Laser micromachining of thin beams for silicon MEMS: Optimization of cutting parameters using the Taguchi method

While thin beams are widely used structural elements in Micro-Electro-Mechanical-Systems (MEMS) there are very few studies investigating the laser machining of clean high aspect ratio silicon beams. This work presents a systematic study of selected influencing cutting parameters with the goal of machining high aspect ratio beams with low side wall surface roughness (Ra) and high cross section verticality, i.e. low taper angle. The Taguchi method was used to find the optimal setting for each of the selected parameters (pulse frequency, laser diode current, pulse overlap, number of patterns to be marked, gap size between patterns) utilizing orthogonal arrays and signalto-noise (S/N) ratio analysis. Double-sided clamped beams of 100μm width and 10mm length were machined in silicon wafers of 525μm thickness using a nanosecond solid-state UV laser system (355nm wavelength). Our experimental results show that beams with an aspect ratio as high as 17.5 can be manufactured. Furthermore, a surface roughness of Ra = 0.37μm and taper angle of a = 2.52 degrees can be achieved. This will make the fast fabrication of MEMS devices with aspect ratios as high as those from deep reactive ion etching possible.