Silicidation Reaction Research Articles

Thermal stability of the Cu/Pd/Si metallurgy

The reaction between Cu and Pd/Si is studied using Cu/Pd/Si structures between 200 and 600 °C. The structural analyses show the reaction between Cu and Pd/Si between 200 and 300 °C, with both Pd and Si diffusing into Cu, and Cu accumulating inside the Si region under the Pd2 Si layer formed. An extensive reaction between Cu and Pd2 Si is observed at 400 °C, forming Cu silicides and Cu3 Pd. The instability of the Cu/Pd2 Si metallurgy is comparable to those of Al/Pd2 Si and Cu/PtSi. In this regard, the difference in stability between the Cu/Pd/Si and Cu/Pt/Si structures is much smaller than that between the Al/Pd/Si and Al/Pt/Si ones. For the latter structures, a difference of about 100 °C is observed, with or without common barrier layers, with Al/Pt/Si being the more stable one. The ready formation of both Cu silicides and Cu3 Pd contributes to the instability of the Cu/Pd/Si structure. The Cu silicide reaction is also responsible for the low thermal stability of the Cu/Pt/Si structures.

The formation of titanium silicides by rapid thermal processing: an XRD and AES study

X-ray diffraction and Auger depth profiling were performed to investigate the reaction of Ti thin films with single crystalline (100) Si by rapid thermal annealing under N 2 and Ar. Within a few seconds titanium silicides are formed at temperatures between 620 and 700°C. The silicides consist of TiSi 2, TiSi and titanium-rich subsilicides like Ti 5Si 4 and Ti 5Si 3. TiSi 2 is the metastable, poorly-conducting C49 modification. Growth of this phase takes place near the substrate interface, and the Ti-rich silicides are situated closer to the surface. The crystallites of the different silicides exhibit a strong texture. Annealing under N 2 leads to the formation of titanium nitrides TiN x near the surface and subsequent growth towards the suicide during prolonged thermal treatments. When the silicide and nitride reaction fronts coalesce the available Ti is consumed for these reactions. Further silicide growth can occur only via complex phase transformations.

L II edge spectroscopy on MoSi x layers

MoSi 2 is one of the main silicides used in semiconductor technology. In the present synchrotron study the local environment around the Mo atom in various Mo-Si systems was investigated by analyzing the EXAFS above the Mo L II edge. Because of the interference of the L II EXAFS with the underlying L III EXAFS all data were compared to simulations to extract the undisturbed L II edge EXAFS information. Differences in electronic structure are visible in the near edge region (XANES). For an in situ evaporated Mo film on Si(100) we found a silicide reaction at room temperature resulting in a Mo-Si distance R = 0.249 nm and a coordination number N = 6.5. Both values are less than those of MoSi 2 ( R = 0.262 nm, N = 10). Using a native silicon oxide as a substrate the evaporated film is not of silicide like structure. The EXAFS of cosputtered MoSi x ( x = 2 − 3) layers, which showed no X-ray diffraction lines after deposition, reveal a short range order with a Mo-Si distance R = 0.256 nm and N = 6.5. There is no evidence of metal clusters in these amorphous layers. Annealing at about 1000 K for 3 min leads to the formation of a local MoSi 2 structure.

Processes in implanted silicon stimulated by silicidation reactions

The extensive application of metal-silicon compounds (silicides and alloys) in solid state technology makes it necessary to study in detail the reactions of metal-silicon interaction and the processes in silicon depth induced by these reactions. The aim of the present study was to examine the effect of silicidation on the implanted silicon surface on the impurity redistribution and structural rearrangement in the silicon bulk. On the basis of the comparision between the electronegativities of the elements in the reaction zone, a rule has been suggested to predict the direction of impurity migration. The enhanced diffusion of impurities during silicide formation has been studied. It has been established that the enhanced diffusion of dopants (P, As, Sb) is observed independently of the silicide formation method — either thermal annealing or ion mixing without heating. Thus the impurity enhanced diffusion is stimulated by silicidation reactions. It has also been found that silicidation the rearrangement of the secondary radiation defects, namely it leads to a drastic decrease in the number of silicon interstitials contained in secondary defects and to the reduction in the average density of secondary defects. The enhanced diffusion and the rearrangement of secondary defects have been suggested to be due to the generation of excess point defects in the silicidation reaction zone and then injection into the silicon bulk. An approach to the description of various effects of surface reactions (silicidation, oxidation, nitridation, etc.) on the processes in the silicon bulk is suggested.

Chemical and physical processes during the formation of MoSi 2 by ion-beam mixing

The formation mechanism of MoSi 2 layers fabricated by implanting arsenic ions through 40–70 nm of molybdenum into silicon was studied. The implantation parameters were varied (dose 5×10 14 to 3×10 16 cm −2, current density 1.1 to 22.6 μA cm −2), and their influence on the formation rate of the silicide was studied. While below 360° C, the thickness of the intermixed molybdenum layer is proportional to the square root of the implanted dose, at higher temperatures, it is proportional to the dose itself. This can be explained by different mechanisms in the different temperature regions. Whereas at low temperatures, physical effects dominate (ballistic mixing), at higher temperatures, molybdenum and silicon react chemically. In the low temperature region, the mixing can be simulated using a dynamic Monte Carlo code (TRIDYN). At high temperatures, the silicidation reaction has an activation energy of 0.3 eV which is much lower than the activation energy of a thermal activated process. This behavior can be explained by the effect of thermal spikes which enhance the temperature by about 1600–1700 K. Rapid annealing in N 2 ambient after implantation resulted in a specific resistivity of 80 μΩ cm .

Silicide formation in metal/Si structures and diffusion barrier properties of CVD tungsten films

Tungsten films were deposited on Si substrates by the H2 or Si reduction of WF6 under various experimental conditions. The composition and structure of as-deposited samples as well as the interfacial reactions and interdiffusion of elements in annealed samples were characterized by nuclear reaction analyses, sheet resistance measurements, x-ray diffraction technique, and Rutherford backscattering spectroscopy. The amount of oxygen at W–Si interfaces was found to be dependent on the cleaning treatment of the Si surface used before WF6–Si interaction. The interfacial oxygen concentration was less than 1 ⊠ 1014 at./cm2 (detection limit of the nuclear reaction analysis) and (2–7) ⊠ 1016 at./cm2 using an HF cleaning and the RCA treatment, respectively. For W/Si samples, the formation temperature of WSi2 was dependent on the dopant level in the Si substrates and the oxygen concentration at W–Si interfaces. The silicidation reaction occurred at 625 °C in “oxygen free” W/Si structures while for structures containing interfacial oxygen atoms, this reaction occurred above 800 °C. In Al/W/Si structures, the intermetallic compound, WAl12, was formed by annealing at 450 °C for 90 min. Furthermore, the formation of WSi2 was observed in structures annealed at a temperature in the range of 550 °C–600 °C regardless of the oxygen concentration at the W–Si interface. A model to explain the effect of interfacial oxygen atoms on the silicidation reaction and the influence of the Al overlayer on the thermal stability of Al/W/Si structures is proposed and discussed in this paper.

Elimination of End‐of‐Range Shallow Junction Implantation Damage during CMOS Titanium Silicidation

Interstitial dislocation loops present in shallow junctions formed following Ge preamorphization and rapid thermal annealing have been eliminated using titanium silicide. The dissolution of these end‐of‐range defects is attributed to the injection of vacancies during silicidation. The size and number of the residual extended defects were reduced in both p+ and n+ junctions after the formation of . The silicide sheet resistance is a measure of the amount of silicide reaction and concomitant vacancy injection. The total elimination of these defects was observed in shallow p+ junctions for sheet resistance of below 3 Ω/□. Leakage current reduction in silicided p+/n junctions, was correlated with the defect reduction.

The influence of an aluminum overlayer on the interaction of tungsten films with silicon substrates

The interactions of tungsten films with silicon and aluminum during annealing treatments of W/Si and Al/W/Si structures have been investigated using sheet resistance measurements, nuclear reaction analyses, x-ray diffraction techniques, Rutherford backscattering, and Auger electron spectroscopy. W(H) and W(Si) films, 100–200 nm thick, were deposited via the H2 reduction of WF6 on undoped single-crystal Si wafers and the Si reduction of WF6 on As implanted Si wafers, respectively. Prior to the metal deposition, As implanted Si wafers were cleaned by dipping in NH4OH:H2O2 followed by HCl:H2O2 at 80 °C for 20 min (RCA cleaning treatment). The oxygen content at the metal–Si interface in W(Si)/As implanted Si structures can reach 3 at. % whereas in W(H) /mono-Si structures, the metal–Si interface is oxygen free. In W/Si samples, the formation temperature of WSi2 was found to be dependent on the interfacial oxygen concentration. For oxygen-free W/Si structures, the silicidation reaction occurred at 625 °C while for structures containing interfacial oxygen atoms, this reaction was observed above 800 °C. In Al/W/Si structures, the intermetallic compound WAl12 was formed by annealing at 450 °C for 90 min. Tungsten atoms diffused in the aluminum overlayer and islandlike structures of WAl12 surrounded by unreacted Al appeared at the surface of samples. Furthermore, the formation of WSi2 was observed in the Al/W/Si structures annealed at a temperature in the range of 550–600 °C regardless of the oxygen concentration at the W–Si interface. The mechanisms of WAl12 and WSi2 formation are analyzed and the role of interfacial oxygen atoms in the silicidation reaction is demonstrated. The origin of the diminution in thermal stability of W–Si interfaces in Al/W/Si structures is also discussed in this paper.

Control of pinholes in epitaxial CoSi2 layers on Si(111)

The growth of ultrathin (<50 Å thick) uniform CoSi2 layers at low temperatures (<450 °C) has been reported recently. Pinholes are formed in these silicide layers when the temperature is raised to above ∼550 °C. An important driving force for the generation of pinholes has been identified as a change of the surface structure from CoSi2-C, stable at low temperature, to the high-temperature stable CoSi2-S. Treatment of the surface of CoSi2 facilitates this transition and prevents the formation of pinholes. A few important parameters in the silicide reaction are shown to govern the morphology of the reacted CoSi2 layers.

The Promotion of Silicide Formation using a Scanned Silicon Ion Beam

ABSTRACTThe promotion of silicide reactions at the interface between silicon and a metal overlayer is described, the reactions being initiated by scanned ion beams. The relative effects of low and high energy Si+ and Si2+ beams are discussed and the results of subsequent annealing are compared with those seen when using low energy (5keV) argon ion beams. The implications for the writing of metallisation lines are also noted.

Simultaneous formation of TiN and TiSi2 by lamp annealing in NH3 ambient and its application to diffusion barriers

The dependence of TiN/TiSi2 bilayer formation on Si by lamp annealing of Ti upon annealing temperature, ambients, and impurity in Si was investigated. In NH3 ambient, nitridation ratio on undoped Si at 680 and 800 °C is 40% and 25%, respectively. When annealing is performed in N2, the nitridation ratio of Ti is only about 25% at both 680 and 800 °C. The above dependence of nitridation ratio on annealing ambient is due to the difference in decomposition energy of gas molecules. The nitridation ratio increases on arsenic-implanted Si because of a retardation effect of the arsenic on the silicidation reaction. The bilayer formation process was applied to the AlSi contact metallization on the n+ diffused layer. From measurements on electrical characteristics of the AlSi/TiN/TiSi2/n+ Si contact system, the following results were obtained: (1) Contact resisitivity is 3.5–5.3×10−7 Ω cm2. (2) The TiN/TiSi2 contact electrode is thermally stable against Al. In particular, when the bilayer is formed in NH3 at comparatively lower temperature, its thermal stability is still further improved. This is due to the formation of the thicker surface TiN layer.

Low Temperature Rapid Thermal Formation of Tin Barrier Layers

ABSTRACTTiN and TiN/TiSi2 films, formed by reacting titanium sputtered onto source-drain and field areas, may be used as local interconnects, low resistance plated junctions, and diffusion barriers. This paper discusses the application of RTP to the low temperature reaction of titanium on silicon and oxide substrates. By adjusting temperature and ambient the nitridation and silicidation reactions may be controlled to provide the desired TiN-TiSix thickness ratio. The advantages of ammonia ambient are discussed.

Trends in ion implantation in silicon VLSI technology

A brief review is presented on the emerging difficulties of implantation in manufacturing small geometry VLSIs of the future. The importance of understanding the physical phenomena associated with the behavior of implanted ions and generated disorders is reflected in the design and fabrication of accurately controlled depth and lateral profiles of shallow and submicron size junctions. As a particular example, the complexity of implantation and mixing involved in a solid phase silicidation reaction is described. New technology fields such as MeV range high-energy implantation, finely focused beam implantation, and heavy dose oxygen or nitrogen ion implantation are also reviewed briefly. Differences between the physical processes involved in the new field of implantation and those of conventional technology are presented with a short description of machine trends in the field.

Enhanced grain growth of phosphorus-doped polycrystalline silicon by titanium silicide formation

The influence of silicide reaction on the grain growth behavior of phosphorus-doped polycrystalline silicon has been studied. Intrinsic and phosphorus-doped polycrystalline silicon, with and without evaporated titanium overlayer, were prepared by low pressure chemical vapor deposition on thermally grown SiO2 on silicon wafer. These samples were then annealed in He ambient at 700 °C for various time. The Rutherford backscattering spectrum showed that TiSi2 formed completely after 30 min anneal. Cross-sectional transmission electron microscopy showed that Kirkendall voids exist at the interface between TiSi2 and polycrystalline silicon, indicating that silicon is the predominant diffusing species. Enhanced grain growth as a result of silicide reaction was observed in phosphorus-doped polycrystalline silicon. The enhanced grain growth of polycrystalline silicon is suggested to be due to the redistribution of dopant, analogous to the alloying and dealloying processes in diffusion induced grain boundary migration.

A self-aligned cobalt silicide technology using rapid thermal processing

The feasibility of a self-aligned silicide technology based upon cobalt has been investigated. Silicidation reactions were performed by means of rapid thermal processing. Phase sequence, layer morphology, and reaction kinetics were studied by XRD, SEM, RBS, AES, and TEM. Extremely smooth, highly conductive (16 μΩ cm) CoSi2 films were formed by direct reaction of Co on Si, without significant lateral silicide formation at oxide edges. Shallow arsenic junctions were successfully silicided and low contact resistances were obtained on n+ and p+ Si.

The formation of titanium silicide by arsenic ion beam mixing and rapid thermal annealing

The reaction of titanium thin films with n+ doped single crystal silicon induced by As+ ion beam irradiation and subsequent rapid thermal annealing has been studied. Arsenic doping in the silicon substrate retards the thermal growth of silicide resulting in an incomplete conversion of the titanium into titanium disilicide during low temperature (∼600 °C) rapid thermal annealing under a nitrogen ambient. High energy As+ion beam irradiation (90–170 keV) prior to the rapid thermal annealing leads to intermixing of the titanium and silicon which, at sufficiently high energy and dosage, results in complete conversion of titanium to a silicide phase. Subsequent rapid thermal annealing of the ion beam irradiated samples induces further uptake of silicon into the silicide phase leading to the formation of the stable C-54 titanium disilicide phase. However, very little additional titanium consumption occurs during the thermal annealing. The silicide formed by the ion beam mixing process shows superior sheet resistance uniformity and has a smoother morphology compared to thermally grown silicide. The uniformity of the silicidation reaction by ion beam mixing also leads to superior n+p junction characteristics.

Thermal instability of Schottky diode barrier heights modified by inert ion sputter-etching damage

The barrier height of Schottky diodes made to dry-etched silicon surfaces differs from those fabricated on wet chemically etched silicon. Some have suggested utilization of this phenomenon to yield diodes for VLSI applications that display barrier enhancement to p-type silicon and barrier reduction to n-type silicon together with good diode ideality factors; however, it is shown here that the modified barrier heights can be dramatically unstable even when exposed to relatively low-temperature cycling, thus rendering this technique of enhancing the barrier height to p-type silicon and reducing the barrier height to n-type silicon inappropriate for conventional VLSI fabrication. It is hypothesized that the barrier instability is due to a localized silicide reaction or surface reconstruction occurring at the metal/silicon interface.

Interaction of Ti with C- and SiC-contaminated Si surfaces

The interaction of Ti with C-contaminated Si surfaces has been studied using Auger electron spectroscopy, x-ray photoemission spectroscopy, and transmission electron microscopy. C contamination due to wet chemistry residues and thin SiC films has been investigated. Our results show that Ti reacts with exposed C to produce TiC upon deposition for both these forms of contamination. The C residues were found to neither impede the silicide reaction nor remain at the interface afterwards. However, SiC thin films (5 Å) prepared by annealing the Si surface in ethylene, blocked silicide formation, resulting in a highly nonuniform interface. The carbide phases formed before and after silicide formation agree with those predicted by a calculated Ti-Si-C ternary-phase diagram.

Influence of P and As implantation of the formation of MoSi2

Molybdenum disilicide films have been formed by a reaction of Mo with polycrystalline silicon. Implantation of phosphorus or arsenic prior to reaction has been shown to have a large effect on the silicidation process. The object of implantation was to enhance the silicide reaction by creating damage at the metal-silicon interface. This, then, results in silicide films with less surface roughness (and a lower electrical resistivity). Molecular P+2 ions were found to be more effective in creating damage than single P+ ions, due to the fact that for P+2 ions two simultaneous collision cascades occur which overlap. Apart from the beneficial effect of implantation damage, the introduction of phosphorus has a deleterious effect on the properties of the silicide films. This phenomenon is explained by the presence of a thin and nonuniform native oxide layer which hinders the silicide reaction. Without addition of phosphorus, new nuclei for the silicide reaction can be formed due to stress introduced by the formation of the silicide, which will act upon the oxide and cause it to break up at some weak spot. The effect of phosphorus is proposed to be that it renders the oxide more viscous which prevents this process from taking place. The x-ray diffraction spectra revealed that the orientation of the grains in the polycrystalline MoSi2 films is influenced by the method of preparation.

Thermal Compatibility Studies of Unirradiated Uranium Silicide Dispersed in Aluminum

Powder metallurgy dispersions of uranium silicides in an aluminum matrix have been developed by the international Reduced Enrichment for Research and Test Reactors program as a new generation of proliferation-resistant fuels. A major issue of concern is the compatibility of the fuel with the matrix material and the dimensional stability of this fuel type. A total of 45 miniplate-type fuel plates were annealed at 400/sup 0/C for up to 1981 h. A data base for the thermal compatibility of unirradiated uranium silicide dispersed in aluminum was established. No tested modification of a standard fuel plate showed any significant reduction in plate swelling. The cause of thermal growth of silicide fuel plates was determined to be a two-step process: (a) the reaction of the uranium silicide with aluminum to form U(AlSi)/sub 3/ and (b) the release of hydrogen and subsequent creep and pillowing of the fuel plate.