Chemical interaction of B4C, B, and C with Mo/Si layered structures

Abstract

To enhance the thermal stability, B 4 C diffusion barrier layers are often added to Mo/Si multilayer structures for extreme ultraviolet optics. Knowledge about the chemical interaction between B 4 C and Mo or Si, however is largely lacking. Therefore, the chemical processes during annealing up to 600°C of a Mo/ B 4 C / Si layered structure have been investigated in situ with hard x-ray photoelectron spectroscopy and ex situ with depth profiling x-ray photoelectron spectroscopy. Mo/ B/Si and Mo/C/Si structures have also been analyzed as reference systems. The chemical processes in these systems have been identified, with two stages being distinguished. In the first stage, B and C diffuse and react predominantly with Mo. MoSi x forms in the second stage. If the diffusion barrier consists of C or B 4 C, a compound forms that is stable up to the maximum probed temperature and annealing time. We suggest that the diffusion barrier function of B 4 C interlayers as reported in literature can be caused by the stability of the formed compound, rather than by the stability of B 4 C itself.

X-ray properties and interface study of B4C/Mo and B4C/Mo2C periodic multilayers

“…The characterization techniques (Grazing X-ray reflectivity and HTEM) do not allow determining the exact chemical composition of this interfacial layer. However, the existence of this B 4 C-on-Mo interdiffusion layer is in good agreement with several published works [28][29][30]. Rooij-Lohmann et al have used thin B 4 C layers (typically less than 1 nm thick) as barrier layer between the silicon and molybdenum layers in Si/Mo multilayers, in order to improve the reflectivity.…”

Hydrogen-induced blistering of Mo/Si multilayers: Uptake and distribution

“…Mo layers deposited on a-Si layers exhibit an amorphous-to-crystalline transition at a thickness of ~2 nm and a scaling of the crystallite size with layer thickness above that point [43]. The polycrystalline structure of the Mo layers [44] with the small size of Mo nano-crystallites (~3 nm) [45], allows for enhanced trapping in vacancies and at grain boundaries and crystallite faces. This represents an example of a material constrained on the nano-scale exhibiting properties substantially different from those of the corresponding "bulk" [46].…”

Interface diffusion kinetics and lifetime scaling in multilayer Bragg optics

“…Due to the much higher formation enthalpy compared to Si 3 N 4 , there is no guarantee that the B 4 C barrier is stable with respect to annealing. In effect, already during deposition of the B 4 C barriers there appears a strong interaction with the Mo and Si layers, where Mo and Si based carbides and borides are formed 14 . In this way, the barrier layer does not act as a physical barrier that slows down interdiffusion, but instead forms a new chemical composition that effectively passivates the interfaces to Mo and Si.…”