Abstract:
Disclosed is an ashing apparatus and its method of manufacture wherein decrease in processing efficiency is suppressed. Specifically, a shower plate is arranged to face a substrate stage on which a substrate is placed, and diffuses oxygen radicals supplied into a chamber. A metal blocking plate is arranged between the shower plate and the substrate stage and has a through hole through which oxygen radicals pass. In addition, the metal blocking plate has a first layer, which is made of a metal same as the one exposed in the substrate, on the surface facing the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ashing device for performing ashing (incineration) to remove organic materials from a substrate. 
     BACKGROUND OF THE INVENTION 
     In the prior art, to form an integrated circuit on a semiconductor substrate, a resist film on which is formed a circuit pattern is arranged on the surface of a semiconductor substrate. Then, layers under the resist film, such as an insulation film, a semiconductor film, or a metal film, are etched through the resist film. The resist film is removed from the substrate surface after ending the etching process. One example of a method for removing the resist film is a dry processing method for ashing (incinerating) the resist film using the plasma of reactive gas, mainly oxygen plasma. 
     The dry processing method causes reaction of active species (radicals), mainly oxygen radicals, generated in the plasma of the reactive gas, in the resist film applied to the substrate, to decompose and vaporize the resist film to CO 2  and H 2 O for removal. Patent document 1 discloses an example of a plasma ashing device for removing a resist film through the dry processing method. This ashing device will be described with reference to  FIG. 7 . 
     As shown in  FIG. 7 , an ashing device includes a chamber (processing chamber)  1 , the upper part of which is coupled to a feed tube  2 . The feed tube  2  is connected to a plasma chamber (not shown) which generates plasma. A shower plate  3 , which includes a plurality of through holes, is arranged at the lower end of the feed tube  2  facing toward a substrate stage  4 . A cylindrical diffusion prevention wall  5  is attached to an upper inner surface of the processing chamber  1  so as to extend around the shower plate  3 . A high frequency power supply  6  is connected to the substrate stage  4 . A ventilation port  7  is formed at the bottom of the chamber  1 . 
     The ashing process performed by the ashing device of  FIG. 7  will now be described. First, a substrate (wafer) W arranged in the chamber  1  is mounted on an upper surface of the substrate stage  4 . The interior of the chamber  1  is depressurized, and high frequency voltage is applied to the substrate stage  4 . Then, gas containing oxygen radicals is supplied to the chamber  1  through the feed tube  2 . The gas containing oxygen radicals flows through the through holes of the shower plate  3  and reaches the substrate W. The gas flowing outward from the shower plate  3  is guided by the diffusion prevention wall  5  towards the substrate W. A resist film (not shown) formed on the upper surface of the substrate W is decomposed and vaporized by the oxygen radicals contained in the gas and then discharged from the ventilation port  7 . 
     In the integrated circuit on the semiconductor substrate, circuit elements such as transistors are connected by a metal wiring of aluminum (Al), copper (Cu), or the like. Some integrated circuits have connection pads of which surfaces are covered by gold (Au) or the like or connection terminals formed from solder. Thus, when manufacturing the semiconductor substrate, during the ashing of the resist film, the metal wiring may be exposed and gold or solder may be formed on the surface. In such a case, the exposed metal material is sputtered by chemical reactions or physical reactions. This scatters metal atoms, and the metal atoms collect on the inner walls of the chamber  1 , that is, the lower surface of the shower plate  3  and the inner circumferential surface of the diffusion prevention wall  5 . If the ashing process is continued in such a state, the metals collected on the inner walls of the chamber  1  bond with the oxygen radicals that should be guided to the substrate W. This oxidizes the metal surface and increases the amount of deactivated oxygen radicals. In other words, the metal collected on the inner wall of the chamber  1  increases the amount of deactivated oxygen radicals. As a result, the amount of oxygen radicals that reaches the substrate W decreases, and the depth (ashing rate) of the resist film that can be processed during the same time decreases. Furthermore, the metal atoms scattered from the substrate W are collected on the inner walls of the chamber  1  in a non-uniform manner. This lowers the uniformity of the ashing rate in the surface of the substrate W. The inventors of the present invention have confirmed that the metals scattered from the substrate W decrease the ashing rate and lowers the in-surface uniformity through experimental results, which are described below. 
       FIGS. 9 and 10  are graphs showing the measurement values of the ashing depth in the substrate W. Referring to  FIG. 8 , the measurement values indicate the ashing depths from the surface of the resist film at forty-nine measurement points on the substrate W, which are set in order from the center of the substrate W in the circumferential direction and the radial direction. In  FIGS. 9 and 10 , the black circles represent the measurement values taken when performing the ashing process after the chamber  1 , the shower plate  3 , and the diffusion prevention wall  5  are all washed. The black squares represent the measurement values taken when performing the ashing process again using the used shower plate  3  and diffusion prevention wall  5 . The black triangles represent the difference between the measurement value represented by the black circles and the measurement values represented by the black squares. 
       FIG. 9(   a ) is a graph showing the measurement results of when a used shower plate  3  and diffusion prevention wall  5 , which were used during a previous ashing are set in a new chamber  1 , and re-ashing is performed on the substrate W from which copper is exposed under a first ashing condition (processing condition A).  FIG. 9(   b ) is a graph showing the measurement result of when the same process as  FIG. 9(   a ) is performed under a second ashing condition (processing condition B), which differs from the first ashing condition.  FIG. 10(   a ) is a graph showing the measurement results of when a used diffusion prevention wall  5 , which were used during a previous ashing are set in a new chamber  1 , and re-ashing is performed on the substrate W from which gold is exposed under processing condition A.  FIG. 10(   b ) is a graph showing the measurement result when the same process as  FIG. 10(   a ) is performed under processing condition B. The processing time is the same for each case (30 seconds). 
     As apparent from  FIGS. 9 and 10 , when the shower plate  3  and the diffusion prevention wall  5  of the ashing device that have processed a substrate, from which metal (copper, gold) was exposed, are set in a chamber  1 , which has been washed, and the ashing process is performed (refer to black squares), the ashing depths all decrease compared to when the ashing process is performed in the ashing device in which the chamber  1 , the shower plate  3 , and the diffusion prevention wall  5  are all washed (refer to black circles). In particular, in  FIG. 9(   a ), the ashing depths of the measurement points  1  to  9  and the measurement points  26  to  49  under the condition represented by the black squares are significantly decreased, and in  FIG. 10 , the ashing depths of the measurement points  26  to  49  under the condition represented by the black squares decrease significantly. In the case of the condition represented by the black squares, a large amount of the oxygen radicals that should reach the measurement points  1  to  9  and  26  to  49  are supplied toward the measurement points via the shower plate  3  or the diffusion prevention wall  5  on which metals are collected. It is thus assumed that the metals collected on the shower plate  3  and the diffusion prevention wall  5  deactivate a large amount of oxygen radicals thereby significantly decreasing the amount of oxygen radicals that reach the measurement points  1  to  9  and  26  to  49  and significantly decreasing the ashing depth at such measurement points. 
     This also shows that the amount of metal collected in the path of the oxygen radicals (shower plate  3 , diffusion prevention wall  5 , etc.) varies the amount of oxygen radicals that reach each measurement point. This, in turn, varies the ashing depth at each measurement point. Actually, as apparent from the results shown by the black squares in  FIGS. 9 and 10 , the ashing depth varies in the surface of the substrate W when the metal distribution state on surfaces facing toward the substrate W is non-uniform, such as when metals are not collected in the chamber 1 but collected on the shower plate  3  and the diffusion prevention wall  5 .
     Patent Document 1: Japanese Laid-Open Patent Publication No. 9-45495   

     SUMMARY OF THE INVENTION 
     The present invention provides an ashing device that prevents the processing efficiency from decreasing over time. 
     One aspect of the present invention is an ashing device for ashing organic material on a substrate including an exposed metal in a processing chamber. The ashing device includes a stage which holds the substrate. A diffuser plate faces toward the stage which diffuses active species supplied to the processing chamber and includes first through holes through which the active species pass. A porous plate is arranged between the stage and the diffuser plate. The porous plate includes a first layer, facing toward the substrate and formed from the same metal as the exposed metal of the substrate, and second through holes, through which the active species pass. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a first embodiment of an ashing device; 
         FIG. 2  is a schematic cross-sectional view of a chamber of  FIG. 1 ; 
         FIG. 3(   a ) is a schematic cross-sectional view of a metal prevention plate of the first embodiment,  FIG. 3(   b ) is a perspective view showing part of the metal prevention plate of  FIG. 3(   a ), and  FIG. 3(   c ) is a schematic cross-sectional view showing a modification of the metal prevention plate; 
         FIG. 4  is a chart showing changes in the ashing rate over time; 
         FIGS. 5(   a ) and  5 ( b ) are charts showing the measurement results of the ashing rate at a plurality of measurement points on the substrate; 
         FIG. 6(   a ) is a schematic cross-sectional view showing a second embodiment of a metal prevention plate,  FIG. 6(   b ) is a schematic cross-sectional view showing a modification of the metal prevention plate, and  FIG. 6(   c ) is a schematic cross-sectional view showing a further modification of a metal prevention plate; 
         FIG. 7  is a schematic diagram of a prior art ashing device; 
         FIG. 8  is a plan view showing a plurality of measurement points on a substrate; 
         FIGS. 9(   a ) and  9 ( b ) are charts showing the measurement results of the ashing depth at each measurement point of  FIG. 8 ; and 
         FIGS. 10(   a ) and  10 ( b ) are charts showing the measurement results of the ashing depth at each measurement point of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Embodiment] 
     A first embodiment of an ashing device according to the present invention will now be discussed with reference to  FIGS. 1 to 5 . 
     As shown in  FIG. 1 , the ashing device includes a chamber (processing chamber)  11 , the upper part of which is connected to a plasma chamber  13  by a feed tube  12 . The plasma chamber  13  is connected to a magnetron  15  by way of a microwave waveguide  14 . A microwave transmissive window  13   a  which is formed from silica or the like, partitions the plasma chamber  13  and the microwave waveguide  14 . A microwave power supply  16  is connected to the magnetron  15 . Microwaves (μ waves) generated in the magnetron  15  are guided to the plasma chamber  13  through the microwave waveguide  14 . 
     The plasma chamber  13  is connected to a plurality of (three in the drawing) mass flow controllers  18   a  to  18   c  by a gas intake tube  17 . The mass flow controllers  18   a  to  18   c  are respectively connected to gas supply sources  19   a  to  19   c . In the first embodiment, the gas supply source  19   a  stores oxygen (O 2 ), the gas supply source  19   b  stores nitrogen (N 2 ), and the gas supply source  19   c  stores carbon tetrafluoride (CF 4 ). The mass flow controllers  18   a  to  18   c  adjust the flow rate of the gas stored in the corresponding gas supply sources  19   a  to  19   c . The oxygen, nitrogen, and carbon tetrafluoride under the predetermined flow rate are mixed to form a reactive gas, which is sent to the plasma chamber  13  through the gas intake tube  17 . 
     The microwaves and reactive gas generates plasma, which contains oxygen, in the plasma chamber  13 , and oxygen radicals, which serve as active species in the plasma, are sent to the chamber  11  through the feed tube  12 . A substrate stage  20  for holding a substrate W is arranged in the chamber  11 . A vacuum auxiliary chamber  22  is connected to the chamber  11  by a gate  21 . The vacuum auxiliary chamber  22  is used to prevent the pressure of the chamber  11  from becoming atmospheric when loading and unloading the substrate W. 
     A ventilation port  23  is formed in the bottom of the chamber  11 . The ventilation port  23  is connected to a ventilation pump (not shown) by a ventilation tube  24 . The ventilation pump reduces the pressure in the chamber  11 . A pressure controller  25  is arranged in the ventilation tube  24  to regulate the pressure in the chamber  11  by driving the ventilation pump. 
     The microwave power supply  16 , the mass flow controllers  18   a  to  18   c , and the pressure controller  25  are connected to a control unit  26 . The control unit  26  includes a storage (not shown). The storage stores information (recipes) on the conditions for processing various types of substrates. When the recipe that is in accordance with the substrate W loaded into the chamber  11  is designated, the control unit  26  controls the microwave power supply  16 , the mass flow controllers  18   a  to  18   c , the pressure controller  25  based on values of the designated recipe. 
     The structure of the chamber  11  will now be discussed with reference to  FIG. 2 . 
     As shown in  FIG. 2 , the feed tube  12  has a lower end coupled to the upper part of the chamber  11 . A shower plate (diffuser plate)  31  is arranged on the lower end. The shower plate  31 , which is disk-shaped and which includes a plurality of through holes (first through hole), faces toward the substrate stage  20 . The shower plate  31  is fixed to the upper part of the chamber  11  by an attachment member  32 . The attachment member  32  spaces the shower plate  31  apart from an upper inner surface  11   a  by a predetermined distance. The predetermined distance, that is, the distance between the upper inner surface  11   a  of the chamber  11  and the shower plate  31 , is set so that oxygen radicals sent into the chamber  11  from the feed tube  12  pass through the through holes formed in the shower plate  31  and pass through the gap formed between the shower plate  31  and the upper part of the chamber  11  to be guided outward. 
     A cylindrical diffusion prevention wall  33  has an upper end attached to the upper inner surface  11   a  of the chamber  11 . The diffusion prevention wall  33  extends around the shower plate  31 . The diffusion prevention wall  33  has an inner diameter set to be slightly larger than the outer diameter of the substrate W held on the substrate stage  20 . 
     A disk-shaped metal prevention plate  34 , which serves as a porous plate and which includes a plurality of through holes (second through holes), is attached in a removable manner to a lower end of the diffusion prevention wall  33  by a fastening member (not shown) such as a screw. The metal prevention plate  34  has an outer diameter that is substantially the same as that of the diffusion prevention wall  33 . Accordingly, the metal prevention plate  34  covers the opening at the lower end of the diffusion prevention wall  33 . The oxygen radicals drawn into the chamber  11  therefore pass through the through holes of the metal prevention plate  34  and are guided toward the substrate W on the substrate stage  20 . 
     The metal prevention plate  34  is arranged in a buffer area  35 , which is defined by the substrate stage  20 , the upper part of the chamber  11 , and the diffusion prevention wall  33 . Furthermore, the metal prevention plate  34  is arranged in a region that is lower than the middle of the buffer area  35 . The metal prevention plate  34  is also spaced apart from the upper surface of the substrate stage  20  so that it does not interfere with the loading and unloading of the substrate W. 
     A substrate guide  36  covers the upper peripheral part of the substrate stage  20 . A lift pin  37  has a distal end arranged in the substrate stage  20  and supported to be movable in upward and downward directions. When the lift pin  37  moves upward, the substrate W can be transferred between the lift pin  37  and a conveying device (not shown). When the lift pin  37  moves downward, the substrate W supported by the lift pin  37  is arranged on the substrate stage  20 . 
     An insulation plate  38  is arranged between the substrate stage  20  and the lower part of the chamber  11 . A high frequency power supply  39  is connected to the substrate stage  20  via a capacitor C. The high frequency power supply  39  supplies a high frequency bias (RF bias) to the substrate stage  20 . Furthermore, a pipe  40  is connected to the substrate stage  20 . The pipe  40  supplies coolant to a coolant passage (not shown), which is formed in the substrate stage  20 . This adjusts the temperature of the substrate stage  20 . 
     As shown in  FIG. 3(   a ), the metal prevention plate  34  includes a plurality of (two as shown in  FIG. 3(   a )) layers  34   a  and  34   b . The upper first layer  34   a , which serves as the oxygen radical entering side, is formed by a layer of metal oxides such as aluminum oxide and ittria (Y 2 O 3 ). The second layer  34   b  located on the side facing toward the substrate W (lower side as viewed in  FIG. 3)  is formed from the same metal as the metal exposed from the substrate W processed in the chamber  11 . For example, if copper is exposed from the substrate W, the second layer  34   b  of the metal prevention plate  34  arranged in the chamber  11  is formed from copper. In other words, the metal mainly exposed from the substrate W is used for the second layer  34   b  of the metal prevention plate  34 . Therefore, in addition to copper (Cu), the second layer  34   b  may also use gold (Au), solder, platinum (Pt), and iridium (Ir). The metal prevention plate  34  may be formed, for example, by applying a metal oxide layer, which serves as the first layer  34   a , on one surface of a metal plate, which serves as the second layer  34   b.    
     The second layer  34   b  of the metal prevention plate  34  is electrically connected to the diffusion prevention wall  33 , which is formed from aluminum or the like, by the fastening member, which is described above. The diffusion prevention wall  33  is electrically connected to the chamber  11 , which is also formed from aluminum or the like, and the chamber  11  is connected to ground. Therefore, the metal prevention plate  34  (specifically, the second layer  34   b  that is formed from a metal) functions as an electrically opposite electrode of the substrate stage  20 , to which is applied the high frequency bias from the high frequency power supply  39 . The second layer  34   b  and the diffusion prevention wall  33  may be electrically connected by removing the first layer  34   a  from the peripheral portion of the second layer  34   b  and then connecting this portion of the second layer  34   b  to the lower end of the diffusion prevention wall  33 . 
       FIG. 3(   b ) is a cross-sectional perspective view showing part of the metal prevention plate  34 . As shown in  FIG. 3(   b ), the metal prevention plate  34  includes a plurality of through holes  41  (second through hole). Each of the through holes  41  has a hole diameter D set to prevent metal atoms, which are scattered from the exposed metal of the substrate W, from entering the buffer area  35 . More specifically, an aspect ratio (H/D) representing the ratio of the plate thickness H of the metal prevention plate  34  and the hole diameter D of the through hole  41  is set to be greater than or equal to 0.5 and less than or equal to 2. This prevents metal atoms, except for those scattered from the substrate W immediately below the through holes  41  in the vertical direction, from passing through the through holes  41 . In other words, even if metal atoms scattered from the substrate W enter the through holes  41 , such metal atoms are efficiently collected on the inner surfaces of the through holes  41 . 
     In  FIG. 3(   a ), instead of using the metal plate, a metal film may be formed on the surface of the metal prevention plate  34  facing toward the substrate W by performing sputtering, plating, spraying, or vapor deposition. In this case, for example, an aluminum plate may be used as a plate material (metal base plate) that serves as the base. As shown in  FIG. 3(   c ), the metal prevention plate  34  may include three layers  42   a  to  42   c . The first layer  42   a  is formed in the same manner as the first layer  34   a . The second layer  42   b  is an aluminum plate, and the third layer  42   c  is a metal film applied to one surface of the second layer  42   b . In the same manner as described above, copper (Cu), gold (Au), solder, platinum (Pt), and iridium (Ir) may be used for the metal film. 
     An ashing process performed with the ashing device of  FIG. 1  will now be described. 
     First, the substrate W is arranged on the substrate stage  20  in the chamber  11  with the surface (processing surface) to which a resist film (organic material) that is to be removed facing upward. Oxygen radicals contained in a plasma are generated in the plasma chamber  13  are sent into the chamber  11 . The oxygen radicals are diffused in the buffer area  35  by passing through the through holes of the shower plate  31  and the gap between the shower plate  31  and the upper inner surface  11   a  of the chamber  11 . The oxygen radicals passing through the gap between the shower plate  31  and the upper inner surface  11   a  of the chamber  11  fall from between the shower plate  31  and the diffusion prevention wall  33 . The diffusion prevention wall  33  restricts movement of the oxygen radicals in the radial direction, that is, unnecessary diffusion of the oxygen radicals. The oxygen radicals in the buffer area  35  then pass through the through holes  41  of the metal prevention plate  34  and reach the substrate W to react with the resist film of the substrate W and remove the resist film. 
     As described above, the metal prevention plate  34  includes a metal oxide layer (first layer  34   a ) serving as a passivation film on the upper surface as viewed in  FIG. 2 , that is, on the surface at the side to which oxygen radicals are supplied. Accordingly, the first layer  34   a  is unlikely to bond with the oxygen radicals since the first layer  34   a , or the path through which the oxygen radicals pass, has already been oxidized. This prevents the oxygen radicals from being deactivated by the metal prevention plate  34  (first layer  34   a ), which has been added. 
     When the ashing process is performed on the substrate W from which a metal material is exposed, metal atoms are scattered from the substrate W when chemical reactions or physical reactions take place on the substrate surface. In this case, the metal prevention plate  34 , which covers the upper side of the substrate W, functions as an opposite electrode of the high frequency bias in the ashing device of the first embodiment. Accordingly, scattered metal atoms are collected and deposited on the lower surface of the metal prevention plate  34  (second layer  34   b ). The lower surface, on which the metal atoms are collected, is arranged in a direction opposite to the supplying direction (advancing direction) of the oxygen radicals that reach the substrate W. Thus, the amount of oxygen radicals deactivated by the metal atoms collected on the lower surface of the metal prevention plate  34  is small. The scattered metal atoms also advance into the through holes  41  formed in the metal prevention plate  34 . However, since the through holes  41  are formed to have the predetermined aspect ratio, the metal atoms become collected on the inner surfaces of the through holes  41 , and the metal atoms subtly pass through the through holes  41 . Furthermore, the metal exposed from the inner surfaces of the through holes  41  in the second layer  34   b  of the metal prevention plate  34 , that is, the metal plate, is the same as the metals that are collected on the inner surfaces. Accordingly, even if metal atoms scattered from the substrate W are collected in the inner surface of the through holes  41  in the second layer  34   b , the area of the metal exposed from the through holes  41  subtly changes. Thus, even if metal atoms are collected on the inner surfaces of the through holes  41 , the amount of oxygen radicals that are deactivated is the same as when the metal atoms are not collected. For this reason, the change in the amount of deactivated oxygen radicals is extremely small regardless of the collection of the metal atoms, that is, the ashing process of the substrate W. In other words, even if the ashing process is performed on a large number of substrates W, the amount of oxygen radicals that reach the substrate W subtly changes. Therefore, the ashing rate subtly changes over time, that is, the processing efficiency is prevented from decreasing. 
     Most of the metal atoms scattered from the substrate W are collected on the lower surface of the metal prevention plate  34 . This prevents the scattered atoms from collecting in the path through which the oxygen radicals pass (e.g., the upper inner surface  11   a  of the chamber  11 , the lower surface of the shower plate  3 , and the diffusion prevention wall  5 ). This maintains uniformity in the distribution of the metal atoms in the path. Furthermore, even if the metal atoms scattered from the substrate W are collected in a non-uniform manner on the lower surface of the second layer  34   b  of the metal prevention plate  34 , the area in which the metal atoms are exposed from the lower surface of the second layer  34   b  subtly changes since the second layer  34   b  is formed from the same metal as the metal atoms scattered from the substrate W. In other words, uniformity of the metal distribution in the planar direction of the lower surface of the second layer  34   b  is maintained regardless of the collection of the metal atoms. Thus, even if the ashing process is performed on a large number of substrates W, the amount of oxygen radicals that reach the substrate W is uniform in the planar direction. In this manner, the in-surface uniformity of the substrate W for the ashing rate is prevented from being decreased. 
       FIG. 4  is a graph showing changes in the ashing rate relative to the processed number of substrates W from which copper is exposed. In  FIG. 4 , the black squares represent the measurement results for when the ashing process is performed by the ashing device of the first embodiment. The black circles represent the measurement results for when the ashing process is performed with the ashing device of the prior art shown in  FIG. 7 . The conditions for processing the substrate W are set so that the flow rates for oxygen, nitrogen, and carbon tetrafluoride are respectively 1750 sccm, 250 sccm, and 500 sccm, the pressure of the chamber  11  is 100 Pa, the power of the microwaves is 2500 W, the RF bias is 300 W, and the processing time is 60 seconds. Here, the ashing rate corresponds to the average value of the ashing rates taken at the measurement points (see  FIG. 8 ) of a single substrate. 
     As apparent from  FIG. 4 , after washing the chamber and the like, the ashing rate for the first substrate W that first undergoes the ashing process the first is substantially the same in the ashing device of the first embodiment and in the ashing device of the prior art. In the prior art ashing device (refer to black circles), the metal collected and deposited on the inner walls of the chamber  1  increases as the processed number increases. This drastically decreases the ashing rate. For the prior art ashing device, the ashing rate was measured for twenty substrates W. It can clearly be understood from the results of this experiment that the ashing rate drastically decreases over time in the ashing device of the prior art. In the prior art ashing device, the ashing rate of the twentieth substrate is decreased by about 30% from the ashing rate of the first substrate. 
     Comparatively, in the ashing device of the first embodiment (see black squares), even if the processed number increases, the ashing rate varies only slightly and the ashing rate remains high. More specifically, the ashing rate was higher when processing 1000 substrates with the ashing device of the first embodiment than when processing 10 substrates with the ashing device of the prior art. This is because the metal prevention plate  34  in the first embodiment prevents the ashing rate from decreasing over time. That is, the metal prevention plate  34  prevents the processing efficiency from changing over time. 
       FIG. 5  shows the measurement result of the ashing rate at each measurement point (see  FIG. 8 ) in the substrate W, from which copper is exposed. In  FIG. 5 , the black circles represent the measurement results for a substrate W that was first ashed by the ashing device of the prior art after the ashing device was washed. The black squares represent the measurement results for a plural ordinal number (e.g., tenth) of substrates W ashed by the ashing device of the first embodiment.  FIG. 5(   a ) shows the measurement results for when the ashing process was performed on the substrate W under processing conditions A. The processing conditions A are set so that the flow rates for oxygen, nitrogen, and carbon tetrafluoride are respectively 2400 sccm, 320 sccm, and 480 sccm, the pressure in the chamber  11  is 125 Pa, the power of the microwaves is 2000 W, the RF bias is 500 W, and the processing time is 30 seconds.  FIG. 5(   b ) shows the measurement result for when the ashing process was performed on the substrate W under processing conditions B. The processing conditions B are set so that the flow rates for oxygen and carbon tetrafluoride are respectively 1700 sccm and 300 sccm, the pressure in the chamber  11  is 85 Pa, the power of the microwaves is 1750 W, the RF bias is 0 W, and the processing time is 30 seconds. 
     As apparent from  FIGS. 5(   a ) and ( b ), in the measurement results for the tenth substrate W obtained with the ashing device of the first embodiment and the measurement results for the first substrate obtained with the prior art ashing device, the ashing rates at each measurement point varied subtly under both processing conditions A and B. In other words, even after the performing the ashing process on a plurality of substrates W, the ashing device of the first embodiment obtains the same ashing rate at each measurement point as would be obtained by a first substrate. This indicates that the metal prevention plate  34  (second layer  34 b) prevents the in-plane uniformity of the ashing rate for the substrate W from being decreased by the metal atoms scattered from the substrate W. 
     The ashing device of the first embodiment has the advantages described below. 
     (1) The metal prevention plate  34 , which serves as a porous plate, is arranged between the shower plate  31  for diffusing oxygen radicals and the substrate stage  20  for holding the substrate W. The metal prevention plate  34  includes the first layer  34   a , which is formed from a metal oxide layer and which is arranged on the oxygen radical entering side, and the second layer  34   b , which is arranged on the side facing toward the substrate W and which is formed from the same metal as the metal exposed from the substrate W that undergoes the ashing process in the chamber  11 . The metal prevention plate  34  includes the through holes  41 , which extend through the first layer  34   a  and the second layer  34   b . The metals scattered from the substrate W by surface reactions collect on the metal prevention plate  34  and do not enter the side of the metal prevention plate  34  from which oxygen radicals are supplied. This prevents the oxygen radicals passing through the metal prevention plate  34  from being deactivated. The second layer  34   b  of the metal prevention plate  34  facing toward the substrate W is formed from metal. Thus, even if the metals scattered from the substrate W collect on the metal prevention plate  34 , the amount of deactivated oxygen radicals in the metal prevention plate  34  varies slightly. Therefore, the amount of oxygen radicals that reach the substrate W is prevented from varying over time. In other words, the processing efficiency when processing a resist film with oxygen radicals is prevented from decreasing over time. 
     Further, the lower surface of the metal prevention plate  34  is made from the same metal as the metal atoms scattered from the substrate W. Thus, even if the metal atoms scattered from the substrate W are collected on the metal prevention plate  34  in a non-uniform manner, the metal distribution at the lower surface of the metal prevention plate  34  is unlikely to become non-uniform. This prevents the in-surface uniformity of the ashing rate for the substrate W from decreasing. 
     (2) The chamber  11  includes the cylindrical diffusion prevention wall  33 , which surrounds the shower plate  31 , for inhibiting unnecessary diffusion of the oxygen radicals. The metal prevention plate  34  is removably attached to cover the lower end opening of the diffusion prevention wall  33 . 
     Therefore, the unnecessary diffusion of the oxygen radicals diffused toward the periphery by the shower plate  31  is inhibited by the diffusion prevention wall  33 , and the oxygen radicals are efficiently supplied to the substrate W. 
     (3) The metal prevention plate  34  is arranged to be lower than the middle part between the upper inner surface  11   a  of the chamber  11  and the upper surface of the substrate stage  20 . Therefore, the metals scattered from the substrate W easily collects on the surface of the metal prevention plate  34  facing toward the substrate W. 
     (4) The metal oxide layer (first layer  34   a ) is formed on the surface of the metal prevention plate  34  arranged on the oxygen radical entering side (upper side as viewed in  FIG. 3 ). In other words, the first layer  34   a , which is the path through which oxygen radicals pass, in the metal prevention plate  34  has been oxidized in advance. Thus, the oxygen radicals are unlikely to bond with the first layer  34   a . Accordingly, the first layer  34   a  optimally prevents the deactivated amount of oxygen radicals from being increased by the metal prevention plate  34 , which has been added. 
     (5) The metal oxide layer is formed from aluminum oxides or ittria. This facilitates the formation of the metal oxide layer on the metal prevention plate  34 . 
     (6) The aspect ratio of the hole diameter of the through hole  41  formed in the metal prevention plate  34  is set to be greater than or equal to 0.5 and less than or equal to 2. Accordingly, metals are prevented from passing through the through holes  41  and being scattered on the side in which oxygen radicals are supplied. 
     (7) The substrate stage  20  is connected to the high frequency power supply  39  for applying high frequency bias, and the metal prevention plate  34  is connected to the chamber  11  (specifically, the diffusion prevention wall  33 ) so as to function as an opposite electrode of the substrate stage  20 . This further ensures that metal atoms scattered from the substrate W are collected on the metal prevention plate  34 . 
     (8) The metal prevention plate  34  is formed by applying to a predetermined metal plate a film of the metal exposed from the substrate W. This facilitates formation of the metal prevention plate  34 . 
     (9) The metal prevention plate  34  is formed by superimposing a metal oxide layer on a plate, which is formed from the metal that is exposed from the substrate W. This facilitates formation of the metal prevention plate  34 . 
     [Second Embodiment] 
     A second embodiment of the present invention will now be discussed with reference to  FIG. 6 . The second embodiment differs from the first embodiment in the structure of the metal prevention plate  34 . The differences from the first embodiment will mainly be discussed below. The ashing device of the second embodiment has substantially the same structure as the ashing device of the first embodiment shown in  FIGS. 1 and 2 . 
     As shown in  FIG. 6(   a ), the metal prevention plate  34  of the second embodiment includes three layers  43   a ,  43   b , and  43   c . In the same manner as the second layer  34   b  of the first embodiment, the third layer  43   b  (lower side as viewed in  FIG. 6)  facing toward the substrate W is a metal plate formed from the same metal as the metal exposed from the substrate W that undergoes ashing in the chamber  11 . The second layer  43   a  is a metal oxide layer formed on an oxygen radical entering side surface of the third layer  43   b . The first layer  43   c  is formed on the oxygen radical entering side surface of the second layer  43   a  and formed from a fluoride layer (fluoride film). The first layer  43   c  is a film formed by performing a fluorination treatment on the upper surface of the second layer  43   a . The fluorination treatment may be performed, for example, raising the temperature of a subject member (second layer  34   a  and third layer  34   b ) and supplying gas that contains fluorine atoms. 
     As another example, fluorine plasma may be produced by using gas containing fluorine atoms, and the subject member may be arranged in such a plasma atmosphere. The gas that is used may contain at least one of CF4, C2F6, C3F8, NF3, and SF6. 
     The metal prevention plate  34 , which includes the three layers  43   a ,  43   b , and  43   c , has a plurality of through holes in the same manner as in the first embodiment. The metal prevention plate  34  is attached in a removable manner to the lower end of the diffusion prevention wall  33  by a fastening member such as a screw. 
     In addition to advantages (1) to (9) of the first embodiment, the ashing device of the second embodiment has the advantages described below. 
     (10) The fluoride layer (first layer  43   c ) is formed on the surface of the metal prevention plate  34  that is located on the oxygen radical entering side. The fluoride layer functions as a passivation film. Thus, the upper surface of the metal prevention plate  34  is less likely to be oxidized compared to when the metal oxide layer of the second layer  43   a  is exposed. The oxygen radicals are thus less likely to bond to the fluoride layer of the first layer  43   c . This effectively prevents the deactivated amount of oxygen radicals from being increased by the metal prevention plate  34 , which is added. As a result, the overall ashing rate is improved. 
     The above embodiments may be modified as described below. 
     In the first embodiment, the first layers  34   a  and  42   a  formed from a metal oxide layer and shown in  FIGS. 3(   a ) and  3 ( c ) may be eliminated. In such a case, the amount of oxygen radicals that reach the substrate W is prevented from being varied over time by the metal plate (second layer  34   b ) or the metal film (third layer  42   c ) formed on the aluminum plate ( 42   b ). That is, the processing efficiency for ashing the resist film with oxygen radicals is prevented from decreasing over time. 
     In the second embodiment, the first layer  43   c  is formed (fluorination treatment) in a device that differs from the ashing device. However, the present invention is not limited in such a manner, and fluorination treatment using fluorine containing plasma may be performed on the metal prevention plate  34  in the ashing device after attaching the metal prevention plate  34 , which includes the second layer  43   a  and the third layer  43   b , to the ashing device. 
     The metal prevention plate  34  in the second embodiment is not limited to a three-layer structure. As shown in  FIG. 6(   b ), a metal plate  44   a  may be formed from the same metal as the metal exposed from the substrate W, and a fluoride layer  44   b  may be formed on the upper surface of the metal plate  44   a  (oxygen radical entering side, that is, the surface facing toward the diffuser plate). 
     Further, as shown in  FIG. 6(   c ), the metal prevention plate  34  may be formed by four layers  45   a  to  45   d.    
     Describing each layer in detail, the layer (third layer)  45   a  is an aluminum plate arranged as a predetermined metal base plate. The layer (fourth layer)  45   b , which is formed on the lower surface of the third layer  45   a  (surface facing toward the substrate W), is a metal film formed, for example, by sputtering the same metal as the metal exposed from the substrate W. The layer (second layer)  45   c  formed on the upper surface of the third layer  45   a  (oxygen radical introducing side) is a metal oxide. The first layer  45   d  formed on the upper surface of the second layer  45   c  is a fluoride layer formed by performing a fluorination treatment on the upper surface of the second layer  45   c.    
     In each of the above embodiments, in addition to removing the resist film from the semiconductor substrate W, the ashing device may remove other films and organic materials, which are removable by plasma or radicals. 
     In each of the above embodiments, instead of using the oxygen plasma, the ashing device may use a different plasma (e.g., hydrogen plasma). 
     In each of the above embodiments, the ashing device is not limited to a plasma ashing device that uses oxygen plasma and may be a light excitation ashing device that generates oxygen radicals by irradiating ultraviolet light on ozone gas. 
     In each of the above embodiments, the types of gases supplied to the ashing device may be increased.