Microstructure of Mo/Si multilayers with B₄C diffusion barrier layers

The growth behavior of B₄C interlayers deposited at the interfaces of Mo/Si multilayers was investigated using x-ray Photoemission spectroscopy, x-ray reflectivity, and x-ray diffraction measurements. We report an asymmetry in the formation of B₄C at the B₄C-on-Mo interface compared to the B₄C-on-Si interface. X-ray Photoelectron spectroscopy (XPS) depth profiling shows that for B₄C-on-Mo the formed stoichiometry is close to expectation (4:1 ratio), while for B ₄C-on-Si it is observed that carbon diffuses from the B₄C interfaces into the multilayer, resulting in nonstochiometric growth (>4:1). As a result, there is a discrepancy in the optical response near 13.5 nm wavelength, where B₄C-on-Mo behaves according to model simulations, while B₄C-on-Sí does not. The as-deposited off-stoichiometric B₄C-on-Si interface also explains why these interfaces show poor barrier properties against temperature induced interdiffusion. We show that the stoichiometry of B₄C at the Mo-Si interfaces is connected to the structure of the layers onto which B₄C is grown. Because of enhanced diffusion into the amorphous Si surface, we suggest that deposited boron and carbon atoms form Si_XB_Y and Si_XCy compounds. The low formation enthalpy of Si_XCy ensures C depletion of any B _XC_Y interlayer. Only after a saturated interfacial layer is formed, does further deposition of boron and carbon atoms result in actual B₄C formation. In contrast to the off-stoichiometric B₄C growth on top of Si, B₄C grown on top of Mo retains the correct stoichiometry because of the higher formation enthalpies for Mo _XB_Y and Mo_XC_Y formation and the limited diffusion depth into the (poly)-crystalline Mo surface.