Silicide Grains Research Articles

Pt Segregation at the NiSi/Si Interface and a Relationship with the Microstructure of NiSi

ABSTRACTNickel monosilicide (NiSi) is used for lowering the parasitic resistances in source/drain. However, NiSi has a disadvantage of lower thermal instability such as NiSi2 nucleation and agglomeration. We first reported Pt segregation was found at a NiSi/Si interface by Atom Probe (AP) analysis.[6] In this study, we found that the scheme of Ni silicide grain growth and the resultant NiSi crystal shape is strongly affected by the existence of Pt by utilizing AP analysis, TEM and FE-SEM. AP observations were carried on a Ni-Pt as sputterd sample and on a Ni-Pt as annealed sample. The depth profile for the sample after silicidation indicates Pt atoms are segregated at a NiSi surface and NiSi/Si interface. For the analysis of the Pt distribution in the sample after silicidation in more details, we thoroughly analyzed the 3D image as follows: a cylindrical part is extracted from the 3D image; it is divided into 5nm thick slices; and 2D images depicting density-distribution of Pt and As. From these 2D images, we found Pt atoms exist around NiSi grains. The Atom Probe result indicates that Pt atoms segregate at the NiSi surface, grain boundaries and NiSi/Si interface. Plane view TEM and FE-SEM observations were carried out on Ni(Pt) silicide and on Ni silicide without Pt to find the influence of the segregated Pt atoms on the microstructure of NiSi. The spindle shape of NiSi(w/o Pt) grains were observed on the former. On the other hand, we observed on the latter that the Pt addition affected the shape of the NiSi grains and changed NiSi grains to round polygonal shape and the average grain size became smaller. It can be said that the Pt addition suppresses a crystal growth along in a longitudinal direction of a grain. We speculate that fine round shape NiSi grains are formed as a result of the suppression of the anisotropic crystal growth by Pt segregation, and provide the improvement of the thermal stability of the NiSi film.

Impact of Ni-silicide grain orientation on the strain and stress fields induced in patterned silicon

The relationship between silicide grain orientation and strain/stress induced in patterned silicon has been experimentally investigated for structures formed by NiSi lines spaced by SiO2∕Si3N4 stacks. The strain and stress fields exhibit a significant asymmetry that has been attributed to the difference in the orientations of silicide grains at the two sides of the stacks. Finite element simulations support and complete the study, indicating that NiSi orientation can strongly affect the stress profile in the silicon between two silicide lines. These results are useful to understand at a fundamental level the buildup of stress and strain in actual transistors.

TEM study of PtSi contact layers for low Schottky barrier MOSFETs

We report investigations of the silicide formation process in the Pt/Si structure of low Schottky barrier MOSFETs on SOI. The silicide layers are used there as source and drain contacts and the high quality of the silicide/Si interface and the silicide structure are essential for the electrical properties of the device. The platinum silicide is formed by the solid-state reaction between 15nm thick Pt metallisation and a Si substrate during the annealing by the rapid-thermal-annealing (RTA) process at various temperatures (300, 400 and 500°C). Cross-sectional transmission electron microscopy (XTEM) specimens were used to study the formation of the platinum silicide. The studies enabled the determination of the silicide layer thickness and morphology, as well as the silicide/Si interface roughness. The silicide layer for all investigated temperatures consists of a single layer of the silicide grains placed side by side. After annealing at 300°C the grains are irregular in shape, while after 400°C the shapes of the most of grains are regular, visible as squares or rectangles in a cross-section. After annealing at 500°C the elongated grains due to the grain coalescence are mostly observed. The thickness of the silicide layer after annealing at 300 and 400°C is about 29–30nm. After annealing at 500°C, the thickness changes and equals from about 27–28nm at grain boundaries, to about 32–35nm in the middle of grains. The roughness of the silicide/Si interface increases with the annealing temperature. The diffraction techniques were used to identify the silicide phase. The diffraction analysis showed that only the PtSi orthorhombic phase is formed in all studied samples.

Impact of an As implant before CoSi 2 formation on the sheet resistance and junction breakdown voltage

Germanium implantation for silicon amorphization before silicide formation, for both titanium and cobalt, and its impact on morphological and electrical properties has been reported in literature. The benefits in terms of improved silicide formation on narrow lines are stress relaxation and silicided junction leakage current reduction. In this work the results of arsenic implantation instead of germanium are reported in the case of Cobalt Salicide technology for its scaling below 90 nm technology node. A baseline process without implant has been compared with arsenic implantation performed at different process conditions. As implantation before CoSi 2 growth can improve the diode breakdown voltage although a slight increase in sheet resistances is observed. Electrical results are in agreement with morphological evidences by TEM, SEM and AFM techniques showing a reduction of the silicide grain size and a smoothening of the silicide by increasing the implant energy.

Surfactant effect of Sb on the growth of MnSi1.7 layers on Si(0 0 1)

Deposition of one monolayer of Sb prior to the deposition of Mn at 600°C is observed to increase the MnSi1.7 island density by about two orders of magnitude as well as to change the crystalline orientation of the silicide grains. The preferential epitaxial orientation of MnSi1.7 grains grown by this process is determined to be MnSi1.7(100)[010]||Si(001)[100]. This growth procedure results in the silicide growth into the Si matrix. For comparison, the same deposition process carried out without Sb leads to silicide formation on top of the substrate surface. The observed morphological changes of the MnSi1.7 layers can be explained by a reduced surface diffusion of the Mn atoms on Si(001) in presence of the Sb monolayer. Additionally, lateral Si diffusion is considered to be nearly suppressed, which is responsible for the observed silicide growth into the substrate.

Thermal stability study on nanoscale polysilicide resistors

In this study, the thermal stability behavior of nanoscale polysilicide resistors with different dopant types was investigated in 90nm complementary metal oxide semiconductor process. It was found that thermal stability performance of polysilicide resistors was impacted not only by the silicide thickness but also by the relation between silicide grain size distribution and the actual poly linewidth. A quantitative equation for thermal stability was derived by the combination of thickness and linewidth. This equation showed good correlation with the activation energy of the degradation for the narrow poly width resistors.

Process and Characteristics of Modified Schottky Barrier (MSB) p-Channel FinFETs

A novel modified Schottky barrier p-channel FinFET (MSB FinFET) has been successfully demonstrated previously. In this paper, the detailed process conditions, especially the formation of MSB junctions, has been presented. Device characteristics as well as the geometry effect are also discussed extensively. In the MSB FinFETs fabricated by the two-step silicidation and implant-to-silicide techniques (ITS), an ultrashort and defect-free source/drain extension (SDE) could be formed at a temperature as low as 600/spl deg/C, resulting in excellent electrical characteristics. The ultrashort SDE could effectively thin out the SB width between source/channel during on-state or broaden and elevate it between drain/channel during off-state. A leakage mechanism of MSB FinFETs similar to the conventional ones was identified by the activation energy analysis. Strong fin width dependence of the electrical characteristics was also found in the proposed devices. When the fin width becomes larger than the silicide grain size, the multigrain structure results in a rough front edge of the MSB junction, which in turn degrades the short-channel device performance. This result indicates that the MSB device is suitable for use as FinFET. The low thermal budget of the MSB FinFET relaxes the thermal stability issue for metal gate/high-/spl kappa/ dielectric integration. It is considered that the proposed MSB FinFET is a very promising nanodevice.

Formation of Nickel Silicide Layer on Strained-Si0.83Ge0.17/Si(001) using a Sacrificial Si Layer and its Morphological Instability

Nickel silicide was formed on strained-Si0.83Ge0.17/Si(001) using a sacrificial Si capping (cap-Si) layer and its morphological characteristics were investigated. Nickel silicide layers were grown by rapid thermal annealing of the samples with the structure of Ni (\\cong14 nm)/cap-Si (\\cong26 nm)/Si0.83Ge0.17/Si(001) at the annealing temperature (TA) range of 400–800°C. The phase formation, surface and interfacial morphologies, and electrical properties of the resulting samples were characterized by various measurement techniques, including X-ray diffraction, atomic force microscopy, scanning electron microscopy, Auger electron spectroscopy, cross-sectional transmission electron microscopy, and the four-point probe method. The results showed the formation of a uniform layer nickel monosilicide (NiSi) with a thickness of \\cong30 nm at 400–550°C and sheet resistance values of 6.5–7.9 Ω/□. The sheet resistance values of the samples annealed at TA≥600°C were found to be increased, however, and this is attributed to the agglomeration of nickel monosilicide leading to discrete large-size NiSi grains. Microstructural and chemical analyses of the samples annealed at elevated temperature, TA≥750°C, indicated the formation of large agglomerated NiSi grains penetrating into the Si0.83Ge0.17/Si(001) structure and the conversion of the cap-Si layer situated in between the nickel silicide grains into an Sil-uGeu layer (u \\cong0.01–0.03), due to the out-diffusion of Ge from the SiGe layer during agglomeration. However, no NiSi2 phase was observed at these elevated annealing temperatures.

The melting diagram of the Ti-corner of the Ti–Zr–Si system and mechanical properties of as-cast compositions

Phase equilibria in the Ti-corner of the Ti–Zr–Si system were studied by the methods of metallography, differential thermal analysis, X-ray and microprobe analyses of as-cast samples. The results are given in the way of the melting diagram and of polythermal sections at 10 and 20at.% Si. At 1300°C invariant four-phase equilibrium of eutectic type L↔〈β-Ti〉(β)+〈Ti5Si3〉(Z)+〈(Ti,Zr)2Si〉 (S2) was shown to occur in the concentration interval studied. The location of the eutectic point E was determined to take place at ∼78Ti–11Zr–11Si. The ternary compound S2 formed from the melt by peritectic reaction L+Z→S2. Microhardness of the primary silicide grains and of eutectics microconstituents has shown that zirconium additions decrease the strength of the Z-phase and of the β+Z eutectic, while they do not change the strength of the S2-silicide and of the β+S2 eutectic. Based on the results of hot hardness measurements, below 550°C the potential for attractive high-temperature structural properties is controlled by refinement of the microstructure, while above this temperature the α↔β transformation temperature of Ti limits the application.

Cu/Si/Cu multilayer structures for Ohmic contact on n-type 4H-SiC substrates

Low temperature processes for Ohmic contacts to n-type 4H-SiC substrates were studied using Cu/Si/Cu multilayer structures as a function of post-annealing and metal deposition conditions. The contact structures were prepared by RF sputtering and electron-beam evaporation on top of the Si face of n-type SiC substrates with doping level of 1.1×10 19 cm −3. The deposited metal thickness was monitored during the deposition, and the ratio of the Cu and Si was varied in the experiment. A two-step annealing technology was performed for decreasing of oxidation problems occurred during high temperature processes. The first step of the annealing was long time process at low temperatures for the intermixing of Si and Cu, while the formation of Cu-silicide phases was achieved at elevated temperatures as the second step. The contact formed after the two-step annealing at 500 °C for 600 s and 850 °C for 60 s demonstrated the lowest specific contact resistivity of 1.2×10 −6 Ω cm 2. The interface between the metal contact and the SiC substrate was observed by transmission electron microscopy, and the interface turned out to have regular distribution of silicide grains with smooth morphology.

Effects of high-dose BF2+ implantation on the formation of Ti-germanosilicide on polycrystalline Si/Si0.87Ge0.13/Si layers

The high-dose BF2+ implantation, which is employed for forming resistors and lowering the extrinsic base resistance in the SiGe heterojunction bipolar transistor (HBT) integrated circuit (IC) fabrication process, is shown to have strong effects on the Ti-silicidation behavior of the polycrystalline Si/Si0.87Ge0.13/Si stack layer. As the dose of BF2+ increases from 4×1014 to 4.4×1015 cm−2, the growth and C49-to-C54 transformation of the silicide/germanosilicide layer and protrusions are highly enhanced, which is consistent with previous observations on the defect-assisted nucleation of silicide grains. However, the final sheet resistance increases with increasing dose of BF2+. Such an inconsistency between the amount of C54 phase and the sheet resistance is probably due to the formation of numerous surface voids on the silicide layer from the fluorine-containing gaseous complexes. As the number of germanosilicide protrusions increases with increasing dose of BF2+, the contact resistance also increases because the protrusions are readily etched away during contact opening and leave behind cavities within the Si/Si0.87Ge0.13/Si layer. To achieve lower values of extrinsic base and contact resistances in SiGe HBT ICs, it is desirable to replace the BF2+ with B+ as well as thicken the Si cap layer on the contact region using the selective growth process of Si.

Heteroepitaxial growth of high-k gate oxides on silicon: insights from first-principles calculations on Zr on Si(0 0 1)

Metal deposition of Zr on a Si(001) surface has been studied by state-of-the-art electronic structure calculations. The energy per Zr adatom as a function of the coverage shows, that Zr forms silicide islands even at low coverages. Adsorbed Zr is thermodynamically unstable against the formation of bulk silicide ZrSi2. The observation that the islands consist of structural elements of the bulk silicide is an indication that silicide grains will form spontaneously.

F-enhanced morphological and thermal stability of NiSi films on BF2+-implanted Si(001)

The morphological and thermal stability of conducting NiSi films formed on Si(001) are significantly enhanced by pre-implantation of the Si wafer with BF2+. In the absence of F, the maximum silicidation temperature Tmax is 650 °C; higher temperatures lead to the formation of the competing high-resistivity NiSi2 phase. Tmax, however, is increased to ⩾750 °C during NiSi formation on Si(001) implanted with 20 keV BF2+ at a dose of 5×1015 cm−2. The observed enhancement in NiSi thermal stability is due to F segregation to the silicide/Si(001) interface and silicide grain boundaries, which retards NiSi grain growth, leading to much smoother layers, and inhibits NiSi2 nucleation.

Copper Thermal Diffusion in TaN Film on Si Substrate

Thermal diffusion of copper in a Cu/TaN/Si-sub sample was investigated after annealing at 630°C for 60 min. The interface morphology, copper silicide grains and SiO2 thin layer were observed using high resolution transmission electron microscope (HRTEM). Depth profiles were obtained with an energy dispersion X-ray spectrometer (EDS) assembled in the TEM system. Based on the grain boundary theory of Fisher and the approximation solution of Le Claire, thermal diffusivity of copper into the tantalum nitride layer was estimated.

Study of CoSi 2 thermal stability improved by interfacial cavities

The interfacial properties of 70-nm CoSi 2 layers have been modulated by implanting argon or nitrogen after silicide reaction. The effect of the implant has been studied by transmission electron microscopy and sheet resistance analyses. The structural stabilisation of the silicide layer has been attributed to the formation of a band of cavities which have been located at the silicide grain boundaries. They have delayed the occurrence of the grooving process and extended the stability window by 75–100 °C. It has been found that the thermal stability improvement is related to the cavity density and size, and depends on the implanted species. The maximum increment of the thermal budget has been found at a cavity size of 22 nm and a cavity density of 2.5×10 10/cm 2.

Electrical properties of TiSi 2 clusters in poly Si

Clusters of TiSi 2 were formed in a polysilicon matrix by thermal annealing of a Ti–Si multilayer structure. The silicide grain size was varied by suitable thermal treatments. The DC electrical resistivity of the composite material has been measured in the 4–500 K temperature range by van der Pauw technique. Structural characterisation was performed by transmission electron microscopy equipped with an electron energy loss spectrometer. The dependence of resistivity on temperature indicated that at low temperature the electric transport occurs by percolation through connected TiSi 2 grains, whilst at high temperature hopping between insulated grain dominates. The volume fraction of percolating grains and the hopping probability has been derived from the fit of the experimental data.

Effects of N-induced heterogeneous nucleation and growth of cavities at the CoSi2/polycrystalline–silicon interface

N-induced cavity nucleation at the CoSi2/Si interface has been investigated for different doses and its influence on improving the thermal stability of the silicide layer up to 1075 °C has been detailed. The N-implant energy and doses have been chosen in such a way that the projected range (Rp) was near the interface and the underlying polycrystalline silicon substrate was completely amorphized. After a thermal treatment of 975 °C, it has been found that the cavity density depends on the dose, but the cavity size is dose independent. The cavity density should be enough to saturate the silicide grain boundaries but not so high to neglect ripening and coalescence. A density of 2.5×1010 cavities/cm2 has been measured at a nitrogen dose of 7×1015/cm2 corresponding to a partial saturation of the silicide grain boundaries and to the higher stability gain (150 °C).

Thermal stability of SiO2/CoSi2/polysilicon multilayer structures improved by cavity formation

The electrical and structural stabilities of SiO2/CoSi2/polysilicon multilayer structures have been improved by 75 °C using 40 keV nitrogen atoms implanted in silicon before silicidation at a dose of 5×1015/cm2. During the reaction process, small cavities have been created within the polysilicon layer and simultaneously pushed ahead by the poly/amorphous silicon interface created by the implant and moving toward the surface. The process has ended up with a band of big cavities located at the silicide/silicon interface. They have an average diameter of 35 nm and are preferentially located at the silicide grain boundaries. Sheet resistance measurements (Rs), Rutherford backscattering spectroscopy and transmission electron microscopy analyses have shown that these interfacial cavities have prevented the agglomeration process to occur up to 1025 °C 30 s annealing process and have reduced the driving force for silicide boundary grooving.

The stability of Nb/Nb5Si3 microlaminates at high temperatures

The microstructural, phase, and chemical stability of Nb/Nb5Si3 microlaminates was investigated at temperatures ranging from 1200 °C to 1600 °C. Freestanding Nb/Nb5Si3 microlaminates were prepared by sputter deposition and their stability was investigated by annealing either in vacuum or in an Ar atmosphere. The microlaminates were generally structurally stable, with no evidence of layer pinchoff, even after annealing at 1600 °C. However, a small volume fraction (<2 pct) of voids formed in the silicide layers at 1500 °C and 1600 °C, which are attributed either to the Kirkendall diffusion of Si or to the growth of silicide grains. In terms of phase stability, there was no discernible dissolution of the Nb5Si3 layers and no silicide precipitates in the Nb layers following anneals at 1400 °C. Annealing at higher temperatures, though, resulted in the formation of non-equilibrium Nb3Si on the Nb/Nb5Si3 interfaces. This phase is thought to precipitate from the supersaturated Nb-Si solid solution on cooling, and is stabilized by the development of tensile stresses in the Nb layers. The most pervasive observed high-temperature breakdown mechanism was chemical in nature, namely, the loss of Si via sublimation to the environment. The Si loss was partially suppressed either by annealing in a Si-rich atmosphere or by annealing in Ar.

The stress-induced escape of migratingaluminium from silicide interconnects

Electromigration of foreign aluminium atoms, arising fromcontact pads, along polycrystalline molybdenum silicide(MoSi2) metallic interconnect lines has been studied. TheMoSi2-lines with aluminium contact pads on both ends were exposed to electric direct current with densities of between106 and 107 A cm-2 over 2000 h. While masstransport of constituent atoms of the line material was notobserved, the atomic migration of aluminium is clearly directedfrom the cathode side toward the anode.Characteristic hillocking along the interconnect line is observed. This result is in accord with the action of apartially grain-blocked polycrystalline via due to clusters,which are present along the line. Compressive stress in frontof blocking silicide grains is found to induce a positive massflux through the line surface. Scanning electron microscopy micrographs, andenergy-dispersive x-ray analyses are applied to identifythe chemical nature of the degradation sites. It is found thataluminium from the cathode pad incorporates into theinterconnect and migrates under the action of the electronwind.