Abstract:
The invention provides a plasma processing apparatus for measuring the etching quantity of the material being processed and detecting the end point of etching using optical interference on the surface of a sample being processed, so as to simultaneously realize long life and ensure sufficient light to be received via a light transmitting unit, to enable long term stable operation and to improve the processing accuracy via accurate etching quantity detection. In a plasma processing apparatus for processing a sample being processed by generating plasma between a shower plate and a lower electrode, a detector for detecting light from a surface of the sample being processed via the shower plate includes a light transmitting unit composed of a light guide into which light is entered and a spectroscope for analyzing the light obtained by the light transmitting unit, wherein the end surface of the light transmitting unit through which light is entered is arranged at a distance of five times or greater of the mean free path of gas molecules within the vacuum reactor from the end surface of the shower plate facing the plasma.

Description:
[0001]    The present application is underlayerd on and claims priority of Japanese patent application No. 2007-327596 filed on Dec. 19, 2007, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the invention 
         [0003]    The present invention relates to a semiconductor manufacturing apparatus for manufacturing semiconductor devices, and more specifically, relates to a dry etching technique for etching semiconductor materials such as silicon and silicon oxide films using plasma into profiles corresponding to a mask pattern formed of a resist material and the like. 
         [0004]    2. Description of the related art 
         [0005]    In the art of dry etching, material gas is introduced into a vacuum reactor having an evacuation means, and the material gas is turned into plasma via electromagnetic waves so as to expose the sample being processed to plasma to etch the areas of the surface of the sample being processed not covered by a mask, to thereby obtain the desired profile. High frequency voltage independent from plasma generation is applied to the sample being processed, and via the high frequency voltage, ions in the plasma are accelerated toward the surface of the sample being processed, by which the etching efficiency is improved and a perpendicular processing profile is obtained (refer for example to Japanese Patent Application Laid-Open Publication No. 2002-184766, hereinafter referred to as patent document 2). 
         [0006]    In the art of dry etching, an endpoint detection for judging whether the etching of a predetermined quantity has been completed or not is normally performed by observing the plasma emission. Actually, the end point detection is performed by monitoring the quantity of emission of the reaction products of the material being etched in the plasma or the underlayer material exposed when etching is completed. However, from viewpoints of improvement of etching accuracy and reduction of costs by simplified processes, there are demands for not completing the etching when the underlayer material is exposed, but for stopping the etching process in midway of etching a single material or immediately prior to completing the etching. 
         [0007]    According to such demands, the end point detection of etching cannot be performed by monitoring the emission from plasma as described above, but must be performed by monitoring either the etching quantity of the material being etched directly or the residual film thickness. A method for monitoring the etching quantity of the material being etched or the residual film thickness includes receiving light reflected on the surface of the sample being processed from plasma or from an independently-disposed light source, so as to analyze the interference pattern of the light accompanying the reduction of the material being etched on the surface of the sample being processed (refer for example to Japanese Patent No. 3643540, hereinafter referred to as patent document 1). 
         [0008]    In etching apparatuses for etching insulating film materials such as silicon oxide films, a shower plate formed of a conductor such as silicon is disposed on an opposite side of the sample being processed, and high frequency power is applied to the whole body of the conductor including the shower plate to generate plasma. Thus, it is necessary to arrange a light transmitting unit to a conductor electrode portion opposed to the sample being processed, so as to monitor the etching quantity by performing analysis of the interference pattern of light accompanying the reduction of the material being etched. In general, a light transmitting unit has a structure to conduct light to the exterior of the vacuum reactor via a light guide rod formed for example of quartz or sapphire, and then to conduct the light via an optical fiber to a light interference pattern analysis unit composed for example of a spectroscope. 
         [0009]    If the light guide formed for example of quartz or sapphire as the light transmitting unit is exposed directly to the shower plate surface formed for example of silicon, the end surface of the light guide rod is consumed by accelerated ions from the plasma or is subjected to deposition, making it impossible to receive light in an extremely short time. In order to overcome the problem, patent document 1 discloses a structure in which a plurality of penetrating holes  115 B through which plasma cannot pass are formed to a portion of the silicon shower plate, and an optical transmitting rod  141  is arranged on the rear side of the shower plate. 
         [0010]    According to the prior art example having the above-described structure, it becomes possible to significantly elongate the life for receiving light compared to when the light guide rod is directly exposed to plasma. 
         [0011]    However, even by adopting the structure illustrated in patent document 1, it becomes difficult to receive light in approximately 100 to 200 hours of discharge time, which is an insufficient life according to the level of production performed in some semiconductor devices. Further, by taking measures such as reducing the diameter of the through holes formed to the shower plate and improving the aspect ratio, it becomes possible to extend the life of the light transmitting unit for some time, but the quantity of light passing therethrough is reduced, and the required accuracy cannot be ensured. 
         [0012]    Further, in volume-production processes of semiconductors, it becomes necessary to replace the light guide rod when the light transmission rate of the rod is deteriorated. However, the prior art method has a drawback in that the replacement operation could not be performed easily. 
       SUMMARY OF THE INVENTION 
       [0013]    The object of the present invention is to provide a plasma processing apparatus for determining the end point of etching by monitoring the etching quantity of the material being processed via light interference on the surface of the sample being processed, wherein a means is provided to realize both longer life of the light transmitting unit and ensured light receiving quantity, and to enable long-term stable operation and improved processing accuracy by accurately detecting the etching quantity. 
         [0014]    The present invention provides a plasma processing apparatus comprising an upper electrode for supplying material gas into a vacuum reactor via a shower plate, a lower electrode opposed to the upper electrode on which is placed a sample being processed, and a detector for detecting light from a surface of the sample being processed via the shower plate, so as to process the sample by generating plasma between the shower plate and the lower electrode; wherein the detector comprises a light transmitting unit including a light guide into which the light is entered and a spectroscope for analyzing the light obtained through the light transmitting unit; and an end surface of the light transmitting unit through which the light is entered is positioned at a distance of five times or greater of a mean free path of a gas molecule within the vacuum reactor from an end surface of the shower plate facing the plasma. 
         [0015]    Further, the present invention provides a light guide rod having a hollow structure in which a space is formed in the interior of the light guide rod. Further, the present invention provides a light guide rod having a convex shape so as to facilitate replacement of the light guide rod. Moreover, the rod may have a cylindrical member disposed within the hollow structure so as to prevent deposits from sticking to the light guide rod. Furthermore, the rod may have an insulating member disposed in the hollow structure so as to prevent abnormal plasma generation in the hollow structure. 
         [0016]    The effects of the present invention are as follows. By arranging the end surface position of the light detecting unit at a distance of five times or greater of the mean free path of the gas within the vacuum reactor from the plasma boundary, it becomes possible to reduce the percentage of ions being accelerated from the plasma reaching the light transmitting unit directly in a collision less manner to 1/100 or smaller. Thus, it becomes possible to significantly suppress the consumption of the end surface of the light transmitting unit, and to elongate the life of the light transmitting unit to 1000 hours of discharge time or longer. Furthermore, by adopting a convex structure to the light guide rod, it becomes possible to reduce the operation time for exchanging rods to 1/10 or shorter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a view showing the basic structure of a plasma processing apparatus according to a first embodiment of the present invention; 
           [0018]      FIG. 2  is a detailed explanatory view showing the structure of a light detecting unit according to the first embodiment of the present invention; 
           [0019]      FIG. 3  is a view showing the multiples of mean free path and the ratio of molecules and atoms passing the distance in a collisionless manner; 
           [0020]      FIG. 4  is a detailed explanatory view of the structure of a light detecting unit according to a first modified example of the present invention; and 
           [0021]      FIG. 5  is a detailed explanatory view of the structure of a light detecting unit according to a second modified example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Now, the preferred embodiments of the present invention will be described with reference to the drawings. 
         [0023]      FIG. 1  is a drawing showing the configuration of a plasma processing apparatus according to a first embodiment of the present invention, which illustrates an example in which the present invention is applied to a magnetic field UHF band electromagnetic radiation discharge-type plasma etching apparatus.  FIG. 1  is a frame format showing the cross-section of the plasma etching apparatus according to the first embodiment. 
         [0024]    In  FIG. 1 , a processing chamber  100  is disposed in the interior of a vacuum reactor capable of achieving a vacuum degree of approximately 10 −6  Torr, and defines therein a space in which a substrate-shaped sample such as a semiconductor wafer is processed via plasma generated therein. An antenna  110  as plasma generating means for radiating electromagnetic waves is disposed on the upper portion in the interior of the vacuum reactor, and a lower electrode  130  on which the sample W such as a wafer is to be placed is disposed below the antenna  110 . 
         [0025]    The antenna  110  and the lower electrode  130  are disposed in parallel and opposed to one another. A magnetic field forming means  101  composed for example of an electromagnetic coil and a yoke is arranged in the circumference of the processing chamber  100 , by which a magnetic field having a predetermined distribution and intensity is formed. By the interaction with the electromagnetic waves radiated from the antenna  110  and the magnetic field formed by the magnetic field forming means  101 , plasma is generated from the processing gas supplied to the interior of the processing chamber, and the generated plasma P is used to process the wafer W placed on the lower electrode  130 . 
         [0026]    The processing chamber  100  is evacuated and pressure-controlled via an evacuation system  104  and a pressure control means  105  connected to the vacuum chamber  103 , and the inner pressure of the chamber can be controlled to a predetermined value, which, for example, is in the range between 0.5 Pa and 4 Pa. The processing chamber  100  and the vacuum chamber  103  are set to earth potential. The temperature of the side wall  102  of the processing chamber  100  is controlled for example to 50° C. via a temperature control means not shown. 
         [0027]    The antenna  110  for radiating electromagnetic waves is composed of a disk-shaped conductor  111 , a dielectric body  112  and a dielectric ring  113 , and supported on a housing  114  which constitutes a portion of the vacuum reactor. A structural body or disk-shaped plate  115  is disposed on one side of the disk-shaped conductor  111  which comes into contact with plasma, which is opposed to a wafer W or a circular sample-mounting plane of the upper surface of the lower electrode  130  described in detail later on which the wafer W is placed. The plate  115  is a circular plate-shaped conductive member, and the plate is fixed to position with respect to the disk-shaped conductor  111  on the outer circumference-side thereof. The diameter of the substantially circular portion of the plate  115  facing the plasma in the processing chamber  100  is either the same or greater than the diameter of the circular wafer W or the sample mounting plane. 
         [0028]    The processing gas for subjecting the wafer W to processes such as etching and film deposition is fed from a gas supply means  116  with a predetermined flow rate and mixing ratio, which is homogenized in the interior of the disk-shaped conductor  111  and fed into the processing chamber through a plurality of holes formed to the plate  115 . The temperature of the disk-shaped conductor  111  is controlled for example to 30° C. via a temperature control means not shown. An antenna power supply system  120  composed of an antenna power supply  121 , an antenna bias power supply  123  and a matching circuit/filter system  122 ,  124  and  125  is connected via an introduction terminal  126  to the antenna  110 . The antenna power supply  121  supplies a UHF-band frequency power preferably between 300 MHz and 900 MHz, so as to radiate UHF-band electromagnetic waves via the antenna  110 . 
         [0029]    The antenna bias power supply  123  applies a bias with a frequency of approximately 100 kHz or a few MHz to 10 MHz, for example, via the disk-shaped conductor  111  to the plate  115 , and controls the reaction on the surface of the plate  115 . Especially, in an oxide film etching process using CF-under layered gas, the material of the plate  115  is preferably formed of high-purity silicon or carbon, so as to enable control of the reaction of F radicals and CFx radicals on the surface of the plate  115  and to control the composition ratio of radicals. In the present embodiment, high-purity silicon is used for forming the plate  115 . 
         [0030]    The disk-shaped conductor  111  and the housing is formed of aluminum, and the dielectric body  112  and the dielectric ring  113  is formed of quartz. The distance between the lower surface of the plate  115  and the wafer W (hereinafter referred to as gap) is 30 mm or greater and 150 mm or smaller, preferably 50 mm or greater and 120 mm or smaller. In the present embodiment, the frequency of the antenna power supply  121  is set to 450 MHz, the frequency of the antenna bias power supply  122  is set to 13.56 MHz, and the gap is set to 70 mm. 
         [0031]    A lower electrode  130  is disposed to face the antenna  110  at the lower portion of the processing chamber  100 . On the upper surface or sample mounting surface of the lower electrode  130  is placed a wafer W, which is fixed thereto via an electrostatic chuck device  131 . A sample stage ring  132  formed for example of high-purity silicon is disposed on an insulating body  133  at the outer circumference portion of the wafer W. A bias power supply  134  for supplying bias power in the range of preferably 400 kHz to 13.56 MHz is connected via a matching circuit/filter system  135  to the lower electrode  130 , by which the bias power applied to the sample W is controlled. 
         [0032]    In the present embodiment, the frequency of the bias power supply  134  is 800 kHz. Furthermore, an evacuation system  104  comprising a vacuum pump such as a turbo molecular pump is connected to the lower portion of the vacuum reactor disposed below the lower electrode  130 , which is communicated with the interior of the processing chamber  100  via a port disposed at the bottom portion of the processing chamber  100 . Through the operation of the evacuation system  104 , the gas, plasma and particles generated by products formed by the processes in the processing chamber  100  are evacuated to the exterior of the processing chamber  100 , and the interior of the processing chamber  100  is set to a pressure of a predetermined vacuum degree. 
         [0033]    Next, a measurement port  140  disposed to measure the surface condition of the sample W, which is the substantial portion of the present embodiment, will be described. In the present embodiment, the measurement port  140  is attached by being inserted to the inner side of the antenna  110  facing the sample W, and through the multiple through holes formed to the plate  115 , the status of the thin film or the like on the surface of the wafer W can be measured from the perpendicular upper direction. Of course, the mounting position of the measurement port is not restricted to the middle area as described above, but can be one or more than two locations arranged in different positions such as on the circumference area. 
         [0034]    An optical transmission means  151  such as an optical fiber or lens is disposed on the opposite side from the wafer W via the plate  115  of the measurement port  140 , and the optical information reflecting the surface condition of the wafer W, such as the direct light from the plasma P or the reflected light or interference light on the wafer W surface of the plasma P or the reference light such as the white light supplied into the processing chamber  100  are transmitted from the plate  115  via the optical transmission means  151  to a measuring equipment  152  composed for example of a camera, an interference film meter or an image processing apparatus for measurement. The measuring equipment  152  is controlled via a measuring equipment control/calculation means  153 , and further connected to an upper system controlling means  154 . The system controlling means  154  monitors and controls the status of the system via a control interface  155 . 
         [0035]    The plasma etching apparatus according to the present embodiment is composed as above, and the actual process for etching silicon oxide films or the like using the present plasma etching apparatus is as follows. 
         [0036]    At first, a wafer W, which is the object being processed, is carried into the processing chamber  100  from a sample transfer mechanism not shown, which is then mounted and attracted to the lower electrode  130 , and the height of the lower electrode is adjusted according to need to set the gap to a predetermined distance. Thereafter, the interior of the processing chamber  100  is vacuumed by the evacuation system  104 , while gases required for the etching process of the wafer W, such as C 4 F 8 , Ar and O 2 , are supplied from the gas supply means  116  with a predetermined flow rate and mixing ratio, such as 1000 sccm Ar, 43 sccm CHF 3  and 10 sccm CF 4 , through the plate  115  of the antenna  110  to the processing chamber  100 . At the same time, the interior of the processing chamber  100  is set to a predetermined processing pressure, such as 2 Pa. 
         [0037]    On the other hand, a substantially horizontal magnetic field of substantially 160 Gauss corresponding to the intensity of an electron cyclotron resonance magnetic field with respect to the antenna power supply  121  with a frequency of 450 MHz is formed in the area below the plate  115 . Then, electromagnetic waves in the UHF band is radiated via the antenna  110  from the antenna power supply  121 , and plasma P is generated in the processing chamber  100  by the interaction with the magnetic field. Processing gas is dissociated and ion radicals are generated in the plasma P, and by further controlling the antenna bias power supply  123  and the bias power supply  134 , the wafer W is subjected to etching and other processes. 
         [0038]    The input power of the respective power supplies are, for example, 300 W for the antenna power supply  121 , 200 W for the antenna bias power supply  123  and 160 W for the bias power supply  141 . Then, at the end of the etching process, the supply of power and processing gas are stopped and the etching is ended. 
         [0039]    Optical information reflecting the plasma emission and the surface condition of the wafer W during the process is transmitted through the measurement port  140  via the optical transmission means  151  to the measuring equipment  152  where measurement is performed, then underlayerd on the measured result, a measuring equipment control/calculation means  153  performs calculation, transmits the result to the upper system control means  154 , and the plasma processing device system is controlled via a control interface  155 . 
         [0040]    Next, the detailed structure of a measurement port  140  will be described with reference to  FIG. 2 . 
         [0041]      FIG. 2  is a cross-sectional view showing in enlarged view a portion of the measurement port  140  attached to the antenna  110  in the embodiment of  FIG. 1 . As already described in  FIG. 1 , the disk-shaped conductor  111  and the dielectric body  112  forming the antenna  110  is supported by the housing  114 , and a plate  115  is attached to the disk-shaped conductor  111 . A number of gas through holes  115 A are formed to the plate  115 , and processing gas is supplied into the processing chamber  100  through gas through holes  111 A formed at corresponding positions to the gas through holes  115 A on a disk-shaped conductor  111  disposed above and adjacently covering the plate  115 . 
         [0042]    The gas through holes  115 A formed to the plate  115  are through holes having a diameter in the range of approximately 0.1 mm to 5 mm, for example, preferably approximately 0.3 mm to 2 mm, and the gas through holes  111 A formed on the disk-shaped conductor  111  are holes having an equal or greater diameter to the gas through holes  115 A, the diameter of which is in the range of approximately 0.5 mm to 5 mm, for example, preferably approximately 2 mm. The thickness of the plate  115  is approximately 3 mm to 20 mm, and in the present embodiment, the thickness thereof is 10 mm. 
         [0043]    A plurality of through holes  115 B for receiving light which are cylindrical pores penetrated through the plate  115  are densely formed to the plate  115  at a position corresponding to the measurement port  140  disposed on the rear side of the plate. Above the opening on a rear side (side opposite from the plasma P) of the through holes  115 B for receiving light on the plate  115  is disposed a light guide  141 , which is placed at a position close to the rear side of the plate  115  either with a given gap therebetween, or with a minute gap therebetween so that the plate and the light guide are substantially considered to be in contact with each other, or mounted on the rear side. 
         [0044]    The light guide  141  according to the present embodiment is composed of two parts that are separable into top and bottom portions, wherein the lower light guide  141 A has its lower end arranged to face or substantially contact the plate  115 , and the upper light guide  141 B is mounted in a vacuum-sealed manner to the housing  114  via a retention means  142  and a vacuum sealing means  143 A such as an O-ring. Then, an optical transmission means  151  such as an optical fiber or lens is disposed at the atmospheric end surface of the light guide  141 . The direct light from the plasma P or the reflected light and interference light form the surface of the wafer W of the plasma P are transmitted through the through holes  115 B for receiving light of the plate  115 , transmitted through the light guide  141  to the optical transmission means  151 , and further transmitted to the measuring equipment  152  for measurement. 
         [0045]    The upper light guide  141 B is positioned between the lower light guide  141 A and the optical transmission means  151  to transmit the transmitted light or the optical information from the light guide  141 A to the optical transmission means  151 . The light guide  141 B is a cylindrical member formed of quartz having a stepped shape in which the lower diameter is greater, wherein the lower large-diameter portion is inserted to the stepped upper surface of the cylindrical opening with multiple steps and having a diameter formed to correspond to the diameter of the lower large-diameter portion, by which the vertical position thereof is determined. The light guide is further covered by a retention means  142  fit thereto from above, and then screwed and attached to the housing  114  being grounded to ground potential. During this attaching operation, an o-ring disposed around the large-diameter portion is pressed against the light guide  141 B by the engagement force by which the retention means  142  is screwed, by which the interior of the vacuum reactor is airtightly sealed from the exterior. 
         [0046]    According to the present embodiment, the light guides  141 A and  141 B are cylindrical rods formed of quartz with steps and having multiple varying diameters. The diameter at the upper portion of the light guide  141 A is preferably between approximately 5 mm and 30 mm, and in the present embodiment, the diameter is 8 mm. The light guide  141 A has a cylindrical hole, in other words, a hollow structure or hollow space  141 C, that is recessed to a predetermined depth in the axial direction of the cylinder from the end surface that faces or opposes to the through holes  115 B for receiving light of the plate  115  when the measurement port  140  is attached thereto. 
         [0047]    In the present embodiment, the inner diameter of the cylindrical hollow space  141 C is 6 mm, and the depth thereof is 15 mm. Similar to the gas through holes  115 A, the through holes  115 B for receiving light has a diameter of approximately 0.1 mm to 5 mm, preferably approximately 0.3 mm to 2 mm, and in the present embodiment, the diameter of the through holes  115 B is 0.5 mm. Further, a multiple number of through holes  115 B for receiving light should be provided so as to improve the measurement sensitivity. Seven through holes are provided in the present embodiment. 
         [0048]    The area in which the through holes  115 B for receiving light are formed is within the opening on the lower end of the hollow space  141 C when the light guide  141 A is attached to the antenna  110 , and the outer edge of the hollow space  141 C of the light guide  141 A is arranged to surround the multiple through holes  115 B for receiving light. The hollow space  141 C can be formed by cutting and hollowing the interior of the cylindrically-shaped quartz material along the axis of the cylinder from one end to the other end, as according to the present invention, or by attaching a cylindrical member to a pipe-like member. 
         [0049]    Further, the light guide  141 A is structured so that the outer diameter of the portion positioned toward the plate  115  is formed greater than the rod diameter (projected or convex structure) so as to facilitate the replacing operation of the light guide  141 A. According especially to the present embodiment, the lower end portion facing the plate  115  of the light guide is extended outward in a flange to form a flange portion  141 D having a diameter of 10 mm and a length of 1.5 mm. In order to prevent supplied gas from directly flowing into the hollow space  141 C of the light guide  141 , vacuum seal means  143 B and  143 C, such as o-rings, are disposed in the circumference of the light guide  141 . 
         [0050]    In other words, an o-ring, which is the vacuum seal means  143 B, is disposed on the outer circumference of the side wall of the upper cylindrical portion or small-diameter portion, sealing the space between the side wall and the gas reservoir space at the inner side of the cylindrical conductor  111 . Further, an o-ring, which is the vacuum seal means  143 C, is fit to the outer circumference of the flanged portion  141 D and the inner wall of the cylindrical recess disposed on the lower surface facing the plasma of the disk-shaped conductor  111 , airtightly sealing the space between the disk-shaped conductor  111  and the gas through holes  111 A and  115 A. The two vacuum seal means  143 B and  143 C prevent the particles of gas supplied to the processing chamber  100  or particles from the gas and plasma in the processing chamber from entering the upper portion of the light guide  141 A and contaminating the interior of the antenna  110  or the surface of the light guides  141 A and  141 B. 
         [0051]    Further, a cylindrical recess is arranged around the through hole at the lower surface of the disk-shaped conductor  111  into which the light guide  141 A is inserted, and when the light guide  141 A is inserted to the through hole of the disk-shaped conductor  111 , the flange portion  141 D is stored in the interior of the recess and the vertical position of the light guide is determined by the upper surface of the stepped portion  111 B of the recess. Moreover, an o-ring which is the vacuum seal means  143 C is fit to the recess of the disk-shaped conductor  111  at the outer circumference portion of the flange portion  141 D. 
         [0052]    As described, when the plate  115  is supported and fixed at the outer circumference to the disk-shaped conductor  111 , the vacuum seal means  143 C is sandwiched and supported by the plate  115 , the upper or side surface of the stepped portion  111 D of the recess and the flange portion  141 D and pressed thereto, so as to seal the space between the interior of the light guide  141 C, the through holes  115 B for receiving light and the gas through holes  111 A and  115 A, and the vertical and horizontal position of the light guide  141 A is determined and fixed thereby. 
         [0053]    Further, the material of the light guide  141 A and  141 B is selected from a group consisting of quartz, sapphire, YAG (yttrium-aluminum-garnet) and yttria crystal (Y 2 O 3 ), preferably sapphire, YAG and Y 2 O 3 . Sapphire, YAG and yttria crystal are expensive but generally not easily sputtered compared to quartz, and therefore, a longer life is expected by using these materials instead of quartz. 
         [0054]    According to the present embodiment, light guide rods  141 A and  141 B are disposed to receive the reflected light from the wafer W via through holes  115 B for receiving light formed to the shower plate  115  and the hollow space  141 C. Further, the length of the hollow space  141 C is set so that the distance from the plasma P side of the shower plate  115  to the upper end of the hollow space  141 C of the light guide rod  141 A, that is, to the opposite end farthest from the plasma P via the plate, is five times or greater of the mean free path of the gas molecules under a gas pressure condition in the plasma generating atmosphere within the vacuum reactor  144 . 
         [0055]    The through holes  115 B for receiving light formed to the shower plate  115  has a function to block plasma P. In the present embodiment, the diameter of each of the through holes  115 B for receiving light is 0.5 mm. This arrangement enables to prevent gas and charged particles in the plasma P from entering the hollow space  141 C. According to the present embodiment, the end surface of the light guide rod  141  disposed at the depth of the hollow space  141 C formed on the rear surface of the plate  115  from the processing chamber is arranged at a position sufficiently spaced apart from the plasma P. In other words, according to the present embodiment, the end surface of the light guide rod is disposed via the hollow space  141 C with a length of 15 mm. 
         [0056]    Accordingly, the distance from the plasma P to the end surface of the light guide rod is 25 mm, which is seven to eight times the mean free path of the gas molecules in a 2 Pa atmosphere. Therefore, the end surface of the rod for introducing light is exposed to very little ion radiation, by which the chances of the end surface of the rod being consumed are reduced, so that the rod can have a longer life. As described, by forming a hollow space  141 C in the light guide rod  141 , the life of the light guide rod  141  can be extended, and since the light guide rod  141  is projected at the outer circumference portion, the operator can easily grip and handle the light guide  141 , by which the time required for the replacing operation of components can be shortened. 
         [0057]    Further, since the inner diameter of the hollow space  141 C is approximately 5 mm or greater, the light guide rod  141 A can be cleaned easily for recycle, so that the cost of replacing the components can be reduced. According further to the present embodiment, the light guide  141  is divided into the lower light guide  141 A and the upper light guide  141 B. The light guides  141 A and  141 B are respectively inserted to the through holes of the disk-shaped conductor  111  and the housing  114  and supported within the antenna  110 , and the light guides are respectively determined of their positions on the surface of the stepped portion  111 B of the recessed portion and the stepped portion  114 A of the housing  114 . 
         [0058]    By screw-engaging the retention means  142 , the upper light guide  141 B is pressed against the upper surface of the stepped portion  114 A by the vacuum seal means  143 A and the retention means  142 , by which the vertical position thereof is determined and retained. Further, the lower light guide  141 A is designed so that when the housing  114  is rotated upward to release the processing chamber  100  by which the plate  115  is separated from the disk-shaped conductor  111 , the lower light guide  141 A can be attached and detached substantially perpendicularly with respect to the antenna  110  or the disk-shaped conductor  111 . When the lower light guide  141 A is inserted to the through hole of the disk-shaped conductor  111  and the flange portion  141 D is fit to the stepped portion  111 B of the recess, and when the vacuum seal means  143 C on the outer circumference is mounted to the recessed portion and the plate  115  is attached, the upper surface of the flange portion  141 D opposing to the disk-shaped conductor  111  is positioned with respect to the surface of the stepped portion  111 B. 
         [0059]    For example, the o-ring or vacuum seal means  143 C is positioned between the plate  115  and the flange portion  141 D to apply a force to press the flange portion  141 D toward the stepped portion  111 B of the disk-shaped conductor  111  (toward the upper direction when the housing  114  is closed), so as to hold the light guide  141 A between the disk-shaped conductor  111  (or the stepped portion  111 B thereof) and the vacuum seal means  143 C (or plate  115 ) and determine the vertical position thereof. 
         [0060]    In this case, a minute gap is formed between the end surface of the flange portion  141 D facing the plate and the rear surface of the plate  115 , and the shapes of the stepped portion  111 B and the flange portion  141 D are designed so that the size of the gap does not cause abnormal electrical discharge by the electric field formed by the supplied high frequency. It is also possible to dispose the flange portion  141 D and the rear surface of the plate  115  to either contact one another or be closely arranged so that they are substantially considered to be in contact with one another, and to form a minute gap between the flange portion  141 D and the stepped portion  111 B of the disk-shaped conductor  111  small enough not to cause abnormal electrical discharge. 
         [0061]    Moreover, the cylindrical light guides  141 A and  141 B having their vertical positions determined respectively are also positioned so that the space between the upper end surface and the lower end surface of the light guides  141 A and  141 B is small enough so as not to cause abnormal electrical discharge by the above-mentioned electric field formed via high frequency. According to such arrangement of positioning, it becomes possible to prevent the occurrence of abnormal electrical discharge in the gap formed between and around the light guides  141 A and  141 B, and also suppress the light in the processing chamber  100  passing through the through holes  115 B for receiving light and the hollow space  141 C and through the light guide  141 A to the light guide  141 B from being attenuated by abnormal reflections or inflections, by which the reliability of the light guide  141  is improved, along with the suppression of optical attenuation caused by contamination and damage of the interior of the hollow space  141 C by particles from the plasma P. 
         [0062]    With reference to  FIG. 3 , a collisionless passage ratio of molecules and atoms with respect to the multiple of mean free path will be described. The collisionless passage ratio of molecules and atoms is reduced exponentially with respect to the multiple of the mean free path. From  FIG. 3 , when molecules and atoms pass approximately five times the distance of the mean free path, the percentage in which the molecules and atoms can pass the distance in a collisionless manner is 1% or less, meaning that most molecules and atoms experience collision within the gas phase and lose their initial kinetic energy. When the distance is approximately seven to eight times the mean free path, the percentage in which the molecules and atoms pass in a collisionless manner is 0.1% or smaller. 
         [0063]    Thus, according to the arrangement illustrated in the present embodiment, the percentage of the ions accelerated in the plasma P and reaching the end surface of the light guide rod in a collisionless manner is 0.1% or smaller. According to the prior art method in which the end surface of the light guide rod is positioned immediately behind the shower plate  115 , the distance is two to three times the mean free path, meaning that according to  FIG. 3 , the percentage of ions reaching the end surface of the light guide rod in a collisionless manner is approximately 5 to 15%. Therefore, according to the arrangement of the present embodiment, the percentage of ions reaching the end surface of the light guide rod in a collisionless manner is 1/50 to 1/150 the percentage thereof according to the prior art arrangement, so that according to the present invention, the life of the end surface of the light guide rod cab be extended significantly. As a result of actual evaluation, the arrangement of the present invention enables to ensure sufficient lighting quantity after a discharge time of 1000 hours, which is five times or greater than the prior art method. 
       MODIFIED EXAMPLE 1 
       [0064]    A modified example of the present invention will now be described with reference to  FIG. 4 . Similar to  FIG. 2  of the first embodiment,  FIG. 4  is a view showing the detailed structure of a measurement port  140 .  FIG. 4  characterizes in that a pipe member  145  is disposed in the interior of the hollow space  141 C of the light guide rod  141 . The pipe member  145  is positioned inside the hollow space  141 C of the light guide rod  141 . 
         [0065]    If the pipe member  145  is not arranged, the ions, molecules and atoms scattered in the hollow space  141 C stick to the side wall of the hollow space  141 C, by which deposits are formed on the side walls. The deposits stuck to the side wall will come off within a time shorter than the life of the end surface of the light guide rod  141 , causing contaminants that may interrupt the production processes of etching, by which the light guide rod  141  may have to be replaced. However, by placing a pipe member  145  in the hollow space  141 C of the light guide rod  141 , it becomes possible to attach the deposits such as scattered ions, molecules and atoms to the inner wall of the pipe member  145 , and to enable the replacement operation to be performed in a short time since only the pipe member  145  must be replaced. 
         [0066]    Further, by creating a multilayered wall surface within the hollow space  141 C by placing the pipe member  145 , and by further forming patterns on the inner wall thereof so as to increase the surface area of the inner wall, it becomes possible to reduce the thickness of the deposits on the inner wall of the pipe member  145 , so as to extend the replacement life of the pipe member  145 . For example, if grooves with a width of 0.1 mm with an inner diameter of 4.75 mm are patterned on the pipe member  145  having an inner diameter of 4.5 mm and a length of 14.5 mm, the surface area of the inner wall will be increased from 230.5 mm 2  to 770 mm 2 , so that the surface area is increased by approximately 3.3 times, and if grooves with a width of 0.01 mm are patterned on the pipe member, the surface area becomes 5508 mm 2 , by which the surface area is increased by approximately seven times, and the life thereof is extended. 
         [0067]    As described, the arrangement of the pipe member  145  enables to extend the life of the light guide rod  141  and to reduce the operation time required for replacing the components when particles are generated. Further, by extending the life of the light guide rod  141 , it becomes possible to reduce the costs of the replaced components. Further, it is possible to position at least one of the multiple through holes  115 B for receiving light to the area between the outer circumference of the inner wall of the hollow space  141 C on the end adjacent to the plate and the outer circumference of the end portion of the pipe member  145 , so as to allow particles from the plasma P to enter the space formed therebetween. 
       MODIFIED EXAMPLE 2 
       [0068]    A second modified example of the present invention will be described with reference to  FIG. 5 .  FIG. 5  is a view illustrating the detailed structure of a measurement port  140 , similar to  FIG. 2  of embodiment  1 .  FIG. 5  characterizes in that an insulating member component  146  formed of a cylindrical quartz extended from the bottom portion of the hollow space  141 C toward the rear surface of the plate  115  is arranged in the interior of and in correspondence with the center axis of the hollow space  141 C which is a cylindrically recessed portion of the light guide rod  141 . 
         [0069]    The cylindrical insulating member component  146  can be formed integrally when forming the main body of the light guide  141 A, or by inserting a separately formed cylindrical component to the hollow space  141 C of the light guide  141 A and assembling the components together. Further, it is preferable that a minute gap is formed between the leading end disposed toward the plate  115  of the insulating member component  146  and the rear surface of the plate  115  opposed thereto, and that no through holes  115 B for receiving light are positioned in this area on the rear surface of the plate  115 . 
         [0070]    If there is no insulating member component  146 , according to some etching conditions (such as when the gas pressure is high, the power of the antenna power supply  121  is high, the power of the antenna high frequency power supply  123  is high, or the power of the bias power supply  141  is high), the electric field formed by the disk-shaped conductor  111  surrounding the light guide rod  141  may create a strong electric field (hollow electric field) in the hollow space  141 C. 
         [0071]    This electric field may accelerate the ions in the hollow space portion  141 C and ionize the gas, by generating plasma in the hollow space  141 C. The electric field intensity is greatest at the center of the hollow space portion, so that by inserting an insulating member component  146  in this area, it becomes possible to suppress acceleration of ions and ionization, and to prevent the generation of plasma. Further, the life of the insulating member component  146  can be extended if no through holes for receiving light are formed immediately below the insulating member component  146 , so as to prevent ions from passing through the through holes  115 B for receiving light formed to the plate  115  toward the insulating member component  146 . Thus, the life of the light guide rod  141  can be extended effectively by inserting an insulating member component  146  and preventing abnormal plasma generation.