Optimized imaging polarimeter for measuring polarization properties of hyper number aperture lithography tools

An imaging polarimeter is developed for measuring polarization properties of hyper number aperture (NA) lithography tools, which can be represented by stokes entrance-pupil, stokes exit-pupil and mueller pupil of projection optics. This imaging polarimeter is optimized from the following three aspects. Firstly, a new method is proposed to measure stokes entrance-pupil of projection optics. It employs a rotating-waveplate and a fixed-polarizer as polarization state analyzer (PSA). Compared to the method proposed by Nomura, the number of measurements is reduced from 16 to 4 times. Additionally, the large incident angle in the mask plane leads to unacceptable retardation error of conventional waveplate, which terribly influent the measurement accuracy of the imaging polarimeter. Therefore, the imaging polarimeter can be optimized by employing a wide-view-angle (WVA) waveplate composed of quartz and sapphire plates. An example of 632.8nm WVA λ/4 waveplate is designed based on least square algorithm, which is firstly introduced to calculate the thicknesses of four crystal plates. Simulation result shows that the retardation error is less than 0.3° for incident angle within 20°. Thirdly, the WVA waveplate is successfully fabricated and its retardation is calibrated by senarmont method. Measurement data is contributed to eliminate the uncertainty of retardation and thus improves the performance of the imaging polarimeter. Because of the lack of lithography equipment, the imaging polarimeter is preliminarily tested in the visible optical system with a He-Ne laser. Stokes entrance-pupil, stokes exit-pupil and mueller pupil of projection optics are all measured with quite enough measurement repeatability.