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7 July 1997 Six degrees of freedom Mag-Lev stage development
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Lithography steppers currently use a combination of mechanical stages to achieve control of a wafer location in six degrees of freedom. In these stages, the wafer is carried on a fine stage which provides six degrees of freedom control with approximately 100 micrometers travel. This fine stage is mounted on a coarse mechanical stage which provides X-Y positioning with approximately 200 mm travel. The fine stage is typically comprised of multiple piezo- actuators and/or voice coil drives which are used to position a flexure mounted platen.. These mechanical stages generally suffer from poor dynamics, a result of the compound flexures used. For these reasons photolithography equipment manufacturers are researching alternate non- contact methods of precise positioning. This research has centered on replacing long travel degrees of freedom with air bearings which carry a fine stage capable of roll, pitch, and vertical displacement (Z). These air bearing/mechanical stages have improved resolution and stability in addition to shorter setting times. An alternate approach is the use of a magnetically levitated (Mag- LevTM) stage that provides six degrees of freedom control without mechanical contact. This type of stage is ideal for clean room use where particle generation from mechanical friction is a major source of contamination. Mag- Lev stages are also mechanically simple, therefore easier and cheaper to fabricate and more reliable than flexure stages. Previous papers have reported the performance of the fine set Mag-Lev stage in a laboratory lithography experiment. Integrated Solutions has chosen to develop a full Mag-Lev stage in three phases. The first Mag-Lev implementation replace all fine mechanical elements with a single suspended structure. This stage will have displacements of 300 micrometers in X,Y,Z and milliradians of rotation in the remaining three degrees of freedom. The stage will be carried over the surface of the wafer by stacked air bearings. This hybrid Mag-Lev/air bearing stage enjoys the benefits of non-contact bearings and will significantly improve stepper performance in both throughput and overlay. The second phase replaces one degree of air bearings with a six degree of freedom Mag-Lev stage with one axis long travel. This geometry eliminates one redundant degree of freedom and five uncontrolled rigid body modes of an air bearing axis. The final phase of development is the implementation of a six degree of freedom planar stage that will consist of a single moving part with two axis long travel (in excess of 200 mm). This stage geometry represents the tightest possible dynamic structure with no redundant or uncontrolled degrees of freedom. Very high bandwidths are possible with Mag-Lev stages which work to reduce settling times and improve disturbance rejection. This paper will focus on the development of two stages; a fine set stage as in phase 1, and a single degree of freedom long travel stage as in phase 2. Step and settle and position stability data for both stages will be presented along with the expected improvement in wafer throughput. In addition to the prototype stages future concepts for a phase three planar stage will be presented with expected performance goals.
© (1997) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Mark Williams, Peter Faill, Paul M. Bischoff, Steven P. Tracy, and Bill Arling "Six degrees of freedom Mag-Lev stage development", Proc. SPIE 3051, Optical Microlithography X, (7 July 1997);

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