Fraunhofer Institute for Microstructure of Materials and Systems IMWSNumerous modern optical devices such as highly reflective laser mirrors or bandpass filters are based on multilayer thin-film structures. Modern surface analysis methods such as ToF-SIMS are essential in order to make statements about the nature of multilayer stacks. This is the only way to avoid impurities or unwanted crystallization and to ensure the most precise architecture of the structures and homogeneity of the layers.
In multilayer thin-film structures for optical components, materials with alternating high and low refractive indices are stacked on top of each other. This makes it possible to control very precisely how much light penetrates the individual layer and how much is reflected. The multilayer stacks usually consist of a large number of individual layers or layer pairs, some of which are only a few nm thick.
The constantly increasing demand for higher reflectance, durability in relation to the source power, longer life cycles and accessibility of shorter wavelength ranges up to the extreme UV part of the spectrum leads to new requirements for these devices and thus for the properties of the thin layers. In particular, a higher purity of the thin-film materials is required to prevent absorption by contaminating elements. New manufacturing processes also aim to achieve very high uniformity of the layer thicknesses, a reduction in the individual layer thicknesses in order to achieve shorter wavelength ranges and an increase in the number of layer pairs within the stack.
In order to make this possible and ensure the necessary quality for new processes and applications, the corresponding analysis techniques must also become increasingly efficient. They provide a detailed insight into the nanostructure of multilayer stacks and thus insights into layer thicknesses, layer homogeneity, possible impurities or unwanted crystallization. At the Fraunhofer IMWS, a combination of analytical high-resolution transmission electron microscopy (abberation-corrected electron microscope FEI TITAN3 80-300 for analysis with sub-nm resolution, including SuperX-EDX system for elemental analysis) and time-of-flight secondary ion mass spectrometry (iontof TOF-SIMS5 for depth profiling with sub-nm resolution for trace element analysis) to investigate the properties of EUV reflective devices based on multilayer stacks of La or LaN and B4C.
The corresponding samples, which consisted of 250 individual layers, were first prepared by magnetron sputtering on 25 mm x 25 mm silicon substrates. Both the entire stack (homogeneity information) and individual layers down to the sub-nm range (layer thickness determination) were imaged in the transmission electron microscope (TEM). The crystallinity can be evaluated by imaging (nano-)crystalline grain structures and electron diffraction analysis for phase determination. The EDX system enables mapping of element distributions with sub-nm accuracy, whereby process-related changes such as oxidation of the uppermost layers (e.g. due to heat treatment) can also be detected. The additional analysis in ToF-SIMS allows the depth profiling of element and compound distributions with sub-nm accuracy and very high detection sensitivity (trace analysis), even for the entire stack. Overall, it was shown that the combination of these methods meets all analytical requirements for the investigation of thin-film multilayers in optical components.
