Plasma apparatus for fabricating semiconductor devices

A plasma apparatus for fabricating a semiconductor device, is provided. This plasma apparatus includes a grounded chamber for providing a space where a predetermined process is to be performed, a chuck mounted within the chamber and insulated from the chamber, a gas injection ring installed around the sidewall of the chuck, an induction plasma power source connected to the chuck, a system controller for controlling the induction plasma power source, and a capacitance compensator for keeping the total chuck capacitance between the chuck and a ground terminal at a constant value. The gas injection ring is separated from the chuck by a predetermined distance and is electrically connected to the chamber.

This application relies for priority upon Korean Patent Application Nos.
 98-39952 and 99-27461, filed on Sep. 25, 1998, and Jul. 8, 1999,
 respectively, the contents of which are herein incorporated by reference
 in their entirety.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an apparatus for fabricating semiconductor
 devices, and more particularly, to an apparatus for fabricating
 semiconductor devices using plasma.
 2. Description of the Related Art
 As the integration of semiconductor devices increases, the demand for a
 deposition technique to form a material film having a uniform thickness,
 or an etching technique showing a uniform etch rate also increases.
 Accordingly, a plasma apparatus for depositing a material film or etching
 the material film by appropriately controlling the type of ions and ion
 energies is being widely used for the manufacture of highly-integrated
 semiconductor devices. To be more specific, the plasma apparatus can be
 applied to dry etching, chemical vapor deposition (CVD) or sputtering
 techniques.
 Meanwhile, a radio frequency generator is widely used as an energy source
 for generating the plasma used in these techniques.
 FIG. 1A is a schematic of a conventional plasma apparatus, and FIG. 1B is a
 plan view of the chuck and the gas injection ring shown in FIG. 1A.
 Referring to FIG. 1A, a chuck 5 is installed on the bottom of a chamber 10,
 which has an opening at the top, and a chuck support 15 made of an
 insulating material is interposed between the chuck 5 and the bottom of
 the chamber 10. A wafer on which a semiconductor device is to be formed,
 is loaded on the chuck 5. A gas injection ring 20 is installed around the
 sidewall of the chuck 5, and is fixed by a fixing means 25 such as a bolt
 in contact with the bottom of the chamber 10. The gas injection ring 20 is
 preferably attached in such a way that it is electrically connected to the
 chamber 10, which has a ground potential. The chuck 5 and the gas
 injection ring 20 are spaced apart from each other by a predetermined
 interval, for example, at an interval of about 1 to 5 mm. Thus, a gap
 exists between the chuck 5 and the gas injection ring 20, such that the
 gas injection ring 20 and the chuck 5 form a chuck capacitor.
 The sidewall of the chamber 10 or a predetermined area of the bottom of the
 chamber 10 is branched and connected to a vacuum pump 30. The opening of
 the chamber 10 is covered by a cover 35, which is formed of a dielectric
 material. An electrode 40 connected to a first plasma power source 45a for
 generating radio frequency power, is installed over the cover 35. Thus,
 when the first plasma power source 45a is turned on, plasma is generated
 within the chamber 10.
 A first radio frequency matching circuit 50a for maximizing the
 transmission efficiency of radio frequency power generated from the first
 plasma power source 45a is interposed between the first plasma power
 source 45a and the electrode 40. The chuck 5 is connected to a second
 plasma power source 45b, which is used to induce plasma generated within
 the chamber 10 by the first plasma power source 45a over the chuck 5.
 A gas injection passage for supplying process gas into the chamber 10 is
 installed within the gas injection ring 20. The gas injection passage is
 connected to a tank 55 which contains a gas source, via a gas inlet. A
 second radio frequency matching circuit 50b having the same function as
 that of 50a is interposed between the second plasma power source 45b and
 the chuck 5. A valve 60 for controlling the flow of gas is installed at a
 predetermined position on the gas inlet between the tank 55 and the gas
 injection ring 20. The valve 60 and the pump 30 are controlled by a valve
 control signal .PHI..sub.v and a pump control signal .PHI..sub.p,
 respectively, which are generated from a system controller 65 that
 controls the operation of the plasma apparatus. The system controller 65
 receives signals from the first and second plasma power sources 45a and
 45b and detects an on/off state of each of the first and second plasma
 power sources 45a and 45b.
 As described above, the total chuck capacitance between the chuck 5 and a
 ground terminal is directly affected by the change in chuck capacitance
 between the chuck 5 and the gas injection ring 20. Thus, when the chuck
 capacitance is changed, the total chuck capacitance is also changed. The
 total chuck capacitance in turn directly affects a plasma process. This
 means that when the total chuck capacitance is changed, the sheath
 potential between the chuck 5 and the plasma induced over the chuck 5 by
 the second plasma power source 45b is changed. When the sheath potential
 is changed as described above, process parameters for the plasma process
 are changed. For example, the process parameters are an etch rate, a
 deposition rate or the like. Hence, the total chuck capacitance between
 the chuck 5 and the ground terminal must be kept constant to obtain
 excellent process uniformity.
 Meanwhile, the plasma apparatus must be periodically pre-maintained as are
 other apparatuses used to fabricate semiconductor devices. This
 pre-maintenance is required since another factor that changes the process
 parameters is generated by the absorption of a contaminant on the inner
 wall of the chamber as the process time or the frequency of processing
 increases. Thus, the plasma apparatus must undergo a pre-maintenance
 operation of disassembling the plasma apparatus, cleaning the respective
 component elements including the chamber, and reassembling the
 disassembled component elements. During the pre-maintenance operation, an
 operation involving measuring and controlling the gap between the chuck 5
 and the gas injection ring 20 using a gage or the like, is performed
 several times to adjust the gap to within an allowable range. This
 operation is manually performed, requiring a great amount of time to
 perform, and making it difficult to accurately adjust the gap to within
 the allowable range. As a result of this, when the chuck 5 and the gas
 injection ring 20 are assembled such that they do not have the same center
 as shown in FIG. 4B, a first gap G.sub.1 on the left side of the chuck 5
 and a second gap G.sub.2 on the right side of the chuck 5 can be
 different. In other words, the gap between the chuck 5 and the gas
 injection ring 20 may not be the same at all positions around the chuck 5.
 When the gap between the chuck 5 and the gas injection ring 20 is not
 uniform as described above, the chuck capacitance may exceed an allowable
 range. Accordingly, as the total chuck capacitance changes, the uniformity
 of the process can be degraded.
 While a process gas is sprayed into the chamber 10 via the gas injection
 ring 20, a physical force due to the pressure or the like of the process
 gas, is applied to the gas injection ring 20. In addition, minute
 vibrations can be generated in the plasma apparatus when it is used for a
 long time. Thus, even when the gas injection ring 20 is fixed to the
 chamber 10 by the fixing means 25, the position of the gas injection ring
 20 can change when the plasma process is performed for a long time. When
 the position of the gas injection ring 20 changes as described above, the
 gap between the chuck 5 and the gas injection ring 20 changes.
 Accordingly, chuck capacitance changes, and the total chuck capacitance
 also changes.
 As described above, the total chuck capacitance between the chuck 5 and the
 grounding terminal of the conventional plasma apparatus cannot be kept
 constant.
 SUMMARY OF THE INVENTION
 To solve the above problems, it is an object of the present invention to
 provide a plasma apparatus including a capacitance compensator for keeping
 constant the total chuck capacitance between a ground terminal and a chuck
 on which a wafer is to be mounted, regardless of a pre-maintenance
 operation.
 It is another object of the present invention to provide a plasma apparatus
 for keeping process parameters constant.
 Accordingly, to achieve the above objects, the present invention provides a
 plasma apparatus including: a grounded chamber having a space where a
 predetermined process may be performed; a chuck mounted in the chamber and
 insulated from the chamber; a chuck plasma power source connected to the
 chuck; a system controller for controlling the chuck plasma power source;
 and a capacitance compensator for keeping the total chuck capacitance
 between the chuck and a ground terminal at a constant value. Also, the
 present invention may further include a gas injection ring installed
 around the sidewall of the chuck. The gas injection ring is separated from
 the chuck by a predetermined distance and is electrically connected to the
 chamber.
 The gas injection ring has a gas injection passage formed therein, and a
 process gas is injected into the chamber through the gas injection
 passage. The gas injection ring is fixed to the chamber by a fixing means
 such as a bolt.
 The capacitance compensator preferably includes: a variable capacitor
 portion interposed between the chuck and the ground terminal; and a
 variable capacitor controller for controlling the capacitance of the
 variable capacitor to keep the total chuck capacitance constant.
 The variable capacitor portion is connected between the chuck and a ground
 terminal, and preferably includes at least one variable capacitor. To be
 more specific, the variable capacitor portion includes at least one
 capacitor selected from the combination of a serial capacitor connected to
 a chuck capacitor in series, and a parallel capacitor connected in
 parallel with the chuck capacitor. Here, the chuck capacitor comprises the
 chuck and the grounded chamber. However, in a case where the present
 invention includes the gas ring, the chuck capacitor comprises the chuck
 and the gas injection ring. Also, at least one of the selected capacitors
 must be a variable capacitor.
 Meanwhile, the variable capacitor controller controls the capacitance of
 the variable capacitor portion by changing the gap between the electrodes
 of at least one variable capacitor that constitutes the variable capacitor
 portion. Thus, when the chuck capacitance between the chuck and the gas
 injection ring changes, the total chuck capacitance between the chuck and
 the ground terminal can be kept at a constant value by appropriately
 controlling the variable capacitance. Consequently, the total chuck
 capacitance can be kept at a constant value without correctly adjusting
 the gap between the chuck and the gas injection ring installed within the
 chamber, but rather by appropriately controlling the variable capacitance.
 Here, in the case where the present invention does not include the gas
 injection ring, the total chuck capacitance is equal to the chuck
 capacitance.
 An aspect of the variable capacitor controller includes: a motive power
 source for changing the gap between the electrodes of the selected
 variable capacitors; a capacitance meter for measuring the total chuck
 capacitance between the chuck and a ground terminal and outputting a
 predetermined signal; an automatic control signal generator for outputting
 an electrical signal corresponding to the difference between the output
 signal of the capacitance meter and a reference signal that corresponds to
 a desired optimal total chuck capacitance input to the system controller;
 and a driver portion for driving the motive power source by amplifying the
 output signal of the automatic control signal generator.
 Preferably, the motive power source is a direct current motor that has a
 rotational shaft connected to an electrode of at least one variable
 capacitor. In this case, the selected electrode is a driven electrode, and
 the driven electrode moves with the rotation of the rotational shaft of
 the direct current motor. As a result, the gap between the electrodes of
 the variable capacitor increases or decreases according to the direction
 of rotation of the rotational shaft.
 The automatic control signal generator preferably includes a comparator,
 most preferably, an analog comparator. The analog comparator amplifies the
 difference between the output signal of the capacitance meter and a
 reference signal corresponding to an optimal total chuck capacitance input
 to the system controller.
 An aspect of the variable capacitor controller further includes a process
 state sensor interposed between the variable capacitor portion and the
 capacitance meter. The process state sensor electrically disconnects the
 capacitance meter from the variable capacitor portion while a
 predetermined process, e.g., an etching process or a deposition process,
 is being performed within the chamber. In other words, the process state
 sensor electrically connects the variable capacitor portion to the
 capacitor meter only when the predetermined process is not being performed
 within the chamber. Thus, the capacitance meter only measures the total
 chuck capacitance when a predetermined process is not being performed.
 It is preferable that an aspect of the variable capacitor controller
 further includes a limit control portion for limiting the range within
 which a driven electrode of the at least one variable capacitor moves. The
 limit control portion preferably includes a voltage limit setting portion
 for setting a negative voltage limit and a positive voltage limit to limit
 the range within the driven electrode of the variable capacitor moves, a
 limit sensor for outputting a predetermined voltage according to the
 position of the driven electrode of the variable capacitor, a digital
 comparator for generating either the first or second output signal or
 generating neither of the first and second output signals, rectifying
 devices for inverting the polarity of the input signal of the driver
 portion according to the first or second output signal of the digital
 comparator, and switches for controlling the connections of the rectifying
 devices.
 An aspect of the variable capacitor controller preferably further includes
 a limit display portion including first and second light emitting devices,
 which are respectively lighted by first and second output signals of the
 digital comparator from the limit control portion.
 An aspect of the variable capacitor controller can further include a
 selection switch interposed between the automatic control signal generator
 and the driver portion, and a manual control signal generator connected to
 the selection switch. The selection switch connects the input terminal of
 the driver portion to the output terminal of the automatic control signal
 generator or connects the input terminal of the driver portion to the
 output terminal of the manual control signal generator. Accordingly, the
 variable capacitor controller in the plasma apparatus according to the
 present invention can be operated in either an automatic mode or a manual
 mode.
 The manual control signal generator includes a power switch for selecting
 either the positive power source or the negative power source, and a
 variable resistor interposed between the power switch and a ground
 terminal. The variable terminal of the variable resistor acts as the
 output terminal of the manual control signal generator, and is connected
 to the selection switch. Thus, in order to operate the variable capacitor
 controller in the manual mode, the selection switch connects the input
 terminal of the driver portion to the output terminal of the manual
 control signal generator, and the variable terminal of the variable
 resistor connected to the power switch controls the position of the driven
 electrode of the variable capacitor.
 Another aspect of the variable capacitor controller includes a motive power
 source for changing the gap between the electrodes of the variable
 capacitor, a capacitance meter for measuring the total chuck capacitance
 between the chuck and a ground terminal and outputting a signal
 corresponding to the measured total chuck capacitance to the system
 controller, a manual control signal generator for generating a desired
 voltage, and a driver portion for driving the motive power source by
 amplifying a voltage generated by the manual control signal generator. The
 variable capacitor controller configured as above can operate only in the
 manual mode, and can further include a limit control portion for limiting
 the range within which the driven electrode of the variable capacitor
 moves. The limit control portion in this alternate embodiment preferably
 has the same configuration and function as those of the limit control
 portion in the variable capacitor controller according to the preferred
 aspect of the present invention.
 Another aspect of the variable capacitor controller preferably further
 includes a limit display portion which is operated by the limit control
 portion. The limit display portion in this alternate embodiment preferably
 has the same configuration and function as those of the limit display
 portion in the variable capacitor controller according to the preferred
 embodiment of the present invention.
 According to the present invention, the total chuck capacitance between a
 chuck and a ground terminal can be maintained at an optimal total chuck
 capacitance without performing any pre-maintenance activities. Thus, the
 uniformity of a predetermined process performed within a chamber can be
 improved. Also, in the present invention, a special operation for
 precisely controlling the gap between a chuck and a gas injection ring is
 not required during pre-maintenance. Therefore, the time required for
 pre-maintenance is significantly reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 A preferred embodiment of the present invention will be described below in
 detail with reference to the attached drawings by taking a plasma etching
 apparatus as an example. However, the present invention is not limited to
 such a plasma etching apparatus, and can be applied to all kinds of
 semiconductor fabricating apparatuses which use plasma. For example, the
 semiconductor fabricating apparatus could be a plasma deposition
 apparatus, or the like.
 FIG. 2 is a schematic of a plasma apparatus according to a preferred
 embodiment of the present invention. Here, the same reference characters
 as those of FIGS. 1A and 1B denote the same elements.
 Referring to FIG. 2, a chuck 5 is installed within a chamber 10 which has
 an opening at the top, and the chamber 10 is grounded. A wafer on which a
 semiconductor device is to be formed is loaded on the chuck 5. The chuck 5
 is insulated from the bottom of the chamber 10 by a chuck support 15
 formed of a dielectric material. A gas injection ring 20 surrounding the
 chuck 5 is installed around the chuck 5. Predetermined gaps G.sub.1 and
 G.sub.2 exist between the gas injection ring 20 and the chuck 5. The gas
 injection ring 20 is fixed to the bottom of the chamber 10 by a fixing
 means 25 such as a bolt. As a result, a chuck capacitance is generated
 between the gas injection ring 20 and the chuck 5, i.e., between the
 chamber 10 and the chuck 5. The gas injection ring 20 has a gas injection
 passage, and a process gas is externally injected into the chamber 10 via
 the gas injection passage. The gas injection passage is connected to a gas
 tank 55 installed outside the chamber 10 through a gas inlet.
 A valve 60 is installed at a predetermined position of the gas inlet, and
 is controlled by a valve control signal .PHI..sub.v output from a system
 controller 65, which controls the operation of the plasma apparatus. The
 bottom of the chamber 10 or a predetermined area of the sidewall of the
 chamber 10 is branched and connected to a vacuum pump 30, which is
 controlled by a pump control signal .PHI..sub.p output from the system
 controller 65.
 The opening of the chamber 10 is covered by a cover 35 that is formed of a
 dielectric material. A plasma electrode 40 connected to a first plasma
 power source 45a is installed over the cover 35. For example, the first
 plasma power source 45a may be a radio frequency power source. The plasma
 electrode 40 is preferably formed of an induction coil or the like. It is
 preferable that a first radio frequency matching circuit 50a, be installed
 between the plasma electrode 40 and the first plasma power source 45a, so
 that it can maximize the transmission efficiency of plasma power generated
 from the first plasma power source 45a. The chuck 5 is also connected to a
 second plasma power source 45b, i.e., to a chuck plasma power source. The
 chuck plasma power source may be an induction plasma power source. The
 second plasma power source 45b induces plasma generated within the chamber
 10 by the first plasma power source 45a over the chuck 5. A second radio
 frequency matching circuit 50b having the same function as that of the
 first radio frequency matching circuit 50a is interposed between the
 second plasma power source 45b and the chuck 5. The second radio frequency
 matching circuit 50b is preferably an induction RF matching circuit. The
 output ports of the first and second plasma power sources 45a and 45b are
 connected to the system controller 65, which detects an on/off state of
 the first and second plasma power sources 45a and 45b. A capacitance
 compensator 70 which is a characteristic element of the present invention,
 is connected between the chuck 5 and the chamber 10.
 The capacitance compensator 70 controls the total chuck capacitance between
 the chuck 5 and the ground terminal using a reference signal
 .PHI..sub.REF, which corresponds to an optimal total chuck capacitance
 applied to the system controller 65, a gas signal .PHI..sub.G for
 controlling a process gas injected into the chamber 10, and a plasma
 signal .PHI..sub.RF. The capacitance compensator 70 measures the total
 chuck capacitance and transmits a signal .PHI..sub.MC to corresponding the
 measured total chuck capacitance, to the system controller 65. The system
 controller 65 then displays the measured total chuck capacitance.
 Referring to FIG. 3, which shows a detailed circuit diagram of the
 capacitance compensator 70 of FIG. 2, the capacitance compensator 70
 includes a variable capacitor portion 105 and a variable capacitor
 controller for controlling the capacitance of the variable capacitor
 portion 105. The variable capacitor portion 105 preferably includes at
 least one capacitor selected among a serial capacitor C.sub.S connected in
 series to a chuck capacitor C.sub.CK, which itself comprises the chuck 5
 and the gas injection ring 20, and a parallel capacitor C.sub.P connected
 in parallel to the combination of the serial capacitor C.sub.S and the
 chuck capacitor C.sub.CK. Here, the selected capacitor C.sub.P is a
 variable capacitor. The serial capacitor C.sub.S in be interposed between
 the chuck capacitor C.sub.CK and a ground terminal (corresponding to a
 node N), or between the chuck capacitor C.sub.CK and the chuck 5
 (corresponding to a node M). However, a variable capacitor portion 105
 comprising only one parallel capacitor C.sub.P, specifically, a parallel
 variable capacitor C.sub.P, will be considered in an embodiment of the
 present invention to be described below, for the convenience of
 explanation.
 Referring to FIG. 3, one electrode of the parallel variable capacitor
 C.sub.P is connected to the node M corresponding to the chuck 5 of FIG. 2,
 and the other electrode of the parallel variable capacitor C.sub.P is
 connected to the node N corresponding to the chamber 10 of FIG. 2. As a
 consequence, the parallel variable capacitor C.sub.P is connected in
 parallel to the chuck capacitor C.sub.CK. In particular, it is preferable
 that one electrode of the parallel variable capacitor C.sub.P is fixed,
 and the other electrode thereof is movable. For example, it is preferable
 that the electrode connected to the node M is a fixed electrode, and the
 electrode connected to the node N is a driven, or movable, electrode.
 The variable capacitor controller includes a motive power source 110, a
 capacitance meter 115, an automatic control signal generator 120, and a
 driver portion 125.
 The motive power source 110 is connected to the driven electrode of the
 parallel variable capacitor C.sub.P, and the position of the driven
 electrode is controlled by the motive power source 110. It is preferable
 that the motive power source 110 is a direct current motor including a
 rotational shaft. In this embodiment, the interval "d" between the driven
 electrode and the fixed electrode increases or decreases with respect to
 the direction of rotation of the rotational shaft of the direct current
 motor that forms the motive power source 110.
 Meanwhile, the fixed electrode of the parallel variable capacitor C.sub.P
 is connected to the capacitance meter 115, which measures the total chuck
 capacitance generated between the chuck 5 of FIG. 2 and a ground terminal.
 Thus, the total chuck capacitance can be changed by the capacitance of the
 parallel variable capacitor C.sub.P. The capacitance meter 115 can be an
 LCR meter, and preferably outputs an electrical signal, such as a voltage,
 corresponding to the value of the measured total chuck capacitance.
 It is preferable that a process state sensor 130 be further installed
 between the parallel variable capacitor C.sub.P and the capacitance meter
 115. The process state sensor 130 receives the plasma signal .PHI..sub.RF
 and the gas signal .PHI..sub.G from the system controller 65 of FIG. 2,
 and electrically connects or disconnects the fixed electrode of the
 parallel variable capacitor C.sub.P to the input port of the capacitance
 meter 115. To be more specific, it is preferable that the process state
 sensor 130 comprise a NOR gate 205, a switching circuit 210, and a relay
 switch 215. The NOR gate 205 outputs a signal corresponding to a logic "0"
 when at least one signal among the plasma signal .PHI..sub.RF and gas
 signal .PHI..sub.G is turned on. The switching circuit 210 generates
 current or generates no current, according to the output signal of the NOR
 gate. The relay switch 215 is interposed between the fixed electrode of
 the parallel variable capacitor C.sub.P and the input port of the
 capacitance meter 115, and is controlled by the output current of the
 switching circuit 210.
 It is preferable that the switching circuit 210 be a darlington circuit,
 comprising two NPN bipolar transistors Q.sub.1 and Q.sub.2. However, the
 switching circuit can be a switching circuit other than the darlington
 circuit, such as, a general switching circuit comprising a MOS transistor.
 If a darlington circuit is used, the collectors of the two NPN bipolar
 transistors that form the darlington circuit are both connected to a
 positive power source +V.sub.cc, and the emitter of the first NPN bipolar
 transistor Q.sub.1 is connected to the base of the second NPN bipolar
 transistor Q.sub.2. The base of the first NPN bipolar transistor Q.sub.1
 is then the input port of the switching circuit 210, and is connected to
 the output port of the NOR gate. The emitter of the second NPN bipolar
 transistor Q.sub.2 is the output port of the switching circuit 210. Thus,
 when at least one of the plasma power sources 45a and 45b (see FIG. 2) is
 turned on or a process gas is injected into the chamber 10, the NOR gate
 outputs a signal corresponding to a logic "0," and no current flows
 between the positive power source +V.sub.cc and the emitter of the second
 NPN bipolar transistor Q.sub.2. Consequently, at this time the relay
 switch 215 is turned off, so that the parallel variable capacitor C.sub.P
 and the capacitance meter 115 are electrically disconnected from each
 other. Conversely, when both the plasma power sources 45a and 45b are
 turned off and no process gases are injected into the chamber, the relay
 switch 215 is turned on, so that the parallel variable capacitor C.sub.P
 and the capacitance meter 115 are electrically connected to each other. As
 a result, the capacitance meter 115 measures the total chuck capacitance
 only when a process is not performed.
 As described above, it is preferable that the process state sensor 130 be
 designed so as to disconnect the chuck 5 from the capacitance meter 115
 while a predetermined process is being performed within the chamber 10.
 This is because the total chuck capacitance cannot be accurately measured
 due to plasma power applied to the chuck 5 or a process gas injected into
 the chamber 10 while a predetermined process is performed within the
 chamber 10.
 Also, the fixed electrode of the parallel variable capacitor C.sub.P and
 the input port of the capacitance meter 115 are preferably connected to a
 coaxial cable 135 having excellent noise immunity.
 The automatic control signal generator 120 preferably includes an analog
 comparator 305 for amplifying the difference between the output signal of
 the capacitance meter 115 and the reference signal .PHI..sub.REF output
 from the system controller 65, and for outputting the resultant compared
 signal. Here, the reference signal .PHI..sub.REF corresponds to an optimal
 total chuck capacitance that an operator inputs to the system controller
 65.
 The analog comparator 305 can include an operational amplifier 320 having
 an inverting input terminal (-) and a non-inverting input terminal (+).
 First and second resistors R.sub.1A and R.sub.2A are connected in series
 to each other between the non-inverting input terminal (+) of the
 operational amplifier 320 and a ground terminal, and a third resistor
 R.sub.3A is interposed between the inverting input terminal (-) of the
 operational amplifier 320 and the reference signal (.PHI..sub.REF) output
 terminal of the system controller 65. A fourth resistor R.sub.4A is
 interposed between the inverting input terminal (-) of the operational
 amplifier 320 and the output terminal of the operational amplifier 320,
 and a node between the first and second resistors R.sub.1A and R.sub.2A is
 connected to the output terminal of the capacitance meter 115.
 Accordingly, the output terminal of the operational amplifier 320 shows an
 output voltage V.sub.oA corresponding to the voltage difference between a
 reference voltage V.sub.REF applied to the inverting input terminal (-)
 and an input voltage V.sub.iA applied to the non-inverting input terminal
 (+). When the input voltage V.sub.iA is lower than the reference voltage
 V.sub.REF, the output voltage V.sub.oA of the operational amplifier 320 is
 negative. Conversely, when the input voltage V.sub.iA is higher than the
 reference voltage V.sub.REF, the output voltage V.sub.oA of the
 operational amplifier 320 is positive. In this circuit, the first through
 fourth resistors R.sub.1A, R.sub.2A, R.sub.3A and R.sub.4A are used to
 appropriately adjust the voltage gain of the analog comparator 305.
 Preferably, the automatic control signal generator 120 further includes a
 first amplifier 310 interposed between the output terminal of the
 capacitance meter 115 and the non-inverting input terminal (+) of the
 analog comparator 305, and a second amplifier 315 interposed between the
 reference signal (.PHI..sub.REF) output terminal of the system controller
 65 and the inverting input terminal (-) of the analog comparator 305. The
 first and second amplifiers 310 and 315 act as voltage controllers for
 applying an appropriate voltage level to the non-inverting input terminal
 (+) and inverting input terminal (-) of the analog comparator 305. For
 example, when the optimal total chuck capacitance input to the system
 controller 65 is consistent with the total chuck capacitance measured by
 the capacitance meter 115, voltages applied to the non-inverting input
 terminal (+) and inverting input terminal (-) of the analog comparator 305
 must be the same. Thus, it is preferable that the first and second
 amplifiers 310 and 315 are further included in the automatic control
 signal generator 120.
 Preferably, both the first and second amplifiers 310 and 315 can include
 operational amplifiers. However, the first and second amplifiers 310 and
 315 can also be formed as other amplifiers that do not use operational
 amplifiers. To be more specific, the first amplifier 310 preferably
 includes an operational amplifier 325, a first resistor R.sub.1B
 interposed between the inverting input terminal (-) of the operational
 amplifier 325 and a ground terminal, and a second resistor R.sub.2B
 interposed between the inverting input terminal (-) and the output
 terminal of the operational amplifier 325. The non-inverting input
 terminal (+) of the operational amplifier 325 is connected to the output
 terminal of the capacitance meter 115. Thus, the voltage gain of the first
 amplifier 310 can be expressed as (R.sub.1B +R.sub.2B)/R.sub.1B. An input
 resistor R.sub.iB can be interposed between the non-inverting input
 terminal (+) of the first amplifier 310 and the capacitance meter 115.
 The output signal of the first amplifier 310, i.e., an amplified signal
 .PHI..sub.MC with respect to the measured total chuck capacitance, can be
 transmitted to the system controller 65. As a result, the system
 controller 65 converts the total chuck capacitance measured by the
 capacitance meter 115 into a decimal number so that the total chuck
 capacitance may be visually detected by an operator. Here, an output
 resistor R.sub.oB such as a variable resistor can be further interposed
 between the output terminal of the first amplifier 310 and the system
 controller 65. Both the input resistors R.sub.iB and the output resistor
 R.sub.oB of the first amplifier 310 are used for the purpose of
 appropriately controlling an analog signal measured by the capacitance
 meter 115.
 The second amplifier 315 preferably includes an operational amplifier 330,
 a first resistor R.sub.1C and an input resistor R.sub.iC sequentially
 connected to each other in series between the inverting input terminal (-)
 of the operational amplifier 330 and a ground terminal, a second resistor
 R.sub.2C interposed between the non-inverting input terminal (+) of the
 operational amplifier 330 and the reference signal (.PHI..sub.REF) output
 terminal of the system controller 65, a third resistor R.sub.3C interposed
 between the inverting input terminal (-) and the output terminal of the
 operational amplifier 330, and a fourth resistor R.sub.4C interposed
 between the non-inverting input terminal (+) of the operational amplifier
 330 and a ground terminal. It is preferable that one resistor among the
 first through fourth resistors, e.g., the first resistor R.sub.1C, be a
 variable resistor, since the offset value of the operational amplifier 330
 can be minimized when the parallel resistance value of the first and
 second resistors is made equal to that of the third and fourth resistors
 by appropriately controlling the first variable resistor R.sub.1C. Here,
 the output voltage V.sub.oc of the second amplifier 315 can be expressed
 as R.sub.2C (V.sub.2 -V.sub.1)/R.sub.1C, where V.sub.1 denotes a node
 voltage between the input resistor R.sub.iC and the first resistor
 R.sub.1C and V.sub.2 denotes the total voltage induced in the second and
 fourth resistors R.sub.2C and R.sub.4C. A variable output resistor
 R.sub.oC can be installed between the output terminal of the second
 amplifier 315 and the analog comparator 305. Thus, voltages applied to the
 input terminals of the analog comparator 305 can be more precisely
 controlled by appropriately adjusting the output resistor R.sub.oC of the
 second amplifier 315. Also, an output resistor R.sub.oA can be further
 included in the output terminal of the analog comparator 305.
 The output signal of the automatic control signal generator 120 is
 preferably amplified by the driver portion 125, and the signal amplified
 by the driver portion 125 drives the motive power source 110.
 The driver portion 125 preferably includes an amplification stage 405 for
 amplifying the output signal of the automatic control signal generator
 120, and a driving capacitor C.sub.d interposed between the output port of
 the amplification stage 405 and a ground terminal. The driving capacitor
 C.sub.d is connected in parallel to the power source terminals of the
 motive power source 110, and removes a noise signal input to the power
 source terminal of the motive power source 110. Thus, the driving
 capacitor C.sub.d operates to stably drive the motive power source 110,
 which is preferably a direct current motor. The rotating direction of the
 rotational shaft of the direct current motor depends on the polarity of a
 signal output by the amplification stage 405. For example, if the rotating
 shaft of the direct current motor rotates clockwise while the output port
 of the amplification stage 405 has a positive voltage, the rotating shaft
 of the direct current motor rotates counterclockwise while the output port
 of the amplification stage 405 has a negative voltage. A predetermined
 power transmitting means for changing the rotational movement into linear
 movement is interposed between the rotating shaft of the direct current
 motor and the driven electrode of the parallel variable capacitor C.sub.P,
 such that the driven electrode of the parallel variable capacitor C.sub.P
 is moved.
 For example, the power transmitting means can be a cylindrical tube having
 a spiral groove formed on its inner surface. In this case, a spiral groove
 to be meshed with the spiral groove formed on the inner surface of the
 tube is formed on the outer surface of the rotating shaft of the direct
 current motor. When the rotating shaft of the direct current motor is
 coupled to fit into the tube, the interval between the electrodes of the
 parallel variable capacitor C.sub.P increases or decreases according to
 the direction of rotation of the rotational shaft.
 Preferably, the amplification stage 405 is a push-pull amplifier including
 an NPN bipolar transistor Q.sub.N and a PNP bipolar transistor Q.sub.P
 connected to each other in series between the positive and negative power
 sources (+V.sub.cc) and (-V.sub.cc). The base electrode of the NPN bipolar
 transistor Q.sub.N and the base electrode of the PNP bipolar transistor
 Q.sub.P are connected to each other and act as the input terminal of a
 push-pull amplifier. The emitter electrodes of the complementary bipolar
 transistors are connected to each other and act as the output terminal of
 a push-pull amplifier. When a positive voltage is applied to the input
 terminal of the push-pull amplifier, the NPN bipolar transistor Q.sub.N
 connected to the positive power source +V.sub.cc is turned on, so that an
 amplified positive voltage is induced at the output terminal. On the other
 hand, when a negative voltage is applied to the input terminal of the
 push-pull amplifier, the PNP bipolar transistor Q.sub.P connected to the
 negative power source -V.sub.cc is turned on, so that an amplified
 negative voltage is induced at the output terminal. Accordingly, the
 capacitance of the parallel variable capacitor C.sub.P increases or
 decreases according to the polarity and level of the voltage applied to
 the input terminal of the amplification stage. Also, an output capacitor
 C.sub.o can be further included between the push-pull amplifier and the
 driving capacitor C.sub.d. The output capacitor C.sub.o operates to remove
 a noise signal from the output signal of the push-pull amplifier and also
 operates to more stably drive the motive power source 110, when it is a
 direct current motor.
 The driver portion 125 can further include an inverter 410 between the
 amplication stage 405 and the automatic control signal generator 120. The
 inverter 410 preferably comprises an operational amplifier 420. The driver
 portion 125 can further include a feed-back resistor R.sub.F interposed
 between the input terminal of the inverter 410 and the output port of the
 amplication stage 405. The feed-back resistor R.sub.F stably drives the
 amplication stage 405. The driver portion 125 can further include a noise
 filter 415, which allows only a direct current signal to pass from the
 automatic control signal generator 120 to the inverter 410. The noise
 filter 415 preferably comprises a filter capacitor C.sub.F interposed
 between the output port of the automatic control signal generator 120 and
 a ground terminal, and a filter inductor L.sub.F interposed between the
 output port of the automatic control signal generator 120 and the input
 port of the inverter 410. The driver portion 125 can further include an
 input resistor R.sub.I between the noise filter 415 and the automatic
 control signal generator 120. The input resistor R.sub.I in the driver
 portion 125 prevents the input signal of the driver portion 125 from
 abruptly changing, by delaying the output signal of the automatic control
 signal generator 120 for a predetermined time.
 As described above, the capacitance compensator 70 in the plasma apparatus
 according to the preferred embodiment of present invention drives the
 motive power source 110 until the total chuck capacitance of the plasma
 apparatus is consistent with the optimal total chuck capacitance, whenever
 a process is not performed, and controls the capacitance of the parallel
 variable capacitor C.sub.P. Thus, the total chuck capacitance can be kept
 at a constant value without periodic pre-maintenance.
 The variable capacitor controller can further include a limit control
 portion 140 for ensuring so that the driven electrode of the parallel
 variable capacitor C.sub.P moves only within a predetermined range.
 The limit control portion 140 includes a voltage limit setting portion 505,
 a limit sensor 510, a digital comparator 515, and a connector 520. The
 voltage limit setting portion 505 generates a desired negative voltage
 limit and a desired positive voltage limit. The limit sensor 510 generates
 a voltage corresponding to the actual position of the driven electrode of
 the parallel variable capacitor C.sub.P. The digital comparator 515
 compares the output voltage of the limit sensor 510 with limit voltages
 set by the voltage limit setting portion 505, and outputs first and second
 signals via first and second output terminals, respectively. The connector
 520 is controlled by the first or second signal of the digital comparator
 515, and electrically connects the input and output terminals of the
 inverter 410 in the driver portion 125 to each other.
 The voltage limit setting portion 505 preferably includes first and second
 variable resistors R.sub.V1 and R.sub.V2 connected to each other in series
 between the negative and positive power sources -V.sub.cc and +V.sub.cc.
 Thus, a desired negative voltage limit can be obtained at a variable
 terminal of the first variable resistor R.sub.V1 by appropriately
 controlling the variable terminal of the first variable resistor R.sub.V1
 connected to the negative power source -V.sub.cc. Similarly, a desired
 positive voltage limit can be obtained at a variable terminal of the
 second variable resistor R.sub.V2.
 The limit sensor 510 preferably includes a variable resistor R.sub.V3
 interposed between the negative and positive power sources -V.sub.cc and
 +V.sub.cc. The variable terminal of the variable resistor R.sub.V3 is
 mechanically connected to the driven electrode of the parallel variable
 capacitor C.sub.P and moves together with the driven electrode. An
 insulator 145 is preferably interposed between the variable terminal of
 the variable resistor R.sub.V3 and the driven electrode to electrically
 insulate them from each other. Accordingly, when the driven electrode
 moves, the voltage of the variable terminal of the variable resistor
 R.sub.V3 also changes. As a consequence, a voltage corresponding to the
 position of the driven electrode can be obtained at the variable terminal
 of the variable resistor R.sub.V3. The variable terminal of the variable
 resistor R.sub.V3 corresponds to the output terminal of the limit sensor
 510.
 The digital comparator 515 preferably comprises first and second
 comparators 530 and 535, first through fourth diodes 540a-540d, and first
 and second induction coils 545 and 550. The first comparator 530 outputs a
 positive voltage only when the output voltage of the limit sensor 510 is
 lower than the voltage of the variable terminal of the first variable
 resistor R.sub.V1 included in the limit setting portion 505. The second
 comparator 535 outputs a positive voltage only when the output voltage of
 the limit sensor 510 is higher than the voltage of the variable terminal
 of the second variable resistor R.sub.V2 included in the limit setting
 portion 505. The first and second diodes 540a and 540b are connected to
 each other in series between the output terminal of the first comparator
 530 and a ground terminal. Similarly, the third and fourth diodes 540c and
 540d are connected to each other in series between the output terminal of
 the second comparator 535 and a ground terminal.
 The P-type electrodes of the first and second diodes 540a and 540b are
 connected directly to the output terminal of the first comparator 530 and
 the ground terminal, respectively. Similarly, the P-type electrodes of the
 third and fourth diodes 540c and 540d are connected directly to the output
 terminal of the second comparator 535 and the ground terminal,
 respectively.
 The first diode 540a is connected in parallel to the first induction coil
 545, and the third diode 540c is connected in parallel to the second
 induction coil 550. Thus, when the output voltage of the limit sensor 510
 is lower than the negative voltage limit due to excessive movement of the
 driven electrode of the parallel variable capacitor C.sub.P, a first
 signal, i.e., a first current, flows only in the first induction coil 545.
 Similarly, when the output voltage of the limit sensor 510 is higher than
 the positive voltage limit due to excessive movement of the driven
 electrode of the parallel variable capacitor C.sub.P, a second signal,
 i.e., a second current, flows only in the second induction coil 550.
 The connector 520 includes a first relay switch 555 controlled by the first
 current flowing in the first induction coil 545, a second relay switch 560
 controlled by the second current flowing in the second induction coil 550,
 a first rectifying device 565, e.g., a first diode interposed between one
 end of the first relay switch 555 and the output terminal of the inverter
 410 included in the driver portion 125, and a second rectifying device
 570, e.g., a second diode interposed between one end of the second relay
 switch 560 and the output terminal of the inverter 410 included in the
 driver portion 125. The other end of each of the first and second relay
 switches 555 and 560 is connected to the input port of the inverter 410.
 Thus, when the output voltage of the limit sensor 510 is lower than the
 negative voltage limit, a current is induced in the first induction coil
 545, causing the first relay switch 555 to turn on. The output voltage of
 the inverter 410 included in the driver portion 125 is thus inverted. As a
 consequence, the rotating direction of the motive power source 110 is
 changed, so that the output voltage of the limit sensor 510 becomes higher
 than the negative voltage limit. Conversely, when the output voltage of
 the limit sensor 510 is higher than the positive voltage limit, a current
 is induced in the second induction coil 550, causing the second relay
 switch 560 to turn on. The output voltage of the inverter 410 included in
 the driver portion 125 is thus inverted. As a consequence, the rotating
 direction of the motive power source 110 is changed, so that the output
 voltage of the limit sensor 510 becomes lower than the positive voltage
 limit.
 The limit control portion 140 can further include an amplifier 525 between
 the limit sensor 510 and the digital comparator 515. The amplifier 525
 preferably has the same configuration and function as the first amplifier
 310 in the automatic control signal generator 120. The amplifier 525 may
 include an operational amplifier 575 and first and second resistors
 R.sub.1D and R.sub.2D. In addition, the limit control portion 140 can
 further include an inductor L.sub.L and a resistor R.sub.L connected in
 series to each other between the limit sensor 510 and the amplifier 525,
 to remove a noise signal generated by the limit sensor 510.
 As described above, the limit control portion 140 limits the range within
 which the driven electrode of the parallel variable capacitor C.sub.P may
 move. This can prevent a collision of the driven electrode of the parallel
 variable capacitor C.sub.P with the fixed electrode by appropriate control
 of the first and second variable resistors R.sub.V1 and R.sub.V2 in the
 voltage limit setting portion 505.
 Meanwhile, the variable capacitor controller can further comprise a limit
 display portion 150, which is controlled by the output signal of the
 digital comparator 515 included in the limit control portion 140. The
 limit display portion 150 generates a signal that allows an operator to
 visually detect when the driven electrode is situated outside of an
 allowable range.
 The limit display portion 150 preferably includes first and second relay
 switches 605 and 610 and at least two light emitting devices 615 and 620.
 Preferably, the light emitting device is a light emitting diode. The first
 relay switch 605 and the first light emitting device 615 are connected to
 each other in series between the positive power source +V.sub.cc and a
 ground terminal, and the second relay switch 610 and the second light
 emitting device 620 are also connected to each other in series between the
 positive power source +V.sub.cc and a ground terminal. The first relay
 switch 605 is controlled by the first induction coil 545 included in the
 limit control portion 140, and the second relay switch 610 is controlled
 by the second induction coil 550 included in the limit control portion
 140. It is preferable that a resistor R.sub.D1 is interposed between the
 first relay switch 605 and the first light emitting device 615 to prevent
 excess current from flowing into the first light emitting device 615 when
 the first relay switch 605 is turned on. Similarly, it is preferable that
 a resistor R.sub.D2 is also interposed between the second relay switch 610
 and the second light emitting device 620. A third light emitting device
 625 and a resistor R.sub.D3 connected to each other in series may be
 further included between the positive power source +V.sub.cc and a ground
 terminal, so that the general operation of the capacitance compensator 70
 can be visually detected. Thus, when the limit control portion 140 is
 further included, an operator may be visually informed when the driven
 electrode of the parallel variable capacitor C.sub.P is outside the
 predetermined range.
 The variable capacitor controller preferably further includes a selection
 switch 155 interposed between the driver portion 125 and the automatic
 control signal generator 120, and a manual control signal generator 160
 connected to the selection switch 155.
 The selection switch 155 operates to select an automatic mode or a manual
 mode, and preferably has three terminals. One terminal of the selection
 switch 155 is connected to the input terminal of the driver portion 125;
 another terminal of the selection switch 155 is connected to the output
 terminal of the automatic control signal generator 120; and the third
 terminal of the selection switch 155 is connected to the output terminal
 of the manual control signal 160. Accordingly, as shown in FIG. 3, the
 capacitance compensator 70 operates in the automatic mode when the
 selection switch 155 is switched in a direction "A," and operates in the
 manual mode when the selection switch 155 is switched in a direction "B."
 The manual control signal generator 160 includes a variable resistor
 R.sub.v4 and a power switch 705. The ends of the variable resistor
 R.sub.v4 are respectively connected to a ground terminal and the power
 switch 705, and the variable terminal of the variable resistor R.sub.v4 is
 connected to the selection switch 155. The power switch 705 selects either
 a positive power source +V.sub.cc or a negative power source -V.sub.cc.
 Accordingly, the capacitance of the parallel variable capacitor C.sub.P
 can be decreased (or increased) by switching the mode of the selection
 switch 155 into a manual mode, connecting the power switch 705 to the
 positive power source +V.sub.cc, and appropriately controlling the
 variable terminal of the variable resistor R.sub.v4. Similarly, the
 capacitance of the parallel variable capacitor C.sub.P can be increased
 (or decreased) by switching the mode of the selection switch 155 into a
 manual mode, connecting the power switch 705 to the negative power source
 -V.sub.cc, and appropriately controlling the variable terminal of the
 variable resistor R.sub.v4.
 FIG. 4A is a graph showing the etch rate of a polysilicon film and that of
 a photoresist film when a conventional plasma apparatus is used. FIG. 4B
 is a graph showing the etch rate of a polysilicon film and that of a
 photoresist film when a plasma apparatus according to a preferred
 embodiment of the present invention is applied. Here, the etch rate of the
 photoresist film was obtained by applying the same etching recipe as that
 used for etching the polysilicon film. In FIGS. 4A and 4B, the horizontal
 axis denotes time (t), and the two vertical axes on the left and right
 denote the etch rate of the polysilicon film and that of the photoresist
 film, respectively. Data in FIG. 4A shows etch rates measured continuously
 for about 60 days, and data in FIG. 4B shows etch rates measured
 continuously for about 20 days. The polysilicon film etching recipe
 applied in the present invention was the same as the polysilicon film
 etching method applied in the prior art.
 Referring to FIGS. 4A, the etch rate of the polysilicon film ranged from
 2800 .ANG./min to 3900 .ANG./min when a conventional plasma etching
 apparatus was used. However, as shown in FIG. 4B, when a plasma etching
 apparatus according to a preferred embodiment of the present invention was
 used, the etch rate of the polysilicon film showed uniform values of about
 3250 .ANG./min to 3350 .ANG./min. Also, when a photoresist film was etched
 using the conventional plasma etching apparatus, the etch rate of the
 photoresist film ranged from 1900 .ANG./min to 3400 .ANG./min. In
 comparison, when a photoresist film was etched using the plasma etching
 apparatus according to the present invention, the etching rate of the
 photoresist film showed uniform values ranging from 2700 .ANG./min to 3000
 .ANG./min.
 FIG. 5 is a graph showing variations in the thickness of a residual oxide
 layer remaining after a gate polysilicon film was etched using a
 conventional plasma etching apparatus and a plasma etching apparatus
 according to a preferred embodiment of the present invention. Here, an
 initial gate oxide film was formed to a thickness of 110 .ANG. to
 120.ANG., and a 1000 .ANG. gate polysilicon film was formed on the initial
 gate oxide film. In FIG. 5, the horizontal axis denotes time t1 and t2,
 the vertical axis denotes the thickness R.sub.ox of a residual oxide film,
 and portions (a) and (b) denote the conventional design and the present
 invention, respectively. Data for the conventional design shows results
 measured for 30 days, while data for the present invention shows results
 measured for 10 days.
 Referring to FIG. 5, the thickness R.sub.ox of a residual oxide film in the
 conventional design ranged from 75 .ANG. to 105.ANG., while the thickness
 R.sub.ox of a residual oxide film in the present invention showed much
 more uniform values, ranging from 87 .ANG. to 94 .ANG..
 FIG. 6 is a graph showing the measured results of the power of a reflected
 wave with respect to the output signal of the second plasma power source
 45b according to the installation position of the capacitance compensator
 70 of FIG. 3. In FIG. 6, the horizontal axis denotes the total chuck
 capacitance between a chuck and a ground terminal, and the vertical axis
 denotes the power of a reflected wave with respect to the output signal of
 the second plasma power source 45b. The center value (X) of the total
 chuck capacitance was 1000 pF. Accordingly, the powers of the reflected
 wave shown in FIG. 6 are measured values when the total chuck capacitance
 changes between 500 pF and 1500 pF. In FIG. 6, data indicated by "x" and
 ".diamond-solid." corresponds to the power of reflected waves measured
 when the capacitance compensator 70 is installed under the chamber 10 and
 over the chamber 10, respectively, and data indicated by ".box-solid." and
 ".tangle-solidup." corresponds to the power of reflected waves measured
 when the capacitance compensator 70 is installed on the left and the right
 sides of the chamber 10, respectively. In this case, the second radio
 frequency matching circuit 50b connected to the chuck 5, e.g., the
 induction RF matching circuit, was installed at a position closest to the
 chuck 5, that is, under the chamber 10.
 Referring to FIG. 6, when the capacitance compensator 70 was installed over
 the chamber 10 or on the right or left side of the chamber 10, the
 reflected wave of an RF signal output by the second plasma power source
 45b greatly changed with a variation of the total chuck capacitance. On
 the other hand, when the capacitance compensator 70 was installed under
 the chamber 10, the reflected wave of an RF signal output from the second
 plasma power source 45b had small changes regardless of a variation in the
 total chuck capacitance.
 When the reflected wave is generated, a plasma process is unstable, so that
 the uniformity of a process parameter such as an etch rate or a deposition
 rate is degraded. Accordingly, when the capacitance compensator 70 is
 installed under the chamber 10, that is, close to the second RF matching
 circuit 50b, a stable plasma process can be performed even when the
 desired optimal total chuck capacitance is changed within a predetermined
 range. Thus, it is most preferable that the capacitance compensator 70 be
 installed close to the second RF matching circuit 50b. Also, it is
 preferable that the output port of the second RF matching circuit 50b be
 connected to the variable capacitor C.sub.P in the capacitance compensator
 70 via a coaxial cable 135, and that the coaxial cable 135 is as short as
 possible. It is also preferable that the capacitance compensator 70
 including the coaxial cable 135 be built in the second RF matching circuit
 50b to ensure an operator's safety and to save space.
 According to the present invention as described above, the total chuck
 capacitance between a chuck and a ground terminal can be controlled so
 that it is always consistent with a desired optimal total chuck
 capacitance. As a result, the uniformity of a plasma process can be
 improved. Also, in the present invention, precise manual adjustment of the
 gap between a chuck and a gas injection ring is not required during
 pre-maintenance. Therefore, the time required for pre-maintenance is
 significantly reduced.
 Although the invention has been described with reference to a particular
 embodiment, it will be apparent to one of ordinary skill in the art that
 modifications of the described embodiment may be made without departing
 from the spirit and scope of the invention.