Patent Publication Number: US-2005115677-A1

Title: Plasma etching apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This is a Continuation-in-Part of Application No. PCT/JP99/06619, filed Nov. 26, 1999. 
    
    
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 10-336886, filed Nov. 27, 1998; and No. 11-308765, filed Oct. 29, 1999, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      The present invention relates to a plasma etching apparatus and method for applying a plasma processing to a substrate such as a semiconductor wafer.  
      In the manufacturing process of a semiconductor device, widely employed is a plasma etching treatment in which a plasma etching is applied to a target object or a semiconductor wafer (including a semiconductor wafer itself and a semiconductor wafer having a single film or a plurality of films formed thereon). Various types of plasma etching apparatuses are employed for the plasma etching treatment. Particularly, a capacitance coupling type parallel plate plasma processing apparatus is mainly used as the plasma etching apparatus.  
      In the capacitance coupling type parallel plate plasma etching apparatus, a pair of parallel plate electrodes (upper and lower electrodes) are arranged within a chamber a predetermined distance apart from each other to face each other. An etching gas is introduced into the chamber and, at the same time, a high frequency power is applied to one of the electrodes to form a high frequency electric field between the two electrodes. A plasma of the etching gas is formed by the high frequency electric field so as to apply a plasma etching to the semiconductor wafer.  
      Where a film formed on the semiconductor wafer, e.g., an oxide film, is etched by using the capacitance coupling type parallel plate plasma etching apparatus of the construction described above, a plasma of an intermediate density is formed by setting up an intermediate pressure within the chamber so as to make it possible to perform the optimum radical control. As a result, a suitable plasma state can be obtained so as to achieve an etching with a high selectivity ratio, with a high stability and with a high reproducibility.  
      To be more specific, it is described in “1997 DRY PROCESS SYMPOSIUM P385- 390 ”, that a high frequency of 27.12 MHz for plasma formation is applied to the upper electrode so as to form a plasma, and a high frequency of 800 kHz is applied to the lower electrode so as to draw the ions generated by the plasma onto the lower electrode, making it possible to perform a satisfactory etching under the pressure of 20 to 100 mTorr.  
      However, miniaturization of the design rule in USLI further proceeds in recent years, leading to demands for a higher aspect ratio in the shape of the hole. Under the conventional conditions, it is difficult to meet the requirement sufficiently.  
      The conventional plasma etching, in which a semiconductor wafer is disposed on the lower electrode, will now be described with reference to  FIG. 8 . A reference numeral  111  shown in the figure represents a resist layer acting as a mask for the selective etching. Reference numerals  112 ,  114  and  117  represent insulating films (SiO 2  films), respectively. Further, reference numerals  113  and  115  represent an aluminum wiring and a gate wiring, respectively.  
      As shown in the figure, that portion of the resist layer  111  which is in the vicinity of a plasma sheath S is charged negative in the etching step, with the result that the electrons supplied from a plasma P are mainly moved in the lateral direction, resulting in failure to enter a contact hole  101  having a large aspect ratio. On the other hand, cations are accelerated by the plasma sheath so as to reach the bottom of the contact hole. As a result, the bottom portion (exposed portion of the aluminum wiring)  103  of the contact hole  101  is charged positive. On the other hand, both electrons and cations are accelerated to arrive at the space portion (exposed portion of the semiconductor wafer)  105  in which the contact hole is not formed without difficulty. As a result, a strong electric field is applied to the thin insulating film  117  positioned below the gate electrode  116  so as to bring about an insulation breakdown called shading damage.  
     DISCLOSURE OF INVENTION  
      An object of the present invention is to provide a plasma etching apparatus and method, which permit obtaining a suitable plasma state, which is capable of coping with the miniaturization of the etching pattern, and which does not bring about a shading damage.  
      As a result of an extensive research on a plasma etching capable of coping with the required miniaturization, the present inventors have found that a plasma can be formed with a lower pressure condition by increasing the frequency of the high frequency power applied to the upper electrode and the lower electrode, e.g., by applying a high frequency power of 60 MHz to the upper electrode and a high frequency power of 2 MHz to the lower electrode, making it possible to form a plasma of a higher density while maintaining a radical dissociation controllability substantially equal to that of the conventional capacitance coupling type parallel plate plasma processing apparatus and, to cope with the further miniaturization.  
      However, it has also been found that, if the frequency of the high frequency power applied to the upper electrode is increased, a new problem is generated that the etching uniformity is rendered poor. To be more specific, as shown by a broken line in  FIG. 1A , the plasma sheath on the side of an upper electrode is rendered very thin in the central portion of the electrode, and the plasma sheath of a lower electrode is rendered thick in the central portion of the electrode. It follows that the plasma is rendered nonuniform in a direction parallel to the electrodes.  
      The difficulty is caused by the phenomenon that higher harmonic waves, which are absolutely generated when a high frequency power is applied, form a standing wave within the plane of the upper electrode. Since the standing wave has a large amplitude in the center of the upper electrode, the standing wave contributes to the plasma generated in the vicinity of the upper electrode, with the result that the sheath in the central portion of the upper electrode is rendered thinner than in the edge portions (peripheral portion). However, where the frequency of the power applied to the upper electrode is relatively low as in the conventional apparatus, e.g., where the frequency is lower than 50 MHz, the plasma density is not high and, thus, the plasma sheath is thick. It follows that the uniformity of the plasma is not greatly affected by the standing wave. Also, where the frequency is lower than 50 MHz, the wavelength of the higher harmonic wave is large compared with the diameter of the upper electrode, with the result that the influence given by the standing wave is diminished.  
      If the frequency of the high frequency power applied to the upper electrode is increased so as to increase the plasma density, the plasma sheath is rendered thin as a whole. Therefore, if the plasma sheath in the central portion of the upper electrode is affected by the standing wave, the plasma sheath is made markedly thin in the central portion of the electrode in the extreme case, as shown in the figure, leading to a poor uniformity of plasma.  
      On the other hand, it should be noted in conjunction with the plasma sheath of the lower electrode that, since the plasma sheath is thin in the central portion of the upper electrode, the capacitance in that portion is increased. As a result, an electric current flows in a concentrated fashion into the filter of 2 MHz so as to increase the thickness of the plasma sheath. It follows that the ions in the plasma in the central portion are further accelerated so as to make the etching rate in the central portion of the wafer higher than that in the peripheral portion of the wafer. As a result, the uniformity of the etching is lowered.  
      As a result of the continued research conducted in an attempt to overcome the new defect described above, the present inventors have found that it is possible to increase the thickness of the plasma sheath by applying a specified high frequency power in addition to the high frequency power described above to the upper and lower electrodes so as to diminish the fluctuation in the thickness of the plasma sheath. It has also been found that, in order to increase the thickness of the plasma sheath so as to diminish the fluctuation in the thickness of the plasma sheath, it is necessary to apply a high frequency power of a specified frequency to each of the upper and lower electrodes, the frequency of the high frequency power applied to the both electrodes being the same, in substantially the reverse phase or in the phase in the vicinity of the reverse phase.  
      It has also been found that, if a high frequency power of a specified frequency is applied to each of the upper and lower electrodes, the frequency of the high frequency power applied to the both electrodes being the same, in substantially the reverse phase or in the phase in the vicinity of the reverse phase, the shading damage is unlikely to take place.  
      According to a first aspect of the present invention, which has been achieved on the basis of the finding described above, there is provided a plasma etching apparatus, comprising: 
          a process chamber in which a target object to be processed is housed;     exhaust means for maintaining a predetermined reduced pressure within the process chamber;     gas introducing means for introducing an etching gas into the process chamber;     first and second electrodes arranged a predetermined distance apart from each other and to face each other within the process chamber, the target object being supported by the second electrode to face the first electrode;     first high frequency apply means for applying a high frequency power having a frequency of 50 MHz or higher to the first electrode so as to generate a plasma of the etching gas between the first electrode and the second electrode; and     second high frequency apply means for applying a high frequency power to the second electrode;     the second high frequency apply means also applying a high frequency power to the first electrode so as to increase the thickness of the plasma sheath formed on the first electrode.        

      According to a second aspect of the present invention, there is provided a plasma etching apparatus, comprising: 
          a process chamber in which a target object to be etched is housed;     first and second electrodes arranged a predetermined distance apart from each other and in a manner to face each other within the process chamber;     first high frequency apply means for applying a first high frequency power having a frequency of 50 MHz or higher to the first electrode;     second high frequency apply means for applying a second and third high frequency powers to the first electrode and the second electrode respectively, the second and third high frequency powers having a frequency not lower than 2 MHz and not higher than that of the first high frequency power, the frequency of the second high frequency power being equal to that of the third high frequency power, and the second high frequency power having a reverse phase relative to the third high frequency power;     exhaust means for maintaining a predetermined reduced pressure within the process chamber; and     gas introducing means for introducing an etching gas into the process chamber;     wherein a plasma of the etching gas is formed by allowing the high frequency powers to form a high frequency electric field between the first and second electrodes with the target object supported on the second electrode, thereby performing an etching treatment of the target object by the plasma, and     the thickness of the plasma sheath formed on the first electrode is increased by the second high frequency power applied from the second high frequency apply means to the first electrode.        

      According to a third aspect of the present invention, there is provided a plasma etching apparatus, comprising: 
          a process chamber in which a target object to be processed is housed;     first and second electrodes arranged a predetermined distance apart from each other in a manner to face each other within the process chamber;     first high frequency apply means for applying a first high frequency power having a frequency not lower than 50 MHz to the first electrode;     second high frequency apply means for applying second and third high frequency powers to the first electrode and the second electrode respectively, the second and third high frequency powers having a frequency not lower than 2 MHz and not higher than that of the first high frequency power, the frequency of the second high frequency power being equal to that of the third high frequency power, and the second high frequency power having a phase difference of 180±45° relative to the third high frequency power;     exhaust means for maintaining a predetermined reduced pressure within the process chamber; and     gas introducing means for introducing an etching gas into the process chamber;     wherein a plasma of the etching gas is formed by allowing the high frequency powers to form a high frequency electric field between the first and second electrodes with the target object supported on the second electrode, thereby performing an etching treatment of the target object by the plasma, and     the thickness of the plasma sheath formed on the first electrode is increased by the second high frequency power applied from the second high frequency apply means to the first electrode.        

      The plasma etching apparatus of the present invention comprises first high frequency apply means for applying a high frequency power having a frequency not lower than 50 MHz to the first electrode and second high frequency apply means for applying a high frequency power not only to the second electrode but also to the first electrode for increasing the thickness of the plasma sheath formed on the first electrode, with the result that, even if the plasma sheath is affected by the standing wave, the ratio in the change of the thickness of the plasma sheath is made relatively small so as to make the plasma more uniform. To be more specific, the second high frequency apply means applies second and third high frequency powers having a frequency not lower than 2 MHz and lower than the frequency of the first high frequency power to the first electrode and the second electrode, the frequency of the second high frequency power being equal to that of the third high frequency power, and the second high frequency power having a reverse phase relative to the third high frequency power. Thus, the second high frequency power having a relatively low frequency is applied from the second high frequency apply means to the first electrode in addition to the first high frequency power applied from the first high frequency apply means to the first electrode. It follows that, as shown in  FIG. 1B , a plasma sheath corresponding to the low frequency, e.g., 2 MHz, of the second high frequency apply means is superposed on the plasma sheath of the frequency, e.g., 60 MHz, of the first high frequency apply means so as to form a thick plasma sheath on the first electrode. Also, the plasma sheath corresponding to 2 MHz is thicker than the plasma sheath corresponding to 60 MHz. It follows that, even if the plasma sheath is affected by the standing wave, the ratio in the change of the thickness of the plasma sheath is small so as to diminish the degree of reduction in the uniformity of the plasma. Also, since the thickness of the plasma sheath of the first electrode is increased, the change in the capacitance caused by the standing wave is small so as to make uniform the current flowing into the second electrode. As a result, the plasma sheath of the second electrode is made substantially uniform, as shown in the figure. It follows that, in the present invention, it is possible to cope with the miniaturization by a high density plasma and to ensure uniformity of the etching by a uniform plasma. Incidentally,  FIG. 1B  shows the state that high frequency powers of reverse phases having a frequency of 2 MHz are applied to the upper electrode (first electrode) and the lower electrode (second electrode) by using a power splitter used in the second high frequency apply means.  
      It should also be noted that, since the second and third high frequency powers applied from the second high frequency apply means to the first and second electrodes are substantially in reverse phases, the intensity of the electric field in the sheath portion is kept at a value higher than a predetermined value so as to make it possible to increase the space potential of plasma. In addition, the ionization rate is increased so as to increase the plasma density. As a result, the ions and electrons are allowed to have a high energy. Also, the ionization rate is increased in the vicinity of the electrode so as to increase the high speed electron flux. It follows that the electrons in the plasma are allowed to reach easily the bottom portion of the contact hole so as to neutralize the positive charge within the hole and, thus, to suppress the shading damage.  
      In the present invention, the prominent effects described above can be produced by applying second and third high frequency powers substantially forming reverse phases to the first and second electrodes. However, these high frequency powers need not form reverse phases for producing these effects as far as the phase difference is close to the reverse phase. To be more specific, desired effects can be obtained if the phase difference between the second and third high frequency powers falls within a range of 180±45°.  
      In the present invention, the first high frequency apply means applies a high frequency power having a frequency not lower than 50 MHz. If the frequency is lower than 50 MHz, it is difficult to obtain a desired high density plasma, resulting in failure to cope with the required miniaturization. Also, if the frequency is lower than 50 MHz, the problem itself to be solved by the present invention does not take place.  
      In the second aspect of the present invention, it is possible for the second high frequency apply means to comprise a high frequency power source having a frequency not lower than 2 MHz and lower than that of the first high frequency apply means and a transformer for distributing the power of the high frequency power source to the first and second electrodes.  
      In the second and third aspects of the present invention, the frequency of the second and third high frequency powers is not lower than 2 MHz. If the frequency is not lower than 2 MHz, the ions are unlikely to follow the high frequency power application, with the result that it is possible to suppress the damage done to the target object when the ions are drawn to the target object.  
      In each of the second aspect and the third aspect of the present invention, it is possible for the second high frequency apply means to comprise a high frequency oscillator having a frequency not lower than 2 MHz and lower than the frequency of the first high frequency apply means, amplifying means for amplifying the high frequency so as to apply a predetermined high frequency power to each of the first electrode and the second electrode, and a phase shift means for shifting the phase of the high frequency applied to the first electrode or the second electrode.  
      Also, in each of the second aspect and the third aspect of the present invention, it is desirable for the frequency of the second and third high frequency powers to fall within a range of between 2 MHz and 27 MHz. For example, it is desirable for the frequency of the first high frequency power to be about 60 MHz and for the frequency of the second and third high frequency powers to be about 2 MHz. Further, it is desirable for the ratio of the power supplied from the second high frequency apply means to the first electrode to the power supplied from the second high frequency apply means to the second electrode to fall within a range of between 6:4 and 4:6.  
      According to a fourth aspect of the present invention, there is provided a plasma etching apparatus, comprising: 
          a process chamber in which a target object to be etched is housed;     exhaust means for maintaining a desired reduced pressure state within the process chamber;     a gas introducing means for introducing an etching gas into the process chamber;     first and second electrodes arranged a predetermined distance apart from each other to face each other within the process chamber, the target object being supported on the second electrode to face the first electrode;     first high frequency apply means for applying a first high frequency power to the first electrode so as to cause the etching gas to form a plasma between the first electrode and the second electrode;     second high frequency apply means for applying a second high frequency power having a frequency lower than that of the first high frequency power to the second electrode;     third high frequency apply means for superposing a third high frequency power having a frequency lower than that of the first high frequency power and higher than that of the second high frequency power on the first high frequency power; and     phase control means for adjusting the phase difference between the second high frequency power and the third high frequency power.        

      According to a fifth aspect of the present invention, there is provided a plasma etching method, comprising the steps of: 
          disposing a target object to be processed on a second electrode in a manner to face a first electrode with a predetermined space provided therebetween, the first and second electrodes being arranged within a process chamber having a reduced pressure;     introducing an etching gas into the process chamber;     applying a high frequency power having a frequency not lower than 50 MHz from a first high frequency apply means to the first electrode to cause the etching gas to generate a plasma between the first electrode and the second electrode; and     applying a high frequency power from a second high frequency apply means to the second electrode;     the second high frequency apply means also applying a high frequency power to the first electrode so as to increase the thickness of the plasma sheath formed on the first electrode.        

      It may be apparent that the method according to the fifth aspect of the present invention produces effects similar to those produced by the invention according to the first aspect of the present invention.  
      Further, according to a sixth aspect of the present invention, there is provided a plasma etching method, comprising the steps of: 
          disposing a target object to be processed on a second electrode in a manner to face a first electrode with a predetermined space provided therebetween, the first and second electrodes being arranged within a process chamber having a reduced pressure;     introducing an etching gas into the process chamber;     applying a first high frequency power to the first electrode to cause the etching gas to generate a plasma between the first electrode and the second electrode;     applying a high frequency power having a frequency lower than that of the first high frequency power to the second electrode;     adjusting the phase of the frequency of at least one of the first high frequency power and the second high frequency power so as to provide a predetermined phase difference between the first and second high frequency powers; and     applying a third high frequency power to the second electrode such that the third high frequency power is superposed on the first high frequency power.        

      Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
       FIG. 1A  is a view for explaining the principle of the prior art;  
       FIG. 1B  is a view for explaining the principle of the present invention;  
       FIG. 2  is a cross sectional view schematically showing an etching apparatus according to a first embodiment of the present invention;  
       FIG. 3  is a view showing the result of simulation in respect of the potential distribution of plasma;  
       FIG. 4  is a graph showing the space potential of plasma;  
       FIGS. 5A and 5B  are graphs showing the ionization rates of the plasma in the vicinity of the electrode and the bulk;  
       FIG. 6  is a view showing another example of a second high frequency power apply mechanism;  
       FIG. 7  is a view for schematically showing an etching apparatus according to a second embodiment of the present invention; and  
       FIG. 8  is a view showing the concept of a shading damage in the conventional apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A first embodiment of the present invention will now be described with reference to the accompanying drawings.  
      Specifically,  FIG. 2  is a cross sectional view schematically showing an etching apparatus  1  according to the first embodiment of the present invention. The etching apparatus  1  is constructed as a capacitance coupling type parallel plate etching apparatus in which upper and lower electrode are arranged in parallel and a power source for plasma formation is connected to one of these upper and lower electrodes.  
      To be more specific, the etching apparatus  1  comprises a cylindrical chamber  2  made of aluminum and having, for example, an anodized aluminum treatment (anodic oxidation treatment) applied to the inner surface. The chamber  2  is connected to the ground for safety. A substantially columnar susceptor table  4  for supporting a target object to be processed, e.g., a semiconductor wafer (hereinafter referred to as a wafer) W, is arranged in the bottom portion of the chamber  2  with an insulating plate  3  made of, for example, a ceramic material, interposed therebetween. A susceptor  5  constituting a lower electrode (plate like electrode) is arranged on the susceptor table  4 . The susceptor  5  is connected to the ground through a high pass filter (HPF)  6 .  
      A cooling chamber  7  is formed within the susceptor table  4 . A coolant such as a liquid nitrogen is introduced into the cooling chamber  7  through a coolant introducing pipe  8  and discharged from within the cooling chamber  7  through a coolant discharge pipe  9 , so as to be circulated within the cooling chamber  7 . During circulation of the coolant within the coolant chamber  7 , the wafer W disposed on the susceptor  5  is cooled by the coolant via the susceptor table  4  and the susceptor  5  arranged on the susceptor table  4  so as to control the process surface of the wafer W at a desired temperature.  
      The susceptor  5  is in the shape of a disc, and an electrostatic chuck  11  substantially equal in shape to the wafer W is formed on the disc-like susceptor  5 . The electrostatic chuck  11  is constructed such that an electrode  12  is interposed between insulating materials. A DC voltage of, for example, 1.5 kV is applied from a DC power source  13  to the electrode  12 , with the result that the wafer W is electrostatically sucked on the electrostatic chuck by the Coulomb force or the Johnson-Rahbeck force.  
      A gas passageway  14  for supplying a heat transfer medium, e.g., a He gas, to the back surface of the wafer W is formed to extend through the insulating plate  3 , the susceptor table  4 , the susceptor  5  and the electrostatic chuck  11 . The low temperature of the susceptor  5  is effectively transferred by the heat transfer medium to the wafer W so as to maintain the wafer W at a predetermined temperature.  
      An annular focus ring  15  is arranged on the upper peripheral portion of the susceptor  5  in a manner to surround the wafer W. The annular focus ring  15 , which is made of a conductive material such as silicon, serves to improve the uniformity of the etching.  
      An upper electrode (plate like electrode)  21  is arranged above the lower electrode  5  in parallel to the susceptor  5  and in a manner to face the lower electrode  5  with a predetermined clearance provided therebetween. The upper electrode  21  is supported in an upper portion of the chamber  2  with an insulating material  22  interposed therebetween so as to constitute a surface facing the susceptor  5 , and includes an electrode plate  24  having a large number of spurting holes  23  and an electrode support member  25  supporting the electrode plate  24 . The electrode plate  24  is formed of, for example, silicon or an amorphous carbon. On the other hand, the electrode support member  25  is made of a conductive material such as an aluminum plate having the surface subjected to an anodized aluminum treatment. Incidentally, the lower electrode  5  and the upper electrode  21  are positioned about 10 to 60 mm apart from each other.  
      A gas introducing port  26  is formed in the center of the electrode support member  25  of the upper electrode  21 . A gas supply pipe  27  is connected to the gas introducing port  26 , and an etching gas supply source  30  is connected to the gas supply pipe  27  via a valve  28  and a mass flow controller  29 . A process gas or etching gas for the etching treatment is supplied from the etching gas supply source  30 . The process gas supplied from the etching gas supply source  30  includes, for example, a CF 4  gas.  
      A discharge pipe  31  is connected to the bottom portion of the chamber  2 . An exhaust apparatus  35  is connected to the exhaust pipe  31 . The exhaust apparatus  35  is equipped with a vacuum pump such as a turbo molecular pump so as to evacuate the chamber  2  to a reduced pressure of, for example about 1 mTorr (0.133 Pa). Also, a gate valve  32  is mounted to the side wall of the chamber  2 . The wafer W is transferred between the chamber  2  and an adjacent load lock chamber (not shown) with the gate valve  32  kept open.  
      The mechanism of forming a plasma will now be described.  
      The apparatus of the first embodiment shown in  FIG. 2  comprises a first high frequency apply mechanism  100  for applying a high frequency power having a relatively high frequency to the upper electrode  21  and a second high frequency apply mechanism  200  for applying high frequency powers having relatively low frequencies of substantially reverse phases to the susceptor  5  acting as the lower electrode and to the upper electrode  21 . The second high frequency apply mechanism  200  is constructed to apply high frequency powers of the same frequency to the susceptor  5  and to the upper electrode  21 .  
      The first high frequency apply mechanism  100  comprises a first high frequency power source  50 , which is connected to the upper electrode  21  via a matching device  51  and a high pass filter (HPF)  52 . The first high frequency power source  50  has a frequency not lower than 50 MHz, preferably between 60 MHz and 200 MHz. A high density plasma is formed within the chamber  2  under a low pressure condition by applying a high frequency power having such a high frequency to the upper electrode  21 . It is more desirable for the first high frequency power source  50  to have a frequency of 60 MHz and an output power of 2,000 W.  
      The second high frequency power source  200  comprises a second high frequency power source  40 , which is connected to the primary side of a transformer  42  constituting a power splitter via a matching device  41 . A ground terminal  43  is connected to the secondary side of the transformer  42 . The secondary side of the transformer  42  is connected to the lower electrode  5  via a low pass filter (LPF)  45  and is also connected to the upper electrode  21  via a low pass filter (LPF)  44 . It follows that the power of the high frequency power source  40  can be distributed with an optional ratio by selecting the opposition of the ground terminal  43 . For example, where the output power of the high frequency power source  40  is 1,000 W, it is possible to distribute 600 W to the lower electrode  5  and 400 W to the upper electrode  21 . Also, high frequency powers substantially forming reverse phases, i.e., differing from each other in phase by 180°, are applied to the lower and upper electrodes  5 ,  21 . The second high frequency power source  40  has a frequency lower than that of the first high frequency power source  50 . Specifically, the second high frequency power source  40  has a frequency of, preferably 800 kHz to 28 MHz, more preferably 2 MHz to 13.56 MHz. If the second high frequency power source  40  has a frequency exemplified above, it is possible to suppress the damage done to the wafer W by the ions drawn to the lower electrode  5 . It is substantially desirable for the second high frequency power source  40  to have a frequency of 2 MHz. As described herein later, it has been experimentally confirmed that, in order to effectively prevent the shading damage, it is desirable for the distribution ratio between the upper electrode  21  and the lower electrode  5  to fall within a range of between 4:6 and 6:4.  
      Where a silicon oxide film (SiO 2  film) formed on a silicon wafer (target object) W is etched by the etching apparatus  1  of the construction described above, the gate valve  32  is opened first and, then, the wafer W is introduced from a load lock chamber (not shown) into the chamber  2  so as to be disposed on the electrostatic chuck  11 . Then, a DC voltage is applied from the high DC power source  13  so as to permit the wafer W to be electrostatically sucked on the electrostatic chuck  11 . Then, the gate valve  32  is closed, and the pressure within the chamber  2  is reduced by the exhaust mechanism  35  to a predetermined degree of vacuum.  
      In the next step, the valve  28  is opened so as to permit, for example, a CF 4  gas to be supplied from the etching gas supply source  30  to the hollow portion of the upper electrode  21  through the process gas supply pipe  27  and the gas introducing port  26  while the flow rate of the CF 4  gas is being controlled by the mass flow controller  29 . Further, the CF 4  passes through the spurting ports  23  of the electrode plate  24  so as to be discharged uniformly toward the wafer W, as denoted by arrows in  FIG. 2 .  
      In the state that the pressure within the chamber  2  is maintained at, for example, 20 mTorr (2.66 Pa), a high frequency of, for example, 60 MHz is applied from the high frequency power source  50  of in the first high frequency apply mechanism  100  to the upper electrode  21 . As a result, a high frequency electric field is formed between the upper electrode  21  and the lower electrode  5  so as to dissociate the process gas to form a plasma. On the other hand, high frequency powers of 2 MHz in substantially reverse phases (differing from each other in phase by 180°) are supplied from the high frequency power source  40  included in the second high frequency apply mechanism  200  to the susceptor  5  and to the upper electrode  21  through the transformer  42 .  
      The component applied from the second high frequency apply mechanism  200  to the upper electrode  21  performs the function of increasing the thickness of the plasma sheath formed on the side of the upper electrode  21  by the high frequency of, for example, 60 MHz supplied from the first high frequency apply mechanism  100  to the upper electrode. As a result, the nonuniformity in the thickness of the plasma sheath formed on the surface of the upper electrode  21  by the higher harmonic wave forming the standing wave is moderated so as to form a uniform plasma.  
      On the other hand, the component applied from the second high frequency apply mechanism  200  to the lower electrode susceptor  5  performs the function of positively drawing mainly the ions in the gaseous molecules converted into a plasma toward the susceptor  5 . By this ion assist, an etching having a higher anisotropy can be applied to the oxide film of the wafer W mounted on the susceptor  5 . In this case, since the frequency is not lower than 2 MHz, a damage is unlikely to be done to the wafer W.  
      As described above, a high density plasma can be formed under a low pressure by applying a high frequency power having a relatively high frequency not lower than 50 MHz, e.g., 60 MHz. In addition, since the etching selectivity and anisotropy can be enhanced by applying a high frequency power having a lower frequency, e.g., 2 MHz, to the lower electrode  5 , it is possible to apply a fine processing to the wafer W. Further, since it is possible to eliminate the nonuniformity of plasma that takes place in the case of applying a high frequency power having a relatively high frequency, e.g., 60 MHz, to the upper electrode  21 , it is possible to ensure the uniformity of the plasma processing.  
      As described previously, high frequency powers of 2 MHz in substantially reverse phases are applied from the second high frequency apply mechanism  200  to both the lower and upper electrodes  5 ,  21  so as to diminish the fluctuation in the sheath thickness. As a result, the intensity of the electric field in the sheath portion bears a value higher than a predetermined value so as to increase the space potential of the plasma. It follows that the ionization rate is increased so as to increase the plasma density and, thus, the ions and the electrons are allowed to have a high energy. It is also possible to increase the ionization rate in the vicinity of the electrode so as to increase the high speed electron flux. It follows that the electrons within the plasma are allowed to reach easily the bottom portion of the contact hole so as to neutralize the positive charge within the hole, thereby suppressing the shading damage.  
      It should also be noted that, since the low pass filter (LPF)  44  is interposed between the secondary side of the transformer  42  and the upper electrode  21  and the low pass filter (LPF)  45  is interposed between the secondary side of the transformer  42  and the lower electrode  5 , it is impossible for the high frequency power of, for example, 60 MHz supplied from the first high frequency source  50  to enter the routes including the low pass filters (LPF)  44  and  45 . Also, since the high pass filter (HPF)  52  is included in the route of supplying the high frequency power of the first high frequency power source  50 , it is impossible for the high frequency power having a low frequency of, for example, 2 MHz supplied from the second high frequency power source  40  to enter the route of supplying the high frequency power of the first high frequency power source  50 . It follows that it is possible to realize a stable process. Incidentally, in view of such a blocking function, it is possible to use another blocking means in place of the low pass filters (LPF)  44 ,  45  and the high pass filter (HPF)  52 .  
      The result of the simulation of the plasma formed by the plasma processing apparatus of the present invention will now be described.  
       FIG. 3  shows graphs showing the potential distribution within the space above the center of the wafer, covering the cases where a high frequency power of 60 MHz was applied to the upper electrode and a high frequency power of 2 MHz was applied to both the upper electrode and the lower electrode with a distribution ratio of 4:6 as in the present invention, and where a high frequency power of 60 MHz was applied to the upper electrode and a high frequency power of 2 MHz was applied to the lower electrode alone as in the comparison example. In each of the graphs shown in  FIG. 3 , the distance in a direction perpendicular to the plane of the electrode is plotted on the abscissa, and the time covering one period of the high frequency power of 2 MHz is plotted on the ordinate. As shown in the graphs, the sheath thickness is stable in the present invention, compared with the comparison example. It is seen that the intensity of the electric field, which is denoted by the gradient of the potential in the sheath portion, has a value higher than a certain value. It should also be noted that the plasma potential for the present invention is higher than that for the comparison example as shown in  FIG. 4 , and the fluctuation in the plasma potential in the present invention is smaller than that in the comparison example. Further, the ionization rate in the present invention is higher than that in the comparison example in each of the bulk (plasma itself) and in the vicinity of the electrode, as shown in  FIGS. 5A and 5B . In other words, the plasma density for the present invention is higher than that for the comparison example. It has also been confirmed by the simulation that the ions and the electrons are allowed to have a higher energy so as to increase the high speed electron flux. Further, the time for electrons within the plasma sheath to be migrated to reach the wafer was examined by means of simulation. It has been found that the electron migrating time for the comparison example was 3.3 nsec in contrast to 3.2 nsec for the present invention. In other words, electrons are migrated in a higher speed in the present invention. It follows that the number of electrons reaching the bottom portion  103  of the contact hole  101  shown in  FIG. 8  is increased in the present invention so as to moderate the positive charge in the bottom portion  103  and, thus, to suppress the shading damage.  
      As described above, it has been confirmed that the thickness of the plasma sheath is unlikely to be fluctuated in the present invention so as to suppress the shading damage.  
      An accelerating experiment of the shading damage was actually conducted. It has been found that the yield was 59% for the comparison example in contrast to 98% for the present invention. Also, it has been confirmed that the shading damage can be prevented particularly effectively in the case where the distribution ratio of the high frequency power to the upper electrode and to the lower electrode falls within a range of between 4:6 and 6:4. It has also been confirmed that, where the distribution ratio of the high frequency power to the upper electrode and to the lower electrode is set at 6:4, the etching selectivity ratio was higher than that in the comparison example and that in the case where the distribution ratio was set at 4:6. It is considered reasonable to understand that, in the case of increasing the power applied to the upper electrode, the radical distribution is increased in the vicinity of the wafer so as to lead to the high etching selectivity ratio noted above.  
      The present invention is not limited to the embodiment described above, making it possible to modify the embodiment described above in various fashions. For example, in the embodiment described above, a so-called power splitter was used in the second high frequency apply mechanism  200 . However, the second high frequency apply mechanism  200  does not include to a power splitter, as far as high frequency powers of reverse phases can be applied to the upper and lower electrodes. For example, it is possible to employ the circuit shown in  FIG. 6 . Specifically, a high frequency oscillator  60  oscillating a high frequency power of, for example, 2 MHz is connected to the upper electrode  21  through an amplifier  61 , a matching device  62  and the low pass filter (LPF)  44  and is also connected to the susceptor or lower electrode  5  through a phase shift circuit  63 , an amplifier  64 , a matching device  65  and the low pass filter (LPF)  45 . The phase of the high frequency power applied to the susceptor  5  is shifted by 180° by the phase shift circuit  63  so as to form a reverse phase. If the frequency is increased, the transformer fails to perform its function. Therefore, the construction shown in  FIG. 6  is particularly effective under a high frequency.  
      In the embodiment described above, high frequency powers having reverse phases are applied to the upper electrode and the lower electrode. However, it is not absolutely necessary for the high frequency powers to have reverse phases, as far as the phases of these high frequency powers are close to the reverse phases. To be more specific, a desired effect can be obtained if the phase shift falls within the range of 180±45°. The particular construction can be achieved by adjusting the shift amount of the phase shift circuit  63  shown in  FIG. 6 .  
      A plasma etching apparatus  140  according to the second embodiment of the present invention will now be described with reference to  FIG. 7 .  
      Specifically, the plasma etching apparatus  140  comprises a conductive and hermetic process vessel  104  connected to the ground. A process chamber  102  is defined within the process vessel  104 . An upper electrode plate (first electrode)  106  and a lower electrode plate (second electrode)  108  are arranged to face each other within the process chamber  102 . These first and second electrodes  106  and  108  are arranged a predetermined distance apart from each other. The lower electrode plate  108  is fixed to the lower wall of the process vessel with an insulator  108   a  interposed therebetween, and performs the function of a susceptor on the upper surface of which is disposed a target object to be processed, e.g., a semiconductor wafer having an SiO 2  film formed thereon. It is possible for the second electrode  108  to be constructed as shown in  FIG. 2 . An inlet port  104   a  connected to an etching gas source  110  is formed in an upper portion of the circumferential wall of the process vessel  104 . On the other hand, an outlet port  104   b  connected to a vacuum pump  109  is formed in a lower portion of the circumferential wall of the process vessel  104 . As a result, during the etching treatment, an etching gas is supplied into the process chamber  102  through the inlet port  104   a  and, at the same time, the process chamber is exhausted by the vacuum pump  109  so as to maintain a predetermined reduced pressure within the process chamber  102 .  
      It is possible for the first electrode  106  and the inlet port  104   a  to be constructed to form a shower head as shown in  FIG. 2 .  
      A first high frequency power source  144  is connected to the upper electrode plate  106  via a low pass filter  146 , a first matching device  148 , and a capacitor  120 . As a result, a first high frequency power of 5 kW having a predetermined frequency, e.g., 50 to 200 MHz, preferably about 60 MHz, is supplied to the upper electrode plate  106 . It should be noted that the capacitor  120  passes the first high frequency power and serves to prevent a third high frequency power, which is to be described herein later, from entering the first matching device  148 .  
      A second high frequency power source  122  is connected to the lower electrode plate  108  via a second matching device  124 . As a result, a second high frequency power of 5 kW having a predetermined frequency, e.g., 800 kHz to 28 MHz, preferably about 2 MHz, is supplied to the lower electrode plate  108 .  
      Further, a third high frequency power source  126  is connected to the wiring between the upper electrode plate  106  and the capacitor  120  via a third matching device  128  and a band pass filter  130 . Therefore, a third high frequency power having a frequency lower than that of the first high frequency power and not lower than that of the second high frequency power is applied to the upper electrode plate  106  in a manner to superpose on the first high frequency power. In this preferred embodiment, the third high frequency power  126  is set at 2 kW with a frequency of about 2 MHz. The band pass filter  130  passes the third high frequency power (2 MHz) and, at the same time, serves to prevent the first high frequency power (60 MHz) from entering the third matching device  128 . It follows that the band pass filter  130  and the capacitor  120  serve to prevent the undesired high frequency power from entering the first and third matching devices  148 ,  128 , thereby preventing malfunctions of the matching devices.  
      The output side of a phase controller  132  is connected to each of the second high frequency power source  122  and the third high frequency power source  126 . The input side of the phase controller  132  is connected to the second matching device  124  and to the third matching device  128 . As a result, the second and third power sources  122 ,  126  are controlled on the basis of the phase of the second high frequency power source and the phase of the third high frequency power source detected by the matching devices  124  and  128 , respectively, so as to permit the phase of the second high frequency power source  122  and the phase of the third high frequency power source  126  to have a predetermined phase difference.  
      The etching treatment using the etching apparatus  140  of the construction described above will now be described.  
      In the first step, the target object (wafer) W is disposed on the lower electrode  108 , followed by supplying an etching gas into the process chamber  102 . At the same time, the process chamber  102  is evacuated to a predetermined pressure, e.g., 20 mTorr, and the reduced pressure is maintained. Under this condition, the first high frequency power is applied from the first high frequency power source  144  to the upper electrode plate  106  so as to generate a plasma of the etching gas between the two electrode plates  106 ,  108 . At substantially the same time, the second high frequency power is applied from the second high frequency power source  122  to the lower electrode plate  108 . Also, the third high frequency power is superposed on the first high frequency power by the third high frequency power source  126  and applied to the upper electrode  106  by detecting the phases of the first and second high frequency powers so as to provide a predetermined phase difference, thereby making the plasma density uniform. Under this condition, the etching of the target object proceeds.  
      A uniform etching treatment can be applied to the SiO 2  film formed on the wafer w with a high etching rate by the plasma of the uniform density thus generated. Also, since the plasma conforms uniformly with the wafer W, it is possible to prevent a damage done to the target object by the nonuniformity of the plasma.  
      In the embodiment described above, a semiconductor wafer W is used as the target object to be processed. However, it is also possible to apply the etching treatment to other substrates such as a glass plate for a liquid crystal display (LCD) device.  
      As described above, each of the etching apparatus according to the first to third aspects and the fifth aspect of the present invention comprises first high frequency apply means for applying a high frequency power of 50 MHz or higher to the first electrode and second high frequency apply means for applying a high frequency power to the second electrode. It should be noted that the second high frequency apply means also applies the high frequency power to the first electrode in order to increase the thickness of the plasma sheath formed on the first electrode. Therefore, even if the plasma sheath is affected by the standing wave, the ratio in the change in the thickness of the plasma sheath is rendered relatively small so as to make the plasma more uniform. To be more specific, since the etching apparatus of the present invention comprises the first high frequency apply means for applying a high frequency power of 50 MHz or higher to the first electrode and the second high frequency apply means for applying a high frequency power having a frequency not lower than 2 MHz and lower than the frequency of the high frequency power applied by the first high frequency apply means to each of the first electrode and the second electrode, the frequency of the high frequency power applied by said second high frequency apply means to the first electrode being equal to that of the high frequency power applied by the second high frequency apply means to the second electrode, and the high frequency power applied by said second high frequency apply means to the first electrode substantially forming a reverse phase relative to the high frequency power applied by the second high frequency apply means to the second electrode, a high frequency power having a relatively low frequency is applied from the second high frequency apply means to the first electrode as well as the high frequency power applied from the first high frequency apply means to the first electrode. It follows that, in the plasma sheath of the upper electrode, the portion corresponding to the high frequency power of the second high frequency apply means having a lower frequency is superposed on the portion corresponding to the high frequency power of the first high frequency apply means. It follows that, even if the plasma sheath is affected by the standing wave, the uniformity of the plasma is scarcely made poor, and the plasma sheath of the lower electrode is made substantially uniform. Therefore, it is possible to cope with the miniaturization performed by a high density plasma, and it is also possible to ensure a uniformity of the plasma processing by a uniform plasma.  
      It should also be noted that, since the fluctuation in the thickness of the plasma sheath is diminished by the application of the high frequency powers to the first and second electrodes, the intensity of the electric field in the sheath portion is kept maintained at a value higher than a predetermined value. As a result, the space potential of the plasma can be increased, and the ionization rate can be increased so as to increase the plasma density. It follows that the ions and the electrons are allowed to have a high energy. It is also possible to increase the ionization rate in the vicinity of the electrode so as to increase the high speed electron flux. It follows that the electrons within the plasma are allowed to reach easily the bottom portion of the contact hole so as to neutralize the positive charge within the hole. AS a result, the shading damage is unlikely to take place.  
      As described above, the prominent effects described above can be produced by applying high frequency powers substantially forming reverse phases to the first and second electrodes. However, desired effects can also be obtained if the phase difference between the high frequency powers applied to the first and second electrodes falls within a range of 180±45°.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.