Tewksbury, MA--Step-and-repeat wafer imaging systems, or steppers, project images onto wafers coated with photoresist. They routinely place as many as 27 image layers atop one another, positioning them with a precision of a few tens of nanometers. Steppers employ two positioning stages to locate wafers beneath a light source and reticule. One, called the coarse stage, performs the longer wafer movements. Mounted on top of the coarse stage, the fine stage has six degrees of freedom. It executes the final, critical adjustment of the wafer's position prior to image exposure. Most state-of-the-art fine stages are mechanical and are driven by piezoelectric motors.
Although they work well, piezoelectric motors require mechanical couplings in order to control independent axes. These couplings greatly increase system complexity and often hinder performance. A typical fine stage that employs piezo motors may comprise 1,000 or more parts.
Working in collaboration with engineers from Sandia National Laboratory and the Massachusetts Institute of Technology, Larry Thompson, Paul Bischoff ,and other members of the development team at Integrated Solutions, Inc. (ISI) have built a magnetically levitated fine stepper stage that improves stage performance and dramatically reduces system complexity. All six degrees of stage freedom are magnetically driven.
Professor David Trumper of MIT's Department of Mechanical Engineering developed the maglev technology used in ISI's new fine stage. The company received an exclusive license from MIT to use the technology in steppers. "The real invention here," says Thompson, ISI's vice president for product development, "is the ability to levitate the structure, and then drive it with an analog signal to unprecedented precision."
Engineers selected aluminum for the base plate of ISI's fine stage, though any non-ferrous material would work. Three permanent magnets are mounted in a triangle on the base, north pole up. Sister permanent magnets on the levitated portion of the stage--called a platen--oppose these magnets. The permanent magnets support 90% of the levitated mass. Sixteen electromagnets, referred to by ISI engineers as actuators, occupy positions on the periphery of the base plate. Pole pieces for the magnets (iron cores) are secured to the levitated platen.
Arranging the actuators in pairs enables them to function in a push-pull mode. Energizing the stage turns on the actuators, thus generating magnetic force vectors. The vertical component of the force lifts the platen 150 [lmu]m above the base plate's permanent magnets. Changing the current delivered to the various actuators can cause the platen to move in all six degrees of freedom.
Feedback from capacitive sensors and a laser interferometer system enables the stepper's DSP-based CPU to continually monitor the fine stage's position. When the system wants the stage to move, actuation signals are calculated and applied to the actuators with the current necessary to move the stage to the commanded position. Because ISI's engineers know the stage's modal characteristics, the DSP can specify actuator currents that eliminate resonances. Thus the time required for the stage to stop oscillating after a move (its settling time) amounts to less than 25 milliseconds. Typically, a mechanical system needs significantly more time to settle.
Nestled into the levitated structure, a theta stage permits coarse wafer rotation. Wafers usually rest on a vacuum chuck mounted on the theta stage. Driven by a side-mounted voice coil, the disc-shaped theta stage carries the wafer to within the maglev stage's one-milliradian capture range. By varying the signal delivered to the magnetic actuators, the system performs fine adjustment of wafer rotation, as well as tilt and tip.
System control involves several different computers. A Sun Microsystems workstation programmed in Cprovides the operator interface to the stepper, while a VME-based CPU called the Instrument Manager controls the stepper in real time. As for the fine stage's actuators, they're controlled by an Ariel Hydra-PlusDSP card. The Instrument Manager instructs the DSP card to apply the appropriate number of millivolts to a given actuator's coil.
In ISI's new stepper system, engineers use the maglev fine stage and a coarse X-Y stage able to execute movements as long as eight inches. The coarse stage carries a wafer into the fine stage's capture range. Much of the software used in ISI's current Model XLS stepper can be imported and used in the new system. New code has been written, of course, to deal with the changeover from a piezo-driven fine stage to maglev technology.
To sense the position of the levitated stage, engineers use a sensor system that includes two Hewlett-Packard interferometers. Made from one piece of very low expansion glass, the interferometers' mirror floats with the stage. By using lambda over 1024 interferometry, the system can theoretically achieve positioning precision of 0.6 nanometers. Errors sum to reduce system precision to 10 to 20 nanometers. Sandia National Laboratory, Sematech, and ARPA all helped develop the maglev stage during projects initiated more than three years ago.
Additional details...Carole Urbano, Marketing Communications Manager, Merrimack Center, 836 North St., Bldg. 5, Tewksbury, MA 01876, (508) 640-2469.
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