Atomic layer deposition of amorphous TiO2/ZnO multilayers for soft x-ray coherent optics

The development of high-reflection mirrors with amorphous metal-oxide multilayers in the "water-window"(lambda = 2.332nm-4.368nm) is desired for soft x-ray coherent optics. One of the authors has already studied and fabricated amorphous Al2O3/TiO2 multilayer for the "water-window" wavelengths by controlled growth with atomic layer deposition (ALD), and then acquired the reflectance of 33.4 % at 2.73nm and at the incidence angle of 18.2 degrees from the normal incidence. In this study, we proposed Al2O3/TiO2/Al2O3/ZnO multilayer mirrors. Al2O3 layers grown as amorphous layers were inserted between TiO2 and ZnO layers. The Al2O3, ZnO and TiO2 thin films were grown on Al2O3 (0001) substrate by controlled growth with atomic layer deposition (ALD) methods at 450 degrees C. Experimental results indicated that the growth of crystalline rutile TiO2 (100) and wurtzite ZnO (0001) were prevented. Thus, inserting amorphous Al2O3 layers, the results indicated that the crystalline growth was prevented. Moreover, we succeeded fabrication of amorphous TiO2/ZnO mirrors by ALD.