Active Vibration Isolation Design for a Photolithographic Stepper

This paper discusses the design and implementation of a six axis active/passive vibration isolation system on a photohthographic stepper. Passive isolation is used to provide good isolation at high frequencies and to support the static weight of the stepper. Active isolation is used to provide the dynamic control forces to reject external disturbances. The active system has the capabihty to reject both seismic disturbances and reaction forces resulting from wafer stage motions. A description of the system hardware is presented followed by dynamic system model-ing and details of the control approach used. A series of coordinate transformations to implement simple control laws are derived. Active control is established over more than two decades around the mount resonant frequency and about 30-40 dB attenuation of disturbances achieved. Experimental results verify the performance of the system. These results illustrate the effectiveness of the system at rejecting disturbances originating from both seismic sources and from the motion of the wafer stage.