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Un-Inventing the Wheel
Magnetic Levitation for Nano Precision Positioning
for Next Generation Silicon Wafers
As admirable as the wheel’s 7,000-year record of performance has been, it appears that, at long last, it may be falling victim to its own success. Today, within a few minutes of downtown Pittsburgh, researchers at Philips Applied Technologies are collaborating with colleagues in the Netherlands to perfect a magnetic levitation positioning system for future silicon chip fabrication devices.


Philips’ efforts are driven by the need to produce more, smaller, denser chips on increasingly large silicon wafers, a task that requires nanoscale precision. Recognizing that a nanometer is several hundred times smaller than a single wavelength of visible light, it is not hard to imagine that even the most subtle mechanical vibration, atmospheric turbulence or electromagnetic disturbance, such as those produced by currently available roller and air-bearing technologies, can make nanoscale positioning of a silicon wafer nearly impossible. Philips’ answer to the problem is wafer isolation; i.e., detachment from all rollers, hoses, jets, beds…even gravity.


For starters, the Philips system counters gravity with an opposing electromagnetic force. When I touched the levitated stage of the 4-foot square by 5-foot tall machine, the tip of my index finger met with less resistance than that of a helium-filled balloon.


I glanced quizzically at my guide, who smiled knowingly, “It’s weird, isn’t it?” Then, after a few keystrokes on the control console, he said, “Try it now.” As though somebody had tightened the screws, the bed would not budge. “You put it back down,” I said. “No,” he responded, “magnetic force is holding it suspended in place. There’s still a space beneath the bed, see?” Indeed the space was there, and neither of us could budge the stage from its position.


To conceptualize a maglev positioning system, imagine a marionette dancing on a parade float that’s rolling across a theatrical stage. Unseen stagehands pull the float back and forth with ropes while a puppeteer manipulates the strings that animate the puppet from above. Once you have that picture in mind, substitute electromagnets for the ropes and strings, and you have a ridiculously large, but imaginatively helpful, magnetic levitation stage.


Meanwhile, back in the laboratory, Philips’ maglev stage is comprised of three physical subsystems – transport (ropes), sensing (puppeteer’s eyes), and control (brains and fingers). The three subsystems work together in what mechatronics engineers call Six Degrees of Freedom Control (6DoF), which allows the stage to move, dip, shift, tilt, roll and yaw, like an airplane or submarine.


The transport system employs two types of electromagnetic motors for nanoscale movement: two linear Lorentz motors to move the stage back and forth (stagehands); and six electromagnetic devices, called variable reluctance actuators, for levitation (puppeteer). Put simply, Lorentz motors work like conventional rotating electric motors except, their essential parts have been rolled out flat, so the electromagnetic force runs in a straight line, rather than in a circle. The second type, variable reluctance actuators, are based on the characteristic of some materials (frequently iron) to respond to an electrical current by aligning their atomic spins in a common direction, resulting in a magnetic force, or flux. More electrical current means more aligned spins; less means fewer, hence variable “pulling” power.


For control, six laser beams detect the position of the wafer carrier, or “stage,” and feed position data back to the control system, which compares it with ideal position data, and adjusts the current to compensate for any discrepancy. In operation, this process would have happened more than 300,000 times while you were reading this column.


Although achieving nanoscale positioning accuracy is an amazingly complex task, un-inventing the wheel appears to have been easier than reinventing it might have been.


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This story first appeared in Tom Imerito’s TEQ column, Innovation Chronicles. You can read it on the Pittsburgh Technology Council’s website.

©Copyright 2007 Thomas P. Imerito/ dba Science Communications


 
©2009 Science Communications
thomas@science-communications.com