Short- and ultra-short-period multilayer (ML) structures play a crucial role in wavelength dispersive x-ray fluorescence (WD-XRF). In WD-XRF a ML serves as an analyzer crystal to disperse emission lines of light elements in the O-Kα – Al-Kα range (λ=2.36 – 0.834nm). For these reasons, MLs with periods ranging from 1.0 to 2.5nm are very interesting. Due to the short period, the reflectance of such MLs is extremely sensitive to interface imperfections.
Our research focuses on synthesis and characterization of MLs with d-spacing between 2.5 nm and 1.0 nm, combining tungsten (W) absorber with B4C, Si and Al spacers. These combinations show high theoretical reflectance in the full range from C-Kα (4.48nm) all the way down to S-Kα (0.54nm).
By optimizing the ion polishing process: ion species, energy, and polishing frequency, we show that a major improvement in reflectivity can be achieved: with the most optimal ion polishing process, a factor 2x in reflectivity was achieved for 1.0 and 1.1nm MLs, with a record reflectivity of almost 10% at lambda=0.84nm for 1.1nm W/Si.
Nanometer-thin multilayers are crucial in various optical applications, from lithography to x-ray instruments. The interface sharpness between layers determines reflectivity losses. Metrology is vital for understanding the interface forming mechanisms at an atomic level. Single techniques like TEM and XRR provide atomic or electronic density resolution, while XPS data about the compound formation. We extended our metrology portfolio with two in-house customized techniques: XSW and LEIS. The XSW technique is used for the analysis of thin film atomic profiles. The specific analysis of the background in LEIS spectra was used to analyze the interface with sub-nm resolution. A hybrid metrology approach combining these techniques is essential for efficient multilayer characterization. The metrology-driven multilayer growth optimization will be illustrated as an example of W/Si multilayers. By analyzing x-ray reflectivity and XSW data with a single model, we revealed the formation of WSix at W/Si interfaces, leading to poor performance. The introduction of 0.1 nm B4C diffusion barriers improved reflectivity, showing their direct contribution to enhanced performance.
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