ADHESIVE SHEET FOR STAMPER INSPECTION AND STAMPER INSPECTION
METHOD
[0001]
BACKGROUND OF THE INVENTION Field of invention
The present invention relates to a method of inspecting a stamper for optical disks and an adhesive sheet used for inspecting a stamper.
[0002] Description of the Related Art
Data recorded on a Blu-ray disk is reproduced by focusing light on, for example, a cover layer having a thickness of 0.1 mm formed on a substrate. Generally, in order to focus on a stamper face when inspecting a groove signal of a stamper, it is necessary to form a cover layer having a thickness of 0.1 mm on the stamper face or to prepare a special-purpose pickup.
[0003]
It is impractical in view of costs to specifically design a special-purpose pickup for blue laser. In actual fact, commercially available stamper inspection devices employ a pickup similar to that used in common disk-recording and disk-reproduction devices.
[0004]
Examples of a method for forming such a cover layer on a disk include a method that includes attaching a sheet provided in advance with an adhesive layer, a method that includes attaching a sheet by using an adhesive agent, and a method that includes forming a layer of 0.1 mm in thickness only with a UV-curable resin. In consideration of the lack of a need for a large-scale apparatus such as a spin coater or a UV-irradiating apparatus, a method that includes forming a cover layer (adhesive sheet) on a stamper is thought to be simple and effective.
[0005]
Various methods of inspecting stampers are known in the art (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 6-259817). Among these, considering that Blu-ray disks are provided with a cover layer, an adhesive sheet having the same thickness as that of the cover layer may be provided for inspection. However, while it is necessary in the case of disk stamper inspection to remove the cover layer and form a substrate after inspection, adhesive components of common adhesive sheets for disks often remain on a

stamper surface, causing adhesion of foreign matter and dust.
[0006]
SUMMARY OF THE INVENTION
The present invention, which has solved the above problems, provides an adhesive sheet for stamper inspection that ensures a clean surface after inspection and leads to smaller fluctuations in the signal from the formed substrate, and a method of inspecting a stamper.
[0007]
After intensive studies to solve the above problems, the inventors have found that the following invention can solve the above problems.
[0008]
A first aspect of the invention is an adhesive sheet for stamper inspection including an optical film and an adhesive layer that includes an adhesive and is formed on the optical film, the adhesive having an adhesion of 3 (N/25 mm) or less.
[0009]
A second aspect of the invention is a method of inspecting a stamper including: adhering, onto a surface of a stamper to be inspected, the adhesive sheet for stamper inspection; irradiating laser light onto the surface of the stamper through the adhesive sheet for stamper inspection; and detecting light reflected by the surface of the stamper. In the inspection method, it is preferable to use a roll for adhering the stamper-inspecting adhesive sheet, and set an actual roll pressure of the roll to be in a range of 100 kMPa to 4 MPa. The "actual pressure" is the average pressure when pressure (cylinder pressure) is applied uniformly to a roll that has an elastic deformation width of 1 mm.
[0010]
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an explanatory view showing a method of determining the adhesion of an adhesive sheet for stamper inspection.
Fig. 2 is an explanatory view illustrating the principal components of a stamper inspection machine.
[0011]
DETAILED DESCRIPTION OF THE INVENTION Adhesive sheet for stamper inspection

The stamper inspection adhesive sheet of the invention has an optical film and an adhesive layer that contains at least an adhesive and is formed on the optical film. The adhesion of the adhesive is 3 (N/25 mm) or less.
[0012]
The stamper inspection adhesive sheet is used when inspecting a stamper in a stamper inspection device known in the art. It can be applied to a stamper that cannot be inspected by conventional apparatuses, such as a stamper for molding Blu-ray disk substrates.
[0013]
When an adhesive sheet is simply attached to the face of a stamper that is to be inspected, the sheet may leave gaps in the grooves of the stamper. On the other hand, when an adhesive sheet coated with an adhesive having high adhesive strength is attached thereto, part of the adhesive may remain on the stamper during separation of the adhesive sheet after inspection, leaving staining and defects.
[0014]
In view of the foregoing, the adhesion of the adhesive is set to 3 (N/25 mm) or less in the invention. When the adhesion is more than 3 (N/25 mm), the adhesive may remain in the grooves of a stamper. The adhesion is preferably in a range of 0 to 1 (N/25 mm). The "adhesion" as referred to herein can be measured by the following method. Firstly, as shown in Figure 1, a stamper inspection adhesive sheet 4 cut in advance into a size of 25 mm in width X 200 mm is attached to a plate 2 formed from the same material as the stamper (for example, Ni). A composite of the stamper inspection adhesive sheet 4 and the plate 2 bonded to each other is formed by abrading the composite under a pressure higher than that actually needed for bonding. The composite is left at room temperature (namely, 23°C at RH of 50%) for 24 hours, the adhesive sheet 4 is then peeled off from the plate 2 up to the 50-mm from the edge of the composite and bent to an angle of 180°, and the edge of the adhesive sheet 4 is pulled at a velocity of 10 mm/minute in a tensile tester 6 in order to determine, as the adhesion of the adhesive sheet 4, the load needed to separate the adhesive sheet from the Ni plate.
[0015]
An adhesive having an adhesion of 3 (N/25 mm) or less is used as the adhesive. An example thereof is an acrylic adhesive. The main component thereof is a copolymer having at least an acrylic ester monomer as its constituent. Examples of the monomer constituting the copolymer include ethyl acrylate, n-butyl acrylate, methyl acrylate, methyl methacrylate.

vinyl acetate, acrylic acid, methacrylic acid, and the like. In addition to the main component, a curing agent can be further included in the adhesive. Examples of the curing agent include tolylene diisocyanate, xylylene diisocyanate, hexamethylene diisocyanate and the like. It is possible to control the adhesion by properly selecting the molecular weight of the main component, the kind and ratio of the copolymer, and the kind of the curing agent. The adhesion can be controlled more easily by using an acrylic component having a molecular weight lower than that of the main component copolymer as a plasticizer.
[0016]
Examples of the optical film used include polycarbonate film, triacetate cellulose film, cyclic polyolefin film, and the like. The thickness of the adhesive layer of adhesive is preferably from 5 to 50 µm. The thickness of the optical film is preferably from 50 to 95 jam.
[0017] Stamper Inspection Method
In the stamper inspection method of the invention, a stamper is inspected by irradiating laser light and analyzing the reflected light. More specifically, the stamper inspection method of the invention includes adhering the stamper inspection adhesive sheet onto a surface of a stamper to be inspected, irradiating laser light onto the surface of the stamper through the stamper inspection adhesive sheet, and detecting light reflected by the surface of the stamper.
[0018]
The stamper inspection method may be performed by using any known optical disk stamper inspection device. The method of analyzing signal reproduction characteristics, such as of the HF signals of the pit and the tracking error signals of the groove formed on the optical disk stamper will be described below with reference to Figure 2. The configuration shown in Figure 2 is partly exaggerated, for easier understanding.
[0019]
An optical disk stamper 10 to be inspected is fixed on a revolving table 12; a laser light is irradiated from an optical head 16 through the stamper-inspecting adhesive sheet 30 onto the optical disk stamper 10, while the stamper is rotated by a spindle motor 14; and the amount of the reflected light is detected. In the optical head 16, a diffused beam is emitted from a semiconductor laser 18, converted to parallel light in a coUimate lens 20, concentrated after transmission through a beam splitter 22 by an object lens 24, and sent to the signal face of the optical disk stamper 10.
[0020]

The stamper inspection adhesive sheet 30 is the adhesive sheet of the invention described above, and provision of the adhesive sheet enables inspection of the stampers with the presumption of production of substrates for Blu-ray disks. As a result, the inspected face remains clean after inspection, reducing fluctuations of the signal on the formed substrate.
[0021]
The laser spot irradiated onto the signal face of the optical disk stamper 10 is reflected, while being modulated by the shape of pit or groove, sent again through the stamper-inspecting adhesive sheet 30 and the object lens 24, reflected by the beam splitter 22, and sent to the photodiode 28. The light intensity of the reflected laser light is detected in the photodiode 28. The incident face of the photodiode 28 is divided into two regions, which respectively independently perform current voltage conversion and amplification, forming two separate channels.
[0022]
The HF signal is obtained by adding together the output signals from the respective channels, while preserving high frequency bands. The tracking error signal is obtained by subtracting the output signals from these two chaimels. The object lens 24 used may be a lens with a design specification similar to that of lenses installed in optical disk drives.
[0023]
Here, the stamper inspection adhesive sheet is adhered to the stamper by, for example, processing in a vacuum or using a roll. In consideration of the convenience and productivity of, for example, the cycle time of the apparatus, it is preferable to use a roll for the adhering and set the actual pressure of the roll within a range of 100 kPa to 4 MPa.
[0024]
An actual roll pressure of 100 kPa to 4 MPa is effective in eliminating contamination by air bubbles and preventing cockling. More preferably, the actual roll pressure is set to be in a range of 200 kPa to 2 MPa.
[0025]
EXAMPLES Example 1
A stamper having a spiral groove having a track pitch of 320 nm and a depth of 45 nm was prepared. The adhesive layer of a stamper inspection adhesive sheet was adhered to the groove-formed surface of the stamper with a roll. The roll pressure was set to 500 KPa. The adhesive agent for the adhesive layer contained a copolymer of butyl methacrylate as its

primary component and, in addition, tolylene diisocyanate as the curing agent, and the adhesion thereof was adjusted to 0 (N/25 mm) by addition of a low-molecular-weight acrylic resin (Mn: 3,000).
[0026]
The composite of stamper and sheet was placed, and the stamper was inspected, in a stamper inspection machine manufactured by Pulstec Industrial Co., Ltd. (laser wavelength: 405 nm, mounted with a pickup with a lens having a NA of 0.85). Specifically, defects on the composite stamper were examined by irradiating laser light on the stamper-mounted face and detecting fluctuations of the signal in the reflected light. After inspection, the adhesive sheet (cover sheet) on the composite was separated, and inspected for the presence of visible deposits (visible residuals) thereon after separation of the adhesive sheet. A 1.1-mm Blu-ray substrate was prepared with polycarbonate (product name Panlite AD5503, manufactured by Teijin Chemicals Ltd.) by injection molding and by using the above stamper obtained by separating the adhesive sheet from the composite. A reflective layer of 100 nm in thickness was prepared on the substrate by Ag sputtering; then, the cover sheet described in the foregoing was adhered thereto; and signal fluctuations on the substrate thus prepared were measured. The results are summarized in the following Table 1.
[0027] Examples 2 to 5 and Comparative Examples 1 to 3
As shown in the following Table 1, a stamper was inspected and evaluated in a similar manner to Example 1, except that the adhesion of the adhesive layer was altered by changing the roll pressure or the monomer component of the main component of the adhesive layer (decrease in the addition amount of 2-ethylhexyl acrylate/tolylene diisocyanate-based low-molecular weight acryl component). Results are summarized in the following Table 1.

As is apparent from Table 1, inspection of the stampers obtained in Examples 4 and 5, which are included within the scope of the invention, was practically possible although there were some problems with inspection feasibility. The surface of the stampers in the Examples that are included in the scope of the invention remained clean even after inspection, and the signal fluctuation of the formed substrate was favorable at 4% or less.

WHAT IS CLAIMED IS:
1. An adhesive sheet for stamper inspection, comprising an optical film and an
adhesive layer that comprises an adhesive and is formed on the optical film, wherein the
adhesive has an adhesion of 3 (N/25 mm) or less.
2. The adhesive sheet for stamper inspection of claim 1, wherein the adhesion of
the adhesive is in a range of from 0 to 1 (N/25 mm).
3. The adhesive sheet for stamper inspection of claim 1, wherein the adhesive
comprises a copolymer comprising an acrylic ester as its constituent.
4. The adhesive sheet for stamper inspection of claim 3, wherein the copolymer
comprises at least one monomer selected from the group consisting of ethyl acrylate, n-butyl
acrylate, methyl acrylate, methyl methacrylate, vinyl acetate, acrylic acid, and methacrylic
acid.
5. The adhesive sheet for stamper inspection of claim 4, wherein the adhesive
comprises, as a curing agent, at least one selected from the group consisting of tolylene
diisocyanate, xylylene diisocyanate, and hexamethylene diisocyanate.
6. The adhesive sheet for stamper inspection of claim 1, wherein the optical film is
selected from the group consisting of a polycarbonate film, a triacetate cellulose film, and a
cyclic polyolefin film.
7. A stamper inspection method comprising:
adhering, onto a surface of a stamper to be inspected, the adhesive sheet for stamper inspection of claim 1;
irradiating laser light onto the surface of the stamper through the adhesive sheet for stamper inspection; and
detecting light reflected by the surface of the stamper.
8. The stamper inspection method of claim 7, wherein a roll is used in the adhering,
and an actual pressure of the roll is set within a range of from 100 kPa to 4 MPa.
9. The stamper inspection method of claim 7, wherein a roll is used in the adhering,
and an actual pressure of the roll is set within a range of from 200 kPa to 2 MPa.