Abstract:
A defect repair apparatus for an EUV mask includes: a gas field ion source that generates a hydrogen ion beam; 
     an ion optical system that scans and irradiates the hydrogen ion beam by focusing the hydrogen ion beam onto the EUV mask; a sample stage on which to place the EUV mask; a detector that detects secondary charged-particles generated from the EUV mask; and an image forming unit that forms an observation image of the EUV mask on the basis of an output signal from the detector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a defect repair apparatus for an EUV mask that repairs a defect in an EUV mask using a charged-particle beam. 
         [0003]    2. Background Art 
         [0004]    In the related art, there is a technique of repairing a defect in a photo-mask employed for a lithographic technique using a focused ion beam. A lithographic technique using EUV (Extreme Ultra Violet) as an exposing source has been developed recently. A mask employed for EUV lithography is formed of a reflection layer made up of ultra-thin films having a multi-layer structure and an absorption layer of a pattern shape. A focused ion beam is irradiated to a defect in the pattern shape of the absorption layer to repair the defect by etching processing or deposition processing. An example is disclosed, for example, in JP-A-2009-210805. 
         [0005]    According to the disclosed technique as above, it becomes possible to repair a defect in the EUV mask. 
         [0006]    The technique in the related art, however, has a problem that anion beam irradiated to the reflection layer in the EUV mask causes damage to the multi-layer structure, which deteriorates reflectance markedly. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention was devised in view of the foregoing and therefore has an object to provide a defect repair apparatus for an EUV mask that repairs a defect without markedly deteriorating reflectance of the reflection layer in the EUV mask. 
         [0008]    In order to provide the defect repair apparatus for an EUV mask as above, it is preferable to use an ion beam that causes less irradiation damage to the EUV mask. In addition, a considerable variance in amount of current of an ion beam during a defect repair causes excessive or insufficient processing. It is therefore necessary to irradiate an ion beam with a stable amount of current to the EUV mask. Further, because the EUV mask has a fine pattern, it is preferable to repair a defect with a high degree of accuracy. In view of the foregoing, the invention provides the following. 
         [0009]    A defect repair apparatus for an EUV mask according to one aspect of the invention includes: a gas field ion source that generates a hydrogen ion beam; an ion optical system that focuses the hydrogen ion beam onto the EUV mask; a sample stage on which to place the EUV mask; a detector that detects secondary charged-particles generated from the EUV mask; and an image forming unit that forms an observation image of the EUV mask on the basis of an output signal from the detector. When configured in this manner, an observation image can be obtained by irradiating a hydrogen ion beam to the EUV mask. It thus becomes possible to lessen damage to the multi-layer structure of ultra-thin films forming a reflection layer in the EUV mask. 
         [0010]    It is preferable that the defect repair apparatus for an EUV mask further includes a hydrogen gas supply source that supplies a hydrogen gas to the gas field ion source and a purifier that purifies the hydrogen gas. When configured in this manner, a high-purity hydrogen gas can be supplied to the gas field ion source. It thus becomes possible to irradiate a hydrogen ion beam with a stable amount of beam current. 
         [0011]    It is preferable that the defect repair apparatus for an EUV mask further includes an ion generation chamber in which to generate ions and an intermediate chamber provided between the ion generation chamber and a vacuum sample chamber. When configured in this manner, it becomes possible to reduce an inflow of an impurity gas from the vacuum sample chamber to the ion generation chamber. 
         [0012]    It is preferable that the defect repair apparatus for an EUV mask further includes a current measuring electrode that is provided between the gas field ion source and a focusing lens electrode and measures an amount of current of the hydrogen ion beam. When configured in this manner, it becomes possible to measure an amount of current of the hydrogen ion beam irradiated from the gas field ion source. Accordingly, it becomes possible to control ion beam irradiation conditions on the basis of the measured amount of current for the gas field ion source to irradiate a hydrogen ion beam with a stable amount of current. 
         [0013]    It is preferable for the defect repair apparatus for an EUV mask that the gas field ion source and the ion optical system irradiate the hydrogen ion beam having a beam diameter of 5 nm or less onto the EUV mask. Accordingly, it becomes possible to repair a fine pattern of the EUV mask with a high degree of accuracy. 
         [0014]    It is preferable for the defect repair apparatus for an EUV mask that an upper limit value of an amount of irradiation of the hydrogen ion beam onto the EUV mask is 4×10 16  ions/cm 2 . When configured in this manner, it becomes possible to further lessen damage to the multi-layer structure of the ultra-thin films forming the reflection layer in the EUV mask. 
         [0015]    It is preferable that the defect repair apparatus for an EUV mask further includes a deposition gas supply system that supplies a deposition gas to the EUV mask. When configured in this manner, by supplying a deposition gas and irradiating a hydrogen ion beam to the EUV mask, a deposition film can be formed locally. It thus becomes possible to repair a defect portion of the absorption layer in the EUV mask with the deposition film. 
         [0016]    It is preferable that the defect repair apparatus for an EUV mask further includes an etching gas supply system that supplies an etching gas to the EUV mask. When configured in this manner, by supplying an etching gas and irradiating a hydrogen ion beam to the EUV mask, etching processing can be applied locally at a high speed. It thus becomes possible to repair a defect by effectively applying etching processing to a defect portion of the absorption layer in the EUV mask. 
         [0017]    A defect repair method for an EUV mask according to another aspect of the invention includes: obtaining an observation image by scanning and irradiating a hydrogen ion beam onto the EUV mask; setting a defect repair position from the observation image; and repairing a defect by irradiating the hydrogen ion beam to the defect repair position. When configured in this manner, an observation image can be obtained by scanning and irradiating a hydrogen ion beam to the EUV mask. It thus becomes possible to lessen damage to the multi-layer structure of the ultra-thin films forming the reflection layer in the EUV mask. 
         [0018]    It is preferable for the defect repair method for an EUV mask that an upper limit value of an amount of irradiation of the hydrogen ion beam is 4×10 16  ions/cm 2 . When configured in this manner, it becomes possible to further lessen damage to the multi-layer structure of the ultra-thin films forming the reflection layer in the EUV mask. 
         [0019]    It is preferable for the defect repair method for an EUV mask that a deposition gas is supplied to the defect when the defect is repaired. When configured in this manner, by supplying a deposition gas and irradiating a hydrogen ion beam to the EUV mask, a deposition film can be formed locally. It thus becomes possible to repair a defect portion of the absorption layer in the EUV mask with the deposition film. 
         [0020]    It is preferable for the defect repair method for an EUV mask that an etching gas is supplied to the defect when the defect is repaired. When configured in this manner, by supplying an etching gas and irradiating a hydrogen ion beam to the EUV mask, etching processing can be applied locally at a high speed. It thus becomes possible to repair a defect by effectively applying etching processing to a defect portion of the absorption layer in the EUV mask. 
         [0021]    According to the defect repair apparatus for an EUV mask configured as above, it becomes possible to repair a defect in the EUV mask without markedly deteriorating reflectance of the reflection layer in the EUV mask. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a view showing the configuration of a defect repair apparatus for an EUV mask according to one embodiment of the invention; 
           [0023]      FIG. 2  is a view showing the configuration of an ion source in the defect repair apparatus for an EUV mask according to one embodiment of the invention; 
           [0024]      FIG. 3  is a cross section of the configuration of an EUV mask according to one embodiment of the invention; 
           [0025]      FIG. 4  is a flowchart according to one embodiment of the invention; and 
           [0026]      FIG. 5  is a surface view showing the configuration in a part of an EUV mask according to one embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Hereinafter, embodiments of a defect repair apparatus for an EUV mask of the invention is explained. 
       (1) Defect Repair Apparatus for EUV Mask 
       [0028]    As is shown in  FIG. 1 , a defect repair apparatus  100  for an EUV mask according to one embodiment includes an ion beam column  1  provided with an ion optical system having a focusing lens electrode  16  and an objective lens electrode  17  that focus ions generated in an ion source  12  onto a sample  3  placed within a vacuum sample chamber  11 . 
         [0029]    The defect repair apparatus  100  for an EUV mask also includes a secondary electron detector  5 , which is a secondary charged-particle detector that detects secondary electrons  4  generated when an ion beam  2  is irradiated to the sample  3  from the ion beam column  1 . Herein, a secondary ion detector is used as the secondary charged-particle detector in a case where secondary ions generated from the sample  3  are detected. 
         [0030]    The defect repair apparatus  100  for an EUV mask also includes a gas supply system  6  that supplies a gas to the surface of the sample  3 , a sample holder  7  that fixes the sample  3 , and a sample stage  8  that moves the sample  3 . 
         [0031]    The defect repair apparatus  100  for an EUV mask uses a reflected ion detector as the secondary charged-particle detector in a case where reflected ions generated from the sample  3  are detected. The defect repair apparatus  100  for an EUV mask further includes an image forming unit  9  that forms an observation image from a scan signal of an ion beam  2  and a detection signal from the secondary electron detector  5  and a display unit  10  that displays an observation image thereon. 
       (2) Ion Source 
       [0032]    The ion source  12  is a gas field ion source and, as is shown in  FIG. 2 , includes an ion generation chamber  21 , an emitter  22 , an extraction electrode  23 , and a cooling device  24 . 
         [0033]    The cooling device  24  is provided to a wall portion of the ion generation chamber  21 . The emitter  22  of a needle shape is attached to the cooling device  24  on a surface facing the ion generation chamber  21 . 
         [0034]    The cooling device  24  cools the emitter  22  with a cooling medium, such as liquid nitrogen and liquid helium, stored inside thereof. As the cooling device  24 , a GM or pulse tube closed cycle refrigerator or a gas flow refrigerator is available. 
         [0035]    The extraction electrode  23  having an opening at a position opposing a tip end  22   a  of the emitter  22  is provided in the vicinity of an opening end of the ion generation chamber  21 . 
         [0036]    The ion generation chamber  21  maintains the interior thereof in a high-vacuum state using an air exhauster. A hydrogen gas supply source  40  is connected to the ion generation chamber  21  via gas introduction tubes  43 ,  44 , and  45 , so that a slight amount of a hydrogen gas is supplied into the ion generation chamber  21 . 
         [0037]    The emitter  22  is a member formed by coating a needle-shaped base material made of tungsten or molybdenum with precious metal, such as platinum, palladium, iridium, rhodium, and gold. The tip end  22   a  is of a pyramidal shape pointed sharply at the atomic level. 
         [0038]    Alternatively, the emitter  22  may be the one formed of a needle-shaped base material made of tungsten or molybdenum with the tip end  22   a  pointed sharply at the atomic level by introducing a nitrogen gas or an oxygen gas to the base material. 
         [0039]    The emitter  22  is maintained at a temperature as low as or lower than about 100 K by the cooling device  24  while the ion source  12  is in operation. A voltage is applied between the emitter  22  and the extraction electrode  23  by a power supply  27 . 
         [0040]    When a voltage is applied between the emitter  22  and the extraction electrode  23 , an extremely large electric field is developed at the sharply pointed tip end  22   a  while hydrogen molecules  25  are polarized and attracted to the emitter  22  and turn into hydrogen ions as they lose electrons through tunneling at a high field position of the tip end  22   a . The hydrogen ions are repelled by the emitter  22  maintained at positive potential and come flying out toward the extraction electrode  23 . Hydrogen ions  28  ejected to an ion optical system  1   b  through the opening in the extraction electrode  23  form an ion beam  2 . In the case of a gas field ion source, hydrogen ions include hydrogen molecular ions and hydrogen atomic ions and a ratio thereof varies with a voltage. Under normal use conditions, most of the hydrogen ions are hydrogen molecular ions. 
         [0041]    The tip end  22   a  of the emitter  22  is of an extremely pointed shape. Because hydrogen ions fly out from the tip end  22   a , an energy distribution width of an ion beam  2  emitted from the ion source  12  is extremely narrow. For example, in comparison with a plasma gas ion source or a liquid metal ion source, an ion beam with a smaller beam diameter at higher intensity can be obtained. 
         [0042]    When an applied voltage to the emitter  22  is too large, constituent elements (tungsten and platinum) of the emitter  22  fly out toward the extraction electrode  23  together with hydrogen ions. Accordingly, a voltage to be applied to the emitter  22  during operation (while an ion beam is being emitted) is maintained to a voltage small enough for constituent elements of the emitter  22  not to fly out from the emitter  22 . 
         [0043]    Meanwhile, the shape of the tip end  22   a  can be adjusted by taking advantage of the capability of engineering the constituent elements of the emitter  22  as described above. For example, the ion beam diameter can be increased by broadening a region to ionize a gas by intentionally removing elements positioned at the leading edge of the tip end  22   a.    
         [0044]    In addition, by heating the emitter  22 , precious metal elements on the surface can be repositioned while preventing the precious metal elements from flying out. It thus becomes possible to restore the sharply pointed shape of the tip end  22   a  that has become dull with use. 
         [0045]    Incidentally, when water molecules are contained in a hydrogen gas supplied to the ion generation chamber  21 , water adheres to the emitter  22  and forms a protrusion. Hydrogen ions are then released in a direction different from the optical axis of an ion beam  2 . Because adhesion of water molecules occurs randomly, an amount of current of an ion beam  2  released in the optical axis direction may possibly vary considerably. In order to avoid such an inconvenience, a hydrogen gas to be supplied to the ion generation chamber  21  is purified. 
         [0046]    Metal tubes are used as the gas introduction tubes  43 ,  44 , and  45 . It is particularly preferable to use SUS-EP tubes having a surface roughness made finer by electropolishing. By pre-heating the gas introduction tubes  43 ,  44 , and  45  to several hundred degrees centigrade, adhesion of water onto the inner surface of the tubes can be lessened. 
         [0047]    Also, purifiers to purify a hydrogen gas supplied from the hydrogen gas supply source  40  are provided. A first purifier  41  purifies a hydrogen gas by allowing an impurity gas to be absorbed into a getter material made of more than one type of activated metal or to pass through a heated palladium thin film. A second purifier  42  eliminates an impurity with a cold trap using liquid nitrogen. 
         [0048]    Consequently, a high-purity hydrogen gas at  9 N purity (99.9999999%) or above can be supplied. Either the first purifier or the second purifier alone may be used as the purifiers. Also, the purifiers may be incorporated into the hydrogen gas supply source  40 . 
         [0049]    Further, in order to reduce an inflow of an impurity gas from the vacuum sample chamber  11  to the ion generation chamber  21 , an intermediate chamber  13  in a vacuum state is provided inside the ion beam column  1 . The interior of the intermediate chamber  13  is evacuated by a vacuum pump  14  different from the air evacuator used to evacuate the ion generation chamber  12 . 
         [0050]    An ion beam  2  generated in the ion generation chamber  21  passes through a small-diameter hole between the vacuum chambers to be irradiated to the vacuum sample chamber  11 . A hole  111  is provided between the ion generation chamber  21  and the intermediate chamber  13  and a hole  112  is provided between the intermediate chamber  13  and the vacuum sample chamber  11 . Herein, it may be configured in such a manner that the intermediate chamber  13  accommodates the objective lens electrode  17  and the hole  112  is positioned closer to the vacuum sample chamber  11  than to the objective lens electrode  17 . In particular, in a case where a deposition material gas or an etching gas for a deposition film is used for a defect repair in the vacuum sample chamber  11 , an inflow of an impurity gas in the deposition material gas or the etching gas into the ion generation chamber  21  can be reduced. 
       (3) Ion Optical System 
       [0051]    The ion optical system includes, sequentially from the ion source  12  to the vacuum sample chamber  11 , the focusing lens electrode  16  that converges an ion beam  2  and the objective lens electrode  17  that focuses the ion beam  2  onto the sample  3 . 
         [0052]    With the ion beam column  1  configured as above, because a source size can be set to 1 nm or smaller and energy dispersion of an ion beam  2  can be set to 1 eV or lower, a beam diameter can be reduced to 5 nm or smaller. Although it is not shown in the drawing, the ion beam column  1  may be provided with a mass filter, such as an E×B mass filter, to discriminate the atomic number of ions. 
       (4) Current Amount Measurement 
       [0053]    The ion beam column  1  includes a current measuring electrode  18  to measure an amount of current of an ion beam  2  between the ion source  12  and the focusing lens electrode  16 . An ammeter  19  connected to the current measuring electrode  18  measures an amount of current of an ion beam  2  irradiated to the current measuring electrode  18 . 
         [0054]    The extraction electrode  23  in the ion source  12  is controlled so that an amount of current measured by the ammeter  19  stays constant. It thus becomes possible to irradiate an ion beam  2  with a stable amount of current to the sample  3 . 
       (5) Gas Supply System 
       [0055]    The gas supply system  6  is configured to supply a deposition material gas of a deposition film (for example, a carbon-based gas, such as phenanthrene and naphthalene, a metal compound gas containing metal, such as platinum and tungsten) onto the surface of the sample  3  through a gas nozzle from a deposition material container. 
         [0056]    In a case where etching processing is applied, the gas supply system  6  is capable of supplying an etching gas (for example, xenon fluoride, chlorine, iodine, chlorine trifluoride, fluorine monoxide, and water) through the gas nozzle from the deposition material container. 
       (6) EUV Mask 
       [0057]    An EUV mask used as the sample  3  is formed of, as is shown in  FIG. 3 , a reflection layer  33  of an Mo/Si multi-layer structure, a buffer layer  32 , an absorber  31  (pattern shape), which are provided sequentially on a glass substrate  34 . According to EUV lithography, EUV light is irradiated on the EUV mask and a mask pattern is transferred using reflected light. When there is a defect in the pattern shape of the absorber  31  of the EUV mask, the pattern shape together with the defect is transferred. In order to avoid such an inconvenience, it is necessary to repair the defect. 
         [0058]    Regarding the pattern dimension of the EUV mask, for example, in a case of a process corresponding to a 22-nm node, a half pitch of a line and a space is 88 nm and required defect repair accuracy is about 2 nm or smaller for three sigmas. 
       (7) Ion Beam Irradiation Damage on EUV Mask 
       [0059]    The inventors of the present application conducted a simulation and an experiment for ion beam irradiation damage on an EUV mask. In the experiment, a damaged state of the EUV mask by irradiation of an ion beam to the Mo/Si multi-layer structure, which is the reflection layer, was examined. 
         [0060]    Helium ion beams with amounts of beam irradiation of 4×10 15  ions/cm 2 , 4×10 16  ions/cm 2 , and 4×10 17  ions/cm 2  were irradiated at an acceleration voltage of 30 kV and sectional TEM (Transmission Electron Microscopy) images of the Mo/Si multi-layer structure after irradiation were obtained. 
         [0061]    Then, it was found that mixing had occurred in the Mo/Si multi-layer structure to a depth of 280 nm from the sample surface in the EUV masks irradiated by helium ions with amounts of beam irradiation of 4×10 16  ions/cm 2  and 4×10 17  ions/cm 2 . A simulation in the process of ion implantation by the Monte Carlo method was also conducted and the ion implantation depth was in agreement with the experiment result. 
         [0062]    Reflectance of EUV masks for EUV light after irradiation of ion beams was measured. Then, it was found that reflectance was deteriorated markedly by irradiation of ion beams with amounts of beam irradiation of 4×10 16  ions/cm 2  and 4×10 17  ions/cm 2 . 
         [0063]    In view of the foregoing, it can be said that mixing occurs in the multi-layer structure by irradiating an ion beam onto the reflection layer and reflectance of the EUV mask for EUV light is consequently deteriorated. Also, damage becomes larger as an amount of beam irradiation increases. With a helium ion beam, it was found that deterioration in reflectance is small when an amount of beam irradiation was 4×10 15  ions/cm 2 , which is, however, insufficient to repair a defect portion. 
         [0064]    Given these circumstances, the inventors of the application paid attention to hydrogen ions having a smaller atomic number than helium ions. A simulation in the process of ion implantation by the Monte Carlo method was conducted and a comparison was made as to the numbers of repelled atoms within the sample when incident ions were implanted. Then, it was found that when incident ions were hydrogen ions, the number of repelled ions was about 1/10 of that when incident ions were helium ions. On the basis of this finding, the inventors achieved an idea that a hydrogen ion beam is effective as a beam used to repair a defect in an EUV mask. 
         [0065]    Hence, the inventors of the application conducted an experiment to check a relation between an amount of irradiation of a hydrogen ion beam and a damaged state of the reflection layer in the EUV mask. Hydrogen ion beams with amounts of beam irradiation of 4×10 16  ions/cm 2  and 4×10 17  ions/cm 2  were irradiated at an acceleration voltage of 30 kV and sectional TEM (Transmission Electron Microscopy) images of the Mo/Si multi-layer structure after irradiation were obtained. 
         [0066]    Then, it was found that a damaged state in the sectional TEM image in the case of an amount of beam irradiation of 4×10 16  ions/cm 2  was substantially equal to a damaged state in the case of a helium ion beam with an amount of beam irradiation of 4×10 15  ions/cm 2 . In other words, a hydrogen ion beam with an amount of beam irradiation of 4×10 16  ions/cm 2  is practical because even when it is irradiated to the reflection layer, deterioration in reflectance of the reflection layer is small and an amount of irradiation is sufficient to repair a defect portion. 
         [0067]    Also, it was found that a region where mixing occurred was 140 nm deep from the sample surface. This result is in agreement with the result of the simulation in the process of ion implantation by the Monte Carlo method. 
         [0068]    In view of the foregoing, it can be said that it becomes possible to repair a defect in an EUV mask while lessening damage to the EUV mask by using a hydrogen ion beam. 
       First Embodiment 
       [0069]    One embodiment of the application will now be described along the flowchart of  FIG. 4 . A defect position in an EUV mask, which is the sample  3  placed on the sample holder  7 , is moved onto the sample stage  8  to be in an irradiation region of an ion beam  2 . 
         [0070]    A hydrogen ion beam is scanned and irradiated to the EUV mask from the ion beam column  1  and secondary electrons  4  generated from the EUV mask are detected by the secondary electron detector  5 . An observation image of the EUV mask is obtained in the image forming unit  9  from a scan signal of the hydrogen ion beam and a detection signal from the secondary electron detector  5  (observation image obtaining step S 1 ). 
         [0071]    Subsequently, the obtained observation image is displayed on the display unit  10  and a repair position setting to set the ion beam irradiation region on a defect portion is carried out (repair position setting step S 2 ). The ion beam irradiation position is thus determined. 
         [0072]    Subsequently, in a case where the defect is a redundant defect of the absorber pattern, xenon fluoride as an etching gas is supplied to the surface of the sample  3  from the gas supply system  6  and defect repair processing is applied by irradiating a hydrogen ion beam to the ion beam irradiation region (defect repair step S 3 ). The processing is ended when the redundant defect portion is etched away. 
         [0073]    In a case where the defect is a missing defect of the absorber pattern, a phenanthrene gas as a deposition gas is supplied to the surface of the sample  3  from the gas supply system  6  and the defect repair processing is applied by irradiating a hydrogen ion beam to the ion beam irradiation region. The defect is repaired as a deposition film is deposited on the defect portion. 
         [0074]    The repair result is confirmed (S 4 ) and the flow ends when the repair is completed. In a case where the repair is not completed, the flow returns to the observation image obtaining step S 1  to repair the defect portion again. 
         [0075]    By using hydrogen ions as the ion seed of an ion beam, ion beam irradiation damage on the reflection layer caused by irradiation of an ion beam when repairing a defect portion can be lessened more considerably than in a case where helium ions are used. It thus becomes possible to lessen deterioration in reflectance of the reflection layer for EUV light. 
       Second Embodiment 
       [0076]    An embodiment where the upper limit value of an amount of hydrogen ion beam irradiation to an EUV mask is set to 4×10 16  ions/cm 2  will be described.  FIG. 5  is a surface view of a part of an EUV mask and the pattern of the absorber  31  is provided on the reflection layer  33 . 
         [0077]    An ion beam  2  is scanned and irradiated on an observation region  52  including a defect  51  in the EUV mask. Secondary electrons  4  being generated are detected by the secondary electron detector  5  and an observation image of the observation region  52  is obtained. A repair position is set on the obtained observation image and the defect is repaired. 
         [0078]    In a case where the repair is not completed, an ion beam  2  is scanned and irradiated again on the observation region  52  to obtain an observation image. Then, a repair position for additional processing is set on the observation image. 
         [0079]    The upper limit value of an amount of hydrogen ion beam irradiation referred to herein means the upper limit value of an amount of ions irradiated onto a given irradiation region. In the repair processing above, it is controlled so that a total amount of ions irradiated onto the defect  51  does not reach the upper limit value. 
         [0080]    The defect  51  is of a dimension ranging from several nm to one micron meter and a processing region is set depending on the size of a defect. A current of 5 pA or below is used as an ion beam current and the ion beam current is measured in advance. A dose of irradiation per unit area is calculated on the basis of the processing region and the current to indicate the maximum time over which irradiation is allowed. This enables an operator to perform a repair operation by paying attention to the upper limit value of an amount of irradiation. 
         [0081]    In a case where an amount of irradiation has reached the upper limit value, it is possible to perform control to inhibit irradiation of an ion beam  2  onto the same observation region. It thus becomes possible to prevent an ion beam  2  being irradiated onto the EUV mask with an amount of irradiation exceeding the upper limit value.