Optics: measuring and testing – Inspection of flaws or impurities – Transparent or translucent material
Reexamination Certificate
1998-03-09
2002-05-28
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Transparent or translucent material
C356S239800, C250S559400, C250S559410
Reexamination Certificate
active
06396579
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method, apparatus, and system for inspecting the surface or interior of transparent objects, such as quartz substrates for semiconductor devices, photomasks formed of quartz, and pellicle membranes used as dust protectors in the manufacture of semiconductor devices, such as LSIs an VLSIS, or liquid crystal display panels.
2. Description of the Related Art
In the manufacture of semiconductor devices, such as LSIs and VLSIs, or liquid crystal display panels, a semiconductor wafer or a liquid crystal material panel is irradiated, via a photomask, with light to thereby form circuit patterns on the wafer or liquid crystal material panel. If dust adheres to the photomask, the dust absorbs or deflects light, causing a transferred pattern to deform, a pattern edge to become rugged, or a white background to be blackishly smudged, and thus impairing dimensional accuracy, quality, or appearance. As a result, the manufacture of semiconductor devices or liquid crystal panels has involved a problem of impairment in the performance of manufactured semiconductor devices or liquid crystal panels or a decrease in manufacturing yield. Thus, the irradiation of a photomask with light is usually performed in a clean room. However, even in a clean room, keeping a photomask completely clean is difficult. Hence, there is employed a method of bonding a pellicle membrane having good transmission of exposure light onto the surface of a photomask for the purpose of protecting the mask surface from dust.
Through use of such a pellicle membrane, dust, if any, does not directly adhere to the surface of a photomask, but adheres to the pellicle membrane. Thus, by focusing light on the surface of the photomask where a circuit pattern exists, the dust adhering on the pellicle membrane has no effect on the transfer.
A transparent pellicle membrane for the above application is formed from a material having good transmission of visible light, such as nitrocellulose, cellulose acetate, modified polyvinyl alcohol, or a fluorine polymer, and is bonded onto a frame of aluminum alloy, stainless steel, polyethylene, or a like material. In bonding, a good solvent for the pellicle membrane is applied onto the frame, and then the pellicle membrane is placed on the frame, followed by air drying. Alternatively, the pellicle membrane is bonded onto the frame through use of adhesive such as an acrylic resin or epoxy resin. Further, on the opposite side of the frame are formed an adhesive layer formed from a polybutene resin, polyvinyl acetate resin, acrylic resin, or the like for adhesion to a photomask and a release layer for protection of the adhesive layer.
In view of the above-mentioned application of a pellicle membrane, both side surfaces and interior of the pellicle membrane must be free of foreign matter. Accordingly, the pellicle membrane undergoes strict inspection.
Also, quartz substrates for semiconductor devices and quartz photomasks bearing circuit patterns formed thereon must have transparency and be free of defects to a degree equal to or higher than in the case of pellicle membranes. Thus, the surfaces or interior of such transparent objects are strictly inspected for foreign matter.
Among conventional inspection methods for inspecting the surfaces or interior of transparent objects, a conventional inspection method for pellicle membranes is exemplified in FIG.
5
. As shown in
FIG. 5
, a frame to which a pellicle membrane A is bonded is attached to a handling jig. In a darkroom for inspection, an inspector holds the jig and exposes the surface of the pellicle membrane A to a spot-light from a convergence lamp B, to thereby visually check for foreign matter adhering to the membrane, foreign matter or defects present inside the membrane, and wrinkles or scratches in the membrane.
In the conventional method, the spot-light from a convergence lamp reflected from the surface of the pellicle membrane or from the pellicle frame impinges on and dazzles the inspector's eyes, thus imposing a burden on the eyes. Accordingly, an adverse effect is imposed on the inspector's eyes, and the inspector cannot continue the inspection for a long period of time. Also, inspection efficiency is impaired, and the inspector is apt to overlook foreign matter, resulting in a possible impairment in detection accuracy. These unfavorable phenomena are observed not only in the inspection of pellicle membranes but also in the inspection of quartz substrates and photomasks. Thus, there has been eager demand for a solution to the problems.
Also, conventional inspection methods for inspecting the surfaces or interior of transparent objects other than the above-mentioned visual inspection method include a method in which a transparent object is irradiated with a laser beam, and light scattered by foreign matter present on the surface or in the interior of the object is detected by a photomultiplier (a first method), and a method in which foreign matter itself is magnified and detected through use of a charge coupled device camera (hereinafter referred to as a CCD camera) equipped with a microscope (a second method).
Further, there has been proposed an inspection method for inspecting a transparent object for defects through use of a CCD camera (Japanese Patent Application Laid-Open (kokai) No. 4-344447) (a third method). In this method, a transparent object is irradiated with light in three directions, and light scattered by a defect such as scratch or smudge is detected by a CCD camera.
The first method has an advantage of very high repeatability of measurement, but has the following disadvantage. For example, for the portion (several millimeters wide) of a pellicle membrane in the vicinity of or along a pellicle frame, measurement is disabled due to interference of the frame with a laser beam, scatter of the skirt portion of intensity distribution (usually Gaussian distribution) of a laser beam caused by the frame, the diffraction effect of a laser beam, or the like.
The second method is not practicable for the following reason. In order to detect foreign matter having a small grain size, the magnifying power of the microscope must be increased, and thus inspection takes a very long time.
The third method is suited for inspecting a transparent object for two-dimensional defects such as scratches and smudges, but cannot properly inspect a transparent object for three-dimensional defects, for example, foreign matter, because three-dimensional defects are significantly different from two-dimensional defects in terms of scattering of light. Further, in the case of inspecting a pellicle membrane, since the pellicle membrane has a frame attached at the periphery thereof, the portion of the pellicle membrane along the frame cannot be inspected by methods, such as the above-described third method, in which a transparent object is irradiated with light in three directions concurrently. Thus, this method is not suited for inspection of pellicles.
Accordingly, inspection apparatuses which carry out the methods described above involve the same problems.
SUMMARY OF THE INVENTION
The present invention has been accomplished in view of the problems, and an object of the invention is to provide an inspection method for transparent objects that enables easy and efficient visual inspection of a transparent object with a reduced burden on the inspector's eyes, as well as accurate judgment.
Another object of the present invention is to provide an inspection apparatus and inspection system which do not involve visual inspection and employ reflected light, and which is free from the above problems involved in conventional inspection apparatuses.
According to a first aspect of the present invention, there is provided an inspection method for a transparent object comprising the steps of irradiating a transparent object with light and inspecting the surface or interior of the transparent object, wherein transmitted light is observed on the side of the transparent o
Hayamizu Mitsuru
Nagata Yoshihiko
Font Frank G.
Hogan & Hartson L.L.P.
Shin-Etsu Chemical Co. , Ltd.
Smith Zandra
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