Abstract:
A plasma processing apparatus includes a vacuum vessel with a sample stage having a mounting surface disposed in a process chamber, and a plate having substantially uniform thickness and electric power applied thereto constituting a ceiling of the chamber. The plate is disposed opposite to and substantially parallel with the sample stage so as to cover the whole area of the stage mounting surface and has a through-hole therein. An optical transmitter with a diameter larger than a diameter of the though-hole is disposed inside of the vacuum vessel and has an end face at a position above and spaced a small distance a back surface of the plate so as to receive light from the chamber via the through-hole. The optical transmitter is independently detachable with respect to the back surface of the plate.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuation of U.S. application Ser. No. 10/810,598, filed Mar. 29, 2004, which is a continuation of U.S. application Ser. No. 09/788,463, filed Feb. 21, 2001, the subject matter of which is incorporated by reference herein and is copending with U.S. application Ser. No. 10/732,285, filed Dec. 11, 2003 and U.S. application Ser. No. 10/732,286, filed Dec. 11, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a plasma processing system and a sample processing method, and particularly to a plasma processing system and sample processing method suited for forming a fine pattern in the semiconductor production process. It relates more particularly to an apparatus or a sample processing method to measure plasma emission in a process chamber and the state of thin film on the surface of such a sample as wafer.  
         [0004]     2. Related Background Art  
         [0005]     A plasma processing apparatus is extensively used in the fine working phase such as etching, film formation and ashing of the semiconductor production process. In the plasma processing apparatus, the process gas introduced into a vacuum chamber (reactor) is converted into plasma by a plasma generating means, and is made to react on the surface of a semiconductor wafer to provide such treatment as processing of fine holes and grooves or film formation. Furthermore, the required treatment is provided by discharging volatile reaction products  
         [0006]     In this plasma processing apparatus, attempts have been made to detect the end point of the etching process by detecting the radiation from plasma during processing, and to measure the film thickness and etching/film forming rate from the reflective light and interference signal on the thin film of wafer surface for plasma radiation in real time, thereby improving the plasma processing accuracy. For example, Official Gazette of Japanese Patent Laid-Open NO. 136098/1993 discloses a parallel plate plasma etching system where two or more plasma receiving sensors are installed on the surface of the electrode opposite to the wafer, and uniform plasma density is ensured by obtaining information on the rate, film thickness uniformity and distribution from plasma radiation strength at multiple points of the wafer.  
         [0007]     Official Gazette of Japanese Patent Laid-Open NO. 148118/1991 discloses a parallel plate plasma etching system where laser beam is applied to the wafer from the upper position through a top plate electrode, and the volume of etching is measured through the reflected laser beam, thereby detecting the end point. To avoid contamination of the top electrode according to said Gazette, this system has a hole with a diameter of about 10 mm formed on the portion of the quartz-made electrode cover where laser beam passes, and measures the volume of etching with high accuracy without laser beam being damped even if the electrode cover is contaminated; this ensures stable detection of an end point.  
         [0008]     The methods described above, however, have the following problems: For monitoring of the state of thin film or the like on the wafer surface, it is preferred to measure from the upper position opposite to the wafer or from the upper position at an angle of about 45°. The plasma processing apparatus which allows measurement in such a manner is restricted in the measurement method and structure. In the microwave ECR method and inductively coupled plasma processing apparatus, for example, a quartz-made transparent window and plate may be installed above the wafer in order to apply microwaves inside the process chamber or to introduce inductive electric field. In this case, the state of the wafer surface can be measured from above. However, in the so-called capacitatively coupled parallel plate plasma processing apparatus, the top electrode opposite to the wafer is made of conductive metal such as aluminum, so it is not structured to allow the surface of the wafer to be directly penetrated. For this reason, measurement of the wafer surface requires a sensor for a plasma radiation to be installed on the surface of the electrode opposite to the wafer, as indicated in the Official Gazette of Japanese Patent Laid-Open NO. 136098/1993. However, reaction products are deposited on the sensor for a plasma radiation according to repeated electric discharges. This makes it very difficult to ensure stable measurement for a long time.  
         [0009]     One of the attempts to solve this problem is the method disclosed in the Official Gazette of Japanese Patent Laid-Open NO. 148118/1991. A hole having a diameter of about 10 mm is formed on the measurement portion of the quartz-made electrode cover directly exposed to plasma where laser beam passes, thereby eliminating an adverse effect on measurement, despite the deposit membrane formed on the quartz-made cover surface. However, this method does not ensure stable measurement. To get the specified plasma density required for plasma processing, high-frequency power amounting to several kilowatts of high power is applied to the top electrode. If an about 10 mm-diameter hole is formed on the electrode and electrode cover as disclosed in said Official Gazette, local abnormal discharge will occur to the hole, or plasma will enter the hole, and the top electrode and electrode cover may be damaged. Furthermore, bias is applied to the top electrode, the top electrode will be sputtered by ions in plasma through the hole on the electrode cover. Since the top electrode is made of such a metal as aluminum, it may be damaged or entry of particle contamination may result.  
         [0010]     Needless to say, the wafer surface can be measured in principle from the side wall of the process chamber at a smaller inclination, instead of from the upper position opposite to the wafer. However, an opposite plate type where such a plate as silicon is placed opposite to the sample at a position several tens of millimeters away is often adopted especially in the oxide film etching system, in order to control excessive dissociation of process gas and to improve process reproducibility. In this case, the angle of measuring the wafer has to be about 10° in practice. It is also difficult to ensure satisfactory measuring accuracy. Under these circumstances, it has been hoped that the state of wafer surface can be measured from the top opposite to the wafer in the opposite plate type plasma processing apparatus as well.  
         [0011]     With reference to the microwave ECR system and inductively coupled plasma processing apparatus, authors of the present invention have described that wafer surface can be measured from a quartz-made transparent window above the wafer. However, reaction products are deposited on the quartz-made window surface as discharge is repeated, and transmittance is reduced. Conversely, surface is etched and is made rough, so stable measurement for a long time is difficult. For this reason, this system has failed to meet practical requirements.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention has been made to solve said problems. It is intended to provide a plasma processing apparatus and a sample processing method which ensure stable measurement of the sample surface and plasma state from the external vacuum chamber or the wall state of the vacuum chamber with high accuracy for a long time, without abnormal discharge or occurrence of particle contamination.  
         [0013]     Authors of the present invention have studied the above problems from the view point of ensuring practicability and reliability, and have found out the following solutions:  
         [0014]     The present invention provides a plasma processing apparatus wherein process gas is supplied into a vacuum chamber, plasma is generated by a plasma generator and a sample placed on a sample bench is processed by said plasma; said plasma processing apparatus further characterized in that  
         [0015]     an optical reflector is arranged within said vacuum chamber,  
         [0016]     at least one or more through-holes with depth-to-diameter ratios ranging from 5 up to 100 are formed at a position opposite to said optical reflector of said vacuum chamber, and  
         [0017]     a means of measuring optical information from the surface state of said optical reflector via said through-hole is provided.  
         [0018]     The present invention is characterized in that a plasma processing apparatus wherein process gas is supplied into a vacuum chamber, plasma is generated by a plasma generator and a sample placed on a sample bench is processed by said plasma, provides;  
         [0019]     an optical reflector which is arranged within said vacuum chamber,  
         [0020]     at least one or more through-holes with depth-to-diameter ratios ranging from 5 up to 100 which are formed at a position opposite to an optical reflector of said vacuum chamber and on a structure in contact with said plasma,  
         [0021]     an optical transmitter which is installed on the back of said through-hole so that one end face thereof will be almost in contact with said structure,  
         [0022]     an optical transmission means which is laid out on the other end face of said transmitter, and  
         [0023]     a means of measuring optical information from the surface state of said optical reflector via said optical transmitter and said optical transmission means.  
         [0024]     The present invention is further characterized in that the diameter of said through-hole is 0.1 mm to 5 mm, desirably 0.3 mm to 2 mm.  
         [0025]     The present invention is still further characterized in that the total of the opening area of said through-hole is made 5 to 50% of the total area of the area in which a plural of said through-holes are formed.  
         [0026]     A further characteristic of the present invention is that quartz or sapphire is used as said optical transmitter.  
         [0027]     A still further characteristic of the present invention is the structure which enables easy replacement of said optical transmitter by removing one set of holding means and vacuum sealing means which holds said optical transmitter in position, when said vacuum chamber is released to the atmosphere.  
         [0028]     The present invention is still further characterized in that a plasma processing apparatus wherein process gas is supplied into a vacuum chamber, plasma is generated by a plasma generator and a sample placed on a sample bench is processed by said plasma, provides;  
         [0029]     an optical reflector which is arranged within said vacuum chamber,  
         [0030]     at least one or more through-holes with depth-to-diameter ratios ranging from 5 up to 100 which are formed at a position opposite to an optical reflector of said vacuum chamber,  
         [0031]     a means of measuring optical information from the surface state of said optical reflector via said through-hole, and  
         [0032]     a means of determining whether some foreign substance has generated, based on the variations of said optical information.  
         [0033]     The present invention is still further characterized in that a plasma processing apparatus wherein process gas is supplied into a vacuum chamber, plasma is generated by a plasma generator and a sample placed on a sample bench is processed by said plasma, provides;  
         [0034]     an optical reflector which is arranged within said vacuum chamber,  
         [0035]     at least one or more through-holes with depth-to-diameter ratios ranging from 5 up to 100 which are formed at a position opposite to an optical reflector of said vacuum chamber and on a structure in contact with said plasma,  
         [0036]     a means of measuring optical information from the surface state of said optical reflector via said through-hole, and  
         [0037]     a means of determining to what extent said structure has been consumed, based on the variations of said optical information.  
         [0038]     The present invention is still further characterized in that a sample processing method whereof process gas is supplied into a vacuum chamber, plasma is generated by a plasma generator and a sample placed on a sample bench is processed by said plasma, is performed to process said sample while;  
         [0039]     measuring optical information from the surface state of said sample via at least one or more through-holes with depth-to-diameter ratios ranging from 5 up to 100 which are formed on the wall of said vacuum chamber at a position opposite to said sample of said vacuum chamber, and while  
         [0040]     measuring the state of the thin film on the surface of said sample based on the variations of said optical information.  
         [0041]     According to the present invention, the plasma processing apparatus is so designed as to provide an optical reflector which is arranged within the vacuum chamber, at least one or more through-holes with depth-to-diameter ratios (aspect ratio) ranging from 5 up to 100 which are formed at a position of a vacuum chamber opposite to an optical reflector, and a means of measuring optical information from the surface state of said optical reflector via said through-hole. So the plasma processing apparatus cannot reduce light transmission characteristics due to adhesion of reaction productions on the end face of an optical transmitter even if making repeated discharge for a long period of time.  
         [0042]     Furthermore, the through-hole diameter is so small and aspect ratio is so large that plasma cannot enter internal through-holes to generate abnormal discharge.  
         [0043]     What is more, if the quartz or sapphire having an excellent light transmittance and great resistance against plasma is used as an optical transmitter, deterioration of optical performance due to the damage on the end face of the optical transmitter can be sufficiently reduced, thereby ensuring a stable measurement for a long time.  
         [0044]     In addition, sample surface and plasma radiation can be measured with sufficient sensitivity and accuracy by compact arrangement of multiple through-holes with an exposed area ratio of 5 to 50%.  
         [0045]     What is more, the down time in wet cleaning of the plasma processing apparatus can be minimized by adoption of the structure permitting easy replacement of an optical transmitter  141 . This prevents the availability factor of the plasma processing apparatus from being reduced.  
         [0046]     In addition, if reaction productions, causing occurrence of particle contamination, which have accumulated at the periphery of the susceptor or on the sidewall inside the vacuum chamber, peel off from the optical reflector, changing a quantity of light from the optical reflector takes place. So detecting this change enables a warning to be generated for the purpose of prevention of frequent occurrence of particle contamination. This warning leads to decide a proper time of cleaning out the internal plasma processing apparatus, enabling to prevent abnormality from occurring during operation of the plasma processing apparatus.  
         [0047]     Furthermore, monitoring a quantity of radiation measured with the through-hole on the plate leads to detect consumption of the plate, enabling to prevent abnormality from occurring during operation of the plasma processing apparatus.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]      FIG. 1  is a schematic diagram representing the cross section of the plasma etching apparatus as an embodiment of the present invention;  
         [0049]      FIG. 2  represents the through-hole section as a major portion of the present invention;  
         [0050]      FIG. 3  represents one embodiment of through-hole arrangement according to the present invention;  
         [0051]      FIG. 4  represents another embodiment of through-hole arrangement according to the present invention;  
         [0052]      FIG. 5  represents a dependency of the aspect ratio to the solid angle of the through-hole of the present invention;  
         [0053]      FIG. 6  represents an example of a experimental signal in the embodiment.  
         [0054]      FIG. 7  is a drawing used to explain an example using an image forming optical system as another embodiment according to the present invention;  
         [0055]      FIG. 8  is a drawing used to explain operations in the embodiment of  FIG. 6 ;  
         [0056]      FIG. 9  represents an example of using a hollow structure in the optical transmitter as another embodiment according to the present invention;  
         [0057]      FIG. 10  is a drawing to explain an example of diagnosing the apparatus by installing a reflector on the susceptor or on the sidewall of the vacuum chamber;  
         [0058]      FIG. 11  is a sectional view of a gas effluence port to explain an example of diagnosing the apparatus with respect to consumption detection of the plate for supplying gas in other embodiments of the present invention; and  
         [0059]      FIG. 12  is a drawing to explain the changes in emission and solid angle in the embodiment of  FIG. 11 .  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0060]     The following describes the embodiments according to the present invention with reference to the drawings:  
         [0061]      FIG. 1  shows an embodiment where the present invention is applied to the magnetic field UHF band electromagnetic wave radiation/discharge type plasma etching apparatus. It is a cross sectional view of said plasma etching apparatus in schematic form.  
         [0062]     The process chamber  100  in  FIG. 1  is a vacuum vessel providing a vacuum of about 10 −6  Torr. An antenna  110  which emits electromagnetic wave as a plasma generating means is provided on the top thereof, and a bottom electrode  130  to mount such sample W as wafer is installed on the bottom thereof. Antenna  110  and bottom electrode  130  are installed in parallel opposite to each other. A magnetic field forming means  101  consisting of an electromagnetic coil and yoke, for example, is installed around the process chamber  100  to provide a magnetic field having a specified distribution and strength. Process gas introduced into the process chamber is made into plasma by interaction between the electromagnetic wave radiated from antenna  110  and magnetic field produced by the magnetic field forming means  101 , and plasma P is generated to treat the sample W on the bottom electrode  130 .  
         [0063]     Vacuum exhaustion and pressure control of process chamber  100  is provided by the vacuum exhaust system  104  connected to vacuum chamber  103  and pressure control means  105 , and the internal pressure can be controlled to a specified value, for example, within the range from 0.5 Pa to 4 Pa. The process chamber  100  and vacuum chamber  103  provide ground potential. The side wall  102  of process chamber  100  has its temperature controlled, for example, to 50° C. by a temperature control means (not illustrated).  
         [0064]     Antenna  110  radiating electromagnetic waves consists of a disk formed conductor  111 , dielectric  112  and dielectric ring  113 , and is held by a housing  114  as part of the vacuum vessel. Furthermore, a plate  115  is installed on the surface of the disk formed conductor  111  on the side in contact with plasma. Process gas used to provide sample etching and film formation is supplied at a specified flow-rate and mixing ratio from the gas supply means  116 . It is made uniformed inside the disk formed conductor  111  and is supplied to the process chamber  100  through numerous holes provided on plate  115 . The temperature of the disk formed conductor  111  is controlled, for example, to 30° C. by a temperature control means (not illustrated). The antenna  110  is connected through the incoming end  126  to antenna power supply system  120  comprising an antenna power supply  121 , antenna bias power supply  123  and matching circuit/filter systems  122 ,  124  and  125 . The antenna power supply  121  supplies the UHF frequency power preferably in the range from 300 MHz to 900 MHz to radiate UHF band electromagnetic waves through the antenna  110 .  
         [0065]     The antenna bias power supply  123  applies to the plate  115  the bias of a frequency of, for example, 100 kHz or several MHz to several tens of MHz through the disk formed conductor  111 , thereby controlling the reaction on the surface of plate  115 . Especially in the oxide film etching process using CF based gas, reaction of F radical and CFx radical on the surface of the plate  115  can be controlled and the percentage of radicals can be adjusted by using high purity silicon and carbon as the material of plate  115 . In the present embodiment, the plate  115  is made of highly pure silicon. Aluminum is used as material for the disk formed conductor  111  and housing, and quartz is used for the dielectric  112  and dielectric ring  113 . Distance between the bottom of the plate  115  and wafer W (hereafter called “gap”) is 30 mm to 150 mm, or preferably 50 mm to 120 mm. In the present embodiment, the frequency of the antenna power supply  121  is 450 MHz, and that of antenna bias power supply  122  is 13.56 MHz; gap is set at 70 mm.  
         [0066]     A bottom electrode  130  is mounted opposite to the antenna  110  on the bottom of the process chamber  100 . A sample W such as wafer is mounted on the top surface of the bottom electrode  130 , namely, on the sample mounting surface, and is held in position by an electrostatic chucking unit  131 . On the peripheral area of the sample W, a sample bench ring  132  made of highly pure silicon, for example, is mounted on the insulator  133 . A bias power supply  134  to supply bias electric power preferably in the range from 400 kHz to 13.56 MHz is connected to the bottom electrode  130  through matching circuit/filter system  135 , thereby controlling the bias applied to sample W. In the present embodiment, the frequency of the bias power supply  134  is 800 kHz.  
         [0067]     The following describes the measuring ports  140 A and  140 B installed to measure the state of the surface of the sample W as a major portion in the present embodiment. In the present embodiment, measuring ports  140 A and  140 B are mounted on the antenna  110  opposite to the sample W. As will be described later, the state of thin film on the surface of the sample W can be measured from the top in the vertical direction through numerous through-holes formed on plate  115 . Information on distribution inside the surface of the sample W can be obtained by placing the measuring port  140 B at the position where the peripheral portion of the sample is measured, and by mounting the measuring port  140 A at the position intermediate between the peripheral portion and the center of the sample W. It goes without saying that the position for installation of the measuring port is not restricted to two positions; peripheral and intermediate positions, as described just above. The measuring port can be installed at one position or three positions. Or other arrangement may be used; for example, the port may laid out on the circumference.  
         [0068]     For example, optical transmission means  151 A and  151 B such as the optical fiber and lens are provided on the measuring ports  140 A and  140 B. Optical information reflecting the surface state of the wafer W such as light directly coming from the plasma P or reflected light or interference light on the wafer W surface of the plasma P is sent, for example, to the camera and interference thin film gauge or measuring instrument  152  consisting of the image processing apparatus, where measurement is performed. The measuring instrument  152  is controlled by the measuring instrument control/operation means  153 , and is connected to a higher-order system. The system control means  154  monitors and controls the state of the equipment and system through the control interface  155 .  
         [0069]     The plasma etching apparatus according to the present embodiment is configured as described above. The following describes a specific process to etch the silicon oxide film, for example, using this plasma etching apparatus:  
         [0070]     Firstly, wafer W as an object for treatment is fed into the process chamber  100  from a sample introduction mechanism (not illustrated). It is then mounted and chucked on the bottom electrode  130 , and the height of the bottom electrode is adjusted as required. The gap is then adjusted to a specified value. Vacuum-exhausting is made by the vacuum exhaust system  106  inside the process chamber  100 . Meanwhile, gases required for sample W etching treatment, for example, C 4 F 8 , Ar and O 2  are fed into the process chamber  100  from the plate  115  of the antenna  110  from the gas supply means  116  at a specified flow rate and mixing ratio (for example, Ar at the rate of 400 sccm, C 4 F 8  at the rate of 15 sccm and O 2  at the rate of 5 sccm). At the same time, the pressure inside the process chamber  100  is adjusted to a specified processing pressure: for example, it is adjusted to reach 2 Pa. Meanwhile, almost horizontal magnetic field of approximate 160 gausses corresponding to the electronic cyclotron resonant magnetic field strength for frequency of 450 MHz of the antenna power supply  121  is formed close to the bottom of the plate  115  by the magnetic field forming means  101 . The UHF band electromagnetic wave is radiated from the antenna  110  from the antenna power supply  121 , and plasma is generated inside the process chamber  100  by interaction with magnetic field. Process gas is dissociated by this plasma P to generate ion and radical. Furthermore, antenna radio frequency power supply  123  and bias power supply  134  are controlled to provide the wafer W with such treatment as etching.  
         [0071]     For example, electric power of antenna power supply  121  is 1000 watts, that of antenna radio frequency power supply  123  is 300 watts, and that of the bias power supply  134  is 800 watts. Supply of electric power and process gas is terminated upon termination of etching. Then etching process completely terminates.  
         [0072]     The optical information reflecting the state of plasma radiation and wafer surface during this treatment is transmitted by the optical transmission means  151 A and  151 B through the measuring ports  140 A and  140 B, and is measured by the measuring instrument  152 . The operation is processed by the measuring instrument control/operation means  153  based on the result of measurement. Then the result is transmitted to the high-order system control means  154 , and the plasma processing apparatus is controlled through the control interface  155 .  
         [0073]     The following describes the detailed structure of the measuring port  140  with reference to FIGS.  2  to  4 :  
         [0074]      FIG. 2  is an enlarged cross sectional view representing the portion of the measuring port  140  mounted on the antenna  110  in the embodiment shown in  FIG. 1 . As described with reference to  FIG. 1 , the disk formed conductor  111  and dielectric  112  forming the antenna  110  are held by the housing  114 , and a plate  115  is installed on the disk formed conductor  111 . Numerous gas permeation holes  115 A are provided on the plate  115 . In the disk formed conductor  111 , process gas is fed into the process chamber  100  through gas permeation hole  111 A located at the position corresponding to that of gas permeation hole  115 A. The gas permeation hole  115 A provided on the plate  115 , for example, is a through-hole having an approximate diameter of 0.1 mm to 5 mm, preferably, 0.3 mm to 2 mm. The gas permeation hole  111 A provided on the disk formed conductor  111  is equal to or greater in size than the hole  115 A; for example, it has an approximate of diameter of 0.5 mm to 5 mm, preferably, 2 mm. Approximate thickness of the plate  115  is 3 mm to 20 mm. According to the present embodiment, it is 6 mm.  
         [0075]     Numerous closely packed through-holes  115 B are formed on the portion of the plate  115  corresponding to measuring port  140 , and optical transmitter  141  is installed almost in contact with the back of the plate  115  (on the surface opposite to the plasma P). It is vacuum-sealed with the housing  114  by the holding means  142  and vacuum sealing means  143  such as an O-ring and is mounted in position. Such optical transmission means  151  as an optical fiber and lens, for example, are provided on the end face of the optical transmitter  141  on the atmospheric side. Light  145 P directly coming from the plasma P, reflected light from the surface of the sample W of the plasma P, and interference light  145 W pass through the through-hole  115 B of the plate  115  such as the optical path  144  indicated by a broken line, and reaches the optical transmission means  151  through the optical transmitter  141 . They are then sent to the measuring instrument  152 , where measurement is performed.  
         [0076]     As described later, the aspect ratio of through-hole  115 B is preferred to range approximately 5 and more through within  100 .  
         [0077]     According to this embodiment, the optical transmitter  141  is made of a quartz-made cylindrical rod. The suitable diameter of the optical transmitter  141  is 5 mm to 30 mm. The present embodiment uses a diameter of 10 mm. Similarly to the gas permeation hole  115 A, the diameter of the through-hole  115 B is 0.1 mm to 5 mm, or preferably 0.3 mm to 2 mm. The diameter of the present embodiment is 0.5 mm. Multiple through-holes  115 B, or several tens of them are preferred to be installed to improve the measuring sensitivity. As will be described below, this embodiment uses about 40 holes.  
         [0078]      FIG. 3  represents one embodiment of the layout of through-holes  115 B. In this embodiment, about 40 through-holes  115 B are arranged at a pitch of 1.5 mm in the area corresponding to the end face of the optical transmitter  141  so that a regular triangle is formed at an equally spaced interval. As described above, the diameter of the through-hole  115 B is 0.5 mm in this embodiment. The exposed area ratio (percentage of the total apertures of the through-hole  115 B for the area of the end face of the optical transmitter  141 ) is about 10% (=(0.5 2 (mm 2 )×40)/(10 2 )(mm 2 )). This can provide a sufficient measuring sensitivity. It goes without saying that the through-hole arrangement is not restricted to  FIG. 3  alone. For example, these holes can be laid out so that they will cross one another, as shown in  FIG. 4 . Or they can be laid out in concentric arrangement. As suggested, holes can be laid out in various arrangements.  
         [0079]     Some space (1 mm or more, for example) must be provided between adjacent through-holes. So the exposed area ratio will be reduced as the through-hole diameter is smaller. For example, if holes having a diameter of 0.3 mm are to be laid out at a pitch of 1.3 mm (a width of 1 mm between apertures) in the area having a diameter of 10 mm, the exposed area ratio is about 5%. Measurement is also possible when the exposed area ratio is about 5%. For in-situ measurement of etching rate and others, the exposed area ratio is preferred to be about 5% or more. So from the view point of measurement sensitivity, the through-hole diameter is preferred to be about 0.3 mm or more. Meanwhile, the diameter of the through-hole  115 B is preferred to be set sufficiently smaller than the mean free path of the molecule, as will be described later. In order not to allow abnormal discharge to take place, the through-hole diameter is 0.1 mm to around 5 mm, or is preferred to be 0.3 mm to around 2 mm.  
         [0080]     If the through-hole  115 B diameter is made the same as that of gas permeation hole  115 A, there is an advantage that the cost increase can be controlled without increasing the number of processing steps of plate  115 . It goes without saying that the hole diameters need not always be made the same. The optimum value can be set according to the sensitivity and stability in measurement. Furthermore, all the diameters of the through-holes  115 B need not be made the same. For example, the hole diameter may be larger on the periphery.  
         [0081]     The optical transmitter  141  need not be completely “transparent”, namely, it need not be transmissive in the entire visible light area. Only a sufficient transmittance is required in the wavelength area to be measured. For example, use of quartz or sapphire is preferred to measure from 200 nm ultraviolet area to 800 nm near-infrared area holding the visible light area in-between. Silicon or such an optical material as ZnS which provides excellent transmittance in the infrared area can be used to measure in the infrared area. Furthermore, a thin film of Al 2 O 3  such as sapphire, for example, may be formed on the end face of the optical transmitter  141  in order to improve resistance to ion sputter and to reduce reflection factor.  
         [0082]     The measuring port  140  has the configuration described above. This configuration avoids abnormal discharge, occurrence of particle contamination or deterioration of optical performance such as transmittance at the measuring port  140 , and ensures stable measurement for a long time. The following describes the reasons:  
         [0083]     If a hole having a diameter of about 10 mm is formed on the top electrode, local abnormal discharge will occur to the hole due to the hollow cathode or plasma will enter the hole to damage the interior, as referred to above as a problem of the prior art. By contrast, the through-hole diameter is set to as small as 0.5 mm in the present embodiment. This does not allow abnormal discharge to occur in the through-hole or the plasma to enter the through-hole. After experiments, the present inventors have verified that measurement can be made without above-mentioned abnormal discharge by setting the diameter of the through-hole  115 B to about 5 mm or less, or more preferably to about 2 mm or less. Furthermore, the optical transmitter  141  is installed at the position almost in contact with the back of the through-hole  115 B; therefore, there is no space between the through-hole  115 B and optical transmitter  141  which may allow abnormal discharge to occur. Thus, abnormal discharge does not occur in this position, either.  
         [0084]     The present embodiment ensures stable measurement for a long time without reaction product depositing on the end face of the optical transmitter  141 , or transmittance being deteriorated by repeated discharge.  
         [0085]     This is because the diameter of the through-hole  115 B is set at a value sufficiently smaller than mean free path of molecules, in the first place. Working pressure in the process chamber is about 0.5 Pa to 4 Pa. The mean free path of molecules in this case is about 5 mm to 30 mm (Ar molecule at 25° C.). For this, the diameter D h  of the through-hole  115 B is about 0.5 mm, so the ratio with the mean free path of the molecule λ, namely, the value of D h /λ is about D h /λ=0.02 to 0.1. As has been described, the diameter D h  of the through-hole  115 B is set at a value sufficiently smaller than the mean free path of the molecule: thus, the possibility is very small that gas molecule in plasma P enters the through-hole  115 B.  
         [0086]     Secondly, the diameter of the through-hole  115 B is 0.5 mm in the present embodiment. By contrast, depth as a plate thickness is set at 6 mm. As described above, the aspect ratio (=depth/diameter) is 10 or more and the hole has a sufficient depth. So the probability of the radical passing through the through-hole  115 B and depositing on the end face of the optical transmitter  141  is kept at a sufficient small rate.  
         [0087]     The possibility of which the radical accumulates on the end face of the optical transmitter  141  is proportional to solid angle dΩ making allowance of the through-hole  115 B (hole diameter: D, length: L) on the end face.  FIG. 5  shows dependency of the aspect ratio (AR=L/D) of solid angle dΩ. The solid angle dΩ is inversely proportional to the square of AR, as shown in  FIG. 5 . If the solid angle dΩ is aspect ratios of 5 and more, the solid angle dΩ is below 1/100 of solid angle π on the plane surface. So the possibility of the radical reaching onto the end face of the optical transmitter  141  is kept at a sufficient small rate. In order to get impurity prevention effect on the end face of the optical transmitter  141 , the aspect ratio of the through-hole  115 B is preferred to range from 5 to 100.  
         [0088]     In addition, the plate  115  is heated by plasma, and the surface temperature rises to 100° C. or more. So the possibility of reaction product depositing inside the through-hole  115 B is small. Deposits do not attach and grow inside the through-hole  115 B to reduce the actual transmission area of the through-hole.  
         [0089]     Thirdly, bias voltage of several tens of volts to hundreds of volts is applied to the plate  115 ; therefore, ion in plasma will be drawn toward the depth of the through-hole  115 B. Thus, ion having an energy of several tens of eV to hundreds of eV may reach the end face of the optical transmitter  141 , although this probability is low. So even if reaction products have deposited on the end face of the optical transmitter  141 , they will be removed quickly by the ion sputtering effect. Optical performance due to damaged end face of the optical transmitter  141  can be reduced sufficiently by manufacturing the optical transmitter  141 , for example, with the quartz or sapphire which is highly resistant to plasma.  
         [0090]     The following can be said as an overall result of these effects: The optical transmitter  141  does not have reaction products deposited on the end face or the surface roughened, and light transmission is kept constant, despite repeated discharge. This ensures stable measurement for a long time.  
         [0091]     Regarding above-mentioned three factors, authors of the present invention have continued experimental studies, and have verified that stable measurement can be made without abnormal discharge by setting the through-hole diameter to 0.5 mm and plate thickness to 6 mm, with respect to the case where the oxide film on the surface of example W is subjected to etching processing, as explained in the present embodiment.  FIG. 5  is a schematic diagram of the signal waveforms gained in the present experiment. Interference signals resulting from changes in interference due to light reflected from the oxide film surface and underlying layer are obtained with the progress of etching treatment. In-situ measurement of the etching rate is possible from this cycle. In addition, plasma emission signal as light directly coming from plasma is also gained. These interference signals and plasma emission signals are changed simultaneously at the end point of the etching processing. It is clear that the state of the surface and changes of plasma composition at the end point of the etching processing can be detected. These signals can be detected with sufficiently high precision over at least several tens of hours of discharge. Furthermore, the number of particle contamination having occurred during this time does not exceed 20 (0.2 microns or more). Thus, stable measurement has been confirmed.  
         [0092]     As can be seen from  FIG. 2 , the optical transmitter  141  is held and is vacuum sealed by the holding means  142  and vacuum sealing means  143  alone. This allows easy replacement by removing the holding means  142  when the process chamber is released to the atmosphere. Consequently, if deposits are gradually accumulated on the end face of the optical transmitter  141  or the surface is roughened by ion sputtering, depending on process conditions, then easy replacement of the optical transmitter  141  is possible when wet cleaning is made by releasing the plasma processing apparatus to the atmosphere. This minimizes the down time at the time of wet cleaning (complete cleaning).  
         [0093]     Using  FIGS. 6 and 7 , the following describes another embodiment, the detection optical system which provides measurement through the through-hole  115 B. In the embodiment shown in  FIG. 2 , the optical transmission means  151  is made of an optical fiber. Light  145 P directly coming from plasma P located in the middle of the optical path  144 , and reflected light and interference light  145 W on the surface of sample W of plasma P all enter the optical fiber for measurement. Especially when radical components in plasma are changed with the progress of etching processing, this arrangement is suited to detect such a change. Meanwhile, for sensitive detection of the change in the state of surface such as thickness of a thin film of sample W, light directly emitted from plasma P forms a noise component for measurement. So failure of its detection may be preferable to the measuring system. In such cases, an image forming optical system using a lens or the like is preferably used for the optical system.  
         [0094]      FIG. 7  shows one embodiment of such an optical system. In the present embodiment, the optical transmission means  151  uses a lens  151 A as an image forming means. Optical information from the surface of the sample W is made to form an image on the detecting element  152 A of the measuring instrument  152  such as the camera and image processing apparatus. Light  145 P directly coming from plasma P is cut off by arranging the filter  152 B including the diaphragm and pin hole immediately before the detecting element  152 A. Only the optical information  145 W from the surface of sample W can be transmitted to the detecting element  152 A. This improves the sensitivity of detection and measurement of the state on the surface of sample W.  
         [0095]     In the present embodiment, a through-hole  115 B is located at some midpoint in the optical system. It may appear difficult to measure the surface of sample W because the optical path is cut off. However, when the aspect ratio (=depth/diameter) of the through-hole  115 B is set to an appropriate value in relation to the expansion of light from the surface of sample W, measurement is possible without the through-hole  115 B cutting off the optical path.  
         [0096]     This will be described in greater details with reference to  FIG. 8 .  FIG. 8  is a schematic diagram showing only the portion related to the measurement and optical system of the embodiment shown in  FIG. 7 . The following shows the symbols in the Figure.  
         [0097]     D h : Diameter of through-hole  115 B  
         [0098]     L h : Depth of through-hole  115 B (equal to the thickness of plate  115 )  
         [0099]     L g : Distance between sample W and plate  115  (equivalent to gap described in embodiment of  FIG. 1 )  
         [0100]     L z : Distance from sample W to image forming means  151 A  
         [0101]     (L z -L g  is equivalent to thickness of antenna portion described in embodiment of  FIG. 1 )  
         [0102]     D z : Effective diameter of image forming means (lens according to the present embodiment)  151 A  
         [0103]     (almost equivalent to the diameter D r  of optical transmitter  141 )  
         [0104]     As already described, each of the actual values in the present embodiment is as follows: D h =0.5 mm (diameter), L h =6 mm, L g =70 mm and D r =D z =10 mm (diameter). Furthermore, the thickness of antenna portion is L z −L g =80 mm, so L z =150 mm.  
         [0105]     Here the divergence angle  0  from the surface of sample W is expressed by θ z =tan−1 ((D z /2)/L z ), based on ratio L z /D z  between distance L z  of the image forming means  151 A from the sample W and effective diameter D z . In the present embodiment, θ z =1.90. L z /D z  equivalent to the divergence angle of beam from the surface of sample W is about 15. By contrast, prospective angle θ h  based on the aspect ratio L h /D h  of the through-hole  115 B is defined as θ h =tan−1 ((D h /2)/L h ); then θ h =2.3°. This value is a little smaller than θ z =1.9°. As described above, if the beam divergence angle θ z  from the surface of sample W is set to a value a little smaller than the prospective angle θ h  of the through-hole  115 B, beam from the surface of sample W will reach the image forming means  151 A without being cut off by the through-hole  115 B, and will form an image on the detecting element  152 A.  
         [0106]      FIG. 8  shows how this was verified by experiment. Image Img 1  of a character of several mm square was depicted on the surface of sample W. The surface of sample W was observed during plasma processing. Then it was found out that Image Img 1  was optically transmitted to the surface of sample, and image Img 2  is displayed on the display screen  152 C of measuring instrument  152 . This image Img 2  was slightly affected on its periphery by the eclipse resulting from by the through-hole  115 B (expressed by the broken line in concentric circle in  FIG. 8 ), but information on the original image Img 1  is sufficiently retained. It has a sufficient quality to measure the state of thin film on the surface of sample W. Oxide film of the surface of sample W was subjected to etching processing by plasma P. Then interference signals due to light reflected from the oxide film surface and underlying layer were obtained corresponding to changes in the thickness of oxide film according to the progress of etching treatment, similarly to what was shown in  FIG. 6 . This experimentally verified that in-situ measurement of the etching rate is possible.  
         [0107]     Incidentally, the embodiments mentioned above use the optical transmitter  141  made of a quartz-made rod. This is only one example. It goes without saying that other arrangements are also possible. The following describes another embodiment with reference to  FIG. 9 .  FIG. 9  shows the arrangement where the rod as an optical transmitter  141  was made hollow with its interior removed, and an optical fiber was inserted therein as an optical transmission means  151 . In  FIG. 9 , a gas supplier  111 B is provided on the portion corresponding to the through-hole  115 B of the disk formed conductor  111 . So even under the process conditions where reaction products are likely to deposit on the end face of the optical transmitter  141 , it is possible to prevent reaction products from depositing since process gas is also supplied from gas supplier  111 B. What is more, the optical path for the light passing through the optical transmitter  141  can be shortened as shown in  FIG. 9 . This has the effect of reducing loss of optical information.  
         [0108]     Next, description will be made of the embodiment of detecting the change of a quantity of reaction productions, causing the occurrence of particle contamination, accumulating at the periphery of the susceptor and on the sidewall inside the vacuum chamber, with reference to  FIG. 10 . This description will be omitted of the identical parts to those in  FIG. 1 . The optical reflectors  169 A and  196 B are installed on the sidewall of the insulator  133  covering the susceptor or on the sidewall inside the vacuum chamber  102 . A measuring port  160 A or  161 A having the through-hole of the present invention is installed at a position opposite to these reflectors. Measurement is made by transmitting the change of reflected or interfered light from the reflectors to an optical measuring instrument  162  via the optical transmission means  161 A or  161 B. The optical measuring instrument  162  is controlled by a measuring instrument control means and calculation means  163 . When measured amount of reflected or interfered light has changed substantially, a display means  164  issues a warning.  
         [0109]     According to this embodiment, if reaction productions, causing occurrence of foreign substance, which have accumulated at the periphery of the susceptor or on the sidewall inside the vacuum chamber, peel off from the optical reflector, changing a quantity of light from the optical reflector takes place. So detecting this change enables a warning to be generated for the purpose of prevention of frequent occurrence of foreign substance. This warning leads to decide a proper time of cleaning out the plasma processing apparatus, enabling to prevent abnormality from occurring during operation of the plasma processing apparatus.  
         [0110]     Next, description will be made of the embodiment of detecting consumption of the plate  115  for diagnosing the system during a continuous etching processing, with reference to  FIG. 11  and  FIG. 12 .  
         [0111]     High frequency power is applied to the plate  115  through the antenna power supply  121  during wafer-etching processing, so the plate is etched and consumed.  FIG. 11  shows the processing-time dependency of the cross section of gas effluence port  115 A on the plate  115 . As shown in  FIG. 11 , longer time of etching processing decreases the thickness of the plate and increases the diameter of the hole on the vacuum chamber side. When further etching processing is continued, the gas effluence port  115 A becomes a through-hole of approximately 4.5 mm in thickness and of approximately 1.3 mm in hole diameter. Such a gas effluence port increases area by 10.6 times in initial etching processing. So this makes a supply state of etching gas change significantly, makes abnormal discharge induce in the gas effluence port, or makes occurrence of foreign substance more possible, causing a serious damage to continuous etching processing.  
         [0112]     In  FIG. 12 , the solid line represents changes in solid angle dΩ, which are disposed under the shape of gas effluence port. In addition, the changes in a quantity of radiation at this time are given by marking some black circles.  FIG. 12  shows that a solid angle dΩ does not indicates a great change within 400 hours of processing time, but it increases rapidly when processing time exceeds 500 hours. It is also found that the changes in a quantity of radiation has a similar tendency. This proves that, when processing time exceeds 500 hours, this makes more possible occurrence of a serious damage to continuous etching processing.  
         [0113]     According to this embodiment, monitoring a quantity of radiation measured through a measuring port with the through-hole of the present invention detects consumption of plate  115 , enabling to prevent abnormality from occurring during operation of the plasma processing apparatus.  
         [0114]     All of said embodiments have referred to the magnetic field UHF band electromagnetic wave radiation/discharge type plasma processing apparatus. However, for example, 2.45 GHz microwave or VHF band wave ranging from several tens of MHz to 300 MHz in addition to the UHF band wave can be radiated as electromagnetic wave. Magnetic field strength has been explained for the case of 160 gausses which signify electronic cyclotron resonant magnetic field strength for 450 MHz. The resonant magnetic field need not necessarily be used; a stronger magnetic field or weak magnetic field on the order of several tens of gausses or more may be used. Furthermore, it goes without saying that the present invention can also be applied to capacitatively coupled parallel plate plasma processing apparatus and magnetron type plasma processing apparatus or inductively coupled plasma processing apparatus, in addition to electromagnetic wave radiation/discharge type plasma processing apparatus.  
         [0115]     The structure of the upper plate opposite to the wafer is comparatively simple especially in the plasma processing apparatus where radio frequency is applied to the bottom electrode and a ground plate is mounted on the top. This makes it easy to install a measuring port similar to that of the present invention. Furthermore, in the parallel plate plasma processing apparatus where radio frequency is applied to the top electrode to generate plasma, several kilowatts of high power radio frequency is applied to the top electrode. So abnormal discharge can occur if a hole or void is provided on the top electrode. According to the structure of the present invention, however, abnormal discharge or other failure does occur to the measuring port. Especially in the narrow electrode type parallel plate plasma system, the space between the top and bottom electrodes is small, and so it is very difficult to get from the side the information on the wafer surface and the plasma between top and bottom electrodes. This indicates that the present invention provides considerable advantages.  
         [0116]     Meanwhile, in the inductively coupled plasma processing apparatus (ICP), the state of wafer surface can be measured to some extent when a transparent quartz is used for the top plate. The measuring port according to the present invention can be applied when an alumina-made dome and silicon plate or the like is used. To put it more specifically, those skilled in the art can easily design the arrangement where multiple closely packed holes are formed on an alumina-made plate or the like as shown in  FIG. 3 , and a vacuum sealing quartz plate is installed on the back. In the ICP type plasma processing apparatus, the top plate may have to be heated to a high temperature of 150° C. or more, for example, in order to get the process characteristic and reproducibility. It goes without saying that the present invention is applicable even under such temperature conditions.  
         [0117]     The object to be treated was a semiconductor wafer in each of said embodiments, and etching of the semiconductor wafer was mentioned. However, the present invention is not restricted to them. For example, the present invention is also applicable when the object for treatment is a liquid crystal substrate. In addition, treatment is not restricted only to etching. For example, the present invention is also applicable to sputtering and CVD treatment.  
         [0118]     As described above, the present invention ensures a stable and a long-term measurement of the state of the sample surface and plasma or the state of the wall of the vacuum chamber with high accuracy from the external vacuum chamber, without allowing abnormal discharge or particle contamination to occur.  
         [0119]     For example, the present invention ensures a stable and precision measurement of the state of thin film of the plasma and sample surface from above the sample W or from the upwardly inclined position for a long time even on a mass production level, in the opposite plate type structure where the antenna and electrode are installed opposite to the wafer surface, without allowing abnormal discharge or particle contamination to occur. As a result, it enables detection of the end point in etching process and in-situ monitoring of etching/film forming rate and uniformity. Thus, the present invention permits a more advanced process control and improves reproducibility of treatment and stability in treatment, and provides a plasma processing apparatus contributing to improvement of the apparatus availability factor and productivity.