Research On6-degree-of-freedom Measurement System Of Wafer Stage

Lithography is a significant procedure in ULSI manufacturing, where the overlayaccuracy is one of the key performance indicators. As the development trend oflithography, wafer stage plays an important role in improving overlay accuracy.Wafer stage is a ultraprecise motion platform with6degree of freedom, whosefunction is based on obtaining its position information exactly. This measurementsystem is different from ordinary geometric measurement system. First of all, thismeasurement system is required to meet the accuracy of nanometer level, but also, it hasa long range. Secondly, the stage’s speed is very high and there’re strong couplingbetween the freedoms. According to characteristics of wafer stage, this research focuson modeling the measurement system and the algorithm of measurement model. Systemmodeling is the key to guarantee sufficient accuracy, while the method of calculating isthe key to meet real-time requirements. In this paper, the geometrical relationship anderror component in the measurement layout was represented by vector and coordinate,and the global interferometer model was derived based on this mathematical description.Thus the measurement system can be described perfectly and accurately.Because that The expressions of global interferometer model are complex andimplicit, solving this expressions maybe need vast CPU Cycle, and this will becontradict with system’s real-time requirements. So, a kind of linearization calculatingmethod was brought forward in this paper, it mathod transform interferometers’readings into wafer stage’s position coordinates directly, and this procedure only costlimited computing resource. While the coefficients of the linear model can be updated ata lower rate. In addition, the error components out of the measurement model wasresearched and relevant offset method was put forward in this paper.The interferometers are incremental measurement systems. To make the positionmeasurement absolute, the system is equipped with zero sensors. The zeroing procedureis performed in order to determine the interferometer offsets. Zeroing procedure is animportant process in this measurement system. Relevant measurement models andzeroing procedure were determined in this paper.