Thin films of rare-earth metal silicides in microelectronics

Structure, phase composition and physical properties of thin films of rare-earth metal (REM) silicides (of Y-subgroup) have been studied. The films were formed on (100) and (111) single-crystal Si substrates. X-ray diffraction analysis reveals the formation of a crystalline silicide phase of LnSi_2−x composition with hexagonal structure of the AIB₂-type for all metals except Sc, Gd, Lu. It is established that the crystalline silicide phase formation is determined by the crystallographic orientation of the Si substrate and that there is a relation between crystallographic parameters of REM silicide (REMS) and Si hexagonal lattices: the critical value of lattice parameter mismatch is ± 1.3%, above which REMS has a disordered structure. The kinetics of the silicide phase growth was determined by measuring the conductance of thin-film structures. A model of REMS formation in thin-film structures on an Si substrate is proposed. Based on the model, conditions have been found for the formation of either quasi-amorphous, polycrystalline or monocrystalline REMS layers. The formation of a REMS amorphous film is most probable on a (100) substrate for such metals as Gd and Lu. Epitaxial growth of REMS films is typical of Tm, Er, Ho on (111) substrates. Also, investigation of the current transport in surface barrier diodes of the REM-p-Si and REMS-p-Si types shows that the potential barrier lowers as a result of silicide formation in REM-Si contacts and, accordingly, the silicide work function increases as compared with that of the initial metal. The photoelectric measurements indicate that the silicide-Si contact photosensitivity shifts to a longer optical wavelength as compared to that of the metal-Si contacts.     

Thermal oxidation of transition metal silicides

A survey has been made of most publications on the oxidation of transition metal silicide films on either silicon or an SiO₂ substrate.On silicon substrates the general trend is that SiO₂ forms on the sample surface in preference to metal oxides, with the silicide layer being morphologically preserved. Thermodynamics, in terms of heats of formation and ternary phase diagrams, has been used successfully to explain the general absence of metal oxides and also to explain the exceptions to that rule. Kinetics also plays a part in the determination of the reaction products. The growth rate of SiO₂ on silicon substrates obeys the linear-parabolic law. The parabolic rate constant of silicide oxidation is essentially the same as that of silicon oxidation, indicating that the oxide and its diffusivity for the oxidant are the same for silicon and silicides. However, the linear rate constant of silicides exceeds that of silicon, and its value varies with the silicide. The differences among silicides might be attributed to differences in the atomic transport processes within the silicide; the enhancement with respect to silicon has been ascribed to the metallic nature of the silicides.On SiO₂ substrates, the oxidation ultimately leads to the formation of metal oxides as well. Instabilities of structure and loss of material can occur.The properties of the grown SiO₂ are reviewed and directions for further studies are outlined.     

Ion-induced phase formation in metal-silicon systems

By using megaelectronvolt ⁴He ion backscattering techniques and transmission electron microscopy, we have investigated the interactions of ion beams with thin film structures in a number of silicide-forming systems. The mixed layer was found to be an equilibrium compound for near-noble metals and an amorphous phase for refractory metals. Differences in behavior have also been observed in near- noble metal systems. For palladium, the Pd₂Si phase grew with ion dose and remained crystalline up to high dose. For nickel, the compound Ni₂Si was formed initially and became amorphous on prolonged irradiation. All the results indicate the significance of atomic mobility at target temperatures in determining the phase formation and in explaining the sensitivity of the silicides to ion bombardment.     

Metal silicide-templated growth of quality Si films for Schottky-diodes

Quality Si films were grown on a metal silicide template and fabricated for a Schottky-diode. The thin metal was firstly deposited and reacted to the supplying Si and then formed the silicide layer, which is a template to grow quality Si film above it due to the lattice affinity to Si. Various types of metal (Co, Ni, and mixture of Co and Ni) were used as catalyst species. The morphological changes of Si grain sizes were systematically investigated. Two steps of Si supply condition were applied and revealed the formation of metal silicide phases and Si film growth.During the Si supply, Co was stable to form CoSi₂ and grew a crystalline Si (c-Si) film above it. However Ni firstly formed Ni rich silicide phases at low Si supply due to the fast Ni diffusion in Si. By increasing the Si supply, Ni diffusion has been staggered and formed NiSi₂ layer to grow a c-Si film above it. It has been also revealed that the NiSi₂ migration produced a c-Si film behind. Mixing of Co with Ni showed a stable silicide phase without a serious metal migration and improved the Si crystallinity providing an enhanced Schottky-diode performance.The investigation of silicide formation and quality Si film growth is presented. Transmission electron microscope analysis proves the volume growth of c-Si film above a metal disilicide of NiSi₂ or CoSi₂.     

Au/Ni bilayers on n-type silicon: Metallurgical and electrical behaviour

Diffusion effects during the formation of silicides in the Ni-Au-Si system were investigated by means of ⁴He⁺ MeV Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar⁺ ion sputtering and X-ray diffraction as a function of the heat treatment temperature (280–350°C) and time (10–1000 min). Schottky barrier heights were used to identify the type of metal present at the silicon surface. Au/Ni/Si and Ni/Au/Si structures were prepared by electron gun deposition of thin gold and nickel films onto n-type Si〈111〉 single crystals. After thermal treatment only Ni₂Si and NiSi compounds were observed and their formation follows the phase order confirmed by previous investigations on the Ni/Si system, with a growth controlled by a lattice diffusion process. In the Ni/Au/Si〈111〉 structure the diffusion of the silicon through the gold film was detected during the formation of nickel silicide and the kinetics of growth of Ni₂Si and NiSi were similar to those studied in the Ni/Si〈100〉 system. A diffusion of gold towards the Si-NiSi interface was observed during the growth of NiSi in the Au/Ni/Si〈111〉 structure. The Schottky barrier height measurements confirm these findings.     

Impurity effects in molybdenum silicide formation

The reaction between molybdenum thin films and single-crystal Si〈111〉 substrates was studied as a function of the concentrations of impurities (mainly oxygen) in the metal film. At a low oxygen concent (1–2 at.%), only the silicide phase MoSi₂ was observed, and a thickness proportional to the square root of time corresponding to an average activation energy of 3 eV in the temperature range 545–600 °C was found. During the formation of the silicide the oxygen originally present in the molybdenum films accumulates at the interface between the silicon and the MoSi₂.In contrast, a higher oxygen content (4–5 at.%) prevents the formation of any silicide phases in the above temperature range and leads to the formation of MoSi₂ and Mo₅Si₃ phases at temperatures near 800 °C. MoSi₂ was always observed at the inner interface with Mo₅Si₃ on the surface.The oxygen segregates from the silicides and accumulates at the Si-MoSi₂ and MoSi₂-Mo₅Si₃ interfaces to form a non-uniform layer of SiO_x(x ? 2).     

Thermal oxidation of transition metal silicides: The role of mass transport

Thin films of almost all transition metal silicides on a silicon substrate oxidize and form SiO₂ on their surface when they are annealed in an oxidizing ambient atmosphere. In applications of these silicides as interconnects in integrated circuits, the oxidation characteristics of the silicide and the SiO₂ growth rate are very important.We review the four major steps controlling silicide oxidation: (1) oxidant transport through the oxide; (2) reaction at the silicide-oxide interface: (3) net transport of silicon atoms with respect to metal atoms in the silicide; (4) reaction at the silicide-silicon interface. The oxidant transport is shown to be the same for all silicides. The reaction at the silicide-oxide interface is explored using equilibrium thermodynamic arguments. The transport through the silicide is discussed and experimental results of inert marker experiments are presented.The diffusing species during the oxidation of PdSi, Pd₂Si, NiSi₂, CoSi₂, PtSi, CrSi₂ and TiSi₂ are discussed. The diffusing species during oxidation correlate with the moving species in silicide formation. A discussion of a mechanism that explains why the oxidation rate of some silicides on a silicon substrate is faster than that of the bare substrate is presented.     

Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si

The influences of stress on the interfacial reactions of Ti and Ni metal thin films on (0 0 1)Si have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the growth of Ti and Ni silicide thin films. On the other hand, the tensile stress present in the silicon substrate was found to enhance the formation of Ti and Ni silicides. For Ti and Ni on stressed (0 0 1)Si substrates after rapid thermal annealing, the thicknesses of TiSi₂ and NiSi films were found to decrease and increase with the compressive and tensile stress level, respectively. The results clearly indicated that the compressive stress hinders the interdiffusion of atoms through the metal/Si interface, so that the formation of metal silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted.     

Preparation and characterization of ultra-thin cobalt silicide for VLSI applications

Ultra-thin cobalt silicide (CoSi₂) was formed from 10 nm cobalt film by solid phase reaction of Co and Si by use of rapid thermal annealing (RTA). The Ge⁺ ion implantation through Co film caused the interface mixing of the cobalt film with the silicon substrate and resulted in a homogeneous silicide layer. XRD was used to identify the silicide phases that were present in the film. The metallurgical analysis was performed by RBS. XRD and RBS investigations showed that final RTA temperature should not exceed 800°C for thin (< 50 nm) CoSi₂ formation.     

Formation and microstructure of various thin film chromium silicide phases

The formation of the various chromium silicide phases, as predicted by the Cr-Si phase diagram for bulk materials, was studied when thin Cr/a-Si (where a-Si is amorphous silicon) bilayers were annealed in situ in a transmission electron microscope using a limited supply of a-Si. These results are compared with the silicide formed when unlimited a-Si or single-crystal silicon was used. The specimens were analysed using electron micrographs and diffraction patterns.The detailed studies of the bilayers were preceded by studies of single layers of chromium and a-Si. The chromium single layers proved to be continuous for films as thin as approximately 2 nm. The as-deposited films with thickness in excess of about 5 nm were crystalline with b.c.c. structure and comprised very small grains. A phase change occured at about 450°C from the b.c.c to a simple cubic lattice structure. The silicon single layers were completely amorphous in their as-deposited state and crystallized around 600°C. Very large grains formed.The self-supporting Cr/a-Si bilayers with a typical total thickness of about 50 nm, where the relative film thickness were adjusted to yield Cr:a-Si atomic ratios of 1:2, 1:1, 5:3 and 3:1, were prepared by sequential electron gun vacuum deposition of chromium and silicon onto photoresist-covered glass slides. In the early stages of phase formation, when both unreacted chromium and silicon were present, the CrSi₂ phase was formed at about 450°C for all of these specimens. This was the end phase for the 1:2 ratio specimen. For all the other specimens the metal- rich Cr₅Si₃ phase was next to grow at about 550°C. For the 5:3 ratio specimen this was the end phase.Upon further heating of specimens with a Cr:a-Si atomic ration of 3:1, a more chromium-rich phase of Cr₃Si was formed at about 650°C. In the 1:1 ratio specimen, however, the next and end phase observed was CrSi, also growing at about 650°C. The end phase was thus determined by the availability of chromium and silicon during the reactions and could be predicted from the phase diagram.When using an unlimited supply of a-Si (or of single-crystal silicon), instead of limiting it to the thickness necessary for the predetermined ratios mentioned above, the only phase that ever formed was CrSi₂.The grain sizes observed in the various final phase specimens were as follows: CrSi₂, 25 nm; Cr₃Si, 40 nm; Cr₅Si₃, 50 nm; CrSi, 100 nm.     

Hot Ta filament resistance in-situ monitoring under silane containing atmosphere

Monitoring of the electrical resistance of the Ta catalyst during the hot wire chemical vapor deposition (HWCVD) of thin silicon films gives information about filament condition. Using Ta filaments for silane decomposition not only the well known strong changes at the cold ends, but also changes of the central part of the filament were observed. Three different phenomena can be distinguished: silicide (stoichiometric Ta_XSi_Y alloys) growth on the filament surfaces, diffusion of Si into the Ta filament and thick silicon deposits (TSD) formation on the filament surface. The formation of different tantalum silicides on the surface as well as the in-diffusion of silicon increase the filament resistance, while the TSDs form additional electrical current channels and that result in a decrease of the filament resistance. Thus, the filament resistance behaviour during ageing is the result of the competition between these two processes.     

Growth of long aligned carbon nanotubes on amorphous hydrogenated silicon nitride by thermal chemical vapor deposition

Vertically aligned long carbon nanotubes in the range of 80-100 µm have been synthesized on amorphous hydrogenated silicon nitride (a-SiN_x:H) coated silicon substrate by thermal chemical vapor deposition of ferrocene and xylene. It is observed that high temperature annealing in oxygen ambient results in formation of crystalline silicon dioxide in the matrix of amorphous silicon nitride due to out diffusion of hydrogen. It is suggested that active sites created on silicon dioxide and a-SiN_x:H clusters provide mechanical support for the alignment of long carbon nanotubes. It is proposed that a thin layer of a-SiN_x:H prevents silicide formation between the catalyst (Fe) and silicon thus lengthening the catalyst life.     

Silicide formation during heat treatment of thin Ni-Pt and Ni-Pd solid-solution films and Pt/Ni bilayers on (111)Si

It is shown that isothermal heat treatment of (Ni-Pt)/Si and Pt/Ni/Si heterostructures leads to the formation of oriented Ni-and Pt-based silicide solid solutions. Owing to the three equivalent azimuth orientations in the basic lattice orientation relationship for the Si-Ni_1?x Pt_xSi system, the resulting silicides have a nanocrystalline substructure. The stability of the substructure is due to the optimal interfacial lattice match and near-special grain-boundary misorientations. The silicide phases Ni_1?x Pt_xSi and Pt_1?y Ni_ySi (or Ni_1?x Pd_xSi and Pd_1?y Ni_ySi) may undergo segregation, having the same lattice orientation. In both systems, the segregation is associated with the predominant Ni diffusion. The second component (Pt) is shown to stabilize the orthorhombic Ni-based silicide and to prevent NiSi₂ formation. Photon processing accelerates diffusion and leads to the formation of phase-pure Ni_1?x Pt_xSi solid solutions.     

Refractory metal silicide formation by ion implantation

A systematic study of interface mixing of transition metal-Si structures by ion implantation was carried out. High resolution Auger analyses at the mixed region of the implanted samples showed an energy shift of the Auger Si LVV transition and a change in the spectrum relative to that of elemental silicon. The elemental depth distribution obtained using Auger electron spectroscopy combined with ion sputtering illustrated a composition plateau for the ion-implanted structures. The X-ray diffraction data show the presence of silicides. These results illustrated metal silicide formation and a reduction in the thermal reaction barrier for forming refractory metal silicides by ion implantation.     

Substrate and annealing temperature effects on the crystallographic and optical properties of MoO3 thin films prepared by laser assisted evaporation

MoO₃ thin Films were prepared using the assisted laser evaporation technique. Samples were grown on glass and silicon substrates at different substrates temperatures. The effect on structural and optical properties of the substrate and on annealing temperatures was evaluated. A phase transition was found around 200 °C in all samples from the amorphous to the β phase with a small percentage of α phase, and another one was found around 500 °C from the α + β to the α phase. The percentage errors between the lattice parameter a₀ of the crystallographic index card for the MoO₃ alpha phase and the indexed lattice parameters were 1.4% and 0.3% for the samples deposited on glass and silicon respectively, indicating the crystalline structure of the silicon substrate favors the formation of the MoO₃ alpha orthorhombic phase. The spectral variation of the refractive index and the absorption coefficient were theoretically determined. The amorphous samples presented a constant gap of 3.2 eV while the optical properties critically depended on the substrate and annealing temperatures.     

Growth, structure and luminescence properties of multilayer Si/β-FeSi2NCs/Si/…/Si nanoheterostructures

Multilayer structures (up to 15 layers) with β-FeSi₂ nanocrystallites (NCs) buried in silicon crystalline lattice were grown by successive repetition of reactive deposition epitaxy (RDE) or solid phase epitaxy (SPE) of thin iron film on Si(100) or Si(111) substrates and silicon molecular beam epitaxy (MBE) (100-200 nm). Cross-section high resolution transmission electron microscopy (HR TEM) images and ex situ optical and Raman spectroscopy data prove that NCs formed in silicon matrix have the structure and optical properties of β-FeSi₂. The growth conditions provide no dislocations in silicon lattice were found in the course of TEM analysis. Two types of NCs depth distribution were observed: (i) layered that corresponds to iron RDE and (ii) uniform that occurs in the case of iron SPE. The uniform NCs distribution points out the fact that during a growth process NCs moves up to the surface. In spite of small nanocrystallites size (5-50 nm) and their distribution in silicon cap layers the significant photoluminescence (PL) signal at 0.8 eV was observed for all grown samples.     

Reversibility of silicidation of Ta filaments in HWCVD of thin film silicon

If tantalum filaments are used for the hot wire chemical vapour deposition (HWCVD) of thin film silicon, various types of tantalum silicides are formed, depending on the filament temperature.Under deposition conditions employed for device quality amorphous and microcrystalline silicon (T_wire ≈ 1750 °C) a Ta₅Si₃ (as determined by XRD) shell is formed around the Ta core. After 8 h of accumulated deposition time this shell has a thickness of around 20 μm. Upon annealing of the filament in vacuum at 2100-2200 °C the tantalum silicide shell becomes thinner, while a Ta layer is reappearing at the surface of the wire. After 4 h of annealing the silicide is completely removed, whereas the total diameter of the wire has not significantly changed. The resistance of the filament has been monitored and after the annealing procedure, it completely recovered to that of a fresh wire. This regeneration procedure greatly helps to avoid frequent replacement of the filaments.     

Formation and oxidation mechanisms in two semiconducting silicides

Marker experiments have been used to study the formation and the oxidation of two semiconducting silicides with large band gaps. Both Ru₂Si₃ and Ir₃Si₅ have been shown to form by mechanisms dominated by the motion of the silicon atoms. The same is true for the growth of SiO₂ over these two silicides under conditions which ensure the integrity of the silicide layers. IrSi has also been shown to grow mostly through the motion of silicon.     

Preparation of rare earth CeO<Subscript>2</Subscript> thin films using metal organic decomposition method for metal-oxide–semiconductor capacitors

Investigation of metal organic decomposed rare earth cerium oxide thin films deposited on Si substrate by sol–gel spin coating technique was carried out. The structural properties have been examined by using XRD, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The XRD confirms the cubic phase of CeO₂ thin films with (111) plane observed at 28.54°. The FTIR and EDAX spectra confirm the formation of CeO₂ films with atomic percentage of 19.39 and 54.82% of Ce and O₂, respectively. Thickness of 60.11 nm of CeO₂ film measured by cross sectional FESEM image, the average roughness of ~0.6 nm of 400?°C annealed CeO₂ films were observed from AFM micrograph. The MOS capacitors were fabricated by using Ti/Au bilayer metal contact depositing by E-beam evaporator on CeO₂/Si thin film for electrical measurements. Capacitance and conductance voltage measurement was carried out to determine the effective oxide charges (Q_eff), interface trap density (D_it) and dielectric constant (k) and are 2.48?×?10¹² cm^?2, 1.26?×?10¹² eV^?1cm^?2 and ~39, respectively. The effective metal work function of 5.68 for Ti/Au bilayer is observed to be higher than the work function of Ti or Au metals in vacuum.     

The diffusion barrier effect of a vanadium layer in the formation of nickel silicides

The formation of nickel silicides and vanadium silicides and the diffusion barrier effect of a vanadium layer in the formation of nickel silicides were studied for annealed metal films deposited on silicon substrates by depth profiling using secondary ion mass spectrometry and X-ray diffraction. Nickel films more than 2500 Å thick react with silicon after annealing at 400 °C for 30 min in a vacuum. A vanadium layer 250 Å thick between nickel and silicon shows a barrier effect in the nickel silicide formation after annealing at 400 °C for 30 min. The barrier effect of a vanadium layer 250 Å thick becomes imperfect after annealing at 500 °C for 30 min.