Patent Publication Number: US-9412565-B2

Title: Temperature measuring method and plasma processing system

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a temperature measuring method and a plasma processing system. 
     DESCRIPTION OF THE RELATED ART 
     In a plasma processing apparatus that performs a plasma process such as etching, variations in the temperature of a member arranged within a chamber of the plasma processing apparatus may cause variations in processing results of a plasma process performed on a substrate, for example. Accordingly, techniques are known for monitoring the temperature of a member arranged within the chamber and controlling the temperature of the member to a predetermined temperature (e.g. see International Patent Publication No. WO/2013/099063). 
     The temperature of a member arranged within the chamber may be measured by various types of thermometers using a contact method or a non-contact method. For example, a radiation thermometer may be used in the case of measuring the temperature of a member using a non-contact method. The radiation thermometer measures radiated light (e.g. infrared light) emitted from a member corresponding to a measuring object arranged within the chamber via a measurement window that is arranged at the chamber and converts the measured amount of radiation into a corresponding temperature. 
     The amount of radiation measured by the radiation thermometer may vary depending on the temperature of the measurement window and the transparency of the measurement window, for example. Accordingly, when the measurement window does not have adequate transparency and the temperature of the measurement window varies depending on heat input from plasma, measurement errors due to such influences that are included in the temperature measurements of the radiation thermometer cannot be ignored, and it may be difficult to accurately measure the temperature of the member corresponding to the measuring object. 
     On the other hand, the measurement window is exposed to plasma, and as such, a material that is resistant plasma has to be selected for the measurement window. Thus, restrictions are imposed on the materials that can be used as the measurement window in view of the demands for both adequate transparency and adequate plasma resistance in the measurement window, and it is rather difficult to select a suitable material having adequate transparency without due consideration to the plasma resistance of the material. 
     SUMMARY OF THE INVENTION 
     In view of the above, an aspect of the present invention relates to providing a temperature measuring method and a plasma processing system that are capable of accurately measuring the temperature of a member corresponding to a measuring object that is arranged within a plasma processing apparatus via a measurement window. 
     According to one embodiment of the present invention, a temperature measuring method for measuring a temperature of a member corresponding to a measuring object arranged within a chamber of a plasma processing apparatus is provided. The temperature measuring method includes a function obtaining step of obtaining a function (f) for correcting a correction target temperature (T meas ) according to a measurement window temperature (T w ), the function (f) being computed based on the correction target temperature (T meas ) corresponding to a temperature of the measuring object measured via a measurement window arranged at the chamber, a reference temperature (T obj ) corresponding to a temperature of the measuring object measured without using the measurement window, and the measurement window temperature (T w ) corresponding to a temperature of the measurement window. The temperature measuring method further includes a first measuring step of measuring the correction target temperature (T meas ) a second measuring step of measuring the measurement window temperature (T w ), and a correction step of correcting the correction target temperature (T meas ) measured in the first measuring step according to the measurement window temperature (T w ) measured in the second measuring step based on the obtained function (f). 
     According to another embodiment of the present invention, a plasma processing system is provided that includes a plasma processing apparatus and a temperature measuring apparatus for measuring a temperature of a member corresponding to a measuring object arranged within a chamber of the plasma processing apparatus. The temperature measuring apparatus includes a function obtaining unit configured to obtain a function (f) for correcting a correction target temperature (T meas ) according to a measurement window temperature (T w ), the function (f) being computed based on the correction target temperature (T meas ) corresponding to a temperature of the measuring object measured via a measurement window arranged at the chamber, a reference temperature (T obj ) corresponding to a temperature of the measuring object measured without using the measurement window, and the measurement window temperature (T w ) corresponding to a temperature of the measurement window. The temperature measuring apparatus further includes a measuring unit configured to measure the correction target temperature (T meas ) and the measurement window temperature (T w ), and a correction unit configured to correct the correction target temperature (T meas ) according to the measurement window temperature (T w ) measured by the measuring unit based on the function (f) obtained by the function obtaining unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an overall configuration of a plasma processing system according to an embodiment of the present invention; 
         FIG. 2  illustrates a temperature measuring method for measuring the temperature of a member corresponding to a measuring object according to an embodiment of the present invention; 
         FIG. 3  illustrates the influence of a window temperature on temperature measurement by a radiation thermometer according to an embodiment of the present invention; 
         FIGS. 4A and 4B  illustrate examples of temperature measurement processes performed upon model creation and during plasma processing according to an embodiment of the present invention; 
         FIGS. 5A and 5B  illustrate exemplary data used for regression analysis upon creating a model according to an embodiment of the present invention; 
         FIG. 6  illustrates a specific example of data used for regression analysis upon creating a model according to an embodiment of the present invention; 
         FIGS. 7A and 7B  illustrate an exemplary effect achieved by a temperature measuring method according to an embodiment of the present invention; 
         FIG. 8  illustrates an exemplary testing apparatus used upon creating a model according to an embodiment of the present invention; 
         FIG. 9  illustrates an example of applying a temperature measuring method according to an embodiment of the present invention to endpoint detection of a heating process; and 
         FIGS. 10A-10D  illustrate examples of members corresponding to measuring objects according to embodiments of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention are described with reference to the accompanying drawings. Note that in the following descriptions and accompanying drawings, elements having substantially the same functions or features may be given the same reference numerals and overlapping descriptions thereof may be omitted. 
     [Overall Configuration of Plasma Processing System] 
     First, overall configurations of a plasma processing apparatus and a plasma processing system according to an embodiment of the present invention are described with reference to  FIG. 1 .  FIG. 1  illustrates exemplary overall configurations of a plasma processing system  2  and a plasma processing apparatus  1  according to an embodiment of the present invention. 
     Note that in the following descriptions of the present embodiment, a capacitively-coupled plasma etching apparatus is illustrated as an example of the plasma processing apparatus  1 . Also, in the following descriptions of the present embodiment, an exemplary case is illustrated in which a radiation thermometer  100  is configured to measure the temperature of a focus ring  18  via a measurement window  102 . 
     The plasma processing system  2  of the present embodiment includes the plasma processing apparatus  1  and a temperature measuring apparatus including a computer  150 . The plasma processing apparatus  1  and the temperature measuring apparatus are interconnected via a wireless or wired network. 
     The plasma processing apparatus  1  includes a cylindrical vacuum chamber (processing chamber)  10  (simply referred to as “chamber” hereinafter) made of aluminum having an alumite-treated (anodized) surface, for example. The chamber  10  may be grounded, for example. 
     A mounting table  12  is arranged within the chamber  10 . The mounting table  12  may be made of aluminum (Al), titanium (Ti), or silicon carbide (SiC), for example, and is supported on a cylindrical support  16  via an insulating cylindrical holder  14 . The cylindrical support  16  extends vertically upward from a bottom of the chamber  10 . 
     An exhaust path  20  is formed between a sidewall of the chamber  10  and the cylindrical support  16 . A ring-shaped baffle plate  22  is arranged in the exhaust path  20 . An exhaust port  24  is formed at a bottom portion of the exhaust path  20  and is connected to an exhaust device  28  via an exhaust pipe  26 . The exhaust device  28  includes a vacuum pump such as a turbo-molecular pump or a dry pump (not shown) and is configured to reduce the pressure of a processing space within the chamber  10  to a predetermined level of vacuum. A gate valve  30  configured to open/close an entry/exit port for a semiconductor wafer (referred to as “wafer W” hereinafter) is arranged at the sidewall of the chamber  10 . 
     A first high frequency power supply  31  for plasma excitation is connected to the mounting table  12  via a matching unit  33 , and a second high frequency power supply  32  for drawing ions toward the wafer W is connected to the mounting table  12  via a matching unit  34 . For example, the first high frequency power supply  31  may be configured to apply to the mounting table  12  a first high frequency power having a frequency that is suitable for generating a plasma within the chamber  10  (e.g. 60 MHz). The second high frequency power supply  32  may be configured to apply to the mounting table  12  a second high frequency power of a lower frequency that is suitable for drawing ions from within the plasma onto the wafer W placed on the mounting table  12  (e.g. 0.8 MHz). In this way, the mounting table  12  is configured to hold the wafer W and also act as a lower electrode. 
     An electrostatic chuck  40  configured to hold the wafer W by an electrostatic attraction force is arranged on a top surface of the mounting table  12 . The electrostatic chuck  40  includes an electrode  40   a  that is made of a conductive film and is arranged between a pair of insulating layers  40   b  (or insulating sheets). A DC voltage supply  42  is connected to the electrode  40   a  via a switch  43 . The electrostatic chuck  40  electrostatically attracts and holds the wafer W by a Coulomb force that is generated when a voltage is applied thereto from the DC voltage supply  42 . To improve in-plane etching uniformity, a focus ring  18  that may be made of silicon or quartz, for example, is arranged around an outer edge portion of the electrostatic chuck  40 . 
     A shower head  38  that acts as an upper electrode at a ground potential is arranged at a ceiling portion of the chamber  10 . In this way, the high frequency power from the first high frequency power supply  31  may be capacitively applied between the mounting table  12  and the shower head  38 . 
     The shower head  38  includes an electrode plate  56  having multiple gas holes  56   a  and an electrode supporting body  58  configured to detachably hold the electrode plate  56 . A gas supply source  62  is configured to supply gas to the shower head  38  via a gas supply pipe  64 , which is connected to a gas inlet  60   a . In this way, the gas may be introduced into the chamber  10  from the multiple gas holes  56   a . A magnet  66  is arranged to extend annularly or concentrically around the chamber  10  so that the plasma generated within a plasma generation space between the upper electrode and the lower electrode may be controlled by the magnetic force of the magnet  66 . 
     A coolant path  70  is formed within the mounting table  12 . A coolant cooled to a predetermined temperature is supplied to the coolant path  70  from a chiller unit  71  via pipes  72  and  73 . Also, a heater  75  is embedded within the electrostatic chuck  40 . Note that the heater  75  may alternatively be attached to a backside surface of the electrostatic chuck  40  instead of being embedded within the electrostatic chuck  40 . A desired AC voltage is applied to the heater  75  from an AC power supply  44 . A heat transfer gas supply source  52  is configured to supply a heat transfer gas such as He gas between a backside surface of the wafer W and a top surface of the electrostatic chuck  40  through a gas supply line  54 . 
     With such a configuration, the temperature of the wafer W may be adjusted to a desired temperature through cooling by the circulation of a coolant from the chiller unit  71  through the coolant path  70  and heating by the heater  75 . Note that such temperature control may be performed based on a command from a control unit  80 . 
     The control unit  80  is configured to control the components arranged in the plasma processing apparatus  1  such as the exhaust device  28 , the AC power supply  44 , the DC voltage supply  42 , the switch  43  for the electrostatic chuck, the first high frequency power supply  31 , the second high frequency power supply  32 , the matching units  33  and  34 , the heat transfer gas supply source  52 , the gas supply source  62 , and the chiller unit  71 . 
     In the plasma processing apparatus  1 , variations in the temperature of a member arranged within the chamber  10  may cause variations in processing results of a plasma process performed on the wafer W. Accordingly, it is important to monitor the temperature of the member arranged within the chamber  10  and control the temperature of the member to a predetermined temperature. In the following descriptions, the focus ring  18  is illustrated as an example of a member corresponding to a measuring object according to the present embodiment. 
     The radiation thermometer  100  measures the amount of infrared light (an example of radiated light) that is emitted by the focus ring  18  via a measurement window  102  that is arranged at the chamber  10 . The computer  150  acquires the measured amount of infrared light and converts the measured amount of infrared light into a corresponding temperature. In this way, the temperature of the focus ring  18  may be calculated. 
     In  FIG. 1 , the temperature of the measurement window  102  is measured using a thermocouple  120 . The computer  150  acquires the temperature of the measurement window  102  measured by the thermocouple  120 , which is attached to the measurement window  102 . An AD converter  140  is configured to convert the measured temperature of the measurement window  102  from an analog signal to a digital signal. The computer  150  acquires the digital signal of the temperature of the measurement window  102  after the conversion. 
     As described below, the computer  150  is configured to correct the temperature of the focus ring  18  measured by the radiation thermometer  100  according to the temperature of the measurement window  102 . Note that the computer  150 , the radiation thermometer  100 , the thermocouple  120 , the AD converter  140  are exemplary elements of a temperature measuring apparatus according to the present embodiment. Note that the temperature measuring apparatus according to the present embodiment may include a thermometer other than a radiation thermometer and a thermocouple, for example. Also, the computer  150  may be implemented by a personal computer or some other information processing apparatus, for example. 
     The control unit  80  acquires the corrected temperature of the focus ring  18  corrected by the computer  150  and controls the temperature within the chamber  10  based on the acquire temperature. The control unit  80  includes a CPU (Central Processing Unit) and a storage area such as a ROM (Read-Only Memory), a RAM (Random Access Memory), and a HDD (Hard Disk Drive), for example (not shown). The CPU is configured to execute processes such as a plasma process on a wafer W and a temperature control process on a member within the chamber  10  according to relevant recipes specifying process procedures and process conditions, for example. 
     A storage area for storing the recipes may be configured as a RAM or a ROM using a semiconductor memory, a magnetic disk, or an optical disk, for example. The recipes may be stored in a storage medium and loaded in the storage area via a driver (not shown), for example. Alternatively, the recipes may be downloaded from a network (not shown) and stored in the storage area, for example. Also, note that a DSP (digital signal processor) may be used instead of the CPU to perform the above functions. The functions of the control unit  80  may be implemented by software, hardware, or a combination thereof. 
     [Temperature Measuring Method] 
     In the following, a temperature measuring method according to an embodiment of the present invention is described with reference to  FIG. 2 . In the plasma processing apparatus  1 , variations in the temperature of a member arranged within the chamber  10  may cause variations in processing results of a plasma process performed on the wafer W. That is, reactions occurring at the interfaces between a plasma and a member arranged within the chamber  10  such as the focus ring  18 , the electrode plate  56 , or the mounting table  12  may vary depending on the surface temperature of the member. As a consequence, variations may occur in the composition of the plasma to thereby cause variations in processing results of a plasma process performed on the wafer W. In view of the above, the temperature of the member within the chamber  10  is typically monitored such that the temperature of the member may be controlled to a predetermined temperature. 
     For example, the radiation thermometer  100  of  FIG. 2  may measure the amount of radiated light emitted from a member corresponding to a measuring object such as the focus ring  18  or the electrode plate  56  (ceiling member) within the chamber  10  via the measurement window  102 . The amount of radiated light measured by the radiation thermometer  100  may vary depending on the temperature of the measurement window  102  and the transparency of the measurement window  102 , for example. 
     The above effect is described in greater detail with reference to  FIG. 3 . As illustrated in  FIG. 3 , the intensity of radiated light emitted from a member corresponding to a measuring object may be expressed as follows:
 
ε obj   ×R   obj ( T   obj )
 
     In the above expression, ε at  represents the emissivity of the measuring object, R obj  represents the intensity of black body radiation at the temperature T obj  of the measuring object. The intensity R obj  of black body radiation at the temperature T obj  is a function of the temperature T obj  of the measuring object Tobj member, and may be expressed as R obj  (T obj ). 
     The intensity of the radiated light emitted light from the measuring object is absorbed by the measurement window  102  and is attenuated upon passing through the measurement window  102 . The intensity of the radiated light after attenuation may be expressed as follows:
 
τ w ε obj   ×R   obj ( T   obj ).
 
In the above expression, τ w  represents the transmittance of the measurement window  102 .
 
     Also, an intensity of radiated light from the measurement window  102  is generated separately from the intensity of the radiated light described above, and the intensity of the radiated light from the measurement window  102  may be expressed as follows:
 
ε w   ×R   w ( T   w )
 
In the above expression, ε w  represents the emissivity of the measurement window  102 , R w  represents the intensity of black body radiation at the temperature T w  of the measurement window  102 . The intensity R w  intensity of black body radiation at temperature T w  is a function of the temperature T w  of the measurement window  102  and may be expressed as R w (T w ).
 
     As illustrated in  FIG. 3 , the intensity of radiated light measured by the radiation thermometer  100  corresponds to the sum of the attenuated intensity of the radiated light emitted from the measuring object and the intensity of the radiated light from the measurement window  102 . In a case where the transmittance of the measurement window  102  is adequately high, the transmittance of the measurement window may be approximately equal to one (τ w ≈1), and the emissivity of the measurement window may be approximately equal to zero (τ w ≈0). Accordingly, the intensity of radiated light measured by the radiation thermometer  100  may be substantially equal to the intensity of the radiated light from the measuring object. That is, in the case where the transmittance of the measurement window  102  is adequately high, the radiation thermometer  100  may accurately measure the temperature of the measuring object without being affected by variations in the temperature T w  of the measurement window  102 . 
     However, the measurement window  102  is exposed to plasma in the plasma processing apparatus  1 . Therefore, a material that is resistant to plasma has to be selected for the measurement window  102 . For this reason, restrictions are imposed on the materials that may be used for the measurement window  102 , and it is rather difficult to select a material having adequate transparency as the material of the measurement window  102 . Thus, the temperature measuring method according to the present embodiment relates to a method of accurately measuring the temperature of a measuring object by taking into account the variations in the temperature T w  of the measurement window  102 . In the temperature measuring method according to the present embodiment, the computer  150  corrects the temperature of the focus ring  18  measured by the radiation thermometer  100  according to the temperature of the measurement window  102 . To perform such a correction, the computer  150  creates a model for performing the correction and defines a function f for the correction. In the following descriptions, the focus ring  18  is illustrated as an example of a member corresponding to the measuring object for temperature measurement according to the present embodiment. 
     (Model Creation) 
       FIG. 4A  illustrates an exemplary temperature measurement process upon creating a model. As illustrated in  FIG. 4A , at the model creation stage, the chamber  10  is exposed to the atmosphere, and three temperatures are measured in such a state. A first temperature of the temperatures measured corresponds to the temperature of the member corresponding to the measuring object that is measured without using the measurement window  102 . Note that in the following descriptions, the temperature of the measuring object that has been measured without using the measurement window  102  is also referred to as “reference temperature T obj ”. In the present example, the reference temperature T obj  corresponds to the temperature of the focus ring  18  that is directly measured by a radiation thermometer  110  without using the measurement window  102 . Note that the thermometer used for measuring the reference temperature T obj  may be a radiation thermometer, a fluorescent thermometer, or some other type of thermometer. The thermometer may be a non-contact type or a contact type thermometer. 
     A second temperature of the temperatures measured corresponds to the temperature of the measuring object that is measured via the measurement window  102 . In the following descriptions, the temperature of the measuring object that is measured via the measurement window  102  is also referred to as “correction target temperature T meas ”. In the present example, the correction target temperature T meas  corresponds to the temperature of the focus ring  18  that has been indirectly measured by the radiation thermometer  100  via the measurement window  102 . Note that the thermometer used for measuring the correction target temperature T meas  is a non-contact type thermometer. 
     A third temperature of the temperatures measured corresponds to the temperature T w  of the measurement window  102 . In the present example, the temperature T w  of the measurement window  102  is measured using the thermocouple  120 . The thermometer for measuring the measurement window  102  may be a radiation thermometer, or a fluorescent thermometer, for example. The thermometer may be a contact type or a non-contact type thermometer. 
     The computer  150  gathers measurement data and computes a function f for correcting the measurement data (modeling) in the following manner. 
     1. The radiation thermometer  110  measures the reference temperature T obj  while the chamber  10  is open to the atmosphere. The reference temperature T obj  is sent to the computer  150 . 
     2. At the same time or in parallel with the above measurement  1 , the radiation thermometer  100  measures the correction target temperature T meas  of the focus ring  18 . The correction target temperature T meas  is sent to the computer  150 . 
     3. Temperature control mechanisms arranged at the focus ring  18  and the outer wall of the chamber  10  are respectively used to adjust the temperature of the focus ring  18  and the temperature of the measurement window  102 . The temperature of the focus ring  18  and the temperature of the measurement window  102  are adjusted using separate temperature control mechanisms. The radiation thermometers  100  and  110 , and the thermocouple  120  repeatedly measure the correction target temperature T meas  of the focus ring  18 , the reference temperature T obj  of the focus ring  18 , and the temperature T w  of the measurement window  102 . The temperature T w  of the measurement window  102  is converted from an analog signal to a digital signal by the AD converter  140  and sent to the computer  150 . 
     4. The computer  150  stores the measured reference temperature T obj , the measured correction target temperature T meas , and the measured temperature T w  of the measuring window  102  in a storage area such as a RAM. 
     5. The computer  150  recursively determines a function f where T obj =f(T meas , T w ) based on measurement data of a plurality of sets of the stored temperature measurements (reference temperature T obj , correction target temperature T meas , measurement window temperature T w ). The function f is a model formula for correcting the correction target temperature T meas  according to the measurement window temperature T w  such that the correction target temperature T meas  may be equal to or approximate the reference temperature T obj . The function f may be determined based on the measurement data of the correction target temperature T meas , the measurement window temperature T w , and the reference temperature T obj . In the following descriptions, a corrected temperature that is obtained by applying the function f to the correction target temperature T meas  is referred to as “corrected temperature T meas ′”. In other words, the relationship between the correction target temperature T meas  (output of the radiation thermometer  100 ) and the corrected temperature T meas ′ may be expressed as follows.
 
 T   meas   ′=f ( T   meas   ,T   w )
 
     In the following, as an exemplary method of recursively calculating the function f, an example implementing multiple regression analysis using the least squares method is described. However, the method of recursively calculating the function f is not limited thereto and any known recursive calculation method including those listed below may be used.
         Polynomial approximation model
 
 y=c+a 1× x 1+ b 1× x 2+ a 2× x 1 2   +b 2× x 2 2 +
   Exponential approximation model
 
 y=c+a ×exp( a′×x 1)+ b ×exp( b′×x 2)+
   Logarithmic function approximation model
 
 y=c+a ×log( x 1)+ b ×log( x 2)+
   Neural Network Model       

       FIGS. 5A and 5B  illustrate exemplary data used in recursive calculation for creating a model according to the present embodiment. As illustrated in  FIG. 5A , in the following example, it is assumed that target variable y represents the reference temperature T obj , variable x 1  represents the correction target temperature T meas , and variable x 2  represents the measurement window temperature T w . The computer  150  may compute the following regression formula (1) based on the measurement data of the three temperatures.
 
 y=β   0 +β 1   x   1 +β 2   x   2   (1)
 
     In this case, using the least squares method, the coefficients β 0 , β 1 , β 2  of the above formula (1) may be calculated based on the following formulas (2)-(5). In the present example, it is assumed that n sets of measurement data of the three temperatures indicated in  FIG. 5B  are used for multiple regression analysis. 
     
       
         
           
             
               
                 
                   
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     The function f may be obtained by assigning the values calculated above to the coefficients β 0 , β 1 , β 2  in the above formula (1). 
     Example 
     In the following, a specific example of recursively determining the function f is described with reference to  FIG. 6 . In the example described below, it is assumed that nine sets of measurement data of the three temperatures as indicated in  FIG. 6  are used for multiple regression analysis. The computer  150  assigns the measured temperatures of the measurement data to the corresponding variables of the formulas 2)-(5) to obtain the coefficients β 0 , β 1 , β 2  of the formula (1). In the case where the nine sets of measurement data of the measured temperatures indicated in  FIG. 6  are used, the following recursive formula may be obtained.
 
 y= 3.93+0.12 x   1 +0.72 x   2  
 
By modifying the above formula, the following formula may be obtained.
 
 x   1 =8.30 y− 6.01 x   2 −32.6
 
In this way, the function f may be obtained by assigning the calculated values to the coefficients β 0 , β 1 , β 2  in the formula (1).
 
     That is, the function f may be expressed by the following formula (6).
 
 f ( T   meas   ,T   w )=−32.6+8.30 T   meas −6.01 T   w   (6)
 
T meas : correction target temperature; T w : measurement window temperature.
 
     (During Process) 
     When the radiation thermometer  100  measures the temperature of the focus ring  18  via the measurement window  102  while a process is performed, the computer  150  uses the function f to correct the measured correction target temperature T meas  of the focus ring  18  according to the temperature T w  of the measurement window  102 . In this case, the computer  150  assigns the measured values to the correction target temperature T meas  and the measurement window temperature T w  in the formula representing the function f obtained in the above model creation stage to calculate the corrected temperature T meas ′. Note that the corrected temperature T meas ′ corresponds to the measured correction target temperature T meas  that is corrected to be equal to or approximate the reference temperature T obj . The corrected temperature T meas ′ may be obtained as follows based on the above formula (6).
 
 T   meas   ′=f ( T   meas   ,T   w )
 
     Note that during a process, the reference temperature T obj  may be set to a predetermined temperature of the focus ring  18  during processing that is prescribed in a recipe, for example. In such case, the corrected temperature T meas ′ that is obtained by applying the function f to the correction target temperature T meas  may be equal to or approximate the predetermined temperature of the focus ring  18  during processing that is prescribed in the recipe (reference temperature T obj ). Note that a temperature control mechanism is arranged at the focus ring  18 , and based on the corrected temperature T meas ′, the temperature control mechanism of the focus ring  18  may adjust the temperature of the focus ring  18  through heating and cooling such that the correction target temperature T meas  may be equal to or approximate the predetermined temperature of the focus ring  18  prescribed in the recipe. 
     In the present embodiment, the computer  150  measures the correction target temperature T meas  and computes the corrected temperature T meas ′ in the following manner. 
     1. Before starting the process, the radiation thermometer  110  is removed and the pressure within the chamber  10  is reduced to vacuum as illustrated in  FIG. 4B . 
     2. The computer  150  monitors the temperature T w  of the measurement window  102 . At the same time or in parallel, the computer  150  monitors the correction target temperature T meas  of the focus ring  18  measured by the radiation thermometer  100  via the measurement window  102 . 
     3. The computer  150  assigns the acquired temperature T w  of the measurement window  102  and the correction target temperature T meas  measured by the radiation thermometer  100  to the corresponding variables in the function f to calculate the corrected temperature T meas ′. In this way, the accuracy of the temperature of a member (measuring object) measured by the radiation thermometer  100  may be improved. 
     [Material of Measurement Window] 
     In one example, yttria (Y 2 O 3 ) may be used in the measurement window  102 . In this case, the measurement window  102  may include at least one layer made of yttria. That is, the measurement window  102  may be made of a bulk of yttria, or the measurement window  102  may be formed by coating a yttria thin film on an infrared light (radiated light) transparent material, for example. In other examples, the measurement window  102  may be made of calcium fluoride (CaF 2 ), sapphire, quartz, silicon, germanium, or any combination of the above materials. 
     [Effects] 
     As described above, a material with adequate plasma resistance has to be selected for the measurement window  102 . Thus, when adequate transparency cannot be achieved in the measurement window  102  owing to the demand for a plasma resistant material, temperature measurements of the radiation thermometer  100  may be affected by variations in the temperature of the measurement window  102 . Particularly, when measuring the temperature of a member arranged within the plasma processing apparatus  1 , the temperature of the measurement window  102  is affected by heat input from plasma and varies depending on the number of wafers processed (processing time). As a result, the temperature of a member within the chamber  10  may not be accurately measured by the radiation thermometer  100  via the measurement window  102  while a plasma process is being performed, for example. 
     In view of the above, in a temperature measuring method according to the present embodiment, the temperature of the measurement window  102  is constantly measured. When the radiation thermometer  100  measures the correction target temperature T meas  via the measurement window  102 , the computer  150  assigns the acquired temperature T w  of the measurement window  102  and the correction target temperature T meas  to corresponding variables in the function f to calculate the corrected temperature T meas ′ corresponding to the corrected value of the correction target temperature T meas . In this way, the correction target temperature T meas  may be corrected to be equal to or approximate the reference temperature T obj , and the accuracy of the measured temperature may be improved. 
       FIGS. 7A and 7B  illustrate an exemplary effect achieved by the temperature measuring method according to the present embodiment.  FIG. 7A  is a graph including a curve representing an exemplary measurement of the correction target temperature T meas  (radiation thermometer  100  output) that is measured via the measurement window  102  and a curve representing the corrected temperature T meas ′ of the correction target temperature T meas . As can be appreciated, the correction target temperature T meas  varies depending on the number of wafers processed. The corrected temperature T meas ′ is obtained by correcting the correction target temperature T meas  according to the measurement window temperature T w  based on a predetermined function f.  FIG. 7B  illustrates an exemplary measurement of the measurement window temperature T w . 
     The correction target temperature T meas  is corrected to remove the influences of variations in the temperature T w  of the measurement window  102 . Note that the curve representing the corrected temperature (T meas ′) in  FIG. 7A  is not obtained by simply shifting the curve representing the correction target temperature (T meas ) by a predetermined amount. That is, the correction target temperature T meas  measured by the radiation thermometer  100  includes not only measurement errors specifically attributed to properties of the radiation thermometer  100 , but also errors due to temperature variations of the measurement window  102  caused by heat input from plasma. The temperature T w  of the measurement window  102  varies depending on the number of wafers processed. That is, the errors included in the correction target temperature T meas  include errors based on variations in the temperature T w  of the measuring window  102 , and the errors based on variations in the temperature T w  of the measurement window  102  change over time. 
     Thus, the variations in the correction target temperature T meas  does not correspond to the variations in temperature T w  of the measurement window  102 . Therefore, in the temperature measuring method according to the present embodiment, the measurement errors included in the correction target temperature T meas  which varies depending on the temperature T w  of the measurement window  102 , is corrected based on the function f. In this way, even in a case where the temperature T w  of the measuring window  102  constantly changes, the temperature of a member corresponding to a measuring object arranged within the plasma processing apparatus  1  may be accurately measured according to the temperature T w  of the measurement window  102 . 
     Note that when plasma processing of wafers included in one lot of wafers is completed, for example, the chamber  10  may be reheated. Thus, the temperature T w  of the measurement window  102  constantly changes. Accordingly, in a preferred embodiment, the correction target temperature T meas  and the temperature T w  of the measurement window  102  are constantly measured not only during processing but before and after processing and during transfer of the wafers, for example, and the correction target temperature T meas  is constantly corrected using the function f. 
     (Model Creation: Modification) 
     In the embodiment described above, the focus ring  18  within the chamber  10  of the plasma processing apparatus  1  corresponds to the measuring object, and the plasma processing apparatus  1  is used to measure the temperature of the focus ring  18 . However, in other embodiments, instead of actually using the plasma processing apparatus  1 , a testing apparatus for model creation may be configured and such a testing apparatus may be used to create a model for temperature measurement of a measuring object. 
       FIG. 8  illustrates a testing apparatus  300  for creating a model according to a modified example. The testing apparatus  300  of  FIG. 8  includes the radiation thermometer  100 , a simplified window  202  made of yttrium, the thermocouple  120  that is connected to the window  202 , a material sample  118  corresponding to a measuring object that is arranged on a stage, a thermocouple  210  that is connected to the material sample  118 , and a heat gun  200  for adjusting the temperature of the window  202 . The heat gun  200  is an example of a temperature control mechanism for adjusting the temperature of the window  202 . The thermocouple  120  and the thermocouple  210  are configured to measure the temperatures of the window  202  and the material sample  118 , respectively. 
     In the present modified example, the simplified testing apparatus  300  as described above is used to measure the reference temperature y (=T obj ), the correction target temperature x 1  (=T meas  (output of the radiation thermometer  100 )), and the window temperature x 2  (=T w ). The computer  150  recursively calculates the function f defined by T obj =f(T meas , T w ) based on a plurality of sets of measurement data obtained by the testing apparatus  300 . 
     By using such a simplified testing apparatus  300 , a model for correcting a temperature measurement may be created even when the actual plasma processing apparatus  1  in which the member corresponding to the measuring object is arranged does not include the measurement window  102  or a temperature control mechanism for controlling the temperature of the member corresponding to the measuring object. 
     [Application] 
     In the following, an exemplary application of the temperature measuring method according to the present embodiment is described with reference to  FIG. 9 . As illustrated in  FIG. 9 , owing to heat input from plasma, the temperature of a member corresponding to a measuring object gradually changes according to the number of wafers processed. Accordingly, a plasma is preferably generated in the plasma processing apparatus  1  prior to wafer processing and the member corresponding to the measuring object is pre-heated by the plasma so that variations in the temperature of the measuring object during wafer processing may be minimized. 
     In this case, when the temperature of the member corresponding to the measuring object cannot be accurately measured, the appropriate amount of time the chamber  10  should be heated by the plasma (heating time t) to minimize variations in the temperature of the measuring object cannot be properly calculated. 
     In this respect, by implementing the temperature measuring method according to the present embodiment, the temperature of the measuring object may be corrected in real time. Accordingly, the heating time t for heating the wafer W prior to plasma processing may be accurately calculated. That is, the temperature measuring method according to the present embodiment can be used to detect an endpoint for heating the member corresponding to the measuring object. 
     Specifically, a saturation temperature of the measuring object that is generally unlikely to fluctuate may be preset as a target temperature. The saturation temperature may be set to a target temperature determined by the plasma processing apparatus  1  upon processing a previous lot of wafers, for example. The saturation temperature may be set to a temperature during processing or before/after processing of a plasma process performed on the previous lot by the plasma processing apparatus  1 , for example. 
     For example, when the corrected temperature T meas ′ corresponding to the correction target temperature T meas  that is corrected by the function f reaches a target temperature corresponding to a temperature during processing or before/after processing of a last wafer of a previous lot of wafers processed by the plasma processing apparatus  1 , the control unit  80  may determine that the temperature of the measuring object has reached the saturation temperature and control the plasma processing apparatus  1  to start processing a present lot of wafers. 
     As described above, the temperature measuring method according to the present embodiment can be used to determine the heating time t for heating the interior of the chamber  10  with plasma before starting a plasma process. Note, however, that application of the temperature measuring method according to the present embodiment is not limited to the above example. That is, the present invention can generally be applied to any temperature correction to be implemented upon measuring the temperature of a member of the plasma processing apparatus  1  via the measurement window  102 . 
     [Measuring Object] 
     In the following, examples of members that may be measuring objects of the temperature measuring method according to the present embodiment are described. In the embodiment described above, the focus ring  18  is illustrated as an example of a measuring object. Temperature adjustment of the focus ring  18  is important because temperature variations in the focus ring  18  may have a substantial impact on processing results of processing a peripheral portion of the wafer W. Thus, by controlling the temperature of the focus ring  18  to a constant temperature using the temperature measuring method according to the present embodiment, favorable plasma processing results may be obtained upon processing the wafer W, for example. 
     Note, however, that a member corresponding to a measuring object of the temperature measuring method according to the present embodiment is not limited to the above. For example, the electrode plate  56  arranged on the ceiling portion of the chamber  10  as illustrated in  FIG. 10A  may correspond to the measuring object. Also, a liner  103  arranged near an inner wall portion of the chamber  10  to prevent corrosion of the inner wall of the chamber  10  as illustrated in  FIG. 10B  may correspond to the measuring object. Also, the mounting table  12  (electrostatic chuck) or some other member of the plasma processing apparatus  1  may correspond to the measuring object. 
     As illustrated in  FIG. 10C , in some embodiments, the measurement window  102  may be arranged at a ceiling portion of the chamber  10 . In this case, the radiation thermometer  100  can measure a ceiling side temperature of the electrode plate  56 . The temperature of the electrode plate  56  measured by the radiation thermometer  100  may be corrected by the function f in this case as well. 
     As illustrated in  FIG. 10D , in some embodiments, the measurement window  102  may be arranged to penetrate through the chamber  10  and the mounting table  12  at the bottom side of the chamber  10 . In this case, the radiation thermometer  100  can measure a bottom side temperature of the focus ring  18 . The temperature of the focus ring  18  measured by the radiation thermometer  100  may be corrected by the function f in this case as well. 
     In the temperature measuring method and the plasma processing system  2  according to the embodiments described above, the temperature of a member corresponding to a measuring object arranged in the plasma processing apparatus  1  can be accurately measured via the measurement window  102 . Also, by constantly monitoring the temperatures of the measuring object within the chamber  10  and the measurement window  102  during plasma processing and correcting the measured temperature of the member corresponding to the measuring object, temperature variations may be controlled upon repeatedly performing a wafer process. In this way, plasma processing may be effectively performed on a wafer. 
     Although illustrative embodiments of a temperature measuring method and a plasma processing system according to the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to these embodiments. That is, numerous variations and modifications will readily occur to those skilled in the art, and the present invention includes all such variations and modifications that may be made without departing from the scope of the present invention. 
     For example, in the above-described embodiments, the corrected temperature T meas ′ is calculated as T meas ′=f(T meas , T w ) based the above formula (6). That is, the function f includes the correction target temperature T meas  and the measurement window temperature T w  as variables. However, the above method of calculating the corrected temperature T meas ′ is merely one example. That is, the function f may include other variables as long as it includes the correction target temperature T meas  and the measurement window temperature T w  as variables. For example, an environmental temperature T env  may be taken into account, and the function f may include the correction target temperature T meas , the measurement window temperature T w , and the environmental temperature T env  as variables to be recursively defined as f(T meas , T w , T env ). 
     Also, a plasma processing apparatus according to the present invention may be any apparatus that utilizes the action of plasma including an etching apparatus, a film deposition apparatus, an ashing apparatus, and a cleaning apparatus, for example. Also, examples of means used by the plasma processing apparatus to generate a plasma may include a capacitively coupled plasma (CCP) generating unit, an inductively coupled plasma (ICP) generating unit, a helicon wave plasma (HWP) generating unit, a microwave surface wave plasma generating unit for generating a microwave plasma such as a slot plane antenna (SPA) plasma or a microwave plasma generated from a radial line slot antenna, an electron cyclotron resonance plasma (ECR) generating unit, a remote plasma generating unit using the above plasma generating units, and the like. 
     Also, note that when a thermometer is arranged within the chamber of a plasma processing apparatus, the thermometer may be affected by high frequency waves and microwaves and may be exposed to plasma, for example. As a result, the thermometer may be prone to breakdown, and the interior of the chamber may be contaminated by the thermometer. In view of the above, in the temperature measuring method according to the present invention, the temperature of a member within the chamber corresponding to a measuring object is measured from outside the chamber via a measurement window. In this way, breakdown of the thermometer and/or contamination of the chamber may be avoided, for example. Also, manufacturing costs may be reasonably low. 
     Also, in the above-described embodiments, modeling (model creation) and temperature correction are performed by the computer  150 . However, the present invention is not limited thereto, and in other embodiments, a part or all of the modelling and temperature correction processes may be performed by the control unit  80 , for example. Although the computer  150  and the control unit  80  are configured as separate units in the above-described embodiments, they may also be integrated into one unit, for example. 
     Also, a workpiece that is subject to a plasma process in the present invention is not limited to a semiconductor wafer but may be a large substrate for a flat panel display (FPD), an electroluminescence (EL) element, or a substrate for a solar battery, for example. 
     The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2014-019634 filed on Feb. 4, 2014, the entire contents of which are herein incorporated by reference.