Articles you may be interested inIndex-coupled surface porous grating distributed feedback quantum cascade laser In photolithography, wafer alignment marks are used to align the current masking layer to the previous layer. Conventional marks are usually gratings with equal width of lines and spaces. In order to achieve more reliable alignment signal, higher order of the reflected beam is required to define the edges precisely. However, the strength of reflected beam for conventional marks decreases as the order number increases. In this article, enhanced order marks, which are able to give better diffraction efficiency at higher reflected order, are studied. A model based on rigorous coupled wave analysis is adopted and developed to analyze different types of step grating alignment marks. As the period-to-wavelength and depth-to-period ratios for alignment mark are acceptable, this model is adequate to generate reliable results. When small periodic gratings of 5.33, 3.20, and 2.29 µm are incorporated into 16 µm periodic marks, the reflected diffraction efficiency is enhanced at third, fifth, and seventh orders, respectively. Forbidden mark depth, which gives almost zero diffraction efficiency for higher reflected order, is also identified for both 633 and 532 nm alignment wavelength. The results also show that, irregardless of the type of marks and alignment wavelength, whenever the zeroth order reflected diffraction efficiency drops to minimum, it gives the strongest higher order reflected diffraction efficiency. When transverse magnetic polarization is considered, it is able to give higher reflected diffraction efficiency if compared to transverse electric polarization. The experimental results demonstrated that overlay as low as 10 nm can be achieved when fifth order enhanced mark was used for 90 nm and 0.11 µm technology node process.

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In this paper, a model for electromagnetic scattering of line structures is established based on high frequency approximation approach - ray tracing. This electromagnetic ray tracing (ERT) model gives the advantage of identifying each physical field that contributes to the total solution of the scattering phenomenon. Besides the geometrical optics field, different diffracted fields associated with the line structures are also discussed and formulated. A step by step addition of each electromagnetic field is given to elucidate the causes of a disturbance in the amplitude profile. The accuracy of the ERT model is also discussed by comparing with the reference finite difference time domain (FDTD) solution, which shows a promising result for a single polysilicon line structure with width of as narrow as 0.4 wavelength.

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