Effect of Molecular Weight and UV Illumination on Optical Constants of PMMA Thin Films

The refractive index of poly(methyl methacrylate) (PMMA) thin films on glass substrates, prepared by spin coating method has been studied using a spectroscopic ellipsometry. Spectroscopic ellipsometry measurements of PMMA thin films were carried out at three angles of incidence, over the wavelength range of 400-700 nm. Optical Cauchy model was used to obtain thickness of the prepared films along with some optical constants. Each PMMA thin film shows a characteristic dispersion relation, although they all behave similarly. Refractive indices were found to be in the range 1.49-1.53 with the maximum wavelength at 400 nm, and found to increase with increasing molecular weight of PMMA, and consequently causing an increase in film thickness. It was found that changing the molecular weight of PMMA and as a result the thickness of the films prepared made significant changes in both real (epsilon(1)) and imaginary (epsilon(2)) parts of the dielectric constant as a function of wavelength. The epsilon(1) and epsilon(2) peaks near 570 and 500 nm, respectively, were shifted towards lower energy upon increasing molecular weight. The refractive index variation and the appearance of surface roughness after UV illumination were evident. The roughness is modelled after fitting the spectroscopic ellipsometry data by using Bruggeman effective medium approximation assuming 50% PMMA and 50% voids. It was found that the UV illumination for one hour decreases the refractive index by 2 x 10(-3). The refractive index changes upon UV illumination might be attributed to different factors namely: (i) microstructural defects; (ii) decomposition of PMMA molecules; (iii) carbon-oxygen polarization; and (iv) dipole-dipole intermolecular interactions.