In the past, the multi-axis heterodyne laser interferometer system is the main tool for the six-degree-of-freedom (DOF) displacement measurement with nanometer-level accuracy of the wafer stage in a lithography scanner [1-2]. However, as the semiconductor process is scaling down continuously, the accuracy enhancement of this system is becoming more and more difficult due to its high environmental sensitivity. Hence, an environmental robustness grating encoder system is more favorable to meet the ever-increasing measurement requirement of wafer stage in non-vacuum environment [1-2]. In addition, a special grating encoder which can simultaneously measure a long stroke with hundreds of millimeters along in-plane direction and a short stroke with hundreds of micrometers along out-of-plane direction with nanometer-level accuracy is one of the key components for six-DOF displacement measurement grating encoder system of wafer stages [3].
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