Silicide Surface Research Articles

Methylsilane on Cu(1 1 1): A MDS study of the formation of the surface silicide

The adsorption of methylsilane on Cu(1 1 1) has been investigated by metastable deexcitation spectroscopy. The deexcitation process for the clean copper and silicide surfaces was demonstrated to occur via resonance ionization followed by Auger neutralization, while upon the adsorption of methylsilane at 295 K it changes to Auger deexcitation. Accompanied by ultraviolet photoelectron spectroscopy, the MD spectrum at 295 K reveals the presence of a methyl group on the surface, supporting the assertion that the majority of the surface is covered with methylsilane fragments. Annealing the surface above 420 K reveals the presence of clean copper sites on a partially silicide surface.

Methylsilane on Cu(1 1 1), a STM study of the [formula omitted]R30°-Cu 2Si surface silicide

Scanning tunnelling microscopy has been used to investigate the surface structure of the ( 3 × 3 ) R30°-Cu 2Si surface silicide formed on Cu(1 1 1) after adsorption of methylsilane at 595 K. The STM images have shown the presence of a domain wall network on the surface, in the form of a 0.1 Å variation in height on the lateral scale of a minimum of 26 Å. The interpretation of the STM images has indicated that the areas between the domains walls are associated with silicon and copper atoms both residing in either fcc or hcp three-fold hollow sites, whilst the domain wall is a result of an abrupt change enhanced with some electronic contribution.

Growth and electronic structure of holmium silicides by STM and STS

The growth of holmium on the Si(1 1 1) surface in the sub-monolayer region was investigated using STM. Several metastable reconstructions, including 5 × 2 and ( 2 3 × 2 3 ) R30° were observed during the nucleation of the 1 × 1 2D rare earth (RE) silicide surface. The authors attempt to combine this data into a detailed picture of the development of the surface, and identify future directions for improving interface quality. Scanning tunnelling spectroscopy data were taken from the 1 × 1 surface, and compared with published theoretical and photoemission results from other 2D RE silicides.

Study of surface segregation of Si on palladium silicide using Auger electron spectroscopy

The transformation of Pd/Si to Pd2Si/Si is studied using Auger electron spectroscopy over a wide temperature range of 370–1020 K. The Pd film gets totally converted to Pd2Si upon annealing at 520 K, and beyond 570 K, Si starts segregating on the surface of silicide. It is found that the presence of surface oxygen influences the segregation of Si. The time evolution study of Si segregation reveals that segregation kinetics is very fast and the segregated Si concentration increases as the temperature is increased. Scanning electron microscopy measurements show that Pd2Si is formed in the form of islands, which grow as the annealing temperature is increased.

Suppression of silicidation in polycrystalline-Si/ high-κ insulator/SiO2∕Si structure by helium through process

Suppression of silicidation in polycrystalline-Si (poly-Si)/ high-κ insulators (ZrO2,HfO2)∕SiO2∕Si structure by helium (He) through process which adds He gas during a poly-Si gate process was demonstrated. In Si deposition on ZrO2 and HfO2 by SiH4 flow diluted by N2 or He, surface silicidation can be suppressed by lowering of initial deposition temperature below 600 and 650°C, respectively, but thin SiO2 films are formed between Si films and these high-κ insulator surfaces without being dependent on the dilution gases. Moreover, it is found that He through process of low-temperature SiH4 flow diluted by He and high-pressure He postannealing is the most effective means of suppressing not only the thermal degradation of high-κ insulator/SiO2∕Si interface but also the silicidation of poly-Si∕SiO2/ high-κ insulator interface, whereas conventional N2 through process cannot suppress both. These results mean that high-concentration He atoms are indispensable for upper and lower SiO2∕Si interfaces. It is supposed that many He atoms physically obstruct SiO creation through the quenching of atomic vibration at both SiO2∕Si interfaces, thus impeding the first step of silicidation reaction. In addition, by comparing with SiO2 single film, it is found that the suppression efficiency of He atoms in the high-κ insulator/SiO2∕Si system is higher than that in the SiO2∕Si system. This phenomenon can be rationalized by presupposing that the efficiency reflected the degradation-prone site density in the interfacial SiO2 layer.

Effects of Ti/Co and Co/Ti systems on the germanosilicidation of poly-Si capped poly-Si 1−xGe x substrate

Ti/Co and Co/Ti bilayered systems were used for the formation of Co germanosilicide on polysilicon buffered poly-Si 1−xGe x ( x=0.2, 0.3) gate stacks. In both cases, substantial Ge was found to segregate to the grain boundaries at the surface of the polycrystalline silicide (i.e., CoSiGe) films, resulting in a substantial degradation in the sheet resistance and a poor quality of the silicide films. It is shown that Ge is probably being expelled from the poly-SiGe grain and segregates to the grain boundaries during the annealing process, leading to the undesirable sheet resistance and poor film morphology observed from the Co germanosilicide films.

Field Electron Emission from Inkjet-Printed Carbon Black

We demonstrate the field electron emission from a carbon black dot array prepared by inkjet printing. A commercially available inkjet printer and drawing software were used to pattern the carbon-black-containing ink on the tungsten silicide surface which was deposited on a Si wafer. Field electron emission characteristics were measured using a diode configuration. It was found that oxygen reactive ion etching (RIE) of the printed carbon black is effective for obtaining the field emission. The turn-on voltage was 500 V and an emission current over 100 µA was obtained at a voltage of 1400 V with a 25 µm-wide gap between anode and cathode. It was also found that a rapid increase of the voltage is effective in increasing the emission current and the number of emission sites.

Electronic structure and Fermi surface of two-dimensional rare-earth silicides epitaxially grown on Si(111)

The electronic structure and the Fermi surface of two-dimensional rare-earth silicides epitaxially grown on Si(111), ${\mathrm{YSi}}_{2}$ and ${\mathrm{GdSi}}_{2},$ have been studied by a combination of angle-resolved ultraviolet photoemission spectroscopy and density functional theory calculations. Both silicides present a very similar electronic structure, with two characteristic electronic bands below the Fermi energy. One crosses the Fermi energy near the $\overline{\ensuremath{\Gamma}}$ point of the surface Brillouin zone (hole pocket) and the other one close to the $\overline{M}$ point (electron pocket). These two bands arise from surface (localized) states and are responsible for all the Fermi surface features. The theoretical calculations are in good qualitative agreement with the experimental results, and also allow to examine the nature of the bonding between the rare earth and the neighboring silicon atoms. We have found a combination of sp metallic type bond together with covalent bonds involving the rare-earth d states and Si $3p$ states.

Suppression of Ni Silicide Formation by Se Passivation of Si(001)

ABSTRACTValence mending of a semiconductor surface, such as the Se-passivated Si(001) surface, improves the chemical and thermal stability of the surface as compared to the bare Si(001) surface. In this paper, we report the suppression of Ni silicide formation between Ni and Si(001) through monolayer passivation of Si(001) by Se. Ni was deposited on both Se-passivated and bare Si(001) surfaces. The samples were annealed at temperatures from 400°C to 700°C. Cross-sectional TEM (Transmission Electron Microscopy) revealed that Ni on bare samples reacted with Si at 400°C and formed silicide, whereas Ni on Se-passivated samples did not react with Si at 500°C. Surface composition analysis by XPS (X-Ray Photoelectron Spectroscopy) showed pure Ni surface on Se-passivated samples annealed at 400°C and 500°C, but silicide surface on bare samples annealed at the same temperatures. Hence, Se passivation suppresses the formation of Ni silicide on the Si(001) surface by over 100°C as compared to the bare Si(001) surface. These results may have important implications in source/drain engineering in sub-100 nm Si CMOS (Complementary Metal Oxide Semiconductor) devices.

Effects of annealing temperature and surface preparation on the formation of cobalt silicide interconnects

Cobalt silicide films have been prepared by rapid thermal annealing of cobalt layers sputter deposited on silicon substrates. We report on the evolution of silicide phases, surface and interface roughness as a function of the annealing temperature and silicon surface preparation. The characterizations are carried out by atomic force microscopy, X-ray diffraction, X-ray reflectivity, Raman spectroscopy, and transmission electron microscopy. The cleaning procedure of the silicon substrate affects the interface roughness and the silicide thickness, whereas little effects are found on the surface. On the other hand, surface roughness increases with annealing temperature.

Electrochemical and surface characterization of platinum silicide electrodes and their use as stable platforms for electrogenerated chemiluminescence assays

Electrochemical methods, including cyclic voltammetry and scanning electrochemical microscopy, as well as surface techniques, including conductive atomic force microscopy (AFM) and X-ray photoelectron spectroscopy, were utilized to evaluate and characterize the extent of an oxide layer on platinum silicide (PtSi) surfaces that were pretreated by a variety of approaches; piranha solution (1:4 H2O2+H2SO4), hydrofluoric acid (HF), chemical reduction in NaBH4 and after mechanically polishing the surface. Electrochemical methods showed that in the presence of an oxide layer, the rate of electron transfer depended upon the charge of the redox couple: the more negative the charge, the slower is the rate of electron transfer. Additionally, the current levels observed in the presence of an extensive oxide layer were considerably lower than those observed after the oxide layer was removed either with HF acid or by mechanically polishing. Surface analysis and depth profiles obtained using Auger electron spectroscopy demonstrated that PtSi surfaces pretreated with piranha contained the largest amounts of surface oxides. AFM topographic scans along with localized surface conductivity showed that in the presence of this oxide layer, electron transfer occurred at nanoscale domains located between the PtSi grains with the rest of the surface, which most likely contains an oxide layer, being non-conductive. The surface oxide layer was used to attach the electrogenerated chemiluminescent (ECL) label, Ru(bpy)32+ covalently, either directly or via single-stranded DNA. Emission during oxidation in the presence of the co-reactant tri-n-propylamine was observed, illustrating the possible use of PtSi as a platform for ECL-based bioassays.

Thermal reaction of iron with a Si(111) vicinal surface: Surface ordering and growth of CsCl-type iron silicide

The structural evolution of a vicinal silicon surface, developing upon low coverage deposition of Fe [0.33 and 2 monolayers (ML)], has been carefully studied as a function of annealing temperature by means of scanning tunneling microscopy. The reaction between the deposited material and the substrate, which occurs at room temperature, leads to surface amorphization. Successive annealing induces substantial changes of the surface structure. The onset of 2\ifmmode\times\else\texttimes\fi{}2 reconstruction expansion of the Si surface, covered with 0.33 ML Fe, is observed after annealing at 400 \ifmmode^\circ\else\textdegree\fi{}C. Complete reconstruction of the whole surface is found at the same temperature for a coverage of 2 ML. Upon annealing at 700 \ifmmode^\circ\else\textdegree\fi{}C three-dimensional iron silicide islands grow epitaxially on the Si substrate. Their size, shape, and location depend on the amount of the deposited material. For 0.33 ML coverage elongated crystallites are distributed randomly on the vicinal substrate surface. Larger crystallites, grown upon deposition of 2 ML of Fe, take the shape of truncated pyramids and show a tendency to nucleate along vicinal surface terrace edges, forming a self-organized array of nanometer size dots. For both Fe coverages studied they are found to grow in a CsCl-type structure, containing an increasing amount of vacancies with increasing crystallite size. The electronic properties of iron silicide surface are probed with a spatial atomic resolution by means of tunneling spectroscopy. The interface between iron silicide crystallite and silicon substrate displays features characteristic of Schottky barriers.

Formation of CaMgSi at Ca 2Si/Mg 2Si interface

The structural property and interfacial morphology have been investigated for the Ca 2Si/Mg 2Si interface. The Ca 2Si layers were grown on Mg 2Si layers by heat treatment of the Mg 2Si/Si substrates in Ca vapor. It is found that the CaMgSi phase is formed at the Ca 2Si/Mg 2Si interface. The structural property of the resultant silicides was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical composition of the layers was examined by energy dispersion spectroscopy (EDS). In addition, the chemical analysis by X-ray photoelectron spectroscopy (XPS) revealed the Mg segregation on the silicide surface. The growth mechanism of Ca 2Si on Mg 2Si/Si is also discussed.

Nickel silicidation techniques for strained Si1−xGex, Si1−x−yGexCy, and Si1−yCy alloys material-device applications

A nickel silicide process for Si1-xGex, Si1-x-yGexCy, and Si1-yCy alloy materials compatible with Si technology has been developed. Low-resistivity-phase (12–20 µΘ cm) nickel silicides have been obtained for these alloys with different low sheet-resistance temperature windows. The study shows that thin (15–18 nm) silicide layers with high crystalline quality, smooth silicide surface, and smooth interface between silicide and the underlying material are achievable. The technique could be used to combine the benefits of Ni silicide and Si1-xGex, Si1-x-yGexCy, and Si1-yCy alloys. The technique is promising for Si or Si1-xGex, Si1-x-yGexCy, and Si1-yCy alloy-based metal-oxide semiconductor, field-effect transistors (MOSFETs) or other device applications.

Tracing the Ti-silicide formation by in situ ellipsometric measurements

In situ multi-wavelength ellipsometry is employed to trace the reaction of Ti/Si to C49-TiSi 2 as well as the polymorphic C49-to-C54-TiSi 2 transformation by thermal annealing up to 900°C for Ti layers of different thickness (10– 80 nm ) on Si (1 0 0). Ti deposition, annealing and ellipsometry were performed in the same ultrahigh vacuum system. In this way, the silicidation process could be studied without the influence of oxygen and carbon contamination. Characteristic changes in the ellipsometric angles as a function of temperature were correlated with phase changes in the Ti/Si system by halting the heating ramp at the corresponding temperatures and performing structural analysis by X-ray diffraction and Raman spectroscopy. The first change, completed at around 530–600°C, is due to the reaction of Ti with Si to form C49-TiSi 2, whereas the one shortly afterwards is caused by the polymorphic transformation of C49–TiSi 2 to the C54 phase at 600–650°C. After C54-TiSi 2 formation both ellipsometric angles keep changing slowly but continuously. At these temperatures, the silicide surface undergoes serious roughening. With increasing ramp rates the transformation temperatures are shifted to higher temperatures. In addition, the C49–C54 transformation is retarded relative to the C49 phase formation with decreasing Ti thickness. An as yet unassigned phase is observed to appear with C49-TiSi 2 formation and to be consumed by C49-TiSi 2 in a narrow temperature interval. Additionally, the optical constants of the constituent phases Ti, C49- and C54-TiSi 2 were calculated from spectroscopic ellipsometry measurements in the range from 415 to 750 nm .

Influence of carbon on the electron stimulated desorption from titanium silicide surfaces

The influence of carbon on the electron stimulated desorption from titanium silicide thin film surfaces has been studied by means of electron-stimulated-desorption (ESD) and Auger electron spectroscopy. The superficial carbon was due to exposure to air. The main ions ejected were O +, H +, and minor amounts of OH +. Ion emission intensity versus incident electron energy showed threshold energies corresponding to the Ti 3p and Si 2p core levels, in agreement with the Knotek–Feibelman model. Intensity and shape of the H + ion yield curves depended markedly on the amount of carbon contamination remaining after Ar + ion cleaning. Ion yield measurements on the low carbon concentration surfaces showed both thresholds due to excitation of the Ti 3p and Si 2p core levels. On the contrary, for high surface carbon concentrations, ions are only ejected by excitation of the Ti 3p levels. The differences between the ESD yield for H + and O + after exposure to air have been studied.

Structural and Electronic Properties of Barium Silicide on Si(100)

Barium silicides formed on the Si(100) surface have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The structure of the silicide surface depends on the preparation procedure; three kinds of silicide phases coexist on the surface when 2 monolayers (MLs) of Ba are deposited on Si(100) at the specimen temperature of 850 K, while two of the three phases appear when 2 MLs of Ba are deposited on Si(100) at room temperature, followed by annealing at 850 K. The STS result indicates that all the phases show semiconducting behavior with a band gap of ∼ 1.3 eV and that one of the three phases has local density of states (LDOS) at 2.5 eV below the Fermi energy, which coincides with the reported results of metastable de-excitation spectroscopy (MDS) and ultraviolet photoelectron spectroscopy (UPS).

Structural studies of two- and three-dimensional dysprosium silicides using medium-energy ion scattering

Medium-energy ion scattering has been used to determine the atomic structure of two-dimensional and three-dimensional (3D) dysprosium silicide films on the Si(1 1 1) surface. A quantitative study of the ion yield from the dysprosium has enabled the positions of the atoms in the top three layers of the Si(1 1 1)1×1–Dy to be precisely determined. For the case of the Si(1 1 1)(√3×√3) R30°–Dy 3D silicide surface the experimental blocking curves are in agreement with simulations for the structural model determined by surface X-ray diffraction for the Si(1 1 1)(√3×√3) R30°–Er 3D silicide surface. A contraction of 2.0±0.6% in the c-axis lattice constant compared to bulk dysprosium disilicide is found.

Surface and interface morphology of CoSi 2 films formed by multilayer solid-state reaction

CoSi 2 is a promising material for self-aligned silicide (salicide) applications in sub-0.25 μm complementary metal–oxide–semiconductor (CMOS) technology. In conventional salicide technology, silicides are formed by a solid-state reaction (SSR) after source/drain formation. With the continued scaling down of junction depths, surface and interface roughness of silicides is a growing concern. In this work, a comparative study has been made to investigate the morphology and thermal stability of CoSi 2 formed by SSR of different structures, i.e. Co/Si, TiN/Co/Si, Ti/Co/Si, Co/Ti/Si and Ti/Co/Ti/Si. Atomic force microscopy and other techniques were used to characterize the morphology and thermal stability. Compared with the Co/Si reaction, TiN or Ti capping reduces the roughness and improves the thermal stability. The reaction with a Ti interfacial layer shows epitaxial growth of CoSi 2 on Si (100). The morphology and thermal stability of epitaxial CoSi 2 were significantly improved. The epitaxial CoSi 2 may be useful for contact in deep submicrometer CMOS devices.

Contact stability of C60 molecules on pure and doped (100) tungsten surface

The influence of the metal surface doping on the contact stability of adsorbed fullerene molecules at room temperature was observed for the first time. The C60 fullerene molecules decompose at room temperature both on a pure (100)W surface and on a tungsten surface containing carbide (WC), while the presence of a surface silicide of the same stoichiometry (WSi) produces passivation of the metal surface, thus preventing the fullerene molecules from decomposition.