Use of a flux-vector drive was key in fielding a photolithography system that employs an excimer laser. Cymer Inc., in San Diego, devised its next-generation system, the NanoLith 7000, with a 20-W laser pulsing at 4 kHz. Billed as the world's narrowest bandwidth production laser, it features a spectral bandwidth of ±0.35 picometers (full width, half max.).
A blower-fan system circulates excimer gases — Fluorine mixed with Argon or Krypton — presenting a fresh, nonthermally excited, nonelectrically excited load to the electrode gap. The fan plus the rotor of the dc motor driving it sits in a conductive, hermetically sealed chamber. Torque couples between the rotor and the stator through the chamber wall.
Typically, blower-motor controls have employed Hall-effect sensors located outside the chamber. One difficulty is that eddy currents and magnetic fields created within the chamber can make such schemes unreliable. The Hall devices are also difficult to line up and, once running, seem to leave some motor power untapped.
States Richard Ujazdowski, a member of Cymer's technical staff, "We had to wrap some of our logic around the Hall devices which was cumbersome and did not let us control speed very well."
The search for an alternative led to an integrated flux vector drive called DriveBlok made by the Automation Div. of MTS Systems Corp. Besides eliminating the need for Hall sensors, DriveBlok directly controls the speed of the motor, off-loading that task from the laser computer. It communicates with the laser computer via serial link and has a small physical footprint, taking up little space in the control cabinet.
Cymer worked with MTS on an accelerated program to get a motor/drive configuration able to hit the 4-kHz rate. Out of this effort came a fan designed to be driven by dual water-jacketed dc motors and powered by a special master/slave configuration of two MTS DriveBloks. The dual 3-hp drives operate in exact synchronization at opposite ends of the blower shaft. An MTS-developed sensorless algorithm applies control without mechanical clocking of the permanent-magnet rotors — no encoders, no specified rotation of the rotors or the stators.
"Power was never an issue, nor was our precision or speed," states MTS Director of Engineering Karl Konecny. "But we had to increase the speed of our I/O so the communication signal between drives has less than a millisecond delay. This assures the two motors start simultaneously."
The custom-designed configuration for the NanoLith 7000 also incorporates an MTS CapBlok, an electrolytic filter capacitor module, as well as a heat sink, an input filter for CE and an in-rush suppression filter.