Optical characterisation of thin organic films by analysing transmission measurements with the Forouhi-Bloomer model

Abstract

The application of the Forouhi-Bloomer model, designed to describe the refractive index dispersion in solid inorganic insulators, in order to obtain film thickness and (complex) refractive index dispersion of organic films of the type encountered in microlithographic and micromachining applications using only transmission measurements is presented. Films with various compositions of the miscible polymers, cresol epoxy novolac (EPNOV) and poly hydroxy styrene (PHS), were prepared on quartz substrates by the standard spin coating techniques and their transmission spectra were recorded. Theoretical transmission spectra were generated simulating the optical dispersion of these films with the aid of the Forouhi-Bloomer model and fitted to the experimental ones using standard regression analysis techniques. The fit (adjustable) parameters were those describing the optical dispersion and the film thickness. The fits were very satisfactory permitting the determination of the film thickness and of the dispersion of the real and of the imaginary parts of the refractive index of the samples. The dispersions of the real and of the imaginary parts of the refractive index were found to vary from one film to the other with a way dependent on film composition. Information with respect to the electronic structure of the films was obtained using this analysis.