Ex situ spectroscopic ellipsometry investigation of the layered structure of polycrystalline diamond thin films grown by electron cyclotron resonance-assisted chemical vapor deposition

Abstract

Polycrystalline diamond thin films deposited by electron cyclotron resonance-assisted chemical vapor deposition on Si (111) were investigated using spectroscopic phase-modulated ellipsometry from the near IR to UV range (830–270 nm). Analysis of the raw ellipsometry data [ψ(λi), Δ(λi)] by applying the conventional Bruggeman effective medium theory and linear regression analysis provided details about the film microstructure: (i) the multilayer structure and the component layer thickness of the films; (ii) the volume fraction of the constituents (sp3- and sp2- bonded carbon) and of voids (fv) in the bulk layer (L2); (iii) the inhomogeneity of the structure along the growth axis and its variation with the seeding density; and (iv) the surface roughness layer thickness (dS). A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed that simulates the ellipsometry data reasonably well. The results obtained through ellipsometry modeling, such as surface roughness and overall film thickness, were compared with those from atomic force microscopy and profilometry, respectively, in order to validate the model employed. Typically, high surface roughness values around 60 nm were found for films grown under different substrate temperatures and oxygen-to-carbon ratios. It was also found that a combination of relatively high substrate temperature and O/C ratio can be used to reduce the surface roughness to around 25 nm. In general, the void fraction (fv) of the bulk layer decreases as a function of seeding density, indicating the formation of a denser film. The sp2-bonded carbon fraction (fsp2 C) also varies with the process parameters. These results (fv and fsp2 C) for the bulk layer and its behavior with respect to process parameters are discussed.