embodiments of the present invention will now be described with reference to the accompanying drawings . first , a description will be given of the first embodiment wherein the present invention is applied to an electron beam excitation ion source . as is shown in fig1 the ion source 1 comprises an electron generation chamber 2 which is located in the upper region thereof and which is like a rectangular container whose sides are of the order of several centimeters in length . the electron generation chamber 2 is made of a high - melting point conductive material , such as conductive ceramics , and has an insulating plate 3 which closes opening at one side of that member . a u - shaped wire 4 , formed of e . g . tungsten , is arranged inside the electron generation chamber 2 , such that its end portions are supported by the insulating plate 3 . a discharge gas - introducing port 5 is formed in the ceiling of the electron generation chamber 2 so as to introduce a discharge gas ( e . g ., ar ) into the electron generation chamber 2 . a circular hole 6 is formed in the bottom of the electron generation chamber 2 so as to extract electrons from the plasma generated in the electron generation chamber 2 . a plate - like insulating member 8 , having a through - hole 7 , is arranged below the electron generation chamber 2 such that the through - hole 7 is located under the circular hole 6 . a plasma cathode chamber 9 formed by a high - melting point conductive member , such as conductive ceramics , is located below the insulating member 8 in such a manner that the plasma cathode chamber 9 is spaced from the electron generation chamber 2 by a certain distance . the bottom of the plasma cathode chamber 9 ia made by a porous electrode 11 , and a plurality of through - holes 10 are formed in the porous electrode 11 , with the through - holes 10 corresponding in location to the through - hole 7 . located under the porous electrode 11 is an ion generation chamber 13 . the ion generation chamber 13 is spaced from the plasma cathode chamber 9 by means of an insulating member 12 . the ion generation chamber 13 is in the form of a container and formed of a high - melting point conductive material , such as conductive ceramics . the internal space of the ion generation chamber 13 defines a cylindrical region whose diameter and height are of the order of several centimeters . a bottom plate 15 is secured to the bottom of the ion generation chamber 13 , with an insulating ring member 14 interposed . a processing gas - introducing port 16 is formed in one of the side walls of the ion generation chamber 13 . through the processing gas - introducing port 16 , a processing gas ( e . g ., bf 3 ) is supplied into the interior of the ion generation chamber 13 , so as to generate desirable ions inside the ion generation chamber 13 . an ion - extracting slit aperture 17 is formed in such a manner as to oppose the processing gas - introducing port 16 . the through - holes 10 formed in the porous electrode 11 are located closer to the ion - extracting port 17 than the central axis of the cylindrical internal region of the ion generation chamber 13 , so that the plasma generated in the internal region is most active in the neighborhood of the outlet port 17 . as the conductive ceramics for forming the ion generation chamber 13 , the electron generation chamber 2 and the plasma cathode chamber 9 , it is preferable to use either &# 34 ; bn composite ec &# 34 ; ( trade name ) commercially available from electrochemical industry co ., ltd . or conductive ceramics containing sic , tic , tib 2 and zrb 2 as major ingredients . the filament 4 mentioned above is connected to a filament power source vf and heated thereby . the electron generation chamber 2 is connected to a discharge power source vd via a resistor r , and the porous electrode 11 is connected directly to the discharge power source vd . the region between the porous electrode 11 and the ion generation chamber 13 is connected to an acceleration power source va , which is arranged in series with the discharge power source vd . desirable ions are produced by the above - mentioned ion source as follows . by a magnetic field - generating means ( i . e ., magnet 19 ), a magnetic field is generated such that it serves to guide electrons in the electron extraction direction , as indicated by arrow bz in fig1 . simultaneously , the filament power source vf , the discharge power source vd and the acceleration power source va apply predetermined voltages to the respective portions of the ion source . a discharge gas ( e . g ., ar ) is introduced from the discharge gas - introducing port 5 into the electron generation chamber 2 at a flow rate of 0 . 4 sccm , for example . plasma is generated , with electric discharge being caused in the electron generation chamber 2 . the electrons contained in the plasma are accelerated by an electric field , and are supplied into the plasma cathode chamber 9 after passing through the circular hole 6 and through - hole 7 . as a result , plasma is generated in the plasma cathode chamber 9 as well . the electrons contained in the plasma generated in the plasma cathode chamber 9 are accelerated by electron acceleration voltage va , pass via the through - holes 10 of the electrode 11 , and are guided to the region in the vicinity of the slit aperture 17 . the inside of the ion generation chamber 13 is supplied with a raw gas ( e . g ., bf 3 ) beforehand . the raw gas is supplied through the port 16 at a predetermined flow rate ( e . g ., 0 . 7 sccm ). the electrons flowing into the ion generation chamber 13 efficiently collide against the molecules of the raw gas by means of the reflecting electrode 15 and magnetic field , thus generating dense plasma . the ions are guided from the ion generation chamber 13 through the slit aperture 17 by means of an ion extraction electrode ( not shown ), so as use them for such processing as ion implantation of semiconductor wafers . inside the ion generation chamber 13 , a high - corrosive raw gas ( e . g ., bf 3 ) is excited upon electron irradiation , so that the plasma generated in the ion generation chamber 13 is made more reactive . in the ion source of the first embodiment , however , the ion generation chamber 13 is formed with conductive ceramics , as mentioned above . therefore , the walls of the ion generation chamber 13 are not much damaged , in comparison with the conventional case . further , the ion extraction electrode is not much contaminated , due to flying particles . accordingly , the apparatus need not be maintained frequently , and a high rate of operation of the apparatus and satisfactory productivity are ensured . in the first embodiment mentioned above , not only the ion generation chamber 13 but also the electron generation chamber 2 and the plasma cathode chamber 9 are formed of conductive ceramics . however , the electron generation chamber 2 and the plasma cathode chamber 9 need not be formed of conductive ceramics ; they may be formed of a high - melting point conductive metal , such as molybdenum . the second embodiment of the present invention will be described , with reference to fig2 a through 2c . of the structural components employed in the second embodiment , those which are substantially similar to the structural components will be referred to by using the same reference numbers , and explanation of such structural components will be omitted . in the second embodiment , the peripheral wall of the ion generation chamber 13 is of a double structure made up of an outer cylinder 13a formed of a metal such as molybdenum , and an inner cylinder ( liner ) 13b formed of conductive ceramics . the bottom wall of the ion generation chamber 13 is formed of conductive ceramics , but may be of a double structure , just like the peripheral wall . as is shown in fig2 c , the porous electrode 11 employed in the second embodiment has a large - diameter through - hole 10a located in the center , and a plurality of small - diameter through - holes 10b ( e . g ., eight through - holes ) located around the large - diameter through - hole 10a . the small - diameter through - holes 10b are spaced from each other at predetermined intervals . the third embodiment wherein the present invention is applied to a freeman type ion source will be described , with reference to fig3 . as is shown in fig3 the freeman type ion source 20 comprises a cylindrical chamber 21 , and a rod - shaped filament 23 extends through the interior of the cylindrical chamber 21 . the two ends of the rodshaped filament 23 are supported by insulating support members 22 . an ion extraction opening 24 is formed in the front wall of the chamber 21 , and an ion extraction electrode 25 is arranged outside the chamber 21 such that the ion extraction electrode 25 opposes the ion extraction opening 24 . a raw gas - introducing opening 26 is formed in the rear wall of the chamber 21 . the inner wall of the chamber 21 is covered with a liner 27 formed of conductive ceramics . the liner 27 is formed of either &# 34 ; bn composite ec &# 34 ; ( trade name ) commercially available from electrochemical industry co ., ltd . or conductive ceramics containing sic , tic , tib 2 and zrb 2 as major ingredients . in the freeman type ion source 20 mentioned above , the filament 23 is applied with a predetermined voltage by means of a filament power source ( not shown ), such that a current in the range of e . g . 50 to 200a flows through the filament 23 . further , a discharge voltage of the order of e . g . 80 to 100v is applied between the filament 23 and the chamber 21 by means of a discharge power source 28 . a predetermined raw gas ( e . g ., bf 3 ) is introduced into the chamber 21 through the raw gas - introducing opening 26 , so that the raw gas is ionized due to the electric discharge occurring in the chamber 21 . the ions produced by the electric discharge are extracted from the ion extraction opening 24 by means of the ion extraction electrode 25 , so as use them for such processing as ion implantation of semiconductor wafers . in the third embodiment , the discharge power source 28 applies a discharge voltage such that the potential of the wall of the chamber 21 is higher than that of the filament 23 . therefore , the ions are attracted to the inner wall of the chamber 21 . however , the inner wall of the chamber 21 is not damaged by a corrosive gas ( bf 3 ) since it is covered with the liner 27 formed of conductive ceramics , as mentioned above . thus , the advantages of the third embodiment are similar to those of the first and second embodiments . in the third embodiment , the entirety of the chamber 21 may be formed of conductive ceramics , instead of covering the inner wall with the liner 27 . the fourth embodiment wherein the present invention is applied to a plasma etching apparatus will be described , with reference to fig4 . as is shown in fig4 the plasma etching apparatus 30 comprises a chamber 31 which is formed of e . g . quartz and the interior of which can be hermetically sealed . inside the chamber 31 , upper and lower electrodes 32 and 33 , which are in the form of disks , are arranged in parallel in such a manner as to face each other . the lower electrode 33 is adapted to hold a semiconductor wafer 34 ( i . e ., an object to be treated ) mounted thereon . the upper and lower electrodes 32 and 33 are formed of conductive ceramics similar to those mentioned with reference to the first to third embodiments , and are connected to a high - frequency power source 35 . a gas - introducing pipe 36 and a gas - discharging pipe 37 are connected to the chamber 31 , so that a gaseous atmosphere of predetermined pressure can be produced inside the chamber 31 . the semiconductor wafer 34 is etched by the procedures below . with the chamber 31 opened by means of an opening / closing mechanism ( not shown ), the semiconductor wafer 34 is placed on the lower electrode 33 . subsequently , the chamber 31 is closed by the opening / closing mechanism , so as to hermetically seal the chamber 31 . thereafter , the gas in the chamber 31 is discharged through the gas - discharging pipe 37 , and a predetermined processing gas ( i . e ., an etching gas ) is introduced into the chamber 31 through the gas - introducing pipe 36 , until the pressure in the chamber 31 reaches a predetermined value . power having a predetermined frequency ( e . g ., 2 . 45 ghz , 13 . 75 mhz , 435 khz , etc .) is applied between the upper electrode 32 and the lower electrode 33 by the high - frequency power source 35 , so that the processing gas in the chamber 31 is converted into plasma . the semiconductor wafer 34 is etched , with the plasma acting thereon . in the fourth embodiment mentioned above , the upper and lower electrodes 32 and 33 are formed of conductive ceramics similar to those mentioned with reference to the first to third embodiments . therefore , they are prevented from being damaged by ions or a corrosive gas . thus , the advantages of the fourth embodiment are similar to those of the first to third embodiments . the fifth embodiment wherein the present invention is applied to a plasma cvd apparatus will be described , with reference to fig5 . as is shown in fig5 the plasma cvd apparatus 40 comprises a cylindrical chamber 41 which is formed of e . g . quartz and the interior of which can be hermetically sealed . inside the chamber 31 , electrode plates 42 , which are in the form of disks , are arranged in parallel like shelves . the electrode plates 42 are connected to a high - frequency power source 43 in such a manner that the adjacent two electrode plates 42 are connected to the opposite - polarity terminals of the power source 43 . more specifically , if the uppermost electrode plate 42 is connected to the positive terminal of the power source 43 , the second electrode plate 42 is connected to the negative terminal of the power source 43 , the third electrode plate 42 to the positive terminal , the fourth electrode plate 42 to the negative terminal , and the like . the electrode plates 42 are formed of conductive ceramics , as in the first to fourth embodiments . inside the chamber 41 , a processing gas - introducing region 44 and a gas - discharging region 45 are defined such that they oppose each other , with the electrode plates 42 located therebetween . as indicated by the arrows in fig5 a predetermined processing gas ( i . e ., a cvd gas ) is made to flow through the region between the adjacent electrode plates 42 . in the fifth embodiment mentioned above , a semiconductor wafer 46 ( which is an example of an object to be treated ) is placed on each electrode plate 42 , except on the uppermost one . a gas in the chamber 41 is discharged from the gas - discharging region 45 , and the processing gas is introduced from the processing gas - introducing region 44 . with the processing gas flowing through the region between the adjacent electrode plates 42 , high - frequency power is applied between the adjacent plates 42 by the high - frequency power source 43 . due to the application of the high - frequency power , the processing gas flowing between the electrode plates 42 is converted into plasma , with the result that a cvd film is formed on each semiconductor wafer 46 . the electrode plates 42 employed in the fifth embodiment are formed of conductive ceramics similar to those mentioned with reference to the first to fourth embodiments . since , therefore , they are prevented from being damaged by ions , the advantages of the fifth embodiment are similar to those of the foregoing embodiments . when the foregoing embodiments were described , reference was made to the case where the present invention was applied to an electron beam excitation ion source , a freeman type ion source , a plasma etching apparatus , and a plasma cvd apparatus . however , the present invention is in no way limited to such embodiments . it can be applied to any type of apparatus as long as the apparatus utilizes plasma . it can be applied to an x - ray source , for example . as has been described , the damage due to ions can be suppressed in a plasma processing apparatus embodying the present invention . since the apparatus need not be frequently maintained , a high rate of operation of the apparatus and satisfactory productivity are ensured . additional advantages and modifications will readily occur to those skilled in the art . therefore , the invention in its broader aspects is not limited to the specific details , and representative devices , shown and described herein . accordingly , various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents .