CoSi2 Layers Research Articles

Enhancing superconductivity in CoSi2 films with laser annealing

Laser annealing was employed to trigger the solid-state reaction of a thin Co film (2.5 nm) with undoped Si. A metastable disilicide layer was obtained after one laser pulse close to the melt threshold. Its diffraction pattern, relaxed lattice parameter, and residual resisitivity are consistent with the formation of the defective CsCl structure. The CoSi2 phase was found after prolonging the thermal treatment with additional pulses or rapid thermal annealing. Because CoSi is skipped in the phase sequence, CoSi2 layers are more uniform in thickness, have an increased superconductivity and a reduced formation temperature. This approach is compatible with the SALICIDE process and can be used to form smooth contacts in superconducting or regular transistors.

Co contamination of Si: Plating & aging

Cobalt has emerged as a vital material in 10 nm technology for localized interconnect layers, potentially offering a compelling alternative to Cu-based interconnects. In this study, we subjected the contamination arising from the presence of cobalt atoms in silicon to comprehensive investigation, employing electron transmission electron microscopy (TEM) observations in conjunction with first-principles calculations. The results show that a dense CoSi layer with a thickness of a few nanometers is formed at the interface of cobalt and Si. The CoSi layer blocks the diffusion of Co atoms into Si. This is due to the semiconducting nature of the covalent bond formed between Co and Si, leading to the emergence of a forbidden zone at the Co/CoSi interface. The diffusion of Co into CoSi is governed by the atomic exchange mechanism, however, the local distortion of the periodic atomic potential due to the presence of the forbidden zone at the Co/CoSi interface hinders the diffusion of Co into Si. Therefore, the deposition of a Co metal layer on a Si chip does not require an additional barrier layer.

Influence of CoSi Oxidation and Passivation During Silicide Selective Etching on Junction Leakage: Applications to Schottky Diodes Devices

For new analogic microelectronic circuits development based on non-linear devices such as Schottky diodes formed in Si active regions, new Co-silicide integrations are required to reduce junction leakages. To gather targeted device requirements, precise Co silicide/Si interface optimization and a limited silicide formation at the active edges is needed. The selective etching during the “Salicide” process plays a real role in the oxidation and/or passivation of the silicide layer. Here, we propose a systematic study including a very large spectrum of experiments around the main parameters of CoSi selective etching. The main conclusions are (1) diode leakages are directly linked to SiO2 layer thickness formed during the SC1 dispense or by air exposure over the CoSi layer, (2) significant effect of dispense flow on SiO2 formation is measured through X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry characterizations; (3) optimized diode leakages together with contact resistances are then demonstrated for low SC1 delivery flow and long dispense time; and (4) major changes in final CoSi2 layer morphology and silicide/silicon interface are observed by transmission electron microscopy-energy-dispersive X-ray analyses for different selective etching processes, which are potentially explained by enrichment in Co atoms at CoSi/Si during SiO2 overlayer growth.

Enhanced thermoelectric properties of cobalt silicide-silicon heterostructured nanowires

Silicon nanowires (SiNWs) have attracted attention as promising high efficiency thermoelectric materials by significantly improving the thermoelectric efficiency of silicon. Here, we have fabricated cobalt silicide/silicon heterostructure on both ends of nanowires using self-aligned silicide (salicide) process. By forming cobalt silicide (CoSi2) layer on SiNWs, the thermal conductivity of SiNWs with diameters of 200, 350, and 500 nm decrease to 25.1, 31.3, and 38.1 W·m−1·K−1, respectively, which is about 8% reduction on average. Since the phonon nanoinclusion scattering is influenced by the density of the nanoinclusions, the thermal conductivity tends to decrease as the volume fraction of CoSi2 in SiNWs increases. The Seebeck coefficient of the heterostructured nanowires increases to 255 μV/K, which is mainly attributed to the low-energy charge carrier filtering effect due to the Schottky barrier at the CoSi2/Si interfaces. The measurements show that the figure-of-merit ZT of the heterostructured nanowires is improved by 10% on average compared with the conventional SiNWs. Consequently, the CoSi2/Si heterostructured nanowires enhance the thermoelectric properties of SiNWs effectively by suppressing phonon transport and improving electron flow.

Scanning tunneling microscopy study of the early stages of epitaxial growth of CoSi2 and CoSi films on Si(111) substrate: Surface and interface analysis

We report scanning tunneling microscopy observations of solid-phase epitaxial growth of ultra-thin CoSi2 and CoSi films at heating temperatures in the range of 350–600°C using a disordered ‘(1×1)’ phase of Co/Si(111) as a pre-existing template. It is found that depending on the growth conditions two types of epitaxial cobalt silicide films can be formed. At coverages below ~2 ML (1 ML=7.83×1014atoms/cm2) of Co, CoSi2 phase grows in the form of flat two-layer patches spreading over the surface at elevation of the temperature. It consists of precisely two the completely filled Co layers and four Si layers above the interface. At coverages higher than ~2 ML of Co, the layer-by-layer growth of CoSi phase with the CsCl-type crystal structure also takes place. The CoSi phase consists of one or more Co-Si double layers, which are located on top of the second Si-Co-Si triple layer of CoSi2, i.e., it has а coherent double interface CoSi/CoSi2/Si(111). According to experimental findings and ab initio total-energy calculations, the structures with the eightfold coordination of interfacial Co atoms and lateral unit cells of the silicide layer and substrate mutually rotated by 180° prove to be the most stable ones.

Helimagnetic order in bulk MnSi and CoSi/MnSi superlattices

Skyrmions are nanoscopic whirls of spins that reside in chiral magnets. It is only fairly recent that a plethora of applications for these quasiparticles emerges, especially in data storage. On the other hand, spin spirals are the periodic analogs of skyrmions, and are equally imperative in the course of exploration to enhance our understanding of helimagnetism. In this study, a new infrastructure based on the B20 compound, MnSi is propounded as a hosting material for spin spirals; alternating thin layers of CoSi and MnSi in the superlattice form provides a facile way of varying the properties of the spin spirals across a continuum. Using first-principles calculations based on full-potential linearized augmented plane-wave (FLAPW)-based density functional theory (DFT), the spin order of bulk MnSi, MnSi film, and the CoSi/MnSi superlattice is investigated. Spin dispersion plots as a function of propagation vectors show that the spiral size changes in the presence of CoSi – we find that the size of the spiral is reduced in the superlattice with thin CoSi layers (CoSi:MnSi=1:1 thickness ratio), whilst at a larger CoSi:MnSi=2:1 thickness ratio, the material behaves as a ferromagnet. In a similar fashion, the spin moment and orbital occupancy depend significantly on the thickness of the CoSi layers. However, the exchange interaction between Mn atoms appears to be generally impervious to the presence of CoSi. Succinctly, the CoSi/MnSi superlattice could be an excellent functional material in data storage applications.

Cobalt silicide formation on a Si(1 0 0) substrate in the presence of an interfacial (Fe90Zr10) interlayer

The reaction between a thin film (126nm) of Co and Si has been studied at 450°C for 24h under high vacuum conditions, in the presence of a FeZr barrier layer. Without a diffusion barrier layer between Co and Si, Co2Si forms at 350°C as the initial phase while CoSi2 forms at 550°C. The FeZr barrier layer changed the flux of atoms arriving at the reaction interface. Co reacted with the Si from the substrate and formed a mixed layer of CoSi and CoSi2 in the interlayer region. The use of the FeZr diffusion barrier has been demonstrated to lower the temperature formation of CoSi2 to 450°C. The reactions were characterised by Rutherford backscattering spectrometry, Auger electron spectroscopy depth profiling, X-ray diffraction using CoKα radiation and scanning electron microscopy.

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.

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe3Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ∼200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (FexCo1 − xSi, FexCo2 − xSi).

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.

Resistance Switching Memory Characteristics of Si/CaF2/CdF2 Quantum-Well Structures Grown on Metal (CoSi2) Layer

A novel scheme of resistance switching random access memory (ReRAM) devices fabricated using Si/CaF2/CdF2/CaF2/Si quantum-well structures grown on metal CoSi2 layer formed on a Si substrate has been proposed, and embryonic write/erase memory operation has been demonstrated at room temperature. It has been found that the oxide-mediated epitaxy (OME) technique for forming the CoSi2 layer on Si dramatically improves the stability and reproducibility of the current–voltage (I–V) curve. This technology involves 10-nm-thick Co layer deposition on a protective oxide prepared by boiling in a peroxide-based solution followed by annealing at 550 °C for 30 min for silicidation in ultrahigh vacuum. A switching voltage of lower than 1 V, a peak current density of 32 kA/cm2, and an ON/OFF ratio of 10 have been observed for the sample with the thickness sequence of 0.9/0.9/2.5/0.9/5.0 nm for the respective layers in the Si/CaF2/CdF2/CaF2/Si structure. Results of surface morphology analysis suggest that the grain size of crystal islands with flat surfaces strongly affects the quality of device characteristics.

CoSi2 ultra-thin layer formation kinetics and texture from X-ray diffraction

In this study we investigated the influence of cobalt thickness (from 50nm to 10nm) on the kinetics and texture of CoSi2 layers. In-situ X-ray diffraction measurements were performed during isothermal annealing to determine CoSi2 kinetics as explained in a previous publication. Decreasing the initial cobalt thickness (50nm, 30nm, and 10nm) induces a slowdown of the formation rate, especially for the 10nm Co layer. A model based on CoSi2 nucleation is used to explain this phenomenon.

Field electron emission based on resonant tunneling in diamond/CoSi2/Si quantum well nanostructures

Excellent field electron emission properties of a diamond/CoSi2/Si quantum well nanostructure are observed. The novel quantum well structure consists of high quality diamond emitters grown on bulk Si substrate with a nanosized epitaxial CoSi2 conducting interlayer. The results show that the main emission properties were modified by varying the CoSi2 thickness and that stable, low-field, high emission current and controlled electron emission can be obtained by using a high quality diamond film and a thicker CoSi2 interlayer. An electron resonant tunneling mechanism in this quantum well structure is suggested, and the tunneling is due to the long electron mean free path in the nanosized CoSi2 layer. This structure meets most of the requirements for development of vacuum micro/nanoelectronic devices and large-area cold cathodes for flat-panel displays.

Formation of CoSi2/CoSixNy Nanocrystals for Nonvolatile Memory Application

Nonvolatile nanocrystal memories recently have been one of the promising candidates to replace traditional floating gate nonvolatile memory because the discrete nanocrystal as the charge storage media have effectively improved data retention under endurance test for the device scaling down. In this study, CoSi2 nanocrystal surrounded with CoSixNy shell structure was proposed by rapid thermal annealing a nitrogen incorporated CoSi layer, which was deposited by sputtering CoSi target in the mixture gas of Ar+N2 or Ar+NH3, to improve data retention. The structures and elements composition of nanocrystal were analyzed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), and the electrical characteristics of nanocrystals memories were defined by C-V curves. Because of the extra CoSixNy shell, the proposed memory can exhibit good data retention to 10 years.

Complementary metal-oxide-semiconductor-compatible and self-aligned catalyst formation for carbon nanotube synthesis and interconnect fabrication

We have for the first time developed a self-aligned metal catalyst formation process using fully CMOS (complementary metal-oxide-semiconductor) compatible materials and techniques, for the synthesis of aligned carbon nanotubes (CNTs). By employing an electrically conductive cobalt disilicide (CoSi2) layer as the starting material, a reactive ion etch (RIE) treatment and a hydrogen reduction step are used to transform the CoSi2 surface into cobalt (Co) nanoparticles that are active to catalyze aligned CNT growth. Ohmic contacts between the conductive substrate and the CNTs are obtained. The process developed in this study can be applied to form metal nanoparticles in regions that cannot be patterned using conventional catalyst deposition methods, for example at the bottom of deep holes or on vertical surfaces. This catalyst formation method is crucially important for the fabrication of vertical and horizontal interconnect devices based on CNTs.

Formation and investigation of cobalt silicide ultrathin layers in Ti/Co/Ti-, TiN/Ti/Co-, and TiN/Co-on-silicon structures

The formation of ultrathin CoSi2 layers in Ti(8 nm)/Co(10 nm)/Ti(5 nm), TiN(18 nm)/Ti(2 nm)/Co(8 nm), and TiN(18 nm)/Co(8 nm) systems magnetron-sputtered on the Si(100) surface is studied. The systems are subjected to two-step rapid thermal annealing. In between the annealing steps, the “sacrificial” layer is chemically removed and the second and third systems are additionally covered by a 17-nm-thick amorphous silicon (α-Si) layer. In the course of the fabrication process, the structures are examined using time-of-flight secondary-ion (cation) mass spectrometry, Auger electron spectroscopy, and scanning electron microscopy combined with X-ray energy dispersion microanalysis. It is shown that the above complex of analytical investigation provides efficient physical control of ultrathin silicide layer formation.

Control of the formation of ultrathin CoSi2 layers during the rapid thermal annealing of Ti/Co/Ti/Si(100) structures

The initial Ti(8 nm)/Co(10 nm)/Ti(5 nm) structures formed on the Si(100) substrate by magnetron sputtering were subjected to two-stage rapid thermal annealing (RTA) in the nitrogen ambient. The samples of the structures were controlled using the time-of-flight SIMS, the Auger spectroscopy, scanning electron microscopy, X-ray dispersion microprobe analysis, and measurements of the layer resistance at each stage of annealing. At the RTA-1 stage (550°C, 45 s), a sacrificial layer formed on the surface. This layer consisted of the titanium (oxy)nitride coating, into which the residual impurities (O, C, and N) were forced out, and the transient Co-Si-Ti(TiO,TiN) layer with a high cobalt content and a low (trace) titanium content. After the selective removal of this sacrificial layer, the surface composition corresponded to monosilicide CoSi, which transformed into the highly conductive CoSi2 phase at the RTA-2 stage (830°C, 25 s).

Investigation of Magnetic Properties and Microstructure of Ultrathin Co Films Grown on Si(111)-7 × 7 Surface

Magnetic properties and growth mechanism of ultrathin Co films on Si(111)-7 x 7 surface have been studied by using both surface magneto-optic Kerr effect (SMOKE) and scanning tunneling microscopy (STM), respectively. STM results show that the growth mechanism of ultrathin Co films on Si(111)-7 x 7 surface at room temperature belongs to Stranski-Krastanov (SK) growth mode. Due to formation of CoSi2 layer, no magnetic signal could be detected by SMOKE for 1-4 ML Co deposited on Si(111) surface. Because of rougher surface, both longitudinal and perpendicular magnetic anisotropy configuration appear for 4.2-10 ML Co/Si(111) films. When the Co thickness is increased to 10 ML, only longitudinal anisotropy configuration is found, resulting from the contribution to the volume anisotropy. Furthermore, in-plane coercivity increases with Co coverage because of enhancement of ferromagnetic coupling with Co thickness, out-of-plane coercivity increases with Co coverage due to the increment of demagnetized field, induced by the rougher Co surface and pinhole structures.

Features of CoSi2 phase formation by two-stage rapid thermal annealing of Ti/Co/Ti/Si(100) structures

A method of cobalt disilicide (CoSi2) layer formation proceeding from a Ti(8 nm)/Co(10 nm)/Ti(5 nm)/Si(100) (substrate) structure prepared by magnetron sputtering is described. The initial structure was subjected to two-stage rapid thermal annealing (RTA) in nitrogen, and the samples after each stage were studied by the time-of-flight secondary-ion mass spectrometry, Auger electron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The RTA-1 stage (550°C, 45 s) resulted in the formation of a sacrificial surface layer of TiNxOy, which gettered residual impurities (O, C, N) from inner interfaces of the initial structure. After the chemical removal of this TiNxOy layer, the enrichment with cobalt at the RTA-2 stage (830°C, 25 s) led to the formation of a low-resistance CoSi2 phase.

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.