Silicide Grains Research Articles

Characterization of Phase Transformation by Fe-SEM and Fe-TEM Analysis

The formation of a silicon alloy has been achieved by electron beam irradiation of a nickel-silicon bilayer. This process, called SiDWEL1 , has been developed by Quantiscript. The nickel-silicon bilayer is very thin and the electron beam operates at low energy and current. Silicide formation occurred around the interaction volume of the electrons within the material. This paper describes the nature of the structures produced and the origin of the difficulties encountered with the characterization. The necessity of further analysis with TEM based techniques is highlighted.Figure 1 presents a cross-section of a typical sample. The growth of silicide grains is induced by the e-beam irradiation of the stack for few microseconds. Nucieation is initiated by the diffusion of silicon atoms in nickel grains, induced by the heating arising from the electron beam interaction with the solid. A new phase is revealed by FE-SEM observation, after wet etching of the nickel film, as shown in figures 2 and 3.

Imaging of the structure of ultra-thin cobalt silicide films by inelastically backscattered electrons

We have investigated the initial growth of cobalt silicide films in the Co/Si(111) system by incoherent medium-energy electron diffraction providing real-space imaging of the subsurface atomic structure. It is shown that deposition of the first few (1–3) monolayers of Co results in formation of cobalt silicide islands with composition and structure depending on the temperature. While, the clusters of B-type CoSi2 grow at elevated temperatures (T>300°C), the room temperature deposits consist of silicide grains of A- and B-type orientation. Further increase of the Co coverage results in both an enrichment of the probed layer with the metal and its disordering. Annealing of such samples at T>250°C gives rise to solid-phase reaction of Co and Si leading to gradual formation of the epitaxial disilicide layer and disappearance of A-type domains.

AFM measurement of the grain size in polycrystalline titanium silicides

In titanium silicide, the transition from the high (C49) to the low resistivity phase (C54) is difficult in small areas. This has been attributed in the literature to the lack of the nucleation sites for phase transition in small areas [1]. The mean grain size is directly related to the nucleation point density and its evaluation is important in obtaining data on nucleation mechanism of a new crystalline phase. TEM and SEM cross sections show that a silicon ridge is formed on the silicon substrate at the silicide grain boundaries, due to the equilibrium between the forces related to the grain–grain and to the grain–silicon interface energies [2,3]. AFM measurements have been performed on the interface between silicon and silicide after the removal of the silicide film. The grain boundaries of the silicide were imaged using the silicon ridges, allowing us to obtain unambiguous and statistically relevant data on the grain size.

Structural relationship of polycrystalline cobalt silicide lines to (001) silicon substrate and their thermal stability

We have investigated the crystalline quality of 0.6–1.2 μm poly-CoSi 2 lines, formed on (001) silicon by a standard salicide process. The microstructure of silicide grains has been investigated by TEM and HRTEM techniques and related to their location inside the strip. The grains at the centre of the line were randomly oriented with respect to the substrate unlike at the edges, where some interesting epitaxial relationships were detected. Selected area diffraction (SAD) analyses on the line section indicated the presence of both A- and B-type epitaxial regions even inside the same grain, whose interface to silicon lies along the (1, 1 ̄ ,1) planes. In these regions, high-resolution transmission electron microscopy (HRTEM) analyses have shown misfit dislocations inserted along the (111) planes to compensate the lattice mismatch. High temperature anneals mainly deteriorate the line edges with serious damage to their electrical performance.

Formation of (Nd,Y)-silicides by sequential channeled implantation of Y and Nd ions

A buried hexagonal Nd0.32Y0.68Si1.7 layer is formed by a sequential implantation of Y and Nd ions into (111)-oriented silicon wafers. The orientation relationship between the epitaxial Nd0.32Y0.68Si1.7 and the silicon is (0001)Nd0.32Y0.68Si1.7//(111)Si with 〈112̄0〉Nd0.32Y0.68Si1.7//〈11̄0〉Si. High temperature annealing (1000°C) results in a gradual transition into an orthorhombic ternary (Nd,Y)-silicide. Between the orthorhombic (Nd,Y)-silicide and the Si a preferential orientation relationship exists: (110)orth//(11̄0)Si with 〈001〉orth//〈111〉Si. However, as not all orthorhombic silicide grains follow this epitaxial relationship, the minimum yield in the Rutherford backscattering spectrometry (RBS) spectrum increases compared to the results after a low temperature annealing.

Influence of TiSi 2 formation temperature on film thermal stability

This work is addressed to investigate thermal stability of a thin TiSi 2 film, that is its ability to resist degradation due to heat treatments at high temperatures. The study was carried out as a function of the formation RT treatment (675–750°C) at the end of a common process flow. Sheet resistance measurements were employed in order to evaluate this degradation. Electrical measures were performed on large and narrow poly-Si lines, on Van Der Pauw structures and on doped mono-Si substrates. An increase in sheet resistance value of an order of magnitude for silicide formed at temperatures below 700°C with respect to the one formed at temperatures above 700°C was found, particularly on poly-Si lines. The effect is detectable independently of the structure: it was observed also on 0.75-μm wide poly-Si lines, increasing when line width decreases. Different morphological analyses were carried out for investigating the influence of the formation temperature. We explain the increase of the final sheet resistance decreasing the formation temperature as a lower thermal stability of the TiSi 2 film, leading to a thermal grooving of the silicide grains.

Thermal stability of cobalt silicide stripes on Si (001)

The effect of lateral scaling on the thermal stability of cobalt disilicide has been investigated on Si (001) substrates. Resistance measurements on four terminal resistors, with linewidth dimensions ranging from 1.3 to 0.5 μm, have been performed after annealing in the range between 900 and 1000 °C. The resistance increases faster in the stripes than in blanket regions. This degradation process has been correlated to the morphological change in the strip cross section, as analyzed by transmission electron microscopy and atomic force microscopy. Grain growth, degradation processes at the line edge, and surface and interface roughness have been observed. These analyses show that the lateral constraints of the silicide lines are mainly responsible for the lowering of the silicide thermal stability compared with blanket regions. Moreover, from measurements of agglomerated silicide grains in equilibrium with the substrate, the silicide surface free energy is 2.3 times the free energy of the CoSi2/Si interface.

Modeling of the surface roughness of thin TiSi 2 films at the point of rupture

When a polycrystalline thin film on a substrate is annealed, grooving at grain boundaries will eventually lead to film rupture, followed by island formation. Two models were developed to predict the surface roughness at the point of rupture (interface roughening effect included) of polycrystalline thin films of TiSi 2 on Si based on 2-D and 3-D film analysis. The modeling results predicted that the rms roughness at rupture (RMS*) is essentially linear with film thickness (verified for thickness up to 150 nm), and that RMS* is not sensitive to grain size, 2 R (for R=1–10 μm). Comparison is made with experimental data obtained with the atomic force microscope from thermal stability testing of two TiSi 2 thin film specimens on (111)-Si, for annealing treatments carried out at 950°C in argon gas. The two specimens had the same thickness of about 150 nm but were formed under different silicide formation schemes. The experimental rms roughness values at rupture were approximately two times higher than predictions of the 3-D film model. The positive deviation is attributed to one or more of the following possibilities: initial film roughness due to intrinsic roughness of the silicide grains and presence of film defects like hillocks, faceting and relative subsidence of grains due to anisotropy and grain orientation effects, and hole formation and impingement.

Effects of Ion Metal Plasma (IMP) Titanium Deposition on Ti Silicide Formation

AbstractTitanium disilicide obtained by direct interaction between Si and a deposited Ti layer is a choice for low- resistance gate interconnections and source and drain areas. The properties of the TiSi2 film can be influenced by many factors; such as substrate nature and doping, the depositing Ti layer, structure dimensions [1][2][3][4]. This work is addressed to study the properties of TiSi2 film as obtained from titanium deposited by Ion Metal Plasma (IMP), which has recently been introduced in high aspect ratio contact and via applications.Its suitability for titanium silicide formation is investigated here in comparison to standard PVD deposition. The study was carried out on flat and patterned samples. Titanium silicide formed on mono- Si substrates were characterized as a function of RTP temperature in terms of sheet resistance, tilm morphology, crystallography and phase evolution. It was found that the TiSi2 film obtained from IMP- Ti is very similar to the one obtained from PVD standard deposition. However, for annealing below 700°C, an increase in the sheet resistance of the TiSi2 C49 phase from IMP- Ti compared to the one from PVD- Ti was found, and is explained by different silicide grain size. Analyses performed on patterned samples with doped silicon and poly- Si lines show similar electrical results for TiSi2 from IMP and PVD deposition; however, fbr p+ poly-Si lines, the IMP samples displayed correct TiSi2 formation down to 0.18µm line width, while the PVD wafers showed discontinuous results at these minimum feature sizes.

Effect of Fluorine on Surface Morphology in Selective Epitaxial TiSi2 Growth by Plasma‐Enhanced Chemical Vapor Deposition

We studied the effect of gas addition on the surface morphology of titanium silicide films grown by plasma‐enhanced chemical vapor deposition using a low‐density plasma of gases. We examined the use of , and as additional gases and found the F‐containing gases to be effective for forming continuous films. Deposition with SiCl4 and without additional gases results in films with slits among the silicide grains, showing that the effect on the surface morphology is due to fluorine. The surface morphology of the film indicated that the fluorine suppresses atom movement. Although gas addition caused etch pits in the substrate, it also resulted in a continuous epitaxial film with a C49‐structure being selectively deposited on a (001)‐oriented Si surface with a mask.

Low-temperature intermediate Au-Si wafer bonding; eutectic or silicide bond

The actual mechanism involved in Au-Si wafer bonding is controversial. Usually a titanium or chromium layer is deposited in between the (oxidized) silicon substrate and the gold layer to ensure adhesion. The resulting bond of two such wafers after annealing is generally considered to be eutectic, however, the bond temperature required is higher than would be expected from the Au-Si eutectic temperature. Moreover, silicide grains are formed at the bonding interface. In this paper it is proposed that the actual bonding is initiated by the dissolution of the oxide layer by silicidation of the titanium adhesion/barrier layer. The subsequent direct Au-Si contact enables the formation of the euteetic phase. The silicidation is required to obtain the eutectic alloy with 19 at.% Si despite the Ti diffusion barrier. The bonding temperature required is, therefore, set by the silicidation process rather than by the eutectic phase. Several experiments have been designed to support this theory. AI-Si eutectic bonding has been investigated, as it is not complicated by an adhesion metal and experiments demonstrate reliable bonding close to its eutectic temperature. Moreover, a Ti/Au/Si/Au stack has been fabricated to be used as a eutectic solder, giving bonding at a temperature not affected by silicidation. Keywords Eutectic bonding Gold Silicide bonding Silicon Wafer bonding

Microstructural studies of Fe-silicide films produced by metal vapor vacuum arc ion implantation of Fe into Si substrates

Iron-silicide films have been formed by metal vapor vacuum arc (Mevva) ion implantation of iron into (111) and (100) oriented silicon wafers. In the as-implanted samples with a dose of 1 × 10 17 ions/cm 2, metastable γ-FeSi 2 is the dominant phase accompanied by a small fraction of the CsCl-derived defect phase Fe 1 − x Si. The silicide grains form a surface layer on (111) silicon, whereas they are embedded in (100) silicon substrate at a depth of 30 nm. In the samples with a higher dose of 4 × 10 17 ions/cm 2, a primitive orthorhombic FeSi 2 phase is formed. Some grains with this new structure contain stacking faults which are lying on (100) planes.

Cause of Aligned‐Orientation Growth of Titanium Silicide in Plasma Enhanced Chemical Vapor Deposition

Growth characteristics in initial stages of an aligned‐orientation growth of epitaxial C49‐structure grains selectively deposited by plasma‐enhanced chemical vapor deposition (PECVD) are studied and compared with polycrystal growth of silicides in low‐pressure chemical vapor deposition (LPCVD). PECVD was performed using gases with and without at about 700°C on (001)‐oriented Si substrates. Ti atoms diffused into the silicon from the surface in the initial growth stages. The density of Ti atoms increased and epitaxial silicide grains were first formed when the density reached that of the disilicide. In LPCVD using and , island grains of a Ti‐rich silicide and a disilicide with multiple orientations were formed in the initial stages. The Ti‐rich silicide was deposited due to the high rate of Cl reduction from by Si of the substrate and low decomposition rate. In the succeeding deposition, the decomposition rate increased and the Ti‐rich silicide was changed into the C54‐structure . The difficulty of epitaxial growth in LPCVD results from the growth of multiple silicides, island growth, and stoichiometry change. Above difference in growth features between PECVD and LPCVD is caused by a difference in behaviors of Ti diffusion into silicon. The Ti‐atom diffusion plays an important role in the aligned‐orientation growth in PECVD.

TiSi2 Phase Transformation by Amorphization Techniques

AbstractULSI packing density calls for sub-micron line widths, but on n-type polysilicon this can lead to incomplete titanium silicide C49 to C54 phase transformation after a conventional two step rapid thermal anneal (RTA). This study compares three different ion beam amorphization techniques: preamorphization, ion beam mixing and silicide amorphization, aiming at a complete phase transformation for submicron silicide lines. For preamorphization, an arsenic implantation at moderate energies (35–75 keV) was used to amorphize the top layer of the polysilicon prior to the titanium deposition. Ion beam mixing used a high-energy (200 keV) arsenic implantation after the titanium deposition to create an amorphous mix of silicon and titanium. These two methods did, each by themselves, lead to an increased fraction of C54 silicide grains already during the low temperature RTA, and a complete phase transformation during the subsequent high temperature RTA. Both methods lowered the thickness difference between titanium silicide on p- and n-type silicon. Silicide amorphization with 75 keV arsenic or 100 keV antimony, applied before the second RTA, did not significantly improve the silicide phase transformation.

Room-temperature oxidation of silicon in the presence of Cu 3Si

The dependence of room-temperature oxidation of silicon catalyzed by Cu 3Si on silicide thickness, silicide grain size and the substrate orientation has been investigated by transmission electron microscopy (TEM), X-ray diffractometry and Auger electron spectroscopy. The extent of oxidation was found to depend critically on the starting film thickness of Cu 3Si. The oxidation was found to be more restricted on (111)Si than that in (001)Si samples. The SiO 2 layer thickness was found to decrease with the average grain size of the starting Cu 3Si layer. High-resolution TEM revealed that the oxidation is initiated at the grain boundaries. An oxide film as thick as 4.5 μm was grown at room temperature over a period of two weeks in (001) samples. The growth of the thick oxide film was achieved by minimizing the grain size of Cu 3Si through a reaction between Cu and an intermediate amorphous silicon layer at 200 °C. Mechanisms for catalytic oxidation of silicon in the presence of Cu 3Si are discussed.

TEM investigation of ion beam synthesized semiconducting FeSi2

Abstract Both as-implanted and annealed semiconducting FeSi 2 samples fabricated using ion beam synthesis technique were studied by transmission electron microscopy. A continuous buried silicide layer with larger precipitates and sharper interfaces formed during annealing at 900 °C for 18 h. Different orientation relationships between the silicide grains and the silicon substrate were found. The existence of a large variety of parallel lattice plane pairs with the available small mismatches between the two phases results in these preferred orientation relationships.

Heteroepitaxy of CoSi2 on Patterned Si(100) Substrates

AbstractThis study has addressed the effect of starting surface topography on the nucleation and growth of epitaxial siicide layers. CoSi2 layers were grown via the template technique on one-dimensionally patterned Si (100) substrates. These substrates contained mesa stripes, running parallel to Si[011], and exhibited either smoothly varying sinusoidal profiles, or a number of well defined Si Ihkl) facets. Conventional plan view and high resolution cross section transmission electron microscopy were used to analyze the films grown on these substrates. The orientation and morphology of the CoSi2 grains depend on the angle (θ), between the CoSi2 / Si interface normal and Si (100). High quality (100) oriented CoSi2 grew on the tops and bottoms of mesa structures, where θ < 5°, and formed atomically sharp interfaces with the substrate. In contrast, CoSi2 (110) and CoSi2 (221 ) grains nucleated along the sidewalls of the mesa structures. The CoSi2 (110) grains formed rough interfaces with the substrate and were terminated by regions of step bunching at the grain boundary / substrate triple points. CoSi2 (110) grains were most highly concentrated in regions where θ varied from 5° to 12°. Similarly, the CoSi2 (221 ) grains formed faceted (111) b-type silicide / substrate interfaces, and were most highly concentrated in regions where θ > 10°. These data suggest that double height steps, step bunches and facets on the substrate are related to the nucleation of misoriented silicide grains.

Assembling three-dimensional microstructures using gold-silicon eutectic bonding

An assembly method for three-dimensional microelements is presented. The assembly is done in situ with a micromanipulator in an SEM using Au-Si eutectic bonding. Microblocks bonded to larger silicon substrates are used for evaluation of the mechanical strength and a microarch is presented to demonstrate the possibilities of the technique. The microelements are fabricated by bulk micromachining, and sputter deposited with chromium and gold. Etched (111) faces have been successfully bonded. TEM investigation of samples from vacuum furnace experiments show large gold grains with smaller chromium silicide grains in the bonded region. The silicon in the eutectic liquid precipitates epitaxially on both silicon faces. Mechanical bending tests on the microblocks give sufficiently high fracture stresses for the intended applications in microrobotic systems. Average fracture stresses of 65 MPa are measured for one set of parameters. Problems encountered are misalignments of the microelements during processing and void formation in the bonds. It is believed this is connected to the experimental equipment and set-up. The microarch, which consists of three assembled microblocks, reaches a tensile stress of 16 MPa, encouraging further development. In conclusion, strong microbonds are achieved using a solidified gold-silicon eutectic melt as an adhesive, and it is demonstrated for the first time that three-dimensional microassembly by means of eutectic bonding of micromachined elements can be performed by manipulation and processing on a locally heated specimen table.

The Effect of Amorphous Layer at NiCr/Ni Silicide Interface on the Sheet Resistivity of Ni Silicides

Two sets of samples with different interfacial amorphous layers were prepared: one with Pt impurity (Pt process) and the other without Pt impurity (Pt-free process) at NiCr/Si interfaces. Following annealing, Cr-rich amorphous layers (a-layers) with and without Pt impurity formed at NiCr/Ni silicide interfaces, respectively. These a-layers control the formation and the growth of Ni silicides. The Pt impurity seems to move with the a-layers and suppress the growth of the a-layers. The a-layers without Pt impurity are therefore thicker and thermally more stable. The thicker a-layers result in larger silicide grains and more uniform silicide layers. Thus, Ni silicides with low sheet resistivity can be obtained by this Pt-free process.

The dependence of room-temperature oxidation of silicon catalyzed by Cu3Si on the silicide grain size

The dependence of room-temperature oxidation of silicon catalyzed by Cu3Si on the silicide grain size has been investigated by transmission electron microscopy and x-ray diffractometry. The thickness of the SiO2 layer was found to decrease with the average grain size of the starting Cu3Si layer. High-resolution transmission electron microscopy revealed that oxidation is initiated at the grain boundaries. Oxide film as thick as 4.5 μm, compared to a previous record of about 2 μm, was grown at room temperature over a period of two weeks in (001) samples. The growth of thick oxide films was achieved by minimizing the grain size of Cu3Si through a reaction between Cu and an intermediate amorphous silicon layer at 200 °C.