Air Bearings Enable Micro-inch-Accuracy Stage

Westboro, MA-The National Institute of Standards and Technology (NIST) defines accuracy, literally. Unsurprisingly, the organization has certain very special needs.

For a project requiring precision area measurement of minute standard apertures needed for defining still-smaller apertures for radiometry, NIST assembled a detailed specification for an ultra-high-accuracy positioning system. It consisted of an open-frame X-Y stage assembly on a granite base, and a Z-axis slide assembly on a granite bridge located directly above the stage. The X-Y stage would carry and position the aperture or other device to be measured over a 4 by 4-inch area. The Z axis would carry a high power microscope used for recording edge locations, and could carry the objective up or down for focusing and to accommodate varying part heights.

The most difficult requirement to meet: planar accuracy to plus or minus 0.5 aem over four inches of travel in both X and Y directions. "The specifications were beyond the capability of most mechanical stages," explains Steve Strandberg, a mechanical project engineer at Dover Instrument Corp. Drawing upon its extensive experience in designing and building high-precision stages and machine tools that incorporate frictionless air bearings, the company designed and assembled a solution in just eight weeks.

Because of certain other NIST requirements, primarily strict weight limits and microscope illumination through the bottom of the stage, Dover took a systems approach to the design. Its stage orchestrates five separate vacuum-preloaded air bearing surfaces, two brushless dc linear servomotors, a lead-screw/servomotor drive for the Z axis, and a laser interferometer feedback system for the stage's X and Y motions.

The laser interferometer and a high accuracy linear encoder on the Z axis provide position and velocity information for the company's DMM 2008 controller. In turn, the controller is responsible for DSP-based motion control for all three axes. Each axis exhibits straightness of less than four microinch/inch of travel, and delivers less than 0.5 arcsecond of yaw over its four inches of travel.

Before design work started, Strandberg, electrical engineer Jeff Lowe, and the rest of the project team constructed an error budget, a list of all the system's anticipated sources of error. These consisted of straightness of travel of the axes, roll, pitch, and yaw errors for each axis, squareness between X and Y, and servo stability. "The overall accuracy and traceability requirement dictated laser-interferometer feedback for the X-Y motions," says Lowe. "So we also had to consider the flatness of the system's mirrors and the alignment accuracy between them."

The team then set about minimizing each error source within the weight and motion constraints of the contract. Its design reflects each component working in harmony (see diagram). For example, the team chose an open-frame, single-plane configuration for the X-Y motion. Unlike a conventional stacked-axis stage, this approach allows unobstructed illumination from below. Also, it allows each axis motion to be referenced off the same plane (the granite base), reducing pitch, roll and Abbe errors.

Further, the planar X-Y stage allows alignment of the laser interferometer's beam path in a single plane with the stage motion at the same height as the workpiece to be measured. Placing the feedback plane at the work plane reduces Abbe offset error to virtually zero. The designers carefully laid out heat-generating components such as the motors and the laser away from the optical path, avoiding thermal gradients.

To further reduce the chance of thermally induced error, they included an optical wavelength compensator in their design that measures changes in the optical index of refraction of the ambient air. These changes, caused primarily by changes in temperature, pressure or humidity, are measured by the compensator and transmitted to the system controller for error correction.

As noted, the stage's base and bridge components are granite, chosen for its inherent stiffness and ability to be lapped to microinch flatness. Even so, the design uses custom manifolding for air and vacuum within the axis components to minimize stress from supply tubing.

Dover's DMM 2008 controller, augmented with a Delta Tau PMAC motion-control board, drives the stage. Using A-quad-B feedback from the laser interferometer and encoder, the controller coordinates all axis motion, performs valve sequencing, and supplies an operator host computer interface.

Additional details...Contact Michael Townsend, Dover Instrument Corp., 200 Flanders Rd., Box 200, Westboro, MA 01581, (508) 366-1456.