AVS 61st International Symposium & Exhibition
    Plasma Science and Technology Tuesday Sessions
       Session PS-TuP

Paper PS-TuP7
Development of a Compact Microwave Plasma Density Sensor for Processing Plasma Monitoring

Tuesday, November 11, 2014, 6:30 pm, Room Hall D

Session: Plasma Science and Technology Poster Session
Presenter: JinSheng Chiou, National Tsing Hua University, Taiwan, Republic of China
Authors: J.S. Chiou, National Tsing Hua University, Taiwan, Republic of China
W.C. Chen, National Tsing Hua University, Taiwan, Republic of China
C.H. Hsieh, National Tsing Hua University, Taiwan, Republic of China
K.C. Leou, National Tsing Hua University, Taiwan, Republic of China
Correspondent: Click to Email
Plasma density is a key parameter that control the property of processing plasmas and hence the processing results. It is thus of great interest to develop a sensor not only for characterization of the plasmas but also for monitoring of the plasma based processes1. In this study, a compact microwave plasma density probe was developed and the effect of plasma sheath and pre-sheath was also investigated. The principle of the probe is based on the resonant absorption of microwave of a coaxial antenna immersed in the plasma. In this study, the probe has a outer diameter 0.9 mm, designed to minimize the perturbation to the plasma. It consists of a monopole antenna formed by a short section of the center conductor of a semi-rigid coaxial cable and a glass enclosure. 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 compact probe was also investigated numerically by employing a full wave electromagnetic simulation using a finite element code (COMSOL). 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 and the surface plasma wave resonance frequency [1]. The effect of plasma sheath and presheath was also studied by the numerical simulation where the sheath (floating) was modeled by an air gap of thickness twice the Debye length while a linear plasma density distribution was assumed for the presheath region. The resonance frequency was also found to be dependent of the probe diameter and antenna length. 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.

1. H. Kokura, et al, Jpn. J. Appl. Phys., Pt 1, 38, 5262, 1999.

* Work supported by the National Science Council and the National Center for High Performance Computing of R. O. C. (Taiwan).