Patent Number: 056169272
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is described above, the frame-supported pellicle of the invention comprises, as the essential elements, (a) a pellicle frame, (b) a pellicle membrane and (c) a layer of a pressure-sensitive adhesive, of which the pellicle frame and pellicle membrane are rather conventional and the most characteristic feature of the inventive pellicle consists in the use of a very specific pressure-sensitive adhesive which can be imparted with a reduced adhesive bonding strength when it is heated or irradiated with a radiation such as ultraviolet light. The reduction in the adhesive bonding strength of a pressure-sensitive adhesive can be caused by several different mechanisms including crosslink formation between polymeric molecules, degradation of the polymeric molecules and foaming within the adhesive layer. At any rate, it is essential that a decrease is caused in the stickiness of the surface of the adhesive layer when it is subjected to a heat treatment or to an irradiation treatment with a radiation such as ultraviolet light to such an extent that the adhesive layer left adherent to the photomask surface after removal of the pellicle frame therefrom can be easily removed to bring the surface of the photomask into a completely clean condition suitable for mounting of another frame-supported pellicle without using any risky or harmful means mentioned above. Various kinds of pressure-sensitive adhesives are used on conventional frame-supported pellicles including those based on a polybutene resin, polyvinyl acetate resin, acrylic resin, silicone resin and the like for the purpose of securing the flame-supported pellicle mounted on a photomask but none of these conventional pressure-sensitive adhesives exhibits a reduction in the adhesive bonding strength when subjected to a heat treatment or irradiation treatment with a radiation. It is necessary accordingly that these adhesive polymers are chemically modified so as to introduce a specific functional structure susceptible to heating or irradiation with light to cause crosslinking or decomposition resulting in reduction of the adhesive bonding strength. Alternatively, the pressure-sensitive adhesive is admixed with a blowing agent so that foaming takes place in the adhesive layer to form a foamed layer having decreased surface stickiness when the layer is heated or irradiated with light. These pressure-sensitive adhesives can be applied as such to the end surface of the pellicle frame. It is optional that a foamed sheet of a rubber such as a polyurethane rubber impregnated or coated with the pressure-sensitive adhesive is attached to the end surface of the pellicle frame. It is further optional according to need that the pressure-sensitive adhesive is admixed with a filler or a pigment. It is a desirable condition that the pressure-sensitive adhesive retains the adhesive bonding strength prolongedly with stability when the adhesive layer is at a temperature, e.g., room temperature, lower than a critical temperature or under irradiation with light of an intensity lower than a critical intensity but the adhesive bonding strength thereof is rapidly decreased when the adhesive layer is heated at a temperature higher than the critical temperature or irradiated with light of an intensity exceeding the critical intensity because otherwise the adhesive bonding strength of the adhesive layer is gradually but steadily decreased even under normal conditions eventually to cause inadvertent falling of the frame-supported pellicle from the photomask. As to the temperature of the heat treatment, the above mentioned critical temperature should be in the range from 40.degree. C. to 300.degree. C. or, preferably, from 70.degree. C. to 150.degree. C. because, when the critical temperature is too low, the decrease in the adhesive bonding strength of the adhesive layer proceeds even at room temperature while, when the critical temperature is too high, a high-temperature heat source must be brought into the clean room to uneconomically put an excessive load on the air-conditioning system of the clean room. When the adhesive layer is irradiated with light in order to be imparted with a reduced adhesive bonding strength, the wavelength of the light is not particularly limitative provided that the desired photochemical reaction can be induced thereby in the pressure-sensitive adhesive. Since the light is desirably ultraviolet in view of the high photochemical efficiency, suitable light sources include high-pressure and low-pressure mercury lamps, metal halide lamps and the like available at low costs. It is desirable to use an ultraviolet-curable pressure-sensitive adhesive of which the adhesive bonding strength can be definitely decreased by the irradiation with ultraviolet light in an irradiation dose of 500 mJ/cm.sup.2 or larger or, preferably, 200 mJ/cm.sup.2 or larger. Electron beams and X-rays are, though effective for the purpose, not preferable because they can be generated only by using very expensive and large instruments. In the following, the frame-supported pellicle of the present invention is described in more detail by way of examples. EXAMPLE 1 A membrane having a thickness of 0.815 .mu.m prepared from an amorphous fluorocarbon resin (Cytop, a product by Asahi Glass Co.) was spread over an end surface of a surface-anodized duralumin-made rectangular frame having outer side lengths of 120 mm by 98 mm, thickness of 2 mm and height of 5.8 mm and adhesively bonded thereto in a slack-free fashion by using a solution of the same fluorocarbon resin as the adhesive. A heat-curable pressure-sensitive adhesive tape (Liva-alpha No. 3195, a product by Nitto Denko Co.) was attached and bonded to the other end surface of the pellicle frame. This adhesive tape can be imparted with a decreased adhesive bonding strength when it is heated as is illustrated by the graphs shown in FIGS. 1 and 2 of the accompanying drawing showing the adhesive bonding strength in g per 20 mm width as a function of the temperature at-which the adhesive tape is contacted with a metallic surface for 1 minute (FIG. 1) and as a function of the heating time in seconds when the adhesive tape is heated at 100.degree. C.. As is understood from these graphs, the adhesive tape is stable at room temperature but rapidly loses the adhesive bonding strength when it is heated at 80.degree. C. or higher resulting in complete loss of the bonding strength by heating for i minute at 90.degree. C. or higher and only 1 second of heating is sufficient to cause complete disappearance of the adhesive bonding strength when the heating temperature is 100.degree. C. or higher. The above prepared frame-supported pellicle with the adhesive tape on one end surface of the pellicle frame was mounted on a photomask used in the patterning work for LSIs to find that reliable mounting could be obtained by virtue of the adhesive tape. The assembly of the photomask and the flame-supported pellicle mounted thereon was heated at 100.degree. C. for 3 seconds to find that the pellicle frame could be lifted from the photomask surface without any sticking resistance due to the substantially complete loss of the adhesive bonding strength on the adhesive tape. The surface of the photomask after removal of the frame-supported pellicle therefrom was so clean with no fragments of the adhesive tape left adherent thereto that the photomask was ready for mounting of another frame-supported pellicle. In contrast thereto, an attempt was made to separate the photomask and the frame-supported pellicle without undertaking the heating treatment at 100.degree. C. for 3 seconds to find that the attempt failed because the frame-supported pellicle could not be removed from the photomask without deformation of the pellicle frame. EXAMPLE 2 A frame-supported pellicle was prepared in substantially the same manner as in Example 1 excepting replacement of the heat-curable pressure-sensitive adhesive tape with an ultraviolet-curable pressure-sensitive adhesive tape (UC 1827, a product of Furukawa Denko Co.). FIG. 3 of the accompanying drawing shows that the ultraviolet-curable pressure-sensitive adhesive tape exhibited a decrease in the adhesive bonding strength given in g per 25 mm width when the tape attached to a mirror-polished surface of a semiconductor silicon wafer was irradiated with ultraviolet light from a metal halide lamp of an output of 80 watts/cm at 20.degree. C. in an atmosphere of 65% relative humidity and then peeled off the substrate surface in the direction to make an angle of 90.degree. at a pulling velocity of 50 mm/minute. The frame-supported pellicle with the pressure-sensitive adhesive tape on one end surface of the frame was mounted on a photomask for patterning of a liquid crystal display panel and the pressure-sensitive adhesive tape in the thus prepared assembly of the frame-supported pellicle and photomask was irradiated with ultraviolet light from a metal halide lamp in a dose of 500 mJ/cm.sup.2 to cause a decrease in the adhesive bonding strength between the pellicle frame and the photomask to about 1/5 of the strength before irradiation so that the flame-supported pellicle could be easily demounted from the photomask leaving absolutely no fragments of the adhesive tape remaining on the photomask. When the frame-supported pellicle was removed from the photomask without undertaking the ultraviolet irradiation, in contrast thereto, fragments of the adhesive tape were left adherent to the photomask surface which could be removed only after swelling with an organic solvent.