Nanoimprint lithography for honeycomb texturing of multicrystalline silicon

We investigated a novel process chain based on Nanoimprint Lithography (NIL) for realising defined textures. As application we chose the honeycomb texturing of multicrystalline silicon (mc-Si), which so far was applied using elaborate photolithographic processing for reaching the highest efficiencies on this type of substrate material up to now. The developed process chain contains the single steps of interference lithography (master origination of the hexagonal pattern) and cast moulding of silicone (stamp fabrication) as preliminary steps and the repetitive steps of NIL (structuring of the etching mask) and plasma etching (pattern transfer into the mc-Si substrate). First characterization steps of these textured substrates included reflection measurements to evaluate their optical properties as well as determination of emitter saturation current densities to gain information about potential plasma induced damages. These tests were conducted on high-quality float zone monocrystalline silicon to be not affected by material quality issues. Solar cells fabricated out of these test samples showed short circuit current densities exceeding 40 mA/cm2, revealing the potential of this texturing scheme. To further increase the feasibility of this process chain we developed a roller-NIL tool, which allows the patterning of UV-curable resists on non transparent, rough and brittle mc-Si substrates in a continuous process flow. Results of imprint quality using this novel tool are also shown in this work.