Phase shifting interferometry is really critical of measurement .To adjust the focus with highest contrast to get the fringes in the figure 3. The first fringes will determent the inference of the light see the bottom of flat surface. The figure fringes will give sample surface(figure 4).Move the curse at higher point to adjust higher point in surface, and adjust until fringes again. The system will calculation the difference based on the changing the thickness of fringes. In the test, using the one location from the Silicon oxidation to get the accuracy and repeatable test in the future.