Abstract:
A plasma etch reactor for plasma enhanced etching of a workpiece such as a semiconductor wafer includes a housing defining a process chamber, a workpiece support configured to support a workpiece within the chamber during processing and comprising a plasma bias power electrode. The reactor further includes a first process gas inlet coupled to receive predominantly or pure oxygen gas and a second process gas inlet coupled to receive a polymerizing etch process gas. The reactor has a ceiling plasma source power electrode including a center circular gas disperser configured to receive a process gas from the first process gas inlet and to distribute the process gas into the chamber over the workpiece, and an inner annular gas disperser centered around the center gas disperser configured to receive the process gas from the second process gas inlet and to distribute the process gas into the chamber over the workpiece through an inner plurality of injection ports.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/793,785, filed Apr. 21, 2006 entitled PLASMA ETCH REACTOR WITH DISTRIBUTION OF ETCH GASES ACROSS A WAFER SURFACE AND A POLYMER OXIDIZING GAS IN AN INDEPENDENTLY FED CENTER GAS ZONE, by Jong Moon Kim, et al. 
    
    
     BACKGROUND OF THE INVENTION 
     Fabrication of integrated circuits with feature sizes on the order of nanometers requires etch processes that are extremely uniform across the entire surface of a semiconductor wafer, typically about 300 millimeters in diameter. Such uniform etch processes are typically realized in a plasma etch reactor such as a capacitively coupled plasma reactor with an overhead electrode having a high aspect ratio (e.g., an electrode-to-wafer gap of only 2.5 cm for a 300 mm wafer). The etch process gas, which may be a fluorocarbon or fluorohydrocarbon gas, is of the type that forms a protective polymer layer over photoresist or other thin film surfaces that are not to be etched. Such a protective thin film also forms on the opening sidewalls formed during the etch process. Formation of such a protective polymer layer enhances etch selectivity and provide the means for etch profile control. 
     Such processes exhibit a low etch rate and/or tapered etch profile (due to excess sidewall polymer) at the center of the wafer and a high etch rate at the wafer periphery. Such a nonuniform radial distribution of etch rate and profile across the wafer surface has appeared to be unavoidable for a number of reasons. First, the process gas is introduced either from the side of the wafer or over the top of the wafer. Evacuation of gas from the reactor chamber requires the gas to flow in a radially outward direction across the surface of the wafer, so that gases fed to the wafer center pass over the wafer periphery before being evacuated. Therefore, the residency time of the gas increases with wafer radius, so that the minimum residency time (and hence the minimum gas dissociation) occurs at the wafer center. This effect is particularly pronounced due to the high aspect ratio of the reactor chamber. This aspect ratio arises from the small electrode-to-wafer gap (e.g., about 2 cm) and the large wafer diameter (e.g., 300 mm). The low dissociation of plasma ions at the wafer center results in more complex (e.g., more carbon-rich) fluorocarbon or fluorohydrocarbon plasma species at the wafer center, which tend to etch dielectric material on the wafer more slowly while depositing etch-stopping polymer more quickly on the bottom floors and side walls of the etched openings or structure, thereby depressing the etch rate at the wafer center and tapering the etch profile. In contrast, the high dissociation of plasma ions at the wafer edge produces simpler (more active) etch species that are relatively high in fluorine content (the extreme example of such reactive species being free fluorine) and produce low-carbon content polymer films that accumulate more slowly on photoresist surfaces. At the wafer center, the effect of low dissociation is most noticeable when etching extremely narrow openings through a dielectric thin film. At the wafer center, the polymer accumulates on the side walls of the narrow openings and eventually, as the opening depth increases, pinches off the opening and stops further etching of the opening before the required depth is reached, a phenomenon referred to as etch stop. 
     These problems are exacerbated when attempting to increase the etch rate by the expedient of increasing the process gas flow rate into the reactor chamber. Such an increase in gas flow rate does not solve the problem of non-uniform residency time across the wafer surface (and hence the non-uniform dissociation across the wafer surface), and may even worsen the problem, thereby worsening the etch rate non-uniformity. One approach to improving the etch rate at the wafer center might seem to be decreasing the process gas flow rate over the wafer center or increasing it at the wafer periphery (or both). However, such a technique is not enough since the polymer composition distribution across the wafer surface is not significantly changed. Thus, there appears to be no solution to the problem. 
     Another reason for such problems is the process gas content. Such problems have not been pronounced for many processes involving fluorocarbon or hydrocarbon etch process gases in a plasma etch reactor chamber. However, we have found that these problems arise with great severity when using particular combinations of such gases that otherwise produce the best possible process results, such as (for example) a process gas that has, for its etchant component, C4F6+CH2F2. Another example can include CHF3 and/or CF4. These processes gas have been found to produce desired results (with the exception of the tendency for etch stop discussed above) when etching such dielectric materials as silicon dioxide or silicon nitride or low dielectric constant materials such as porous organo-silicate glass or nitrogen-doped silicon-carbon compounds, for example. Using other process gases compromises etch performance when etching such materials as silicon dioxide, silicon nitride, porous organo-silicate glass or nitrogen-doped silicon-carbon compounds. It has seemed that the only way of avoiding center-low etch rate distribution or the related etch stop problems is to employ other (less desirable) process gas mixtures. 
     Another cause for the center-low etch rate distribution across the wafer surface arises in a particular type of capacitively coupled etch reactor. In the beginning, a capacitively coupled etch reactor employed a single RF bias power supply coupled to the wafer. In such a reactor, the etch rate could only be increased (to enhance productivity) by increasing the RF power. Such an increase unfortunately increases the ion energy, causing more bombardment damage to photoresist and thereby reducing etch selectivity. This problem was circumvented by introducing magnetic fields at the sides of the chamber (in lieu of increasing the RF power) to improve the etch rate, in which case the reactor is called a magnetically enhanced reactive ion etch (MERIE) reactor. This approach was successful in improving the etch rate without damaging the photoresist or reducing the etch selectivity. The magnetic field boosts the etch rate by increasing ion dissociation. Recently, the RF source power has been decoupled from the ion energy by applying VHF source power that contributes primarily (or almost exclusively) to ion density while applying independently a low frequency (or HF frequency) bias power that contributes primarily (or almost exclusively) to ion energy. This permits ion density to be increased, without increasing ion bombardment damage to photoresist, by increasing the VHF source power without increasing the lower frequency bias power. Nevertheless, even with such dual or triple frequency approaches for decoupling control of ion density and ion energy, MERIE magnets are found to be an essential feature for enhancing etch performance. The problem is that the MERIE magnetic fields tend to have their greater etch rate-enhancing effect near the wafer edge. This produces a center-low etch rate distribution across the wafer surface, which has seemed to be an unavoidable characteristic of MERIE reactors. Typically, the MERIE reactor also suffers from the effects (discussed above) of low gas residency time over the wafer center, that causes center-low etch rate distribution. The relatively high dissociation achieved in such a reactor, through the use of VHF source power and MERIE magnets makes the non-uniformity of the dissociation (due to non-uniform gas residency time across the wafer) more critical. 
     SUMMARY OF THE INVENTION 
     A plasma etch reactor for plasma enhanced etching of a workpiece such as a semiconductor wafer includes a housing defining a process chamber, a workpiece support configured to support a workpiece within the chamber during processing and comprising a plasma bias power electrode. The reactor further includes a first process gas inlet coupled to receive predominantly or pure oxygen gas and a second process gas inlet coupled to receive a polymerizing etch process gas. The reactor has a ceiling plasma source power electrode including a center circular gas disperser configured to receive a process gas from the first process gas inlet and to distribute the process gas into the chamber over the workpiece, and an inner annular gas disperser centered around the center gas disperser configured to receive the process gas from the second process gas inlet and to distribute the process gas into the chamber over the workpiece through an inner plurality of injection ports. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are side and top views of a plasma reactor having features for carrying out a process of the invention. 
         FIG. 1C  depicts a gas flow splitter employed in the reactor of  FIGS. 1A and 1B . 
         FIGS. 2A and 2B  depict center and edge etch profiles obtained in a conventional process and obtained in a process of the invention, respectively. 
         FIGS. 3 and 4  depict different gas distribution apparatus for the reactor of  FIG. 1A  for carrying out different embodiments of a process of the invention. 
         FIGS. 5A and 5B  depict oxide and photoresist etch rate radial distributions obtained in a first embodiment of the invention. 
         FIGS. 6A and 6B  depict oxide and photoresist etch rate radial distributions obtained in a second embodiment of the invention. 
         FIG. 7  is a graph depicting the effect of MERIE magnetic fields upon etch rate distribution. 
         FIG. 8  is a graph of gas distribution patterns in a first embodiment of the invention. 
         FIG. 9  is a graph of gas distribution patterns in a second embodiment of the invention. 
         FIGS. 10A ,  10 B and  10 C depict successively improved etch rate distributions obtained in accordance with successive steps for adjusting the etch process in accordance with an aspect of the invention. 
         FIG. 11  is a block flow diagram of a first embodiment of a process of the invention. 
         FIG. 12  is a block flow diagram of a second embodiment of a process of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention solves the problem of center-low etch rate and tapered etch profile at the wafer center that has plagued capacitively coupled plasma etch processes for etching dielectric thin films. The invention is based upon our surprising discovery that the problem can be completely solved without requiring any increase in etch process gas flow rate over the wafer center. It is further based upon our discovery that removing or slowing deposition of polymer on sidewalls of narrow openings at the wafer center during the etch process is the only effective solution, rather than attempting to change the etch gas flow rate at the wafer center. Finally, the invention is based upon our surprising discovery that a gas such as oxygen which does not etch dielectric material, but which does reduce the amount of deposited polymer, is an essential part of this solution. Such a gas is referred to herein as a polymer-retarding gas. The invention is realized by providing a novel independent gas injection orifice or zone at the center of the overhead electrode that is supplied with the polymer retarding gas (pure oxygen) with a flow rate adjustment independent of the polymerizing etch process gas flow to the inner/outer gas zones of the ceiling electrode/gas showerhead. The purpose of this new feature is to inject the polymer retarding gas (pure oxygen) primarily (or exclusively) onto the wafer center, without necessarily having to alter the gas distribution of the etchant (fluorocarbon or fluorohydrocarbon) gases in the chamber. The surprising result is that the additional gas flow over the wafer center (i.e., the added oxygen gas), which would be expected to reduce etch rate at the wafer center by reducing gas residency time, actually increases etch rate as high as desired, depending upon the oxygen gas flow rate to the center. In fact it was discovered that the dielectric etch rate at the wafer center can be increased to a level higher than any other location on the wafer, if desired, by simply increasing the oxygen flow (influx) rate at the wafer center. Thus, the effect of the oxygen gas in suppressing polymer growth at the wafer center during etch (thereby solving the etch-stop problem) outweighs the effect of increased gas flow on residency time or dissociation. Another advantage is that the photoresist etch rate (caused by ion bombardment during the dielectric etch operation) can be made more uniform by adjusting the polymer retarding gas (oxygen) flow rate over the wafer center. 
       FIG. 1A  depicts a capacitively coupled plasma reactor that includes features for carrying out the process of the invention. The reactor of  FIG. 1A  includes a chamber  100  defined by a cylindrical side wall  105  supporting an overhead electrode  110 . The overhead electrode  110  is both an electrode and a gas distribution showerhead having an outer annular zone  115  of gas injection orifices  116  in the electrode bottom surface  110   a  fed from a common outer manifold  117  inside the electrode  110 , and an inner annular zone of  120  of gas injection orifices  121  in the electrode bottom surface  110   a  fed from a common inner manifold  122  inside the electrode  110 . 
     An electrostatic chuck (ESC)  125  supports the semiconductor wafer  130  that is to be etched. The ESC  125  has an insulating layer  126  containing a chucking electrode or grid  127  and a lower conductive base layer  129  that may be cooled. The ESC surface supporting the wafer is about one inch below the ceiling electrode bottom surface  110   a , which establishes a high aspect ratio for a large (300 mm) wafer diameter. A D.C. chuck voltage supply  128  is connected to the ESC grid  127 . Plasma bias power is applied by a high frequency (HF) RF generator  135  and/or a low frequency (LF) RF generator  140  through an impedance match circuit  145  through an isolation capacitor  146  to the ESC grid  127 . Plasma source power is applied by a very high frequency (VHF) RF generator  150  through an impedance match element  155  to the ceiling electrode  110 . 
     The ESC  125  and the side wall  105  define a pumping annulus  106  between them through which gas is evacuated from the chamber interior by a vacuum pump  107 . A throttle valve  108  regulates the evacuation rate and chamber pressure. 
     In one type of such a reactor, the impedance match element  155  is a coaxial tuning stub. This type of reactor is disclosed in U.S. Pat. No. 6,838,635 by Daniel Hoffman, et al. and assigned to the present assignee. Preferably, the tuning stub has a resonant frequency near the frequency of the VHF generator  150  which is near a resonant frequency formed by plasma in the chamber  100  and the ceiling electrode  110 . For this purpose, the ceiling electrode reactance may be tuned to provide a resonance with the plasma near the VHF generator frequency. 
     In another type of capacitively coupled reactor, the impedance match element  155  is a conventional circuit. In either type of reactor, etch performance can be enhanced by magnetically enhanced reactive ion etch (MERIE) electromagnets  161 ,  162 ,  163 ,  164  (shown in  FIG. 1B ) disposed around the sidewall  105  and driven by successively phased low frequency currents to produce a slowly rotating magnetic field in the chamber  100 . The currents are provided by an MERIE current source  160 . The sidewall  105  may be formed of a non-magnetic conductor such as aluminum. 
     A process gas supply  170  furnishes a process gas that includes, as its main component, polymerizing etch gases (e.g., a fluorocarbon and/or fluorohydrocarbon gases) to a flow ratio controller (FRC)  175  which apportions the gas flow to the inner and outer gas manifolds  122 ,  117  of the inner and outer gas orifices  120 ,  115  of the ceiling electrode  110 . (The FRC  175  is depicted in  FIG. 1C  and consists of a gas flow divider or splitter  176   b  and a pair of control valves  177 ,  178  coupling the splitter  176  to the two manifolds  117 ,  122  respectively.) 
     Conventionally, the problem of low etch rate or etch stop at the wafer center was addressed by decreasing process gas flow to the inner zone  120  or increasing gas flow at the outer zone  115 . Such an expedient is supposed to increase dissociation at the wafer center and/or reduce it at the edge. However, the change in dissociation at the wafer center (for example) is not enough to overcome the non-uniform distribution of polymer composition (i.e., carbon-rich polymer at the center and carbon-lean polymer at the edge). The problem is observed in the etch profile of a deep narrow opening depicted in  FIG. 2A . It is desired to form the same identical opening at both the wafer center (left side of  FIG. 2A ) and at the wafer periphery (right side of  FIG. 2A ). However, as described above, the gas residency time and (hence) the dissociation are much greater at the wafer periphery, so that more active species (simpler compounds of fluorine) are present at the wafer edge, thereby slowing the polymer deposition at the wafer periphery and preventing it from filling in the narrow opening before the opening is etched to the desired depth and dimension. The result at the wafer edge is a top critical dimension (TCD) at the top of the opening and a bottom critical dimension (BCD) at the bottom of the opening that are within the desired range, the opening being of the desired depth (right side of  FIG. 2A ). In contrast, at the wafer center (left side of  FIG. 2A ), the high carbon content of the polymer deposited on the side wall of the high aspect ratio opening causes the opening to be very narrow at its bottom, forming a BCD that is too small, and the hole depth to be insufficient, due to etch stop at the wafer center. 
     The problem is solved by flowing pure oxygen gas to the wafer center through a center gas injection zone  180 . The center gas injection zone  180  may be a single gas injection orifice (or plural orifices), in the center of the ceiling electrode  110 . The center gas injection zone  180  receives the oxygen gas from an oxygen gas supply  185  that is independent or separate from the polymerizing etch gas supply  170 . A control valve  190  controls the flow rate of the oxygen gas to the center gas injection zone  180 .  FIG. 2B  depicts the successful results obtained using the center gas injection zone  180 , in which the deep narrow opening at the wafer center (left side of  FIG. 2B ) has the approximately same BCD and depth as the narrow opening at the wafer periphery (right side of  FIG. 2B ). The flow rate of the pure oxygen through the center gas injection zone  180  is increased using the control valve  190  until reaching a threshold at which the etch process produces a BCD at the wafer center is about the same as the BCD at the wafer edge. This threshold is determined prior to performing the process by etching successive test wafers at successively different flow rates of oxygen gas through the center zone  180  and measuring the BCD&#39;s at the center and periphery of each of the successive wafers, and noting which one had little or no deviation in BCD between the wafer periphery and center. Alternatively (or in addition), anyone or all of the following measurements may be taken to compare the following etch parameters between the wafer periphery and center: etch profile, etch depth, bottom critical dimension (BCD). The oxygen flow rate threshold deduced from the data is one that renders uniform the radial distribution of any one or a combination of BCD, etch depth or etch profile, depending upon the measurements selected. Using the threshold oxygen flow rate determined in this manner, non-uniformities are eliminated (or reduced) in the invention. An additional advantage is the elimination of excess over-etching near the wafer edge, which (prior to the present invention) was a necessary consequence of extending the etch process time sufficiently to gain a slight increase in BCD at the wafer center. The overall production throughput is therefore increased. 
     In one successful implementation, it was unnecessary to flow process gas to the outer zone  115 , so that only the inner zone  120  and the center gas injection zone  180  were used. 
     In the implementation depicted in  FIG. 3 , some of the oxygen from the oxygen supply  185  may be diverted through a valve  200  to the inner gas zone  120 . 
     In the implementation of  FIG. 4 , some of the oxygen from the oxygen supply  185  may be diverted through a valve  205  to the outer gas zone  115  (in addition to being diverted to the inner gas zone  120 ). 
     Thus, in the implementation of  FIG. 3  or  FIG. 4 , oxygen gas flow rate radial distribution can be broadened or narrowed, as desired, to increase or decrease the oxygen gas flow rate to the inner and/or outer gas zones  120 ,  115 . Specifically, in each of the inner and outer gas injection zones  120 ,  115 , a different ratio between the polymerizing etch process gas and the pure oxygen gas may be selected to optimize uniformity of the radial distribution of a selected etch parameter (BCD, etch depth or etch profile). In one embodiment, the oxygen gas is injected through only the inner and outer annular gas injection zones  120 ,  115  and none through the center orifice  180 . In this case, the increase in oxygen gas flow rate sufficient to render uniform the radial distribution of the selected etch parameter (BCD, etch depth, etch profile) is carried out in the inner annular gas injection zone  120 . This latter step has the effect of increasing the ratio of oxygen gas to polymerizing etch process gas in the inner gas injection  120  zone over the same ratio in outer gas injection zone  115 . We have found that this increase varies depending upon the reactor design, the composition of the thin film being etched, the process recipe, RF power levels, etc., and therefore can only be determined empirically by the etching of successive test wafers at different oxygen flow rates. Such an empirical determination may be carried out by initially setting the oxygen flow rates in the inner and outer gas injection zones  120 ,  115  to the same level. For each test wafer, the inner zone oxygen gas flow rate is increased to a different level, the etch process carried out, and the selected etch parameter measured at center and periphery of that wafer. We have found that the increase in oxygen flow rate to the inner gas injection zone  120  relative to the flow rate in the outer gas injection zone  115  sufficient to reach process uniformity (of the selected etch parameter) is readily determined, and corresponds to a definite ratio between the final (higher) inner zone oxygen flow rate and the outer zone oxygen flow rate. 
       FIG. 5A  is a graph qualitatively comparing radial distributions of dielectric etch rates obtained in plasma etch processes with and without the invention. The results of  FIG. 5A  were obtained by etching a dielectric thin film layer of silicon dioxide in a capacitively coupled plasma reactor of the type depicted in  FIG. 1A . The curve labeled  300  depicts a typical silicon dioxide etch rate as a function of radial location on the wafer in the case in which a polymerizing etch gas is fed through the inner zone gas injection orifices  120  with no oxygen gas (or any other gas) fed through the center gas injection zone  180 . The curve  300  indicates that the etch rate is depressed at the wafer center, as expected in such a conventional capacitively coupled etch process. The curve labeled  305  depicts the oxide etch rate as a function of radial location under the same process conditions in which the curve  300  was obtained, with the addition of a modest flow rate of oxygen gas at the center gas injection zone  180 . The curve  305  indicates that the oxygen gas injected through the center gas injection zone  180  causes the etch rate at the wafer center to increase somewhat, although the etch rate remains somewhat depressed at the wafer center. The curve labeled  310  depicts the oxide etch rate as a function of radial location under the same process conditions but with a significantly higher oxygen gas flow rate through the center gas injection zone  180 . The curve  310  indicates that the higher oxygen flow rate through the center orifice is sufficient to raise the etch rate at the wafer center to at least nearly equal the etch rate near the wafer periphery, so that the etch rate distribution is far more uniform. The curve  315  depicts the oxide etch rate as a function of radial location under the same process conditions except that the oxygen gas flow rate through the center gas injection zone  180  is increased even further (above that used to obtain the curve  310 ). The curve  315  indicates that the etch rate radial distribution is center high when the oxygen gas flow rate through the center gas injection zone is very high. These results indicate that the user may increase the oxygen gas flow rate through the center zone  180  to an ideal level at which the most uniform etch rate radial distribution is attained, corresponding to the curve  310 . The uniform dielectric etch rate distribution of the curve  310  corresponds to the ideal etch profile distribution depicted in  FIG. 2B  in which the bottom critical dimension (BCD) at the wafer center is nearly as great as the BCD at the wafer edge. 
     The dielectric plasma etch process is performed with a photoresist mask overlying the dielectric film, and the photoresist mask is etch by ion bombardment in the plasma (simultaneously with the etching of the dielectric thin film layer), which removes some of the photoresist during the etch step. As long as this removal merely reduces the photoresist mask thickness without removing the mask, it is acceptable. However, because partial photoresist removal can affect TCD and BCD, it is desirable that the photoresist etch rate have a radial distribution that is as uniform as possible.  FIG. 5B  is a graph depicting the photoresist etch rate distribution obtained in the process conditions corresponding the dielectric etch rate curves  300 ,  305 ,  310  and  315  of  FIG. 5A . The curves labeled  300   a ,  305   a ,  310   a  and  315   a  of  FIG. 5B  depict the photoresist etch rate distributions obtained in the etch steps represented by the dielectric etch rate curves  300 ,  305 ,  310  and  315 , respectively, of  FIG. 5A .  FIG. 5B  shows that the photoresist etch rate distribution is much more responsive to the oxygen flow rate through the center gas zone  180 . Specifically, an ideal oxygen flow rate through the center zone  180  corresponds to the uniform dielectric etch rate distribution represented by the curve  310  of  FIG. 5A , and to the center high photoresist etch rate represented by the curve  310   a  of  FIG. 5B . A higher oxygen gas flow rate through the center gas zone  180  produces a slightly center high dielectric etch rate represented by the curve  320  of  FIG. 5 , and to the much more center high photoresist etch rate represented by the curve  320   a  of  FIG. 5B . The results illustrated in  FIGS. 5A and 5B  show that the response of the etch rate distribution to the flow rate oxygen gas through the center gas zone  180  is very sensitive, so that the center zone oxygen gas flow rate should be adjusted with great care to obtain the most uniform dielectric etch rate distribution without undue non-uniformity in the photoresist etch rate. 
     A less sensitive response by the etch rate distribution to oxygen gas flow over the wafer center may be obtained in another embodiment of the invention in which the entire gas distribution pattern is shifted radially outwardly. Specifically, the oxygen gas is fed through the inner gas zone  120  while the polymerizing etch process gas is fed through the outer gas zone  115 . In this case, very little (or no) oxygen gas is fed through the center gas injection zone  180 . This may be accomplished by employing the embodiment of  FIG. 3 , in which the only gas fed to the inner zone is oxygen from the oxygen gas supply  185  while the only gas fed to the outer zone is the etch process gas (e.g., a fluorocarbon or fluorohydrocarbon gas) from the etch process gas supply  170 . For this purpose, the FRC  175  can shut off the flow of etch gas to the inner gas zone  120 , and the center zone gas flow is likewise turned off at the valve  190 . This embodiment retains the same approach as the preferred embodiment of  FIG. 1A  in that oxygen gas flow is directed to the wafer center to reduce the deposition of polymer at the wafer center, while the polymerizing etch process gas is directed to the outer portion of the wafer. However, in this alternative embodiment, the oxygen gas distribution is not as concentrated at the wafer center. The resulting response of the dielectric etch rate distribution, illustrated in  FIG. 6A , is a more moderate behavior, in which the successive dielectric etch rate distributions  300   b ,  305   b ,  310   b ,  315   b  arising from successively higher oxygen gas flow rates exhibit more gradual increase in dielectric etch rate at the wafer center. Similarly, in  FIG. 6B  the corresponding photoresist etch rate distributions  300   c ,  305   c ,  310   c ,  315   c  arising from the successively higher oxygen gas flow rates exhibit a more gradual increase in dielectric etch rate at the wafer center, with only the highest oxygen gas flow rate resulting in a center high photoresist etch rate distribution (the curve  315   c  of  FIG. 6B ). 
     The sensitivity of the etch process to adjustments in the oxygen flow rate to the wafer center may be adjusted between the highly sensitive behavior of  FIG. 5A  and the moderate behavior of  FIG. 6A  by combining the features of the two embodiments. Specifically, some of the oxygen gas may be directed to the center gas zone  180  and the remainder directed to the inner gas zone  120 , while some of the polymerizing etch process gas may be directed to the outer gas zone  115  and the remainder directed to the inner gas zone  120 . The apportionment of these gases between the alternative paths determines how closely the process follows the sensitive behavior of  FIG. 5A  or the moderate behavior of  FIG. 6A . 
       FIG. 7  depicts how the pronounced center-low etch rate distributions of the curves  300  and  300   a  of  FIGS. 5A and 6A  are affected by the MERIE fields of the reactor of  FIG. 1A . Without the MERIE fields, the etch rate is low and the etch rate distribution is moderately center low (the curve  400  of  FIG. 7 ) because of the greater process gas residency time at the wafer periphery referred to above. Application of the MERIE field raises the overall etch rate while at the same time increasing the non-uniformity (center-low shape) of the etch rate distribution (the curve  405  of  FIG. 7 ), due to the proximity of the MERIE magnets to the wafer periphery. The center low etch rate distribution of the curve  405  is corrected in the manner described above by feeding pure oxygen gas to the center gas injection zone  180  and increasing its flow rate until a uniform etch rate distribution is provided. 
       FIG. 8  is a graph illustrating the radial distribution of gas concentration in the embodiment of  FIG. 1A  in which no gas is fed to the outer zone  115 . In  FIG. 8 , the oxygen gas distribution (labeled “O2” in  FIG. 8 ) is concentrated at the wafer center while the polymerizing etch process gas distribution (labeled “process gas” in  FIG. 8 ) is fairly uniform but is somewhat center low and edge low. 
       FIG. 9  is a graph illustrating the radial distribution of gas concentration in the embodiment of  FIG. 1A  in which the oxygen gas is fed to the center zone  180 , while polymerizing etch process gas is fed to both the inner zone  120  and the outer zone  115  independently.  FIG. 9  shows that different gas distributions are obtained over the wafer center, middle and periphery, because each of the gas injection zones  115 ,  120 ,  180  has a different gas flow distribution pattern. These patterns are labeled in  FIG. 9 , respectively, “center gas distribution pattern”, “inner zone pattern” and “outer zone pattern”. Applying different gas flow rates to the three zones  115 ,  120 ,  180  enables the gas distribution of the oxygen gas and etch process gas to be optimized to provide the most uniform etch rate distribution. For example, a center low etch rate distribution (depicted in  FIG. 10A ) may be improved to a distribution having a center peak and an edge peak (depicted in  FIG. 10B ), by increasing the oxygen gas flow rate at the center gas zone  180 . The etch rate distribution of  FIG. 10B  may have a dip or depression over the wafer middle, which may be lessened by adjusting the proportion of polymerizing etch process gas flow at the inner and outer zones  120 ,  115  (e.g., by increasing the inner zone flow rate relative to the outer zone flow rate) to obtain the more uniform distribution depicted in  FIG. 10C . 
     The polymerizing etch process gases that can be employed in the process of the invention can be a mixture of C4F6, CH2F2. This mixture may be combined with O2 and Ar. Other polymerizing etch species that can be employed in similar mixtures include CHF3, CF4 and CH3F. With such gases, the process of the invention can be applied to the etching of silicon dioxide thin films and silicon nitride thin films, organo-silicate glass and nitrogen-doped silicon-carbon compounds, for example. 
       FIG. 11  illustrates one process of the invention employing the reactor of  FIG. 1A  and using only the center and inner gas injection zones  180 ,  120 . A polymerizing etch process gas is fed through the inner gas injection zone  120  of the ceiling electrode  110  (block  500  of  FIG. 11 ). Oxygen gas is fed through the center gas injection zone  180  of the ceiling electrode  110  (block  505  of  FIG. 11 ). VHF plasma source power (30 MHz to 400 MHz) is applied to the ceiling electrode  110  (block  510  of  FIG. 11 ). RF plasma bias power is applied to the ESC grid  127  (block  515  of  FIG. 11 ). The RF bias power may be either one or a combination of HF power (7 MHz to 30 MHz) and LF power (0.1 MHz to 4 MHz). If MERIE magnets are present, then the MERIE magnetic field is adjusted to provide a desired (enhanced) overall etch rate (block  520  of  FIG. 11 ). The flow rate of the oxygen gas to the center gas zone  180  is adjusted (increased) to optimize or maximize the uniformity of the radial distribution of the dielectric etch rate (block  525  of  FIG. 11 ). Specifically, the oxygen flow rate to the center zone  180  is increased until at level is reached at which dip at the wafer center in the radial distribution of etch rate or BCD (for example) is removed. 
     In an alternative embodiment that employs the apparatus of  FIG. 3 , some oxygen gas may be diverted from the center gas injection zone  180  and fed through the inner zone  120  (block  530  of  FIG. 11 ). In this case, the ratio between the oxygen and etch gas flow rates in the inner zone may be adjusted to further enhance the uniformity of the dielectric etch rate radial distribution (block  535  of  FIG. 11 . 
       FIG. 12  illustrates a process of the invention involving all three gas injection zones  115 ,  120 ,  180 . It includes the steps of blocks  500  through  530  of  FIG. 11 . In addition, it includes the following steps: introducing a polymerizing etch process gas through the outer gas injection zone  115  (block  501  of  FIG. 12 ); diverting some of the oxygen gas from the center zone  180  to the outer zone  115  (block  531 ); adjusting the inner and outer gas zone flow rates to improve etch rate uniformity across the wafer (block  560  of  FIG. 12 ); and, adjusting the oxygen-to-etch process gas ratios in each of the center, inner and outer gas injection zones  180 ,  120 ,  115  to improve the dielectric etch rate radial distribution uniformity (block  565  of  FIG. 12 ). 
     In order to carry out the foregoing processes, a process controller  610  individually controls the chucking voltage supply  128 , the vacuum pump  107  and valve  108 , the RF power levels of the RF generators  135 ,  140 ,  150 , each of the valves  177 ,  178  of the flow ratio controller  175 , the valve  190 , the valve  200  in the embodiment of  FIG. 3 , and the valve  205  in the embodiment of  FIG. 4 . The process controller  610  may be programmable in that instructions for carrying out the process of  FIG. 11  or  FIG. 12  may be stored in the controller  190  for sequential execution by the controller  190 . 
     While the invention has been described in detail by specific references to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.