form a first trench 30 a a second trench 30 b and a plurality of isolation trenches 30 c having a width of approximately 1 5 2 u03bcm and a depth of for instance 10 u03bcm the shape of which may be seen in fig6 in detail as is shown in fig5 and 6 the first trench 30 a has the shape of a circumference with a first diameter and externally delimits a central supporting region 29 a extending next to a surface 28 a of the substrate 29 the second trench 30 b has the shape of a circumference with a second diameter greater than the first diameter and is concentric with the first trench 30 a so as to delimit laterally together with the first trench 30 a an annular supporting region 29 b arranged next to the surface 28 a of the substrate 29 the isolation trenches 30 c not visible in fig5 and represented each only by a line in fig6 have a square or in general polygonal shape are arranged outside the second trench 30 b and delimit respective portions of the substrate 29 designed to form through electric connection regions 29 c also adjacent to the surface 28 a in particular the isolation trenches 30 c are arranged externally with respect to the second trench 30 b the first and second trench 30 a 30 b have the purpose of mechanically separating movable portions and fixed portions of the substrate 29 as will be explained.