Full wafer charge coupled device interferometry is a new technique that can be used for in situ monitoring of plasma etching. The use of this technique to optimize an electron cyclotron resonance etch process for TaSiN and its application to x-ray mask fabrication are presented. Fine tuning of the etch process was performed by optimizing the microwave power and the collimating magnet current. Under optimized conditions, etch rate uniformity (3 sigma) across a 4 in. substrate was as low as 2%. The full wafer interferometer enables in situ uniformity measurements, thus minimizing process development time. The etch process has been used to successfully fabricate sub-0.25 mu m x-ray masks.