Femtosecond laser machining of fluidic microchannels for miniaturized bioanalytical systems

Femtoseoond laser ablation of borosilicate (Borofloat(TM)) glass has been studied to machine fluidic micro-channel geometries (high aspect ratio and variable depth) not possible through traditional micro-lithographic techniques. Utilizing a 1 kHz repetition rate femtosecond laser system (Positive Light Spitfire) and a long-working distance 5x objective lens, groove patterns 10 mum wide and as deep as 30 mum have been produced. The experiments were performed in air and the samples were cleaned after the ablation with sodium hydroxide dissolved in water to remove the debris. The substrates were mounted on a computer controlled x-y translation stage. The quality of the micro-channels showed dependency on the scanning speed of the sample. The surrounding area of the channels was smooth at scanning rates greater than 400 mum/s and smaller than 10 mum/s. Whereas, cracks appeared around the channels at scanning rates between 200 to 50 mum/s. Surface morphology is studied using optical, electron and atomic force microscopies. For a quantitative evaluation of ablation threshold and ablation rates, single-shot experiments in vacuum were performed. We found that the damage threshold for borosilicate glass is around 1.7 J/cm(2). With single pulse laser fluence of 30 J/cm(2), a 600 nm deep crater could be ablated. A ring, higher than the surface, appeared around the craters and was most probably created by adiabatic compression of glass due to the high-pressure plasma generated in the early stages of the process.