Abstract:
An electron gun includes an electron source configured to emit electrons. The electron source includes an electron emission region configured to emit the electrons and an electron emission restrictive region configured to restrict emission of the electrons. The electron emission restrictive region is located on a side surface of the electron source except an electron emission surface on a tip of the electron source and is covered with a different material from the electron source. The electron gun emits thermal field-emitted electrons by applying an electric field to the tip while maintaining a sufficiently low temperature to avoid sublimation of a material of the electron source. The material of the electron source may be lanthanum hexaboride (LaB 6 ) or cerium hexaboride (CeB 6 ). The electron emission restrictive region may be covered with carbon.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of prior International Patent Application No. PCT/JP2006/321991, filed Nov. 2, 2006, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron gun used in a lithography process for manufacturing a semiconductor device, an electron beam exposure apparatus provided with the electron gun, and to an exposure method. 
     2. Description of the Prior Art 
     In order to enhance throughput, a recent electron beam exposure apparatus is provided with a mask having a variable rectangular aperture formed therein, or with multiple mask patterns. One of the mask patterns is firstly selected by means of beam deflection, and the selected pattern is then transferred onto a wafer through exposure. An exposure method employing the above-mentioned multiple mask patterns is achieved by an electron beam exposure device that is configured to perform block exposure. In the block exposure, patterns are transferred onto a sample surface by the following procedures. Specifically, a beam is irradiated on a pattern region selected from multiple patterns disposed on a mask by means of beam deflection so that a cross section of the beam is formed into a shape of the pattern. Furthermore, the beam that has passed through the mask is deflected back by a deflector located on a later stage and is transferred onto the sample surface after the scale of the pattern is reduced to a certain reduction ratio which is determined by an electron optical system. 
     Incidentally, ensuring linewidth accuracy is also an important factor for the above-mentioned type of exposure apparatus in order to enhance throughput. To ensure linewidth accuracy, intensity of an electron beam emitted from an electron gun must not be changed over time. In a case where the intensity of the electron beam is weakened due to the change over time, the degree of exposure is gradually reduced. An increase in exposure time to supplement the weakened electron beam due to the change over time may lead not only to complication of control but also to deterioration in the throughput. 
     In general, methods of emitting electrons from an electron gun are classified broadly into a thermal electron emission type method and a field emission type method. The thermal electron emission type electron gun includes a cathode configured to emit electrons by heating, a Wehnelt cylinder configured to generate an electron beam flux with converging the electrons emitted from the cathode, and an anode configured to accelerate the converged electron beam. 
     With the use of the above-described thermal electron emission type electron gun, a material constituting an electron source (a chip) used in the electron gun loses its quantity by sublimation or evaporation along with the emission of the electrons. Accordingly, a phenomenon of deformation of an electron emitter occurs. Various countermeasures have been studied to prevent the above-mentioned phenomenon. For example, Japanese Patent Application Publication No. Hei 8-184699 discloses an electron gun in which a chip is covered by a two-layer structure consisting of tungsten (W) and rhenium (Re) to suppress depletion of a chip. 
     As described above, with the use of the thermal electron emission type electron gun, not only the chip that constitutes the electron gun may emit the electrons but also the materials themselves constituting a chip may sublimate in some cases. The above mentioned sublimation of a chip is considered to occur because the temperature is heated up to the extent equal to or higher than the sublimation start temperature of the electron generating material. 
     Due to this sublimation, the shape of the chip for emitting the electrons is changed. Accordingly, such deformation prevents uniform irradiation of a variable rectangular beam or a block pattern beam, which causes the intensity reduction of the emitted electron beam. For example, with the use of a thermal electron emission type electron gun constituting a chip made of lanthanum hexaboride (LaB 6 ), at a temperature equal to 1500° C., the chip sublimates in an amount of 10 μm in one month. 
     Moreover, the chip material such as LaB 6  or cerium hexaboride (CeB 6 ) is attached to a rear surface of a grid as a result of the above-mentioned sublimation. The attached material may grow to a whisker. Then with the charged electrons thereon, a micro discharge may occur. Occurrence of this micro discharge may cause instabilities of the electron beam quantity and of a radiating position, thereby making it impossible to properly use an electron beam exposure apparatus. Moreover, the throughput may be deteriorated due to the time consumption for adjustment or other operations. The most serious problem is that the pattern drawn during the occurrence of the micro discharge may lead a loss of reliability. Therefore, the eradication of the micro discharge around the electron gun is indispensable to establish a high reliability of the electron beam exposure apparatus. In other words, the reduction in quantity of the material sublimation for the electron gun is the indispensable factor for the development to establish a high reliability as well as a high stability. 
     Although Japanese Patent Application Publication No. Hei 8-184699, discloses the technique to suppress depletion of the chip by covering the surface of the chip with the two-layer structure made of tungsten (W) and rhenium (Re), this technique is unable to prevent the change in the shape of a chip attributable to sublimation of an electron emission surface not covered with the two-layer structure. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the aforementioned problem of the conventional technology. An object of the present invention is to provide an electron gun which is capable of reducing a quantity of sublimation attributable to heat from an electron source for emitting electrons and thereby allowing stable use for a longer period of time, and to provide an electron beam exposure apparatus and an exposure method using this electron gun. 
     The above-described problem is solved by an electron gun including an electron source configured to emit electrons, in which the electron source includes an electron emission region configured to emit the electrons and an electron emission restrictive region configured to restrict emission of the electrons. Here, the electron emission restrictive region is located on a side surface of the electron source except an electron emission surface on a tip of the electron source and is covered with a material different from the electron source. Moreover, thermal field-emitted electrons are emitted through application of an electric field to the tip with a sufficiently low temperature being maintained to avoid sublimation of a material of the electron source. 
     In the electron gun of this aspect, the material of the electron source may be any one of lanthanum hexaboride (LaB 6 ) and cerium hexaboride (CeB 6 ). Meanwhile, the electron emission restrictive region may be covered with carbon. Here, the temperature may be set in a range from 1100° C. to 1300° C. 
     Moreover, in the electron gun of this aspect, the electron emission restrictive region may be defined as a region including a rear surface and the side surface of the electron source except the electron emission surface and a part sandwiched by a carbon chip used for heating by energization. 
     In the present invention, only the electron emission surface at the tip of a chip as an electron generating material is exposed while the remaining side surface part is covered with the different material. When LaB 6  is applied to the electron generating material, the different material is carbon (C), for example. Since the electron gun is operated at a low temperature, the chip hardly sublimates. In this way, the electron gun can be used stably for a long period of time while avoiding deformation of the electron emission surface. Moreover, as the side surface of the chip is covered with carbon, no electrons are emitted from the side surface of the chip even if an intense electric field is applied. Accordingly, a change in the shape of an electron beam can be avoided and a phenomenon of degradation in vacuum attributable to a rise in temperature of an unwanted part can be prevented. Not only the side surface is covered, but also the entire chip is preferably covered except the electron emission part on the tip with a protective film. This configuration is effective, not in terms of the electron emission, but in terms for suppressing sublimation and reducing an amount of an attached material onto a Wehnelt cylinder. 
     Meanwhile, the above-described problem is solved by an exposure method applying an electron gun provided with an electron source configured to emit electrons, an extraction electrode located at a given distance away from an electron emission surface of the electron source and configured to extract the electrons, a suppressor electrode located above the extraction electrode and the electron emission surface and configured to suppress emission of the electrons from a side surface of the electron source, and an acceleration electrode located below the extraction electrode and configured to accelerate the electrons. The electron gun is configured to emit thermal field-emitted electrons by applying an electric field to a tip of the electron source while maintaining a sufficiently low temperature to avoid sublimation of a material of the electron source, and then to render a sample exposed to the thermal field-emitted electrons. Here, the method includes the steps applying the voltage for a given time period so as to render electric potential at the extraction electrode lower than electric potential at the tip of the electron source, and performing exposure by applying the voltage so as to render the electric potential at the extraction electrode higher than electric potential at the tip of the electron source. 
     In the present invention, the electric potential at the extraction electrode is set lower than the electric potential at the electron source prior to performing the exposure. In this way, no electrons are extracted from the electron source. Accordingly, the electron source can be prevented from melting or breakage when conditioning is carried out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electron beam exposure apparatus according to the present invention. 
         FIG. 2  is a block diagram of an electron gun according to the present invention. 
         FIG. 3  is a chart showing an example of electric field intensity distribution of the electron gun according to the present invention. 
         FIG. 4  is a block diagram of an electron source and electrodes constituting the electron gun shown in  FIG. 2 . 
         FIGS. 5A and 5B  are cross-sectional diagrams showing shapes of a tip of the electron source. 
         FIG. 6  is a block diagram of an electron source and electrodes according to another embodiment of the electron gun shown in  FIG. 2 . 
         FIG. 7  is a cross-sectional diagram of the electron source for explaining a region for restricting emission of electrons. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, embodiments of the present invention will be described below with reference to the accompanying drawings. 
     First, a configuration of an electron beam exposure apparatus will be described. Subsequently, a configuration of an electron gun will be described and then a configuration of an electron source in the electron gun, which constitutes a characteristic part of the present invention, will be described. Next, an exposure method for an exposure apparatus applying the electron gun of the present invention will be described. Then, a method of forming a region for controlling electron emission on a surface of the electron source will be described. Lastly, effects of using the electron gun of the embodiment of the present invention will be described. 
     (Configuration of Electron Beam Exposure Apparatus) 
       FIG. 1  is a block diagram of an electron beam exposure apparatus according to an embodiment of the present invention. 
     The electron beam exposure apparatus is broadly divided into an electron optical column  100  and a controller  200  for controlling respective units of the electron optical column  100 . Here, the electron optical column  100  includes an electron beam generator  130 , a mask deflector  140 , and a substrate deflector  150 . Pressure inside the electron optical column  100  is reduced. 
     In the electron beam generator  130 , an electron beam EB generated from an electron gun  101  is subjected to a converging action by a first electromagnetic lens  102  and is then passed through a rectangular aperture  103   a  of a beam forming mask  103 . Accordingly, a cross-section of the electron beam EB is formed into a rectangle. 
     Then, the electron beam EB is imageformed on an exposure mask  110  by use of a second electromagnetic lens  105  of the mask deflector  140 . Then, the electron beam EB is deflected into a specific pattern Si formed on the exposure mask  110  by use of first electrostatic deflectors  104  and second electrostatic deflectors  106 , whereby a cross-sectional shape thereof is formed into the shape of the pattern Si. 
     Here, the exposure mask  110  is fixed on a mask stage  123 . The mask stage  123  is movable on a horizontal plane. When using a pattern S located outside deflections range (beam deflection ranges) of the first electrostatic deflectors  104  and the second electrostatic deflectors  106 , the pattern S is moved inside the beam deflection ranges by shifting the mask stage  123 . 
     Third electromagnetic lenses  108  and fourth electromagnetic lenses  111  located above and below the exposure mask  110  have a function to imageform the electron beam EB on a substrate W by adjusting the amounts of current applied thereto. 
     The electron beam EB passed through the exposure mask  110  is deflected back to an optical axis C by deflection operation attributable to third electrostatic deflectors  112  and fourth electrostatic deflectors  113 . Then, the size of the electron beam EB is reduced by use of a fifth electromagnetic lens  114 . 
     The mask deflector  140  is provided with first correction coils  107  and second correction coils  109 . Beam deflection aberration caused by the first to fourth electrostatic deflectors  104 ,  106 ,  112 , and  113  is corrected by the first correction coils  107  and second correction coils  109 . 
     Thereafter, the electron beam EB is passed through an aperture  115   a  of a closure plate  115  constituting the substrate deflector  150 , and is projected onto the substrate W by use of first projection electromagnetic lenses  116  and second projection electromagnetic lenses  121 . In this way, an image of the pattern on the exposure mask  110  is transferred to the substrate W at a given reduction ratio such as 1/10. 
     The substrate deflector  150  is provided with a fifth electrostatic deflector  119  and an electromagnetic deflector  120 . The electron beam EB is deflected by these deflectors  119  and  120  so that the image of the pattern on the exposure mask  110  is projected onto a given position on the substrate W. 
     Furthermore, the substrate deflector  150  is provided with third correction coils  117  and fourth correction coils  118  for correcting deflection aberration of the electron beam EB on the substrate W. 
     The substrate W is fixed on a wafer stage  124  which is horizontally movable by a drive  125  such as a motor. Accordingly, exposure on the entire surface of the substrate W can be performed by shifting the wafer stage  124 . 
     Meanwhile, the controller  200  includes an electron gun controller  202 , an electron optical system controller  203 , a mask deflection controller  204 , a mask stage controller  205 , a blanking controller  206 , a substrate deflection controller  207 , and a wafer stage controller  208 . Among these controllers, the electron gun controller  202  controls an acceleration voltage of the electron beam EB, beam emission conditions, or the like by controlling the electron gun  101 . In the meantime, the electron optical system controller  203  controls amounts of current applied to the electromagnetic lenses  102 ,  105 ,  108 ,  111 ,  114 ,  116 , and  121 , and thereby adjusting a magnification, a focal position, and other parameters of the electron optical system configured by these electromagnetic lenses. The blanking controller  206  deflects the electron beam EB generated prior to start of exposure onto the closure plate  115  by controlling a voltage applied to a blanking electrode  127 , and thereby preventing the irradiation of the electron beam EB onto the substrate before starting the exposure. 
     The substrate deflection controller  207  deflects the electron beam EB to the given position on the wafer W by controlling a voltage applied to the fifth deflector  119  and an amount of current applied to the electromagnetic deflector  120 . The wafer stage controller  208  shifts the substrate W in the horizontal direction by adjusting a driving amount of the drive  125  so as to irradiate the electron beam EB onto the desired position on the substrate W. The respective constituents  202  to  208  described above are integrally controlled by an integrated control system  201  such as a workstation. 
     (Configuration of Electron Gun) 
       FIG. 2  is a block diagram of the electron gun  101 . In the embodiment of the present invention, the electron gun  101  applied is a thermal electric field emission type. The electron gun  101  includes an electron source  20 , an extraction electrode  21 , a heat generator  22  for heating the electron source which is made of carbon and located on both sides of the electron source  20 , a holder  23  for holding the electron source  20  and the heat generator  22  for heating the electron source, and a suppressor electrode  24  for holding and surrounding the holder  23 . The electron source applied is a single crystal of LaB 6  or CeB 6 , for example. 
     In the electron gun  101  thus configured, the electron gun controller  202  heats the electron source  20  up to 1300° C. by continuously applying an electron source heating current to the heat generator  22  for heating the electron source. Then, the electron gun controller applies an intense electric field into a space between the suppressor electrode  24  and the extraction electrode  21  while maintaining the electron source  20  to a constant temperature, thereby extracting electrons from the electron source  20 . Moreover, the electron gun controller  20  applies a voltage to an acceleration electrode  25  located below the extraction electrode  21  to extract an electron beam  29  holding a predetermined energy. Then, the electron beam  29  is irradiated on the photoresist coated substrate W fixed onto the wafer stage  124  so as to effectuate electron beam exposure. 
     Here, the voltage to be applied to the suppressor electrode  24  is set in a range from −0.1 to −0.5 kV while the voltage to be applied to the extraction electrode  21  is set in a range from 3.0 to 6.0 kV. These voltages are set to the values corresponding to electric potential at the electron source  20 , which is adjusted by adding −50 kV because the electron source  20  usually has the electric potential at −50 kV relative to true earth ground. 
     In the embodiment of the present invention, the electrons are emitted by heating the electron source  20  and applying the intense electric field thereto at the same time. Accordingly, absorption of gas molecules on the surface of the electron source  20  can be avoided, and eventually deterioration in luminance of the electron beam can be avoided as well. 
     The disposition of an electrostatic lens electrode  26  in the space between the extraction electrode  21  and the acceleration electrode  25  is also conceivable in addition to the above-described electrodes. 
     The above-mentioned electrostatic lens electrode  26  is configured to adjust an opening angle of the electronic irradiation irradiated from the electron source  20  and to apply a voltage so as to avoid irradiation of the electrons onto the acceleration electrode  25 . 
       FIG. 3  is a chart showing an example of electric field intensity distribution. In  FIG. 3 , the horizontal axis indicates the distance between the electron source and an electron emission surface. The vertical axis indicates the electric potential. In  FIG. 3 , x 1  indicates the position of the extraction electrode  21  and x 2  indicates the position of the electrostatic lens electrode  26 . Moreover,  FIG. 3  shows a case where the electric potential at the acceleration electrode  25  is set equal to 0 [kV] and the electric potential on the electron emission surface of the electron source  20  is set equal to −50 [kV]. 
     As shown in  FIG. 3 , by forming an electron lens bearing the voltage slightly higher than a cathode voltage on the electron emission surface in the position of the electrostatic lens electrode  26 , the opening angle of the irradiated electrons is reduced thereby. In this way, the electron exposure on the acceleration electrode  25  is avoided. As a consequence, heat generation attributable to irradiation of the electron beam onto the acceleration electrode  25  is eliminated, and degradation in vacuum inside the exposure apparatus is also controlled. 
     (Configuration of Electron Source) 
     Now, a configuration of the electron source  20  used in the embodiment in the present invention will be described below. 
       FIG. 4  is a cross-sectional diagram showing part of the electron source  20  and the electrodes which are both constitutions of the electron gun  101 . 
     A tip of the electron source  20  is formed into a cone shape and a surrounding area is covered with carbon  30 . This carbon  30  is formed on a surface of the electron source  20  in accordance with the chemical vapor deposition (CVD) method, for example. The material of the electron source  20  is exposed on the tip of the electron source  20  and the exposed part is planarized. 
     The tip of the electron source  20  is located in a position between the suppressor electrode  24  and the extraction electrode  21 . A voltage equal to 0 or a negative value is applied to the suppressor electrode  24 , so that the suppressor electrode  24  functions to block the electrons emitted from parts other than the tip of the electron source  20 . The electric field intensity is determined by the following factors; a voltage difference between the extraction electrode  21  and the suppressor electrode  24 , a height and an angle of the tip of the electron source  20 , and a diameter of the planarized part of the tip. The planarized part at the tip of the electron source  20  is arranged parallel to the suppressor electrode  24  and the extraction electrode  21 . 
     The tip of the electron source  20  is formed into a cone shape, and the electron emission surface  20   a  for emitting the electrons is planarized. The surrounding area of the cone-shaped electron source  20  is covered with a material which is different from the material consisting of the electron source  20 . The cone-shaped part preferably has a cone angle equal to or below 50°. The diameter of the electron emission surface is set preferably in a range from 10 μm to 100 μm. Usually, the diameter is preferably set to 40 μm. Meanwhile, a thickness of the material covering the surrounding area of the electron source  20  is preferably set equal to 10 μm. Note that the covering with the different material have following two objectives; (1) to avoid emission of the electrons from the electron source  20  and (2) to suppress sublimation and evaporation of the material at a base body of the electron source  20 . The thickness value of the covering material depends on the electric field intensity and the material used therein. The thinner covering material is preferable in light of increasing the electric field intensity as long as less covering material evaporates or is depleted at an operating temperature. 
     The temperature applied to the electron source  20  is set lower than a temperature causing sublimation of the material constituting the electron source  20 . In case that a high temperature is applied to the electron source  20  to emit thermal electrons, the electron source  20  may sublimate so that the electron emission surface  20   a  is depleted and deformed. From this point of view, the temperature is regulated to an appropriate level so as not to cause the sublimation. Even when lowering the temperature, the current density and luminance are required to achieve the equivalent levels to those obtained in the case of applying the high temperature. For this reason, the intense electric field is applied to the tip of the electron source  20  so as to extract the electrons. For example, in case that a work function can be reduced by 0.3 eV when lowering the temperature from 1500° C. to 1300° C., then the electron beam luminance equivalent to the luminance that is supposed to be obtained by thermal electron emission can be obtained while maintaining the temperature at 1500° C. In this context, the high electric field is applied to the electron source  20  for causing emission of the electrons even when reducing the work function by 0.3 eV. 
     In this case, application of the high electric field causes extraction of the electrons not only from the tip part of the electron source  20  constituting the electron emitting part but also from a side surface part of the electron source  20  formed into the cone shape. As a consequence, the electron beam in a desired amount and in a desired shape may be difficult to obtain, or the luminance of the electron beam generated from a central part may be reduced by occurrence of a space-charge effect due to the unnecessary electrons in the surrounding area. To avoid these problems, the area other than the electron emission part of the electron source  20  is covered with the material different from the material of the electron source  20 . As for this different material, a substance having a larger work function than that of the material constituting the electron source  20  should be selected. 
     When using LaB 6  as the electron source  20 , the use of carbon (C) is preferable because carbon does not react with LaB 6  and has a larger work function than that of LaB 6 . Since the carbon reacts with oxygen, the carbon is forecasted to be evaporated and eventually disappear in the form of carbon dioxide (CO 2 ) in case that the carbon film is too thin. Accordingly, it is preferable to set the thickness of the carbon film in a range from 2 μm to 10 μm. Likewise, the carbon material is also suitable for the covering material in the case where CeB 6  is used as an electron source, as the CeB 6  has similar properties to LaB 6 . 
       FIGS. 5A and 5B  are cross-sectional diagrams showing examples of the electron source  20  that has different cone angles at the tip part thereof. In general, a smaller radius or a smaller angle at the tip of the cone-shaped electron source  20  causes stronger electric field concentration on the tip part, whereby the electrons inside the electron source  20  can break through a work function barrier on the surface more easily by a tunneling phenomenon. However, in case that the tip part is formed too thin, the electron source  20  loses its strength. Therefore, the tip angle of the electron source  20  is determined in consideration of the strength of the electron source  20  and the electric field intensity. 
       FIG. 5A  shows a case of setting the cone angle at the tip part of the electron source  20  approximately equal to 90°. Meanwhile,  FIG. 5B  shows a case of reducing the cone angle at the tip part of the electron source  20  as compared to  FIG. 5A . Conventionally, the cone angle at the tip part of the electron source  20  has been set approximately equal to 90° as shown in  FIG. 5A . As the tip angle is reduced as shown in  FIG. 5B , the higher electric field is easily obtained and the electrons is emitted easily. In addition, particulates such as ions existing inside a lens barrel collide with the tip part of the electron source less frequently. Therefore, the depletion and the deformation effects on the surface of the electron source attributable to ions and the like can be reduced. 
     In the embodiment of the present invention, the angle at the tip of the electron source  20  is set approximately equal to 30°. Although a product life depends on the material and sizes such as the length or the width of the electron source  20 , the electron source  20  of the embodiment of the present invention can be used stably for a longer period than the conventional electron source. 
     (Exposure Method) 
     Next, an exposure method applying the exposure apparatus including the electron gun of the embodiment in the present invention will be described. 
     In general, the electron beam exposure apparatus carries out conditioning when starting operation in order to clean up an electron gun chamber (not shown) for housing the electron gun  101 , the suppressor electrode  24 , the extraction electrode  21 , the electrostatic lens electrode  26 , and the acceleration electrode  25 . In this conditioning operation, a high voltage such as a voltage 1.6 times higher (80 kV) than a voltage during a usual operation (50 kV) is applied between the electrodes constituting the electron gun  101  (including the electron source  20 , the suppressor electrode  24 , the extraction electron  21 , and the electrostatic lens electrode  26 ) and the acceleration electrode  25  to cause a discharge. In this way, dust inside the electron gun chamber is removed. 
     In the conditioning operation, in case that the extraction electrode  21  and the electrostatic lens electrode  26  are omitted or otherwise not provided such that the electron source  20  is configured to be directly opposed to the acceleration electrode  25 , then the discharge maybe initiated from the electron beam  20 . Such a discharge may cause melting or breakage of the electron source  20 . 
     To avoid this problem, the extraction electrode  21  is provided and the electric potential at this extraction electrode is set lower than the electric potential at the electron source  20  so as to avoid extraction of the electrons from the electron source during the conditioning operation. 
     After a lapse of a predetermined period of time or upon completion of the conditioning operation for a period ranging from one to several tens of hours, for example, the electric potential at the extraction electrode  21  is set higher than the electric potential at the electron source  20  to establish a usual operating state. 
     As described above, since the electric potential at the extraction electrode  21  is set lower than the electric potential at the electron source  20  during the conditioning operation of applying the high voltage to the electrodes, the extraction of the electrons from the electron source  20  can be suppressed and thereby the electron source  20  is prevented from melting. 
     (Method of Forming Region for Restricting Electron Emission on Surface of Electron Source) 
     Next, a method of forming a region for restricting the above-described emission of the electrons on the electron source  20  will be described. 
     Here, the electron source having the structure shown in  FIG. 5B  will be used as an example. Moreover, single-crystal LaB 6  is assumed to be used for the electron source  20 . 
     Firstly, the LaB 6  single crystal is processed to form the tip in the cone shape. 
     Next, the carbon  30  is coated on the surface of the LaB 6  single crystal in order to form the region for restricting emission of the electrons. For the coating operation, any methods among a CVD (chemical vapor deposition) method, a vacuum evaporation method, a sputtering method, and so forth may be applicable. In this case, the coated film only needs to have a sufficient thickness for adequately changing the work function on the surface of the electron emission surface (changing the work function larger than that of LaB 6 ) and for preventing evaporation of the LaB 6  material. When using carbon, the thickness of carbon is preferably set in the range from 2 μm to 10 μm taking into consideration that carbon reacts with oxygen and then evaporates in the form of CO 2 . 
     Next, the tip of the electron source  20  is polished together with the coated film so as to form the planarized surface having the diameter in the range from 1 μm to 200 μm. 
     (Effects) 
     As described above, according to the embodiment in the present invention, only the tip of a chip of the electron source  20  is exposed and the remaining side surface part thereof is covered with the different material. When using LaB 6  as the material of the electron source  20 , for example, carbon is used as the different material. 
     In the embodiment in the present invention, the chip hardly sublimates because the electron gun is operated at a low temperature. Accordingly, the electron gun  101  can be used stably for a long period of time while avoiding deformation of the electron emission surface  20   a  of the electron source  20 . Moreover, since the side surface of the electron source  20  is covered with the carbon  30 , no electrons are emitted from the side surface of the electron source  20  even when the intense electric field is applied thereto. In this way, the deformation of the shape of the electron beam can be prevented and the phenomenon of degradation in vacuum due to an increase in the temperature in an unnecessary position is avoided thereby. 
     Further, the surface of LaB 6  is virtually exposed only on the central part at the tip of the electron gun. Therefore, the attachment of LaB 6  on an inner surface of a Wehnelt cylinder as observed in the conventional technology, which is attributable to a large area of exposure of LaB 6  including side wall parts and a rear surface, can be prevented. 
     By using the electron gun  101  of the embodiment in the present invention, the sublimation of the electron source  20  can be suppressed and the attachment of the material constituting the electron source  20 , such as LaB 6  or CeB 6 , on a rear surface of a grid can be prevented. In case such a material is attached to the rear surface of the grid, the attached material may form a whisker, charge the electrons thereon, and cause a micro discharge. In that case, the micro discharge may cause instabilities of the electron beam quantity and of a radiating position in the use of the electron beam exposure apparatus. Accordingly, even in case the deformation of the electron source  20  of the electron gun  101  is nominal, such a micro discharge makes stable operation of the electron beam exposure apparatus difficult. 
     In the case of a conventional electron gun, such a micro discharge has been deemed to emerge 100 hours to 500 hours after starting to use the electron gun. In contrast, as described above, the electron source  20  hardly sublimates when using the electron gun  101  of the embodiment in the present invention. Therefore, the interval of time until the micro discharge emerges can be extended almost as one hundred times as long as the conventional technology. This is because the electron gun of the embodiment in the present invention is used at the temperature which is lower by 200° C than the conventional technology so that the degree of sublimation of the electron source is reduced to 1/100. In this way, the period for stable use of the electron beam exposure apparatus is accomplished. 
     Moreover, in the case of a multicolumn electron beam exposure apparatus configured to expose on a single wafer, by the use of multiple electron guns  101 , the period of stable use is dramatically extended compared to the conventional technology, by applying the electron guns  101  of the embodiment in the present invention. In the use of the conventional electron guns, the adjustment in short cycles after the use should be performed because micro discharges will occur within 100 to 500 hours as described above. Therefore, even when using the multiple electron guns, the entire apparatus must be stopped once when any of the electron guns becomes unstable, thereby leading to a drop in the operation rate and incapability of enhancing the throughput. On the other hand, such a drop in the operation rate is avoided and thereby the throughput of exposure processes is enhanced virtually by applying the electron guns of the embodiment in the present invention to the multicolumn electron beam exposure apparatus. 
     In the above-described embodiment, the tip of the electron gun  101  is planarized, and the electron emission surface  20   a  and the different material for covering the side surface of the electron source  20  are formed to be on the same plane. This structure is applied to the embodiment in the present invention as the heat applied to the electron source  20  is low enough to avoid sublimation of the material constituting the electron source  20  regarding that the electron source  20  would not be deformed by emission of the electron beam. 
     However, even when the heat reduced to a predetermined temperature sufficient for avoiding sublimation is applied, the temperature may still exceed the predetermined temperature for any reason. In such a case, maintaining the planarized surface may be difficult due to actual depletion of the material of the electron source in excess of the forecast, and eventually, the central part of the planarized surface may gradually subside with time. In consideration of this problem, the tip including the electron emission surface  20   a  can be formed so as to protrude from the planarized surface of the different material as shown in  FIG. 6  instead of forming the electron emission surface  20   a  of the electron source  20  and the surface of the surrounding different material on the same plane. 
     Moreover, in the embodiment in the present invention, the side surface of the electron source is described as the region for restricting emission of the electrons. Instead, as shown in  FIG. 7 , the following parts can be covered with the different material: a rear surface  81   b  and a side surfaces ( 81  and  81   a ) of an electron source  80  excluding an electron emission surface  80   a  and a part sandwiched by a carbon chip  82  to be energized for heating. In this way, sublimation of the electron source  80  can be reduced and an amount of attached materials to the Wehnelt cylinder and other components can be reduced as well.