The development of high-reflection mirrors with amorphous metal-oxide multilayers in the
"water-window"(λ=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° 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°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.
A novel TiO2/ZnO multilayer deposited by atomic layer epitaxy technique has been fabricated to achieve a high
reflective mirror and an attosecond chirped mirror in soft-x-ray "water-window" (λ=2.332-4.368 nm) wavelengths
region. The technique in this study is able to satisfy reguirements for atomic layer control through epitaxial growth using
sequential surface reaction and self-limiting nature. In preliminary experimental studies, both rutile TiO2 (200) and
wurtzite ZnO (0001) thin films were grown epitaxially on the same sapphire (0001) substrates at 450°C and moreover a
high reflectivity of 29.8% was obtained at around 2.734 nm and a grazing angle of 2θ=10°.
The authors conducted the ALE experiment of TiO2/ZnO multilayer using a ZnO buffer layer. As a result, the multilayer
using a buffer layer was able to be grown epitaxially on not only sapphire (0001) but also Si (100). In addition,
reflectivity of multilayer remained to be 24.6% even on Si (100) in contrast with that about 27.5% on sapphire (0001)
at grazing angle of 2θ
= 8°. Thus, the ZnO buffer layer becomes the key layer to fabricate the TiO2/ZnO multilayer on
various substrates.
In the presentation, ALE of TiO2/ZnO multilayer mirrors using buffer layer will be shown in detail.
Novel metal-oxide multilayer mirrors for water-window wavelengths have been already studied and then
fabricated by atomic layer deposition (ALD) or atomic layer epitaxy (ALE) methods which have the
self-limiting nature of the surface reactions and can control thickness on an atomic scale over large areas.
The reason why metal-oxide multilayer mirrors are effective in the water-window wavelength is that they
can prevent the formation of various alloys at the interface resulting in scattering loss, and the absorption
of oxygen in oxides is negligible at the wavelength.
In this study, high and low refractive materials were chosen to be TiO2 and Al2O3 respectively, because
they can be fabricated by ALD or ALE methods and Ti L-absorption edge is located at 2.73nm. We
investigated the atomic-scale growth of these films and then found that the growth rates could be constant.
Moreover, Al2O3/TiO2 multilayer mirrors were fabricated by the ALE method. As a result, the soft x-ray
reflectivity of the 10-bilayer mirror was 1.54%, approximately.
Atomic layer epitaxy of zizc oxide (ZnO) and titanium dioxide (TiO2) have been applied to compose
superlattice film devices such as soft x-ray multilayers. The reason why oxide films are chosen, is that
oxygen of oxide films has transparency for the "water-window" (λ=2.332-4.368 nm) wavelengths.
Actually, we researched the fabrication of TiO2/ZnO multilayer mirrors and then found that these
multilayer films provided high reflection in the wavelength region. Our theoretical calculation indicated
that multilayer mirror could have the high reflectance of nearly 50% at the wavelength of 2.73 nm and at
the incidence angle of 18.2° from the normal incidence.
TiO2/ZnO films were grown on the c-plane (0001) sapphire substrate by means of atomic layer epitaxy
(ALE) technique, which involved the alternate reactions of Zn(CH2CH3)2 (DEZ) and H2O, and Ti(Cl)4
(TCT) and H2O.
Our experimental results indicated that thin Wurtzite ZnO (0001) and Rutile TiO2(200) films were
grown epitaxially on c-plane (0001) sapphire substrates at 450°C with self-limiting mechanism. The
10-bilayer TiO2/ZnO multilayer indicated high soft X-ray reflectivity of around 30%.
The development of high-reflection multilayer mirrors in the "water window" (λ=2.3-4.4nm) is desired
for attosecond soft x-ray optics. TiO2/ZnO multilayer mirrors were proposed in this study as highly
reflective coatings for the water window wavelength region. The theoretical calculation on the layer
combination indicated that the high reflectivity of approximately 50% at 2.73 nm was obtainable at the
incidence angle of 18.2° from the normal incidence.
The ZnO and TiO2 thin films were grown using atomic layer epitaxy (ALE) methods at 450°C.
Experimental results indicated that both the crystalline rutile TiO2 (200) and wurtzite ZnO (0001) thin
films both were grown epitaxially on Al2O3 (0001) substrates by ALE. Moreover, a 10-bilayer TiO2/ZnO
multilayer showed the soft X-ray reflectivity of around 10%.
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