Plasma process uniformity across a wafer by controlling a variable frequency coupled to a harmonic resonator
A method of processing a workpiece in a plasma reactor chamber includes coupling RF power via an electrode to plasma in the chamber, the RF power being of a variable frequency in a frequency range that includes a fundamental frequency f. The method also includes coupling the electrode to a resonator having a resonant VHF frequency F which is a harmonic of the fundamental frequency f, so as to produce VHF power at the harmonic. The method controls the ratio of power near the fundamental f to power at harmonic F, by controlling the proportion of power from the generator that is up-converted from f to F, so as to control plasma ion density distribution.
1. A method of processing a workpiece in a plasma reactor chamber, comprising:
coupling RF power from a generator via an electrode to plasma in the chamber, said RF power being of a variable frequency in a frequency range that includes a fundamental frequency f;
coupling the electrode to a resonator having a resonant VHF frequency F which is a harmonic of the fundamental frequency f, so as to produce VHF power at the harmonic from said RF power; and
controlling the ratio of power near the fundamental f to power at harmonic F by controlling the proportion of power from the generator that is up-converted from f to F so as to control plasma ion density distribution.
2. The method of claim 1 wherein said controlling the proportion of power up-converted comprises:
controlling the difference between the variable output frequency of the VHF generator and the fundamental frequency f.
3. The method of claim 2 wherein said controlling comprises varying the frequency of the variable frequency generator.
4. The method of claim 1 wherein said controlling comprises:
reducing center-high plasma ion density distribution by decreasing the ratio of power at F relative to power at the frequency f.
5. The method of claim 1 wherein said controlling comprises:
reducing edge-high plasma ion density distribution nonuniformity by decreasing the ratio of power at the frequency f relative to power at the frequency F.
6. The method of claim 1 further comprising:
providing a first center return path for the harmonic frequency F and a second center return path for a lower frequency near f;
providing an edge return path.
7. The method of claim 6 further comprising:
adjusting the impedance of the first center return path relative to the impedance of the edge return path to promote current flow at the frequency F to the center return path.
8. The method of claim 6 further comprising:
adjusting the impedance of the second center return path relative to the impedance of the edge return path to promote current flow at the f frequency to the side wall.
9. The method of claim 1 further comprising:
adjusting the impedances of either or both the first and second center return paths to either: (a) channel more current toward the edge in order to suppress a center-high non-uniformity or (b) channel more current toward the center to suppress an edge-high non-uniformity.
10. The method of claim 1 further comprising:
adjusting the plasma ion density distribution by adjusting a gap between the workpiece and the ceiling electrode.
11. A method of processing a workpiece in a plasma reactor chamber, comprising:
coupling RF power from a generator via an electrode to plasma in the chamber, said RF power being of a variable frequency in a frequency range that includes a fundamental frequency f, said fundamental frequency being sufficiently low to produce a center-low edge-high non-uniformity in plasma ion density distribution;
generating from said RF power a harmonic of the fundamental frequency f at a harmonic frequency F that is sufficiently high to produce a center-high edge-low non-uniformity in plasma ion density distribution; and
adjusting the plasma ion density distribution on the workpiece between a center-high and a center-low distribution by controlling the proportion of power from the generator that is up-converted from f to F so as to control plasma ion density distribution.
12. The method of claim 11 wherein said controlling the proportion of power up-converted comprises:
controlling the difference between the variable output frequency of the VHF generator and the fundamental frequency f.
13. The method of claim 12 wherein said controlling comprises varying the frequency of the variable frequency generator.
14. The method of claim 11 wherein said controlling comprises:
reducing center-high plasma ion density distribution by decreasing the ratio of power at the frequency F relative to power at the frequency f.
15. The method of claim 11 wherein said con trolling comprises:
reducing edge-high plasma ion density distribution nonuniformity by decreasing the ratio of power at the frequency f relative to power at the frequency F.
16. The method of claim 11 further comprising:
providing a first center return path for the harmonic frequency F and a second center return path for a lower frequency near f;
providing an edge return path.
17. The method of claim 16 further comprising:
adjusting the impedance of the first center return path relative to the impedance of the edge return path to promote current flow at the frequency F to the center return path.
18. The method of claim 16 further comprising:
adjusting the impedance of the second center return path relative to the impedance of the edge return path to promote current flow at the f frequency to the edge ground return path.
19. The method of claim 16 further comprising:
adjusting the impedances of either or both the first and second center return paths to either: (a) channel more current to the edge of the chamber in order to suppress a center-high non-uniformity or (b) channel more current toward the center of the chamber to suppress an edge-high non-uniformity.
20. The method of claim 11 further comprising:
adjusting the plasma ion density distribution by adjusting a gap between the workpiece and the ceiling electrode.