Adaptive multilayer optics for extreme ultraviolet wavelengths

In this thesis we describe the development of a new class of optical components to enhance the imaging performance by enabling adaptations of the optics. When used at extreme ultraviolet (EUV) wavelengths, such ‘adaptive optics’ offers the potential to achieve the highest spatial resolution in imaging known to date, i.e., a resolution down to several nanometers. These optics are discussed, e.g., in the field of photolithography for the fabrication of more energy efficient and faster electronic equipment. The topics in this thesis can be subdivided into two parts. The first part includes a mirror design that can be used to enhance the spectral quality of EUV light sources. Our design consists of a diffractive Fresnel zone plate patterned in the surface of a curved mirror to achieve spectral purification, i.e., removing undesired wavelengths of the EUV light sources. The removed light is recycled to produce more EUV light. This design achieves four orders of magnitude purification and provides a 35% improvement in efficiency. The second part of the thesis describes a new class of adaptive multilayer mirrors that can be used to correct wavefront distortions, reflectance degradations or wavelength deviations. These adaptive optics contain integrated crystalline piezoelectric thin films of which the film thickness can be adjusted by an externally applied voltage. Because the optics substrates typically have an amorphous structure, so far it was not possible to grow high quality crystalline piezoelectric films on them. We utilized crystalline nanosheets on these substrates to grow the highest quality, i.e., epitaxial, piezoelectric films (PbZr0.52Ti0.48O3, PZT). The PZT films show a record high piezoelectric coefficient of 98 pm/V, although the piezoelectric response in this case is still reduced due to clamping by the substrate. We minimized the clamping effect by depositing films with a columnar structure, which enhances the voltage-controlled change in film thickness. In these films we measured a piezoelectric response of 280 pm/V, the highest value reported on glass substrates. Importantly, the developed films can produce a stroke of 25 nm. This order of magnitude is useful when considering to correct EUV wavefront distortions.