Measurement of the thickness and dispersion of the oxide is a common application because the optical properties depend on the deposition condition. Measurement of the Si oxide with the known dispersion, like thermal Si dioxide, is one of the basic and most simple applications. However, there are many types and stoichiometries of Si oxide and they can have a different dispersions. In this case, it is important to measure the thickness and dispersion together. SiOx is a good example to illustrate the approach to this measurement. In this example, we use  Si oxide sample step-etched to different thticknesses. 

1. Use the filmstack model with the standard SiO2 and check if the good fit to the measured data can be achieved (One can never achieve a good fit if the dispersion does not match)
2. Use Cauchy representation of the SiO2 dispersion and let both the thickness and Cauchy coefficients set as calculation parameters. (Note. the best results select the thicker available film. Thicknesses above 400nm are preferred)
3. Add the “scale” as a calculation parameter and recalculate again. Scale is correcting for the difference in light intensity between calibratiojn and measurement configuration. This difference will be very small with correct setting but when measuring dispersion it is helpful to have more accurate results.
4. Now, when we have a good fit, the dipersion can be fixed. Other, thinner samples, can be measuremed using dispersion determined from the first measurment. In this case, only thickness and scale parameters will be measured.
5. How do we know that the correct dispersion was measured? The practical test is to show that the same dispersion give good fit to the measured data for both thick and thin films.