Raman scattering studies of Si/B4C periodic multilayer mirrors with an operating wavelength of 13.5 nm

In order to obtain mirrors with a minimum value of residual stress, periodic multilayer mirrors composed of Si/B4C were deposited by magnetron sputtering with change in pressure of sputtering Ar gas. The microstructure and phase of Si and B4C was manipulated by the pressure of Ar gas which overall affected the stress in the mirrors. The minimum stress was obtained at higher pressure of sputter Ar gas, which showed the formation of amorphous boron, amorphous B4C, free carbon atoms and amorphous carbon structure in the B4C layers, investigated by Raman scattering spectroscopy. In Raman spectroscopy, a transverse optical (TO) mode of amorphous Si was shifted to lower frequency with increase in Ar gas pressure, which indicated relaxation of stress, also confirmed by the curvature measurement of mirrors. However, in the case of high residual stress, the amorphous B4C was a prominent phase in this layer and the frequency of the TO mode of amorphous Si was blue-shifted. Microstructure and stress affected the interfaces and modulation of the periodicity of the Si/B4C mirrors, investigated by secondary ion mass spectroscopy, which influenced the reflectivity of the mirrors.