Patent Publication Number: US-2009229759-A1

Title: Annular assembly for plasma processing, plasma processing apparatus, and outer annular member

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an annular—assembly for plasma processing, a plasma processing apparatus, and an outer annular member, and in particular relates to an annular assembly for plasma processing, which is disposed such as to surround an outer periphery of a substrate that is mounted on a mounting stage and subjected to plasma processing. 
     2. Description of the Related Art 
     In general, a plasma processing apparatus that subjects a disk-shaped wafer as a substrate to plasma processing has a processing chamber in which a wafer is accommodated, a showerhead that supplies a process gas into the processing chamber, and a mounting stage that is disposed in the processing chamber and on which the wafer is mounted. The showerhead is connected to an upper radio frequency power source and acts as an upper electrode that applies radio frequency electrical power into the processing chamber. The mounting stage is connected to a lower radio frequency power source and acts as a lower electrode that applies radio frequency electrical power into the processing chamber. In the plasma processing apparatus, radio frequency electrical power is applied to a process gas supplied into the processing chamber, whereby the process gas is turned into plasma so as to produce ions and radicals. The wafer is subjected to the plasma processing by the ions and the radicals. Moreover, the mounting stage has in an upper portion thereof an electrostatic chuck that attracts and holds the wafer through a Coulomb force or a Johnsen-Rahbek force, and the electrostatic chuck is cooled so as to control the processing temperature of the wafer attracted to and held on a surface of the electrostatic chuck. 
       FIG. 5A  is a cross-sectional view showing an electrostatic chuck and its vicinity in a conventional plasma processing apparatus. 
     Referring to  FIG. 5A , around the electrostatic chuck  21 , an annular focus ring  23  is disposed such as to surround a wafer W mounted on the electrostatic chuck  21 , and an annular cover ring  50  is disposed such as to surround an outer periphery of the focus ring  23 . The focus ring  23  is made of a conductive material such as silicon, and the cover ring  50  is made of an insulating material such as quartz. The focus ring  23  focuses plasma toward the wafer W, and the cover ring  50  protects a mounting stage  51  from plasma. 
     In the case that the focus ring  23  is disposed such that the level of the upper surface thereof is substantially the same as the level of a to-be-processed surface of the wafer W, the wafer W and the focus ring  23  are at substantially the same potential, and hence ions and radicals tend to enter a gap between the outer periphery of the wafer W and an inner periphery of the focus ring  23 . In general, in the case that etching processing as plasma processing is carried out on a silicon oxide film (SiO 2  film) formed on the to-be-processed surface of the wafer W, CF-based gas is used as a process gas, and hence CFx radicals produced from the CF-based gas enter the gap between the outer periphery of the wafer W and the inner periphery of the focus ring  23 . The CFx radicals reach an outer periphery of the electrostatic chuck  21  under the wafer W, and because the electrostatic chuck  21  is cooled as described above, the CFx radicals cause an attracting reaction on the outer periphery of the electrostatic chuck  21  and turn into CF type deposit D, which becomes attached to the outer periphery of the electrostatic chuck  21  ( FIG. 5A ). 
     Conventionally, to remove the above described deposit D or the like, the plasma processing apparatus carries out dry cleaning processing using oxygen gas after processed wafers W are transferred out (see, for example, Japanese Laid-open Patent Publication (Kokai) No. 2007-214512). 
     However, in the above described dry cleaning processing using oxygen gas, the CF type deposit i.e. deposit D containing fluorine resists being dissolved by oxygen radicals, and it is thus difficult to completely dissolve the deposit D. For this reason, even if the dry cleaning processing is carried out in the processing chamber at the time of mass production of wafers W during which the plasma processing is repeatedly carried out, the deposit D accumulates in the gap between the outer periphery of the electrostatic chuck  21  and the inner periphery of the focus ring  23 , and the accumulated deposit D may project out from a surface of the electrostatic chuck  21  ( FIG. 5B ). At this time, the deposit D inhibits the wafer W from coming into contact with the surface of the electrostatic chuck  21 , and the wafer W is brought to a state in which it floats above the surface of the electrostatic chuck  21 . 
     When the wafer W is brought to a state in which it floats above the surface of the electrostatic chuck  21 , helium gas as a heat transfer gas supplied into a gap between the wafer W and the electrostatic chuck  21  leaks from the gap. Upon detecting the leakage of the helium gas, the plasma processing apparatus recognizes the poor attraction of the wafer W and stops operating. Thus, to resume the operation of the plasma processing apparatus, maintenance for removing the above described deposit D is required, and hence there is the problem that the rate of operation of the plasma processing apparatus considerably decreases. 
     SUMMARY OF THE INVENTION 
     The present invention provides an annular assembly for plasma processing, a plasma processing apparatus, and an outer annular member, which can prevent poor attraction of a substrate. 
     Accordingly, in a first aspect of the present invention, there is provided an annular assembly for plasma processing, comprising a focus ring that is mounted on a mounting stage and disposed such as to surround an outer periphery of a substrate subjected to plasma processing and an outer annular member that is disposed such as to surround an outer periphery of the focus ring, wherein the outer annular member comprises an exposed surface that is exposed into a plasma producing space in which plasma is produced, and the exposed surface is covered with yttria. 
     According to the first aspect of the present invention, the exposed surface of the outer annular member, which is exposed into a plasma producing space, is covered with yttria. In the case that CF-based gas is turned into plasma, and the substrate is subjected to plasma processing by the plasma, CFx radicals are produced in the plasma producing space. However, the yttria in the outer annular member pulls out fluorine in the CFx radicals, and hence radicals that enter a gap between the outer periphery of the substrate and an inner periphery of the focus ring and reach an outer periphery of the mounting stage hardly contains fluorine, and hence deposit arising from the radials is carbon-rich deposit. The carbon-rich deposit can be easily removed by dry cleaning processing using oxygen. As a result, if the dry cleaning processing is carried out at the time of mass production of substrates, no deposit accumulates in a gap between the outer periphery of the mounting stage and the inner periphery of the focus ring, and as a result, poor attraction of the substrates can be prevented. 
     The first aspect of the present invention can provide an annular assembly for plasma processing, comprising an inner annular member that is disposed such as to surround the outer periphery of the focus ring, and is closer to the focus ring than the outer annular member. 
     According to the first aspect of the present invention, the inner annular member is disposed such as to surround the outer periphery of the focus ring and closer to the focus ring than the outer annular member. That is, the focus ring and the inner annular member are interposed between the outer annular member and the substrate mounted on the mounting stage. As a result, the focus ring and the inner annular member can be caused to act as barriers that prevent yttria contamination resulting from dispersion of yttria in the outer annular member from spreading to the substrate, thus preventing the substrate from being contaminated with yttria. 
     The first aspect of the present invention can provide an annular assembly for plasma processing, wherein the inner annular member comprises quartz. 
     According to the first aspect of the present invention, the inner annular member is formed of quartz. Because quartz is plasma resistant, the mounting stage can be reliably protected from plasma. 
     The first aspect of the present invention can provide an annular assembly for plasma processing, wherein the inner annular member is disposed such that an upper surface thereof is at a lower level than an upper surface of the focus ring and at a higher level than an upper surface of the outer annular member. 
     According to the first aspect of the present invention, the inner annular member is disposed such that the upper surface thereof is at a lower level than the upper surface of the focus ring and at a higher level than the upper surface of the outer annular member. That is, the members constituting the annular assembly for plasma processing are disposed in the form of a ladder from the focus ring down to the outer annular member. As a result, the flow of a process gas flowing from above the focus ring to above the outer annular member in the plasma producing space and further to the side of the mounting stage can be smoothed, and hence the process gas can be smoothly discharged. 
     The first aspect of the present invention can provide an annular assembly for plasma processing, wherein the outer annular member comprises an upper surface thereof formed as an inclined surface that is inclined downward toward an outer periphery. 
     According to the first aspect of the present invention, the upper surface of the outer annular member is formed as an inclined surface that is inclined downward toward the outer periphery. As a result, the flow of a process gas supplied into the plasma producing space and discharged downward through the side of the mounting stage is never obstructed by upper surface of the outer annular member, and hence the process gas can be smoothly discharged. 
     Accordingly, in a second aspect of the resent invention, there is provided a plasma processing apparatus comprising a processing chamber in which a substrate is subjected to plasma processing, a mounting stage that is disposed in the processing chamber, and on which the substrate is mounted, and an annular assembly for plasma processing which is disposed such as to surround an outer periphery of the substrate mounted on the mounting stage, wherein the annular assembly for plasma processing comprises a focus ring that is disposed such as to surround the outer periphery of the substrate, and an outer annular member that is disposed such as to surround an outer periphery of the focus ring, and the outer annular member comprises an exposed surface that is exposed into a plasma producing space in which plasma is produced, and the exposed surface is covered with yttria. 
     Accordingly, in a third aspect of the present invention, there is provided an outer annular member that is disposed such as to surround an periphery of a focus ring that is disposed such as to surround an outer periphery of a substrate mounted on a mounting stage and subjected to plasma processing, comprising an exposed surface that is exposed into a plasma producing space in which plasma is produced, and the exposed surface is covered with yttria. 
     The features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing the construction of a plasma processing apparatus to which an annular assembly for plasma processing according to an embodiment of the present invention is applied; 
         FIG. 2  is an enlarged cross-sectional view showing the annular assembly for plasma processing in  FIG. 1  and its vicinity; 
         FIGS. 3A and 3B  are cross-sectional views schematically showing variations of the construction of the annular assembly for plasma processing in  FIG. 1 , in which  FIG. 3A  shows the case that the annular assembly for plasma processing is provided with only an outer cover ring in addition to a focus ring, and  FIG. 3B  shows the case that the outer cover ring is formed in a shape similar to the shape of a conventional cover ring in the construction shown in  FIG. 3A ; 
         FIGS. 4A ,  4 B,  4 C, and  4 D are graphs showing the results of etching processing on an oxide film and a photoresist film in an example  2  of the present invention and a comparative example  2 , in which  FIG. 4A  is a graph showing the distribution of etch rates in the etching processing on the oxide film in the example  2 ,  FIG. 4B  is a graph showing the distribution of etch rates in the etching processing on the photoresist film in the example  2 ,  FIG. 4C  is a graph showing the distribution of etch rates in the etching processing on the oxide film in the comparative example  2 , and  FIG. 4D  is a graph showing the distribution of etch rates in the etching processing on the photoresist film in the comparative example  2 ; and 
         FIGS. 5A and 5B  are cross-sectional views showing an electrostatic chuck in a conventional plasma processing apparatus, in which  FIG. 5A  shows the case that CF type deposit becomes attached to an outer periphery of the electrostatic chuck, and  FIG. 5B  shows the case that CF type deposit projects out from a surface of the electrostatic chuck. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the drawings showing a preferred embodiment thereof. 
     First, a description will be given of a plasma processing apparatus to which an annular assembly for plasma processing according to the present embodiment is applied. 
       FIG. 1  is a cross-sectional view schematically showing the construction of a plasma processing apparatus to which an annular assembly for plasma processing according to the present embodiment is applied. The plasma processing apparatus is constructed such as to carry out etching processing on a silicon oxide film (SiO 2  film) formed on a semiconductor wafer as a substrate. 
     Referring to  FIG. 1 , the plasma processing apparatus  10  has a chamber  11  in which a semiconductor wafer (hereinafter referred to merely as a “wafer”) W having a diameter of, for example, 300 mm is accommodated, and a cylindrical susceptor  12  (mounting stage) on which the wafer W is mounted is disposed in the chamber  11 . In the plasma processing apparatus  10 , a side exhaust path  13  that acts as a flow path through which gas above the susceptor  12  is exhausted out of the chamber  11  is formed between an inner side wall of the chamber  11  and the side face of the susceptor  12 . An exhaust plate  14  is disposed part way along the side exhaust path  13 . 
     The exhaust plate  14  is a plate-shaped member having a large number of holes therein and acts as a partition plate that partitions the chamber  11  into an upper portion and a lower portion. Plasma is produced in a processing space S (plasma producing space) between the susceptor  12  and a showerhead  32 , described later, in the upper portion (hereinafter referred to as the “reaction chamber”)  15  of the chamber  11  partitioned by the exhaust plate  14 . An exhaust pipe  17  through which gas in the chamber  11  is exhausted is connected to the lower portion (hereinafter referred to as the “exhaust chamber (manifold)”)  16  of the chamber  11 . The exhaust plate  14  captures or reflects ions and radicals produced in the processing space S of the reaction chamber  15  to prevent leakage of the ions and the radicals into the manifold  16 . 
     The exhaust pipe  17  has a TMP (turbo-molecular pump) and a DP (dry pump) (both not shown) connected thereto, and these pumps reduce the pressure in the chamber  11  down to a vacuum state. Specifically, the DP reduces the pressure in the chamber  11  from atmospheric pressure down to an intermediate vacuum state (e.g. a pressure of not more than 1.3×10 Pa (0.1 Torr)), and the TMP is operated in collaboration with the DP to reduce the pressure in the chamber  11  down to a high vacuum state (e.g. a pressure of not more than 1.3×10 −3  Pa (1.0×10 −5  Torr)), which is at a lower pressure than the intermediate vacuum state. It should be noted that an APC valve (not shown) controls the pressure in the chamber  11 . 
     A lower radio frequency power source  18  is connected to the susceptor  12  in the chamber  11  via a lower matcher  19 . The lower radio frequency power source  18  supplies predetermined radio frequency electrical power to the susceptor  12 . The susceptor  12  thus acts as a lower electrode. Moreover, the lower matcher  19  reduces reflection of the radio frequency electrical power from the susceptor  12  so as to maximize the efficiency of the supply of the radio frequency electrical power into the susceptor  12 . 
     An electrostatic chuck  21  having an electrostatic electrode plate  20  therein is provided in an upper portion of the susceptor  12 . The electrostatic chuck  21  is formed by placing an upper disk-shaped member, which has a smaller diameter than a lower disk-shaped member having a certain diameter, over the lower disk-shaped member. It should be noted that the electrostatic chuck  21  is made of ceramic. When a wafer W is mounted on the susceptor  12 , the wafer W is disposed on the upper disk-shaped member of the electrostatic chuck  21 . 
     A DC power source  22  is electrically connected to the electrostatic electrode plate  20  in the electrostatic chuck  21 . Upon a positive DC voltage being applied to the electrostatic electrode plate  20 , a negative potential is produced on a surface of the wafer W which faces the electrostatic chuck  21  (hereinafter referred to as “the rear surface of the wafer W”). A potential difference thus arises between the electrostatic electrode plate  20  and the rear surface of the wafer W, and hence the wafer W is attracted to and held on the upper disk-shaped member of the electrostatic chuck  21  through a Coulomb force or a Johnsen-Rahbek force due to the potential difference. 
     Moreover, an annular assembly for plasma processing (assembly)  22  is disposed on an upper portion of the susceptor  12  such as to surround an outer periphery of the wafer W mounted on the electrostatic chuck  21 . 
       FIG. 2  is an enlarged cross-sectional view showing the annular assembly for plasma processing in  FIG. 1  and its vicinity. 
     Referring to  FIG. 2 , the annular assembly for plasma processing  22  is comprised of an annular focus ring  23  that is disposed such as to surround the outer periphery of the wafer W mounted on the electrostatic chuck  21 , an annular inner cover ring  24  (inner annular member) that is disposed such as to surround an outer periphery of the focus ring  23 , and an annular outer cover ring  25  (outer annular member) that is disposed such as to surround an outer periphery of the inner cover ring  24 . The inner cover ring  24  is disposed such that an upper surface thereof is at a lower level than an upper surface of the focus ring  23 , and the outer cover ring  25  is disposed such that an upper surface thereof is at a lower level than the upper surface of the inner cover ring  24  and is formed as an inclined surface that is inclined downward toward the outer periphery. Moreover, the focus ring  23  is mounted on an annular ring spacer  26  mounted on the electrostatic chuck  21 , and the inner cover ring  24  and the outer cover ring  25  are mounted on an annular susceptor cover member  27  that covers the side face of the susceptor  12 . The focus ring  23  is made of a conductive material such as silicon (Si), and the inner cover ring  24  is made of an insulating material such as quartz (Qz) or the like, which is plasma resistant. The outer cover ring  25  is made of aluminum (Al), and an exposed surface of the outer cover ring  25  which is exposed into the processing space S is covered with yttria (Y 2 O 3 ). The focus ring  23  focuses plasma produced in the processing space S toward a front surface of the wafer W, thus improving the efficiency of the etching processing. The inner cover ring  24  and the outer cover ring  25  protect the susceptor  12  from plasma. 
     Referring again to  FIG. 1 , an annular coolant chamber  28  that extends, for example, in a circumferential direction of the susceptor  12  is provided inside the susceptor  12 . A coolant, for example, cooling water or a Galden (registered trademark) fluid, at a low temperature is circulated through the coolant chamber  28  via a coolant piping  29  from a chiller unit (not shown). The susceptor  12  cooled by the low-temperature coolant cools the wafer W and the focus ring  23  via the electrostatic chuck  21 . 
     A plurality of heat transfer gas supply holes  30  are opened to a portion of the upper surface of the upper disk-shaped member of the electrostatic chuck  21  on which the wafer W is attracted and held (hereinafter referred to as the “attracting surface”). The heat transfer gas supply holes  30  are connected to a heat-transmitting gas supply unit (not shown) via a heat-transmitting gas supply line  31 , and the heat-transmitting gas supply unit supplies helium (He) gas as a heat transfer gas into a gap between the attracting surface and the rear surface of the wafer W via the heat transfer gas supply holes  30 . The helium gas supplied into the gap between the attracting surface and the rear surface of the wafer W effectively transfers heat from the wafer W to the electrostatic chuck  21 . 
     The showerhead  32  is disposed in a ceiling portion of the chamber  11  such as to face the susceptor  12 . An upper radio frequency power source  34  is connected to the showerhead  32  via an upper matcher  33  and supplies predetermined radio frequency electrical power to the showerhead  32 . The showerhead  32  thus acts as an upper electrode. It should be noted that the upper matcher  33  has a similar function to the lower matcher  19  described above. 
     The showerhead  32  has a ceiling electrode plate  36  having therein a number of gas holes  35 , a cooling plate  37  that detachably suspends the ceiling electrode plate  36 , and a lid member  38  that covers the cooling plate  37 . Moreover, a buffer chamber  39  is provided inside the cooling plate  37 , and a process gas-introducing pipe  40  is connected to the buffer chamber  39 . The showerhead  32  supplies a process gas supplied to the buffer chamber  39  through the process gas-introducing pipe  40  into the reaction chamber  15  via the gas holes  35 . In the present embodiment, for example, a CF-based gas is supplied as the process gas into the reaction chamber  15 . 
     In the plasma processing apparatus  10 , radio frequency electrical power is supplied to the susceptor  12  and the showerhead  32  to apply radio frequency electrical power to the processing space S so that the process gas supplied from the showerhead  32  is turned into high density plasma to produce ions and radicals, whereby the wafer W is subjected to the etching processing using the ions and the radicals. 
     Operation of the component parts of the above described plasma processing apparatus  10  is controlled in accordance with a program for the etching processing by a CPU of a control unit (not shown) of the plasma processing apparatus  10 . 
     In the case that a CF-based gas is turned into plasma and the wafer W is subjected to the etching processing by the plasma, CFx radicals are produced in the processing space S. Because the exposed surface of the outer cover ring  25  which is exposed into the processing space S is covered with yttria, the yttria pulls out fluorine in the CFx radicals. For this reason, the radicals that enter the gap between the outer periphery of the wafer W and an inner periphery of the focus ring  23  to reach an outer periphery of the electrostatic chuck  21  under the wafer W includes almost no fluorine, and thus deposit arising from the radicals is carbon-rich deposit. In general, carbon-rich deposit is easily removed by oxygen radicals, but in dry cleaning processing that is carried out in the chamber  11  by the plasma processing apparatus  10 , oxygen gas is used as the process gas, and hence the carbon-rich deposit attached to the outer periphery of the electrostatic chuck  21  is easily removed by the dry cleaning processing. 
     According to the present embodiment, because the exposed surface of the outer cover ring  25  which is exposed into the processing space S in the annular assembly for plasma processing  22  is covered with yttria, deposit does not accumulate in a gap between the outer periphery of the electrostatic chuck  21  and the inner periphery of the focus ring  23  if the dry cleaning is carried out at the time of mass production of wafers W, and thus poor attraction of the wafer W can be prevented. 
     According to the present embodiment, in the annular assembly for plasma processing  22 , the focus ring  23  and the inner cover ring  24  are disposed closer to the wafer W attracted to and held on the electrostatic chuck  21  than the outer cover ring  25 . That is, the focus ring  23  and the inner cover ring  24  are interposed between the outer cover ring  25  and the wafer W attracted to and held on the electrostatic chuck  21 . As a result, the focus ring  23  and the inner cover ring  24  can act as barriers that prevent yttria contamination resulting from dispersion of yttria in the outer cover ring  25  from spreading to the wafer W, and hence the wafer W can be prevented from being contaminated with yttria. 
     Moreover, according to the present embodiment, the inner cover ring  24  is disposed such that an upper surface thereof is at a lower level than the upper surface of the focus ring  23  and at a higher level than the upper surface of the outer cover ring  25 . That is, the members constituting the annular assembly for plasma processing  22  are arranged in the form of a ladder from the focus ring  23  down to the outer cover ring  25 . As a result, the flow of the process gas flowing from above the focus ring  23  to above the outer cover ring  25  in the processing space S and further to the side of the susceptor  12  can be smoothed, and hence the process gas can be smoothly discharged. 
     Further, according to the present embodiment, the upper surface of the outer cover ring  25  is formed as an inclined surface that is inclined downward to the outer periphery. As a result, the flow of the process gas supplied into the processing space S and discharged downward via the side of the susceptor  12  is not obstructed by the upper surface of the outer cover ring  25 , and hence the process gas can be quickly discharged. 
     Further, although the annular assembly for plasma processing  22  described above has the inner cover ring  24  that is interposed between the focus ring  23  and the outer cover ring  25 , the annular assembly for plasma processing  22  may not be provided with the inner cover ring  24 . For example, an annular assembly for plasma processing  41  may be provided with only an outer cover ring  43  mounted on a susceptor cover member  42  in addition to the focus ring  23  as shown in  FIG. 3A . As is the case with the outer cover ring  25  described above, the outer cover ring  43  is made of aluminum, and an exposed surface of the outer cover ring  43  is covered with yttria. In this case, as compared with the case that there is provided the annular assembly for plasma processing  22 , the exposed surface of the outer cover ring  43  covered with yttria can be made closer to the processing space S, and hence the yttria can effectively pull out fluorine in the CFx radicals described above. Further, as shown in  FIG. 3B , an outer cover ring  44  that has a construction similar to the construction of the outer cover rings  25  and  43  described above may be formed in a shape similar to the shape of a conventional cover ring  50 . In this case, merely by replacing the cover ring  50  with the outer cover ring  44 , fluorine in the CFx radicals can be pulled out, and hence poor attraction of wafers W can be prevented using an inexpensive construction. 
     Further, in the case of the outer cover rings  25 ,  43 , and  44  described above, when the yttria of the exposed surface pulls out fluorine in the CFx radicals, yttrium chemically reacts with the fluorine. This reaction is an endothermic reaction, and thus the temperatures of the outer cover rings  25 ,  43 , and  44  are preferably high so that the reaction can be promoted. For this reason, in the present embodiment, the outer cover rings  25 ,  43 , and  44  are mounted on the susceptor  12  via susceptor cover members  27 ,  42 , and  45 , respectively, which are made of quartz or the like with low heat transferability. This inhibits the outer cover rings  25 ,  43 , and  44  from being cooled by the susceptor  12 , and enables the temperatures of the outer cover rings  25 ,  43 , and  44  to be more easily increased by heat input from plasma produced in the processing space S. 
     Further, although the outer cover rings  25 ,  43 , and  44  described above are made of aluminum, and their exposed surfaces are covered with yttria, they may be formed of yttria alone (bulk). 
     Although in the above described embodiment, the substrates are semiconductor wafers, the substrate are not limited to them and rather may instead be any of various glass substrates used in LCDs (Liquid Crystal Displays), FPDs (Flat Panel Displays), or the like. 
     Next, a concrete description will be given of examples of the present invention. 
     EXAMPLE 1 
     First, the above described annular assembly for plasma processing  22  was disposed in the plasma processing apparatus  10 . 
     After that, the etching processing was carried out on oxide films of 65 wafers, and after the wafers W were transferred out, the dry cleaning processing were carried out. Then, the state of deposit attachment to the outer periphery of the electrostatic chuck  21  was visually checked, and the outer periphery of the electrostatic chuck  21  was cleaned by wiping it using a BEMCOT (registered trademark). 
     COMPARATIVE EXAMPLE 1 
     The conventional focus ring  23  and cover ring  50  were disposed in the plasma processing apparatus  10 . 
     After that, as is the case with the above described example 1, the etching processing was carried out on oxide films of 65 wafers, and after the wafers W were transferred out, the dry cleaning processing was carried out. Then, the state of deposit attachment to the outer periphery of the electrostatic chuck  21  was visually checked, and the outer periphery of the electrostatic chuck  21  was cleaned by wiping it using a BEMCOT (registered trademark). 
     In the comparative example 1, it was visually confirmed that deposit was attached to the outer periphery of the electrostatic chuck  21 , and a large amount of deposit was attached to the BEMCOT (registered trademark) after the cleaning using wiping, whereas in the example 1, attachment of deposit to the outer periphery of the electrostatic chuck  21  was not visually confirmed, and further, the amount of deposit attached to the BEMCOT (registered trademark) after cleaning by wiping was obviously smaller than in the comparative example 1. In the example 1, it was also confirmed that the exposed surface of the outer cover ring  25  turned black. 
     The reason for this was considered to be that in the comparative example 1, CFx radicals produced in the plasma processing reached the outer periphery of the electrostatic chuck  21 , caused an attracting reaction on the outer periphery of the electrostatic chuck  21 , and turned into CF type deposit to accumulate on the outer periphery of the electrostatic chuck  21 , whereas in the example 1, the exposed surface of the outer cover ring  25  turned black, and hence yttria in the exposed surface pulled out fluorine in the CFx radicals produced in the plasma processing, and radicals that reached the outer periphery of the electrostatic chuck  21  included almost no fluorine, and deposit arising from the radicals was carbon-rich deposit, and the deposit was easily removed by the dry cleaning processing and thus did not accumulate on the outer periphery of the electrostatic chuck  21 . 
     Next, the present inventors checked how the etching processing is affected by the presence of the outer cover ring  25  whose surface exposed into the processing space S is covered with yttria. 
     EXAMPLE 2 
     First, as is the case with the example 1, the annular assembly for plasma processing  22  was disposed in the plasma processing apparatus  10 . 
     After that, etching processing was carried out on an oxide film on a wafer W, and etch rates in the etching processing were measured. Further, by using another wafer, etching processing was carried out on a photoresist film on the wafer, and etch rates in the etching processing were measured. Then, the results of the etching processing on the oxide film were graphed in  FIG. 4A , and the results of the etching processing on the photoresist film were graphed in  FIG. 4B . 
     COMPARATIVE EXAMPLE 2 
     As is the case with the comparative example 1, the conventional focus ring  23  and cover ring  50  were disposed in the plasma processing apparatus  10 . 
     After that, etching processing was carried out on an oxide film on a wafer W, and etch rates in the etching processing were measured. Further, by using another wafer, etching processing was carried out on a photoresist film on the wafer, and etch rates in the etching processing were measured. Then, the results of the etching processing on the oxide film were graphed in  FIG. 4C , and the results of the etching processing on the photoresist film were graphed in  FIG. 4D . 
     As a result of comparison between  FIGS. 4A and 4C  and comparison between  FIGS. 4B and 4D , it was confirmed that the etch rate does not vary regardless of whether the outer cover ring  25  is present or absent in the etching processing on an oxide film and the etching processing on a photoresist film. It was thus found that the presence of the outer cover ring  25  hardly affects the etching processing.