Epitaxial CoSi2 Layers Research Articles

Effects of NH3 flow rate on the epitaxial growth of CoSi2 thin film using a CoN x interlayer deposited by MOCVD

A uniform epitaxial CoSi2 layer could be grown on a Si(100) substrate by deposition of a CoNx interlayer followed by an in situ annealing process. The amorphous Si–N layer which is formed at the CoNx/Si interface plays a role in inhibiting excessive Co incorporation into Si layer and consequently enhancing the epitaxial growth of CoSi2 layer. However, the Si–N layer also limits the growth of thick CoSi2 layer. Therefore, the thickness of the amorphous Si–N layer is an important factor for the epitaxial growth of the CoSi2 layer with uniform structure and low sheet resistance.

Formation of epitaxial cobalt disilicide thin film by solution processing

A precursor obtained by the reaction between cyclopentasilane and dicobaltoctacarbonyl in a toluene solution provides a method for the production of a low-resistivity thin film of cobalt disilicide CoSi2 (ca. 15 µΩ·cm) on a silicon (100) substrate by spin coating and annealing. Several types of analysis have revealed that the films formed on silicon substrates have a double-layer structure consisting of an upper layer (oxidized complex film) and an epitaxial CoSi2 layer with low resistivity along the (100) orientation of the silicon substrate. The former layer can be removed by dry etching or chemical mechanical planarization when used in actual applications.

Formation of an Epitaxial CoSi2 Layer with Reduced Oxygen Contamination

The formation of epitaxial using an oxide buffer layer is known to be sensitive to the oxygen contamination of the Co film. The oxygen contaminants may originate from various oxygen sources such as ambient annealing. The presence of an oxynitride buffer layer and a Ti-capping layer affect the growth of epitaxial , resulting in good epitaxial quality and excellent interface morphology between the Co film and Si substrate. Almost no interference due to oxygen contamination was observed, in spite of intentional exposure of the Co film to air. The oxynitride buffer layer could effectively control the flux of the Co into the Si, resulting in a coherent interface between the and the Si without the formation of additional . Moreover, the Ti-capping layer could absorb oxygen, resulting in less oxygen contamination. This capping layer positively affected the interface morphology between the Co-silicide and the Si substrate.

Crystallinity of CoSi2 nanolayer grown by refractory metal interlayer and cap layer methods

Epitaxial formation of CoSi2 nanolayer by solid state reaction of Co-Si in refractory metal intermediate-layer and cap-layer systems was investigated. Thin films of Ta and W, as the refractory metal intermediate or cap layers of the Co film, were deposited on Si(100) substrate and then heat-treated. The both interlayers resulted in formation of epitaxial CoSi2 with (100) crystallographic orientation at 900°C. However, in the Ta intermediated system, the grown CoSi2 layer was thermally unstable at 1000°C, unlike the W system with a stable silicide layer. We found that use of W cap-layer cannot yield an epitaxial CoSi2 phase. But, a Ta cap-layer resulted in formation of epitaxial CoSi2(100) layer even in a lower temperature (800°C). Again, in the presence of Ta, the grown CoSi2 layer was thermally unstable at 900°C.

Epitaxial growth of CoSi2 on Si(001) by reactive deposition epitaxy: Island growth and coalescence

Epitaxial CoSi2 layers, which are phase pure but contain {111} twins, are grown on Si(001) at 700 °C by reactive deposition epitaxy. Transmission electron microscopy analyses show that the initial formation of CoSi2(001) follows the Volmer–Weber mode characterized by the independent nucleation and growth of three-dimensional islands whose evolution we follow as a function of deposited Co thickness tCo in order to understand the origin of the observed twin density. We find that there are two families of island shapes: inverse pyramids and platelets. The rectangular-based pyramidal islands extend along orthogonal 〈110〉 directions, bounded by four {111} CoSi2/Si interfaces, and grow with a cube-on-cube orientation with respect to the substrate: (001)CoSi2||(001)Si and [100]CoSi2||[100]Si. Platelet-shaped CoSi2 islands are bounded across their long 〈110〉 directions by {111} twin planes (i.e. {111}(001)CoSi2||{111}Si) and their narrow 〈110〉 directions by {511}CoSi2||{111}Si interfaces. The top and bottom surfaces are {22¯1}, with {22¯1}CoSi2||(001)Si, and {1¯1¯1}, with {1¯1¯1}CoSi2||{11¯1}Si, respectively. The early stages of film growth (tCo≤13 Å) are dominated by the twinned platelets due to a combination of higher nucleation rates resulting from a larger number of favorable adsorption sites in the Si(001)2×1 surface unit cell and rapid elongation of the platelets along preferred 〈110〉 directions. However, at tCo≥13 Å island coalescence becomes significant as orthogonal platelets intersect and block elongation along fast growth directions. In this regime, where both twinned and untwinned island number densities have saturated, further island growth becomes dominated by the untwinned islands. A continuous epitaxial CoSi2(001) layer, with a twin density of 2.8×1010 cm−2, is obtained at tCo=50 Å.

The interaction of cobalt with oxidized silicon surface

The initial stages of the growth of cobalt disilicide (CoSi2) on a 2×1 reconstructed Si(100) surface in the presence of oxygen have been studied for the first time by method of high-resolution photoelectron spectroscopy. The evolution of the electron structure of the sample surface was traced in the course of silicon oxidation, cobalt deposition, and subsequent thermal annealing. It is established that cobalt atoms penetrate to the oxide-silicon interface even at room temperature. This phenomenon favors the formation of an epitaxial CoSi2 layer with improved morphology.

Effect of deposition temperature on the formation of CoSi2 through the rapid thermal annealing of CVD cobalt

Epitaxial CoSi2 layers were successfully produced on (100) silicon substrates by rapid thermal annealing of cobalt layer deposited by CVD. The epitaxial layer had a resistivity of 66 μΩcm for a ∼100 nm layer. The relatively high resistivity is attributed to discontinuities and contamination in the layer.Cobalt silicide formation was found to be dependent on the deposition temperature of the CVD process and the thickness of the cobalt layer. The deposition of a PVD Ti/TiN capping layer was found to allow silicide formation at lower temperatures as the titanium reduces the interface oxide.

Strain relaxation of epitaxial CoSi2 and SiGe layers in cap-Si/Si0.83Ge0.17/Si(0 0 1) and epi-CoSi2/Si0.83Ge0.17/Si(0 0 1) structures

Strain relaxation behaviors of the epitaxial CoSi2 (epi-CoSi2) and Si0.83Ge0.17 layers in epi-CoSi2/Si0.83Ge0.17/Si(001) and cap-Si/Si0.83Ge0.17/Si(001) structures were investigated by high-resolution X-ray diffraction (HR-XRD) analyses. Samples were treated at the temperature, TA = 650–900°C by rapid thermal annealing. Comparative measurements showed a different strain relaxation behavior in the SiGe layers with and without CoSi2 layer. And Ge content and lattice mismatch in the SiGe film of the epi-CoSi2/Si0.83Ge0.17/Si(001) are smaller than those in the SiGe layer in cap-Si/SiGe/Si(001) possibly due to the diffusion of Ge into the tensile-stressed epi-CoSi2 layer to reduce the compressive stress in the SiGe layer at elevated temperature. The analyses of high-resolution ω–2θ scan spectra and reciprocal space mapping showed that epi-CoSi2 layer is under tensile residual stress and a significant strain relaxation starts at TA = 900°C indicating of thermal stability up to TA = 850°C.

In situ analysis of epitaxial cobalt silicide reaction on silicon (0 0 1)

Formation process of CoSi2 on Si(0 0 1) was investigated with in situ reflection high-energy electron diffraction analysis for developing a self-aligned silicide (salicide) process of advanced silicon transistors. A thin Co layer deposited onto a wet chemically oxidized Si(0 0 1) surface silicided at 550°C to form epitaxial CoSi2(0 0 1) without developing polycrystalline subsilicide phase. The oxide remained on the surface up to 750°C and kept the surface flat until its evaporation. It was thus revealed that chemically formed thin oxide on Si(0 0 1) surface is an effective reaction control layer that acts as a reaction barrier to Co/Si subsilicide formation and as a kinematical barrier to surface migration of growing species. These conditions are necessary for fabricating flat and thin epitaxial CoSi2 layers.

Formation and Properties of Epitaxial CoSi2 Layers on p-Si0.83Ge0.17/p-Si(001) using a Si Capping Layer by Metal-Organic Chemical Vapor Deposition

Epitaxial CoSi2 layers were in-situ grown using a sacrificial Si capping layer on p-Si0.83Ge0.17/p-Si(001) by metal organic vapor deposition (MOCVD) at 600°C using cyclopentadienyl cobalt, Co(η5-C5H5)(CO)2. The grown epitaxial CoSi2 layers remain thermally stable at the annealing temperature, TA≤800°C. The CoSi2/p-Si0.83Ge0.17/p-Si(001) samples annealed at TA≤850°C showed the sheet resistance values as low as \\cong5.6 Ω/□. At TA=900°C, the observed increase in the sheet resistance value to \\cong14.7 Ω/□ was attributed to the formation of CoSi phase with higher resistivity resulting from the direct interaction of Co with the SiGe alloy layer. The effective Schottky barrier heights (φBp), measured for the first time, ranged from 0.103 to 0.194 eV at the reverse bias of -5–0 V.

Epitaxial cobalt silicide formation using a Co/TiSix bilayer on Si (1 0 0) by sputtering

Annealing a Co/Ti bilayer by sputtering on the Si substrate offers higher probability of epitaxial growth and less Si consumption than annealing of a single Co layer on the Si substrate. Nevertheless, the Co/Ti bilayer epitaxy method does not solve the Si consumption problem satisfactorily. In this communication, a new CoSi2 epitaxial growth technique of annealing a Co/TiSix bilayer on Si (100) is investigated for the first time. Cobalt silicide thin films obtained using the Co/TiSix bilayer have been analyzed using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) techniques to identify the phases and to investigate the surface and interface morphologies and the chemical composition. The XRD spectra show the epitaxial growth of CoSi2 layer with (200) crystallographic orientation on the Si (100) substrate for a Co/TiSix bilayer while the Co–Ti–Si spinel phase is detected over the annealing temperature range of 400–600 °C. The AES analysis indicates that native oxides on the silicon substrates are removed by Ti at the beginning of RTA and Co diffuses inward so that an epitaxial CoSi2 layer could form. The AFM micrographs show smooth surface and the cross-sectional high-resolution TEM analysis reveals that the layer is composed of the CoSi2 epi-layer formed on the Si substrate. Also it is found that Si consumption during silicidation annealing for the Co/TiSix bilayer formed by sputtering has been significantly reduced compared with that for the Co/Ti bilayer formed by sputtering.

Epitaxial CoSi 2-nanostructures: an approach to Si nanoelectronics

CoSi 2-nanostructures were fabricated using a self-assembly process involving local oxidation of silicides (LOCOSI). The nanostructures are generated along the edge of a mask consisting of SiO 2 and Si 3N 4, deposited by plasma enhanced chemical vapor deposition (PECVD) and patterned with conventional optical lithography. The mask induces a stress field near its edges into the underlying CoSi 2/Si-heterostructure. Rapid thermal oxidation (RTO) leads to the separation of the CoSi 2 layer in this region due to the concomitant anisotropic diffusion of the cobalt atoms in the stress field. Using this method, uniform gaps and narrow wires were produced from 20–30 nm-thick single-crystalline, epitaxial CoSi 2-layers grown by molecular beam allotaxy (MBA) on conventional Si(100) and silicon-on-insulator (SOI) substrates. These structures with dimensions down to 40 nm can be used as building blocks for nanoscale metal-oxide-semiconductor field effect transistor (MOSFET) devices. We produced 70 nm gate-length Schottky barrier MOSFETs (SBMOSFETs) on SOI using the silicide nanostructures. These devices can be driven as both p-channel and n-channel MOSFETs without complementary substrate doping and show good I– V characteristics.

Formation of epitaxial CoSi2 films on Si and on Si/Si80Ge20 (100) by reactive deposition epitaxy

Co x Ti 1−x layers were deposited on Si (100) and on Si/Si80Ge20 (100) capped with 30- or 40-nm-thick Si at 650 °C substrate temperature at 1×10−6 Pa pressure. The Co–silicide films grown by reactive deposition epitaxy were characterized by Rutherford backscattering-channeling, x-ray difraction, by depth profile analysis of the components, and by sheet resistance measurements. The Ti content of the deposited Co layers was between 0.1 and 8 at. %. The epitaxy of the layers on Si and on Si/Si80Ge20 improved by increasing the Ti concentration. The minimum yield of the channeling and the full width at half maximum value of the rocking curve of CoSi2 decreased. The sheet resisitance of the formed layers was also minimal in these cases. The method applied is promising to form epitaxial CoSi2 layers on SixGe1−x substrates.

Fabrication of epitaxial CoSi2 nanowires

We have developed a method for fabricating epitaxial CoSi2 nanowires using only conventional optical lithography and standard silicon processing steps. This method was successfully applied to ultrathin epitaxial CoSi2 layers grown on Si(100) and silicon-on-insulator substrates. A nitride mask induces a stress field near its edges into the CoSi2/Si heterostructure and leads to the separation of the CoSi2 layer in this region during a rapid thermal oxidation step. A subsequent etching step and a second oxidation generate highly homogenous silicide wires with dimensions down to 50 nm.

Properties of ion implanted epitaxial CoSi2/Si(1 0 0) after rapid thermal oxidation

Epitaxial CoSi2 layers were grown on Si(100) using molecular beam allotaxy. Boron ion implantations and rapid thermal oxidation (RTO) were performed. During oxidation, SiO2 formed on the surface of the CoSi2 layers, and the silicides was pushed into the substrate. The diffusion of boron was slightly retarded during oxidation for the specimen with a 20 nm epitaxial CoSi2 capping layer as compared to the specimen without CoSi2 capping layer. The electrical measurements showed that the silicide has good Schottky contacts with the boron doped silicon layer after RTO. A nanometer silicide patterning process, based on local oxidation of silicide (LOCOSI) layer, was also investigated. It shows two back-to-back Schottky diodes between the two separated parts of the silicide.

Nondestructive characterization of thin silicides using x-ray reflectivity

X-ray reflectivity (XRR) is a nondestructive method for the characterization of layer thickness, surface and interfacial roughness of thin films. We have applied the XRR technique to the characterization of thin CoSi2 (both epitaxial and polycrystalline) and PtSi layers. The silicide films were prepared by rapid thermal annealing and XRR was used before and after silicidation to measure the layer thickness. The XRR results are compared to results obtained on the same films by Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, profilometry and atomic force microscopy. For silicides which grow by diffusion (CoSi2 formed on amorphous Si and PtSi), we found that XRR may be used to characterize both thin (3 nm) and thick (>50 nm) layers. For polycrystalline CoSi2 formed on single crystalline Si, where not only diffusion but also nucleation is important, roughness was a limiting factor for XRR measurements and only thin layers (<15 nm) could be characterized. For epitaxial CoSi2 layers, which are very smooth, both thin and thick layers could be measured.

Comparison study of nucleation and growth characteristics of chemical-vapor-deposited diamond films on CoSi2 (001) and Si(001)

Epitaxial CoSi2 (001) layers, deposited on Si(001) substrates by molecular beam allotaxy, were used as substrates for diamond deposition in order to realize applications. The nucleation and textured growth of diamond films were compared with those on the Si(001) substrates. The results indicate that in a microwave-plasma chamber diamond can be nucleated with a higher density on CoSi2 at lower temperatures using a bias-enhanced method. High-quality [001]-textured diamond films can be synthesized on CoSi2 (001) using the [001]-textured growth conditions. So far, epitaxial growth of diamond on CoSi2 cannot be observed. Statistically, a rotating angle distribution of diamond grains around the [001] axis in a [001]-textured film shows, however, preferred in-plane orientations of 13°, 22°, 45°, and 77° relative to the CoSi2 [011] axis. The structural and chemical analyses show that no Co and Si element diffusion from the CoSi2 substrate into the diamond film can be detected. The films grown on CoSi2 have similar crystal quality to that of epitaxial films deposited directly on Si.

In-situ Growth and Growth Kinetics of Epitaxial (100) CoSi2 Layer on (100) Si by Reactive Chemical Vapor Deposition

ABSTRACTUniform epitaxial CoSi2 layers have been grown in situ on a (100) Si substrate at temperatures near 600 °C by reactive chemical-vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5-C5H5)(CO)2. The growth kinetics of an epitaxial CoSi2 layer on a Si (100) substrate was investigated at temperatures ranging from 575 to 650 °C. In initial deposition stage, platelike discrete CoSi2 spikes were nucleated along the <111> directions in (100) Si substrate with a twinned structure. The discrete CoSi2 plates with both {111} and (100) planes grew into an epitaxial layer with a flat interface on (100) Si. For epitaxial CoSi2 growth on (100) Si, the activation energy of the parabolic growth was found to be 2.80 eV. The growth rate seems to be controlled by the diffusion of Co through the CoSi2 layer.

Solid solubility of As in CoSi2 and redistribution at the CoSi2/Si interface

The solid solubility of As in CoSi2 and the redistribution of As at the CoSi2/Si interface at temperatures between 650 and 950 °C have been investigated. As was implanted in the cap Si layer of mesotaxy samples (epitaxial CoSi2 layers buried in a Si substrate). The As profiles after annealing were measured by secondary ion mass spectrometry. The solubility of As is lower in the cobalt disilicide than in silicon. A detailed description of thermodynamic equilibrium between the solution of As in CoSi2 and the solution of As in Si shows that the distribution coefficient is dependent on the concentration. Within the experimental accuracy, the enthalpy of solution, deduced from the distribution coefficient, is constant in the temperature range and equal to 0.28 eV. The solution of As in CoSi2 can thus be described by Henry’s law, which is valid for dilute solutions. Accumulation of As at both interfaces of the buried CoSi2 layer has been observed and is discussed in terms of segregation and precipitation. A transient enhancement of the As diffusion in the Si substrate has also been found. Simulation of the redistribution process shows that this enhancement is not due to the diffusion of As in CoSi2 but more likely associated with the excess of point defects resulting from ion implantation or from the silicide film. On the basis of both ternary phase diagram calculations and experimental results, it is shown that there is a ternary equilibrium between Si(As), CoSi2, and SiAs in contrast to the equilibrium between Si(As), CoSi2, and CoAs reported previously by other authors. It is shown that the latter equilibrium should decrease the solubility of As in Si, but such a decrease has never been observed experimentally.

Growth behavior and thermal stability of epitaxial CoSi2 layer from cobalt–carbon films on (100) Si substrate

A uniform epitaxial CoSi2 layer was grown on (100) Si substrate by rapid thermal annealing at 800 °C in N2 ambient without capping layers from an amorphous cobalt-carbon film. The amorphous cobalt-carbon film was deposited on Si substrate by the pyrolysis of cyclopentadienyl dicarbonyl cobalt, Co(η5–C5H5)(CO)2, at 350 °C. The discrete epitaxial CoSi2 layers with {111} and (100) faceted interfaces were formed on (100) Si substrate at the initial stage of reaction between Co and Si. Annealing at elevated temperatures lowered the roughness of the CoSi2/Si interface. The leakage current measured on the junction, fabricated with the epitaxial CoSi2 layer and annealed at 1000 °C for 30 s, was as low as that of the as-fabricated junction without silicide. The result indicates that epitaxial (100) CoSi2 is thermally stable at temperatures even above 1000 °C and has potential application to the salicide process in subhalf micron devices.