Development of photomask linewidth measurement and calibration using AFM and SEM in NMIJ

We developed a measurement method for linewidth patterns of photomasks, and started a calibration service of the photomask linewidth measurement. For the photomask standards, high-quality of chromium film patterns, typical thickness of about 80 nm, sharp edges (edge angles more than 85 degree) and smooth side walls on a quart glass substrate were used. Two kinds of microscopes, an atomic force microscope (AFM) and a scanning electron microscope (SEM), were employed to calibrate the linewidth. At the first, the surface profile of line structures were inspected using the AFM, so that the distance between the left and the right side walls at the edge positions were geometrically-determined. Before the each measurement of photomask patterns by the AFM tips, each of the tip shape was checked using a needle artifact. Then AFM profiles of the photomask patterns were corrected using the tip shape data. The linewidth was calculated using the corrected profiles under a definition of the edge positions, a 10 % level from the top film surface. At the next, the linewidth bias between SEM and AFM were evaluated using the AFM data. Using this method, an uncertainty of the linewidth measurement was evaluated at 60 nm for a linewidth range of 0.5 μm-10 μm.