Abstract:
A gas distribution assembly for the ceiling of a plasma reactor includes a center fed hub and an equal path length distribution gas manifold underlying the center fed hub.

Description:
CROSS-REFERENCE TO REALATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/126,600, filed May 5, 2008. 
    
    
     BACKGROUND 
     In plasma processing of semiconductor wafers, precise feature profile control has become increasingly important during gate etching as the critical dimensions of semiconductor devices continue to scale down below 45 nm. For example, the integrity and critical dimension (CD) control of the hardmask during gate mask definition is critical in gate etch applications. For example, for a polysilicon gate, the hardmask layer overlying the polysilicon layer can be silicon nitride. For etching of the silicon nitride hardmask layer, the CD of greatest criticality is the mask length at the bottom of the hardmask. Likewise, for etching of the polysilicon gate, the CD of greatest criticality is the gate length at the bottom of the polysilicon gate. This length typically defines the all-important channel length of the transistor during later process steps. Therefore, during definition (etching) of the hardmask or of the polysilicon gate, it is important to minimize discrepancy between the required CD and the CD obtained at the end of the etch step. It is also important to minimize the variation in the CD bias, the difference between the CD as defined by the mask and the final CD after the etch process. Finally, it is important to minimize the CD bias microloading, which is the difference between the CD bias in regions in which the discrete circuit features are dense or closely spaced and the CD bias in regions in which the discrete circuit features are isolated or widely spaced apart. 
     Various conventional techniques have been used to meet these requirements. For instance, trial-and-error techniques have been used for determining the optimum gas flow rates for the various gas species in the reactor, the optimum ion energy (determined mainly by RF bias power on the wafer) and the optimum ion density (determined mainly by RF source power on the coil antenna). The foregoing process parameters affect not only CD, CD bias and CD bias microloading but also affect other performance parameters, such as etch rate and etch rate uniformity. It may not be possible to set the process parameters to meet the required performance parameters such as etch rate and at the same time optimize CD and minimize CD bias and CD bias microloading. As a result, the process window, e.g., the allowable ranges of process parameters such as chamber pressure, gas flow rates, ion energy and ion density, may be unduly narrow to satisfy all requirements. 
     A current problem is that CD bias is non-uniform, decreasing near the wafer edge. This problem is becoming more acute as device feature sizes are scaled down to 32 nm and smaller. Part of this problem is the sharp drop in CD bias at the wafer edge. We believe that this sharp drop is due to the lack of etch passivation species to passivate etch by-products. The amount of passivation species affects etch profile tapering and sidewall etch rate in high aspect ratio openings. Typically, the greater the amount of passivation gas present, the greater the etch profile tapering. What is desired is the etch profile or etch profile tapering be uniform across the wafer. This will promote a uniform distribution of CD bias. Because of the lack of passivation gas at the wafer edge, the etch profile taper is small at the wafer edge and large elsewhere. 
     SUMMARY 
     A ceiling gas distribution assembly is provided for use in a plasma reactor for processing a semiconductor substrate. The assembly includes a planar gas injection orifice plate including concentric inner, intermediate and outer annular zones of gas injection orifices. A central gas receiving hub overlies the orifice plate, the hub including three gas supply ports and having a hub bottom surface and three concentric hub channels formed in the hub bottom surface and internally coupled to respective ones of the three gas supply ports. A translation gas manifold underlies the hub and includes three sets of internal gas flow channels associated with the inner, intermediate and outer zones, respectively, each of the internal gas flow channels including: (a) a gas input opening at a top surface of the translation gas manifold in registration with a respective one of the concentric hub channels and (b) a gas output opening at a bottom surface of the translation gas manifold at a radius corresponding to a respective one of the inner, intermediate and outer zones. The assembly further includes an equal path length (EPL) manifold between the translation gas manifold and the orifice plate and providing gas flow paths of equal path lengths from the gas output openings of individual ones of the zones to the gas injection orifices of the same zones. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depicts a plasma reactor in accordance with a first embodiment. 
         FIGS. 2A ,  2 B and  2 C are different cross-sectional side views of a ceiling of the reactor of  FIG. 1  revealing a gas distribution assembly within the ceiling. 
         FIG. 2D  is a top view of a gas feed hub in the reactor of  FIG. 1 . 
         FIG. 2E  is an enlarged cross-sectional side view of a portion of the ceiling of the reactor of  FIG. 1 . 
         FIG. 3A  is a view of the bottom surface of an equal path length manifold in the gas distribution assembly of  FIGS. 2A-2C . 
         FIG. 3B  is an enlarged portion of the view of  FIG. 3A . 
         FIG. 4  is a view of the bottom surface of a gas distribution orifice plate in the gas distribution assembly of  FIGS. 2A-2C . 
         FIG. 5  depicts a plasma reactor in accordance with a second embodiment including a gas distribution assembly in the ceiling of the reactor. 
         FIG. 6  is a view of the bottom surface of an equal path length manifold in the gas distribution assembly of  FIG. 5 . 
         FIG. 7  is a bottom view of a gas distribution orifice plate in the gas distribution assembly of  FIG. 6 . 
         FIG. 8A  is an enlarged view of  FIG. 7 , illustrating an embodiment in which each individual orifice of  FIG. 7  consists of seven miniature orifices. 
         FIG. 8B  is a cross-sectional view corresponding to  FIG. 8A . 
         FIG. 9  is a flow diagram depicting a hard mask etch process in accordance with one embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     DETAILED DESCRIPTION 
       FIG. 1  depicts a plasma reactor for processing a workpiece or semiconductor wafer in accordance with a first embodiment. The reactor has a chamber  100  defined by a cylindrical sidewall  102 , a ceiling  104  and a floor  106 . An RF plasma source power applicator  108  is provided and may be an inductive coil antenna overlying the ceiling  104 . The coil antenna  108  may consist of an inner coil  112  and an outer coil  114  surrounding the inner coil. RF power to each of the coils  112 ,  114  may be independently controllable and may be furnished from a common power generator or (as depicted in  FIG. 1 ) from separate RF power generators  116 ,  118  coupled to the respective coils  112 ,  114  through respective impedance matches  120 ,  122 . The chamber is evacuated by a vacuum pump  124  through the floor  106 . A wafer support pedestal  126  supported at the floor  106  holds a workpiece  128  such as a semiconductor wafer. An RF plasma bias power generator  130  (or plural RF bias power generators of different frequencies) may be coupled through an impedance match  132  (or plural respective impedance matches) to an electrode  134  within the pedestal  126 . 
     In embodiments described below, the gas distribution apparatus within the ceiling  104  may distribute process gases in three gas distribution zones that receive process gas from three independent gas supply lines  141 ,  142 ,  143 . These three zones are, in one embodiment, annular concentric zones including inner, middle and outer zones. The gas mixtures and flow rates in each of the lines  141 ,  142 ,  143  may be independently controlled. For example, each line  141 ,  142 ,  143  may be supplied with process gas from a respective gas source  144 ,  145 ,  146 . As will be described below, the gas supply lines  141 ,  142 ,  143  supply process gas for injection in respective inner, middle and outer gas injection zones below the ceiling. The gas furnished by the gas supplies  144  and  145  to the inner and middle gas injection zones is, in one embodiment, a mixture of an etch species precursor gas and a passivation species precursor gas, and etch rate distribution across the wafer may be controlled by the ratio of the flow rates from the gas supplies  144 ,  145 . Gas furnished by the gas supply  146  to the outer gas injection zone may be a pure or nearly pure passivation species precursor gas, and radial distribution of CD bias or etch profile taper may be controlled by varying the gas flow rate from the gas supply  146 . This latter adjustment is independent or nearly independent of the adjustment of the etch rate distribution. Typically, the CD bias distribution is non-uniform because it decreases near the wafer edge, and uniformity is achieved by increasing the passivation species precursor gas flow rate to the outer gas injection zone. In this way, two etch performance parameters, namely (a) distribution of etch rate and (b) distribution of CD bias, are controlled simultaneously and nearly independently of one another in the reactor of  FIG. 1 . 
     The ceiling  104  in one embodiment includes a showerhead orifice plate  150  having an array of gas injection orifices  152  extending through it. In the illustrated embodiment of  FIG. 2A , the orifices  152  are located in three concentric radial zones, namely an inner zone  154 , an annular middle zone  156  and an annular outer zone  158 . A multipath lid  160  overlies the orifice plate  150 . A hub  170  may overlie the lid  160 . As depicted in  FIG. 2A through 2E , the hub  170  has three concentric channels  171 ,  172 ,  173  in its bottom surface  174 . The hub  170  further has three gas supply ports  175 ,  176 ,  177  coupled to the gas supply lines  141 ,  142 ,  143  respectively, the ports  175 ,  176 ,  177  being coupled to respective ones of the concentric channels  171 ,  172 ,  173 . Each channel  171 ,  172 ,  173  receives process gas from a particular one of the supply lines  141 ,  142 ,  143 . The hub  170  may have a passageway or hole (not shown) extending axially through the hub  170  to enable installation of an optical interferometric sensor for process end-point detection. 
     In the illustrated embodiment, the lid  160  consists of an equal path length manifold  162  whose top surface  162   b  contacts the hub  170 . Referring to  FIG. 2E , the equal path length manifold  162  has an array of equal path length channels  180 ,  190 ,  200  formed in its bottom surface  162   a . As shown in  FIG. 2E , the equal path length manifold  162  has a radial translation layer  164  overlying the equal path length channels  180 ,  190 ,  200 . The radial translation layer  164  has radial channels  220 ,  230 ,  240  providing communication between individual hub channels  171 ,  172 ,  173  and respective ones of the equal path length channels  180 ,  190 ,  200 , as will be described in greater detail below. The radial translation layer  164  and the equal path length manifold constitute an integral structure. Alternatively, they may be formed as separate pieces that are joined together. The equal path length channels  180 ,  190 ,  200  communicate between individual ones of the radial channels  220 ,  230 ,  240  and respective ones of the gas injection zones  154 ,  156 ,  158 . The cross-sectional side views of  FIGS. 2A ,  2 B and  2 C are taken at different angles around the axis of symmetry to reveal different internal features. In the view of  FIG. 2A , the communication between the inner hub channel  171  and the inner gas injection zone  154  is exposed. In the view of  FIG. 2B , the communication between the middle hub channel  172  and the middle gas injection zone  156  is exposed. In the view of  FIG. 2C , the communication between the outer hub channel  173  and the outer gas injection zone  158  is exposed. 
       FIGS. 3A and 3B  are top views of equal path length manifold (EPLM)  162  showing the different equal path length channels  180 ,  190 ,  200  correspond to three different groups or types of gas flow channels, namely the inner zone channels  180 , the middle zone channels  190  and the outer zone channels  200 . In the implementation of  FIGS. 3A and 3B , there are eight inner zone channels  180 , eight middle zone channels  190  and eight outer zone channels  200 , the channels of each type being azimuthally distributed in periodic fashion. 
       FIG. 4  is a bottom view of the gas distribution orifice plate  150  showing how the plural gas injection orifices  152  may be grouped in different circular zones corresponding to the inner, middle and outer zones  154 ,  156 ,  158  referred to above, including a set of inner zone orifices  152   a , first and second sets of middle zone orifices  152   b - 1 ,  152   b - 2 , and first and second sets of outer zone orifices  152   c - 1 ,  152   c - 2 . A subset of the overlying equal path length channels  180 ,  190 ,  200  is depicted in hidden line in  FIG. 4  to show their alignment with the various orifices  152 . 
     In the illustrated embodiment of  FIGS. 3B and 4 , each of the eight inner zone channels  180  consists of a pair of legs  181 ,  182  forming an acute angle and joined together at an apex  183  from which the legs  181 ,  182  radiate toward terminations  184 ,  185 . A gas inlet hole  186  extends from the apex  183  to the opposite (top) surface  162   b  ( FIG. 2E ) of the EPLM  162 . Each termination  184 ,  185  is aligned with a corresponding one of the orifices  152   a  of the inner zone  154  of the orifice plate  150 . In this manner, each of the orifices  152   a  of the inner zone  154  is aligned with one of the terminations  184 ,  185  of the eight inner zone channels  180 . 
     Referring again to  FIGS. 3B and 4 , each of the middle zone channels  190  consists of a radial main leg  191  extending from an apex  192  and terminating in the middle of a transverse leg  193  forming a “T” with the main leg  191 , the two ends of the transverse leg  193  terminating in the middle of each of respective radial legs  194 - 1 ,  194 - 2 , each of the radial legs  194 - 1 ,  194 - 2  having a radially inward end  195  and a radial outward end  196 , each radial leg  194 - 1 ,  194 - 2  terminating in the middle of a transverse leg  197  at its radially outward end  196  to form a “T”. Each transverse leg has a pair of opposite ends  198 - 1 ,  198 - 2 . A gas inlet hole  199  extends from the apex  192  to the opposite (top) surface  162   b  ( FIG. 2A ) of the EPLM  162 . The first set of orifices  152   b - 1  in the middle zone  156  of the orifice plate  150  face the channel ends  195 . The second set of orifices  152   b - 2  of the middle zone  156  face respective ones of the channel ends  198 - 1 ,  198 - 2 . 
     Referring yet again to  FIGS. 3B and 4 , each of the outer zone channels  200  consists of a radial main leg  201  extending from an apex  202  and terminating in the middle of a transverse leg  203  forming a “T” with the main leg  201 , the two ends of the transverse leg  203  terminating in the middle of each of respective radial legs  204 - 1 ,  204 - 2 , each of the radial legs  204 - 1 ,  204 - 2  extending radially to a radial outward end  206 , each radial leg  204 - 1 ,  204 - 2  terminating in the middle of a transverse leg  207  at its radially outward end  206  to form a “T”. Each transverse leg  207  has a pair of opposite ends  208 - 1 ,  208 - 2  terminating in the middle of each of respective radial legs  210 . Each radial leg  210  has a pair of opposite termination ends  211 ,  212 . Each outer channel  200  has a total of four channel ends  211  and four channel ends  212 . A gas inlet hole  209  extends from the apex  202  to the opposite (top) surface  162   b  ( FIG. 2E ) of the EPLM  162 . The first set of orifices  152   c - 1  in the outer zone  158  of the orifice plate  150  face the channel ends  211 . The second set of orifices  152   c - 2  of the outer zone  158  face the channel ends  212 . 
     In accordance with one feature, the array of channels  180 ,  190 ,  200  in the bottom surface  162   a  of the EPLM manifold  162  are configured so that the distances traveled within the EPLM  162  by process gas to different orifices within inner zone  154  are uniform. In the illustrated embodiment, the distances traveled within the EPLM  162  by process gas to different orifices  152  within the middle zone  156  are uniform. In this same embodiment, the distances traveled within the EPLM  162  by process gas to different orifices  152  within the outer zone  158  are uniform. Another feature is that the arc distances subtended by the various equal path length channels within the EPLM are all not more than fractions of a circle, which prevents or minimized inductive coupling to the gases therein. 
     Referring to  FIGS. 2A-2E , the radial translation layer  164  of the EPLM  162  provides the gas communication from the inner, middle and outer concentric channels  171 ,  172 ,  173  of the hub  170  to the inner zone, middle zone and outer zone gas inlets  186 ,  199 ,  209  of the EPLM  162 . Specifically, the radial translation layer  164  provides gas communication between the inner hub channel  171  and the inner zone gas inlets  186  through the radial channels  220 , between the middle hub channel  172  and the middle zone gas inlets  199  through the radial channels  230 , and between the outer hub channel  173  and the outer zone gas inlets  209  through the radial channels  240 . 
     As shown in  FIGS. 2A through 2E , the radial translation layer  164  may have its plural inner zone channels  220  tilted at a first acute angle A relative to the axis of symmetry. Each inner zone axial channel  220  has a first end  221  open at the top surface  162   b  and facing the inner concentric hub channel  171 . Each inner zone axial channel  220  further has a second end in registration with one of the inner zone gas inlets  186  of the EPLM  162 . In this manner, eight inner zone axial channels  220  provide gas flow from the inner hub channel  171  to the eight inner zone gas inlets  186  of the EPLM  162 . 
     The radial translation layer  164  may have its plural middle zone axial channels  230  tilted at a second acute angle B relative to the axis of symmetry. In the illustrated embodiment, each middle zone axial channel  230  may have a first end  231  open at the top surface  162   b  and facing the middle concentric hub channel  172 . Each middle zone axial channel  230  further may have a second end in registration with one of the middle zone gas inlets  199  of the EPLM  162 . In this manner, eight middle zone axial channels  230  may provide gas flow from the middle hub channel  172  to the eight middle zone gas inlets  199  of the EPLM  162 . 
     The radial translation layer  164  may have its plural outer zone axial channels  240  tilted at a third acute angle C relative to the axis of symmetry. Each outer zone axial channel  240  has a first end  241  open at the top surface  162   b  and facing the outer concentric hub channel  173 . Each outer zone axial channel  240  further may have a second end in registration with one of the outer zone gas inlets  209  of the EPLM  162 . In this manner, eight outer zone axial channels  240  may provide gas flow from the outer hub channel  173  to the eight outer zone gas inlets  209  of the EPLM  162 . 
     The first, second and third acute angles A, B, C may be progressively smaller to accommodate the different radial locations of the inner zone gas inlets  186 , the middle zone gas inlets  199  and the outer zone gas inlets  209 . In the implementation of  FIGS. 1-3 , the radial distance of the middle and outer zone gas inlets  199 ,  209 , from the axis of symmetry are the same so that the second and third acute angles B and C are nearly the same. The middle and outer zone gas inlets  199 ,  209  have different azimuthal locations in alternating sequence, as shown in the drawings. 
       FIGS. 5 and 6  depict another embodiment employing an EPLM manifold  462  and an orifice plate  450 . In  FIG. 5 , the three gas supply lines  141 ,  142 ,  143  are coupled directly to the EPLM manifold  462 . An advantage of the embodiment of  FIGS. 5 and 6  is that the hub  170  and radial translation layer  164  of  FIG. 1  are eliminated. 
     In the illustrated embodiment of  FIGS. 5 and 6 , the bottom surface of the EPLM  462  has gas distribution channels including inner, middle and outer zone gas input channels  301 ,  302 ,  303  coupled to the gas supply lines  141 ,  142 ,  143 , respectively. The gas input channels  301 ,  302 ,  303  may be formed in a radial extension  464  of the circular EPLM  462 . Gas connections (not shown) are provided at the outer terminations of the channels between the gas supply lines  141 ,  142 ,  143  and respective ones of the input channels  301 ,  302 ,  303 . 
     In the illustrated embodiment of  FIGS. 5 and 6 , the inner zone input channel  301  in the extension  464  merges with a radial supply channel  305  within the main circular portion of the manifold  462 . The radially inward termination of the supply channel  305  is coupled to the middle of a half-circular channel  310 . The opposite ends of the half-circular channel  310  are coupled to the middle of a respective quarter-circular channel  314  through respective radial short transition channels  312 . Each of the opposite ends or terminations of the quarter-circular channels  314  is coupled through a respective short radial transition channel  316  to the middle of a respective arcuate channel  318  having opposite first and second ends or terminations  318   a ,  318   b . The terminations  318   a ,  318   b  may have a common radial location as shown in  FIG. 6 , and are aligned with respective ones of a set of inner zone orifices  452 - 1  of the orifice plate  450  shown in  FIG. 7 . 
     In the illustrated embodiment of  FIGS. 5 and 6 , the middle zone input channel  302  in the extension  464  merges with a radial supply channel  306  within the main circular portion of the manifold  462 . The radially inward termination of the supply channel  306  is coupled to one end of a half-circular channel  332 . The opposite end of the half-circular channel  332  is coupled through a short radial transition channel  334  to the middle of a half-circular channel  336 . The opposite ends of the half-circular channel  336  are each coupled through a respective short radial transition channel  338  to the middle of a respective quarter-circular channel  340 . Each of the opposite ends or terminations of the quarter-circular channels  340  is coupled through a respective short radial transition channel  342  to the middle of a respective arcuate channel  344 . Each of the opposing ends or terminations of the arcuate channels  344  is coupled through a respective short radial transition channel  346  to the middle of a respective arcuate channel  348  having opposite first and second ends or terminations  348   a ,  348   b . The terminations  348   a ,  348   b  may have a common radial location as shown in  FIG. 6 , and are aligned with respective ones of a set of middle zone orifices  452 - 2  of the orifice plate  450  shown in  FIG. 7 . 
     In the illustrated embodiment of  FIGS. 5 and 6 , the outer zone input channel  303  in the extension  464  merges with one end of an outer half-circular supply channel  360  within the main circular portion of the manifold  462 . The opposite end or termination of the outer supply channel  360  is coupled radially inwardly through a short radial transition channel  362  to the middle of an inner half-circular channel  364  concentric with and inside the radius of the outer supply channel  360 . Each one of the opposite ends of the half-circular channel  364  is coupled radially inwardly through a respective short radial transition channel  366  to the middle of a respective quarter-circular channel  368 . The quarter-circular channel  368  is encircled by the half-circular channel  364 . Each opposite end of each quarter-circular channel  368  is coupled through a respective short radial transition channel  370  to the middle of a respective arcuate channel  372 . Each of the opposite ends or terminations of the arcuate channels  372  is coupled through a respective short radial transition channel  374  to the middle of a respective arcuate channel  376 . Each of the opposing ends or terminations of the arcuate channels  376  is coupled through a respective short radial transition channel  378  to the middle of a respective arcuate channel  380  having opposite first and second ends or terminations  380   a ,  380   b . The terminations  380   a ,  380   b  may have a common radial location as shown in  FIG. 6 , and are aligned with respective ones of a set of outer zone orifices  452 - 3  of the orifice plate  450  shown in  FIG. 7 . 
     Referring to  FIGS. 8A and 8B , each of the orifices  452  in one embodiment may form a single hole or opening in the top surface  450 a of the orifice plate, but branch radially outwardly into seven smaller holes  453 - 1 ,  453 - 2 ,  453 - 3 ,  453 - 4 ,  453 - 5 ,  453 - 6  and  453 - 7  in the bottom surface  450   b  of the orifice plate.  FIG. 8A  depicts this feature in the group of inner zone orifices  452 - 1 . 
       FIG. 9  is a flow diagram depicting a process in accordance with one embodiment that can be carried out in the reactor of  FIG. 1  (or in the reactor of  FIG. 5 ). The process of  FIG. 9  begins by flowing a first process gas mixture of an etchant species precursor gas and a passivation species precursor gas to an annular inner zone of gas dispersers in the ceiling at a first flow rate (block  610  of  FIG. 9 ). The process includes flowing a second process gas mixture of an etchant species precursor gas and a passivation species precursor gas to an annular middle zone of gas dispersers in the ceiling surrounding the inner zone at a second flow rate (block  615 ). The process further includes flowing a process gas which is a pure or nearly pure passivation species precursor gas to an annular outer zone of gas dispersers in the ceiling surrounding the middle zone at a third flow rate (block  617 ). RF plasma source power is applied at first and second independently controlled power levels to respective inner and outer coil antennas overlying the ceiling (block  620 ). The radial distribution of etch rate across the entirety of the wafer is obtained by controlling the ratio of the first and second power levels in the inner and outer coil antennas and (or, alternatively) by controlling the ratio of the inner and outer zone (first and second) gas flow rates (block  625 ). Uniformity of the radial distribution of either etch critical dimension (CD) bias or etch profile taper is controlled by controlling the third flow rate, i.e., the flow rate of the passivation species precursor gas to the third gas injection zone (block  630 ). 
     The process may be applied to etching a silicon nitride or silicon oxide hard mask prior to a gate etch step. In this case the etchant species precursor may be CF 4  and the passivation species precursor may be CHF 3 . In general, the etchant species precursor gas is a fluorocarbon (i.e., a species containing no hydrogen) while the passivation species precursor gas is a fluoro-hydrocarbon (i.e., a species containing a significant proportion of hydrogen). More generally, the etchant species precursor gas contains a high proportion of fluorine and a low proportion (less than a few percent atomic ratio) or zero amount of hydrogen, while a significant fraction (20% atomic ratio) of the passivation species is hydrogen. The gas mixtures flowed to the inner and middle zones may be identical, while their flow rates are different and independently controlled. 
     The etch critical dimension (CD) bias and the etch profile taper tend to be less at the wafer edge. In order to improve uniformity of radial distribution of either or both the CD bias and the etch profile tapering, the third gas flow rate (the flow rate of the pure passivation species precursor gas to the outer zone of gas dispersers) is increased until the nonuniformity in distribution of CD bias or profile taper has been minimized. An overcorrection that raises the CD bias or etch profile taper at the wafer edge above the average value across the wafer requires a corresponding reduction in the pure passivation species precursor gas in outer zone of gas dispersers. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.