Abstract: A non destructive method of spectroscopic ellipsometry adapted to measure the width of features in periodic structures, particularly those features which are less than one micron wide. The method is also adapted to make comparisons between a known reference structure and a sample structure, and to control the fabrication of periodic structures in a plasma etching reactor. Peaks in functions DELTA and PSI versus wavelength are monitored and correlated against reference curves, permitting etching conditions to be modified. This technique avoids the need for use of scanning electron microscopy to measure the linewidth, which is a destructive method. It also posses an advantage over scatterometry which requires several detectors arrayed at different angles from an incident beam to measure the different diffracted orders.