A rigid mask protects selective portions of a chip including a plurality of wells for biochemical reactions. The rigid mask includes a supporting portion and a plurality of legs, where each leg is provided with a rigid stem and a plate. The plurality of legs are arranged and fixed with respect to the supporting portion in a way aligned to the spatial arrangement of the wells, and are configured in such a way that, when each leg is inserted into the corresponding well, the respective plate covers at least in part the bottom of the well, protecting it during a chemical/physical treatment of side walls of the wells.