Frequency tuning for pulsed radio frequency plasma processing
This disclosure describes systems, methods, and apparatus for pulsed RF power delivery to a plasma load for plasma processing of a substrate. In order to maximize power delivery, a calibration phase using a dummy substrate or no substrate in the chamber, is used to ascertain a preferred fixed initial RF frequency for each pulse. This fixed initial RF frequency is then used at the start of each pulse during a processing phase, where a real substrate is used and processed in the chamber.
1. A method of selecting a fixed initial RF frequency for each of a plurality of RF pulses provided to a plasma load, the method comprising:
providing pulsed RF power to a plasma load, where each pulse comprises RF power having a controllable frequency, and where a plasma having the plasma load interacts with a dummy substrate, or no substrate, in a plasma chamber;
tuning an initial RF frequency for each pulse in a string of pulses, pulse to pulse, in a calibration process, by minimizing a difference between a characteristic indicative of the pulsed RF power at a start of each pulse and a desired characteristic of the pulsed RF power at a start of each pulse until the difference between the characteristic for a start to two consecutive pulses is below a threshold; then
selecting the initial RF frequency as a fixed initial RF frequency for use in a processing run using a real substrate; and
tuning the RF frequency during processing within each pulse but returning to the fixed initial RF frequency at a start of each pulse.
2. The method of claim 1 , wherein the characteristic indicative of the pulsed RF power is reflected power.
3. The method of claim 1 , wherein the characteristic indicative of the pulsed RF power is load impedance power.
4. The method of claim 1 , wherein the characteristic indicative of the pulsed RF power is a density of the plasma.
5. The method of claim 1 , wherein the processing run includes providing pulsed RF power to the plasma load where an initial RF frequency at a start of each pulse is set to the fixed initial RF frequency.
6. A non-transitory, tangible computer readable storage medium, encoded with processor readable instructions to perform a method for frequency tuning a power source to optimize delivered power to a plasma load, the method comprising:
in a calibration phase,
tuning an initial RF frequency at a start of each pulse in a string of RF pulses, pulse to pulse, until a difference between a characteristic indicative of power at the start of each pulse and a desired characteristic of the power at the start of each pulse, for a start to two consecutive pulses, is below a threshold; then
selecting the initial RF frequency as a fixed initial RF frequency to be used during a processing phase; and
in the processing phase,
setting an initial RF frequency for all pulses in the processing phase equal to the fixed initial RF frequency selected during the calibration phase; and
tuning the RF frequency of pulsed RF power during each pulse, but returning to the fixed initial RF frequency at a start of each pulse.
7. The non-transitory, tangible computer readable storage medium of claim 6 , wherein the characteristic indicative of the pulsed RF power is reflected power.
8. The non-transitory, tangible computer readable storage medium of claim 6 , wherein the characteristic indicative of the pulsed RF power is load impedance power.
9. The non-transitory, tangible computer readable storage medium of claim 6 , wherein the characteristic indicative of the pulsed RF power is a density of the plasma.
10. A power delivery system comprising:
a power source configured to provide pulsed RF power to a plasma load via a matching network;
a sensor configured to sample a characteristic indicative of the pulsed RF power;
a controller in communication with the sensor and the power source and executable to:
in a calibration phase,
tuning an initial RF frequency at a start of each pulse in a string of RF pulses, pulse to pulse, until a difference between a characteristic indicative of power at the start of each pulse and a desired characteristic of the power at the start of each pulse, for a start to two consecutive pulses, is below a threshold; then
selecting the initial RF frequency as a fixed initial RF frequency to be used during a processing phase; and
in the processing phase,
setting an initial RF frequency for all pulses in the processing phase equal to the fixed initial RF frequency selected during the calibration phase; and
tuning the RF frequency of pulsed RF power during each pulse, but returning to the fixed initial RF frequency at a start of each pulse.
11. The power delivery system of claim 10 , wherein the sensor is a reflected power sensor.
12. The power delivery system of claim 11 , wherein the sensor is a directional coupler.
13. The power delivery system of claim 10 , wherein the sensor is a delivered power sensor.
14. The power delivery system of claim 10 , wherein the sensor is an impedance sensor.
15. The power delivery system of claim 10 , wherein the sensor is a plasma density sensor.
16. The power delivery system of claim 10 , wherein the power source is in communication with a switching circuit that converts RF power from the power source to the pulsed RF power.
17. The power delivery system of claim 10 , wherein the sensor is part of an oscilloscope.