Thin film thickness measurement by double laser interferometry

When the thickness and the refractive index of a thin film (around 1000nm) growing onto a substrate is measured, two experimental data have to be measured. In the Experimental Astrophysics Laboratory of the "Universidad Politecnica de Valencia", we use an experimental setup that allows us to obtain both, thickness and refractive index of thin films grown onto a substrate, from the values of reflectance at two different angles. The experiments are performed in a vacuum chamber operating at a pressure of 10(-7) mbar and a temperature which can be set between 17 and 300 K. When initial conditions are established, the substances that will form the film are prepared in gas form in a prechamber and goes trough a needle valve to the chamber. The proportions of a particular composition are controlled by their partial pressures. To monitor the thickness of the film during the accretion, two He-Ne laser beam (632.8 nm) are reflected at two different incidence angles. From the interferometric patterns we can obtain the refractive index from the quotient between the periods corresponding to each pattern. This arrangement makes up the double laser interferometric technique and it is argued on the basis as the grown rate is always the same so the quotient of periods is constant and it only depends on incidence angles (its values were chosen to optimize the measure process) and refractive index of the film, whose value can be obtained from experimental data.