Spot size measurements for ultra-small laser welds

Abstract

A desire to weld devices where significant feature sizes are on the order of microns requires laser spot sizes of similar dimensions. Since the very high power densities typically found in such tightly focused beams may cause abrupt welding-to-drilling transitions, knowledge of the focused spot sizes is essential for adequate process control. While determination of the focused beam diameter becomes more critical, it is harder or impossible to measure with common beam characterization equipment at such small spot sizes. We sought a simple method for process control analogous to the commonly-used Kapton film method; however, at diameters of <50 microns, typical Kapton films are "thick". Instead, we used glass microscope slides deposited with a sub-micron thick carbon film. A Nd:YAG 100 W pulsed laser with a 1.01mm intra-cavity aperture focused through a 50mm focal length plano-convex microscope lens was used to produce spots down to ∼25 microns (∼2.5x diffraction limit). We discuss the merits of the carbon-on-glass technique relative to Kapton film and CCD camera techniques.