Two and Three Dimensional Extreme Ultraviolet Holographic Imaging with a Nanometer Spatial Resolution

Abstract

The word “hologram” (from the greek “holos”: whole, complete and “graphos”: writing, drawing) means “total recording”. Holography is a well known technique originally proposed in 1948 by Gabor, who also coined the name, as a new microscopy alternative. He realized that the interference of two mutually coherent waves, one called the reference wave and the second one the object wave, allows for recording of information consisting of both amplitude and phase of diffracted or scattered beam from an object (Gabor, 1948). This coding of the amplitude and phase of the object beam into an interference pattern allowed him to demonstrate that from this complicated holographic pattern, ultimately the image of the original object can be obtained. Several years after the appearance of Gabor’s paper, Baez (Baez, 1952) suggested extension of this idea to the X-ray region, but it remained as an interesting proposal till the early 1960s, when holography started to be widely applied. It was after the paper by Leith and Upatnieks, who proposed the off-axis holography scheme which overcomes many of Gabor configuration drawbacks (Leith & Upatnieks, 1962). Since that time holography was widely used in numerous applications, some of them requiring increased spatial resolution. On this path, reducing the illumination wavelength is a direct way to improve spatial resolution both in nanopatterning (Solak et al., 1999; Wachulak et al., 2008a) and holographic imaging, described herein. This is the reason why short wavelength sources such as synchrotrons, extreme ultraviolet (EUV) and soft X-ray (SXR) lasers, high harmonics generation sources (HHG), etc., became an interesting alternative for high resolution imaging. This chapter is devoted to 2-D and 3-D holographic imaging using a capillary discharge EUV laser. The chapter is organized as follows. In section 2 recent developments in high resolution holographic imaging will be briefly presented including different imaging techniques and short wavelength sources. In section 3 some general information about Gabor in-line EUV holography will be presented with detailed analysis of the resolution limitations due to coherence of the EUV source and digitization process. Starting from section 4 through 6 recent developments in holographic 2-D and 3-D imaging will be