Abstract: A dense and stable dielectric layer of silicon nitride and silicon dioxide suitable for use in transistors of ULSI circuits is fabricated by a high pressure process in which a nitride layer is first formed on a surface of a silicon substrate and then a silicon dioxide layer is formed on the silicon surface under the nitride layer. By placing the nitride layer above the silicon dioxide and next to a doped polysilicon gate, diffusion of dopant ions such as boron from the gate into the silicon dioxide is reduced. As semiconductor devices are scaled down, the thermal budget required for the process steps is reduced.