Modeling Process-Dependent Thermal Silicon Dioxide (SiO2) Films on Silicon
Though extensive work has been done in the Si/SiO2 system, no process-dependent two-layer SiO2 film model has ever been established, due largely to the lack of motivation for such a model. This study attempts to model correctly the process dependence of thermal SiO2 film physical structures and their associated densities, as well as high frequency dielectric constants, so as to provide a foundation for a ULSI process-dependent device reliability simulator. By exploring the characteristic signature of ellipsometric data reduced using a one-layer film model, and comparing it to a two-layer model, we establish a process-dependent, two-layer model for thermal SiO2 films. Internal consistency in this model is demonstrated using three intrinsic-stress-related phenomena in thermal SiO2 films on Si. Both the interfacial layer and bulk film are characterized quantitatively for 38 samples, dry-oxidized at four temperatures, leading to three empirical equations describing interlayer thickness, bulk layer density, and bulk layer optical frequency dielectric constant, as functions of oxidation temperature. The interfacial layer refractive index is taken to be independent of oxidation time, and found to be independent of oxidation temperature. The oxidation-temperature-dependent index of refraction of bulk SiO2 films obtained using the proposed model agrees well with independent one-layer model data on oxides which have thicknesses around the first half-cycle of ellipsometry thickness, for which the interlayer effect is minimal. It is also found that interlayer thickness has a relatively weak dependence on oxidation temperature, which supports the strain energy model for interlayer formation. Application of the thermal SiO2 film model to Si-device dielectric characterization using fixed index ellipsometry is also discussed, based on recent, new understanding of the ellipsometry equation.
KeywordsSiO2 Dioxide Anisotropy Assured Refraction
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