Applied Surface Science 288 (2014) 673–676 Contents lists available at ScienceDirect Applied Surface Science j ourna l ho me page: www.elsevier.com/locate/apsusc Decomposition of SnH 4 molecules on metal and metal–oxide surfaces D. Ugur a,b , A.J. Storm a , R. Verberk a , J.C. Brouwer b , W.G. Sloof b,∗ a TNO, Stieltjesweg 1, 2628 CK Delft, The Netherlands b Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands a r t i c l e i n f o Article history: Received 23 July 2013 Received in revised form 13 October 2013 Accepted 14 October 2013 Available online 24 October 2013 a b s t r a c t Atomic hydrogen cleaning is a promising method for EUV lithography systems, to recover from sur- face oxidation and to remove carbon and tin contaminants. Earlier studies showed, however, that tin may redeposit on nearby surfaces due to SnH 4 decomposition. This phenomenon of SnH 4 decomposi- tion during tin cleaning has been quantified for various metallic and metal-oxide surfaces using X-ray photoelectron spectroscopy (XPS). It was observed that the metal oxide surfaces (TiO 2 and ZrO 2 ) were significantly less contaminated than metallic surfaces. Tin contamination due to SnH 4 decomposition can thus be reduced or even mitigated by application of a suitable metal-oxide coating. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Extreme ultraviolet lithography (EUVL), employing a wave- length of 13.5 nm, enables sub 30 nm feature sizes to be written on semiconductor chips. However, a thin contamination or an oxide film on the optics significantly reduces the reflectivity [1], which should be removed for the optimal operation of the instrument. A thin capping layer can be applied to protect the multilayer mir- ror against oxidation, provided that it exhibits a very low extinction coefficient in the EUV domain [2,3]. Ru, Rh, TiO 2 and ZrO 2 are among the strongest candidates to be used as a capping layer [2]. Never- theless, contamination of capping layers is still possible [1] and an active strategy to mitigate oxidation is required. Exposure of the mirror to molecular [4] or atomic hydrogen [5–10] is a prospective method to remove carbon and oxide contaminants. A dense plasma of Sn can be used to generate the EUV radiation with a wavelength of 13.5 nm [11]. However, Sn debris from the EUV source can deposit on surfaces within the vacuum chamber [11–13], including the collector mirror near the Sn plasma [14]. This metallic contamination can be removed by atomic hydrogen, which creates volatile metal-hydrides [12–16]. However, the cleaning rate can decrease significantly for thin Sn layers due to the re-deposition of Sn on the mirror surface [15]. Moreover, the volatile reaction product SnH 4 may decompose elsewhere in the system, leading to subsequent Sn contamination. Although the Sn contamination phenomenon may be critical for EUV optics lifetime, there is a lack of knowledge on the decom- position behavior of SnH 4 at different surfaces [15–18]. Hence, in this study, various metal surfaces, viz. Ni (1 1 1), Ru (0 0 0 1), ∗ Corresponding author. E-mail address: w.g.sloof@tudelft.nl (W.G. Sloof). polycrystalline Ru film, Rh (1 1 1), Au (1 1 1), and metal-oxide sur- faces, viz. TiO 2 and ZrO 2 films of Si wafer, were simultaneously exposed to a known flux of SnH 4 to study their catalytic effect on the metal-hydride decomposition. The total Sn contamination at these surfaces was evaluated with XPS. In the following sections, first the experimental details of this research are discussed. Next, the results of this research will be presented and subsequently the differences between the exposed materials will be addressed. 2. Experimental A quartz crystal with a fundamental resonance frequency of 6 MHz (Inficon) is used to quantify the generation rate of the SnH 4 species (henceforth denoted as the source crystal). The quartz crys- tal with a diameter of 12 mm is AT cut [19] to provide stability in frequency against temperature fluctuations. The source crystal was coated by Ar + sputtering (Leica EM QSG 100) with a 450 nm thick Sn film. The top surface of the Sn film was cleaned by a differentially pumped Ar + ion gun (PHI model 04-303) in a UHV chamber, before the source crystal was mounted in another UHV chamber, directly facing the atomic hydrogen gun (Veeco Atomic Hydrogen Source); see Fig. 1. The total time that the sputter cleaned source crystal was exposed to ambient conditions was less than 15 min. The surface of the source crystal was etched by the hydrogen radicals [16] forming volatile SnH 4 (stannane). This etching reac- tion is exothermic and proceeds spontaneously: Sn + 4H → SnH 4 G o = -625.5 kJ mol -1 (1) The source crystal was maintained at 60 ◦ C, because at that tem- perature the microbalance was found to be the most stable against temperature variations. Further details regarding the quartz crystal 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.096