Patent Publication Number: US-2021175051-A1

Title: Edge ring and substrate processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority to Japanese Patent Application No. 2019-220661 filed on Dec. 5, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an edge ring and a substrate processing apparatus. 
     BACKGROUND 
     For example, Patent Document 1 describes a focus ring that contacts a member of a lower electrode, and a surface roughness of at least one of the contact surface of the focus ring and the contact surface of the member of the lower electrode is equal to or greater than 0.1 μm, in order to stabilize attraction characteristics of the focus ring. 
     RELATED ART DOCUMENT 
     Patent Document 
     [Patent Document 1] Japanese Laid-open Patent Application Publication No. 2017-050509 
     SUMMARY 
     The present disclosure provides a technique for reducing leakage of a heat transfer gas supplied between an edge ring (also referred to as a focus ring) and a mounting surface of an electrostatic chuck on which the edge ring is placed, thereby improving heat transfer characteristics. 
     According to one aspect of the present disclosure, an edge ring that is placed on an electrostatic chuck of a substrate processing apparatus so as to surround a periphery of a substrate is provided. Multiple contact portions are provided on a lower surface of the edge ring, and each of the contact portions is of a ring shape. Each of the contact portions is in line contact with a mounting surface of the electrostatic chuck. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a substrate processing apparatus according to an embodiment; 
         FIGS. 2A and 2B  are diagrams illustrating movement of electric charge between a conventional edge ring and an electrostatic chuck; 
         FIG. 3A  is a diagram illustrating the configuration of an edge ring according to the embodiment; 
         FIG. 3B  is a diagram illustrating the configuration of an edge ring according to a first variation of the embodiment; 
         FIG. 4  is a graph illustrating experimental results of measuring flow rates of a heat transfer gas when the edge ring according to the embodiment was used and when the edge ring according to a comparative example was used; and 
         FIG. 5A  illustrates the configuration of an edge ring according to a second variation of the embodiment; and 
         FIG. 5B  illustrates the configuration of an edge ring according to a third variation of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of the present disclosure will be described with reference to the drawings. Note that in the following drawings, elements having identical features are given the same reference symbols and overlapping descriptions may be omitted. 
     [Overall Configuration of Substrate Processing Apparatus] 
       FIG. 1  is a cross-sectional view illustrating a schematic configuration of a substrate processing apparatus  1  according to an embodiment. The present embodiment describes a case in which the substrate processing apparatus  1  disclosed in the present embodiment is a reactive-ion etching (RIE) type substrate processing apparatus. However, the substrate processing apparatus  1  may be a plasma etching apparatus, a plasma CVD apparatus, or the like that utilizes surface wave plasma. 
     The substrate processing apparatus  1  includes a cylindrical chamber  10  made of metal, such as aluminum or stainless steel. The interior of the chamber  10  is a processing chamber in which a plasma process, such as plasma etching or plasma CVD, is performed. The chamber  10  is grounded. 
     A disc-shaped stage  11 , on which a substrate W is placed, is provided in the chamber  10 . The stage  11  also functions as a lower electrode. The stage  11  includes a base  11   a  and an electrostatic chuck  25 , and the electrostatic chuck  25  is disposed on the base  11   a . The base  11   a  is made of aluminum for example, and is supported by a cylindrical support  13  that extends vertically upward from the bottom of the chamber  10 , via an insulating cylindrical support member  12 . 
     The electrostatic chuck  25  includes a disc-shaped central portion  25   a  on which the substrate W is placed, and an annular peripheral portion  25   b . The height of the central portion  25   a  is higher than the height of the peripheral portion  25   b . An edge ring  30  surrounding the periphery of the substrate W is placed on the upper surface of the peripheral portion  25   b . In some cases, the height of the central portion  25   a  may be equal to the height of the peripheral portion  25   b.    
     The central portion  25   a  is formed by sandwiching an electrode  25   c  made of a conductive film between a pair of dielectric films. The peripheral portion  25   b  is constructed by sandwiching an electrode  25   d  made of a conductive film between a pair of dielectric films. The electrode  25   d  is a bipolar type electrode structure having an inner peripheral electrode  25   d   1  and an outer peripheral electrode  25   d   2 . 
     A power supply  26  is electrically connected to the electrode  25   c  via a switch  27 . A power supply  28   a   1  is electrically connected to the electrode  25   d   1  via a switch  29   a   1 . A power supply  28   a   2  is electrically connected to the electrode  25   d   2  via a switch  29   a   2 . The electrostatic chuck  25  generates Coulomb force by voltage supplied from the power supply  26  to the electrode  25   c  (hereinafter, also referred to as “HV voltage”), thereby electrostatically attracting and holding the substrate W onto the electrostatic chuck  25 . 
     The electrostatic chuck  25  generates Coulomb force by voltage supplied to the electrode  25   d   1  from the power supply  28   a   1  and voltage supplied to the electrode  25   d   2  from the power supply  28   a   2 , thereby electrostatically attracting and holding the edge ring  30  onto the electrostatic chuck  25 . In a bipolar type electrode, charge of different polarity can be supplied to each of the electrode  25   d   1  and the electrode  25   d   2 . However, the electrode  25   d  may be a unipolar type electrode structure in which the electrode  25   d   1  and the electrode  25   d   2  are integrated. 
     Inside the base  11   a , for example, an annular or spiral refrigerant chamber  31  extending circumferentially is provided. A temperature control medium at a predetermined temperature, such as cooling water, is supplied to the refrigerant chamber  31  from a chiller unit  32 , and the temperature control medium circulates through the refrigerant chamber  31  and pipes  33  and  34 . The temperature of the substrate W placed on the electrostatic chuck  25  is controlled by the temperature of the temperature control medium. 
     A heat transfer gas supply  35  supplies a heat transfer gas to a space between the central portion  25   a  of the electrostatic chuck  25  and the substrate W via a heat transfer gas line  36 . The heat transfer gas supply  35  also supplies the heat transfer gas to a space between the peripheral portion  25   b  of the electrostatic chuck  25  and the edge ring  30  via a heat transfer gas line  37 . As the heat transfer gas, a gas having heat conductivity, such as helium gas, may preferably be used. A pressure adjustable flow meter  23  is attached to a heat transfer gas inlet at the base  11   a  that is connected to the heat transfer gas line  37 , and is configured to measure a flow rate of the heat transfer gas supplied to the space between the peripheral portion  25   b  and the edge ring  30 . As attractive force for attracting the edge ring  30  to the electrostatic chuck  25  decreases and an amount of leakage of the heat transfer gas from the space between the peripheral portion  25   b  and the edge ring  30  increases, the flow rate of the heat transfer gas measured by the flow meter  23  increases. The flow meter  23  may be disposed at any location at which the flow rate of the heat transfer gas flowing through the heat transfer gas line  37  can be measured as the flow rate of the heat transfer gas supplied to the space between the peripheral portion  25   b  and the edge ring  30 . 
     A first radio frequency power supply  21  for plasma generation is electrically connected to the stage  11  via a matcher  21   a . The first radio frequency power supply  21  supplies, for example, radio frequency power at 40 MHz to the stage  11 . In the present embodiment, radio frequency power supplied from the first radio frequency power supply  21  is referred to as “HF power”. A second radio frequency power supply  22  for biasing is also electrically connected to the stage  11  via a matcher  22   a . The second radio frequency power supply  22  supplies radio frequency power having a lower frequency than the HF power to the stage  11 . In the present embodiment, radio frequency power supplied from the second radio frequency power supply  22  is referred to as “LF power”. The frequency of the LF power is, for example, 3 MHz. 
     An exhaust path  14  is formed between the inner side wall of the chamber  10  and the outer peripheral wall of the cylindrical support  13 . An annular baffle plate  15  is provided at the inlet or midway of the exhaust path  14 , and an exhaust port  16  is provided at the bottom of the exhaust path  14 . The exhaust port  16  is connected to an exhaust device  18  via an exhaust pipe  17 . The exhaust device  18  includes a vacuum pump to reduce the pressure in a processing space in the chamber  10  to a predetermined vacuum level. In addition, the exhaust pipe  17  includes an automatic pressure control valve (APC; not illustrated in the drawing), which is a variable butterfly valve. The automatic pressure control valve automatically controls the pressure in the chamber  10 . 
     A loading/unloading port  19  is provided at the side wall of the chamber  10 , and is opened and closed by a gate valve  20  when the substrate W is loaded into or unloaded from the chamber  10 . A gas showerhead  24  is attached to an upper opening of the chamber  10  via an insulating member  44 , and the upper opening of the chamber  10  is occluded by the gas showerhead  24 . The gas showerhead  24  also functions as an upper electrode. With such a configuration, the HF power is supplied from the first radio frequency power supply  21  to a space between the stage  11  and the gas showerhead  24 . 
     The gas showerhead  24  has a ceiling plate  40  and an electrode support  38  that detachably supports the ceiling plate  40 . The ceiling plate  40  includes a large number of gas holes  40   a . Inside the electrode support  38 , a buffer chamber  39  is provided, and a gas inlet  38   a  is connected to a through-hole that penetrates the electrode support  38  from the buffer chamber  39 . Gas supplied from a gas supply  45  passes through the gas supply line  41 , and is introduced into the buffer chamber  39  from the gas inlet  38   a . The gas introduced into the buffer chamber  39  passes through the gas holes  40   a , and is introduced into the chamber  10  from the lower surface of the gas showerhead  24 . 
     In the present embodiment, the central axis of the stage  11  is defined as the Z-axis. The gas showerhead  24 , the electrostatic chuck  25 , the base  11   a , and the electrode  25   c  are formed in a generally circular shape concentric to the Z-axis. The edge ring  30 , the electrodes  25   d   1  and  25   d   2 , and the cylindrical support member  12  are formed in a cylindrical or annular shape concentric to the Z-axis. 
     Components of the substrate processing apparatus  1  are connected to a controller  43 . The controller  43  controls each of the components of the substrate processing apparatus  1 . Examples of the components include the exhaust device  18 , the first radio frequency power supply  21 , the second radio frequency power supply  22 , the switches  27 ,  29   a   1 , and  29   a   2  for the electrostatic chuck, the power supplies  26 ,  28   a   1 , and  28   a   2 , the chiller unit  32 , the heat transfer gas supply  35 , and the gas supply  45 . 
     The controller  43  includes a CPU  43   a  and a memory  43   b , and controls desired substrate processing performed in the substrate processing apparatus  1 , by the CPU  43   a  reading out a program and a processing recipe stored in the memory  43   b  and executing the program. The controller  43  also controls a process for controlling electrostatic attraction of the substrate W and the edge ring  30 , a process for supplying a heat transfer gas, and the like, along with the substrate processing. 
     Annular or concentric magnets  42  are disposed around the chamber  10 . By the magnets  42 , a unidirectional horizontal magnetic field is formed in the chamber  10 . A vertical radio frequency electric field is formed by radio frequency power supplied between the stage  11  and the gas showerhead  24 . This causes magnetron discharge through a process gas in the chamber  10 , and generates a high density plasma from the process gas near the surface of the stage  11 . 
     When performing substrate processing in the substrate processing apparatus  1 , the gate valve  20  is first opened, a substrate W is loaded into the chamber  10  through the loading/unloading port  19 , and the substrate W is placed on the electrostatic chuck  25 . A gas output from the gas supply  45  is introduced into the chamber  10 , and the HF power and the LF power are supplied to the stage  11  from the first radio frequency power supply  21  and the second radio frequency power supply  22 , respectively. Further, voltage is applied from the power supply  26  to the electrode  25   c  to attract the substrate W to the mounting surface of the electrostatic chuck  25 , and voltage is applied from the power supplies  28   a   1  and  28   a   2  to the electrodes  25   d   1  and  25   d   2  respectively to attract the edge ring  30  to the mounting surface of the electrostatic chuck  25 . A heat transfer gas is supplied into the space between the substrate W and the mounting surface of the electrostatic chuck  25  and into the space between the edge ring  30  and the mounting surface of the electrostatic chuck  25 . The gas introduced from the gas showerhead  24  is formed into a plasma, and a predetermined plasma process is applied to the surface of the substrate W by means of radicals or ions in the plasma. 
     [Movement of Electric Charge Between Conventional Edge Ring and Electrostatic Chuck] 
     Movement of electric charge between an edge ring and an electrostatic chuck that occurs when using the conventional edge ring  130  is described with reference to  FIGS. 2A and 2B .  FIGS. 2A and 2B  are diagrams illustrating the movement of electric charge between a conventional edge ring  130  and an electrostatic chuck  25 . 
     The lower surface  130   g  of the conventional edge ring  130  is flat, and is brought into surface contact with the mounting surface  25   f  of the peripheral portion  25   b  of the electrostatic chuck  25 , at portions other than a heat transfer gas supply groove  25   e  of the heat transfer gas line  37  provided on the electrostatic chuck  25 . In such a configuration, an amount of leakage of a heat transfer gas from a space between the edge ring  130  and the mounting surface  25   f  of the peripheral portion  25   b  gradually increases. The reason will be described below. 
     Normally, when the HV voltage is supplied to the electrodes  25   d   1  and  25   d   2 , as illustrated in  FIG. 2A , Coulomb force is generated by electric charge accumulated near the lower surface  130   g  of the edge ring  130  and electric charge of opposite polarity accumulated in the electrodes  25   d   1  and  25   d   2 . This causes the edge ring  130  to be attracted to the electrostatic chuck  25 . 
     In this case, the lower surface  130   g  of the edge ring  130  is in surface contact with the mounting surface  25   f  of the electrostatic chuck  25 . Because a contact area when the lower surface  130   g  is in surface contact with the mounting surface  25   f  is larger than a contact area between the lower surface  130   g  and the mounting surface  25   f  when the lower surface  130   g  is in line contact with the mounting surface  25   f , electric charge is easily moved from between the edge ring  130  and the electrostatic chuck  25 . Accordingly, as illustrated in  FIG. 2B , as processing time of the substrate increases, the electric charge gradually moves from between the edge ring  130  and the electrostatic chuck  25  made of ceramic, thereby reducing attractive force for attracting the edge ring  130 . Therefore, as processing time of the substrate increases and a period of time when the edge ring  130  is attracted to the electrostatic chuck  25  becomes longer, decrease in the attractive force for attracting the edge ring  130  becomes greater. 
     Occurrence of the above-described movement of electric charge from between the edge ring  130  and the electrostatic chuck  25  is not limited to the case in which the processing time of the substrate is long. The above-described movement of electric charge from between the edge ring  130  and the electrostatic chuck  25  is also likely to occur when the temperature of the edge ring  130  and the electrostatic chuck  25  is high or when magnitude of the LF power is high. In an etching process with a high aspect ratio in recent years, because time for the etching process tends to be longer, movement of electric charge occurs easily. Also, in substrate processing in recent years, magnitude of the LF power tends to be controlled to be high, to increase an amount of ions drawn to the stage  11 , and in this case, movement of electric charge occurs easily. Furthermore, as the HF power tends to be supplied at high power, the temperature of the edge ring  130  tends to be high due to heat input from a plasma. This also facilitates movement of electric charge. Because of these factors, attractive force for attracting the edge ring  130  decreases, so that the heat transfer gas leaks from the space between the edge ring  130  and the mounting surface  25   f  of the peripheral portion  25   b , and the leakage amount of the heat transfer gas gradually increases. 
     In areas where heat transfer leaks, the pressure decreases, and the temperature of the edge ring  130  becomes locally high. Thus, controllability of the temperature of the edge ring  130  deteriorates. As a result, temperature distribution becomes non-uniform in the circumferential direction of the substrate W, and this exerts an adverse effect on the substrate processing. 
     Accordingly, in order to suppress movement of electric charge from between the edge ring  30  and the electrostatic chuck  25 , the edge ring  30  according to the present embodiment is configured such that the lower surface of the edge ring  30  is brought into line contact with the mounting surface of the electrostatic chuck  25  rather than surface contact. This allows the edge ring  30  to be stably attracted to the electrostatic chuck  25  even in substrate processing with a high aspect ratio or substrate processing using the high LF power. 
     &lt;Edge Ring According to the Present Embodiment&gt; 
     Hereinafter, the configuration of the edge ring  30  according to the present embodiment will be described with reference to  FIG. 3A .  FIG. 3A  is a diagram illustrating the configuration of the edge ring  30  according to the present embodiment. The lower surface  30   a  of the edge ring  30  according to the present embodiment is provided with a contact portion C 1  and a contact portion C 2  that contact the mounting surface  25   f  of the electrostatic chuck  25 . Each of the contact portion C 1  and the contact portion C 2  contacts the mounting surface  25   f  by ring-shaped line contact. 
     The lower surface  30   a  of the edge ring  30  includes a lower surface  30   a   1  and a lower surface  30   a   2 , each of which is formed horizontally. The lower surface  30   a   2  is located below the lower surface  30   a   1 . At the center of the lower surface  30   a , which is between the lower surface  30   a   1  and the lower surface  30   a   2 , a vertical section  30   b  is formed. In the following description, the vertical section  30   b  may also be referred to as a “riser  30   b”.    
     The lower surface of the edge ring  30  is processed to form steps (i.e., lower surfaces  30   a   1  and  30   a   2 ). The edge ring  30  before being processed has an inclined surface  131  at its bottom. The inclined surface  131  of the lower surface  30   a , to which the steps are formed, is inclined downward toward the outer periphery of the edge ring  30  at a tilt angle θ with respect to the horizontal plane. For example, the tilt angle θ is approximately within a range from 0.03° to 0.06° downward with respect to the horizontal direction. In the present embodiment, the horizontal lower surfaces  30   a   1  and  30   a   2  are formed by processing the inclined surface  131 . However, a method of forming the lower surfaces  30   a   1  and  30   a   2  is not limited thereto. By processing the lower surface  30   a  of the edge ring  30  as described above, the lower surface  30   a  is formed into steps lowering toward the outer periphery. The height of the riser  30   b  may be approximately 15 μm to 30 μm. 
     The contact portion C 1  is formed into a ring shape, at an inner edge of the lower surface  30   a   1  arranged at the inner side of the lower surface  30   a  of the edge ring  30 , and the contact portion C 2  is formed into a ring shape, at an inner edge of the lower surface  30   a   2  arranged at the outer side of the lower surface  30   a  of the edge ring  30 . The contact portion C 1  and the contact portion C 2  are concentrically formed with respect to the Z-axis (see  FIG. 1 ), which is the central axis. Multiple heat transfer gas supply grooves  25   e  are formed in the mounting surface  25   f . In addition, supply channels as paths for a heat transfer gas may be formed. The contact portion C 1  and the contact portion C 2  are configured to contact the mounting surface  25   f  by ring-shaped line contact, while avoiding the contact portions C 1  and C 2  being disposed at the heat transfer gas supply grooves  25   e  or the supply channels as possible. If part of the contact portion C 1  or the contact portion C 2  is located at the heat transfer gas supply grooves  25   e  or the supply channels, the ring-shaped line contact is partially interrupted. However, the ring-shaped line contact of the contact portion C 1  and of the contact portion C 2  may include a case in which the line contact of the contact portion C 1  or of the contact portion C 2  is partly interrupted. Line contact of the contact portion C 1  and the contact portion C 2  has a width on the order of micrometers (μm) (e.g., a single digit number of micrometers). That is, the contact portion C 1  and the contact portion C 2  are brought into line contact with the mounting surface  25   f  at a width on the order of μm. 
     The contact portion C 1  may be disposed inward relative to the heat transfer gas supply grooves  25   e  in the radial direction, and the contact portion C 2  may be disposed outward relative to the heat transfer gas supply grooves  25   e  in the radial direction. The contact portion C 2  is located below the contact portion C 1 . The contact portions C 1  and C 2  are edges provided on the lower surface  30   a  of the edge ring  30 . The contact portion C 1  is an example of a first contact portion. The contact portion C 2  is an example of a second contact portion. 
     Thus, the edge ring  30  is in ring-shaped line contact with the electrostatic chuck  25 , and has a smaller contact area than the conventional edge ring  130  that is in surface contact with the electrostatic chuck  25 . Thus, electric charge does not move substantially from between the edge ring  30  and the electrostatic chuck  25  made of ceramic. As described above, in the edge ring  30  according to the present embodiment, the attractive force for attracting the electrostatic chuck  25  of the edge ring  30  does not decrease even if attracting time is increased. Accordingly, leakage of the heat transfer gas supplied between the edge ring  30  and the electrostatic chuck  25  is suppressed, and stable and good heat transfer characteristics can be obtained. Accordingly, because heat can be stably removed from the edge ring  30 , controllability of the temperature of the edge ring  30  can be improved. That is, locally high temperature portions are not formed in the edge ring  30 , and radical supply distribution can be uniform in the circumferential direction of the substrate W. As a result, uniformity of substrate processing can be increased. 
     [Experimental Results] 
     Next, measurement results of the flow rate of the heat transfer gas when the heat transfer gas was supplied from the heat transfer gas line  37  during processing of the substrate using the substrate processing apparatus  1  illustrated in  FIG. 1  will be described with reference to  FIG. 4 .  FIG. 4  is a graph illustrating experimental results of measuring flow rates of the heat transfer gas when the edge ring  30  according to the present embodiment was used and when the edge ring  130  according to a comparative example was used. The edge ring  30  according to the present embodiment has the structure of the edge ring  30  illustrated in  FIG. 3A , and the edge ring  130  according to the comparative example has the structure of the conventional edge ring  130  illustrated in  FIG. 2A or 2B . 
     The horizontal axis of  FIG. 4  indicates time (s), and the vertical axis indicates a measured value of the flow rate of the heat transfer gas (a.u.). The measured value of the flow rate of the heat transfer gas was obtained from the flow meter (see  FIG. 1 ) by measuring the flow rate of the heat transfer gas flowing through the heat transfer gas line  37 . 
     As a result of the experiments, in a case in which the edge ring  130  according to the comparative example was used, the measured value of the flow rate of the heat transfer gas increased over time. This is because the lower surface  130   g  of the edge ring  130  is in surface contact with the mounting surface  25   f , so that electric charge moves from between the edge ring  130  and the electrostatic chuck  25 , and attractive force for attracting the edge ring  130  decreases over time. Therefore, the amount of leakage of the heat transfer gas from the space between the lower surface  130   g  of the edge ring  130  and the mounting surface  25   f  increased. 
     In contrast, in the edge ring  30  according to the present embodiment, the measured value of the flow rate of the heat transfer gas was generally constant even after a lapse of time. This is because the lower surface  30   a  of the edge ring  30  is in line contact with the mounting surface  25   f . Thus, electrical charge does not move from between the edge ring  30  and the electrostatic chuck  25 , and the attractive force for attracting the edge ring  30  does not change over time. Therefore, the amount of leakage of the heat transfer gas from the space between the lower surface  30   a  of the edge ring  30  and the mounting surface  25   f  was constant. 
     From the above-described results, in the edge ring  30  according to the embodiment, as the leakage amount of the heat transfer gas generally does not change, heat removal of the edge ring  30  can be performed stably, and controllability of the temperature of the edge ring  30  can be improved. As a result, uniformity of the substrate processing can be improved. 
     &lt;Variations&gt; 
     (First Variation) 
     Next, the configuration of the edge ring  30  according to a first variation of the present embodiment will be described with reference to  FIG. 3B .  FIG. 3B  illustrates the configuration of the edge ring  30  according to the first variation of the present embodiment. 
     The lower surface  30   a  of the edge ring  30  according to the first variation includes a contact portion C 1  and a contact portion C 2  that contact the mounting surface  25   f  of the electrostatic chuck  25 . Each of the contact portion C 1  and the contact portion C 2  is brought into ring-shaped line contact with the mounting surface  25   f.    
     The lower surface  30   a  of the edge ring  30  is formed to a shape of a sawtooth wave in the cross-sectional view. The lower surface  30   a  of the edge ring  30  includes a lower surface  30   a   1  and a lower surface  30   a   2 , each of which is inclined upward toward the outer periphery of the edge ring  30 . At the center of the lower surface  30   a , which is between the lower surface  30   a   1  and the lower surface  30   a   2 , a vertical section  30   b  is formed. The lower surface  30   a   1  is located inward relative to the vertical section  30   b  in the radial direction, and the lower surface  30   a   2  is located outward relative to the vertical section  30   b  in the radial direction. The tilt angle θ of the lower surface  30   a   1  may be the same as the tilt angle θ of the lower surface  30   a   2 , or may be different. For example, the tilt angle θ is approximately within a range between 0.03° and 0.06° upward with respect to the horizontal direction. By processing a horizontal plane, the lower surfaces  30   a   1  and  30   a   2 , each of which is an inclined surface with the tilt angle θ, are formed. 
     The contact portion C 1  is formed into a ring shape, at an inner edge of the lower surface  30   a   1  arranged at the inner side of the lower surface  30   a  of the edge ring  30 , and the contact portion C 2  is formed into a ring shape, at an inner edge of the lower surface  30   a   2  arranged at the outer side of the lower surface  30   a  of the edge ring  30 . The contact portion C 1  and the contact portion C 2  are brought into ring-shaped line contact with the mounting surface  25   f . The contact portion C 1  and the contact portion C 2  are brought into line contact with the mounting surface  25   f  at a width on the order of micrometers. The contact portion C 1  may be disposed inward relative to the heat transfer gas supply grooves  25   e  in the radial direction, and the contact portion C 2  may be disposed outside relative to the heat transfer gas supply grooves  25   e  in the radial direction. The contact portion C 2  is located at the same level as the contact portion C 1 . The contact portions C 1  and C 2  are edges provided on the lower surface  30   a  of the edge ring  30 . 
     The edge ring  30  according to the first variation can also suppress leakage of the heat transfer gas supplied between the edge ring  30  and the mounting surface  25   f  of the electrostatic chuck  25 , and can improve heat transfer characteristics. 
     Next, the configuration of the edge ring  30  according to second and third variations of the present embodiment will be described with reference to  FIGS. 5A and 5B .  FIG. 5A  illustrates the configuration of the edge ring  30  according to the second variation of the present embodiment.  FIG. 5B  illustrates the configuration of the edge ring  30  according to the third variation of the present embodiment. 
     (Second Variation) 
     The lower surface  30   a  of the edge ring  30  according to the second variation illustrated in  FIG. 5A  includes a contact portion C 1  and a contact portion C 2  that contact the mounting surface  25   f  of the electrostatic chuck  25 . The contact portion C 1  and the contact portion C 2  are brought into ring-shaped line contact with the mounting surface  25   f.    
     The lower surface  30   a  of the edge ring  30  is formed as a slope descending toward the outer periphery of the edge ring  30 , and includes a protrusion  30   c . The tilt angle θ of the lower surface  30   a  of the edge ring  30  with respect to the horizontal plane is approximately within a range from 0.03° to 0.06° downward with respect to the horizontal direction. 
     The protrusion  30   c , which is constituted by multiple protrusions, includes an inward protrusion  30   c   1  having a semicircular cross section, and an outward protrusion  30   c   2  having a semicircular cross section. The protrusions  30   c   1  and  30   c   2  are concentrically formed in the circumferential direction, with respect to the central axis Z of the substrate processing apparatus  1  (see  FIG. 1 ). 
     Accordingly, the contact portion C 1  of the second variation is the protrusion  30   c   1 , and is brought into ring-shaped line contact with the mounting surface  25   f  at a tip of the protrusion  30   c   1 . Also, the contact portion C 2  of the second variation is the protrusion  30   c   2 , and is brought into ring-shaped line contact with the mounting surface  25   f  at a tip of the protrusion  30   c   2 . The contact portion C 1  and the contact portion C 2  are brought into line contact with the mounting surface  25   f  at a width on the order of μm. The contact portion C 1  may be disposed inward relative to the heat transfer gas supply grooves  25   e  in the radial direction, and the contact portion C 2  may be disposed outward relative to the heat transfer gas supply grooves  25   e  in the radial direction. The contact portion C 2  is located below the contact portion C 1 . The contact portions C 1  and C 2  are protrusions provided on the lower surface  30   a  of the edge ring  30 . 
     (Third Variation) 
     The lower surface  30   a  of the edge ring  30  according to the third variation illustrated in  FIG. 5B  has a contact portion C 1  and a contact portion C 2  that contact the mounting surface  25   f  of the electrostatic chuck  25 . The contact portion C 1  and the contact portion C 2  are brought into ring-shaped line contact with the mounting surface  25   f  of the electrostatic chuck  25 . 
     The lower surface  30   a  of the edge ring  30  is horizontal toward the outer periphery of the edge ring  30 , and has a protrusion  30   c . The protrusion  30   c , which is constituted by multiple protrusions, includes an inward protrusion  30   c   1  having a semicircular cross section, and an outward protrusion  30   c   2  having a semicircular cross section. The protrusions  30   c   1  and  30   c   2  are concentrically formed in a circumferential direction, with respect to the central axis Z of the substrate processing apparatus  1  (see  FIG. 1 ). 
     Accordingly, the contact portion C 1  of the third variation is the protrusion  30   c   1 , and is brought into ring-shaped line contact with the mounting surface  25   f  at a tip of the protrusion  30   c   1 . Also, the contact portion C 2  of the third variation is the protrusion  30   c   2 , and is brought into ring-shaped line contact with the mounting surface  25   f  at a tip of the protrusion  30   c   2 . The contact portion C 1  and the contact portion C 2  are brought into line contact with the mounting surface  25   f  at a width on the order of micrometers. The contact portion C 1  may be disposed inward relative to the heat transfer gas supply grooves  25   e  in the radial direction, and the contact portion C 2  may be disposed outward relative to the heat transfer gas supply grooves  25   e  in the radial direction. The contact portion C 2  is located at the same height as the contact portion C 1 . The contact portions C 1  and C 2  are protrusions provided on the lower surface  30   a  of the edge ring  30 . 
     A cross-sectional shape of the protrusion  30   c  according to the second or third variation is not limited to a semicircle. Any shape can be employed for the shape of the protrusion  30   c , as long as the protrusion  30   c  is in line contact with the electrostatic chuck  25 . For example, the cross section of the protrusion  30   c  may be a triangle, or may be of other shapes such as polygons. 
     As described above, because the edge ring  30  according to the present embodiment and the first to third variations is in ring-shaped line contact with the electrostatic chuck  25 , the edge ring  30  has a smaller contact area than the conventional edge ring  130  that is in surface contact with the electrostatic chuck  25 . Thus, even if processing time of a substrate increases, movement of electric charge from between the edge ring  30  and the electrostatic chuck  25  is less likely to occur. Therefore, attractive force for attracting the edge ring  30  does not decrease. This prevents a heat transfer gas from being leaked from the space between the edge ring  30  and the electrostatic chuck  25 , thereby improving heat transfer characteristics. 
     Further, in the edge ring  30  according to the present embodiment and the first to third variations, the number of contact portions that are brought into line contact with the electrostatic chuck  25  is not limited to two, and multiple contact portions may be provided in the edge ring  30 . The number of contact portions may be three or more. 
     The edge ring and the substrate processing apparatus according to the embodiment and its variations that have been disclosed herein should be considered exemplary in all respects and not restrictive. The above embodiment and its variations may be modified and enhanced in various forms without departing from the appended claims and spirit thereof. Matters described in the above embodiment and its variations may take other configurations to an extent not inconsistent, and may be combined to an extent not inconsistent. 
     The substrate processing apparatus of the present disclosure is applicable to any of the following types of processing apparatuses: an atomic layer deposition (ALD) type processing apparatus, a capacitively coupled plasma (CCP) type processing apparatus, an inductively coupled plasma (ICP) type processing apparatus, a processing apparatus using a radial line slot antenna (RLSA), an electron cyclotron resonance plasma (ECR) type processing apparatus, and a helicon wave plasma (HWP) type processing apparatus. 
     The substrate processing apparatus is not limited to an etching apparatus in which an etching process is applied to a substrate. The substrate processing apparatus may be a deposition apparatus in which a film forming process is applied, an asking apparatus, a cleaning apparatus, or the like.