Ultra-Thin Silicon Wafer Fabrication And Performance Characterization

The metal-assisted chemical etching（MACE）has been widely used for fabricating nano-texturing on silicon wafer,and there has been great interest in using this nano-texturing silicon wafer as an antireflection（AR）coating for Si solar cells because of its excellent AR effect.The common MACE process has a preferential etching orientation,such as,the preferential etching direction is along（100）orientation for single crystal silicon substrate covered with noble metal nanoparticles.This research proposed the introduction of cheaper metal copper instead of the traditional precious metal（Au-Ag-Pt）to assist chemical etching.The copper assisted chemical etching（CACE）is more promising for commercial fabrication of Si solar cells because of its much lower cost.Recently,copper had been employed as electrode material instead of Ag in order to reduce the cost of silicon solar cells.The etching effect and mechanism of copper nitrate concentration,hydrogen peroxide concentration,reaction temperature and etching time on the morphology of silicon wafer are researched in this thesis.The inverted pyramid and thin silicon wafer were investigated.The main research contents and conclusions are as follows:（1）Comparing with the properties of silicon nanowires structure and inverted pyramids structure,it is found that the inverted pyramids structure is more excellent in reflectivity and minority carrier lifetime.Not only the superior anti-reflection performance and excellent saw marks elimination ability show that the formed small surface area inverted pyramid by metal copper catalytic etching,but also the small surface area inverted pyramids can more perfectly balance anti-reflection and surface passivation.（2）The effects of amount of Cu（NO₃）₂,H₂O₂ concentration,reaction temperature and etching time on the nano-texturing structures,anti-reflective ability and minority carrier lifetime were systematically investigated.After etching at the solution of 0.06 mol/L copper nitrate（Cu（NO₃）₂）,3 mol/L hydrogen peroxide（H₂O₂）concentration and 2 mol/L hydrofluoric acid（HF）at 60°C for 5 min.by metal copper assisted chemical etching,resulting the thin silicon wafer under 40μm and an inverted pyramids structure were obtained.The standard inverted pyramids structure with wide of 0.8¹.2μm and depth of1³μm were obtained.The average reflectivity of textured sample decrease from 26.8%to 4.3%in the wavelength range of 300¹000 nm and the roughness of the surface is reduced.In the later treatment of silicon wafer,metal copper is easily removed to avoid the formation of impurities and new composite centers.Compared with the metal silver assisted chemical etching,the cost of texturing is lower.（3）The formation mechanism of the inverted pyramid structure is discussed and can be attributed to the moderate catalytic activity of nanocopper particles that induce etching preferentially along the non-<111>directions of the silicon.The nano-texturing structures,with the excellent anti-reflectivity and minority carrier lifetime,have potential application in the fabrication of solar cells compatible with the semiconducting industry.