  Laser drilling becomes of increasing importance when hole diameter is in the
range of 10 to 50 mum, for which conventional alternative approaches are
becoming difficult and cost inefficient. Furthermore, it is viewed as the
technique of choice for a number of composite materials and hard materials
which are not readily processed at the microscopic level by contact mechanical
tools. We have demonstrated that suitable experimental conditions are capable
of producing microholes with record aspect-ratio (up to 600) in pure polymers
like PET, PI, PC, PS, PMMA, PEEK,... For example holes of diameter typically 30
mum can be as long as 18 mm, depth at which the drilling rate is getting nearly
zero and the profile stationary. Other materials (metals, ceramics) which can
be similarly laser microdrilled do not exhibit such very high aspect-ratio. The
mechanisms of the drilling process have been studied in details and an original
analytical model has been constructed recently. The various experimental
results, obtained with the KrF laser, will be reviewed with emphasis on the
parameters leading to formation of good holes with high aspect-ratio. For the
application it is also important to note that such high values of aspect-ratio
are obtained with regular configuration of the KrF laser giving a standard
divergence of 3 mrad. However as shown by the model there is still room for
improvement by using a beam with a lower divergence (theoretical limit is 0.2
mrad). Further experimental work is now in progress.
