Lithium niobate surface structuration for phononic crystal fabrication

Reactive ion etching (RIE) has been used to etch micron-sized holes which are several microns in depth in lithium niobate, mostly on Y- and Y-rotated cut substrates, the underlying idea being the realization of phononic crystal devices. The etching process is based on the use of sulfur hexafluoride as the etching gas. Photoresist and sputtered or electroplated metals masks were used and compared to ensure high process selectivity and good sidewall verticality. Maximum mask selectivity was found to be of the order of 20. Etched depths larger than 10 mum and aspect ratios above 1.5 have been achieved. Sidewall verticality higher than 73deg is also reported. The technique has been applied to the fabrication of phononic SAW devices designed to operate at a frequency around 200 MHz. The phononic structure consists of periodical arrays of 9 mum diameter, with a 10 mum period etched in Y-cut lithium niobate.