Polarization is an important and useful degree of freedom to explore defect SNR (Signal-to-Noise Ratio) improvement on repetitive memory devices. Sub-wavelength repetitive memory cell structures, especially in process layers of high-refractive-index dielectric and conducting materials, act as polarizer, resulting in a strong dependency of the optical response on polarization direction. In this study, STI layers of two typical memory products, Dynamic and Flash RAM, were selected to investigate defect detection capabilities with different polarization state of illumination light for different layouts. Several defect types, including void and scratch, are investigated. SNR improvement is observed primarily through linear polarization that is parallel to the pattern layout. Flash memory devices exhibit stronger birefringence than DRAM devices.