RF power control device for RF plasma applications

Abstract

There is provided by this invention an improved rf power control device for plasma applications for optimization of the feedback control voltage in the presence of harmonic and non-harmonic spurious frequencies. In this system, an oscillator and mixer, similar to those normally used in radio receiver applications are placed at the sampled output of the solid state rf signal source used for plasma ignition. The sampled output is mixed to a low frequency and filtered to remove the spurious frequencies that is created in the non-linear plasma. In this way, the feedback power control essentially ignores the spurious frequencies. In this application, the oscillator and mixer do not interfere with other desirable system characteristics and effectively isolate the feedback control voltage from changes in plasma spurious content. This allows rf power to be delivered to the plasma with greater accuracy than would otherwise be possible with conventional power control device and methods.

1. A VHF generator for delivering rf power to a plasma, comprising,
 a) a variable rf signal generator including a power amplifier connected to a directional coupler; 
 b) the directional coupler having one output connected to a matching network wherein power is delivered to plasma in a processing chamber; 
 c) at least one output of the directional coupler disposed to sample a forward power signal of the rf signal generator and at least one output of the directional coupler disposed to sample a reflected power signal of the rf signal generator; 
 d) each of the sampled forward and reflected signals is connected to mixers for mixing with an intermediate frequency of an oscillator; 
 e) the mixed forward and reflected signals are passed through low pass filters; 
 f) the filtered forward and reflected signals are connected to amplifiers and detectors; and 
 g) the detected forward and reflected signals are fed back to a power control circuit wherein the power delivered to the plasma is monitored without interference from spurious frequency signals generated by the plasma. 
 
     
     
       2. A VHF generator for delivering rf power to a plasma as recited in  claim 1  wherein forward and reflected bandpass filters are connected between the output of the directional coupler and the mixers for removing harmonics and spurious low frequency signals. 
     
     
       3. A VHF generator for delivering rf power to a plasma as recited in  claim 2  wherein the detectors are diode detectors. 
     
     
       4. A VHF generator for delivering rf power to a plasma as recited in  claim 2  wherein the detectors are RMS detectors. 
     
     
       5. A VHF generator for delivering rf power to a plasma, comprising,
 a) an rf signal generator including a power amplifier connected to a directional coupler; 
 b) the directional coupler having one output connected to a matching network wherein power is delivered to plasma in a processing chamber; 
 c) at least one output of the directional coupler disposed to sample a forward power signal of the rf signal generator and at least one output of the directional coupler disposed to sample a reflected power signal of the rf signal generator; 
 d) each of the sampled forward and reflected signals is connected to a first mixer; 
 e) a first oscillator connected to a second mixer for mixing a sampled output of the variable rf signal generator with a first intermediate frequency; 
 f) the output of the second mixer is connected to a first band pass filter and then connected to a third mixer for mixing with a second intermediate frequency of a second oscillator; 
 g) the output of the third mixer is connected to a second band pass filter and connected to the first mixer; 
 h) the mixed forward and reflected signals are passed through low pass filters; 
 i) the filtered forward and reflected signals are connected to amplifiers and detectors; and 
 j) the detected forward and reflected signals are fed back to a power control circuit wherein the power delivered to the plasma is monitored without interference from spurious frequency signals generated by the plasma. 
 
     
     
       6. A VHF generator for delivering rf power to a plasma as recited in  claim 5  wherein forward and reflected bandpass filters are connected between the output of the directional coupler and the first mixer for removing harmonics and spurious low frequency signals. 
     
     
       7. A VHF generator for delivering rf power to a plasma as recited in  claim 6  wherein the detectors are diode detectors. 
     
     
       8. A VHF generator for delivering rf power to a plasma as recited in  claim 6  wherein the detectors are RMS detectors. 
     
     
       9. An apparatus for delivering RF power to a plasma, comprising:
 a. an RF power source that delivers power to a plasma in a processing chamber; and   b. an output sampler disposed between the RF power source and the plasma, the output sampler disposed to sample at least one output characteristic of the RF power source and thereby generate at least one output signal; wherein the output signal is mixed with an oscillator frequency to generate a mixed signal, the mixed signal processed to generate a filtered mixed signal, and the filtered mixed signal fed back to a control circuit for controlling the RF power source.   
     
     
       10. The apparatus of claim 9 wherein the output sampler is a directional coupler. 
     
     
       11. The apparatus of claim 9 wherein the output characteristic is one of forward or reflected power. 
     
     
       12. The apparatus of claim 9, further comprising a matching network disposed between the output sampler and the plasma. 
     
     
       13. The apparatus of claim 9 wherein the RF power source is a VHF generator. 
     
     
       14. The apparatus of claim 9 wherein the RF power source is a variable frequency RF power source. 
     
     
       15. The apparatus of claim 10, further comprising forward and reflected band pass filters disposed at the output of the directional coupler for removing harmonics and spurious low frequency signals. 
     
     
       16. The apparatus of claim 15 wherein the filters are connected to detectors that monitor the power delivered to the plasma. 
     
     
       17. The apparatus of claim 16 wherein the detectors are RMS detectors. 
     
     
       18. A method of delivering RF power to a plasma, comprising,
 a. providing an RF power source that delivers power to a plasma in a processing chamber;   b. providing an output sampler between the RF power source and the plasma;   c. sampling at least one output characteristic of the RF power source using the output sampler, thereby generating at least one output signal;   d. mixing the output signal with an oscillator frequency to generate a mixed signal;   e. processing the mixed signal to generate a filtered mixed signal; and   f. using the filtered mixed signal to control the RF power source.   
     
     
       19. The method of claim 18 wherein the output sampler is a directional coupler. 
     
     
       20. The method of claim 18 wherein the output characteristic is one of forward or reflected power. 
     
     
       21. The method of claim 18 wherein the RF power source is a VHF generator. 
     
     
       22. The method of claim 18 wherein the RF power source is a variable frequency RF generator. 
     
     
       23. The method of claim 19, further comprising providing forward and reflected band pass filters disposed at the output of the directional coupler for removing harmonics and spurious low frequency signals. 
     
     
       24. The method of claim 23 wherein the filters are connected to detectors to monitor the power delivered to the plasma. 
     
     
       25. The method of claim 24 wherein the detectors are RMS detectors.