Abstract:
A scanning electron microscope is disclosed. The primary electron beam is radiated on a reticle (specimen), and an observation image of the reticle is obtained using the electrons secondarily released. The microscope comprises a lamp for radiating the vacuum ultraviolet light having the wavelength of not more than 172 nm on the reticle in the atmosphere, a radiation chamber for hermetically sealing the reticle so that the vacuum ultraviolet light can be radiated on the reticle, and a specimen holder for holding the reticle in the radiation chamber and capable of adjusting the distance between the lamp and the reticle.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a scanning electron microscope used for inspection of a reticle to fabricate a semiconductor integrated circuit, or in particular to a scanning electron microscope for inspecting, without contaminating the reticle surface, the dimensions and shape of the pattern formed on the reticle. 
   In fabrication of a semiconductor integrated circuit, the photolithography is used to form a circuit having fine shapes of various films on the surface of a silicon wafer. According to this technique, a pattern of a predetermined shape preformed on a quartz substrate is transferred by an exposer to a photosensitive resin (photoresist) film formed on the silicon wafer. The quartz substrate thus formed with the pattern is called a reticle. Generally, the reticle is made of a transparent quartz substrate of molten quartz glass or the like having the surface thereof formed with a pattern of an opaque film of a metal (hereinafter sometimes referred to as “the opaque film pattern”). The reticle is fabricated through the steps of forming an opaque film on a quartz substrate, forming a resist pattern of a photosensitive resin on the opaque film, and etching the opaque film through the resist pattern and forming an opaque pattern. 
   With the advance of micronization of a semiconductor integrated circuit, the resist pattern and the opaque film pattern formed on the reticle have become more and more difficult to inspect with the conventional inspection apparatus using the light. To overcome this difficulty, a technique using the scanning electron microscope for reticle inspection is under development. This is an attempt of using, for reticle inspection, the conventional scanning electron microscope with the measuring function (hereinafter sometimes referred to as “the measuring SEM”) for dimensional measurement and shape inspection of the semiconductor integrated circuit. 
   The measuring SEM includes a transport means for transporting a silicon wafer formed with a circuit at a high speed in the vacuum chamber of the scanning electron microscope, a moving means for moving a predetermined point on the silicon wafer with a high accuracy to a point just under the optical system of the scanning electron microscope, a calculation means for processing the specimen image formed from a secondary electron signal based on the secondary electrons released from the silicon wafer and thus calculating the dimensions of a predetermined portion, and a control means for executing the series of operation of each means automatically and continuously according to a predetermined procedure. An application of the measuring SEM to the reticle inspection, however, encounters the problem described below. Specifically, when the primary electron beam to release the secondary electrons is radiated on the reticle portion to be observed, the molecules of hydrocarbon and water existing on the reticle surface or in the surrounding space acquire the energy of the primary electron beam and are polymerized. The hydrocarbon, etc. thus polymerized is deposited on the reticle surface. This is the phenomenon called the contamination and conspicuous especially in the reticle inspection using the conventional measuring SEM. More specifically, the reticle inspection is accompanied by a conspicuous contamination due to the fact that the quartz substrate is charged and polarized by the primary electron beam and the resulting local electric field formed on the quartz substrate induces the molecules of hydrocarbon, etc. The contamination not only is deposited on the opaque film pattern to be observed and changes the dimensions thereof but also reduces the light transmittance of the transparent part of the reticle (the part other than the opaque film pattern), thereby often reducing the fabrication yield of the semiconductor integrated circuit. For using the measuring SEM for reticle inspection, therefore, it is necessary to take a measure to reduce the contamination. 
   A conventional measuring SEM having an ozone generator in a specimen observation chamber is known (e.g. G. W. B. Schulter et. al., Proceedings of SPIE, Vol. 5567 [2004], pp. 876-886). In this measuring SEM, ozone having a large mean free path is introduced into the specimen observation chamber in a low vacuum. The substances causing the contamination (hydrocarbon, etc.) in the specimen observation chamber are decomposed by reaction with ozone having large energy thereby to purify the interior of the specimen observation chamber. 
   Also, a conventional technique for purifying the surface of the glass substrate of a liquid crystal display element is known, in which ozone having a small mean free path in the atmosphere or comparatively small energy is caused to act on the glass substrate (Hishinuma Norikore: “2001 FPD Technology Daizen”, published by Electronic Journal, 2000″). According to this technique, a vacuum ultraviolet lamp having a wavelength of not more than 200 nm is turned on in the atmosphere thereby to ozonize the oxygen molecules in the surrounding atmosphere. The vacuum ultraviolet light lamp is turned on in the immediate neighborhood of the glass substrate and therefore the vacuum ultraviolet light is radiated on the surface of the glass substrate. As a result, hydrocarbon, etc. is decomposed not only by ozone but also by the optical energy of the vacuum ultraviolet light and the surface of the specimen is purified. 
   An attempt to inspect the resist pattern of a photosensitive resin of the reticle under the measuring SEM described above in the reference of Schulter et. al., however, would fail due to the fact that the resist pattern to be observed is decomposed easily by reaction with ozone having a large energy, and therefore the reticle cannot be arranged in the specimen observation chamber during the purification process of the interior of thereof. In this measuring SEM, therefore, the specimen cannot be observed during the purification process, thereby posing the problem of an extremely low reticle inspection rate. 
   Also, in this measuring SEM, ozone cannot be used in the step of observing the resist pattern formed on the opaque film which is considered essential in reticle fabrication. In other words, this measuring SEM cannot purify the contamination caused in the reticle fabrication process. Further, this measuring SEM poses the problem that ozone is diffused in the specimen observation chamber and therefore the hydrocarbon on the reticle surface playing the controlling role in the contamination cannot be effectively removed. The problem of the conventional measuring SEM, therefore, is that the reticle cannot be inspected with high accuracy due to the contamination of the reticle. 
   In the technique described in the reference of Hishinuma, on the other hand, the decomposition of the resist pattern can be suppressed due to the comparatively low ozone energy, but the resist pattern is liable to be decomposed by the optical energy of the vacuum ultraviolet light. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of this invention to provide a scanning electron microscope in which a specimen can be inspected with high accuracy and the specimen inspection rate is not reduced. 
   In order to achieve the object described above, according to this invention, there is provided a scanning electron microscope for radiating the primary electron beam on a specimen and acquiring an observation image of the specimen using the electrons released secondarily, comprising an ultraviolet light source for radiating the vacuum ultraviolet light having the wavelength of not more than 172 nm on the specimen in the atmosphere, a radiation chamber for hermetically sealing the specimen to permit the radiation of the vacuum ultraviolet light on the specimen, and a specimen holder for holding the specimen in the radiation chamber and capable of adjusting the distance between the ultraviolet light source and the specimen. 
   In this scanning electron microscope, the radiation of a vacuum ultraviolet light having the wavelength of not more than 172 nm on a specimen like the reticle for fabrication of a semiconductor integrated circuit generates excited oxygen atoms and ozone with high efficiency from the oxygen in the atmosphere in the radiation chamber, and the hydrocarbon considered a substance causing the reticle contamination is decomposed by the excited oxygen atoms and ozone. 
   In the case where the reticle has a resist pattern in this scanning electron microscope, the reticle is kept at a distance from the ultraviolet light source by the specimen holder. As a result, the vacuum ultraviolet light is rapidly attenuated in the atmosphere after generating the excited oxygen atoms and ozone. In other words, this scanning electron microscope can prevent the decomposition of the resist pattern by the vacuum ultraviolet light. 
   In the case where the reticle has no resist pattern in this scanning electron microscope, on the other hand, the ultraviolet light source and the reticle are brought close to each other by the specimen holder. As a result, the vacuum ultraviolet light reaches the reticle. In this scanning electron microscope, therefore, the hydrocarbon is efficiently decomposed by the excited oxygen atoms, ozone and the vacuum ultraviolet light. Thus, a still higher purification effect can be exhibited. 
   Also, in this scanning electron microscope, the hydrocarbon causing the contamination is purified, and therefore, the contamination of the reticle is suppressed in the case where the reticle is inspected by being irradiated with the primary electron beam. As a result, the reticle can be inspected with high accuracy. 
   Further, in this scanning electron microscope, the reticle is purified in the radiation chamber, and therefore the specimen observation chamber for inspecting (observing) the reticle can be arranged independently. As a result, the reticle can be purified and inspected at the same time. As compared with the conventional measuring SEM (the reference of Schulter et. al., for example), therefore, the reticle inspection rate can be increased. 
   This scanning electron microscope desirably uses the reticle for fabrication of a semiconductor integrated circuit having an insulating material as the specimen described above. 
   With the scanning electron microscope according to this invention, the contamination of the specimen caused by the radiation of the primary electron beam on the specimen is reduced, and therefore the specimen can be inspected with high accuracy. Also, according to this invention, the decomposition of the specimen is suppressed. Further, according to this invention, the specimen inspection rate can be increased as compared with the conventional measuring SEM. 
   Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram for explaining the configuration of a scanning electron microscope according to an embodiment of the invention. 
       FIG. 2  is a schematic diagram showing the manner in which the vacuum ultraviolet light is radiated in a radiation chamber in which a reticle having no resist pattern is arranged. 
       FIG. 3  is a schematic diagram showing the manner in which the vacuum ultraviolet light is radiated in a radiation chamber in which a reticle having a resist pattern is arranged. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Now, an embodiment of the invention is explained in detail with reference to the accompanying drawings. In the drawings,  FIG. 1  is a diagram for explaining the configuration of a scanning electron microscope according to this embodiment. In this case, an explanation is made about a scanning electron microscope for inspecting a reticle used for fabrication of a semiconductor integrated circuit. 
   As shown in  FIG. 1 , the scanning electron microscope includes a specimen observation chamber  105 , a front chamber  108 , a radiation chamber  123 , a specimen holder  125 , a transport robot  117  and a control unit  104 . 
   The specimen observation chamber  105  is for observation of the reticle  107  for inspection. The reticle  107  corresponds to the “specimen” as described in the appended claims and is similar to the known reticle used for fabrication of the semiconductor integrated circuit. The reticle  107  includes an opaque film formed on a transparent quartz film and a resist pattern formed on the opaque film, or includes an opaque film pattern formed on a quartz substrate with a resist pattern removed. 
   The specimen observation chamber  105  includes an electron gun  101  for radiating the primary electron beam and a stage  106  for moving the reticle  107  accurately just under a column  118  having an electronic optical system  102 . The column  118  has a detector  103  for detecting the secondary electrons released from the reticle  107  irradiated with the primary electron beam. The detector  103  transmits the secondary electron signal based on the secondary electrons to the control unit  104 . The specimen observation chamber  105  is connected with an exhaust unit  111 . The specimen observation chamber  105  is maintained at high vacuum by the exhaust unit  111  to allow the passage of the primary electron beam and the secondary electrons therethrough. 
   The front chamber  108  is arranged in the front stage of the specimen observation chamber  105  and has an exhaust unit  112  to vacuumize the front chamber  108 . The front chamber  108  includes a gate valve  109  for partitioning the front chamber  108  and the specimen observation chamber  105 , and also a gate valve  110  arranged on the side of the front chamber  108  far from the gate valve  109  for partitioning the interior and the exterior of the front chamber  108 . The reticle  107  is adapted to be moved into and out of the front chamber  108  by opening the gate valve  110 . The specimen observation chamber  105  is vacuumized by the exhaust unit  111 , and after the front chamber  108  is vacuumized by the exhaust unit  112 , the gate valve  109  is opened, so that the reticle  107  becomes movable between the front chamber  108  and the specimen observation chamber  105 . In other words, the front chamber  108  makes it possible transport the reticle  107  quickly to the highly vacuumized specimen observation chamber  105 . 
   The radiation chamber  123  is for generating excited oxygen atoms and ozone and has a gate valve  124  for partitioning the inside and the outside of the radiation chamber  123 . The reticle  107  is moved into and out of the radiation chamber  123  by opening the gate valve  124 . By closing the gate valve  124 , the reticle  107  arranged in the radiation chamber  123  is hermetically closed. 
   A lamp  114  of dielectric barrier discharge type for generating the vacuum ultraviolet light 172 nm in wavelength is mounted above the radiation chamber  123 . This lamp  114  corresponds to the “ultraviolet light source” described in the appended claims. The lamp  114  is configured of a quartz glass tube with xenon gas sealed therein, which xenon gas is ionized by a high frequency signal from an external source and the vacuum ultraviolet light having the wavelength of 172 nm is radiated when two excited xenon atoms are dissociated. The vacuum ultraviolet light thus generated is radiated on the reticle  107  in the radiation chamber  123  through a synthetic quartz window  120 , and at the same time, quickly attenuated while generating excited oxygen atoms and ozone in the radiation chamber  123 . Nitrogen is filled up in a casing  113  for accommodating the lamp  114 , so that the vacuum ultraviolet light is not attenuated in the casing  113 . 
   The specimen holder  125  holds the reticle  107  in the radiation chamber  123  and thus adjusts the distance between the lamp  114  and the reticle  107 . The specimen holder  125  is configured mainly of a specimen table  115  and a lift mechanism  116 . The specimen table  115  is to hold the reticle  107  in the radiation chamber  123 . The lift mechanism  116  is for moving the specimen table  115  up and down in the radiation chamber  123 . The vertical range of movement of the specimen table  115  is controlled by a low-level controller  119 . According to this embodiment, the specimen table  115  is movable in the vertical range of about 30 mm. At the highest position of the specimen table  115 , the interval  121  between the reticle  107  and the window  120  is set to not more than 2 mm. The radiation chamber  123  is connected with an ozone remover  122  mainly including a catalyst filter (not shown) and an exhaust fan (not shown), so that excited oxygen atoms and ozone, described later, generated in the radiation chamber  123  are removed from within the radiation chamber  123  gradually with time by the ozone remover  122 . 
   The transport robot  117  is adapted to retrieve the reticle  107  from a storage (not shown) and transport it between the front chamber  108  and the radiation chamber  123 . The operation of the transport robot  117  is controlled by the low-level controller  119 . 
   The control unit  104  is configured to image the profile of the surface of the reticle  107  by the well-known image processing of the secondary electron signal input from the detector  103 . The surface profile of the reticle  107  imaged based on the secondary electron signal is displayed on a monitor  104   a . The control unit  104  measures, by the well-known process, the dimensions of the surface profile of the reticle  107  by the image processing described above. Thus, the control unit  104  corresponds to the “measuring means” described in the appended claims. The control unit  104  controls the low-level controller  119  for controlling the operation of the specimen holder  125  and the transport robot  117 . 
   The control unit  104  holds the information on the type of the reticle  107  inspected. The control unit  104  according to this embodiment is so configured as to control the low-level controller  119  as described below in accordance with whether the reticle  107  has a resist pattern existing thereon or removed therefrom. At the time of inspecting the reticle  107  with a resist pattern, the control unit  104  according to this embodiment outputs a command signal to the low-level controller  119  to activate the lift mechanism  116  to keep the interval  121  of not less than 20 mm between the reticle  107  and the window  120 . At the time of inspecting an opaque pattern of the reticle  107  with the resist pattern removed therefrom, on the other hand, the control unit  104  according to this embodiment outputs a command signal to the low-level controller  119  to activate the lift mechanism  116  to space the reticle  107  and the window  120  from each other with the interval  121  of not more than 2 mm. In this way, the control unit  104  doubles as the “distance adjust means” described in the appended claims. The control unit  104  according to this embodiment, based on a command input by the operator by way of a keyboard (not shown) connected to the control unit  104 , determines whether the specimen to be inspected, i.e. the reticle  107  has a resist pattern or the resist pattern removed therefrom. Alternatively, the control unit  104 , based on a detection signal from an optical sensor (not shown) arranged in the radiation chamber  123  for confirming the presence or absence of the resist pattern, may determine whether the specimen to be inspected, i.e. the reticle  107  has a resist pattern or the resist pattern removed therefrom. 
   Next, the operation of the scanning electron microscope  1  according to this embodiment is explained with reference to the accompanying drawings.  FIG. 2  is a schematic diagram showing the manner in which the vacuum ultraviolet light is radiated into the radiation chamber in which the reticle having no resist pattern is arranged.  FIG. 3  is a schematic diagram showing the manner in which the vacuum ultraviolet light is radiated into the radiation chamber in which the reticle having a resist pattern is arranged. 
   First, an explanation is made about a case in which as shown in  FIG. 2 , the reticle  107  as a specimen for inspection is configured of an opaque pattern  206  and a quartz substrate  107   a , which has no organic matter such as a resist pattern on the surface thereof. 
   As shown in  FIG. 1 , the reticle  107  is retrieved from a storage, not shown, by a transport robot  117  and mounted on a specimen table  115  in the radiation chamber  123  through an open gate valve  124 . Then, the gate valve  124  is closed not to disperse the subsequently generated excited oxygen atoms and ozone out of the radiation chamber  123 . 
   The control unit  104  instructs the low-level controller  119  to make sure that the vacuum ultraviolet light reaches the reticle  107  directly, i.e. to keep an interval  121  of not more than 2 mm between the reticle  107  and the window  120 . As a result, the low-level controller  119  moves up the specimen table  115  by the lift mechanism  116  and sets the interval  121  of not more than 2 mm between the reticle  107  and the window  120 . After the reticle  107  moves up, the lamp  114  emits light and the vacuum ultraviolet light from the lamp  114  reaches the reticle  107  through the window  120 . 
   As shown in  FIG. 2 , the vacuum ultraviolet light  202  radiated from the lamp  114  is absorbed into the oxygen in the atmosphere after passing through the window  120  thereby to generate the excited oxygen atoms (O*: oxygen radical) and ozone (0 3 )  204 . In the process, the interval  121  between the reticle  107  and the window  120  is so narrow that part of the vacuum ultraviolet light  202  reaches the surface of the reticle  107  and cuts off the intramolecular bondage of the hydrocarbon  201  existing on the surface of the reticle  107 . As a result, the hydrocarbon  201  is decomposed into hydrocarbon  203  having a smaller molecular weight. This photodecomposition is strong and affects the purification capability. The decomposed hydrocarbon  203  small in molecular weight further reacts with the excited oxygen atoms and ozone  204  into carbon dioxide (CO 2 ) and water (H 2 O)  205 . In this way, with this scanning electron microscope  1  ( FIG. 1 ), the hydrocarbon  201 ,  203  existing on the surface of the reticle  107  are removed in the form of carbon dioxide and water. As a result, the contamination is suppressed which otherwise might be caused by the radiation of the primary electron beam on the reticle  107  in the specimen observation chamber  105  ( FIG. 1 ). As shown in  FIG. 2 , even in the case where the reticle  107  is exposed to the excited oxygen atoms, ozone  204  and the vacuum ultraviolet light  202  in the radiation chamber  123 , the opaque film pattern  206  on the surface of the reticle  107  is hardly affected. 
   The reticle  107  thus purified, as shown in  FIG. 1 , is recovered from the radiation chamber  123  through the open gate valve  124  by the transport robot  117 . The reticle  107  is transported into the front chamber  108  through the open gate valve  110  by the transport robot  117 . In the front chamber  108 , the gate valve  110  is closed, and the preliminary exhaust operation up to an appropriate vacuum degree is carried out by the exhaust unit  112 . In the process, the gate valve  109  is closed, and the specimen observation chamber  105  is exhausted to high vacuum degree by the exhaust unit  111 . After that, the reticle  107  is mounted on the stage  106  in the specimen observation chamber  105  through the open gate valve  109 . The reticle  107  discharges the secondary electrons upon receipt of the primary electron beam radiated from the electron gun  101 . The control unit  104 , on the other hand, executes the image processing based on the secondary electron signal input through the detector  103 . The control unit  104  then inspects the reticle  107  by measuring the size of the surface profile of the reticle  107 . In this scanning electron microscope  1 , the hydrocarbon  201 ,  203  which otherwise would cause the contamination are removed before the inspection of the reticle  107 , and therefore, only a very small contamination occurs upon radiation of the primary electron beam on the reticle  107 . 
   Next, an explanation is given about a case in which as shown in  FIG. 3 , the reticle  107  as a specimen for inspection has an opaque film  206   a  formed on a quartz substrate  107   a  and a resist pattern  301  formed on the opaque film  206   a.    
   As shown in  FIG. 1 , once the reticle  107  is mounted on the specimen table  115  in the radiation chamber  123 , the control unit  104  instructs the low-level controller  119  to set the interval  121  between the reticle  107  and the window  120  to not less than 20 mm, or 30 mm in the case under consideration, to reduce the amount of the vacuum ultraviolet light directly reaching the reticle  107 . As a result, the low-level controller  119  moves up the specimen table  115  by the lift mechanism  116  and sets the interval  121  between the reticle  107  and the window  120  to 30 mm. After moving up the reticle  107 , the lamp  114  emits light. 
   As shown in  FIG. 3 , the vacuum ultraviolet light  202  from the lamp  114  generates excited oxygen atoms and ozone  204  like in the foregoing case. The interval  121  between the reticle  107  and the window  120  is so large that the vacuum ultraviolet light  202  is absorbed into oxygen in the atmosphere before reaching the reticle  107 . As a result, no substantial photodecomposition of the hydrocarbon  301  is caused by the vacuum ultraviolet light  202 . The hydrocarbon  201  is thus gradually purified by the excited oxygen atoms and ozone  204  generated. Although the excited oxygen atoms and ozone  204  act also on the resist pattern  301  of the reticle  107 , the purification action thereof is so slow that only a small amount of the resist pattern  301  is removed by the excited oxygen atoms and ozone  204 . 
   As described above, in this scanning electron microscope  1 , the vacuum ultraviolet light  202  having the wavelength of not more than 172 nm ( FIGS. 2 ,  3 ) is quickly attenuated after generating the excited oxygen atoms and ozone  204  with high efficiency from the oxygen in the atmosphere. This vacuum ultraviolet light  202  has the linear absorption coefficient of about 0.26 in the normal atmosphere and is attenuated to about 0.6% over the distance of 20 mm. Specifically, in this scanning electron microscope  1 , the effect of the vacuum ultraviolet light  202  on the resist pattern  301  ( FIG. 3 ) can be avoided by keeping the reticle  107  at a distance from the lamp  114 . 
   Also, in this scanning electron microscope  1 , as shown in  FIG. 1 , the radiation chamber  123  can be arranged on the route of transportation of the reticle  107  to the specimen observation chamber  105 . Therefore, the observation of the reticle  107  in the specimen observation chamber  105  and the purification of the reticle  107  in the radiation chamber  123  can be carried out in parallel at the same time. Unlike the conventional measuring SEM (see the reference of G. W. B. Schulter et. al., for example), therefore, the inspection rate of the reticle  107  is not reduced in the scanning electron microscope  1 . 
   Also, in this scanning electron microscope  1 , in the case where the reticle  107  having only the opaque film pattern  206  and no resist pattern  301  ( FIG. 3 ) as a specimen for inspection is used as shown in  FIG. 2 , the photodecomposition action of the hydrocarbon  201  due to the vacuum ultraviolet light  202  can be effectively utilized by bringing the lamp  114  and the surface of the reticle  107  closer to each other. As a result, his scanning electron microscope  1  can exhibit a higher purification effect. 
   Also, in this scanning electron microscope  1 , the distance between the lamp  114  and the reticle  107  is automatically set by the control unit  104  shown in  FIG. 1  in accordance with the type of the reticle  107 , and therefore the inspection process can be completely automated. Further, in this scanning electron microscope  1 , the radiation chamber  123  and the specimen observation chamber  105  are arranged independently of each other, and therefore the reticle  107  can be observed while at the same time processing the ozone of the reticle  107 . In the scanning electron microscope  1 , therefore, unlike in the conventional measuring SEM (see the reference of G. W. B. Schulter et. al., for example), the inspection rate of the reticle  107  is not reduced. 
   This invention is not limited to the embodiments described above and can be embodied in various manners. 
   Although the embodiments described above refer to the scanning electron microscope  1  to inspect the reticle  107  for fabrication of the semiconductor integrated circuit, this invention is applicable to the observation and inspection of a specimen having other resist patterns as well as the reticle  107 . Also, this invention can be used for purification as a specimen preprocessing for observation and inspection of a specimen having a fine structure of an organic material on the surface thereof. 
   It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.