Anomalous plasma event detection and mitigation in semiconductor processing
In particular embodiments, anomalous plasma events, which may include formation of an electric arc in a semiconductor processing chamber, may be detected and mitigated. In certain embodiments, a method may include detecting an optical signal emitted by a plasma, converting the optical signal to a voltage signal, and forming an adjusted voltage signal. Responsive to determining that the changes associated with the adjusted voltage signal exceed a threshold, an output power of an RF signal coupled to the chamber may be adjusted. Such adjustment may mitigate formation of the anomalous plasma event occurring within the chamber.
1. A method of detecting an anomalous plasma event in a semiconductor processing chamber, the method comprising:
detecting an optical signal emitted by a plasma in the semiconductor processing chamber, wherein the plasma is formed in response to an RF signal from an RF generator;
converting the optical signal to an analog voltage signal;
filtering the analog voltage signal to form a low-pass filtered voltage signal;
comparing the low-pass filtered voltage signal with the analog voltage signal to determine whether changes associated with the low-pass filtered voltage signal exceed a threshold; and
adjusting, based at least in part on the determination, an output parameter of the RF signal from the RF generator.
2. The method of claim 1 , wherein the filtering comprises taking a first derivative of the analog voltage signal.
3. The method of claim 1 , further comprising:
determining, based at least in part on light intensity within the semiconductor processing chamber or process parameters, a first amount of RF power reduction, and
reducing an output power level of the RF generator by the first amount, wherein
the process parameters include one or more process parameters selected from the group consisting of: DC power level, RF bias power level, station-to-station RF power variations, frequency tuning parameters, pressure, and temperature.
4. The method of claim 1 , wherein:
the comparing comprises comparing the changes of the analog voltage signal to the adjusted analog voltage signal.
5. The method of claim 4 , wherein the filtering further comprises applying an offset to the low-pass filtered voltage signal.
6. The method of claim 1 , wherein the adjusting of the output parameter of the RF generator comprises lowering an output power of the RF generator from a first power level to a second power level.
7. The method of claim 6 , further comprising:
maintaining, after lowering the output power of the RF generator from the first power level to the second power level, the output power of the RF generator at the second power level for a first time period; and
after the maintaining, increasing the output power of the RF generator from the second power level to the first power level for a second time period.
8. A semiconductor processing tool comprising:
a semiconductor processing chamber;
an RF generator configured to provide RF power to the semiconductor processing chamber to generate and maintain a plasma;
a photodetector configured to detect an optical signal indicative of a plasma's luminescence in the semiconductor processing chamber, the photodetector configured to convert the optical signal to an analog voltage signal; and
a slope change detection unit configured to:
receive the analog voltage signal,
filter the analog voltage signal to form a low-pass filtered voltage signal,
compare the low-pass filtered voltage signal with the analog voltage signal to determine whether changes associated with the low-pass filtered voltage signal exceed a threshold, and
in response to the determination, cause a signal to be sent to the RF generator that is configured to cause an adjustment to the RF generator.
9. The semiconductor processing tool of claim 8 , further comprising:
a lens on the semiconductor processing chamber configured to allow light within the semiconductor processing chamber to pass through the lens, and
a fiber optic cable between the lens and the photodetector and configured to convey at least a portion of the light that passes through the lens to reach the photodetector.
10. The semiconductor processing tool of claim 8 , wherein the slope change detection unit further comprises:
a comparator to compare changes between the low-pass filtered voltage signal with the analog voltage signal.
11. The semiconductor processing tool of claim 8 , wherein the RF generator is configured to reduce the RF power from a first power level to a second power level in response to receiving the signal from the slope change detection unit.
12. The semiconductor processing tool of claim 8 , wherein the slope change detection unit further comprises:
a differentiator configured to take a derivative of the analog voltage signal to convert the analog voltage signal to an adjusted analog voltage signal, and
a comparator configured to determine whether the changes of the adjusted analog voltage signal exceed the threshold.
13. The semiconductor processing tool of claim 8 , further comprising:
a fiber optic cable between the semiconductor processing chamber and the photodetector are configured to transmit light generated by the plasma in the semiconductor processing chamber to reach the photodetector.
14. The semiconductor processing tool of claim 8 , wherein:
the photodetector is electrically connected to the RF generator, and
the optical signal from the semiconductor processing chamber is fed from the semiconductor processing chamber to the RF generator.
15. The semiconductor processing tool of claim 8 , wherein:
the photodetector and the slope change detection unit are positioned outside the semiconductor processing chamber or on the semiconductor processing chamber.
16. A method of detecting an anomalous plasma event comprising:
forming a plasma utilizing a signal from an RF generator;
detecting a fluctuating optical signal generated by the plasma;
computing a spectral density of the fluctuating optical signal; and
determining that the spectral density of the fluctuating optical signal differs from one or more reference spectral densities of a fluctuating optical signal, generated by the plasma, by a threshold amount, wherein computing the spectral density of the fluctuating optical signal comprises determining the spectral density of the fluctuating optical signal at a frequency of about 400 kHz or between about 400 kHz and about 4 MHz.
17. The method of claim 16 , wherein computing the spectral density of the fluctuating optical signal comprises utilizing a Fast Fourier Transform.
18. The method of claim 16 , wherein the one or more reference spectral densities of a fluctuating optical signal correspond to spectral densities of a fluctuating optical signal generated by a plasma maintained under nominal conditions.
19. The method of claim 18 , wherein the threshold amount corresponds to one standard deviation relative to the spectral density of the fluctuating optical signal from the plasma maintained under nominal conditions.