Scatterometry is presented as an optical metrology technique potentially capable of determining the critical parameters of a phase etched diffraction grating test structure (sidewall profile, etch depth, and linewidth). The technique is noncontact, rapid and nondestructive. The test grating structure is illuminated by a laser beam and the intensities in the different diffracted orders are measured as the angle of incidence of the laser beam is varied over a certain range. A phase shift mask consisting of an array of chrome and chromeless phase etched gratings was fabricated at AT&T Bell Labs using e-beam techniques. The grating linewidths varied from nominal 0.5 micrometers to 5.0 micrometers , while the etch depths varied from a nominal 190 nm to 400 nm depths. Both the chrome and the quartz gratings were measured, although only data for the quartz gratings is presented here. The measurements of the diffracted orders were made using the two theta scatterometer located at the University of New Mexico. The shape of the diffraction curves obtained in this manner has been shown to be sensitive to the grating structure parameters (sidewall profile, etch depth, linewidth, etc.). An estimate of the quarts phase etched structure parameters was obtained through a combination of rigorous coupled wave theory (CWT) and minimum mean square error (MMSE) analysis. Additionally, each grating was measured using an AFM located at AT&T. Comparison of the scatterometer and AFM measurements are presented along with their absolute differences. Finally, the long term and short term repeatabilities of the scatterometer measurements are shown to be excellent.