Impedance monitoring system and method
An apparatus ( 14 ) for and method of measuring impedance in a capacitively coupled plasma reactor system ( 10 ). The apparatus includes a high-frequency RF source ( 150 ) in electrical communication with an upper electrode ( 50 ). A first high-pass filter ( 130 ) is arranged between the upper electrode and the high-frequency RF source, to block low-frequency, high-voltage signals from the electrode RF power source ( 66 ) from passing through to the impedance measuring circuit A current-voltage probe ( 140 ) is arranged between the high-frequency source and the high-pass filter, and is used to measure the current and voltage of the probe signal with and without the plasma present. An amplifier ( 250 ) is electrically connected to the current-voltage probe, and a data acquisition unit ( 260 ) is electrically connected to the amplifier. A second high-pass filter ( 276 ) is electrically connected to a lower electrode ( 56 ) and to ground, so as to complete the isolation of the high-frequency circuit of the impedance measurement apparatus from the low-frequency, high-voltage circuit of the capacitively coupled plasma reactor system. A method of measuring the plasma impedance using the apparatus of the present invention is also disclosed.
1. An apparatus for measuring impedance in a capacitively coupled plasma reactor system having an upper and lower electrode capable of forming a plasma therebetween when a plasma generating RF signal is coupled to at least one of the upper and lower electrodes, comprising:
a) a high-frequency RF source in electrical communication with the upper electrode and capable of generating an electrical probe signal having a higher frequency than said plasma generating RF signal;
b) a first high-pass filter arranged between the upper electrode and said high-frequency RF source, for passing high-frequency components of the electrical probe signal to said upper electrode and isolating said high frequency RF source from said plasma generating RF signal; and
c) a current-voltage probe arranged between said high-frequency source and said high-pass filter, for measuring the current and voltage of the probe signal.
2. The apparatus as claimed in claim 1 , further comprising:
an amplifier electrically connected to said current-voltage probe.
3. The apparatus as claimed in claim 2 , further comprising:
a data acquisition unit electrically connected to said amplifier.
4. An apparatus according to claim 3 , wherein said data acquisition unit is an analog-to-digital converter.
5. An apparatus according to claim 2 , wherein said amplifier is a lock-in amplifier.
6. The apparatus as claimed in claim 1 , further comprising:
a second high-pass filter electrically connected to the lower electrode and to ground.
7. An apparatus according to claim 1 , wherein said high-frequency RF source and said current-voltage probe are connected by a coaxial line, and wherein said current-voltage probe is formed in said coaxial line.
8. An apparatus according to claim 1 , wherein said high-frequency RF source is capable of generating electrical signals having different frequencies.
9. An apparatus according to claim 1 , further comprising:
an upper electrode RF power source separate from the high-frequency RF source and configured to generate said plasma generating RF signal; and
a frequency-specific path to ground, wherein the frequency-specific path to ground acts as a low impedance path to ground for the high-frequency components of the electrical probe signal but as a high impedance path to ground for power provided by the upper electrode RF power source.
10. An apparatus according to claim 1 , further including a computer electrically connected to said data acquisition unit.
11. An apparatus according to claim 10 , wherein said computer is also electrically connected to the capacitively coupled plasma reactor system.
12. An apparatus according to claim 1 , wherein said first high-pass filter passes electrical signals having a frequency of at least 100 MHz.
13. A method for measuring the impedance in a capacitively coupled plasma processing system having an upper and lower electrode, comprising the steps of:
a) ensuring no plasma exists between the upper and lower electrodes and transmitting a high-frequency probe signal to the upper electrode through an electrical line connected thereto, said probe signal having a higher frequency than a plasma generating signal applied to said plasma processing system;
b) measuring, in said electrical line, a first current and a first voltage of the probe signal;
c) calculating a no-plasma-present impedance Z np from said first current and said first voltage;
d) forming a plasma between the upper and lower electrodes using said plasma generating signal; and
e) calculating a system impedance Z sys in the presence of the plasma.
14. The method as claimed in claim 13 , wherein the calculating step e) comprises measuring a second current and a second voltage of the probe signal passing to the upper electrode through said electrical line.
15. The method as claimed in claim 14 , further comprising:
measuring a third voltage of the plasma generating signal passing to the upper electrode through said line.
16. The method as claimed in claim 15 , further comprising:
determining a sheath thickness d s and sheath impedance Z sheath .
17. The method as claimed in claim 16 , further comprising:
calculating the plasma electron density n e and electron-neutral collision frequency γ.
18. A method according to claim 17 , further comprising:
adjusting at least one control parameter of the plasma processing system based on the step of calculating the plasma electron density n e and the electron-neutral collision frequency γ.
19. A method according to claim 13 , wherein said step b) includes the step of blocking low-frequency electrical signals transmitted from the upper electrode.
20. A method according to claim 13 , wherein said step b), said measuring is performed using a current-voltage probe formed directly in said electrical line.
21. A method according to claim 13 , further comprising:
electrically connecting a high-pass filter to the lower electrode and to ground.
22. A method according to claim 21 , wherein said step b) further includes modulating said probe signal and detecting said probe signal with a lock-in amplifier tuned to said modulated probe signal.
23. A method according to claim 13 , wherein said step b) further includes the step of transmitting said first current and said first voltage to a data acquisition unit and storing said first current and said first voltage therein.
24. A method according to claim 13 , wherein said step b) includes the step of selecting the probe frequency to be between a harmonic of a fundamental RF frequency used to create the plasma.
25. A method according to claim 13 , wherein said step h) includes modeling the sheath resistance.
26. A method according to claim 13 , further comprising:
measuring the first current and the first voltage over a range of probe signal frequencies; and
selecting a minimum value for the plasma impedance Z p in the range of the probe signal frequencies.
27. A method according to claim 26 , further comprising:
adjusting at least one control parameter of the plasma processing system based on the step of selecting.