Independent control of RF phases of separate coils of an inductively coupled plasma reactor
Plasma distribution is controlled in a plasma reactor by controlling the phase differences between different RF coil antennas, in accordance with a desired or user-selected phase difference, by a phase-lock feedback control loop.
1. A plasma reactor for processing a workpiece, comprising:
a vacuum chamber comprising a ceiling and a side wall, a workpiece support pedestal in said chamber having a workpiece support surface, and plural coil antennas;
plural impedance matches and plural RF power amplifiers coupled to respective ones of said plural coil antennas through respective ones of said plural impedance matches, one of said plural RF amplifiers being a reference RF amplifier;
a clock signal source coupled to said reference RF power amplifier, and respective phase shifters coupled between said clock signal source and at least respective ones of the RF power amplifiers other than said reference RF amplifier, each of said phase shifter having a phase shifter control input;
plural RF sensor probes, respective ones of the plural RF sensor probes coupled or adjacent to respective ones of said plural RF coil antennas, one of said RF sensor probes being a reference RF sensor probe;
respective phase detectors each having a phase detector output and a respective pair of phase detector inputs, one phase detector input in each of said respective pair of phase detector inputs being coupled to a respective one of said plural RF sensor probes other than the reference RF sensor probe, the other phase detector input in each of said respective pair of phase detector inputs being coupled to said reference RF sensor probe;
a user interface having a plurality of user interface outputs defining a user-selected phase difference between said reference RF sensor probe and a respective one of the remaining RF sensor probes; and
a feedback controller stage coupled to (a) respective ones of said phase detector outputs and (b) respective ones of said user interface outputs, said feedback controller stage further comprising respective controller outputs coupled to respective ones of said phase shifter control inputs.
2. The reactor of claim 1 wherein said feedback controller stage comprises a pair of respective feedback controllers having respective pairs of inputs coupled to (a) a respective one of said phase detector outputs and (b) a respective one of said user interface outputs, each of said respective feedback controllers comprising a respective one of said respective controller outputs.
3. The reactor of claim 1 wherein said feedback controller stage comprises a single feedback controller, said reactor further comprising a multiplexer for causing respective ones of said phase detector outputs and respective ones of said user interface outputs to reach said single feedback controller during respective time windows, and a demultiplexer coupled between an output of said single feedback controller and respective ones of said phase shifter control inputs.
4. The reactor of claim 1 wherein each said phase detector comprises:
a frequency down conversion stage having respective inputs coupled to said RF sensor probes and respective outputs; and
a phase comparator having an output and a pair of inputs coupled to the respective outputs of said frequency down conversion stage.
5. The reactor of claim 1 further comprising an integrator coupled between said controller stage and said phase shifter control input.
6. The reactor of claim 5 wherein:
said feedback controller stage is adapted to produce successive correction signals;
said integrator is adapted to provide to said phase shifter control input an average over n of the previous successive correction signals.
7. The reactor of claim 6 wherein n is an integer in a range up to 5.
8. The reactor of claim 6 wherein n is an integer in a range up to 100.
9. The reactor of claim 6 wherein n is an integer in a range up to 1000.
10. The reactor of claim 6 wherein said successive correction signals correspond to a sampling period T, and wherein T is less than a settling time of one of said impedance matches by a factor greater than 10.
11. The reactor of claim 4 wherein each of said phase comparators comprises:
respective sine wave-to-square wave converters coupled to said respective outputs of said frequency down conversion stage;
a phase lock loop phase comparator coupled to said respective sine wave-to-square wave converters.
12. The reactor of claim 4 wherein said phase comparator comprises an IQ demodulator.
13. The reactor of claim 1 wherein said workpiece support comprises an electrostatic chuck (ESC), said ESC comprising an insulating puck layer having a top surface for supporting a workpiece, an ESC electrode embedded in said insulating puck layer, and a D.C. chucking voltage supply coupled to said ESC electrode.
14. The reactor of claim 1 wherein said plural coil antennas comprise plural helically wound conductors in respective cylindrical solenoids.
15. The reactor of claim 14 wherein each one of said plural helically wound conductors comprises plural parallel interleaved conductors.
16. The reactor of claim 14 wherein said respective cylindrical solenoids comprise inner and intermediate solenoids overlying said ceiling and an outer solenoid below said ceiling and surrounding a portion of said side wall.
17. The reactor of claim 1 wherein said coil antennas each comprise respective conductors concentrically wound in flat spirals.