A method for measuring the dimensions of patterned metal features on a lithographic photomask during iterative-etch processing has been developed. Features are inspected using reflected light from the unpatterned side of the mask; a specially corrected 0.55-NA objective lens provides diffraction limited resolution at 488-nm wavelength for mask thicknesses up to 5 mm. The method provides more accurate process control than conventional transmitted or reflected light metrology: Experimental data show nearly three times smaller measurement error due to the presence of resist overhanging the metal edges. The method may also have benefit for process control in the manufacture of certain types of rim-shifting phase-shift photomasks.
Ian Smith, Ian Smith,
"Dimensional metrology for partially etched photomasks in iterative-etch fabrication processes," Optical Engineering 31(8), (1 August 1992). https://doi.org/10.1117/12.58724