Adaptive state observer and PD control for dynamic perturbations in optical systems

High-performance objectives pose strict limitations on residual errors present in the system. External mechanical influences can induce structural vibrations in the optical system which causes a displacement of the lenses present in the system. This will influence the imaging performance, causing degraded images or broadened structures in a lithographic processes. In this paper an adaptive state observer for the detection of structural vibrations of the optical elements of an imaging system based on a series of wavefront tilt measurements is introduced. The observer output is used as an input for a closed-loop PD control to mitigate the lens displacements directly.