Ruthenium and Ruthenium-Dioxide Surface Chemistry. Herbert Over

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1 Founded 1607 Ruthenium and Ruthenium-Dioxide Surface Chemistry IEUVI Optics TWG Meeting November 10, 2005 Herbert Over Physical Chemistry Department Justus Liebig Universität Gießen TransMIT Centrum for Model Catalysis and Surface Chemistry 1

2 Challenges to be met by Ru-based Capping Layers No degradation of the optical reflectivity Blocking H 2 /O 2 diffusion Chemical Stability Thermal Stability Suppress Carbon Uptake 2

3 Main Reasons for the degradation of optical reflectivity Carbon Uptake Heterogeneous Surface Composition due to Oxidation of Ru into Ru/RuO 2 : Optical Roughness Note: Catalytic Activity of the Ru-based Capping layers may facilitate both carbon uptake and carbon removal. 3

4 Physico-chemical Properties of Ru versus RuO 2 Ru RuO 2 Hcp crystal structure Rutile crystal structure (TiO 2 ) Electronic conductivity: high Good Hydrogenation Catalyst Medium Dehydrogenation Catalyst Poor Oxidation Catalyst Electronic Conductivity: high. Not photo catalyst! Poor Hydrogenation Catalyst Medium Dehydogenation Catalyst Good Oxidation Catalyst Two Strategies: Removal of Carbon containing Molecules by REDUCTION or by OXIDATION 4

5 Grain Boundaries facilitate H/O diffusion through the capping layer 2nm Ru/RuO 2 capping layer: Ru Grain size: about 2nm, mostly oriented along the (0001) direction (S. Bajt: LLNL) Polycrystalline 2nm Ru/RuO 2 capping layer Si/Mo multilayers Overcome problem with grain boundaries: Use an amorphous RuO 2 layer (Intel: Dr. M. Fang, Dr. M. Chandhok) 5

6 Surface Chemistry: Model Systems for the Ru Capping Layer 2nm thick RuO 2 (110) single crystalline films supported on Ru(0001): Study the Complex Redox Surface Chemistry under well-defined Conditions. Supported Ru/RuO 2 Nano-particles (size: 2-3nm): Models the Ru grains in the Ru capping layer. Polycrystalline Ru/RuO 2 powder: 1000nm particle size 6

7 Chemical Stability of Ru versus RuO 2 Ru RuO 2 - Is not stable under EUVL conditions: Ru oxidizes - Uptake of Carbon: by Dehydrogenation - Formation of a single Graphite Layer possible (e.g. by alkines) - Oxide is stable under EUVL conditions - Uptake of Carbon: by Dehydrogenation and Oxidation - No single Graphite Layer is stabilized - reducible by CO,MeOH,H 2, Exposure (Note EUVL chamber is in oxidizing conditions) EUV Environment is Net-Oxidizing for Ruthenium Heterogeneous Ru/RuO 2 may be even more prone to take up carbon RuO 2 is stable under EUVL conditions 7

8 Thermal Stability of RuO 2 (110) under UHV conditions J. Phys. Chem. B 105 (2001) Below 800K: RuO 2 is thermally stable - At 1000K also the volatile (and toxic) RuO 4 and RuO 3 are leaving the Surface. 8

9 Understanding the oxidation of Ru on Atomic Scale - Oxidation of Ru(0001): active RuO 2 (110) - Oxidation of supported Ru nano-particles 9

10 Complex Oxidation Chemistry of Ruthenium Autocatalytic oxidation of Ru(0001) metal Science 297 (2002)

11 STM image of oxidized Ru(0001) 2 x 10 6 L O 2 at 650K (under UHV conditions) 1L = about 10-6 mbar x 1 second (exposure) RuO 2 (110) Ru(0001)-(1x1)O 100 nm x 100 nm, 1.21 V, 0.46 na Science 287 (2000) Heterogeneous Surface Optical roughness -Use RuO 2 directly as capping layer instead of Ru 11

12 The Rutile Structure of RuO 2 (110) Oxidic Ru Ru cus O 3f O br STM, quantitative LEED High Resolution XPS Surf. Sci.465 (2000) 1. Chem. Lett. 342 (2001) 467. Surf. Sci. 504 (2002) L196 12

13 SXRD: Oxidation of Ru(0001) under p(o 2 ) = 10-4 mbar, T=357 C (630K) H (r. l. u.) t (min) Intensity (a. u.) Intensity (a. u.) (0.73, 0, L) p(o 2 +CO)= C p(co):p(o 2 )=1: L (r. l. u.) Self-limiting Growth of RuO 2 (110) film on Ru(0001): Thickness = 1.6 nm. Depth control of the oxidation process Y. B. He, M. Knapp, E. Lundgren, H. Over: JPC in press 13

14 Oxidation of Ru by O 2 exposure Single crystalline Ru(0001): T>530K; p(o 2 ) = mbar DRIFTS: Infrared Spectroscopy 100 Ru/SiO 2 CO / O 2 = 0.5 CO:2073cm cm cm -1 Ru Nano-particles (2-3nm): T > room temperature; P(O 2 ) = mbar DRIFTS: 2073 cm -1 of CO characteristic for RuO 2 Reflection / % CO g Assmann et al. JPC B 108 (2004) Wavenumber / cm -1 Supported Ru catalyst: P(O 2 )=2P(CO)=10mbar 14

15 Understanding the Reduction of RuO 2 - Reduction of Ru Nano particles by H 2 - Reduction of single-crystalline RuO 2 (110) by H 2 15

16 Reduction of Ru/RuO 2 Nano Particles by H 2 H 2 consumption and CH 4 formation / mol s K 410 K 380 K 410 K 10-7 RuO 2 Ru/SiO 2 CH 4 * 4 Ru/MgO Temperature Programmed Reduction: RuO 2 particles reduce completely to Ru particles beyond 400 K, the desorption temperature of water Temperature / K J. Phys. Chem. B 108 (2004)

17 Reduction of 1.6nm thick RuO 2 (110) by H 2 : In-Situ SXRD (a) Intensity (arb. units) P(H 2 )= 10-5 mbar, T=415K Integral inten. (arb. units) Exposure time (min) (0.872, 0.872, 1.3) 0 min 69 min H+K-scan Complete reduction of the RuO 2 (110) film after 50 minutes Open problem: Hydrogen may diffuse through the Ru capping layer 17

18 Interaction of Methanol and Water with RuO 2 - Oxidation of Methanol: over RuO 2 (110) - Oxidation of Methanol over polycrystalline RuO 2 - Passivation of RuO 2 (110) by water 18

19 Partial Methanol Oxidation: Introduction - Attention: Formaldehyde may polymerize on the surface. - On RuO 2 : Dehydrogenation forms water rather than H 2 RuO 2 becomes reduced. 19

20 Oxidation of Methanol over RuO E E E E E E-07 a. Methanol/RuO 2 (110) CO 2 x 1 m/e 30 x 10 m/e 31 x b. Methanol/RuO 2 powder m/e 30 x 25 m/e 44 x 1/2 m/e 31 x CH 2 O 1500 CH 3 OH E E E E E-06 CH 2 O 8.00E E-07 CH 3 OH 4.00E E-07 Total Oxidation requires temperatures of 500K even on polycrystalline RuO 2 powder. Formaldehyde CH 2 O may be bad since it can polymerize on the surface! 0.00E E Temperature (K) Temperature (K) J. Catal. 202 (2001)

21 Exposure of 0.1L corresponds to P(MeOH)=10-7mbar for 10s Alcohols interact strongly with RuO2: Without oxygen: Methanol reduces the RuO2(110) surface 21

22 Interaction of water with RuO 2 (110) Interaction with Ru Interaction with RuO 2 - Water: weak (250K) -Hydrogen: medium (300K) Ted Madey - Water: strong (420K) - Hydrogen: strong (600K) -H 2 O: able to passivate RuO 2 (110) Dissociative adsorption of Water on RuO 2 (110). Passivation: The bridging O atoms from O br -H and the cus-ru atom are capped by O ot -H. control chemical activity J. Am. Chem. Soc. 127 (2005)

23 Carbon Uptake: Ru versus RuO 2 Interaction with Ru Interaction with RuO 2 - Alkanes: weak (100K) - Alkanes: weak (100K) - Alkenes, Alkines: strong (400K) -Alkenes (320K, desorbs intact), Alkines: medium -Alcohols: medium - Conjugated double bonds: medium (Benzene: 300K) - Carboxyl Groups: medium -Carbonyl groups: medium - Alcohols: strong (400K) - Conjugated double bonds: weak -Carboxyl groups: medium - Carbonyl groups: (300K) 23

24 Carbon Removal: Ru versus RuO 2 Ru RuO 2 Hydrogenation of hydrocarbons, in particular alkenes and alkines Oxidation of hydrocarbons Complete Oxidation of hydrocarbons is quite universal, producing CO 2 and water. Hydrogenation is mild way but might have a limitation. Oxidation of hydrocarbons can form reactive intermediates which are even more difficult to remove than the original hydrocarbons. 24

25 Summary Ru is not stable under EUVL conditions RuO 2 is stable under EUVL conditions Ru: up-take/removal of hydrocarbon by dehydrogenation/hydrogenation make EUVL environment reducing by adding H 2. RuO 2 : up-take/removal of hydrocarbons by dehydrogenation/total oxidation make EUVL environment even more oxidizing by adding O 2. Water adsorption passivates RuO 2 : good for preventing carbon uptake but bad for removal of hydrocarbons. Worst situation: heterogeneous Ru/RuO 2 surface optical roughness and enhanced carbon uptake. 25

26 Acknowledgements - Intel Coop.: Dr. M. Fang, Dr. M. Chandhok - EU: NanO2: Oxidation of Nano Materials - DFG: Priority program Bridging Pressure and Materials gap Dr. G. Mellau, A. Farkas, M.Knapp, D. Crihan, Dr. Y.B. He: JLU Giessen we expect that insights gained from selected surface science studies will allow engineers to find suitable solutions to minimize mirror degradation in working EUVL systems. Ted Madey 26

27 Oxidation of 100nm thick Ru films and Ru particles p(o 2 ) = 10-4 mbar, T= 645 K Scanning Electron Microscopy: - Preparation Pulse Laser Deposition - Ru-film on MgO(100): 100nm thick - Grain size in film: 20-40nm - Ru particles: 500nm 3000nm - Grains in Ru particles: nm 27

28 Oxidation of 100nm Ru film and Ru particles Oxidation of Ru films and particles p(o 2 )= p(o -1 mbar, T=357 C 2 ) = 10-4 mbar, T= 645 K Ru 3d Ru (oxide) Oxi. Film Oxi. Particle Ru (metal) O 1s Ru 3d 5/2 intensity (a. u.) Clean Film Clean Particle Ru 3d 5/2 intensity (a. u.) Ru (oxide) Ru (metal) Binding energy (ev) Binding energy (ev) Scanning PhotoElectron Microscopy (Synchrotron Trieste): Ru film more oxidized than Ru particle due to grain boundaries 28

29 Reduction of RuO2(110) by COHigh-Resolution exposure XPS - The surface roughens considerably - Below the metallic Ru film there is still RuO2 Surf. Sci. 515 (2002)