CoSi2 Layers Research Articles

Nanoscale CoSi2 contact layer growth from deposited Co/Ti multilayers on Si substrates

In this letter, we describe procedures for forming continuous, planar, and thermally stable 12-nm-thick CoSi2 layers via Co/Si interaction through an interfacial Ti(O) diffusion barrier layer. Three Co and three Ti layers were deposited sequentially on Si-(100) substrates by dual source thermal evaporation with Ti as the first layer. Oxygen was found to be selectively incorporated into all Ti layers during deposition. Following a 550 °C, 2 h anneal the morphology of the silicide layer depended strongly on the thickness of the initial Ti(O) layer. For an initial Ti(O) layer of ∼5 nm, both Co and Si readily diffused to form a Co silicide interfacial layer with a very rough, faceted interface. Increasing the Ti(O) thickness to ∼10 nm stopped Si out diffusion and reduced Co in diffusion such that a uniform 6 nm CoSix interfacial layer formed. Selective removal of the upper layers and a 750/800 °C annealing produced a 12 nm CoSi2 layer with a resistivity of ∼28 μΩ cm.

Co diffusion and growth of buried single-crystal CoSi2 in Si(111) by endotaxy

At a growth temperature of 800 °C, Co deposited on a Si capping layer exhibits oriented growth on buried CoSi2 grains on Si(111), a process referred to as endotaxy. This occurs preferentially to surface nucleation of CoSi2, provided the thickness of the Si cap is less than a critical value of ≊100 nm for deposition rates of 0.003–0.01 nm/s. The steady-state process is modeled using known values of the Co diffusion coefficient and solid solubility in Si, allowing some conclusions to be drawn regarding parameters relevant to CoSi2 epitaxy. Using this technique, single-crystal continuous layers of CoSi2 can be formed under a high-quality Si capping layer.

Epitaxial growth and electrical conductance of metal(CoSi2)/insulator(CaF2) nanometer-thick layered structures on Si (111)

Epitaxial growth of a metal (CoSi2) /insulator (CaF2) nanometer-thick layered structure on Si(111) was demonstrated and the resistivity of CoSi2 epilayer in this structure was investigated. An epitaxial CoSi2 layer on CaF2 was obtained by the two-step growth technique,i.e. solid phase epitaxy with the epitaxial Si layer grown in the first step and Co deposited in the second step. This technique was shown to be effective to avoid the Co agglomeration on CaF2 layer observed in the co-evaporation of Si and Co. An epitaxial CaF2 layer was formed on CoSi2/CaF2 at low substrate temperature (450°C) with partially ionized and accelerated CaF2 beam, to avoid Co agglomeration in the CoSi2/CaF2 underlayer as well. Obtained results showed a single-crystalline nature in reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) observations. The resistivity of a few nm-thick CoSi2 epilayers embedded by CaF2 has been investigated. We studied thickness and annealing temperature dependence of resistivity and showed that a minimum resistivity of 30 μΩ cm was obtained in a 2 nm-thick CoSi2 sample annealed at 860°C.

New method for epitaxial heterostructure layer growth

A new method for the fabrication of compound thin films inside single-crystalline matrices is demonstrated. It differs from all known techniques by the combination of epitaxial growth of the host material and the controlled deposition of a precipitating compound. A subsequent thermal treatment leads to a coalescence of the precipitates, resulting in a continuous buried layer. We call this method allotaxy. As an example we demonstrate the growth of buried epitaxial CoSi2 layers in Si(100).

Ion beam synthesis of epitaxial CoSi2 layers and the redistribution of dopants within them

The move within the semiconductor industry towards ultra-large-scale integration (ULSI) has prompted the evolution of new technologies. Ion beam synthesis (IBS) is one such technology in which high doses of energetic ions are used to form compound layers within a target substrate. This paper studies the evolution of IBS structures produced by high dose cobalt implantation, during both implantation and annealing. It also examines the redistribution of dopants in the as implanted and annealed CoSi2 structures.

Formation of Bilayer Shallow MoSi2/CoSi2 Salicide Contact Using W/Co-Mo Alloy Metallization

A very simple process to form a bilayer shallow MoSi2/CoSi2/Si silicided contact using W/Co54Mo46/Si bilayer alloy metallization has been developed. Two-step annealing in a conventional flowing-nitrogen furnace is employed. The first annealing is performed at a temperature of 500-600°C, with the W film passivating the underlying Co-Mo alloy layer from being oxidized and consuming some Co. A shallow contact is feasible by selectively etching away the remaining outer layer of the Mo-dominant Mo-Co alloy (or W-Co alloy) phase to deduct the metal source which would finally be transformed into silicides. Silicide lateral growth which is observed during the silicidation of Co on Si does not occur in the present system. This is presumably because the formation of CoSi2 is not preceded by Co2Si or CoSi due to the nonrich Co source in the Co-Mo alloy system. The second annealing performed at a higher temperature of 700-950°C is to further transform the CoSi2 and mixed-silicide layers obtained after the first annealing into separate layers of MoSi2 and CoSi2.

Mössbauer and channeling measurements on buried layers of CoSi2 in Si

Single crystalline multilayered structures of Si/CoSi2/Si were made by high dose implantation of Co into a Si wafer which was subsequently annealed. These structures were then investigated with both Mossbauer spectroscopy and channeling measurements. The experiments show that a change occurs in the structure of the CoSi2 at a temperature between 150 K and 220 K.

Observation and analysis of epitaxial growth of CoSi2 on (100) Si

CoSi2 layers formed by the thermal reaction of vapor-deposited Co films on Si(100) substrates have been studied by transmission electron microscopy, and x-ray diffraction. It is shown that first a layer of CoSi is formed between Co and Si. Only thereafter is the formation of CoSi2 initiated at the Si/CoSi interface. In view of the similarity of the crystal structure and the small lattice mismatch between the Si and the CoSi2, epitaxy of aligned (100) CoSi2 is expected to occur. However, in addition to an aligned (100) orientation, CoSi2 occurs in a number of orientations, including a (110) preferential orientation. Many individual grains are composed of subgrains, slightly rotated with respect to each other and connected by small-angle boundaries. It is shown that the observations can be largely attributed to the geometrical lattice match between CoSi2 and Si. A computer program has been developed that searches systematically for a large number of possible geometrical matches. It allows us to calculate epitaxial relationships between CoSi2 and the Si(100) substrate. The probability of various fits is estimated on the basis of their strain energy and coincidence site density, showing good correspondence with the experimental observations.

Surface CoSi2 layers by moderate dose cobalt implantation for self-aligned contacts

CoSi2 layers produced by implantation of cobalt into silicon at 20 keV with doses 3–6×1016 cm−2 were investigated. Room temperature and 355 °C implants produced continuous layers of silicides with similar electrical properties. The minimum sheet resistivity of annealed samples was 9.2 Ω/sq. and the films were thermally stable up to about 900 °C. Low leakage currents (estimated <1×10−14 Å per μm of a typical side-wall spacer length) in a SiO2 film which was similarly implanted with Co demonstrate the usefulness of this method for the formation of thin (≂20 nm) cobalt silicides for self-aligned source-drain contacts in shallow junction complimentary metal-oxide-semiconductor technology.

Current/voltage characteristics of ion beam synthesised CoSi 2 /Si Schottky barrier diodes

The current/voltage characteristics of single crystal CoSi2 layers fabricated in (100)n-type silicon by high energy ion implantation have been investigated using mesa device structures prepared by wet chemical etching. The characteristics, measured at temperatures down to 100 K, are typical of Schottky barrier diodes. From the measurements the barrier height was found to be 0.64 ± 0.01 eV. The ideality factor n (∼1.05 for T>250K) was found to increase as the temperature decreased suggesting that bulk recombination is taking place. The reverse breakdown voltage is found to vary but is in excess of 20 V.

Correlation of Optical Properties and Structural Analysis for Cobalt Silicide Layers Buried in Silicon

ABSTRACTThe optical reflectivity from buried and exposed CoSi2 layers produced by Co ion implantation of Si substrates has been measured over the energy range 0.012–6.2 eV (0.2- 100 μm). Both unannealed and annealed layers were investigated. The complex dielectric constant of the bare CoSi2 layer was obtained from a Kramers-Kronig analysis. The plasma frequency, carrier concentration and relaxation time of the buried CoSi2 layers were obtained from classical dispersion analyses of the multicomponent reflectivity data. The effective carrier concentration derived from the optical data and the measured chaneling from Rutherford back scattering correlated with anneal temperature.

Investigation of an in-situ probe for phase transformations during RTP silicidation

ABSTRACTIn-situ emissivity measurements at a wavelength of 2.4 μα were used to monitor RTP Co silicidation on crystalline and polycrystalline silicon substrates. The influence of various parameters influencing the silicidation reaction was extensively studied. It is shown that particularly the phase transformation from CoSi to the final suicide phase, COSi2, strongly depends on parameters such as background doping level and type of substrate. This is illustrated for As-doped substrates. The method is extremely sensitive for the in-situ detection of the thermal degradation of thin COSi2 films at high temperatures, which is demonstrated for 25 nm COSi2 layers on highly As-doped c-Si substrates.

Fabrication and Evaluation of Ternary Co-Fe-Si Structures Produced by Ion Beam Synthesis

ABSTRACTDual implantation of cobalt and iron into silicon (100) wafers and subsequent annealing has been used to form layers containing mixtures of CoSi2 and FeSi2. The structure and properties of the layers were followed by Secondary Ion Mass Spectrometry (SIMS), cross-sectional transmission electron microscopy (XTEM), Transmission Electron Diffraction (TED), Rutherford Backscattering Spectroscopy (RBS), and photoluminescence (PL). When a high dose of both species was implanted, segregation of the cobalt and iron occurred which for 1000°C anneals, resulted in an epitaxial layer of αFeSi2 upon a CoSi2 layer. The epitaxial quality of both of these layers was superior to those previously fabricated by single species implants. For a low dose cobalt implant followed by a high dose iron implant, a single phase solid solution was formed and segregation did not occur. Photoluminescence at 1.54 urn was observed from this layer, but with a much lower intensity and a broader line width than that from a pure βFeSi2 layer.

Solid State Interaction and Nano-Scale Silicide Formation for Co/Ti Multilayers on Silicon

ABSTRACTThe formation of a 12nm thick, continuous and thermally stable COSi2 layer was described in our previous work [MRS Proc. 238, 587 (1992)]. Interdiffusion in the Co/Ti-Si multilayer system has been further studied and the initial Ti(O) thickness is shown to be a critical parameter in controlling its effectiveness as a diffusion barrier, and in modulating the Co-Si and Ti-Si compctctive reactions. Three Ti(O) and three Co layers with thickness from ∼5nm 20nm were deposited sequentially, with Ti(O) as the first layer, on Si-(100) substrates by dual source thermal evaporation. The morphology of the CoSix/Si interface was strongly influenced by Ti(O) thickness from ∼5nm to ∼10nm, and a 12nm thick uniform CoSi2 layer with ∼28μΩ-cm resistivity was produced as decribed previously. When the initial Ti(O) thickness was increased to ∼20nm and the Co thickness set at -10nm, Co diffusion was suppressed and Ti reacted with Si yielding an ∼10nm amorphous TiSix layer at 550°C. This amorphous layer transformed to a 15nm thick uniform C-54 TiSi2 layer after selective removal of upper layers and a 750°C plus 800°C annealing. A flat silicide/Si interface and a ∼58μΩ-cm resistivity were obtained. The significance of both thermodynamic and kinetic factors in the compctetive reactions is discussed.

Ion Beam Synthesis of Buried CoSi2 Layers in SiGe Alloys

ABSTRACTSynthesis of buried, epitaxial CoSi2 layers in Si1−xGex alloys (x =0.48 and x = 0.09) by 100 and 150 keV Co+ ion implantation and subsequent rapid thermal annealing was studied by X-Ray diffraction, Rutherford backscattering spectroscopy, He ion channeling, Auger Eectron Spectroscopy and Transmission Electron Microscopy. Buried single-crystal CoSi2 layers in the Si0.91Ge0.09 alloy containing ≈ 1 at% Ge were formed. The suicide formation causes an outdiffusion of Ge leading to an increase in the Ge concentration of the adjacent SiGe layers. In contrast, in the Si0.52Ge0.48 alloy no buried suicide layers could be produced.

Columnar and Subsurface Silicide Growth with Novel Molecular Beam Epitaxy Techniques

ABSTRACTMolecular beam epitaxy of silicides is conventionally carried out at temperatures ≤500°C, with stoichiometric Si:metal flux ratios or deposition of pure metal. We have found novel growth modes for epitaxial CoSi2 at high temperatures coupled with Si-rich flux ratios or low deposition rates. In the first of these modes, codeposition of metal and Si at 600-800°C with excess Si leads to the formation of epitaxial silicide columns surrounded by single-crystal Si. During the initial stages of the deposition, the excess Si grows homoepitaxially in between the silicide, which forms islands, so that the lateral growth of the islands is confined. Once a template layer is established by this process, columns of silicide form as a result of selective epitaxy of silicide on silicide and Si on Si. This growth process allows nanometer control over silicide particles in three dimensions. In the second of these modes, a columnar silicide seed layer is used as a template to nucleate subsurface growth of CoSi2. With a 100 nm Si layer covering CoSi2 seeds, Co deposited at 800°C and 0.01 nm/s diffuses down to grow on the buried seeds rather than nucleating surface silicide islands. For thicker Si caps or higher deposition rates, the surface concentration of Co exceeds the critical concentration for nucleation of islands, preventing this subsurface growth mode from occurring. Using this technique, single-crystal layers of CoSi2 buried under single-crystal Si caps have been grown.

Mössbauer and channeling measurements on buried layers of CoSi 2 in Si

Single crystalline multilayered structures of Si/CoSi 2/Si were made by high dose implantation of Co in a Si wafer which was subsequently annealed. These structures were then investigated with both Mössbauer spectroscopy and channeling measurements. The experiments show that a change occurs in the structure of the CoSi 2 at a temperature between 150 and 220 K.

Growth of Single Crystal Si/NiSi2/Si(100) and Si/CoSi2/Si(100) Structures by Molecular Beam Epitaxy and Furnace Annealing

ABSTRACTUniform, single crystal Si/NiSi2/Si(100) and Si/CoSi2/Si(100) structures were fabricated by molecular beam epitaxy and furnace annealing. A Si template technique and a post-growth anneal at ∼960°C led to high quality buried NiSi2 layers with thickness ∼1000Å. The conditions for fabricating high quality buried CoSi2 layers by the recently proposed allotaxy process were studied and discussed. The endotaxial growth of buried CoSi2 layers in Si(100) was also studied.

Structural Phase Transition in Buried CoSi2 Layers

ABSTRACTHeteroepitaxial Si/CoSi2/Si<111> structures were made by high dose implantation of Co in a Si wafer. These single crystaline layers were investigated with 57Co source Mössbauer spectroscopy, 4He ion channeling and X-ray scattering. All results show there is a structural phase transition in the buried CoSi2 layer at a temperature around 200 K.

Influence of Low-Dose Ion-Beam Mixing on CoSi2 Formation

ABSTRACTIn this study, the critical dose for ion-beam mixing of Co and Si with Ge-ions which results in homogenous CoSi2 formation after rapid thermal annealing was found. For this purpose, Co was deposited by sputtering on chemically cleaned, <100>-oriented Si and subsequently mixed with Ge ions at doses in the range of 2. 1014 to 1. 1015 cm−2. Silicidation was performed in a rapid thermal annealing (RTA) system at temperatures between 700° and 100°C. Rutherford backscattering measurements showed that annealing at 700°C results in an incomplete reaction when ion-beam mixing at a dose of 2.1014 cm−2 or no ion-beam mixing was performed. After annealing at 1000°C, TEM samples revealed an inhomogeneous CoSi2 film consisting of large grains embedded in the Si. Mixing at doses at or above 5.1014 cm−2 and subsequent RTA at 700°C resulted in uniform CoSi2 layers. Higher annealing temperatures cause larger grains and resistivity values as low as 18 μΩcm. Therefore, we demonstrated that the critical dose leading to complete formation of smooth CoSi2 films with abrupt interface is 5.1014 cm−2 which is nearly the same value as the amorphization dose of Ge in Si.