We investigated a high-precision grating interferometer displacement measurement system that can be applied to high-end immersion lithography machines and explained its composition and measurement algorithm. By analyzing the optical path variation caused by the installation error of the grating interferometer, we calculated the influence of the installation error on the displacement measurement. A displacement measurement model of a grating interferometer using three read heads was established, and the influence of the model coefficients on the displacement measurement model was analyzed. The simulation results show that, under the condition that the error of the measurement model in the X direction and the Y direction is <0.1 nm, the translation error of the read head should be within ±100 μm, and the relative rotation deviation between the two read heads or two gratings placed along the diagonal should be within ±50 μrad. The methods and results of studying the influence of grating interferometer installation error on the displacement measurement provide a theoretical basis for the application of a grating interferometer displacement measurement system in immersive high-end lithography scanners.
A high-precision six-degree-of-freedom (6-DOF) displacement measurement system with four one-dimensional gratings was investigated to satisfy the displacement measurement requirement of the wafer stage of a high-end immersion photolithography scanner. A 6-DOF system that was capable of measuring the three-degree-of-freedom (3-DOF) translational displacement motions of the wafer stage along the X, Y, and Z directions (X, Y, and Z, respectively), and the 3-DOF angular motions about the x, y, and z-axes (Rx, Ry, and Rz, respectively), was essential for measuring displacements. The optical path structure of this system employed the interference of secondary diffracted beams. The displacement measurement model for recording the displacements of photolithography scanners in real time is constructed, and a simulation verification is performed. The results show that the measurement model errors of X and Y are 0.01 and 0.02 nm, respectively, and the model errors of Rz, Rx, and Ry are 0.37, 1.29, and 0.74 nrad, respectively, without considering the grating and read head installation errors. Furthermore, the measurement model errors of X and Y are 0.06 and 0.09 nm, respectively, and the model errors of Rz, Rx, and Ry are 0.47, 1.36, and 0.78 nrad respectively, considering installation errors. The model error of Z is small and can be ignored. Simulation methods are used to verify the feasibility of the measurement model. The simulation results also show that this model satisfies the requirements for the measurement errors of the wafer stages of photolithography scanners.