UV excimer laser drilled high aspect ratio submicron via hole

We have demonstrated UV excimer laser drilled a submicron via hole with an entrance diameter of 300 nm inside a via hole with an entrance diameter of 5 μm. The smaller via hole formation was due to the refocusing of the reflected UV light from the tapered side-wall of the bigger via hole and the wave-guide effect of the light trapped inside the smaller via hole. The aspect ration of the smaller vias hole was >200. This method could be used to fabricate microfilters or nanopores.