The Tencor FLX-2320 determines wafer curvature by measuring the angle of deflection of a laser beam off the surface of 6”, 4” and 3” substrates (no chips). Film stress is determined by comparing the change in radius of curvature of the substrate, with and without the film, i.e., before and after a deposition.

The substrate and the film must be optically reflective in the wavelength used. Because semi-transparent films may absorb light (depending on the incident wavelength/angle and the film thickness/refractive index) the FLX-2320 has two wavelengths (670 nm and 750 nm) to accommodate various films.

In order to calculate the film stress correctly, one needs to know the substrate thickness, which can be measured with a manual thickness gauge, and the film thickness, which can be measured with an ellipsometer.