2500 angstrom using nh 3 and sih 4 the silicon nitride layer 14 can also be deposited using lpcvd or pecvd procedures using a reactant gas mixture such as dichlorosilane sicl 2 h 2 and amonia nh 3 photolithographic techniques and reactive ion etching rie are then used to create the desired pattern in the layer 16 of silicon nitride the silicon nitride layer 14 can be wet etched using a phosphoric acid solution the silicon nitride can also be etched using anisotropic rie using chf 3 or sf 6 u2014 o 2 as an etchant the silicon nitride layer 14 can also be dipped into phosphoric acid h 3 po 4 to be removed the pattern of silicon nitride that has been highlighted with 14 in fig2 aligns with surface areas of substrate 10 over which cmos devices are to be created another way of stating this is to say that the surface regions of substrate 10 that are exposed in fig2 and that are located between the pattern 14 of silicon nitride are the surface regions into which the sti trenches are to be etched fig3 shows a cross section of the silicon substrate after a layer 16 of photoresist has been deposited over the surface of the substrate layer 16 has been patterned and etched using the p well mask in order to.