| Ti/Al soft X-ray multilayers (Λ=9.25 nm,Γ=0.3,N=20) used at the wavelength of～18 nm have been designed according to the recursive optical model and the principle of material selection. The structure and thermal stability of Ti/Al multilayers deposited by a facing-target sputtering system at argon pressures of 0.5 Pa and 2.0 Pa were studied.It was found that the structure and thermal stability are the function of sputtering pressure. The multilayers prepared under lower sputtering pressure of 0.5 Pa exhibit relatively high quality, textured crystalline layers, low surface roughness and high thermal stability. On the other hand, higher sputtering pressure of 2.0 Pa results in random orientation, high surface roughness, unstable and poor modulation structure.The systematic investigation on the structure and thermal stability of the Ti/Al multilayers deposited at the sputtering pressure of 0.5 Pa was also carried out. The multilayer shows a superior moderation structure, sharp and smooth surface with relatively low roughness, in which the Al and Ti layers are both strongly textured with the selective orientation of Ti(002) and Al(111). The thickness of the mixed layers at Ti-Al interfaces is thicker than that at Al-Ti interfaces. At the annealing temperature of 290 oC, the lateral correlation at interfaces is improved, and the surface roughness reduces. Thermal annealing studies on these multilayers show that the superlattice modulation was stable up to 320 oC. At 320 oC, the formation of amorphous Al-Ti alloy was detected by transmission electron microscopy (TEM). According to the theory of the effective heat of formation, the predicted phase is TiAl3. The modulation structure of the multilayers are destroyed, and the surface roughness increases at the annealing temperature of 380 oC. Either the gradual period shrink below 290 oC and abve 350 oC and the saturation of the period shrink between 290 oC and 350 oC observed are caused mainly by the competition between thermally induced interdiffusion and reaction at interfaces.In order to improve the thermal stability of Ti/Al multilayers, the DC and RF-magnetron sputtering were adapted to fabricate the Ti/SiO2(2.0 nm)/Al/SiO2(2.0 nm) multilayers. The experimental results reflect the worse lateral correlation and surface morphology compared with Ti/Al multilayers. This is mainly due to the amorphous state of Ti, Al and SiO2 layers and the easier interdiffusion between Ti(Al) and SiO2 layers. At the annealing temperature of 320 oC, the lateral correlation at interfaces is improved, and the surface roughness decreases. Annealed at 590 oC, the multilayers well keep the original modulation structure. At the annealing temperature of 420 oC, the period expansion tends to saturation. The facts above indicate that the inserting of SiO2 is an effective approach in improving the thermal stability of Ti/Al multilayers.
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