Ti-Si Multilayers Research Articles

In-situ spectroscopic ellipsometry for monitoring the TiSi multilayers during growth and annealing

The TiSi x formation and properties depend strongly on the deposition and annealing process. We employ spectroscopic ellipsometry (SE) to monitor both processes as well as transmission electron microscopy (XTEM) to verify the above results. The Ti and Si layers were deposited by magnetron sputtering on (100)Si. The multilayer thickness is about 930 Å and the Si Ti ratio ≈ 2.2 , while the thickness of each TiSi bilayer ≈ 120 A ̊ . After deposition of each Ti or Si layer and during annealing of TiSi multilayers up to 680 °C we obtain the SE spectra in the energy region 1.5–5.5 eV. Their analysis and XTEM observations show that during deposition an intermixing of 20 Å width at each TiSi interface occurred. By using SE we found that during annealing below 150 °C there is no drastic intermixing or reaction between Ti and Si. A fast interdiffusion of Ti and a-Si and their reaction is observed between 200 and 580 °C. A phase transition occurs at 580 °C from the amorphous Ti 5Si 3 compound to the C49 TiSi 2 structure and above this temperature XTEM depicts that only structural modification within the interdiffused layers of TiSi is observed.

In situ X-ray diffraction study of the formation of TiSi 2-C49 phase from Ti-Si multilayers on Si(100)

The kinetics of the formation of a metastable TiSi 2-C49 phase from amorphous Ti-Si multilayers grown on Si(100) substrate has been studied using a time-resolved X-ray diffraction technique at elevated temperatures ranging from 270 to 310 °C. A linear position-sensitive proportional counter (PSPC) was used to simultaneously collect the (131) and (150) Bragg peaks of the C49 phase. The kinetics data were analyzed using the Johnson-Mehl-Avrami (JMA) equation. The Avrami exponent was found to be 2.0 ± 0.1; the reaction rate constants follow a familiar Arrhenius-type behavior resulting in an activation energy of 2.5 eV. Comparison of our X-ray results with kinetics data obtained by other means is discussed.

Interdiffusion in metal/amorphous Si multilayers

Metal (Ni, Ti, Mo) and amorphous Si (a-Si) compositionally modulated multilayers (metal/a-Si) were prepared by ion beam sputtering. An in situ X-ray diffraction technique was applied to investigate the interdiffusion phenomena in these multilayer systems. The temperature-dependent interdiffusivities were obtained by monitoring the decay of the first-order modulation peak as a function of annealing time. It was found that both positive and negative interdiffusions took place in TiSi multilayers during annealing, while only positive interdiffusion occurred in Mo-Si multillayers. Accompanying interdiffusion, the process of solid state amorphization played a dominant role in TiSi multilayers; on the contrary, the crystallization process was the main process in MoSi multilayers. The effective interdiffusivities of Ni-Si can be expressed in terms of an Arrhenius relation.

Kinetics of the formation of C49 TiSi2 from Ti-Si multilayers as observed by in situ stress measurements

The formation of C49 TiSi2 from sputter-deposited amorphous Ti-Si multilayers on Si(100) substrates was studied during isochronal and isothermal heat treatment. The formation process was analyzed for multilayer periodicities (i.e., the thickness of one Ti and one Si layer) of 1, 5, and 12 nm, respectively, by in situ stress measurements and differential scanning calorimetry (DSC). A strong correlation between the evolution of the stress and the formation of the C49 TiSi2 phase was found. During heat treatment an abrupt change of the stress in the film was observed. DSC and x-ray diffraction experiments revealed that this change of the stress coincides with the formation process of the C49 TiSi2 phase. The transition temperature of the Ti-Si multilayer to the C49 TiSi2 phase shifts from ∼295 to ∼550 °C with increasing multilayer periodicity. From kinetic analysis of the experiments the activation energy of the C49 TiSi2 phase formation process was acquired. For the smallest periodicity of 1 nm an effective activation energy of 170±25 kJ/mol was found, for multilayers with 12 nm periodicity the activation energy was 280±40 kJ/mol. The difference in activation energy is explained by different rate-limiting steps during silicide formation as a function of the multilayer periodicity. DSC measurements revealed two processes during the formation of the C49 TiSi2 phase: mixing and crystallization. The enthalpy for mixing for Ti-Si multilayers with periodicities of 1 and 12 nm was −38 and −33 kJ/mol, respectively. The crystallization enthalpy was comparable for both periodicities: −5 kJ/mol.

Comparison of Interdiffusion Behavior in MoSi and TiSi Multilayers

The interdiffusion of Mo-Si and Ti-Si multilayers is investigated by an in situ X-ray diffraction technique. It is found that the behavior of the interdiffusion in both multilayers is quite different. Both positive and negative interdiffusion take place in Ti-Si multilayers during annealing, while only positive interdiffusion occurs in Mo-Si multilayers. Accompanying interdiffusion, the process of solid state amorphization reaction plays a dominant role in Ti-Si multilayers, on the contrary, the crystallization process was the main process in Mo-Si multilayers. In addition, a transient amorphization process at early stage of annealing is observed in Mo-Si multilayers for the first time.

Elastic constants and thermal expansion coefficient of metastable C49 TiSi2

The elastic constants and thermal expansion coefficient of C49 TiSi2 thin films have been investigated by in situ curvature measurement during heat treatment and ex situ x-ray diffraction measurements. The C49 TiSi2 compound was formed from Ti-Si multilayers deposited on monocrystalline silicon and sapphire substrates. The films were polycrystalline without any evident texture. Young’s modulus (142 GPa), Poisson’s ratio (0.27), and the thermal expansion coefficient (10.9×10−6 K−1) have been determined. Note that these values are averages over random crystal orientations. Directly after formation C49 TiSi2 films exhibit tensile stress. This stress relaxes considerably above 375 °C. Below this temperature the thermal expansion is found to be independent of the state of relaxation of the C49 TiSi2 film.

Structure of an amorphous TiSi alloy formed by thermal reaction

The amorphous nature of a-Ti 60Si 40 formed by interdiffusion of sputtered TiSi multilayers has been confirmed by the calculation of a radial distribution function (RDF) from scattering patterns obtained by electron diffraction. This material shows the characteristics of an amorphous metallic alloy with a coordination number of 11.4. The atomic pair distribution function shows some similarity to the crystalline silicide closest to it in composition — Ti 5Si 4. The likely presence of chemical short-range order in the alloy is consistent with the driving force for the formation of the amorphous state by interdiffusion in the TiSi system, which is the formation of TiSi bonds.

In Situ Stress Measurements During the Formation of TiSi2 C49 On Si Wafers

In the microelectronics industry titanium disilicide (TiSi2) is used as a material for metallization and interconnection on silicon based integrated circuits. In the C54 structure (face centred orthorhombic) TiSi2 has important properties for application in electronic devices: low resistivity (16 μΩ cm), stability up to 900°C and compatibility with silicon processing. In thin films this phase is formed above approx. 700°C. However before this phase is formed, a metastable TiSi2 phase with the C49 (or ZrSi2) structure [1] is already formed at lower temperature. This C49 phase is unfavourable as metallization in IC applications because of the high resistivity (60–300 μΩ cm). From a technological point of view however it is important to realize that during the C49 formation the thin film is subject to a large change of the intrinsic stress. The occurrence of this stress can cause problems during semiconductor device fabrication. Gate oxides in MOSFETs (and other IC microstructures) may be deteriorated by stress. Also focussing problems in lithographic steps can arise because of wafer warpage. In this paper we present in situ stress measurements of the formation of TiSi2 C49 from Ti-Si multilayers. From these measurements the kinetics of the formation process is analyzed.

Stresses in sputtered TiSi multilayers and polycrystalline silicide films

Stresses in sputtered TiSi multilayers and polycrystalline silicide films

High-resolution and in situ tem studies of annealing of Ti-Si multilayers

The solid state amorphization reaction in sputtered amorphous-siliconpolycrystalline-titanium multilayers has been studied in cross-section by high-resolution transmission electron microscopy (TEM). Heat treatments included rapid thermal annealing and in situ heating in the microscope. The bilayer spacing of the sample is 25.0 nm and the composition is 50at.%Ti-50at.%Si. Growth of the alloy is strictly planar. The amorphization does not occur preferentially along titanium grain boundaries. The reaction is diffusion controlled with an activation energy of about 2.0 eV atom −1. When the layers are partially reacted, the amorphous alloy composition is silicon rich, with an estimated Ti:Si atomic ratio of 1:2. Ónce all of the elemental silicon is consumed the alloy incorporates more titanium, reaching a composition of about Ti 45Si 55. In situ studies reveal that Kirkendall voids form at this point in place of the silicon layers. In situ annealing studies duplicate the microstructure observed in the bulk-annealed samples; from this we conclude that bulk diffusion dominated in the thin TEM foil.

Stresses in sputtered Ti-Si multilayers and polycrystalline silicide films

Titanium silicon multilayers have been produced by alternating sputter deposition. The stress in the as-deposited layers is a function of the period of the multilayer structure. The multilayers were subsequently annealed to form silicide films. From stress and strain measurements on these films Poisson’s ratio is determined. After annealing, the films exhibit a tensile stress which can be attributed to the difference in thermal expansion coefficient between substrate and silicide film.

Amorphous Phase Formation During Ion Mixing of Ti/Si Bilayers at Elevated Temperature

ABSTRACTThe amorphous phaseformation in Ti-Si bilayers upon ion mixing at elevated temperatures and in Ti-Si multilayers upon thermal treatment was studied. In the case of ion mixing with 5×1015 cm−2 Xe atoms at temperatures around 240°C a 100nm thick amorphous Ti-Si alloy is formed with a very homogeneous Ti:Si=3 :4 composition. Thermal treatment of the Ti-Si multilayer structure at similar temperatures also yields amorphous silicide layers. The results are interpreted according to the evolution in a planar binary diffusion couple, where the Si and Ti concentrations in the reacted layer are dictated by thermodynamic and kinetic arguments.

Stress in Titanium Disilicide Layers, During and After Formation.

AbstractDuring and after formation of a thin layer of titanium disilicide (TiSi2) on a silicon substrate stress is caused in several ways: Intrinsic stresses are due to the deposition process or to phase transformations and grain growth of the deposited material. Extrinsic stresses are caused by thermal effects: the difference in linear thermal expansion coefficients of the layer and the substrate respectively. Problems related to stresses can occur in semiconductor device fabrication. Stresses can deteriorate gate oxides in MOSFETs and can cause cracks in interconnect lines. Also, focusing problems in lithographic steps can occur because of wafer warpage. In this paper some examples of the different types of stress that can occur are shown and discussed. Both multilayer and self aligned Ti-Si samples have been studied: The advantage of the use of Ti-Si multilayers to produce TiSi2 is that diffusion has to proceed only over a short distance i.e., the multilayer period. So the annealing time can be short. In the self aligned silicidation process, where a layer of a titanium layer on top of a silicon substrate is annealed, the diffusion length is equal to the thickness of the Ti layer. Because longer annealing times are needed, the latter type is used to monitor stress during formation.

Amorphous Ti-Si alloy formed by interdiffusion of amorphous Si and crystalline Ti multilayers

Reactions upon rapid thermal annealing of sputtered Ti-Si multilayers have been studied by cross-section and through-foil transmission electron microscopy, glancing-angle Rutherford backscattering, and x-ray diffraction. The compositions of the samples are 40 at. % Ti, 60 at. % Si and 60 at. % Ti, 40 at. % Si, and the bilayer periodicity is about 10 nm. The silicon layers in the as-deposited films are amorphous; the titanium layers are polycrystalline hcp. After a 30-s anneal at 455 °C, significant interdiffusion occurs and we observed the formation of an amorphous Ti-Si alloy by interfacial reaction. The metastable disilicide, C49 TiSi2, nucleated along with a small amount of TiSi in the sample with higher silicon content (60%) upon annealing at 550 °C for 10 s, but the amorphous alloy remained as the only product of reaction in the 40-at. % Si sample.

Interfacial Reactions in Titanium - Silicon Multilayers

ABSTRACTWe have used cross-section and through-foil TEM, RBS, and x-ray diffraction to investigate reactions upon rapid thermal annealing of sputtered Ti-Si multilayers of various compositions and bilayer spacings. This study addresses the behavior of the Ti-Si interface at low annealing temperatures (T< 600δC) to determine the extent to which interdiffusion occurs prior to suicide formation, and to detect the formation of the first suicide at an early stage. Whereas most previous work studied a single Ti-Si interface, we have used multilayer films because a reaction occurs at many interfaces, allowing detection of a phase even if it is formed to a very limited extent.The silicon layers in the as-deposited films are amorphous; the titanium layers are polycrystalline hep. The compositions of the samples are 40 at.% Ti, 60 at.% Si and 60 at.% Ti, 40 aL% Si, and the bilayer periodicity is about 10 nm. After a 30 second anneal at 455°C, significant interdiffusion occurs and we observed the formation of an amorphous Ti-Si alloy by interfacial reaction. The metastable disilicide, C49 TiSi2, nucleated along with a small amount of TiSi in the sample with higher silicon content (60%) upon annealing at 550°C for 10 seconds, but the amorphous alloy remained as the only product of reaction in the 40 at. % Si sample. Cross-section TEM of a sample fabricated at 60°C suggests that interdiffusion had begun during deposition of the films.