Papers

AVS2002 Session TF-ThA: Ultra Thin Films

Thursday, November 7, 2002 2:00 PM in Room C-101

Thursday Afternoon

Start	Invited?	Item
2:00 PM		TF-ThA-1 Oxidation of InP, Si and SiC Surfaces Y.J. Chabal, O. Pluchery, F. Amy, M. Frank, K. Raghavachari (Agere Systems) Passivation of semiconductor surfaces by thermal oxidation or low temperature oxide deposition is critical for both microelectronics (e.g. gate oxides on Si) and optoelectronics (e.g. epi-growth and wet chemistry of InP-based laser structures). Yet, the amorphous or microcrystallline nature of most oxides has made it difficult to understand and therefore to control oxide growth and composition. We have developed in situ infrared (IR) absorption methods to characterize ultra-thin oxides and to understand their growth in various environments (vacuum, gas ambient, and solution.¹) In this talk, we contrast the multiphase and layered nature of a variety of oxides (native, wet-chemical, plasma and anodic) grown on InP(100) surfaces to the much simpler stoichiometric SiO₂ films on silicon, and investigate oxide regrowth on acid-etched oxide-free InP surfaces. We also highlight the role of dangling bonds and strain on molecular oxygen dissociation and incorporation into Si(100)-(2x1), 4H-SiC(0001)-(3x3)² and 3C-SiC(100)-(3x2) surfaces. Finally, we discuss the role that hydrogen plays during the growth of ultra-thin oxides, and its ability to passivate (or create) interfacial defects. ¹ O. Pluchery, J. Eng Jr., R. L. Opila, Y. J. Chabal, Surf. Sci. (April 2002). ² F. Amy et al., Phys. Rev. Lett. 86, 4342 (2001).
2:20 PM		TF-ThA-2 The Kinetics of Ultrathin Aluminum Oxide Film Growth on Aluminides J.F. Moore, A. Zinoviev, M.J. Pellin (Argonne National Laboratory) Oxide films grown on metals at high temperature under high vacuum meet the criterion for near-equilibrium growth, since the diffusion rate of atoms in the metal is high enough to prevent kinetically trapped phases from forming. In this study, a number of aluminide single crystal and polycrystalline metals are oxidized at temperatures near their melting point under high vacuum (less than 10^-5mBar O₂). The film growth kinetics are measured continuously with x-ray photoelectron spectroscopy (XPS) during oxidation. Phenomena including surface segregation of intermetallic components, diffusion, nucleation, and island formation are elucidated during these experiments. Self-limited growth of aluminum oxide at 3-5nm was observed in some cases, as well as dramatic reduction in cracking due to thermal cycling or lattice mismatch. This last effect is of considerable interest since compounds with a wide range of bulk properties could be used for a given application rather than a more limited set with the appropriate surface structure. The kinetics of the oxidation process in these studies will be discussed, including critical parameters such as temperature, aluminum concentration, and oxygen dose rate (pressure).
2:40 PM		TF-ThA-3 Stability of Hafnium Oxide and Silicate Ultrathin Films on Si I.J.R. Baumvol (Universidade Federal do Rio Grande do Sul, Brazil) Among the potential replacements for SiO₂ or SiO_xN_y as gate dielectric in Si-based advanced MOSFET devices, hafnium oxide (HfO₂) and silicates (Hf_xSi_yO_z) seem to be the most promising materials, combining high dielectric permittivity with low leakage current due to a reasonably high barrier height that limits electron tunnelling. In order to be effectively incorporated into ultra-large scale integration (ULSI) fabrication technology, the gate dielectric material must conserve its integrity in further processing steps. In particular, rapid thermal annealing of source and drain dopants, usually performed at and above 1000 °C, has been indicated as the most aggressive step. The main deleterious consequences of post gate dielectric deposition annealing reported so far include thickening of the SiO₂ interlayer and chemical reactions at both gate electrode/dielectric and dielectric/Si interfaces, with the consequent low! ering of the capacitance equivalent thickness, and diffusion of Hf into Si which reduces electron and hole mobilities in the transistor channel. Furthermore, these studies pointed out the need for controlling the effects of annealing in intentionally or unintentionally O₂-containing atmospheres (even at very low O₂ partial pressures), which renders post-deposition annealing even more aggressive to gate dielectric integrity, especially in the near dielectric-semiconductor interface region. The present work describes the results of composition, atomic transport and chemical reaction investigations following rapid thermal annealing in Ar, N₂ and O₂ of ultrathin HfO₂ and Hf_xSi_yO_z films deposited on Si in the temperature range 800 to 1000 °C. Film structures and compositions were established by high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, nuclear reaction analysis, and X-ray photoelectron spectroscopy. Isotopic substitution and sub-nanometric depth resolution profiling using narrow nuclear resonant reaction profiling revealed that O migrates by means of a propagating front from the surface that reacts with the hafnium oxide or silicate networks as it advances, the main reaction channels being O-O and O-N exchanges. O penetration, incorporation in the bulk of the ultrathin film structures, and oxidation of the substrate forming SiO₂ here observed were significantly smalle! r than in previously studied Al, Zr and Gd oxides and silicates. Hf penetration into Si was observed, amounting between 10¹² and 10¹³ Hf/cm², which would degrade electron and hole mobilities in MOSFET transistor channels. Pre-annealing in non-reactive atmospheres like Ar and N₂ increase the stability and resistance to oxidation of the films.
3:20 PM		TF-ThA-5 Electrical and Physical Properties of Very Thin, Nearly Epitaxial Cu (100) Sputter-deposited at Room Temperature C. Detavernier, S.M. Rossnagel, C. Noyan, C. Lavoie (IBM T.J. Watson Research Center); T.S. Kuan (SUNY-Albany); D. Deduytche, R.L. Van Meirhaeghe (Universiteit Gent, Belgium) The sputter deposition of high purity Cu onto HF-cleaned Si(100) substrates at room temperature or below results in very highly oriented, nearly epitaxial Cu (100) films with good electrical properties. Using XRD, we could only observe the Si(400) and Cu(200) peaks, indicating that the Cu is strongly textured. The resistivity of these films is found to be much smaller than the typical resistivity values obtained for unannealed polycrystalline PVD Cu films; approximately 1.9 micro-ohm-cm for films > 100 nm thickness, dropping to 1.7 micro-ohm-cm for >200 nm films deposited at 90C. The electrical resistivity increases as the film thickness is decreased, consistent with the non-specular electron-surface scattering. The resistivity increase exceeds the size-effect alone for very thin films which may be indicative of the presence of line defects or low angle grain boundaries. These defects would also explain the broadening of the XRD peaks that we observed for thinner films. The films were stable with time, unlike companion films deposited on silicon dioxide which undergo grain growth on the scale of days resulting in reduced electrical resistivity. TEM analysis shows a high dislocation density in the films, but good evidence of epitaxy. The grain size for 30 nm films is many hundreds of nm. The Cu(100) films are ideal for the study of size effects, specular scattering and any electrical anisotropies due to their near-bulk resistivity, lack of grain boundary effects, and crystalline stability.
3:40 PM		TF-ThA-6 Epitaxial Growth of Au and Ag Films on Si(111) using Cu Buffer Layers K. Pedersen (Aalborg University, Denmark); P. Morgen (University of Southern Denmark); T.G. Pedersen (Aalborg University, Denmark); Z.S. Li, S.V. Hoffmann (Aarhus University, Denmark) Deposition of metals on semiconductor surfaces often lead to films of poor quality due to lattice mismatch or chemical reaction between metal and the substrate. For both Au and Ag growing on Si(111)-7x7 the films are imperfect (111) metal layers. In the case of Au the deposition also creates a disordered, Si-rich layer that floats on top of the Au layer. Furthermore, the surface states usually found on (111) faces of noble metals are absent for these Au and Ag films. Though Cu reacts with Si, growth of a well ordered Cu(111) film starts after a thin reacted phase around the contact layer has been created. The reacted layer thus serves as buffer layer between crystalline Cu and the Si substrate. In synchrotron radiation photoemission experiments we demonstrate how the reacted Cu layer also serves as a buffer layer improving the properties of Au and Ag films on Si(111). Core level spectra show that the buffer layer prevents the reaction between Au and Si. This leads to an ordered surface of the film with a surface state. Thin Ag films grown directly on Si(111)-7x7 form quantum well states that are seen as peaks in valence band spectra. These peaks appear broader and weaker than for Ag grown epitaxially on for instance Fe(001). Furthermore, strains caused by mismatch between Ag and Si lattices shift the surface state above the Fermi level. The thin Cu buffer layer improves on both problems. The sharpness of the quantum well levels increases considerably and the surface state appears just below the Fermi level. From the sharpness of quantum well levels it is concluded that the optimum buffer layer thickness is 6 - 8 atomic layers.
4:00 PM		TF-ThA-7 Ag Ultra Thin Film Stability on GaAs (110): An ab-initio Study D.L. Irving, S.B. Sinnott (University of Florida); R.F. Wood (Oak Ridge National Laboratory) Metal-semiconductor interfaces are widely utilized in modern device applications. As device sizes are drastically reduced it is essential to have a fundamental understanding of the stability of the ultra thin metallic overlayer on a semiconducting substrate. The Ag(111)/GaAs(110), a system that is known to be non-wetting, has been widely studied in recent years because it has been shown that stable metallic overlayers could be generated by means of a low temperature deposition followed by an anneal to room temperature. The stability of the Ag film was also found to be dependent on the amount of Ag initially deposited onto the GaAs substrate. This research uses density functional theory under the pseudopotential approximation to study the construction of an accurate computational model of this heterogeneous interface. The goal is to determine how strain in the unit cell affects the adhesion of the Ag layer to the GaAs substrate.
4:20 PM		TF-ThA-8 Synthesis of Ultrasmooth CN_x Overcoats for 1 Tb/in² Magnetic Storage Applications D.-J. Li, Y.-W. Chung (Northwestern University) Nitrogenated carbon (CN_x) is now being used in protective overcoats of commercial hard disk systems, due to its compatibility with existing lubricants as well as desirable tribological and corrosion protection performance. The current goal of the disk drive industry is to increase the areal storage density to 1 Tb/in² by reducing the head-disk spacing. Modeling calculations show that the protective overcoat thickness has to reduce from 4-5 nm in current dirves to 1.0 nm. In this case, producing an atomically smooth and dense CN_x coating with low defect density becomes crucial. The purpose of this work is to synthesize smooth and pinhole-free CN_x overcoats over large areas by controlling magnetron sputtering process parameters. Effects of sputter gas composition, target power, substrate bias, substrate tilt and rotation speed on film growth and properties were explored. One important finding from this study is the combined use of substrate tilt and rotation. AFM scans over large sampling areas show that thin CN_x films obtained at -100 V substrate bias with 45 degree substrate tilt and 20 rpm rotation have r.m.s roughness almost four times lower than those prepared without substrate tilt and rotation. These 1-nm thick ultrasmooth coatings reduced corrosion damage by a factor four compared with coatings of the same thickness grown without substrate tilt and rotation. This improved performance is likely a result of more efficient and uniform momentum transfer from energetic species in the plasma to surface atoms parallel to the surface during deposition in this configuration.
4:40 PM		TF-ThA-9 Ultra-thin Silicon- and Aluminum Oxides on Silicon formed Layer-by-layer P. Morgen, T. Jensen, C. Gundlach (University of Southern Denmark (SDU)); K. Pedersen, N. Skiversen, P. Kristiansen (Aalborg University, Denmark); S.V. Hoffman, Z.S. Li (Aarhus University, Denmark) At the end of the "road" for silicon based micro- and nano-electronics the gate oxide layers must shrink to their ultimate limits of thickness. We have developed some new layer-by-layer assembly procedures to form the thinnest possible oxides (0.3 - 5nm). These are then subjected to various types of measurements of their structure, chemical-, physical-, and electrical properties. We report details of the different processes to form these oxides, and their nano-chemical composition at - and vertically above - the interface, including a comparison of oxides grown on the Si (111) and Si (001) faces, in studies primarily using synchrotron-induced photoemission. We have established a procedure to monitor the field across ultra-thin layers of silicon oxide, with deposition of Ag on top of the oxide, ranging in coverage from dispersed atoms to above a full layer. Such a field exists for the Si (111)/silicon oxide interface but not for the Si (001)/silicon oxide interface after our processes. In both cases a Schottky barrier is formed under a thicker Ag layer, with the silicon oxide between the metal and semiconductor. Aluminum oxide has recently attracted a lot of interest as a substitute for silicon oxide on silicon, as it has a much higher dielectric strength than for a comparable thickness of silicon oxide. With our method, the layer-by-layer growth of aluminum oxide on silicon is very easy to control and to extend to other substrates than silicon.
5:00 PM		TF-ThA-10 Characterization of Ultra-Thin Films using Angle Resolved XPS and Maximum Entropy Methods R. White, R. Champaneria, J. Wolstenholme, P. Mack (Thermo VG Scientific, UK) The Thermo VG Scientific Theta Probe has the unique ability to collect angle resolved XPS (ARXPS) spectra in parallel without the need to tilt the sample. The collection of ARXPS data (free from artifacts associated with sample tilting) is now routine for samples ranging in size from 20 µm to 300 mm. This has made it necessary to implement a consistent method for the generation of depth profiles from ARXPS data. The conversion of ARXPS data into depth profiles is problematic as no single transformation from ARXPS data to depth dependent concentrations exists. As real data contain noise, a weighted sum squares of error between measured and calculated data is not sufficient to determine accurate depth profiles. A usable solution can be found by the inclusion of an entropy term that allows a profile to be generated with the minimum amount of structure. Characterisation of ultrathin high K dielectric overlayers on SiO₂ is one application where ARXPS shows great promise, potentially yielding thickness, distribution, dosage and chemical state information. A maximum entropy approach based on the methods developed by Smith and Livesey has been adopted in order to interpret Theta Probe data. This approach may be thought of as a least squares fitting of the data, with an in-built 'smoothness' parameter to prevent overfitting to noise in the data, for example. Maximum entropy calculations are not restricted to small numbers of chemical state components. This makes this method particularly suitable for ARXPS where low concentration and multi chemical state components need to be profiled. This paper describes the method and results from various systems where this approach has been adopted.