| AVS 63rd International Symposium & Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS-WeM |
| Session: | Plasma Sources and Novel Mechanisms for Generating Plasmas |
| Presenter: | Guillaume Fischer, Institut Photovoltaïque d'Ile-de-France (IPVF), France |
| Authors: | G. Fischer, Institut Photovoltaïque d'Ile-de-France (IPVF), France E. Drahi, Total MS-Energies Nouvelles, France G. Poulain, Total MS-Energies Nouvelles, France B. Bruneau, LPICM, Ecole Polytechnique, France E.V. Johnson, LPICM, Ecole Polytechnique, France |
| Correspondent: | Click to Email |
The nanotexturing of the surface of a crystalline silicon (c-Si) wafer for improved photovoltaic performance can be achieved through the use of an SF6/O2 reactive ion etching (RIE) capacitively coupled plasma (CCP). The resulting surfaces typically consist of nano-sized structures resembling cones (sizes ranging from 30 to 500nm) with little preferential crystallographic orientation. The process occurs through competing mechanisms involving Si etching by fluorine radicals, formation of in-situ micro-masking species, and physical etching by ions, all these mechanisms being strongly influenced by plasma conditions.
As has been done for previous processes and chemistries, we attempt to decouple the influence of various plasma properties through the use of Tailored Voltage Waveforms (TVWs), and thus obtain insights into the mechanisms involved in the dry nanotexturing of silicon. TVW excitation consists of adding harmonic frequencies with controlled amplitudes and phase-shifts to the RF (13.56 MHz) driving voltage, and allows one to quasi-independently control parameters such as species flux and ion bombardment energy (IBE). Furthermore, in an electronegative chemistry such as the SF6/O2 mixture, waveforms resembling "sawtooths" induce high ionization asymmetries due to plasma sheath dynamics, and may impact the type of reactive species arriving at the surface.
In this study, the phase-shift of the harmonic frequencies of the TVW excitation is varied at constant discharge power in an SF6/O2 mixture, therefore modifying (by an up to fourfold increase in absolute value) the self-bias voltage (VDC) at the powered substrate holder and therefore the maximum IBE. The impact of varying the TVW shape is observed through both the plasma properties (VDC, optical emission) and the morphological and optical properties of the obtained nanotexture. The effectiveness of the texture is quantified by the surface effective reflectance (Reff) which is the average reflectance weighted by the solar spectrum irradiance.
It is here shown that the use of TVW excitation allows, at constant discharge power, to switch from a regime with no etching (almost no change of Reff, no nanostructures observed) to a texturing regime where the decrease of Reff scales with the IBE (Reff decreasing up to 30% and nanocones observed). Moreover, new types of nanostructures have been observed for some particular etching regimes, showing partial dependence on the crystallographic orientation of the substrate.