Six axis active vibration isolation and payload reaction force compensation system

"Successful precision engineering-is the balance of robustness of the machine and how benign the environment can be made through isolation to minimize the strains caused by vibration that compromises a machine's accuracy."[1] Photolithography equipment precision positioning capabilities have recently improved to the point where seismic level base disturbances comprise a much larger percentage of the total machine error budget. Additionally, the adverse effects of payload induced reaction forces limit the allowable stage acceleration and thus the wafer throughput of the machine. Current wafer stepper manufacturers have chosen to isolate the system from seismic disturbances and simultaneously profile stage motions to reduce reaction forces. This approach has allowed manufacturers to improve throughput, but is still limited in its capabilities. We will present an integrated active seismic isolation system with the additional capability of compensating for payload reaction forces, in real time. in six degrees of freedom. The system is comprised of both active and passive elements to allow the isolation or control of disturbance forces in a band which extends from frequencies below 0.5 Hz and beyond any frequency of interest. These two distinct and separate sources of stepper vibration each require a unique controls approach. The vibration induced by stage motions are deterministic in nature and are easily compensated for with feedforward techniques. During wafer exposure, the primary source of vibration is from seismic level base disturbances. These vibrations are usually stochastic and broadband in nature. The fact that the stage motions are deterministic and that the isolation from ground is crucial only during wafer alignment and exposure allow us to hold the system reasonably stationary in inertial space; In summary, this paper will present the mechanical design and the dual control approach implemented. Ground vibration transmissibility data will also be presented. Additionally, the performance of the system during stage accelerations will be presented. As lithography tools improve in performance, and the industry moves toward very high throughput platforms, these precision isolation and disturbance rejection issues become increasingly more critical.