# Patent

## Title
Plasma processing apparatus and plasma processing method

## Abstract

A plasma processing apparatus includes a plasma-generation high-frequency power supply which generates plasma in a processing chamber, a biasing high-frequency power supply which applies high-frequency bias electric power to an electrode on which a sample is placed, a monitor which monitors a peak-to-peak value of the high-frequency bias electric power applied to the electrode, an electrostatic chuck power supply which makes the electrode electrostatically attract the sample, a self-bias voltage calculating unit which calculates self-bias voltage of the sample by monitoring the peak-to-peak value of the high-frequency bias electric power applied to the electrode, and an output voltage control unit which controls output voltage of the electrostatic chuck power supply based on the calculated self-bias voltage.

## Background
BACKGROUND OF THE INVENTION
The present invention relates to a plasma processing method and a plasma processing apparatus (e.g. plasma etching apparatus) having a mechanism for holding a sample or wafer (semiconductor wafer, liquid crystal substrate, etc.) on an electrode (sample stage) by means of the so-called electrostatic chuck (electrostatic attraction), and in particular, to a plasma processing method and a plasma processing apparatus suitable for reducing damage to the inner wall of the processing chamber of the plasma processing apparatus caused by a rise in plasma potential.
In a plasma processing apparatus such as a plasma etching apparatus, a method called “electrostatic chuck” or “electrostatic attraction” is widely used for holding a wafer in the processing chamber. The electrostatic chuck (electrostatic attraction) is a method of holding a wafer on an electrode (sample stage) by electrostatic force which is caused by the potential difference between the wafer and the electrode. The electrostatic chuck has advantages over mechanical holding methods (using a holding member such as a clamp) in that wafer contamination due to contact can be avoided and wafer temperature control is easy (since the whole of the back of the wafer is attracted). Methods for the electrostatic chuck generally include a monopole method (applying electrostatic chuck voltage to one electrode) and a dipole method (using two or more electrodes and generally applying electrostatic chuck voltages of different polarities to the electrodes, respectively).
FIG. 2 is a schematic diagram showing the general composition of a plasma processing apparatus.
An electrostatic chuck power supply 111 is capable of applying DC voltage to an electrode 108 which is embedded in a sample stage 109. The value of the DC voltage will hereinafter be referred to as “ESC voltage (output voltage) of the electrostatic chuck power supply 111”. Average electric potential of the wafer 113 caused by application of output of a biasing high-frequency power supply 110 to the electrode 108 via a capacitor will hereinafter be referred to as “self-bias voltage” (Vdc). The self-bias voltage Vdc is a negative DC voltage. In this case, the potential difference between the electrode 108 and the wafer 113 (i.e. the difference between the ESC voltage and the self-bias voltage Vdc) is the electrostatic chuck voltage (Vchuck). The self-bias voltage Vdc is dependent on the peak-to-peak value (Vpp) of the high-frequency bias power applied to the wafer 113 in the following relationship:

α=|Vdc/Vpp|≦0.5

where α is a constant which can vary depending on the plasma processing apparatus (0.3-0.45 in a standard plasma processing apparatus).

The peak-to-peak value Vpp can be monitored with a Vpp monitor 112.
FIG. 3 is a graph showing an example of the relationship among the ESC voltage, the self-bias voltage Vdc, the peak-to-peak value Vpp and the electrostatic chuck voltage Vchuck.
The self-bias voltage Vdc appears in the negative region of the graph with an absolute value α×Vpp (positive). As shown in FIG. 3, there exist two ESC voltages that cause the same Vchuck with respect to the value of Vdc. The ESC voltages on the positive side and on the negative side of Vdc will hereinafter be expressed as VESC+ and VESC−, respectively.
In this case, the relationship among the ESC voltages (VESC+, VESC−), the electrostatic chuck voltage Vchuck and the self-bias voltage Vdc can be expressed by the following equations:

VESC+=Vdc+Vchuck 

VESC−=Vdc−Vchuck 

where Vdc is negative and Vchuck is positive.

SUMMARY OF THE INVENTION
The existing plasma processing apparatuses (related arts) involve the following problems regarding the holding of the wafer by means of electrostatic chuck (electrostatic attraction).
First, when the electrostatic chuck voltage Vchuck is too low, the wafer, not sufficiently attracted to the electrode, can peel off from the electrode (sample stage) during the plasma processing. On the other hand, when the electrostatic chuck voltage Vchuck is too high, chuck force (attracting force) becomes too high and the wafer can be cracked. Therefore, the peeling off and cracking of the wafer have to be prevented by properly adjusting the value of the electrostatic chuck voltage Vchuck.
Further, when resistance at the inner wall of the processing chamber making contact with the plasma is high, abnormal electric discharge occurs between the plasma and the inner wall.
For example, when plasma 106 for the processing of the wafer 113 is generated by applying positive ESC voltage to the electrode 108 by the monopole method, minute leak current I (indicated with a broken line in FIG. 2) flows into the plasma. When the resistance R of the inner wall of the processing chamber 105 is high, the plasma is electrically charged up to a voltage V=IR (positive), that is, the plasma potential takes on a positive value. When the plasma potential exceeds a certain level, abnormal electric discharge occurs due to electric breakdown of an insulator layer on the inner wall of the processing chamber. Due to foreign materials caused by the abnormal electric discharge, contamination of the wafer and the inside of the processing chamber becomes a problem.
In order to suppress the abnormal electric discharge and the accompanying generation of foreign materials, various techniques for suppressing the rise in the plasma potential (the cause of the abnormal electric discharge) have been proposed.
For example, in a technique described in JP-A-2007-73309, the potential of the wall is measured and the absolute value of the ESC voltage is reduced when abnormal electric discharge is likely to occur.
However, this technique involves problems such as the peeling off of the wafer due to the reduced absolute value of the ESC voltage, unevenness of the plasma processing due to nonuniformity in wafer temperature (caused by changes in the chuck force during the process due to the changing (adjustment) of the ESC voltage during the process), etc.
Meanwhile, a technique described in JP-A-2006-210726 suppresses the rise in the plasma potential by negatively shifting the ESC voltage.
With this technique, however, the voltage applied to the electrode becomes extremely high especially when Vpp and Vdc are both high, which is undesirable in consideration of dielectric strength of a flame-sprayed film of the electrode head and the transmission system, costs, etc.
It is therefore the primary object of the present invention to provide a plasma processing apparatus and a plasma processing method capable of resolving the above problems, such as the wafer peeling (peeling off of the wafer) caused by insufficient chuck force, the wafer cracking (cracking of the wafer) caused by excessive chuck force, the unevenness of plasma processing, the abnormal electric discharge (due to the rise in the plasma potential) and the accompanying generation of foreign materials, power loss caused by excessive supply voltage, and the problem related to dielectric strength and costs.
In accordance with an aspect of the present invention, there is provided a plasma processing apparatus including a plasma-generation high-frequency power supply which generates plasma in a processing chamber, a biasing high-frequency power supply which applies high-frequency bias electric power to an electrode on which a sample is placed, a monitor which monitors a peak-to-peak value of the high-frequency bias electric power applied to the electrode, an electrostatic chuck power supply which makes the electrode electrostatically attract the sample, a self-bias voltage calculating unit which calculates self-bias voltage of the sample by monitoring the peak-to-peak value of the high-frequency bias electric power applied to the electrode, and an output voltage control unit which controls output voltage of the electrostatic chuck power supply based on the calculated self-bias voltage.
Preferably, when the absolute value of the self-bias voltage is smaller than electrostatic chuck voltage necessary for the attraction of the sample, the output voltage control unit sets the output voltage at (self-bias voltage)−(electrostatic chuck voltage).
Preferably, when the absolute value of the self-bias voltage is larger than electrostatic chuck voltage necessary for the attraction of the sample, the output voltage control unit sets the output voltage at (self-bias voltage)+(electrostatic chuck voltage).
Preferably, when the obtained output voltage is smaller than a prescribed value, the output voltage control unit sets the output voltage at a value which is on the negative side of the self-bias voltage and capable of achieving the electrostatic chuck voltage necessary for the attraction of the sample only at plasma ignition.
Preferably, the plasma processing apparatus further includes a control unit which stops supply of gas to the back of the sample while processing of the sample shifts from a step to the next step in cases where the processing of the sample includes multiple steps which are executed successively and electrostatic chuck voltage varies from step to step.
In accordance with another aspect of the present invention, there is provided a plasma processing method using a plasma processing apparatus equipped with a plasma-generation high-frequency power supply which generates plasma in a processing chamber, a biasing high-frequency power supply which applies high-frequency bias electric power to an electrode on which a sample is placed, a monitor which monitors a peak-to-peak value of the high-frequency bias electric power applied to the electrode, and an electrostatic chuck power supply which makes the electrode electrostatically attract the sample. In the plasma processing method, the electrostatic chuck of the sample is carried out by setting output voltage of the electrostatic chuck power supply at a negative value which is on the positive side of self-bias voltage of the sample.
In accordance with another aspect of the present invention, there is provided a plasma processing method using a plasma processing apparatus equipped with a plasma-generation high-frequency power supply which generates plasma in a processing chamber, a biasing high-frequency power supply which applies high-frequency bias electric power to an electrode on which a sample is placed, a monitor which monitors a peak-to-peak value of the high-frequency bias electric power applied to the electrode, and an electrostatic chuck power supply which makes the electrode electrostatically attract the sample. In the plasma processing method, the electrostatic chuck of the sample is carried out by setting output voltage of the electrostatic chuck power supply at a negative value which is on the negative side of self-bias voltage of the sample at plasma ignition and thereafter setting the output voltage at a negative value which is on the positive side of the self-bias voltage of the sample.
Preferably, supply of gas to the back of the sample is stopped while processing of the sample shifts from a step to the next step in cases where the processing of the sample includes multiple steps which are executed successively and electrostatic chuck voltage varies from step to step.
With the plasma processing apparatus and the plasma processing method in accordance with the present invention, appropriate electrostatic chuck force can be achieved under a condition avoiding the problems with the conventional techniques (abnormal electric discharge due to a rise in the plasma potential and the accompanying generation of foreign materials, the problem related to the dielectric strength of the electrode substrate and the transmission system caused by the application of high voltage).
Further, stable electrostatic chuck force can be achieved without wasting electric power since the wafer can be attracted to the electrode by use of low applied voltage within a permissible range.
Incidentally, while inductively coupled plasma is used as an example in the plasma processing apparatus in the following embodiments, the present invention achieves the same effects even when a generally-used plasma generating unit of a different type (magneto-microwave plasma, etc.) is used.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a chart showing a method which is used for determining ESC voltage in embodiments in accordance with the present invention.
FIG. 2 is a schematic diagram showing the general composition of a plasma processing apparatus.
FIG. 3 is a graph showing an example of the relationship among ESC voltage, self-bias voltage Vdc, a peak-to-peak value Vpp and electrostatic chuck voltage Vchuck.
FIG. 4 is a graph showing an example of the relationship between plasma potential and the ESC voltage in a case where the absolute value of the self-bias voltage Vdc is smaller than the electrostatic chuck voltage Vchuck.
FIG. 5 is a graph showing an example of the relationship between the plasma potential and the ESC voltage in a case where the absolute value of the self-bias voltage Vdc is larger than the electrostatic chuck voltage Vchuck.
FIG. 6 is a graph showing still another example of the relationship between the plasma potential and the ESC voltage in a case where VESC+ is negative, |VESC+|<β and additional voltage is added to the ESC voltage only at plasma ignition.
FIG. 7 is a graph showing an example of the relationship among the ESC voltage, the self-bias voltage Vdc, the peak-to-peak value Vpp, the electrostatic chuck voltage Vchuck and gas pressure on the back of the wafer.



DETAILED DESCRIPTION OF THE EMBODIMENTS
Referring now to the drawings, a description will be given in detail of preferred embodiments in accordance with the present invention.
In order to resolve the problems with the related art, the present inventors conducted various experiments for studying the relationship between the plasma potential and the ESC voltage (DC voltage outputted by the electrostatic chuck power supply 111 to be applied to the electrode 108).
FIG. 4 is a graph showing an example of the relationship between the plasma potential and the ESC voltage in a case where the absolute value of the self-bias voltage Vdc is smaller than the electrostatic chuck voltage Vchuck.
In FIG. 4, Vplasma+ represents the plasma potential when VESC+ (ESC voltage on the positive side of Vdc) is used, and Vplasma− represents the plasma potential when VESC− (ESC voltage on the negative side of Vdc) is used. Under the condition shown in FIG. 4, VESC+ is positive and VESC− is negative.
In this case, a rise in the plasma potential was observed when the positive ESC voltage VESC+ was used, and it was found that abnormal electric discharge occurs when the plasma potential gets high in the positive region.
Thus, in order to prevent the abnormal electric discharge, the ESC voltage employed has to be changed depending on the behavior of the plasma potential.
In the following, the setting of ESC voltage that is capable of achieving appropriate electrostatic chuck force without causing the abnormal electric discharge (occurring dependent on each value of VESC+) will be explained with reference to FIGS. 4, 5 and 6.
First, a value of the electrostatic chuck voltage Vchuck that can be regarded as optimum is set. The electrostatic chuck voltage Vchuck should be set at a value that causes no wafer peeling (peeling off of the wafer) or wafer cracking (cracking of the wafer). The electrostatic chuck voltage “Vchuck” in the following explanation means this value (a constant which is set here).
Subsequently, the peak-to-peak value Vpp of the high-frequency bias electric power applied to the electrode 108 is monitored and the value of the self-bias voltage Vdc is calculated from the monitored peak-to-peak value Vpp and the preset coefficient α (α=|Vdc/Vpp|≦0.5).
In order to realize the electrostatic chuck (electrostatic attraction) of the wafer 113 by use of Vchuck (set value) with respect to the calculated Vdc, it is necessary to select which of VESC+ or VESC− should be used.
The absolute value of VESC+ is constantly smaller than that of VESC− since Vdc is negative. Thus, necessary electric power can be reduced by using VESC+ instead of using VESC−. However, the plasma potential Vplasma+ becomes positive when VESC+ is positive as shown in FIG. 4, which involves a possibility of abnormal electric discharge.
On the other hand, the plasma potential Vplasma− is constantly negative since VESC− is constantly negative. Thus, the use of VESC− involves no danger of abnormal electric discharge.
Based on the above consideration, VESC− is used in the following embodiments when VESC+ is positive (i.e. when |Vdc|<Vchuck).
FIG. 5 is a graph showing an example of the relationship between the plasma potential and the ESC voltage in a case where the absolute value of the self-bias voltage Vdc is larger than the electrostatic chuck voltage Vchuck.
In this case where VESC+ is negative (i.e. |Vdc|>Vchuck), VESC+ and VESC− are both negative, which involves no danger of abnormal electric discharge irrespective of whether VESC+ or VESC− is used. Therefore, the use of VESC+ (having a smaller absolute value than VESC−) is suitable for realizing smaller output voltage and is advantageous in consideration of the dielectric strength of the electrode substrate and the transmission system, electric power consumption, etc.
In the case where VESC+ is negative, however, an experiment showed that the plasma potential behaves in two ways (having a positive high-voltage peak or no peak when the plasma is ignited) depending on the absolute value of the employed VESC+.
In the example shown in FIG. 5, the plasma potential Vplasma+ shows a positive high-voltage peak at the plasma ignition and thereafter remains at a negative level (stationary state).
It was found that the positive high-voltage peak at the plasma ignition appears when |VESC+| is small and disappears when |VESC+| is increased (with VESC+ in the negative region). From this result, it can be considered that there exist a threshold value of VESC+ (hereinafter referred to as “β”) for eliminating the positive high-voltage peak at the plasma ignition. It was also found that the threshold value β can be estimated by previously conducting an experiment.
Therefore, when the plasma potential Vplasma+ has no positive high-voltage peak at the plasma ignition (i.e. when |VESC+|>β), the plasma potential smoothly shifts to the negative region since the plasma ignition and thereafter remains negative, with no problem occurring with the use of VESC+. Thus, VESC+ is used in this case.
On the other hand, when the plasma potential Vplasma+ has a positive high-voltage peak at the plasma ignition (i.e. when |VESC+|<β), there is a possibility of the aforementioned abnormal electric discharge (even if the positive high-voltage peak appears only at the plasma ignition), which is problematic.
Thus, in order to resolve the problem of the positive high plasma potential at the plasma ignition, an additional voltage, for increasing the absolute value of the ESC voltage (VESC+) within the negative region, is added to the ESC voltage at the plasma ignition in the following embodiments.
FIG. 6 is a graph showing an example of the relationship between the plasma potential and the ESC voltage in the case where VESC+ is negative, |VESC+<β and the additional voltage (for increasing the absolute value of the ESC voltage (VESC+) within the negative region) is added to the ESC voltage only at the plasma ignition.
Thanks to the additional voltage for increasing the absolute value of VESC+ within the negative region, the positive high plasma potential peak at the plasma ignition (which was seen in FIG. 5) was suppressed successfully and the plasma potential remained constantly in the negative region. Thus, there is no possibility of abnormal electric discharge in this case. A (negative) voltage of approximately VESC− or lower can be considered to be sufficient for the additional voltage used at the plasma ignition.
FIG. 1 is a chart showing a method which is used for determining the ESC voltage in the following embodiments.
First, the value of the electrostatic chuck voltage Vchuck is set and the self-bias voltage Vdc is calculated from the monitored peak-to-peak value Vpp and the coefficient α predetermined experimentally or theoretically.
Subsequently, whether VESC+ is positive or not (i.e. whether |Vdc|<Vchuck or not) is judged.
When VESC+ is positive (i.e. |Vdc|<Vchuck), VESC− is used as the ESC voltage. When VESC+ is negative (i.e. |Vdc|>Vchuck), VESC+ is used as the ESC voltage.
In the case where VESC+ is used as the ESC voltage, if |VESC+|<β, the additional voltage (for increasing the absolute value of the ESC voltage within the negative region) is added to the ESC voltage VESC+ at the plasma ignition and thereafter the ESC voltage is set at VESC+. If |VESC+|>β, the ESC voltage is set constantly at VESC+ from the plasma ignition. Incidentally, β is a preset positive constant.
By determining the ESC voltage according to the above chart, it is possible to provide a plasma processing apparatus and a plasma processing method capable of achieving appropriate electrostatic chuck force without causing abnormal electric discharge.
Embodiment 1
A plasma processing apparatus and a plasma processing method in accordance with a first embodiment of the present invention will be described below with reference to FIGS. 1 and 2.
Referring to FIG. 2, the plasma processing apparatus of this embodiment includes a plasma-generation high-frequency power supply 101 for generating plasma in a processing chamber 105, a biasing high-frequency power supply 110 for applying high-frequency bias electric power to an electrode 108 on which a wafer 113 is placed, a Vpp monitor 112 for monitoring the peak-to-peak value Vpp of the high-frequency bias electric power applied to the electrode 108, an electrostatic chuck power supply 111 for making the electrode 108 (sample stage 109) electrostatically attract the wafer 113, a self-bias voltage calculating unit 114 for calculating the self-bias voltage Vdc of the wafer 113 by monitoring the peak-to-peak value Vpp of the high-frequency bias electric power applied to the electrode 108, and an output voltage control unit 115 for controlling the output voltage of the electrostatic chuck power supply 111 (ESC voltage) based on the calculated self-bias voltage Vdc. The wafer 113 held on the electrode 108 (sample stage 109) by the electrostatic chuck force is treated with plasma (plasma treatment). In the figure, a reference numeral 103 depicts an antenna, 104 an inductive coupling window and 107 a ground electrode.
The value of the output voltage of the electrostatic chuck power supply 111 (ESC voltage) is determined by use of the monitored peak-to-peak value Vpp, the electrostatic chuck voltage Vchuck which has previously been set, and the coefficients α and β which have previously been measured or set.
A case where Vchuck=300 V, α=0.4 and β=200 V will be described below as an example.
When the measured peak-to-peak value Vpp is 300 V, the self-bias voltage calculating unit 114 calculates the self-bias voltage Vdc as −120 V. Since |Vdc|<Vchuck (i.e. VESC+>0) holds, the output voltage control unit 115 sets the ESC voltage at VESC−=Vdc−Vchuck=−420 V.
When the measured peak-to-peak value Vpp is 1000 V, the self-bias voltage calculating unit 114 calculates the self-bias voltage Vdc as −400 V. Since |Vdc|>Vchuck (i.e. VESC+<0) holds, the output voltage control unit 115 uses VESC+ (=Vdc+Vchuck=−100 V) as the ESC voltage. In this case, |VESC+|<β holds (since VESC+=−100 V and β=200 V), and thus the output voltage control unit 115 executes the control to add the additional voltage (approximately VESC−=Vdc−Vchuck=−700 V) to the ESC voltage VESC+only at the plasma ignition.
When the measured peak-to-peak value Vpp is 2000 V, the self-bias voltage calculating unit 114 calculates the self-bias voltage Vdc as −800 V. Since |Vdc|>Vchuck (i.e. VESC+<0) holds, the output voltage control unit 115 uses VESC+(=Vdc+Vchuck=−500 V) as the ESC voltage. In this case, the additional voltage at the plasma ignition is unnecessary since |VESC+|>β holds (since VESC+=−500 V and β=200 V), and thus the output voltage control unit 115 sets the output voltage (ESC voltage) at VESC+=−500 V from the plasma ignition.
By controlling the ESC voltage as in the above examples, the electrostatic chuck voltage is maintained constantly at the set value Vchuck, by which the wafer peeling and the wafer cracking are avoided. Further, the abnormal electric discharge is eliminated since the aforementioned rise in the plasma potential due to positive leak current is prevented.
Embodiment 2
A plasma processing apparatus and a plasma processing method in accordance with a second embodiment of the present invention will be described below with reference to FIG. 7.
In this embodiment, a case where the processing of the wafer includes multiple steps which are executed successively and the peak-to-peak value Vpp (of the high-frequency bias electric power applied to the electrode 108) varies from step to step will be described.
FIG. 7 is a graph showing an example of the relationship among the ESC voltage, the self-bias voltage Vdc, the peak-to-peak value Vpp, the electrostatic chuck voltage and gas pressure on the back of the wafer.
In the case where multiple steps differing in the peak-to-peak value Vpp are executed successively, the electrostatic chuck (electrostatic attraction) of the wafer has to be carried out in each step by use of an ESC voltage suitable for the peak-to-peak value Vpp in the step, that is, an ESC voltage capable of achieving appropriate electrostatic chuck force without causing the abnormal electric discharge.
In this case, a problem can occur when the employed ESC voltage is changed from VESC+ to VESC− or from VESC− to VESC+ in the transition between two consecutive steps.
For example, a case where Vchuck=300 V, α=0.4 and β=200 V (the aforementioned condition) will be considered below.
When two successive steps in which the peak-to-peak value Vpp changes from 300 V to 1000 V are executed, the self-bias voltage Vdc changes from −120 V to −400 V. According to the chart of FIG. 1, the ESC voltage changes continuously from −420 V to −100 V in this case. Since the plasma changes continuously, no positive high plasma potential (like that appearing at the plasma ignition) occurs in the transition between the two steps. Thus, in the process in which the ESC voltage changes, there is a point in time when the ESC voltage becomes equal to Vdc or the difference between Vdc and the ESC voltage becomes smaller than Vchuck. At this point, the electrostatic chuck (electrostatic attraction) of the wafer becomes insufficient, and in the plasma processing apparatus in which gas (e.g. He) is supplied to the back of the wafer (for the purpose of temperature control, chuck force measurement, etc.), trouble such as wafer peeling or wafer flying can be caused by the pressure of the gas (e.g. He).
In this embodiment which is designed to resolve the above problem, when the employed ESC voltage is changed from VESC+ to VESC− or from VESC− to VESC+ in the transition between steps, a control unit 116 (unshown) stops the supply of the gas (e.g. He) to the back of the wafer (a step of removing the gas on the back of the wafer) before the transition as shown in FIG. 7.
By the above step, the gas on the back of the wafer is removed when the ESC voltage is changed. Consequently, the ESC voltage can be changed safely without the danger of wafer peeling, wafer flying, etc.
It should be further understood by those skilled in the art that although the foregoing description has been on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

## Claims

1. A plasma processing method of involving electrostatic chucking a sample in a plasma processing apparatus equipped with a plasma-generation high-frequency power supply which generates plasma in a processing chamber, a biasing high-frequency power supply which applies high-frequency bias electric power to an electrode on which the sample is placed, a monitor which monitors a peak-to-peak value of the high-frequency bias electric power applied to the electrode, and an electrostatic chuck power supply which makes the electrode electrostatically attract the sample, the method comprising steps of:
setting an output voltage of the electrostatic chuck power supply at a first negative value during a predetermined period after plasma ignition until plasma potential becomes stable;
determining a second negative value for the output voltage of the electrostatic chuck power supply based upon an output from the monitor, the determined second negative value being greater than a self-bias voltage of the sample and greater than the first negative value; and
after setting the output voltage at the first negative value during the predetermined period after the plasma ignition until the plasma potential becomes stable, setting the output voltage at the determined second negative value for plasma processing of the sample.

2. The method according to claim 1, wherein the first negative value of the output voltage of the electrostatic chuck power supply is set, during the predetermined period after the plasma ignition, to be less than the plasma potential.