| AVS 59th Annual International Symposium and Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS2-WeA |
| Session: | Plasma Surface Interactions during PECVD and Plasma Surface Modification |
| Presenter: | J. Palmans, Eindhoven University of Technology, the Netherlands |
| Authors: | J. Palmans, Eindhoven University of Technology, the Netherlands A.C. Bronneberg, Eindhoven University of Technology, the Netherlands X. Kang, Eindhoven University of Technology, the Netherlands M.C.M. van de Sanden, Eindhoven University of Technology, the Netherlands W.M.M. Kessels, Eindhoven University of Technology, the Netherlands M. Creatore, Eindhoven University of Technology, the Netherlands |
| Correspondent: | Click to Email |
Thin-film silicon solar cells extensively use hydrogenated microcrystalline silicon (µc-Si:H) where large area and fast deposition processes are considered key issues. Both requirements can be fulfilled when operating in the so-called high pressure depletion (HPD) regime in a parallel plate capacitively coupled plasma (CCP). This latter involves high plasma powers (~0.5 W/cm2), enabling high growth rates through high silane (SiH4) depletion, and high pressures (> 0.75 Torr) enhancing the material properties by suppressing the ion bombardment. In this work direct ion measurements obtained in a parallel plate CCP are presented for two collisional pressure regimes enabling a quantitative comparison of the role of ions during the plasma-surface interaction and its relation to the amorphous-to-microcrystalline silicon phase transition as determined in both the HPD (10.50 Torr) and a low pressure (0.45 Torr) regime by varying the SiH4 flow rate, while preserving hydrogen (H2) flow rate and power density. By relating Raman and infrared absorption spectroscopy data the absence of the narrow high stretching modes and oxidation processes for the solar-grade µc-Si:H thin films has been demonstrated. Next to material analysis, this contribution will address selected aspects of plasma-surface interaction during µc-Si:H deposition. A capacitive probe has been implemented to study the ion flux for a range of parameters, i.e. silane flow rate and plasma power, whereas ion energies are determined with a retarding field energy analyzer (limited to pressures < 0.75 Torr), both implemented in the substrate holder. In the HPD regime an increase of SiH4 flow rate (0-10 sccm) is found to induce only a moderate increase in ion flux, accounting for 30% of the growth flux for solar-grade material [1]. With ion energies limited to below 19 eV (in collisionless H2 plasma), less than 6 eV are estimated to be available per deposited Si atom, suggesting that we are either in a regime of Si surface atom displacement, or more likely that a thermal spike is induced at the surface by the arrival of ions, enhancing the radical surface diffusion. Since the ion energy measurements are compatible only with low pressure, this study supports the direct correlation between material quality and plasma-surface interaction in terms of ion energy. The novel insights obtained can lead to the further development of deposition techniques in order to meet the stringent requirements of solar cells in terms of efficiency and production costs.
[1] A.C. Bronneberg, X. Kang, J. Palmans, P.H.J. Janssen, T. Lorne, M.C.M. van de Sanden and M. Creatore,
Submitted to J. Appl. Phys. (2012)