Cobalt Silicide Films Research Articles

An in situ and ex situ TEM study of the formation of thin cobalt silicide films by molecular beam epitaxy on the silicon (100) surface

The cobalt disilicide/silicon system has potential applications as a metal-base and as a permeable-base transistor. Although thin, low defect density, films of CoSi2 on Si(111) have been successfully grown, there are reasons to believe that Si(100)/CoSi2 may be better suited to the transmission of electrons at the silicon/silicide interface than Si(111)/CoSi2. A TEM study of the formation of CoSi2 on Si(100) is therefore being conducted. We have previously reported TEM observations on Si(111)/CoSi2 grown both in situ, in an ultra high vacuum (UHV) TEM and ex situ, in a conventional Molecular Beam Epitaxy system.The procedures used for the MBE growth have been described elsewhere. In situ experiments were performed in a JEOL 200CX electron microscope, extensively modified to give a vacuum of better than 10-9 T in the specimen region and the capacity to do in situ sample heating and deposition. Cobalt was deposited onto clean Si(100) samples by thermal evaporation from cobalt-coated Ta filaments.

I n s i t u study of the molecular beam epitaxy of CoSi2 on (111) Si by transmission electron microscopy and diffraction

The growth of CoSi2 has been studied by deposition and reaction of Co on clean Si (111) surfaces in situ in a modified ultrahigh vacuum transmission electron microscope. On deposition of nominally 20 Å Co at room temperature, strong interaction between Si and Co occurs yielding an epitaxial film which is believed to be a hexagonally distorted form of the Co2Si structure. On heating to ∼350 °C the growth of epitaxial CoSi is observed, which transforms to CoSi2 at ∼450 °C. Silicon-rich phases propagate by lateral motion of phase boundaries. Substantial pinhole density arises in films only after annealing at higher temperatures. The B orientation dominates under similar annealing conditions. In contrast to NiSi2, the growth of cobalt silicide films on Si is dominated by bulk phase formation, with interfacial energy minimized in all cases by epitaxy.

The Effect of Formation Conditions on the Structural and Electrical Properties of Ultrathin Cobalt Silicide Films

ABSTRACTWe have studied the dependence of the structural and electrical properties of ultrathin cobalt silicide films on the annealing temperature and deposited Co thickness.We recently reported that coherent, electrically continuous films of CoSi2 could be grown on Si(111) with thicknesses as small as 1 nm.1 The thinnest of these films had a high density of pinholes which covered about 20% of the area of the substrate. The resistivity of the films increased dramatically with decreasing thickness in a manner which was qualitatively consistent with quantum size effects. In order to investigate the dependence of both the pinhole density and the resistivity of ultrathin CoSi2 films on Si(111), we have deposited different thicknesses of Co onto near room temperature (<100°C) Si(111) substrates and have annealed the samples at different temperatures in order to study the effect of silicide formation conditions on the structural and electrical properties of the resulting films.Co layers were deposited onto Si(111) wafers in a molecular beam epitaxy apparatus as described previously.1 In our earlier work, CoSi2 was formed by heating the sample to ∼600°C for a few seconds after Co deposition. Because higher temperature anneals are known to lead to pinhole formation in thicker films,2 annealing temperatures in the present experiments were constrained to be 600° or less. The reaction time was on the order of 5 minutes. The layers were examined using plan-view transmission electron microscopy (TEM) in order to avoid beam heating and ion beam mixing effects which might cause artefacts in cross-section TEM observation.The results of our experiments are summarized in Figure 1. The figure indicates the silicide phases found in a film as a function of deposited Co thickness and annealing temperature. The numbers in each square correspond to the residual resistivity, ρo, measured for each film. Perhaps the most striking finding of this study is the abrupt change in the structure of the films between 0.7 and 1.4 nm of deposited Co. For all temperatures investigated (including near room temperature), films formed by depositing less than 1 nm of Co form CoSi2 immediately upon deposition. (For films approaching this thickness, there is also a small amount of Co2Si which persists at all temperatures

Study of solid phase and molecular beam epitaxial cobalt silicide films on Si〈111〉 using electron energy loss spectroscopy

Very thin cobalt silicide films of several atomic compositions from Co to CoSi2 have been grown by thermal reaction of Co deposited at room temperature and by molecular beam epitaxy on Si〈111〉 substrates under ultrahigh vacuum conditions. They were analyzed by electron energy loss spectroscopy. Loss spectra characteristic of the Co‐rich silicides, CoSi, and CoSi2 phases were found to be quite different, first by the energy of the bulk plasmons, around 22 eV for Co‐rich silicides, 20.6 and 19.5±0.7 eV for CoSi and CoSi2, respectively, second by the interband and surface‐state transitions. In the CoSi and CoSi2 spectra, two transitions were interpreted as related to the excitation of bonding Co(3d)–Si(3p) and nonbonding Co(3d) electronic states. A transition at 1.7 eV obtained on CoSi2 grown both by the solid phase and molecular beam epitaxy techniques was found to be sensitive to the surface conditions and was attributed to a surface state. This result was correlated to the surface morphology and film cry...