Epitaxial CoSi2 Research Articles

Fabrication of 2D ordered arrays of cobalt silicide nanodots on (0 0 1)Si substrates

Two-dimensional (2D) periodic arrays of Co metal and Co silicide nanodots were successfully fabricated on (0 0 1)Si substrate by using the polystyrene (PS) nanosphere lithography (NSL) technique and thermal annealing. The epitaxial CoSi 2 was found to start growing in samples after annealing at 500 °C. The sizes of the Co silicide nanodots were observed to shrink with annealing temperature. From the analysis of the selected-area electron diffraction (SAED) patterns, the crystallographic relationship between the epitaxial CoSi 2 nanodots and (0 0 1)Si substrates was identified to be [0 0 1]CoSi 2//[0 0 1]Si and (2 0 0)CoSi 2//(4 0 0)Si. By combining the planview and cross-sectional TEM examination, the epitaxial CoSi 2 nanodots formed on (0 0 1)Si were found to be heavily faceted and the shape of the faceted epitaxial CoSi 2 nanodot was identified to be inverse pyramidal. The observed results present the exciting prospect that with appropriate controls, the PS NSL technique promises to offer an effective and economical patterning method for the growth of a variety of large-area periodic arrays of uniform metal and silicide nanostructures on different types of silicon substrates.

Epitaxial growth of CoSi2 on Si(001) by reactive deposition epitaxy: Island growth and coalescence

Epitaxial CoSi2 layers, which are phase pure but contain {111} twins, are grown on Si(001) at 700 °C by reactive deposition epitaxy. Transmission electron microscopy analyses show that the initial formation of CoSi2(001) follows the Volmer–Weber mode characterized by the independent nucleation and growth of three-dimensional islands whose evolution we follow as a function of deposited Co thickness tCo in order to understand the origin of the observed twin density. We find that there are two families of island shapes: inverse pyramids and platelets. The rectangular-based pyramidal islands extend along orthogonal 〈110〉 directions, bounded by four {111} CoSi2/Si interfaces, and grow with a cube-on-cube orientation with respect to the substrate: (001)CoSi2||(001)Si and [100]CoSi2||[100]Si. Platelet-shaped CoSi2 islands are bounded across their long 〈110〉 directions by {111} twin planes (i.e. {111}(001)CoSi2||{111}Si) and their narrow 〈110〉 directions by {511}CoSi2||{111}Si interfaces. The top and bottom surfaces are {22¯1}, with {22¯1}CoSi2||(001)Si, and {1¯1¯1}, with {1¯1¯1}CoSi2||{11¯1}Si, respectively. The early stages of film growth (tCo≤13 Å) are dominated by the twinned platelets due to a combination of higher nucleation rates resulting from a larger number of favorable adsorption sites in the Si(001)2×1 surface unit cell and rapid elongation of the platelets along preferred 〈110〉 directions. However, at tCo≥13 Å island coalescence becomes significant as orthogonal platelets intersect and block elongation along fast growth directions. In this regime, where both twinned and untwinned island number densities have saturated, further island growth becomes dominated by the untwinned islands. A continuous epitaxial CoSi2(001) layer, with a twin density of 2.8×1010 cm−2, is obtained at tCo=50 Å.

Tailoring of epitaxial CoSi2 /Si nanostructures by low temperature wet oxidation

We have investigated a process for tailoring of epitaxialCoSi2/Si nanostructures using low temperature wet oxidation. A separation between twoCoSi2 layers on a Si substrate in the range of 60 nm is generated by aself-assembly process. During subsequent low temperature wet oxidation,SiO2 formation on top of the silicide layers pushes the latter into the substrate. At theedges of the gap, the silicide layers are shifted in both and directions, leading to an effective reduction of the separation width todimensions below 20 nm and eventually to merging of the two layers. Thesignificantly lower oxidation rate of the silicon in the initial gap compared with theCoSi2 provides the excess Si for the shift in the direction. The structures were investigated using transmission electron microscopy (TEM)and scanning electron microscopy (SEM).

Oxide-mediated formation of epitaxy silicide on heavily doped Si surfaces and narrow width active region

Oxide-mediated epitaxy (OME) is a highly promising means of forming epitaxial CoSi 2. In this work, various chemical treatments were applied to grow chemical oxides on heavily doped Si substrates and narrow width active region. The effects of the treatment on the OME performance were investigated. Both the thickness and the quality of the oxide varied with the chemicals and dopants, resulting in various levels of junction leakage current. The junction leakage current was minimized by choosing the most favorable chemical treatment and optimizing the annealing temperature.

In situ resistance measurements of epitaxial cobalt silicide nanowires on Si(110)

We have performed in situ resistance measurements for individual epitaxial CoSi2 nanowires (NWs) (approximately 60 nm wide and 5μm long) formed on a Si(110) surface. Two- and four-point probe measurements were done with a multitip scanning tunneling microscope at room temperature. The NWs were well isolated from the substrate by a Schottky barrier with zero-bias resistance of 107Ω. The resistivity of the NWs was 30μΩcm, which is similar to that for high-quality epitaxial films. The NW resistance was essentially unchanged after exposure to air.

Resistance measurements of metallic silicide nanowires on a Si substrate with a four-tip scanning tunneling microscope

We have measured electrical resistance of individual epitaxial CoSi2 nanowires (NWs) formed on a Si(110) surface in situ in ultrahigh vacuum, by two-, three-, and four-point probe methods using a multi-tip scanning tunneling microscope at room temperature. The NWs were electrically isolated from the substrate by a Schottky barrier in-between with zero-bias resistance of approximately 107 Ω. Contact resistance between a W tip and a NW was as small as several tens Ω. The resistivity of the NWs was ca. 30 μΩcm, which is similar to that of high-quality epitaxial CoSi2 films. The oxidation of the NW surface had negligible influence on the resistance, meaning marginal carrier scattering at the NW surface. [DOI: 10.1380/ejssnt.2005.362]

Nanopatterning of epitaxial CoSi2 using oxidation in a local stress field and fabrication of nanometer metal-oxide-semiconductor field-effect transistors

A patterning method for the generation of epitaxial CoSi2 nanostructures was developed based on anisotropic diffusion of Co∕Si atoms in a stress field during rapid thermal oxidation (RTO). The stress field is generated along the edge of a mask consisting of a thin SiO2 layer and a Si3N4 layer. During RTO of the masked silicide structure, a well-defined separation of the silicide layer forms along the edge of the mask. The technique was used to make 50-nm channel-length metal-oxide-semiconductor field-effect transistors (MOSFETs). These highly uniform gaps define the channel region of the fabricated device. Two types of MOSFETs have been fabricated: symmetric transistor structures, using the separated silicide layers as Schottky source and drain, and asymmetric transistors, with n+ source and Schottky drain. The asymmetric transistors were fabricated by an ion implantation into the unprotected CoSi2 layer and a subsequent out diffusion to form the n+ source. The detailed fabrication process as well as the I–V characteristics of both the symmetric and asymmetric transistor structures will be presented.

The interaction of cobalt with oxidized silicon surface

The initial stages of the growth of cobalt disilicide (CoSi2) on a 2×1 reconstructed Si(100) surface in the presence of oxygen have been studied for the first time by method of high-resolution photoelectron spectroscopy. The evolution of the electron structure of the sample surface was traced in the course of silicon oxidation, cobalt deposition, and subsequent thermal annealing. It is established that cobalt atoms penetrate to the oxide-silicon interface even at room temperature. This phenomenon favors the formation of an epitaxial CoSi2 layer with improved morphology.

Effect of deposition temperature on the formation of CoSi2 through the rapid thermal annealing of CVD cobalt

Epitaxial CoSi2 layers were successfully produced on (100) silicon substrates by rapid thermal annealing of cobalt layer deposited by CVD. The epitaxial layer had a resistivity of 66 μΩcm for a ∼100 nm layer. The relatively high resistivity is attributed to discontinuities and contamination in the layer.Cobalt silicide formation was found to be dependent on the deposition temperature of the CVD process and the thickness of the cobalt layer. The deposition of a PVD Ti/TiN capping layer was found to allow silicide formation at lower temperatures as the titanium reduces the interface oxide.

Strain relaxation of epitaxial CoSi2 and SiGe layers in cap-Si/Si0.83Ge0.17/Si(0 0 1) and epi-CoSi2/Si0.83Ge0.17/Si(0 0 1) structures

Strain relaxation behaviors of the epitaxial CoSi2 (epi-CoSi2) and Si0.83Ge0.17 layers in epi-CoSi2/Si0.83Ge0.17/Si(001) and cap-Si/Si0.83Ge0.17/Si(001) structures were investigated by high-resolution X-ray diffraction (HR-XRD) analyses. Samples were treated at the temperature, TA = 650–900°C by rapid thermal annealing. Comparative measurements showed a different strain relaxation behavior in the SiGe layers with and without CoSi2 layer. And Ge content and lattice mismatch in the SiGe film of the epi-CoSi2/Si0.83Ge0.17/Si(001) are smaller than those in the SiGe layer in cap-Si/SiGe/Si(001) possibly due to the diffusion of Ge into the tensile-stressed epi-CoSi2 layer to reduce the compressive stress in the SiGe layer at elevated temperature. The analyses of high-resolution ω–2θ scan spectra and reciprocal space mapping showed that epi-CoSi2 layer is under tensile residual stress and a significant strain relaxation starts at TA = 900°C indicating of thermal stability up to TA = 850°C.

Structural and electrical characteristics of epitaxial CoSi2 grown on n-Si0.83Ge0.17/n-Si(001) by reactive chemical vapor deposition using a Si capping layer

Epitaxial cobalt disilicide (CoSi 2 ) layers are grown on n-Si 0.83 Ge 0.17 /n-Si(001) using a sacrificial Si capping layer at the growth temperature T s =650 °C by reactive chemical vapor deposition using cyclopentadienyl dicarbonyl cobalt (Co(η 5 -C 5 H 5 )(CO) 2 ). Structural and electrical properties of epi-CoSi 2 /Si 0.83 Ge 0.17 /Si(001) were measured by transmission electron microscopy, X-ray diffraction, Auger electron spectroscopy and sheet resistance measurement as a function of annealing temperature. The combined results showed that the epitaxial CoSi 2 phase by the reaction of Co with the Si capping layer was formed in the as-grown layers. Rapid thermal anneals for the investigation of thermal stability of the as-grown layers showed good thermal stability of the epitaxial CoSi 2 layers with the low sheet resistance value as low as ≅4.4 Ω/cm up to the annealing temperature as high as 850 °C without the formation of other cobalt silicide phases. At T A =900 °C, Co 2 Si and CoSi phases with higher resistivity were formed due to the reaction of remaining Co on the surface with the diffusing Si or Ge from the SiGe layer and as a result the increase in the sheet resistance value to ≅26.6 Ω/cm was observed.

Structural and electrical characteristics of epitaxial CoSi 2 grown on n-Si 0.83Ge 0.17/n-Si(001) by reactive chemical vapor deposition using a Si capping layer

Epitaxial cobalt disilicide (CoSi 2) layers are grown on n-Si 0.83Ge 0.17/n-Si(001) using a sacrificial Si capping layer at the growth temperature T s =650 °C by reactive chemical vapor deposition using cyclopentadienyl dicarbonyl cobalt (Co(η 5-C 5H 5)(CO) 2). Structural and electrical properties of epi-CoSi 2/Si 0.83Ge 0.17/Si(001) were measured by transmission electron microscopy, X-ray diffraction, Auger electron spectroscopy and sheet resistance measurement as a function of annealing temperature. The combined results showed that the epitaxial CoSi 2 phase by the reaction of Co with the Si capping layer was formed in the as-grown layers. Rapid thermal anneals for the investigation of thermal stability of the as-grown layers showed good thermal stability of the epitaxial CoSi 2 layers with the low sheet resistance value as low as ≅4.4 Ω/cm up to the annealing temperature as high as 850 °C without the formation of other cobalt silicide phases. At T A=900 °C, Co 2Si and CoSi phases with higher resistivity were formed due to the reaction of remaining Co on the surface with the diffusing Si or Ge from the SiGe layer and as a result the increase in the sheet resistance value to ≅26.6 Ω/cm was observed.

In situ analysis of epitaxial cobalt silicide reaction on silicon (0 0 1)

Formation process of CoSi2 on Si(0 0 1) was investigated with in situ reflection high-energy electron diffraction analysis for developing a self-aligned silicide (salicide) process of advanced silicon transistors. A thin Co layer deposited onto a wet chemically oxidized Si(0 0 1) surface silicided at 550°C to form epitaxial CoSi2(0 0 1) without developing polycrystalline subsilicide phase. The oxide remained on the surface up to 750°C and kept the surface flat until its evaporation. It was thus revealed that chemically formed thin oxide on Si(0 0 1) surface is an effective reaction control layer that acts as a reaction barrier to Co/Si subsilicide formation and as a kinematical barrier to surface migration of growing species. These conditions are necessary for fabricating flat and thin epitaxial CoSi2 layers.

Uniform Void-Free Epitaxial CoSi[sub 2] Formation on STI Bounded Narrow Si(100) Lines by Template Layer Stress Reduction

Void free epitaxial-CoSi 2 with nano-thickness has been successfully fabricated on narrow Si(100) substrates surrounded by shallow trench isolation (STI) using a capped titanium mediated epitaxy method. The silicide is epitaxial with a CoSi 2 (110)∥Si(100) crystal orientation. Void growth in the narrow silicon lines under the film edges due to an anomalous creep effect in the presence of a localized tensile stress between CoSi 2 and Si was suppressed completely by optimizing the initial rapid thermal annealing (RTA) thermal budget, and ensuring that no voids nucleated prior to the selective wet clean and second higher-temperature RTA process. The epitaxial Co-silicided n/p metal oxide semiconductor field effect transistors show excellent device performance.

Surface and Interface Smoothing of Epitaxial CoSi2 Films by Solid-Phase Epitaxy Using Adsorbed Oxygen Layers and Two-Step Growth on Si(001) Surfaces

The oxygen-mediated two-step growth of high-quality CoSi2(001) films on Si(001) is demonstrated. In the first-step growth, uniform 6-monolayers (ML)-thick CoSi2(001) films with atomically flat surfaces and interfaces are grown by solid-phase epitaxy (SPE) at 470°C of 3 ML Co on O-adsorbed Si and O-covered 3 ML Co on clean Si. In both cases, O atoms improve the surface and interface flatness in SPE growth. In the second-step growth, 5.6 nm Co is solely deposited at 470°C on the O-covered CoSi2 template. We successfully achieve the growth of 20-nm-thick CoSi2(001) films with smooth surfaces and abrupt interfaces. In the O-mediated two-step growth, O atoms segregating to the film growth front suppress the lateral migration of Co atoms. Furthermore, in the second-step growth, the uniform template promotes homogeneous growth and keeps the surfaces and interfaces of CoSi2 films smooth. This novel technique, applicable to salicide processes, is highly suitable for the formation of epitaxial CoSi2 contacts in future ULSI devices.

Fabrication of Schottky barrier MOSFETs using self-assembly CoSi 2 nanopatterning and spacer gate technologies

A self-assembly patterning method for generation of epitaxial CoSi 2 nanostructures was used to fabricate 50 nm channel-length MOSFETs. The transistors have either a symmetric structure with Schottky source and drain or an asymmetric structure with n +-source and Schottky drain. The patterning technique is based on anisotropic diffusion of Co/Si atoms in a strain field during rapid thermal oxidation. The strain field is generated along the edges of a mask consisting of 20 nm SiO 2 and 300 nm Si 3N 4. During rapid thermal oxinitridation (RTON) of the masked silicide structure, a well-defined separation of the silicide layer forms along the edge of the mask. These highly uniform gaps define the channel region of the fabricated device. The separated silicide layers act as metal source and drain. A poly-Si spacer was used as the gate contact. The asymmetric transistor was fabricated by ion implantation into the unprotected CoSi 2 layer and a subsequent out-diffusion process to form the n +-source. I–V characteristics of both the symmetric and asymmetric transistor structures have been investigated.

Formation and Properties of Epitaxial CoSi2 Layers on p-Si0.83Ge0.17/p-Si(001) using a Si Capping Layer by Metal-Organic Chemical Vapor Deposition

Epitaxial CoSi2 layers were in-situ grown using a sacrificial Si capping layer on p-Si0.83Ge0.17/p-Si(001) by metal organic vapor deposition (MOCVD) at 600°C using cyclopentadienyl cobalt, Co(η5-C5H5)(CO)2. The grown epitaxial CoSi2 layers remain thermally stable at the annealing temperature, TA≤800°C. The CoSi2/p-Si0.83Ge0.17/p-Si(001) samples annealed at TA≤850°C showed the sheet resistance values as low as \\cong5.6 Ω/□. At TA=900°C, the observed increase in the sheet resistance value to \\cong14.7 Ω/□ was attributed to the formation of CoSi phase with higher resistivity resulting from the direct interaction of Co with the SiGe alloy layer. The effective Schottky barrier heights (φBp), measured for the first time, ranged from 0.103 to 0.194 eV at the reverse bias of -5–0 V.

Effect of Al interlayers on two-step epitaxial growth of CoSi 2 on Si(1 0 0)

We have performed the two-step epitaxial growth of CoSi 2 on Si(1 0 0) surfaces with Al interlayers. Microstructures of the surface and the interface during the growth have been investigated by scanning tunneling microscopy and transmission electron microscopy (TEM). At the first-step growth, the introduction of an 1-monolayer (ML)-thick Al interlayer prior to a 3-ML-thick Co layer deposition on the Si clean surface results in the formation of two dimensional islands of epitaxial CoSi 2 with higher coverage, compared to the case without the Al interlayer. Cross-sectional TEM observations revealed atomically-flat CoSi 2–Si interfaces. At the second step, CoSi 2 is epitaxially grown on these islands that acts as a template layer. In-depth growth of CoSi 2 domains is also observed to occur forming {1 1 1} facets at the CoSi 2–Si interfaces. Effects of the Al interlayer on the growth morphology of epitaxial CoSi 2 are discussed.

Epitaxial cobalt silicide formation using a Co/TiSix bilayer on Si (1 0 0) by sputtering

Annealing a Co/Ti bilayer by sputtering on the Si substrate offers higher probability of epitaxial growth and less Si consumption than annealing of a single Co layer on the Si substrate. Nevertheless, the Co/Ti bilayer epitaxy method does not solve the Si consumption problem satisfactorily. In this communication, a new CoSi2 epitaxial growth technique of annealing a Co/TiSix bilayer on Si (100) is investigated for the first time. Cobalt silicide thin films obtained using the Co/TiSix bilayer have been analyzed using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) techniques to identify the phases and to investigate the surface and interface morphologies and the chemical composition. The XRD spectra show the epitaxial growth of CoSi2 layer with (200) crystallographic orientation on the Si (100) substrate for a Co/TiSix bilayer while the Co–Ti–Si spinel phase is detected over the annealing temperature range of 400–600 °C. The AES analysis indicates that native oxides on the silicon substrates are removed by Ti at the beginning of RTA and Co diffuses inward so that an epitaxial CoSi2 layer could form. The AFM micrographs show smooth surface and the cross-sectional high-resolution TEM analysis reveals that the layer is composed of the CoSi2 epi-layer formed on the Si substrate. Also it is found that Si consumption during silicidation annealing for the Co/TiSix bilayer formed by sputtering has been significantly reduced compared with that for the Co/Ti bilayer formed by sputtering.

Epitaxial growth of CoSi2 on hydrogen-terminated Si(001)

We demonstrate that CoSi2 grows epitaxially on H-terminated Si(001) and present the growth mechanism. It was found that direct reaction of Co with Si is suppressed on H-terminated Si below 400 °C. Thus, the hydrogen at the Co/Si interface hinders the formation of Co2Si and CoSi. Upon thermal desorption of hydrogen at around 400–550 °C, CoSi2, which is closely lattice-matched to Si(001), grows on Si(001) and thus, thin epitaxial CoSi2 films are formed on Si(001). The {111}-faceting was completely suppressed in the epitaxial CoSi2/Si(001), leading to the atomically flat interface.