Slope of the groove profile (α) as a function of the number of deposited periods N (resp. number of deposited layers) measured by TEM and by AFM for (a) the 16-period and (b) the aperiodic ML gratings. For TEM, the error bars on the slope (α) are calculated by averaging the relative standard deviation at each N. For AFM, the error bars correspond to the standard deviations on the measurements over 6 grating periods (i.e., 12 different α values) for each N.