Design and characterisation of an active mirror for EUV-lithography

Abstract

Due to the absorption of extreme ultraviolet (EUV) light in the projection optical system of an EUV lithography machine, its mirrors thermally deform resulting in wavefront errors (WFEs) that deteriorate the imaging process. In order to compensate and correct for these mirror deformations, this paper proposes an adaptive optics (AO) system that uses an active mirror (AM) to counteract these WFEs. For this purpose, a list of system and mirror requirements is composed as well as a list of design choices, which motivate the AM design. In order to asses the specifications of this AM, finite element method (FEM) and experimental analyses are carried out to obtain and validate the general properties of the AM. For assessing linearity, the formal definition of linearity is used and verified in the experimental set-up. The instrument transfer function (ITF) is obtained by actuating the AM with different spatial frequencies fx and measuring the deformation amplitude. The AM is proven to be linear given a linear coefficient of thermal expansion (CTE) and the ITF shows a trend for both the time constant and the amplitude of the deformation. By using the ITF it is shown that the characteristics of the AM is suitable for an AO system for EUV lithography.