A compact plasma absorption probe for plasma density measurements

Measurement of plasma density, a key parameter that control the property of processing plasmas and hence the processing results, has been the focus of extensive studies in recent years, not only for characterization of the plasmas but also for development of tools for monitoring of the plasma based processes1.In this study, a miniature Plasma Absorption probe (mini-PAP)[2] of outer diameter 0.9 mm, consisting of a monopole antenna formed by a section of the center conductor of a semi-rigid coaxial cable and a glass enclosure has been developed so that the perturbation to the plasma can be minimized. Experimental measurements were carried out in an inductively coupled plasma and the results show that the resonance frequency increase linearly with the source rf power, as expected. The results are also consistent with that from measurements by a Langmuir probe. The mini-PAP was also investigated numerically by employing a full wave electromagnetic simulation using a finite element code, High Frequency Structure Simulation (HFSS, ANSOFT CORP). In the simulation analysis, a coaxial line was attached to the probe head and the “driven mode” was adopted, i.e., the wave was fed into the coaxial line and the absorption spectrum, reflection coefficient vs frequency, was then analyzed. The plasma surrounding the probe was characterized by the collisional electromagnetic plasma permittivity, determined by plasma density, wave frequency and momentum transfer collisional frequency. The simulation results show that the absorption spectrum exhibits a resonance, occurring between the plasma frequency (ω
_p
) and the surface plasma wave resonance frequency ω
_p
/(1 + εd)^0.5 [2], where εd being the dielectric constant of the dielectric (glass). The effect of plasma sheath was also studied by the numerical simulation where the sheath (floating) was modeled by an air gap (ε
_r
= 1) of thickness twice the Debye length. The resonance frequency was also found to be dependent of the probe diameter and antenna length. For example, under the same plasma conditions, the resonance frequency obtained by the mini-PAP is ~30% higher than that from a PAP of conventional size (diameter 6 mm) [2]. These results are consistent with the results obtained by experimental measurements. This indicates that one can use the result from numerical simulation as a calibration for the plasma resonance probe measurement.