Abstract:
A plasma source for use in, for example, semiconductor processing contains a radio-frequency generator, an impedance matching network, and a coil that encloses a tube. The coil is bifilar, i.e., the turns of one are interlaced with the turns of a second winding. The matching network supplies only a single coil in the plasma source, unlike conventional arrangements wherein a single matching network supplies multiple coils in the plasma source.

Description:
This application claims the priority of U.S. Provisional Application No. 60/391,373, filed Jun. 25, 2002. 

   FIELD OF THE INVENTION 
   This invention relates to the formation of plasmas for use in semiconductor processing and in particular to the formation of an inductively-coupled plasma. 
   BACKGROUND OF THE INVENTION 
   Plasmas are widely used in the semiconductor industry for depositing thin films, for etching thin films and the underlying semiconductor material, and for dry-cleaning wafers. The plasma is typically formed in a cylindrical vessel or tube around which a wire is coiled. The reactant gases are introduced into one end of the tube and the atoms or ions that are generated by the plasma exit the other end of the tube and flow towards the wafer. Power is applied to the plasma by means of the coil. The mechanism by which the electrical power is transferred to the plasma can be either capacitive or inductive. Normally, both modes are present but one of the modes is predominant. In many applications the inductive mode is preferred because it produces a plasma having a higher ion density and because the ions in the plasma do not bombard the walls of the tube as much as they do when the plasma operates in the capacitive mode. This reduces wear on the tube and increases its life. 
   One structure for generating a plasma is described in U.S. Pat. No. 6,007,675 ( FIGS. 2 ,  2 B,  5 A,  5 B and  7 ) and U.S. Pat. No. 6,224,680 ( FIGS. 2 ,  2 B,  5  and  6 ). Both of these arrangements include a plurality of plasma tubes that generate plasmas used in a corresponding plurality of processing stages. As shown in  FIG. 1 , the coils are arranged in pairs  10 ,  12 , and each pair of coils is connected to an RF generator  14  through a single impedance-matching network  16 , which contains a phase angle detector  22 , a control motor  24  and a variable capacitor  26 . Phase angle detector  22  detects the phase difference between the signal produced by RF generator  14  and the signal in coils  10 ,  12  and actuates motor  24  and in turn variable capacitor  26  so as to drive the phase difference towards zero. As shown, coils  10 ,  12  are interconnected, and the coil around each of the tubes  18 ,  20  is interlaced, i.e., each coil consists of two windings, with the individual turns of each winding embedded between turns of the other winding. Ideally, the power supplied through the matching network  16  would be shared equally by the two tubes  18 ,  20 . 
   In practice, it has been found that this structure has several defects. The plasma operates primarily in the capacitively coupling mode when the power supplied to each tube is less than about 500W. For example, when the plasma is used to strip bulk photoresist, it has been found that the coupling mode changes abruptly from capacitive to inductive when the power supplied to each tube increases beyond about 600-700W. This is shown in  FIG. 2 , which is a graph of ion density of the plasma versus power per tube. The ion density levels off at about 6.0E+11 ions/cm 3  at 600-700W and increases rapidly above 750W, indicating that the coupling in this region becomes inductively coupling rich. 
   Moreover, in the arrangement shown in  FIG. 1  the power supplied to the tubes  18 ,  20  is not in fact balanced. Typically, one of the tubes receives more power through normal perturbations, and this condition worsens as the power is delivered to the path of least resistance. Also, the reaction rate (e.g., the photoresist stripping rate) is limited by the available power. 
   SUMMARY OF THF INVENTION 
   These problems are overcome in an arrangement according to this invention, wherein a single tube is supplied with electrical power through a separate impedance match network. There is no direct electrical connection between the coils which enclose different tubes. Each of the coils is bifilar, i.e., each coil contains at least two windings that have turns that are interlaced or interdigitated with the turns of the other winding. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic circuit diagram of a prior art plasma source. 
       FIG. 2  is a graph of the ion density of the plasma versus the power per tube. 
       FIG. 3  is a schematic diagram of a plasma source in accordance with this invention. 
       FIG. 4  shows a plasma source similar to the plasma source shown in  FIG. 3  but with a larger tube. 
       FIG. 5  shows how the turns of the coil may be positioned so as to improve the uniformity of the plasma within the tube. 
       FIG. 6  shows a Faraday shield positioned between the coil and the tube. 
       FIG. 7  shows a recombination mechanism positioned between the tube and the vacuum chamber. 
       FIGS. 8 and 9  illustrate that the voltage difference between adjacent turns of the coil (delta voltage) is symmetrical. 
   

   DESCRIPTION OF THE INVENTION 
     FIG. 3  is a schematic diagram of a plasma source  30  in accordance with this invention. Plasma source  30  includes a coil  32  which is wrapped around a tube  34 . 
   Coil  32  is supplied by an RF generator  40 , which operates at 13.56 MHz and which supplies a signal to coil  32  through an impedance-matching network  50 . Impedance-matching network  50  includes a phase angle detector  502  and a control motor  504 , which drives a load capacitor  506  and a phase capacitor  508  in an LC circuit  514 . Circuit  514  also includes inductances  510  and  512 , which are connected in series with phase capacitor  508 . Matching network  50  is tuned to the impedance of coil  32  by minimizing the reflected power as seen by phase angle detector  502 . The minimal reflected power is achieved through a tuning algorithm in which the positions of capacitors  506  and  508  are controlled by motor  504 . To minimize the internal losses in matching network  50 , capacitors  506  and  508  are preferably vacuum capacitors. 
   It should be understood that matching network  50  is only illustrative. Numerous forms of matching networks may be employed in other embodiments of this invention. 
   Coil  32  includes a first winding  36  (solid lines) and a second winding  38  (dotted lines). Windings  36  and  38  are interlaced or interdigitated, e.g., turn  38   y  of winding  38  is interposed between adjacent turns  36   x  and  36   z  of winding  36 . Output line  55  from matching network  50  connects to a first end of winding  36 , which is located at a first end of coil  32 . The other (second) end of winding  36  is located at a second end of coil  32  and is connected via a return line  39  to a first end of winding  38  that is located at the first end of coil  32 . The other (second) end of winding  38  is located at the second end of coil  32  and is connected to ground. 
   The reactor in which coil  32  is housed typically contains other plasma tubes (not shown) that are connected to impedance-matching networks separate from impedance-matching network  50 . This allows independent control of the plasma in each tube, and the plasma in a given tube will not be impacted by the conditions in other tubes. 
   As shown in  FIG. 4 , a different size tube may require a coil with a different number of turns to strike and sustain a uniform plasma. Normally, a larger tube requires fewer turns of inductive coil for the same coil length or uses a lower frequency generator (according to the equation c=λf, discussed below, where λ is the wavelength of the electromagnetic radiation within the tube, f is the frequency of the RF generator and c is the velocity of light). Thus coil  62  around tube  60  shown in  FIG. 4  has fewer turns than coil  32  in FIG.  3 . 
   The length of the coil may be set so as to provide a helical resonator, with standing waves in the tube. This requires that the length of the coil be a multiple of the wavelength of the electromagnetic radiation within the tube (λ=c/f). 
   The pressure within the tube may be not uniform from top to bottom, particularly if the tube has a diameter that is relatively small in comparison to its length. The distribution of the plasma inside the coil can be improved by adjusting the position of the coil.  FIG. 5 , for example, shows a coil  70  wherein the gaps between turns  72   a ,  72   b  and  72   c  are smaller than the gaps between turns  72   x ,  72   y  and  72   z.    
   The plasma may be further moved in the direction of the inductive coupling mode by positioning a Faraday shield between the coil and the tube.  FIG. 6 , for example, shows a tubular Faraday shield  82  between coil  80  and plasma tube  86 . Slots  84   a ,  84   b  and  84   c  in Faraday shield  82  run perpendicular to the turns of coil  82 . An insulating liner could be incorporated into Faraday shield  82  to avoid overheating. 
   If a more neutral species is required for the particular process (e.g., stripping), a recombination mechanism may be placed between the plasma tube and the reaction chamber. In the reactor  90  shown in  FIG. 7 , a recombination mechanism  96  is attached to an adapter  94  between a plasma tube  92  and a vacuum chamber  98 . A wafer  100  is placed on a stage  102  in vacuum chamber  98 . In this embodiment, recombination mechanism  96  is a grounded aluminum plate with openings  96   a  which allow the neutral atoms or molecules to enter vacuum chamber  98 . Openings  96   a  could be in the form of circular holes or elongated slots, for example. 
   The plasma source of this invention has numerous benefits and advantages. The plasma is easy to strike and sustain. For example, it is possible to strike and sustain a plasma at a power level of only 1W per tube. On the other hand, the power may vary widely, up to 3000W per tube, for example. The reaction rate may greatly increased by using higher power levels. For example, in stripping processes more power may be used to dissociate a higher  02  flow rate. As shown in  FIG. 2 , this wider range of power allows operation in the inductive coupling mode above 600W without any design changes in the reactor. The plasma can be very dense. As shown in  FIG. 2 , at power levels above 900W the density of the plasma is greater than E+12 ions/cm 3 . As shown in  FIGS. 8 and 9 , the voltage difference between adjacent turns of the coil (delta voltage) is symmetrical, whether the-root mean-square (V rms ) or peak-to-peak (V p-p ) voltage is considered. This provides another electrical path which minimizes capacitive coupling into the plasma. 
   The foregoing embodiments are illustrative only and not limiting. Numerous alternative embodiments will be apparent to persons of skill in the art. The broad scope of the invention is limited only by the following claims.