A tantalum pentoxide substrate 34 immersed in a liquid ambient (e.g. 10% hydrofluoric acid 30) and illuminated with radiation (e.g. collimated visible/ultraviolet radiation 24) produced by a radiation source (e.g. a 200 Watt mercury xenon arc lamp 20). A window 26 which is substantially transparent to the collimated radiation 24 allows the radiated energy to reach the Ta.sub.2 O.sub.5 substrate 34. An etch mask (e.g. organic photoresist 32) may be positioned between the radiation source 20 and the substrate 34. The Ta.sub.2 O.sub.5 substrate 34 and liquid ambient 30 are maintained at a nominal temperature (e.g. 25.degree. C.). Without illumination, the Ta.sub.2 O.sub.5 is not appreciably etched by the liquid ambient. Upon illumination the etch rate is substantially increased.