March 2015
Spotlight Summary by Holger Wittel
Actuation profiles to form Zernike shapes with a thermal active mirror
Improving lithography is a key factor for producing more powerful microprocessors, and lithography using extreme ultraviolet (EUV) radiation is being developed for this purpose. This paper aims to overcome one of the challenges associated with EUV lithography using an adaptive optics approach with a deformable mirror.
Adaptive optical systems have been used in many different fields, from astronomy to interferometry. They are used to correct distorted wavefronts, caused, for example, by turbulence in the atmosphere or by imperfect optics. But even perfectly polished mirrors in vacuum will still absorb a fraction of the light incident on them, and this leads to thermal deformation and usually worse imaging properties. For some applications this effect will induce a strong distortion of the wavefront, and EUV lithography is one of them. Due to the extremely short wavelength of EUV light, the optics will absorb a significant fraction of it and will distort as a consequence. The authors present and test a thermally deformable mirror that can be used to straighten distorted wavefronts.
The presented setup is rather nice, and has some potential advantages to the commercially available, mechanically actuated deformable mirrors. It consists of a mirror with an absorptive coating layer underneath its reflective coating. A standard video projector is used to intentionally heat the absorptive coating, thus inducing a thermal deformation to the mirror surface.
Using their system, the authors demonstrate a set of orthogonal mirror surface shapes, and present the spatial heating profiles necessary to achieve those specific shapes. An FEM simulation has been used to find the heating profile for each of the desired mirror shapes. Furthermore, the generated mirror shape and its deviation from the desired shape were measured in order to gauge how well the process of forming the mirror has performed. It is also shown that linear combinations of the heating patterns lead to a linear combination of the resulting mirror shapes, which implies that an arbitrary mirror shape can be obtained by an appropriate combination of orthogonal actuation profiles.
Apart from a possible application in EUV lithography, this approach has great potential for the use in optics labs, and the supplied thermal actuation patterns will help with the implementation of similar systems.
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Adaptive optical systems have been used in many different fields, from astronomy to interferometry. They are used to correct distorted wavefronts, caused, for example, by turbulence in the atmosphere or by imperfect optics. But even perfectly polished mirrors in vacuum will still absorb a fraction of the light incident on them, and this leads to thermal deformation and usually worse imaging properties. For some applications this effect will induce a strong distortion of the wavefront, and EUV lithography is one of them. Due to the extremely short wavelength of EUV light, the optics will absorb a significant fraction of it and will distort as a consequence. The authors present and test a thermally deformable mirror that can be used to straighten distorted wavefronts.
The presented setup is rather nice, and has some potential advantages to the commercially available, mechanically actuated deformable mirrors. It consists of a mirror with an absorptive coating layer underneath its reflective coating. A standard video projector is used to intentionally heat the absorptive coating, thus inducing a thermal deformation to the mirror surface.
Using their system, the authors demonstrate a set of orthogonal mirror surface shapes, and present the spatial heating profiles necessary to achieve those specific shapes. An FEM simulation has been used to find the heating profile for each of the desired mirror shapes. Furthermore, the generated mirror shape and its deviation from the desired shape were measured in order to gauge how well the process of forming the mirror has performed. It is also shown that linear combinations of the heating patterns lead to a linear combination of the resulting mirror shapes, which implies that an arbitrary mirror shape can be obtained by an appropriate combination of orthogonal actuation profiles.
Apart from a possible application in EUV lithography, this approach has great potential for the use in optics labs, and the supplied thermal actuation patterns will help with the implementation of similar systems.
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Article Information
Actuation profiles to form Zernike shapes with a thermal active mirror
Rudolf Saathof, Gerrit Jan M. Schutten, Jo W. Spronck, and Robert H. Munnig Schmidt
Opt. Lett. 40(2) 205-208 (2015) View: Abstract | HTML | PDF