Deep Wet-Etched Silicon Cavities for Micro-Optical Sensors: Influence of Masking on $\{111\}$ Sidewalls Surface Quality

In this paper, we investigate the influence of different masking parameters onto the surface quality of the {111} sidewalls in order to generate specifically deep cavities by wet-anisotropic-etching of bulk silicon, for optical sensors using cavity sidewalls as reflectors. Mask alignment with crystal planes prior to wet-etching is found to be essential in order to avoid the appearance of additional planes during long etching. Mask deposition processes have been also evaluated. Among the different employed mask materials, Cr/Au gives the best results. It is then shown that cavities as deep as 1 mm with low roughness sidewalls can be simply produced with potassium hydroxide solution with periodic piranha cleaning.