PUB - Publikationen an der Universität Bielefeld

Since the first invention of optical microscopes, science made constant progress in developing techniques to image smaller and smaller objects. For optical microscopy, the fundamental resolution limit was described by Ernst Abbe in 1873. Nowadays, various charged particle methods are used for high resolution imaging. Especially the scanning electron microscope (SEM2 is widely distributed due to its ease of use and intuitively understandable images. It uses a finely focused beam of electrons that are scanned pixel by pixel over a surface. Electrons emitted from the surface are detected and an image is built. Obviously, the size of the probing beam is an important resolution limit for all scanning microscopes using charged particles. The smallest possible spot diameter depends on several factors, but the wavelength of the incident radiation is crucial. This causes fundamental limitations of electron-based microscopy that the long-lasting development of SEM is now facing. A promising development possibility is changing the primary particle. Ions of the same energy have a much smaller wavelength, and using helium ions in the same way provides one solution for the "next generation microscopy". The wavelength for He+ is ~100 times smaller than for electrons, so this is no limiting factor anymore. This allows spot sizes smaller than 0.35 nm and a much smaller semi-angle, so the depth of field is ~ 10 times higher than in SEM in comparable circumstances. So far, these are the advantages originating directly from the optical system. But for the final image the beam-sample interaction is also highly important. Ion beams promise a more localized interaction than electrons and thus a higher resolution and better surface sensitivity. In addition, it is possible to neutralize charge accumulation on insulating samples with the built-in electron floodgun. The technological challenge of actually building such a microscope was the source of a helium ion beam. It is based on the effect of field emission. From the apex of a small tip (the "gas field ion source", GFIS) only the emission originating from one single atom is directed on the sample. Some years ago, it was possible to obtain a stable and reliable source, so in 2007 Zeiss commercialized the first helium ion microscope (HIM) and is still the only manufacturer. Right now, the third generation (Orion Nanofab) is available with the possibility to irradiate samples with helium, neon, gallium and electrons in one chamber. In this work the previous model "Orion Plus" is used, which exclusively employs helium and has a better resolution specification (0.35 instead of 0.5 nm spot size for helium). As we are using the technology build by Zeiss, it is not our focus to enhance the technology of the microscope itself. Overview of this work The goal of this work is to explore and analyze the capabilities of helium ion microscopy for today’s scientific applications. In the course of this work, a huge variety of samples were investigated. To show the unique possibilities in practice, a number of scientifically challenging sample systems were chosen to be presented in this thesis. The outline is as following: The next chapter describes the fundamentals and technology of the helium ion microscope. Its focus is the beam-sample interaction and the imaging process, providing all necessary information for a correct interpretation of micrographs. Chapter 3 presents several aspects of imaging with this microscope. Ultrathin carbon nanomembranes (CNMs) are a fascinating new material produced from self-assembled monolayers. Due to the high surface sensitivity, HIM is perfectly suited to image this kind of material. In addition, it is insulating and not very stable in conventional electron based microscopy. High resolution images are used to thoroughly study the porosity on the nm-scale of these membranes. For another sample system, i.e. soot, the high resolution of the microscope is used for a better understanding and improvement of combustion processes. Soot is a matter consisting of carbon-based nanoparticles generated in flames. The knowledge about soot growth at very early stages was enhanced with this work, where HIM settled a new lower size boundary for imaging such particles. Staying close to combustion, catalytically active films were analyzed. Transition metal oxide (TMO) films were produced by pulsed spray evaporation chemical vapor deposition (PSE-CVD). With its high depth of focus at full resolution the HIM is well suited to investigate the morphology of these functional films. To obtain additional information about the chemistry, X-ray photoelectron spectroscopy (XPS) was used on both combustion-related sample systems. Limited spectroscopic information is also available within the HIM: Rutherford backscattered ions (RBI) could be detected and analyzed. However, the second detector mode is used here only to measure the backscatter rate, as the spectrometer is not available in Bielefeld. However, it is providing the HIM with a second imaging channel to distinguish elements from each other. The combination of RBI and voltage contrast enables interesting application in the analysis of nanowires. In addition to imaging, it is possible to use the helium beam to modify samples. In chapter 4 examples are shown where the HIM provides new possibilities. Cutting of nanometer wide gaps in carbon nanotubes (CNTs) and producing extremely well defined gold nanostructures for plasmonic applications is presented. Finally, helium ion lithography is used to directly pattern (crosslink) a carbon nanomembrane. A dose dependent crosslinking process is visualized for the first time.