Atomic force microscopy (AFM) and optical diffraction microscopy (ODM) are used to measure the profiles of grating grooves with depths much larger than their widths. Gratings with these features are essential in numerous optical devices such as spectrometers, monochromators and for the production of many fibre Bragg gratings. However, measurement of the physical shape is inherently difficult but necessary for the understanding of their function and in order to improve the manufacturing process. After a thorough calibration of an AFM and by tilting the plane of the grating by up to 17° relative to the symmetry axis of the sensing probe we measured accurately and traceably the sidewall angle and the sidewall profile in a non-destructive way. ODM is a new method where the intensity of the optical field diffracted is measured as a function of the frequency and an inverse algorithm is used to reconstruct the surface profile. It is fast, non-destructive, and it gives height and filling degree of a grating very accurately. As example a high aspect ratio grating with period p of 220 nm, depths d of ≈300 nm, and sidewall angles
γ of approximately ≈90° and filling degree f of ≈40 % were examined. Standard uncertainties as low as u(d) = 3 nm, u( α) = 0.4° and u(f) = 3.1 % were achieved. Despite the fact that the AFM responds to the physical surface and ODM responds to the optical
properties of the material we find that the results are in very good agreement and consistent with (destructive) scanning electron microscopy measurements of the filling degree.