Abstract:
A plasma processing apparatus capable of attracting and holding a semiconductor wafer reliably once the processing of the semiconductor wafer is started includes: a vacuum chamber; an electrode arranged inside the vacuum chamber; a dielectric film formed on a surface of the electrode; a gas supply port leading to the vacuum chamber; a high-frequency electric power supply connected to the electrode; a memory operation unit which depends on a processing condition for producing a desired plasma, to calculate and output the voltage value corresponding to the sum of a value of a minimal actual attract and hold voltage required to be applied between one surface of the semiconductor wafer mounted on the dielectric film and a surface of the dielectric film to attract and hold one surface of the semiconductor wafer on the surface of the dielectric film and a value of a self-bias voltage generated at the other surface of the semiconductor wafer when the desired plasma is produced; and an electrostatic chuck power supply for applying the voltage corresponding to the voltage value calculated in the memory operation unit to the electrode. An electrostatic attract and hold vacuum chucking method employed in the plasma processing apparatus is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to plasma processing apparatuses and electrostatic attract-and-hold vacuum chucking methods employed therein, and in particular to plasma processing apparatuses electrostatically attracting and holding semiconductor wafers to fix the semiconductor wafers and electrostatic attract-and-hold vacuum chucking methods employed therein. 
     2. Description of the Background Art 
     In recent years, electrostatic chuck technology has been increasingly used for apparatuses which process semiconductor wafers as desired, such as plasma etching apparatuses, plasma film-forming apparatuses. Electrostatic chuck technology can prevent deposition of foreign matters at the perimeter of a semiconductor wafer that have been conventionally often produced at a wafer clamp clamping the perimeter of the semiconductor wafer. This ensures that the most outer peripheral portion of a semiconductor device fabricated on the semiconductor wafer can be provided as a product to increase yield. Electrostatic chuck technology is a technology that can be utilized for various semiconductor manufacturing apparatuses in the future. 
     Referring to FIG. 1, a conventional plasma processing apparatus  60  which employs electrostatic chuck technology includes a vacuum chamber  21  blocking the external atmosphere from the internal for maintaining the internal atmosphere. 
     Vacuum chamber  21  includes a lower electrode  24 , a dielectric film  23  formed on a surface of lower electrode  24  to attract a semiconductor wafer  22  through electrostatic force, a gas supply port  25  for introducing a desired gas into vacuum chamber  21  from e.g. a gas cylinder (not shown), an upper electrode  26  arranged opposite to lower electrode  24  for diffusing the gas introduced via gas supply port  25  to introduce the gas into vacuum chamber  21  and also functioning as an electrode, an exhaust port  27  provided to exhaust the gas in the vacuum chamber  21  by means of a vacuum pump (not shown), and an insulator  33  formed on lower electrode  24  to maintain the insulation between lower electrode  24  and the gas in vacuum chamber  21 . 
     Plasma processing apparatus  60  also includes an electrostatic chuck power supply  31  for applying a desired voltage to dielectric film  23  via lower electrode  24 , a control signal unit  32  receiving a value of an electrostatic chuck voltage Vs (described hereinafter) stored in a processing-condition memory unit  62  described hereinafter to control a voltage output from electrostatic chuck power supply  31  and thus apply electrostatic chuck voltage Vs from electrostatic chuck power supply  31  to lower electrode  24 , a high-frequency power supply  29  for applying high-frequency electric power to lower electrode  24 , a high-frequency cutting filter  30  provided to prevent the high-frequency electric power from sneaking from high-frequency power supply  29 , and a matching transformer  28  for achieving the matching/integrity between high-frequency power supply  29  and lower electrode  24 . 
     A desired gas introduced into vacuum chamber  21  is electromagnetized by high-frequency power supply  29  to produce a plasma  34 . 
     Plasma processing apparatus  60  also includes a processing-condition memory unit  62  for storing the conditions for producing plasma  34  desired, such as gas flow, the pressure in vacuum chamber  21 , the magnitude of high-frequency electric power (referred to as “processing conditions” hereinafter), and the voltage applied from electrostatic chuck power supply  31  to lower electrode  24 , or electrostatic chuck voltage Vs. 
     A plasma  34  producing operation effected in plasma processing apparatus  60  will now be described briefly and electrostatic attract-and-hold vacuum chuck operation will then be described. 
     Plasma Producing Operation 
     Semiconductor wafer  22  is transported into vacuum chamber  21  via a transport device (not shown) and mounted on lower electrode  24  with dielectric film  23  interposed therebetween. Depending on the processing conditions stored in processing-condition memory unit  62 , a predetermined amount of gas is introduced from gas supply port  25  via upper electrode  26  into vacuum chamber  21 . Simultaneously, a predetermined amount of gas is exhausted from exhaust port  27 . Thus, the pressure inside vacuum chamber  21  is adjusted to have the value of a pressure determined by the processing conditions. Then, high-frequency power supply  29  applies high-frequency electric power to lower electrode  24  via matching transformer  28 . Associated with the application of high-frequency electric power, plasma  34  is produced inside vacuum chamber  21 . Then, desired processes, such as etching, film-forming, are applied to semiconductor wafer  22 . 
     Electrostatic Attract-and-Hold Vacuum Chucking Operation 
     When semiconductor wafer  22  is mounted on dielectric film  23  and plasma  34  is produced in vacuum chamber  21 , an equivalent circuit, such as shown in FIG. 2, is formed. 
     The equivalent circuit shown in FIG. 2 includes electrostatic chuck power supply  31  having one end connected to the ground and the other end connected to lower electrode  24  for applying electrostatic chuck voltage Vs to lower electrode  24 , dielectric film  23  formed on lower electrode  24 , semiconductor wafer  22  mounted on dielectric film  23 , and an equivalent plasma resistance  70  having one end connected to semiconductor wafer  22  and the other end connected to the ground, and formed of plasma  34 . 
     When electrostatic chuck power supply  31  applies negative (−) direct current voltage to lower electrode  24 , positive (+) and negative (−) electric charges are induced at an interface between lower electrode  24  and dielectric film  23  and between dielectric film  23  and semiconductor wafer  22 . As a result, the attraction referred to as Coulomb force or Johnsen-Rahbeck force is caused between semiconductor wafer  22  and dielectric film  23  and semiconductor wafer  22  is thus attracted onto dielectric film  23 . Thus, conventional plasma process apparatus  60  can reliably attract semiconductor wafer  22  onto dielectric film  23  when the characteristics of plasma  34  formed are constant. 
     In plasma processing apparatus  60 , the difference between the electron current and iron current that flow onto semiconductor wafer  22  causes a self-bias voltage Vdc. The value of self-bias voltage Vdc varies depending on the condition of plasma  34 . 
     Referring to FIG. 3, the relation represented as equation (1) is established between self-bias voltage Vdc, a voltage V1 caused between semiconductor wafer  22  and dielectric film  23 , and electrostatic chuck voltage Vs: 
     
       
         Vs=V1+Vdc  (1) 
       
     
     As has been mentioned above, the value of self-bias voltage Vdc varies depending on the condition of plasma  34 . In conventional plasma processing apparatus  60 , however, the value of electrostatic chuck voltage Vs is fixed. Accordingly, for conventional plasma processing apparatus  60 , the value of voltage V1 decreases as the value of self-bias voltage Vdc increases. Thus, the force to attract and hold semiconductor wafer  22  is reduced this disadvantageously. 
     FIG. 4 shows respective experiment results of a self-bias voltage Vdc and a minimal voltage Vmin required to attract and thus hold wafer  22  of 8″φ on dielectric film  23  when the high-frequency electric power output from high-frequency power supply  29  is varied. Minimal voltage Vmin is a voltage applied from electrostatic chuck power supply  31  to lower electrode  24  to attract and hold semiconductor wafer  22  on dielectric film  23 . The graph shows that as that self-bias voltage Vdc has a more negative value, minimal voltage Vmin also has a more negative value. For example, when electrostatic chuck voltage Vs is set at −450V, it is understood that semiconductor wafer  22  can be attracted and held for a high-frequency electric power of no more than 400 W whereas semiconductor wafer  22  cannot be attracted or held for a high-frequency electric power of 500 W. It is thus understood that determining the value of electrostatic chuck voltage Vs depending on self-bias voltage Vdc is important in stabilizing the force to attract and hold semiconductor wafer  22 . 
     For a plasma processing apparatus disclosed in Japanese Patent Laying-Open No. 8-124913, a computer is employed to observe self-bias voltage Vdc. Depending on the value of self-bias voltage Vdc observed, the value of electrostatic chuck voltage Vs is corrected and thus applied to an electrode to stabilize the force to attract and hold a semiconductor wafer. 
     There is a time delay caused, however, in observing self-bias voltage Vdc and then feeding the observed value back to electrostatic chuck voltage Vs. Furthermore, the value of self-bias voltage Vdc varies from time to time, since the plasma is not stable at the start of process. 
     Thus, electrostatic chuck voltage Vs corrected can fail to provide the voltage sufficient to attract and hold a semiconductor wafer. This results in a disadvantage that the force to attract and hold the semiconductor wafer is not stabilized. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the disadvantages described above. 
     One object of the present invention is to provide a plasma processing apparatus capable of reliably attracting and holding a semiconductor wafer once a semiconductor wafer process is started, and an electrostatic attract-and-hold vacuum chucking method employed in the plasma processing apparatus. 
     Another object of the present invention is to provide a plasma processing apparatus wherein simply inputting process conditions allows the same to reliably attract and hold a semiconductor wafer once the processing of the semiconductor wafer is started, and an electrostatic attract-and-hold vacuum chucking method employed for the same. 
     A plasma processing apparatus in one aspect of the present invention includes a vacuum chamber blocking the external atmosphere from the interior thereof for maintaining the atmosphere therein, an electrode arranged in the vacuum chamber, a dielectric film formed on a surface of the electrode, a gas supply port for supplying a desired gas into the vacuum chamber, a plasma production unit for allowing a gas to change into a plasma, a memory operation unit for calculating and outputting depending on a process condition for producing a desired plasma the voltage value equal to the sum of the value of the actual minimal attract and hold voltage required to be applied between one surface of a semiconductor wafer mounted on the dielectric film and a surface of the dielectric film to attract and hold one surface of the semiconductor wafer on the surface of the dielectric film and the value of a self-bias voltage generated at the other surface of the semiconductor wafer when the desired plasma is produced, and an electrostatic chuck power supply for applying the voltage corresponding to the voltage value calculated in the memory operation unit to the electrode. 
     Since the voltage corresponding to the value of an actual attract and hold voltage plus the value of a plasma-generated, self-bias voltage is applied to the electrode, the value of the voltage generated between one surface of the semiconductor wafer and the dielectric film can be equal to the value of the actual attract and hold voltage. Thus, once the processing of the semiconductor wafer is started, the semiconductor wafer is kept attracted and thus held reliably on a surface of the dielectric film. 
     Preferably, the memory operation unit includes a circuit for storing the relation between a processing condition and the value of a self-bias voltage, externally receiving a processing condition and the value of an actual attract and hold voltage, calculating the value of a self-bias voltage depending on the externally input processing condition, and adding the calculated value of the self-bias voltage to the actual attract and hold voltage for output. 
     The relation between a processing condition and the value of a self-bias voltage are stored previously. The value of a self-bias voltage calculated depending on a processing condition externally input is add to the value of an actual attract and hold voltage externally input. The voltage corresponding to the value resulting from the summation is applied to the electrode. Thus, simply entering a processing condition and the value of an actual attract and hold voltage from the external allows a semiconductor wafer to be attracted and held on a surface of the dielectric film reliably once stably the processing of the semiconductor wafer is started. 
     Still preferably, the memory operation unit includes a circuit for storing the relation between a processing condition, and the value of a self-bias voltage and the value of an actual attract and hold voltage, externally receiving a processing condition, calculating the value of a self-bias voltage and the value of an actual attract and hold voltage depending on the processing condition externally input, and adding the both values together for output. 
     The relation of a processing condition, and the value of a self-bias voltage and the value of an actual attract and hold voltage is stored previously. Depending on an externally input processing condition, the value of a self-bias voltage and the value an actual attract and hold voltage are calculated and the value corresponding to the summation of the both values is applied to the electrode. Thus, simply entering a processing condition form the external allows a semiconductor wafer to be attracted and thus held on a surface of the dielectric film reliably once the processing of the semiconductor wafer is started. 
     Still preferably, the plasma processing apparatus also includes a measuring instrument for measuring and outputting to the self-bias voltage, and the memory operation unit includes a determination unit connected to the measuring instrument for determining whether the value of the self-bias voltage is stable, a memory device for storing the relation between a processing condition and the value of a self-bias voltage, and a circuit connected to the measuring instrument, the determination unit and the memory device, responsive to an output from the determination unit for selecting one of the self-bias voltage determined by the processing condition and the self-bias voltage measured by the measuring instrument and adding the selected self-bias voltage to an actual attract and hold voltage input externally or stored in the memory device for output. 
     The relation between a processing condition and the value of a self-bias voltage is stored previously. The measuring instrument measures the value of a self-bias voltage. If it has been determined depending on the measurement that the value of the self-bias voltage is not stable at e.g. the initiation of a process, the value of a self-bias voltage is calculated depending on a processing condition externally input. The value of the self-bias voltage calculated and the value of an actual attract and hold voltage externally input are added together. The voltage corresponding to the value obtained from the summation is applied to the electrode. When determination is made that the value of a self-bias voltage is stabilized, the value of the self-bias voltage actually measured and that of an actual attract and hold voltage externally input are added together. The voltage corresponding to the value obtained from the summation is applied to the electrode. Thus, if the value of a self-bias voltage is not stable at e.g. the initiation of a processing, the value of a self-bias voltage previously stored is used to determine the value of a voltage applied to the electrode. If the value of a self-bias voltage is stable, the value of the self-bias voltage actually measured is used to determine the value of a voltage applied to the electrode. Thus, a semiconductor wafer can be attracted and thus held precisely on a surface of the dielectric film reliably once the processing of the semiconductor wafer is started. 
     A plasma processing apparatus in another aspect of the present invention includes a vacuum chamber, an electrode arranged inside the vacuum chamber, a dielectric film formed on a surface of the electrode, a gas supply port leading to the vacuum chamber, a high-frequency power supply connected to the electrode, a memory operation unit calculating depending on a processing condition for producing a desired plasma the value of the voltage corresponding to the summation of the value of a minimal actual attract and hold voltage required to be applied between one surface of a semiconductor wafer mounted on the dielectric film and a surface of the dielectric film to attract and thus keep one surface of the semiconductor wafer held on the surface of the dielectric film the value of a self-bias generated at the other surface of a semiconductor wafer when the desired plasma is produced and outputting the calculated voltage value, and an electrostatic chuck power supply for applying the voltage corresponding to the voltage value calculated in the memory operation unit to the electrode. 
     Since the voltage corresponding to the value of an actual attract and hold voltage plus that of a plasma-generated, self-bias voltage is applied to the electrode, the value of the voltage generated between one surface of a semiconductor wafer and the dielectric film can be equal to the value of the actual attract and hold voltage. Thus, once the processing of the semiconductor wafer is started, the semiconductor wafer can be attracted and held reliably on a surface of the dielectric film. 
     An electrostatic attract and hold vacuum chucking method in still another aspect of the present invention is employed in a plasma processing apparatus including a vacuum chamber blocking the external atmosphere from the interior thereof for maintaining the atmosphere therein, an electrode arranged inside the vacuum chamber, a dielectric film formed on a surface of the electrode, a gas supply port for supplying a desired gas into the vacuum chamber, a plasma producing unit for changing a gas into a plasma, a memory operation unit for calculating depending on a processing condition for producing a desired plasma the value of the voltage corresponding to the sum of the value of a minimal actual attract and hold voltage required to be applied between one surface of a semiconductor wafer mounted on the dielectric film and a surface of the dielectric film to attract and thus keep one surface of the semiconductor wafer held on the surface of the dielectric film and the value of a self-bias voltage generated at the other surface of the semiconductor wafer when the desired plasma is produced and outputting the calculated voltage value, and an electrostatic chuck power supply for applying the voltage corresponding to the voltage value calculated in the memory operation unit to the electrode. The electrostatic attract-and-hold vacuum chucking method includes the step of externally receiving a processing condition for producing a desired plasma, and the step of calculating, depending on the processing condition, the value of the voltage corresponding to the summation of the value of a minimal actual attract and hold voltage required to be applied between one surface of a semiconductor wafer mounted on the dielectric film and a surface of the dielectric film to attract and thus keep one surface of the semiconductor wafer held on the surface of the dielectric film and the value of a self-bias voltage generated at the other surface of the semiconductor wafer when the desired a plasma is produced and of providing the calculated voltage value to the electrostatic chuck power supply. 
     Since the voltage corresponding to the value of an actual attract and hold voltage plus the value of a plasma-generated, self-bias voltage is applied to the electrode, the value of the voltage generated between one surface of the semiconductor wafer and the dielectric film can be equal to the value of the actual attract and hold voltage. Thus, the semiconductor wafer can be attract and thus held on a surface of the dielectric film reliably once the processing of the semiconductor wafer is started. 
     Preferably, the memory operation unit includes a circuit for storing the relation between a processing condition and the value of a self-bias voltage, receiving a processing condition and the value of an actual attract and hold voltage from respective externals, calculating the value of a self-bias voltage depending on the externally input processing condition, adding the calculated value of the self-bias voltage to the value of the actual attract and hold voltage together for output. The step of calculating and provided includes the step of externally receiving the value of an actual attract and hold voltage, the step of calculating the value of a self-bias voltage depending on a processing condition, and the step of adding the value of the actual attract and hold voltage to the value of the self-bias voltage and applying the value of obtained from the summation to the electrostatic chuck power supply. 
     The relation between a processing condition and the value of a self-bias voltage is stored previously. The value of a self-bias voltage calculated depending on a processing condition externally input is added to the value of an actual attract and hold voltage externally input. The voltage corresponding to the value resulting from the summation is applied to the electrode. Thus, simply entering a processing condition and the value of an actual attract and hold voltage from the external allows a semiconductor wafer to be attracted and thus held on a surface of the dielectric film reliably once the processing of the semiconductor wafer is started. 
     Still preferably, the memory operation unit includes a circuit for storing the relation between a processing condition, and the value of a self-bias voltage and the value of an actual attract and hold voltage, externally receiving a processing condition, calculating the value of a self-bias voltage and the value of an actual attract and hold voltage depending on an externally input processing condition, and adding the both values together for output. The step of calculating and providing includes the step of calculating the value of a self-bias voltage and the value of an actual attract and hold voltage depending on a processing condition and the step of adding the value the self-bias voltage and the value of the actual attract and hold voltage together and applying the value obtained from the summation to the electrostatic chuck power supply. 
     The relation between a processing condition, and the value of a self-bias voltage and the value of an actual attract and hold voltage is stored previously. The value of a self-bias voltage and the value of an actual attract and hold voltage are calculated depending on a processing condition externally input, and the voltage corresponding to the value obtained from adding the both values together is applied to the electrode. Thus, simply entering a processing condition from the external allows a semiconductor wafer to be attracted and thus held on a surface of the dielectric film reliably once the processing of the semiconductor wafer is started. 
     Still preferably, the plasma processing apparatus also includes a measuring instrument for measuring and outputting a self-bias voltage, and the memory operation unit includes a determination unit connected to the measuring instrument for determining whether the value of a self-bias voltage is stable, a memory device for storing the relation between a processing condition and the value of a self-bias voltage, and a circuit connected to the measuring instrument, the determination unit and the memory device, responsive to an output from the determination unit for selecting one of a self-bias voltage determined by a processing condition and a self-bias voltage measured by the measuring instrument and adding the selected self-bias voltage to an actual attract and hold voltage externally input for output. The step of calculating and providing includes the step of externally receiving the value of and an actual attract and hold voltage, the step of determining whether the value of a self-bias voltage output from the measuring instrument is stable, the step of calculating a self-bias voltage depending on a processing condition and providing to the electrostatic chuck power supply the value corresponding to the summation of the calculated self-bias voltage and an actual attract and hold voltage externally input or stored in the memory device if the value of the measured self-bias voltage is not stable, and the step of providing to the electrostatic chuck power supply the value corresponding to the summation of a self-bias voltage measured with the measuring instrument and an actual attract and hold voltage input externally or stored in the memory device if the value of the measured self-bias voltage is stable. 
     The relation between a processing condition and the value of a self-bias voltage is stored previously. The measuring instrument measures the value of a self-bias voltage. When determination is made depending on the measurement that the value of the self-bias voltage is not stable at e.g. the initiation of a processing, the value of a self-bias voltage is calculated depending on a processing condition externally input. The calculated value of the self-bias voltage is added to the value of an actual attract and hold voltage externally input. The voltage corresponding to the value resulting from the summation is applied to the electrode. If determination is made that the value of a self-bias voltage is stable, the value of the self-bias voltage actually measured is added to the value of an actual attract and hold voltage externally input. The voltage corresponding to the value resulting from the summation is applied to the electrode. Thus, if the value of a self-bias voltage is not stable at e.g. the initiation of a processing, the value of a self-bias voltage previously stored is used to determine the value of a voltage applied to the electrode. If the value of a self-bias voltage is stable, the value of the self-bias voltage actually measured is used to determine the value of a voltage applied to the electrode. Thus, a semiconductor wafer can be attracted and thus held precisely on a surface of the dielectric film reliably once the processing of the semiconductor wafer is started. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a structure of a conventional plasma processing apparatus. 
     FIG. 2 shows an equivalent circuit formed in vacuum chamber  21  when plasma  34  is produced. 
     FIG. 3 shows a relation between self-bias voltage Vdc, voltage V1 generated between a semiconductor wafer and the dielectric film, and electrostatic chuck voltage Vs. 
     FIG. 4 represents a relation between high-frequency electric power and self-bias voltage Vdc and a relation between high-frequency electric power and a minimal voltage to be applied to an electrode so as to attract and keep a semiconductor wafer held. 
     FIG. 5 is a block diagram showing a configuration of a plasma processing apparatus according to a first embodiment of the present invention. 
     FIG. 6 is a flow chart representing an operation of the plasma processing apparatus according to the first embodiment. 
     FIG. 7 represents a relation between high-frequency electric power and an actual attract and hold voltage VESC. 
     FIG. 8 illustrates a process for obtaining self-bias voltage Vdc. 
     FIG. 9 represents a one-dimensional, linear interpolation process. 
     FIG. 10 is a block diagram showing a configuration of a plasma processing apparatus according to a second embodiment of the present invention. 
     FIG. 11 is a flow chart representing an operation of the plasma processing apparatus according to the second embodiment. 
     FIG. 12 represents a process for obtaining actual attract and hold voltage VESC and self-bias voltage Vdc. 
     FIG. 13 shows a configuration of a plasma processing apparatus according to a third embodiment of the present invention. 
     FIG. 14 is a flow chart representing an operation of the plasma processing apparatus according to the third embodiment. 
     FIG. 15 represents self-bias voltage Vdc varying with time. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Hereinafter, a plasma processing apparatus according to a first embodiment of the present invention will be described with reference to the drawings. It should be noted that in the description provided hereinafter, identical portions are denoted by the same reference characters. The name and function thereof are also identical and a description thereof will not be repeated, as appropriate. 
     Referring to FIG. 5, a plasma processing apparatus  20  according to the first embodiment includes a vacuum chamber blocking the external atmosphere from the interior thereof for maintaining the atmosphere therein. 
     Vacuum chamber  21  includes a lower electrode  24 , a dielectric film  23  formed on a surface of lower electrode  24  to attract and keep a semiconductor wafer  22  held thereon through electrostatic force, a gas supply port  25  for introducing a desired gas from e.g. a gas cylinder (not shown) into vacuum chamber  21 , an upper electrode  26  arranged opposite to lower electrode  24  for diffusing the gas introduced via gas supply port  25  to introduce the diffused gas into vacuum chamber  21  and also serving as an electrode, an exhaust port  27  provided to exhaust the gas inside the vacuum chamber by means of a vacuum pump (not shown), and an insulator  33  formed on lower electrode  24  to maintain the insulation between lower electrode  24  and the gas inside vacuum chamber  21 . 
     Plasma processing apparatus  20  also includes an electrostatic chuck power supply  31  for applying a desired voltage to dielectric film  23  via lower electrode  24 , a control signal unit  32  provided to control an output voltage from electrostatic chuck power supply  31  and apply an electrostatic chuck voltage Vs from electrostatic chuck power supply  31  to lower electrode  24 , a high-frequency power supply  29  provided to apply high-frequency electric power to lower electrode  24 , a high-frequency cutting fielder  30  provided to prevent high-frequency electric power from sneaking from high-frequency power supply  29 , and a matching transformer  28  provided to achieve the matching between high-frequency power supply  29  and lower electrode  24 . 
     A desired gas introduced into vacuum chamber  21  is electro-magnetized by high-frequency power supply  29  to produce a plasma  34 . The operation of producing plasma  34  is similar to that associated with the background art described hereinbefore and a description thereof will thus not be repeated. 
     Plasma processing apparatus  20  also includes a processing-condition memory unit  35  for storing the conditions for producing plasma  34  as desired, such as gas flow, the pressure inside vacuum chamber  21 , the magnitude of high-frequency electric power (referred to as “processing conditions” hereinafter), and an actual attract and hold voltage VESC corresponding to the voltage required for attracting and keeping semiconductor wafer  22  held on dielectric film  23 , and a memory operation unit  36  for calculating the value of the voltage applied from electrostatic chuck power supply  32  to lower electrode  24  depending on the processing conditions and actual attract and hold voltage VESC stored in processing-condition memory unit  35 . 
     Referring to FIG. 6, the various portions of plasma processing apparatus  20  operate as described below. Initially, the user inputs a processing condition and actual attract and hold voltage VESC via an input portion (not shown) (S 1 ). The input processing condition and actual attract and hold voltage VESC are stored in processing-condition memory unit  35 . The processing condition stored in processing-condition memory unit  35  is transmitted to a control device (not shown) of vacuum chamber  21  (S 6 ). Depending on the transmitted processing condition, the control device introduces a desired gas into vacuum chamber  21  to set a desired pressure in vacuum chamber  21  (S 7 ). 
     In parallel with the steps of S 6  and S 7 , the steps from S 2  to S 5  are provided as described below. Memory operation unit  36  is loaded with the processing condition and actual attract and hold voltage VESC stored in processing-condition memory unit  35  (S 2 ). Memory operation unit  36  compares the loaded processing condition with the internal data stored in memory operation unit  36  and calculates self-bias voltage Vdc generated at a surface of semiconductor wafer  22  (S 3 ). The process of calculation of self-bias voltage Vdc will be described hereinafter. Memory operation unit  36  adds self-bias voltage Vdc and actual attract and hold voltage VESC together and transmits the value obtained from the summation to control signal unit  32  provided in electrostatic chuck power supply  31  (S 4 ). According to an instruction from control signal unit  32 , the voltage corresponding to a magnitude of (Vdc+VESC) is applied from electrostatic chuck power supply  31  to lower electrode  24  (S 5 ). The process provided so far allows semiconductor wafer  22  to be attracted and held on dielectric film  23 . The reason why electrostatic chuck power supply  31  applies the voltage corresponding to the magnitude of (Vdc+VESC) to lower electrode  24  will be described hereinafter. 
     Then, high-frequency power supply  29  applies high-frequency electric power to lower electrode  24 . Thus, plasma  34  is produced in vacuum chamber  21  (S 8 ). Then, semiconductor wafer  22  is processed as desired to form a semiconductor device on semiconductor wafer  22  (S 9 ). 
     The reason why electrostatic chuck power supply  31  applies the voltage corresponding to the magnitude of (Vdc+VESC) to lower electrode  24  will now be described with reference to FIG.  7 . As has been described hereinbefore, there are the relations as shown in FIG. 4 between self-bias voltage Vdc and the high-frequency electric power applied from high-frequency power supply  29  to lower electrode  24  and between an electrostatic chuck power supply or minimal voltage Vmin required to be applied from electrostatic chuck power supply  32  to lower electrode  24  to attract and hold semiconductor wafer  22  and the high-frequency electric power applied from high-frequency power supply  29  to lower electrode  24 , respectively. 
     According to the relations, the relation as shown in FIG. 7 is obtained between minimal value Vmin minus self-bias voltage Vdc (Vmin−Vdc) and the high-frequency electric power. That is, the value of (Vmin−Vdc) is fixed regardless of the value of the high-frequency electric power. This value corresponds to actual attract and hold voltage VESC. Thus, the relation as represented by expression (2) is established: 
     
       
         VESC=Vmin−Vdc  (2) 
       
     
     From equation (2), equation (3) is derived: 
     
       
         Vmin=VESC+Vdc  (3) 
       
     
     Thus, the voltage corresponding to the magnitude of (VESC+Vdc) applied from electrostatic chuck power supply  32  to lower electrode  24  allows semiconductor wafer  22  to be attract and held reliably on lower electrode  24 . 
     The S 3  process in FIG. 6, i.e. how memory operation unit  36  calculates self-bias voltage Vdc, will now be more specifically described with reference to FIG.  8 . 
     Memory operation unit  36  stores the table as shown in FIG. 8 for comparing processing conditions. The table for comparing processing conditions represents a relation between a processing condition and self-bias voltage Vdc that has been obtained experimentally. 
     For example, when gas pressure, gas type and high-frequency electric power are provided as processing conditions A1, A2 and A3, respectively, these values are compared with the values presented in the table for comparing processing conditions and the value of self-bias voltage Vdc is obtained as X. 
     If the self-bias voltage corresponding to a processing condition is not stored in the table for comparing processing conditions, memory operation unit  36  calculates a self-bias voltage depending on the two processing conditions closest to the processing condition and the respective self-bias voltages corresponding to the two processing conditions. The self-bias voltage is calculated through a linear interpolation process. 
     The linear interpolation process in one dimension will now be described with reference to FIG.  9 . Let us now assume that the high-frequency electric power applied has a value E3 which is larger than A3 and smaller than B3. Since self-bias voltage Vdc has a value of X for high-frequency electric power A3 and a value of Y for high-frequency electric power B3, a value XY of self-bias voltage Vdc for high-frequency electric power E3 is represented by equation (4): 
     
       
         
           
             
               
                 
                   XY 
                   = 
                   
                     
                       
                         
                           Y 
                           - 
                           X 
                         
                         
                           B3 
                           - 
                           A3 
                         
                       
                        
                       
                         ( 
                         
                           E3 
                           - 
                           A3 
                         
                         ) 
                       
                     
                     + 
                     X 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     That is, XY is obtained as the value for the y coordinate when the value for the x coordinate is E3 in the two-dimensional straight line passing coordinates (A3, X) and (B3, Y). 
     Thus, plasma processing apparatus  20  according to the first embodiment applies the voltage corresponding to the value of actual attract and hold voltage VESC plus the value of self-bias voltage Vdc determined by a processing condition to lower electrode  24 . Thus, the voltage generated between dielectric film  23  and semiconductor wafer  22  has the same value as actual attract and hold voltage VESC. Thus, semiconductor wafer  22  is attracted and held reliably on the dielectric film  23 . While a one-dimensional linear interpolation process is employed for calculating self-bias voltage Vdc in the present embodiment, a two- or more dimensional interpolation process or general approximate formula techniques may also be employed therefor. 
     Second Embodiment 
     Referring FIG. 10, a plasma processing apparatus  40  according to a second embodiment is almost similar in configuration to the FIG. 5 plasma processing apparatus  20  according to the first embodiment. The portions differing between them will now be described and a description of the remaining portions will not be repeated. 
     Plasma processing apparatus  40  employs a processing-condition memory unit  41  and a memory operation unit  42  in place of processing-condition memory unit  35  and memory operation unit  36  of plasma processing apparatus  20 . 
     Processing-condition memory unit  41  stores processing conditions, such as gas flow, the pressure inside vacuum chamber  21 , the magnitude of high-frequency electric power and the like, as the conditions for producing plasma  34  as desired. That is, processing-condition memory unit  41  does not store actual attract and hold voltage VESC stored in processing-condition memory unit  35 . 
     Memory operation unit  42  determines the magnitude of the voltage applied from electrostatic chuck power supply  32  to lower electrode  42  depending on the processing conditions. 
     Referring FIG. 11, the various portions of plasma processing apparatus  40  operate as described below. The user inputs a processing condition via an input portion (not shown) (S 11 ). The input processing condition is stored in processing-condition memory unit  41 . The processing condition stored in processing-condition memory unit  41  is transmitted to a control unit (not shown) of vacuum chamber  21  (S 6 ). Depending on the processing condition, the control device introduces a desired gas into vacuum chamber  21  and sets a desired pressure in vacuum chamber  21  (S 7 ). 
     In parallel with the S 6  and S 7  steps, the S 12  to S 13  and S 4  to S 5  steps are provided as described below. Memory operation unit  42  are loaded with a processing condition stored in processing-condition memory unit  41 (S 12 ). Memory operation unit  42  compares the loaded process condition with the internal data stored therein and calculates self-bias voltage Vdc and actual attract and hold voltage VESC that are generated at semiconductor wafer  22  (S 13 ). The process for calculating self-bias voltage Vdc and actual attract and hold voltage VESC will be described hereinafter. Memory operation unit  42  adds self-bias voltage Vdc and actual attract and hold voltage VESC together and transmits the obtained value to control signal unit  32  provided in electrostatic chuck power supply  31  (S 4 ). According to an instruction from control signal unit  32 , electrostatic chuck power supply  31  applies the voltage corresponding to a value of (Vdc+VESC) to lower electrode  24  (S 5 ). As with plasma processing apparatus  40  according to the first embodiment, the process provided so far allows semiconductor wafer  22  to be attracted and held on dielectric film  23 . 
     Then, high-frequency power supply  29  applies high-frequency electric power to lower electrode  24 . Thus, plasma  34  is produced in vacuum chamber  21  (S 8 ). Then, semiconductor wafer  22  is processed as desired to form a semiconductor device on semiconductor wafer  22  (S 9 ). 
     Referring to FIG. 12, the S 13  step in FIG. 11, i.e. how memory operation unit  42  calculates actual attract and hold voltage VESC and self-bias voltage Vdc, will now be described more specifically. 
     Memory operation unit  42  stores such a table for comparing processing conditions as shown in FIG.  8 . The table represents a relation between processing conditions, and actual attract and hold voltage VESC and self-bias voltage Vdc that has been obtained experimentally. 
     For example, when gas pressure, gas type and high-frequency electric power are provided as processing condition A1, A2 and A3, respectively, these values are compared with the values presented in the table for a comparing processing conditions. The value of actual attract and hold voltage VESC is obtained as α and the value of self-bias voltage Vdc is obtained as X. 
     If the respective values of actual attract and hold voltage VESC and self-bias voltage Vdc corresponding to a processing condition are not stored in the table for comparing processing conditions, a process is provided as described below. That is, memory operation unit  42  applies two processing conditions closest to the processing condition and the corresponding values of actual attract and hold voltage VESC and the corresponding values of self-bias voltage Vdc to provide the linear interpolation process and thus calculate self-bias voltage Vdc. The linear interpolation process applied here is similar to that described with reference to FIG. 9 and a description thereof will not be repeated. 
     Thus, plasma process apparatus  40  according to the second embodiment applies the voltage corresponding to the summation of the values of actual attract and hold voltage VESC and self-bias voltage Vdc determined by a processing condition to the lower electrode. Thus the magnitude of the voltage thus generated between dielectric film  23  and semiconductor wafer  22  is equal to the value of actual attract and hold voltage VESC. Thus, semiconductor wafer  22  can be attracted and held reliably on the dielectric film  23 . 
     Since the values of actual attract and hold voltage VESC and self-bias voltage Vdc are stored previously, processing conditions are only required to be input from the external. 
     Third Embodiment 
     Referring FIG. 13, a plasma processing apparatus  50  according to a third embodiment has the configuration of FIG. 5 plasma processing apparatus  20  according to the first embodiment plus a measuring instrument  51  and a measurement operation unit  52 . The different portions will now be described and a description of the remaining portions will not be repeated. 
     Measuring instrument  51  is provided at vacuum chamber  21  to measure self-bias voltage Vdc generated at semiconductor wafer  22 . Measuring instrument  51  is provided in the form of an electrostatic probe, a high-pressure probe, a device which observes the light-emission intensify of plasma  34 , or the like. 
     Measurement operation unit  52  is connected to processing-condition unit  35 , memory operation unit  36  and measuring Instrument  51  and determines whether self-bias voltage Vdc measured with measuring instrument  51  has a stable value. If any unstable condition of plasma  34  caused in starting a process, changing a processing condition or the like results in self-bias voltage Vdc having an unstable value, measurement operation unit  52  outputs the value corresponding to the summation of the actual attract and hold voltage VESC and self-bias voltage Vdc output from memory operation unit  36 . If the self-bias voltage Vdc is otherwise stable, measurement operation unit  52  outputs the value corresponding to the summation of actual attract and hold voltage VESC stored in processing-condition memory unit  35  and self-bias voltage Vdc obtained from measuring instrument  51 . Control signal unit  32  receives the value output from measurement operation unit  52  and electrostatic chuck power supply  31  thus controls the voltage applied to lower electrode  24 . 
     Referring to FIG. 14, the various portions of plasma processing apparatus  50  operate as described below. The S 1 -S 8  steps are similar to those effected in the FIG. 6 plasma processing apparatus  20  according to the first embodiment and the description thereof will not be repeated. 
     Then, measuring instrument  51  measures self-bias voltage Vdc (S 21 ). Measurement operation unit  52  determines whether self-bias voltage Vdc is stable (S 22 ). This steps will be described hereinafter. If measurement operation unit  52  determines that self-bias voltage Vdc is not stable (NO at S 22 ), measurement operation unit  52  outputs a value obtained through calculation in memory operation unit  36 . Electrostatic chuck power supply  31  applies the voltage corresponding to the value output from measurement operation unit  52 , i.e. (VESC+Vdc), to lower electrode  24 . Then, semiconductor wafer  22  is processed as desired (S 23 ). When the S 23  step completes, the control returns to S 22  and measurement operation unit  52  again determines whether self-bias voltage Vdc is stabilized. 
     If measurement operation unit  52  determines that self-bias voltage Vdc is stabilized (YES at S 22 ), measuring instrument  51  outputs the value corresponding to actual attract and hold voltage VESC stores in processing-condition memory unit  35  plus self-bias voltage Vdc obtained from measuring instrument  51 . Electrostatic chuck power supply  31  applies the voltage corresponding to the value output from measurement operation unit  52 , i.e. VESC+Vdc, to lower electrode  24 . Then, semiconductor wafer  22  is processed as desired (S 24 ). 
     Referring to FIG. 15, the S 22  step will now be described more specifically. FIG. 15 represents self-bias voltage Vdc varying with time. It is assumed that a processing condition is input at time 0 and the processing condition is changed at time T2. The condition of plasma  34  is unstable from time 0 through time T1 and from time T2 through time T3. Thus the value of self-bias voltage Vdc also varies. When the condition of plasma  34  is stabilized, self-bias voltage Vdc has a constant value. Thus, measurement operation unit  52  determines that self-bias voltage Vdc is stabilized if the value of self-bias voltage Vdc is constant during a determined period prior to the present. For example, measurement operation unit  52  may be adapted to determine that self-bias voltage Vdc is stabilized if the difference between the maximum value and minimum value of self-bias voltage Vdc does not exceed a predetermine value during the predetermine period. Measurement operation unit  52  may also be adapted to determine that self-bias voltage Vdc is stabilized if the inclination of the graph does not exceed a predetermine value, with the horizontal axis representing time and the vertical axis representing self-bias voltage Vdc. 
     Thus, for plasma processing apparatus  50  according to the third embodiment, measuring instrument  51  is employed to measure self-bias voltage Vdc generated at semiconductor wafer  22 . If the self-bias voltage Vdc is unstable, plasma processing apparatus  50  applies the voltage corresponding to the summation of the value of actual attract and hold voltage VESC and that of self-bias voltage Vdc output from memory operation unit  36  to lower electrode  24 . If self-bias voltage Vdc is stable, plasma processing apparatus  50  applies the voltage corresponding to the summation of the value of self-bias voltage Vdc actually measured by means of measuring instrument  51  and the value of actual attract and hold voltage VESC stored in processing-condition memory unit  35  to lower electrode  24 . Thus, the voltage generated between dielectric film  23  and semiconductor wafer  22  has the same value that actual attract and hold voltage VESC has. Thus, semiconductor wafer  22  is reliably attracted and thus held on dielectric film  23 . If the value of self-bias voltage Vdc is stable, the value of self-bias voltage Vdc actually measured is used to precisely attract and thus hold semiconductor wafer  22  on directive film  23 . 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.