R. Saathof, G. Schutten, J. Spronck, R. M. Schmidt:
"Design and characterisation of an active mirror for EUV-lithography";
Precision Engineering (eingeladen), 41 (2015), S. 102 - 110.

Kurzfassung englisch:
Due to the absorption of extreme ultraviolet (EUV) light in the projection optical system of an EUV lithog-raphy machine, its mirrors thermally deform resulting in wavefront errors (WFEs) that deteriorate theimaging process. In order to compensate and correct for these mirror deformations, this paper proposesan adaptive optics (AO) system that uses an active mirror (AM) to counteract these WFEs. For this pur-pose, a list of system and mirror requirements is composed as well as a list of design choices, whichmotivate 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. Forassessing linearity, the formal definition of linearity is used and verified in the experimental set-up. Theinstrument transfer function (ITF) is obtained by actuating the AM with different spatial frequencies fxand measuring the deformation amplitude. The AM is proven to be linear given a linear coefficient ofthermal expansion (CTE) and the ITF shows a f−2xtrend for both the time constant and the amplitudeof the deformation. By using the ITF it is shown that the characteristics of the AM is suitable for an AOsystem for EUV lithography.

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