Active damping elements for improving the accuracy of a microlithography machine

High-precision machines are usually designed according to a limited number of well-known design principles. The dynamic behaviour is optimised mainly by means of proper stiffness management: the design of the mechanical structure is aimed at minimisation of the mass, and maximisation of the stiffness. Damping management is not yet a mature design principle. This is due to the difficulties in designing passive damping mechanisms that do not endanger accuracy. As an example of a vibration problem within an industrial high-precision application, in this paper the need for active damping management in a microlithography machine is discussed. In future, vibrations of the lenses of this machine may pose a practical limit to the accuracy of the lithography process. For that reason, active structural elements have been developed for supporting the lenses. The active elements, consisting of a piezoelectric actuator and a collocated piezoelectric force sensor, are especially suited for implementing robust active damping. The purpose of the present paper is to discuss the conflicting requirements in the mechanical design of the active elements. The discussion is illustrated by means of experimental active damping results that have been obtained on the microlithography machine.