Abstract:
A mask inspection apparatus includes: an electron gun for generating an electron beam; an exposure mask for shaping the electron beam into a predetermined cross-sectional shape; means for scanning the electron beam shaped by the exposure mask; means for selecting and transmitting part of the shaped electron beam, which selecting means includes a thin film having a small transmission aperture transmitting the electron beam scanned by the scanning means and includes a thick substrate having an opening larger than the small transmission aperture and a thickness greater than that of the thin film; and means for detecting the electron beam passed through the selecting means and outputting a current signal. The detecting means includes: a reflective body for reflecting the electron beam selected by the selecting means; and a detector for detecting the electron beam reflected by the reflective body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority of Japanese Patent Application No. 2004-279675, filed on Sep. 27, 2004, the entire contents of which are being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mask inspection apparatus, a mask inspection method, and an electron beam exposure system, which inspect an electron beam exposure mask. 
     2. Description of the Prior Art 
     In lithography processes for semiconductor integrated circuits and the like, patterns formed on masks are exposed to light and transferred onto wafers. If a mask has a defect, the shape and the like of a pattern transferred onto a wafer becomes abnormal. Accordingly, the mask needs to be inspected. 
     Heretofore, a pattern of an exposure mask has been inspected by making a comparison with a predetermined reference image using an optical microscope or making a comparison between actual image data on the mask pattern observed using an SEM or the like and a predetermined reference image. 
     Further, Japanese Unexamined Patent Publication No. 2001-227932 discloses a technique for performing an inspection by applying a converged electron beam to a mask, converting the electron beam passed through the mask into light, and converting an optical image obtained by conversion to light into an image signal using a CCD camera to obtain an image of the mask. 
     Incidentally, in the case where an exposure mask is inspected using a microscope or the like, the exposure mask is detached from an electron beam exposure system, set on the microscope or the like, and then inspected. Accordingly, an inspection process takes a long time, and there is a risk of damaging the mask when the mask is detached or mounted or a risk of allowing dusts and the like to adhere to the mask. 
     Moreover, the method disclosed in Japanese Unexamined Patent Publication No. 2001-227932, a mask is inspected using a dedicated mask inspection apparatus. The mask cannot be inspected in the state where the mask is attached to an electron beam exposure system. Thus, there are similar problems such as damage to the mask and the contamination of the mask. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in order to solve the above-described problems. An object of the present invention is to provide a mask inspection apparatus for an electron beam exposure, a mask inspection method, and an electron beam exposure system, which can perform a defect inspection of a pattern of an exposure mask with high precision using an exposure system actually used. 
     The above-described object is achieved by a mask inspection apparatus including: an electron gun for generating an electron beam; an exposure mask for shaping the electron beam into a predetermined cross-sectional shape; means for scanning the electron beam shaped by the exposure mask; means for selecting and transmitting part of the shaped electron beam, which selecting means includes a thin film having a small transmission aperture transmitting the electron beam scanned by the scanning means and includes a thick substrate having an opening larger than the small transmission aperture and a thickness greater than that of the thin film; and means for detecting the electron beam passed through the selecting means and outputting a current signal. 
     Here, the detecting means includes: a reflective body for reflecting the electron beam selected by the selecting means; and a detector for detecting the electron beam. The reflective body is made of any one of metal and a semiconductor. The detector is a PIN diode. 
     Further, the aforementioned object is achieved by an electron beam exposure system including the above-described mask inspection apparatus. 
     Moreover, the aforementioned object is achieved by a mask inspection method including: shaping an electron beam into a predetermined cross-sectional shape using an exposure mask to be inspected; scanning the electron beam shaped by the exposure mask; selecting part of the electron beam using a thin film and a thick substrate to pass the part of the electron beam through the thin film and the thick substrate, and obtaining a first signal waveform corresponding to a pattern of the exposure mask, which thin film has a small transmission aperture transmitting the electron beam, which thick substrate has an opening larger than the small transmission aperture and a thickness greater than that of the thin film; comparing the first signal waveform with a second signal waveform corresponding to a nondefective pattern of the exposure mask; and inspecting the exposure mask for a defect based on a result of the comparison. 
     In the present invention, an exposure mask is inspected in the state where the exposure mask is mounted on an electron beam exposure system and where an electron beam exposure is actually performed. Thus, the mask can be prevented from being damaged and dusts can be prevented from adhering to the mask during the time that the mask is being detached or mounted for an inspection. 
     Further, a transmissive electron beam detector  160  used in the present invention has a constitution in which the only electron beam EB passed through a small transmission aperture  27  of an electron beam transmission unit  21  is detected by an electron beam detection unit  22 . This improves an S/N ratio in the detection of an electron beam and makes it possible to more accurately inspect a mask. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of the constitution of an electron beam exposure system used in an embodiment of the present invention. 
         FIG. 2  is a diagram of the constitution of a transmissive electron beam detection device used in the embodiment of the present invention. 
         FIG. 3  is an explanatory diagram showing an overview of a method of inspecting an exposure mask. 
         FIG. 4  is a flowchart showing a procedure of the method of inspecting an exposure mask. 
         FIG. 5  is a plan view showing small transmission apertures of the electron beam detection device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to drawings. 
     (Constitution of Electron Beam Exposure System) 
       FIG. 1  is a diagram of the constitution of an electron beam exposure system according to this embodiment. 
     This electron beam exposure system is broadly divided into an electro-optical system column  100  and a control unit  200  which controls each unit of the electro-optical system column  100 . Of these, the electro-optical system column  100  includes an electron beam generation unit  130 , a mask deflection unit  140 , and a substrate deflection unit  150 , and the inside of the electro-optical system column  100  is decompressed. 
     In the electron beam generation unit  130 , an electron beam EB generated in an electron gun  101  is converged in a first electromagnetic lens  102 , and then passes through a rectangular aperture  103   a  of a beam-shaping mask  103 , whereby the cross section of the electron beam EB is shaped into a rectangular shape. 
     After that, an image of the electron beam EB is formed onto an exposure mask  110  by a second electromagnetic lens  105  of the mask deflection unit  140 . Then, the electron beam EB is deflected by first and second electrostatic deflectors  104  and  106  to a specific pattern S formed in the exposure mask  110 , and the cross-sectional shape thereof is shaped into the shape of the pattern S. 
     Incidentally, though the exposure mask  110  is fixed to a mask stage  123 , the mask stage  123  can be moved in a horizontal plane. In the case where a pattern S is used which lies over a region exceeding the deflection range (beam deflection region) of the first and second electrostatic deflectors  104  and  106 , the pattern S is moved to the inside of the beam deflection region by moving the mask stage  123 . 
     Third and fourth electromagnetic lenses  108  and  111 , which are respectively placed over and under the exposure mask  110 , have the role of forming an image of the electron beam EB onto a substrate W by adjusting the amounts of currents flowing therethrough. 
     The electron beam EB passed through the exposure mask  110  is returned to an optical axis C by the deflection functions of the third and fourth electrostatic deflectors  112  and  113 , and then the size of the electron beam EB is reduced by a fifth electromagnetic lens  114 . 
     In the mask deflection unit  140 , first and second correction coils  107  and  109  are provided. These correction coils  107  and  109  correct beam deflection errors generated in the first to fourth electrostatic deflectors  104 ,  106 ,  112 , and  113 . 
     After that, the electron beam EB passes through an aperture  115   a  of a shield plate  115  partially constituting the substrate deflection unit  150 , and projected onto the substrate W by first and second projection electromagnetic lenses  116  and  121 . Thus, an image of the pattern of the exposure mask  110  is transferred onto the substrate W at a predetermined reduction ratio, e.g., a reduction ratio of 1/60. 
     In the substrate deflection unit  150 , a fifth electrostatic deflector  119  and an electromagnetic deflector  120  are provided. The electron beam EB is deflected by these deflectors  119  and  120 . Thus, an image of the pattern of the exposure mask is projected onto a predetermined position on the substrate W. 
     Furthermore, in the substrate deflection unit  150 , provided are third and fourth correction coils  117  and  118  for correcting deflection errors of the electron beam EB on the substrate W. 
     The substrate W is fixed to a wafer stage  124  which can be moved in horizontal directions by a driving unit  125  such as a motor. The entire surface of the substrate W can be exposed to light by moving the wafer stage  124 . 
     Moreover, a transmissive electron beam detection device  160  is placed on the wafer stage. This allows the electron beam exposure system to function as a mask inspection apparatus, and makes it possible to inspect whether there is a defect in the pattern of the exposure mask. 
     (Explanation for Control Unit) 
     On the other hand, the control unit  200  has an electron gun control unit  202 , an electro-optical system control unit  203 , a mask deflection control unit  204 , a mask stage control unit  205 , a blanking control unit  206 , a substrate deflection control unit  207 , and a wafer stage control unit  208 . Of these, the electron gun control unit  202  controls the electron gun  101 , and controls the acceleration voltage of the electron beam EB, beam radiation conditions, and the like. Further, the electro-optical system control unit  203  controls the amounts of currents flowing and the like into the electromagnetic lenses  102 ,  105 ,  108 ,  111 ,  114 ,  116 , and  121 , and adjusts the magnification, focal point, and the like of the electro-optical system constituted by these electromagnetic lenses. The blanking control unit  206  deflects the electron beam EB generated before the start of an exposure onto the shield plate  115  by controlling the voltage applied to a blanking electrode  127 , thus preventing the electron beam EB from being applied to the substrate W before an exposure. 
     The substrate deflection control unit  207  controls the voltage applied to the fifth electrostatic deflector  119  and the amount of a current flowing into the electromagnetic deflector  120  so that the electron beam EB is deflected onto a predetermined position on the substrate W. The wafer stage control unit  208  moves the substrate W in horizontal directions by adjusting the driving amount of the driving unit  125  so that the electron beam EB is applied to a desired position on the substrate W. The above-described units  202  to  208  are integrally controlled by an integrated control system  201  such as a workstation. 
     (Explanation for Transmissive Electron Beam Detection Device) 
       FIG. 2  is a diagram showing the constitution of a transmissive electron beam detection device  160  used in this embodiment. This transmissive electron beam detection device  160  includes an electron beam transmission unit  21  which transmits only the electron beam EB passed through a small transmission aperture  27  formed in a thin film  23  made of silicon and which is configured not to transmit an electron beam scattered by the thin film  23  made of silicon, and an electron beam detection unit  22  which detects the electron beam EB passed through the electron beam transmission unit  21 . 
     As the electron beam transmission unit  21 , used is a silicon-on-insulator (SOI) substrate including a substrate  25  made of a silicon single crystal, a buried oxide layer  24 , and a thin film  23  made of silicon. In this embodiment, the thickness of the thin film  23  made of silicon is approximately 2 μm. Further, the thickness of the buried oxide layer  24  (SiO 2 ) is approximately 1 μm, and that of the substrate  25  made of a silicon single crystal is approximately 500 μm. Thus, the substrate  25  is formed to have a thickness greater than that of the thin film  23  made of silicon. Incidentally, the thickness of the thin film  23  made of silicon is preferably 1 μm to 10 μm, and that of the buried oxide layer  24  is preferably 0.1 μm to 4 μm. Further, the thickness of the substrate  25  is preferably 100 μm to 1000 μm. 
     In the substrate  25  made of a silicon single crystal, an opening  26  having a diameter of 10 μm is formed. Further, in this embodiment, in the thin film  23  made of silicon, the small transmission aperture  27  having a diameter of 50 nm is formed with the center thereof at the center of the opening  26  provided in the substrate  25  made of a silicon single crystal. Incidentally, the diameter of the opening  26  is preferably 1 μm to 100 μm, and that of the small transmission aperture  27  is preferably 10 nm to 500 nm. 
     By forming the electron beam transmission unit  21  as described above, the electron beam EB scattered by the thin film  23  cannot pass through the opening  26  of 10 μm under the thin film  23 , and is absorbed by the silicon of the substrate  25 . Accordingly, the electron beam EB scattered by the thin film  23  cannot pass through the substrate  25 , but only the electron beam EB passed through the small transmission aperture  27  formed in the thin film  23  can pass through the substrate  25 . 
     The electron beam detection unit  22  includes a reflective body  28  made of a metal which reflects the electron beam EB passed through the electron beam transmission unit  21  in lateral directions, and a detection unit  30  which detects the electron beam EB. In this embodiment, a PIN diode is used as the detection unit  30  (detector). The reflective body  28  may be made of metal or a semiconductor. Alternatively, the reflective body  28  may be one in which gold or the like is plated on a portion of a base  29  from which the electron beam EB is reflected. 
     Incidentally, in this embodiment, the electron beam EB is not applied directly to the detection unit  30 , but is applied to the detection unit  30  after having been reflected from the reflective body  28 . This is because the life of the PIN diode is shortened if the electron beam EB is applied directly to the PIN diode. In practice, if the electron beam EB continues to be applied to one position on the PIN diode, the life of the PIN diode is shortened to several tens of hours. However, the inventors have confirmed that if the PIN diode receives the electron beam EB after the electron beam EB has been reflected from the reflective body  28 , the life of the PIN diode is prolonged compared to that for the case where the PIN diode directly receives the electron beam EB. 
     As described above, only the electron beam EB passed through the electron beam transmission unit  21  is detected by the detection unit  30  of the electron beam detection unit  22 . Accordingly, an unnecessary electron beam is not detected, and an S/N ratio is improved. This makes it possible to inspect a mask pattern with high precision. 
     Further, since the electron beam is applied to the PIN diode after having been reflected from metal, the life of the PIN diode is prolonged compared to that for the case where the PIN diode directly receives the electron beam. 
     (Method of Inspecting Exposure Mask) 
       FIG. 3  is a diagram for explaining an overview of a method of inspecting an exposure mask using the transmissive electron beam detection device  160 . Here, an exposure mask pattern (partial batch pattern)  31  shown in  FIG. 3  is inspected. 
     First, an exposure mask is mounted on the mask stage of the electron beam exposure system and irradiated with an electron beam. The electron beam EB passed through the exposure mask is deflected by the fifth electrostatic deflector  119  and the electromagnetic deflector  120  of the substrate deflection unit  150 . Thus, an image of a pattern  32  of the exposure mask is projected onto a predetermined position on the transmissive electron beam detection device  160 . The electron beam EB passed through the exposure mask is scanned using these deflectors, and the electron beam is detected by the electron beam detector  22  of the transmissive electron beam detection device  160  placed on the wafer stage. For the detection of the electron beam, electrons passed through the small transmission aperture  27  are received by the PIN diode, and a current waveform  35  generated in the PIN diode is obtained. 
     Next, a comparison is made between the current waveform  35  (first signal waveform) obtained in the transmissive electron beam detection device  160  and a reference current waveform  36  (second signal waveform). Here, the reference current waveform is an ideal waveform which is expected to be obtained for a nondefective pattern. For example, a reference current waveform corresponding to a portion indicated by dotted lines in the exposure mask pattern  31  becomes like the waveform  36  because currents are generated in portions  31   a  to  31   f  where the exposure pattern and the lines indicated by the dotted lines intersect. 
     As can be seen from the current waveform  35 , there is a portion A in which the current waveform  35  is different from the reference current waveform  36 . Accordingly, it is revealed that the exposure mask  31  has a defect. Further, a waveform  37  can be obtained by performing waveform matching between the reference current waveform  36  and the actual current waveform  35 , whereby a defect occurrence position is clarified. 
       FIG. 4  is a flowchart in which a procedure for inspecting an exposure mask is summarized. 
     First, in step S 11 , an exposure mask to be inspected is selected. 
     Next, in step S 12 , scan widths in the X and Y directions of the exposure mask are determined. 
     Next, in step S 13 , a scanned portion of the exposure mask to be inspected is extracted, and a reference current waveform is created. 
     Next, in step S 14 , the electron beam exposure system is actually operated, and an electron beam passed through the exposure mask is deflected by the substrate deflection unit to scan the electron beam. Further, a current waveform is obtained which is generated by electrons obtained by detecting the scanned electron beam using the transmissive electron beam detection device  160 . 
     Next, in step S 15 , a comparison is made between the reference current waveform found in step S 13  and the actual current waveform obtained in step S 14 . If the waveforms match with each other, there is no defect in the scanned portion of the exposure mask inspected. Then, the process goes to step S 17 . 
     In step S 17 , a determination is made as to whether the entire mask to be inspected has been scanned or not. If the entire mask has not been scanned, the process goes back to step S 12 . Then, the scanned portion is shifted in the Y direction, and the same process is performed. 
     On the other hand, if the current waveforms do not match with each other in step S 15 , a determination is made that there is a defect in the scanned portion of the exposure mask. In this case, a portion determined to have a defect in step S 16  is recorded in a storage device or the like. Then, the process goes to step S 17 . 
     The above-describe process is performed until the scanning of the entire exposure mask to be inspected is completed. 
     Incidentally, in this embodiment, as shown in  FIG. 5 , a dot  52 , a vertical line  53  (line which is long in the Y direction), and a horizontal line  54  (line which is long in the X direction) are prepared as shapes of the small transmission aperture  27  of the transmissive electron beam detection device  160 . Further, a shape  55  is also prepared which is formed by combining the dot  52  and the vertical and horizontal lines  53  and  54 . 
     Moreover, the four types of small transmission apertures  27  shown in  FIG. 5  are formed in an area  51 , and various sizes of transmission apertures are prepared for each area  51 . For example, dots are prepared which have diameters of 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, and 200 nm. Further, vertical and horizontal lines are prepared which have long sides of 7 μm and have short sides of 40 nm, 50 nm, 60 nm, 80 nm, 100 nm, and 300 nm. Thus, a width in which the electron beam is scanned can be appropriately determined depending on what is selected as the shape of the small transmission aperture  27 . For example, in the case where a vertical line  53  which is long in the Y direction is selected, a scan can be performed in a shorter time by setting a scan width in the Y direction to the length of the vertical line. Further, in the case where the shape of a pattern cannot be accurately detected only using the vertical line  53  depending on the shape of the pattern, the electron beam EB can be scanned by selecting not only the vertical line  53  but also a dot  52 . Thus, the mask to be inspected can be accurately scanned without excess or deficiency. 
     As described above, in this embodiment, an exposure mask is inspected in the state where the exposure mask is mounted on the electron beam exposure system and where an electron beam exposure is actually performed. Thus, the mask can be prevented from being damaged and dusts can be prevented from adhering to the mask during the time that the mask is being detached or mounted for an inspection. 
     Moreover, the transmissive electron beam detector  160  used in this embodiment has a constitution in which the only electron beam EB passed through the small transmission aperture  27  of the electron beam transmission unit  21  is detected by the electron beam detection unit  22 . This improves an S/N ratio in the detection of an electron beam and makes it possible to more accurately inspect a mask.