Local molten regions formation on silicon surfaces by laser annealing at the melting threshold

Nanosecond (NLA) and microsecond laser annealing (μLA) are promising techniques for contact formation, crystallization, or dopant activation in semiconductors, especially when integrating with thermal budget limitations. When exposed to NLA or μLA, at the boundary between solid-phase and liquid-phase processing conditions, Si or Ge surfaces can be roughened by the formation of local molten regions (LMRs). In this work, Si surfaces were irradiated after various preparations using NLA and μLA, to better understand the formation of LMRs. During the exposure of a Si surface to NLA, LMRs nucleate at the Si/surface oxide interface through the formation of a liquid Si droplet and then extend in depth and laterally. Due to the anisotropy of the melting rate, the LMRs’ edges are close to a square or an equilateral triangle on (100) and (111) surfaces, respectively, with edges along crystalline directions. In the case of (100) Si surfaces exposed to ultraviolet NLA, the molten volume forms a square-based inverted pyramid, truncated at 20-30 nm depth. Upon solidification, some topography is introduced to the initial flat surface. On (100) Si surfaces, the smallest LMRs are monocrystalline pyramids of 12 to 25 nm height, while the largest ones exhibit a castle-like shape with 4 peaks. The presence of a thin surface oxide is mandatory to obtain LMRs with a geometrically well-defined base, and the thickness of this oxide impacts the shape and size of the LMRs. The evolution of the LMRs size as a function of the annealing duration suggests that thermal diffusion limits their extent.