Light wave interference during laser drilling of polymer coatings

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Abstract
A CO2 laser has been used to drill holes in a 150-μm-thick, UV-curable, modified acrylate, polymer coating a copper substrate. A typical hole is 100–150 μm in diameter. The holes in this study were each made with a single laser pulse of 10.6-μm wavelength, duration 100 or 200 μs, and 4–20 mJ energy. Two superimposed sets of periodic ripples have been observed on the hole walls. The shorter wavelength varies from 4.0 μm at the top of the hole to 5.3 μm at the bottom of the hole. The longer wavelength appears to be 13.2 μm and is attenuated as the wave propagates towards the copper substrate. The experimental data are compared with values calculated using a model that considers the interference of a standing wave inside the hole with radiation propagating through the dielectric surrounding the hole. The amplitude (trough-to-peak distance) of the waves in the hole wall is about half the wavelength of the standing waves. The long-wavelength waves (13.2 μm) yield ripples in the wall of 6.5-μm amplitude. These ripples give 13.0 μm (0.5 mils) as an ultimate lower limit for laser drilling holes using the 10.6-μm wavelengths obtained with a CO2 laser. Chemical etching of the polymer causes all of the holes to have thin rims surrounding the top of the hole. This is due to accelerated etching of the less cured polymer material inside the hole. A chemical etching process etches away some of the ripple pattern, particularly near the top of the hole.

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