Simple technique for linewidth measurement of gratings on photomasks

Abstract

A novel laser scatterometer linewidth measurement tool has been developed for the CD metrology of photomasks. Calculation of the linewidth is based on a rigorous theoretical model, thus eliminating the need for any calibrations. In addition, the effect of the glass slab on which the grating is placed, is explicitly taken into account. The experimental arrangement consists of a chrome-on-glass diffraction grating illuminated with a converging spherical wave from a He-Ne laser. A photodiode mounted in the Fourier plane of the scatterer measures the scattered power in each diffracted order. A rigorous theoretical model is used to provide a lookup table giving the 0-order transmitted power as a function of the linewidth for a fixed pitch of the grating. This table is then used to associate a linewidth with the experimentally measured value of the power in the 0 transmitted order. A local company manufactured various photomask gratings having a 2 micron pitch and varying linewidths. The 0-order transmitted power for each of these gratings was measured by the scatterometer, and a prediction of the linewidth was made based on the theoretical model. The linewidth measured by the scatterometer system represents an average of the linewidths over the total lines illuminated by the laser. All present CD measurement systems however, measure the linewidth of a single line. If the variation of linewidth is assumed to be small, comparable results should be obtained from the two procedures. The predicted linewidth values were compared to those obtained using commercial optical linewidth measurement systems and excellent agreement was obtained.