| AVS 61st International Symposium & Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS1-WeM |
| Session: | Plasma Based Ion Implantation and Ion-Surface Interactions |
| Presenter: | Bastien Bruneau, Ecole Polytechnique, France |
| Authors: | B. Bruneau, Ecole Polytechnique, France T. Novikova, Ecole Polytechnique, France T. Lafleur, Ecole Polytechnique, France J.-P. Booth, Ecole Polytechnique, France E.V. Johnson, Ecole Polytechnique, France |
| Correspondent: | Click to Email |
The use of Tailored Voltage Waveforms (TVWs) to manipulate the Electrical Asymmetry Effect (EAE) in a capacitively coupled plasma (CCP) chamber has been shown to be an effective technique for varying ion bombardment energy (IBE) at the surface of an electrode. It stems mainly from an amplitude asymmetry, i.e. from waveforms with different maximum and minimum. We present herein a new plasma asymmetry, obtained by sawtooth-like waveforms.
We use Particle-in-Cell (PIC) simulations to study an argon plasma excited by sawtooth-like waveforms. Using a waveform with slow rise and fast fall, we show that a fast fall leads to fast sheath expansion in front of the powered electrode, and therefore high ionization at this sheath edge. On the other hand, the slow fall leads to slow sheath expansion in front of the grounded electrode, and therefore to weak ionization at this sheath edge. This ionization asymmetry subsequently leads to an ion flux asymmetry, with up to twice higher flux on powered electrode. Because of this ion flux asymmetry, a positive self-bias develops in this plasma, leading to smaller IBE on the powered electrode compared to the grounded electrode. Therefore, the high-flux electrode also corresponds to the low-energy electrode. This property is unique, as it cannot be obtained with any of the geometrical asymmetry, amplitude asymmetry or with any mono-frequency RF excitation. We show that the ion flux asymmetry effect increases both with the number of frequencies composing the waveform, as the slope-asymmetry of the waveform then increases, and with pressure, as diffusion from one electrode to the other is hindered at high pressure. Waveforms optimizing the slope-asymmetry effect and allowing a fine and continuous control over the asymmetry are presented.
This slope asymmetry effect can be of great interest for any process using RF-CCP plasma, as one can control independently the ion flux on each electrode. For instance, one can imagine using a sawtooth-like waveform in a deposition (or etching) process. The deposition (or etching) rate can then be increased on the substrate of interest, while benefitting from a low IBE, and while keeping the up-time of the reactor high by keeping the maintenance-time low, thanks to the low deposition (or etching) rate on the other electrode.