Raman scattering study of nanoscale Mo/Si and Mo/Be periodic multilayer structures

Abstract

Investigation of the microstructure and phase analysis of the periodic Mo/Si and Mo/Be multilayers are essential for depositing high reflective multilayers that operate at soft x-ray to extreme ultraviolet radiations. Raman spectroscopy revealed the presence of an amorphous phase of silicon (Si) in the Mo/Si multilayers. Furthermore, the disorder of the amorphous Si phase was increased with decreasing the periodic thickness of the Si layers in the nanoscale Mo/Si multilayers. The polycrystalline Mo periodic layers coexisted with the amorphous silicon layers in the Mo/Si multilayers. In contrast, both the Mo and Be layers in the Mo/Be periodic multilayers were condensed into the polycrystalline phases. At higher annealing temperatures, the polycrystalline and amorphous phases in both the Mo/Si and Mo/Be multilayers were destroyed due to the extensive interdiffusion process. However, the amorphous Si phase was partially preserved due to the formation of crystalline intermetallic h-MoSi2 and t-MoSi2 phases, for critically thicker Si layers in periodic Mo/Si multilayers.

Microstructural Transformation of Nanoscale Be Layers in the Mo/Be and Be/Mo Periodic Multilayer Mirrors Investigated by Raman Spectroscopy

“…This coupling may induce a possible effect on phonon excitation using Raman scattering. The E 2g mode of Be nanolayers in the Mo/Be multilayers became broader, and it is red-shifted compared to crystalline bulk Be . Such a modification of the E 2g mode in the Be nanolayers is associated with stretching of Be–Be bonding due to defects, surface bonds, and formation of dimers .…”

“…Recently, Raman scattering showed that Be nanospacer layers in the Mo/Be multilayer were condensed into the crystalline structure. 13 At normal temperature and pressure, the bulk Be existing in hexagonal close-packed (hcp) lattice, which has two atoms per unit cell with a 0 = 0.22866 nm and c 0 = 0.35833 nm, 15 located at D 3h symmetry. The six normal modes of zero wavevectors belong to the irreducible representations of A 2u + B 1g + E 1u + E 2g modes.…”

“…The E 2g mode of Be nanolayers in the Mo/Be multilayers became broader, and it is red-shifted compared to crystalline bulk Be. 13 Such a modification of the E 2g mode in the Be nanolayers is associated with stretching of Be−Be bonding due to defects, surface bonds, and formation of dimers. 22 Interplanar spacing in the Be structure is greatly influenced by the local atomic coordination, which also modifies the electronic properties and phonon dispersion.…”

“…However, the structural properties of Mo are only investigated for a few model Mo/Si multilayer systems, − while condensing it into the body-centered cubic (bcc) polycrystalline phase. The microstructure and bonding properties of the spacer layers such as Si and Be for the Mo/Si, − and Mo/Be, respectively, are rarely reported. Interphase formation in the multilayer structures is well investigated, which is manifested by the thermodynamic driving force and effective heat of formation .…”

“…However, the investigation of Mo layer using Raman spectroscopy is not possible due to the absence of available phonon states at the center of the Brillouin zone. Recently, Raman scattering showed that Be nanospacer layers in the Mo/Be multilayer were condensed into the crystalline structure . At normal temperature and pressure, the bulk Be existing in hexagonal close-packed (hcp) lattice, which has two atoms per unit cell with a 0 = 0.22866 nm and c 0 = 0.35833 nm, located at D 3 h symmetry.…”