Power limits for non-destructive laser ablation of contaminants on micromachined structures

The minimum and maximum laser irradiances that are effective in cleaning etch-released membranes and microstructure was measured for carbon microparticles on LPCVD silicon nitride thin films and suspended platforms. A 1064nm Nd:Yag laser was scanned across the samples to ablate the contaminants. Microscope images of the membranes showed mass removal for an irradiance as low as 600W cm^-2. More thorough cleaning was achieved by increasing the irradiance. For 2.5×2.5mm, 205nm thin silicon nitride membranes, 79% of the contaminated area was removed with an exposure of 3.7kW cm^-2. Catastrophic damage was seen at a power level of 8.4kW cm^-2. Ablation effects were also measured as a change in optical absorption using a photo thermal common-path interferometer. Peak absorption values were decreased from over 100,000ppm to less than 20,000ppm. Silicon nitride platforms were also tested. Despite significant substrate heating, the platforms survived intact up to power levels of 8.4kW cm^-2 with near perfect cleaning of carbon particles from their surfaces.