The principle of measuring the stress in thin films
Laserbeam schematics

The above illustrations demonstrate the principle of all our stress measurement systems:

The beam of a laser is split into two parallel ones. These beams are directed onto the sample. As one side of the sample is (at least 3%) reflecting the beams they are redirected back onto a detector where the actual distance of the beams is measured.
Before the coating the measured distance Xbefore equals the inital laser beams distance after the splitting optic. After the coating process, when the sample is covered with a stressed film(system), the stress can lead to a bending of the sample. This occurs all the more, as the sample is thinner. The laser beams are not reflected parallel anymore. The measured distance of the beams on the detector, Xafter, can then be smaller or greater than the initial value.

By the following Stoney-formula the stress in the thin film can be calculated by using the change in the measured laser beams distances.

Stoney Formel


SIGMAfilm Stress in the film
Esubstrate Young's modulus of the substrate
vsubstrate Poisson ratio of the substrate
dsubstrate Thickness of the substrate
dfilm Thickness of the film
L Distance sample-detector
a Distance of the laser beams on the sample