Solving the inverse grating problem by white light interference Fourier scatterometry

Scatterometry is a well-established, fast and precise optical metrology method used for the characterization of sub-lambda periodic features. The Fourier scatterometry method, by analyzing the Fourier plane, makes it possible to collect the angle-resolved diffraction spectrum without any mechanical scanning. To improve the depth sensitivity of this method, we combine it with white light interferometry. We show the exemplary application of the method on a silicon line grating. To characterize the sub-lambda features of the grating structures, we apply a model-based reconstruction approach by comparing simulated and measured spectra. All simulations are based on the rigorous coupled-wave analysis method. Combining white light interferometry with Fourier scatterometry provides an optical metrology tool for characterizing subwavelength structures. Valeriano Ferreras Paz and co-workers from Stuttgart University in Germany have experimentally shown that this approach is capable of accurately measuring the profile of a silicon line grating, which has a height of around 75 nm, a critical dimension of 200 nm and a pitch of 400 nm. The technique uses a CCD camera to image interfering pupil images that are generated by mixing white light travelling down a reference arm with white light that has illuminated a sample. The profile of the sample is reconstructed by comparing the measured and simulated pupil images for each position of a scanned mirror in the reference arm until the best match is found.