CoSi2 Layers Research Articles

Interfacial Reactions of Co and Si/Co Films on GaAs

ABSTRACTInterfacial reactions of Co and Si/Co films on GaAs have been investigated using X-ray diffraction(XRD), Auger electron spectroscopy(AES), cross-sectional transmission electron microscopy(XTEM), microdiffraction, and energy dispersive spectroscopy(EDS). Cobalt starts to react with GaAs at 380°C by formation of ternary phase, most probably Co2GaAs. At 420°C, CoGa nucleates at the Co and Co2GaAs interface and grows with Co2GaAs. At higher temperature, Co2GaAs disappears and CoGa/GaAs layer structures are formed. Annealing of the layer-deposited Si/Co films at 380°C results in the formation of Co2GaAs. At 420°C, the entire layer of Co is consumed, while Co2Si transforms to CoSi, and binary phases, CoGa and CoAs, are formed on top of the ternary phase beneath the CoSi layer. In the subsequent reaction, CoSi grows at the expense of the decompositions of CoGa and CoAs at 460°C. At 600°C, ternary phase is decomposed, and CoSi forms interface with GaAs. This finding can be understood from the calculated Si-Co-Ga-As quaternary phase diagram.

Interdiffusion and phase formation during thermal processing of Co/Ti/Si(100) structures

Rapid thermal processing of Co/Ti/Si(100) structures has been investigated over a wide temperature range, from 300°C to 1100°C. Titanium and cobalt layers, 10nm and 20nm thick respectively, were consecutively deposited on Si(100) substrates using an e-beam evaporator. X-ray diffraction, Secondary Ion Mass Spectrometry and Rutherford Backscattering Spectrometry were employed to analyse interdiffusion and phase formations. The present results revealed the formation of CoSi2 when annealing at 750°C. The CoSi2 was formed epitaxially and yielded a minimum channelling yield of approximately 8.5% for layers annealed at 1100°C. Evidence for the presence of a CoSi layer, when annealing between 650°C and 800°C, is presented.

Microstructural studies of epitaxial CoSi2 layers on silicon produced by ion beam synthesis and rapid thermal annealing

Structures of cobalt disilicide layers fabricated by ion beam synthesis on (001) silicon wafers have been studied by cross-sectional transmission electron microscopy. Implantation at 350 °C with doses of 5 and 7×1017 cm−2 of 200 keV Co+ ions was used, followed by rapid thermal annealing. For the as-implanted wafer with the lower dose, a CoSi2 layer in a parallel (A-type) epitaxial orientation was formed, and below this there were CoSi2 precipitates, some in twinned (B-type) orientations, and {113} defects. With the higher dose, polycrystalline CoSi was also present at the surface and there was substantial surface roughening. For the annealed wafers, as the annealing temperature increased from 700 to 1100 °C, the CoSi2 layer progressively increased in thickness, and the CoSi at the surface of the CoSi2 layer was eliminated. In the silicon beneath the silicide layer, the CoSi2 precipitates were greatly reduced in number and the {113} defects were eliminated.

Formation of ultrathin CoSi2 films using a two-step limited reaction process

The formation of ultrathin (≤20 nm) and smooth CoSi2 layers on c–Si substrates has been studied by using a one- and a two-step RTP silicidation method. Pinhole-free silicide layers with a thickness down to ∼10–12 nm were formed on n−, n+, and p+ crystalline Si substrates in the one-step RTP process by sputtering of Co films as thin as 4 nm and subsequent silicidation at 750 °C for 5 or 30 s. The two-step RTP silicidation method is based on the consumption of only a small fraction of a thick sputtered Co film to form Co2Si or CoSi during a first RTP step at 400–500 °C. A selective etch follows to remove the unreacted Co film. During a second, higher temperature, RTP step CoSi2 is formed. Pinhole-free and smooth CoSi2 films with a thickness down to 20 nm were formed in this way on both n+ and p+ monocrystalline Si substrates.

Epitaxial growth of CoSi2 on Si wafer using Co/Ta bilayer

A CoSi2 epitaxial layer was grown by thermal annealing of Co/Ta bilayer on a Si(100) wafer in the NH3 ambient. During thermal annealing an intermediate Ta layer between a Co layer and a Si wafer played two important roles for epitaxial growth of CoSi2. One was a reduction of a native oxide on a Si substrate, which resulted in an atomically clean Si surface. The other was a limitation of a flux of Co atoms to the Si surface, which made CoSi2 the most stable phase among several cobalt silicide ones. Both roles of the Ta layer made it possible to produce an epitaxial layer of CoSi2 with a coherent interface with a Si substrate, even at the early stage of thermal annealing.

Structure and defect characterization of epitaxial CoSi2 on Si(001) formed using an amorphous Co75W25 sputtered layer

The structure evolution and the defects present in epitaxial CoSi2 formed using the solid-state reaction between an amorphous Co75W25 sputtered layer and Si(001) have been studied by transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), positron annihilation and resistivity measurements. By employing anneals between 500 and 600 °C, Co diffuses out of the amorphous alloy into the substrate to form a CoSi2 layer. After the anneal, the remaining amorphous alloy on top of the silicide has been removed by a selective wet etch. It has been found that the greater part (∼68%) of the so-formed CoSi2 film is epitaxial and that a high density of vacancy-type defects (2.6×1019/cm3) is present in the material. A second anneal at a higher temperature has been performed to improve the quality of the silicide. This results in a decrease of the RBS minimum yield and the residual resistivity to values of ∼25% and 2.6 μΩ cm, respectively. Only a few grains of twinned CoSi2 could be detected after the second anneal. By plan-view TEM and RBS dechanneling energy dependence measurements, a total projected length of dislocation lines of ∼1.7×106 cm/cm2 has been estimated. By positron annihilation a density of 1.1×1018 vacancies/cm3 has been measured.

Z-contrast imaging of an ordered interface structure in the Si/CoSi2/Si system

We show the atomic structure of buried CoSi2/Si(001) boundaries involves a 2×1 ordering of the interfacial Co atoms. The ability to directly image and interpret this unforeseen structure is possible through the Z-contrast technique, and presents a new level of insight into the important and controversial relationship between the atomic structure and electronic properties, such as the energy barrier for electron transport across metal-semiconductor interfaces.The buried CoSi2 layer was produced by implanting Co+ ions (200 keV, 2 × 1017 ions/cm2) in a Si(001) substrate heated to 350°C. The substrate was capped with 200 nm of SiO2 and then given a two-step anneal in high purity argon (750°C for 30 s + 1150°C for 10 s). This treatment results in a continuous buried CoSi2 layer ~70 nm thick, ~90 nm below the substrate surface. The layer and the substrate are oriented such that the cubic CaF2 unit cell of CoSi2 is aligned parallel to the cubic unit cell of Si.

Study of Co silicide formation by multiple implantation

The annealing behavior of 〈111〉 Si implanted with cobalt to doses between 2.5×1016 and 2×1017 cm−2 at energies between 30 and 250 keV has been investigated. The silicide formation during postannealing has been found to proceed in two phases characterized by activation energies of 0.8±0.3 and 2.9±0.3 eV. During the first phase a fast Co redistribution and pronounced nucleation and growth of the CoSi2 precipitates at defects has been observed. The investigation of the dose dependence proved that the amount of Co redistributed during this phase depends on the initial Co and defect concentration. However, the processes contributing to this fast Co redistribution require further investigation. The second step is well characterized by Ostwald ripening. Multiple implantations of Co at different energies have been applied to modify the Co concentration profile. In this way, it is possible to form Si/CoSi2/Si/CoSi2-layer systems on Si substrate with different layer thicknesses. All Si layers and the deeper lying CoSi2 layer are A type, whereas the crystalline orientation of the near surface layer depends on the procedure of formation. If the dose of the second implantation is too high CoSi nucleates during implantation. The CoSi precipitates are dissolved during annealing and a CoSi2 surface layer of type A is formed. Additional Si implantations offer the possibility to modify the defect profile without changing the Co distribution. During the subsequent annealing a redistribution of Co into the region of the additional defects was observed. Thus, implantation defects act as sinks for the metal atoms and play an important role during the first phase of the annealing process.

Investigation of the defect structure of thin single-crystalline CoSi2 (B) films on Si(111) by transmission electron microscopy

Thin epitaxial single-crystalline B-type CoSi2 films (twin-oriented) have been grown in ultrahigh vacuum by stoichiometric codeposition of Co and Si on slightly misoriented (0.1°–0.3°) Si(111) substrates. The microstructure as well as the nature of interfacial defects has been investigated in detail by transmission electron microscopy. The defect structure is found to depend closely on the initial deposition parameters, annealing temperature, and the topography of the Si substrate. It will be shown that even during the early stages of layer growth, loss of coherence is obtained and lattice strain already starts to occur with the introduction of misfit dislocations with Burgers vector b=a/2〈110〉 inclined to the interface or with Burgers vector b=a/6〈112〉 parallel to it. It is demonstrated that ultrathin CoSi2 films with thickness of about 1 nm grown on slightly misoriented substrates with parallel surface steps, exhibit quite different defect structures at annealing temperatures between 300 °C and 550 °C. Control of the dislocation density has been obtained by applying a two-step growth procedure. CoSi2 layers grown to a thickness <hc (4–5 nm) exhibit line defects with Burgers vector b=a/6〈112〉 associated with interfacial misorientation-related steps. Above this thickness additional dislocations in the three equivalent directions are formed, indicating biaxial strain relaxation. In addition, calculations of the critical thickness hc of biaxial strain relaxation based on thermodynamic equilibrium theory are presented. It is shown that the observed critical thickness hc is in qualitative agreement with theoretical predictions.

Allotaxial growth of single-crystal Si/CoSi 2/Si(100) heterostructures

Monocrystalline buried layers of CoSi 2 in a Si(100) matrix have been grown by a new technique, named allotaxy. This is a two-step process, including the epitaxial growth of the Si host matrix with CoSi 2 precipitates and a high-temperature anneal. Si/CoSi 2/Si(100) heterostructures of different thickness were produced using various growth parameters and annealing temperatures and were characterized by RBS, ion channeling, TEM, X-ray diffraction and resistivity measurements. The quality of the produced epitaxial buried CoSi 2 layers is comparable with the best layers produced by ion-beam synthesis.

Formation of epitaxial CoSi2 on Si(100): Role of the annealing ambient

With a thin Ti layer interposed between a Si(100) substrate and a Co overlayer, the inversion of the Co and Ti films and the formation of a partly relaxed epitaxial CoSi2 layer on Si(100) can be obtained by steady-state annealing in inert as well as reactive ambients. A reactive ambient chemically binds the Ti near the surface as an oxide or nitride layer, which preserves the bilayer structure during a high temperature treatment. In a nonreactive ambient, the Ti and CoSi2 layers react further, resulting in a uniform layer of Co0.25Ti0.75Si2 and CoSi2. An eptiaxial orientation of CoSi2 is retained even in that case.

Negative differential resistance of metal (CoSi2)/insulator (CaF2) triple-barrier resonant tunneling diode

The electron transport and negative differential resistance in metal–insulator nanometer-thick heterostructures are reported for the first time. The structure of the samples is a resonant tunneling diode with three-barriers of 0.9-nm-thick CaF2 layers and two wells of 1.9- and 2.8-nm-thick CoSi2 layers. These layers were grown by means of partially ionized beam epitaxy for CaF2 and a two step growth technique for CoSi2. In the current-voltage characteristics at 77 K, negative differential resistance was observed in the significant number of samples and the typical peak-to-valley ratio was as high as 2. The negative differential resistance observed here can be attributed to the electron transport through the resonant levels in metal/insulator multilayered heterostructures.

Agglomeration-Free Nanoscale Cobalt Silicide Film Formation Via Substrate Preamorphization

AbstractWe have used preamorphization of silicon substrates as a process modification to suppress agglomeration during cobalt disilicide film formation. Planar, continuous and low resistivity (<21 μΩ-cm) silicide films less than 35 nm thick have been produced both on single crystal silicon and polysilicon. Nanoscale(<35 nm) silicide films are more susceptible to islanding phenomena since agglomeration is dependent onthe ratio of grain size to film thickness. Preamorphization prior to silicidation favorsa large increase in silicide nucleation rate, as well as reduction in critical nuclei size, both of which aid the formation of silicide with small grains. The resulting small grain silicides enable nanoscale films to remain below the critical grain size to thickness ratio for which thin films become morphologically unstable. An interphase void band which occurs between CoSi and CoSi2 layers, acts as a convenient diffusion marker and aids interpretation of the complex stability issues. Preamorphization prior to silicidation was also extended to heavily doped substrates to study the applicability of this approach for junctions and gate contacts. Silicidation of amorphized heavily boron doped substrates produces non uniform layers due to the collision of the advancing silicidation and SPE interfaces. A comparision of concurrent processing, i.e. simulatneous dopant activation and silicide formation, with conventional silicidation of Si+ preamorphized heavily doped(B) substrates is also presented.

Thickness Dependence of Electrical Transport in Buried CoSi2 Films Fabricated by Ion Beam Synthesis

ABSTRACTWe have performed electrical transport measurements on thin epitaxial buried CoSi2 layers in (111) and (100)Si with thicknesses ranging from 11.5 to 110 nm. The resistivity as a function of temperature exhibits metallic behaviour. The increase of residual resistivity with decreasing thickness can be explained by quantum mechanical weak localization effects induced by the interface roughness. This is supported by magnetoresistance measurements which provide long phase coherence lengths of 1Փ, ≈ 0.75 μm in (111)Si and 1Փ, ≈ 2.3 μm in (100)Si at 4.2 K.

The Influence of Implantation-Induced Non-Stoichionetry on the Epitaxial Recrystallization of CoSi2

ABSTRACTEpitaxial CoSi2 layers on Si substrates have been amorphized with Co and/or Si ion implantation. The influence of nonstoichiometry on the rate of solid-phase epitaxial growth (SPEG) of amorphized CoSi2 has been investigated with time-resolved reflectivity, Rutherford backscattering spectrometry and Mossbauer spectrometry, the latter with radioactive 57Co probes. A decrease in SPEG rate was apparent with an increase in nonstoichiometry. For a given ion dose, the decrease was greater following Co implantation. The means by which non-stoichiometry is accommodated in a crystalline CoSi2 lattice - either through phase separation or defect formation - has been considered. SPEG rate retardation was also evident in samples implanted with both Si and Co ions with a Si:Co dose ratio of 2:1. Additional mechanisms may thus also contribute to the observed SPEG rate reduction.

Growth of Buried CoSi2 Layers in Si(100) by Molecular Beam Allotaxy

ABSTRACTBuried single crystalline CoSi2 layers in Si(100) have been grown using molecular beam allotaxy. In this paper, we investigated the diffusive interaction of two buried silicide precipitate layers, one with a small Co peak concentration of 10 at% and another one with 26 at%. Annealing causes first local coarsening in each layer, and then dissolution of the thinner precipitate layer. The accumulation of the Co atoms at the thicker layer is described by a simple model for the diffusional redistribution.

Observation of a structural phase transition in a CoSi2 layer buried in ?111? Si

Single crystalline multilayered structures of Si/CoSi2/Si〈111〉 made by high dose implantation of Co in a Si wafer were investigated with57Co source Mossbauer spectroscopy, channeling measurements and X-ray diffraction. The results point to a structural phase transition in the CoSi2 buried layer between 180 and 220 K.

Self-Aligned Epitaxial Cosi2 Formation From Multilayer Co/Ti-Si(100) by a Two-Step Rta Process

ABSTRACTWe have extended our recent work on nano-scale CoSi2 formation from Co/Ti(O)multilayers on Si(100) to a self-aligned epitaxial CoSi2 structure produced by a two-step RTA annealing process. Parallel oxide stripes/Si windows were produced on a 4-inch Si(100) wafer by thermal oxidation and patterning. Six layers of 20nm Co and 10nm Ti were deposited sequentially on the patterned wafers with Ti as the first layer. The wafers were then annealed at 550°C to 700°C in N2 using a lamp RTA system. XTEM and RBS showed that a 25nm CoSi layer formed at the interface after a 650°C, 60sec annealing. The unreacted layers above it and oxide were selectively removed leaving a residual amorphous layer and CoSi intact on the patterned Si substrates. A second annealing at 900'C for 10sec produced 20nm of epitaxial CoSi2 covered with an ∼10nm CoxTiy(O)Siz surface layer. The epitaxial CoSi2 layer was thermally stable up to 1000°C, had a resistivity of ∼20μΩ-cm, and consumed ∼300Å of Si, thereby satisfying the most stringent deep submicron device contact scaling requirements.

Buried single-crystal CoSi2 layers in SiGe alloys made by ion beam synthesis

The synthesis of buried CoSi2 layers in Si0.91Ge0.09 alloys by the implantation of high doses of 100 and 150 keV Co+ ions and subsequent rapid thermal annealing at T≥1000 °C is studied by x-ray diffraction, Rutherford backscattering spectroscopy, He ion channeling, Auger electron spectroscopy, and transmission electron microscopy. After annealing, a buried single-crystal CoSi2 layer containing ≊1 at. % Ge is formed in the SiGe alloy. Silicide formation causes an outdiffusion of Ge leading to an increase in the Ge concentration of the adjacent SiGe layers. The remaining Ge within the silicide layer is found to be present as Si-Ge islands with the composition of the virgin Si0.91Ge0.09 alloy.

30 nm CoSi2 surface layers for contact metallization in complementary metal-oxide-semiconductor processes

Thin, low resistivity CoSi2 and TiSi2 surface layers have been realized through room-temperature ion implantation of Co+ and Ti+, respectively, followed by low-temperature annealing. TEM studies show that the layers are polycrystalline with large, uniformly thick grains. The results of leakage current measurements performed on junctions fabricated with and without CoSi2 surface layers illustrate the potential of this technique for contact metallization in sub-half-micron CMOS processes.