The controlling of the polarization state of light is required for various photonic applications, e.g. for biomedical imaging, lithography, microscopy or ellipsometry. Major advantages ofmicro- and nanostructures for polarization control are realization of elements for spectralbands, where no alternatives exist (e.g. polarizers in the UV wavelength range) and betterintegration with optical elements or sensors. Nano-optical polarizers and wave plates can beused to fully manipulate and convert the state of polarization. The fabrication of sub-wavelength grating quarter-wave plates for applications in the visible and near infraredwavelength regime is challenging. In this work major grating structure deviations, namelygrating ridge tilt, chamfers on top of the ridges, grating displacement and their influence onphase retardation are investigated. Basing on this we present theoretical investigations andexperimental results for an in-situ process control. Thereby, the impact of structure deviationscan be compensated and a fine tuning of the phase retardation becomes feasible. Wedemonstrate this approach by fabrication of a wave plate for 532nm wavelength. This work isthe foundation for future development of such an in-situ process control.