Patent Publication Number: US-2007113787-A1

Title: Plasma process apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a division of U.S. application Ser. No. 10/496,361, filed May 21, 2004, which is the National Phase of International Application PCT/JP2002/013093, filed Dec. 13, 2002. This application claims priority from Japanese patent application Serial No. 2001-380168 filed Dec. 13, 2001, the entire contents of which are expressly incorporated herein by reference. 
    
    
     TECHNICAL FIELD  
      The present invention relates to plasma process apparatus that carries out processes such as film formation and etching to workpieces such as semiconductor wafers.  
     BACKGROUND ART  
      Plasma process apparatus is used in the fabrication processes of such as semiconductor substrates and liquid crystal substrates. The apparatus carries out surface treatment on those substrates using plasma. Plasma process apparatus includes, for example, plasma etchers that carry out etching on substrates, and plasma deposition reactors that carry out the process of chemical-vapor deposition (CVD). Among these kinds of plasma process apparatus, those of parallel-plate type are vastly used because they can carry out processes homogeneously and make the structure of equipment relatively simple.  
      The plasma process apparatus of parallel-plate type has a pair of parallel plate electrodes in the upper and lower sides of a chamber. The lower electrode has a pedestal to hold a workpiece, whereas the upper electrode has multiple gas outlets on the bottom side. The upper electrode is connected to the source of process gases, and process gases are supplied to the space between the two electrodes (plasma-generating space) through the gas outlets during processing. The process gases supplied through the gas outlets are ionized by the radio frequency (RF) electric power applied to the upper electrode. The generated plasma is then pulled near the lower electrode by another RF electric power applied to the lower electrode, the frequency of which is lower than the former. Then, the workpiece located adjacent to the lower electrode is processed with a certain surface treatment by the pulled plasma.  
      With regard to the plasma process apparatus of parallel-plane type described above, the concentration of plasma produced near the upper electrode is reduced until it reaches the workpiece adjacent to the lower electrode. This reduction of concentration is a major problem because the efficiency of processing deteriorates.  
      Besides, it is difficult to install pipes for process gases or coolant, the latter of which is for chamber temperature control, through the upper electrode.  
      The present invention has been made in consideration of the above. And an object thereof is to provide a plasma process apparatus that has high efficiency in plasma processing and that has simple structures.  
     DISCLOSURE OF INVENTION  
      In order to achieve the above object, according to the first aspect of the present invention, there is provided a plasma process apparatus, comprising a chamber ( 2 ) having multiple components and inside of which a workpiece is treated with a certain process, first electrode ( 15   a ) installed as one of the components and electrically grounded, second electrode ( 15   b ) installed as one of the components and supplied with first and second radio frequency electric powers, and a certain area of the chamber ( 2 ) containing plasma produced between the first and second electrodes by applying the second radio frequency power to the second electrode ( 15   b ).  
      In the above structure, plasma is mainly produced near the second electrode ( 15   b ), since both the first and the second RF power are applied to the second electrode ( 15   b ) and the first electrode ( 15   a ) is grounded. Therefore, by putting a workpiece near the second electrode ( 15   b ), plasma process is carried out without moving plasma and the deterioration of process efficiency due to reduction of plasma concentration is prevented.  
      Besides, since the first electrode ( 15   a ) is grounded and the installation of RF power generators or filters is not necessary, the structure of the plasma process apparatus becomes simple. Therefore, it is easy to have a structure in which pipes for process gases and coolant penetrates through the first electrode ( 15   a ).  
      The above structure may further comprise: a low-pass filter ( 14 ) connected between the second electrode ( 15   b ) and the first external power generator that distributes the first RF power, a high-pass filter ( 23 ) connected between the second electrode ( 15   b ) and the second external power generator that distributes the second RF power, and wherein the high-pass filter ( 23 ) substantially prevents the first RF power, which is supplied by the first power generator, from passing through, and the low-pass filter ( 14 ) substantially prevents the second RF power, which is supplied by the second power generator, from passing through.  
      By further having this structure, the malfunction of the RF power generators and loss of power are prevented, both of which are due to the leakage of the first RF power of the first RF power generator into the second RF power generator, or vice versa. Therefore, further efficiency of plasma processing is achieved.  
      The low-pass filter ( 14 ) has capacitors (C 1  and C 2 ) that are connected in parallel to the first RF power generator and a inductor (L) that passes through the first RF power that is distributed to the second electrode. When the inductor (L) makes parallel resonance circuit with its parasitic capacitance, and the resonant frequency of which is around the frequency of the second RF power, it efficiently blocks the second RF power and prevents the loss of the second RF power, keeping the volume of the inductor (L) small.  
      According to the second aspect of the present invention, there is provided a plasma process apparatus, comprising a chamber ( 2 ) having components and inside of which a workpiece is treated with a certain process, first electrode ( 15   a ) installed as one of the components and electrically grounded second electrode ( 15   b ) installed as one of the components and supplied with first radio frequency power, a chuck (ESC) that mounts the workpiece adjacent to the second electrode ( 15   b ) and used to heat the workpiece cooling channels made of conductor and capacitively coupled to the second electrode ( 15   b ) and used to pass through coolant for cooling the chuck (ESC) and a certain area of the chamber ( 2 ) containing plasma produced between the first and second electrodes by applying second radio frequency power to the second electrode ( 15   b ) via the cooling channels.  
      In the above structure, plasma is also mainly produced near the second electrode ( 15   b ), since both the first and the second RF power is applied to the second electrode ( 15   b ) and the first electrode ( 15   a ) is grounded. Therefore, by putting a workpiece near the second electrode ( 15   b ), plasma process is carried out without moving plasma and the deterioration of process efficiency due to reduction of plasma concentration is prevented.  
      Besides, since the first electrode is grounded and the installation of RF power generators or filters is not necessary, the structure of the plasma process apparatus becomes simple. Therefore, it is easy to have a structure in which pipes for process gases and coolant penetrates through the first electrode ( 15   a ).  
      In addition, in the above structure, the second RF power is distributed to the second electrode ( 15   b ) without using wire made of high melting point metal, which generally has high resistivity. Therefore, loss of the second RF power is reduced and process with high efficiency in use of RF power is achieved.  
      The above structure may further comprise a low-pass filter ( 14 ) connected between the second electrode ( 15   b ) and the first external power generator that distributes the first RF power, a high-pass filter ( 23 ) connected between the cooling channels and the second external power generator that distributes the second RF electric power, and wherein the high-pass filter ( 23 ) substantially prevents the first RF electric power, which is distributed by the first power generator, from passing through, and the low-pass filter ( 14 ) substantially prevents the second RF electric power, which is distributes by the second power generator, from passing through.  
      By further having this structure, power loss is prevented, which is due to the leakage of the first RF power of the first RF power generator into the second RF power generator, or vice versa. Therefore, further efficiency of plasma processing is achieved.  
      In addition, in the above structure, the low-pass filter ( 14 ) has capacitors (C 1  and C 2 ) that are connected in parallel to the first RF power generator and an inductor (L) that passes through the first RF power that is distributed to the second electrode. When the inductor (L) makes parallel resonance circuit with its parasitic capacitance, and the resonant frequency of which is around the frequency of the second RF power, it efficiently blocks the second RF power and prevents the loss of the second RF power, keeping the volume of the inductor (L) small.  
      As described above, the second RF power is distributed to the second electrode ( 15   b ) without using wire made of high melting point metal. Besides, the melting point of the conductor used in the cooling channels can be lower than that of the conductor used in the second electrode ( 15   b ) or that of the wire used to distribute the first RF power to the second electrode ( 15   b ). Therefore, the resistivity of the conductor used in the cooling channels is generally lower than that of the conductor used in the second electrode ( 15   b ).  
      According to the third aspect of the present invention, there is provided a plasma process apparatus, comprising a chamber ( 2 ) having multiple components and inside of which a workpiece is treated with a certain process, an electrode installed as one of the components an impedance matching circuit surface-mounted on the electrode and connecting the electrode with the external radio frequency power generator and a certain area of the chamber ( 2 ) contains plasma produced between the electrodes by applying radio frequency power to the electrodes.  
      In the above structure, loss of the RF power that is distributed by the RF power generator is reduced, because the impedance matching circuit is surface-mounted on the electrode. Therefore, the process applied to the workpieces can be made efficient. Besides, since the impedance matching circuit is surface-mounted on the electrode, extra equipment such as boxes to store the circuit is not needed. Thus, the structure of the plasma process apparatus becomes simple, and it is easy to have a structure in which pipes for process gases and coolant penetrates through the electrode.  
      The impedance matching circuit includes surface-mounted passive elements such as capacitors and inductors (L). 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      These objects and other objects and advantages of the present invention will become more apparent upon reading of the following detailed description and the accompanying drawings in which:  
       FIG. 1  shows the structure of the plasma process apparatus for the first embodiment of the present invention.  
       FIG. 2  shows an example of the low-pass filter installed in the plasma process apparatus of  FIG. 1 .  
       FIG. 3  shows the baffle of the plasma process apparatus of  FIG. 1 .  
       FIG. 4  shows a variation of the low-pass filter.  
       FIG. 5  shows the structure of the plasma process apparatus for the second embodiment of the present invention.  
       FIG. 6  shows a part of the structure of the plasma process apparatus for the third embodiment of the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The plasma process apparatus of the present invention comprises: a chamber ( 2 ) includes multiple components and inside of which a workpiece is treated with a certain process; the first electrode ( 16   a ) that is installed as one of the components and is electrically grounded; the second electrode ( 15   b ) that is installed as one of the components and is supplied with the first and the second RF electric power; and wherein a certain area of the chamber ( 2 ) contains the plasma produced between the first and the second electrodes by applying the second RF power to the second electrode ( 15   b ).  
     First Embodiment  
      Details of an embodiment of the present invention will be described below using attached figures. In this embodiment of the present invention, plasma deposition reactors that carry out the process of chemical-vapor deposition (CVD) will be described as an example of the plasma process apparatus (equipment).  
       FIG. 1  shows the structure of the plasma process apparatus for the first embodiment of the present invention. The plasma process apparatus  1  for the first embodiment of the present invention is constructed as that of parallel-plate type, which has a pair of parallel plate electrodes in the upper and lower sides of a chamber. The equipment has a function to form films, e.g. of SiOF, on the surface of semiconductor wafers (hereafter referred to as the wafer W).  
      As shown in  FIG. 1 , the plasma process apparatus has a cylindrical chamber  2 . The chamber  2  is made of conductive materials such as aluminum processed with anodic oxide coating (Alumite). The chamber  2  is electrically grounded.  
      There is a vent  3  at the bottom part of the chamber  2 . The vent  3  is connected to an exhaust system  4  equipped with vacuum pumps such as turbo-molecular pumps. The exhaust system  4  evacuates the chamber  2  to a certain pressure, for example less than 0.01 Pa. Besides, a gate valve  5  is installed in the sidewall of the chamber  2 . With the gate valve  5  opened, the wafer W is carried between the chamber  2  and the load-lock chamber, which is located next to the chamber  2  (not shown).  
      A pseudo-cylindrical susceptor holder  6  is put on the bottom of the chamber  2 . On the susceptor holder  6  lies a susceptor  8  to put the wafer W. The interface between the susceptor holder  6  and the susceptor  8  is insulated with an insulator  7  such as aluminum nitride. In addition, the susceptor holder  6  is connected to an elevator, which is installed in the bottom part of the chamber  2  (not shown), via a shaft  9 , and it can move up and down.  
      The center-top part of the susceptor  8  is molded into a convex disk, upon which the high-temperature electrostatic chuck ESC is mounted. The high-temperature electrostatic chuck ESC has the shape similar to the wafer W, and it has the lower electrode  15   b  and a heater H 1  therein. The lower electrode  15   b  is made of a conductor with high melting point, such as molybdenum. The heater H 1  consists of, for example, Nichrome wire.  
      The lower electrode  15   b  is connected to a direct-current power generator HV via wire made of a conductor with high melting point such as molybdenum. The wafer W put on the susceptor  8  is held against the high-temperature electrostatic chuck ESC by an electrostatic force, by applying the direct-current voltage generated by the direct-current power generator HV to the lower electrode  15   b.    
      In addition, the lower electrode  15   b  is connected to the first RF power generator  13  via the low-pass filter  14  and the second RF power generator  22  via the high-pass filter  23 . Both RF power generators are connected to the direct-current power generator HV in parallel.  
      The frequency of the first RF power generator  13  has range of 0.1˜13 MHz. The application of this frequency band is effective, for example, in reducing damage to the workpieces.  
      The frequency of the second RF power generator  22  has range of 13˜150 MHz. By applying these high frequencies, plasma can be produced in preferable dissociation state and in high density within the chamber  2 .  
      The low-pass filter  14  substantially prevents the second RF electric power, which is distributed by the second power generator  22 , from passing through. Therefore, leakage of the second RF power generated by the second RF power generator  22  into the first RF power generator  13 , and subsequent power loss, can be prevented.  
      Specifically, the low-pass filter  14  consists of, for example, a capacitor C 1  and an inductor L. As shown in  FIG. 2 , one end of the inductor L is connected to the first RF power generator  13 , and the other end of it is connected to the lower electrode  15   b  via a coupling capacitor C 2 . Besides, one end of the capacitor C 1  is connected to the joint of the inductor L and the first RF power generator  13 , and the other end of it is grounded.  
      The high-pass filter  23  consists of, for example, a capacitor placed between the second RF power generator  22  and the lower electrode  15   b . The high-pass filter  23  substantially prevents the first RF power, which is generated by the first power generator  13 , from passing through. Therefore, the leakage of the first RF power generated by the first RF power generator  13  into the second RF power generator  22 , and subsequent power loss, can be prevented.  
      A heater H 1  is connected to a heater power generator H 2  that consists of, e.g. commercial power generator, via a low-pass filter H 3 . The high-temperature electrostatic chuck ESC is heated by applying voltage generated by the heater power generator H 2 . Here, the low-pass filter H 3  is used to prevent the RF electric power generated by the first or the second RF power generator from leaking into the heater power generator H 2 .  
      The center-bottom part of the susceptor holder  6  is covered by, for example, a bellows  10  made of stainless steel. The bellows  10  separates into two parts: one is a vacuum part in the chamber  2 ; the other is an atmosphere-exposed part. The upper and the lower part of the bellows  10  are screwed to the bottom surface of the susceptor holder  6  and to the floor of the chamber  2 , respectively.  
      Inside of the susceptor holder  6  is a lower cooling channels  11 . The lower cooling channels  11  circulate coolant such as Fluorinert. By this procedure, the temperature of the susceptor  8  and that of the surface of the wafer W is controlled preferably.  
      The lower cooling channels  11  are made of conductors. The upper part of them, which is near the susceptor  8 , constitutes a jacket  11 J that circulates coolant around the interface of the susceptor holder  6  and the insulator  7 .  
      There are lift pins  12  at the susceptor holder  6 . The lift pins  12  are used for delivering the semiconductor wafer W, and that can be raised or lowered by a cylinder (not shown).  
      The upper electrode  15   a  is located above the susceptor  8 , being parallel with it. The upper electrode  15   a  is grounded, and the lower side of it has a plate electrode  16 , which is made of e.g. aluminum and has multiple gas outlets  16   a . The ceiling of the chamber  2  supports the upper electrode  15   a , via the insulator  17 . There are upper cooling channels  18  inside the upper electrode  15   a . The upper cooling channels  18  circulate coolant such as Fluorinert, controlling the temperature of the upper electrode  15   a  preferably.  
      In addition, the upper electrode  15   a  is equipped with the gas outlet  20 , which is connected to the process gas source  21  located outside the chamber  2 . Process gases from the process gas source  21  are distributed via the gas outlet  20  to the hollow space inside the upper electrode  15   a  (not shown). The supplied process gases disperse in the hollow space, and then they flow out of the gas outlets  16   a  toward the wafer W. Various kinds of gases can be used as process gases. In the case of SiOF film forming, the following conventionally used gases can be used: SiF4, SiH4, O2, NF3, NH3 as reaction gases, and Ar as a dilution gas.  
      The sidewall of the chamber  2  is equipped with a baffle  24 . The baffle  24  is made of a conductor such as aluminum processed with anodic oxide coating (Alumite). It is a disk-shaped component with a hole at the center, and it has a structure that the susceptor  8  penetrates through the center hole.  
       FIG. 3  shows the top view of the baffle  24 . As shown in  FIG. 3 , there is a hole  24   b  at the center of the baffle  24 , and in the circumference of the hole lies multiple radial slits  24   a . Now, the slit  24   a  is a rectangle-shaped slit that is bored vertically through the baffle  24 . The width of the slit  24   a  is set to 0.8˜1.0 mm, in order to block plasma while making gases pass through. The hole  24   b  has nearly the same area as that of the wafer W.  
      During processing, the inner edge of the hole  24   b  is located immediately adjacent to the outer edge of the wafer W. In addition, the slits  24   a  of the baffle  24  are located below the bottom surface of the wafer W (i.e. in vent side). Therefore, the treatment surface of the wafer W is exposed to the plasma produced between the susceptor  8  and the upper electrode  15   a  through the hole  24   b  of the baffle  24 . At this point, the space where plasma is produced is determined by the upper part of the chamber  2  and the plate electrode  16  for the upper boundary, and by the wafer W and the baffle  24  for the lower boundary. Then, the plasma concentration is kept constant.  
      The baffle  24  also has a function to return a part of the RF power applied to the lower electrode  15   b , to the first and the second RF power generators  13  and  24 , respectively. Specifically, the return current, which originates in the RF power applied to the lower electrode  15   b  by the first and the second RF power generators  13  and  22 , returns to the respective RF power generator via the baffle  24  and the grounded sidewall of the chamber  2 .  
      The behavior of the plasma process apparatus in the above structure will be described below using  FIG. 1 , in the case of being used for forming SiOF film on the wafer W.  
      At first, the susceptor holder  6  is moved to the position where the wafer W can be carried in, by the elevator that is not shown. After the gate valve is opened, a carrier arm that is not shown carries the wafer W in the chamber  2 . The wafer W is put on the lift pin  12  that is protruding from the susceptor  8 . Then the lift pin  12  retracts, and the wafer W is put on the susceptor  8 , being clamped in place by an electrostatic force of the high-temperature electrostatic chuck ESC. After the gate valve  5  is closed, the exhaust system  4  evacuates air from the chamber  2  until a certain degree of vacuum is achieved. Then, the elevator that is not shown lifts up the susceptor holder  6 .  
      In this condition, the temperature of the susceptor  8  is kept at a certain level, for example 50° C., by circulating coolant through the lower cooling channels  11 , and/or supplying electric power to the heater H 1  from the heater power generator H 2 . On the other hand, the exhaust system  4  further evacuates air from the chamber  2  via the vent  3 , and it brings the chamber into high vacuum state, for example 0.01 Pa.  
      Then, process gases such as SiF4, SiH4, O2, NF3, NH3 and a dilution gas of Ar are distributed into the chamber  2  from the process gas source  21 , with their flow controlled at a certain flow rate. The process gases and the carrier gas that are distributed to the upper electrode  15   a  flow out of the gas outlets  16   a  of the plate electrode  16 , and uniformly spread over the wafer W.  
      After that, RF power with frequency of, e.g., 50˜150 MHz is applied to the lower electrode  15   b  by the second RF power generator  22 . By this procedure, RF electric field is generated between the upper electrode  15   a  and the lower electrode  15   b , and the process gases provided via the upper electrode  15   a  are ionized and plasma is created. On the other hand, RF power with frequency of, e.g., 1˜4 MHz is applied to the lower electrode  15   b  by the first RF power generator  13 . As a result, ions in the plasma are pulled toward the susceptor  8 , and the concentration of the plasma adjacent to the surface of the wafer W increases. As described above, plasma of the process gases are created by the generation of RF electric field between the upper electrode  15   a  and the lower electrode  15   b . Subsequently, SiOF film is formed on the surface of the wafer W, by chemical reactions occurred on the wafer surface due to plasma.  
      As described above, in the plasma process apparatus of the first embodiment of the present invention, both of the RF power generated by the first and the second RF power generators are applied to the lower electrode  15   b , while the upper electrode  15   a  is grounded. Therefore, plasma is produced mainly near the lower electrode, and reduction of the plasma concentration until it reaches the wafer W can be prevented. As a result, deterioration of the film-forming process efficiency can be prevented.  
      Besides, since the first electrode  15   a  is grounded and any RF power generators or filters are not installed around the first electrode, the structure of the plasma process apparatus becomes simple. Therefore, it is easy to have a structure in which pipes for process gases and coolant penetrates through the first electrode  15   a.    
      By the way, the structure of the plasma process apparatus  1  is not limited to the one described above.  
      For example, the baffle  24  may have a structure in which an insulator such as ceramics is installed between the outer side of the baffle and the inner wall of the chamber  2 . In this case, by limiting electrical contact between the baffle and the inner wall of the chamber  2 , further reduction of RF power loss can be achieved.  
      In addition, the material of the baffle  24  is not limited to the aluminum processed with anodic oxide coating (Alumite). Other materials such as alumina and yttria may be used, provided that they are conductors and have high plasma resistance. By meeting these conditions, baffle  24  acquires high plasma resistance and the plasma process apparatus  1  as a whole achieves high maintainability.  
      In the above embodiment of the present invention, the plasma process apparatus of parallel-plate type for forming SiOF film on semiconductor wafers is described. However, workpieces are not limited to semiconductor wafers, and this equipment can be used to make other devices such as liquid crystal display. Besides, films to be formed may be other materials such as SiO2, SiN, SiC, SiCOH, and CF.  
      The plasma processing applied to workpieces is not limited to the film forming. Other processes such as etching can be carried out by the present invention. Furthermore, suitable plasma process apparatus is not limited to that of parallel-plate type. Other plasma process apparatus such as magnetron type thereof is also applicable, provided that it has electrodes inside the chamber.  
      As shown in  FIG. 4 , the inductor L of the low-pass filter may form a parallel resonant circuit with the wiring capacitance (or other parasitic capacitances) Cp created by the coils of the inductor L. In this case, the resonance frequency of the parallel resonant circuit must be nearly equal to that of the RF electric power generated by the second RF power generator  22 .  
      By applying the structure of the low-pass filter  14  shown in  FIG. 4 , power loss can be prevented by efficiently limiting the leakage of the RF power generated by the second RF power generator  22 , keeping the volume of the inductor L small.  
     Second Embodiment  
      The second embodiment of the present invention will be described below using  FIG. 5 . The symbols in  FIG. 5  are the same as those of  FIG. 1  for the same components.  
      As shown in  FIG. 5 , the structure of the plasma process apparatus  1  is practically the same as that of the first embodiment of the present invention, except those points described below. The structure of the low-pass filter  14  can be the same as, e.g., that shown in  FIG. 4 .  
      In the plasma process apparatus  1  shown in  FIG. 5 , the jacket  11 J and the lower electrode  15   b  that is embedded in the high-temperature electrostatic chuck ESC are capacitively coupled. In other words, the jacket  11 J and the lower electrode  15   b  constitute the electrodes of a capacitor.  
      The second RF power generator  22  is connected to the lower cooling channels  11  through the high-pass filter  23 . The RF power generated by the second RF power generator  22  is applied to the lower electrode  15   b  via the capacitor composed of the jacket  11 J and the lower electrode  15   b.    
      In the plasma process apparatus of the second embodiment of the present invention shown in  FIG. 5 , the RF power generated by the second RF power generator  22  is distributed to the lower electrode  15   b  without using wire made of high melting point metal, which generally has high resistivity. Therefore, loss of the RF power can be reduced, and plasma processing with further high efficiency in use of RF power can be achieved.  
     Third Embodiment  
      The third embodiment of the present invention will be described below using  FIG. 6 .  FIG. 6  shows a cross section of a part of the plasma process apparatus for the third embodiment of the present invention. The symbols in  FIG. 6  are the same as those of  FIG. 1  for the same components.  
      The structure of the plasma process apparatus  1  in  FIG. 6  is practically the same as that of  FIG. 1 , except those points described below. As shown in  FIG. 6 , in this plasma process apparatus  1 , the upper electrode  15   a  is not grounded. Alternatively, it is connected to the second RF power generator  22  via the matching circuit  25 , which is surface-mounted on the upper side (opposite to the inside of the chamber  2 ) of the electrode  15   a . In addition, there is a gap between the upper electrode  15   a  and the chamber  2  to store the matching circuit  25 . The matching circuit  25  consists of variable capacitors VC 1  and VC 2 , and an inductor L, as shown in  FIG. 6 .  
      Each of the variable capacitors VC 1  and VC 2  consists of a rotor and a stator. The stator of the variable capacitor VC 1  is mounted on the inner wall of the insulator  17 . The rotor of the variable capacitor VC  1  is connected to that of the variable capacitor VC  2 , via the inductor L. The stator of the variable capacitor VC  2  is surface-mounted on the center part of the upper electrode  15   a , without using lead wire. The first RF power generator  13  is connected to the joint of the variable capacitor VC 1  and the inductor L.  
      The variable capacitor VC 2  is not necessarily mounted on the center part of the upper electrode  15   a . However, it is desirable to mount the variable capacitor VC 2  on the center part of the upper electrode  15   a , in order to make the RF power that is generated by the second RF power generator  22  uniformly applied on the first electrode  15   a.    
      The rotor of the variable capacitor VC 1  has a shaft S 1 , which corresponds to the axis of the rotor. The shaft S 1  is connected to a motor M 1 , which is used to rotate the shaft S 1 . The capacitance of the variable capacitor VC 1  can be varied, by operating a control circuit (not shown) to drive the motor M 1  to rotate the shaft S 1 .  
      Similarly, the rotor of the variable capacitor VC 2  has a shaft S 2 , to which a motor M 2  is connected. The capacitance of the variable capacitor VC 2  can be varied, by operating a control circuit (not shown) to drive the motor M 2  to rotate the shaft S 2 .  
      In addition, the upper cooling channels  18  include an upper coolant outlet-pile  18   a  and an upper coolant drainpipe  18   b . As shown in  FIG. 6 , both of the upper coolant outlet-pipe  18   a  and the upper coolant drainpipe  18   b  are installed in the gap described above, connecting the inside of the upper electrode  15   a  and the outside of the chamber  2 . The gas outlet  20  is also installed in the gap, connecting the inside of the upper electrode  15   a  and the process gas source  21 .  
      When forming SiOF films using the plasma process apparatus with the structure shown in  FIG. 6 , the operator manipulates the above mentioned control circuits to drive the motors M 1  and M 2 . Then, by adjusting the capacitances of the variable capacitors VC  1  and VC 2 , the operator carries out impedance matching.  
      Then, the process gases and the carrier gas are supplied into the upper electrode  15   a , and they flow out of the gas outlets  16   a  of the plate electrode  16  towards the wafer W. With the gases flowing, the RF power with frequencies of, e.g., 50˜150 MHz distributed from the second RF power generator  22  is applied to the upper electrode  15   a . By this procedure, RF electric field is created between the upper electrode  15   a  and the lower electrode  15   b , and the process gases supplied from the upper electrode  15   a  is ionized, producing plasma. On the other hand, the RF electric power with frequencies of, e.g., 1˜4 MHz is applied to the lower electrode  15   b  from the first RF power generator  13 . By this procedure, active species in the plasma is pulled near the susceptor  8 , increasing the plasma concentration adjacent to the surface of the wafer W. As described above, plasma of the process gases are created by the generation of RF electric field between the upper electrode  15   a  and the lower electrode  15   b . Subsequently, SiOF film is formed on the surface of the wafer W, by chemical reactions occurred on the wafer surface due to plasma.  
      With regard to the plasma process apparatus  1  shown in  FIG. 6 , loss of the RF power generated by the second RF power generator  22  can be reduced and the plasma process becomes more efficient, because the matching circuit  25  is surface-mounted on the upper electrode  15   a . Besides, since the matching circuit  25  is surface-mounted, extra equipment such as boxes to store the matching circuit  25  is not needed. Thus, the structure of the plasma process apparatus becomes simple, and it is easy to install pipes for process gases and coolant penetrating through the electrode.  
      The present invention provides plasma process apparatus that has high efficiency in plasma processing and that has simple structure. This application is based on Japanese Patent Application No. 2001-380168 filed on Dec. 13, 2001 and including specification, claims, drawings and summary. The disclosure of the above mentioned Japanese Patent Application is incorporated herein by reference in its entirety.  
     INDUSTRIAL APPLICABILITY  
      The present invention relates to plasma process apparatus to conduct plasma processes such as film forming and etching, which is applied to workpieces such as semiconductor wafers.