Control of Nanocrystalline Silicon Growth Phase and Deposition Rate through Voltage Waveform Tailoring during PECVD
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Control of Nanocrystalline Silicon Growth Phase and Deposition Rate through Voltage Waveform Tailoring during PECVD E.V.Johnson†, S. Pouliquen‡, P.A. Delattre†,‡, J.P. Booth‡ † LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, France ‡ LPP-CNRS, Ecole Polytechnique, 91128 Palaiseau, France ABSTRACT The use of Voltage Waveform Tailoring (VWT) – that is the use of non-sinusoidal waveforms with a period equivalent to RF frequencies – is shown to be effective in modifying the electric field distribution in a parallel plate, capacitively coupled laboratory plasma deposition reactor, and thus in changing the growth mode of silicon thin films from amorphous to nanocrystalline. The use of the VWT technique allows one to decouple the power injected into the plasma from the ion-bombardment energy at the film surface without changing any other deposition parameters, such as pressure or gas mixture. Material results are presented for an H2/SiH4 gas composition. A “peaks” type waveform increases the ion-bombardment energy at the RF electrode and reduces it at the substrate, resulting in more nanocrystalline growth. The use of a “valleys”-type waveform has the opposite effect, and results in more amorphous growth. We show the dependence of the process on silane dilution and pressure, including results on changes to the deposition rate when changing the excitation voltage waveform. INTRODUCTION The electrical asymmetry effect (EAE) in laboratory-scale plasma systems is a topic of much current interest due to the potential for its practical application in microelectronics processing steps. Using numerical simulations, it has been shown [1,2,3] showed that when a symmetric plasma reactor is excited with a sinusoidal voltage at frequency f, mixed with second signal that is an even multiple of f (for example 2f or 4f), the time-averaged voltage is no longer necessarily divided equally between the two sheaths. Furthermore the same group has shown experimentally that, using excitation with equal voltage amplitudes at f and 2f, it is possible to shift the majority of the time-averaged voltage drop from one sheath to the other simply by adjusting the relative phase of the two signals [4]. In another study, Wang and Wendt [5] demonstrated that, in a decoupled plasma source (helicon plasma), the ion energy distribution function (IEDF) at the substrate can be controlled by applying tailored waveforms to the substrate holder. Nearly monoenergetic IEDF’s were obtained by manipulating the applied waveform such that sharp positive voltage peaks occur at the substrate surface. Using such a helicon plasma source, control over the etch-selectivity through modification in the ion bombardment energy (IBE) was also shown using this technique [6]. Broadly, these investigations have demonstrated that it is possible to control the IBE at the substrate surface with the shape of the excitation voltage waveform, and not just the peak-to-peak value (VPP). In this work, we combine these two approaches and apply them to the plasma enhanced chemical vapour depo
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