Method of operating a plasma reactor having an overhead electrode with a fixed impedance match element

Abstract

A method is provided for processing a workpiece in a plasma reactor chamber having a ceiling electrode and a workpiece support pedestal within the chamber facing the ceiling electrode and supporting the workpiece. The method includes providing a fixed impedance match element between a generator of VHF plasma source power and the ceiling electrode. The method further includes introducing a process gas into the chamber and supplying VHF plasma source power from the generator to the ceiling electrode to generate a plasma having a plasma ion density in the chamber. The reactance of the ceiling electrode is selected to establish a plasma-electrode resonant frequency of a resonance formed between the plasma and the ceiling electrode at least nearly equal to the frequency of the VHF plasma source power.

1 . A method of processing a workpiece in a plasma reactor chamber having a ceiling electrode and a workpiece support pedestal within the chamber facing the ceiling electrode and supporting the workpiece, comprising: 
 providing a fixed impedance match element between a generator of VHF plasma source power and said ceiling electrode;    introducing a process gas into the chamber and supplying VHF plasma source power from said generator to said ceiling electrode to generate a plasma having a plasma ion density in said chamber; and    selecting the reactance of said ceiling electrode to establish a plasma-electrode resonant frequency of a resonance formed between the plasma and the ceiling electrode at least nearly equal to the frequency of said VHF plasma source power.    
   
   
       2 . The method of  claim 1  further comprising coupling RF plasma bias power of an HF or LF frequency to said workpiece through the workpiece support pedestal.  
   
   
       3 . The method of  claim 1  wherein said frequency of said VHF plasma source power is offset from said plasma-electrode resonant frequency by a fraction of said plasma-electrode resonant frequency.  
   
   
       4 . The method of  claim 3  further comprising selecting a resonant frequency of the impedance match element to be at least near or equal to said frequency of said VHF plasma source power.  
   
   
       5 . The method of  claim 4  wherein said resonant frequency of said impedance match element is offset from said frequency of the VHF plasma source power by a fraction of said frequency of said VHF plasma source power.  
   
   
       6 . The method of  claim 4  wherein said resonant frequency of said impedance match element is offset from the plasma-electrode resonant frequency by a fraction of said plasma-electrode resonant frequency.  
   
   
       7 . The method of  claim 6  further comprising selecting the frequency of VHF plasma source power to lie between said plasma-electrode resonant frequency and said resonant frequency of said match element.  
   
   
       8 . The method of  claim 1  wherein the step of providing an impedance matching element comprises connecting the generator to the impedance match element at a tap point at least near an ideal point at which a voltage-to-current standing wave ratio is equal to an output impedance of said generator.  
   
   
       9 . The method of  claim 8  further comprising offsetting said tap point from said ideal point by about 5% of a wavelength of said VHF plasma source power.  
   
   
       10 . The method of  claim 8  further comprising offsetting said tap point from said ideal location by an amount sufficient to realize an increase of 6:1 in impedance match space.  
   
   
       11 . The method of  claim 8  further comprising offsetting said tap point from said ideal location by an amount that is sufficient to realize an addition of current at said tap point whenever the load impedance at said ceiling electrode falls below a nominal level and a subtraction of current at said tap point whenever the load impedance at said ceiling electrode exceeds the nominal level.  
   
   
       12 . The method of  claim 1  further comprising maintaining plasma ion density in said chamber between about 10 9  per cc and 10 12  per cc.  
   
   
       13 . A method of processing a workpiece in a plasma reactor chamber having a ceiling electrode and a workpiece support pedestal within the chamber facing the ceiling electrode and supporting the workpiece, comprising: 
 coupling a coaxial tuning stub to the ceiling electrode as a fixed impedance match element, said coaxial tuning stub having a stub resonant frequency;    introducing a process gas into the chamber;    applying VHF plasma source power through the coaxial tuning stub to said electrode to generate a plasma having a plasma ion density in said chamber;    selecting the reactance of said ceiling electrode to establish a plasma-electrode resonant frequency of a resonance formed between the plasma and the ceiling electrode at least nearly equal to the frequency of said VHF plasma source power; and    selecting a length of said coaxial tuning stub to provide a stub resonant frequency at least nearly equal to the frequency of said VHF source power.    
   
   
       14 . The method of  claim 13  further comprising coupling RF plasma bias power of an HF or LF frequency to said workpiece through the workpiece support pedestal.  
   
   
       15 . The method of  claim 13  wherein said frequency of said VHF plasma source power is offset from said plasma-electrode resonant frequency by a fraction of said plasma-electrode resonant frequency.  
   
   
       16 . The method of  claim 15  wherein the resonant frequency of said coaxial tuning stub is offset from the frequency of the VHF plasma source power by a fraction of the frequency of said VHF plasma source power.  
   
   
       17 . The method of  claim 16  wherein the resonant frequency of said coaxial tuning stub is offset from the plasma-electrode resonant frequency by a fraction of said plasma-electrode resonant frequency.  
   
   
       18 . The method of  claim 17  further comprising selecting the frequency of VHF plasma source power to lie between said plasma-electrode resonant frequency and said resonant frequency of said coaxial tuning stub.  
   
   
       19 . The method of  claim 13  wherein the step of applying VHF source power through the coaxial tuning stub comprises applying the VHF source power to a tap point of the coaxial tuning stub at least near an ideal point at which a voltage-to-current standing wave ratio is equal to an output impedance of said generator.  
   
   
       20 . The method of  claim 19  further comprising offsetting said tap point from said ideal point by about 5% of a wavelength of said VHF plasma source power.