Fabrication of vertical silicon microwire arrays for enhanced light trapping using Deep Reactive Ion Etching

Abstract

Silicon microwire array is one of the promising platforms as a means for developing highly efficient solar cells thanks to the enhanced light trapping efficiency. Among the various fabrication methods of microstructures, Deep Reactive Ion Etching (DRIE) process has been extensively used. In this presentation, we show precisely controlled vertical Si microwire arrays by tuning the DRIE process conditions. A periodic microdisks were patterned on 4 inch Si wafer (p-type, 1-10 Ωcm) using photolithography. After developing the pattern, 150-nm-thick Al was deposited and lifted-off to leave Al microdisk arrays on the starting Si wafer. Periodic Al microdisk arrays (diameter of 2 μm and periodic distance of 2 μm) were used as an etch mask. A DRIE process (Tegal 200) is used for anisotropic deep silicon etching. During the process, SF6 and C4F8 gases were used for the etching and surface passivation, respectively. The length and shape of microwire arrays were controlled by etching time and SF6/C4F8 ratio. After DRIE process, the residual polymer and etching damage on the surface of the microwires were removed using piranha solution followed by thermal oxidation (1000 °C, 3 hour). The oxide layer formed through the thermal oxidation was etched by diluted hydrofluoric acid (1 wt% HF). The surface morphology of a Si microwire arrays was characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Optical reflection measurements were performed over 300-1100 nm wavelengths using a UV-Vis/NIR spectrophotometer (Cary 5000, Agilent). The total reflection by the microwire arrays sample was reduced from 20 % to 10 % of the incident light over the visible region when the length of the microwire was increased from 10 μm to 40 μm.