Reduction in heat affected zone and recast layer in laser materials processing using a photon sieve lens

Abstract

A laser ablation study was carried out by focusing optical energy via a photon sieve planar diffractive lens and the results are compared with traditional refractive lenses. It was found that the materials processed by the photon sieve show a significant reduction in the lateral spread and thickness of the oxidized surface when compared to the refractive lens. This oxide layer is primarily attributed to the heat affected zone and recast layer in the silicon substrate. This difference is attributed to the pinhole diffraction of the photon sieve, which produces a reduced 1/e2 diameter compared to the f-theta lens for a given full-width at half-maximum value. This reduced spot size compared to a refractive lens, for a given numerical aperture, translates to less material removal/redeposition and a narrower ablation region in the case of the photon sieve, without any sacrifice in depth of focus. The numerical apertures achievable with photon sieves are comparable to those of commercial microscope objectives, but with a sub-Airy disk focal spot size. This increased resolution, combined with a longer depth of focus for a given spot size, reduced material redeposition, and a narrower ablation region show a significant improvement over the current state of the art. The results of this study could have a large impact on laser materials processing in the aerospace, medical, semiconductor, and automotive industries, among others.