Etching Techniques for Thinning Silicon Wafer for Ultra Thin High Efficiency Interdigitated Back Contact Solar Cells

Abstract

High efficiency Interdigitated back contact (IBC) solar cells help reduce the area of solar panels needed to supply sufficient amount of energy for household consumption. We believe that a properly passivated IBC cell with the aid of light trapping schemes can maintain an efficiency of 20% even with thickness under 20 μm. In this work, photolithography and etching techniques are used for deep etching of crystalline Silicon (c-Si) wafer to a thickness less than 20 μm. Tetramethylammoniumhydroxide (TMAH) wet anisotropic etching and plasma based Reactive ion etching (RIE) are used with SPR 220-7.0 and SU-8 photoresists. SiO2 is used as making layer for TMAH etching. TMAH etch of a 4-inch c-Si wafer is done at a temperature of 80°C for 8 hours. RIE of a quarter of a 4-inch c-Si wafer is done for 3 hours using SF6 as reactive gas. A baseline photolithography process flow for SU-8 photoresist deposition was developed. The etch rates of TMAH etch techniques fall within the range of 0.3 – 0.45 μm/min and etch rates for RIE fall within the range of 1.2 – 1.8 μm/min. The RIE shows capability of achieving smaller thickness sizes with greater advantages than the TMAH etching technique.