Comparison of laser micro-inscribed pillars on flat versus tilted substrates

When laser micromachining is performed at a non-zero angle of incidence, it is possible to fabricate microstructures with tilted sidewalls that could not be achieved using a perpendicular beam. In this report, we examine the influence of the angle of incidence (0–70°), feature size and spacing (10–150 µm), lens focal length (25 and 100 mm), and spot radius (15–50 µm) on the minimum achievable feature width ((W_{{{ ext{half}}}})) of periodic arrays of square micropillars. A new empirical model is developed that is applicable to both flat and tilted substrates, and accurately predicts the pillar height as a function of the feature width. By comparing structures inscribed at different tilt angles, we found that when the angle of incidence is increased (W_{{{ ext{half}}}}) also rises, such that it becomes more difficult to fabricate thin microstructures on angled substrates. Based on this knowledge, we establish a correlation that predicts (W_{{{ ext{half}}}}) as a function of the angle of incidence and the beam’s spot radius. Therewith, we can predict the minimum feature width that can be achieved over a wide range of laser processing parameters, with an average error of only 4.5% within the bounds of this investigation. Finally, by testing lenses with different focal lengths, we found that a shorter focal length reduces (W_{{{ ext{half}}}}) (improves the spatial resolution) when the angle of incidence is zero, but increases (W_{{{ ext{half}}}}) when the angle of incidence is high. We attribute this discrepancy to the angular spread of the beam cone, which can interact significantly with adjacent pillars, leading to degradation of the intended microstructure and a reduction in pillar height.