A method and apparatus for etching a photomask substrate with enhanced process monitoring is provided. In one embodiment, a method of determining an etching endpoint includes performing an etching process on a first tantalum containing layer through a patterned mask layer, directing a radiation source having a first wavelength from about 200 nm and about 800 nm to an area uncovered by the patterned mask layer, collecting an optical signal reflected from the area covered by the patterned mask layer, analyzing a waveform obtained the reflected optical signal reflected from the substrate from a first time point to a second time point, and determining a first endpoint of the etching process when a slope of the waveform is changed about 5 percent from the first time point to the second time point.