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In order to obtain the mirrors with minimum value of residual stress, periodic multilayer mirrors with the composition of Si/B4C was deposited by magnetron sputtering with change in pressure of sputtering Ar gas. The microstructure and phase of Si and B4C was manipulated by 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 TO mode of amorphous Si was blue-shifted. Microstructure and stress affected the interfaces and modulation of periodicity of the Si/B4C mirrors investigated by secondary ion mass spectroscopy which influenced the reflectivity of mirrors.

The ambient atmospheric thermal stability of beryllium (Be) layers in Be/Mo and Be/W multilayer mirrors was investigated by Raman scattering. The physical characteristic of the transverse optical (TO) mode was considered for structural analysis of the Be layers in the multilayers. With an increase in thermal annealing temperature, two important modifications of this mode were noticed: the TO mode of Be was found to shift to a lower frequency and the peak width became wider. These two facts are related to the deterioration of the crystalline quality of the Be layers upon thermal annealing. The TO mode of the crystalline Be phase completely vanished and high-intensity peaks at the shoulder were detected for the Be/Mo multilayer thermally annealed at 723 K. This evidence is associated with the transformation of polycrystalline into an oxidized amorphous Be phase. In this case, the diffusion of oxygen to the inner period and the destruction of the modulation of the periodic structure of the multilayer was investigated by a secondary ion mass spectrometer. However, the TO mode of Be embedded within W in Be/W multilayers was stable in similar annealing conditions, which revealed the thermal stability.