Patent Publication Number: US-2019198298-A1

Title: Plasma etching apparatus and plasma etching method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority to Japanese Patent Application No. 2017-245362 filed on Dec. 21, 2017, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a plasma etching apparatus and a plasma etching method. 
     2. Description of the Related Art 
     In a processing chamber of a plasma etching apparatus, a focus ring is disposed on a stage so as to surround a periphery of a wafer, in order to direct plasma toward a surface of the wafer. During plasma processing, as the focus ring is exposed to plasma, the focus ring becomes worn. 
     As a result, because a level difference is generated on a sheath at an edge of the wafer and an incident angle of ions is tilted, tilting occurs on an etching profile. Also, as an etching rate at the edge of the wafer varies, an etching rate in a wafer becomes uneven. Accordingly, when a focus ring is worn to a certain amount, the focus ring is replaced with a new one. However, a time for replacing a focus ring is one of factors for degrading productivity. 
     Some technologies have been developed in order to alleviate the problem. For example, Patent Document 1 discloses a technique for controlling distribution of an etching rate of a surface by applying DC (direct current) voltage to a focus ring from a DC power source. Patent Document 2 discloses a technique for measuring degree of abrasion of a focus ring based on a change of temperature of the focus ring according to passage of time. Patent Document 3 discloses a technique for controlling DC voltage to be applied to a focus ring, in accordance with a measured result of a thickness of the focus ring. 
     However, appropriate DC voltage to be applied to a focus ring varies depending on degree of abrasion of the focus ring and a process condition. Thus, it is difficult for the techniques disclosed in Patent Document 1 or Patent Document 2 to appropriately control DC voltage to be applied to a focus ring in accordance with degree of abrasion of the focus ring and the like. 
     Although, in the technique disclosed in Patent Document 3, DC voltage applied to a focus ring is controlled in accordance with a thickness of the focus ring, because abrasion of a focus ring occurs in not only a thickness direction but also a width direction, it is difficult for the techniques disclosed in Patent Document 3 to appropriately control DC voltage to be applied to a focus ring in accordance with degree of abrasion of the focus ring and the like. Also, as it is difficult to directly measure a thickness of a focus ring installed inside a plasma etching apparatus, enabling the techniques disclosed in Patent Document 3 requires high cost. 
     CITATION LIST 
     Patent Document 
     
         
         [Patent Document 1] Japanese Patent No. 5281309 
         [Patent Document 2] Japanese Patent No. 6027492 
         [Patent Document 3] Japanese Laid-open Patent Application Publication No. 2005-203489 
       
    
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a plasma etching apparatus is provided to solve the above problem. The plasma etching apparatus includes a processing vessel capable of being evacuated; a lower electrode provided in the processing vessel that is configured to place a substrate; an upper electrode provided in the processing vessel that is arranged in parallel with the lower electrode so as to face each other; a process gas supply unit configured to supply process gas to a processing space between the upper electrode and the lower electrode; a high frequency power supply unit configured to supply high frequency electric power for generating plasma from process gas; a focus ring surrounding a periphery of the substrate; a direct current (DC) power source configured to output DC voltage applied to the focus ring; a heating unit configured to heat the focus ring; and a temperature measurement unit configured to measure temperature of the focus ring. 
     According to another aspect of the present invention, a plasma etching method including an etching step of etching a substrate by using the above described plasma etching apparatus is provided. In the etching step, the DC voltage applied to the focus ring is controlled based on temperature of the focus ring measured by the temperature measurement unit, by accessing a memory unit storing information indicating a relationship between temperature rising rate of the focus ring and DC voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a plasma etching apparatus according to an embodiment; 
         FIG. 2A  and  FIG. 2B  are diagrams illustrating a variation of an etching rate and occurrence of tilting that are caused by abrasion of a focus ring; 
         FIG. 3  is a diagram illustrating an example of a cross section of a peripheral structure of the focus ring according to the embodiment; 
         FIG. 4  is a flowchart of an example of a process for calculating a relationship between temperature rising rate and DC voltage according to the embodiment; 
         FIG. 5  illustrates an example of a graph representing the relationship between temperature rising rate and DC voltage according to the embodiment; 
         FIG. 6  is a flowchart illustrating an example of a DC voltage control process according to the embodiment; 
         FIG. 7  is a diagram illustrating an example of a cross section of a peripheral structure of a focus ring according to modified example; and 
         FIG. 8  is a diagram illustrating an example of a system for controlling DC voltage according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the drawings. Note that in the following descriptions and the drawings, elements having substantially identical features are given the same reference symbols and overlapping descriptions may be omitted. 
     [Plasma Etching Apparatus] 
     First, an example of a plasma etching apparatus  1  according to an embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating a cross section of the plasma etching apparatus  1  according to the embodiment. The plasma etching apparatus  1  according to the present embodiment is a plasma etching apparatus of a reactive ion etching (RIE) type. 
     The plasma etching apparatus  1  includes a cylindrical processing vessel  10  capable of being evacuated. The processing vessel  10  is formed of metal, such as aluminum or stainless steel. The inside of the processing vessel  10  is a processing chamber for performing a plasma process such as plasma etching or plasma CVD. The processing vessel  10  is grounded. 
     A disc shaped stage  11  is provided in the processing vessel  10 . An example of a workpiece to be placed on the stage  11  includes a semiconductor wafer W (hereinafter referred to as a “wafer W”). The stage  11  is supported by a cylindrical supporting member  13  that extends upward from the bottom of the processing vessel  10 , via a cylindrical holding member  12  formed of aluminum oxide (Al 2 O 3 ). 
     The stage  11  includes an electrostatic chuck  25 . The electrostatic chuck  25  includes a base  25   c  formed of aluminum, and a dielectric layer  25   b  disposed on the base  25   c.  On an outer circumference side of an upper surface of the base  25   c,  a focus ring  30  is disposed so as to surround a periphery of a wafer W. An outer circumference of the base  25   c  and an outer circumference of the focus ring  30  are covered by an insulator ring  32 . 
     An attracting electrode  25   a  made from conductive film is embedded in the dielectric layer  25   b.  A direct current (DC) power source  26  is connected to the attracting electrode  25   a  via a switch  26   a.  The electrostatic chuck  25  generates electrostatic force (Coulomb force) by DC voltage applied from the DC power source  26  to the attracting electrode  25   a,  and a wafer W is attracted to and held by the electrostatic chuck  25  by the generated electrostatic force. 
     The focus ring  30  is made from silicon. In the base  25   c,  a heater  52  is embedded at a position close to a bottom surface of the focus ring  30 . An alternate current (AC) power source  58  is connected to the heater  52 . When electric power is supplied from the AC power source  58  to the heater  52 , the heater  52  is heated and the focus ring  30  is also heated. Temperature of the bottom surface of the focus ring  30  can be measured by a radiation thermometer  51 . 
     A variable DC power source  28  is connected to an electrode  29  via a switch  28   a.  Because the electrode  29  is in contact with the focus ring  30 , DC voltage output from the variable DC power source  28  is applied to the focus ring  30 . 
     Further, as will be described below, in the present embodiment, by appropriately controlling magnitude of DC voltage applied from the variable DC power source  28  to the focus ring  30  in accordance with degree of abrasion of the focus ring, a thickness of sheath generated above an upper surface of the focus ring  30  is controlled. Accordingly, occurrence of tilting is suppressed, and distribution of an etching rate of a surface can be controlled. The variable DC power source  28  is an example of a DC power source for outputting DC voltage to be applied to the focus ring  30 . 
     A first high frequency power source  21  is connected to the stage  11  via a matching unit  21   a.  The first high frequency power source  21  supplies, to the stage  11 , high frequency electric power of a first frequency (such as a frequency of 13 MHz) for generating plasma or for RIE (hereinafter, the high frequency electric power of the first frequency may also be referred to as “first high frequency electric power”). Also, a second high frequency power source  22  is connected to the stage  11  via a matching unit  22   a.  The second high frequency power source  22  supplies high frequency electric power of a second frequency lower than the first frequency (such as a frequency of 3 MHz) for generating bias voltage (hereinafter, the high frequency electric power of the second frequency may also be referred to as “second high frequency electric power”). That is, the stage  11  functions as a lower electrode. 
     In the base  25   c,  a coolant chamber  31  of an annular shape, which extends in a circumferential direction, is provided. From a chiller unit, coolant at a predetermined temperature, such as cooling water, is supplied to the coolant chamber  31 , and the coolant circulates in the coolant chamber  31  via pipes  33  and  34 , in order to cool the electrostatic chuck  25 . 
     To the electrostatic chuck  25 , a heat transmitting gas supply unit  35  is connected via a gas supply line  36 . The heat transmitting gas supply unit  35  supplies heat transmitting gas to a space between the upper surface of the electrostatic chuck  25  and a bottom surface of a wafer W. Gas having good heat conductivity, such as He gas, may preferably be used as a heat transmitting gas. 
     Between a side wall of the processing vessel  10  and the cylindrical supporting member  13 , an exhaust path  14  is formed. At an entrance of the exhaust path  14 , an annular baffle plate  15  is provided. At a bottom of the exhaust path  14 , an exhaust port  16  is provided. An exhaust device  18  is connected to the exhaust port  16  via an exhaust pipe  17 . The exhaust device  18  includes a vacuum pump, and can reduce pressure of a processing space in the processing vessel  10  to a desirable quality of vacuum. Also, the exhaust pipe  17  includes an automatic pressure control valve (hereinafter referred to as an “APC”) of a variable butterfly valve. The APC automatically controls pressure in the processing vessel  10 . Further, a gate valve  20  is provided at the side wall of the processing vessel  10 , which is used for opening and/or closing a loading/unloading port  19  for a wafer W. 
     A gas shower head  24  is mounted to a ceiling of the processing vessel  10 . The gas shower head  24  includes an electrode plate  37 , and an electrode supporting member  38  that detachably supports the electrode plate  37 . The electrode plate  37  includes a large number of gas holes  37   a.  The gas shower head  24  is arranged in parallel with the stage  11  such that the gas shower head  24  faces the stage  11  which also acts as the lower electrode. The gas shower head  24  also acts as an upper electrode. 
     A buffer chamber  39  is provided in the electrode supporting member  38 . The buffer chamber  39  includes a gas inlet port  38   a,  and a process gas supply unit  40  is connected to the gas inlet port  38   a  via a gas supplying pipe  41 . The process gas supply unit  40  supplies process gas to the processing space between the gas shower head  24  and the stage  11 , through the gas holes  37   a.  Further, at a periphery of the processing vessel  10 , annular magnets  42  are provided coaxially. 
     Each component of the plasma etching apparatus  1  is connected to a control unit  43 . The control unit  43  controls each of the components of the plasma etching apparatus  1 . Examples of the component include the exhaust device  18 , the matching units  21   a  and  22   a,  the first high frequency power source  21 , the second high frequency power source  22 , the switches  26   a  and  28   a,  the DC power source  26 , the variable DC power source  28 , the heat transmitting gas supply unit  35 , and the process gas supply unit  40 . 
     The control unit  43  is a computer having a CPU  43   a  and a memory  43   b.  The CPU  43   a  reads out a control program for the plasma etching apparatus  1  and a process recipe that are stored in the memory  43   b,  to control an etching process performed in the plasma etching apparatus  1 . 
     The control unit  43  also maintains, in the memory  43   b,  a table storing information indicating a relationship between temperature rising rate of the focus ring  30  and DC voltage that is calculated in a pre-process of a DC voltage control process of the focus ring  30  to be described below. The memory  43   b  is an example of a memory unit storing a relationship between temperature rising rate and DC voltage. 
     For example, when an etching process is performed in the plasma etching apparatus  1 , the gate valve  20  is opened first. Next, a wafer W is loaded into the processing vessel  10  and placed on the electrostatic chuck  25 . Subsequently, DC voltage from the DC power source  26  is applied to the attracting electrode  25   a  to attract the wafer W to the electrostatic chuck  25 . 
     Also, heat transmitting gas is supplied to a space between the upper surface of the electrostatic chuck  25  and a bottom surface of the wafer W. Next, process gas from the process gas supply unit  40  is supplied to the inside of the processing vessel  10 , and pressure in the processing vessel  10  is reduced by the exhaust device  18  and the like. Further, first high frequency electric power and second high frequency electric power are supplied to the stage from the first high frequency power source  21  and the second high frequency power source  22  respectively. 
     In the processing vessel  10  of the plasma etching apparatus  1 , a magnetic field of a horizontal direction is created by the magnets  42 , and an RF electric field of a vertical direction is generated by high frequency electric power applied to the stage  11 . Because of the generated magnetic field and the generated electric field, the process gas introduced from the gas shower head  24  is changed to plasma, and a given etching process is applied to the wafer W by radicals or ions in the plasma. 
     The first high frequency power source  21  is an example of a high frequency power supply unit that supplies, to the stage  11 , high frequency electric power for generating plasma from process gas. However, the high frequency power supply unit may supply high frequency electric power for generating process gas plasma to the gas shower head  24 , instead of the stage  11 . 
     The heater  52  is an example of a heating unit that heats the focus ring  30 . The heating unit is not limited to the heater  52 , and another heating medium may be used. The radiation thermometer  51  is an example of a temperature measurement unit for measuring temperature of the focus ring  30 . However, the temperature measurement unit is not limited to a specific type of thermometer. For example, a fiber optical thermometer such as Luxtron or a thermocouple may be used as the temperature measurement unit. 
     [Abrasion of Focus Ring] 
     Next, a change of sheath, a variation of an etching rate, and occurrence of tilting, which are caused by abrasion of the focus ring, will be described with reference to  FIGS. 2A and 2B . As illustrated in  FIG. 2A , when the focus ring  30  is new, a thickness of the focus ring  30  is designed such that an upper surface of a wafer is positioned at the same height as the height of the upper surface of the focus ring  30 . In such a state, during plasma processing, the sheath above the wafer W and the sheath above the focus ring  30  are at the same height, and an incident angle of ions from plasma above the wafer W and the focus ring  30  is vertical. As a result, an etching profile of a hole or the like formed on the wafer W becomes vertical. That is, tilting, in which an etching profile tilts, does not occur. Also, an etching rate becomes uniform on an entire surface of the wafer W. 
     However, by plasma processing being performed, as the focus ring  30  is exposed to plasma, the focus ring  30  abrades. Thus, as illustrated in  FIG. 2B , because the focus ring  30  becomes thinner, the height of the upper surface of the focus ring  30  becomes lower than the height of the upper surface of the wafer W, and the sheath above the focus ring  30  becomes lower than the sheath above the wafer W in height. 
     At an edge of the wafer W in which a height difference of sheath occurs, an incident angle of ions becomes slanted (departs from a vertical angle), and tilting may occur in an etching profile. In addition, an etching rate at the edge of the wafer W varies, and the etching rate may become uneven on the surface of the wafer W. 
     On the other hand, in the present embodiment, by applying DC voltage output from the variable DC power source  28  to the focus ring  30 , distribution of an etching rate on a surface and tilting can be controlled. 
     However, as the focus ring  30  is exposed to plasma during plasma processing, the focus ring  30  abrades gradually. Thus, appropriate magnitude of DC voltage to be applied from the variable DC power source  28  varies in accordance with degree of abrasion of the focus ring  30 . Also, as illustrated in  FIG. 2B , the abrasion of the focus ring  30  includes not only a decrease in the focus ring  30  in a thickness direction, but also a decrease in a width direction and deterioration of quality of material. Accordingly, in a case in which degree of abrasion of the focus ring  30  is estimated by measuring a thickness of the focus ring  30  and in which DC voltage to be applied from the variable DC power source  28  is calculated based on the estimated degree of abrasion, because the estimated degree of abrasion deviates from an actual degree of abrasion, it is difficult to calculate appropriate DC voltage. 
     To solve the above problem, in the present embodiment, degree of abrasion of the focus ring  30  is estimated based on heat capacity, and DC voltage to be applied to the focus ring  30  is controlled based on the estimated heat capacity. In the present embodiment, as a physical quantity corresponding to heat capacity, temperature rising rate of the focus ring  30  is measured while the focus ring  30  is heated by the heater  52  to which a constant electric power is supplied. Based on the measured temperature rising rate, degree of abrasion of the focus ring  30  is estimated, and magnitude of DC voltage to be applied is controlled. The above mentioned heat capacity estimated is heat capacity of not only the focus ring  30  but also of peripheral members of the focus ring  30 . That is, the above mentioned temperature rising rate of the focus ring  30  corresponds to the heat capacity of the focus ring  30  and the peripheral members of the focus ring  30 . 
     [Focus Ring Peripheral Structure] 
     In order to estimate degree of abrasion of the focus ring  30  based on temperature rising rate of the focus ring  30  and to appropriately control magnitude of DC voltage applied to the focus ring  30 , information about a relationship between the temperature rising rate and the appropriate DC voltage is obtained first. In the following, a peripheral structure of the focus ring  30  concerning temperature measurement of the focus ring  30 , which is used for obtaining information indicating a relationship between temperature rising rate and appropriate DC voltage, will be described with reference to  FIG. 3 .  FIG. 3  is a diagram illustrating an example of a cross section of the peripheral structure of the focus ring according to the present embodiment. 
     The focus ring  30  is a ring-shaped member, and is disposed on an outer circumference side of the upper surface of the base  25   c  of the electrostatic chuck  25 . In the base  25   c,  a heater  52  coated with an insulator  52   a  is provided at a position close to the bottom surface of the focus ring  30 . When electric power is supplied from the AC power source  58  to the heater  52 , the heater  52  is heated and the focus ring  30  is also heated. The radiation thermometer  51  measures temperature of the bottom surface of the focus ring  30 . A tip of the radiation thermometer  51  is disposed in a vicinity of an antireflection-treated glass piece  54  made from material such as Ge. From a tip of the radiation thermometer  51 , infrared light or visible light is emitted. The emitted infrared light or visible light passes through a space formed in an insulator  56 , reaches the bottom surface of the focus ring  30 , and reflects. In the present embodiment, in order to measure temperature of the focus ring  30 , intensity of the reflected infrared light or visible light is measured. An O-ring  55  seals the insulator  56  so as to separate a space in the insulator  56  at an atmospheric pressure from a vacuum space in the processing vessel  10 . The variable DC power source  28  is connected to the electrode  29  coated with an insulator  29   a.  From the variable DC power source  28 , DC voltage of a magnitude in accordance with degree of abrasion of the focus ring  30  is applied to the electrode  29 . When DC voltage is to be applied to the focus ring  30 , the control unit  43  determines an optimal DC voltage value to be applied to the focus ring  30 , based on the temperature of the focus ring  30  measured by the radiation thermometer  51  and based on the information about the relationship between temperature rising rate of the focus ring  30  and appropriate DC voltage, in order to control the variable DC power source  28  to output the appropriate DC voltage. 
     [Pre-Process of DC Voltage Control Process] 
     Next, a process for obtaining the information indicating a relationship between temperature rising rate of the focus ring  30  and DC voltage will be described with reference to  FIG. 4  and  FIG. 5 .  FIG. 4  is a flowchart of an example of a process for calculating a relationship between temperature rising rate and DC voltage according to the present embodiment (hereinafter, this process may simply be referred to as a “calculation process”). An example of a graph representing a relationship between temperature rising rate and DC voltage according to the present embodiment is illustrated in  FIG. 5 . The calculation process is performed as a pre-process of the DC voltage control process in which an optimal DC voltage is applied to the focus ring  30 . 
     When starting the calculation process in  FIG. 4 , a new focus ring  30  is used. The control unit  43  first heats the focus ring  30  by supplying constant electric power (such as 100 W) from the AC power source  58  to the heater  52 , and measures temperature of the bottom surface of the focus ring  30  at every heating interval (heating time) (step S 10 ). 
     Next, each time the focus ring  30  has been used for a predetermined period (for example, each time the focus ring  30  has been used for 100 hours), the control unit  43  heats the focus ring  30  by supplying constant electric power (such as 100 W) from the AC power source  58  to the heater  52 , and measures temperature of the bottom surface of the focus ring  30  at every heating interval (heating time) (step S 12 ). 
     An example of a graph representing an execution result of step S 10  and step S 12  is illustrated in  FIG. 5  (a graph (a) of  FIG. 5 ). In the graph (a) of  FIG. 5 , a horizontal axis represents a heating time, and a vertical axis represents temperature of the focus ring  30  when the focus ring  30  has been heated for a period corresponding to the heating time. The graph (a) of  FIG. 5  represents relationships between heating time and temperature of the focus ring  30 , in cases in which the focus ring  30  is new, the focus ring  30  has been used for 100 hours, the focus ring  30  has been used for 200 hours, the focus ring  30  has been used for 300 hours, the focus ring  30  has been used for 400 hours, and the focus ring  30  has been used for 500 hours. From this example (the graph (a)), it is found that an amount of increase in temperature of the focus ring  30  per a certain amount of heating time becomes higher as a usage time of the focus ring  30  increases and degree of abrasion of the focus ring  30  increases, because the focus ring  30  has been exposed to plasma. 
     Referring back to  FIG. 4 , each time the focus ring  30  has been used for a predetermined period (for example, each time the focus ring  30  has been used for 100 hours); determination of an optimal magnitude of DC voltage to be applied to the focus ring  30  at this time is performed (by performing experiment using the focus ring  30 , or by using other conventional methods) (step S 14 ). 
     Next, the control unit  43  calculates temperature rising rate of the focus ring  30  (when power supplied to the heater  52  is constant) for each usage time of the focus ring  30 , and determines a relationship between temperature rising rate of the focus ring  30  (when power supplied to the heater  52  is constant) and an optimal magnitude of DC voltage to be applied to the focus ring  30  (step S 16 ). Next, the control unit  43  records the calculated relationship into the table in the memory  43   b  (step S 18 ), and the process terminates. 
     An example of a graph representing a relationship between temperature rising rate and DC voltage, which is obtained by performing the above mentioned calculation process, is illustrated in a graph (b) of in  FIG. 5 . The usage time of the focus ring  30  corresponds to degree of abrasion of the focus ring  30 . As the focus ring  30  abrades, heat capacity of the focus ring  30  becomes smaller and temperature rising rate increases. Thus, by calculating an optimal magnitude of DC voltage corresponding to temperature rising rate, as illustrated in the graph (b) of  FIG. 5 , an optimal magnitude of DC voltage corresponding to degree of abrasion of the focus ring  30  is estimated. Based on information of the calculated relationship between temperature rising rate and DC voltage, control for applying, to the focus ring  30 , an optimal DC voltage corresponding to degree of abrasion of the focus ring  30 , is realized. In the following, the DC voltage control process according to the present embodiment, in which an optimal DC voltage corresponding to degree of abrasion of the focus ring  30  is applied to the focus ring  30 , will be described with reference to  FIG. 6 .  FIG. 6  is a flowchart illustrating an example of the DC voltage control process according to the present embodiment. 
     [DC Voltage Control Process] 
     In the DC voltage control process, temperature of the focus ring  30  is measured while raising temperature of the heater  52  disposed under the focus ring  30 , by supplying constant electric power (such as 100 W) from the AC power source  58  to the heater  52 , in order to calculate temperature rising rate of the focus ring  30 . The DC voltage control process is started when a certain period of time has passed since the heating of the focus ring  30 , by supplying constant electric power to the heater  52 , was started. Note that the DC voltage control process may preferably be started except for a period while a process of etching a wafer W is being performed. However, the DC voltage control process may be performed at any time after the above mentioned calculation process has been performed. 
     When starting the DC voltage control process, the control unit  43  determines whether a predetermined number of wafers W have been processed or not (step S 20 ). The predetermined number wafers W may be a single wafer W, or may be one lot of (for example, 25 pieces of) wafers W. Further, although the present embodiment describes a case in which the number of processed wafers W is used for determination, other determination methods may be used. For example, the control unit  43  may determine whether or not an etching process has been performed for a given period of time, and if the given period of time has elapsed, the process may proceed to step S 22 . 
     The control unit  43  repeats step S 20  until it is determined that the predetermined number of wafers W have been processed. If it is determined that the predetermined number of wafers W have been processed, the control unit  43  measures temperature rising rate of the focus ring  30  (step S 22 ). The temperature rising rate can be obtained based on temperature measured by the radiation thermometer  51 , after heating the focus ring  30  while supplying constant electric power to the heater  52 . 
     Next, the control unit  43  refers to the table storing information indicating the relationship between temperature rising rate and DC voltage that has been calculated in the pre-process of the DC voltage control process, to specify an optimal DC voltage value corresponding to the temperature rising rate measured at step S 22  (step S 24 ). For example, as illustrated in the graph (b) of  FIG. 5 , a DC voltage value corresponding to the measured temperature rising rate is uniquely specified. 
     Referring back to  FIG. 6 , the control unit  43  next controls output of the variable DC power source  28  so that the variable DC power source  28  applies DC voltage at the specified magnitude to the focus ring  30  (step S 26 ), and the process terminates. 
     According to the DC voltage control process in the present embodiment, by calculating an optimal magnitude of DC voltage corresponding to temperature rising rate, an optimal magnitude of DC voltage corresponding to degree of abrasion of the focus ring  30  is estimated. Also, by applying DC voltage at the estimated optimal magnitude corresponding to the degree of abrasion of the focus ring  30 , sheath above the focus ring  30  and sheath above a wafer W are made to be at the same height. Thus, at least one of an occurrence of tilting and variation of an etching rate can be suppressed. For example, in a case in which the calculated optimal DC voltage value is 100 V, by applying DC voltage of 100 V to the focus ring  30 , even if the abraded focus ring  30  is used, similar etching characteristics to that in a state in which the focus ring  30  is new (such as a vertical etching profile and uniform distribution of an etching rate) can be obtained. 
     As described above, even if the focus ring  30  has abraded, because similar etching characteristics to that in the state in which the focus ring  30  is new can be obtained by applying DC voltage to the focus ring  30 , a replacement cycle of the focus ring  30  can be extended. A time required for replacing a focus ring  30  includes a time for opening the processing vessel  10 , a time for replacing the focus ring  30  with a new one, a time for closing the processing vessel  10  after replacing the focus ring  30 , a time for cleaning inside the processing vessel  10 , and a time for making a condition inside the processing vessel  10  adequate by performing seasoning. Thus, by extending a replacement cycle of the focus ring  30 , productivity is increased. 
     MODIFIED EXAMPLE 
     Next, a modified example of a peripheral structure of the focus ring  30  concerning temperature measurement of the focus ring  30  will be described with reference to  FIG. 7 .  FIG. 7  is a diagram illustrating an example of a cross section of the peripheral structure of the focus ring according to the modified example. 
     In the example illustrated in  FIG. 3 , the radiation thermometer  51  is disposed so as to measure temperature at an outer circumferential side of the bottom surface of the focus ring  30 . However, in the modified example illustrated in  FIG. 7 , the radiation thermometer  51  is disposed so as to measure temperature at a middle point between an outer circumference and an inner circumference of the bottom surface of the focus ring  30 . Therefore, in the modified example illustrated in  FIG. 7 , the heater  52  coated with the insulator  52   a  and a heater  62  coated with an insulator  62   a  are respectively provided at locations on the base  25   c  corresponding to the inner circumference and the outer circumference of the bottom surface of the focus ring  30 . 
     According to the above described structure, a position, in which temperature is measured by the radiation thermometer  51  according to the modified example, is closer to the heater  52  or  62  as compared with a position in which temperature is measured by the radiation thermometer  51  according to the first-described embodiment, and the radiation thermometer  51  according to the modified example tends to measure temperature at a middle point between an outer circumference and an inner circumference of the bottom surface of the focus ring  30 . However, a positional relationship between the heater  52  or  62  and the radiation thermometer  51  is not limited to a specific one. The radiation thermometer  51  may be close to the heater  52  or  62  or be apart from the heater  52  or  62 . Also, a position of the radiation thermometer  51  is not limited to the outer circumferential side of the bottom surface of the focus ring  30  or the middle point between the outer circumference and the inner circumference of the focus ring  30 . For example, the radiation thermometer  51  may be disposed so as to measure temperature at the inner circumferential side of the bottom surface of the focus ring  30 . In any case, by performing the pre-process of the DC voltage control process for obtaining the information indicating a relationship between temperature rising rate of the focus ring  30  and DC voltage, DC voltage at an optimal magnitude can be applied to the focus ring  30 . 
     Lastly, an example of a system utilizing the information stored in the memory  43   b  by the control unit  43  which indicates the relationship between temperature rising rate and DC voltage, and an example of control performed by a server  2  in the system, will be described with reference to  FIG. 8 .  FIG. 8  is a diagram illustrating an example of a system for controlling DC voltage applied to the focus ring of the plasma etching apparatus according to the above-described embodiment. 
     The system to be described below includes two types of plasma etching apparatuses, which are a plasma etching apparatus A (hereinafter simply referred to as an “apparatus A”), and a plasma etching apparatus B (hereinafter simply referred to as an “apparatus B”). The system also includes control units  1   a,    1   b,  and  1   c  each of which controls a corresponding apparatus A, and includes control units  2   a,    2   b,  and  2   c  each of which controls a corresponding apparatus B. The control units  1   a  to  1   c  and the control units  2   a  to  2   c  are connected to the server  2  via a network. 
     Examples of the apparatus A are, but are not limited to, a plasma etching apparatus  1 A, a plasma etching apparatus  1 B, and a plasma etching apparatus  1 C. The plasma etching apparatuses  1 A,  1 B, and  1 C are controlled by the control units  1   a,    1   b,  and  1   c  respectively. 
     Examples of the apparatus B are, but are not limited to, a plasma etching apparatus  2 A, a plasma etching apparatus  2 B, and a plasma etching apparatus  2 C. The plasma etching apparatuses  2 A,  2 B, and  2 C are controlled by the control units  2   a,    2   b,  and  2   c  respectively. 
     Each of the control units  1   a,    1   b,    1   c,    2   a,    2   b,  and  2   c  transmits, to the server  2 , information indicating the relationship between temperature rising rate and DC voltage which is stored in its memory. The server  2  receives the information ( 3   a,    3   b,  and  3   c ) indicating the relationship between temperature rising rate and DC voltage respectively from the control units  1   a,    1   b,  and  1   c  controlling the apparatus A. The server  2  also receives the information ( 4   a,    4   b,  and  4   c ) indicating the relationship between temperature rising rate and DC voltage respectively from the control units  2   a,    2   b,  and  2   c  controlling the apparatus B. In  FIG. 8 , the information indicating the relationship between temperature rising rate and DC voltage is represented as a symbol of a graph, for convenience. 
     The server  2  classifies the received information into a category with respect to the apparatus A and a category with respect to the apparatus B. The information  3   a,    3   b,  and  3   c  indicating the relationship with respect to the apparatus A belongs to the category with respect to the apparatus A, and the information  4   a,    4   b,  and  4   c  indicating the relationship with respect to the apparatus B belongs to the category with respect to the apparatus B. 
     The server  2  calculates an optimal value of DC voltage corresponding to temperature rising rate of the apparatus A, based on the information  3   a,    3   b,  and  3   c  classified to the category with respect to the apparatus A. For example, when obtaining one optimal value of DC voltage corresponding to a certain temperature rising rate of the apparatus A, the server  2  may obtain the optimal value by calculating an average of each DC voltage value corresponding to the certain temperature rising rate of the apparatus A, based on the information  3   a,    3   b,  and  3   c.  The server  2  may also obtain the optimal value by calculating a median of each DC voltage value corresponding to the certain temperature rising rate of the apparatus A. Alternatively, the server  2  may obtain the optimal value by calculating a maximum or a minimum of each DC voltage value corresponding to the certain temperature rising rate of the apparatus A. In another embodiment, the server  2  may obtain the optimal value by calculating a specific value among each DC voltage value corresponding to the certain temperature rising rate of the apparatus A, based on the information  3   a,    3   b,  and  3   c.    
     Similarly, the server  2  calculates an optimal value of DC voltage corresponding to temperature rising rate of the apparatus B, based on the information  4   a,    4   b,  and  4   c  classified to the category with respect to the apparatus B. For example, when obtaining one optimal value of DC voltage corresponding to a certain temperature rising rate of the apparatus B, the server  2  may obtain the optimal value by calculating an average, a median, a maximum, or a minimum of each DC voltage value corresponding to the certain temperature rising rate of the apparatus B, based on the information  4   a,    4   b,  and  4   c.  Alternatively, the server  2  may obtain the optimal value by calculating a specific value among each DC voltage value corresponding to the certain temperature rising rate of the apparatus B, based on the information  4   a,    4   b,  and  4   c.    
     The server  2  calculates an optimal value of DC voltage corresponding to temperature rising rate, from DC voltage corresponding to temperature rising rate of each plasma etching apparatus, and feeds the calculated optimal value of DC voltage corresponding to the temperature rising rate, back to the control units  1   a,    1   b,    1   c,    2   a,    2   b,  and  2   c.  Accordingly, the control units  1   a  to  2   c  can control DC voltage applied to the focus ring  30  by using the optimal value of DC voltage corresponding to degree of abrasion of the focus ring  30 , which is obtained by using information of not only its own plasma etching apparatus but also other plasma etching apparatuses. 
     According to the above description, information about DC voltage corresponding to temperature rising rate measured in multiple plasma etching apparatuses belonging to the same category can be collected by the server  2 . Thus, based on the information about DC voltage corresponding to temperature rising rate collected from the multiple plasma etching apparatuses, an optimal value of DC voltage corresponding to temperature rising rate can be calculated without variation. Accordingly, DC voltage of an optimal magnitude corresponding to degree of abrasion of the focus ring  30  can be applied to the focus ring  30  more precisely. Note that the server  2  may be implemented by a cloud computing environment. 
     As described above, according to the above-described embodiment, by applying, to the focus ring  30 , appropriate DC voltage corresponding to degree of abrasion of the focus ring  30 , at least one of occurrence of tilting and variation of an etching rate can be suppressed. Therefore, because a cycle of replacement of the focus ring  30 , which is caused by abrasion of the focus ring  30 , can be extended, productivity in the plasma etching apparatus is increased. 
     In the above embodiments, the plasma etching apparatus and the plasma etching method have been described. However, a plasma etching apparatus and a plasma etching method according to the present invention are not limited to the above embodiments. Various changes or enhancements can be made hereto within the scope of the present invention. Matters described in the above embodiments may be combined unless inconsistency occurs. 
     The plasma etching apparatus according to the present invention can be applicable to any type of plasma processing apparatuses, such as a capacitively coupled plasma (CCP) type, an inductively coupled plasma (ICP) type, a radial line slot antenna type, an electron cyclotron resonance plasma (ECR) type, and a helicon wave plasma (HWP) type. 
     In this specification, the semiconductor wafer W is referred to as an example of a workpiece. However, the workpiece is not limited to the semiconductor wafer. Examples of the workpiece may include various types of substrates used in an LCD (Liquid Crystal Display) or a FPD (Flat Panel Display), a CD substrate, or a printed circuit board.