Our MProbe system measures light reflected and/or transmitted from layer structure in a specified wavelength range. Light reflected from the different interfaces can be in or out of phase, resulting in constructive(adding intensities) or destructive (subtracting intensities) interference i.e. causing intensity oscillation that are characteristic of the thin film structure.
Spectroscopic reflectance measurement, like other optical methods, is indirect: the measurement data need to be analyzed in order to determine the filmstack parameters (thickness, optical constants, surface roughness, etc.) There are two basic approaches that are being used to analyze the data:
The corollary is refractive index can be measured very accurately if refractive index of the substrate is known
Curve Fitting is a very powerful method of data analysis that allows to determine many physical properties of the layers but there are situation where it is not suitable. In many cases, films are not perfect, they have some artifacts like coloring, non-uniformity, residual absorption that are of no interest i.e. does not need to be determined. However, in order to use curve fitting all these artifacts need to be taken into account. This is especially true in cases of the thick films (films >2um). As we discussed earlier, information about film thickness is contained in the period of intensity oscillation.
FFT based method is based on determining only film thickness using intensity oscillation frequency and ignoring all the other properties. Of course, optical constants still need to be supplied in order to get correct results.
FFT based method has several other advantages:
These advantages makes it a preferred method for inline measurement in manufacturing environment.