Abstract:
A method for controlling the voltage distribution of the standing wave impressed upon the coil of an inductively coupled plasma generator includes the steps of impressing a radio frequency voltage across the coil to establish a standing wave thereacross. A voltage profile is selected for the standing wave so as to control the location and amount of capacitive coupling. A circuit parameter is controlled to achieve the selected voltage profile. Proper selection of the voltage profile enhances process capabilities, decreases the time between cleans, minimizes component wear, and minimizes cleaning time. An apparatus for carrying out the disclosed method is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    [0001] 1 . Field of the Invention  
           [0002]    The present invention is directed generally to plasma processing chambers and, more particularly, to inductively coupled processing chambers.  
           [0003]    2. Description of the Background  
           [0004]    Plasma processing chambers are used in a number of different industries. For example, plasma processing chambers are used in the fabrication of integrated circuits, for coating medical devices, and for coating mirrors. Plasma processing chambers may be either inductively coupled or capacitively coupled. In the capacitively coupled systems, electrodes comprised of parallel plates are energized to produce the plasma. In the inductively coupled systems, an inductive coil is energized to produce the plasma. In both systems, varying the parameters of the mechanism used to generate the plasma provides some ability to control the characteristics of the generated plasma.  
           [0005]    For example, in inductively coupled systems, the coil has a time varying radio frequency voltage impressed thereupon. The electron heating zone in the inductively coupled plasma is shaped like a torroid whose diameter is affected by the coil geometry. The value of the capacitor between the coil and ground may also effect the size and location of the electron heating zone.  
           [0006]    In the semiconductor industry, a plasma chamber may be used to carry out a variety of processes such as etching, deposition, sputtering, and annealing. Many of those processes leave contaminant depositions throughout the processing chamber. Such contaminants may adversely impact the process step being performed which, in turn, can adversely impact device yield. The adverse impact on device yield becomes more pronounced as device size decreases.  
           [0007]    Another problem is the wear out of the dielectric parts (the plate) within the processing chamber. Wear out of the plate is a particular problem when the chamber is used for etching. For example, an etching process can result in the deposition of a polymer on the plate. A cleaning step is required to remove that polymer. The efficacy of the cleaning step depends on a number of parameters, one of which is the value of the capacitive coupling between the coil and plasma. That capacitive coupling is defined by a voltage standing wave present on the inductive coil while powered. The higher the voltage, the greater the capacitive coupling. The cleaning efficacy at different radial and azimuthal locations on the parts varies with the value of the capacitive coupling. Also, the wear out of the plate is concentrated in one location determined by the value of the capacitive coupling.  
           [0008]    Because of the need to keep expensive process equipment such as plasma chambers in service, it is desirable to operate the plasma chamber in a way which improves process capability, minimizes part wear, reduces time between cleans, and minimizes cleaning time. Thus, the need exists for a method and apparatus which achieves those goals.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention solves the problems encountered in the prior art by controlling the voltage distribution of the standing wave impressed upon the coil, thereby controlling the location and amount of capacitive coupling. In the presently preferred embodiment, that is accomplished by controlling the value of the capacitor in the RF circuit between the coil and ground. Controlling the value of the coil to ground capacitor during an etching step controls the location of the deposit that occurs during an etching process. The capacitor can be fixed at an optimal value or varied from the beginning of the etch process (where the capacitor&#39;s value may be less important) to the end (where the capacitor&#39;s value can be very important). That permits the more critical end of the etch to occur in an optimally shaped electron heating zone and in a chamber that has an optimal deposition pattern of the polymer resulting from the capacitor value during the early portion of the step.  
           [0010]    The invention also contemplates changing the value of the capacitor from the etch to the clean step and changing the value of the capacitor during the clean step. Changing the value of the capacitor during the clean step makes that step more efficient and eliminates the concentration of wear in one place. The value of the coil to ground capacitor may be changed stepwise or continuously during the etch step or the clean step.  
           [0011]    More broadly, the present invention is directed to a method comprised of the steps of:  
           [0012]    impressing a radio frequency voltage across a coil to establish a standing wave thereacross;  
           [0013]    selecting a voltage profile for the standing wave so as to achieve optional capacitive coupling of at at least one predetermined position across the coil; and  
           [0014]    controlling a circuit parameter, such as the value of a capacitor or inductor, to achieve the selected voltage profile.  
           [0015]    An apparatus for carrying out the method is also disclosed. By controlling the voltage distribution of the standing wave, process capabilities are enhanced, the time between cleans is maximized, component wear is minimized, and cleaning time is minimized. Those advantages and benefits of the present invention, and others, will become apparent from the Description of the Preferred Embodiments hereinbelow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures wherein:  
         [0017]    [0017]FIG. 1 is a diagram illustrating certain of the components of a plasma processing apparatus with which the present invention may be used;  
         [0018]    [0018]FIG. 2 is an electrical schematic illustrating the apparatus of the present invention; and  
         [0019]    [0019]FIG. 3 is a flow chart illustrating the steps of the method of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 1 illustrates a plasma processing apparatus  10  with which the present invention may be used. It is to be understood that the apparatus  10  has been simplified to illustrate only those components which are relevant to an understanding of the present invention. Those of ordinary skill in the art will recognize that other components are required to produce an operational processing apparatus  10 . However, because such components are well known in the art, and because they do not further aid in the understanding of the present invention, a discussion of such components is not provided.  
         [0021]    In FIG. 1, the apparatus  10  may be comprised of a cold wall reaction chamber  12  constructed of aluminum. The bottom and sides of the reaction chamber  12  may be lined with quartz to protect the walls from film deposition during the processing steps. The walls of the chamber  12  may be cooled by a circulating water jacket (not shown) in conjunction with a heat exchanger (not shown). The walls are typically maintained at or above 100° C., because lower temperatures may induce the deposition of films on the walls of the reaction chamber  12 . Such depositions can be undesirable because they remove etchant from the plasma and result in material which must be eventually cleaned from the surface to maintain productivity of the chamber. Such depositions may cause temperature gradients which adversely affect the processing steps and alter the stability of processing chemistries. Furthermore, depositions on walls may flake and produce particulate that can contaminate a wafer in the chamber  12 .  
         [0022]    A wafer support table  14  or the like is located near the bottom of the chamber  12 , and is used for supporting a wafer  16 . The support table  14  is a flat surface, typically having three or more vertical support pins  15  with low thermal mass.  
         [0023]    A wafer handling system  18  is adjacent to the chamber  12 , and includes a wafer cassette  20  and a wafer handling robot  22 . The wafer cassette  20  holds a plurality of wafers, and the wafer handling robot  22  transports one wafer at a time from the wafer cassette  20  to the wafer support table  14 , and back again. A door  24  isolates the wafer handling system  18  from the chamber  12  when the wafers are not being transported to and from the wafer support table  14 .  
         [0024]    A gas source  26  is in fluid communication with the chamber  12 . More than one type of gas may be available from gas source  26 , and gases may be provided individually or in combination. The gases may, for example, be used to deposit films on the wafer  16 , flush gases from the chamber  12 , or cool the chamber  12  and the wafer  16 .  
         [0025]    Gases are removed from the chamber  12 , and a vacuum may be created within the chamber  12 , by a gas exhaust and vacuum system  28 , as is well known in the art. Also present are sensors  30 , such as a pyrometer, which are used to measure process parameters.  
         [0026]    A power supply  32  supplies power to a coil  34  connected to ground through a variable capacitor  36 . The coil  34  is a flat spiral coil that is positioned proximate to a window  38  in the chamber  12 . The position of the coil  34  and the material comprising the window  38  are selected as is known in the art such that RF energy produced by the coil  34  is efficiently coupled to the gases in the chamber  12  to produce a plasma  40 .  
         [0027]    The electron heating zone in the inductively coupled plasma  40  is shaped like a torroid whose diameter is effected by the coil&#39;s  34  geometry. The size of the heating zone and its location relative to the wafer have important effects on the process results. In addition, the deposition thickness of polymer on the window  38  varies with radial and azimuthal position in a way that depends on the value of the capacitor  36 .  
         [0028]    Completing the description of FIG. 1, a control computer  42  controls the various components which comprise the plasma processing apparatus  10 .  
         [0029]    In FIG. 2, an electrical schematic illustrating the apparatus of the present invention is illustrated. The power supply  32  of FIG. 1 is illustrated in FIG. 2 as an RF generator  44 . The RF generator  44  supplies power to the coil  34  through a matchbox circuit  46 . The matchbox circuit  46  is known in the art and is used for impedance matching to ensure maximum coupling of power from the RF generator  44  to the coil  34 . As seen in FIG. 2, the coil  34  is connected at a first end  47  to the RF generator  44  through the matchbox circuit  46  and is connected at a second end  48  to ground through a variable component  50 . In the preferred embodiment of the present invention, the variable component  50  may be the variable capacitor  36  as shown in FIG. 1.  
         [0030]    In operation, the RF generator  44  is used to impress a radio frequency voltage across the coil  34 . As is known in the art, that causes a standing wave to be created across the coil  34 . By graphing the magnitude of the voltage as a function of coil length, a voltage profile can be generated. By varying the value of the variable component  50 , that voltage profile can be varied. The advantage of varying the voltage profile is that it varies where the point or points of maximum or minimum voltage appear across the coil  34 . The points of maximum voltage correspond to areas of maximum capacitive coupling to the plasma whereas the points of minimum voltage correspond to areas of minimum capacitive coupling to the plasma. “Capacitive coupling to the plasma” refers to the region under the coil in the plasma where the voltage on the coil is high enough to cause capacitive etching. That is etching with a sputter component. That is the type of etching that is most efficient to keep a surface clean. By varying these areas of maximum and minimum coupling to the plasma, various desirable characteristics can be obtained as described more fully hereinbelow.  
         [0031]    [0031]FIG. 3 is a flowchart illustrating the steps of the method of the present invention. One aspect of the present invention is illustrated by block  56 . Block  56  represents the step in the process wherein the value of the variable component  50  is selected so that contaminant polymer depositions will be deposited on window  38  in locations wherein it will be the easiest to clean with subsequent cleaning steps. Thereafter, the apparatus  10  is used for its desired purpose.  
         [0032]    Step  56  also contemplates that the value of the variable component  50  may be changed during the process step. For example, the value of the variable component  50  may be fixed at a first value during the beginning of a process such as an etch process. The value of the variable component  50  may be selected so as to produce a desirable deposition pattern. Thereafter, later in the etch process, the value of the variable component  50  may be changed so as to produce an optimally shaped electron heating zone in the chamber  12  that has an optimal deposition pattern of the polymer resulting from the earlier value of the variable component  50 .  
         [0033]    The present invention also contemplates changing the value of the variable component  50  from the process step to the clean step as shown by block  58  in FIG. 3. According to that aspect of the present invention, the voltage profile of the standing wave impressed upon the coil  34  is adjusted so as to locate regions of maximum capacitive coupling proximate to those regions of the window  38  which require the most cleaning. Thereafter, the clean cycle is run at step  60 .  
         [0034]    The present invention also contemplates changing the value of the variable component  50  during the clean step. That is, after the contaminant polymer begins to be removed from the window  38 , it will be removed first from those areas having maximum capacitive coupling. After those areas are clean, it is desirable to relocate the area of maximum capacitive coupling to those portions of the window  38  which are not yet clean. That may be accomplished by either continually varying the value of the variable component  50  or varying the value of the variable component  50  in a step-wise manner.  
         [0035]    By controlling the voltage distribution of the standing wave, process capabilities are enhanced, the time between cleanings is maximized, component wear is minimized, and the cleaning time is minimized as discussed above.  
         [0036]    While the present invention has been described in conjunction with preferred embodiments thereof, many modifications and variations will be apparent to those of ordinary skill in the art. The foregoing description and the following claims are intended to cover all such modifications and variations.