fig3 is a schematic perspective plan view showing a photomask unit including a photomask and a pellicle covering the photomask , which is a preferred embodiment of the present invention and fig4 is a schematic cross sectional view taken along a line b - b in fig3 . as shown in fig3 and 4 , a photomask unit 1 according to this preferred embodiment includes a photomask 2 , a photomask stage 3 onto which the photomask 2 is to be fixed , a pellicle membrane 6 and a pellicle frame 7 having a frame - like shape and attached with the pellicle membrane 6 on the one surface thereof , and a pellicle 5 is constituted by the pellicle membrane 6 and the pellicle frame 7 . in this embodiment , the pellicle frame 7 is made of aluminum . as shown in fig3 and 4 , the size of the pellicle frame 7 is larger than that of the photomask 2 and the pellicle frame 7 is fixed onto the photomask stage 3 at a region outside of a region of the photomask stage 3 onto which the photomask 2 is fixed . thus , the photomask 2 is accommodated in a closed space defined by the pellicle membrane 6 and the pellicle frame 7 . in this embodiment , the photomask 2 is fixed by an electrostatic chuck ( not shown ) onto the photomask stage 3 . on the other hand , in this preferred embodiment , although the pellicle frame 7 is also fixed by an electrostatic chuck onto the photomask stage 3 , the pellicle frame 7 may be fixed onto the photomask stage 3 using a mechanical means such as a clamp , a thread or the like . further , the pellicle frame 7 may be fixed onto the photomask stage 3 using an agglutinant agent containing an organic material such as a silicone agglutinant agent . the thus fabricated photomask unit 1 is set in an exposure chamber ( not shown ). then , an exposure light is projected via the photomask 2 onto a photoresist film coated on a semiconductor wafer or a liquid crystal display panel mother substrate and a pattern of the photomask 2 is transferred onto the photoresist film , thereby forming a pattern of a semiconductor wafer or the liquid crystal display panel mother substrate . in this preferred embodiment , the photomask 2 and the pellicle frame 7 are fixed onto the photomask stage 3 by the electrostatic chuck and the size of the pellicle frame 7 is larger than that of the photomask 2 . thus , the pellicle frame 7 is fixed by an electrostatic chuck onto the photomask stage 3 at a region outside of a region of the photomask stage 3 onto which the photomask 2 is fixed . therefore , even when the photoresist film of the semiconductor wafer or the liquid crystal display panel mother substrate ( not shown ) is exposed to an euv light while keeping an exposure chamber to be vacuum , an outgas cannot be generated . accordingly , even in the case of forming a microscopic pattern of 32 nm or smaller on the photoresist film while keeping an exposure chamber to be vacuum , it is possible to improve the yield ratio of the semiconductor device or the liquid crystal display panel . further , according to this preferred embodiment , since the pellicle frame 7 is fixed onto the photomask stage 3 at a region outside of a region of the photomask stage 3 onto which the photomask 2 is fixed and is not fixed onto the surface of the photomask 2 unlike the conventional photomask unit , it is possible to suppress the lowering of the flatness of the photomask 2 caused by attaching the pellicle 5 to the minimum . fig5 is a schematic perspective plan view showing a photomask unit including a photomask and a pellicle attached to the photomask stage , which is another preferred embodiment of the present invention and fig6 is a schematic cross sectional view taken along a line c - c in fig5 . as shown in fig5 and 6 , similarly to the photomask unit 1 according to the previous embodiment , a photomask unit 1 according to this preferred embodiment includes a photomask 2 , a photomask stage 3 onto the one surface of which the photomask 2 is to be fixed , a pellicle membrane 6 and a frame - like shaped pellicle frame 7 onto which the pellicle membrane 6 is attached , and a pellicle 5 is constituted by the pellicle membrane 6 and the pellicle frame 7 . in this embodiment , the pellicle frame 7 is also made of aluminum . as shown in fig5 and 6 , the size of the pellicle frame 7 is larger than that of the photomask 2 and the pellicle frame 7 is fixed onto the photomask stage 3 at a region outside of a region of the photomask stage 3 onto which the photomask 2 is fixed . like the previous embodiment , in this embodiment of the present invention , the photomask 2 is fixed by an electrostatic chuck onto the photomask stage 3 . in the photomask unit 1 according to this preferred embodiment , as shown in fig5 and 6 , the pellicle frame 7 is fixed onto the photomask stage 3 by a mechanical clamp 9 . the thus fabricated photomask unit 1 is set in an exposure chamber ( not shown ). then , an exposure light is projected via the photomask 2 onto a photoresist film formed on a semiconductor wafer or a liquid crystal display panel mother substrate and a pattern of the photomask 2 is transferred onto the photoresist film , thereby forming a pattern of a semiconductor wafer or a liquid crystal display panel mother substrate . in this preferred embodiment , the size of the pellicle frame 7 is larger than that of the photomask 2 and the pellicle frame 7 is fixed by mechanical clamp 9 onto the photomask stage 3 at a region outside of a region of the photomask stage 3 onto which the photomask 2 is fixed . therefore , even when the photoresist film of the semiconductor wafer or the liquid crystal display panel mother substrate ( not shown ) is exposed to an euv light while keeping an exposure chamber to be vacuum , an outgas cannot be generated . thus , even in the case of forming a microscopic pattern of 32 nm or smaller while keeping an exposure chamber to be vacuum , it is possible to improve the yield ratio of the semiconductor device or the liquid crystal display panel . further , according to this preferred embodiment , since the pellicle frame 7 is fixed onto the photomask stage 3 at a region outside of a region onto which the photomask 2 is fixed and is not fixed onto the surface of the photomask 2 unlike the conventional photomask unit , it is possible to suppress the lowering of the flatness of the photomask 2 caused by attaching the pellicle 5 to the minimum . hereinafter , working examples and a comparative example will be set out in order to further clarify the advantages of the present invention . a square shaped photomask substrate whose one side length was 152 mm and thickness was 6 mm and which was made of quartz was prepared and a chromium film was evaporated on one surface of the photomask substrate and the other surface of the photomask substrate was fixed onto a photomask stage by an electrostatic chuck . then , a rectangle shaped pellicle frame whose inner frame length was 160 mm , outer frame length was 166 mm and wall thickness was 3 mm and which was made of aluminum was prepared and the pellicle frame was fixed by an electrostatic chuck onto the photomask stage at a region outside of a region of the photomask stage onto which the photomask substrate was fixed , thereby fabricating a photomask unit . next , in order to simulate the behavior of the photomask unit when it would be subjected to a high speed scanning operation in an actual manufacturing line , the photomask stage was vibrated for ten minutes so that acceleration of about 5 g was applied to the photomask stage . as a result , the displacement of the pellicle frame from its initial position was not observed although the pellicle frame was fixed onto the photomask stage only by the electrostatic chuck . further , an euv light having a wavelength of 13 . 5 nm was intermittently projected onto the photomask unit for three minutes in total and the reflection ratio of the photomask was measured after the irradiation with the euv light . the thus measured reflection ratio of the photomask was compared with the reflection ratio of the photomask before the irradiation with the euv light . as a result , the reduction in reflection ratio of the photomask was not observed . a square shaped photomask substrate whose one side length was 152 mm and thickness was 6 mm and which was made of quartz was prepared and a chromium film was evaporated on the one surface of the photomask substrate and the other surface of the photomask substrate was fixed onto a photomask stage by an electrostatic chuck . then , a pellicle frame made of aluminum whose inner frame length was 160 mm , outer frame length was 166 mm and wall thickness was 3 mm was prepared and the pellicle frame was fixed by a mechanical clamp onto the photomask stage at a region outside of a region of the photomask stage onto which the photomask substrate was fixed , thereby fabricating a photomask unit . next , in order to simulate the behavior of the photomask unit when it would be subjected to a high speed scanning operation in an actual manufacturing line , the photomask stage was vibrated for ten minutes so that acceleration of about 5 g was applied to the photomask stage . as a result , the displacement of the pellicle frame from its initial position was not observed although the pellicle frame was fixed onto the photomask stage only by the mechanical clamp . further , an euv light having a wavelength of 13 . 5 nm was intermittently projected onto the photomask unit for three minutes in total and the reflection ratio of the photomask was measured after the irradiation with the euv light . the thus measured reflection ratio of the photomask was compared with the reflection ratio of the photomask before the irradiation with the euv light . as a result , the reduction in reflection ratio of the photomask was not observed . a square shaped photomask substrate whose one side length was 152 mm and thickness was 6 mm and which was made of quartz was prepared and a chromium film was evaporated on the one surface of the photomask substrate and the other surface of the photomask substrate was fixed onto a photomask stage by an electrostatic chuck . then , a pellicle frame whose inner frame length was 160 mm , outer frame length was 166 mm and wall thickness was 3 mm and which was made of aluminum was prepared and the pellicle frame was fixed by a silicone agglutinant agent onto the photomask stage at a region outside of a region of the photomask stage onto which the photomask substrate , thereby fabricating a photomask unit . next , in order to simulate the behavior of the photomask unit when it would be subjected to a high speed scanning operation in an actual manufacturing line , the photomask stage was vibrated for ten minutes so that acceleration of about 5 g was applied to the photomask stage . as a result , the displacement of the pellicle frame from its initial position was not observed although the pellicle frame was fixed onto the photomask stage only by the silicone agglutinant . further , an euv light having a wavelength of 13 . 5 nm was intermittently projected onto the photomask unit for three minutes in total and the reflection ratio of the photomask was measured after the irradiation with the euv light . the thus measured reflection ratio of the photomask was compared with the reflection ratio of the photomask before the irradiation with the euv light . as a result , the reduction in reflection ratio of the photomask was not observed . a square shaped photomask substrate whose one side length was 152 mm and thickness was 6 mm and which was made of quartz was prepared and a chromium film was evaporated on the one surface of the photomask substrate and the other surface of the photomask substrate was fixed onto a photomask stage by an electrostatic chuck . then , a pellicle frame whose inner frame length was 143 mm , outer frame length was 149 mm and wall thickness was 3 mm and which was made of aluminum was prepared and the pellicle frame was fixed by a silicone agglutinant agent onto the photomask substrate , thereby fabricating a photomask unit . next , in order to simulate the behavior of the photomask unit when it would be subjected to a high speed scanning operation in an actual manufacturing line , the photomask stage was vibrated for ten minutes so that acceleration of about 5 g was applied to the photomask stage . as a result , the displacement of the pellicle frame from its initial position was not observed . further , an euv light having a wavelength of 13 . 5 nm was intermittently projected onto the photomask unit for three minutes in total and the reflection ratio of the photomask was measured after the irradiation with the euv light . the thus measured reflection ratio of the photomask was compared with the reflection ratio of the photomask before the irradiation with the euv light . as a result , it was observed that the reflection ratio of the photomask was reduced by about 0 . 5 % by the irradiation with the euv light . it would be reasonable to understand that this reduction in the reflection ratio of the photomask was caused by outgas generated from the silicone agglutinant agent used for fixing the pellicle frame . the present invention has thus been shown and described with reference to the preferred embodiments and the working examples . however , it should be noted that the present invention is in no way limited to the details of the described arrangement but changes and modifications may be made without departing from the scope of the appended claims . for example , the pellicle frame 7 is fixed onto the photomask stage 3 by the electrostatic chuck in the preferred embodiment shown in fig3 and 4 and the pellicle frame 7 is fixed onto the photomask stage 3 by the mechanical clamp 9 in the preferred embodiment shown in fig5 and 6 . however , it is not absolutely necessary to fix the pellicle frame 7 onto the photomask stage 3 by the electrostatic chuck or the mechanical clamp 9 and the pellicle frame 7 may be fixed onto the photomask stage 3 by an agglutinant agent containing an organic material such as a silicone agglutinant agent . according to the present invention , even in the case of fixing the pellicle frame 7 onto the photomask stage 3 using an agglutinant agent containing an organic material such as a silicone agglutinant agent , since the pellicle frame 7 is fixed onto the photomask stage 3 at a region outside of a region of the photomask stage 3 onto which the photomask 2 is fixed , when the photoresist film of a semiconductor wafer or a liquid crystal display panel mother substrate ( not shown ) is exposed to an euv light while keeping an exposure chamber to be vacuum , even if an outgas is generated from the agglutinant agent containing an organic material , it is possible to suppress the adverse effect of the outgas to the minimum . further , although the pellicle frame 7 is fixed onto the photomask stage 3 by the mechanical clamp 9 in the preferred embodiment shown in fig5 and 6 , in the case of fixing the pellicle frame 7 onto the photomask stage 3 by a mechanical means , it is not absolutely necessary to fix the pellicle frame 7 onto the photomask stage 3 using the mechanical clamp 9 but the pellicle frame 7 may be fixed onto the photomask stage 3 using other mechanical means such as a thread . moreover , in the above described preferred embodiments and working examples , although the pellicle frame 7 made of aluminum was used , it is not absolutely necessary to use a pellicle frame 7 made of aluminum but it is possible to use a pellicle frame 7 made of other metal such as stainless steel instead of aluminum . further , it is not absolutely necessary for a pellicle frame 7 to be made of metal but it is possible to make a pellicle frame 7 of plastics such as polyethylene . furthermore , in working examples , although the photomask substrate having a square shape and the pellicle frame whose inner frame length was 160 mm , outer frame length was 166 mm having a rectangle shape were used , it is not absolutely necessary for each of the photomask substrates to have a square shape and it is not absolutely necessary for each of the pellicle frames to have a size whose inner frame length is 160 mm , outer frame length is 166 mm rectangle shape . it is possible to use the photomask substrate or the pellicle frames having an arbitrary shape as occasion demands .