A method of etching a layer of electrical insulating material including a layer of strontium titanate on a surface of a semiconductor substrate. The layer of strontium titanate is immersed in a passivated etching solution including an acid and HF and focused light is directed onto the surface of the layer of strontium titanate at areas to be etched, so as to depassivate the passivated surface and to etch the layer of strontium titanate only at the surface receiving collimated light. In a preferred embodiment, the passivated etching solution includes HCl and less than 1000 ppm of HF.