Theory of Midinfrared Absorption Microspectroscopy: I. Homogeneous Samples

摘要

Midinfrared (IR) microspectroscopy is widely employednfor spatially localized spectral analyses. A comprehensiventheoretical model for the technique, however, has notnbeen previously proposed. In this paper, rigorous theorynis presented for IR absorption microspectroscopy by usingnMaxwell’s equations to model beam propagation. Focus-ning effects, material dispersion, and the geometry ofnthe sample are accounted to predict spectral responsenfor homogeneous samples. Predictions are validatednexperimentally using Fourier transform IR (FT-IR) mi-ncrospectroscopic examination of a photoresist. The resultsnemphasize that meaningful interpretation of IR micro-nspectroscopic data must involve an understanding of thencoupled optical effects associated with the sample, sub-nstrate properties, and microscopy configuration. Simula-ntions provide guidance for developing experimental meth-nods and future instrument design by quantifying distortionsnin the recorded data. Distortions are especially severe forntransflection mode and for samples mounted on certainnsubstrates. Last, the model generalizes to rigorouslynconsider the effects of focusing. While spectral analysesnrange from examining gross spectral features to assessingnsubtle features using advanced chemometrics, the limita-ntions imposed by these effects in the data acquisition onnthe information available are less clear. The distortingneffects are shown to be larger than noise levels seen innmodern spectrometers. Hence, the model provides anframework to quantify spectral distortions that may limitnthe accuracy of information or present confounding effectsnin microspectroscopy.