Abstract:
The objective of the present invention is to provide a charged particle beam device such that a tip part can be effectually maintained in a clean state, while the frequency of valve body replacements is also reduced. To achieve the objective, a charged particle beam device is offered, comprising: a partition that is positioned between a charged particle source-side vacuum space and a specimen stage-side vacuum space, said partition further comprising an opening for a charged particle beam to pass through; a driver mechanism that moves a shutter member between a first location within the optical axis of the charged particle beam and a second location outside the optical axis of the charged particle beam; and a control device that controls the driver mechanism. The first location is a location wherein the shutter member is distanced from the partition, and the control device carries out a control that opens a valve between the specimen chamber and the exchange chamber when the shutter member is in a state of being located in the first location.

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2011/003213, filed on Jun. 8, 2011, which in turn claims the benefit of Japanese Application No. 2010-168773, filed on Jul. 28, 2010, the disclosures of which Applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present invention relates to a charged particle beam device, and particularly relates to a charged particle beam device in which a valve body is provided between a charged particle source of the charged particle beam device and a specimen chamber in which a specimen on which a charged particle beam is irradiated is disposed. 
     BACKGROUND ART 
     A charged particle beam device represented by an electron scanning microscope needs to maintain an optical system and a specimen chamber in a vacuum, and when an electron beam is irradiated, an electron optical system and the specimen chamber is evacuated. Furthermore, an atmosphere surrounding an electron source (charged particle source) emitting an electron beam needs to maintain an extremely high degree of vacuum. The technology of blocking an atmosphere on the specimen chamber side in which the degree of vacuum is lowered or could be lowered from an atmosphere on the electron source side when the specimen is replaced, is disclosed in Patent Literatures 1 to 3. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-03-22333 
     Patent Literature 2: JP-A-05-203123 
     Patent Literature 3: JP-A-06-294481 
     SUMMARY OF INVENTION 
     Technical Problem 
     A valve body disclosed in Patent Literatures 1 to 3 is disposed in two vacuum spaces having different pressure and used to maintain the pressure difference between them, so that the valve body needs to be strongly pressed to an opening between the vacuum spaces. However, in this case, as the valve body with a seal function while being made of a soft material compared to an inner structure of an electron microscope has to be strongly pressed, its contact surface is deteriorated, so that replacement of the corresponding member has been required with a proper frequency. 
     While one original reason of providing the valve body is to maintain a tip part of the electron source in clean, according to the review of the present inventors, it is found that cleanliness is deteriorated when gas component and moisture remaining in the vacuum chambers linearly pass a portion near a beam optical axis to be attached to a chip. 
     Hereinafter, a charged particle beam device of which object is such that a chip part can be effectually maintained in a clean state, while the frequency of valve body replacements is also reduced will be explained. 
     Solution to Problem 
     As one aspect to achieve the above-described object, there is suggested a charged particle beam device including: a charged particle source; a specimen stage that maintains a specimen on which a charged particle beam emitted from the charged particle source is irradiated; a specimen chamber that sets an atmosphere in which the specimen exists as a vacuum state; and an exchange chamber that evacuates a specimen atmosphere to be introduced to the specimen chamber, the charged particle beam device further including a partition that is located between a charged particle source-side vacuum space and a specimen stage-side vacuum space, the partition further including an opening for the charged particle beam to pass through, a drive mechanism that moves a shutter member between a first location within an optical axis of the charged particle beam and a second location outside the optical axis of the charged particle beam and a control device that controls the drive mechanism, wherein the first location is a location in which the shutter member is distant from the partition, and the control device carries out control for opening a valve between the specimen chamber and the exchange chamber when the shutter member is in a state of being located in the first location. 
     Advantageous Effects of Invention 
     According to the above configuration, a passage route of gas component etc. communicating between the charged particle source-side space and the specimen stage-side space can be blocked without the need of pressing the shutter member to other members, which achieves not only a long-life shutter member but also maintenance of cleanliness of a surface of the chip of the charged particle source. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an electron scanning microscope. 
         FIG. 2  is a schematic configuration diagram of a Schottky emission electron source. 
         FIG. 3  is a detailed explanation diagram of a gate valve. 
         FIG. 4  is a plain view of a distribution system of the electron scanning microscope. 
         FIG. 5  is a flowchart explaining control steps of the distribution system of the electron scanning microscope and the gate valve. 
         FIG. 6  is a drawing explaining a summary of the distribution system of the electron scanning microscope and a control device that controls the gate valve. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Recently, miniaturization of a semiconductor device is advanced, and along with this situation, a higher resolution and a higher throughput have been required for en electron scanning microscope that measures or tests the semiconductor device. In addition, enhancing an operation rate of the device is also important. To improve the operation rate of the device, the stop time of the device caused by replacement or maintenance of an electron source etc. has to be shortened as much as possible. 
     Since the electron source is an expendable item, a stable and long-time extraction of its output is desired. Among the electron sources, a Schottky emission electron source includes a hairpin filament made of a W &lt;100&gt; single-crystal chip and a W polycrystalline line, a terminal of stainless steel, etc. in which a filament is spot-welded, and a ceramic insulator. It is configured such that an oxide supply source such as Zr whose work function is lower than that of the W single-crystal chip is attached a central portion, a root or a filament of the W single-crystal chip. By heating the electron source at a temperature of 1,500 to 1,900K, its oxide is thermally diffused along the W single-crystal chip. The metal oxide diffused on a tip portion of the W single-crystal chip is adsorbed on a tip portion of the W single-crystal chip as respective nearly-monoatomic layers of the oxide and the metal. At this time, the adsorption is selectively performed on a specific crystal surface (100) having a high activation energy of surface diffusion and desorption. By using the W single-crystal line in which the (100) crystal surface is a tip portion of the W single-crystal chip, only the tip portion of the W single-crystal chip can be maintained in a state of low work function. Consequently, a high emission electronic current density can be obtained from that portion. 
     As such Schottky electron sources, Zr/O/W is disclosed in Journal of Vacuum Science Technology, B3(1), 1985, p. 220. 
     On the other hand, in a length measurement-type electron scanning microscope for measuring a pattern width of a semiconductor device (Critical Dimension Scanning Electron Microscope: CD-SEM) and an electron scanning microscope for reviewing a defect based on positional information of the defect (Defect Review SEM: DR-SEM) etc., measurement or testing of many specimens for unit time is required, however, if a replacement frequency of wafers becomes higher, the number of times of opening and closing of a gate valve and an exchange chamber is increased. Moreover, for each opening and closing, pressure in a vacuum chamber could fluctuate. In a case where a turbopump is used as an air displacement pump that evacuates the vacuum chamber, gas remains in the vacuum chamber etc. unless baking is performed. Also, to discharge remaining moisture, baking is needed with high frequency (Vacuum Handbook (new edition), ULVAC, Inc., 1.6 vacuum gauge and potentiometer, P.38 to P.39, Residual Gas Component of Vacuum System C). 
     When such moisture and the like remain in the vacuum chamber for a long period of time, it is decomposed into hydrogen and oxygen. On the other hand, it is known that zirconium (Zr) as a catalyst of an electron source used for an electron gun has such a high affinity for oxygen that it is used as an oxymeter. The electron source existing on the uppermost portion of the beam optical axis of the electron scanning microscope is attached to a surface of the single-crystal chip when the above decomposed molecules pass the shortest route of a mean free path along the beam optical axis to thereby affect surface diffusion and a desorption phenomenon. 
     Since the Schottky emission electron source has to be maintained in a high vacuum atmosphere while it can take out an electron beam with high brightness, it is separated from a specimen chamber and is differentially evacuated. However, an opening for an electron beam to pass has to be provided, so that a molecule could pass through the opening to be attached to the electron source. In particular, in response to requirement of high throughput of recent years, replacement frequency of a specimen (for example, semiconductor wafer) has been increased. As a result, pressure of the specimen chamber is frequently fluctuated. Along with this pressure fluctuation, the electron source also receives the above influence. 
     The influence on the electron source based on the pressure fluctuation of the specimen chamber can be mitigated to some extent by providing a shutter mechanism that blocks an atmosphere in the electron source-side space from an atmosphere in the specimen chamber-side space, however, frequent opening and closing of the shutter mechanism for securing the pressure difference between both spaces provide concern for deterioration of the shutter mechanism. Accordingly, in the following explanation, a charged particle beam device which achieves both inhibition of deterioration of the shutter mechanism and inhibition of influence on the electron source accompanied by pressure fluctuation of the specimen chamber will be explained in detail with reference to the drawings. 
     In this embodiment, an electron scanning microscope in which a shutter member is located such that it is not pressed to an opening provided on a partition that separates an electron source-side vacuum atmosphere from a specimen chamber-side vacuum atmosphere and that it is located on an electron beam optical axis will be explained. 
     According to one aspect of this embodiment, a charged particle beam device by which replacement frequency due to protection of a chip of an electron gun and longevity of a valve O-ring is reduced, the device stop time due to the replacement can be reduced, and the operation time is increased can be provided. 
     As a result of the research by the present inventors, the inventors have newly found that most of molecules having a possibility of reaching the electron source move linearly along the electron beam optical axis. That is, the molecules can be prevented from reaching the electron source by locating the shutter mechanism at a location that the opening of the partition and the shutter member are overlapped seen from the direction of the electron source, instead of pressing the shutter member to the partition. 
     Hereinafter, the electron scanning microscope that can locate the shutter member to a location that does not contact the partition provided between the electron source-side vacuum atmosphere and the specimen chamber-side vacuum atmosphere in the electron beam optical axis will be explained with reference to the drawings. Additionally, while an electron scanning microscope having a Schottky emission electron source will be explained as an example of the charged particle beam device in this embodiment, this is applicable to any device in which a beam source could be influenced by molecules from the specimen chamber. As other examples of the charged particle beam device, an electron scanning microscope having a field-emission electron source and a focused ion beam system etc. can be listed. 
       FIG. 1  is a schematic configuration diagram of an electron scanning microscope. An electron beam emitted from an electron gun  1  including an electron source is focused on a wafer  6  by a condenser lens  3  and an objective lens  4  in a condensed manner. A deflector (not shown) scans the electron beam on the wafer  6 . From a scanning area of the electron beam on the wafer  6 , a secondary electron (SE) or a backscattered electron (BSE) is emitted, and in the electron scanning microscope, a detector that detects these electrons is provided (not shown). The electron detected by the detector is converted into an optical signal and amplified by a not-shown amplifier. This amplified signal is displayed on a display as a two-dimensional image or stored in a frame memory by synchronizing with a scanning signal of the not-shown deflector. Moreover, the detected signal forms a profile waveform and can be used to measure the dimension of a pattern formed on a specimen based on the waveform. 
     To perform dimension measurement or defect inspection of a fine pattern formed on the wafer  6  using the electron scanning microscope, a measuring position or a position of an object to be inspected on the wafer  6  should correspond to a beam irradiating position. A specimen stage  7  is held by a driving mechanism that moves the wafer  6  at least in the X and Y directions (in a case where the electron beam optical axis is defined as a Z axis) to thereby achieve the movement. 
     Since a foreign object deteriorates the wafer  6  due to a fine pattern engraved thereon, it is transferred from a production device and the like to the electron scanning microscope in a state of being housed in a wafer casing  13  in which high cleanliness is maintained compared to a circumferential environment. The wafer  6  is taken out from the wafer casing  13  by a robot  12  placed in a local exhaust chamber  11  subjected to local ventilation and delivered to a device (not shown) that determines the direction of a wafer  6   a . The wafer  6   a  whose direction is adjusted by the device waits on the robot  12  until it is introduced to an exchange chamber  10  for preliminary evacuation. 
     In the exchange chamber  10  that enables specimen delivery between an aerial space and a vacuum space, two valves of a specimen chamber valve  8  and an exchange chamber valve  14  are provided to be selectively opened during the specimen delivery. When the wafer  6   a  is introduced to the exchange chamber  10 , firstly, after an atmospheric state of the exchange chamber  10  is made, the exchange chamber valve  14  is opened, and the wafer  6   a  is introduced into the exchange chamber  10  by the robot  12 . After a wafer  6   b  is introduced in the exchange chamber  10 , an exchange chamber valve  14  is closed to evacuate the exchange chamber  10 . The evacuation is performed such that, firstly, a not-shown roughing pump is used to evacuate some degree of pressure, and the roughing pump is switched to an exchange chamber pump  10   a  to continue the evacuation to approach the degree of vacuum of a specimen chamber  9 . 
     At this time, considering throughput, when the pressure difference between the specimen chamber  9  and the exchange chamber  10  becomes some degree of value (for example, the pressure difference of 10 2  Pa or less (triple-digit pressure difference is the maximum)), the specimen chamber valve  8  is opened to replace the wafer  6   b  in the exchange chamber  10  with a wafer  6   c  in the specimen chamber  9 . After the replacement, the specimen chamber valve  8  is closed. After the specimen chamber valve  8  is closed, a specimen chamber pump  9   a  is opened to maintain the vacuum pressure in the specimen chamber  9 . 
     As described above, since the specimen chamber valve  8  is opened before the pressure of the specimen chamber  9  corresponds to the pressure of the exchange chamber  10  considering throughput, the pressure of the specimen chamber  9  is temporarily increased (the degree of vacuum is deteriorated). 
     A partition for maintaining an atmosphere around an electron source  1   a  in high vacuum is provided between the electron source  1   a  the specimen chamber  9 , and to this partition, an opening (aperture)  17  that allows an electron beam to pass is provided. Due to existence of the aperture  17 , the specimen chamber  9  is in the state in which a space reaching the electron gun  1  is communicated therewith. While the atmosphere of the electron source  1   a  has the pressure difference from that of the specimen chamber  9  by a differential evacuation structure, a location of the chip of the electron source  1   a  and the a location of the aperture  17  are provided on the electron beam optical axis (an ideal optical axis when the electron beam is not deflected) and the specimen chamber  9  exists on the electron beam optical axis. That is, influence of vacuum fluctuation of the specimen chamber  9  could reach the electron source  1   a.    
       FIG. 2  shows an example of the configuration of the electron source  1   a . The electron source  1   a  includes a tungsten &lt;100&gt; single-crystal chip  20 , a hairpin filament  21  made of a tungsten single-crystal line, a terminal  22  made of stainless etc. in which the filament is spot-weld and a ceramic insulator  23  fixing these components. The electron source  1   a  is used as a Schottky emitter that reduces work function of tungsten by covering a surface of a specific tungsten single-crystal chip having high activation energy with an adsorption layer of zirconium  24  and oxygen  25 . 
     Since the Schottky emission electron source as shown in  FIG. 2  has a high brightness and its energy width is small, the probe diameter can be squeezed even in a low accelerating voltage, so that it is frequently used as an electron source for surface observation of a semiconductor material or device. 
     On the other hand, as described above, when the specimen chamber valve  8  is opened relatively early for improvement of throughput, a pressure fluctuation occurs in the specimen chamber  9 . The main cause of this pressure fluctuation is pressure difference caused when the wafer  6  is introduced into the specimen chamber  9  (see  FIG. 1 ), moisture adsorbed by the wafer  6  or a material applied to the wafer  6 . Especially, when the moisture is left in vacuum for a long period of time, it is decomposed into hydrogen and oxygen. When this oxygen is attached to a surface of the tungsten single-crystal chip, a covering phenomenon occurring in the adsorption layers of zirconium and oxygen is prevented, which could deteriorate a function of the Schottky emission electron source. 
     In this embodiment, a shutter member which can be disposed on the electron beam optical axis and does not contact the aperture etc. and an application method of the shutter member are explained. 
       FIG. 3  is a drawing explaining an example of the shutter member. In this embodiment, an example that a gate valve for vacuum sealing is used as the shutter member will be explained. The gate valve includes a valve body drive assembly  18  that is a driving mechanism that drives the shutter member, a shaft  33  that is linearly moved in the vertical direction to the electron beam optical axis by the valve body drive assembly, a pressing metal piece  30  having an inclined surface on its tip portion, a valve body  16  that has an inclined surface abutting the inclined surface provided on the pressing metal piece  30  and moves downward (to the side of specimen chamber) by pressing operation of the shaft  33 , a sealing member  32  that blocks an atmosphere of upper and lower spaces of the aperture, a valve body receptacle  15  that converts the pressing force of the shaft  33  into a downward pressing force, and a spring  34  having a sufficient spring force to lift the valve body  16 . 
     Moreover, the gate valve shown in  FIG. 3  can be switched among at least three modes of an open state  37 , a vacuum sealing state  35  and an intermediate state  36 , and is controlled by a control device as shown in  FIG. 6 . The control device shown in  FIG. 6  mainly includes a pressure monitoring unit that monitors pressure data obtained by a vacuum gauge  42  (see  FIG. 4 ) provided on the exchange chamber  10 , a time counting unit that counts the time until it reaches a predetermined degree of vacuum, a vacuum pump controlling UNIT that controls a vacuum pump based on a predetermined condition, a specimen valve opening and closing control unit that controls the opening and closing of the valve (the specimen chamber valve  8  and the exchange chamber valve  14 ) at a predetermined timing, the robot  12  that delivers a specimen between the local exhaust chamber  11  and the exchange chamber  10 , a specimen delivering robot controlling unit that controls a robot  41  that delivers the specimen between the exchange chamber  10  and the specimen chamber  9 , a valve body controlling portion that controls the valve body drive assembly  18  shown in  FIG. 3 , and a condition storing unit that stores control conditions of these controlling units. 
     The valve body controlling unit controls the valve body drive assembly such that a location of the valve body  16  is maintained in at least the three states as described above, and a driving signal etc. corresponding to a valve body position is stored in the condition storing unit. The three modes are as follows. 
     The first is a first mode in which, when an electron beam emitted from the electron source  1   a  is irradiated to the wafer  6   c , the valve body  16  is removed from the aperture  17  to completely open a passage for the electron beam (an open state  37 : the second location). The valve body drive assembly  18  may be constituted by a motor. 
     Next is a second mode in which, when the specimen chamber  9  is open to the air in the case where an electron beam is not irradiated or the specimen chamber  9  is repaired, to thereby generate a large pressure difference with the electron source, a sealing surface is pressed to the side of the aperture  17  by nipping the pressing metal piece  30  with the valve body  16  and the valve body receptacle  15  (a vacuum sealing state  35 ). 
     The last is a third mode in which, when the specimen is delivered between the specimen chamber  9  and the exchange chamber  10 , while the valve body  16  is located in the electron beam optical axis, the valve body  16  is not pressed to the aperture  17  (a partition on which the aperture  17  is provided) as in the second mode (an intermediate state  36 : the first location). In the third mode, by providing a clearance between the sealing member  32  and the partition, the valve body  16  floats from the partition. This third mode is to block molecules etc. linearly moving from the side of the specimen chamber to the electron source without pressing the valve body to the partition as described above, in order to eliminate influence generated at the time of the opening of the specimen chamber valve  8 , and accordingly, the third mode should be maintained until such influence is eliminated. Since the time to maintain the third mode varies depending on a specimen to be handled, an arbitrary time setting can be desirably performed. In this way, the time to maintain the third mode is also stored in the condition storing unit of the control device shown in  FIG. 6 . 
     Next, a concrete control method of the control device shown in  FIG. 6  will be explained with reference to a flowchart in  FIG. 5 . Additionally,  FIG. 4  is a plain view of a specimen delivery system unit of  FIG. 1 , and in the following explanation,  FIG. 4  will also be cited. 
     Firstly, after the exchange chamber valve  14  is controlled to be opened by the specimen valve opening and closing controlling unit, by the robot  12  controlled by the specimen delivering robot controlling unit, the wafer  6   a  is delivered to the exchange chamber  10  (step  501 ). Next, the exchange chamber pump  10   a  is controlled by the vacuum pump controlling unit to evacuate the exchange chamber  10  (step  502 ). Next, through the vacuum gauge  42  as shown in  FIG. 4 , pressure in the exchange chamber  10  is monitored at the pressure monitoring unit, and if the pressure in the exchange chamber  10  reaches a predetermined value, it goes to the next step (step  503 ). In step  504 , whether the evacuation time at the exchange chamber  10  exceeds a predetermined value is determined based on the vacuum reaching time measured by the time counting unit (step  504 ), and if it is determined to exceed the vacuum reaching time, by the control by the valve body controlling unit, mode switching is performed based on a predetermined rule to move the valve body  16  (step  505 ). Needless to say, if switching to a mode to be switched has been already made, the valve position is not changed to maintain that state. 
     The reason for judging a threshold value of the evacuation time in step  504  is that, because it can be understood that, when a long-time evacuation is made, there are many molecules emitted from the specimen and the like, which greatly affects the electron source, while, when the evacuation is terminated in a short time, there are little molecules emitted, which rarely affects the electron source; corresponding different modes are used to these conditions. For example, when the evacuation time is longer than the threshold value, it is predicted that there are many molecules emitted, and therefore the valve body is completely closed (the second mode), and on the other hand, when the evacuation time is the threshold value or less, from the viewpoint that only molecules linearly moving toward the side of the electron source may be inhibited, the intermediate state (the third mode) is adopted. With such switching, life extension of the valve body can be achieved while inhibiting the influence of the molecules emitted to the electron source. Moreover, not only switching of the two modes, but also two threshold values may be provided to perform switching of three modes depending on the degree of the evacuation time. 
     Further, to switch between the first and third modes, the determination based on the threshold value may be performed. In this case, since the valve body does not contact the partition, a further life extension of the valve body can be achieved. 
     Next, the specimen chamber valve  8  is opened by the control according to the specimen valve opening and closing control unit (step  506 ), and by the control according to the specimen delivering robot controlling unit, the wafer  6   b  is delivered to the specimen chamber  9  (step  507 ). It is important that, when the second or third mode is selected, the valve body is in the vacuum sealing state  35  or the intermediate state  36  at the time of opening the specimen chamber valve  8 . 
     While this state is kept, after the specimen chamber valve  8  is closed (step  508 ), the valve body  16  is switched to the first mode (step  509 ) to open the electron beam passage. Additionally, since a valve body removal step (step  509 ) may be performed in a state in which the degree of vacuum is sufficiently increased, the removal step may be performed simultaneously with the step  507  or step  508 . In this case, the step  509  may be performed when the degree of vacuum obtained by the vacuum gauge  42  or a vacuum gauge (not shown) provided on the side of the specimen chamber  9  exceeds a predetermined threshold value (when a predetermined high vacuum or more is reached). 
     Moreover, the step  509  may be performed when a predetermined time has been elapsed or when the degree of vacuum reaches a predetermined value or more, after the specimen chamber valve  8  is closed. Even when a waiting time is provided until the valve body removal in this way, the degree of vacuum that is once lowered can be increased using two pumps of the specimen chamber pump  9   a  and the exchange chamber pump  10   a , so that the evacuation time can be shortened compared to the case where the specimen chamber valve  8  is opened after the vacuum is sufficiently increased in the exchange chamber  10 . 
     Additionally, a clearance to be provided between the valve body  16  and the partition on which an electron beam passage  31  is provided is balanced with the spring  34  that lifts the weight of the valve body  16 , which achieves life extension of the sealing member  32  without contacting a surface of the aperture  17 . As explained before, in the electron source  1   a , the zirconium  24  has a high affinity for the oxygen  25 . The probability that moisture included in pressure fluctuation of the specimen chamber  9  or the oxygen  25  sublimed from the moisture attached to the wafer  6   c  moves from the aperture  17  to reach the electron source  1   a  without a single collision with other molecules etc. during the movement can be obtained as follows. 
     In the case where N 0  gas molecules advance in the same direction, it is assumed that at the point advanced from a starting point by x, the number of molecules that have not yet collided be N(x) and the number of molecules that collide during advancement by dx from the starting point be dN (x), the relational expression is as follows.
 
 dN ( x )=μ N ( x ) dx  . . . an expression obtaining distribution of mean free path
 
 dN ( x ): the number of molecules that collide while advancing by  dx  
 
 N ( x ): the number of molecules that have not collided at the time of advancing by  x  
 
μ: a constant showing probability of collision  [Expression 1]
 
     Since this is the sum of distances of all the N 0  molecules that have advanced by x until each molecule is collided and scattered, this is represented by the following expression. 
     
       
         
           
             
               
                 
                   
                     
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                     sum 
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                     straight 
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                     distances 
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                     of 
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                     mean 
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     Since the mean free path is the average of distances that the molecules have advanced, if the sum of the straight distances of the mean free path is divided by N 0 , the expression representing mean free path (I) can be obtained.
 
1=1/μ
 
1: mean free path
 
μ: probability of collision  [Expression 3]
 
     When this is substituted in the expression to obtain the distribution of mean free path, the expression is as follows.
 
 N ( x )= N   0 exp(− x/ 1)  [Expression 4]
 
     When what percent of the initial molecules has not yet collided at the point that has advanced by mean free path  1  is obtained, if the advanced distance is the same as the mean free path, from x=1, the following expression can be obtained.
 
 N (1)/N 0 =exp(−1)=1/ e =0.367  [Expression 5]
 
     Accordingly, it is found to be 36.7 percent. 
     By setting the valve body  16  shown in  FIG. 3  to the intermediate state  36 , the probability that the oxygen  25  that has passed through the electron passage  31  directly reaches the electron source  1   a  can be reduced. 
     This prevents the contact of the sealing member  32  of the valve body  16  even when throughput is improved, and replacement frequency is reduced by extending longevity, which in turn reduces the device stop time. Moreover, generation of foreign matters can be prevented since the sealing member  32  does not contact even when throughput is improved, and the electron source  1   a  is also stabilized, whereby reliability is improved. 
     REFERENCE SIGNS LIST 
     
         
           1  electron gun 
           2  mirror body 
           3  condenser lens 
           4  objective lens 
           5  gate valve 
           6  wafer 
           7  specimen stage 
           8  specimen chamber valve 
           9  specimen chamber 
           10  exchange chamber 
           11  local exhaust chamber 
           12  robot 
           13  wafer casing 
           14  exchange chamber valve 
           15  valve body receptacle 
           16  valve body 
           17  aperture 
           18  valve body drive assembly