Epitaxial CoSi2 Layers Research Articles

Effect of an epitaxial CoSi2 layer on diffusion of B and Sb in Si during annealing and oxidation

The diffusion of B and Sb in Si, with and without a 20-nm-thick epitaxial CoSi2 layer on top, is investigated, during annealing and oxidation, using doping superlattices (DSLs). CoSi2 layers were grown on Si by molecular beam allotaxy. DSLs were grown by molecular beam epitaxy. They consisted of six spikes with peak concentrations of 1018 cm−3(B) and about 1019 cm−3(Sb) with peak centers spaced 100 nm apart. The shallowest spike was capped with 100 nm of Si followed by 20 nm of CoSi2. Annealing in pure N2 and oxidation in pure O2 were performed at temperatures ranging from 800 to 1200 °C. Concentration depth profiles were measured by secondary ion mass spectrometry. The results showed that the dopant diffusion in Si was markedly different with or without a CoSi2 layer. For specimens without CoSi2 layer, we observed oxidation enhanced diffusion of B and oxidation retarded diffusion of Sb in accordance with the literature. However, the effect of CoSi2 layer was a strong retardation of B diffusion and an enhancement of Sb diffusion. The B diffusivity was retarded by a factor of 2–10 as compared to the thermal diffusivity and by a factor of 20–100 as compared to the corresponding diffusivity for oxidation of Si without a CoSi2 layer. Sb diffusivity was enhanced by a factor of 2 with respect to thermal diffusivity and by about a factor of 5 as compared to the case without a CoSi2 layer.

In situ growth of an epitaxial CoSi2 layer on a Si (100) substrate by reactive chemical-vapor deposition using a cobalt metallorganic source

Uniform epitaxial CoSi2 layers have been grown in situ on a (100) Si substrate at temperatures above 600 °C by reactive chemical-vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5-C5H5)(CO)2. Co-rich phases such as Co2Si and CoSi were suppressed during cobalt metallorganic chemical-vapor deposition at substrate temperatures above 500 °C. A thin carbon layer was found on the top of the epitaxial CoSi2 layer grown on the Si substrate due to incomplete decomposition of the cobalt metallorganic source and diffusion of Co into the Si substrate. In spite of the existence of a surface carbon layer, an ion channeling minimum yield, χmin, of 8% in Rutherford backscattering/channeling spectrometry has been achieved in the epitaxial layer, indicating a nearly perfect epitaxial order. The carbon pileup on the surface of the CoSi2 layer at the initial stage of Co deposition seems to play the role of a cobalt diffusion barrier, avoiding the formation of Co-rich phases.

Epitaxial growth of a (100) CoSi2 layer from carbonic cobalt films deposited on (100) Si substrate using an organometallic source

We report the epitaxial growth of a (100) CoSi2 layer on Si (100) substrate by the diffusion of Co from an amorphous carbonic cobalt film. The employment of an intermediate buffer layer, usually required between Si and pure Co, was eliminated in this experiment. The amorphous carbonic cobalt film was prepared by the organometallic chemical vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5–C5H5)(CO)2 at 350 °C. The carbonic cobalt film was capped by a sputtered Ti layer to avoid oxidation of Co during annealing. A CoSi2 layer was epitaxially grown on Si (100) by ex situ rapid thermal annealing at 800 °C in N2 ambient. The supply of Co by diffusion in the carbonic cobalt film seemed to be low enough to form an epitaxial CoSi2 layer.

Nanometer patterning of epitaxial CoSi2/Si(100) for ultrashort channel Schottky barrier metal–oxide–semiconductor field effect transistors

A nanometer patterning method, based on local oxidation of silicide layers, was used to pattern epitaxial CoSi2 layers. A feature size as small as 50 nm was obtained for 20 nm epitaxial CoSi2 layers on Si(100) after patterning by local rapid thermal oxidation in dry oxygen. A Schottky source/drain metal–oxide–semiconductor field effect transistor with epitaxial CoSi2 on p-Si(100) was fabricated using this nanopatterning method to make the 100 nm gate. The device shows good I–V characteristics at 300 K.

Epitaxial Growth and Thermal Stability of CoSi2 Layer on (100) Si from Co-C Films without Capping Layer

AbstractA 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 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. indicating that epitaxial (100) CoSi2 is thermally stable at temperatures even above 1000°C and has a potential applicability to the salicide process in sub-half micron devices.

Oxide Mediated Epitaxial Growth of CoSi2 in a Single Deposition Step

AbstractA number of modifications of the oxide-mediated epitaxy (OME) technique are presented which have enabled the growth of thick (∼25–40nm) epitaxial CoSi2 layers in a single deposition sequence. The uses of (a) a thin Ti cap, (b) a thin Ti blocking layer, (c) the codeposition of Co-rich CoSix, and (d) the co-deposition of Col−xTix. have all been shown to lead to improved epitaxial quality over the pure Co OME process, for Co thickness greater than 6nm. Essentially uniform, single crystal silicide layers of over 25nm have been grown in a single deposition step. These results are supportive of the proposed role of a diffusion barrier/kinetics retarder on the part of the interlayer in the OME and the Ti-interlayer mediated epitaxy processes.

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.

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.

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.

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.

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.

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.

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.

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.

Electrical characterization and physical analysis of epitaxial CoSi2 grown from the Si〈100〉/Ti/Co system

The electrical and structural properties of epitaxial CoSi2 layers grown on Si by solid-state interaction between Ti/Co bimetallic layers and Si〈100〉 substrates have been investigated. The Schottky barrier height (SBH) of the CoSi2–Si contact determined by current–voltage characteristics at room temperature varies between 0.64 and 0.71 eV on the n-type substrates and between 0.47 and 0.43 eV on the p-type substrates. The variation of the SBH is found to be related to the interfacial properties at the CoSi2–Si contact which is in turn determined by the heat treatment used for the CoSi2 formation. The formation of polycrystalline CoSi2 is found to be responsible for the low SBH deviated from 0.71 eV on the n-type substrates and the high SBH deviated from 0.43 eV on the p-type substrates. The formation of a ternary compound, identified as Co16Ti6Si7, within the epitaxial CoSi2 does not seem to affect the SBH on the n-type substrates. A permeable base transistor (PBT) was fabricated using the epitaxial CoSi2 for self-aligned contact metallization. The current–voltage characteristics of the PBTs are presented.

In situ x-ray diffraction study of CoSi2 formation during annealing of a Co/Ti bilayer on Si(100)

X-ray diffraction was performed in situ during annealing of a Co/Ti/Si(001) multilayer, which produced an epitaxial CoSi2 layer. The results indicate that the Ti layer did not stay intact during the reaction, and thus could not act like a membrane, moderating Co/Si interdiffusion. Strongly textured phases (M) formed prior to CoSi2 nucleation, and was unobservable upon completion of the anneal. Nucleation and growth of CoSi2 on Si(001) took place in the presence of M, new Co-Ti-(O) phases that were located at the metal/Si interface, and thus M might play an important role in the perfection of the silicide.

Epitaxial growth of a metal(CoSi2)/insulator(CaF2) nanometer‐thick heterostructure and its application to quantum‐effect devices

Metal(CoSi2)/insulator(CaF2) nanometer‐thick heterostructures are grown on a Si(111) substrate, and applied to quantum‐effect electron devices. In the epitaxial growth of a CoSi2/CaF2 multilayer, 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. An epitaxial CaF2 layer was formed on CoSi2/CaF2 at the low substrate temperature of 450 °C with a partially ionized and accelerated CaF2 beam. Co agglomeration in nanometer‐thick CoSi2 layer was avoided by using these techniques. A resonant tunneling transistor with double CoSi2 quantum wells was fabricated, and a transistor action with the negative differential resistance was observed at 77 K. The transfer efficiency of electrons from the emitter to the collector through the resonant energy levels is estimated to be close to unity. A resonant hot electron transistor structure was also fabricated, and multiple negative d...

Formation of Epitaxial CoSi2 Layers Grown from the Interaction of Co/Ti Bilayers with Si <100> Substrates

AbstractThe redistribution of titanium during the growth of epitaxial CoSi2 from the reaction of Co(20nm)/Ti(lOnm)/Si<100> structures has been investigated. The concentration of Ti in the CoSi2 layers versus annealing temperature has been determined. Emphasis is placed on the formation of inhomogeneities in the epitaxial CoSi2 layers, and the role of Ti with respect to the thermal stability of the layers. The fundamental mechanism for the development of inhomogeneities in the epitaxial CoSi2 layers will be discussed.

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.

Formation of YBa2Cu3O7 films on silicon/silicide/oxide structures formed via amorphous CoZr alloys

Thin films of the superconductor YBa2Cu3O7 have been grown on Si(111)/CoSi2/ZrO2 structures by inverted cylindrical magnetron sputter deposition at 700 degrees C. The CoSi2/ZrO2 bilayer structure was formed simultaneously by annealing an amorphous CoZr alloy layer in O2 at 650 degrees C. The amorphous alloy was formed by solid state reaction of crystalline Co and Zr under long anneals at a temperature below the crystallization temperature of the CoZr alloy. The bilayer formation gave a polycrystalline ZrO2 and an epitaxial CoSi2 layer aligned with the Si(111) substrate, with a channelling yield of 40%. YBa2Cu3O7 films grown on this multilayered structure had a Tc of 81 K and a transition width, Delta T, of 3 K as measured by resistivity readings.