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IR-SE metrology is an emerging technology in semiconductor production environment. Infineon Technologies SC300 started the first worldwide development activities for production applications. One application part of our IRSE development roadmap is shallow trench isolation (STI) monitoring. Depth below silicon, film thickness and STI profile are the parameters of interest. A set of wafers with different lines/spaces structures (1D gratings) was prepared. These structures were characterized with atomic force microscope (AFM) for reference data. Extensive IRSE mapping measurements were taken, for 2 different wafers orientations, 0° and 90°. Preliminary outcomes are: A clear form birefringency and sensitivity to trench depth and lines density. To take advantage of this sensitivity to gratings depth, 2 concepts based on effective medium theory (EMT) and rigorous coupled wave analysis (RCWA) method were implemented. Excellent correlation results to AFM are presented for EMT for high lines density and for RCWA for low and high lines density.