| AVS 59th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Wednesday Sessions |
| Session EN+PS-WeM |
| Session: | Plasmas for Photovoltaics and Energy Applications |
| Presenter: | E.V. Johnson, LPICM-CNRS, Ecole Polytechnique, France |
| Authors: | E.V. Johnson, LPICM-CNRS, Ecole Polytechnique, France S. Pouliquen, LPP-CNRS, Ecole Polytechnique, France P.A. Delattre, LPP-CNRS, Ecole Polytechnique, France J.-P. Booth, LPP-CNRS, Ecole Polytechnique, France |
| Correspondent: | Click to Email |
The use of non-sinusoidal, radio-frequency (RF) « tailored » voltage waveforms (TVW’s) to drive plasma processes in a capacitively coupled plasma reactor allows one to decouple the injected power from the mean ion bombardment energy (IBE) at the substrate. Also known as the Electrical Asymmetry Effect (EAE), this decoupling stems from a controllable division of the sheath voltage between the two electrodes when an asymmetric voltage waveform is applied to one of the electrodes. In a symmetric reactor, this effect manifests itself through the presence of a self-bias voltage (VDC), and as dramatic changes in this parameter in an asymmetric one. For example, the application of a “peaks” waveform to the RF electrode - consisting sharp pulses separated by plateaus - results in a large, negative VDC, and thus a reduction in the IBE at a substrate on the grounded electrode. A “valleys” waveform results in the opposite, while both waveforms inject the same power into the plasma.
Such independent control over the sheath voltages is very useful when applied to the deposition by PECVD of thin-films of materials needing a high radical flux but low IBE, such as hydrogenated microcrystalline silicon (µc-Si:H). By controlling the growth conditions of the µc-Si:H thin films through the shape of the waveform, one can control many aspects of the film properties (Raman crystallinity, density, hydrogen bonding, surface morphology, and electronic properties) without changing any other process conditions. In particular, the optical response of the films (and film surfaces) can be observed in-situ during growth using spectroscopic ellipsometry, thus linking the redistribution of the sheath voltages to the growth dynamics. Furthermore, we show that when TVW’s are used to decrease the IBE during the growth of the absorber layer of thin-film solar cells, good device properties at acceptable deposition rates are obtained.
A promising aspect of this technique is the prospect of achieving process control without modifying the core of an existing reactor chamber. However, the counteracting challenge is that of efficiently coupling multiple harmonics to the reactor simultaneously, and these two facets will also be discussed.