Abstract:
The disclosure concerns a wafer support for use in a plasma reactor chamber, in which the wafer support has a wafer edge gas injector adjacent and surrounding the wafer edge.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure concerns a plasma reactor chamber for processing a workpiece such as a semiconductor wafer to produce integrated circuits. Specifically, the disclosure concerns independent process gas injection at the ceiling and at the wafer edge in such a reactor chamber. 
       BACKGROUND 
       [0002]    In a plasma reactor chamber for etching silicon or polysilicon thin films on a semiconductor wafer, a uniform distribution of etch rate across the wafer is needed. Non-uniform distribution of etch rate across the wafer is indicated by non-uniformity in critical dimension (CD). The critical dimension may be a width of a typical line in the thin film circuit pattern. The CD is less in areas on the wafer surface experiencing a higher etch rate and greater in areas of lower etch rate. 
         [0003]    In silicon etch chambers in which the process gas is injected from the ceiling, it has been found that the CD is very small at the wafer edge compared to other areas on the wafer surface. The effect of a small CD is typically confined to the outer or peripheral 1% of the wafer surface. This problem was not solved using conventional techniques. Specifically, etch uniformity can be improved by dividing the gas distribution into independent inner and outer gas injection zones at the ceiling and maximizing uniformity by adjusting the gas flow rates to the inner and outer zones. However, adjustment of the inner and outer gas injection zone flow rates does not solve the problem of small CD at the outer 1% of the wafer surface. Specifically, adjustment of the inner and outer gas injection zone flow rates at the ceiling can produce fairly uniform CD across the wafer, with the exception of a region at the wafer edge whose width is about 1% of the wafer diameter. 
         [0004]    Therefore, there is need to independently control the CD at the outer 1% of the wafer edge without detracting from etch rate distribution uniformity achieved for the other areas of the wafer. 
       SUMMARY 
       [0005]    A workpiece support is provided for supporting a workpiece such as a semiconductor wafer during processing in a plasma reactor. The workpiece support comprises a pedestal having a workpiece support surface. A processing ring overlies a periphery of the pedestal. The processing ring is adjacent a peripheral boundary of the workpiece support surface. A wafer edge gas injector is formed by the process ring and has a gas injection opening generally facing a workpiece location overlying the workpiece support surface. A process gas supply is coupled to the wafer edge gas injector. 
         [0006]    In one embodiment, the wafer edge gas injector comprises an annular slit opening. In a further embodiment, a liner surrounds a side of the pedestal and has a top surface underlying the process ring. Plural axial channels inside the liner extend through the liner to the top surface of the liner. An annular feed channel is defined between the process ring and the liner. Each of the plural axial channels is coupled to the annular feed channel and the wafer edge gas injector is coupled to the annular feed channel. 
         [0007]    In a yet further embodiment, the liner further comprises a bottom surface and a base underlying the bottom surface, the base containing an annular plenum. The plural axial channels are coupled to the annular plenum. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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. 
           [0009]      FIG. 1  depicts a plasma reactor in accordance with one embodiment. 
           [0010]      FIG. 2  illustrates internal structural features of a cathode liner of the reactor of  FIG. 1 . 
           [0011]      FIG. 3  is a cross-sectional view taken along lines  3 - 3  of  FIG. 2 . 
           [0012]      FIG. 4  is a cross-sectional view taken along lines  4 - 4  of  FIG. 2 . 
           [0013]      FIG. 5  is a detailed view of a portion of the process rings and cathode liner of one embodiment. 
           [0014]      FIG. 6  is a side view corresponding to  FIG. 5 . 
           [0015]      FIG. 7  is a graph depicting radial distribution of SiCl 2  in the reactor of  FIG. 1  with and without gas flow through the wafer edge injector slot. 
           [0016]      FIG. 8  illustrates a method in accordance with one embodiment. 
           [0017]      FIG. 9  illustrates a method in accordance with another embodiment. 
       
    
    
       [0018]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale. 
       DETAILED DESCRIPTION 
       [0019]    Referring to  FIG. 1 , a plasma reactor includes a vacuum chamber  100  enclosed by a cylindrical side wall  108 , a ceiling  110  and a floor  115 . A wafer support  125  supports a semiconductor wafer  130  during wafer processing. The wafer support  125  includes a cathode electrode  135  that also serves as an electrostatic chuck (ESC) electrode. The support  125  includes an insulating layer  137  separating the electrode  135  from the wafer  130  and an insulating layer  139  separating the electrode  135  from underlying components of the wafer support  125 . The upper insulating layer  137  has a top wafer-supporting surface  137   a . The reactor further includes an inductively coupled source power applicator or coil antenna  140  overlying the ceiling  110 . An RF plasma source power generator  145  is coupled to the coil antenna  140  through an RF impedance match  150 . An RF plasma bias power generator  155  is coupled to the cathode electrode  135  through an RF impedance match  160 . A D.C. chucking voltage supply  161  is connected through a control switch  162  to the ESC electrode  135 . An isolation capacitor  163  blocks D.C. current from the supply  161  from the RF bias power generator  155 . 
         [0020]    Process gas is delivered into the chamber interior by a gas distribution injector  165  on the ceiling  110 . The injector  165  consists of an inner zone injector  170  and an outer zone injector  175 . Each one of the inner zone injector  170  and the outer zone injector  175  may be implemented with plural injection holes or, alternatively, as a slit. The inner zone injector  170  is oriented to direct process gas toward a center region of the chamber. The outer zone injector  175  is oriented to direct process gas toward a peripheral region of the chamber. The inner zone injector  170  is coupled through a valve  180  to a gas distribution panel  185 . The outer zone injector  175  is coupled through a valve  190  to the gas distribution panel  185 . Different process gas supplies  101 ,  102 ,  103 ,  104 ,  105  supply different process gases to the gas distribution panel  185 . As indicated in the drawing of  FIG. 1 , in one embodiment, each gas supply may be connected separately to different ones of the inner and outer valves  180 ,  190  through independent valves  195 . In the embodiment of  FIG. 1 , the gas supply  101  contains a fluoro-hydrocarbon gas such as CH 2 F 2  or CHF 3 , the gas supply  102  contains hydrogen bromide gas, the gas supply  103  contains chlorine gas, the gas supply  104  contains argon gas and the gas supply  105  contains oxygen gas. The gases referred to here are examples. Any suitable process gas may be used. 
         [0021]    The wafer support  125  is surrounded by a ring-shaped cathode liner  200 . The cathode liner  200  may be formed of a process-compatible material such as quartz, for example. A process ring  205  covers the top of the cathode liner  200  and covers a peripheral portion of the wafer support surface  137   a . The process ring  205  is formed of a process-compatible material such as quartz. The wafer support  125  may contain materials such as metal that are incompatible with plasma processing, and liner  200  and the ring  205  isolate the wafer support  125  from the plasma. The radially inner edge  205   a  of the process ring  205  is adjacent the edge of the wafer  130 . In one embodiment, the process ring may provide improved distribution of RF electrical fields. 
         [0022]    A silicon or polysilicon etch process employs silicon etch gases such as HBr and Cl 2  to etch silicon material and employs a polymerizing species such as CH 2 F 2  or CHF 3  to improve etch profile. The polymer deposits on sidewalls of deep aspect ratio openings in a polymer deposition reaction that competes with the etch reaction. 
         [0023]    The reactor of  FIG. 1  can have a problem of poor critical dimension (CD) control at the wafer edge. Typically, the CD is the width a selected line in the circuit pattern. The CD tends to be smaller at the wafer edge than elsewhere on the wafer  130 . The problem of a small CD tends to occur in an annular zone at the edge of the wafer  130  whose width (extending inwardly from the wafer edge) is about 1% of the wafer diameter. (This narrow zone will hereafter be referred to as the wafer edge zone  130   a  shown in  FIG. 5 , which is discussed later in this specification.) Over the remainder of the wafer  130 , such problems are minimized or prevented by adjusting the valves  180  and  190  to obtain an optimum ratio of process gas flow rates to the inner and outer gas ceiling injectors  170 ,  175 . However, such an optimum adjustment does not solve the problem of poor CD control at the wafer edge zone  130   a . The small CD at the wafer edge zone  130   a  is indicative of a higher etch rate at the wafer edge zone than elsewhere. 
         [0024]    We have discovered that the gas flow velocity over the wafer edge zone  130   a  is extremely low relative to gas flow velocity over most other portions of the wafer. For instance, in certain applications, while gas flow velocity over the majority of the wafer surface is between about 10 and 20 meters per second, gas flow over the wafer edge zone approaches zero. If gas flow over the wafer edge zone is thus stagnant, then the gas residency time over the wafer edge zone is extremely high, yielding correspondingly high dissociation of the process gas species. Such high dissociation can increase the population at the wafer edge zone of highly reactive species. Such highly reactive species may include radicals or neutrals that either (a) etch extremely fast or (b) inhibit polymer deposition. A highly reactive etch species generated by such dissociation may include atomic HBr and/or atomic Cl 2 , for example. The result is a higher etch rate and a correspondingly smaller CD. 
         [0025]    In one embodiment, a new gas is injected at the wafer edge to address the non-uniform etch rate at the wafer edge. The new gas may be an inert gas such as argon, for example. In one embodiment, the injection of the new gas increases the gas flow velocity over the wafer edge zone and decreases process gas residency time over the wafer edge zone. The decrease in residency time reduces the population over the wafer edge zone of highly reactive species such as radicals or neutrals. The velocity or flow rate at which the new gas is injected at the wafer edge can be sufficiently low to avoid influencing the etch rate beyond the narrow wafer edge zone. Typically, the wafer edge zone is about 3 mm wide. 
         [0026]    In one embodiment, a polymerizing gas is injected at the wafer edge to address the non-uniform etch rate at the wafer edge. The polymerizing gas may be CH 2 F 2  or CHF 3 , for example. The addition of the polymerizing species increases the polymer deposition rate the wafer edge zone, which decreases the etch rate. The velocity or flow rate at which the polymerizing species gas is injected at the wafer edge can be sufficiently low to avoid influencing the etch rate beyond the narrow wafer edge zone. Typically, the wafer edge zone is about 3 mm wide. 
         [0027]    In one embodiment, the process ring  205  is divided into an upper process ring  210  and a lower process ring  212 , leaving a narrow circular slit  220  between them facing (almost touching) the edge of the wafer  130 . The circular slit  220  is separated from the edge of the wafer by a very small distance in the range of 0.6 mm to 3 mm, e.g., about 1% of the wafer diameter. A desired gas (such as an inert gas or a polymerizing species gas) is supplied so as to be ejected from the circular slit  220  radially inwardly and directly at the wafer edge. This new gas or polymerizing species gas may be supplied from the gas distribution panel  185 . 
         [0028]    In one embodiment, an annular gas plenum  225  is provided at the bottom of the cathode liner  200 . A cathode gas flow control valve  227  controls gas flow from the gas distribution panel  185  to the plenum  225  through a conduit  229 . Gas is conducted from the plenum  225  to the circular slit  220  at the wafer edge by vertical passages  240  inside the cathode liner  200 . 
         [0029]      FIG. 2  illustrates an exemplary internal structure of the cathode liner  200 . The cathode liner  200  was described with reference to  FIG. 1  as being formed of an insulator such as quartz. In the embodiment of  FIG. 2 , the cathode liner  200  is formed of metal, and, as shown in  FIG. 5 , a quartz liner  126  separates the metal cathode liner  200  from the wafer support  125 . The cathode liner  200  includes a cylindrical wall  201  having an annular top surface  201   a . An annular base  215  supports the cylindrical wall  201 . A shoulder  235  extends in the radially outward direction from the base  215  and houses a gas supply inlet  230 . The plenum  225  shown in  FIG. 1  is formed within the cathode ring annular base  215  of  FIG. 2 , as depicted in the cross-sectional view of  FIG. 3 . An internal channel  232  extends radially through the shoulder  235  and is coupled at one end to the gas supply inlet  230  and is coupled at an opposite end to the plenum  225 , as depicted in the cross-sectional view of  FIG. 4 . As shown in  FIG. 2 , the vertical passages  240  extend axially through the cylindrical wall  201  and are spaced azimuthally around the cylindrical wall  201 . The bottom end of each vertical passage  240  is coupled to the plenum  225  and the top end of each vertical passage  240  opens at the annular top surface  201   a  of the cylindrical wall  201 . In one embodiment, the cylindrical wall  201  is about 0.25 inch thick, and each of the vertical passages  240  is an axial 0.05 inch hole within the cylindrical wall  201 . 
         [0030]    In the embodiment of  FIG. 1 , the cylindrical wall  201  supports the lower process ring  212  and the upper process ring  210  is supported on the lower process ring  215 . 
         [0031]    As shown in  FIG. 5 , the interior quartz liner  126  surrounds the workpiece support  125  and is surrounded by the cathode liner cylindrical wall  201 . As shown in  FIG. 5 , the interior liner  126  supports the lower process ring  212 , while the cathode liner cylindrical wall  201  supports the upper process ring  210 . An annular gas feed chamber  260  is bounded by the cylindrical wall top surface  201   a , the upper process ring and the lower process ring  212 . An annular feed passage  262  is formed as a gap between the upper and lower process rings  210  and  212 . An outer annular protrusion  210   a  in the bottom surface of the upper process ring  210  faces an outer annular recess  212   a  in the top surface of the lower process ring  212 . An inner annular recess  210   b  is provided in the bottom surface of the upper process ring  210 . The inner annular recess  210   b  faces a raised shoulder  212   b  of the lower process ring  212  to form the gas injection slit  220 . The protrusion  210   a , the recess  212   a , the recess  210   b  and the shoulder  212   b  provide the feed passage  262  with a meandering path, as shown in  FIG. 5 . Gas supplied through the valve  227  of  FIG. 1  flows to the cathode or wafer support  125  and enters the inlet  230  shown in  FIG. 4 , and then flows through the internal channel  232  to the plenum  225 . From the plenum  225 , the gas flows up through the vertical channels  240  into the feed chamber  260  of  FIG. 5 , and then flows through the feed passage  262  into the injection slit  220 . 
         [0032]    As shown in the side view of  FIG. 6 , the end or exit port of the injection slit  220  is within a very short distance D of the edge of the wafer  130 , where D is on the order of between 0.6 mm to 3 mm. Given such a short distance, the effect of gas flow from the injection slit  220  may be highly localized so as to not affect processing beyond the 3 mm-wide wafer edge zone  130   a . Such localization may be realized by establishing a very low gas flow rate within the injection slit  220 . For example, the gas flow rate through the valve  227  (to the wafer edge injection slit  220 ) may be between 1% and 10% of the gas flow rate through the valves  180  and  190 . In this way, the gas flowing out of the injection slit  220  affects processing (e.g., etch rate) only in the narrow wafer edge zone  130   a , without affecting processing on the remaining portion of the wafer  130 . 
         [0033]      FIG. 7  is a graph depicting the density of SiCl 2  over the wafer surface as a function of radial position in a process in which a polymerizing gas, such as CH 2 F 2  or CHF 3  is introduced through the wafer edge injection slit  220  of  FIGS. 1-6 , while an etch process gas such as HBr and Cl 2  is introduced through the ceiling injectors  170 ,  175 . The density of SiCl 2  is an indicator of the degree of polymerization in such a process. The graph in  FIG. 7  shows that, in the absence of any gas flow from the injection slit  220 , polymerization is relatively depressed at the wafer edge (curve A). With the polymerizing gas being supplied through the injection slit  220 , the degree of polymerization at the wafer edge increases significantly (curve B). The polymerization gas flow through the wafer edge injection slit  220  is limited to a low rate. This limitation of the injection slit flow rate confines the increase in polymerization to the outer 1% of the wafer diameter, the wafer edge zone. In one example, the etch process gas flow rate through the ceiling injector nozzles  170 ,  175  was about 150 sccm while the polymerization gas flow through the wafer edge injector slot  220  was about 5 sccm. 
         [0034]      FIG. 8  illustrates an exemplary method of operating the plasma reactor of  FIGS. 1-6  so as to increase CD in the wafer edge zone. A silicon etchant species gas, such as HBr and Cl 2 , is injected through the inner zone ceiling injector  170  at a first gas flow rate (block  400  of  FIG. 8 ), and through the outer zone ceiling injector  175  at a second gas flow rate (block  405  of  FIG. 8 ). Gas flow through the inner and outer zone ceiling injectors  170 ,  175  is sufficient to attain a desired average etch rate across the wafer surface. Etch rate distribution is adjusted over all but the peripheral 1% of the wafer surface by independently adjusting the gas flow rates through the inner and outer ceiling injectors  170 ,  175  until etch rate distribution uniformity is optimized (block  410  of  FIG. 8 ). This typically leaves the etch rate too high (or the CD too low) in the wafer edge zone or the outer 1% of the wafer surface. Etch rate is adjusted downwardly (or CD is adjusted upwardly) in the wafer edge zone by reducing gas residency time over the wafer edge zone (exclusively) in order to reduce dissociation over the wafer edge zone. In one embodiment, reducing the gas residency time over the wafer edge zone is done by flowing through the wafer edge injection slit  220  a suitable gas, such as an inert gas or oxygen, to stir up gas flow over the wafer edge (block  415  of  FIG. 8 ). The increase in gas flow, or decrease in gas residency time, is confined to the wafer edge zone by limiting the gas flow rate through the wafer edge injector slit to a small flow rate. This small flow rate is chosen to attain the most uniform CD distribution, which may be affected by the choice of process gas species, and may be in the range of 1-20 sccm, for example. 
         [0035]      FIG. 9  illustrates another exemplary method of operating the plasma reactor of  FIGS. 1-6  so as to increase CD in the wafer edge zone. A silicon etchant species gas, such as HBr and Cl 2 , is injected through the inner zone ceiling injector  170  at a first gas flow rate (block  420  of  FIG. 9 ), and through the outer zone ceiling injector  175  at a second gas flow rate (block  425  of  FIG. 9 ). Gas flow through the inner and outer zone ceiling injectors  170 ,  175  is sufficient to attain a desired average etch rate across the wafer surface. Etch rate distribution is adjusted over all but the peripheral 1% of the wafer surface by independently adjusting the gas flow rates through the inner and outer ceiling injectors  170 ,  175  until etch rate distribution uniformity is optimized (block  430  of  FIG. 9 ). This typically leaves the etch rate too high (or the CD too low) in the wafer edge zone or the outer 1% of the wafer surface. Etch rate is adjusted downwardly (or CD is adjusted upwardly) in the wafer edge zone by increasing polymerization over the wafer edge zone (exclusively) in order to reduce etch rate over the wafer edge zone. In one embodiment, increasing polymerization over the wafer edge zone is done by flowing through the wafer edge injection slit  220  a polymerization gas, such as CH 2 F 2  or CHF 3  (block  435  of  FIG. 9 ). The resulting increase in polymer deposition rate increases the CD. This increase is confined to the wafer edge zone by limiting the gas flow rate through the wafer edge injector slit to a small flow rate. This small flow rate is chosen to attain the most uniform CD distribution, which may be affected by the choice of process gas species, and may be in the range of 1-20 sccm, for example. 
         [0036]    In either one of the methods of  FIG. 8  or  9 , further optimization is achieved by adjusting the gas flow rates through the ceiling injectors  170  and  175  and/or adjusting the gas flow rates through the wafer edge slit  220 . For example, the etchant gas flow through the ceiling injectors  170 ,  175  may be reduced while increasing inert or polymerization gas flow through the wafer edge slit  220  to further increase CD at the wafer edge zone. However, the flow rate through the wafer edge slit can be sufficiently low in order to confine the effects to the wafer edge zone. However, the etchant gas flow rate through the ceiling injectors  170 ,  175  may be decreased as low as desired (e.g., to zero). And conversely, the etchant gas flow through the ceiling injectors  170 ,  175  may be increased while decreasing inert or polymerization gas flow through the wafer edge slit  220  to decrease CD at the wafer edge zone. 
         [0037]    While the invention has been described with reference to embodiments in which a selected gas is injected next to wafer edge through a continuous slit injector, the injector at the wafer edge may assume other forms, such as an array or succession of many gas injection orifices around the wafer edge. 
         [0038]    While the foregoing is directed to embodiments of the 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.