Scatterometry: a metrology for subwavelength surface-relief gratings

Phase-modulation scatterometry is a metrology technique for determining the parameters of gratings using as a key device a phase modulator. For measurement purposes the phase modulator requires a complicated calibration procedure that is analyzed here in detail. The main source of error to be dealt with are the fluctuations of the phase modulation amplitude. The measurables are the direct term and the first two harmonics of the output. For the fitting of the experimental data we used the ratio of the harmonics to the direct term because it improves significantly the accuracy. A sensitivity analysis was performed for two samples, one real and one theoretical, to find the measurement configuration that insures optimum determination precision for the grating parameters. For the real sample, comparisons of the theoretical predictions for sensitivity with the actual values showed a good agreement. For both samples the sensitivity analysis indicated sub-nanometric precision for the critical dimension (grating linewidth).