Study Of Key Techniques On2D Measurement In Six Degrees Of Freedom Meaurement

In the manufacture of lithographies, the wafer stage is one of the keycomponents. Six degrees of freedom of the wafer stage are measured by amulti-axis laser interferometer system, and before the work of the interferometer,a absolute reference zero position of the wafer stage must be provided. Thereforea rapid and effective measuring method is needed to provide the reference zeroposition. So far there are a variety of six degrees of freedom measuring methods.In the coordinate measuring method, the wafer stage shall be placed on theCoordinate Measuring Machine (CMM) stage and it cannot meet therequirements of online measurements. The measuring method based on imageprocessing is usually limited by the camera size and it′s difficult to meet thesmall-scale measurement. Ultrasonic measurement method is mainly used inindustrial areas, so it is obvious that the precision of the ultrasonic measurementis not very high. There are also installation difficulties and other issues in theholographic technology method. Because the spot position detection method canmeet dynamic, large-scale or small-scale measurements, it is the most suitablemethod for this project.Based on the spot detection method a simple and rapid method formeasuring six degrees of freedom is proposed. In the method thetwo-dimensional PSD is used as measuring components and the Retro-reflector isused as sensitive components. This method has many characteristics such assimple structure, validity, high accuracy, large range, and to be suitable for staticand dynamic measurements, etc. The main work of this paper is as follows:(1) Firstly, The mathematical model of the method is established. Then thesix-dimensional information is measured through the three sets oftwo-dimensional measurement system composed by PSD and Retro-reflector, andthis information can be converted to the six degrees of freedom of the wafer stageby the model matrix. Theoretical analysis shows that, the two-dimensionaldisplacement measurement based on PSD and Retro-reflector is the keytechnology of the above methods. Furthermore, index requirements of thetwo-dimensional measurement are proposed by the measuring model.(2) All possible errors of the two-dimensional measurement are analyzed andfrom all portions of the system considered, the errors come from the light sourceportion, the Retro-reflector portion and the PSD measurement portion, whereinthe PSD′s installation error and the laser drift error is the most negative to themeasurement. For the PSD installation error, a correction method is proposed that a light source is used as a benchmark and the pentaprism and two-dimensionaltarget are used to test PSD installation error. Theoretical analysis shows that, afterusing the method the PSD installation error can be controlled to less than39.38′,and the measurement error caused by PSD can be reduced to0.0164μm. For thelaser drift error, a double laser beams difference compensation method is putforward to eliminate the influence of laser drift. The laser drift is measured by thereference light path, then it can be converted by drift errors model to compensatethe displacement measurement. Theoretical analysis shows that, after using themethod the PSD installation error can be reduced to less than9.8%.(3) All the design works of the two-dimensional displacement measuringsystem is completed in this paper, specifically including the optic devicesselection, the design of circuits, software and mechanical structures. Then anexperimental platform is built and the corresponding experimental program isdesigned. Based on them a lot of related experiments are tested fortwo-dimensional displacement measuring system.The experimental results show that, by the double laser beams differencecompensation method, in8hours the X-axis drift reduction and Y-axis driftreduction are55.1%and73.3%. In the full-scale range of±250μm, the X-axisand Y-axis linear error are0.2%F.S. and0.22%F.S.. The X-axis and Y-axisresolution can both reach45nm. The repeatability errors of X-axis and Y-axis are23.3nm and19.1nm. The system frequency response can be proved by indirectexperiments to be higher than8kHz. The experimental results show that thedesired design specifications can be reached through the design oftwo-dimensional displacement measurement system based on PSD andRetro-reflector. Therefore, the above-described two-dimensional displacementmeasurement technology is effective, and it plays a key role for the finalimplementation of the six degrees of freedom measuring technology.