IR Spectroscopy


Fourier Transform IR spectrometer (FTIR) based on a Michelson Interferometer


The application of IR spectroscopy to the analysis of thin films differs markedly from the application to liquids and gases where large sample cells are used, the extinction of the sample is small, interference can be neglected, polarization does not matter, and Lambert's law is valid.
The application of IR-spectroscopy to thin films (nm - some mm) on absorbing substrates (e.g. metals) or substrates with a rough backside (e.g. standard Si wafer) requires:

  • measurement in reflection mode
  • polarization of the IR-radiation either parallel (p) or nornal (s) to the plane of incidence
  • measurement at well known angle of incidence with known angular spread of the IR-beam
  • exact knowledge of the optical properties of the substrate
  • optical multi-layer model calculation to extract reliable information (similar to ellipsometry)
Fourier-Transform IR (FTIR) spectrometers are widely used and offer high scan rates (some spectra / s) and good signal to noise ratio.


  • Fourier Transform Infrared Spectrometer (FTIR), NICOLET Magna IR 750

  • variable angle of incidence reflection accessory with wire grid polarizer