Atomic transport during growth of ultrathin dielectrics on silicon

Atomic transport in thermal growth of thin and ultrathin silicon oxide, nitride, and oxynitride films on Si is reviewed. These films constitute the gate dielectrics, the “heart” of silicon metal-oxide-semiconductor field-effect transistor (MOSFET) and dynamic random-access memory (DRAM) devices, which are usually thermally grown on the active region of the semiconductor Si substrate. The drive of ultra-large scale integration towards the 0.18 μm channel length and below requires gate dielectrics with thicknesses of 3–4 nm and less, establishing new and very strict material requirements. Knowledge on an atomic scale of dielectric film growth promoted by thermally activated transport mechanisms is essential to the engineering of this fabrication step. In the case of thermal growth of silicon oxide films on Si in dry O₂, the mobile species is O₂ and growth is essentially a diffusion–reaction phenomenon. The thermal growth of silicon nitride and oxynitride films on Si in NH₃, NO and N₂O, on the other hand, involves catalytic dissociation of the original gas molecules at the surfaces and interfaces and diffusion–reaction of different resulting species, like NH₂, NH, H, N, NO, O, and O₂. Hydrogen transport and incorporation is a crucial, ubiquitous issue in thermally grown dielectric films on Si which is also addressed here. A recall is made of the physico-chemical constitution of the involved surfaces and interfaces for each different dielectric material, as well as complementary studies of the gas, gas-surface, and solid phase chemistry. An outline of the unique tools of isotopic substitution and high resolution depth profiling is included.