SURFACE COMPOSITION ANALYSIS OF ULTRA-SHALLOW SURFACE REGIONS IN COMPLEX MULTILAYERS AND MULTICOMPONENT ALLOYS BY ION SCATTERING AND PHOTOELECTRON SPECTROSCOPY
ABSTRACT: Quantitative compositional analysis at ultra-shallow depth scales is important for the understanding and optimization of processes that involves modification of the topmost layers of surfaces to subsurfaces using plasmas and ion beams. A thorough and precise understanding of the behavior of the surface under exposure to plasmas and ion beams is important in applications such as the mirror optics facing the tin plasma in an extreme ultra-violet lithography (EUVL) tool. Moreover, such understanding is also essential in developing a predictive model for the self-organization behavior of nanoscale structures on surfaces upon low energy ion irradiation, a phenomenon that is both very intriguing from a physics point of view and promising for controllable, one-step, bottom-up fabrication of nanostructures. For low energy ions, the modification is confined within ~10 nm below the surface. Therefore, probing the plasma- and ion-induced changes requires extremely surface-sensitive characterization techniques. In this dissertation, x-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS) have been utilized for the compositional analysis and depth profiling of complex multicomponent materials. XPS is the most widely used technique in obtaining chemical state information at the top <10 nm of the surface, and LEISS is the most surface-sensitive technique capable of probing the composition of the topmost monolayer. Combining XPS and LEISS analysis, researchers are then able to compare and contrast the compositional and chemical state information at different depth scales within the first 10 nm of the surface.
Two applications involving complex and technologically important materials have been chosen as demonstrations of combining XPS and LEISS in surface compositional analysis. The first application involves utilizing LEISS analysis combined with carefully tuned sputter depth profiling to map the depth distribution of tin (Sn) particles in ZrO2-capped Mo/Si ultra-thin multilayer mirrors (MLMs) exposed to tin ions from a Sn plasma, which is the source of extreme ultra-violet (EUV) light in EUV lithography, the only lithography technique that can produce next generation sub-10 nm node semiconductor chips in high volumes. The results have shown that LEISS depth profiling, when optimized, was able to produce compositional depth profiles that agreed well with simulations, and more accurate than conventional XPS depth profiling due to its extreme surface sensitivity. The measured Sn concentration and distribution were also consistent with XPS analysis, demonstrating the reliability of LEISS. Elemental detection sensitivity with LEISS is also satisfactory and comparable with XPS, being able to detect a Sn concentration as low as 0.08 ± 0.05 at-%. In addition, by complementing with XPS analysis and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) it allows characterization of structural defects and chemical changes due to Sn ion exposure as well.
The second application involves utilizing a combination of XPS and LEISS analysis to characterize the compositional changes of Zr52.5Cu17.9Ni14.5Al10.0Ti5.0 (Vit 105) bulk metallic glass (BMG), a glassy alloy that exhibits superior mechanical properties compared to typical metal alloy, upon low energy Kr+ ion beam irradiation and correlate the changes to highly-ordered nanopattern formation on the surface. Utilizing in-situ XPS and LEISS analysis, this work has found that highly-ordered patterns can be induced on Vit 105 only if the amount of W impurities has exceeded a certain threshold. This information is essential in formulating a theoretical description for pattern formation, which is intrinsically extremely complicated involving many synergistic physical and chemical mechanisms. In addition, XPS and LEISS analysis have revealed a very different composition and amount of W impurities at the subsurface and at the top surface respectively. This indicates the existence of a heterogeneous composition at the surface. These results have emphasized the importance of utilizing a multitude of surface characterization techniques to extract information from different depth scales and provide a more complete picture of the ion-induced changes at the surface.