Comparing flat-top and Gaussian femtosecond laser ablation of silicon at normal and oblique angles of incidence

The optical intensity profile of a laser beam is crucial in laser micromachining and laser ablation mass spectrometry (MS). For the latter, Gaussian laser beams are usually used to probe surfaces of material and obtain two-dimensional (2D) maps of their compositions. Such 2D MS analysis can be extended into the third dimension (3D) if the probe is able to remove homogeneously thin layers of material by each laser shot. This high depth resolution of the probe is difficult to achieve with Gaussian beams because they cannot produce flat bottoms of ablated craters. To better understand laser ablation MS experiments aimed at enabling and optimizing this capability, studies of laser ablation of silicon under ambient conditions with flat-top and Gaussian beams generated by a femtosecond 800 nm laser with increasing incidence angles and numbers of laser shots were conducted. The resultant craters were characterized via optical 3D microscopy. When the beam profile was flat-top, the ablation rate was homogenized over the laser beam spot so that flat crater bottoms were achieved over a wide range of incidence angles. This enables 3D MS analysis with laser ablation probes, with ablation rates approaching <1 nm per shot, significantly steeper crater walls and minimal surface damage in comparison to the Gaussian craters. Flat bottom near-cylindrical and "splash-like" conical crater geometries observed in these experiments indicate that ablation regimes for the flat-top and Gaussian profiles, respectively, were different despite using similar laser energies.