Optical properties and microstructure of plasma deposited Ta2O5 and Nb2O5 films

Abstract

Advanced optical filter applications require an appropriate control of the optical constants, as well as of other suitable film properties such as mechanical performance, thermal and environmental stability, absence of refractive index inhomogeneities, and others. In the present work we studied the characteristics of two high index optical materials, namely amorphous tantalum pentoxide (Ta2O5) and niobium pentoxide (Nb2O5) prepared by plasma enhanced chemical vapor deposition, using penta-ethoxy tantalum Ta(OC2H5)5 and penta-ethoxy niobium, Nb(OC2H5)5, precursors. We particularly investigated the effect of energetic conditions on the film growth by using different modes of plasma excitation, namely rf, microwave, and dual-mode microwave/radio frequency discharges. Under sufficient ion bombardment, controlled by the rf-induced negative substrate bias, the dense Ta2O5 and Nb2O5 films exhibited a refractive index of 2.16 and 2.26 (at 550nm), respectively, while the extinction coefficient was below 10−5, as determined by spectroscopic ellipsometry, and spectrophotometry. We found that increasing ion bombardment during the film growth leads to an appreciable increase of carbon concentration, as indicated by a strong double peak at 1400 and 1500cm−1 in the Fourier transform infrared spectra. Elastic recoil detection measurements revealed an atomic concentration of 2.5% and 5.5% of carbon in the bulk of the Ta2O5 and Nb2O5 films. The presence of carbon did not appear to negatively affect the film optical and mechanical performance and stability. We discuss the relationship between the optical properties and microstructure, and the possible mechanism of carbon bonding in the form of chelate and bridging groups.