Abstract:
A method is provided for processing a workpiece supported on a support surface in a chamber of a plasma reactor. A process gas is introduced into the chamber and a plasma is generated with pulse-modulated RF power. The method comprises successively repeating the following cycle: (a) concentrating the plasma in the chamber in a center-high plasma ion distribution for a first on-time duration; (b) permitting plasma to drift during a first off-time duration away from the center-high plasma ion distribution; (c) concentrating the plasma in the chamber in an edge-high plasma ion distribution for a second on-time duration; and (d) permitting plasma to drift during a second off-time duration away from the edge-high plasma ion distribution. The method further comprises adjusting a plasma process rate near a center of the workpiece by adjusting a duty cycle of the first on-time and first off-time. The method also comprises adjusting a plasma process rate near a periphery of the workpiece by adjusting a duty cycle of the second on-time and second off-time.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure concerns the processing of a workpiece or semiconductor wafer in a plasma reactor having plural overhead coils for applying RF plasma source power, and in particular a method for controlling and improving uniformity of the radial distribution of plasma ion density. 
       BACKGROUND 
       [0002]    A plasma reactor that generates an inductively coupled plasma is capable of etching thin films on a workpiece such as a semiconductor wafer at a relatively high etch rate. Such a reactor has an inductively coupled plasma source power applicator, typically a coil antenna, coupled to an RF power generator. As the wafer diameter has increased in recent years, the chamber size has increased accordingly, requiring larger coil antennas, which greater inductance and more concentrated power deposition profiles. Power deposition tends to peak in narrow annular regions underlying the coil antenna or underlying inner and outer coil antennas. Such concentrated profiles cause large peaks in the plasma ion density distribution that are difficult to compensate, leading to reduced process uniformity across the wafer. Some improvement in process uniformity can be achieve using two (or more) concentric coil antennas over the reactor ceiling, one antenna overlying the wafer periphery and the other being closer to the wafer center. Even though such a configuration can improve process uniformity, the concentrated peaks in the power deposition profiles of the inner and outer coil antennas lead to process non-uniformities that are difficult to reduce. 
       SUMMARY 
       [0003]    A method is provided for processing a workpiece supported on a support surface in a chamber of a plasma reactor. A process gas is introduced into the chamber and a plasma is generated with pulse-modulated RF power. The method comprises successively repeating the following cycle: (a) concentrating the plasma in the chamber in a center-high plasma ion distribution for a first on-time duration; (b) permitting plasma to drift during a first off-time duration away from the center-high plasma ion distribution; (c) concentrating the plasma in the chamber in an edge-high plasma ion distribution for a second on-time duration; and (d) permitting plasma to drift during a second off-time duration away from the edge-high plasma ion distribution. The method further comprises adjusting a plasma process rate near a center of the workpiece by adjusting a duty cycle of the first on-time and first off-time. The method also comprises adjusting a plasma process rate near a periphery of the workpiece by adjusting a duty cycle of the second on-time and second off-time. 
         [0004]    In one embodiment, the adjustment of the plasma process rate near a center of the workpiece comprises reducing the plasma process rate near the center of the workpiece and the adjusting the first duty cycle comprises reducing the first duty cycle. In one embodiment, the adjustment of a plasma process rate near a periphery of the workpiece comprises reducing the plasma process rate near the periphery of the workpiece and the adjusting the second duty cycle comprises reducing the second duty cycle. 
         [0005]    In one embodiment, the reduction in plasma process rate near the center of the workpiece and the reduction in plasma process rate near the periphery of the workpiece reduces non-uniformity in distribution of process rate across the workpiece. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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. 
           [0007]      FIG. 1  illustrates a plasma reactor adapted to carry out processes disclosed herein. 
           [0008]      FIGS. 2A ,  2 B and  2 C illustrate a chronological sequence depicting how plasma ion distribution spreads out over time following a trailing edge of pulsed RF power applied to the inner coil only in the reactor of  FIG. 1 . 
           [0009]      FIGS. 3A ,  3 B and  3 C illustrate a chronological sequence depicting how plasma ion distribution spreads out over time following a trailing edge of pulsed RF power applied to the outer coil only in the reactor of  FIG. 1 . 
           [0010]      FIGS. 4A and 4B  are contemporaneous timing diagrams of pulse waveforms that pulse-modulate RF power applied to the inner and outer coils, respectively, of the reactor of  FIG. 1 . 
           [0011]      FIGS. 5A ,  5 B,  5 C,  5 D,  5 E and  5 F illustrate a chronological sequence depicting how plasma ion distribution alternately (a) concentrates during pulse on times below one or the other of the inner and outer coils of  FIG. 1 , and (b) spreads out during off times between pulses. 
           [0012]      FIGS. 6A ,  6 B,  6 C,  6 D,  6 E and  6 F depict radial distributions of etch rate corresponding to  FIGS. 5A ,  5 B,  5 C,  5 D,  5 E and  5 F, respectively. 
           [0013]      FIG. 7  illustrates a process in accordance with one embodiment. 
       
    
    
       [0014]    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 
       [0015]      FIG. 1  illustrates a plasma reactor adapted to carry but certain processes in accordance with embodiments disclosed herein. The reactor includes a vacuum chamber  100  enclosed by a cylindrical side wall  105  and a disk-shaped ceiling  110 . A wafer support pedestal  120  within the chamber  100  has a top insulating layer  121 , a conductive base  122 , a cylindrical side wall  123  and a mesh electrode  124  within the insulating layer  121 . The top of the insulating layer  121  defines a wafer support surface  125  that is separated from the mesh electrode  124  by a very thin portion of the insulating layer  121 . A semiconductor wafer  126  can be supported on the wafer support surface  125 . A process gas supply  130  furnishes process gases to the chamber  100  through gas injection apparatus  135  of any suitable type, such as individual gas injectors  135   a ,  135   b . Chamber pressure is controlled by a vacuum pump  140  and by the flow rate of process gases from the injection apparatus  135 . The reactor of  FIG. 1  further includes an inductively coupled plasma source power applicator consisting of an inner coil antenna  150  and an outer coil antenna  155  concentric with the inner coil antenna  150 . Each coil antenna  150 ,  155  may consist of an individual conductor or wire helically wound in a torus (as shown in  FIG. 1 ), or may be a flat winding helical winding. In order to enhance inductive coupling of RF power from the antennas  150 ,  155  into the chamber  100 , the ceiling  110  may be formed of an insulating material. The side wall  105  may be metal that is coupled to RF ground. The inner coil antenna  150  receives RF power from a first RF power generator  160  through a first pulse gate  162  and through a first RF impedance match circuit or element  164 . The outer coil antenna  155  receives RF power from a second RF power generator  165  through a second pulse gate  167  and through a second RF impedance match circuit  169 . A programmable controller  170  or other suitable device controls the pulse gates  162 ,  167 . RF plasma bias power from an RF generator  180  is coupled to the wafer  126  by applying it to the mesh electrode  124  (or alternatively to the conductive base  122 ) through an RF impedance match element  182 . Optionally, a second power generator  184  may be coupled to the mesh electrode  124  (or alternatively to the conductive base  122 ) through another impedance match element  186 . The RF bias generator  180  may be a low frequency generator while the RF generator  184  may be a high frequency or very high frequency generator. The two source power generators  160 ,  165  may be of similar frequencies (e.g., LF or HF) that are sufficiently offset from one another to avoid coupling between them, for example. 
         [0016]    It is our discovery that pulse-modulating the RF power applied to the coil antennas  150 ,  155  can be performed or controlled in such a way as to solve the problem non-uniformity caused by the concentrated power deposition profile of the coil antennas  150 ,  155 . In one embodiment, this is accomplished by causing the plasma to alternate between different ion density distributions, so that the plasma processing results reflect a time average of the different distributions. During each pulse cycle, RF power is turned off at the trailing edge of the pulse, which permits the plasma to drift away from a concentrated profile to a more diffuse profile. This movement in plasma distribution provides an time-averaged plasma distribution that has better uniformity. 
         [0017]    In one embodiment,  FIGS. 2A ,  2 B and  2 C depict plasma distribution across the process region overlying the wafer  126  in chronological sequence. The regions labeled  190  and  191  in each of  FIGS. 2A ,  2 B and  2 C correspond to zones of concentrated plasma ion density. For example, the region  190  may have on the average of about 10 11  ions/cc, while the region  191  may have on the average of about 1.5·10 10  ions/cc. The remainder of the chamber  100  outside of both regions  190 ,  191  has a much lower plasma ion density, on the average less than 10 10  ions/cc.  FIG. 2A  represents the distribution during the time that RF power is applied to the inner coil antenna  150  only.  FIG. 2B  illustrates the distribution shortly after power has been turned off and  FIG. 2C  illustrates the distribution after power has been turned off for a somewhat longer time. The time differences between  FIGS. 2A ,  2 B and  2 C may be on the order of 0.01-100 milliseconds. In  FIG. 2A , plasma ions concentrate over the center of the wafer. In  FIG. 2B , removal of RF power causes the plasma to drift out of the concentrated profile of  FIG. 2A  and begin to distribute outwardly away from the center. In  FIG. 2C , the continue drift of the plasma results in any even greater radial spreading of plasma ion distribution away from the center and toward the periphery of the process region. 
         [0018]    In one mode, the inner coil RF power is pulse modulated by the gate  162  with a desired repetition rate and duty cycle in which the plasma ion distribution corresponds to  FIG. 2A  during the “ON” time and during the time between pulses from the distribution of  FIG. 2A  to that of  FIG. 2B  and later to that of  FIG. 2C . The spreading of the distribution depicted in  FIGS. 2B and 2C  is halted at the beginning of the next cycle when RF power is again applied to the inner coil  150 . 
         [0019]    In another embodiment,  FIGS. 3A ,  3 B and  3 C depict plasma distribution across the process region overlying the wafer  126  in another chronological sequence involving the outer coil  155 . The regions labeled  193  and  194  in each of  FIGS. 3A ,  3 B and  3 C correspond to zones of concentrated plasma ion density. For example, the region  193  may have on the average of about 10 11  ions/cc, while the region  194  may have on the average of about 1.5.10 10  ions/cc. The remainder of the chamber  100  outside of both regions  193 ,  194  has a much lower plasma ion density, on the average less than 10 10  ions/cc.  FIG. 3A  represents the distribution during the time that RF power is applied to the outer coil antenna  155  only.  FIG. 3B  illustrates the distribution shortly after power has been turned off and  FIG. 3C  illustrates the distribution after power has been turned off for a somewhat longer time. The time differences between  FIGS. 3A ,  3 B and  3 C may be on the order of 0.01-100 milliseconds. In  FIG. 3A , plasma ions concentrate over or near the periphery of the wafer. In  FIG. 3B , removal of RF power causes the plasma to drift out of the concentrated profile of  FIG. 3A  and begin to distribute inwardly toward the center and away from the periphery. In  FIG. 3C , the continue drift of the plasma results in any even greater radial spreading of plasma ion distribution away from the periphery and toward the center of the process region. 
         [0020]    In one mode, the outer coil RF power is pulse modulated by the gate  167  with a desired repetition rate and duty cycle in which the plasma ion distribution corresponds to  FIG. 3A  during the “ON” time and during the time between pulses from the distribution of  FIG. 3A  to that of  FIG. 3B  and later to that of  FIG. 3C . The spreading of the distribution depicted in  FIGS. 3B and 3C  is halted at the beginning of the next cycle when RF power is again applied to the outer coil  155 . 
         [0021]    In one mode, pulses of RF power are applied alternately to the inner and outer coils  150 ,  155 .  FIGS. 4A and 4B  are contemporaneous time domain diagrams of enabling signals applied to the pulse gates  162 ,  167  by the controller  170  in such a mode.  FIGS. 5A through 5F  depict a chronological sequence of changing plasma ion distributions in the process zone over one cycle corresponding to  FIGS. 4A and 4B . Reference is now made to  FIGS. 4A ,  4 B and  5 A through  5 F. From time T 1  to time T 2 , RF power is applied to the inner coil  150  only, so that plasma distribution is concentrated over the center of the process region ( FIG. 5A ). At time T 2 , power is turned off, and the plasma begins to drift or progressively spread outward and away from the center to become less concentrated. This trend ( FIGS. 5B and 5C ) continues until time T 3 , when RF power is applied to the outer coil  155 . The causes the plasma to concentrate near the edge of the process zone ( FIG. 5D ). At time T 4 , power is turned off, and the plasma begins to drift toward the center of the process region, spreading more over time as depicted in  FIGS. 5E and 5F . This continues until time T 5 , when a new cycle is begun and power is again applied to the inner coil  150 , at which point the distribution returns to that depicted in  FIG. 5A . 
         [0022]    The resulting change in etch rate distribution is depicted in the chronological sequence of  FIGS. 6A through 6F . In  FIG. 6A , etch rate peaks at the wafer center (time T 1  through time T 2 ) during application of RF power to the inner coil  150 . In  FIGS. 6B and 6C , etch rate begins to decrease over the center and increase away from the center (time T 2  through T 3 ). In  FIG. 6D , progression of etch rate away from the center and toward the edge results in concentration of the maximum etch rate at the periphery (time T 3  through T 4 ) while RF power is applied to the outer coil  155 . In  FIGS. 6E and 6F , RF power is turned off and etch rate distribution drifts back away from the periphery and toward the center. The foregoing cycle repeats itself at time T 5 . 
         [0023]    Etch results on the wafer at the end of the etch process are the time-average of all of the etch rate distributions (samples of which are depicted in  FIGS. 6A through 6F ) over the entire etch process. The progression of etch rate distribution actually consists of a continuum of distributions, not merely the six discrete distributions of  FIGS. 6A through 6F . The time average spans this continuum of distributions. This time average of etch rate (or ion density) distribution is far more uniform than can be achieved by the conventional methods in which RF power is continuously applied to one or both of the inner and outer coils  150 ,  155 . The plasma drift resulting in the progression of etch rate distribution as depicted in  FIGS. 6A-6F  provides a far more continuous movement of etch rate across the wafer, which tends to reduce or eliminate areas of lower etch rate or areas of peak etch rate. 
         [0024]    In an exemplary process, a polysilicon film overlying a thin gate oxide layer is to be etched to form polysilicon gates. A silicon etch gas, such a fluorine-containing species, is introduced into the chamber  100  of  FIG. 1  with the wafer  126  supported on the pedestal  120 . RF bias power is applied to the electrode  124  from the RF generator  180 . The controller  170  causes RF power to be applied alternately to the inner and outer coils  150 ,  155  in accordance with the waveforms of  FIGS. 4A and 4B . The duty cycles of the pulses applied to the inner coil gate  162  ( FIG. 4A ) and of the pulses applied to the outer coil gate  167  ( FIG. 4B ) are adjusted to maximize uniformity of the time average of the progression of etch rate distributions realized over many cycles of the waveforms of  FIGS. 4A and 4B . 
         [0025]    For example, if the etch rate distribution is too concentrated at the center during the inner coil on-time (times T 1 -T 2 ) and moreover is too concentrated at the edge during the outer coil on-time (times T 3 -T 4 ), then this excessive concentration is compensated by reducing the duty cycles of both the pulses applied to the inner coil gate  162  ( FIG. 4A ) and the pulses applied to the outer coil gate  167  ( FIG. 4B ). This allows greater time during which no RF power is applied to either coil  150 ,  155  and the plasma drifts away from its more concentrated distribution states, and provides a more uniform time-averaged etch rate distribution. The duty cycles of the control pulses governing the pulsed RF on the two coils  150 ,  155  ( FIGS. 4A and 4B ) may be the same or may be different depending upon the differences in design or performance of the two coils  150 ,  155 . In the illustrated example of  FIGS. 4A and 4B , the duty cycles of the pulse waveform of  FIG. 4A  and the pulse waveform of  FIG. 4B  are approximately the same and are on the order of about ⅙. However, other choices of duty cycle may be made depending upon a particular reactor design and process recipe. In another embodiment, the duty cycle may be increased, so as to decrease the power off interval (e.g., from time T 2  to time T 3 ) to a lesser time period. In the example of  FIGS. 4A and 4B , the pulse widths of the two control signals are depicted as being the same. However, the pulse widths (duty cycles) of the pulse signals of  FIGS. 4A and 4B  may be chosen independently and differ significantly from one another. 
         [0026]    As one example involving different duty cycles applied to the inner and outer coils  150 ,  155 , if etch rate is higher over the center and weaker at the periphery, then the duty cycle of the pulse waveform of  FIG. 4A  applied to the inner coil gate  162  may be decreased and/or the duty cycle of the pulse waveform of  FIG. 4B  applied to the outer coil gate  167  may be increased. Conversely, if etch rate is predominant over the periphery and weak at the center, then the duty cycle of the pulse waveform of  FIG. 4A  applied to the inner coil gate  162  may be increased and/or the duty cycle of the pulse waveform of  FIG. 4B  applied to the outer coil gate  167  may be decreased. 
         [0027]    In some applications, the duty cycles of the pulsing of the gates  162 ,  167  may be set to relatively high values. For example, both duty cycles may exceed 50%, in which case the “on” time periods of the two coils  150 ,  155  will be partially contemporaneous or overlapping. In other words, each coil will be turned off after the other coil has been turned on. In this case, RF coupling between the two coils can be minimized by offsetting the frequencies of the two RF generators  160 ,  165 . As one possible example of this, the two RF generators may have respective frequencies of 2.75 MHz and 2.25 MHz. 
         [0028]    The period or length of one cycle in the pulse waveforms of  FIGS. 4A and 4B  (i.e., the period from time T 1  to time T 5 ) is in one embodiment relatively short, for example on the order of 0.01-100 milliseconds. Shorter values of this time period provide the best continuity of etch performance and minimize fluctuations within a single etch process or step. 
         [0029]    A process in accordance with one embodiment is depicted in  FIG. 7 . The wafer  126  is placed in the chamber  100  and a process gas is introduced (block  200  of  FIG. 7 ). RF power is applied to the inner and outer coils  150 ,  155  through the respective pulse gates  162 ,  167  (block  210 ). The controller  170  enables power flow through the respective gates  162 ,  167  during alternate time windows defined by applying alternating pulses waveforms to the gates  162 ,  167  (block  215 ). The duty cycles of the respective pulse waveforms are adjusted to optimize uniformity of radial ion distribution over the wafer (block  220 ). If the inner coil  150  produces an excessively concentrated or peak ion distribution or etch rate over the wafer center, then the duty cycle of the pulse waveform applied to the inner coil  150  is reduced (block  221 ). This decrease in inner coil duty cycle allows more time for plasma drift following temporary RF power removal to spread or even out the ion distribution or counter the non-uniform distribution created during the preceding pulse of RF power. Alternatively (or in addition) to decreasing the inner coil duty cycle, the outer coil duty cycle may be increased, provided that this increase does not result in excessive ion concentration by the outer coil  155 . If the outer coil  155  produces an excessively concentrated or peak ion distribution or etch rate over the wafer periphery, then the duty cycle of the pulse waveform applied to the outer coil  155  is reduced (block  222 ). This decrease in outer coil duty cycle allows more time for plasma drift following temporary RF power removal to spread or even out the ion distribution or counter the non-uniform distribution created during the preceding pulse of RF power. Alternatively (or in addition) to decreasing the outer coil duty cycle, the inner coil duty cycle may be increased provided that this increase does not result in excessive ion concentration by the inner coil  150 . In one embodiment, if a higher overall plasma ion density (or higher process rate or higher etch rate) is desired, then the duty cycle of one or both waveforms may be increased up to a point at which uniformity may be compromised (block  223 ). 
         [0030]    While foregoing embodiments have been described with reference to RF generators  160 ,  165  with pulsed gates  162 ,  167  for pulse modulating the RF outputs of the generators  160 ,  165 , the generator  160  and corresponding gate  162  may be combined in one unit as a commercially available pulse-modulated RF generator. Likewise, the generator  165  and corresponding gate  167  may be combined in another similar unit. 
         [0031]    While the foregoing description of embodiments having at least two coils (e.g., the inner and outer coils  150 ,  155 ) have been described with reference to operational modes in which the RF power to both coils is pulse-modulated, in another embodiment both coils  150 ,  155  are driven with RF power but only one of the two coils is driven with pulse-modulated RF power. In such an embodiment, for example, RF power to the inner coil  150  would be pulse modulated in accordance with the sequence of  FIG. 4A , while RF power to the outer coil would be applied continuously. Alternatively, RF power to the outer coil  155  would be pulsed modulated in accordance with the sequence of  FIG. 4B , while RF power to the inner coil would be applied continuously. The controller  170  may be configured to implement either of these embodiments by pulsing one of the two gates  162 ,  167  while continuously enabling (holding “on”) the other of the two gates. This corresponds to an operational mode in which the pulse duty cycle to one of the coils is 100%. 
         [0032]    While the foregoing description of embodiments having at least two coils (e.g., the inner and outer coils  150 ,  155 ) have been described with reference to separate independent RF power generators for each coil (e.g., the RF power generators  160 ,  165 ), in another embodiment only a single RF generator is employed and has its RF output power apportioned among the different coils. For example, as indicated in dashed line in  FIG. 1 , the RF generator  160  may have its output coupled to both gates  162 ,  167 . In this case, the individual gates  162 ,  167  perform the pulse-modulation functions governed by the controller  170  as described above, and in addition each includes conventional RF circuitry that enables the controller  170  to control the amount of RF power admitted through each of the gates  162 ,  167 . With this latter feature, the controller  170  in this embodiment apportions the RF power from the generator  160  to the two coils  150 ,  155 . 
         [0033]    While foregoing embodiments have been described with reference to an inductively coupled RF power applicator consisting of two coils, an inner coil  150  and an outer coil  155 , in another embodiment there may be only a single coil (e.g., either the inner coil  150  or the outer coil  155  or a single coil at an intermediate location). Alternatively, more than one coil may be present, but only a single coil is driven by RF power, the remaining coil (or coils) being inactive. The single coil would be driven by a single RF power generator (e.g., the generator  160 ) with pulse modulation of the RF power being performed by a pulsed gate (e.g., the gate  162 ) pulsed by the controller  170  in accordance with a chosen pulsing sequence. 
         [0034]    While embodiments having more than one coil have been described above with reference to two coils (i.e., the inner and outer coils  160 ,  165 ), such embodiments may have more than two coils, e.g., three or four or more coils. Generally at least some or all of such coils may be concentric as in the embodiment of  FIG. 1 . 
         [0035]    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.