Simulating an electromagnetic response of a semiconductor structure, such as a metrology target, for diffraction based optical metrology is described. Known techniques for simulating an electromagnetic response of a metrology target are typically applied to metrology targets comprising a multi-layer structure such as a grating, wherein grating lines have the same pitch. This involves rigorously simulating the electromagnetic response of the metrology target using Maxwell solvers, e.g. using Rigorous Coupled-Wave Analysis (RCWA), for various parameters of light incident on the metrology target and for different critical dimensions (CD) and pitches of the metrology target. However, when the grating lines of the upper grating and the lower grating are not equal, i.e. have different pitches, the computational complexity of the simulation increases such that using known Maxwell solving techniques take a much greater length of time, making the use of known Maxwell solving techniques for such a metrology target design impractical or sometimes even impossible.A new approach for using an RCWA to simulate reflectivity of the semiconductor structure layer by layer, unit cell by unit cell, and determine an optical property such as a reflection coefficient for the semiconductor structure, is described. With a proper choice of a division point (e.g., a midpoint), the semiconductor structure can be separated into first (e.g.. top) and second (e.g., bottom) portions that can be further sub-divided into smaller, identical unit cells in an X-Y plane. These unit cells can be simulated individually, and then the simulations can be combined together to obtain the reflection coefficient of the entire semiconductor structure. The presently disclosed RCWA applied in a unit cell simulation approach advantageously decreases required simulation time, docs not require calculation of a full scattering matrix, and has other advantages compared to prior approaches.According to an embodiment, there is provided a method for simulating an electromagnetic response of a semiconductor structure to determine an optical property of the semiconductor structure for diffraction based optical metrology. The method comprises separating an electronic representation of the semiconductor structure into first and second portions. The method comprises separately simulating an electromagnetic response of the first portion and the second portion using a layer based simulation. The method comprises combining separate simulations of the electromagnetic response of the first portion and the second portion into a combined simulation. Only partial (e.g., reflectivity) portions of scattering matrices determined by the layer based simulation for the separate simulations arc combined for the combined simulation of the electromagnetic response such that the combined simulation comprises only the partial portions. The method comprises determining the optical properly based on the combined simulation of the electromagnetic response.In some embodiments, the partial portions of scattering matrices comprise reflectivity portions and/or other portions.In some embodiments, the layer based simulation comprises or is based on rigorous coupled wave analysis (RCWA), and the separate simulations are performed layer by layer, from top to bottom, bottom to top, or from outer layers to inner layers of the first and second portions of the semiconductor structure.In some embodiments, the optical property comprises a transmissive and/or reflective property of the semiconductor structure in response to incident radiation.In some embodiments, the optical property comprises a reflection coefficient In some embodiments, the first portion has first repeating structures, and the second portion has second, different repealing structures.In some embodiments, the first and second repeating structures are in an X and/or a Y direction of the semiconductor structure, the first portion is a top portion of the semico
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