Interferometer for phase measurements in phase shift masks

The etch depth of phase shift masks is typically measured by means of profilomctry and the expected phase shift is calculated from a knowledge of the refractive index at the lithographic wavelength of interest. In the case of masks utilizing deposited films the index may differ from values for bulk materials and commonly varies to some degree with method of deposition. The interferometer offers a method for the measurement of phase directly and hence for a means to obtain refractive index values for phase shift films on quartz blank substrates. Arrangements are described for direct phase measurements using visible, 632.8 nm. and UV 257 nm radiation.