Hurdles prompt design innovations

Talk about design challenges!

Rene Schnetzler, president of NUTEC Components, and NUTEC engineers Eddy Gabis and Larry Boas had to design a 300- x 300- x 16-mm x 10-deg travel precision motion platform within a very compact 800- x 800- x 200-mm envelope to fit within a vacuum chamber in 4 weeks.

And, the engineers had to deliver a functioning motion system ready to be used in less than 4 months.

They opted for a 4-axis system with a 300- x 300-mm XY table. The vertical Z-axis had to be compact enough to be partially buried into the XY stage but achieve high stability for supporting a 300-mm wafer chuck that also needed limited range of rotation for alignment.

Oh, and the system would hang inverted with a guide and drive system of considerable stiffness to deal with the combined load of the 280-lb stage and the customer payload of 12 lbs (5.4 kg).

The four-axis positioning platform design held several design challenges for the engineering team. Among them was the small footprint it had to fit into.

The theta axis was to have a limited 10-deg rotary range with a 0.36 arc sec resolution. Space restrictions and tribology issues precluded a worm drive. The engineering team chose a direct-drive brushless servo torque motor coupled with a direct-reading high-resolution encoder to not increase system height and to reduce Abbé error, the entire theta axis would be buried within the Z-axis stage envelope. A highly pre-loaded in-line custom roller leadscrew drive offered the characteristics and compactness the limited space demanded.

All of these challenges were manageable, says Schnetzler. But, the team had to think way out of the box to meet one major criteria: the 10^-7 torr vacuum environment. Dealing with the vacuum environment itself posed no great new challenges, but coupled with the foot-print restriction to achieve a compact vacuum chamber made the task interesting.

Vacuum, size, and complexity dictated mechanical bearings and a compound (stacked Y on X) architecture. However, says Schnetzler, achieving 5 micron accuracy over the 300-mm travel and a 2 micron flatness required the team to consider numerous design elements, among them structural stability and structural stiffness of stage body, inverted attitude and autonomous body, material selection, optimized cross sections, stiffness vs. mass, and thermal issues. The latter is relevant not because of accuracy but because of the guide system preload forces acting against the stage body.

True geometry of critical stage structure surfaces has a direct influence on performance specifications of the stage. Engineers had to address design-for-manufacturing issues right out of the starting gate. Choosing the right linear guide rail system was of paramount importance. "We settled on a customized version of a double V-lock rail with near frictionless needle bearings offering extreme stiffness," says Schnetzler.

Position sensing is the single most critical function on a metrology-grade positioning device. Assuming the digital servo control is capable of commanding the exact translation, it is principally the linear encoder system providing the key function of generating position information. The main features of the linear encoders selected were non-contacting electro-optical, all solidstate read heads, specially prepared to operate in vacuum environments. To have a reliable signal reaching the motion controller via three different cable sections with vacuum feed-through junctions, bulkhead and interface connectors, engineers amplified the microcurrents of the optoelectronic circuitry and generated a digital signal in the read head, permitting amplified square wave signals to be transmitted. They equipped all four axes with such direct reading encoders.

Holding position. Wafer level inspections involve a process with durations up to several minutes during which the positioning system must hold position, without dithering, drifting, or continually servoing back and forth. To achieve this on a nanometer level requires extreme mechanical stability through highly preloaded components, fast-cycling digital servo control, powerful servo amplifiers, ample proportioned motors, stable electrical power supplies, an appropriate mechanical support system, high resolution position sensors, servo algorithms addressing these specific operating conditions, absence of electrical noise and mechanical vibration, and stable environmental conditions and temperature.

An added complication involved the theta axis. During wafer testing, probes interact with the silicon wafer and exert a force onto the theta axis, possibly triggering a servo action which could change position and resettle. To avoid this effect, engineers installed a holding brake, locking the theta axis in place during the test cycle. The design of choice, says Schnetzler, was a magnetic brake module with a solenoid release generating ample force against a steel brake plate, similar to an automotive disc brake, but single sided.

Since the positioner has to stay in position for an extended time interval during testing, and because of the considerable mass being moved, the engineering team applied a fine-pitch screw drive to optimize drive characteristics with resolution, essentially achieving the largest angular displacement per linear resolution increment. They selected a time-proven precision single roller leadscrew drive with a 16-mm diameter stainless steel screw and an ABEC 7 duplex angular ball bearing.

Digital servo control. Matching a suitable control to a high-precision positioning platform is crucially important. The criteria to meet include the ability to commutate the brushless servo motors sinusoidally to achieve control over the smallest motion increments and permitting the high resolution to be usefully implemented. Proper control of all three phases of the servo motors with corrections for phase shift angles is essential. A fast cycling CPU is required to allow a reasonably fast servo update rate for all four axes operating simultaneously as well as in coordinated motion. They used their own NUTEC MICROMATIC-5 digital servo control. It's a DSP-based motion controller.

The most difficult aspect of the project turned out to be the fact that no vacuum-compatible servo motors were available on the market. The only options available were to wait twelve weeks for a purchased custom motor or design and build their own. They opted for the latter and proceeded on a crash program to conceptualize, engineer, design, and build a vacuum-compatible motor. The choice was made for a brushless kit motor with the power, winding option, and envelope matching the application. The windings still needed customization for vacuum rating as well as the ball bearings for the motor shaft. The inch-size bearing facilitated the availability of a bearing cage material having extremely low outgassing.

The moral of the story: The design required a multi-disciplined engineering team. The short design cycle left no room for a learning curve. Design-for-manufacturing was critical, as were close relationships with vendors.

For more information on positioners from NUTEC, enter 547 at www.designnews.com/info.