Ellipsometry is a truly powerful technique to measure and analyze thin films.It is often possible tomeasure the film thickness with a resolution in the order of 1(A)(0.1nm).However,ultra-thin films with thickness in nanoscale which are used more and more in practical application is still very difficult to characterize accurately by ellipsometry,since ellipsometric parameters of the sample close to the limits of resolution so that the deduction of sample parameters becomes very sensitive to errors in ellipsometric parameters.