We describe a technique for surface and sub-surface micro-machining of glasssubstrates using tightly focused femtosecond laser pulses at a wavelength of1660 nm. Although silicate glass is normally transparent at this wavelength,the extremely high intensity of the focused beam causes multi-photonabsorption, resulting in localized ablation of the glass substrate. Ablation isstrictly confined to the vicinity of focus, leaving the rest of the substrateunaffected. We exploit this phenomenon to drill a micro-hole through a thinvertical wall that separates two adjacent pits machined by the same laser in aglass plate. A salient feature of pulsed laser micro-machining, therefore, isits ability to drill sub-surface tunnels and canals into glass substrates, aprocess that requires multiple steps in standard lithography. To demonstrate apotential application of this micro-machining technique, we have fabricatedsimple micro-fluidic structures on a glass plate. To prevent the evaporation ofliquids in open micro-channels and micro-chambers thus fabricated requires acover plate that seals the device by making point-to-point contact with theflat surface of the substrate. This point-to-point contacting is essential ifthe fluids are to remain confined within their various channels and chambers onthe chip, without leaking into neighboring regions. Methods of protecting andsealing the micro-machined structures for microfluidic applications are alsodiscussed.
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