Curvature of laser-machined grooves in Si3N4

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Abstract
In laser machining, the article to be machined is moved with respect to the laser beam resulting in the formation of a groove in its surface by vaporization. Grooves formed in hot‐pressed Si3N4 with a CO2 laser beam at normal incidence are observed to have cross sections that curve out of the plane defined by the velocity direction of the article and the direction of the laser beam. The curved cross sections change from one side of this plane to the other if the velocity direction is reversed. The effect is most pronounced at the lowest velocities and highest incident powers. The cross‐section shape is shown to vary systematically with the angle between the velocity direction and the direction of the electric vector of the incident beam, which was found to be partially polarized. A mechanism for the curved cross‐section effect is proposed based on the existence of a significant difference in reflectance between the transverse electric (TE) and transverse magnetic (TM) reflections from groove walls in Si3N4 for large angles of incidence. Calculations of values of reflectance for the TM and TE reflections as a function of the angle of incidence show the existence of a large difference for the CO2 wavelength of 10.6 × 10−4 cm.

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