Apparatus and methods for manipulating radio frequency power at an edge ring in plasma process device

1. A method of operating a process chamber, wherein the process chamber comprises an electrostatic chuck having a substrate electrode to feed radio frequency (RF) power to a substrate, an edge ring disposed over the electrostatic chuck, and an edge ring electrode located underneath the edge ring and configured to develop voltage and current at the edge ring, the method comprising:

monitoring an amplitude ratio and phase difference between voltages of the edge ring and a substrate by monitoring an amplitude ratio and phase difference between the voltages of the edge ring electrode and the substrate electrode;

adjusting an RF power source such that the substrate electrode maintains a constant amplitude of voltage by adjusting an RF circuit including at least one variable capacitor coupled to the edge ring electrode; and

tuning the at least one variable capacitor to obtain a target amplitude of the voltage at the edge ring.

2. The method of claim 1 , further comprising:

calibrating the RF circuit to find a relation between voltages at the substrate and the substrate electrode; and

calibrating the RF circuit to find a relation between voltages of the edge ring and the edge ring electrode.

3. The method of claim 2 , further comprising: calibrating the RF circuit to find a relation between the voltages of the substrate and a baseplate.

4. The method of claim 1 , wherein monitoring the amplitude ratio and the phase difference further comprises: measuring the voltages, currents, and their phases at the edge ring electrode, the substrate electrode, and a point between an inductor and the at least one variable capacitor.

5. The method of claim 1 , wherein the method further comprises: operating a first feedback loop to adjust the RF power source.

6. The method of claim 1 , wherein the method further comprises: operating a second feedback loop to tune the at least one variable capacitor.

7. The method of claim 1 , further comprising:

calibrating the RF circuit to find a condition for a minimal or zero voltage at the edge ring; and

under the condition for the minimal or zero voltage at the edge ring, measuring the current going to the substrate.

8. A process chamber comprising:

a chamber body and a lid defining an inner volume;

a substrate support assembly disposed in the inner volume that includes a substrate electrode and an edge ring electrode;

an edge ring;

an inductively coupled plasma apparatus; and

a controller that includes:

a central processing unit; and

a memory storing instructions that when executed performs a method comprising:

monitoring an amplitude ratio and phase difference between voltages of the edge ring and a substrate by monitoring an amplitude ratio and phase difference between the voltages of the edge ring electrode and the substrate electrode;

adjusting an RF power source such that the substrate electrode maintains a constant amplitude of voltage by adjusting an RF circuit including at least one variable capacitor coupled to the edge ring electrode; and

tuning the at least one variable capacitor to obtain a target amplitude of the voltage at the edge ring.

9. The process chamber of claim 8 , wherein the substrate support assembly includes:

a first RF power source, and

a second RF power source.

10. The process chamber of claim 9 , wherein the substrate electrode is coupled to the first RF power source at a first frequency.

11. The process chamber of claim 9 , wherein the edge ring electrode is coupled to the second RF power source at a second frequency.

12. The process chamber of claim 8 , wherein the inductively coupled plasma apparatus includes a first coil and a second coil disposed above the lid.

13. The process chamber of claim 12 , wherein a relative position, ratio of diameters, and number of turns in the first coil and the second coil is adjusted to control density of plasma.

14. The process chamber of claim 13 , wherein the first coil and the second coil are coupled to a RF power source through a matching network via an RF feed structure.

15. The process chamber of claim 8 , wherein the method further comprises:

calibrating the RF circuit to find a relation between voltages at the substrate and the substrate electrode; and

calibrating the RF circuit to find a relation between voltages of the edge ring and the edge ring electrode.

16. The process chamber of claim 8 , wherein the method further comprises:

calibrating the RF circuit to find a condition for a minimal or zero voltage at the edge ring; and

under the condition for the minimal or zero voltage at the edge ring, measuring the current going to the substrate.

17. The process chamber of claim 8 , wherein the RF circuit includes a parallel LC resonant circuit.

18. The process chamber of claim 17 , wherein the parallel LC resonant circuit includes a variable capacitor and inductor that are coupled to ground and an electrode.

19. A method comprising:

positioning a substrate on a substrate support assembly, wherein an edge ring is positioned around the substrate;

supplying process gases;

applying power from a radio frequency (RF) power source to ignite the process gases into a plasma; and

manipulating an RF circuit to obtain a target amplitude and phase of voltage at the edge ring to maintain a constant ratio of voltage between the edge ring and the substrate support assembly.

20. The method of claim 19 , wherein the RF circuit is a portion of a tuning circuit connected to a bias RF power source coupled to the edge ring and the substrate support assembly.