FTIR spectroscopic system with improved sensitivity

The problem of the determination of the intensity of weak infrared (IR) absorption bands by differential IR spectroscopy is considered for the case of a noise-limited sensitivity. A spectroscopic system based on a Fourier transform IR (FTIR) spectrometer which improves the sensitivity of the IR method by at least 30 times, is described. In the conventional differential FTIR measurements, only a single pair of spectra (from the analyzed sample and from the reference) is taken during the whole acquisition time. In contrast, in our system, we take the data interchangeably from the analyzed pair of sample and reference during the same acquisition time. This “modulation” of samples suppresses the contribution of the low-frequency noise as well as long-term instabilities in the differential spectrum. The described system consists of an FTIR spectrometer, a computer-controlled optimized sample changer, and software for fully automated multiple measurements. The main steps of data processing as well as an example of application of multiple measurements for a diagnostics of thin wafers are presented.