Depth Profiling by Confocal Raman Microspectroscopy: Semi-empirical Modeling of the Raman Response

J. PABLO TOMBA, LUIS M. ARZONDO, JOSÉ M. PASTOR

APPLIED SPECTROSCOPY

Año: 2007 vol. 61 p. 177 - 185

0003-7028

It has been well documented that the use of dry optics in depth profiling byconfocal Raman microspectroscopy significantly distorts the laser focalvolume, thus negatively affecting the spatial resolution of the measurements.In that case, the resulting in-depth confocal profile is an outcome ofseveral contributions: the broadening of the laser spot due to instrumentalfactors and diffraction, the spreading of the illuminated region due torefraction of the laser beam at the sample surface, and the influence of theconfocal aperture in the collection path of the laser beam. Everall andBatchelder et al. developed simple models that describe the effect of thelast two factors, i.e., laser refraction and the diameter of the pinholeaperture, on the confocal profile. In this work, we compare thesetheoretical predictions with experimental data obtained on a series ofwell-defined planar interfaces, generated by contact between thinpolyethylene (PE) films (35, 53, 75, and 105 lm thickness) and a muchthicker poly(methyl methacrylate) (PMMA) piece. We included tworefinements in the above-mentioned models: the broadening of the laserspot due to instrumental factors and diffraction and a correction for theoverestimation in the decay rate of collection efficiency predicted byBatchelder et al. These refinements were included through a semiempiricalapproach, consisting of independently measuring the Ramanstep-response in the absence of refraction by using a silicon wafer and theactual intensity decay of a thick and transparent polymer film. With theseimprovements, the model reliably reproduces fine features of the confocalprofiles for both PE films and PMMA substrates. The results of this workshow that these simple models can not only be used to assist datainterpretation, but can also be used to quantitatively predict in-depthconfocal profiles in experiments carried out with dry optics.