Abstract:
A plasma reactor for processing a workpiece includes a process chamber comprising an enclosure including a ceiling and having a vertical axis of symmetry generally perpendicular to said ceiling, a workpiece support pedestal inside the chamber and generally facing the ceiling, process gas injection apparatus coupled to the chamber and a vacuum pump coupled to the chamber. The reactor further includes a plasma source power applicator overlying the ceiling and comprising a radially inner applicator portion and a radially outer applicator portion, and RF power apparatus coupled to said inner and outer applicator portions, and tilt apparatus capable of tilting either the workpiece support pedestal or the outer applicator portion about a radial axis perpendicular to said axis of symmetry and capable of rotating said workpiece support pedestal about said axis of symmetry. In a preferred embodiment, the reactor further includes apparatus for effecting axially symmetrical adjustments of plasma distribution, which may be either (or both) elevation apparatus for changing the location of said inner and outer portions relative to one another along said vertical axis of symmetry, or apparatus for apportioning the RF power levels applied to the inner and outer applicator portions.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     In semiconductor device fabrication involving plasma processing to form nanometer-scale feature sizes across a large workpiece, a fundamental problem has been plasma uniformity. For example, the workpiece may be a 300 mm semiconductor wafer or a rectangular quartz mask (e.g., 152.4 mm by 152.4 mm), so that maintaining a uniform etch rate relative to nanometer-sized features across the entire area of a 300 mm diameter wafer (for example) is extremely difficult. The difficulty arises at least in part from the complexity of the process. A plasma-enhanced etch process typically involves simultaneous competing processes of deposition and etching. These processes are affected by the process gas composition, the chamber pressure, the plasma source power level (which primarily determines plasma ion density and dissociation), the plasma bias power level (which primarily determines ion bombardment energy at the workpiece surface), wafer temperature and the process gas flow pattern across the surface of the workpiece. The distribution of plasma ion density, which affects process uniformity and etch rate distribution, is itself affected by RF characteristics of the reactor chamber, such as the distribution of conductive elements, the distribution of reactances (particularly capacitances to ground) throughout the chamber, and the uniformity of gas flow to the vacuum pump. The latter poses a particular challenge because typically the vacuum pump is located at one particular location at the bottom of the pumping annulus, this location not being symmetrical relative to the either the workpiece or the chamber. All these elements involve asymmetries relative to the workpiece and the cylindrically symmetrical chamber, so that such key parameters as plasma ion distribution and/or etch rate distribution tend to be highly asymmetrical.  
         [0002]     The problem with such asymmetries is that conventional control features for adjusting the distribution of plasma etch rate (or deposition rate) across the surface of the workpiece are capable of making adjustments or corrections that are symmetrical relative to the cylindrical chamber or the workpiece or the workpiece support. (Examples of such conventional features include independently driven radially inner and outer source-power driven coils, independently supplied radially inner and outer gas injection orifice arrays in the ceiling, and the like.) Such features are, typically, incapable of completely correcting for non-uniform distribution of plasma ion density or correcting for a non-uniform distribution of etch rate across the workpiece (for example). The reason is that in practical application, such non-uniformities are asymmetrical (non-symmetrical) relative to the workpiece or to the reactor chamber.  
         [0003]     There is, therefore, a need to enable conventional control features for adjusting distribution of plasma process parameters (e.g., distribution across the workpiece of either etch rate, or etch microloading, or plasma ion density, or the like) to correct the type of asymmetrical or non-symmetrical non-uniformities that are encountered in actual plasma process environments.  
       SUMMARY OF THE INVENTION  
       [0004]     A plasma reactor for processing a workpiece includes a process chamber comprising an enclosure including a ceiling and having a vertical axis of symmetry generally perpendicular to said ceiling, a workpiece support pedestal inside the chamber and generally facing the ceiling, process gas injection apparatus coupled to the chamber and a vacuum pump coupled to the chamber. The reactor further includes a plasma source power applicator overlying the ceiling and comprising a radially inner applicator portion and a radially outer applicator portion, and RF power apparatus coupled to said inner and outer applicator portions, and tilt apparatus capable of tilting either the workpiece support pedestal or the outer applicator portion about a radial axis perpendicular to said axis of symmetry and capable of rotating said workpiece support pedestal about said axis of symmetry. In a preferred embodiment, the reactor further includes apparatus for effecting axially symmetrical adjustments of plasma distribution, which may be either (or both) elevation apparatus for changing the location of said inner and outer portions relative to one another along said vertical axis of symmetry, or apparatus for apportioning the RF power levels applied to the inner and outer applicator portions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  depicts a reactor of a first preferred embodiment.  
         [0006]      FIGS. 2A and 2B  depict the operation of a tilt adjustment mechanism in the embodiment of  FIG. 1 .  
         [0007]      FIGS. 3A, 3B  and  3 C depict successive steps in the operation of the embodiment of  FIG. 1 .  
         [0008]      FIGS. 4A, 4B  and  4 C depict the etch rate distribution across the surface of a workpiece obtained in the respective steps of  FIGS. 3A, 3B  and  3 C.  
         [0009]      FIG. 5  depicts a reactor of a second preferred embodiment.  
         [0010]      FIG. 6  depicts a reactor in accordance with an alternative embodiment.  
         [0011]      FIG. 7  is a block flow diagram depicting a first method of the invention.  
         [0012]      FIG. 8  is a block flow diagram depicting a second method of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The present invention is based upon the inventors&#39; discovery that spatial distribution across the workpiece surface of a plasma process parameter (such as etch rate) may be transformed from an asymmetrical distribution (relative to the workpiece or to the chamber) to a more symmetrical distribution. Following such a transformation, the distribution (e.g., etch rate distribution) readily may be corrected to a uniform (or nearly uniform) distribution by employing adjustment features that operate symmetrically relative to the workpiece or relative to the chamber. In a preferred embodiment, the spatial distribution of etch rate (for example) across the workpiece is transformed from an asymmetrical distribution to a symmetrical one by tilting an overhead plasma source power applicator relative to the workpiece at such an angle that the etch rate distribution becomes symmetrical with respect to the cylindrical symmetry of the chamber or of the workpiece. For example, the etch rate, which was initially distributed in a non-symmetrical fashion, may be transformed to a center-high or center-low etch rate distribution across the workpiece. The resulting center-high or center-low etch rate distribution is then rendered perfectly uniform (or nearly uniform) by adjusting an inner portion of the overhead source power applicator relative to an outer portion of the overhead source power applicator. In a preferred embodiment, the source power applicator is an inductively coupled source power applicator consisting of (at least) a radially inner symmetrically wound conductor coil and a radially outer symmetrically wound conductor coil concentric with the inner coil. In one implementation, the adjustment of the inner coil relative to outer coil is performed by adjusting the different heights of the inner and outer coils relative to the workpiece.  
         [0014]     Referring to  FIG. 1 , a plasma reactor for processing a workpiece consists of a vacuum chamber  100  defined by a cylindrical side wall  105 , a ceiling  110  and a floor  115 . A workpiece support pedestal  120  on the floor  115  can hold a workpiece  125  that is either a semiconductor wafer or a quartz mask (for example). A process gas supply  130  furnishes a process gas at a desired flow rate into the chamber  100  through gas injection devices  135  which may be provided either in the side wall  105  as shown or in the ceiling  110 . A pumping annulus  140  is defined between the workpiece support pedestal  120  and the side wall  105 , and gas is evacuated from the chamber  100  through the pumping annulus  140  by a vacuum pump  145  under the control of a throttle valve  150 . Plasma RF source power is coupled to the gases inside the chamber  100  by an RF plasma source power applicator  160  overlying the ceiling  110 . In the preferred embodiment illustrated in  FIG. 1 , the source power applicator  160  consists of an inner RF coil or helical conductor winding  162  and an outer RF coil or helical conductor winding  164 , driven by respective RF source power generators  166 ,  168  through respective impedance matches  170 ,  172 . RF plasma bias power is coupled to the plasma by an electrode or conductive grid  175  inside the workpiece support pedestal  120  with bias power applied by an RF bias power generator  180  through an impedance match  185 .  
         [0015]     In order to adjust the distribution of plasma process non-uniformities across the surface of the workpiece  125 , the outer coil  164  can be rotated (tilted) about any selected radial axis (i.e., an axis extending through and perpendicular to the chamber&#39;s cylindrical or vertical axis of symmetry  190 ). As one advantage of this feature, we have discovered that such a rotation (or “tilt”) of the outer coil  164 , if performed about an optimum radial axis and through an optimum angle, will transform a non-symmetrical non-uniform spatial distribution of a plasma process parameter (e.g., etch rate) to a symmetrical non-uniform distribution (i.e., symmetrical about the vertical or cylindrical axis of symmetry  190 ). The “optimum” radial axis and the “optimum” angle for this tilt rotation depends upon the individual characteristics of the particular reactor chamber, among other things, and are determined empirically prior to processing of a production workpiece, e.g., by trial and error testing.  
         [0016]     Once the etch rate distribution is rendered symmetrical in this manner, its non-uniformities are readily corrected by adjusting the effect of the inner coil  162  relative to the outer coil  164 . In a preferred embodiment, this adjustment can be made by changing the height above the ceiling of one of the coils  162 ,  164  relative to the other one. For this purpose, the inner coil  162  is translatable along the cylindrical axis of symmetry  190  relative to the outer coil  164  (and relative to the workpiece  125  and the entire chamber  100 ). If for example the etch rate distribution has been transformed from the typical non-symmetrical distribution to a symmetrical center-high distribution, then the non-uniformity is decreased (or eliminated) by translating the inner coil  162  vertically upward (away from the ceiling  110 ) to decrease plasma ion density over the center of the workpiece  125 . Conversely, if for example the etch rate distribution has been transformed from the typical non-symmetrical distribution to a symmetrical center-low distribution, then this non-uniformity is decreased (or eliminated) by translating the inner coil  162  vertically downward (toward the ceiling  110 ) to increase plasma ion density over the center of the workpiece  125 .  
         [0017]     In an alternative embodiment, adjustment of the effect of the inner coil  162  relative to the outer coil can be made by adjusting the relative RF power levels applied to the different coils  162 ,  164 . This can be in addition to or in lieu of vertical translation of the inner coil  162 .  
         [0018]     In the preferred embodiment, the tilt rotation of the outer coil  164  is performed with very fine control over extremely small rotation angles by a pair of eccentric rings  200 , namely a top ring  202  and a bottom ring  204 , best shown in  FIGS. 2A and 2B . The outer coil  164  is supported by the top ring  202  and (preferably) rotates with the top ring  202 . The upper and lower rings  202 ,  204  may be thought of as having been formed from a single annular ring which has been sliced in a plane  206  that is slanted at some angle “A” relative to the horizontal. As one of the two rings  202 ,  204  is rotated relative to the other about the cylindrical axis  190 , the top surface of the top ring  202  tilts from the initial level orientation of  FIG. 2A  to the maximum rotation of  FIG. 2B . For this purpose, the two rings  202 ,  204  are rotated independently of one another about the cylindrical axis  190  by respective top and bottom rotation actuators  210 ,  215 . Either ring  202 ,  204  may be rotated in either direction (clockwise, counter-clockwise) about the axis  190  while the other ring is held still. Or, the two rings may be rotated simultaneously in opposite rotational directions for the fastest change in tilt angle. Also, in order to adjust the orientation of the tilt direction, the two rings  202 ,  204  may be rotated simultaneously in unison by the actuators  210 ,  215  either before or after a desired tilt angle is established. Thus, a typical sequence may be to establish a desired tilt angle by rotating the rings  202 ,  204  in opposite rotational directions until the desired tilt angle is reached, and then establishing the azimuthal direction of the tilt angle (e.g., “north”, “south”, “east” or “west” or any direction therebetween) by rotating the rings  202 ,  204  simultaneously in unison—or non-simultaneously—in the same rotational direction until the tilt direction is oriented as desired.  
         [0019]     While in the preferred embodiment of  FIG. 1  only the outer coil  164  is coupled to the top ring  202 , in an alternative embodiment both the inner and outer coils  162 ,  164  are coupled to the top ring  202  so as to be tilted by the tilt actuators  210 ,  215 .  
         [0020]     The axial (vertical) translation (up or down) of the inner coil  162  is performed by a mechanical actuator, such as the screw-drive actuator  220  that is depicted in  FIG. 1 . The screw drive actuator  220  may be formed of non-conducting material and may consist of a threaded female rider  222  coupled to the inner coil  162  and a rotatable threaded screw  223  threadably engaged with the rider  222 . The screw  223  is rotated clockwise and/or counter-clockwise by a vertical translation motor  224 . Alternatively, the actuator  220  may be mounted on support structure overlying the coil  162  (not shown).  
         [0021]     In an alternative (but not preferred) embodiment, the top ring  202  supports both the inner and outer coils  162 ,  164 , so that the inner and outer coils  162 ,  164  tilt simultaneously together.  
         [0022]      FIGS. 3A-3C  and  4 A- 4 C depict a basic process of the invention. Initially, the outer coil  164  is essentially level relative to the plane of the ceiling  110  and of the workpiece support  120 , as depicted in  FIG. 3A . The etch rate distribution tends to have a non-symmetrical pattern of non-uniformity, as depicted in  FIG. 4A . The outer coil  164  is then tilted ( FIG. 3B ) about a particular radial axis by a particular angle that is sufficiently optimum to transform the non-symmetrical pattern of etch rate non-uniformities of  FIG. 4A  to the symmetrical distribution of non-uniformities of  FIG. 4B . Such an axially symmetrical distribution ( FIG. 4B ) reflects an etch rate distribution that is either center-high or center-low (for example). This non-uniformity is reduced or eliminated to produce the perfectly uniform distribution of  FIG. 4C  by translating the inner coil  162  either up or down along the vertical axis  190 , as indicated in  FIG. 3C . Preferably, the inner coil  162  is not tilted with the outer coil  164 . However, if both coils  162 ,  164  are tilted together, then the up/down translation of the inner coil  162  may be along a trajectory that is at a slight angle to the cylindrical axis  190 .  
         [0023]     In order to enable a versatile selection of all modes or combinations of all possible rotations of the top and bottom rings  162 ,  164  (i.e, for tilting and/or rotation about the cylindrical axis of the outer coil  164 ) and the vertical translation of the inner coil  162 , a process controller  250  independently controls each of the rotation actuators  210 ,  215  and the translation actuator  220 , as well as the RF generators  166 ,  168 ,  180 .  
         [0024]      FIG. 5  depicts another alternative embodiment in which the outer coil  164  is suspended from the bottom of a support  255  coupled to the top ring  202  (rather than resting on the top ring  202  as in  FIG. 1 ).  
         [0025]      FIG. 6  depicts another embodiment in which an intermediate coil  260  is introduced that lies between the inner and outer coils  162 ,  164 , the intermediate coil being independently driven by an RF source power generator  262  through an impedance match  264 . This embodiment may be employed in carrying out certain steps in a process of the invention in which each of the three coils  162 ,  164 ,  260  are driven with different RF phases (and possible the same RF frequency) to set up different maxima and minima in the RF power density distribution in the plasma generation region. This in turn is reflected in different patterns in etch rate distribution across s the surface of the workpiece  125 . For example, the intermediate coil  260  may be driven 180 degrees out of phase from the inner and outer coils  162 ,  164 .  
         [0026]     Returning now to  FIG. 1 , while the preferred embodiments have been described with reference to apparatus and methods in which the outer coil  164  (at least) is rotated (“tilted”) about a radial axis relative to the workpiece  125  and relative to the entire chamber  100 , the converse operation could be performed to achieve similar results. Specifically, the workpiece  125  and workpiece support  120  could be tilted relative to the source power applicator  160  (and relative to the entire chamber  100 ) rather than (or in addition to) tilting the outer coil  164 . For this purpose, a pair of concentric eccentric rings  360  (identical to the rings  162 ,  164  of  FIG. 1 ), consisting of a top ring  362  and a bottom ring  364 , are provided under and supporting the wafer support pedestal  120 , so that the pedestal  120  can be tilted in the manner previously described with reference to the outer coil  164 . Respective top and bottom actuators  366 ,  368  separately control rotation of the top and bottom rings  362 ,  364  about the cylindrical axis  190 .  
         [0027]      FIG. 7  is a block flow diagram depicting a first method of the invention. The first step (block  400 ), is to tilt the RF source power applicator  160  (or at least its outer portion or coil  164 ) relative to the chamber  100  or relative to the workpiece  125  so as to transform the non-uniform distribution of a plasma process parameter (e.g., etch rate) from a non-symmetrical non-uniform distribution ( FIG. 4A ) to an axially symmetrical non-uniform distribution ( FIG. 4B ). The second step (block  402 ) is to vertically translate the inner RF source power applicator (e.g., the inner coil  162 ) relative to the outer RF source power applicator (e.g., the outer coil  164 ) or relative to the ceiling  110  or workpiece  125 , so as to transform the axially symmetrical non-uniform distribution of the process parameter (e.g., etch rate) ( FIG. 4B ) to a uniform distribution ( FIG. 4C ).  
         [0028]      FIG. 8  is a block flow diagram depicting another method of the invention that can subsume a number of different versions. The first step (block  404 ) is to rotate (tilt) the RF source power applicator  160  (or at least its outer coil  164 ) about a radial axis. In one version, this step is performed initially, i.e., prior to processing a production workpiece (block  404   a ). This step may be performed to level the source power applicator  160  (or outer coil  164 ) relative to a datum plane of the chamber  100  (block  404   a - 1 ). Or this step may be performed, as discussed previously in this specification, to make the etch rate distribution symmetrical (or at least nearly so) about the cylindrical axis  190  (block  404   a - 2 ). Or, this step may be performed to orient the plane of coil  164  relative to a plane of the workpiece  125  (block  404   a - 3 ). In another version, this step may be performed continuously during processing (block  404   b ). Alternatively, this step may be performed non-continuously or sporadically (block  404   c ).  
         [0029]     In an alternative embodiment, the purpose of the step of block  404  is to tilt the plane of the source power applicator  160  (or at least its outer coil  164 ) relative to the plane of the workpiece  125 , in which case either the coil  164  is tilted (using the rotation actuators  210 ,  215  of  FIG. 1 ) or the workpiece support  120  is tilted (using the rotation actuators  366 ,  368 ). Or, it is possible to simultaneously tilt both the outer coil  164  and the workpiece support  120  until the desired relative orientation of the plane of one relative to the plane of the other one is achieved. As described above, the optimum orientation is one in which the distribution across the workpiece  125  of a plasma parameter such as etch rate is at least nearly symmetrical relative to the vertical axis of symmetry  190 . This enables a symmetrical adjustment in plasma distribution to render the plasma process parameter distribution at least nearly uniform. Such a symmetrical adjustment may be a change in the relative heights of the inner and outer coils  162 ,  164 , or a change in the relative RF power levels applied to the two coils, for example, or a change in respective process gas flow rates to the inner and outer portions of the process region overlying the workpiece  125 . Such adjustments are carried out in some of the steps that are described below.  
         [0030]     A next step is to adjust the vertical levels of the inner and/or outer RF source power applicators  162 ,  164  relative to one another or relative to the workpiece  125  (block  406 ). This step may be carried out for the purpose of transforming a cylindrically symmetrical non-uniform etch rate distribution across the workpiece  125  to a uniform distribution (or nearly uniform), as discussed above in this specification.  
         [0031]     A next step is to rotate the RF source power applicator  160  (or at least its outer coil or portion  164 ) about the vertical axis during processing (block  408 ). As mentioned previously in this specification, such a step may be carried out by rotating the two eccentric rings  202 ,  204  simultaneously in unison. This step may be carried out continuously during processing (block  408   a ). Alternatively, this step may be carried out non-continuously or sporadically (block  408   b ), depending upon the desired effects during processing. Such a step may average out non-uniform effects of the source power applicator  160  across the surface of the workpiece  125  over a number of rotations during a given plasma process step. The rotation of the source power applicator  160  (or at least its outer portion  164 ) may be carried out before, during or after the tilting operation. The difference is that tilting requires relative rotational motion about the axis of symmetry  190  of the top and bottom rings  202 ,  204 , whereas pure rotational motion about the axis of symmetry by the outer applicator portion  164  requires rotation in unison of the two rings  202 ,  204  with no relative motion between the two rings  202 ,  204 . These two modes of motion may be performed simultaneously by combining the two types of relative ring motions. Although the outer applicator portion  164  may already be tilted so that its axis of symmetry does not coincide with the vertical axis  190 , its rotational motion (when the rings  202 ,  204  rotate in unison) is nevertheless defined in this specification as occurring about the vertical axis  190 .  
         [0032]     A next step (block  410 ) may be to adjust the respective levels of RF power delivered to the inner and outer coils  162 ,  164  independently, in order to control the radial distribution of a plasma processing parameter (e.g., etch rate) or the effective area of the RF source power applicator  160 . As one possible example, this step may be carried out to correct a symmetrical non-uniform etch rate distribution across the workpiece surface. As such, this step may be supplementary to (or in lieu of) the vertical translation of the inner coil  162  referred to above.  
         [0033]     Another step (block  412 ) may be to adjust the RF phase differences between the different (inner/outer) source power applicator portions (e.g., multiple concentric coils  162 ,  164 ,  260  of  FIG. 6 ) to control the radial distribution of a plasma processing parameter (e.g., etch rate). Different RF power distributions may be achieved with different phase relationships between the multiple coils, and some may be optimum for certain desired processing effects in particular instances.  
         [0034]     In a further step that is optional (block  414  of  FIG. 8 ), the process gas flow rates from process gas supplies  130 ,  131  to inner and outer gas inlets  130   a,    131   a  (shown in  FIG. 6 ) may be adjusted relative to one another to adjust plasma ion density radial distribution. The adjustments of block  406  (adjusting the relative axial locations of the inner and outer coils  162 ,  164 ), block  410  (adjusting the relative RF power levels applied to the inner and outer coils  162 ,  164 ) and block  414  (adjusting the relative gas flow rates to the inner and outer gas inlets  131   a,    130   a ) are all symmetrical relative to the vertical axis  190  ( FIG. 1 ) and may be used to render the etch rate distribution (for example) uniform, provided that the etch rate distribution has been transformed to a symmetrical one by the tilting step of block  404 .  
         [0035]     While the invention has been described in detail by specific reference 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.