Fault Detection

Radio-Frequency (RF) voltage-current (VI) probes mounted between the matching network and the plasma can detect plasma impedance changes at the fundamental and harmonic RF frequencies. The plasma impedance is very sensitive to any electrical, chemical, mechanical and/or geometrical changes that may occur in the reactor. The nonlinear nature of the plasma impedance generates a rich harmonic spectrum, accurate measurement of which can be used to precisely detect faults that affect the workpiece being processed, often leading to a scrappage event.

Figure 1 Schematic of the RF spectrometer attached to the window of a plasma chamber and monitoring plasma harmonics emitted from the plasma (Figure taken from Moduli RF Spectrometer Application note MI01)

The RF harmonic spectrum can be measured noninvasively by putting a radio antenna outside the plasma chamber (for example, on an optical window normally used for Optical Emission Spectroscopy (OES)) to detect the RF waveform which can then be digitized and analyzed with a VI probe. By applying statistical methods to analyze changes in the harmonic spectrum, many of the in-line VI probe applications can be achieved from a non-intrusive location. The harmonic emission comes from electron motion in plasma. They don’t move perfectly in sync with the powered frequency due to the non-linear nature of the plasma impedance, which is impacted by factors including: particle collisions, DC bias, the sheath lengths and the dynamics and properties of the sheath electric fields (which control the plasma density). Therefore, the RF harmonics are strong indicators of plasma, chamber and substrate conditions.