A demonstrator adaptive optics-system with a thermally actuated active mirror (AM) is presented to pre-study feasibility of sub-nm wavefront control in extreme ultraviolet (EUV) lithography. The AM is thermally actuated by selective heating using a spatial controllable heat source. Four different methods have been implemented to control the deformation of the AM. First thermal feedforward using estimated state feedback (ESF), second thermal feedback using proportional integral (PI) control, third their combination and fourth deformation feedback using PI control. To support ESF, a thermo-elastic finite element model is employed that describes the thermal deformation of the AM. ESF shows satisfying performance with a time constant of 10 s and a residual error of 0.7 nm. Thermal feedback shows large fluctuations of 12 nm for spherical aberrations of due to feedback of noise from the thermal camera. By applying deformation feedback the RMS-error is reduced to a satisfying 0.25 nm. This study shows that deformation control of this AM can be realised using thermal feedforward and deformation feedback to meet the requirements for EUV lithography.@en

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.@en

In EUV lithography, the absorption of EUV light causes wavefront distortion that deteriorates the imaging process. An adaptive optics system has been developed [“Adaptive optics to counteract thermal aberrations,” Ph.D. thesis (TU Delft, 2013)] to correct for this distortion using an active mirror (AM). This AM is thermally actuated by absorbing an irradiance profile exposed by a projector onto the AM. Due to thermal conductivity and bimorph-like deformation of the AM, the relation between actuation profile and actuated shape is not trivial. Therefore, this Letter describes how actuation profiles are obtained to generate Zernike shapes. These actuation profiles have been obtained by a finite-element-based optimization procedure. Furthermore, these actuation profiles are exposed to the AM, and the resulting deformations are measured. This Letter shows actuated Zernike shapes with purities higher than 0.9 for most actuation profiles. In addition, superimposed actuation profiles resulted in superimposed Zernike shapes, showing linearity needed to apply modal wavefront correction. Therefore, this approach can be used to obtain actuation profiles for this AM concept, which can be used for highly precise wavefront correction.@en