Film Thickness Measurement
method principles

A commonly well-known effect, which occurs for example with soap bubbles or on a thin oil film on water, is used here for the determination of the film thickness. You can see many colors which change according to the layer thickness, e.g. when a soap bubble is blown up (see Soap Bubble Website for some really nice pictures and detailed explanation of this effect).

These "colors at thin layers" are based on the interference phenomenon, i.e. on the superposition of light waves, which have been reflected at the front and back side of the layer (at two boundaries with different optical densities):
 

The undisturbed superposition of the two reflected light rays 1 and 2 leads to periodical amplifications and extinction in the spectrum of a white continuum light source (such as a halogen spectral lamp as a pseudo white-light source). Since the superposition of the two light rays is not purely additive, a so-called interference occurs. The figure below shows the interference spectrum of a 1 µm and 2 µm thick layer:
 

Such an interference spectrum is captured and analyzed by our TranSpec spectrometers, from which the film thickness result is computed then.