Ablation and cutting of planar silicon devices using femtosecond laser pulses

Although lasers are generally able to machine silicon, the major material in many microsystems applications, doing so without influencing the physical properties of the bulk material remains an important challenge. Ultrafast lasers, in particular, with their potential to precisely ablate all kinds of solid materials, are able to perform such processes with high efficiency and accuracy. This article starts with an overview of the general interaction of ultrafast laser radiation with semiconductors, explaining the absorption processes and different fluence regimes for the ablation of silicon. Major parameter influences, especially for cutting processes in thin silicon, are described. By varying pulse energies, beam shaping methods, the beam polarization, and temperatures, the cutting quality and speed can be significantly influenced. One important quality aspect, besides kerf widths and surface roughness, is the amount of back-side chipping when cutting brittle materials. Achievements in speed enhancement using linear focus shapes are presented, with cutting speeds up to five times higher than by conventional spot-focusing. On the other hand, laser processes that cut with a spot focus offer the possibility of free-shape cutting, which is explained using the example of wafers carrying silicon chips with highly increased package densities.