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

Wavefront aberrations are commonly expressed in a series of orthogonal Zernike polynomial terms [1]. Each of these Zernike polynomials mathematically describes a certain typical deviation from an ideal wavefront. To correct for optical aberrations a deformable mirror must be able to actuate the shapes corresponding to these terms. This paper presents the results of experiments to actuate a deformable mirror surface, using a non-contact thermal actuation principle. Thermal radiation induces a temperature distribution on the mirror substrate which causes mirror surface deformation. In the experiments, shapes are actuated up to Z3³ with amplitudes ~ 60 [nm] and ~ 3 [nm] root mean square (RMS) shape error.@en

For an adaptive optics system, a non-contact actuated deformable mirror has been developed. It is based on the principle of thermal expansion; the required energy is applied with a spatially controllable heat source and absorbed by an absorptive layer on top of the mirror substrate, underneath the reflective surface [1]. For the experimental validation of three relevant design parameters sample mirrors have been manufactured. The motivation of the design parameters and the experimental results are presented in this paper.@en