Consider the "No Touch" Solution

Air bearings may be the right answer in automation settings that demand high precision and low upkeep

Lawrence D. Maloney -- Design News, November 6, 2005

While still dominant in the semiconductor industry, air bearings are now tackling positioning challenges in applications ranging from food processing to medical.

More engineers are choosing air bearing systems, particularly in designs that call for zero-maintenance, long-life performance. "Unless an air bearing is inadvertently damaged or contaminated, no service should ever be required," notes Walter Silvesky, senior business development manager of Newport Corp.

Other designers like the positioning advantages of air bearings. Without friction, air bearing stages don't need to overcome resistance when making small point-to-point moves or when maintaining precise speeds, as in scanning applications.

Not that air bearings will ever supplant precision roller stages in most applications. "I encounter air bearings in perhaps 5 percent of my motion control applications," says Todd Shearer, senior application engineer for Galil. "Typically, you'll see air bearings on one axis of a multi-axis system where you need smooth, low-speed motion."

What applications stand to benefit most from air bearing technology? To find out, Design News got the following reports from leading companies in the field.

Newport: Semiconductor Proving Ground

Semiconductor settings ranging from lithography to memory repair

Newport's family of DynamYX air-bearing stages features extensive use of ceramic moving elements that provide a light-weight but rigid structure. Typical accuracy requirements: below: 0.5 micron.

have driven the evolution of air bearing stages, says Newport's Silvesky. He notes that many such applications demand XY accuracy below 0.5 micron, repeatability in the range of 0.25 micron, angular deviations smaller than 15 micro-radians, and constant velocity specifications better than 0.1 percent.

Newport engineers also see increasing use of air bearing stages in flat panel inspection, where travel ranges (larger than 2.5m in a gen-7 system) and micron-level accuracies suggest air bearing designs. Another new opportunity, according to Newport, is wafer bump inspection, which has traditionally relied on recirculating mechanical bearing stages. For example, one recent customer chose air bearings based on their superior dynamic flatness. Vertical deviations of the stage carriage are substantially smaller with air bearings because of the averaging effect of their large contact surface versus the individual contact points of a ball or crossed-roller bearing architecture.

"Most high-performance stages use bearings with reference surfaces that are precision ground and sometimes lapped to be as uniform and smooth as possible," explains Silvesky. "However, the topography is not perfect and even tiny imperfections can cause dynamic disturbances as the balls and rollers in a mechanical stage encounter these surface features."

Newport attributes the high level of performance in its family of DynamYX air bearing stages in large part to extensive use of ceramic moving elements. These components provide a light-weight and rigid structure with a density similar to that of aluminum and a Young's Modulus of steel. Because the ceramic material allows for a finely lapped and stable surface, the air bearing elements may be directly formed into the moving stage.

Integration of the air bearing elements (pads) results in a low-profile stage, with an XY carriage mounting surface that is only 66 mm above the granite base. This design provides high accuracy by minimizing the lateral (abbe) errors at the mounting surface. The low center of gravity, coupled with the rigid and highly damped properties of the structure, deliver excellent step-and-settle performance, say Newport engineers.

In addition, the ceramic materials provide a very low thermal expansion coefficient and a very high thermal conductivity. Result: 10 nm repeatability and the ability to use error maps to further improve the absolute accuracy of the system. In a recent lithography application, the DynamYX GT, equipped with linear encoders, replaced an older stage that used interferometer feedback. The new design saved costs and reduced system complexity.

Aerotech: A Systems Approach

Aerotech has distinguished itself in air bearing applications by integrating stages, motors, controllers, and other components in a systems solution.

Al Ciez, manager of Aerotech's positioning systems division, gives the example of the company's ABL9000, a planar style air bearing. He believes that this design offers better geometric stage performance due to minimum tolerance stack-up of parts, coupled with lower mass and higher stiffness.

Aerotech's air-bearing stages typically employ direct-drive linear or rotary brushless motors with high-resolution encoder feedback. Some linear designs also use laser interferometers for secondary feedback. Aerotech controls air-bearing stages and systems with its Automation 3200 motion controller. It provides single- and dual-loop operation, interfaces with all major-brand laser interferometers, and offers up to 2 MHz encoder input frequency capability.

Besides semiconductor and flat panel applications, Aerotech finds increased air-bearing use in diamond turning and optics finishing processes. These applications typically require linear and rotary air-bearing combinations to achieve stringent form error and surface finish requirements.

Direct-write applications also rely on the multi-axis contouring accuracy of air-bearing stages. Ciez explains that this application benefits from the high-speed stage position synchronization of Aerotech's motion controllers. Here, the stage position triggers external events, such as laser firing, at rates up to 20 MHz. Aerotech uses proprietary techniques to dynamically correct system errors, providing the high performance levels that such applications demand.

Anorad: Pick-and-Place Dynamo

Rockwell Automation's Anorad subsidiary has been providing engineers with air bearing and direct-drive linear technologies for more than 30 years, but new ideas are constantly being developed, notes David Broich, senior applications engineer.

The Cyclone ZT from Anorad offers air-bearing positioning in a tiny package that is just 30 mm wide. Among the target applications: pick-and-place in electronics assembly.

Broich points to the company's Cyclone ZT Positioning Module, a high-performance dual-axis miniature air bearing that replaces bulky electromechanical actuators in pick-and-place applications. Only 30 mm wide, the module combines brushless servo motor, optical encoder, and air bearing technologies in one compact package. The small footprint allows multiple units to fit on a small mounting surface, which enhances throughout and design options. What's more, the non-contacting design reduces particles, making the module a good option in clean room environments.

The Cyclone ZT is typically mounted to the cross axis of a gantry machine, creating an integral X-Y-Z-T platform. In one recent application, engineers ganged multiple units side by side for picking and placing odd-shaped components in a relay contactor assembly line. Among the module's performance capabilities: acceleration up to 2 g's, radial resolution to 0.05 degree, and linear resolution to 0.4 µm.

As for Anorad's traditional air bearing line, Broich observes that the higher spring rate of the company's tuned orifice air bearings offers superior static and dynamic stiffness versus porous-style air bearings. Higher stiffness translates to a quicker damping reaction, whether the shocks come from motive accelerating forces, load changes, or vibration. In addition, Anorad often specifies magnetically preloaded orifice air bearings for greater stiffness in high-precision applications. This type of bearing delivers both tight servo control during stepping motion and superior damping in constant-velocity scanning applications.

New Way Air Bearings: Broader Application Mix

At New Way Air Bearings, company founder Drew Devitt reports a surge of new orders from applications outside the precision marketplace, such as "no-oil" environments. The food industry—in particular the processing of flour, wheat, and sugar—see air bearings as delivering significant advantages, says Devitt.

Traditional bearing surfaces require oil to maintain function, but dry process dust is attracted to that oil, creating a troublesome slurry where the processed material meets the lubricant. Result: a maintenance nightmare. With continuous air flow through New Way's porous media, the bearing surface is self-cleaning, which reduces down time and maintenance costs.

Another key application area is Free Abrasive Lithographic Patterning (FALP), a high-precision process for producing semiconductor components. CoorsTek, Golden and CO, use FALP to manufacture patterned ceramic components for the LCD and semiconductor industries. "These products are manufactured from high-purity ceramics, using the most advanced precision machining and grinding equipment available," notes Steve Williams, sales manager for CoorsTek. "Precision, speed, and repeatability are critical to our process, and air bearings deliver on each of these requirements."

He adds that air bearings represent the only viable solution for such a dry, abrasive, oil-free environment. "Without the air bearing, the free abrasive media would quickly destroy the machine," says Williams.

Devitt sees the pairing of ceramics with porous media air bearings as a natural fit. "CoorsTek also manufactures metrology and machine components," he explains, "so they understand accuracy. Combined with our air bearings, the high stiffness-to-weight ratio and micron-level precision of CoorsTek ceramic beams and stage components provide unsurpassed accuracy and reliability."

Web Resources
//For more information check out the links below//
Newport Corp.: http://rbi.ims.ca/4418-504	Aerotech, Inc.: http://rbi.ims.ca/4418-505	Anorad, Rockwell Automation: http://rbi.ims.ca/4418-506
New Way Air Bearings: http://rbi.ims.ca/4418-507