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Silicides have found application as high conductivity, high temperature, and corrosion resistance materials that form good electrical contacts to silicon and good low resistivity cladding on polysilicon films used as gate metal. Of various silicides investigated in past CoSi2 offers several advantages including lowest resistivity, self-aligned formation, low lattice mismatch with silicon, stability in presence of dopants and on SiO2, Si3N4, or Sioxynitrides, and reliability to process temperatures ≤900°C even when used in thicknesses as thin as 50-60 nm. Thus, CoSi2 has found an application in VLSI and ULSI. In this paper, the properties, formation and processing, reliability, and applicability of CoSi2 will be reviewed. It will be shown that CoSi2 is only silicide that offers properties and reliability for continued use in sub-0.25 pm VLSI and ULSI integrated circuits.
Titanium silicides are used as source, gate and drain contacts and local interconnections in CMOS integrated circuits. In these applications, it is important that the titanium silicide phase have a low resistivity (< 20μΩ-cm) and not agglomerate during high temperature processing. The Ti/Si system has two silicide phases that are useful for electronic applications, high resistivity C49-TiSi2 (60-70 μΩ-cm) which forms at 600 - 700°C and low resistivity (15-20 μΩ-cm) C54-TiSi2 which forms from 700 to 850°C. This paper will review how the size of the thermal annealing process window for forming low resistivity C54-TiSi2 from high resistivity C49-TiSi2 without having the silicide agglomerate varies with annealing treatments, electronic dopants, and contact size. In addition, processing methods to improve the size of the process window will be discussed.
This paper discusses the attributes of metal silicides as they are applied to detection of infrared photons. These materials have a long history in the silicon community as interconnects and are easily integrated into manufacturing. The technology is currently only sensitive to 10 μm in the infrared. New results obtained from silicon germanium alloys are discussed that will help overcome these spectral limitations.
State of the art DRAM and logic processes widely make use of silicides formed by refractory metals like Ta, Mo, W, Ti, Co, or others. The methods for silicide formation range from sputtering, reaction with predeposited metal, to selective CVD deposition.
The most common application of silicides in state of the art CMOS processes is to use them as a conducting layer that can withstand high temperature processes and that has a significantly lower sheet resistivity than doped mono or poly crystalline silicon. Besides their role as a temperature stable conductor silicides have further interesting properties when using them as a diffusion source for impurities to generate shallow junctions, or as an alternative gate material with a work function in the middle of the silicon band gap.
Based on the example of the self aligned silicidation with titanium silicide the limitations of the state of the art technology will be shown and requirements for future applications will be discussed.
This study investigates characteristics of p-n junction diodes fabricated by silicidation through a silicon buffer layer and dopant drive-out process. The purpose of using the buffer silicon layer is to reduce silicon consumption from the Si substrate during silicidation, and thus reduce the effective junction depth. The resulting structure is suitable for elevated CoSi2 source/drain contact in a metal-oxide-semiconductor field effect transistor or a silicided polysilicon emitter in a bipolar junction transistor. It was found that boron diffusion is enhanced by these buffer layers comparing to silicided diodes without silicon buffer layers. The sheet resistance of the CoSi2/polysilicon/Si structure does not degrade as seriously as CoSi2/polysilicon/oxide structure. The diode leakage current density is higher compared to diodes without buffer layers, especially when thinner buffer layers and high temperature 1000°C anneal are used.
This paper reports a titanium salicide process capable of fabricating low resistance salicide (<5 ohms/sq.) on narrow polysilicon leads (line widths less than 0.35 μm) which are heavily doped with arsenic and boron. The process utilizes conventional processing but avoids excessive vertical scaling of the titanium silicide film. The process has been demonstrated on a 0.35 μm CMOS technology and results show that a process window exists which is suitable for technologies of 0.35 μm and below. The most serious scaling issue for titanium salicide appears to be the silicide film thickness.
In this paper the electrical properties of epitaxial CoSi2 on Si obtained by solid-state reaction of a Ti/Co bimetallic layer are investigated. Low temperature resistivity, magnetoresistance and Hall data are presented. The CoSi2ISi Schottky diodes are characterised by current - voltage and capacitance - voltage measurements at temperatures varying between - 100°C and 60°C.
A new silicide/silicon IR detector is presented which has the potential for multicolour detection due to the tunability of its photoresponse. This tunable internal photoemission sensor (TIPS) fabricated using the Ir/Si/ErSi2 system, consists of two back–to–back Schottky diodes separated by a thin undoped Si layer. The two metals have different Schottky barrier heights so that the depleted Si forms an asymmetrical potential barrier to the carriers photocreated in each metallic film. The photocurrent flowing between the two metallic films is therefore strongly dependent on the shape and height of the effective potential barrier that can be varied by a bias applied between the two metallic electrodes. The Ir/Si/ErSi2 photoresponse and cut–off wavelength are indeed dramatically modulated when a small bias (less than 1 volt) is applied between the Ir and ErSi2 electrodes. The quantum efficiencies, measured in the 1 to 3 μm range, are comparable to the best obtained in Schottky and SiGe/Si internal photoemission detectors. A quantitative model derived from the Fowler formalism (by taking into account (i) the hole and electron photocurrents and (ii) the wavelength dependence of the photon absorption in each metallic film) fits all the experimental data over the whole range of photon energy and applied biases. The effective barrier heights thus measured as a function of applied bias are in good agreement with those deduced from activation energy analysis of the TIPS dark current and show that the cut–off wavelength can be modulated from 2.5 μm to more than 6 μm. Finally, electrical and photoresponse measurements on Cr/Si/SiGe(p+) structures (using the same TIPS mode of operation) also demonstrate the photoresponse tunability, thus combining the TIPS tunability with the extended wavelength range of operation (up to 10 μm) of SiGe/Si detectors.
We discuss the properties of semiconducting iron silicides, grown epitaxially on Si(001) and Si(111) by molecular beam epitaxy. The growth on Si (111) involves phase transitions from epitaxially stabilized metallic phases, leading to larger epitaxial β-FeSi2 grains than most other deposition procedures. The structural and electric properties of β-FeSi2/Si(001) are improved considerably for growth temperatures above 650 °C. Hall mobilities of p—conducting films reach values up to 600 cm2/Vsec at 100 K, at carrier densities below 1017 cm−3. Despite of the high majority carrier mobility and low carrier density, the photoelectric response of p-β-FeSi2/n-Si(001) diodes does not yield any significant contribution from the silicide, however, in accordance with the expected band structure diagram.
An investigation of the influence of an intermediate titanium thin film on the silicidation reaction between an overlying iron film and the (100)-oriented silicon substrate is presented. The Fe-Ti bilayers were obtained by consecutive sputtering of a Ti layer and an Fe layer on Si substrates. In addition, single iron layers were made by sputtering for comparison with the bilayers. By subsequent rapid-thermnal processing (RTP), depending on the annealing conditions, both the semiconducting β- and the metallic α-FeSi2 thin films could be formed. The phase formation has been investigated as a function of the thickness of the titanium layer, the annealing time and temperature. In this paper the first results on iron disilicide formation through Fe-Ti-Si diffusion couples are shown. Characterisation of the FeSi2 layers using Rutherford backscattering spectrometry (RBS), channelling RBS, X-ray diffraction (XRD), sheet resistivity measurements will be presented.
Thin films of semiconducting iron sulicide β-FeSi2 have been synthetized by Low Pressure Chemical Vapor Deposition in a cold wall reactor, starting from iron chloride and silane. Optimum experimental conditions for both iron chlorination and iron disilicide deposition have been determined by classical thermodynamic calculations. Despite the narrow range of as predicted deposition parameters, it has been possible to obtain mirror like thin films of pure polycrystalline β-FeSi2 on SiO2 substrate. The structural characteristics of the as deposited layers observed by SEM, ABS and RBS are presented together with their electronic properties.
Rutherford backscattering spectrometry and transmission electron microscopy were used to compare thermally induced solid phase epitaxy (SPE) with ion-beam induced epitaxial crystallization (IBIEC) of Fe-implanted Si (001). It was found that thermal annealing leads to both Si SPE and β-FeSi2 precipitation at 520°C, but has no visible effect at 320°C. In contrast, Si SPE and FeSi2 precipitation occur at both 320 and 520°C, when ion irradiation is introduced. The precipitates grow epitaxially as γ-FeSi2 at 320°C, but consist of both β-FeSi2 and γ-FeSi2 at 520°C. It was also found that thermal annealing at 520°C results in Fe segregation toward the surface, while IBIEC basically retains the as-implanted Fe profile.
This study focuses on the characterization of iron silicide grown, for the first time, by pulsed laser deposition on Si(111). Silicide growth was attempted both by deposition of pure Fe followed by annealing, and congruent deposition of Fe and Si from a stoichiometric FeSi2 target. The films formed by deposition of pure Fe and annealing did not grow epitaxially on Si(111) and contained a number of phases including β-FeSi2. Films grown by congruent deposition of Fe and Si did grow epitaxially on Si(111) and contained either pure β-FeSi2 or a mixture of both FeSi and β-FeSi2, depending on deposition conditions. The following epitaxial orientations were observed: β-FeSi2(001)//Si(111), β-FeSi2//Si<110> with three variants, and FeSi(111)//Si(111), FeSi//Sit[112^. Films of various thicknesses were analyzed with conventional transmission electron diffraction and microscopy.
By fitting orthogonal tight binding parameters to the ab inlio bands of Calciumfluorite FeSi2 (γ-phase) and Cesiumcloride FeSi, we calculate the electronic structure (bands and density of states) and the total-energy of the semiconductive, orthorombic β-phase and the disordered, cubic one. The latter, the γ and the β nfigurations, have been recently observed at different annealing temperatures in thin films grown on Si (111) by Molecular Beam Epitaxy. The transferability of our method among different phases allows for a comparison of the cohesive energy curves which, in turn, supplies an interpretation of the relative stability and the growth kinetics.
We fabricated α-FeSi2 and α-FeSi2 layers by using two methods: Ion Beam Synthesis (IBS) and Molecular Beam Allotaxy (MBA). In the latter technique a trapezoidal-shaped Fe profile was embedded in the Si matrix by codeposition of Si and Fe at temperatures of about 650°C. A rapid thermal anneal of the IBS and MBA samples at 1150°C for 10 s is necessary to obtain continuous α-FeSi2 layers. The Fe vacancy concentration of the α-FeSi2 layers was varied by a further anneal at lower temperatures. Resistivity measurements indicate a decrease of the resistivity with decreasing Fe vacancy concentration. The α-FeSi2 was transformed to a continuous β-FeSi2 layer by an anneal at 800°C for several hours. To investigate the nature of the band gap we performed absorption measurements at room temperature and 77 K. The analysis of the room temperature data revealed a direct transition at 0.84 eV and an additional indirect transition at 0.78 eV. At 77 K the direct transition shifts to ≈ 0.875 eV and the indirect to ≈ 0.86 eV.
Measurements of the electrical resistivity and magnetoresistivity are reported for 100 nm buried α-FeSi2 in the temperature range 1.2 to 300 K, and in magnetic fields up to 5 Tesla.The metallic α-FeSi2 phase, formed by ion-beam synthesis and subsequent rapid-thermal annealing, is found to have a high residual resistivity of about 227 μΩ2 cm. Furthermore, a minimum in the electrical resistivity is found at 40 K. Combined with mnagnetore si stance measurements it is concluded that this minimum cannot be attributed to the Kondo effect, but is caused by weak localisation and spin-orbit interaction due to the very strong elastic scattering in the material. From the magnetoresistance measurements, at temperatures below 40 K, the relaxation times due to inelastic and spin-orbit scattering are deduced. The inelastic scattering rate is found to saturate below 4.2 K, and follow a T3 power law between 4.2 K to 40 K.
Transmittance, reflectance and Raman measurements have been performed from 50 to 700 cm−1 on different thin films of β-FeSi2 epitaxially grown by MBE on Si substrates. Vibrational spectra show much more structures than the five phonons usually observed in this material; the dependence on the crystalline orientation, the thickness, the growth and the annealing temperature has been studied.
Fe-sulicides were formed by annealing MBE-deposited thin 57Fe layers with thicknesses between 20 Å and 60 Å on (7x7) reconstructed Si(111) substrates. During the growth the substrate was held at room temperature. The silicide formation upon annealing in the temperature range of 200°C to 900°C was studied in-situ with RH-EED. Samples were studied with in-situ CEMS (Conversion Electron Mossbauer Spectroscopy) as well as with ex-situ CEMS after covering with Ag to prevent oxidation. RBS/Channeling was used to study the epitaxial quality and the structure of these layers.
Using the 57Fe nuclear probe, it is shown that silicide formation occurs at the interface already at room temperature. Metastable silicides with cubic structure are observed in the low temperature annealing range, and characterized by their Mossbauer parameters. At higher temperatures stable ε-FeSi and β-FeSi2 are formed.
Heteroepitaxy of β-FeSi2 on (100) and (111) silicon surfaces has been achieved by gas source molecular beam epitaxy (GSMBE). Fe(CO)5 and SiH4 are used as sources for the silicide growth in the substrate temperature range 400 – 750°C. Depending on growth temperature different growth modes are observed. Concerning morphology two best temperatures were identified for the growth of β-FeSi2 on Si(111). Tg = 550°C - the epitaxial relationship with the substrate is β-FeSi2(100)||Si(111) as shown by High Resolution Transmission Electron Microscopy (HRTEM) and Low Energy Electron Diffraction (LEED); a thin γ-FeSi2 (metallic cubic phase) layer is seen to be stabilized at the interface acting as a buffer layer for the further growth of the orthorhombic semiconducting phase. Tg = 700°C - the same epitaxial relationship as in SPE grown layers is observed: β-FeSi2(101) or β-FeSi2(110)||Si(111) and no γ-FeSi2 is present at the substrate interface. In accord with the lower growth temperature, the silicide layers show slightly higher surface roughness. On Si(100) the epitaxial relationship with the substrate is β-FeSi2(100)||Si(100) in the whole temperature range. On both Si substrates domains with different azimuthal orientations are observed by LEED. In-situ electronic characterization is performed by photoelectron and electron energy loss spectroscopies. Electrical characterization at room temperature shows relatively high mobility values (up to 70cm2/Vs) but the complex temperature behaviour of the Hall constant suggests the presence of both carrier types in the β-FeSi2 grown layers. Measurements of the absorption coefficient at RT show an indirect minimum gap for β-FeSi2.
The electrical conductivity of a number of the semiconducting alloys of iron disilicide with the iron group transition metals and some elements of Vb group have been investigated in detail. Wide temperature regions of linear temperature dependence of resistance were found. The temperature coefficient of resistance (TCR) in these regions is low enough. There is an opportunity of varying the resistivity of these alloys by means of modification of the alloy composition.