Plasma polymer films for 532 nm laser micromachining

Laser micromachining with a frequency doubled Nd: YAG laser (532 nm) can replace more complex microlithographic processes for rapid turnaround in the development of prototype application-specific integrated circuits. Plasma polymerization is a rapid, dry, environmentally friendly process that yields crosslinked pinhole-free films. Plasma polymerized films of ethylene and an additional gas [PP(gas/E)] were investigated for their micromachining potential. The deposition rates, molecular structures, physical properties and optical properties of the polymers were characterized. PP(Ar/E), with relatively little oxygen and no nitrogen, with superior substrate adhesion and with no debris generated on laser micromachining was chosen as the optimal laser micromachining film. The PP(Ar/E) coefficient of optical absorption at 532 nm (α₅₃₂), related to unsaturated group concentration, increased with the ratio of plasma power to ethylene mass flow rate [W/F_m(E)]. α₅₃₂ reached an asymptote of 2.9 μm^-1 at high W/F_m(E) and could be enhanced slightly using postpolymerization ultraviolet exposure. The optimum conditions were using Ar/E=1/1 and 75 W to produce a 0.6 μm thick film for micromachining at 2 J/cm² focused 0.25 μm beneath the surface. The laser pulse in a 1.2 μm thick film was not fully developed at 2 J/cm² and exhibited rounded corners at 4 J/cm², indicating that multiple low energy pulses would be preferable. A complicated and densely packed pattern with several different pulse sizes in which neighboring holes from pulses in close proximity do not merge was accurately reproduced in PP(Ar/E) using laser micromachining.