CoSi2 Layers Research Articles

An infrared study of H8Si8O12 cluster adsorption on Si(100) surfaces

Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO2 interfaces derived from the adsorption of H8Si8O12 spherosiloxane clusters on Si(100) surfaces, we have studied the adsorption geometry of the H8Si8O12 clusters on deuterium-passivated and clean Si(100) surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm−1 was made possible through the use of specially prepared Si(100) samples which have a buried metallic CoSi2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si(100) surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si(100) surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si(100) surface and show evidence that the clusters are adsorbed on the Si(100) via attachment by one vertex. A complete assignment of the observed vibrational features, for both physisorbed and chemisorbed clusters, has been made based upon comparisons with the results obtained in ab initio calculations using gradient-corrected density functional methods.

Oxygen and the thermal stability of thin CoSi2 layers

The thermal stability of thin CoSi2 layers is demonstrated to improve with the use of oxygen-containing annealing ambients. Pinhole formation observed in 11–27 nm thick CoSi2 layers after anneals at 800 °C in nitrogen and vacuum is found to be eliminated when oxygen is used as the annealing ambient. A thin SiO2 layer grown during oxygen anneals, which curbs surface diffusion and reduces the rates of kinetic processes, is thought to be the primary reason for the retardation of layer agglomeration. The beneficial effect of air exposure and wet etches to the integrity of thin CoSi2 layers is also shown. These findings suggest the inclusion of oxygen in certain Co salicide processing steps.

Hot filament chemical vapor deposition diamond growth kinetics on an epitaxial CoSi2 surface monitored by three electron spectroscopies

Diamond deposition on an epitaxial CoSi2 layer over Si(111) is preceded by the formation of a 4 nm thick silicon carbide layer. The steps of carbide formation, diamond nucleation, and diamond growth are monitored in situ by three electron spectroscopies (x-ray photoelectron spectroscopy, Auger electron spectroscopy, and electron-loss spectroscopy). By comparison with our previous studies of diamond growth on clean Si(100) and Si(111), the time required to stabilize the SiC composition is much longer. This slow step is interpreted by a strong carbon diffusion into the bulk which goes together with silicon enrichment of the carbide phase. The lack of carbon saturation at the surface induces a large delay for the subsequent diamond nucleation process by a time scale factor of 10. In addition, the electron spectroscopy measurements reveal the appearance of C–C sp3 species before the diamond nucleation starts. These species probably correspond to carbon aggregates which act as precursors of the diamond phase.

Reduction of boron penetration for a p + polycide gate by using cobalt silicides as a boron diffusion source

A scheme for forming a p + polycide gate is described, by using BF 2 implantation into CoSi/poly-Si bilayer films and subsequent annealing. Instead of the conventional BF 2 implantation into thin poly-Si films, this scheme employs the CoSi layer as an implant barrier as well as a boron diffusion source to retard the boron diffusion. The improved gate oxide integrity and the reduced flat-band voltage shift can be obtained without causing other side effects. In addition, it is found that the stack layer of CoSi/a-Si can serve as an excellent implant barrier for suppressing the boron penetration through a gate oxide.

Improved Thermal Stability of Ultrathin CoSi2 Layers by Oxygen Annealing

ABSTRACTThe thermal stability of thin CoSi2 layers is demonstrated to improve with the use of oxygen-containing annealing ambients. Layer agglomeration in epitaxial and polycrystalline CoSi2 layers grown on single crystal Si and polycrystalline CoSi2 layers grown on α-Si was retarded by oxygen annealing. A thin SiO2 layer grown during oxygen anneals which curbs surface diffusion and reduces the rates of kinetic processes is thought to be the primary reason for the improvement in CoSi2 thermal stability.

Oxide Mediated Epitaxy on Planar and Non-Planar Si

ABSTRACTThe recently developed oxide mediated epitaxy (OME) technique involves the growth of epitaxial CoSi2 through a thin layer of silicon oxide grown in peroxide baths. Here we report improvements of the OME technique from the use of in-situ grown thermal oxide as the barrier layer and post-growth oxygen anneals. Growth of uniform epitaxial CoSi2 layers by OME on non-planar surfaces of selectively deposited CVD Si layers with oxide and nitride patterns was also demonstrated. Both the epitaxial orientation of CoSi2 and the thickness of the CoSi2 layer, measured along the deposition direction, remained unchanged across facet boundaries. OME appears well suited for shallow-junction silicidation application in devices involving raised source/drain structures.

Growth, Patterning and Microelectronic Applications of Epitaxial Cobaltdisilicide

ABSTRACTWe report on the epitaxial growth of CoSi2 on silicon using ion beam synthesis and molecular beam allotaxy. The latter process uses molecular beam epitaxy to grow a silicide precipitate distribution embedded in single crystalline silicon and thermal annealing to form the epitaxial layer. Both, ion beam synthesis and molecular beam allotaxy, are capable to grow epitaxial buried and surface CoSi2 layers on Si(100) and Si(111) of high quality.

Observation of quantum interference effects in submicron CoSi2 wires in Si(100)

We report on the structural and electrical characterization of submicron wide cobalt disilicide wires with widths down to 170 nm fabricated by ion beam synthesis. This versatile technique consists of implantation of cobalt through a special implant mask followed by a rapid thermal annealing step. We observed quantum interference effects in CoSi2 wires, which we attribute to weak localization with strong spin-orbit scattering and superconducting fluctuations. Cross-sectional transmission electron microscopy micrographs showed that the thinnest wires have diamondlike shapes due to energetically preferred {111} facets. Analysis of the magnetoresistance at temperatures between 2 and 4 K yielded relatively large phase coherence lengths of 0.5–2.5 μm very similar to values obtained for epitaxial CoSi2 layers indicating the high crystalline quality of the wires.

Growth kinetics of CoSi formed by ion beam irradiation at room temperature

Growth kinetics of cobalt silicide layers formed by ion beam irradiation was investigated at a temperature between room temperature and 100 °C. The CoSi phase was identified by x-ray diffraction of Co/Si samples irradiated with 25 keV argon ions to a dose of 2.0×1015 cm−2. The number of intermixed silicon atoms in the CoSi layers was evaluated as a function of dose, dose rate, and nuclear energy deposition rate at the Co/Si interface for samples irradiated with 40 keV focused silicon ion beams. The growth is shown to be diffusion-limited and attributed to radiation-enhanced diffusion with an activation energy of 0.16 eV. The number of intermixed silicon atoms is approximately proportional to the nuclear energy deposition rate at the initial Co/Si interface, while it is independent of dose rate, which shows that the CoSi phase is formed without contribution of the sample heating caused by irradiation.

Submicrometer patterning of cobaltdisilicide layers by local oxidation

We investigated the diffusion and reaction processes occurring during local silicide oxidation. We patterned epitaxial CoSi 2-layers also by local oxidation. This novel method for patterning of silicide layers allows the realization of deep submicrometer spacings between silicide layers. Epitaxial CoSi 2-silicide layers were grown by a special molecular beam epitaxy method on Si(100) and Si(111) with thicknesses of 40 nm. The SiO 2/Si 3N 4 oxidation mask was patterned photolithographically with line widths of ∼ 1 μm. During thermal oxidation SiO 2 forms on the unprotected regions of the silicide layer. The silicide is pushed into the substrate in these regions. By this process the silicide layer thins near the edges of the oxidation mask and at a critical oxide thickness the oxidized part of the silicide layer separates from the protected part. Using this patterning method we fabricated metallic CoSi 2 layers separated by distances as small as ∼ 200 nm on Si(100) and Si(111). Spacings below 100 nm should be feasible.

Interfacial reaction and formation mechanism of epitaxial CoSi2 by rapid thermal annealing in Co/Ti/Si(100) system

A ternary compound of Co3Ti2Si is suggested as a reaction barrier for the formation of epitaxial CoSi2 in the Co/Ti/Si system when adopting the rapid thermal annealing process. It controls Co diffusion to the Si substrate, followed by formation of epitaxial CoSi2. After the epitaxial CoSi2 was formed, the interfacial morphology of the upper layer/ CoSi2 interface was very different according to silicidation temperature, that is, the interface was planar at 800 °C, but rough at 900 °C. This was attributed to the reaction between the upper layer consisting of Co–Ti–Si and the CoSi2 layer at 900 °C, which resulted in Ti-rich precipitates at the surface. The Ti-rich precipitates acted as a diffusion sink of dopant, thus, the leakage current density for the silicidation temperature of 900 °C was much higher than that for the temperature of 800 °C. These results suggest that the silicidation temperature is one of the most critical factors in determining the leakage current of the p+n junction diode.

Formation of Single-crystal CoSi2 Buffer Layers on Si(100) Substrates by High Dose Co Ion Implantation for the Deposition of YBa2Cu3O7−x Thin Films

High quality single-crystal CoSi2 layers have been successfully formed on Si(100) using low energy high dose Co ion implantation followed by subsequent annealing method as a buffer layer for the deposition of YBa2Cu3O7−x (YBCO) thin films. Rutherford backscattering spectrometry with channeling (RBS-C) measurements showed that CoSi2 layers after annealing at temperatures between 850 and 950 °C had a minimum yield Xmin of about 3%. X-ray diffraction (XRD) spectra revealed that CoSi2 layers had the same orientation as the Si(100) substrates. Phi scan XRD spectra proved that CoSi2 layers epitaxially grew in the cube-on-cube epitaxial growth mode with respect to the Si(100) substrates. YBCO films and CeO2/YSZ buffer layers were deposited on CoSi2/Si(100) substrates via laser ablation and electron beam evaporation, respectively. θ-2θ, ω, and φ scan XRD spectra illustrated that YBCO films and CeO2/YSZ buffer layers had the epitaxial structure both in a-b plane and along the c-axis. YBCO films grown on this multilayered structure demonstrated excellent superconducting properties with the zero resistance transition temperature Tc0 of 87–90 K. The transition width (ΔTc) was about 1 K. Orientation and epitaxial crystalline quality of YBCO films and CeO2/YSZ buffer layers were confirmed by XRD and RBS-C characterization. All films consisted of c-axis oriented grains. RBS-C spectra indicated a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 8%, and interdiffusion between the YBCO film and buffer layers or between the YBCO film and the substrate was limited. This multilayer system shows the possibility for the application of YBa2Cu3O7−x thin films on technical Si substrates in the field of hybrid superconductor-semiconductor technology.

Continuous CoSi2 layers in silicon synthesized by Co-ion implantation

CoSi2 thin layers were fabricated by Co-ion implantation into Si(100) and subsequent annealing. The ion implantation was carried out in a metal vapour vacuum are (MEVVA) source implanter. Various doses of Co ions were implanted using doses of 1.6 x 10(17) to 7.5 x 10(17) Co/cm(2) at an extraction voltage of 40 kV and ion current densities of 60 and 150 mu A/cm(2). It was found that the thicknesses of continuous CoSi2 layers after annealing within Si(100) increased with increasing implantation dose, and ranged from 43 to 60 nm for the implantation doses used in this work. The specific resistivity of implanted samples after annealing was between 12.1 and 13.8 mu Omega cm at room temperature. The results indicated that the ion current density played an important role in the structure of the CoSi2 layer, and annealing can significantly improve the electrical property of the CoSi2 layers.

A vibrational study of ethanol adsorption on Si(100)

The adsorption of ethanol-d0, -d3, and -d6 on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range (<1450 cm−1) is made possible by using specially prepared Si(100) wafers which have a buried metallic CoSi2 layer that acts as an internal mirror. We find that ethanol dissociatively adsorbs across the Si(100) dimers near room temperature to form surface bound hydrogen and ethoxy groups. Furthermore, the ethoxy groups are oriented such that the C3v axis of the methyl group is nearly perpendicular to the surface, unlike the case for ethoxy groups bound to metal surfaces. This adsorption geometry is deduced on the basis of the surface dipole selection rule, which applies to these Si(100) samples with a buried CoSi2 layer. Ab initio cluster calculations using gradient-corrected density functional methods confirm the proposed adsorption geometry for ethoxy on Si(100) and accurately reproduce the observed normal mode frequencies.

Study of phase separation in Ti-Co-N thin films on silicon substrate

Co27 and Ti73Co27- N during thermal annealing has been studied by SIMS, AES and XRD methods. It has been shown that in case of Ti73Co27 the CoSi2 layer was not formed and the formation of ternary silicide compounds CoTiSi and Co3Ti2Si took place. At the same time in case of Ti73Co27- N the bottom layer CoSi2 and the upper layer based on TiN were formed. The interaction behaviour has been found to depend on nitrogen concentration in initial film. For high amount of nitrogen the diffusion of Si atoms into upper layer and Si3N4 phase formation were observed. The possible variants of solid-phase interaction between silicon and the alloys containing intermetallic compounds and influence of nitrogen on this process are discussed.

Co silicide formation on SiGeC/Si and SiGe/Si layers

The reaction of Co with epitaxial SiGeC/Si layers is investigated and compared to the reaction of Co with SiGe/Si layers. The sequence of phase formation is the same as the reaction of Co with monocrystalline Si, however, cobalt disilicide is formed at much higher temperatures. The presence of C further delays the disilicide formation, as a result of C accumulation at the silicide/substrate interface during the reaction, which blocks the Co diffusion paths. The CoSi2 layers thus formed exhibit a preferential (h00) orientation. The slow supply of Co atoms to the silicide/Si interface, due to the blocking of Co diffusion paths by Ge and C, is believed to be the reason for this epitaxial alignment.

A Novel Technique for Ultrathin CoSi2 Layers: Oxide Mediated Epitaxy

A novel technique, oxide mediated epitaxy (OME), was recently described for the growth of single crystal CoSi2 layers on the (100), (110), and (111) surfaces of silicon. Deposition of a thin layer of cobalt (1–3 nm) onto Si surfaces covered with a thin peroxide-grown SiOx layer and annealing at 500–700° C led to the growth of essentially uniform, epitaxial CoSi2 layers. The thin SiOx layer remained largely on the surface of the OME grown CoSi2 layers as a cap. Deposition of cobalt at elevated temperature onto OME grown CoSi2 layers led to a significant re-evaporation of cobalt from the SiOx surface. Presently, the OME effect was demonstrated on heavily doped p+ and n+-Si and on narrow (0.22 µ m) Si lines. On all surfaces, thin (10–30nm), excellent quality, CoSi2 single crystal thin films were grown by repeated growth sequences involving depositions of cobalt at room temperature and anneals at 600–700° C. It was also shown that faceting at CoSi2/Si(100) interfaces could be significantly reduced by a high temperature anneal.

Fabrication of buried epitaxial CoSi2 layer through selective diffusion

The incorporation of metallic layers into the bulk of semiconductors is gaining tremendous attention for device applications. This is mainly achieved by ion beam synthesis. In the ultrathin film regime, however, this technique is not practical due to the damage incurred. Here we report a technique by which fabrication of buried epitaxial CoSi2 is achieved by making use of the selective diffusion behavior of Co. Co atoms diffuse through a Ge overlayer on a Si(111) substrate and are terminated by reaction with the Si atoms underneath. Ion scattering as well as high resolution microscopy results confirm that the CoSi2 layer thus formed is in epitaxial form. This method would help in providing functionality to nanostructure based devices.

Cobalt disilicide buffer layer for YBCO film on silicon

The CoSi2 films were used as buffer layers of YBCO/CoSi2/Si(100), YBCO/ZrO2/CoSi2/Si(100) and YBCO/CeO2/YSZ/CoSi2/epi-Si/Al2O3 heterostructures in this work. Transition temperatures of YBCO films were obtained up to 86K for YBCO films deposited by laser ablation on the top of CeO2/YSZ/CoSi2/Si/Al2O3 structure. Local nucleation on the crystal defects of silicon, the phenomenon of lateral directed growth (DLG) and agglomeration of CoSi2 phase are responsible for grain boundaries (GB) position in CoSi2 layer and its roughness. The roughness was decreased using an additional Zr film on the top structure.

Electrical properties of CoSi2 precipitates in cobalt-implanted silicon: a conducting atomic force microscopy study

Ion beam synthesis of cobalt disilicide CoSi2 in silicon has been the subject of a number of studies [1±3]. The peculiar properties of CoSi2 which make it a suitable candidate for wide application in microelectronic devices such as contact materials, gate electrodes or interconnects are its low resistivity, its high thermal stability and its cubic CaF2 structure with a lattice constant only 1.2% smaller than that of Si. The buried CoSi2 thin ®lm with high quality can be formed after post-implantation annealing due to the latter factor. For as-implanted samples, a large amount of CoSi2 precipitates distribute on the surface and in the bulk of silicon; the details of the distribution pro®le depend on dose, energy and substrate temperature during implantation. Previous investigations were focused mainly on classi®cation and identi®cation of the type of CoSi2 precipitates, formation and characterization of thin buried CoSi2 layer, and electrical properties such as Hall coef®cient and temperature-dependent resistance. The invention of scanning tunnelling microscopy (STM) in 1981 and subsequently atomic force microscopy (AFM) stimulated the development of many novel scanning probe microscopes [4]. Combined investigation of surface morphology and simultaneous electrical imaging can be realized by modifying the commercial AFM, i.e. adding a preampli®er and using a conducting probe tip. Conducting AFM has been used to study the delineation of semiconductor doping [5], local electrical properties of metal surfaces [6] and quantization of the conductance of alkane layers on graphite [7]. Recently, preliminary results of mapping the conductance of Ni±SiO2 composite have been reported by using conducting AFM [8]. A substantial change of conductance for the Ni composition below and above the critical value was observed. However, owing to its low resolution, it is impossible to probe the electron transport of a single Ni cluster or connected conducting channel. Consequently, the conductance from the experiment is the sum of surface metal particles that are in contact with the probe. We report here the ®rst mapping of the CoSi2 precipitates in silicon media, and their electron transport properties on the nanometre scale by using conducting AFM. Commercial silicon nitride cantilevers coated with Ti=Au or Cr=Au by evaporation were used. These tips show good conductivity and reproducibility during scanning. The Co implantation was performed with a metal vapour vacuum arc (MEVVA) ion source into (1 0 0) p-type silicon wafers with resistivity 10±20 Ucm at an extraction voltage of 70 kV to a dose of 2 3 10 cmy2. The substrate temperature during the implantation ranged from 210 to 700 8C. Details of the implantation process and characterization of asand post-implanted samples by transmission electron microscopy (TEM) and electrical measurements have been reported elsewhere [2]. TEM and high resolution electron microscopy (HREM) revealed that Aand B-type CoSi2 precipitates distribute from the surface down to 280 nm in the bulk. The density of the precipitates is correlated with the depth pro®le of the Co centres at low substrate temperature, and the large density position shifts to the surface with an increase in substrate temperature due to coarsening and coalescence effect. At suf®ciently high substrate temperature (above 650 8C) a continuous layer is formed near the surface. Generally the typical size of the precipitates increases with the substrate temperature. When an external voltage is applied between the conducting tip and the sample, there is a current owing in the electrical circuit. The tip may be in contact with the silicon surface or a CoSi2 precipitate on the surface during scanning. (A thin silicon dioxide layer was present on the silicon surface because no chemical treatment was performed before the measurement.) A signi®cant current increase is expected when the probe tip is in contact with the surface precipitates due to different contact resistance between Au=CoSi2 and Au=SiO2 contact. The main transport mechanism in the sample is via tunnelling since a barrier of about 0.6 eV exists between CoSi2 and the surrounding silicon media. Of course, the electron may transport directly through the silicon media. There is also the possibility for the electron to transport via precipitate networks formed, especially for samples implanted at higher dose. The physical mechanism of the system is similar to that of metal±insulator composite, which is beyond the scope of this letter. The surface morphology and simultaneous current image for a sample at substrate temperature of 210 8C with bias V ˆ 100 mV is presented in Fig. 1. The surface is rather at, with root-mean-square