A hydraulic X–Y stage system for application in electron beam exposure systems

Abstract

A hydraulic X–Y stage system for application in electron beam lithography machines is described. The X–Y table is supported on hydrostatic bearings and is driven hydraulically. This system operates in a vacuum and is used in the AT&T Bell Laboratories’ EBES4 electron beam exposure system. The X–Y stage system consists of a yoke assembly mounted on the end of a large hydraulic piston and cylinder assembly. The X–Y table is mounted on a movable carriage that rides on a smaller hydraulic piston fixed in the yoke assembly. Carriage motion in the yoke is orthogonal to the motion of the large hydraulic cylinder, providing X and Y motions, respectively. Both hydraulic axes use hydrostatic bearings to minimize sliding friction and to prevent undesired motion normal to the travel directions. The hydrostatic bearings, appropriately shaped, also prevent rotation. All sliding portions of the X–Y drive are enclosed with flexible metal bellows. The entire hydraulic system, including the pump, is sealed and evacuated to about 50 mTorr. The hydraulic fluid is a narrow distillate fraction of a perfluorinated polyether vacuum pump oil. The position of the X–Y table is measured to within (1)/(64) μm with a standard two‐axis laser interferometer system. Stage velocity can be as high as 10 cm/s but is typically less than 5 cm/s during pattern writing. Oil temperature is actively controlled to keep the stage head temperature constant within 0.1 °C under all operating conditions. The position of each axis is independently controlled by a 16‐bit microcomputer through an analog control board. The analog board drives a standard two‐stage electrohydraulic servovalve that controls oil flow to and from the hydraulic actuator. The analog gains and offsets are set digitally. Position feedback and positioning strategies, including acceleration and velocity limits, are implemented in the microcomputer.