Precision Motion Technology: Understanding the Four Performance Levels

Crossed-roller bearings are available for linear and rotary stages. They provide a step toward smoother, more highly constrained precision motion. In the graphic, this is the second tier from the bottom, offering high precision and good stiffness, with additional axes still added as separate stages.

Instead of recirculating balls, they use cylindrical rollers arranged alternately at 90-degree orientations between precision bearing surfaces. This geometry provides better stiffness, since the line contact of a roller is stiffer than a point contact of a ball and supports loads and moments in multiple directions.

The bearing arrangement is especially useful for compact positioning stages where mechanical stability and precise guidance are priorities.

Compared with typical recirculating-bearing systems, crossed-roller stages can offer smoother travel because they avoid disturbances associated with ball recirculation. Their high stiffness also makes them attractive for applications involving demanding payloads or moment loads.

However, crossed-roller bearings still depend on mechanical contact. Friction and small geometric variations in the rolling elements remain considerations when extremely smooth scanning motion is required.

Traditional non-recirculating crossed-roller bearings also have practical limits on travel range and cage migration may be an issue with repetitive motion.

Air bearings eliminate mechanical contact between the moving platform and its guide surfaces. Instead, a thin, pressurized air film supports and guides the moving components. In the graphic, this is the second tier from the top, with very high precision and near-frictionless motion, though it needs an air supply and separate ancillary axes.

Removing rolling contact virtually eliminates bearing friction, wear, and mechanical disturbances caused by rolling elements.

The result is exceptionally smooth motion with excellent straightness, flatness, and repeatability potential.

Air-bearing stages are particularly valuable in semiconductor inspection, metrology, high-resolution scanning, and applications where friction, and particle generation affect measurement quality.

When combined with direct-drive motors and high-resolution feedback, air-bearing stages can deliver high velocities, accelerations, and precise trajectory control. 

The absence of mechanical bearing contact also eliminates rolling-element disturbances and greatly reduces mechanical wear and associated particle generation.

The performance comparison becomes particularly interesting when applications require more than basic X and Y motion.

Consider an inspection machine that must position a substrate in X and Y, adjust its focus in Z, correct angular alignment in tip and tilt, and rotate the substrate around its vertical axis.

That represents six degrees of freedom.

With conventional recirculating-bearing, crossed-roller, or air-bearing stages, the system may use separate modules for these functions.

Such modular systems are flexible and well established, but every additional axis introduces design considerations:

• Increased stack height and moving mass
• More mechanical interfaces and mounting hardware
• Additional drive electronics, feedback devices, and cables
• Greater potential for accumulated geometric errors
• Structural dynamics that can influence settling behavior
• Additional installation, alignment, and calibration effort

Stacking stages also moves the working point farther away from some position measurement references, potentially increasing Abbe errors when small angular deviations occur. MagLev can address several of these challenges by integrating motion and active correction into a common platform.

The main benefit is not simply having more axes. It is the ability to control multiple degrees of freedom without necessarily adding separate mechanical positioning assemblies. In practice, available travel ranges and control authority differ substantially between axes. A MagLev stage designed for long XY travel may provide only relatively small Z and angular correction ranges.

That can still be sufficient for focus adjustment, wafer leveling, trajectory correction, or compensation of small alignment errors.

There is no universally best bearing technology. The right choice depends on the performance characteristics that matter most to the application.

Recirculating bearings offer a strong combination of practicality, long travel, robustness, and cost-effectiveness.

Crossed-roller bearings are attractive when higher mechanical stiffness and smooth precision guidance are required within a compact stage design.

Air bearings are preferred when extremely smooth, nearly frictionless motion and excellent geometric performance justify the additional compressed-air infrastructure.

Magnetic levitation becomes particularly attractive when the application requires contactless motion combined with multi-axis positioning, active error correction, and a highly integrated stage architecture.

One important distinction is that positioning resolution alone does not determine a stage's performance level. A mechanically guided stage with a high-resolution encoder can achieve extremely small, commanded steps, but resolution does not necessarily describe repeatability of step size, motion smoothness, constancy of velocity, dynamic tracking accuracy, or behavior under changing loads.

Engineers should evaluate performance at the working point, including geometric performance, such as straightness, flatness, angular errors, and other factors, such as settling time, vibration and thermal stability, and the interaction between axes.

For advanced inspection and metrology applications, these characteristics can be more important than a single headline resolution specification.