is performed using spray equipment and finally etching of the first insulating layer 33 is carried out using the same etching solution as previously and the same machine for example for the etch the machine mxp manufactured by applied materials inc may be used at a low pressure for example comprised between 10 u2212 2 and 1 torr in particular approximately 0 2 torr with application of a magnetic field of 10 gauss and using a gas of cf 4 ar chf 3 and o 2 according to an embodiment of the present process for the etching step a flow of cf 4 is used that is greater than for example three times that of chf 3 in particular the flow of cf 4 may be comprised between 40 and 50 sccm and the flow of chf 3 may be comprised between 10 and 20 sccm during the plasma etching reaction as is known polymeric by products are produced with a base of c and f the majority of which is generally expelled and removed the structures being defined by an appropriate choice of the pressure and of the flow of the etching gases in the process described instead the parameters are studied so that said by products during the expulsion process deposit with a particular pattern on the walls of the structure just defined in fact with the values of flow indicated as the trench 41 is formed on the.