Patent Publication Number: US-9887109-B2

Title: Plasma etching method and plasma etching apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional patent application of, and claims the benefit of and priority to, U.S. patent application Ser. No. 14/238,552 filed on Feb. 12, 2014, which is a National Stage of International Application No. PCT/JP2012/071723 filed on Aug. 28, 2012, claiming priority based on Japanese Patent Application No. 2011-188600 filed on Aug. 31, 2011, and U.S. Provisional Application No. 61/534,973, filed on Sep. 15, 2011, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a plasma etching method and a plasma etching apparatus. 
     BACKGROUND ART 
     In a semiconductor device manufacturing process, a plasma etching apparatus that etches a substrate such as a semiconductor wafer (simply referred to as “wafer” hereinafter) by irradiating plasma on the wafer may be used as an apparatus for processing the wafer. 
     In plasma etching, a gas containing fluorine, chlorine, and oxygen, for example, may be used as a processing gas that is turned into plasma. Plasma includes active species such as charged particles (referred to as “ion” hereinafter) and neutral particles (referred to as “radicals” hereinafter), for example. The surface of a wafer reacts with the ions and radicals contained in the plasma to prompt the generation of reaction products, and etching of the wafer progresses as the reaction products are volatized. 
     In recent years, the diameter of wafer holes are becoming larger. As the wafer hole diameter is enlarged, it becomes increasingly difficult to ensure in-plane uniformity of the depth and line width (critical dimension: CD) of bottom portions of holes (and trenches) within a wafer plane in an etching process. 
     In this respect, Patent Document 1 discloses a technique for controlling the concentration distribution of radicals at a center region and a peripheral region within a wafer plane by adjusting the supply rate of processing gas supplied from an upper electrode. 
     Patent Document 1: Japanese Patent No. 4358727 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, because radical diffusion is not uniform at the center region and the peripheral region within a wafer plane, it is difficult to ensure in-plane uniformity with the plasma etching apparatus disclosed in Patent Document 1. 
     In light of the above problems, one object of the present invention is to provide a plasma etching method and a plasma etching apparatus that can improve in-plane uniformity of the depth and line width of bottom portions of holes (and trenches) within a wafer plane. 
     Means for Solving the Problem 
     According to one embodiment of the present invention, a plasma etching apparatus that etches a substrate using a processing gas converted into plasma is provided. The plasma etching apparatus includes a processing chamber; a holding unit that is arranged within the processing chamber and is configured to hold the substrate; an electrode plate that is arranged to face the holding unit within the processing chamber; a plurality of supply parts for supplying processing gas to a space between the holding unit and the electrode plate, the supply parts being arranged at different positions with respect to a radial direction of the substrate; a high frequency power supply that converts the processing gas supplied to the space from the plurality of supply parts into plasma by supplying a high frequency power to at least one of the holding unit and the electrode plate; an adjustment unit that adjusts a supply condition for supplying processing gas with respect to each of the plurality of supply parts; and a control unit that controls the adjustment unit to vary the supply condition to be adjusted between a first position and a second position. The first position is where an effect of diffusion of supplied processing gas is greater than an effect of flow of supplied processing gas on an active species concentration distribution of active species contained in the plasma-converted processing gas at the substrate, and the second position is where the effect of flow of supplied processing gas is greater than the effect of diffusion of supplied processing gas on the active species concentration distribution at the substrate. 
     Advantageous Effect of the Invention 
     According to an aspect of the present invention, a plasma etching method and a plasma etching apparatus that can improve in-plane uniformity of the depth and line width at bottom portions of holes (and trenches) within a wafer plane may be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary configuration of a plasma etching apparatus according to an embodiment of the present invention; 
         FIG. 2  illustrates an exemplary configuration of a gas supply device of the plasma etching apparatus illustrated in  FIG. 1 ; 
         FIG. 3  illustrates an exemplary configuration of a shower head of  FIG. 1 ; 
         FIG. 4  illustrates the Peclet number at radial positions of a wafer in an embodiment of the present invention; 
         FIG. 5  illustrates an example of a change in the etch rate when a processing gas supply condition is altered; 
         FIG. 6  illustrates another example of a change in the etch rate when a processing gas supply condition is altered; 
         FIG. 7  illustrates another example of a change in the etch rate when a processing gas supply condition is altered; 
         FIG. 8  illustrates another example of a change in the etch rate when a processing gas supply condition is altered; 
         FIG. 9  illustrates how the depth of a trench may be controlled under plasma etching conditions according to an example of the present embodiment and a comparative example; and 
         FIG. 10  illustrates how the bottom critical dimension of a trench may be controlled under plasma etching conditions according to an example of the present embodiment and a comparative example. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10 , W wafer 
           105  susceptor (holding unit) 
           106  temperature distribution adjustment unit 
           120  upper electrode (electrode plate) 
           122  bellows 
           130  gas supply condition adjustment unit (adjustment unit) 
           140  shower head 
           143  buffer chamber 
           145  annular partition wall member 
           150  gas supply device 
           190  device control unit 
           200  upper electrode drive unit (gap adjustment unit) 
       
    
     EMBODIMENTS FOR IMPLEMENTING THE INVENTION 
     In the following, embodiments of the present invention are described with reference to the accompanying drawings. 
     (Plasma Etching Apparatus Configuration) 
     First, an exemplary configuration of a plasma etching apparatus according to an embodiment of the present invention is described with reference to  FIG. 1 . 
       FIG. 1  schematically illustrates an exemplary configuration of a plasma etching apparatus according to an embodiment of the present invention.  FIG. 2  schematically illustrates an exemplary configuration of a gas supply device  150  of the plasma etching apparatus of  FIG. 1 . Note that the illustration of the gas supply device  150  in  FIG. 1  is simplified, and the gas supply device  150  is described in greater detail in  FIG. 2 . 
     The plasma etching apparatus  100  of the present embodiment is a parallel plate type plasma etching apparatus. 
     The plasma processing apparatus  100  includes a cylindrical chamber (processing chamber)  102  made of aluminum having an alumite-treated (anodized) surface, for example. The chamber  102  is grounded. 
     A substantially cylindrical susceptor support  104  is arranged at a bottom portion within the chamber  102  via an insulating plate  103  made of ceramic, for example. A susceptor  105  corresponding to a lower electrode is arranged on the susceptor support  104 . A high pass filter (HPF)  105   a  is connected to the susceptor  105 . 
     The susceptor  105  has an upper side center portion arranged into a convex circular plate shape. An electrostatic chuck  111  having substantially the same shape as a wafer W corresponding to a processing object is arranged on this circular plate shaped portion. The electrostatic chuck  111  is made of an insulating material and has an electrostatic electrode  112  interposed between the insulating material. The electrostatic chuck  111  is a circular plate shaped ceramic member, and the electrostatic chuck  112  is connected to a DC power supply  113 . 
     When a positive DC voltage is supplied to the electrostatic chuck  112 , a negative electric potential is generated at a face of the wafer W toward the electrostatic chuck  111  (referred to as “backside” of the wafer W hereinafter). In this way, a difference in potential is created between the electrostatic chuck  112  and the backside of the wafer W. The wafer W is electrostatically attracted to the electrostatic chuck  111  by a Coulomb force or a Johnsen-Rahbek force that is generated as a result of such a difference in potential. At this point, a DC voltage of 1.5 kV, for example, may be supplied to the electrostatic chuck  111  from the DC power supply  113  connected to the electrostatic electrode  112 . 
     A first high frequency power supply  114  and a second high frequency power supply  116  are connected to the susceptor  105  via a first matching unit  115  and a second matching unit  117 , respectively. The first high frequency power supply  114  supplies to the suceptor  105  a high frequency power for biasing having a relatively low frequency of 13.6 MHz, for example. The second high frequency power supply  115  supplies to the suscpetor  105  a high frequency power for plasma generation having a relatively high frequency of 40 MHz, for example. In this way, the susceptor  105  can supply power for plasma generation within the chamber  102 . 
     A gas passageway  118  for supplying a heat transfer medium (e.g., backside gas such as He gas) to the backside of the wafer W is arranged to extend through the insulating plate  103 , the susceptor support  104 , the susceptor  105 , and the electrostatic chuck  111 . Heat may be exchanged between the suceptor  105  and the wafer W via such a heat transfer medium so that the wafer W may be maintained at a predetermined temperature. 
     An annular focus ring  119  is arranged on an upper edge portion of the susceptor  105  to surround the wafer W that is placed on the electrostatic chuck  111 . The focus ring  119  may be made of a dielectric material such as ceramic or quartz, or a conductive material such as a single crystal silicon. For example, the focus ring  119  may be made of the same conductive material as the wafer W. 
     By expanding a plasma distribution range to the focus ring  119 , the plasma density at a periphery portion of the wafer W may be maintained substantially the same as the plasma density at a center portion of the wafer W. In this way, plasma etching uniformity within a wafer plane may be improved. 
     An upper electrode  120  is arranged to face the susceptor  105  and be substantially parallel to the susceptor  105 . The upper electrode  120  is connected to a DC power supply  123 . The upper electrode  120  is also connected to a low pass filter (LPF)  124 . 
     The upper electrode  120  may be driven by an upper electrode drive unit  200  to move in the vertical direction, for example. By arranging the upper electrode  120  to be movable in the vertical direction, a distance between the upper electrode  120  and the susceptor  105  (referred to as “gap G” hereinafter) may be adjusted. As described below in connection with a plasma etching method according to an embodiment of the present invention, the gap G is a parameter that has a substantial effect on the diffusion and flow of processing gas. Accordingly, as described in detail below, by enabling adjustment of the gap G, the plasma distribution at a space between the upper electrode  120  and the susceptor  105  within the chamber  102  may be controlled. 
     The displacement of the upper electrode  120  that is driven by the upper electrode drive unit  200  is not particularly limited. For example, the displacement of the upper electrode  120  may be 70 mm, and the gap G may be adjusted to be within a range of 20 mm to 90 mm. However, the present invention is not limited to such an example. Also, the orientation of the plasma etching apparatus  100  is not limited to that illustrated in  FIG. 1 . For example, the plasma etching apparatus  100  of  FIG. 1  may be turned 90 degrees or turned upside down. 
     The upper electrode  120  is supported by an upper inner wall of the chamber  102  via bellows  122 . The bellows  122  are attached by fixing means such as bolts to the upper inner wall of the chamber  102  via an annular upper flange  122   a . Also, the bellows  122  are attached by fixing means such as bolts to the surface of the upper electrode  120  via an annular upper flange  122   b.    
     In the following, the upper electrode drive unit  200  for adjusting the gap G is described. The upper electrode drive unit  200  includes a substantially cylindrical support member  204  that supports the upper electrode  120 . The support member  204  is attached to an upper center portion of the upper electrode  120  by fixing means such as a bolt. 
     The support member  204  is arranged to be movable in and out of a hole  102   a  formed around a center portion of an upper wall of the chamber  102 . An outer peripheral face of the support member  204  is supported by a slide mechanism  210  inside the hole  102   a  of the chamber  102 . 
     The slide mechanism  210  includes a fixing member  214  that is L-shaped in cross section, a guide member  216  that is fixed to a vertical portion of the fixing member  214  at an upper part of the chamber  102 , and a rail part  212  arranged in one direction (vertical direction in the present example) on the outer peripheral face of the support member  204  to be slidably supported by the guide member  216 . 
     The fixing member  214  that fixes the guide member  216  of the slide mechanism  210  is fixed to the upper part of the chamber  102  via a horizontal adjustment plate  218  having an annular horizontal portion. A horizontal positioning of the upper electrode  120  may be adjusted by the horizontal adjustment plate  218 . 
     The horizontal adjustment plate  218  may be fixed to the chamber  102  by bolts that are arranged equidistantly around the horizontal adjustment plate  218 , for example. A tilt of the horizontal adjustment plate  218  with respect to the horizontal direction may be adjusted by adjusting how much the bolts are protruding, for example. By adjusting the tilt of the horizontal adjustment plate  218  with respect to the horizontal direction and adjusting a tilt of the guide member  216  of the slide mechanism  210  with respect to the vertical direction, a tilt of the upper electrode  120  in the horizontal direction may be adjusted. That is, the upper electrode  120  may be constantly maintained horizontal. 
     An air pressure cylinder  220  for driving the upper electrode  120  is attached to the upper side of the chamber  102  via a cylindrical body  201 . That is, the bottom edge of the cylindrical body  201  is hermetically sealed to the upper side of the chamber  102  by bolts, for example, to cover the hole  102   a . The top edge of the cylindrical body  201  is hermetically sealed to the bottom edge of the air pressure cylinder  220 . 
     The air pressure cylinder  220  includes a rod  202  that may be driven to move in one direction. The bottom end of the rod  202  may be connected to an upper center portion of the support member  204   b  a bolt, for example. When the rod  202  is driven, the upper electrode  120  is driven by the support member  204  to move along the slide mechanism  210 . The rod  202  may have a cylindrical structure, for example, and an internal space of the rod  202  communicates with a center hole formed at a center portion of the support  204  and is released to the atmosphere. In this way, wiring that is grounded via the upper electrode  120  and the low pass filter (LPF)  124 , and a power supply line for supplying a DC voltage to the upper electrode  120  from the DC power supply  123  may be connected to the upper electrode  120  via the internal space of the rod  202  and the center hole of the support member  204 . 
     Also, a position detector such as a linear encoder  205  for detecting the position of the upper electrode  120  is arranged at a side of the air pressure cylinder  220 . A top end member  207  including an extending portion  207   a  extending sideways from the rod  202  is arranged at the top end of the rod  202 . The extending portion  207   a  of the top end member  207  and a position detection unit  205   a  of the linear encoder  205  are arranged to be in contact with each other. The top end member  207  moves in conjunction with the upper electrode  120 . Thus, the linear encoder  205  may detect the position of the upper electrode  120 . 
     The air pressure cylinder  220  includes a cylinder main body  222 , an upper support plate  224 , and a lower support plate  226 . The cylinder main body  222  is interposed between the upper support plate  224  and the lower support plate  226 . An annular partition member  208  for partitioning the internal space of the air pressure cylinder  220  into an upper space  232  and a lower space  234  is arranged around an outer peripheral face of the rod  202 . 
     Compressed air is introduced into the upper space  232  of the air pressure cylinder  220  from an upper port  236  of the upper support plate  224 . Compressed air is introduced into the lower space  234  of the air pressure cylinder  220  from a lower port  238  of the lower support plate  226 . By controlling the amount of air introduced into the upper space  232  and the lower space  234  from the upper port  236  and the lower port  238 , the rod  202  may be driven and controlled to move in one direction (e.g., vertical direction). The amount of air introduced into the air pressure cylinder  220  is controlled by an air pressure circuit  300  arranged near the air pressure cylinder  220 . 
     The upper electrode drive unit  200  also includes a control unit  290 , which is connected to a device control unit  190 . A control signal from the device control unit  190  is transmitted to the control unit  290 , and in turn, the control unit  290  controls drive operations of various components of the upper electrode drive unit  200 . 
     A temperature distribution adjustment unit  106  for adjusting an in-plane temperature distribution of the wafer W is arranged within the susceptor support  104 . The temperature distribution adjustment unit  106  includes heaters  106   a  and  106   b , heater power supplies  106   c  and  160   d , thermometers  106   e  and  106   f , and coolant paths  107   a  and  107   b.    
     The susceptor support  104  has the heater  106   a  arranged at the center side and the heater  106   b  arranged at the outer side. The center side heater  106   a  is connected to the center side heater power supply  106   c , and the outer side heater  106   b  is connected to the outer side heater power supply  106   d . The center side heater power supply  106   c  and the outer side heater power supply  106   d  are capable of independently adjusting the power supplied to the center side heater  106   a  and the outer side heater  106   b , respectively. In this way, a temperature distribution may be created at the susceptor support  104  and the susceptor  105  along the radial direction of the wafer W. That is, the temperature distribution along the radial direction of the wafer W may be adjusted. 
     Also, the susceptor support  104  has the thermometer  106   e  arranged at the center side and the thermometer  106   f  arranged at the outer side. The center side thermometer  106   e  and the outer side thermometer  106   f  measure temperatures of the susceptor support  104  at the center side and the outer side, respectively. In this way, the center side thermometer  106   e  and the outer side thermometer  106   f  may derive temperatures at the center side and the outer side of the wafer W. The temperatures measured by the center side thermometer  106   e  and the outer side thermometer  106   f  are transmitted to the device control unit  190 . The device control unit  190  adjusts outputs of the center side heater power supply  106   c  and the outer side heater power supply  106   d  so that the temperatures of the wafer W derived from the measured temperatures reach their target temperatures. 
     The susceptor support  104  may also have the coolant path  107   a  arranged at the center side and the coolant path  107   b  arranged at the outer side. The center side coolant path  107   a  and the outer side coolant path  107   b  may be arranged to circulate coolants such as cooling water or fluorocarbon coolants at different temperatures, for example. To circulate the coolants, a coolant is introduced into the coolant path  107   a  via a center side introduction pipe  108   a  and is discharged via a center side discharge pipe  109   a . Also, a coolant is introduced into the outer side coolant path  107   b  via an outer side introduction pipe  108   b  and is discharged via an outer side discharge pipe  109   b.    
     The temperature of the susceptor  105  is adjusted through heating by the heaters  106   a  and  106   b , and cooling by the coolants. Accordingly, the wafer W is adjusted to a predetermined temperature by heat from plasma radiation and irradiation of ions included in the plasma, and heat exchange with the susceptor  105 . Note that because the susceptor support  104  has the center side heater  106   a  (and center side coolant path  107   a ) and the outer side heater  106   b  (and outer side coolant path  107   b ), the temperatures of the wafer W at the center side and the outer side may be independently adjusted. 
     Also, although not illustrated in  FIG. 1 , a heat insulating layer such as a heat insulating material or a space may be provided between the center side heater  106   a  and the outer side heater  106   b  or the center side coolant path  107   a  and the outer side coolant path  107   b . By providing such a heat insulating layer, heat insulation may be achieved between the center side heater  106   a  and the outer side heater  106   b  or the center side coolant path  107   a  and the outer side coolant path  107   b . That is, a greater heat distribution may be created between the center side and the outer side of the wafer W. 
     An exhaust pipe  131  is connected to a bottom portion of the chamber  102 , and an exhaust device  135  is connected to the exhaust pipe  131 . The exhaust device  135  includes a vacuum pump such as a turbo-molecular pump for adjusting the internal pressure within the chamber  102  to a reduced-pressure atmosphere (e.g., 0.67 Pa or lower). Also, a gate valve  132  is arranged at a side wall of the chamber  102 . The gate valve  132  may be opened to allow the wafer W to be transferred into and out of the chamber  102 . Note that a transfer arm may be used to transfer the wafer W, for example. 
     (Configuration of Adjustment Unit for Adjusting Processing Gas Supply Conditions) 
     In the following, referring to  FIGS. 2 and 3 , an exemplary configuration of a gas supply condition adjustment unit  130  for adjusting gas supply conditions for supplying plasma gas to the wafer W supported by the susceptor  105  is described. Note that the gas supply condition adjustment unit  130  is an exemplary embodiment of a plurality of supply parts for supplying processing gas and an adjustment unit for adjusting a supply condition for supplying processing gas with respect to each of the plurality of supply parts. 
       FIG. 2  illustrates an exemplary configuration of the gas supply device  150  of the plasma etching apparatus of  FIG. 1 .  FIG. 3  illustrates an exemplary configuration of the shower head  140  of the plasma etching apparatus of  FIG. 1 . 
     The gas supply condition adjustment unit  130  includes the shower head  140 , which is integrated with the upper electrode  120 , and the gas supply device  150 . 
     The shower head  140  is configured to spray a predetermined processing gas (e.g., mixed gas) on the wafer W that is held by the susceptor  105 . The shower head  140  includes a circular electrode plate  141  (upper electrode  120 ) having multiple gas spray holes and an electrode support  142  that detachably supports an upper face (front side) of the electrode plate  141 . The electrode support  142  is arranged into a circular disk shape having the same diameter as the electrode plate  141 . A circular buffer chamber  143  is formed within the electrode support  142 . The electrode plate  141  has gas spray holes for supplying gas such as processing gas to the wafer W (referred to as “gas spray holes  141   x ” hereinafter, with x being variable). 
     As illustrated in  FIG. 3 , the buffer chamber  143  has one or more annular partition wall members  145  forming O-rings. The annular partition wall members  145  are arranged at different positions with respect to the radial direction of the shower head  140 . In  FIG. 3 , a first annular partition wall member  145   a , a second annular partition wall member  145   b , and a third annular partition wall member  145   c  are concentrically arranged from the center side to the periphery side with respect to the radial direction of the shower head  140 . In this way, the buffer chamber  143  is divided into a first buffer chamber  143   a , a second buffer chamber  143   b , a third buffer chamber  143   c , and a fourth buffer chamber  143   d  from the center side to the periphery side. 
     The number of annular partition wall members  145  arranged within the buffer chamber  143  is not particularly limited as long as at least one is provided. For example, three may be provided as illustrated in  FIG. 3 . In a case of performing plasma etching using a wafer W with a diameter of 300 mm, three annular partition wall members  145  are preferably provided (i.e., the buffer chamber  143  is preferably divided into four zones) for facilitating control of the processing gas and securing in-plane etching uniformity at the same time by the plasma etching method described below. Note that by providing N annular partition wall members  145 , the buffer chamber  143  may be divided into N+1 buffer chambers. 
     The gas supply device  150  supplies a predetermined processing gas to the first through fourth buffer chambers  143   a ,  143   b ,  143   c , and  143   d.    
     One or more gas spray holes  141   x  are arranged to communicate with the first through fourth buffer chambers  143   a ,  143   b ,  143   c , and  143   d  at the bottom side of the buffer chamber  143 . A predetermined processing gas may be sprayed on the wafer W via these gas spray holes  141   x . The number and layout of the gas spray holes  141   x  are preferably arranged such that processing gas may be evenly sprayed on the wafer W. 
     In the following, an exemplary layout of the gas spray holes  141   x  is described in a case where a wafer W with a diameter of 300 mm is used and the buffer chamber  143  is divided into four zones by three annular partition wall members  145 . In this case, the first buffer chamber  143   a  has four gas spray holes  141   a  arranged (e.g., equidistantly) along the circumference of a 11-mm-radius circle with its center point at the center of the shower head  140 , and twelve gas spray holes  141   b  arranged (e.g., equidistantly) along the circumference of a 33-mm-radius circle with its center point the center of the shower head  140 . The second buffer chamber  143   b  has twenty-four (24) gas spray holes  141   c  arranged (e.g., equidistantly) along the circumference of a 55-mm-radius with its center point the center of the shower head  140 , and thirty-six (36) gas spray holes  141   d  arranged (e.g., equidistantly) along the circumference of a 77-mm-radius circle with its center point at the center of the shower head  140 . The third buffer chamber  143   c  has forty-eight (48) gas spray holes (not shown) arranged (e.g., equidistantly) along the circumference of a 99-mm-radius circle with its center point at the center of the shower head  140 , and sixty (60) gas spray holes (not shown) arranged (e.g., equidistantly) along the circumference of a 121-mm-radius circle with its center point at the center of the shower head  140 . The fourth buffer chamber  143   d  has eighty (80) gas spray holes (not shown) arranged (e.g., equidistantly) along the circumference of a 143-mm-radius circle with its center point at the center of the shower head  140 , and one hundred (100) gas spray holes (not shown) arranged (e.g., equidistantly) along the circumference of a 165-mm-radius circle with its center point at the center of the shower head  140 . 
     In the following, referring to  FIG. 2 , a valve mechanism and a flow rate adjustment mechanism of the gas supply device  150  for individually supplying processing gas to the buffer chambers are described. Note that  FIG. 2  also illustrates an exemplary case where the buffer chamber  143  is divided into four zones by three annular partition wall members  145 . However, the present invention is not limited to such an example. 
     The gas supply device  150  includes a first gas box  161  and a second gas box  160 . The first gas box  161  accommodates a plurality of gas supply sources and a first valve  303 . The second gas box accommodates a second valve  302 , a flow rate controller  301  such as a mass flow controller, and a third valve  303 . 
     In the present embodiment, the gas supply sources include fluorocarbon-based fluorine compound (CF) processing gases such as CF 4  gas, C 4 F 6  gas, C 4 F 8  gas, CH 2 F 2  gas, and CHF 3  gas, for example. Also, the gas supply sources include gas for controlling adhesion of reaction products of the CF gases such as oxygen (O 2 ) gas. Further, the gas supply sources include carrier gases such as Ar gas, N 2  gas, and He gas, for example. 
     In the first gas box  161 , the gas supply sources are connected to pipes. The pipes are arranged into a branching structure so that each gas from the gas supply sources may be supplied to each of the buffer chambers. In the example illustrated in  FIG. 2 , pipes  170 - 173  each branch out into four branches at the gas supply source side. Also, the pipes are connected to the first valve  303  so that gases to be supplied may be switched according to a desired process. With such a structure, a new gas supply source may be added or the supply of a processing gas that may not be needed for a process may be stopped, for example, through simple operations. 
     The pipes  170 - 173  are connected to the first through fourth buffer chambers via the second gas box  160 , and a predetermined processing gas may be supplied to each of the buffer chambers. 
     The second gas box  160  accommodates a second valve  302 , a flow rate controller (or mass flow controller)  301 , and a third valve  300  arranged in this order from the gas supply source side. The second valve  302 , the third valve  300 , and the flow rate controller  301  are arranged at each of a plurality of pipes for supplying different types of gas to the buffer chambers. In this way, in addition to adjusting the flow rate of processing gas supplied to a specific buffer chamber from the gas supply sources, a supply of a specific processing gas may be stopped by closing a specific valve so that the processing gas may only be supplied to a specific buffer chamber (i.e., a specific zone). 
     The branches of the pipes  170 - 173  merge at the downstream side of the third valve  300  so that a mixed processing gas may be supplied to each of the first through fourth buffer chambers. Note that although not shown, additional valves may be arranged at the downstream side of the merging points of the processing gases, and in this way, whether to supply the processing gas to each buffer chamber may be controlled by opening/closing each of the valves. 
     The second gas box  160  may also include a pressure gauge as a pressure adjustment unit (not shown) and an additional valve, and the flow rate of processing gas may be controlled based on the measurement result of the pressure gauge, for example. 
     Operations of the flow rate controller  301  within the second gas box  160  may be controlled by the device control unit  190  of the plasma etching apparatus  100 , for example. Accordingly, the device control unit  190  may start/stop the supply of different types of gasses from the first gas box  161  and the second gas box and control the amount of the gases supplied. 
     As described above, the plasma etching apparatus  100  includes the device control unit  190 . The device control unit  190  includes a processing unit such as a CPU (not shown) and a recording medium such as a hard disk (not shown). The device control unit  190  controls operations of units including the first high frequency power source  114 , the second high frequency power source  116 , the temperature distribution adjustment unit  106 , the upper electrode drive unit  200 , and the gas supply condition adjustment unit  130 , for example. To control the operations of the above units, the CPU of the device control unit  190  may implement corresponding programs stored in the hard disk of the device control unit  190  for prompting the units to execute an etching process, for example. 
     Note that the device control unit  190  is an exemplary embodiment of a control unit of the present invention. 
     (Plasma Etching Method) 
     In the following, an exemplary plasma etching method using the above plasma etching apparatus  100  is described. 
     When gas is supplied from the gas spray holes to a space between the upper electrode  120  and the susceptor  105 , the gas is diffused while being flown in an exhaust gas flow direction (direction toward the exhaust device  135 ). Depending on the gas spray hole position, for example, the effect of “flow” or the effect of “diffusion” on the concentration distribution of gas components (e.g., radicals) transported may vary. The Peclet number is a dimensionless number that qualitatively indicates the degree of dependency on the “diffusion” or “flow”. The Peclet number may be expressed by Formula (1) indicated below, where u represents the gas velocity (m/s), D AB  represents the interdiffusion coefficient of gases (m 2 /s), and L represents the characteristic length (m).
 
 Pe=uL/D   AB   (1)
 
     When the Peclet number is less than one (1), the effect of “diffusion” is greater than the effect of “flow” on gas transportation. When the Peclet number is greater than one (1), the effect of “flow” is greater than the effect if “diffusion” on gas transportation. 
     As one specific example,  FIG. 4  includes a graph indicating the Peclet number at different radial positions on a wafer in the present embodiment (see upper graph of  FIG. 4 ). The upper graph of  FIG. 4  indicates a Peclet number obtained by calculating the velocity u in a case where a gas mixture of Ar gas and C 4 F 8  gas (interdiffusion coefficient D AB =1.23×10 −1  m 2 /s) is used and the characteristic length L (i.e., gap G between the susceptor  105  and the upper electrode  120 ) is 0.03 m. Note that in the graph of  FIG. 4  indicating the Peclet number, the horizontal axis represents radial positions of a 300-mm-diameter wafer with the center of the wafer set equal to 0 mm. 
     It can be appreciated from the upper graph of  FIG. 4  that the wafer is divided into a region where the effect of “diffusion” is greater than the effect of “flow” and a region where the effect of “flow” is greater than the effect of “diffusion” at a 86-mm radial position with respect to the center of the wafer. 
     Also,  FIG. 4  includes a graph indicating etch rate ratios at various wafer positions in a case where a 300-mm-diameter wafer is used (see lower graph of  FIG. 4 ). Specifically, the lower graph of  FIG. 4  indicates an etch rate ratio at a wafer position in a case where the buffer chamber is divided into four zones (i.e., “Center”, “Middle”, “Edge”, and “Very Edge” zones) by three annular partition wall members and plasma etching is performed on a 300-mm-diameter wafer by spraying gas from each of these zones. Note that in the present example, four (4) gas spray holes arranged along the circumference of a 11-mm-radius circle around the center of the shower head and twelve (12) gas spray holes arranged along the circumference of a 33-mm-radius circle around the center of the shower head are provided at the “Center” zone. Twenty-four (24) gas spray holes arranged along the circumference of a 55-mm-radius circle around the center of the shower head and thirty-six (36) gas spray holes arranged along the circumference of a 77-mm-radius circle around the center of the shower head are provided at the “Middle” zone. Forty-eight (48) gas spray holes arranged along the circumference of a 99-mm-radius circle around the center of the shower head and sixty (60) gas spray holes arranged along the circumference of a 121-mm-radius circle around the center of the shower head are provided at the “Edge” zone. Eighty (80) gas spray holes arranged along the circumference of a 143-mm-radius circle around the center of the shower head and one hundred (100) gas spray holes arranged along the circumference of a 165-mm-radius circle around the center of the shower head are provided at the “Very Edge” zone. The following descriptions relating to supplying gas from the “Center”, “Middle”, “Edge”, and “Very Edge” zones refer to supplying gas from the above gas spray holes arranged at the respective zones. 
     Note that the vertical axis of the lower graph of  FIG. 4  represents a standardized etch rate ratio with the etch rate of a maximum etch rate position set equal to one (1). 
     According to the lower graph of  FIG. 4 , in cases where gas is supplied from the “Center” zone and the “Middle” zone, the etch rate is higher at positions corresponding to the positions where gas is supplied. This is because at the “Center” and “Middle” zones, the effect of “diffusion” is greater than the effect of “flow” on gas transportation (see upper graph of  FIG. 4 ). Also, it is believed that gas supplied from the “Center” and “Middle” zones also affect the etch rates at the “Edge” zone and the “Very Edge” zone. 
     On the other hand, in cases where gas is supplied from the “Edge” zone and the “Very Edge” zone, the effect of supplied gas on the etch rate is shifted to the outer side. That is, because the effect of “flow” of the supplied gas is greater than the effect “diffusion” on gas transportation at the “Edge” (and “Very Edge”) zone (see upper graph of  FIG. 4 ), gas supplied from the “Edge” zone is believed to flow toward the periphery. Note that gas supplied from the “Edge” zone and the “Very Edge” zone has very little impact on the etch rate at the “Center” and “Middle” zones. 
     Based on the above, in controlling the supply of processing gas, it is important to vary the gas supply condition to be adjusted between a position where the effect of diffusion of supplied processing gas is greater than the effect of flow, and a position where the effect of flow of supplied processing gas is greater than the effect of diffusion. That is, with respect to the position where the effect of diffusion of supplied processing gas is greater than the effect of flow, a gas supply condition of gas spray holes corresponding to (right above) this position may be adjusted, and with respect to the position where the effect of flow of supplied processing gas is greater than the effect of diffusion, a gas supply condition of gas spray holes located toward the center with respect to this position may be adjusted to improve in-plane uniformity upon plasma etching. For example, based on the values u, L, and D AB , if the effect of diffusion is greater than the effect of flow of processing gas supplied from the “Edge” (or “Very Edge”) zone, a gas supply condition for supplying processing gas from the “Edge” (or “Very Edge”) zone may be adjusted, and if the effect of flow is greater than the effect of diffusion, a gas supply condition for supplying processing gas from the “Center” (or “Middle”) zone may be adjusted. 
     In the following, the effects of gas supply conditions on gas transportation are described. That is, the impact of supply gas parameters on the in-plane etching uniformity of a wafer is described. 
     Diffusion of supply gas depends on the mean free path l (m) of diffusion molecules (gas molecules) and the gas flow rate u (m/s). Assuming the supply gas is in an ideal gas state and the Maxwell distribution applies to the molecular speed of the diffusion molecules, the mean free path l of the diffusion molecules may be expressed by Formula (2) indicated below.
 
 l =( T×C   1 )/( d   2   ×P )  (2)
 
     In the above Formula (2), C 1  represents a constant, d represents the molecular collision diameter (m) of the diffusion molecules, P represents the pressure in the system, and T represents the temperature (K) in the system. 
     Also, assuming the supply gas is in an ideal gas state, the supply gas flow rate u may be expressed by Formula (3) indicated below.
 
 u =( Q×C   2 )/ PV   (3)
 
     In the above Formula (3), C 2  represents a constant, Q represents the flow rate (m 3 /s) at 1 atmosphere, P represents the pressure in the system, and V represents the volume (m 3 ) in the system 
     In this case, because a supply gas diffusion area d area  is proportionate to the ratio of the mean free path l to the flow rate u (l/u), Formula (4) indicated below may be derived from Formula (2) and Formula (3).
 
 d   area   ∝   l/u =( T×V×C   3 )/( d   2   Q )  (4)
 
     In the above Formula (4), C 3  represents a constant. 
     It can be appreciated from the above that the supply gas diffusion area depends on the volume in the system, the supply gas flow rate, the temperature in the system, and the molecular collision diameter. Note that the volume in the system approximates the volume of the space between the upper electrode  120  and the susceptor  105  in the present embodiment, and because the diameter of a processing object does not change during plasma etching, the distance of the space between the upper electrode  120  and the susceptor  105  (gap G) may be used to represent this parameter. Also, the supply gas flow rate has a correlation with the pressure in the system. Further, because the molecular collision diameter depends on the type of supply gas (i.e., molecular weight of the supply gas), the supply gas diffusion area may also depend on the molecular weight of the supply gas. 
     In the following, referring to  FIGS. 5-8 , experiments are described that indicate how the supply gas diffusion area depends on parameters (supply conditions) such as the supply gas flow rate (and supply gas pressure), the supply gas molecular weight, and the gap G. 
       FIG. 5  illustrates an example of a change in the etch rate when a processing gas supply condition is changed.  FIG. 6  illustrates another example of a change in the etch rate when a processing gas supply condition is changed.  FIG. 7  illustrates another example of a change in the etch rate when a processing gas supply condition is changed.  FIG. 8  illustrates another example of a change in the etch rate when a processing gas supply condition is changed. Note that the supply condition subject to change corresponds to the supply gas flow rate in  FIG. 5 , the supply gas pressure in  FIG. 6 , the supply gas molecular weight in  FIG. 7 , and the gap G in  FIG. 8 . Also, as with the previously described examples, the buffer chamber was divided into four zones (“Center”, “Middle”, “Edge”, and “Very Edge”) by three annular partition wall members, and the partial pressure of gas supplied from the gas spray holes was arranged to be constant (see description of etching conditions below). Further, additional gas was supplied from gas spray holes arranged along the outermost circle at the “Very Edge” zone (165-mm radius circle around the center of the shower head) under the conditions indicated below, and the etch rates at various wafer positions were plotted under these conditions. Note that the vertical axis of  FIG. 5  represents the etch rate of silicon oxide at a BEOL (Back End of Line) trench pattern formed on a processing object including a silicon wafer with silicon oxide deposited thereon as a hard mask. 
     The vertical axis of  FIG. 6  represents a standardized etch rate ratio with the etch rate of a maximum etch rate position (outermost position) set equal to 1. 
     Detailed etching conditions are indicated below. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Etching apparatus 
                 80 mTorr 
               
               
                   
                 internal pressure 
                 (Pressure change range: 
               
               
                   
                   
                 30-150 mTorr) 
               
               
                   
                 Gap G 
                 30 mm 
               
               
                   
                   
                 (Gap change range: 22-50 mm) 
               
               
                   
                 High frequency power 
                 700/1000 W 
               
               
                   
                 supply power (40 MHz/13 MHz) 
               
               
                   
                 Upper electrode potential 
                 0 V 
               
               
                   
                 Processing gas flow rate 
                 C 4 F 8 /Ar/N 2 /O 2  = 
               
               
                   
                 (total pressure) 
                 30/1200/70/17 sccm 
               
               
                   
                   
                 (Additional C 4 F 8  (O 2  or 
               
               
                   
                   
                 CH 2 F 2  upon molecular 
               
               
                   
                   
                 weight change) at 20 sccm 
               
               
                   
                   
                 supplied to outermost 
               
               
                   
                   
                 position; Flow rate 
               
               
                   
                   
                 change range: 0.33-1.5 
               
               
                   
                   
                 times the above flow 
               
               
                   
                   
                 rate) 
               
               
                   
                 Processing time 
                 60 seconds 
               
               
                   
                   
               
            
           
         
       
     
     It can be appreciated from the graphs plotting the etch rates in  FIGS. 5-8  how each of the parameters affects the diffusion of supply gas. For example, by decreasing the supply gas flow rate, decreasing the supply gas molecular weight, increasing the pressure in the system, and increasing the gap G, the supply gas may be diffused to a greater extent. In other words, by controlling these parameters, the concentration distribution of gas (i.e., radicals) may be controlled, and in this way, in-plane uniformity of a wafer plane shape may be improved upon performing plasma etching. 
     (Experiments Confirming Effects of Plasma Etching Apparatus and Plasma Etching Method of Present Embodiment) 
     In the following, experiments confirming the effects of the plasma etching apparatus and plasma etching method of the present embodiment are described with reference to  FIGS. 9 and 10 . 
       FIG. 9  illustrates an exemplary manner of how the depth of a line portion and the line width of a bottom portion (bottom CD) can be controlled to achieve substantial uniformity under plasma etching conditions according to an example of the present embodiment and plasma etching conditions according to a comparative example.  FIG. 10  illustrates another exemplary manner of how the depth of a line portion and the line width of a bottom portion (bottom CD) can be controlled to achieve substantial uniformity under plasma etching conditions according to an example of the present embodiment and plasma etching conditions according to a comparative example. As with the previously-described examples, the buffer chamber was divided into four zones (“Center”, “Middle”, “Edge”, and “Very Edge”) by three annular partition wall members. In the comparative example, the partial pressure (see description of etching conditions below) of gas supplied from the gas spray holes was arranged to be constant. In the example of the present embodiment, the partial pressure of processing gas (C 4 F 8 ) supplied from the gas spray holes at the “Edge” zone was increased, and the partial pressure of processing gas (C 4 F 8 ) supplied at the “Very Edge” zone was set equal to zero (0). Note that the vertical axes of the graphs shown in  FIGS. 9 and 10  represent the trench depth and the bottom CD of a BEOL (Back End of Line) trench pattern formed by depositing a silicon oxide on a silicon wafer as a hard mask and etching the silicon wafer. 
     Detailed etching conditions are indicated below. 
     &lt;&lt;Common&gt;&gt; 
                                                    Etching apparatus   80   mTorr           internal pressure           Gap G   30   mm           High frequency power   400/200   W           supply power (40 MHz/13 MHz)           Upper electrode potential   700   V           Processing time   95   seconds                        
&lt;&lt;Processing Gas Flow Rate of Example&gt;&gt;
 
(Note that the Partial Pressure from Gas Spray Holes is Maintained Constant within Each Zone)
 
                                                “Center” zone partial   C 4 F 8 /Ar/N 2 /O 2  =           pressure total   1.3/53/3.1/1.0 sccm           “Middle” zone partial   C 4 F 8 /Ar/N 2 /O 2  =           pressure   4.9/198/12/3.8 sccm           “Edge” zone partial   C 4 F 8 /Ar/N 2 /O 2  =           pressure   13.4/356/21/6.8 sccm           “Very Edge” zone partial   C 4 F 8 /Ar/N 2 /O 2  = 0/593/35/11 sccm           pressure                        
&lt;&lt;Processing Gas Flow Rate of Comparative Example&gt;&gt;
 
(Note that the Partial Pressure from Gas Spray Holes is Maintained Constant Throughout all the Zones)
 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 “Center” zone partial 
                 C 4 F 8 /Ar/N 2 /O 2  = 
               
               
                   
                 pressure total 
                 1.3/53/3.1/1.0 sccm 
               
               
                   
                 “Middle” zone partial 
                 C 4 F 8 /Ar/N 2 /O 2  = 
               
               
                   
                 pressure 
                 4.9/198/12/3.8 sccm 
               
               
                   
                 “Edge” zone partial 
                 C 4 F 8 /Ar/N 2 /O 2  = 
               
               
                   
                 pressure 
                 8.9/356/21/6.8 sccm 
               
               
                   
                 “Very Edge” zone partial 
                 C 4 F 8 /Ar/N 2 /O 2  = 
               
               
                   
                 pressure 
                 14.8/593/35/11 sccm 
               
               
                   
                   
               
            
           
         
       
     
     As can be appreciated from  FIG. 9 , in the case where the partial pressure of processing gas supplied from the gas spray holes is maintained constant throughout all the gas spray holes as in the comparative example, the trench depth within a wafer plane has a variation of 20 nm. That is, the in-plane depth substantially decreases at the “Very Edge” zone in the comparative example. On the other hand, in the case where the partial pressure at the “Edge” zone is increased and the partial pressure at the “Very Edge” zone is decreased as in the example of the present embodiment, the in-plane depth variation is 10 nm. That is, the in-plane depth at the “Very Edge” zone may be controlled to be substantially the same as the in-plane depth at the “Center” zone and the “Middle” zone meaning in-plane uniformity of the in-plane depth can be improved. Also, as can be appreciated from  FIG. 10 , in the case where the partial pressure of processing gas supplied from the gas spray holes is maintained constant throughout all the gas spray holes as in the comparative example, the in-plane bottom CD variation is 15 nm. However, in the case where the partial pressure at the “Edge” zone is increased and the partial pressure at the “Very Edge” zone is decreased as in the example of the present embodiment, the in-plane bottom CD variation may be reduced to 3 nm meaning in-plane uniformity of the in-plane bottom CD can be improved. 
     As described above with reference to  FIG. 4 , when gas is supplied from the “Very Edge” zone, the effect of supplied gas on the etch rate is shifted toward the periphery. That is, in this zone, the effect of “flow” is greater than the effect of “diffusion” on gas transportation and gas supplied from the “Very Edge” zone flows further outward. By increasing the amount of gas supplied from the “Edge” zone, which is at the inner side of the “Very Edge” zone, the etch rate at the “Very Edge” zone may be increased and in-plane uniformity of the in-plane depth may be improved as a result. 
     Although the present invention is described above with reference to certain illustrative embodiments, the present invention not limited to these embodiments but includes numerous variations and modifications that may be made without departing from the scope of the present invention. For example, a processing object to be etched by the plasma etching apparatus of the present invention is not particularly limited. In one specific example, a wafer made of a silicon substrate and having a silicon dioxide (SiO 2 ) film, an etching film made of a polysilicon film, a single-layer or multi-layer mask film, a BARC (Bottom Anti-Reflective Coating) film, and a photo resist film formed thereon may be used. In this case, the resist film may be exposed and developed beforehand to have a predetermined pattern formed thereon.