Measurement of plasma electron density using a non-invasive microwave resonance spectroscopy through a chamber window in a low-pressure plasma

Plasma monitoring technology measures changes in plasma according to the conditions of plasma process. This enables the detection of errors to be detected during the process and the various losses to be prevented in advance.



Promising microwave probes for the plasma monitoring technology include cutoff, absorption, and hairpin probes insert one or two tips into the plasma chamber to obtain information on the plasma electron density via microwave spectra and intrinsic resonance peak.



However, previous invasive type probes that inserted their this into the plasma caused disturbance to the plasma and contamination problems. Therefore, they cannot be used for real-time monitoring in sensitive semiconductor processes. To overcome these limitations, research is being conducted into non-invasive planar microwave probes that do not require direct insertion into the plasma.



In this study, we developed a new probe design that is essentially non-invasive and allows electron density to be measured outside the chamber. When microwaves entered the chamber through the window, we found that a resonance peak appeared near the plasma electron oscillation frequency, in a manner similar to that of an invasive cutoff probe.

We analyzed the characteristics of the resonance peak formation mechanism in response to changes in the probe’s shape (tip size and distance) and plasma variables (pressure, power, etc.). We believe that this non-invasive type microwave resonance sensor would be useful technique for measuring plasma processing diagnostics without disturbance.