, diisocetyl sebacate, trioleyl trimellitate, and triisocetyl trimellitate. Among these, it is preferable to use isopropyl myristate, isopropyl palmitate, and 2-ethylhexyl stearate, and it is particularly preferable to use isopropyl myristate and/or 2-ethylhexyl stearate.

Examples of esters of unsaturated fatty acids having 14 to 18 carbon atoms with difunctional to tetrafunctional alcohols include esters of unsaturated fatty acids such as myristoleic acid, oleic acid, linoleic acid, linolenic acid, isopalmitic acid, and isostearic acid with alcohols such as ethylene glycol, propylene glycol, glycerin, trimethylolpropane, pentaerythritol, and sorbitan.

The content of fatty acid ester (E) in the adhesive composition of this embodiment is preferably 3 to 18 mass%, and more preferably 5 to 15 mass%. When the content of fatty acid ester (E) is 3 mass% or more, the adhesive strength of an adhesive sheet using the cured product of the adhesive composition as an adhesive layer falls within a preferred range for an adhesive sheet, and the lamination property and bubble releasing property of the adhesive layer become good. When the content of fatty acid ester (E) is 18 mass% or less, adhesive residue containing fatty acid ester (E) is unlikely to be left when an adhesive sheet using the cured product of the adhesive composition as an adhesive layer is peeled off from an adherend, which is preferable.

(solvent)
The pressure-sensitive adhesive composition of the present embodiment may contain a solvent, but is more preferably a solvent-free composition that is substantially free of solvent.
When the pressure-sensitive adhesive composition of the present embodiment contains a solvent, the solvent can be used, for example, as a leveling agent and/or a softening agent.

When the adhesive composition of the present embodiment is solvent-free, the step of heating and drying the solvent can be omitted when using the adhesive composition to form an adhesive layer of an adhesive sheet, resulting in excellent productivity. In particular, when using the adhesive composition of the present embodiment to manufacture an adhesive sheet having an adhesive layer with a thickness of more than 50 μm, the productivity improvement effect of omitting the step of heating and drying the solvent is significant, so it is preferable that the adhesive composition is solvent-free.

In the present invention, the adhesive composition being "substantially free of solvent" means that the content of the solvent in the adhesive composition is 0 to 1 mass %, preferably 0 to 0.5 mass %, and more preferably 0 to 0.1 mass %.

(others)
The pressure-sensitive adhesive composition of the present embodiment may contain other additives as necessary within the range that does not impair the effects of the present invention. Examples of the additives include plasticizers, surface lubricants, antioxidants, antiaging agents, light stabilizers, ultraviolet absorbers, polymerization inhibitors, light stabilizers such as benzotriazole-based stabilizers, phosphate ester-based and other flame retardants, antistatic agents such as surfactants, dyes, etc.

(Gel Fraction)
The pressure-sensitive adhesive composition of the present embodiment is photocurable, and the gel fraction of the cured product cured at an irradiation dose of 1000 mJ/cm ² to a thickness of 150 μm after curing is within the range of 50 to 65 mass %. The gel fraction of the cured product cured at an irradiation dose of 1000 mJ/cm ² can be adjusted by controlling the content of the chain transfer agent (C) and/or the photopolymerization initiator (D) in the pressure-sensitive adhesive composition of the present embodiment containing polyurethane (A), (meth)acrylic monomer (B), chain transfer agent (C), and photopolymerization initiator (D). More specifically, increasing the content of the chain transfer agent (C) and/or the photopolymerization initiator (D) reduces the gel fraction, and decreasing the content of the chain transfer agent (C) and/or the photopolymerization initiator (D) increases the gel fraction.

The cured product having a gel fraction of 50 to 65% by mass obtained by curing the pressure-sensitive adhesive composition of this embodiment at an irradiation dose of 1000 mJ/ ^cm2 has sufficient adhesive strength, is unlikely to leave adhesive residue, and has excellent unevenness absorbency. Therefore, the pressure-sensitive adhesive composition of this embodiment is suitable as a material for the pressure-sensitive adhesive layer in a pressure-sensitive adhesive sheet used for applications in which the sheet is attached to an adherend having uneven surfaces and then peeled off. The cured product obtained by curing the pressure-sensitive adhesive composition of this embodiment at an irradiation dose of 1000 mJ/ ^cm2 preferably has a gel fraction of 52% by mass or more. In addition, the gel fraction of the cured product is preferably 63% by mass or less.

In contrast, adhesive sheets using a cured material with a gel fraction of less than 50% by mass as an adhesive layer are prone to leaving adhesive residue when peeled off after application to an adherend. Furthermore, cured materials with a gel fraction of more than 65% by mass have insufficient fluidity when used as an adhesive layer of an adhesive sheet. For this reason, adhesive sheets using a cured material with a gel fraction of more than 65% by mass as an adhesive layer are prone to creating gaps between the adhesive layer and the uneven surface of the adherend when the adhesive sheet is applied to an adherend with uneven surfaces.

(Measurement of Gel Fraction)
In the present invention, the gel fraction of the cured product obtained by curing the pressure-sensitive adhesive composition at an irradiation dose of 1000 mJ/cm ² is measured as follows.
First, the adhesive composition is applied to a 75 μm-thick release PET film (manufactured by Higashiyama Film Co., Ltd., product name: Clean Sepa (trademark) HY-S10-2) using an applicator so that the thickness after curing is 150 μm. Next, the surface to which the adhesive composition is applied is covered with a 75 μm-thick silicone-based ultra-light release PET film (manufactured by Toyobo Co., Ltd., product name: E7006).

Next, using an ultraviolet irradiation device (manufactured by iGraphics Co., Ltd., UV irradiation device 3 kW, high-pressure mercury lamp), ultraviolet rays are irradiated through the ultra-light release PET film under conditions of an irradiation distance of 25 cm, a lamp movement speed of 1.0 m/min, and an irradiation dose of 1000 mJ/ ^cm2 , thereby curing the adhesive composition to form a cured product (adhesive layer).

Next, a square sheet of 8 cm long and 8 cm wide is cut out from the sheet having the cured product (adhesive layer), and the release PET film and the super light release PET film are peeled off from the adhesive layer of the obtained square sheet. The adhesive layer peeled off from the square sheet is used as a measurement sample, and its mass is measured. Next, the measurement sample is immersed in 50 ml of toluene and left at room temperature for 72 hours. Then, the measurement sample is taken out of the toluene, dried at 80 ° C for 5 hours, and the mass is measured again. Then, the gel fraction is measured based on the following formula.
Gel fraction (%) = [A/B] x 100
A: Mass of the measurement sample after immersion in toluene (excluding the mass of toluene)
B: Mass of the measurement sample before immersion in toluene

(Storage modulus and loss tangent)
The pressure-sensitive adhesive composition of the present embodiment is cured at an irradiation dose of 1000 mJ/ ^cm2 so that the thickness after curing is 150 μm. The cured product preferably has a storage modulus of 1.0×10 ⁴ to 1.0×10 ⁵ at 25° C. measured at a frequency of 1 Hz and a loss tangent of 0.25 to 0.55.
The storage modulus and loss tangent at 25°C measured at a frequency of 1 Hz for a cured product cured at an irradiation dose of 1000 mJ/ ^cm2 can be adjusted by controlling the content of the chain transfer agent (C) and/or the photopolymerization initiator (D) contained in the pressure-sensitive adhesive composition. More specifically, when the content of the chain transfer agent (C) and/or the photopolymerization initiator (D) is increased, the storage modulus decreases and the loss tangent increases. On the other hand, when the content of the chain transfer agent (C) and/or the photopolymerization initiator (D) is decreased, the storage modulus increases and the loss tangent decreases.

The cured product obtained by curing the pressure-sensitive adhesive composition of this embodiment at an irradiation dose of 1000 mJ/ ^cm2 has a storage modulus of 1.0× ¹⁰⁴ to 1.0× ¹⁰⁵ and a loss tangent of 0.25 to 0.55, and has good flexibility and flowability. Therefore, when an adhesive sheet using this cured product as an adhesive layer is attached to a workpiece (adherend) such as a semiconductor wafer having uneven parts such as bumps, it is possible to prevent the occurrence of voids between the uneven parts and the adhesive sheet.

The storage modulus at 25° C. of the cured product obtained by curing the pressure-sensitive adhesive composition of the present embodiment at an irradiation dose of 1000 mJ/cm ² and measured at a frequency of 1 Hz is more preferably 2.0×10 ⁴ or more, and even more preferably 3.0×10 ⁴ or more. Moreover, the storage modulus is more preferably 9.0×10 ⁴ or less, and even more preferably 8.0×10 ⁴ or less.

The cured product having a storage modulus of 1.0×10 ⁴ or more is not too soft when used as an adhesive layer of an adhesive sheet. Therefore, an adhesive sheet using a cured product having a storage modulus of 1.0×10 ⁴ or more as an adhesive layer is less likely to leave adhesive residue even when peeled off after being attached to an adherend, which is preferable. In addition, a cured product having a storage modulus of 1.0×10 ⁵ or less has good flexibility when used as an adhesive layer of an adhesive sheet. Therefore, when an adhesive sheet using a cured product having a storage modulus of 1.0×10 ⁵ or less as an adhesive layer is attached to an adherend having an uneven surface, voids are less likely to occur between the adhesive layer and the uneven part of the adherend, which is preferable.

The loss tangent at 25° C. of the cured product obtained by curing the pressure-sensitive adhesive composition of the present embodiment at an irradiation dose of 1000 mJ/cm ² and measured at a frequency of 1 Hz is more preferably 0.30 or more. Moreover, the loss tangent is more preferably 0.50 or less.

A cured product having a loss tangent of 0.25 or more has good fluidity when used as an adhesive layer of an adhesive sheet. Therefore, when an adhesive sheet using a cured product having a loss tangent of 0.25 or more as an adhesive layer is applied to an adherend having an uneven surface, gaps are unlikely to occur between the adhesive layer and the uneven surface of the adherend, which is preferable. Furthermore, a cured product having a loss tangent of 0.55 or less does not have too high fluidity when used as an adhesive layer of an adhesive sheet. Therefore, an adhesive sheet using a cured product having a loss tangent of 0.55 or less as an adhesive layer is unlikely to leave adhesive residue when peeled off after application to an adherend, which is preferable.

(Measurement of storage modulus and loss tangent)
In the present invention, the storage modulus and loss tangent at 25° C., measured at a frequency of 1 Hz, of the cured product obtained by curing the pressure-sensitive adhesive composition at an irradiation dose of 1000 mJ/cm ² are measured by the following method.
First, in the same manner as in the above-mentioned method for measuring the gel fraction, the adhesive composition is cured to prepare a sheet having a cured product (adhesive layer) of 150 μm in thickness. The release PET film and the super-light release PET film are peeled off from the adhesive layer of the sheet having the cured product (adhesive layer). Then, 10 peeled adhesive layers of 150 μm in thickness are laminated to obtain a laminated sheet of 1.5 mm in thickness. A square sheet of 8 mm in length and 8 mm in width is cut out from the obtained laminated sheet to obtain a measurement sample.

Then, the storage modulus and loss modulus of the obtained measurement sample are measured in shear mode at a frequency of 1 Hz (6.28 rad/sec) from -20 to 120°C using a rotational viscoelasticity measuring device (rheometer) (TA Instruments, product name "AR2000"), and the storage modulus and loss tangent at 25°C are calculated.

<Method of producing pressure-sensitive adhesive composition>
Next, the method for producing the pressure-sensitive adhesive composition of the present embodiment will be described in detail with reference to examples.
Among the components contained in the pressure-sensitive adhesive composition of the present embodiment, the polyurethane (A) will be described below by way of an example of a preferred synthesis method. The components contained in the pressure-sensitive adhesive composition of the present embodiment, such as the (meth)acrylic monomer (B), the chain transfer agent (C), the photopolymerization initiator (D), and the fatty acid ester (E), except for the polyurethane (A), are readily available as commercially available products, and the synthesis method varies depending on the type of compound used as each component, so that the explanation of the synthesis method is omitted.

<Method for synthesizing polyurethane (A)>
Hereinafter, an example of a preferred synthesis method for the polyurethane (A) contained in the pressure-sensitive adhesive composition of the present embodiment will be described. Note that the synthesis method for the polyurethane (A) is not limited to the synthesis method shown below, and can be appropriately changed depending on conditions such as raw materials and equipment used in the synthesis.

In the synthesis method of polyurethane (A) shown below, the reaction between hydroxyl groups and isocyanato groups in each step is carried out using a urethane catalyst such as dibutyltin dilaurate, dibutyltin diethylhexoate, or dioctyltin dilaurate in the presence of an organic solvent inactive to isocyanato groups. In each step, the reaction between hydroxyl groups and isocyanato groups is preferably carried out at 30 to 100°C for 1 to 5 hours. The amount of the urethane catalyst used is preferably 50 to 500 ppm by mass based on the total mass of the reactants (raw materials).

To synthesize polyurethane (A), first, polyoxyalkylene polyol and polyisocyanate are charged in a ratio such that the amount of isocyanato groups (based on the number of molecules, the same applies below) is greater than the amount of hydroxyl groups (based on the number of molecules, the same applies below). The polyoxyalkylene polyol and polyisocyanate are then reacted to synthesize polyurethane having isocyanato groups at its terminals as a precursor of polyurethane (A). Specific examples of polyoxyalkylene polyols and polyisocyanates used as raw materials are as exemplified in the section on polyurethane (A).

At this time, the molecular weight (degree of polymerization) of the polyurethane having isocyanato groups at its terminals can be adjusted by adjusting the ratio of the amount of isocyanato groups to the amount of hydroxyl groups contained in the raw material. Specifically, the smaller the excess amount of isocyanato groups relative to the amount of hydroxyl groups, the higher the molecular weight of the polyurethane having isocyanato groups at its terminals. Also, the greater the excess amount of isocyanato groups relative to the amount of hydroxyl groups, the lower the molecular weight of the polyurethane having isocyanato groups at its terminals. In this embodiment, the weight average molecular weight of the target polyurethane (A) is adjusted by adjusting the molecular weight of the polyurethane having isocyanato groups at its terminals.

Next, the polyurethane having an isocyanato group at the end is reacted with a compound having a hydroxy group and a (meth)acryloyl group to produce polyurethane (A) having a skeleton including a polyoxyalkylene polyol-derived structure and a polyisocyanate-derived structure, and including polyurethane (a1) having (meth)acryloyl groups at multiple ends. The (meth)acryloyl groups at the ends of the produced polyurethane (A) are preferably part of a (meth)acryloyloxy group.

The compounds having a hydroxy group and a (meth)acryloyl group are not particularly limited, but include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; monools having a (meth)acryloyl group derived from various polyols such as 1,3-butanediol mono(meth)acrylate, 1,4-butanediol mono(meth)acrylate, 1,6-hexanediol mono(meth)acrylate, and 3-methylpentanediol mono(meth)acrylate. These compounds having a hydroxy group and a (meth)acryloyl group may be used alone or in combination of two or more types. Among these compounds having a hydroxy group and a (meth)acryloyl group, it is preferable to use 2-hydroxyethyl (meth)acrylate in terms of reactivity with the isocyanato group of polyurethane having an isocyanato group at the end and photocurability of the adhesive composition.

Alternatively, the polyurethane (A) may be produced by reacting a compound having a hydroxy group and a (meth)acryloyl group with an alkyl alcohol having no (meth)acryloyl group and one hydroxy group, together with a polyurethane having an isocyanato group at its terminal.
The alkyl alcohol is not particularly limited as long as it does not have a (meth)acryloyl group and has one hydroxyl group, and may be a linear, branched, or alicyclic alkyl alcohol, etc. The alkyl alcohol may be used alone or in combination of two or more kinds.

By reacting a compound having a hydroxy group and a (meth)acryloyl group with an alkyl alcohol having one hydroxy group but no (meth)acryloyl group, and a polyurethane having an isocyanato group at its terminal to produce polyurethane (A), it is possible to adjust the amount of (meth)acryloyl groups introduced into the polyurethane having an isocyanato group at its terminal.

More specifically, the above reaction produces polyurethane (A) containing multiple types of polyurethanes with different amounts of (meth)acryloyl groups introduced at the ends. The multiple types of polyurethanes include polyurethane (a1) having multiple (meth)acryloyl groups at their ends. Furthermore, the multiple types of polyurethanes include not only polyurethane (a1) but also polyurethanes in which at least some of the multiple ends have a structure derived from the above alkyl alcohol. Therefore, the multiple types of polyurethanes produced include polyurethanes in which at least some of the multiple ends do not have (meth)acryloyl groups. Furthermore, the multiple types of polyurethanes produced may also include polyurethane (a2) having a (meth)acryloyl group at only one end.

<Another Example of the Method for Synthesizing Polyurethane (A)>
Next, another preferred example of the synthesis method for the polyurethane (A) will be described.
In the synthesis method of polyurethane (A) shown below, as in the above synthesis method example, the reaction between hydroxyl groups and isocyanato groups is carried out in the presence of an organic solvent inactive to isocyanato groups in any step using a urethanization catalyst such as dibutyltin dilaurate, dibutyltin diethylhexoate, or dioctyltin dilaurate. In any step, the reaction between hydroxyl groups and isocyanato groups is preferably carried out continuously for 1 to 5 hours at 30 to 100° C. The amount of the urethanization catalyst used is preferably 50 to 500 ppm by mass based on the total mass of the reactants (raw materials).

When polyurethane (A) is synthesized using this synthesis method, unlike the above-mentioned example synthesis method, a polyurethane having a hydroxyl group at its terminal is synthesized as a precursor of polyurethane (A).
Specifically, first, a polyoxyalkylene polyol and a polyisocyanate are charged in a ratio such that the amount of hydroxyl groups (based on the number of molecules, the same applies below) is greater than the amount of isocyanato groups (based on the number of molecules, the same applies below).Then, the polyoxyalkylene polyol and the polyisocyanate are reacted to synthesize a polyurethane having a hydroxyl group at its terminal as a precursor of polyurethane (A).

At this time, the molecular weight (degree of polymerization) of the polyurethane having hydroxyl groups at its terminals can be adjusted by adjusting the ratio of the amount of hydroxyl groups to the amount of isocyanato groups contained in the raw material. Specifically, the smaller the excess amount of hydroxyl groups relative to the amount of isocyanato groups, the higher the molecular weight of the polyurethane having hydroxyl groups at its terminals. Also, the greater the excess amount of hydroxyl groups relative to the amount of isocyanato groups, the lower the molecular weight of the polyurethane having hydroxyl groups at its terminals. In this embodiment, the mass average molecular weight of the target polyurethane (A) is adjusted by adjusting the molecular weight of the polyurethane having hydroxyl groups at its terminals.

Next, the polyurethane having a hydroxyl group at the end is reacted with a compound having an isocyanato group and a (meth)acryloyl group to produce polyurethane (A) having a skeleton including a polyoxyalkylene polyol-derived structure and a polyisocyanate-derived structure, and including polyurethane (a1) having (meth)acryloyl groups at multiple ends. The (meth)acryloyl groups at the ends of the produced polyurethane (A) are preferably part of a (meth)acryloyloxy group.

The compound having an isocyanato group and a (meth)acryloyl group is not particularly limited, but includes 2-(meth)acryloyloxyethyl isocyanate, 2-(meth)acryloyloxypropyl isocyanate, 1,1-bis(acryloyloxymethyl)ethyl isocyanate, etc. In addition, examples of commercially available compounds having an isocyanato group and a (meth)acryloyl group include Karenz MOI (registered trademark) and Karenz AOI (registered trademark) manufactured by Showa Denko K.K. These compounds having an isocyanato group and a (meth)acryloyl group may be used alone or in combination of two or more types. Among these compounds having an isocyanato group and a (meth)acryloyl group, it is preferable to use 2-(meth)acryloyloxyethyl isocyanate in terms of reactivity with the hydroxy group of polyurethane having a hydroxy group at the end and the photocurability of the adhesive composition.

Alternatively, the polyurethane (A) may be produced by reacting a compound having an isocyanato group and a (meth)acryloyl group with an alkyl isocyanate having no (meth)acryloyl group and one isocyanato group, together with a polyurethane having a hydroxy group at its terminal.
The alkyl isocyanate is not particularly limited as long as it does not have a (meth)acryloyl group and has one isocyanato group, and may be a linear, branched, or alicyclic alkyl isocyanate, etc. The alkyl isocyanate may be used alone or in combination of two or more kinds.

By reacting a compound having an isocyanato group and a (meth)acryloyl group with an alkyl isocyanate having no (meth)acryloyl group but one isocyanato group, and a polyurethane having a hydroxyl group at its terminal to produce polyurethane (A), it is possible to adjust the amount of (meth)acryloyl groups introduced into the polyurethane having a hydroxyl group at its terminal.

More specifically, the above reaction produces polyurethane (A) containing multiple types of polyurethanes with different amounts of (meth)acryloyl groups introduced at the ends. The multiple types of polyurethanes include polyurethane (a1) having multiple (meth)acryloyl groups at their ends. Furthermore, the multiple types of polyurethanes include not only polyurethane (a1) but also polyurethanes in which at least some of the multiple ends have a structure derived from the alkyl isocyanate. Therefore, the multiple types of polyurethanes produced include polyurethanes in which at least some of the multiple ends do not have a (meth)acryloyl group. Furthermore, the multiple types of polyurethanes produced may also include polyurethane (a2) having a (meth)acryloyl group at only one end.

<Method of mixing components contained in pressure-sensitive adhesive composition>
The pressure-sensitive adhesive composition of the present embodiment can be produced by a method of mixing the polyurethane (A) obtained by the above-mentioned synthesis method, a (meth)acrylic monomer (B), a chain transfer agent (C), a photopolymerization initiator (D), and, if necessary, a fatty acid ester (E) and other additives.
The method for mixing the components contained in the pressure-sensitive adhesive composition of the present embodiment is not particularly limited, and can be carried out using, for example, a stirring device equipped with stirring blades such as a homodisper or a paddle blade.

The adhesive composition of the present embodiment includes polyurethane (A), (meth)acrylic monomer (B), chain transfer agent (C), and photopolymerization initiator (D), and polyurethane (A) has a skeleton including a structure derived from polyoxyalkylene polyol and a structure derived from polyisocyanate, and includes polyurethane (a1) having (meth)acryloyl groups at a plurality of ends, and the gel fraction of the cured product photocured at an irradiation dose of 1000 mJ/cm ² is 50 to 65% by mass. Therefore, by curing the adhesive composition of the present embodiment, a cured product having sufficient adhesive strength, which is unlikely to leave adhesive residue, and which is excellent in unevenness absorption properties can be obtained. Therefore, the adhesive composition of the present embodiment is suitable as a material for the adhesive layer in an adhesive sheet used in applications in which the adhesive sheet is attached to an adherend having uneven portions on the surface and then peeled off.

<Method of manufacturing pressure-sensitive adhesive sheet>
Next, a method for producing the pressure-sensitive adhesive sheet of this embodiment will be described.
The method for producing the pressure-sensitive adhesive sheet of the present embodiment is not particularly limited, and the sheet can be produced using a known method.
For example, a pressure-sensitive adhesive composition is applied onto a sheet-like substrate, and a separator is laminated to form a laminate. Then, the pressure-sensitive adhesive composition is irradiated with ultraviolet light through the separator to photocure the pressure-sensitive adhesive composition. This provides a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer formed of a cured product of the pressure-sensitive adhesive composition on the substrate.

The method of applying the adhesive composition to the substrate is not particularly limited and can be selected as appropriate. For example, methods of applying the adhesive composition to the substrate include methods using various coaters such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater, spray coater, comma coater, and direct coater, and screen printing methods.

Examples of light sources for photocuring the pressure-sensitive adhesive composition include black lights, low-pressure mercury lamps, high-pressure mercury lamps, extra-high-pressure mercury lamps, metal halide lamps, and xenon lamps.
The light irradiation intensity may be any condition that allows the pressure-sensitive adhesive composition to be sufficiently cured and the gel fraction of the cured product to be within the range of 50 to 65% by mass, and is preferably, for example, 50 to 3000 mW/ ^cm2 . If the light irradiation intensity is weak, curing takes a long time and productivity decreases. In addition, it is more preferable to adjust the light irradiation intensity so that the storage modulus and loss tangent of the cured product at 25°C measured at a frequency of 1 Hz are within the desired range.

In this embodiment, the pressure-sensitive adhesive composition is irradiated with ultraviolet light through a transparent separator, but if the substrate and separator are transparent, ultraviolet light may be irradiated from either the separator side or the substrate side.

<Applications and required performance of adhesive sheets>
The pressure-sensitive adhesive sheet of the present embodiment can be used for applications in which it is attached to an adherend having an uneven surface and then peeled off. Specifically, it can be suitably used as a pressure-sensitive adhesive sheet that is attached to a surface of a semiconductor wafer on which bumps are formed to protect the surface of the semiconductor wafer and peeled off after a predetermined processing step.

When the adhesive sheet of this embodiment is used to protect the bump-formed surface of a semiconductor wafer, the peel strength (adhesive strength) of the adhesive sheet must be strong enough to firmly fix the semiconductor wafer to the adhesive sheet during, for example, the back grinding process in the semiconductor device processing step, while being strong enough not to damage the components of the semiconductor device when the adhesive sheet is peeled off from the semiconductor wafer after a predetermined processing step.
From these viewpoints, the peel strength of the pressure-sensitive adhesive sheet used for the above applications is preferably 10 to 300 gf/25 mm, more preferably 15 to 200 gf/25 mm, and even more preferably 20 to 150 gf/25 mm, when the peel speed is 0.3 m/min. and the thickness of the pressure-sensitive adhesive layer is 50 to 200 μm. A specific method for measuring the peel strength of the pressure-sensitive adhesive sheet will be described later in the examples.

The adhesive sheet of this embodiment has an adhesive layer made of a cured product of the adhesive composition of this embodiment on one side of a sheet-like substrate. Therefore, the adhesive sheet of this embodiment has sufficient adhesive strength, is unlikely to leave adhesive residue when the adhesive layer is transferred to the adherend after the adhesive sheet is peeled off, and has excellent unevenness absorption properties. Therefore, the adhesive sheet of this embodiment is suitable for applications in which it is attached to an adherend having uneven portions on its surface and then peeled off.

The adhesive sheet of this embodiment can be suitably used, for example, as an adhesive sheet that is applied during the backgrinding process of a semiconductor wafer having uneven portions made of bumps on its surface, and is peeled off after the backgrinding process. In this case, the adhesive sheet of this embodiment fixes the semiconductor wafer with sufficient adhesive strength, and is less likely to cause gaps between the adhesive sheet applied to the semiconductor wafer and the periphery of the bumps. This prevents water used in the backgrinding process from penetrating the gap between the adhesive sheet and the periphery of the bumps, thereby contaminating the semiconductor wafer. In addition, after the backgrinding process, adhesive residue is less likely to occur around the bumps of the semiconductor wafer when the adhesive sheet is peeled off, which is preferable.

The present invention will be described in more detail below with reference to examples and comparative examples. Note that the present invention is not limited to the following examples.

<Synthesis of Polyurethane (A-1)>
A reactor equipped with a thermometer, a stirrer, a dropping funnel, and a cooling tube with a drying tube was charged with 0.55 kg (2.1 mol) of a hydrogenated product of diphenylmethane diisocyanate (Desmodur W, manufactured by Sumika Covestro Urethane Co., Ltd.), 4.01 kg (2.0 mol) of polypropylene glycol having a hydroxyl group at its end and a hydroxyl value of 56 mg KOH/g (Actocol D-2000, manufactured by Mitsui Chemicals, number average molecular weight 2000), and 0.8 g of a urethanization catalyst dioctyltin (Neostan U-810, manufactured by Nitto Kasei Co., Ltd.).

Then, the reactor was heated to 60°C and reacted for 4 hours to obtain a polyurethane having isocyanato groups at both ends as a precursor of polyurethane (A). Next, 23.22 g (0.2 mol) of 2-hydroxyethyl acrylate was added to the reactor, heated to 70°C, and reacted for 2 hours to obtain 4.58 kg of polyurethane (A-1) with a mass average molecular weight of 67,000.

The resulting polyurethane (A-1) was analyzed using infrared absorption spectroscopy (IR). As a result, no peaks derived from isocyanato groups were observed. Therefore, it was confirmed that polyurethane (A-1) was polyurethane (a1) in which acryloyloxy groups had been introduced to all of the terminals.

<Synthesis of Polyurethane (A-2)>
A polyurethane (A-2) having a mass average molecular weight of 66,000 was obtained in the same manner as in the synthesis of polyurethane (A-1), except that 2.1 mol of isophorone diisocyanate (Desmodur I, manufactured by Sumika Covestro Urethane Co., Ltd.) was used instead of the hydrogenated product of diphenylmethane diisocyanate.

The resulting polyurethane (A-2) was analyzed using infrared absorption spectroscopy (IR). As a result, no peaks derived from isocyanato groups were observed. Therefore, it was confirmed that polyurethane (A-2) was polyurethane (a1) in which acryloyloxy groups had been introduced to all of the terminals.

<Preparation of Pressure-Sensitive Adhesive Composition>
The polyurethane (A-1) or (A-2) thus obtained was mixed with the (meth)acrylic monomer (B), the chain transfer agent (C), the photopolymerization initiator (D) and the fatty acid ester (E) shown in Table 1 or 2 in the ratios shown in Table 1 or 2, and mixed at 25°C using a disper to obtain pressure-sensitive adhesive compositions of Examples 1 to 5 and Comparative Examples 1 to 5.

The following symbols shown in Tables 1 and 2 represent the compounds shown below.
2EHA: 2-ethylhexyl acrylate (manufactured by Toagosei Co., Ltd.)
BUA: n-butyl acrylate (manufactured by Toagosei Co., Ltd.)
ACMO: Acryloylmorpholine (manufactured by Shin-Nakamura Chemical Co., Ltd.)
IBOA: Isobornyl acrylate (manufactured by Osaka Organic Chemical Industry Ltd.)
TMPTA: Trimethylolpropane triacrylate (manufactured by Toagosei Co., Ltd.)

PE1: Pentaerythritol tetrakis(3-mercaptobutyrate) (manufactured by Showa Denko K.K.)
NR1: 1,3,5-tris(3-mercaptobutyloxyethyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (manufactured by Showa Denko K.K.)
TPO (Omnirad TPO H): 2,4,6-trimethylbenzoyldiphenylphosphine oxide (manufactured by IGM Resins B.V.)
Exepar IPM: Isopropyl myristate (Kao Corporation)
Exepar EH-S: 2-ethylhexyl stearate (manufactured by Kao Corporation)

<Measurement of Gel Fraction>
Measurement samples were prepared using the pressure-sensitive adhesive compositions of Examples 1 to 5 and Comparative Examples 1 to 5 by the above-mentioned method, and the gel fractions of the cured products photocured at an irradiation dose of 1000 mJ/ ^cm2 were measured. The results are shown in Tables 1 and 2.

<Measurement of storage modulus and loss tangent>
Measurement samples were prepared using the pressure-sensitive adhesive compositions of Examples 1 to 5 and Comparative Examples 1 to 5 by the above-mentioned method, and the storage modulus and loss tangent at 25°C were measured at a frequency of 1 Hz for the cured products photocured at an irradiation dose of 1000 mJ/ ^cm2 . The results are shown in Tables 1 and 2.

<Preparation of adhesive sheet>
A 50 μm thick PET film (manufactured by Toyobo Co., Ltd., product name: Ester (trademark) film E5100) was prepared as a sheet-like substrate. The pressure-sensitive adhesive composition of Example 1 was applied to the corona-treated surface of the substrate using an applicator so that the thickness after curing would be 150 μm.

Next, a 75 μm thick silicone-based ultra-light release PET film (product name: E7006, manufactured by Toyobo Co., Ltd.) was attached as a separator to the surface coated with the pressure-sensitive adhesive composition using a rubber roller.
Thereafter, the pressure-sensitive adhesive composition was irradiated with ultraviolet light through the separator using an ultraviolet irradiator (manufactured by iGraphics Co., Ltd., UV irradiator 3 kW, high-pressure mercury lamp) under conditions of an irradiation distance of 25 cm, a lamp moving speed of 1.0 m/min, and an irradiation amount of 1000 mJ/ ^cm2 , thereby photocuring the pressure-sensitive adhesive composition. This resulted in the production of a pressure-sensitive adhesive sheet of Example 1 in which a pressure-sensitive adhesive layer, which was a cured product of the pressure-sensitive adhesive composition, and a separator were laminated on the substrate.

Next, the adhesive compositions of Examples 2 to 5 and Comparative Examples 1 to 5 were used instead of the adhesive composition of Example 1, and adhesive sheets were produced in the same manner as the adhesive sheet of Example 1. As a result, adhesive sheets of Examples 2 to 5 and Comparative Examples 2 to 4 were obtained.
However, for the adhesive compositions of Comparative Examples 1 and 5, due to insufficient curing, an adhesive layer made of a cured product of the adhesive composition could not be formed.

Next, the pressure-sensitive adhesive sheets of Examples 1 to 5 and Comparative Examples 2 to 4 were evaluated for the following items.
<Peeling force (peeling strength)>
The adhesive sheet was cut into a size of 25 mm long and 150 mm wide, and the separator was peeled off to expose the adhesive layer. The entire surface of the exposed adhesive layer was then laminated onto a glass plate, and a rubber roller (diameter: 85 mm, width: 50 mm) with a mass of 2 kg (load 19.6 N) was rolled once to obtain a measurement sample. The obtained measurement sample was left for 30 minutes in an environment of a temperature of 23°C and a relative humidity of 50% RH. Then, a tensile test was performed in the 180° direction at a peel speed of 0.3 m/min. according to JIS K 6854-2 to measure the peel strength (gf/25 mm) against the glass plate. The results are shown in Table 1 or 2.

<Irregularity absorption>
The adhesive sheet was cut into a size of 25 mm long and 50 mm wide, and the separator was peeled off to expose the adhesive layer. The surface of the exposed adhesive layer was then placed facing the bumps of a bumped wafer (WALTS-TEG FC150SCJY LF (PI), bump height: 75 μm, bump size: diameter 90 μm, manufactured by Waltz Corporation). A rubber roller (diameter: 85 mm, width: 50 mm) with a mass of 2 kg (load 19.6 N) was then moved back and forth three times at a speed of 10 mm/sec on the base material of the adhesive sheet to bond the adhesive sheet and the bumped wafer.

The bumped wafer attached to the adhesive sheet was observed from the substrate side of the adhesive sheet using a digital optical microscope (RH-2000, manufactured by Kaihyou Hirox Co., Ltd.), and the bumps were evaluated for their ability to absorb unevenness according to the following criteria. The results are shown in Tables 1 and 2.
"standard"
◯ (Fair): There is no gap between the adhesive layer of the adhesive sheet and the periphery of the bumps on the bumped wafer.
× (unacceptable): There is a gap between the adhesive layer of the adhesive sheet and the periphery of the bumps on the bumped wafer.

<Glue residue>
The adhesive sheet was cut into a size of 25 mm long and 50 mm wide, and the separator was peeled off to expose the adhesive layer. The exposed surface of the adhesive layer was then placed facing the bumps of a bumped wafer (WALTS-TEG FC150SCJY LF (PI), bump height: 75 μm, bump size: diameter 90 μm, manufactured by Waltz Corporation). A rubber roller (diameter: 85 mm, width: 50 mm) with a mass of 2 kg (load 19.6 N) was then reciprocated three times at a speed of 10 mm/sec on the base material of the adhesive sheet to bond the adhesive sheet and the bumped wafer.

The bumped wafer attached to the adhesive sheet was left at 23°C for 24 hours, and then the adhesive sheet was peeled off by hand at a speed of about 2 m/min. The surface of the bumped wafer was then observed with a digital optical microscope (RH-2000, manufactured by Hirox Co., Ltd.) and the presence or absence of adhesive residue was evaluated according to the following criteria. The results are shown in Tables 1 and 2.
"standard"
○ (Acceptable): No adhesive residue around the bump.
× (Unacceptable): There is adhesive residue around the bump.

As shown in Tables 1 and 2, the adhesive sheets of Examples 1 to 5, which have a gel fraction of 50 to 65% by mass, all had a peel strength of 10 gf/25 mm or more and sufficient adhesive strength. In addition, the adhesive sheets of Examples 1 to 5 were all rated as "○ (fair)" for unevenness absorption and adhesive residue.

In contrast, the pressure-sensitive adhesive sheets of Comparative Example 2 and Comparative Example 4, which had a gel fraction of less than 50 mass%, were both rated as "○ (acceptable)" for unevenness absorbency, but were rated as "× (unacceptable)" for glue residue.
Furthermore, the adhesive sheet of Comparative Example 3, which had a gel fraction of more than 65% by mass, was rated as "◯ (good)" for adhesive residue, but was rated as "× (bad)" for unevenness absorbency.

The present invention provides an adhesive sheet that has sufficient adhesive strength, is unlikely to leave adhesive residue on the adherend when the adhesive layer is transferred to the adherend after the adhesive sheet is peeled off, and has excellent unevenness absorption properties.

Claims (10)

The composition comprises 20 to 50% by mass of a polyurethane (A), a (meth)acrylic monomer (B) composed of a compound having a (meth)acryloyloxy group, 0.5 to 5% by mass of a chain transfer agent (C), and 0.01 to 5% by mass of a photopolymerization initiator (D),
the polyurethane (A) includes a polyurethane (a1) having a skeleton including a structure derived from a polyoxyalkylene polyol and a structure derived from a polyisocyanate and having (meth)acryloyl groups at two or more ends;
The gel fraction of the cured product photocured at an irradiation dose of 1000 mJ/ ^cm2 to a cured thickness of 150 μm is 50 to 65% by mass,
The (meth)acrylic monomer (B) contains a monofunctional (meth)acrylate and a polyfunctional (meth)acrylate,
A pressure-sensitive adhesive composition comprising 85 to 99 mol % of the monofunctional (meth)acrylate and 1 to 15 mol % of the polyfunctional (meth)acrylate when the total amount of the (meth)acrylic monomers (B) is 100 mol %. The cured product has a storage modulus of 1.0×10 ⁴ to 1.0×10 ⁵ at 25° C. as measured at a frequency of 1 Hz;
The pressure-sensitive adhesive composition according to claim 1, which has a loss tangent of 0.25 to 0.55 at 25° C. measured at a frequency of 1 Hz. The pressure-sensitive adhesive composition according to claim 1 or 2, wherein the chain transfer agent (C) is a multifunctional thiol. The pressure-sensitive adhesive composition according to claim 1 or 2, which contains 49 to 79 mass% of the (meth)acrylic monomer (B). The pressure-sensitive adhesive composition according to claim 1 or 2, further comprising a fatty acid ester (E). The pressure-sensitive adhesive composition according to claim 1, which contains 45 to 79% by mass of the (meth)acrylic monomer (B). The pressure-sensitive adhesive composition according to claim 1, which contains 53 to 73 mass% of the (meth)acrylic monomer (B). The pressure-sensitive adhesive composition according to claim 1, which contains 25 to 45% by mass of the polyurethane (A). The pressure-sensitive adhesive composition according to claim 1, which contains 30 to 40% by mass of the polyurethane (A). The adhesive composition according to claim 1, which is used as a material for an adhesive layer in an adhesive sheet.