Belonging to the PhD thesis of Deniz Uğur
Molecular contamination phenomena in EUVL and mitigation methods with hydrogen
1. In cleaning oxidic contamination on a ruthenium surface, atomic hydrogen is more effective than molecular hydrogen. (This thesis, Chapters 2 and 3)
2. Part of a RuO2 film will be buried below the metallic Ru surface after reduction
above 200 °C with hydrogen. (This thesis, Chapters 2 and 3)
3. Despite the decrease in the dissociation efficiency, increasing the hydrogen flow through a thermal H2 cracking gun enhances the atomic hydrogen flux. (This
thesis, Chapter 4)
4. Regarding the resistance against contamination due to decomposition of metal hydrides, metal-oxide capping layers on EUV mirrors outperform pure metallic capping layers. (This thesis, Chapter 6)
5. It is harder to manage your PhD project than to conduct the research itself.
6. Carl Sagan was wrong in his saying* “The beauty of a living thing is not the atoms that go into it, but the way those atoms are put together”. Beauty is just a vision in the observer’s mind, and it is only determined by the way the observer’s neurons are put together.
*
From “Cosmos: A personal voyage”, Episode 5.
7. It is hard –even for Paul, the German oracle octopus– to predict the next move of a Dutch biker on sunny days.
8. Those who are eligible for the 30 % tax rule in the Netherlands, are exempt from taking a driving test, since their eligibility for the 30 % rule is assumed to certify their driving skills. This assessment might as well be based on one’s astrological sign.
9. Nerds and supercomputers are rather alike; they perform remarkably well in computational tasks but lack the common sense of a six year-old.
10.It is ironic to note that there is no designated section for a personal philosophical note in a thesis submitted for the degree of Doctor of Philosophy.
These propositions are regarded as opposable and defendable, and have been approved as such by the supervisors prof. dr. B. J. Thijsse and dr. ir. W. G. Sloof.
Behorende bij het proefschrift van Deniz Uğur
Molecular contamination phenomena in EUVL and mitigation methods with hydrogen
1. Atomaire waterstof is effectiever dan moleculaire waterstof voor het verwijderen van oxide op een ruthenium oppervlak. (Dit proefschrift, hoofdstukken 2 en 3) 2. Een deel van de RuO2 film blijft na reductie boven 200 °C met waterstof achter
onder het metallische oppervlak. (Dit proefschrift, hoofdstukken 2 en 3)
3. De atomair waterstof flux na thermische dissociatie neemt toe met toenemende moleculaire waterstofstroom, ondanks dat de efficiëntie van de dissociatie afneemt. (Dit proefschrift, hoofdstuk 4)
4. De mate van contaminatie door dissociatie van metaalhydriden van de toplaag van EUV spiegels is geringer als deze toplaag niet metallisch is maar een oxide. (Dit proefschrift, hoofdstuk 6)
5. Het is moeilijker om je PhD project te managen, dan om het onderzoek uit te voeren.
6. Carl Sagan was verkeerd in zijn zeggen "De schoonheid van een levend wezen komt niet door de atomen waaruit zij bestaat, maar de manier waarop de atomen gerangschikt zijn". Schoonheid is slechts de visie van de waarnemer, en dat wordt alleen bepaald door de manier waarop de neuronen van de waarnemer zijn gerangschikt.
*
Van “Cosmos: A personal voyage”, Aflevering 5.
7. Het is moeilijk, zelfs voor Paul (de Duitse orakel octopus), om de volgende stap van een Nederlandse fietser op zonnige dagen te voorspellen.
8. Degenen die in aanmerking komen voor de 30% belastingregel in Nederland, zijn vrijgesteld van het afleggen van het rijexamen, omdat wordt verondersteld dat zij beschikken over voldoende rijvaardigheid als zij in aanmerking komen voor de 30% regel. Deze vaststelling kan net zo goed worden gebaseerd op hun astrologisch sterrenbeelden.
9. Nerds en supercomputers komen met elkaar overeen; ze presteren opvallend goed bij rekentaken, maar missen het gezonde verstand van iemand die zes jaar oud is. 10.Het is ironisch om vast te stellen dat er in een dissertatie, voor het verkrijgen van
de graad van doctor in de filosofie, geen plaats is voor een persoonlijke filosofische beschouwing.
Deze stellingen worden opponeerbaar en verdedigbaar geacht en zijn als zodanig goedgekeurd door de promotoren prof. dr. B. J. Thijsse en dr. ir. W. G. Sloof.
Molecular contamination phenomena in
EUVL and mitigation methods with
hydrogen
Front cover art: “Window cleaners with buckets and ladders”, by Stephen Finn, Shutterstock Back cover art: “Industrial Cleaning Services”, by Leremy, Shutterstock
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op woensdag 19 december 2012 om 10:00 uur
door
Deniz
UĞUR
Master of Science in Mechanical Engineering, Boğaziçi Üniversitesi (Turkije)
Copromotor: Dr. ir. W.G. Sloof
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. dr. B.J. Thijsse, Technische Universiteit Delft, promotor Dr. ir. W.G. Sloof, Technische Universiteit Delft, copromotor Prof. dr. H. Terryn, Vrije Universiteit Brussel, België
Prof. dr. G.C.A.M. Janssen, Technische Universiteit Delft Prof. dr. E. Neyts, Universiteit Antwerpen, België
Prof. dr. F. Bijkerk, Universiteit Twente
Dr. D. Ehm, Carl Zeiss SMT GmbH, Duitsland
This research was supported by the FP7-PEOPLE program of Marie Curie Initial Training Networks (ITN), under the project name “Surface Physics for Advanced Manufacturing” (S.P.A.M.), within the grant no: 215723.
ISBN: 9789461915504
Copyright © 2012 by D. Uğur
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior written permission of the copyright owner.
Çocukken kucağıma büyük bir lahana verip
yapraklarını teker teker açmamı sabırla izleyen anneme ve babama,
To mom and dad, who gave me a big cabbage when I was a kid, and waited patiently for me to peel each layer, one by one,
i
Contents
1. Introduction 3
1.1.Historical background 3
1.2.Extreme Ultra-Violet Lithography (EUVL) 4
1.3.Scope of this thesis 7
1.4.Outline of the thesis 8
References 10
2. Kinetics of reduction of a RuO (110) film on Ru(0001) by 2 H 2 15
2.1.Introduction 16
2.2.Experimental 17
2.3.Results and discussions 20
2.3.1. Oxidation of the Ru(0001) surface 20
2.3.2. Interaction of H with RuO2 2(110) 20
2.3.3. Kinetics of RuO reduction by 2 H 2 25
2.3.4. Effect of H pressure 2 32
2.4.Conclusions 33
Acknowledgments 34
References 35
3. Kinetics of reduction of a RuO2(110) film on Ru(0001) by atomic hydrogen a41 3.1.Introduction 42
3.2.Materials and Methods 42
ii
3.2.2. Oxidation of the Ru(0001) surface 44
3.2.3. Interaction of the H radicals with RuO2(110) 44
3.2.4. Stability of the RuO2 film 47
3.3.Results and discussions 47
3.3.1. Mechanism of RuO2 reduction 47
3.3.2. Kinetics of RuO2 reduction 51
3.4.Conclusions 54
Acknowledgments 55
References 56
4. Atomic hydrogen sensor and quantification of the H radical flux 61
4.1.Introduction 62
4.2.Theory 63
4.3.Experimental 66
4.4.Results and Discussions 68
4.4.1. Elements of the heat balance 68
4.4.1.1. Radiative and conductive heat losses 68
4.4.1.2. Heat losses by convection in the gas 69
4.4.1.3. Radiative heat input 70
4.4.1.4. Recombinative heat input 73
4.4.1.5. Quantification of the atomic hydrogen flux 74
4.4.2. Computational Fluid Dynamics (CFD) simulations 76
4.4.3. Recursive dissociation phenomena at the gun 82
4.5.Conclusions 83
Acknowledgments 84
Appendix: Resolving the radiative viewing factors from the experimental data 85
References 86
5. Generation and decomposition of volatile SnH4 monitored by in situ QCM 91
5.1.Introduction 92
iii
5.3.QCM theory 94
5.4.Results and Discussions 95
5.4.1. Generation of the SnH molecules 4 95
5.4.2. Transport of the SnH molecules 4 98
5.4.3. Decomposition of the SnH molecules 4 99
5.5.Conclusions 101
Acknowledgments 101
Appendix: Method to quantify the Sn atoms through the PZL photoelectron
intensities of a Sn reference and the target crystal 102
References 103
6. Decomposition of SnH4 molecules on metal and metal oxide surfaces 107
6.1.Introduction 108
6.2.Experimental 109
6.3.Results and Discussions 111
6.3.1. Generation of the SnH4 molecules 111
6.3.2. Decomposition of the SnH4 molecules 114
6.4.Conclusions 116
Acknowledgments 117
References 118
7. Conclusions and the Future Outlook 123
7.1.What is at hand 123
7.1.1. Oxidic contamination phenomena 123
7.1.2. Radical flux considerations 124
7.1.3. Metal contamination phenomena 125
7.2.Short term lithography prospects 126
7.3.The ultimate dream 127
iv
Summary 133
Samenvatting 139
Acknowledgments 145
About the author 149
List of Publications 151
3
1.
Introduction
1.1.
Historical background
When the world’s first electronic general-purpose computer, known as Electronic Numerical Integrator And Computer (ENIAC) was developed in 1946, the concept of a computer was rather “lofty” with the size of a room, weighing 27 tons and consuming 175 kW of power during operation0F
1
. Similarly, the first supercomputer (Cray 1), presented to the world in 1976, had only 8 megabytes of main memory and was installed in Los Alamos National Laboratory for 9 million dollars1F
2
. Nowadays it is customary for a mobile phone to have 64 gigabytes of built-in memory2 F
3
, and two terabytes of information can be rapidly stored in a 3 x 1.5 x 0.3 cm size gadget3F
4
. We are at the verge of decreasing a personal computer merely to a flexible sheet of organic light-emitting diode (OLED) and the key element lying behind that progress is the technology roadmap for semiconductors laid by Gordon Moore in 19654F
5
.
Moore’s law –though scientifically speaking is not actually a law– states that the number of transistors that can be incorporated on an integrated circuit will increase exponentially over time. This would result in a reduction of the manufacturing costs, in turn enabling the production of more complex circuits on a single semiconductor substrate as stipulated in the International Technology Roadmap for Semiconductors5F
6
. Since the introduction of the Moore’s law, the number of components per chip has doubled every two years (see Figure 1.1) and this was only realized by a continuous decrease in the critical dimension (i.e., the half pitch6) of the integrated circuits.
The limit in the resolution (R) of a lithography system, that is the smallest feature size that the system can print, is determined by the Rayleigh criterion:
A
R k
N
λ
= (1.1)
Here k is the instrumental factor, NA is the numerical aperture and λ is the wavelength of the light that is used to write the features6F
7
. In order to increase the resolution of the current systems, either the k factor needs to be decreased (where the physical limit is 0.257) or the NA needs to be increased. The latter of these methods is exploited in water immersion lithography (the highest achievable NA is 1.357F
8
4
Figure 1.1: Exponential increase in the number of transistors incorporated on a microprocessor as proposed by Moore’s law (after Ref. 8F9)
Nevertheless a significant leap in the improvement of the resolution can only be achieved with a decrease in the wavelength of the incident light. This is the main motivation behind the design of extreme ultra violet lithography (EUVL) systems.
1.2.
Extreme Ultra-Violet Lithography (EUVL)
EUVL, employing a wavelength of 13.5 nm, leads to a new era with sub 30 nm resolution on semiconductor chips. Some fundamental steps of the EUVL printing process are illustrated in Figure 1.2. The main ingredient of the semiconductor industry is silicon, found abundantly as sand in the earth’s crust9F
10
[Figure 1.2 (a)]. It is first purified until electronic grade silicon, then melted and cast into a single crystal ingot from which the individual Si wafers are cut [Figure 1.2 (b)]. Next the wafer is coated with a photoresist and exposed to EUV light through a mask
5 [Figure 1.2 (c)]. Upon EUV exposure, the masked areas remain intact, whereas the exposed areas of the photoresist melt away. The unit shown in Figure 1.2 (c) is the base element of a transistor, of which about 30 million could fit on the head of a pin10. After cleaning the molten photoresist, an etchant is applied to the surface [Figure 1.2 (d)] to produce grooves in the Si bulk [Figure 1.2 (e)]. Through several sequences of photoresist application, EUV exposure, etching, ion implantation, electroplating and polishing (not shown here), finally a transistor is made [Figure 1.2 (f)]. The process continues with laying down numerous metal layers to provide the interconnection between the transistors [Figure 1.2 (g)], which leads to the formation of the microprocessor structure eventually.
Since all matter absorbs EUV radiation, the imaging process has to be conducted in vacuum and it is no longer possible to employ refractory optics to focus the beam10F
11
. Instead, defect free Mo/Si thin film multilayer mirrors are used as collimator optics, which are tailored for maximum reflectivity (70 %) at the 13.5 nm wavelength11F
12
. The multilayer structure consists of alternating layers of high mass (Mo) and low mass (Si) materials. Each bilayer has a thickness equal to half the wavelength of light to be reflected12F
13
. Constructive interference between the scattered light from each of these bilayers will allow the mirror to reflect the EUV light at 13.5 nm. An EUVL mirror is composed of about 50 Mo/Si bilayer stacks13F
14
(see Figure 1.3), where a thin protective layer (1-2 nm) is added on top to provide resistance against oxidation14F
15,
15F
16,
16F
17
. An ideal capping layer (i.e., the protective layer) must have a very low extinction coefficient in the EUV domain, along with resistance against contamination. The strongest candidates for such protective layers are Ru, Rh,
2
TiO and ZrO214,17, where both of these criteria are fulfilled.
To generate the EUV light, Sn, Xe or Li plasma could be used17F
18,
18F
19
. Nevertheless, significant intensity loss is experienced as the light travels through the EUVL chamber; only about 4 to 11 % of the incident light intensity can be utilized for the actual pattern printing process12. Therefore, it is of crucial importance that the light source is sufficiently bright (about 200 W19F
20
) and the mirrors are extremely smooth within Angstrom scale (about 0.3 nm20F
21
). The flatness of the mirrors is depicted with an analogy21F
22
:
“If one of the mirrors was to be blown up to the size of Germany, the biggest bump would be less than 1 millimeter high”.
6
Figure 1.2: Manufacturing steps of the most basic part of a transistor. The process starts with the purification of the sand (a) until electronic grade Si, which is later melted and cast into a single crystal ingot, from which the Si wafers are cut (b). After coating the wafer with photoresist, it is subjected to the light (e.g., EUV) through a mask (c). The parts that are not exposed remain intact and protect the structure during etching (d). As a result, grooves are manufactured in the etched areas (e). Through several sequences of photoresist application, light exposure, etching, ion implantation, electroplating and polishing (not shown here), a transistor is made (f). A number of these transistors are interconnected with metal layers to serve as a part of a microprocessor (g). Images reproduced with the courtesy of Intel10.
7 The targeted lifetime of EUV optics in the lithographic exposure systems is specified as 30,000 hours of operation22F
23,
23F
24
. This stringent condition enforces many technical challenges to be tackled before EUVL is put into general practice. Some important issues awaiting resolution are related with the EUV sources (regarding the power and life time), reticles (particle contamination on photomasks) and in general with the vacuum and resist technologies (sensitivity and line edge roughness problems)24. The last but not the least, is the EUV optics topic (imaging, flare, life time and molecular contamination)24, where significant time and effort is put to develop various mitigation strategies for mirror surfaces.
Figure 1.3: (a) Cross sectional TEM image of the Mo/Si multilayer mirror, (b) magnified to indicate the interfaces between the Mo and Si layers (after Ref. 24F25). In an ideal EUVL mirror, the
surface corrugations shall not exceed about 0.3 nm21.
1.3.
Scope of this thesis
The aim of this research was to contribute to the fundamental knowledge established in the contamination-mitigation research in EUVL studies. To this end, the topics24 molecular contamination phenomena and prospective contamination mitigation techniquesare the leitmotivs of this study.
Since the last decade it was known that even capped mirrors are prone to oxidation during prolonged use25F
26
(i.e., a RuO film growing on top of a Ru capped 2 mirror) and hydrogen (atomic or molecular) is a prospective agent to remove such contaminants26F
27,
27F
28
8
of the quantitative expression of the process kinetics and the reduction mechanisms. Moreover, another type of contamination problem is being experienced recently; when hydrogen radicals are in contact with certain metallic species (e.g., Sn) in the EUVL chamber, volatile metal hydride species are created, which can in turn decompose on the vital optics components and reduce the overall reflectivity28F
29,
29F
30
. To come up with preventive measures, it is imperative to understand the fundamentals of the generation and decomposition processes. Therefore, the principal aim of this research was to answer these five questions:
- What is the kinetics of the various hydrogen cleaning processes for EUVL mirrors?
- How does the reduction of RuO proceed upon exposure to molecular or 2 atomic hydrogen?
- How can the atomic hydrogen flux on a surface be quantified accurately, and how can it be optimized?
- What are the fundamentals of the volatile metal hydride generation and decomposition phenomena?
- What are the effects of different surfaces on the metal hydride decomposition reaction?
1.4.
Outline of this thesis:
In Chapter 2, an overview is presented concerning the oxidation phenomena at the surface of the EUV mirrors, and the first cleaning method is introduced: exposure of the oxidized mirrors to molecular hydrogen. The reduction mechanism of the oxide layer is unraveled by employing structural and chemical characterization techniques. Following these fundamental issues, Chapter 3 focuses on the effect of atomic hydrogen species on the reduction mechanism and discusses the main differences between the two methods. In Chapter 4, the details of a novel atomic hydrogen sensor developed during this research are presented, and the radical flux incident on the sample surface is quantified as a function of gas flow and cracking filament temperature. Chapter 5 is focused on a different contamination phenomenon that is arising due to atomic hydrogen presence in the chamber: the volatile metal hydride
9 contamination problem. The rates of metal hydride generation and decomposition reactions are quantified precisely via a quartz crystal microbalance pair and the reaction efficiencies are described per H atom involved in the reaction. In Chapter 6, the dissociation characteristics of the volatile metal hydrides on different surfaces is studied. The main findings of this research are revisited in Chapter 7, along with a prospect for the short and long term future. A summary of the work completes the thesis.
10
References
1
G. C. Farrington, Popular Science, 248, (1996) 74.
2
Cray History, obtained from http://www.cray.com/About/History.aspx in August 2012.
3
Apple iPhone 4S technical specifications, obtained from: http://www.apple.com/iphone/specs.html in August 2012.
4
US 2012/0057867 A1, C-H Lin, Y-H Sun, “Optical USB thin card”, 8 March 2012.
5
G. E. Moore, Electronics, 38 (1965) 114.
6
The International Technology Roadmap for Semiconductors, obtained from: http://public.itrs.net/ in August 2012.
7
C. P. Ausschnitt, Microelectron. Eng. 41/42 (1998) 41.
8
Y. Wei, R. L. Brainard, Advanced Processes for 193-Nm Immersion Lithography, SPIE Press (2009).
9
“Transistor Count and Moore's Law”, from freely licensed media file repository of Wikipedia, obtained in August 2012 from:
http://en.wikipedia.org/wiki/File:Transistor_Count_and_Moore%27s_Law_-_2011.svg
10
From Sand to Silicon: the Making of a Chip, obtained from Intel Newsroom http://newsroom.intel.com/docs/DOC-2476 in August 2012.
11
C. Wagner, N. Hamed, Nat. Photonics, 4 (2010) 24.
12
V. Bakshi, EUV Lithography, SPIE Press (2009).
13
D. G. Stearns, R. S. Rosen, and S. P. Vernon, Appl. Opt., 32 (1993) 6952.
14
S. Bajt, Opt. Eng. 41 (8) (2002) 1797.
15
N. S. Faradzhev, S. B. Hill, T. B. Lucatorto, B. V. Yakshinskii, T. E. Madey, Bulletin of the Russian Academy of Sciences: Physics, 74 (2010) 28.
11
16
T. E. Madey, N. S. Faradzhev, B. V. Yakshinskiy, N. V. Edwards, Appl. Surf. Sci., 253 (2006) 1691.
17
S. Bajt, N. V. Edwards, T. E. Madey, Surf. Sci. Rep., 63 (2008) 73.
18
V. Bakshi, “EUV source technology: challenges and status” in EUV Sources for Lithography (Ed. V. Bakshi), SPIE Press (2006).
19
J. P. Allain, M. Nieto, A. Hassanein, V. Titov, P. Plotkin, M. Hendricks, E. Hinson, C. Chrobak, M.H.L. van der Velden, B. Rice, Proc. SPIE 6151 (2006) 31.
20
K. Bourzac, Nature 487 (2012) 419.
21
E. Louis, A. E. Yakshin, P. C. Gorts, S. Oestreich, R. Stuik, E. L. G. Maas, M. J. H. Kessels, F. Bijkerk, M. Haidl, S. Mullender, M. Mertin, D. Schmitz, F. Scholze, G. Ulm, Proc. SPIE, 3997 (2000) 406.
22
ASML: EUV Questions and Answers;
http://www.asml.com/asml/show.do?ctx=41905&rid=41906, accessed in August 2012.
23
S. Wurm, C. Gwyn, “Chapter 8: EUV Lithography” in Microlithography, (Ed. K. Suzuki), CRC Press/Taylor and Francis Informa Group, (2007) 2nd edition.
24
B. Mertens, M. Weiss, H. Meiling, R. Klein, E. Louis, R. Kurt, M. Wedowski, H. Trenkler, B. Wolschrijn, R. Jansen, A. van de Runstraat, R. Moors, K. Spree, S. Plöger, R. Van de Kruijs, Microelectron. Eng., 73-74 (2004) 16.
25
S. Y. Lee, H. J. Kim, J. Ahn, I. Y. Kang, Y-C Chung, J. Korean Phys. Soc., 41 (2002) 427.
26
B. Mertens, M. Weiss, H. Meiling, R. Klein, E. Louis, R. Kurt, M. Wedowski, H. Trenkler, B. Wolschrijn, R. Jansen, A. Van de Runstraat, R. Moors, K. Spee, S. Plöger, R. Van de Kruijs, Microelectron. Eng., 73–74 (2004) 16.
27
I. Nishiyama, H. Oizumi, K. Motai, A. Izumi, T. Ueno, H. Akiyama, A. Namiki, J. Vac. Sci. Technol. B, 23 (2005) 3129.
12
28
Y. Matsui, M. Hiratani, S. Kimura, J. Mater. Sci. 35 (2000) 4093.
29
N. Faradzhev, V. Sidorkin, J. Vac. Sci. Technol. A, 27 (2009) 306.
30
M. M. J. W. van Herpen, D. J. W. Klunder, W. A. Soer, R. Moors, V. Banine, Chem. Phys.Lett. 484 (2010) 197.
15
2.
Kinetics of reduction of a RuO
2
(110) film on
Ru(0001) by H
2
*
The kinetics and mechanism of the full reduction of a 2.3 nm RuO2 film with a
stoichiometric (110) surface on Ru(0001) by H2 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 RuO2 into
metallic Ru. The temperature dependent reduction rate increases linearly with H2
pressure. In the temperature range of 100 up to 200 °C, initially hydrogenation of the RuO2(110) oxygen atoms at the surface occurs. Next, oxygen vacancies are created
due to the 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 RuO2 by H2 already occurs in
the initial period with an activation energy of 0.48 eV and is followed by a faster reduction, but the reduction rate is slower than in the lower temperature range of 100 to 200 °C. In the temperature range of 300 to 400 °C, the reduction of RuO2 starts
immediately when exposed to H2 and the activation energy (0.48 eV) is similar to the
activation energy in the lower temperature range (100 to 200 °C), but the reduction is not as fast as in the low temperature range. Apparently, the annihilation of oxygen vacancies during reduction is more prominent with increasing temperature.
*
This chapter is based on:
D. Ugur, A. J. Storm, R. Verberk, J. C. Brouwer, W. G. Sloof, “Kinetics of reduction of RuO2(110) film on Ru(0001) by H2”, accepted for publication in The Journal of Physical
16
2.1.
Introduction
Since RuO2 is an excellent and versatile oxidation catalyst, its interaction with
many molecules has received much attention1. RuO2 is a promising catalyst for low
temperature dehydrogenation of small molecules such as: NH3, HCl, methanol and
other hydrocarbons. While the interaction of these molecules with RuO2 involves the
release of hydrogen onto its surface, understanding of the reduction mechanism and kinetics of RuO2 reduction by hydrogen is of fundamental importance.
The kinetics of reduction of RuO2 with H2 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 light5.
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. Extreme ultraviolet lithography (EUVL), employing a wavelength of 13.5 nm, can meet those size requirements6. The lifetime specification of EUV optics in the lithographic exposure systems is 30,000 h7. The main challenge here is to prevent carbon contamination and oxidation of the optical surfaces6-9.
Ruthenium is a promising material as a protective capping layer10, since it provides oxidation resistance and has a very low extinction coefficient in the EUV domain10,11. Oxidation of ruthenium capped mirrors is still possible though6, and an active strategy to mitigate oxidation is thus required.
Reversibility of ruthenium oxidation is possible with chemical reduction agents such as carbon monoxide12-14, molecular hydrogen2-4 and atomic hydrogen9,15,16. Chemical reduction with molecular hydrogen is advantageous due to its simplicity in implementation17, prevention of the further contamination of the optical system and reliability in the case of over-exposure18,19. Although reduction of ruthenium oxide layers with molecular hydrogen has been reported previously, 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 as well17-22. The
17 reduction mechanism and kinetics of the following reduction stages has received minor attention, and thus is the main topic of this study.
As a model system for the Ru capping layer, a closed packed Ru(0001) surface of a 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 with molecular hydrogen pressures in the range of 10-4 to 10-2 Pa. The oxidation of the Ru(0001) surface resulted in a thin RuO2 layer with its (110) plane of the rutile crystal
structure parallel to the surface23,24. Since the surface energy of the RuO2(110) is the
lowest among the primary surfaces17, it is anticipated that this orientation will be the most abundant in a polycrystalline RuO2 film which may be present on top of a Ru
capping layer. Spectroscopic ellipsometry, thermal desorption spectroscopy (TDS), and X-ray photoelectron spectroscopy (XPS) were used to study the mechanism and the kinetics of reduction. First, the experimental details will be described. Next, the results will be presented, and subsequently the reduction mechanism and kinetics will be discussed.
2.2.
Experimental
A Ru(0001) surface of a high-purity (4N) single crystal with a diameter of 10 mm and a thickness of 1 mm was used for all oxidation and reduction experiments. Prior to each oxidation, the Ru(0001) surface was cleaned by a differentially pumped Ar+ ion gun (PHI model 04-303); initially for 30 min at 4 keV on a 7x7 beam deflection area, followed by a secondary sputtering step of 30 min at 2 keV on a 5x5 beam deflection area. The cleanliness of the surface was analyzed with XPS by observing the O 1s photoelectron line. The sample was subsequently annealed at 600 °C for 30 min to allow for the full recovery of any damage induced to the crystal structure by the ion sputtering process, as evidenced by low energy electron diffraction24.
The crystal was mounted on a molybdenum sample holder, containing two gold contacts for a K-type (i.e., NiCr/NiAl wires with a diameter of 0.1 mm) or an R-type thermocouple (i.e., Pt/PtRh13 wires with a diameter of 0.1 mm). Thermocouple wires were spot-welded to the back-side of the crystal. Electrical contact with the thermocouple was made through an extractable fork, sliding onto the
18
gold contacts of the sample holder. The temperature of the crystal is regulated by a control unit (Eurotherm 902 S with PID characteristics), connected to the power supply of a 150 W halogen lamp (Delta S/15-18).
The experiments were carried out in a UHV reaction chamber (base pressure less than 5×10−8 Pa), coupled to an adjoining UHV chamber (base pressure less than
8
1 10× − Pa) for the XPS analysis. The total pressure inside the processing chamber is measured with a Bayart Alpart vacuum gauge (Granville Phillips Series 307). The UHV condition was achieved via three pumping stages: initially a dry-scroll roughing pump evacuates the chamber to a medium vacuum (Varian 300 Triscroll, 12 m /h ), 3 creating the environment for the secondary turbomolecular pumps to operate (Pfeiffer TMU071 and TPU180, 70 and 180 l/s , respectively). A titanium sublimation pump (Varian Combivac 300 l/s ) was used to finally, to achieve the desired UHV level.
The gas composition in the chamber was continuously monitored using mass spectrometers during the experiments (Pfeiffer QMS200 during oxidation, Pfeiffer QMS422 during reduction). Oxygen or hydrogen gas (both 99.999 % pure) was fed to the chamber by the designated leak valves (Pfeiffer UDV 146), operated by PI thermovalve control units (Pfeiffer RVG050). To maintain the high purity of the oxygen, an additional water vapor filter (‘hydrosorb’ of Messer Griesheim) was added on the line. Oxidation of the Ru(0001) surface was done at 400 °C of the Ru single crystal temperature and at an oxygen pressure of 1×10−2 Pa for 70 min, yielding an oxide film thickness of about 2.3 nm.
The change in the oxide thickness during the oxidation and reduction experiments was measured in situ with spectroscopic ellipsometry (Woollam M2000L, rotating compensator ellipsometer). The ellipsometer was operated with a 75 W Xe light source and the polarizing elements were made of Glan-Taylor type calcite. The incident light beam stroke the sample at an angle of 15° with respect to the sample surface and the reflected light was observed at the same angle. The light in the wavelength range of 245 to 900 nm (with a spectral resolution of 0.8 nm and a bandwidth of 2.4 nm) was detected with a 4 quadrant photodiode, where the Ψ and ∆ values were extracted for the determination of Fresnel reflection coefficients. The
19 instrument was calibrated24 in situ with a Si/SiO reference sample that has a 25 nm 2 thick oxide layer.
The ellipsometric model applied in the oxidation and reduction experiments on Ru(0001) consisted of a substrate and two oxide layers (i.e., the clean metal layer, the intermediate oxide layer, and the RuO layer, respectively). For the substrate, a four 2 oscillators Drude-Lorentz model24 was used to describe the Ru(0001) metal. The oscillator model was fitted to ellipsometric data for each experiment, recorded from the clean metal surface after the annealing process. Prior to each oxidation, the surface was stabilized at the specified reduction temperature and the optical parameters regarding the Ru(0001) substrate was updated at the reduction model for that temperature. Later the sample was heated to the oxidation temperature (400 °C), and the optical parameters were updated this time at the oxidation model. During the oxidation and reduction sequences, the set of optical parameters of the metal layer was unaltered, only the oxide thicknesses were fitted.
The next layer consisted of a ruthenium oxide layer, however this layer was not RuO2. To reduce the measurement noise for this extremely thin layer (about 1
ML), the optical constants were fitted to a large number of measurement points. For all measurement points associated with this layer, the thickness was assumed constant but unknown. This resulted in a reasonable, albeit noisy, set of optical constants that fulfilled the Kramers-Kronig relations24.
The topmost layer was modelled with a 3 oscillators Lorentz model, fitted to a 40 nm thick reference RuO layer that was grown on a sintered Ru metal by oxidizing 2
it for 24 hours at 500 °C and at 1 10× 5 Pa oxygen pressure24.
The reduction experiments were performed at 1×10−2, 1×10−3 and 1×10−4 Pa
molecular hydrogen pressure, respectively, and at a constant Ru single crystal temperature in the range of 100 to 400 °C. The reduction experiments were continued until the ellipsometer spectra to observe the oxide thickness did not change anymore.
The chemical nature of the sample surface was analyzed with XPS (PHI 5400 ESCA) after the oxidation and reduction experiments. The photoelectron spectra of the oxidized and reduced states were recorded using a non-monochromatic Mg anode X-ray source (Mg Kα1,2 = 1253.6 eV), operated at 200 W and 13 kV. The constant
20
analyzer pass energy was set to 71.55 eV for the sample cleanliness assessments and 35.75 eV for the analysis of the oxidized and reduced surface. The energy scale of the spherical capacitor analyzer spectrometer was calibrated with respect to Ref. 25. The photoelectrons were observed at 45° with respect to sample surface. The resulting spectra were analyzed with the PHI MultiPak V8.0 software.
2.3.
Results and Discussions
2.3.1.
Oxidation of the Ru(0001) surface
After the oxidation of a clean Ru(0001) surface at 400 °C and 10-2 Pa O2 for
70 minutes, a continuous and uniform RuO2 film is formed with a quasi saturation
thickness of about 2.3 nm24 due to self-limited oxidation. Under these conditions, the RuO2 film with a rutile crystal structure grows epitaxially on Ru(0001) with its (110)
plane parallel to the surface23,24. The oxide film studied comprises of about 7 monolayers of RuO2.
In an oxygen containing ambient, a RuO2(110) surface is terminated by rows
of differently coordinated O atoms in the [001] direction13,17,21; see Figure 2.1 (a). Namely, 2-fold uncoordinated O atoms on top (Oot) of 5-fold coordinated Ru atoms,
2-fold coordinated O atoms (Obr) bridging two 4-fold coordinated Ru atoms, and
3-fold coordinated O atoms (O3f) similar to the bulk O atoms. The 4-fold coordinated
Ru atoms in RuO2 are denoted as Rubr while the bulk Ru atoms are 6-fold coordinated.
When oxygen is absent in the gas phase, i.e., in UHV, the Oot atoms will
desorb from the RuO2(110) surface in the temperature range of 100-200 °C with a
second order kinetics26. Then the surface is next to the Obr and O3f, terminated with
5-fold coordinated Ru atoms, i.e., 1-fold coordinatively unsaturated sites (Rucus); see
Figure 2.1 (a). In this study, the Oot are considered absent and thus the RuO2(110)
surface is stoichiometric.
2.3.2.
Interaction of H
2with RuO
2(110)
Exposure of RuO2 to H2 at elevated temperatures leads to full reduction by
forming water according to:
21 (a)
(b)
(c)
(d)
Figure 2.1: (a) Stoichiometric RuO2(110) surface with Obr, O3f and Rucus species, (b) Hydrogenated surface with the Obr-H groups, (c) Hydrogen transfer reaction forming adsorbed water vapor species (Oot-2H) and an oxygen vacancy (Vbr) due to the displacement of the Obr atom, (d) Formation of a vacancy at the bulk (V3f) due to the place exchange of an O3f atom with the surface vacancy (Vbr).
22
The Gibbs free energy for this reaction equals -204.5 kJ/mol (2.12 eV) [cf. Ref. 27]. In the course of the reduction process with H2, the differently coordinated O
atoms are consumed. The reaction involves the donation of electrons by H2 to the O
atoms, which in turn donates electrons to the Ru atoms.
The mechanism of reduction at a RuO2(110) surface has already been studied
in detail [e.g., in References 1,12-14,17,21-23,26,28,29] with, e.g., temperature controlled reaction (TPR), scanning tunneling microscopy (STM), high resolution core level shift spectroscopy (HR CLS), surface x-ray diffraction (SXRD), low energy electron diffraction (LEED) etc. in combination with DFT calculations. These studies demonstrated that at a stoichiometric RuO2(110) surface, H2 may adsorb
non-dissociatively at the Rucus atoms28, however, these sites are not necessary for the
adsorption of H229. Since the interaction of molecular hydrogen with O3f atoms at the
RuO2(110) surface is energetically unfavorable, it will also not be dissociated at these
O3f atoms28. At room temperature, exposure of RuO2(110) to H2 leads first to the
formation of OH groups, thereby passivating the uncoordinated Obr atoms17 [see
Figure 2.1 (b)]. The OH terminated RuO2(110) surface is stable at room temperature,
however more hydrogen on RuO2(110) can be accommodated than required for the
complete passivation of the bridging O atoms. At high H2 exposures, H2O is forming
on the surface by moving Obr-H to the Rucus, i.e., the Obr atom becomes an Oot atom,
thereby picking up a second hydrogen atom to form H2O21,30 [see Figure 2.1 (c)]. The
hydrogen uptake on RuO2(110) may be mediated by the adsorption of molecular
hydrogen on the Rucus sites29. In this hydrogen transfer reaction, an oxygen vacancy
(Vbr) is created at the Obr site [see Figure 2.1 (c)] as has been observed by STM30. The
adsorbed water species (Oot-2H) over Rucus atoms are (by 0.19 eV) more stable than
hypothetic Obr-2H species30.
Next, H2O molecules above the Rucus sites desorb at temperatures beyond
about 400 K (147 °C), thereby the RuO2 surface becomes depleted of oxygen and
vacancies are created. The desorption of H2O is a first-order thermally activated
process with an activation energy of 6 kJ/mol (0.06 eV)29.
Full reduction of RuO2 requires also the removal of O3f atoms from the lattice.
Since the interaction of RuO2(110) with H2 creates vacancies at the Obr sites, it is
23 binding energy of an O3f (5.8 eV) and an Obr (4.6 eV) atom in RuO2 is 1.2 eV26.
Consequently, a vacancy V3f will be created at the O3f site [see Figure 2.1 (d)]. Then,
an oxygen concentration, as well as an oxygen vacancy gradient perpendicular to the surface occurs, which may drive the diffusion of oxygen atoms from the bulk of the RuO2 layer to the surface. Ultimately, this depletion of oxygen leads to the reduction
of RuO2, forming metallic Ru according to Eq. (2.1). The crucial role of vacancies in
the reduction of metal oxides has been recognized in other studies, e.g., the reduction of NiO by H231. The relation between the concentration of surface oxygen vacancies
and the reduction rate has been established considering Fickian diffusion32.
In summary, the mechanism proposed for the reduction of RuO2 with a
stoichiometric RuO2(110) surface by H2 is as follows: Initially hydrogenation of the
RuO2(110) surface occurs, thereby forming Obr-H. Next, a hydrogen transfer reaction
takes place in which an oxygen vacancy Vbr is formed on top of a coordinatively
unsaturated Rubr atom, according to:
2(Obr-H)→(Oot-2H)+Obr +Vbr (2.2)
At elevated temperatures (>100 °C) desorption of water may occur:
Oot-2H→(H2O)gas
(2.3)
Further reduction of RuO2 requires diffusion of the coordinatively saturated oxygen
atoms to the oxygen bridge positions:
O3f+Vbr→Obr+V3f (2.4)
Subsequently, the subsurface O atoms (O3f) may diffuse towards the surface by
exchanging place with the induced oxygen vacancies (V3f) until the oxygen atoms are
removed from the oxide lattice. Finally, the remaining Ru atoms transform into a metallic phase.
24
Figure 2.2: Reduction of a RuO2(110) film on Ru(0001) surface with
-4
1×10 Pa H2 at 175, 200 , 250 and 300 °C, respectively. The oxide thickness was measured in situ with spectroscopic ellipsometry.
25
2.3.3.
Kinetics of RuO
2reduction by H
2The reduction by H2 of a thin film of RuO2 with a thickness of about 2.3 nm as
obtained after dry thermal oxidation of a Ru(0001), was studied in the temperature range of 100 to 400 °C; see Section 2.2. When the RuO2 film was heated to
temperatures between 100 and 400 °C in UHV, it remained stable and no oxygen or any other species were observed with mass spectrometry. In this temperature range, H2O was the only species observed with mass spectrometry that desorbed from the
RuO2 surface upon exposure to H2.
The reduction of the RuO2 film by H2, as observed with spectroscopic
ellipsometry at temperatures below 200 °C, can be characterized by an initial period followed by a fast reduction regime until a stage where a constant amount of oxide remains; see Figure 2.2 (a). At 200 °C and up to about 300 °C, some reduction is already observed during the initial stage, followed by a fast reduction regime until no oxide is left; see Figure 2.2 (b) and Figure 2.2 (c). At 300 °C and above, the reduction starts immediately when the oxide surface is exposed to H2, see Figure 2.2 (d).
26
The initial period below 200 °C is associated with hydrogenation of the RuO2(110) surface, i.e., H atoms are bonded to Obr atoms (Obr-H); cf. Eq. (2.2). This
initial period (τ) decreases with increasing temperature; see Figure 2.3. An activation energy of 22 kJ/mol (0.23 eV) was derived from the experimental data for this hydrogenation process. Considering that first hydrogenation of the Obr occurs and that
the density of the Obr sites on a RuO2(110) plane equals 5.05×1018 m-2 [Ref. 13], a
sticking coefficient for H2 on RuO2(110) can be derived with the Hertz-Knudsen
equation33:
(
2)
2 H H 1 2 2 p J mkT π = (2.5) where 2 HJ is the number of H2 molecules hitting the RuO2(110) surface, pH2 the total
pressure of H2, m the mass of a H2 molecule, k the Boltzmann constant and T the
absolute gas temperature. The probability of the H2 molecules to adsorb at the
RuO2(110) surface is rather low, but increases linearly with temperature; see Figure
2.4.
27 At the end of the initial period, evolution of water vapor occurs (see Figure 2.5) and vacancies at the Obr sites (Vbr) are generated; see Eq. (2.2) and (2.3). Then the
reduction of RuO2 commences. For continued reduction, also vacancies at the O3f sites
(V3f) are created and diffusion of oxygen atoms to the surface occurs; see Eq. (2.4).
The rate of this autocatalytic reduction process depends on the oxygen vacancy concentration at the surface [e.g., Ref 32]. The activation energy of this fast reduction process equals 44 kJ/mol (0.45 eV).
Figure 2.5: Upon reduction at 150 °C and 10-4 Pa H2, water vapor is generated as observed with quadrupole mass spectrometry.
The remaining oxide after reduction below 200 °C has also been observed with XPS; see Figure 2.6. The binding of the O 1s photoelectrons from the RuO2 film
shifted from 529.3 to 529.8 eV after reduction, indicating that the remnant oxygen is more dispersed than in RuO2. Since the solubility of O in Ru is extremely low34, the
remnant oxygen is not dissolved in the metal. Apparently, this oxygen could not reach the surface during the reduction by hydrogen. This is probably due to the fact that the reduced RuO2 at the very surface is already transformed into metallic Ru, thereby
28
In the RuO2 lattice, the distances between the nearest neighbor Ru atoms are
0.311 and 0.353 nm in the [001] and [111] directions, respectively; whereas in a Ru metal lattice (hcp), the distances between the nearest neighbor Ru atoms corresponds to 0.267 nm and 0.264 nm in the [100] and [423] directions, respectively. The latter is the most stable configuration when the RuO2 lattice is depleted of oxygen. Thus the
Ru atoms will move towards each other upon transformation to metallic Ru. When comparing the molar volume of RuO2 with the molar volume of Ru, 18.85 and
8.24 cm3 respectively, it is also evident that the lattice shrinks 56 % upon the reduction of the oxide and transformation into metallic Ru.
Figure 2.6: O 1s X-ray photoelectron spectrum, recorded after oxidation of a Ru(0001) surface at 400 °C and 10-2 Pa O2 for 70 minutes (black) and after reduction of the RuO2(110) film at 150 °C and 10-4 Pa H2 (red).
In the temperature range of 200 to 300 °C, the reduction of RuO2 by H2 starts
already during the initial period, see Figure 2.2 (b) and Figure 2.2 (c). Then water vapor slowly desorbs form the surface creating oxygen vacancies, first at the Obr sites
(Vbr) and later at the O3f sites (V3f). In the second stage the reduction accelerates,
however, the rate of reduction at this stage is slower than below 200 °C and decreases with temperature; see Figure 2.7.
29 The decrease in the reduction rate is due to simultaneous formation of metallic Ru, probably as islands35-37. Then, as explained above, the oxygen vacancy concentration decreases rapidly. Since the diffusion of oxygen atoms from the bulk to the surface depends on this vacancy concentration, the rate of reduction reduces. As the transformation of Ru into the stable metallic configuration progresses with temperature and correspondingly the annihilation of oxygen vacancies, the rate of RuO2 reduction also decreases with increasing temperature in the temperature range of
200-300 °C; see Figure 2.7.
Figure 2.7: Reduction kinetics as a function of temperature. Measurements were executed at a H2 pressure of 10 Pa. -4
The desorption of water, the creation of oxygen vacancies as well as their annihilation, and the diffusion of oxygen are all thermally activated phenomena. Only the annihilation of oxygen vacancies reduces the reduction rate, while the other thermally activated phenomena enhance the reduction rate. Apparently, in the temperature range considered here (200 to 300 °C), the effect of oxygen vacancies annihilation surpasses the effect of the oxygen concentration gradient on the reduction rate.
30
Figure 2.8: Activation energy of the reduction of RuO2(110) with H2.
In the temperature range of 300 to 400 °C, the reduction of RuO2 starts
immediately when exposed to H2; see Figure 2.2 (d). Thus, the H2 molecules that are
adsorbing at the RuO2(110)surface directly react with the Obr atoms and desorb as
H2O, while creating Vbr vacancies; cf. Eq. (2.2) and Eq. (2.3). Subsequently, these
vacancies induce new oxygen vacancies (V3f) via place exchange with O3f; see
Eq. (2.4). This depletion of oxygen at the very surface induces a large oxygen concentration gradient. As for the reduction of RuO2 at lower temperatures, due to
transformation into metallic Ru, the oxygen vacancies are annihilated. However, in this temperature range (300 to 400 °C), the reduction rate increases with temperature, although still not as fast as during the second stage in the temperature range of 100 to 200 °C; see Figure 2.7. Here, the reduction of RuO2 by H2 is governed by the oxygen
gradient in the near surface region, driving the diffusion of oxygen. The activation energy for the reduction of RuO2 by the diffusion of oxygen to the surface in the
temperature range of 250 to 400 °C corresponds to 46 kJ/mol (0.48eV) (see Figure 2.8). This is close to the activation energy in the lower temperature range of 100 to 200 °C, viz. 0.45 eV. This suggests that the mechanism of oxygen transport towards the surface is the same for both temperature regions. At temperatures above 450 °C,
31 the RuO2 film is not stable anymore37, as observed by the reduction in the oxide
thicknesses with the ellipsometer in the absence of H2.
Figure 2.9: Reduction efficiency of a RuO2 film by H2 at 10-4 Pa as a function of temperature. The reduction efficiency is defined as the ratio between the number of O atoms reduced, and the number of H2 molecules arriving at the surface.
The efficiency of reduction of a RuO2(110) film with H2 at different regimes
(cf. Figure 2.7) can be determined from the experimental data. The efficiency (
2
H η ) is defined as the ratio between the number of O atoms reduced, and the number of H2
molecules arriving at the surface [cf. Eq. (2.5)]. The reduction rate r (nm/s) can be translated into the number of O atoms removed from the oxide lattice. Considering that one monolayer of RuO2(110) contains 2.02×1019 at.O/m2 and that the distance
between two monolayers equals 0.317 nm (using lattice constants from Ref. 13 and 17), the efficiency is defined as:
2 2 H H r C J
η
= (2.6)where C is a constant (6.36 10 m nm× 19 −2 −1). The efficiency for the different reduction regimes is displayed in Figure 2.9.
32
The efficiency of the reduction reaction after hydrogenation (100 to 200 °C), as well as during the initial reduction (250 to 400 °C), increases with temperature in a similar manner. However, during the second reduction (200 to 300 °C), the efficiency decreases with temperature. Considering the high efficiency at 175 °C and 10-4 Pa H2
(cf. Figure 2.9), then each O atom removed from the oxide lattice requires about 50 molecules of H2.
2.3.4.
Effect of H
2pressure
The effect of hydrogen pressure on the reduction of RuO2(110) films has been
studied in the two different kinetic regimes shown in Figure 2.7; viz. at 150 and 300 °C, respectively. The reduction experiments were executed at different pressures in the range of 10-4 to 10-2 Pa H2.
Figure 2.10: The rate of RuO2 reduction as a function of H2 pressure, at 150 °C and 300 °C, respectively.
The reduction kinetics at 150 °C scales practically linearly with H2 exposure to
the RuO2 surface within the experimental error; see Figure 2.10. Both the
33 arriving at the surface. Similar observations have been made for the reduction of other transition metal oxides32,38. At this temperature, the hydrogenation period of the RuO2(110) surface corresponds to about 1100 Langmuir of H2 exposure (1 L =
1 µTorr.s).
At 300 °C, where the reduction of RuO2(110) directly starts when exposed to
H2 without an initial period, the reduction rate scales also practically linearly with H2
pressure; see Figure 2.10. The difference between the increase of the reduction rate with hydrogen pressure at 150 and 300 °C may be attributed to the higher H2 sticking
coefficient at 300 °C than at 150 °C.
Since both the reduction rate and the number of H2 molecules that impinge at
the surface are linearly proportional with the H2 pressure [cf. Eq. (2.5)], the efficiency
of the reduction reaction [cf. Eq. (2.6)] is independent of pressure.
2.4.
Conclusions
The reduction of a stoichiometric RuO2(110) film by H2 at 10-4 Pa proceeds in
the temperature range of 100 to 200 °C first by the hydrogenation of the Obr oxygen
atoms at the surface, with an activation energy of 0.06 eV. Once oxygen vacancies (Vbr) are created due to desorption of water vapor, the reduction is accelerated by
place exchange of oxygen bulk atoms (O3f) with the oxygen vacancies, and the
activation energy of this process equals 0.45 eV.
In the temperature range of 200 to 300 °C, slow reduction of RuO2(110) by H2
already occurs in the initial period with an activation energy of 0.48 eV. When a significant amount of oxygen vacancies are created at the bulk O3f sites, the reduction
is accelerated, but the reduction rate is slower than in the lower temperature range of 100 to 200 °C, probably due to the annihilation of oxygen vacancies upon transformation into metallic Ru.
In the temperature range of 300 to 400 °C, the reduction of RuO2(110) starts
immediately when exposed to H2. Apparently, oxygen vacancies are directly created
enabling the diffusion of oxygen towards the surface. The reduction rate increases with temperature and the activation energy equals 0.48 eV. However, the reduction rate is not as fast as in the low temperature range of 100 to 200 °C.
34
The activation energies for reduction after hydrogenation at 100 to 200 °C (0.45 eV) and in the initial stage of reduction at 200 to 400 °C (0.48 eV), are larger than the activation for hydrogenation (0.23 eV) and water vapor desorption (0.06 eV).
The reduction rate of RuO2(110) by H2 at the various stages of the reduction
process (cf. Figure 2.2) scale with H2 pressure, thus the reduction kinetics as a
function of exposure is the same.
The efficiency of the reduction of RuO2(110) by H2, in terms of the number of
H2 molecules needed to impinge at the surface to remove one O atom, increases with
temperature in the two temperature regimes (100-200 and 300-400 °C, respectively), and is independent of H2 pressure.
Acknowledgments
This work was supported by FP7-PEOPLE program of Marie Curie ITN, under the project name “Surface Physics for Advanced Manufacturing” (SPAM), grant no: 215723.
35
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41
3.
Kinetics of reduction of a RuO
2
(110) film on
Ru(0001) by atomic hydrogen
*
The kinetics and the mechanism of reduction of a RuO2(110) film, grown
thermally on a Ru(0001) surface, has been studied in the temperature range of 60 to 200 °C by using an atomic hydrogen flux of 19 -2 -1
2 10 at.H m s× . The reduction kinetics is dominated by the creation of oxygen vacancies at the surface (Vbr) and bulk
lattice positions (V3f), and by the subsequent diffusion of subsurface oxygen species
(O3f) to these vacancies. The activation energy associated with this reduction process
equals 0.43 eV. At 200 °C, about 10 H atoms are required for the removal of an oxygen atom from the RuO2 lattice. This value is about an order of magnitude lower
when compared with the reduction of RuO2 by molecular hydrogen under similar
conditions. Moreover, the reduction proceeds at least 14 times faster when using highly reactive atomic hydrogen species.
*
This chapter is based on:
D. Ugur, A. J. Storm, R. Verberk, J. C. Brouwer, W. G. Sloof, “Kinetics of reduction of a RuO2(110) film on Ru(0001) by atomic hydrogen”, submitted to Microelectronic Engineering.
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3.1.
Introduction
Extreme ultraviolet lithography (EUVL), employing a wavelength of 13.5 nm, enables sub 30 nm feature sizes to be written on semiconductor chips1. Since all matter absorbs EUV radiation, the imaging process has to be conducted in vacuum and it is no longer possible to employ refractory optics to focus the beam2. Instead, defect free Mo/Si thin film multilayer mirrors are used as collimator optics, which are tailored for the maximum reflectivity (about 70 %) at 13.5 nm of wavelength3. These mirrors are composed of about 50 Mo/Si bilayer stacks4, with a thin protective layer (e.g., Ru) added on top to provide resistance against oxidation5-7. Nonetheless, even capped mirrors are prone to oxidation after prolonged use in EUVL1. A thin carbon or oxide film on the optics significantly reduces the reflectivity1,8 and therefore should be removed for the optimal operation of the instrument. Exposure of the contaminated EUVL mirrors to molecular hydrogen9-12 or atomic hydrogen13 -17 is a prospective method to mitigate these contaminants.
Although reduction by atomic hydrogen (H) is a strong candidate for optics cleaning purposes, knowledge is limited about the kinetics13, 18 and the exact mechanism of the reduction19,20. To study these phenomena, a Ru(0001) single crystal surface was used to replicate the Ru capped mirror. This surface was thermally oxidized, yielding a RuO (110) surface structure2 21,22. Afterwards, the oxide layer was reduced by atomic hydrogen exposure. The thickness of the oxide layer was monitored in situ with spectroscopic ellipsometry during the oxidation and reduction processes. The chemical state of the surface at various stages of the reduction process was analyzed with X-ray photoelectron spectroscopy (XPS). In the following sections, first the details of the experiments will be described. Next, the results will be presented and finally the reduction mechanism and kinetics will be discussed.
3.2.
Materials and Methods
3.2.1.
Experimental setup
The Ru(0001) surface of a high-purity (4N) single crystal with a diameter of 10 mm and a thickness of 1 mm was used for all oxidation and reduction experiments. The experiments were carried out in a UHV reaction chamber (base pressure less than
