As we have seen in the previous example the doping level of a semiconductor can be determined from an infrared reflectance spectrum. An undoped layer on a highly doped substrate leads to optical contrast in those spectral regions which carry information on the doping, i.e. the low wavenumber range.

At each interface the radiation is partially transmitted and reflected. The superposition of the partial waves reflected at the vacuum-epilayer interface and the epilayer-substrate interface to the total reflected wave leads to destructive and constructive interference: The spectral position of the maxima and mimina depends mainly on the thickness of the epilayer which determines the travelling time of the light waves through the epilayer and hence their phase shift.

If we can match the simulated interference patterns with the measured ones quantitatively we will get a very reliable thickness value from the simulation.