A semiconductor wafer comprises an SOI comprising a device layer on an oxide layer supported on a handle layer. Micro-mirrors are formed in the device layer, and access bores extend through the handle layer and the oxide layer to the micro-mirrors for accommodating optical fibers to the micro-mirrors. The access bores are accurately aligned with the micro-mirrors, and the access bores are accurately formed of circular cross-section. Each access bore comprises a tapered lead-in portion extending to a parallel portion. The diameter of the parallel portion is selected so that the optical fibers are a tight fit therein for securing the optical fibers in alignment with the micro-mirrors. The tapered lead-in portions of the access bores are formed to a first depth by a first dry isotropic etch for accurately forming the taper and the circular cross-section of the tapered lead-in portions. The parallel portions are formed from the first depth to a second face of the handle layer by a second dry etch, namely, an anisotropic etch carried out using the Bosch process. By so etching the access bores the access bores are accurately formed of circular transverse cross-section and of accurate dimensions.