At least one hollow zone is formed in a stack of at least one upper layer and one lower layer. The upper layer is patterned to form at least a first hollow region passing through said upper layer. The first hollow region is extended by a second hollow region formed in the lower layer by etching through an etching mask formed on the patterned upper layer. The etching mask is formed by a resin layer, positively photosensitive to an optic radiation of a predetermined wavelength, exposed to the said optic radiation through the stack and developed. The lower and upper layers of the stack are respectively transparent and opaque to said predetermined wavelength so that the patterned upper layer acts as exposure mask for the resin layer.