The researchers took advantage of the fact that amorphous silicon layers can already be produced on the wafer holding the circuit at temperatures below 450 ºC and high deposition rates. The laser not only crystallizes this silicon layer, but it also activates the dopants it contains, ensuring suitable electrical conductivity. Subsequently, the sensor units are processed further using classic microelectronic manufacturing processes. When laser radiation is used to crystallize silicon at high temperature, but below its melting point, crystallization occurs spatially, selectively, and very quickly, in the lower millisecond range. This way —  in conjunction with targeted temperature management —  the process minimized mechanical stresses in the layer material but did not damage the sensitive electronics on the underlying substrate. The silicon is crystallized with a focused laser beam and guided by a mirror to scan the entire surface step by step. In this spatially selective process, heat is removed effectively in three spatial directions. This distinguishes the process from alternative photonic processes such as flash exposure, where heat can only be dissipated in one direction because the area to be processed is so large. “Since the energy is quickly introduced into only a small volume, we achieve solid phase crystallization of the silicon with laser processing at temperatures that are actually above the destruction threshold of the underlying circuit. Due to the short local processing time, the circuit is nevertheless not damaged,” said Christian Vedder, head of the Thin Film Processing group at Fraunhofer ILT.