Selective etching of fs-laser inscribed high aspect ratio microstructures in YAG

We report on a systematic investigation of selective etching of fs-laser inscribed microstructures in Y3Al5O12 (YAG). The resulting microchannels are up to 8.9 mm long and exhibit cross sections from below 10 µm to more than 100 µm. Aspect ratios of up to 593 were achieved. Investigations with different structuring and etching parameters revealed that the etching process is mainly diffusion determined. The etching depth depends on the square root of time, similar to the well-known Brownian motion. In addition, we could enhance the etching diffusion constant by a factor of two, reducing the time to etch the longest channel by an order of magnitude, using a 1:1 mixture of sulfuric and phosphoric acid instead of pure phosphoric acid. The observed fundamental time dependence in conjunction with diffusion coefficients up to 160 µm/h1/2 makes the etching behavior highly predictable and paves the way toward arbitrary three-dimensional micro- and nanostructuring over long distances in crystalline materials.