Analysis of Thin-Film Polymers Using Attenuated Total Internal Reflection–Raman Microspectroscopy

Abstract

Two methods commonly employed for molecular surface analysis and thin-film analysis of microscopic areas are attenuated total reflection infrared (ATR-IR) microspectroscopy and confocal Raman microspectroscopy. In the former method, the depth of the evanescent probe beam can be controlled by the wavelength of light, the angle of incidence, or the refractive index of the internal reflection element. Because the penetration depth is proportional to the wavelength of light, one could interrogate a smaller film thickness by moving from the mid-infrared region to the visible region employing Raman spectroscopy. The investigation of ATR Raman microspectroscopy, a largely unexplored technique available to Raman microspectroscopy, was carried out. A Renishaw inVia Raman microscope was externally modified and used in conjunction with a solid immersion lens (SIL) to perform ATR Raman experiments. Thin-film polymer samples were analyzed to explore the theoretical sampling depth for experiments conducted without the SIL, with the SIL, and with the SIL using evanescent excitation. The feasibility of micro-ATR Raman was examined by collecting ATR spectra from films whose thickness measured from 200 to 60 nm. Films of these thicknesses were present on a much thicker substrate, and features from the underlying substrate did not become visible until the thin film reached a thickness of 68 nm.

Extracting interface locations in multilayer polymer waveguide films using scanning angle Raman spectroscopy

“…Other reported methods that are similar to SA Raman spectroscopy, variable‐angle internal reflection, and ATR Raman spectroscopy have been used to measure bilayer polymer films . More recently, a solid immersion lens has been used to collect the near‐field Raman scattering (providing a 30× improvement in sampling depth compared to far‐field illumination and collection) from bilayer films with less than 200 nm polystyrene on a microns thick polycarbonate substrate using an ATR Raman microscope . These studies focused on micron to hundreds of microns thick bilayer films, or in the work by Fumihiko et al and Tran et al, no buried interface location was extracted.…”

Scanning angle Raman spectroscopy: A nondestructive method for simultaneously determining mixed polymer fractional composition and film thickness

“…however, the evanescent wave penetration depth varies across the spectrum complicating the data analysis. [20][21][22][23] Raman spectroscopy is an inelastic optical scattering effect that relies on the polarizability of a molecule. Raman spectroscopy using conventional illumination geometries (e.g., epi-illumination) cannot simultaneously measure the fractional composition and film thickness, and is not well suited for films with thicknesses in the hundreds of nanometer regime.…”