SILICA FILMS ON SILICON - THICKNESS MEASUREMENT BY ELECTRON-MICROSCOPY AND ELLIPSOMETRY

The thicknesses of thin oxide films grown on silicon in oxygen were me asured by transmission electron microscopy (TEM), ellipsometry, and pr ofilometry. For films thicker than about 30 nm the thicknesses measure d with these three techniques were the same within experimental error; the ellipsometric thicknesses were calculated from a model of a singl e oxide film with a refractive index that did not vary with thickness. For films thinner than 30 nm, the ellipsometric thicknesses calculate d from this model were greater than those measured by TEM, as found by others. Ellipsometric DELTA and psi angles were measured for a number of films thinner than 30 nm in four liquids of different refractive i ndex, from 1.33 to 1.495. These results are not consistent with a mode l of a single film of refractive index increasing with decreasing thic kness, as deduced by several other workers from statistical fits to el lipsometer data. Good agreement of measured DELTA and psi values for f ilms whose thicknesses were determined by TEM was found with calculate d DELTA and psi values for a model of two films, including the main ox ide film of constant refractive index, and an interfacial film of cons tant refractive index and thickness of 1.2 nm. These results suggest t hat reliable thicknesses of oxide films of silicon below 30 nm thick c an be calculated from such a model.