  We describe a technique for surface and sub-surface micro-machining of glass
substrates using tightly focused femtosecond laser pulses at a wavelength of
1660 nm. Although silicate glass is normally transparent at this wavelength,
the extremely high intensity of the focused beam causes multi-photon
absorption, resulting in localized ablation of the glass substrate. Ablation is
strictly confined to the vicinity of focus, leaving the rest of the substrate
unaffected. We exploit this phenomenon to drill a micro-hole through a thin
vertical wall that separates two adjacent pits machined by the same laser in a
glass plate. A salient feature of pulsed laser micro-machining, therefore, is
its ability to drill sub-surface tunnels and canals into glass substrates, a
process that requires multiple steps in standard lithography. To demonstrate a
potential application of this micro-machining technique, we have fabricated
simple micro-fluidic structures on a glass plate. To prevent the evaporation of
liquids in open micro-channels and micro-chambers thus fabricated requires a
cover plate that seals the device by making point-to-point contact with the
flat surface of the substrate. This point-to-point contacting is essential if
the fluids are to remain confined within their various channels and chambers on
the chip, without leaking into neighboring regions. Methods of protecting and
sealing the micro-machined structures for microfluidic applications are also
discussed.
