Speaker
Arti Rawat
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.4007.pdf
Influence of electrode area asymmetry on harmonics generated in a direct
coupled radio frequency discharge
A. Rawat, A. Ganguli, R. Narayanan, R.D. Tarey
Indian Institute of Technology Delhi, New Delhi, India
Radio frequency (RF) discharges have been serving modern plasma processing technologies
for several decades [1]. In this context, an important issue that arises is the mechanism of power
coupling in RF discharges. For instance, while at high pressures, collisions are responsible for
the net power transferred to plasma (ohmic heating), it is conjectured that at low pressures
nonlinear RF sheath oscillations may be responsible (stochastic heating) for sustaining the
discharge [2]. However, the latter mechanism is unable to explain fully power absorption at
low pressures. A key feature of low pressure (few mTorr) RF discharges is the presence of
harmonics of the fundamental RF frequency. It is well known that the non-linearity of sheaths
can generate multiple harmonics and so it may be expected that detailed characterization of the
harmonics may provide important signatures of absorption mechanisms. Although numerous
theories and simulations have been proposed so far, experimental studies on nonlinear
characterization of RF discharges are very rare.
In this work, a novel non-invasive plasma diagnostic technique using a Dual Directional
Coupler (DDC) was used to investigate these nonlinear phenomena in RF discharges. Different
electrode geometries were used (AP/AG = 1, 0.91, 0.51, where AP and AG are the area of
powered and grounded electrodes respectively). The technique not only determines the
harmonics present in plasma, but also yields accurately the power in the forward and reflected
waves of the fundamental and each harmonic generated in the plasma. Plasma is ignited using
13.56 MHz RF generator with argon pressure ranging from 5 mTorr to 200 mTorr. The
experiments were performed in a directly coupled capacitive RF discharge (without a blocking
capacitor) since a blocking capacitor induces a large, negative DC self-bias on the powered
electrode, that damages the coupler. Experimental results show the presence of even and odd
harmonics for symmetric geometry (AP/AG = 1) along with a high plasma potential. Both these
facts contradict the theoretical models [3] for AP/AG = 1. Also, the power content of these
harmonics increases with increasing geometrical asymmetry and decreasing pressures. Some
of the results from the ongoing experiments will be presented in the conference.
References
[1] P. Chebert et al, Physics of radio-frequency plasmas, Cambridge University Press (2011).
[2] J. Schulze et al, Plasma Sources Sci. Technol.18, 034011 (2009).
[3] M. A. Lieberman, IEEE Trams. Plasma Sci. 16, 6 (1988).
