Polycrystalline CoSi2 Research Articles

Correlated TKD/EDS - TEM - APT analysis on selected interfaces of CoSi2 thin films

Correlative analysis is a powerful way to relate crystallographic and chemical information to the properties of materials. In this work, a procedure is proposed to select and analyze interfaces of polycrystalline thin film materials through correlative transmission Kikuchi diffraction/energy dispersive X-ray spectroscopy (TKD/EDS), transmission electron microscopy (TEM) and atom probe tomography (APT). TKD provides information on the crystallographic orientation. The EDS analysis performed together with TKD in the scanning electron microscope (SEM) makes chemical information available allowing phases of similar crystal structure, but with a different composition to be distinguished. The information of TKD/EDS can be correlated to successive TEM and APT analysis on selected interfaces for structural and chemical analysis at the atomic scale. An interface of an epitaxial orientated grain of a polycrystalline CoSi2 thin film on (111)Si is selected and analyzed. The selected interface has a twin character and shows facets of different orientation and area. Site-specific segregation of Ge to junctions of the facets is evidenced. The correlation between local strain from misfit (defects) at the interface and segregation is discussed.

Ti segregation at CoSi2 grain boundaries

The localization of Ti at grain boundaries after the phase formation of CoSi2 on Si(111) from a CoTi bilayer is examined. For the chemical analysis at the atomic scale, atom probe tomography (APT) is applied. The structure of grain boundaries is analyzed by high resolution transmission electron microscopy (HRTEM). A polycrystalline CoSi2 film has been formed after annealing. Inside the grains of CoSi2, Ti is determined to be present in low concentration of 0.2 at.%. An excess of Ti is measured at the grain boundaries. The Ti segregation varies for different grain boundaries. HRTEM shows different distances between defects for varying misorientation. The distances between defects are compared with theoretical misfits at the grain boundaries. A relationship between the Ti excess at grain boundaries determined by APT and the defect densities of grain boundaries analyzed by HRTEM is proposed.

Formation of Vertically Aligned Cobalt Silicide Nanowire Arrays Through a Solid-State Reaction

We report for the first time synthesis of high-density arrays of vertically well-aligned cobalt monosilicide (CoSi) nanowires (NWs) in a large area via a solid-state reaction. The vertical arrays of 1-μm-long Si NWs were first grown on a p-type (1 0 0) Si substrate by the aqueous electroless etching (AEE) method, and a 40-nm-thick Co layer was conformally deposited using a thermal atomic layer deposition system as revealed by SEM and transmission electron microscope analyses. The rapid thermal annealing process was carried out at various temperatures ranging from 700 to 1000 °C; the X-ray diffraction analysis confirmed that the polycrystalline CoSi NW arrays were formed at temperatures above 900 °C. The required high driving force for this silicide formation can be attributed to the significant amounts of oxygen-related contaminants at the defect sites of the highly rough surfaces of AEE-grown Si NWs. To demonstrate practical applications, field emitters and Schottky diodes were fabricated using the vertically aligned CoSi NW arrays. The field emission measurements showed a turn-on field of 10.9 V/μm and a field enhancement factor of 328, indicating the feasibility of vertically aligned CoSi NW arrays as promising field emitters. For the Schottky diodes, the measured Schottky barrier height was 0.52 eV and the estimated ideality factor obtained from the I-V characteristic curves was 2.28.

Formations and morphological stabilities of ultrathin CoSi2 films

In this paper we investigate the formations and morphological stabilities of Co-silicide films using 1-8-nm thick Co layers sputter-deposited on silicon (100) substrates. These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450 °C to 850 °C. For a Co layer with a thickness no larger than 1 nm, epitaxially aligned CoSi2 films readily grow on silicon (100) substrate and exhibit good morphological stabilities up to 600 °C. For a Co layer thicker than 1 nm, polycrystalline CoSi and CoSi2 films are observed. The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon (100) substrate. The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.

Enhanced formation and morphological stability of low-resistivity CoSi 2 nanodot arrays on epitaxial Si 0.7Ge 0.3 virtual substrate

We report here the first successful growth of large-area, low-resistivity cobalt disilicide (CoSi 2) nanodot arrays on epitaxial (0 0 1)Si 0.7Ge 0.3 substrates by using the nanosphere lithography (NSL) technique with an interposing amorphous Si (a-Si) thin film serving as the sacrificial layer. For the Co/a-Si bilayer nanodots array on Si 0.7Ge 0.3 samples after annealing, polycrystalline CoSi 2 appears to form as the only silicide phase at an annealing temperature as low as 500 °C The a-Si interlayer with appropriate thickness was found to effectively prevent Ge segregation and maintain the morphological stability in forming CoSi 2 nanodots on Si 0.7Ge 0.3 substrate. The size, interparticle spacing, and triangular shape of the CoSi 2 nanodots remain almost unchanged even after annealing at 950 °C. For the Co/a-Si nanodot samples further annealed at 1000 °C, amorphous SiO x nanowires, 15–35 nm in diameter, were observed to grow from CoSi 2 nanodot regions. The observed results present the exciting prospect that the NSL technique in conjunction with a sacrificial a-Si interlayer process promises to be applicable in fabricating periodic arrays of other low-resistivity silicide nanocontacts with controlled size, shape, and periodicity on Si 1− x Ge x substrates.

Texture of CoSi 2 films on Si(111), (110) and (001) substrates

Synchrotron radiation was used to study the texture of polycrystalline CoSi 2 films that were formed by a solid-state reaction between a 30 nm Co film and Si(111), (110) and (001) substrates. All films were strongly textured, and several texture components were identified. We discuss the simultaneous occurrence of axiotaxy (i.e. alignment of lattice planes across the interface) and several different types of epitaxy in each of the films. Comparison of the different texture components observed on the three substrate orientations suggests a strong preference for the alignment of CoSi 2{110} planes in the film with Si{110} planes in the substrate, and twinning around Si[111] directions.

Two routes to polycrystalline CoSi 2 thin films by co-sputtering Co and Si

Two processes for the fabrication of polycrystalline CoSi 2 thin films based on the codeposition of Co and Si by sputtering were studied and compared. The first process involved “annealing after deposition”, where Co and Si are codeposited at ambient temperature and then crystallized by annealing. This process yielded randomly oriented plate-like CoSi 2 grains with a grain size that is governed by the nanostructure of the as-deposited film. Polycrystalline CoSi 2 thin films were obtained at a process temperature of 170 °C, which was much lower than the annealing temperature of 500 °C needed for Co/Si bilayers. The second process involved “heating during deposition”, where Co and Si are codeposited on heated substrates. This process yielded CoSi 2 grains with a columnar structure, and the grain size and degree of (1 1 1) orientation are temperature dependent. The sheet resistance of the resulting films was determined by the preparation temperature regardless of the deposition process used, i.e. “annealing after deposition” or “heating during deposition”. Temperatures of 500 °C and higher were needed to achieve CoSi 2 resistivity of 40 μΩ cm or lower for both processes.

Ab initio calculations of the structural and elastic properties of CoSi2

Under GGA, the geometry, energy, electronic structure, and elastic properties of the CoSi2 have been investigated by using ab initio plane-wave ultrasoft pseudopotential method. The calculated equilibrium lattice parameter a and elastic stiffness constants c 11, c 12, and c 44 are in better agreement with the experimental values than those obtained by both the VAMP and FLAPW with LDA. For engineering and technological applications, the isotropic elastic properties, including the shear modulus G, the bulk modulus B, Young’s modulus E, and Poisson’s ratio $$ \upsilon $$ , are also calculated for polycrystalline CoSi2 with Voigt, Reuss, and Hill approximations. The lower Poisson’s ratio $$ \upsilon $$ of 0.327 means an increase in the volume is associated with the uniaxial tensile deformation and the higher ratio of bulk modulus to shear modulus B/G of 2.56 indicates the ductility of CoSi2 which is in accordance with the metallic property of CoSi2 obtained by the density of states.

Nanoscale epitaxial cobalt salicide bitlines for charge trapping memory cells

An epitaxial CoSi2 process is presented, which allows the self-aligned formation of bitlines with only a few tens of nanometer width for Twin Flash memory cells in the 63nm generation. The bitlines show a good thermal stability and low resistance for widths down to 35nm, where polycrystalline CoSi2 is known to exhibit a strong narrow linewidth effect. Transmission electron microscopy studies revealed a cube-on-cube epitaxy with only a few twins depending on the annealing conditions. The low bitline resistance results in a linear drain voltage dependence of the programing characteristics and a suppression of secondary electron injection during programing.

Titanium Interlayer Mediated Epitaxy of CoSi 2 on Si 1− xGe x

Titanium Interlayer Mediated Epitaxy (TIME) has been shown to promote the formation of epitaxial CoSi 2 on Si (100). Similarities between Si and Si 1− x Ge x alloys have motivated a study of whether the TIME process could be successful in forming epitaxial CoSi 2 on Si 1− x Ge x . Titanium layers of varying thickness were deposited as interlayers between a Co layer and c-Si/Si 0.8Ge 0.2 grown epitaxially onto Si (100) to investigate their role in the formation of epitaxial CoSi 2 on Si 1− x Ge x alloys. The effect of Ti interlayer thickness on the orientation of CoSi 2 to the Si 1− x Ge x substrate, and the conditions under which a polycrystalline CoSi 2 film has been formed have been studied. It was found that Ti was beneficial in promoting epitaxy to the substrate in all cases. The experimental results indicate that with a Ti interlayer thickness of ∼ 50 Å, the formation of epitaxial CoSi 2 adjacent to the substrate was achieved, and pinhole formation was minimized. It was also observed that for increased interlayer thickness, Ti reacted with Si to form a titanium silicide.

Polycrystalline silicon/CoSi2 Schottky diode with integrated SiO2 antifuse: a nonvolatile memory cell

A Schottky diode consisting of doped polycrystalline silicon (polysilicon) and CoSi2 films is described. When an SiO2 antifuse thin film is grown in between the polysilicon and CoSi2, the film stack can function as a nonvolatile one-time programmable memory cell. The cell is programmed when the SiO2 that insulates the doped polysilicon from the CoSi2 is broken down by applying a large biasing field, and unprogrammed when the antifuse is not broken down. By taking advantage of the ability to grow SiO2 directly on CoSi2, the entire device can made with only two masking steps and relatively simple tool set, while achieving high density.

Magnetization reversal processes in amorphous and polycrystalline Co-Si patterned nanowires

Arrays of amorphous and polycrystalline Co/sub x/Si/sub 1-x/ nanowires have been prepared by electron beam lithography and a lift-off technique. The angular dependence of the magnetooptical transverse Kerr-effect hysteresis loops for both kind of samples has been compared in order to analyze the interplay between material microstructure and element shape at the submicrometric scale. The overall magnetic properties of the Co-Si nanowires are found to be dominated by patterning induced shape anisotropy, with the presence of coherent rotations in the magnetization reversal process in a wide angular range around the hard axis direction. It is found that the more homogenous microstructure of the amorphous samples results in a softer magnetic behavior and on a much better definition of the wires uniaxial anisotropy.

Diamond growth on CoSi2/Si by bias-enhanced microwave plasma chemical vapor deposition method

Abstract Diamond was grown on polycrystalline CoSi 2 /Si substrates by bias-enhanced microwave plasma chemical vapor deposition. Both of the positive and negative biasing effects were investigated by microstructural characterization. It has been found that nucleation density can reach ∼10 9 cm −2 with positive biasing, much higher than with negative biasing. Cross-sectional transmission electron microscopy shows that diamond deposited by positive biasing grows on a relatively smooth CoSi 2 surface, while the etching effect of ion bombardment during negative biasing results in a rough CoSi 2 surface. The diamond morphology obtained with negative bias has a flat surface with a strong (1 0 0) texture.

CoSi 2 nucleation in the presence of Ge

We studied CoSi 2 formation in the presence of Ge. Bilayers of Co/Ge/Si (interlayer) and Ge/Co/Si (capping layer) were annealed and phase formation was determined by sheet resistance and XRD. During the initial CoSi formation (∼400°C), Ge is partly dissolved in the monosilicide. At higher temperature (>600°C), Ge is expelled from the CoSi, although for samples with a thick interlayer, 7–10% of Ge remains dissolved in the CoSi. The expulsion of Ge causes the formation of Ge-rich SiGe at the surface and interface of the monosilicide. With increasing temperature, the amount of SiGe present at the interface increases and the composition evolves towards Si 0.5Ge 0.5. The CoSi 2 phase forms at increasingly higher temperatures for increasing Ge thickness. The delay in nucleation temperature saturates at approximately 750°C for a Ge thickness larger than 3.5 nm. Similar behavior was observed for a Ge capping layer. The presence of Ge also influences the crystallographic properties of the CoSi 2 layer formed. For a Ge interlayer with a thickness smaller than 2 nm, polycrystalline CoSi 2 is formed with a grain size that is larger than for the standard Co/Si reaction. For a Ge interlayer thicker than 2 nm, there is a preferential epitaxial orientation of the CoSi 2. A model is presented to explain the delay in nucleation, the increased grain size and the preferential orientation of the CoSi 2.

Orientation-dependent stress build-up during the formation of epitaxial CoSi 2

The epitaxial CoSi 2 is formed by using a Ti interlayer with variable thickness on a (100) Si substrate (TIME: Ti interlayer mediated epitaxy). These experiments confirm previous work where for an increasing amount of Ti in between the Si substrate and the Co layer, a transition from polycrystalline CoSi 2 over a preferential (220) orientation towards (400) epitaxial CoSi 2 is observed. This work emphasises the impact of the volume fraction of the epitaxial (400) CoSi 2 grains on the mechanical stress induced during the Si/Ti/Co silicidation. It is shown that the mechanical stress increases for increasing epitaxial (400) orientation of the CoSi 2 film. The stress caused by the epitaxial CoSi 2 can have a detrimental impact on the device performance.

The effect of the reaction temperature on the thermal stability of polycrystalline CoSi 2 layers on Si(001)

We investigated the solid state reaction of 30-nm and 100-nm-thick silicide layers on single-crystalline silicon in the temperature range between 650 and 800°C using transmission electron microscopy and atomic force microscopy. Sheet resistance measurements were used to observe the effect of the reaction temperature on the layer properties and to perform a systematic study on the thermal stability. At the lower reaction temperature good quality layers were produced with quite small grains. The specific resistivity is as low as 15 μΩ cm and the interface to the silicon substrate is quite flat. These favourable properties lead to a high thermal stability. The processed layers are stable in the temperature range between 850 and 950°C. When the layer thickness is increased to 100 nm, the stability range extends by about 250°C. The deterioration rate of 30-nm and 100-nm-thick silicide layers was deduced from sheet resistance measurements. The activation energy for the electrical degradation of the layer due to layer agglomeration was found to be nearly independent of the thickness.

Nanometer patterning of thin CoSi 2-films by application of local stress

We have developed a self-assembly method for patterning thin CoSi 2 layers on Si(100) during their formation in a solid state reaction. This technique is based on anisotropic diffusion in a local stress field during rapid thermal processing. The stress is induced by a layer structure consisting of 30 nm SiO 2 and 300 nm Si 3N 4 which is patterned with conventional optical lithography. We have investigated two different silicide formation processes. Firstly, we deposited Co on Si in a UHV MBE chamber. Rapid thermal annealing leads to the formation of polycrystalline CoSi 2. Secondly, we used a titanium oxide mediated epitaxy process. For both processes we observed nanostructures with dimensions of about 100 nm showing wave-like separation edges in the first case and good uniformity in the second case.

Ballistic electron emission microscopy studies of the temperature dependence of Schottky barrier height distribution in CoSi 2/n-Si(1 0 0) diodes formed by solid phase reaction

Ballistic electron emission microscopy (BEEM) and ballistic electron emission spectroscopy have been performed on polycrystalline and epitaxial CoSi 2/n-Si(1 0 0) contacts at temperatures ranging from −144°C to −20°C. The ultra-thin CoSi 2 films (∼10 nm) were fabricated by solid state reaction of a single layer of Co (3 nm) or a multilayer of Ti (1 nm)/Co (3 nm)/amorphous-Si(1 nm)/Ti (1 nm) with a Si substrate, respectively. The spatial distribution of barrier height over the contact area obeys a Gaussian function at each temperature. The mean barrier height increases almost linearly with decreasing temperature with a coefficient of −0.23±0.02 meV/K for polycrystalline CoSi 2/Si diodes and −0.13±0.03 meV/K for epitaxial diodes. This is approximately equal to one or one-half of the temperature coefficient of the indirect energy gap in Si, respectively. It suggests that the Fermi level is pinned to different band positions of Si. The width of the Gaussian distribution is about 30–40 meV, without clear dependence on the temperature. The results obtained from conventional current–voltage and capacitance–voltage ( I– V/ C– V) measurements are compared to BEEM results.

Influence of Ti on CoSi2 nucleation

Evidence is presented that impurities present in the precursor phase may influence the nucleation of a new phase. In case of the CoSi→CoSi2 transition, it is found that the presence of small amounts of Ti (originating from either a Ti capping layer or interlayer) causes an increase in the CoSi2 nucleation temperature. Moreover, for an increasing amount of Ti, we observed a transition from polycrystalline CoSi2 over preferential (220) orientation towards epitaxial (400) CoSi2. The model that we propose entails a new point of view on the mechanism of Ti interlayer mediated epitaxy.

Electrical characteristics of CoSi 2/n-Si(1 0 0) Schottky barrier contacts formed by solid state reaction

The Schottky barrier height (SBH) of CoSi 2 contacts formed by solid state reaction of Co, Co/Ti, Ti/Co and Ti/Co/SiO 2 on n-Si(1 0 0) substrates has been measured in the temperature range from 80 to 300 K with the use of current– and capacitance–voltage techniques. The forward I– V characteristics are analyzed on the basis of the standard thermionic emission model and the assumption of a Gaussian distribution of the barrier heights. The difference in SBHs determined from the I– V and C– V data is temperature dependent. From this difference, the standard deviation and its temperature coefficient are derived and are in the range of 58–78 meV and −0.07 to −0.14 meV K −1, respectively. The Richardson plots, modified according to the Gaussian distribution model, have a good linearity over the whole temperature range for all samples. The corresponding activation energy is in good agreement with the barrier height determined from the C– V data. The SBH of the CoSi 2 contacts grown from Co and Ti bimetallic layers is lower than that grown from a Co layer only. The temperature coefficient of the SBH varies from approximately −0.16 meV K −1 for polycrystalline CoSi 2 to ∼0 meV K −1 for epitaxial CoSi 2 contacts, thus suggesting different interfacial Fermi level pinning at the CoSi 2/Si contacts grown from different multilayer structures.