Abstract:
A plasma reactor includes a vacuum chamber having an interior and a pedestal within the chamber for supporting a semiconductor wafer to be processed. Gas distribution apparatus introduces a process gas into said chamber. Power is applied to the chamber by plural concentric coaxial waveguides outside of said chamber having an axis of propagation pointing toward the interior of said chamber and establishing corresponding annular zones of radiation within said chamber. The reactor further includes apparatus that can apply different levels of electromagnetic radiation power to different ones of the plural concentric coaxial waveguides.

Description:
BACKGROUND  
         [0001]    1. Technical Field  
           [0002]    The invention concerns a plasma reactor for processing a semiconductor wafer using microwaves for plasma source power.  
           [0003]    2. Background Art  
           [0004]    Plasma reactors are employed in semiconductor wafer processing for etching thin films such as silicon dioxide, silicon and aluminum films and for depositing thin films such as an epitaxial silicon layer. For example, in a typical silicon dioxide etch process, a crystalline silicon wafer is placed in a vacuum chamber of the plasma reactor and a process gas containing a fluorine compound is introduced into the reactor. An electromagnetic source of energy, such as microwaves or RF power, is applied to the chamber to ionize the process gas, thereby producing free fluorine. In a chemical vapor deposition process, a similar procedure is followed except that the process gas contains a compound of the species (e.g., silicon) to be deposited as an epitaxial layer.  
           [0005]    In either an etch process or a chemical vapor deposition process, the uniformity with which the process is carried out across the wafer surface determines whether the process succeeds. As device geometries continue to shrink and device densities climb in the microelectronic industry, process uniformity must improve. A principal factor in determining process uniformity is distribution of plasma ion density across the wafer surface. This is because plasma ion density determines various key parameters in a plasma process. For example, plasma ion density determines etch rate in a plasma assisted etch process and determines deposition rate in a plasma assisted chemical vapor deposition process. Such parameters must be uniform across the wafer surface. Otherwise, an etch process, for example, will cause overetching of devices in one zone of the wafer and underetching of devices in other zones of the wafer.  
           [0006]    One of the principal factors determining plasma ion density distribution at the wafer surface is the distribution of plasma source power density within the chamber. A key aspect of plasma ion density distribution is the radial distribution of plasma ion density. In plasma reactors in which plasma source power is applied by microwave power applicators, there is a need to adjust or improve the uniformity of the radial distribution of plasma ion density across the wafer surface. For example in some cases, plasma ion density is greater in one zone (e.g., near the wafer center) and less in another zone (e.g., near the wafer periphery. There is a need to reduce such non-uniformity by adjusting the radial distribution of plasma ion density without having to replace or redesign elements of the reactor chamber with each adjustment.  
         SUMMARY  
         [0007]    A plasma reactor includes a vacuum chamber having an interior with a vacuum pump coupled to the chamber and a pedestal within the chamber for supporting a semiconductor wafer to be processed. Gas distribution apparatus introduces a process gas into said chamber. Power is applied to the chamber by plural concentric coaxial waveguides outside of said chamber having an axis of propagation pointing toward the interior of said chamber and establishing corresponding annular zones of radiation within said chamber. The reactor further includes apparatus that can apply different levels of electromagnetic radiation power to different ones of the plural concentric coaxial waveguides.  
           [0008]    In one embodiment, the apparatus that can apply different levels of electromagnetic radiation power are plural electromagnetic wave power sources coupled to respective ones of the plural concentric waveguides, each of the plural electromagnetic wave power sources being adjustable relative to one another for adjustment of electromagnetic radiation power within each of the annular radiation zones of said chamber.  
           [0009]    In another embodiment, each of the plural concentric waveguides has an annular input end facing away from the chamber for receiving electromagnetic radiation, and the apparatus that can apply different levels of electromagnetic radiation power is a single coaxial waveguide having an input end and an output end, the output end being coupled to the annular input end of each of said plural concentric coaxial waveguides. An electromagnetic wave power source is coupled to the single concentric waveguide. The different amount of radiation apportioned to the different concentric coaxial waveguides is determined by tuning the openings of the input ends of the different waveguides.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a diagram of a plasma reactor of a first embodiment.  
         [0011]    [0011]FIG. 2 depicts a waveguide power applicator of the embodiment of FIG. 1.  
         [0012]    [0012]FIG. 3 is a plan view corresponding to FIG. 2.  
         [0013]    [0013]FIG. 4 depicts a waveguide power applicator in accordance with a second embodiment.  
         [0014]    [0014]FIG. 5 depicts a waveguide power applicator in accordance with a third embodiment.  
         [0015]    [0015]FIG. 6 depicts a waveguide power applicator in accordance with a fourth embodiment.  
         [0016]    [0016]FIG. 7 is a plan view corresponding to FIG. 6.  
         [0017]    [0017]FIG. 8 depicts a waveguide power applicator in accordance with a fifth embodiment.  
         [0018]    [0018]FIG. 9 depicts a waveguide power applicator in accordance with a sixth embodiment.  
         [0019]    [0019]FIG. 10 depicts one implementation of the embodiment of FIG. 8.  
         [0020]    [0020]FIG. 11 depicts a second implementation of the embodiment of FIG. 8.  
         [0021]    [0021]FIG. 12 depicts a third implementation of the embodiment of FIG. 8.  
         [0022]    [0022]FIG. 13 illustrates a mechanical shutter of the type employed in the implementation of FIG. 12.  
         [0023]    [0023]FIG. 14 depicts a waveguide power applicator in accordance with a seventh embodiment.  
         [0024]    [0024]FIG. 15 depicts a waveguide power applicator in accordance with an eighth embodiment.  
         [0025]    [0025]FIG. 16 depicts a waveguide power applicator in accordance with a ninth embodiment.  
         [0026]    [0026]FIGS. 17A, 17B and  17 C illustrate, respectively, plasma radial density distribution produced by a radially uniform power source, a power density radial pattern that the illustrated embodiments are capable of producing, and an improved radial distribution of plasma density produced by the power density distribution of FIG. 17B. 
     
    
     DETAILED DESCRIPTION  
       [0027]    Referring to FIGS. 1, 2 and  3 , a plasma reactor  100  includes a side wall  105  and a ceiling  110  defining a vacuum chamber  115 . The chamber  115  houses a wafer support pedestal  120  for supporting a semiconductor wafer  125  to be processed. A pumping annulus  130  is coupled at the floor of the chamber  100  to a vacuum pump  135 . A gas manifold  140  furnishes process gas from as gas supply  145  to plural orifices or gas injection nozzles  150  that spray gas into the chamber  100 . FIG. 1 shows plural gas injection nozzles  150  extending through the side wall  105 . In other implementations, the process gas may be injected through plural gas injection orifices in another part of the chamber, such as the ceiling  110  for example.  
         [0028]    Microwave power is applied to the chamber interior by a coaxial waveguide  160  facing the ceiling  110 . The axis of the waveguide  160  can be at least nearly parallel and coincident with the axis of the wafer  125 , of the pedestal  120  and of the chamber  100 , as shown in FIG. 1. However, the axis of the waveguide  160  may be non-parallel and/or non-coincident with the axis of the wafer  125 , the pedestal  120  or the chamber  100 .  
         [0029]    The coaxial waveguide  160  consists of plural concentric coaxial waveguides  161 ,  162 ,  163  with an axial conductor  170  along the axis of the coaxial waveguide  160 . The three concentric coaxial waveguides  161 ,  162 ,  163  consist of respective concentric cylindrical conductors  161   a,    162   a,    163   a  terminated at the bottom in a quartz window  175  that constitutes the ceiling  110  of the chamber  100 . The three waveguides  161 ,  162 ,  163  are terminated at the top by flat annular conductors  161   b,    162   b,    163   b.  The axial conductor  170  is the center conductor of the innermost coaxial waveguide  161 . The innermost coaxial waveguide  161  functions as the center conductor of the intermediate coaxial waveguide  162 . The intermediate coaxial waveguide  162  functions as the center conductor of the outermost coaxial waveguide  163 .  
         [0030]    In accordance with an optional feature, the waveguides  161 ,  162 ,  163  can be tuned individually with respective conductive annular plungers  602 ,  604 ,  606  inside the input ends of the respective waveguides  161 ,  162 ,  163  by axially moving any one or all of the plungers  602 ,  604 ,  606 . Each of the plungers  602 ,  604 ,  606  is in electrical contact with the respective waveguide.  
         [0031]    Microwave power is coupled into the waveguides  161 ,  162 ,  163  by respective wire or rod radiators  181 ,  182 ,  183  protruding into the top portion of the corresponding waveguides  161 ,  162 ,  163 . Each of the wire radiators  181 ,  182 ,  183  is connected to an independently adjustable microwave power source  185 ,  186 ,  187 , respectively. The three microwave sources  185 ,  186 ,  187  may be a separate microwave generator, such as a magnetron and may include an impedance matching circuit of the type well-known in the art. Alternatively, the three microwave sources  185 ,  186 ,  187  may be a single microwave generator and a three-way power splitter that produces three microwave outputs whose power levels may be adjusted relative to one another.  
         [0032]    Microwave propagation in the coaxial waveguides  161 ,  162 ,  163  is in the transverse electric mode (TEM) in which the electric field is radial while the magnetic field is circular. The coaxial waveguides  161 ,  162 ,  163  contribute to uniform distribution of microwave power across the wafer surface because their cylindrical symmetry corresponds to the cylindrical symmetry of the chamber  100  and the cylindrical symmetry of the wafer  125 . The coaxial waveguides  161 ,  162 ,  163  provide much greater bandwidth than other types of waveguides, since coaxial waveguides generally can support a broad spectrum of electromagnetic radiation both within and below microwave frequencies. The three waveguides  161 ,  162 ,  163  establish three annular zones over the wafer  125  within which microwave power density is independently adjustable. This feature solves the problem of non-uniform radial distribution of plasma ion density, because it enables the adjustment of the radial distribution of microwave power over the wafer surface in order to adjust or optimize the radial distribution of plasma ion density. For example, if a uniform application of the microwave power to each of the three waveguides  161 ,  162 ,  163  produces a non-uniform center-high radial distribution of plasma ion density at the wafer surface, then a more uniform distribution of plasma ion density may be realized by applying less power to the innermost waveguide  161  and the most power to the outermost waveguide  163 .  
         [0033]    In the embodiment of FIG. 1, the three waveguides  161 ,  162 ,  163  have different axial lengths that increase from the outermost to the innermost waveguide so as to permit free access of each of the wire radiators  181 ,  182 ,  183  to the corresponding waveguide. However, in other embodiments the three waveguides can be of the same length.  
         [0034]    In the embodiment of FIGS.  1 - 3 , each wire radiator  181 ,  182 ,  183  consists of a thin straight conductor  180   a  that passes through an entry hole  161   c,    162   c,    163   c  in the respective waveguide and penetrates into the interior of the waveguide by a selected distance D, a 180 degree loop  180   b  and a second straight conductor  180   c  extending from the loop  180   b  and terminated on the interior surface of the waveguide near the entry hole. The penetration distance D is selected to optimize the impedance match between the wire radiator and the waveguide at the frequency of the power source  185 ,  186 ,  187 .  
         [0035]    The axial lengths of the waveguides  161 ,  162 ,  163  may be varied since the coaxial waveguides are very broadband devices, as noted above. However, their lengths may be selected to achieve resonance and/or impedance match at the frequency of the power source  185 ,  186 ,  187  using conventional analytical techniques. Alternatively or in addition, each source  185 ,  186 ,  187  may have its own impedance match device functioning in the conventional manner to minimize power reflected back to the source.  
         [0036]    If the three power sources  185 ,  186 ,  187  are independent, then the user may select, if desired, a different frequency for each one and select the corresponding waveguide axial length and wire radiator penetration distance D accordingly.  
         [0037]    While FIGS.  1 - 3  illustrate three concentric coaxial waveguides  161 ,  162 ,  163  within the waveguide  160 , other embodiments may include any other number of concentric waveguides from as few as two to as many as desired. A greater number of concentric waveguides enables a finer adjustment of radial distribution of power across the wafer surface.  
         [0038]    While the description of the embodiment of FIGS.  1 - 3  refers to the application of microwave power, frequencies other than microwave frequencies may be employed, since the coaxial waveguide  160  is a broadband device. For example, radio frequencies (UHF, VHF or HF) may be employed instead of microwave frequencies. Such different frequency selections may entail careful selection of various impedance matching measures, such as waveguide axial length, wire radiator penetration distance or use of dynamic impedance matching devices.  
         [0039]    [0039]FIG. 4 illustrates an embodiment in which the loop  180   b  and the second straight section  180   c  of each wire radiator  181 ,  182 ,  183  are eliminated, so that each wire resonator is simply a straight wire.  
         [0040]    [0040]FIG. 5 illustrates and embodiment in which cross-coupling through the quartz window  175  between various ones of the concentric waveguides  161 ,  162 ,  163  and mode generation within the quartz window  175  are prevented (or reduced) by the introduction of thin conductive cylindrical barriers  191 ,  192  within the quartz window  175  that separate the quartz window  175  into separate annular sections. The conductive cylindrical barriers  191 ,  192  coincide with respective ones of the cylindrical conductive walls  161   a,    162   a  of the concentric waveguides  161 ,  162 .  
         [0041]    Referring to FIGS. 6 and 7, power may be selectively apportioned among the three annular zones referred to above while using only a single power source with a single wire radiator to a single waveguide. Specifically, a single wire radiator  180  couples power near the top of a single coaxial waveguide  160 . The coaxial waveguide  160  is coupled at its bottom to three concentric waveguides  165 ,  166 ,  167  that receive respective predetermined portions of the power transmitted through the single waveguide  160 . In the embodiment of FIG. 6, the three concentric waveguides  165 ,  166 ,  167  are conical and consist of respective conical thin conductive walls  165   a,    166   a,    167   a.  The angles A 1 , A 2 , A 3  of the conical walls  165   a,    166   a,    167   a,  respectively, are selected to minimize the change in radiation direction in the transition between the single waveguide  160  and the three conical waveguides  165 ,  166 ,  167 , thereby reducing reflections at the transition or interface. Each waveguide  165 ,  166 ,  167  forms an annular opening  165   b,    166   b,    167   b  facing the waveguide  160 . The areas of the openings  165   b,    166   b,    167   b  determine the apportionment among the three concentric waveguides  165 ,  166 ,  167  of power received from the waveguide  160 . Thus, like the embodiment of FIG. 1, the embodiment of FIG. 6 provides plural radial zones over the wafer  125  beneath the respective waveguides  165 ,  166 ,  167  in which different levels of power may be applied to achieve more nearly uniform radial distribution of power across the entire wafer.  
         [0042]    Referring to FIG. 8, electron cyclotron resonance may be achieved in the embodiment of FIG. 4 by providing concentric ring magnets  201 ,  202 ,  203  and a center pole magnet  204  coinciding, respectively, with the outermost cylindrical conductor wall  163   a,  the intermediate cylindrical conductor wall  162   a,  the innermost cylindrical conductor wall  161   a  and the center conductor  170 . In the embodiment of FIG. 8, these magnets are placed within the quartz window  175 , although they may be placed in other suitable locations near the waveguides  161 ,  162 ,  163 . The skilled worker can readily select the appropriate magnet strengths and microwave frequencies for electron cyclotron resonance in the embodiment of FIG. 8.  
         [0043]    [0043]FIG. 9 illustrates how the concentric waveguides  161 ,  162 ,  163  may be of the same axial length and the wire radiators  181 ,  182 ,  183  may be fed at different angles about the central axis. The wire radiator  181  that excites the innermost waveguide  161  is fed through the intermediate and outermost waveguides  162 ,  163  through a cylindrical conductive shield  901 . The shield  901  prevents or reduces radiation from the wire radiator  161  into all but the innermost waveguide  161 . Similarly, the wire radiator  182  that excites the intermediate waveguide  162  is fed through the outermost waveguide  163  through a cylindrical conductive shield  902 .  
         [0044]    [0044]FIG. 10 illustrates one way the embodiment of FIG. 6 may be adapted to provide variable apportionment of microwave power among the three conical waveguides  165 ,  166 ,  167 . Specifically, the top sections  165   d    166   d  of the conical cylindrical walls of the conical waveguides  165 ,  166  may be articulated at least slightly, with some elastic deformation of the walls, to change the area of any one or all of the openings  165   b,    166   b,    167   b.  Thus, some openings may be made to be larger than others so that different amounts of power may be apportioned among the three conical waveguides  165 ,  166 ,  167 . FIG. 11 illustrates another way of adjusting the areas of individual ones of the openings  165   b,    166   b,    167   b  by providing slidable conical conductors  165   f,    166   f,  around corresponding ones of the top sections  165   d,    166   d.  The slidable conductors may be moved along the surfaces of the respective conical walls to at least slightly change the areas of individual ones of the openings  165   b,    166   b,    167   b.  FIG. 12 illustrates yet another way of mechanically adjusting the areas of individual ones of the openings  165   b,    166   b,    167   b  using shutter-like adjustable openings  301 ,  302 ,  303 . A single shutter opening typical of the three shutter openings  301 ,  302 ,  303  is illustrated in FIG. 13. While the shutter openings  301 ,  302 ,  303  may lie in planes perpendicular to the axis of symmetry, in the embodiment of FIG. 12 they conform to the conical angle of the respective conical walls of the waveguides  165 ,  166 ,  167 , and thereby provide a smoother transition at the boundary between the single waveguide  160  and the conical waveguides  161 ,  162 ,  163 .  
         [0045]    [0045]FIG. 14 illustrates how the three waveguide  165 ,  166 ,  167  of FIG. 6 may elongated by adding three concentric cylindrical waveguides  1401 ,  1402 ,  1403  at the bottom of the three conical waveguides  165 ,  166 ,  167 .  
         [0046]    While the wire radiators  181 ,  182 ,  183  of FIG. 1 extend into the respective waveguides  161 ,  162 ,  163  in a direction perpendicular to the axis of symmetry, FIG. 15 illustrates that the wire radiators  181 ,  182 ,  183  may extend into the respective waveguides  161 ,  162 ,  163  at an angle other than perpendicular to the axis of symmetry. FIG. 16 illustrates how the wire radiators  181 ,  182 ,  183  may extend into the respective waveguides  161 ,  162 ,  163  in a direction parallel to the axis of symmetry but at respective radii corresponding to the annuli defined by the respective waveguides  161 ,  162 ,  163 .  
         [0047]    [0047]FIGS. 17A, 17B and  17 C illustrate how the adjustment of the radial pattern of applied microwave power at the chamber ceiling can improve uniformity of plasma ion density at the wafer surface. FIG. 17A illustrates how plasma ion density at the surface of the wafer  125  tends to have a center-high radial distribution relative to the axis of symmetry of the chamber  100 . This may tend to occur when plasma source power is applied in a fairly uniform radial pattern at the ceiling, as it is in a typical reactor. FIG. 17B illustrates a center-low apportionment of microwave power among the three waveguides  161 ,  162 ,  163  that may be selected to compensate for the nonuniform distribution of plasma ion density illustrated in FIG. 1. In the example of FIG. 17B, the least amount of microwave power is coupled to the innermost waveguide  161 , a moderate amount of microwave power is applied to the intermediate waveguide  162  and the greatest amount of microwave power is applied to the outermost waveguide  163 . The severity of the differences between the respective amounts of microwave power applied to the three waveguides  161 ,  162 ,  163  is selected according to the severity of the nonuniformity of the center-high radial distribution of plasma ion density of FIG. 17A. FIG. 17C illustrates the resulting radial distribution of microwave power across the surface of the wafer  125  produced by the center-low apportionment of microwave power in the waveguides  161 ,  162 ,  163  depicted in FIG. 17B. The radial distribution of microwave power at the wafer surface illustrated in FIG. 17C has better uniformity than that of FIG. 17A. This is because the center-low apportionment of microwave power among the three concentric waveguides  161 ,  162 ,  163  enhances plasma density at the wafer periphery while attenuating it at the wafer center.  
         [0048]    While the invention has been described in detail with 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.