Abstract
Lanthanum and lanthanum nitride thin films were deposited by magnetron sputtering onto silicon wafers covered by natural oxide. In situ and real-time synchrotron radiation experiments during deposition reveal that lanthanum crystallizes in the face-centred cubic bulk phase. Lanthanum nitride, however, does not form the expected NaCl structure but crystallizes in the theoretically predicted metastable wurtzite and zincblende phases, whereas post-growth nitridation results in zincblende LaN. During deposition of the initial 2-3 nm, amorphous or disordered films with very small crystallites form, while the surface becomes smoother. At larger thicknesses, the La and LaN crystallites are preferentially oriented with the close-packed lattice planes parallel to the substrate surface. For LaN, the onset of texture formation coincides with a sudden increase in roughness. For La, the smoothing process continues even during crystal formation, up to a thickness of about 6 nm. This different growth behaviour is probably related to the lower mobility of the nitride compared with the metal. It is likely that the characteristic void structure of nitride thin films, and the similarity between the crystal structures of wurtzite LaN and La2O3, evoke the different degradation behaviours of La/B and LaN/B multilayer mirrors for off-normal incidence at 6.x nm wavelength.
Highlights
Multilayer (ML) mirrors in the soft X-ray and extreme ultraviolet (EUV) regimes are employed for various applications, including EUV telescopes for space research (Culhane et al, 2007; Lemen et al, 2012), beam-transport systems and focusing optics for free-electron lasers (Corso et al, 2012; Nelson et al, 2009), EUV lithography (Louis et al, 2011) and total-reflection X-ray fluorescence analysis (TXRF) (Tiwari et al, 2010).Lanthanum and boron are very promising candidates for mirror applications focusing on wavelengths of around 6.x nm (Hawryluk & Ceglio, 1993; Makhotkin et al, 2012)
To verify the chemical composition and bond structure, X-ray photoelectron spectroscopy (XPS) measurements were performed on thin films deposited under non-reactive and reactive sputter conditions
With the exception of the nitrogen peak, the same features are observed for non-reactive sputter deposition of lanthanum
Summary
Multilayer (ML) mirrors in the soft X-ray and extreme ultraviolet (EUV) regimes are employed for various applications, including EUV telescopes for space research (Culhane et al, 2007; Lemen et al, 2012), beam-transport systems and focusing optics for free-electron lasers (Corso et al, 2012; Nelson et al, 2009), EUV lithography (Louis et al, 2011) and total-reflection X-ray fluorescence analysis (TXRF) (Tiwari et al, 2010). A boron cap thicker than about 2–3 nm is expected to protect La/B and LaN/B multilayers from oxidation This approach works for normal-incidence mirrors (ML periods of around 3.5 nm) but not for grazing-incidence mirrors with larger thickness: while La/B structures with an ML period of 15 nm appeared to be stable to storage for at least six months, LaN/B multilayers with the same period showed strong surface degradation after one week of storage in air (Kuznetsov et al, 2016b). The phase formation and texture development could be determined and related to the simultaneously detected film thickness and roughness This detailed understanding of structure formation during La and LaN thin-film deposition is required for a deeper insight into the structural details and long-term stability of 6.x nm multilayer stacks, for both normal and grazing incidence
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
