Abstract
The influence of B4C incorporation during magnetron sputter deposition of Cr/Sc multilayers intended for soft X-ray reflective optics is investigated. Chemical analysis suggests formation of metal: boride and carbide bonds which stabilize an amorphous layer structure, resulting in smoother interfaces and an increased reflectivity. A near-normal incidence reflectivity of 11.7%, corresponding to a 67% increase, is achieved at λ = 3.11 nm upon adding 23 at.% (B + C). The advantage is significant for the multilayer periods larger than 1.8 nm, where amorphization results in smaller interface widths, for example, giving 36% reflectance and 99.89% degree of polarization near Brewster angle for a multilayer polarizer. The modulated ion-energy-assistance during the growth is considered vital to avoid intermixing during the interface formation even when B + C are added.
Highlights
A vital component in advancing the instrumentation in extreme ultraviolet (EUV) and soft X-ray based lithographic tools, microscopes, solar telescopes, and time-resolved attosecond spectroscopy is normalincidence multilayer X-ray optics
Two facts related to Cr/Sc multilayers are that Cr and Sc do not form any intermetallic phase [11] at the interfaces, and that both Cr and Sc layers attain an amorphous structure below ~ 1.8 nm period [10, 12, 13]
This suggests that instead of forming a covalent network of B-C bonds which act as diffusion barrier against atomic displacements, sputtered carbon and boron atoms preferably bind to metal atoms at the growing surface, even in the case of interleaved B4C layers
Summary
A vital component in advancing the instrumentation in EUV and soft X-ray based lithographic tools, microscopes, solar telescopes, and time-resolved attosecond spectroscopy is normalincidence multilayer X-ray optics. Successful experiments have been made to grow smooth interfaces in Cr/Sc multilayers with periods > 1.8 nm using ion beam deposition techniques as well as by the use of so called “diffusion barrier layers”. The concept was first introduce for Mo/Si multilayers where the presence of sub-nm thin B4C layers, preferably on Mo-on-Si interfaces, was found to effectively hinder molybdenum silicide formation and provide higher optical contrast for EUV reflection [20]. Interface flatness is desired as interface abruptness for high reflectance of the multilayer mirrors, essential measures must be taken, such as e.g. bombardment of the growing film by energetic particles (ions or atoms) to reduce such interface morphology replication. Multiple characterization techniques are used to correlate the growth parameters, the usage of a high flux low energy ion bombardment and the distribution of B4C, with structural morphology and optical performance of the multilayers
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