Kinetics of Reduction of a RuO 2 (110) Film on Ru(0001) by H 2 D. Ugur,* ,†,‡ A. J. Storm, † R. Verberk, † J. C. Brouwer, ‡ and W. G. Sloof ‡ † TNO, Stieltjesweg 1, 2628 CK, Delft, The Netherlands ‡ Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands * S Supporting Information ABSTRACT: The kinetics and mechanism of the full reduction of a thin RuO 2 film with a stoichiometric (110) surface on Ru(0001) by H 2 has been studied in the temperature range of 100 to 400 °C at 10 -2 to 10 -4 Pa. The reduction kinetics is dominated by the creation of oxygen vacancies and their annihilation upon transformation of RuO 2 into metallic Ru. The temperature-dependent reduction rate increases linearly with H 2 pressure. In the temperature range of 100 up to 200 °C, initially hydrogenation of the RuO 2 (110) surface occurs. Next, oxygen vacancies are created due to desorption of water vapor, which accelerates the reduction by place exchange of oxygen bulk atoms with an activation energy of 0.45 eV. In the temperature range of 200 to 300 °C, slow reduction of RuO 2 by H 2 already occurs in the initial period with an activation energy of 0.48 eV and is followed by faster reduction. In the temperature range of 300 to 400 °C, the reduction of RuO 2 starts immediately when exposed to H 2 and the activation energy (0.48 eV) is similar to the activation energy in the lower temperature range (100 to 200 °C). Apparently, the annihilation of oxygen vacancies during reduction is more prominent with increasing temperature. 1. INTRODUCTION Because RuO 2 is an excellent and versatile oxidation catalyst, its interaction with many molecules has received much attention. 1 RuO 2 is a promising catalyst for low-temperature dehydrogen- ation of small molecules such as: NH 3 , HCl, methanol, and other hydrocarbons. While the interaction of these molecules with RuO 2 involves the release of hydrogen onto its surface, understanding of the reduction mechanism and kinetics of RuO 2 by hydrogen is of fundamental importance. The kinetics of reduction of RuO 2 with H 2 is studied here in the context of cleaning a ruthenium capping layer on top of mirrors for extreme ultraviolet lithography (EUVL). 2-4 This ruthenium-capping layer serves as a protection of the Mo/Si thin film multilayer mirror, which is tailored for maximum reflectivity of EUV light. 5 Today’s technological requirements demand smaller feature sizes to be written on the semiconductor chips, and thus it is necessary to go beyond the resolution of ArF lasers. EUVL, employing a wavelength of 13.5 nm, can meet those size requirements. 6 The lifetime specification of EUV optics in the lithographic exposure systems is 30 000 h. 7 The main challenge here is to prevent carbon contamination and oxidation of the optical surfaces. 6-9 Ruthenium is a promising material as a protective capping layer 10 because it provides oxidation resistance and has a very low extinction coefficient in the EUV domain. 10,11 Oxidation of ruthenium-capped mirrors is still possible, though, 6 and an active strategy to mitigate oxidation is thus required. Reversibility of ruthenium oxidation is possible with chemical reduction agents such as carbon monoxide, 12-14 molecular hydrogen, 2-4 and atomic hydrogen. 9,15,16 Chemical reduction with molecular hydrogen is advantageous due to its simplicity in implementation, 17 prevention of the further contamination of the optical system, and reliability in the case of over- exposure. 18,19 Although reduction of ruthenium oxide layers with molecular hydrogen has been previously reported, knowledge about the kinetics of the reduction process is scarce. The explanation of the reduction mechanism is limited to the initial stages of the reaction. 17-22 The reduction mechanism and kinetics of following reduction stages has received minor attention and is the main topic of this study. As a model system for the Ru-capping layer, a closed-packed Ru(0001) surface of single crystal was taken. The reduction kinetics of oxidized Ru(0001) by molecular hydrogen was studied in the temperature range of 100 to 400 °C and molecular hydrogen pressures in the range of 10 -4 to 10 -2 Pa. The oxidation of the Ru(0001) surface resulted in a thin RuO 2 layer with its (110) plane of the rutile crystal structure parallel to the surface. 23,24 Because the surface energy of the RuO 2 (110) is the lowest among the primary surfaces, 17 it is anticipated that this orientation will be most abundant in a polycrystalline RuO 2 film, which may be present on top of a Ru capping layer. Spectroscopic ellipsometry, thermal desorption spectroscopy (TDS), and X-ray photoelectron spectroscopy (XPS) techniques were used to study the mechanism and kinetics of reduction. First, the experimental details will be described. Next, the results will be presented and the reduction mechanism and kinetics will be discussed. Received: October 7, 2012 Revised: November 13, 2012 Published: November 26, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 26822 dx.doi.org/10.1021/jp309905z | J. Phys. Chem. C 2012, 116, 26822-26828