Figure 2.  blue (0.75 J/cm²), light blue (5.5 J/cm²), orange (10 J/cm²)


The observed ablation behavior is affected by a multi-pass  the material properties of silicon or semiconductors in general. In a multi-pass ablation strategy, the beam is guided over the workpiece several times. In every repetition a certain amount of     area is no longer planar, and the incoming laser beam is projected to the walls of the cutting kerf. Therefore, the effective beam  an ablation geometry and the dependency of the projected intensity from the angle of incidence.


The occurring critical angle determines the attainable cutting depth in relation to the cutting width, and thus explains the  the appearance of a saturation level can be explained, because once the critical angle is reached, almost no ablation takes place inside the cutting groove anymore. The steepest point with the critical angle always occurs in the center, due to the highest   effective angle of incidence. Therefore, the cutting kerf may not develop absolutely vertically, but can drift to one side (compare Figure 2, middle image).


In conclusion, dicing thin silicon wafers with an ultra-short pulsed laser system has been investigated as to which cutting depths


www.lia.org 1.800.34.LASER 11


Figure 3. Explanation of the cutting kerf geometry and the progress of the groove depth 


and kerf widths can be reached. According to the experiments, the amount of the groove depth strongly depends on the laser          a saturation level after a certain depth or number of passes. Afterwards the depth no longer increases. This correlation is the result of a multi-pass ablation strategy with ultra-short pulsed laser radiation, in which a critical angle of incidence limits the attainable cutting geometry and depth.


Christian Fornaroli is a research engineer with Fraunhofer ILT.


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