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INTRODUCTION

Metal silicide thin films are integral parts of all microelectronics devices. They have been used as ohmic contacts, Schottky barrier contacts, gate electrodes, local interconnects, and diffusion barriers. With advances in semiconductor device fabrication technology, the shrinkage in line width continues at a fast pace. The International Technology Roadmap for Semiconductors (ITRS) predicted that in 2005, in the 90 nm generation devices, the gate length and thickness of silicide at the contact window would be 32 nm and 20 nm, respectively. In the year 2007, for the 65 nm generation devices, these numbers are predicted to further decrease to 25 nm and 17 nm, respectively.¹

In addition, more transistors will be incorporated in one chip. However, owing to the demand for increased integration level, the surface area will not be adequate to meet the interconnect demand. Multi-level interconnections provide flexibility in circuit design and a substantial reduction in die size and, thus, chip cost. Figure 1 shows a scanning electron microscope (SEM) cross section of a six-level metal backend structure. Electrical connection between the various metal layers is provided by vertical interconnects commonly referred to as vias. See the sidebar for device application details.

The impetus for the study of silicide formation on silicon was stimulated by the expectation of device applications of silicides in the late 1970s and early 1980s. Two review chapters have succinctly summarized the knowledge accumulated up to the early 1980s.^6,7 This article focuses on the most important developments in recent years.

Solid-State Amorphization
In device applications, interfacial reactions of metal thin films with silicon are rather peculiar in that polycrystalline metal film reacts with single-crystal silicon. The substrate is covalently bonded and the thin film is metallic. As a result, the microstructure of the silicide film and orientation of the substrate may play an important role in influencing the reaction. Some silicides can form at a temperature as low as 100°C. The mechanism for the break up of silicon bonds at such a low temperature is rather intriguing.⁷ Furthermore, the silicide phases formed at relatively low temperature are apparently related more to the growth kinetics than they are dictated by the thermodynamic consideration.

The formation of an amorphous interlayer (a–interlayer) by solid-state diffusion in diffusion couples has been one of the most challenging problems in condensed matter physics in recent years. The a-interlayer has been found to occur in all refractory metal/silicon and a number of rare-earth (RE) metal and platinum-group metal and crystalline silicon systems. A systematic survey and review of extensive studies on the subject in the past years showed:

• A negative heat of mixing provides the driving force for the reaction and fast diffusion of one component in the other preempts the formation of crystalline compounds
• The growth follows a linear law at the initial stage with activation energy around 1–1.5 eV for refractory metal/silicon systems and 0.5 eV for RE metal/silicon systems
• The dominant diffusing species is silicon
• The stability of the amorphous interlayer depends on the composition
• Multiphases are present simultaneously in the initial stage of metal/ silicon interaction
• Good correlations exist between physical parameters and kinetic data

From the investigation of amorphous interlayers, mechanisms of roughing of epitaxial RE silicide/(001)silicon interface, formation of stacking faults, and pinholes in RE silicides have gained in basic understanding. The insight led to successful growth of a pinhole-free epitaxial RE silicide layer on (111)Si. Furthermore, the enhanced formation of technologically important C54-TiSi₂ by high-temperature sputtering, a thin interposing molybdenum layer, and tensile stress can all be explained involving some aspects of the amorphous interlayers.⁸

The First Nucleated Phase and Simultaneous Occurrence of Multiphases
In the transition metal-silicon binary phase diagrams, three or more silicide phases usually can be found. However, only selective phases are detected after thermal annealing of metal thin films on silicon. From x-ray diffraction and Rutherford backscattering spectrometry data, it was concluded initially that only one phase grows at a time for a clean system. This is consistent with the assertion that the formation of silicides is determined more by the growth kinetics than by energetics. However, more refined analysis by high-resolution transmission-electron microscopy (HRTEM) in conjunction with the fast Fourier transform analysis as well as auto-correlation function analysis indicated that formation of multiphases occurred in a number of refractory metal/silicon systems.^8–11

In the Ti/Si system, Ti₅Si₃, located at the Ti/a–interlayer interface was identified to be the first nucleated phase.⁹ Ti₅Si₃, Ti₅Si₅, TiSi, and C49-TiSi₂, along with an amorphous interlayer, were observed to be present simultaneously in samples annealed at higher temperatures.¹⁰ Examples are shown in Figures 2 and 3. Similar results were obtained for many refractory metal-silicon systems.⁸ For the near-noble silicides, a complex formation sequence was also found recently. The complex sequence of nickel silicide formation has been observed with the sheet resistance measurements combined with in-situ x-ray and light-scattering measurements in a synchrotron radiation facility.⁵

Growth Kinetics of Silicides
Kinetic data are crucial for a basic understanding of interfacial reactions between metal thin films and silicon. Most silicides are formed at a temperature far lower than the eutectic temperature. The growth is often diffusion controlled or interface-reaction controlled. The thickness of the silicide is proportional to the square root of time t and t, respectively. The presence of contaminating or doping impurities was found to influence the growth rate. For platinum films deposited in ultrahigh vacuum, the growth rate of PtSi was found to increase significantly. However, the growth law remained the same.¹²

Cross-section transmission electron microscopy (XTEM) has been demonstrated to provide direct and accurate kinetic data, such as the sequence of phase formation, the dependence of the phase growth, and morphology of phase and interface structure in the growth of silicides on silicon.¹³

In TiSi₂, CoSi₂, NiSi₂, and a number of RE silicides, the silicide formation took place within a narrow temperature range and nucleation was suggested as a controlling mechanism.¹⁴ The nucleation effects are eliminated when these phases are formed on an amorphous layer.¹⁵ The importance of nucleation effects in silicide formation has been discussed extensively by d’Heurle.¹⁴ The films produced from nucleation-limited reactions are often rather rough.

Dominant Diffusing Species
In the silicide formation, metal atoms diffuse across the metal/silicide interface, silicon atoms diffuse across the silicide/silicon interface, or both. In order to determine the dominant diffusing species, it is common to introduce an inert marker. In thin film reactions, the markers are usually tens of nanometers in size and should not influence the growth kinetics of silicide formation. Ideally, the markers should be inert and remain immobile as the diffusing species streams by. An additional constraint is that the marker should be located in the silicide layer to avoid possible influence due to the presence of the interface.⁷

From the marker experiments, it was revealed for metal-rich silicides such as M₂Si, the dominant diffusing species are mostly metal atoms. On the other hand, in the formation of monosilicide and disilicide, silicon atoms are generally the dominant diffusing species. However, there are exceptions. Important silicides in ultralarge-scale integrated-circuit technology, the dominant diffusing species in the growth of TiSi₂, CoSi₂, WSi₂, and NiSi are Si, Co, Si, and Ni, respectively.^6,7,14 For the TiSi₂ salicide process, if the temperature, time, and ambient for the rapid thermal annealing were not optimized, C49-TiSi₂ and/or C54-TiSi₂, which are not easily removed by ammonia and peroxide solution, are prone to form on the dielectric sidewall between the poly-gate and source/drai. This results in the so-called bridging problem, which may lead to device failure. Since cobalt is the dominant diffusing species in the formation of CoSi₂, the bridging problem is less troublesome in the CoSi₂ salicide technology.

Epitaxial Growth of Silicides
Epitaxial silicides belong to a special class of silicides that exhibit a definite orientation relationship with respect to the silicon substrate. A silicide is expected to grow epitaxially on silicon if the crystal structures are similar and the lattice mismatch between them is small. The impetus for the study of epitaxial silicides mainly stemmed from several favorable characteristics of epitaxial silicides in comparison with their polycrystalline counterparts, including greater stability and a lower stress at the interface, alleviation of grain boundary effects, as well as conductivity enhancement.¹⁶

NiSi₂ and CoSi₂ can be grown in single-crystal form on silicon.¹⁷ Many hexagonal RE silicides have been grown on Si(111) for the almost perfect lattice matches between RE silicide (0001) and Si(111) planes. Furthermore, on top of the silicide layer, a single-crystal silicon layer can be grown. An example of the Si/TbSi₂/Si heterostructure is shown in Figure 4.¹⁸ On the other hand, almost all transition metal silicides can be grown epitaxially on silicon to a certain extent. In particular, FeSi and TiSi₂ can be grown to tens of micrometers in grain size.¹⁶

Initial studies on the epitaxial growth of silicides on silicon were mostly on the growth of silicides on a large area. However, in device applications, silicides were grown on laterally confined silicon. Lateral confinement was found to exert significant influence on the epitaxial growth of NiSi₂ and CoSi₂ on silicon.^19–21 The epitaxial silicides were relevant to the device applications as the contact size shrank to sub-100 nm.

In an Ni/(001)Si system, low-resistivity NiSi is at the center of attention in device applications. In nickel on blank (001)Si, NiSi is formed and stable at 350–700°C.⁶ It has been reported that dopants do not affect NiSi formation.²² However, striking effects of B⁺ and BF₂⁺ implantation on the growth of epitaxial NiSi₂ on silicon were observed. As a result of ion implantation into (001)Si, epitaxial NiSi₂ was found to grow at 200–280°C instead of the usual formation temperature of about 800°C on blank (001)Si. Both boron and fluorine atoms introduced by ion implant into silicon were found to promote the epitaxial growth of NiSi₂ on silicon at low temperatures. Good correlation was found between the atomic size factor and the resulting stress and NiSi₂ epitaxy at low temperatures. The final structure of the silicide layer was found to depend critically on the thickness of the starting nickel overlayer and the annealing temperature. The amorphicity of the substrate apparently played an important role in promoting the formation of polycrystalline NiSi₂ at low temperatures.^23–25

NANOSILICIDES

Nanoscale silicides are named nanosilicides. As the integrated circuit industry moves into the nano-era, metal silicide contacts are naturally falling into this category. On the other hand, many efforts have been made to fabricate nanosilicides employing the bottom-up approach without elaborate microlithography.

Nanodots
Quantum dots are envisioned to be useful in devices such as single-electron transistors, high-density memories, light emission, semiconductor lasers, and tunnel diodes.²⁶ In principle, any ultrathin (~ 1 nm) silicide forming metal film may react with silicon substrate to form silicide nanodots under appropriate annealing conditions. Other means, such as ion implantation of metal ions into silicon nanowires followed by annealing, may also produce silicide nanoparticles.²⁷ To meet the requirements of microelectronics and optoelectronics, it is imperative to control the size, density, and ordering of the dots.

Self-assembly is an attractive nanofabrication technique because it provides the means to precisely engineer structures on the nanometer scale over large sample areas. Self-organizing nanocrystal assemblies have already shown the degree of control necessary to address the challenges of building nanometer-scale technologies.²⁸

Self-Assembled Low-Resistivity Metal Silicide Quantum Dot Arrays on Epitaxial Si_0.7Ge_0.3 on (001)Si
Si_1–xGe_x/Si heterostructures are used to fabricate high-speed transistors that extend the range of applications of silicon technology.²⁹ Self-assembled NiSi quantum-dot arrays have been grown on relaxed epitaxial Si_0.7Ge_0.3 on (001)Si. The formation of the one-dimensional (1-D) ordered structure is attributed to the nucleation of NiSi nanodots on the surface undulations induced by step bunching on the surface of SiGe film. This results from the miscut of the wafers from normal to the (001)Si direction. The two-dimensional (2-D), pseudohexagonal structure was achieved under the influence of repulsive stress between nanodots. Since the periodicity of surface bunching can be tuned with appropriate vicinality and misfit, the undulated templates promise to facilitate the growth of ordered silicide quantum dots with selected periodicity and size.³⁰

Figure 5 shows a planview TEM micrograph of an Ni(2 nm)/a-Si(2 nm)/ Si_0.7Ge_0.3 sample revealing the ordered, equally spaced NiSi dot arrays, oriented along the [110] surface direction. The apparent 1-D alignment and less ordered 2-D arrangement features rule out the direct influence of the misfit-dislocation strain. The average size of nanodots and spacings between adjacent arrays are about 15 nm and 20–40 nm, respectively. In contrast, NiSi nanodots in Ni/a-Si/ Si(001) samples were found to be randomly distributed. It indicated that the use of an Si_0.7Ge_x/Si heterostructure template induces the highly ordered alignment of NiSi dots.

A close look at the Si_0.7Ge_0.3/(001)Si and Ni(2 nm)/a-Si(2 nm)/Si_0.7Ge_0.3 surfaces with HRTEM indeed revealed the presence of the atomic steps, about 5–20 nm in spacing and 10 nm in average spacing. The HRTEM images further showed that the irregularity in step spacing indicating the presence of step bunching. In a particular instance, the nanodot arrays, about 100–800 nm apart, were found to align with the cross-hatch pattern in a 500°C annealed Ni(7 nm)/a- Si(13 nm)/Si_0.7Ge_0.3 sample, as shown in Figure 6. The nanodots tended to be connected along individual arrays. The alignment of nanodots is apparently under the influence of the strain fields associated with the cross-hatch patterns. It is conjectured that the alignment with the cross-hatch pattern is most prominent in places where step bunching is of low density and exerts weak influence on the formation of nanodot patterns. Similarly, CoSi₂ and TiSi₂ nanodot arrays were formed.³¹

Formation of Epitaxial β-FeSi₂ Nanodot Arrays on Strained Si/ Si_0.8Ge_0.2 (001) Substrate
Epitaxial β-FeSi₂ nanodots were grown on strained Si/Si_0.8Ge_0.3 (001) substrates by the solid-phase epitaxy method. High-quality β-FeSi₂ nanodots were grown at 800°C by employing strained Si/Si_0.8Ge_0.2 substrates, owing to a decrease of the in-plane lattice mismatch between the lattice spacing of the β-FeSi₂ [001] and [010] directions and that of a silicon substrate. Ordered β-FeSi₂ arrays along <110> direction were observed to form on surfaces of strained Si/Si_0.8Ge_0.2 substrate. It is shown that dislocation slip originating from compositionally graded Si_1–xGe_x layers can produce local surface-strain and local thickness variation. The surface features are used for the fabrication of epitaxial β-FeSi₂ nanostructures on strained Si/Si_0.8Ge_0.2 substrate.³²

First Nucleated Phase and the Dominant Diffusing Species
Atomic resolution techniques have been successful in studying nanoscale silicides. A particularly pertinent example is seen in the identification of Ti₅Si₄ as the first nucleated phase in submonolayer titanium deposited on the Si(111)-7×7 surface by ultrahigh vacuum scanning tunneling microscopy in conjunction with atomic-resolution TEM. The direct observation of the formation of clusters surrounded by the heavily damaged silicon lattice strongly suggested that silicon is the dominant diffusing species in forming the silicide. An example is shown in Figure 7.³³

Nanowires
One-dimensional building blocks, such as nanowires and nanotubes, are especially attractive candidates around which to develop a bottom-up paradigm for nanotechnology-enabled architectures. As opposed to zero-dimensional nanocrystals, which have been the subject of intense study but are challenging to contact electrically, nanowires and nanotubes can act both as interconnects for the transport of charge carriers as well as active device elements.^34,35 Nanowires are intrinsically suitable as highly sensitive sensor elements, due to their high surface/volume ratio and the extreme sensitivity of 1-D transport to gating fields or adsorbates.

Self-Assembled Nanowires
Self-assembled silicide nanowires are envisioned to possess advantages of perfect single crystallinity, metallic resistivity, compatibility with silicon device processing, and high thermal stability. A large number of self-assembled epitaxial silicide nanowires were investigated in the past.^36–44 Many RE silicide nanowires were grown on silicon substrates. These RE nanowires are commensurate with nearly perfect lattice match in their long direction and are limited in their ability to grow coherently with the substrate in the lateral direction). For PtSi on Si(001), the long direction is aligned with [001]PtSi direction in parallel with the Si(220) plane with smaller lattice mismatch.⁴⁵ On the other hand, for the growth of C49-TiSi₂, the range of structural variants argues against a simple interface-energy explanation.⁴⁰ It is, however, interesting that TiSi₂ nanowires are incommensurate (8%) in their long direction.⁴⁶ However, the interface structure for the nanowires may not be the same as that inferred from the bulk lattices.

For systems of isotropic lattice mismatch, such as Ni/Si and Co/Si systems, the aspect ratio of nanowires in these systems was generally small and unsatisfactory for practical applications.^42–44 Strained epitaxial layers may form while the interface between the overlayer and the substrate is commensurate. These layers are inherently unstable and have interesting properties, which are of importance in semiconductor devices. Two kinds of strained relief mechanism were recognized: one is the formation of dislocations and the other is shape transition.

In recent years, it has been recognized that shape changes such as island formation constitute a major mechanism for strain relief.^42,47,48 Tersoff and Tromp reported that a strain-induced shape transition may occur. Below a critical size, islands have a compact symmetric shape. For larger sizes, they adopt a long thin shape that allows better elastic relaxation of the island’s stress.⁴⁷ Experimental data on silicide island formation [e.g., Au₄Si/Si(111)⁴⁸ and CoSi₂/Si(100)]⁴⁸ also exhibited the elongated island growth. For the Ti/Si system, a series of phase transformations was reported in thin-film reactions.⁴⁹ Titanium silicide islands of various shapes were observed.⁵⁰ The shapes were found to depend on the thickness of titanium deposition and the thermal treatment process. A previous work showed that the formation of CoSi₂ nanowires involved the mechanism of “endotaxy.”⁴⁴ The twinning relationship with the substrate breaks the symmetry of the surface and leads to the asymmetric growth of islands. By combining the methods of reactive deposition epitaxy and nitridemediated epitaxy, the formation of high aspect ratio NiSi₂ nanowires can be achieved. Examples are shown in Figures 8 to 10.⁵¹ The nanowires were successfully grown with high aspect ratios despite the four-fold symmetric epitaxial relationship between NiSi₂ (of cubic CaF₂ structure) and silicon (of diamond cubic structure). Nitride-mediated epitaxy was presented by Chong et al. to complement the use of oxide mediated epitaxy in promoting epitaxial growth of CoSi₂ on (001)Si.^52,53 The thin amorphous interlayer acts as a physical barrier to control the flux of metal atoms on the silicon substrate. Such a concept was used in the growth of self-assembled silicide nanowires to control the kinetic process during the growth. A similar effect is expected to be applicable to other strained epitaxial layer systems.

The challenges for self-assembled silicide nanowires are the control of aspect ratio and location. In addition, the self-assembly of nanowires usually requires that the substrate be crystalline, precluding their use for many potential applications.

Alternative Growth of Silicide Nanowires
Alternative approaches have been adopted to grow nanowires without relying on the mismatch between the nanowires and the substrate.

Wu et al. prepared single-crystal metallic NiSi nanowires using free-standing silicon nanowires as the template. NiSi nanowires were produced by annealing the nickel-metal-coated silicon nanowires at 550°C. They also prepared NiSi/Si nanowire heterostructures with NiSi formed using crossed Si/SiO₂ core-shell nanowires as masks to define the lengths of the unreacted silicon regions. Electrical measurements show that the single-crystal nickel silicide nanowires have ideal resistivities of about 10μΩcm and remarkably high failure current densities. In addition, the nickel silicide/silicon (NiSi/Si) nanowire heterostructures have been used to produce field-effect transistors in which the source–drain contacts are defined by the metallic NiSi nanowire regions.³³ On the other hand, carbon-coated NiSi nanowires were prepared in a radio-frequency-induction heating chemical vapor deposition reactor. The growth of the NiSi nanowires and the coating of the nanowires with carbon layers simultaneously took place in the reaction. The nanowires were more than 10 μm long and with an average diameter of 20–40 nm. The resistivity of individual NiSi nanowire was about 370μΩcm at room temperature, indicating the presence of considerable impurities and/or defects.⁵⁴ Nickel silicide nanowires were also grown on nickel surfaces by decomposition of silane at 320–420°C. Depending on the growth conditions, single-phase Ni₂Si, Ni₃Si₂, and NiSi nanowires were formed. It has been demonstrated that directed growth of silicide nanowires can be achieved with the aid of applied electric field.⁵⁵

Xiang et al. used a vapor-phase deposition method to grow TiSi₂ nanowires on silicon wafers. Field emission and cathodoluminescence measurements reveal the potential applications in vacuum microelectronics.⁵⁶

TaSi₂ nanowires have been synthesized by annealing FeSi₂ thin film and nanodots grown on silicon substrate in an ambient containing tantalum vapor. The TaSi₂ nanowires are formed in three steps: segregation of silicon atoms from the FeSi₂ underlayer to form a silicon base, epitaxial growth of TaSi₂ nanodots on a silicon base, and elongation of the TaSi₂ nanowire along the growth direction. Strong field emission properties promise future electronics and optoelectronics applications.⁵⁷

ACKNOWLEDGEMENTS

The research was supported by the Republic of China National Science Council through grant No. NSC 93-2215- E-007-011 and Ministry of Education grant No. 91-E-FA04-1-4.

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L.J. Chen is Ministry of Education National Chair Professor of the Department of Materials Science and Engineering at National Tsing Hua University in Hsinchu, Taiwan.

For more information, contact L.J. Chen, National Tsing Hua University, Department of Materials Science and Engineering, Hsinchu, Taiwan, +886-3-573-1166; fax +886-3-571-8328; e-mail ljchen@mx.nthu.edu.tw.