XY-Linear Motion – The Difference between Planar XY-Stages, Integrated XY-Stages, and Stacked Stages

In industries such as precision optics, semiconductor test/manufacture, laser processing, flat panel display manufacturing, etc., precision motion in multiple degrees of freedom is often required. When large surfaces need to be processed, the flatness specification (vertical deviations) of the positioning stages involved is especially critical, for example to reduce out of focus errors.   

The differences between stacked, integrated, and planar stage designs are often misunderstood by users. Planar XY-stages or XY-tables by definition use a common reference plane for both X and Y axis motion. True planar motion is not available with cross roller bearings – these are used in stacked, integrated designs.

Stacked, Integrated, and Planar XY Stages

XY motion can be achieved by stacking two linear stages of similar stroke or type (depending on the application requirements, they can also be different). This modular design provides flexibility. The downside is – for long travel ranges – flatness errors and pitch/roll errors increase towards the end of travel, caused by the lack of support for the base of the upper axis.

As the above graphs show, air bearing planar 2-axis stages can achieve flatness of 1µm and better over hundreds of mm of travel. Putting this in perspective with the size of a typical ice-skating rink, we’re talking about deviations on the order of a few hair widths — across the whole area. While this might be the dream of many an athlete, a resurfacing machine with such performance is not available.

Another difference between a true planar XY stage and other 2-axis linear stage designs is evident in the footprint. While 2-axis planar stages are designed to keep the moving platform always inside the area covered by the base plate, this is not possible with stacked designs. They might appear to require less production space at first glance, but their travel range has to be added to the footprint, which may not be obvious from the drawings, when the stage is shown in its centered position.

For short travel ranges, planar flexure guided stages are another option. Depending on the application, out of plane motion of only a few nanometers is tolerable. Semiconductor and AFM related tools require this outstanding performance. More information on piezo nanopositioning stages

Magnetic levitation is yet another way to achieve true planar motion. The moving platform is not restricted by any mechanical restraints, just by magnetic fields. Depending on the complexity of the coils, drive circuitry, and sensors used, motion with up to 6 degrees of freedom can be achieved, with excellent geometric performance.