CoSi2 Films Research Articles

The effect of germanium on the Co-SiGe thin-film reaction.

Ge Co Germanium was found to have a strong influence on the path and products of the Co-SiGe reaction, and on the interfacial stability and crystallographic orientation of the silicide film. The segregation of Ge that occurs during the reaction of blanket Co films with SiGe results in thickness effects not present in the reaction of Co with Si. The thickness effect was modelled in terms of the energy cost of Ge segregation, and good agreement with experimental results was obtained. In s i tu EXAFS experiments on sub-monolayer Co films annealed on SiGe substrates indicate a strong preference for the formation of Co-Si bonds at the silicide-SiGe interface. The implications of these results for the stability of the interface and the epitaxial orientation of co-deposited cobalt disilicide (CoSi2) films will be discussed.

Effects of stress on the growth of TiSi2 thin films on (001)Si

Tensile stress induced by backside CoSi2 films on a silicon substrate has been found to enhance the growth of C54–TiSi2 on (001)Si. In contrast, compressive stress induced by backside oxide films on the silicon substrate was found to retard significantly the growth of C54–TiSi2 on (001)Si. For Ti on stressed (001)Si after rapid thermal annealing at 800 °C for 30 s, the thickness of the C54– TiSi2 films was found to increase and decrease with the tensile and compressive stress levels, respectively. The retarded growth is attributed to the hindrance of the migration of Si through the Ti/Si interface by the compressive stress. On the other hand, the presence of tensile stress promotes the Si diffusion to facilitate the formation of Ti silicide thin films.

Effect of Ti-capping thickness on the formation of an oxide-interlayer-mediated-epitaxial CoSi2 film by ex situ annealing

A modified oxide mediated epitaxy process using a single deposition and ex situ annealing by Ti capping has been suggested in this study. It has been shown that in the case of pure Co on SiOx-covered Si, the reaction between Co and Si did not occur up to 800 °C during ex situ annealing. However, Co silicidation occurred in the case of Ti-capped Co on SiOx-covered Si. The crystalline nature of CoSi2 formed in this case strongly depends on the Ti capping thickness. When a thin Ti capping layer of thickness less than 5 nm was used, Ti oxidation occurred nonuniformly, and the morphology of the surface Ti oxide layer was very rough. This caused an exposure of Co to the oxygen in the ambient, resulting in the formation of polycrystalline CoSi2 due to the suppressed Co diffusion towards the Si substrate. In the case of Ti capping thickness being more than 10 nm, however, a uniform Ti oxide surface layer, which blocks the incoming oxygen retarding Co diffusion, was formed, and it led to uniform Co diffusion into the Si substrate, resulting in epitaxial CoSi2.

Formation and Characterization of Spe Grown Ultra-Thin Cobalt Disilicide Film

AbstractUltra-thin epitaxial CoSi2 films formed by Co(3∼5nm)/Ti(1 nm)/Si(100) and Co(3∼5nm)/Si(lnm)/Ti(Inm)/Si are studied. The multilayers are deposited by ion-beam sputtering. Rapid thermal annealing (RTA) is used for silicidation. XRD, RBS, TEM, AFM, four-point probe, I-V and C-V measurements are carried out for characterization. The XRD spectra show the CoSi2 film formed by Co/Ti/Si or Co/Si/Ti/Si solid phase epitaxy has, epitaxial characteristic. XTEM shows that the film is continuous. RBS/Channeling shows that the formed CoSi2 has sharp interface with a minimum channeling yield of Co signal of 40%. AFM shows that the surface of ultra-thin CoSi2 film is smooth with a roughness of nearly 0.7 nm. The Rs∼T relationship shows that the CoSi2 films formed by Co/Si/Ti/Si reaction have the best thermal stability (stable up to 900°C). Those formed by Co/Ti/Si reaction are stable up to 850°C, while those formed by Co/Si reaction are only stable up to 750°C. By fitting the experimental I-V and C-V curves of the epitaxial CoSi2/Si Schottky diodes, barrier heights of around 0.6 eV and close to unity ideality factors are obtained.

In-Situ Study of the Oxide Mediated Epitaxy of CoSi2 on Si

AbstractOxide Mediated Epitaxy (OME) shows promise as a method to form good quality, thin epitaxial CoSi2 films on most Si surfaces. We have performed an in-situ study of the OME of CoSi2, on the Si (001) surface. Our work was carried out with our specially modified ultra-high vacuum transmission electron microscope (UHV TEM) SHEBA (Surface High Energy Electron Beam Apparatus). With SHEBA we were able to monitor the diffraction pattern and therefore the phase formation throughout the anneal. Our results confirm the suppression of intermediate phases during CoSi2 formation in the OME process. We also see a difference in the as deposited Co film when the oxide coated silicon surface is used rather than a clean substrate. From combined imaging and diffraction studies we will shed some light on the mechanism behind the success of OME.

Introduction to manufacturing processes

Using the Boltzmann approach we have developed one dimensional (1D) model of metal films conductivity for the quantum electron transport. Thickness fluctuation of film has an effect on the electron spectra via carriers scattering in quantum size regime. In the framework of the developed model size dependent conductivity of metal films has been calculated. Here, we employed above mentioned model to explain experimental data on single crystalline CoSi2 films and fine-grained gold films. Wide film thickness ranges applicable by the described model cover also the crossover from quantum to quasiclassical electron transport.

Thermal Studies on Stress-Induced Void-Like Defects in Epitaxial-CoSi2 Formation

AbstractWe report experimental findings on the thermal-induced stress-voiding observed during epitaxial CoSi2 formation on patterned Si(100) wafers associated with the Co/Ti-interposing layer scheme [1–6]. The first rapid thermal process (RTP) in a typical SALICIDE process was studied by varying the temperature and the RTP ramp-up rate. Based on ex-situ cross-sectional transmission electron microscopy (XTEM), crystal defects were observed at shallow trench isolation (STI)/Si and gate spacer/Si edges after annealing at temperatures greater than 520°C. In addition, isothermal anneals for various time periods resulted in increased encroachment of an ultra-thin CoSi2-phase at the STI/Si edge. Higher temperature anneals also resulted in a corresponding increase in void size. In addition, higher ramp-up rates led to a thicker CoSi2 “pocket” film at the tensile-stressed STI/Si edge, aligned epitaxially along the {111}-habit planes. Our attempts to bypass the CoSi formation in order to alleviate the anomalous Si diffusion was found to be irrelevant to the voiding issue, due to the inherent nature of this film stack system to nucleate CoSi2 as the first phase. By comparing our experimental observations of voiding with Hu's analysis of the trench isolation stress fields, we found good agreement with the predicted stress field distribution in the active Si and STI/Si edge with the locations of void nucleation. The anomalously large stress induced is still not known, but can be attributed to the encroaching CoSi2 film which tends to realign epitaxially along the STI/Si interface.

The Influence Of Stress on The Growth of Titanium Silicide Thin Films on (001) Silicon Substrates

AbstractThe influence of stress on the enhanced formation of C54-TiSi2 phase has been investigated. Tensile stress induced by backside CoSi2 film on the silicon substrate has been found to enhance the growth of C54-TiSi2 on (001)Si. The thickness of amorphous interlayers (a-interlayers) between Ti films and silicon substrates was found to be thicker and thinner in the tensilly and compressively stressed samples, respectively. From auto-correlation function analysis, the thicker a-interlayer was found to consist of a higher density of crystallites. The crystallites provide nucleation sites for C49-TiSi2 and facilitate the formation of C49-TiSi2 of small size. The larger total area of C49-TiSi2 grain boundaries supplies more nucleation sites for the phase transformation of C49- to C54-TiSi2. Therefore, the tensile stress present in the silicon substrate promotes the formation of a-interlayer and decreases the grain size of C49- TiSi2, which increases the nucleation density of the C54-TiSi2 phase. As a result, the transformation of C49- to C54-TiSi2 phase is enhanced.

Mechanism of Improved Thermal Stability of Cobalt Silicide Formed on Polysilicon Gate by Nitrogen Implantation

It is found that the thermal stability of CoSi2 films is improved by N2+ implantation (I/I) because the grain size of CoSi2 films with N2+ I/I is much smaller than that without N2+ I/I. Rutherford backscattering spectrometry (RBS) and Auger electron spectroscopy (AES) analyses show that the nucleation of CoSi2 transformation from CoSi is suppressed by N2+ I/I. The nucleation temperature for the sample with 5×1015 cm-2 N2+ I/I is about 25°C higher than that without N2+ I/I. Since the nucleation temperature is increased, CoSi2 films are formed incompletely at the as-formed state; thus, CoSi2 has small grain size. As a result, the thermal stability of CoSi2 films is significantly improved by N2+ I/I due to the small grain size of CoSi2.

Formation of CoTi barrier and increased thermal stability of CoSi2 film in Ti capped Co/Si(100) system

We investigated the formation of CoSi2 for Ti capped Co on (100) Si substrate with emphasis on the Co–Ti interaction and its effect on thermal stability. A 15 nm thick Ti capping layer is shown to improve the interfacial roughness and thermal stability of CoSi2 film grown on Si substrate compared with TiN capping. The increased uniformity of silicide/Si(100) interface is speculated to result from retarded Co–Si reaction by the formation of CoTi binary phase. And the high thermal stability can be explained by the fact that the amount of Ti atoms in CoSi2 film for Ti capping is much higher than what is in TiN capping. It is likely that the surface Ti diffuses rapidly into CoSi2 grain boundaries and slows down the agglomeration process, thereby increasing thermal stability while Ti in TiN capping did not.

Pinhole formation in solid phase epitaxial film of CoSi2 on Si(111)

The long-standing pinhole problem in solid phase epitaxial growth of a CoSi2 film on Si(111) has been revisited with in situ scanning tunneling microscopy. While the as-deposited film with 5 Å of Co at room temperature shows a smooth granular texture with original substrate terraces remaining intact, annealing at 580 °C produces an epitaxial CoSi2 film with large pinholes enclosed by a thin ring CoSi2, exhibiting a volcano feature. Quantitative analysis shows that the formation of pinholes is a result of rapid Si outward diffusion from bulk to surface, and of the subsequent Si reaction with Co on the outer surface. Evidence suggests that inhibiting the Si diffusion channels during the thermal annealing process is the key to solving the pinhole problem.

Recent progress on BEEM

The application of ballistic-electron-emission microscopy (BEEM) and spectroscopy (BEES) to epitaxial CoSi 2/Si interfaces is reviewed. Dislocations in 2–3 nm thick CoSi 2(0 0 1)/Si(0 0 1) films with Burgers vectors b = a/4〈1 1 1〉 are shown to lower the barrier height measured at 77 K by almost 0.1 eV. This corresponds to a decrease of the Schottky barrier height at the metallurgical interface from the value of Φ B=0.74±0.03 eV, characteristic for defect-free regions, to near zero within the range of their strain field of a few nanometers. By contrast, the dislocations of type b = a/6 〈1 1 2 ̄ 〉 present at CoSi 2/Si(1 1 1) interfaces do not affect the barrier height. Films of CoSi 2/Si(1 1 1) are more suitable for studying interfacial scattering by BEEM because of their simpler surface structure and because of band structure effects. Here individual point defects can be resolved. Their spatial distribution indicates diffusion along the interface during film growth. By analysing the shape of BEES spectra obtained on isolated point defects, and by making use of the projected band structure predicting the BEES current to set in ∼0.2 eV above the Schottky barrier, it may be concluded that the defects are located at the interface.

Native point defects in Si with a CoSi2 film: Single crystal versus polycrystalline

We have determined the native point defects concentrations in silicon with single and polycrystalline CoSi2 films by annealing Sb and B doping superlattices at 850 °C/60 min in N2. The polycrystalline film results in enhanced Sb diffusion in Si, indicating a vacancy supersaturation (CV/CV*)∼2.4±0.8, while the single crystal film maintains a vacancy concentration near equilibrium. Boron diffusion is retarded by the same amount by both films, indicating an interstitial undersaturation (CI/CI*∼0.3±0.1). This directly implies that CoSi2 grain boundaries are the cause of the higher vacancy supersaturation that is well known to occur during standard Co silicidation.

Structure of Epitaxial CoSi2 Films on Si(111) Studied with Low-Energy Electron Diffraction (LEED)

Expitaxial films of CoSi 2 on Si(111) were investigated by low-energy electron diffraction. Films of approximately 12 Å thickness were prepared by simultaneous deposition of Co and Si and subsequent annealing. The films were found to crystallize in CaF 2 structure in (111) orientation. Two (1×1) phases of different stoichiometry exist. The surface phase that contains more Co is found to be a CoSi 2(111) bulklike structure terminated by a Si–Co–Si trilayer. The Si-rich phase is terminated by an additional nonrotated silicon bilayer with the lower silicon atoms bound to cobalt in the first CoSi 2 layer. Consequently, these cobalt atoms have an eightfold coordination. Due to the lattice mismatch the silicide films are expanded by 0.5% in the lateral direction and contracted by 1.4% in the vertical direction.

Silicide Technology in Deep Submicron Regime

ABSTRACTSilicide technology using cobalt-titanium alloy has been developed for sub-quarter micron devices. Extremely flat and epitaxial CoSi2 films are successfully grown on a (100) Si substrate. Improved SALICIDE process is not influenced by native oxide on a Si surface. This technology will be very useful in deep sub-quarter micron devices.

Reduction of Electrical Resistance of Nanometer-Thick CoSi 2 Film on CaF 2 by Pseudomorphic Growth of CaF 2 on Si(111)

1.9-nm-thick epitaxial metal (CoSi2) films were grown on relaxed or pseudomorphic CaF2/Si(111) and their electrical resistance was measured. It was found that the electrical resistance of the CoSi2 film on pseudomorphic CaF2 was about half of that on relaxed CaF2. This result can be attributed to the improved of flatness and crystalline quality of the CoSi2 by using of pseudomorphic CaF2 instead of relaxed CaF2 due to the flat pseudomorphic CaF2 surface and the small lattice mismatch between CoSi2 and pseudomorphic CaF2.

Cobalt disilicide buffer layer for YBCO film on silicon

The CoSi2 films were used as buffer layers of YBCO/CoSi2/Si(100), YBCO/ZrO2/CoSi2/Si(100) and YBCO/CeO2/YSZ/CoSi2/epi-Si/Al2O3 heterostructures in this work. Transition temperatures of YBCO films were obtained up to 86K for YBCO films deposited by laser ablation on the top of CeO2/YSZ/CoSi2/Si/Al2O3 structure. Local nucleation on the crystal defects of silicon, the phenomenon of lateral directed growth (DLG) and agglomeration of CoSi2 phase are responsible for grain boundaries (GB) position in CoSi2 layer and its roughness. The roughness was decreased using an additional Zr film on the top structure.

Formation of self-aligned CoSi2 on selective epitaxial growth silicon layer on (001)Si inside 0.1–0.6 μm oxide openings prepared by electron beam lithography

The self-aligned formation of CoSi2 was achieved on the selective epitaxial growth (SEG) silicon layer on (001)Si inside 0.1–0.6 μm oxide openings prepared by electron beam lithography. The uniform, high quality SEG Si layer was grown by ultrahigh vacuum chemical vapor deposition at 560 °C with Si2H6. Self-aligned CoSi2 film without lateral growth of silicide was grown on the SEG Si layer by rapid thermal annealing at 700 °C in N2 ambient. The successful integration of the self-aligned CoSi2 and SEG of Si processes promises to be a viable process technology for the future deep submicron devices.

Formation of metastable epitaxial CoSix (x<2) layers by reactive codeposition on CoSi2(111)

The reaction of Co deposited at 350 °C on epitaxial CoSi2 is investigated by means of low-energy electron diffraction, core-level and angle-resolved valence-band photoemission, and ion scattering spectroscopy. Co is deposited onto ∼100-Å-thick CoSi2(111) films epitaxially grown on Si(111) which exhibit a Si-rich surface. For one Co monolayer equivalent, the Si-rich surface labeled CoSi2(111)–Si is converted into a bulklike terminated one, labeled CoSi2(111). The latter is characterized by a specific surface state located at ∼2.8 eV binding energy at the center of the surface Brillouin zone, which derives from dangling bonds on Co and Si in the topmost planes. Upon increasing the amount of Co, the latter is found to react with the silicide layer up to much larger depths comparable to the photoelectron mean free path (15–20 Å). A thin metastable epitaxial CoSix (x≤2) overlayer with a cubic structure is formed for Co amounts up to the equivalent of 15 monolayers, typically. The electronic as well as crystallographic structure of this thin metastable CoSix layer is compared to that of recently discovered metastable CoSix layers, epitaxially grown on Si(111) by codeposition and found to be closely related. The data strongly suggest a simple picture where the additional Co merely diffuses into the initial CaF2-type CoSi2 layer and occupies part of the interstitial octahedral sites of the face centered cubic Co sublattice. Moreover, it is shown that upon reaction of 6–8 Co monolayers the surface state observed on the ideal-bulk–CoSi2(111) termination still persists but is shifted toward higher binding energies due to changes in the composition of the silicide underneath. This indicates that the surface structure of the CoSix (x≤2) layer is also similar to that of bulk-terminated CoSi2(111). For higher Co coverages, an epitaxial ε-CoSi layer with the stable B20 type structure is achieved, as attested by a ∛×∛ R30° low-energy electron diffraction pattern and specific valence-band features.

Growth of epitaxial CoSi 2 films on strained [formula omitted] heterostructures

Thin epitaxial films of CoSi 2 have been grown on strained Si 1 − x Ge x Si(001) heterostructures with Ge contents between 20 and 25 at% by molecular beam epitaxy (MBE). Single-oriented CoSi 2(001) films of high crystalline quality ( x min = 4.6%) and smooth surface morphology with low resistivities (17 μΩ · cm) could be obtained using a special template method and an additional thin silicon sacrificial cap layer on top of the Si 1 − x Ge x Si(001) heterostructure. The films were characterized using RHEED, LEED, in-situ AES, SIMS, RBS, SEM, TEM and sheet resistance measurements. The results are compared with films grown on mere Si(001) substrates and on Si 1 − x Ge x Si(001) heterostructures grown without the use of a silicon cap layer. It is demonstrated that a silicon sacrificial cap layer is essential for the growth of high-quality single-domain CoSi 2(001) films on Si 1 − x Ge x Si(100) heterostructures.