AVS 50th International Symposium
    Thin Films Thursday Sessions
       Session TF-ThA

Paper TF-ThA3
Numerical Analysis of the Three-phase Problem in Optical Diagnostics

Thursday, November 6, 2003, 2:40 pm, Room 329

Session: In-Situ / Ex-Situ & Real-Time Monitoring
Presenter: K.F. Flock, North Carolina State University
Authors: K.F. Flock, North Carolina State University
D.E. Aspnes, North Carolina State University
Correspondent: Click to Email
One of the major unsolved problems in optical diagnostics is the practical simultaneous determination of n, k, and t, i.e., the real and imaginary parts of the complex refractive index and the thickness, respectively, of a depositing film, ideally at the monolayer or near-monolayer level. This capability is particularly important for purely sample-driven feedback control of deposition processes such as OMCVD. For very thin layers analysis can be done in principle within the three-phase (substrate/overlayer/ambient) model, since the underlying substrate, no matter how complicated, can be represented approximately as a pseudodielectric function and the material deposited between measurements can be considered uniform in composition. Current optical diagnostic tools, such as our PDA-based polarimeter, return three pieces of information, i.e., the p- or s-polarized reflectance and the phase and amplitude of their ratio, and hence are well suited for this approach. Wavelength-by-wavelength analysis of electrochemical modulated-reflectance data, a related application, has been done previously in the three-phase model with marginal results. Here, we use a simple analytic approach to investigate correlations among n, k, and t to gain better insight into the nature of these solutions. We find that the correlation among the three parameters would be exact if the power reflectance were an analytic function. This explains the high sensitivity to experimental uncertainty, which in wavelength-by-wavelength applications would require accuracies of the order of 1 part in 10@super 6@ for consistent results. We present a method that circumvents this difficulty by taking advantage of spectral dependences. Applications discussed include the determination of n, k, and t for sub-nm-scale layers of Ga and AlAs on GaAs.