Active damping within an advanced microlithography system using piezoelectric Smart Discs

Vibrations within high-precision machines are often badly damped, and may easily show up as a major factor in limiting the achievable accuracy. As it is hard to damp these vibrations by passive means, active vibration control may become inevitable within such machines. Robust active damping can be obtained by applying integral force feedback to so-called ‘Smart Discs’, i.e., active structural elements consisting of a piezoelectric position actuator and a collocated piezoelectric force sensor, which are inserted at appropriate locations within a machine frame. A wafer stepper, i.e., the advanced microlithography system that is at the heart of integrated circuit manufacturing, is an excellent example of a high-precision machine with a frame that has low structural damping. Badly damped vibrations of the lens of the wafer stepper may in future limit the attainable line width of the circuit patterns. For this reason an active lens support, based on Smart Discs, is developed. Experiments with the active lens support show that the relative damping of the dominant vibration modes of the lens is increased from 0.2% to 16%, which effectively results in 86% reduction of the vibration amplitude.