Study On Diagnosis Of Fluoro-based Capacitively Coupled Plasma And The Application In Texturing Of Multi-crystalline Silicon Solar Cells

The electronegative fluoro-based plasma texturing technique may be an alternativecandidate for multi-crystalline （mc） silicon surface texturing to replace the conventionalacidic texturing. In most of the works reported in literature, attention has not been paidmuch more to electrical characteristics of fluoro-based plasma and the inner mechanism ofplasma texturing. In this paper, characteristics of fluoro-based dual-frequency capacitivelycoupled plasma （DF-CCP） and the application in texturing of mc-silicon are investigated.Firstly, a floating microwave resonator probe has been proposed to measure electrondensity of capacitively coupled fluoro-based plasma. The experimental results from themeasurement of Ar/SF₆and SF₆/O₂capacitively discharge driven by40.68MHz show thataddition of SF6into Ar plasma reduces the electron density significantly; With furtheraddition of SF6flow rate, electron density shows a gradual decrease. While for the additionof O₂in SF6discharge, the electron density continuously decreases with the increase of O₂flow rate. Additionally, the electron density doesn’t vary with lower frequency input powerfor SF₆/O₂capacitively coupled discharge driven by40.68MHz/13.56MHz.Secondly, Emission intensity of F and O radicals in SF₆/O₂capacitively coupledplasma driven by40.68MHz/13.56MHz are investigated by using optical emissionspectroscopy （OES）. Experimental results show that, with the addition of O₂in SF6discharge, O atom relative concentration is expected to be increased linearly with O₂addition; Whilst, the F atom relative concentration firstly increases with O₂flow rate andthen keeps constant basically;[F]/[O] ratio increases significantly with the addition of O₂. It can be found that F atom concentration increases monotonously with increasingdischarge pressure; However, O atom concentration increases initially and then decreases;[F]/[O] ratio increases significantly with increasing the pressure. With increasing inputpower of upper electrode, F and O atom concentration increase initially and then decreasewith further increasing input power; Whilst the ratio of [F]/[O] shows an increase asincreasing input power and then it keeps a constant as input power is larger than250W.Additionally, F and O atom relative concentration doesn’t vary with bias voltage.Lastly, driven by dual frequency of40.68MHz and13.56MHz was used for plasmatexturing of mc-Si with SF₆/O₂gas mixture. Influence on mc-Si plasma texturing wasinvestigated with different flow rate ratio, pressure and radio-frequency input power.Experimental results show that suitable plasma texturing of mc-silicon occurs only undernarrow window of plasma parameters, where electron density nemust be largerthan6.3×109cm3and ratio of spectral intensity of F and O atom （[F]/[O]） in the plasmamust be between0.8and0.3. Out of this range, no cone-like structure is formed on themc-silicon surface. In our experiments, the lowest reflectance of about7.3%formc-silicon surface texturing is obtained at the [F]/[O] of0.5andne of6.9×109cm3.Moreover，the minority carrier lifetimes decrease with increased O₂flow rate, etching timeand bias voltage. A physical model is developed to explain the evolution of dry plasmatexturing.