Optical Characterization of Attenuated Phase Shifters

The optical properties of an amorphous hydrogenated carbon film utilized as an attenuated phase shifter were characterized using a n&k Analyzer. This novel instrument computes univocal values of the index of refraction n(lambda), extinction coefficient k(lambda), and film thickness d from a reflectance or reflectance/transmittance single scan covering 190 nm to 900 nm. By fitting the scanned curves, values of k as a function of wavelength or energy can be calculated and then the index n is computed by using Kramers-Kronig equations. Since the n&k Analyzer calculates n, k, and thickness from a single reflectance scan, phase angles can be easily calculated at any given wavelength between 900 and 190 nm. To test the accuracy of this instrument we have compared phase angles obtained by using the n&k Analyzer against laser interferometry at 257 nm. In this technique direct phase measurements are obtained by comparing the difference in the optical path of the beams going through the quartz and the film/quartz structure. The agreement between the two techniques was very good to within 1-3degrees for eight of the nine samples analyzed. Interferometer phase errors are conservatively estimated to be around plusmin3degrees. This includes noise levels as well as day to day variations. The agreement between the two techniques resides within the experimental errors. Thus, this analyzer can give phase angle maps of a blank film on quartz substrates in a relatively short itme and nondestructively.