Dual layer etch stop barrier

A method of reactive ion etching SiO2 comprising the steps of:

• providing a silicon substrate comprising a semiconductor device including a gate structure between a first and a second portions of said silicon substrate,
• forming a barrier of silicon nitride directly on said silicon substrate, said barrier of silicon nitride having a first section and a second section superimposed on each other and coextensive entirely with each other with the first section being in contact with said first and second portions of said silicon substrate and the second section being spaced from said silicon substrate, said first and second sections extending continuously from said first portion of said silicon substrate to said second portion of said silicon substrate over said gate structure,
• said second section having a ratio of Si:N of at least about 0.8 and providing desired etch selectivity, said first section having a ratio of Si:N of less than about 0.8 and providing desired resistance to positive mobile ion penetration,
• forming a layer of SiO2 on said barrier of silicon nitride,
• forming at least one opening through reactive ion etching in said layer of SiO2 using said barrier of silicon nitride as an etch stop layer,
• removing an exposed portion of said barrier of silicon nitride in said opening to reveal the substrate, and
• depositing a conductor in said opening and in contact with said substrate.