Normally, each thin film layer is subject to intrinsic mechanical stress, resulting from deposition processes. Examples are adjustments of the film’s lattice structure to the substrate during epitactic film growth and thermally induced stress, caused by different thermal expansion coefficients of thin film and substrate. All types of stress result in film strain. As the film grows on a substrate, it can only compensate this strain to a certain point. The substrate will eventually give in an curve. So the deposition of strained films always results in a curvature of the substrate. Already in the beginning of the 20th century, the Irish physicist G. Stoney discovered a formula that linked film strain to the resulting substrate curvature. The main advantage of this calculation method is that apart from thickness, no mechanical information on the film is needed.
Toho Technology’s Flexus series uses a patented Dual-Wavelength technology – the optical reflection of two laser beams (670 nm and 780 nm) that are parallel scanned over the sample. The extremely accurate scanning translation stage enables high-precision identification of the substrate’s curvature radius (or change thereof), thus allowing the determination of film stress in the range 1 MPa to 4 GPa.
It is further possible to perform measurements from -65 °C to 500 °C and gather information on the temperature-dependent stress development of the examined film.