Abstract:
In a plasma reactor for processing a semiconductor wafer having an overhead inductive coil antenna, automatic compensation for the load impedance shift that accompanies plasma ignition is achieved using fixed elements. This is accomplished by applying RF power to an intermediate tap of the coil antenna that divides the antenna into two portions, while permanently suppressing the inductance of one of the two portions to an at least nearly fixed level.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns impedance matching between an RF power source and a tapped coil source power applicator of a plasma reactor. 
     Referring to FIG. 1, a plasma reactor for processing a semiconductor workpiece (e.g., a silicon wafer for fabrication of computer chips) includes a reactor chamber  100  with a ceiling  110 , the chamber enclosing a wafer support pedestal  115  for holding a semiconductor wafer  120  during processing. Processing gases are introduced into the chamber  100  through gas inlets  125  and are ionized to form a plasma in the chamber  100  by RF power radiated from an overhead coil antenna  130 . The coil antenna  130  consists of at least one coiled conductor wound to form a number of windings. RF power is applied to the antenna  130  by an RF plasma source power generator  135  connected at a tap point  137  on an intermediate winding  140  of the coil antenna  135 . The circuit is completed by connecting the antenna&#39;s top winding  145  to RF return or ground through a capacitor  150  and connecting the antenna&#39;s bottom winding  155  directly to ground. The capacitor  150  is selected to form a resonant circuit with the inductive coil antenna  130  with a resonance near the desired frequency of the RF generator  135 . Generally, the load impedance presented by the combination of the coil antenna  130  and the chamber (both before and after plasma ignition) differs from the output impedance of the RF generator  135 . The greater the difference in impedance, the more RF power is reflected back to the RF generator and the less power is delivered to the chamber. For this reason, the typical RF generator itself has a limited capability to maintain the forward power at a nearly constant level even as more RF power is reflected back to the generator as the plasma impedance fluctuates. Typically, this is achieved by the generator servoing its output power level, so that as an impedance mismatch increases (and therefore reflected power increases), the generator increases its output power level. Of course, this capability is limited by the maximum output power of which the generator is capable of producing. Typically, the generator is capable of handling a maximum ratio of forward standing wave voltage to reflected wave voltage (i.e., the voltage standing wave ratio or VSWR) of not more than 3:1. If the difference between impedances increases (e.g., due to plasma impedance fluctuations during processing) so that the VSWR exceeds 3:1, then the RF generator can no longer control the delivered power, and control over the plasma is lost. As a result, the process is likely to fail. Therefore, at least an approximate impedance match must be maintained between the RF generator  135  and the load presented to it by the combination of the coil antenna  130  and the chamber  100 . This approximate impedance match must be sufficient to keep the VSWR at the generator output within the 3:1 VSWR limit over the entire anticipated range of plasma impedance fluctuations. The impedance match space is, typically, the range of load impedances for which the match circuit can maintain the VSWR at the generator output at or below 3:1. 
     One difficulty with the reactor of FIG. 1 is that when RF power is first applied, there is no plasma in the chamber  100 . Thereafter, the load impedance undergoes a very large abrupt change upon plasma ignition. This is because after plasma ignition the coil antenna induces mirror currents in the plasma which oppose the coil EMF and thereby effectively reduce the coil inductance. This reduction in inductance changes the load impedance of the coil antenna, so that the pre-plasma ignition load impedance significantly differs from the post-plasma ignition load impedance. The difference between the pre-and post-plasma ignition impedances is so great that it is not possible to provide an optimal impedance match prior to and after plasma ignition. This is because, typically, the impedance match space provided by a conventional fixed impedance match circuit is not sufficiently broad to encompass both the pre-ignition load impedance and the post-ignition load impedance. As stated above, the impedance match space is, typically, the range of load impedances for which the match circuit can maintain the VSWR at the generator output at or below 3:1. Even if the match space were sufficiently broad to encompass both the pre- and post-ignition load impedances, the system would have to be carefully tuned since the margin by which the impedance match space could cover both impedances would be relatively narrow. Thus, the useful impedance match space during plasma processing would necessarily be significantly constricted. As a result, the processing window of the reactor is constricted to avoid swings in plasma load impedance which would take the load impedance outside the constricted impedance match space. 
     Some compromise must be made in the selection of RF frequency, capacitance of the capacitor  150  and antenna inductance so that the VSWR limitations of the RF generator  135  are met both prior to and after plasma ignition. This situation is illustrated in the Smith chart of FIG. 2, in which reactance is plotted on the imaginary vertical axis and resistance is plotted on the real horizontal axis. Z 1  is the pre-plasma ignition load impedance and Z 2  is the post-plasma load impedance of an exemplary plasma reactor. Their location is a function of the capacitance of the tuning capacitor  150 , which must be carefully selected. With such a selection, the load impedances Z 1  and Z 2 , together with the RF generator output impedance Z 0  of 50 Ohms, provide reflection coefficients (Z 1 /Z 0  and Z 2 /Z 0 , respectively) that do not exceed the 3:1 VSWR capability of the RF generator  150 . However, this condition is satisfied by a small margin, so that the system is susceptible to failure during processing occasioned by wide swings in the plasma impedance. 
     One compromise that can be made (by an appropriate selection of the tuning capacitor etc. in accordance with conventional techniques) is to center the limited impedance match space around the post-ignition load impedance. This provides an optimum match to the post-ignition load impedance to optimize control during processing. It also provides a correspondingly inferior impedance match to the pre-ignition load impedance which must be, however, sufficient to couple enough power to ignite a plasma. Of course, such an arrangement is unreliable. Alternatively, some type of dynamic impedance matching device must be employed, which would increase system cost and complexity. 
     Therefore, there is a need to provide a fixed impedance match with a sufficiently large match space to accommodate both the pre-ignition load impedance and the post-ignition load impedance. 
     The present invention provides a way of following the abrupt impedance change characteristic of plasma ignition without a dynamic impedance matching device. 
     SUMMARY OF THE INVENTION 
     In a plasma reactor for processing a semiconductor wafer having an overhead inductive coil antenna, automatic compensation for the load impedance shift that accompanies plasma ignition is achieved using fixed elements. This is accomplished by applying RF power to an intermediate tap of the coil antenna that divides the antenna into two portions, while permanently suppressing the inductance of one of the two portions to an at least nearly fixed level. For this purpose, an inductance-suppressing conductive body is held sufficiently close to one of the two portions so as to fix the inductance of the one portion at a suppressed level that is at least nearly constant over plasma ignition, leaving the inductance of the other portion unsuppressed and free to fall when a plasma is ignited and rise when it is extinguished. The resulting change in the ratio of the inductances of the two portions upon plasma ignition automatically compensates for the change in load impedance that occurs upon plasma ignition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a plasma reactor embodying the present invention. 
     FIG. 2 is a graph chart of a complex plane illustrating the match space of a typical reactor. 
     FIG. 3 is an equivalent circuit of the power applicator circuit in the embodiment of FIG. 1 prior to plasma ignition. 
     FIG. 4 is an equivalent circuit of the power applicator circuit in the embodiment of FIG. 1 after plasma ignition. 
     FIG. 5 is a top view of a conductive ring employed in the embodiment of FIG. 1 to render the inductance of certain portions of the inductive power applicator less variant during plasma ignition than other portions. 
     FIG. 6 is a diagram of one alternative embodiment of the invention. 
     FIG. 7 is top view of the conductive body employed in the embodiment of FIG.  6 . 
     FIG. 8 is a diagram of a variation of the alternative embodiment of FIG.  6 . 
     FIG. 9 is a diagram of another alternative embodiment of the invention. 
     FIG. 10 is a diagram of a further alternative embodiment of the invention. 
     FIG. 11 is a diagram of a still further alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, automatic compensation for the change in load impedance occurring upon plasma ignition is achieved without any moving parts by providing a stationary conductive body  160  (preferably a metal ring) adjacent a selected portion of the coil antenna  130 . In the preferred embodiment, the coil antenna  130  is unevenly divided into two portions, namely a longer portion  130   a  and a shorter portion  130   b,  separated at the power input tap  137  on the winding  140 , and the conductive body  160  is adjacent the shorter portion  130   b.  The conductive body  160  is sufficiently close to the antenna portion  130   b  so that the inductive field of the antenna portion  130   b  induces mirror currents in the conductive body  160 . These mirror currents oppose the induced EMF of the antenna portion  130   b,  which reduces the inductance of the lower antenna portion  130   b.  Since the conductive body is relatively far away from the other antenna portion  130   a,  it has relatively no effect thereon. As will be explained below, this feature automatically reduces (or eliminates) the large shift in load impedance presented by the coil antenna that otherwise occurs upon plasma ignition. As a result, there is less degradation in generator-antenna impedance match upon plasma ignition. The system parameters are selected so that an impedance match exists prior to plasma ignition, and this impedance match is not lost upon plasma ignition. Specifically, the frequency of the generator  135  and the capacitance of the capacitor  150  may be selected for an optimum impedance match that obtains both before and after plasma ignition. 
     In one exemplary implementation, the antenna  130  consisted of six windings conformal in shape with a multi-radius dome-shaped ceiling, the tap  137  was located about 45″ (inches) from the bottom coiled conductor end as measured along the length of the conductor, the effective system capacitance including the capacitor  150  was about 63 micro-farads and the RF generator had a frequency of 2 MHz. The conductive body  160  was of copper metal. 
     That this is so will now be shown by an analysis of the embodiment of FIG. 1 with reference to the equivalent circuits of FIGS. 3 and 4. FIG. 3 is the equivalent circuit of the embodiment of FIG. 1 prior to plasma ignition. The coil antenna  130  has a total inductance L. The lower antenna portion  130   b  contributes to that total inductance with its own smaller inductance L 1 . The inductance of the antenna portion  130   b  is smaller because the antenna portion  130   b  is shorter than the remainder of the antenna  130  and is further reduced by the presence of the conductive body  160 . The resistance r in FIG. 3 represents the resistive losses in the system prior to plasma ignition. The capacitance C in FIG. 3 is the capacitance of the capacitor  150  in FIG.  1 . The load impedance Z presented by the coil antenna  130  to the RF generator  135  is determined by the ratio a of the lower antenna portion inductance L 1  to the total coil inductance L, in accordance with the following equation: 
     
       
           Z=α   2   [L/Cr]   (1) 
       
     
     where: 
     
       
         α= L   1 / L.   (2) 
       
     
     FIG. 4 illustrates the situation after plasma ignition. A plasma  170  is now adjacent the coil antenna  130  and acts to a limited extent like a conductor (with a plasma conductivity less than that of metal) in that the inductive field of the coil antenna  130  induces mirror currents in the plasma  170 . These mirror currents reduce the inductance of the coil antenna by an amount dL so that the total coil antenna inductance L becomes L−dL. However, the inductance of the lower antenna portion  130   b  adjacent the conductive body  160  is already reduced by the mirror currents in the conductive body  160 . Since the conductivity of the plasma is less than that of the conductive body  160 , the influence of the plasma on the inductance of the lower antenna portion  130   b  is insignificant or small relative to the effect of the conductive body. Thus, the appearance of the plasma  170  has little or no effect upon the lower antenna portion inductance L 1 , relative to the reduction in the antenna inductance in the other antenna portion  130   a  caused by the plasma. The conductive body  160  is adjacent only to the lower antenna portion  130   b  so that its effects are localized to that portion and do not affect the inductance of the upper portion  130   a.  Thus, at least to a first approximation, L 1  does not change upon plasma ignition, while L experiences a significant change because the other antenna portion  130   a  not covered by the conductivity is more susceptible to a reduction in its inductance by the plasma. As a result, the ratio α=L 1 /L increases upon plasma ignition because L decreases while L 1  remains constant (or at least nearly so). As a result, the decrease in L, which would otherwise cause an abrupt decrease in the load impedance Z, is compensated by the corresponding increase in α. This is because in Equation 1 above the load impedance Z is a function of the product of α 2  and L. Since α is squared in this Equation 1, a small increase in a compensates for a large decrease in L. Thus, L 1  does not necessarily need to remain constant before and after plasma ignition in order to compensate for the reduction in L upon plasma ignition. The result is that the load impedance Z presented to the RF generator  135  remains more constant than in the prior art, a significant advantage, and can be made to remain at least nearly perfectly constant, depending upon the conductivity, size and proximity of the conductive body to the smaller antenna portion  130   b.  Accordingly, the match space of the system encompasses both the pre-plasma ignition load impedance and the post-plasma ignition load impedance. 
     It is preferable that the conductive body  160  not provide a continuous conductive path around the lower antenna portion  130   b  because. Otherwise, it is difficult to strike or maintain a stable plasma. Therefore, as illustrated in FIG. 5, there is at least one radial slot  165  across the width of the conductive body  160 , and additional slots may be provided as indicated in dashed line in FIG.  5 . The conductive body  160  may be a ring of square cross-section as shown in FIGS. 1 and 5. The conductive body  160  should be sufficiently close to the short antenna portion  130   b  to produce the desired mirror currents in the conductive body  160 . 
     In one embodiment, the conductive body  160  was separated from the lower antenna portion  130   b  by one skin depth of the inductive field of the coil antenna  130 . The skin depth is a well-known function of the frequency of the RF generator and other parameters and is readily computed by the skilled worker. In the preferred embodiment, the conductive body  160  was within about one inch of the short antenna portion  130   b  for an RF frequency of about 2 MHz. At this close distance, the effect of the conductive body  160  on the short antenna portion  130   b  is maximized. Moreover, at such a close distance to the lower antenna portion  130   b,  the distance between the conductive body  160  and the longer antenna portion  130   a  is comparatively much longer and therefore its effect on the longer antenna portion  130   a  is relatively insignificant or reduced. Thus, the inductance of the upper antenna portion  130   a  is free to change upon plasma ignition while the inductance of the lower portion  130   b  is fixed at a nearly constant value by the conductive body  160 . 
     While in the preferred embodiment of FIG. 1 the conductive body  160  faces only the lower antenna portion  130   b  to achieve the differential response of the two antenna portions  130   a,    130   b  to the plasma, a differential response may also be achieved in an embodiment in which a conductive body faces both antenna portions  130   a,    130   b.  This is accomplished by sculpting the conductive body so that its spacing relative to the two antenna portions is different. For example, in the alternative embodiment of FIG. 6, a conductive body  510  is a tapered cylindrical sheet that faces or covers nearly the entire coil antenna  130 . However, the conductive body  510  is shaped so that it tapers away from the top of the antenna  130  and towards the bottom of the antenna  130  so that, on average, it is closer to the shorter antenna portion  130   b  at the bottom and farther away from the longer antenna portion  130   a  that extends to the top. In this embodiment, the coil antenna  130  has a dome shape so that the tapered cylindrical shape (a truncated cone shape) of the conductive body  510  achieves the desired differential spacing of the conductive body  510  relative to the two antenna portions  130   a,    130   b.  Referring to the top view of FIG. 7, the conductive body  510  preferably is separated by at least one axial slit  520  extending along its entire axial length, although additional axial slits may be provided as indicated in dashed line. Referring to the side view of FIG. 8, the conductive body  510  may be separated into an array of conductive sub-bodies  531 ,  532 ,  533 , etc., by at least one axial slit  520  and at least one circumferential slit  540 , although more than one axial slit and more than one circumferential slit may be provided. The respective displacements of the individual sub-bodies are each selected to provide the above-described differential responses of the two antenna portions  130   a,    130   b  to the plasma. In the embodiment of FIG. 8, the configuration of the conductive sub-bodies conforms to the tapered cylindrical shape of FIG.  6 . The array of sub-bodies may cover all of the coil antenna  130  (as illustrated in FIG. 8) or may leave selected portions uncovered in other alternative embodiments. 
     The invention is not confined to a coil antenna having a particular shape (such as the dome shape of the foregoing embodiments), but is adaptable to other coil antenna shapes. For example, in the alternative embodiment of FIG. 9, the coil antenna  130 ′ has a tapered cylindrical shape (a truncated cone shape). In this case, a conductive body  610  having a curved cross-sectional shape that curves outwardly away from the top of the antenna  130 ′ achieves the desired differential spacing with respect to the top and bottom antenna portions  130 ′ a,    130 ′ b.  In the alternative embodiment of FIG. 10, a conductive body  710  has a tapered cylindrical shape that is tapered differently from the tapered cylindrical shape of the coil antenna  130 ′. In FIG. 10, the conductive body  710  actually is tapered oppositely from the coil antenna  130 ′, although this may not be necessary in other embodiments. Specifically, the conductive body  710  tapers outwardly from bottom to top, while the coil antenna  130 ′ tapers inwardly from bottom to top. As in the embodiments of FIGS. 6,  7  and  8 , the conductive bodies in the embodiments of FIGS. 9 and 10 may be separated by one or more axial slits and/or circumferential slits in the manner of FIGS. 7 or  8 . 
     The invention is not limited to a coil antenna having a single power input tap (e.g., the tap  137  of FIG.  1 ). For example, a coil antenna may have more than one power input tap, as in the embodiment of FIG.  11 . In FIG. 11, a coil antenna consisting of a single coiled conductor  800  has its top and bottom terminating ends  810 ,  820  connected to RF return (ground) through respective tuning capacitors  830 ,  840 , and one of its intermediate windings  850  connected directly to ground at a middle ground tap  855 . The coil antenna  800  is thus divided into two sections by the middle ground tap  855 , namely an upper section  860  an lower section  870 . RF power is applied to the coil antenna  800  at a power input tap to each of the two sections. Specifically, an RF generator  880  applies RF power to a upper tap  865  connected to a winding in the upper coil section  860  and to a lower tap  875  connected to a winding in the lower coil section  870 . The upper coil section  860  is thus divided into an upper portion  860 a and a lower portion  860   b,  the division preferably being uneven so that the upper portion  860   a  is shorter than the lower portion  860   b.  Similarly, the lower coil section  870  is divided into an upper portion  870   a  and a lower portion  870   b,  the division preferably being uneven so that the upper portion  870   a  is shorter than the lower portion  870   b.  In the manner of FIG. 1, conductive bodies  890 ,  895  are placed adjacent the two shorter portions  860   a,    870   a,  of the respective antenna sections  860 ,  870 . The circuits of the upper and lower sections  860 ,  870  including their respective conductive bodies  890 ,  895  mirror one another and replicates the individual circuit of the embodiment of FIG.  1 . Thus, the upper and lower antenna sections with their respective conductive bodies act to automatically compensate for the shift in plasma impedance that occurs upon plasma ignition in the manner described above with respect to FIG.  1 . 
     The result is that the match space of the system is greatly extended to encompass both the pre-plasma ignition load impedance and the post-plasma ignition load impedance. Thus, no compromise is necessary in providing a match space under either condition. Heretofore, the match space during plasma processes was necessarily limited by the necessity of such a compromise. One advantage of the invention is that, with the elimination of any compromise in match space, the entire match space may be optimized for plasma processing (i.e., for the post-plasma ignition load impedance), so that the process window of the reactor is greatly expanded. With such an expanded process window, the reactor performance is more reliable and versatile and is susceptible of a broader range of process recipes, a significant advantage. 
     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.