J. Holterman

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.

Prediction of the applicability of active damping elements in high-precision machines

The Smart Disc project at the Drebbel Institute of the University of Twente is aimed at the development of active structural elements for high-precision machines. The active elements consist of a piezoelectric position actuator and a collocated piezoelectric force sensor. As the actuators and sensors are collocated, the elements are especially suited for implementing robust active damping. The decision whether or not to incorporate active damping elements in a high-precision machine should ideally be made in an early design stage, i.e., at a time at which only limited knowledge of the vibration problem is available. Despite the uncertainties that may exist at that stage, one would like to be able to roughly predict the amount of damping that could possibly be obtained. For that reason, the present paper is concerned with the development of an analysis tool that may help in predicting the applicability of active damping elements in a mechanical structure of which only a rough model is available. Based on extensive simulations, several practical rules of thumb are given for the requirements for the mechanical structure and the active elements, in order to enable the realisation of relative damping values as high as 10%.