Formation Of Epitaxial CoSi2 Research Articles

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.

Formation of epitaxial CoSi2 by a Cr or Mo interlayer: Comparison with a Ti interlayer

The influence of Cr and Mo on phase formation and preferential orientation of CoSi2 is reported. Three different regimes are distinguished, depending on the thickness of the interlayer. For a thin interlayer or a capping layer, CoSi forms first, as in the standard Co/Si reaction. The remaining Cr or Mo can be considered as a contaminant that is present in the CoSi layer, causing a delay in CoSi2 nucleation and inducing preferential (220) and (400) nucleation. For interlayers with intermediate thickness, epitaxially (400) oriented CoSi2 is formed. For a thick interlayer, a polycrystalline layer of CrSi2 (or MoSi2) is formed first, followed by CoSi formation on top of the CrSi2. At higher temperatures, the CoSi layer is transformed into a polycrystalline, continuous layer of CoSi2 on top of the CrSi2 or MoSi2 layer, while some grains of CoSi2 are formed underneath the CrSi2. A similar behavior for Ti interlayers is observed, although a much thicker Ti layer is needed before the third regime is reached.

Reaction of Co and capping layers and its effect on CoSi2 formation in Si/SiOx/Co system

The role of the reaction between the capping layer and Co on the crystalline nature of CoSi2 films in refractory metal-capped Si/SiOx/Co system has been investigated. The epitaxial CoSi2 film was obtained in the capping layers (Ti, Zr) with high tendency of mixing between Co and the capping layer. Amorphous Ti–Co layer was produced at 450 °C, and its thickness was increased at 550 °C. The formation of amorphous Ti–Co layer during low-temperature annealing may be responsible for the formation of epitaxial CoSi2. Meanwhile, the polycrystalline CoSi2 was formed in the capping layer (Cr, Mo) with low tendency of mixing. These results can be explained by the fact that the mixing layer formed from the reaction between Co and refractory metal control the Co diffusion to the Si substrate as well as the thin SiOx between Co and Si.

Nucleation of single-crystal CoSi2 with oxide-mediated epitaxy

Oxide-mediated epitaxy (OME) has shown promise as a technique for the formation of epitaxial CoSi2 on a variety of Si surfaces. With our in situ ultra-high-vacuum transmission electron microscope we have studied the phase formation sequence of the deposited Co during an anneal on both clean and oxide (OME) -covered Si (001) samples. The striking difference in OME is the absence of polycrystalline CoSi2 nucleation. We discuss the origin and consequences of this observation, and report other details of the phase evolution sequence.

Thermal Studies on Stress-Induced Void-Like Defects in Epitaxial-CoSi2 Formation

AbstractWe report experimental findings on the thermal-induced stress-voiding observed during epitaxial CoSi2 formation on patterned Si(100) wafers associated with the Co/Ti-interposing layer scheme [1–6]. The first rapid thermal process (RTP) in a typical SALICIDE process was studied by varying the temperature and the RTP ramp-up rate. Based on ex-situ cross-sectional transmission electron microscopy (XTEM), crystal defects were observed at shallow trench isolation (STI)/Si and gate spacer/Si edges after annealing at temperatures greater than 520°C. In addition, isothermal anneals for various time periods resulted in increased encroachment of an ultra-thin CoSi2-phase at the STI/Si edge. Higher temperature anneals also resulted in a corresponding increase in void size. In addition, higher ramp-up rates led to a thicker CoSi2 “pocket” film at the tensile-stressed STI/Si edge, aligned epitaxially along the {111}-habit planes. Our attempts to bypass the CoSi formation in order to alleviate the anomalous Si diffusion was found to be irrelevant to the voiding issue, due to the inherent nature of this film stack system to nucleate CoSi2 as the first phase. By comparing our experimental observations of voiding with Hu's analysis of the trench isolation stress fields, we found good agreement with the predicted stress field distribution in the active Si and STI/Si edge with the locations of void nucleation. The anomalously large stress induced is still not known, but can be attributed to the encroaching CoSi2 film which tends to realign epitaxially along the STI/Si interface.

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.

Silicide formation by concentration controlled phase selection

It is proposed that direct formation of epitaxial CoSi2 and NiSi2 as the first phase, is due to the interlayer between the metal and silicon acting as a diffusion barrier, which decreases the metal concentration at the growth interface. Such concentration controlled phase selection (CCPS) is explained thermodynamically by utilizing the effective heat of formation (EHF) model. This approach is also used to explain silicide formation with metal alloys. Concentration controlled phase selection (CCPS) is not only applicable to silicide formation but should in general enable materials scientists to form phases with desirable properties, by controlling the concentrations of the reactants at the growth interface.

Control of Co flux through ternary compound for the formation of epitaxial CoSi2 using Co/Ti/Si system

A ternary compound of Co3Ti2Si is suggested as reaction barrier for the formation of epitaxial CoSi2 in the Co/Ti/Si system. It has a role to control Co diffusion to the Si substrate, followed by formation of CoSi2. After Co3Ti2Si was formed, CoO and Ti oxide were formed at surface, depending on Ti thickness. In the case of Ti oxide being at surface, the outdiffusion of Ti in ternary compound was accelerated. Then, the decomposition of Co3Ti2Si occurred by reaction with Ti oxide, resulting in uniform epitaxial CoSi2. However, in the case of CoO being at surface, the Ti outdiffusion was suppressed, followed by thermally decomposition of Co3Ti2Si. This caused nonuniform Co supply to form nonuniform CoSi2.

On the formation of inhomogeneities in epitaxial CoSi2 layers grown from the interaction of Co/Ti bilayers with Si 〈100〉 substrates

The redistribution of titanium during the formation of epitaxial CoSi2, grown from the reaction of Co(20 nm)/Ti(10 nm) bilayers with Si 〈100〉, has been investigated. Annealing of Co/Ti/Si structures, at temperatures between 850 and 1050 °C, is shown to be associated with the growth of an inhomogeneous CoSi2 layer having Ti-rich surface layer(s) on top. The formation of inhomogeneities in the CoSi2 layer is conclusively attributed to the presence of Ti-rich surface layer(s). It is shown that smooth and morphologically stable CoSi2 layers can be grown by removing these surface layers followed by a high-temperature treatment in nitrogen atmosphere. We propose that the underlying mechanism for the inhomogeneity formation within the CoSi2 layer is a nucleation-controlled process, induced by an anticipated reaction between the CoSi2 layer and Ti-rich phases near the surface.

Defect Generation during Epitaxial CoSi2 Formation Using Co/Ti Bilayer on Oxide Patterned (100)Si Substrate and Its Effect on the Electrical Properties

Self‐aligned formation of epitaxial , using a Co/Ti bilayer, on linear oxide patterned (100)Si substrates has been investigated. Rapid thermal annealing (RTA) at 550°C resulted in lateral encroachment of silicide at the Si under the edge of the oxide. After RTA at 900°C, even though an epitaxial layer was formed on the Si substrate, defects such as lateral encroachment and void were generated under the edge of the oxide. It was found that such defects yield device failures due to deterioration of the gate oxide and the shallow junction.

In situ x-ray diffraction study of the role of annealing ambient in epitaxial CoSi2 growth from Co/Ti bilayers on Si(001)

The reactions during annealing of a Co/Ti bilayer structure on Si(001) were studied in situ to reveal the roles of the Ti interlayer and the annealing ambient on the formation of epitaxial CoSi2. We shown that an oxygen contaminant in the nitrogen annealing gas is needed to form a stable, Co–Ti–O (spinel) membrane at the metal/Si interface that limits diffusion and is crucial for the perfection of epitaxial CoSi2. Annealing in vacuum or otherwise inert environments led to polycrystalline CoSi2 films and no spinel phase.

On the formation of epitaxial CoSi2 from the reaction of Si with a Co/Ti bilayer

In spite of much work, the formation of epitaxial CoSi2 from Ti/Co on (100) Si remains something of a mystery. It has been proposed that epitaxy occurs via the formation of an intermediate phase of CoSi with a (311) preferred orientation. In the absence of sufficient information it is impossible to validate or to invalidate the specific original claim. However, one shows that the formation of preferably oriented CoSi is not a necessary condition for the subsequent growth of epitaxial CoSi2. Careful measurements of diffraction intensities reveal the probable, temporary formation of a metastable form of CoSi2, based on a diamond cubic rather than the usual CaF2 structure.

Defect Generation During Epitaxial Growth of CoSi2 on Miniature Sized (100) Si Substrate and its Effect on Electrical Properties

AbstractSelf-aligned silicide (salicide) formation of epitaxial CoSi2, using a Co/Ti bilayer, on linear oxide (SiO2) patterned (100)Si substrate has been investigated. Rapid thermal annealing (RTA) at 550°C resulted in the lateral encroachment of silicide in the Si under the edge of the oxide. After RTA at 900°C, even though an epitaxial CoSi2 layer was formed on the Si substrate, defects such as lateral encroachment and voids were generated under the edge of the oxide. It was found that such defects lead to device failure due to the deterioration of the gate oxide and the shallow junction.

Laser Induced Epitaxial CoSi2 Formation

ABSTRACTWhile it is possible to grow epitaxial CoSi2 films of a high quality using UHV technology it is much more difficult to produce films of similar quality under high vacuum and non-“clean room” conditions. Tung et al.1 have however shown that films of a good quality can be produced under these poorer conditions with the aid of a laser anneal of an appropriate power followed by a short thermal anneal. In this paper the role of the laser anneal, and of the (post laser) thermal anneal, in the production of epitaxial films is examined. It appears that the principle role of the laser anneal is to induce the epitaxial CoSi2 formation at the silicon / silicide interface, and that this epitaxial layer then acts as a seed for epitaxial film growth during the subsequent thermal anneal.

Direct Solid State Phase Transformation from Co to Epitaxial CoSi2 in Co / Thin Ti / (100) Si Structure and its Application for Shallow Junction Formation

ABSTRACTDirect epitaxial CoSi2 formation from Co, which is contrary to the reported silicidation process : Co→Co2 Si→CoSi→CoSi2, has been found during anneal of Co / Ti / (100) Si system. 2 nm thick Ti and 15 nm thick Co films were sputter deposited, and then annealed for 30 min at temperatures between 375°C and 900°C. At room temperature, the 2 nrm Ti immediately forms an amorphous Ti-Si-Co layer between the Co and Si. Epitaxial CoSi2 begins to form at 400°C, while the amorphous layer continues to act as both a Co diffusion retardant and Si diffusion suppressant even at 900°C. This retarded diffusion of Co reduces the growth rate of the CoSi2 over the entire temperature range studied. Superiority of the epitaxial to polycrystalline silicide has been demonstrated. In self aligned structures, an epitaxial CoSi2 film is formed by a single-step anneal without any overgrowth onto adjacent field oxide areas utilizing the amorphous diffusion controlling layer. A p+/n junction of 40 nm depth with reduced leakage and low ideality factor has been obtained by impurity diffusion from epitaxial CoSi2.

Epitaxial CoSi2 formation on Si(001) from an amorphous Co75W25 sputtered layer

The formation of epitaxial CoSi2 on (001) Si using the solid state reaction between an amorphous Co75W25 sputtered layer and Si has been studied. Auger electron spectroscopy depth profiling, Rutherford backscattering spectrometry, x-ray diffraction, transmission electron microscopy, and resistivity and mobility measurements on Van der Pauw structures have been used to investigate the interaction between the amorphous alloy and Si. By employing anneals in vacuum between 500 and 600 °C for 60 or 120 min, Co diffuses out of the alloy into the substrate to form CoSi2. X-ray diffraction measurements indicate that the greater part (about 75%) of the disilicide film is epitaxial. The CoSi2 film is unstrained at the growth temperature. At room temperature a tetragonal distortion of the silicide lattice is noted, which results from cooling from the growth temperature, and is caused by the difference in the thermal expansion coefficients of Si and CoSi2. After a selective etch to remove the remaining amorphous alloy, a second anneal at a higher temperature has been performed to improve the quality of the silicide. As a result of the second anneal, the Rutherford backscattering channeling minimum yield and the residual resistivity of the film decrease to values of about 25% and 2.6 μΩ cm, respectively. Only the epitaxial (A type) orientation of CoSi2 could be detected after the second anneal by x-ray pole figure measurements. Both the vertical and the lateral distortions in the silicide lattice, measured at room temperature, appear to decrease during this anneal. The Debye temperature, obtained from electrical measurements, increases from 382 K, after Co outdiffusion, to 412 K after the second anneal.

Effects of two-step annealing on the epitaxial growth of CoSi2 on silicon

The formation of epitaxial CoSi2 on silicon by both conventional and two-step annealing of cobalt thin films on silicon was studied by transmission electron microscopy.For two-step annealing, samples were first annealed at 350 °C for 1 h, followed by high temperature annealing at 650–950 °C for 1 h. The scheme was found to be very effective in promoting the epitaxial growth of CoSi2 on silicon as well as eliminating faceted structures on Si(111). The results are discussed in terms of the driving away of impurities from the interfaces.

Effects of Two-Step Annealing on the Epitaxialgrowth of CoSi2 on Silicon

The formation of epitaxial CoSi2 on silicon by both conventional and two-step annealing of cobalt thin films on silicon was studied by transmission electron microscopy.For two-step annealing, samples were first annealed at 350°C for 1 h, followed by high temperature annealing at 650–950°C for 1 h. The scheme was found to bevery effective in promoting the epitaxial growth of CoSi2 on silicon as well aseliminating faceted structures on Si(111). The results are discussed in terms of the driving away of impurities from the interfaces.