Nanostructured Back Surface Amorphization of Silicon with Picosecond Infrared Laser Pulses

Silicon, the backbone material of the electronics industry, shows vastly different properties in the amorphous (a-Si) and the crystalline (c-Si) state. Traditional techniques for producing a-Si show drawbacks as the need for a hot environment or permanently induced impurities. Laser-based silicon amorphization enables a precise and contactless production of an a-Si surface layer. The locally induced state change on a micrometer scale enable a mask-less wet etching process and the inscription of surface waveguides. Currently, this process is only applicable to the front surface [1]. If one attempts to simply transpose this technique to the back surface, the propagation of Gaussian-shaped intense pulses in silicon leads to the formation of micro-filaments due to the low critical power for self-focusing (≈10-20 kW). Consequently, typical filamentation features such as nonlinear focal shift as well as intensity clamping occur simultaneously. These mechanisms hinder energy deposition in a confined volume in the bulk and the back surface of silicon [2,3].