Three-coil inductively coupled plasma source with individually controlled coil currents from a single RF power generator

1. A plasma reactor for processing a workpiece, comprising:

a reactor chamber comprising a cylindrical side wall and a ceiling;

an RF power generator and an impedance match coupled to said RF power generator;

three coil antennas having respective driven ends and return ends, the return ends being connected to a common potential;

a pair of current branches each comprising respective series and parallel reactance elements, the series reactance element of each of the current branches being coupled between said impedance match and the driven end of a respective one of said coil antennas, the parallel reactance element of each of the current branches being coupled in parallel with a respective one of said coil antennas;

a third current branch coupled between said impedance match and the driven end of a third one of said three coil antennas; and

a current apportionment controller governing reactances of said parallel reactance elements in accordance with a user-specified apportionment of currents among said coil antennas.

2. The reactor of claim 1 wherein said third current branch comprises a conductor.

3. The reactor of claim 1 wherein each one of said variable parallel reactance elements comprises a variable capacitor.

4. The reactor of claim 3 wherein each one of said series reactance elements comprises a capacitor.

5. The reactor of claim 1 wherein said current apportionment controller comprises:

a look-up table storing a sequence of pairs of reactance values of said first and second variable parallel reactance elements.

6. The reactor of claim 5 wherein said sequence of pairs of values defines a linear programming sequence.

7. The reactor of claim 6 further comprising a user interface coupled to said current apportionment controller, said user interface adapted to record a user-selected point along said path and transmit said user-selected point to said current apportionment controller.

8. The reactor of claim 6 wherein said sequence of pairs of values defies a path in a 2-dimensional space whose dimensions are the reactance values of said variable parallel reactance elements, respectively.

9. The reactor of claim 8 wherein said path includes:

a reference point at which currents in said three RF coil antennas are at least approximately equal,

a first point at which current in a first one of said coil antennas, relative to currents in second and third ones of said coil antennas, is near a maximum,

a second point at which current in said second coil antenna, relative to currents in said first and third coil antennas, is near a maximum, and

a third point at which current in said third coil antenna, relative to currents in said first and second coil antennas, is near a maximum.

10. The reactor of claim 1 further comprising:

an interface for entering said user-specified apportionment of currents in said three coil antennas;

wherein said current apportionment controller comprises:

a memory containing data representing respective currents in said three RF coil antennas relative to total current as functions of values of said variable parallel reactive elements;

a processor for determining from said data the values of said variable parallel reactance elements at which said respective currents in said three coil antennas at least nearly correspond to said user-specified apportionment.

11. The reactor of claim 1 wherein:

said impedance match comprises a match input and a match output and said RF generator comprises a power terminal coupled to said match input and a return terminal connected to said common potential;

said impedance match further comprises:

a first section comprising:

a first variable tuning capacitor connected to said match input and a first tuning inductor connected in series between said first variable tuning capacitor and said match output;

a second tuning inductor connected in parallel with the combination of said first variable tuning capacitor and said first tuning inductor;

a second section comprising:

a second variable tuning capacitor connected to said match input and a third tuning inductor connected in series between said second variable tuning capacitor and said common potential.

12. The reactor of claim 11 further comprising:

an SWR detector coupled to said power terminal;

a match controller responsive to said SWR detector and governing said first and second variable tuning capacitors.

13. A method of individually controlling currents in three coils of a plasma reactor supplied from an RF generator output node of a single RF power generator through a single RF impedance match, comprising:

providing parallel current divider branches between an output node of said RF generator and respective ones of said coils, each one of at least two of said current divider branches comprising a series reactance element connected between said RF generator output node and a respective one of said coils and a variable parallel reactance element connected in parallel with said respective coil;

providing a model comprising a first set of equations defining voltage drops in said parallel branches in accordance with mutual inductances of said coils and corresponding portions of a plasma in said reactor and a second set of equations defining voltage drops in said corresponding portions of said plasma in accordance with said mutual inductances;

solving said first and second sets of equations for currents in said coils relative to total current as 3-dimensional functions of reactance values of said variable parallel reactance elements in said at least two current divider branches;

correlating information corresponding to said functions to a user-defined apportionment of currents in said coils.

14. The method of claim 13 wherein said correlating comprises:

searching said functions for respective points in a space representing pairs of values of said variable parallel reactance elements at which coil currents are maximum for respective ones of said coils;

defining a linear programming path that includes said respective points;

responding to a user selection of a location on said linear programming path by controlling impedance values of said variable impedance elements in accordance with said location.

15. The method of claim 14 wherein said path further includes a point at which all coil currents are at least nearly the same.

16. The method of claim 15 wherein said path comprises straight segments between said points.

17. The method of claim 13 wherein said correlating comprises:

searching said functions for a set of reactance values of said variable parallel reactance elements corresponding to an apportionment of currents among said coils that at least nearly approximates said user-defined apportionment.

18. A method of individually controlling currents in three coils of a plasma reactor supplied from an RF generator output node of an RF power generator through an RF impedance match, comprising:

providing parallel current divider branches between an output node of said RF generator and respective ones of said coils, at least two of said current divider branches comprising a series reactance element connected between said RF generator output node and a respective one of said coils and a variable parallel reactance element connected in parallel with said respective coil;

correlating information, corresponding to coil currents as 3-dimensional functions of reactance values of said variable parallel reactance elements in said at least two current divider branches, to a user-defined apportionment of currents in said coils.

19. The method of claim 18 wherein said correlating comprises:

defining respective points in a space representing pairs of values of said variable parallel reactance elements at which coil currents are maximum for respective ones of said coils;

defining a linear programming path that includes said respective points;

responding to a user selection of a location on said linear programming path by controlling impedance values of said variable impedance elements in accordance with said location.

20. The method of claim 19 wherein said path further includes a point at which all coil currents are at least nearly the same.