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Etching-enhanced surface stress relaxation during initial ozone oxidation
Surface Science 601 (2007) 1384–1388 www.elsevier.com/locate/susc
Etching-enhanced surface stress relaxation during initial ozone oxidation Tetsuya Narushima a,*,1, Masahiro Kitajima a, Akiko N. Itakura a, Akira Kurokawa b, Shingo Ichimura b, Kazushi Miki a a
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan and 1-2-1 Sengen, Tsukuba 305-0047, Japan b Research Institute of Instrumentation Frontier (RIIF), National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, Tsukuba 305-8568, Japan Received 13 June 2006; accepted for publication 4 January 2007 Available online 12 January 2007
Abstract Initial oxidation via ozone on the Si(1 0 0) surface is investigated by measuring surface stress and observing atomic structure via a scanning tunneling microscopy (STM). A similar investigation is also carried out for molecular oxygen and the results are compared. As a result, monotonic increase of the surface stress to the compressive stress side is obtained up to 0.33 N/m for ozone oxidation at room temperature, while molecular oxygen shows only tiny surface stress growth. From the STM observations, it is found that the difference between ozone and molecular oxygen oxidation is the existence of surface etching. As the origin of the surface stress, therefore, the reduction of the intrinsic tensile surface stress due to the reconstructed surface by the etching process is proposed. 2007 Elsevier B.V. All rights reserved. Keywords: Surface stress measurement; Scanning tunneling microscopy; Silicon; Ozone oxidation
1. Main body Heterostructures play very important roles in many kinds of micro and nano electronic/optoelectronic devices. Here, as the dimensions of the heterostructures are reduced, interface effects between constituent materials cannot be ignored. For example, in the case of metal-oxide semiconductor field-effect transistors (MOSFETs), it is a key technology to control the electronic as well as the atomic structure of the transition layer at the interface for their miniaturization and upgrade. The electronic structure is influenced by the interfacial stress existing in the * Corresponding author. Present address: The Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778581, Japan. Tel.: +81 47 1363323; fax: +81 47 1363474. E-mail address: [email protected] (T. Narushima). 1 Also at CRANN and School of Chemistry, Trinity College Dublin, Dublin 2, Ireland.
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.01.006
transition layer. The periodicity of the Si substrate can be easily modified via tiny stress application. As a result, such modification changes the mobility of carriers. In order to control such interfacial stress artificially, current high performance central processing units (CPUs) utilize lattice constant differences, i.e., a mixed system of Si and Ge. However, a high temperature annealing process is required to mix and/or grow SixGe1x layer. Presently, lower temperature processes are preferable for further integration of nano scale electronic/optoelectronic devices [1], as well as polymer/biological systems, to suppress changes in properties due to thermal diffusion, alteration and denaturation of the proteins. For the purposes of lowering processing temperatures, reactive species such as ozone which is used for oxidation appears to be a promising candidate. By use of such species instead of ordinary molecular oxygen, different transition layer structures and interfacial stress can be expected. In this letter, we report the difference between molecular
T. Narushima et al. / Surface Science 601 (2007) 1384–1388
oxygen and ozone in the surface stress of Si(1 0 0) caused by them and then discuss the origin of the stress from the perspective of surface atomic structure. Consequently, it is found that only ozone exposure shows monotonic increase of the surface stress to the compressive stress side. This is attributed to the fact that surface etching exists even at room temperature in the case of ozone exposure, and consequently reduces the intrinsic tensile stress due to the reconstructed Si(1 0 0)-2 · 1 surface structure. Reducing this tensile stress is advantageous for further oxidation accompanied with volume expansion by oxygen insertion into Si–Si bonds. Therefore, this idea could give us the ability of controlling the interfacial stress in the transition layer of SiOx/Si. We measured the surface stress evolutions of Si(1 0 0) surfaces during oxidation with use of ozone (O3) and molecular oxygen (O2) at room temperature (RT). An optical micro cantilever bending method was used to detect changes in the surface stress (see inset of Fig. 1). A laser beam incident on the back side of the cantilever sample at the free end was reflected through an ultra high vacuum (UHV) window and detected. This system allows us to detect sample bending induced by atomic scale strain. We used a small Si(1 0 0) micro cantilever as a sample with dimension 450 · 50 · 2.0 lm. This system achieves a high resolution of 0.1 nm in the sample deflection, d. Clean Si(1 0 0) surfaces were prepared by heating-up the sample to 750 C under UHV conditions via an infrared light source after dipping the cantilever sample in diluted HF acid [2,3]. During surface stress measurement, the substrate temperature was set to be RT. In order to introduce O3 gas, we used vapor from liquid ozone. To obtain a high concentration and purity, ozone gas was circulated, as it can be easily dissociated at the pipeline walls. The ozone concentration was estimated to be 93% [4]. For O2 exposure, high purity oxygen gas (99.9999%) was introduced with the same set-up. Gas pressures in the pipeline for both O3
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and O2 exposures were set to be 0.2 Torr. During the exposures, pressure increases in the UHV chamber were in the range of 107 Torr or less. To avoid dissociation of each molecule, we turned off all ionization pressure gauges. A detailed experimental setup for O3exposure combined with the surface stress measurement is described elsewhere [5]. To observe the effect of ozone exposure to Si(1 0 0) surfaces, we conducted an in-situ Scanning Tunneling Microscopy (STM) observation. STM can reveal atomic scale structures on the surface where reactions occurred and provide us with information to understand the origin of the measured surface stress. For STM observation, this time, we focused only on the initial stage of O3 exposure. Clean Si(1 0 0) surfaces were prepared by flashing at 1100 C and then confirmed by STM. Following this, the sample was cooled down to RT and was oxidized by introducing the highly concentrated ozone gas into a UHV chamber for a short time (61–2 s) at 107–106 Torr. This amount of O3 roughly corresponds to 0.1–2 L. Although we used the same O3 source as that used for the surface stress measurements, we observed surfaces of a different shaped sample due to experimental limitations. Fig. 1 displays evolutions in the surface stress for two cases, namely, O3 and O2 exposure to Si(1 0 0) surfaces at RT. In the case of O3 exposure, from the initial exposure, the surface stress remarkably and monotonically increased to the compressive stress side as a function of time and then became saturated. The saturated value of the compressive stress was 0.33 N/m. This indicates that O3 oxidation results in surface expansion from the initial exposure. The oxide film thickness was up to 0.6 nm which was estimated by X-ray Photoelectron Spectroscopy (XPS). In contrast, in the case of molecular O2 exposure at RT, only a tiny compressive side surface stress of 0.002 N/m was induced. A huge difference exists in the surface stress evolutions of O3 and O2. This strongly suggests that differences in oxidation species can be detected via surface stress measurement.
Fig. 1. Sample deflection (d) and the surface stress evolution (rt) during ozone (O3) and molecular oxygen (O2) gas exposure. In both cases during measurements, pressures in pipeline and substrate temperatures are set to be 0.2 Torr and RT, respectively. Arrow indicates the point where STM images of Fig. 2 are taken.
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It is known that oxide films thicker than 10–50 nm have an intrinsically compressive stress due to difference in the volume changes between SiO2 and Si [6]. This does not depend on surface orientation but simply difference in the volume changes between Si and SiO2 layers. However, surface stresses on oxide films thinner than this thickness is not so easily understood, because atomic scale effects resulting from the surface reconstruction/orientation cannot be ignored. In an early study performed by Sander and Ibach [7], molecular oxygen exposures to Si(1 1 1) and Si(1 0 0) surface at RT showed different surface stress signatures. Their results indicated that there should exist varieties in the surface stress due to the atomic scale surface structure. Hence it is important to consider the dominant surface structure effect for the oxide film on Si system thinner than 10 nm. Recently, our group carried out surface stress studies of Si(1 0 0) surfaces during plasma oxidation [2,3,8]. Basically, during the initial stages, tensile surface stresses were induced both by pure oxygen plasma [2,3] and a plasma of krypton and oxygen mixture [8]. However ozone oxidation induced a compressive surface stress as previously mentioned. In addition, plasma oxidation processes displayed much more complicate behaviors than those of thicker oxide film growth or ozone oxidation. For example, in the case of oxidation using pure oxygen plasma and applying a positive bias to the sample, three surface stress stages were apparent, namely, compressive-tensile-compressive, for 1 nm oxide growth. To investigate the influence of the surface structure of Si(1 0 0) on the complicate behavior, we intentionally destroyed the surface structure to a depth of 1 nm via low energy Ar ion bombardment and observed surface stress under this same conditions [3]. As a result, the first three surface stress stages disappeared completely. This strongly suggests that the complicate behavior for pure oxygen plasma oxidation under positive sample bias should be caused by the ordered surface structure of Si(1 0 0). In other words, in the case of oxidation, surface stress measurement can detect surface structure changes as well as differences in oxidation species. Fig. 2 displays surfaces exposed at RT to different amounts of O3. The exposure quantity was estimated from the pressure increase, and deemed to represent the surface condition on initial exposure as highlighted in Fig. 1. From Fig. 2a, bright bumps were visible, which typically lie in the middle and on the top of dimer rows. Such features were also observed for Si(1 0 0) on exposure to molecular oxygen of 108 Torr at RT [9]. Along with the features above, significant dimer row buckling was induced, which had not been mentioned in the previous report. The bumps also acted as nucleation sites for elongated islands when the exposure was done at the elevated temperature of over 300 C, while the exposure at 200 C did not produce it at all as shown in Fig. 3. The nucleation of elongated islands from the bumps was also reported in the case of molecular oxygen exposure [9]. Here, the bumps and the elongated islands were thought to consist of ejected Si
Fig. 2. STM images of Si(1 0 0) surface for the initial stage of ozone oxidation at room temperature. These images were obtained under Vsample = 1.5 V and Itunnel = 0.10 nA. Amount of O3 exposures increases from (a) to (c). Dashed circles in (b) indicate positions of the ‘downstairs dimer row’.
atoms from the surface region. Also, although numerous buckling dimer row structures were still observed at 200 C, most of the buckling dimer row structures vanished away at 300 C. Therefore, the induced buckling dimer row structures could relax into symmetric dimers due to the nucleation over 300 C as well as a simple thermal effect. As mentioned above, local structures induced by O3 exposure were similar to those of molecular oxygen. However, the number of the surface Si atoms does not seem to be conserved by O3 exposure, but largely reduced from that of the original clean surface, even with the inclusion of the Si atoms ejected onto the surface. It is hard to determine the exact number of the surface atoms from the STM images, because reacted sites also show depletion in brightness
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Fig. 3. Images of Si(1 0 0) surfaces at room temperature after the ozone exposure at the elevated temperature of 200 C (a) and 300 C (b), which were taken with the tunneling conditions of (1.44 V, 0.16 nA) and (1.7 V, 0.07 nA), respectively. Exposure quantities seem to be slightly less than that of the Fig. 2. Dashed circles denote isolated bumps in (a) and elongated islands in (b). Also, arrows indicate positions of buckling dimer rows.
due to the decrease in the local density of states (DOS). Qualitatively speaking in the case of Fig. 2b, 30% of atoms are removed from the surface region by estimating the area of the dark patches. As a piece of supporting evidence for atomic removal, we could also find newly formed dimer row structures in the removed region, as shown by the dashed circles of Fig. 2b. This structure looked like the ‘downstairs dimer row’ (DDR), having a 1 ML depth. It also implied that these dark patches could not only be due to a decrease of the local DOS by reaction but also represent real topology. In the case of oxidation of Si using molecular O2 at RT, it has been reported that the initial stage of oxide growth only depends on the total number of O2 molecules impinging on the surface [10]. Also, at higher temperature, lower pressure causes a higher desorption rate of Si–O. Although the exposure of O3 is almost equivalent to that of molecular oxygen and also the pressure of O3 was higher than that of molecular oxygen exposure [9], we saw much heavier removal of atoms, i.e., surface etching only in the case of O3. Hence, we concluded that the difference between O3 and O2 exposure was due to the existence of the surface etching. At higher exposure, as shown in Fig. 2c, the surface became invariably rough and any ordered features such as DDR structures can no longer be seen. In general, it is essential to have a flat and/or well-ordered structure in the surface plane for an effective buildup of surface stress. The clean Si(1 0 0) surface is one of the typical examples and shows intrinsic surface shrinkage, i.e., a tensile stress. This is caused by forming 2 · 1 dimer structures at the surface. Dimer formation results in surface shrinkage, with slight expansion occurring along dimer rows. Here, the difference was theoretically determined to be 1.1–3.2 N/m [11,12].
The initial surface stress measured here should originate from the structure which the snapshots of Fig. 2 represent. The main difference between the O3 and O2 exposure was the existence of surface etching as previously mentioned. This surface etching introduces surface roughness, which results in the destruction of the intrinsic surface stress. Therefore, the huge surface stress to the compressive stress side should be basically driven by release of the intrinsic tensile stress due to the 2 · 1 dimer formation. Especially, during the initial stage of O3 exposure, this release mechanism of the intrinsic surface stress can work effectively, because larger amounts of surface shrinkage can be released. Then, an initial steep increase and the following gradual saturation of the surface stress may be basically explained with this mechanism above. On the other hand, the local features which were also observed for molecular O2 exposure [9] should make a tiny contribution to the surface stress growth. It should be also noted that the surface stress evolution for further oxide growth (which could potentially show a compressive stress due to volume expansion via oxygen atom insertion) could not be distinguished properly, as the oxidation rate is extremely low at room temperature. When O3 exists on the Si(1 0 0) surface, the following reaction can be expected as a first step, i.e., Si + O3 ! Si–O + O2. Molecular O2 is created by this process. This reaction emits 357.93 kJ/mol (=3.72 eV) as exothermic energy. The bonding energy of Si–O is assumed to be 464.4 kJ/mol following thermal consideration. Hence, if this exothermic energy is consumed for the subsequent reaction, two possible reaction paths should exist, namely, thermal desorption and additional oxide formation. The activation energy for thermal desorption of SiO into the vapor phase was experimentally determined to be
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4.0 ± 0.37 eV [13,14]. On the other hand, the reaction for the formation of SiO, following molecular O2 creation due to the first step reaction can be written as, Si + O2 ! Si–O + 1/2O2, generating 215.23 kJ/mol as exothermic energy. For this situation, the activation energy is experimentally determined to be 2.7 ± 0.15 eV [13,14]. From an energetic point of view, O3 can provide sufficient energy to promote not only oxidation but also an etching event due to its dissociation reaction step. This etching event is not possible for O2 however, due to energetics limitations. For pure molecular oxygen, only oxidation is achievable. On the contrary, the formation of DDR structures may be explained in the following way. When Si monomers are created at etched sites following the desorption of Si atoms from the surface, these can form stable dimers without any additional activation barrier [15]. Following this, an activation energy ranging from 1.3 to 1.45 eV is required to create dense dimer row structures [15]. As a result, it may be possible to create DDR structures even at RT with help from the exothermic energy created during the dissociation of O3. Such DDR structures are expected to generate a tensile stress component. However the population of these structures is too low to make a noticeable contribution to the total surface stress signal. We have studied the surface stress evolution and atomic scale structure during initial ozone oxidation of the Si(1 0 0) surface, and compared results with oxidation via molecular oxygen. As a result, we found that the surface stress evolutions during ozone exposure showed a monotonic increase to the compressive stress side, while no remarkable surface stress change was observed for molecular oxygen exposure. In addition, heavier surface etching even at RT was observed on the surface exposed to ozone. We have attributed the origin of the compressive stress, not to local Si–O bond formation, but to the reduction of the intrinsic tensile surface stress of the reconstructed Si(1 0 0)-2 · 1 surface via the etching process. Surface etching introduces surface roughness. As a result, excess strain caused by the reconstruction at the surface or the SiOx/Si interface is released. Therefore oxide formation involving volume expansion can proceed under a stress free condition. Ozone can produce very thin SiOx/Si transition layers [16,17] with good electrical
characteristics [17,18], due to the release of the intrinsic stress for small exposures. Its characteristics are much more beneficial that those of molecular oxygen. Using stress control techniques, ozone appears to have potential to be utilized for future nanoscale devices. Acknowledgements We would like to thank Prof. J. J. Boland, Mr. N. T. Kinahan and Mr. M. O’Toole, Trinity College Dublin for their fruitful discussions and useful comments. References [1] T. Narushima, M. Kitajima, K. Miki, J. Phys.: Condens. Matter 16 (2004) L193. [2] A.N. Itakura, T. Narushima, M. Kitajima, Appl. Surf. Sci. 159–160 (2000) 62. [3] T. Narushima, A.N. Itakura, T. Kurashina, T. Kawabe, M. Kitajima, Appl. Surf. Sci. 159–160 (2000) 25. [4] H. Nonaka, A. Kurokawa, S. Ichimura, T. Nishiguchi, Y. Morikawa, M. Miyamoto, J. Vac. Soc. Jpn. 44 (2001) 221. [5] A. Kurokawa, T. Narushima, K. Nakamura, H. Nonaka, S. Ichimura, A.N. Itakura, M. Kitajima, Jpn. J. Appl. Phys., Part 1 43 (2004) 281. [6] E. Kobeta, E.A. Irene, J. Vac. Sci. Technol. B 4 (1986) 720; E. Kobeta, E.A. Irene, J. Vac. Sci. Technol. B 5 (1987) 15; E. Kobeta, E.A. Irene, J. Vac. Sci. Technol. B 6 (1988) 574. [7] D. Sander, H. Ibach, Phys. Rev. B 43 (1991) 4236. [8] A.N. Itakura, T. Narushima, M. Kitajima, J. Vac. Soc. Jpn. 44 (2001) 81. [9] D.G. Cahill, P. Avouris, Appl. Phys. Lett. 60 (1992) 326. [10] F. Lutz, J.L. Bischoff, L. Kubler, D. Bolmont, Phys. Rev. B 40 (1989) 10356. [11] O.L. Alerhand, D. Vanderbilt, R.D. Meade, J.D. Joannopoulos, Phys. Rev. Lett. 61 (1988) 1973. [12] F. Liu, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 3156. [13] U. Memmert, M.L. Yu, Surf. Sci. Lett. 245 (1991) L185. [14] M.L. Yu, B.N. Eldridge, Phys. Rev. Lett. 58 (1987) 1691. [15] T. Yamasaki, T. Uda, K. Terakura, Phys. Rev. Lett. 76 (1996) 2949. [16] K. Nakamura, A. Kurokawa, S. Ichimura, Thin Solid Films 343–344 (1999) 361. [17] K. Nakamura, S. Ichimura, A. Kurokawa, K. Koike, G. Inoue, T. Fukuda, J. Vac. Sci. Technol. A 17 (1999) 1275. [18] T. Nishiguchi, H. Nonaka, S. Ichimura, Y. Morikawa, M. Kekura, M. Miyamoro, Appl. Phys. Lett. 81 (2002) 2190.
Etching-enhanced surface stress relaxation during initial ozone oxidation Tetsuya Narushima a,*,1, Masahiro Kitajima a, Akiko N. Itakura a, Akira Kurokawa b, Shingo Ichimura b, Kazushi Miki a a
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan and 1-2-1 Sengen, Tsukuba 305-0047, Japan b Research Institute of Instrumentation Frontier (RIIF), National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, Tsukuba 305-8568, Japan Received 13 June 2006; accepted for publication 4 January 2007 Available online 12 January 2007
Abstract Initial oxidation via ozone on the Si(1 0 0) surface is investigated by measuring surface stress and observing atomic structure via a scanning tunneling microscopy (STM). A similar investigation is also carried out for molecular oxygen and the results are compared. As a result, monotonic increase of the surface stress to the compressive stress side is obtained up to 0.33 N/m for ozone oxidation at room temperature, while molecular oxygen shows only tiny surface stress growth. From the STM observations, it is found that the difference between ozone and molecular oxygen oxidation is the existence of surface etching. As the origin of the surface stress, therefore, the reduction of the intrinsic tensile surface stress due to the reconstructed surface by the etching process is proposed. 2007 Elsevier B.V. All rights reserved. Keywords: Surface stress measurement; Scanning tunneling microscopy; Silicon; Ozone oxidation
1. Main body Heterostructures play very important roles in many kinds of micro and nano electronic/optoelectronic devices. Here, as the dimensions of the heterostructures are reduced, interface effects between constituent materials cannot be ignored. For example, in the case of metal-oxide semiconductor field-effect transistors (MOSFETs), it is a key technology to control the electronic as well as the atomic structure of the transition layer at the interface for their miniaturization and upgrade. The electronic structure is influenced by the interfacial stress existing in the * Corresponding author. Present address: The Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778581, Japan. Tel.: +81 47 1363323; fax: +81 47 1363474. E-mail address: [email protected] (T. Narushima). 1 Also at CRANN and School of Chemistry, Trinity College Dublin, Dublin 2, Ireland.
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.01.006
transition layer. The periodicity of the Si substrate can be easily modified via tiny stress application. As a result, such modification changes the mobility of carriers. In order to control such interfacial stress artificially, current high performance central processing units (CPUs) utilize lattice constant differences, i.e., a mixed system of Si and Ge. However, a high temperature annealing process is required to mix and/or grow SixGe1x layer. Presently, lower temperature processes are preferable for further integration of nano scale electronic/optoelectronic devices [1], as well as polymer/biological systems, to suppress changes in properties due to thermal diffusion, alteration and denaturation of the proteins. For the purposes of lowering processing temperatures, reactive species such as ozone which is used for oxidation appears to be a promising candidate. By use of such species instead of ordinary molecular oxygen, different transition layer structures and interfacial stress can be expected. In this letter, we report the difference between molecular
T. Narushima et al. / Surface Science 601 (2007) 1384–1388
oxygen and ozone in the surface stress of Si(1 0 0) caused by them and then discuss the origin of the stress from the perspective of surface atomic structure. Consequently, it is found that only ozone exposure shows monotonic increase of the surface stress to the compressive stress side. This is attributed to the fact that surface etching exists even at room temperature in the case of ozone exposure, and consequently reduces the intrinsic tensile stress due to the reconstructed Si(1 0 0)-2 · 1 surface structure. Reducing this tensile stress is advantageous for further oxidation accompanied with volume expansion by oxygen insertion into Si–Si bonds. Therefore, this idea could give us the ability of controlling the interfacial stress in the transition layer of SiOx/Si. We measured the surface stress evolutions of Si(1 0 0) surfaces during oxidation with use of ozone (O3) and molecular oxygen (O2) at room temperature (RT). An optical micro cantilever bending method was used to detect changes in the surface stress (see inset of Fig. 1). A laser beam incident on the back side of the cantilever sample at the free end was reflected through an ultra high vacuum (UHV) window and detected. This system allows us to detect sample bending induced by atomic scale strain. We used a small Si(1 0 0) micro cantilever as a sample with dimension 450 · 50 · 2.0 lm. This system achieves a high resolution of 0.1 nm in the sample deflection, d. Clean Si(1 0 0) surfaces were prepared by heating-up the sample to 750 C under UHV conditions via an infrared light source after dipping the cantilever sample in diluted HF acid [2,3]. During surface stress measurement, the substrate temperature was set to be RT. In order to introduce O3 gas, we used vapor from liquid ozone. To obtain a high concentration and purity, ozone gas was circulated, as it can be easily dissociated at the pipeline walls. The ozone concentration was estimated to be 93% [4]. For O2 exposure, high purity oxygen gas (99.9999%) was introduced with the same set-up. Gas pressures in the pipeline for both O3
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and O2 exposures were set to be 0.2 Torr. During the exposures, pressure increases in the UHV chamber were in the range of 107 Torr or less. To avoid dissociation of each molecule, we turned off all ionization pressure gauges. A detailed experimental setup for O3exposure combined with the surface stress measurement is described elsewhere [5]. To observe the effect of ozone exposure to Si(1 0 0) surfaces, we conducted an in-situ Scanning Tunneling Microscopy (STM) observation. STM can reveal atomic scale structures on the surface where reactions occurred and provide us with information to understand the origin of the measured surface stress. For STM observation, this time, we focused only on the initial stage of O3 exposure. Clean Si(1 0 0) surfaces were prepared by flashing at 1100 C and then confirmed by STM. Following this, the sample was cooled down to RT and was oxidized by introducing the highly concentrated ozone gas into a UHV chamber for a short time (61–2 s) at 107–106 Torr. This amount of O3 roughly corresponds to 0.1–2 L. Although we used the same O3 source as that used for the surface stress measurements, we observed surfaces of a different shaped sample due to experimental limitations. Fig. 1 displays evolutions in the surface stress for two cases, namely, O3 and O2 exposure to Si(1 0 0) surfaces at RT. In the case of O3 exposure, from the initial exposure, the surface stress remarkably and monotonically increased to the compressive stress side as a function of time and then became saturated. The saturated value of the compressive stress was 0.33 N/m. This indicates that O3 oxidation results in surface expansion from the initial exposure. The oxide film thickness was up to 0.6 nm which was estimated by X-ray Photoelectron Spectroscopy (XPS). In contrast, in the case of molecular O2 exposure at RT, only a tiny compressive side surface stress of 0.002 N/m was induced. A huge difference exists in the surface stress evolutions of O3 and O2. This strongly suggests that differences in oxidation species can be detected via surface stress measurement.
Fig. 1. Sample deflection (d) and the surface stress evolution (rt) during ozone (O3) and molecular oxygen (O2) gas exposure. In both cases during measurements, pressures in pipeline and substrate temperatures are set to be 0.2 Torr and RT, respectively. Arrow indicates the point where STM images of Fig. 2 are taken.
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It is known that oxide films thicker than 10–50 nm have an intrinsically compressive stress due to difference in the volume changes between SiO2 and Si [6]. This does not depend on surface orientation but simply difference in the volume changes between Si and SiO2 layers. However, surface stresses on oxide films thinner than this thickness is not so easily understood, because atomic scale effects resulting from the surface reconstruction/orientation cannot be ignored. In an early study performed by Sander and Ibach [7], molecular oxygen exposures to Si(1 1 1) and Si(1 0 0) surface at RT showed different surface stress signatures. Their results indicated that there should exist varieties in the surface stress due to the atomic scale surface structure. Hence it is important to consider the dominant surface structure effect for the oxide film on Si system thinner than 10 nm. Recently, our group carried out surface stress studies of Si(1 0 0) surfaces during plasma oxidation [2,3,8]. Basically, during the initial stages, tensile surface stresses were induced both by pure oxygen plasma [2,3] and a plasma of krypton and oxygen mixture [8]. However ozone oxidation induced a compressive surface stress as previously mentioned. In addition, plasma oxidation processes displayed much more complicate behaviors than those of thicker oxide film growth or ozone oxidation. For example, in the case of oxidation using pure oxygen plasma and applying a positive bias to the sample, three surface stress stages were apparent, namely, compressive-tensile-compressive, for 1 nm oxide growth. To investigate the influence of the surface structure of Si(1 0 0) on the complicate behavior, we intentionally destroyed the surface structure to a depth of 1 nm via low energy Ar ion bombardment and observed surface stress under this same conditions [3]. As a result, the first three surface stress stages disappeared completely. This strongly suggests that the complicate behavior for pure oxygen plasma oxidation under positive sample bias should be caused by the ordered surface structure of Si(1 0 0). In other words, in the case of oxidation, surface stress measurement can detect surface structure changes as well as differences in oxidation species. Fig. 2 displays surfaces exposed at RT to different amounts of O3. The exposure quantity was estimated from the pressure increase, and deemed to represent the surface condition on initial exposure as highlighted in Fig. 1. From Fig. 2a, bright bumps were visible, which typically lie in the middle and on the top of dimer rows. Such features were also observed for Si(1 0 0) on exposure to molecular oxygen of 108 Torr at RT [9]. Along with the features above, significant dimer row buckling was induced, which had not been mentioned in the previous report. The bumps also acted as nucleation sites for elongated islands when the exposure was done at the elevated temperature of over 300 C, while the exposure at 200 C did not produce it at all as shown in Fig. 3. The nucleation of elongated islands from the bumps was also reported in the case of molecular oxygen exposure [9]. Here, the bumps and the elongated islands were thought to consist of ejected Si
Fig. 2. STM images of Si(1 0 0) surface for the initial stage of ozone oxidation at room temperature. These images were obtained under Vsample = 1.5 V and Itunnel = 0.10 nA. Amount of O3 exposures increases from (a) to (c). Dashed circles in (b) indicate positions of the ‘downstairs dimer row’.
atoms from the surface region. Also, although numerous buckling dimer row structures were still observed at 200 C, most of the buckling dimer row structures vanished away at 300 C. Therefore, the induced buckling dimer row structures could relax into symmetric dimers due to the nucleation over 300 C as well as a simple thermal effect. As mentioned above, local structures induced by O3 exposure were similar to those of molecular oxygen. However, the number of the surface Si atoms does not seem to be conserved by O3 exposure, but largely reduced from that of the original clean surface, even with the inclusion of the Si atoms ejected onto the surface. It is hard to determine the exact number of the surface atoms from the STM images, because reacted sites also show depletion in brightness
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Fig. 3. Images of Si(1 0 0) surfaces at room temperature after the ozone exposure at the elevated temperature of 200 C (a) and 300 C (b), which were taken with the tunneling conditions of (1.44 V, 0.16 nA) and (1.7 V, 0.07 nA), respectively. Exposure quantities seem to be slightly less than that of the Fig. 2. Dashed circles denote isolated bumps in (a) and elongated islands in (b). Also, arrows indicate positions of buckling dimer rows.
due to the decrease in the local density of states (DOS). Qualitatively speaking in the case of Fig. 2b, 30% of atoms are removed from the surface region by estimating the area of the dark patches. As a piece of supporting evidence for atomic removal, we could also find newly formed dimer row structures in the removed region, as shown by the dashed circles of Fig. 2b. This structure looked like the ‘downstairs dimer row’ (DDR), having a 1 ML depth. It also implied that these dark patches could not only be due to a decrease of the local DOS by reaction but also represent real topology. In the case of oxidation of Si using molecular O2 at RT, it has been reported that the initial stage of oxide growth only depends on the total number of O2 molecules impinging on the surface [10]. Also, at higher temperature, lower pressure causes a higher desorption rate of Si–O. Although the exposure of O3 is almost equivalent to that of molecular oxygen and also the pressure of O3 was higher than that of molecular oxygen exposure [9], we saw much heavier removal of atoms, i.e., surface etching only in the case of O3. Hence, we concluded that the difference between O3 and O2 exposure was due to the existence of the surface etching. At higher exposure, as shown in Fig. 2c, the surface became invariably rough and any ordered features such as DDR structures can no longer be seen. In general, it is essential to have a flat and/or well-ordered structure in the surface plane for an effective buildup of surface stress. The clean Si(1 0 0) surface is one of the typical examples and shows intrinsic surface shrinkage, i.e., a tensile stress. This is caused by forming 2 · 1 dimer structures at the surface. Dimer formation results in surface shrinkage, with slight expansion occurring along dimer rows. Here, the difference was theoretically determined to be 1.1–3.2 N/m [11,12].
The initial surface stress measured here should originate from the structure which the snapshots of Fig. 2 represent. The main difference between the O3 and O2 exposure was the existence of surface etching as previously mentioned. This surface etching introduces surface roughness, which results in the destruction of the intrinsic surface stress. Therefore, the huge surface stress to the compressive stress side should be basically driven by release of the intrinsic tensile stress due to the 2 · 1 dimer formation. Especially, during the initial stage of O3 exposure, this release mechanism of the intrinsic surface stress can work effectively, because larger amounts of surface shrinkage can be released. Then, an initial steep increase and the following gradual saturation of the surface stress may be basically explained with this mechanism above. On the other hand, the local features which were also observed for molecular O2 exposure [9] should make a tiny contribution to the surface stress growth. It should be also noted that the surface stress evolution for further oxide growth (which could potentially show a compressive stress due to volume expansion via oxygen atom insertion) could not be distinguished properly, as the oxidation rate is extremely low at room temperature. When O3 exists on the Si(1 0 0) surface, the following reaction can be expected as a first step, i.e., Si + O3 ! Si–O + O2. Molecular O2 is created by this process. This reaction emits 357.93 kJ/mol (=3.72 eV) as exothermic energy. The bonding energy of Si–O is assumed to be 464.4 kJ/mol following thermal consideration. Hence, if this exothermic energy is consumed for the subsequent reaction, two possible reaction paths should exist, namely, thermal desorption and additional oxide formation. The activation energy for thermal desorption of SiO into the vapor phase was experimentally determined to be
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4.0 ± 0.37 eV [13,14]. On the other hand, the reaction for the formation of SiO, following molecular O2 creation due to the first step reaction can be written as, Si + O2 ! Si–O + 1/2O2, generating 215.23 kJ/mol as exothermic energy. For this situation, the activation energy is experimentally determined to be 2.7 ± 0.15 eV [13,14]. From an energetic point of view, O3 can provide sufficient energy to promote not only oxidation but also an etching event due to its dissociation reaction step. This etching event is not possible for O2 however, due to energetics limitations. For pure molecular oxygen, only oxidation is achievable. On the contrary, the formation of DDR structures may be explained in the following way. When Si monomers are created at etched sites following the desorption of Si atoms from the surface, these can form stable dimers without any additional activation barrier [15]. Following this, an activation energy ranging from 1.3 to 1.45 eV is required to create dense dimer row structures [15]. As a result, it may be possible to create DDR structures even at RT with help from the exothermic energy created during the dissociation of O3. Such DDR structures are expected to generate a tensile stress component. However the population of these structures is too low to make a noticeable contribution to the total surface stress signal. We have studied the surface stress evolution and atomic scale structure during initial ozone oxidation of the Si(1 0 0) surface, and compared results with oxidation via molecular oxygen. As a result, we found that the surface stress evolutions during ozone exposure showed a monotonic increase to the compressive stress side, while no remarkable surface stress change was observed for molecular oxygen exposure. In addition, heavier surface etching even at RT was observed on the surface exposed to ozone. We have attributed the origin of the compressive stress, not to local Si–O bond formation, but to the reduction of the intrinsic tensile surface stress of the reconstructed Si(1 0 0)-2 · 1 surface via the etching process. Surface etching introduces surface roughness. As a result, excess strain caused by the reconstruction at the surface or the SiOx/Si interface is released. Therefore oxide formation involving volume expansion can proceed under a stress free condition. Ozone can produce very thin SiOx/Si transition layers [16,17] with good electrical
characteristics [17,18], due to the release of the intrinsic stress for small exposures. Its characteristics are much more beneficial that those of molecular oxygen. Using stress control techniques, ozone appears to have potential to be utilized for future nanoscale devices. Acknowledgements We would like to thank Prof. J. J. Boland, Mr. N. T. Kinahan and Mr. M. O’Toole, Trinity College Dublin for their fruitful discussions and useful comments. References [1] T. Narushima, M. Kitajima, K. Miki, J. Phys.: Condens. Matter 16 (2004) L193. [2] A.N. Itakura, T. Narushima, M. Kitajima, Appl. Surf. Sci. 159–160 (2000) 62. [3] T. Narushima, A.N. Itakura, T. Kurashina, T. Kawabe, M. Kitajima, Appl. Surf. Sci. 159–160 (2000) 25. [4] H. Nonaka, A. Kurokawa, S. Ichimura, T. Nishiguchi, Y. Morikawa, M. Miyamoto, J. Vac. Soc. Jpn. 44 (2001) 221. [5] A. Kurokawa, T. Narushima, K. Nakamura, H. Nonaka, S. Ichimura, A.N. Itakura, M. Kitajima, Jpn. J. Appl. Phys., Part 1 43 (2004) 281. [6] E. Kobeta, E.A. Irene, J. Vac. Sci. Technol. B 4 (1986) 720; E. Kobeta, E.A. Irene, J. Vac. Sci. Technol. B 5 (1987) 15; E. Kobeta, E.A. Irene, J. Vac. Sci. Technol. B 6 (1988) 574. [7] D. Sander, H. Ibach, Phys. Rev. B 43 (1991) 4236. [8] A.N. Itakura, T. Narushima, M. Kitajima, J. Vac. Soc. Jpn. 44 (2001) 81. [9] D.G. Cahill, P. Avouris, Appl. Phys. Lett. 60 (1992) 326. [10] F. Lutz, J.L. Bischoff, L. Kubler, D. Bolmont, Phys. Rev. B 40 (1989) 10356. [11] O.L. Alerhand, D. Vanderbilt, R.D. Meade, J.D. Joannopoulos, Phys. Rev. Lett. 61 (1988) 1973. [12] F. Liu, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 3156. [13] U. Memmert, M.L. Yu, Surf. Sci. Lett. 245 (1991) L185. [14] M.L. Yu, B.N. Eldridge, Phys. Rev. Lett. 58 (1987) 1691. [15] T. Yamasaki, T. Uda, K. Terakura, Phys. Rev. Lett. 76 (1996) 2949. [16] K. Nakamura, A. Kurokawa, S. Ichimura, Thin Solid Films 343–344 (1999) 361. [17] K. Nakamura, S. Ichimura, A. Kurokawa, K. Koike, G. Inoue, T. Fukuda, J. Vac. Sci. Technol. A 17 (1999) 1275. [18] T. Nishiguchi, H. Nonaka, S. Ichimura, Y. Morikawa, M. Kekura, M. Miyamoro, Appl. Phys. Lett. 81 (2002) 2190.