Variation of Layer Spacing in Self-Assembled Hafnium-1,10-Decanediylbis(phosphonate) Multilayers As Determined by Ellipsometry and Grazing Angle X-ray Diffraction

Grazing angle X-ray diffraction and ellipsometry have been used to characterize the film thickness, thickness uniformity, density, and index of refraction of multilayer hafnium-l,10-decanediylbis(phosphonate) (Hf-DBP) films grown on silicon wafers. Grazing angle X-ray diffraction is a powerful technique that can be used to assess thickness, density, and thickness uniformity of a wide variety of thin films. Grazing angle diffraction of Hf-DBP multilayer films generally gives a series of peaks (fringes) in the range 0.5-4.0° 20, which arise from interference between X-rays reflected off the front and back of the film. Analysis of the fringes allows an assessment of film thickness which is independent of the physical properties of the films. With less accuracy, these data also permit an assessment of film density and thickness uniformity. Indexes of refraction at optical wavelengths can be determined by comparison of X-ray and ellipsometry data. For a given sample, a plot of film thickness as a function of the number of Hf-DBP layers deposited gives a straight line, indicating that individual layers within a given sample are uniform in thickness. However, large variations in thickness per Hf-DBP layer are observed from sample to sample (ranging from 15 to 21 Å/layer), indicating that substrate effects are important in determining the layer spacing. Estimates of film density indicate that multilayer films are only ~75% as dense as the calculated density of the bulk material. Consistent with this finding, the indexes of refraction of all films studied fall in the 1.485-1.500 range, significantly smaller than the refractive index determined for the bulk compound (1.544). These findings indicate that Hf-DBP multilayers vary substantially in individual layer thicknesses from sample to sample and in general are not as similar to the assumed structure of the bulk compound with respect to the density and mode of metal-phosphonate binding as has been previously suggested for the analogous Zr-DBP films.