Hard X-ray HfO2/Al2O3 multilayers fabricated by atomic layer deposition

Multilayer is a widely used X-ray optics that can be designed and optimized for specific X-ray application. Atomic layer deposition (ALD) has unique advantage in coating homogeneous ultra-thin film on both flat and curved substrates, which shows great potential in the fabrication of X-ray multilayer with small period. In this paper, hard X-ray HfO2/Al2O3 multilayers on Si substrate prepared by ALD were investigated. The X-ray (0.154 nm) reflectivity of HfO2/Al2O3 multilayer with 3.44 nm period, 60 bilayers is 24 %. Atomic force microscope results show that surface roughness of HfO2/Al2O3 multilayer with total thickness more than 200 nm is slightly larger than that of the substrate. High resolution transmission electron microscope images reveal that amorphous HfO2/ Al2O3 material pair forms multilayer with sharp interfaces even for 1.69 nm period. Further study indicates that lowering substrate roughness can reduce the deviation between actual and theoretical X-ray reflectivity. These results demonstrate that for multilayer-based hard X-ray optics, ALD-grown HfO2/Al2O3 is a promising candidate.