M-A. Nicolet

When is thermodynamics relevant to ion-induced atomic rearrangements in metals?

Abstract The problem of ion-induced mixing of metal bilayers is examined in the limit of heavy metals (Z ≳ 20) and heavy energetic ions (E ≳ 100 keV) and in the absence of delayed effects such as radiation enhanced thermal diffusion. Thermochemical effects are shown to play an important role in biasing the random walk process of mixing. A universal mixing equation is derived which predicts the evolution of the concentration profile as a function of ion dose. Finally, a model is presented which allows one to predict what metallurgical phases are formed during the mixing process. Criteria for amorphous phase formation are particularly emphasized.

Principles and applications of ion beam techniques for the analysis of solids and thin films

Abstract This paper reviews in principle and by examples how a collimated mono-energetic and mono-atomic beam incident on a target provides information on its structure and composition when the energy of the back-scattered beam atoms, or of the particles generated by nuclear reactions, is analyzed. Examples are selected with particular emphasis on thin films and Si technology. For convenience, we define three different energy ranges of the incident beam (low energies from 1 to 6 keV, medium energies from 100 to 500 keV and high energies from 1 to 2 MeV) and discuss each range separately, according to the following table of contents.

Reaction of thin metal films with SiO2 substrates

Abstract Thin films of Co, Cr, Cu, Fe, Hf, Mn, Nb, Ni, Pd, Pt, Ti, V and Zr vacuum-deposited on SiO 2 substrates of thermally oxidized Si wafers and/or fused quartz were annealed under vacuum at about 800°C for 3 hr and then analyzed by backscattering spectrometry and scanning electron microscopy. It is found that Hf, Nb, Ti, V and Zr react with SiO 2 . The result is a thin layer of metal silicide sandwiched between the substrate and a top layer of metal oxide. The other investigated metals apparently do not react. A table of the standard heats of formation for metal silicides has been compiled. These values were used to calculate the free energy change during reaction. The thermodynamic predictions are consistent with experimental observation. The results can also be correlated with the mean electronegativity of the metal, which offers a convenient empirical method to predict whether a metal will react with SiO 2 . It is found that metals with an average electronegativity (average of Allred-Rochow, relative compactness and Pauling electronegativities) of less than 1.5 on the Pauling scale react with the SiO 2 substrate.

Structural difference rule for amorphous alloy formation by ion mixing

We formulate a rule which establishes a sufficient condition that an amorphous binary alloy will be formed by ion mixing of multilayered samples when the two constituent metals are of different crystalline structure, regardless of their atomic sizes and electronegativities. The rule is supported by the experimental results we have obtained on six selected binary metal systems, as well as by the previous data reported in the literature. The amorphization mechanism is discussed in terms of the competition between two different structures resulting in frustration of the crystallization process.

Influence of chemical driving forces in ion mixing of metallic bilayers

The effective interdiffusion coefficient of metallic bilayers under ion irradiation has been correlated with the heat of mixing of corresponding binary alloys. The results are interpreted according to Darkens theory of chemically enhanced diffusion.

LOW‐TEMPERATURE MIGRATION OF SILICON IN THIN LAYERS OF GOLD AND PLATINUM

The backscattering method is employed to obtain microscopic information about solid‐solid reactions of Si with thin layers (500–2000 A) of both vacuum‐evaporated Au and sputtered Pt. A remarkable observation is the migration of Si atoms into Au and Pt at relatively low temperatures (150 and 350 °C, respectively). Migration of Si in Pt induces first the formation of Pt2Si‐like compounds and then PtSi. In the Au–Si system, on the other hand, Si moves through and accumulates on the Au surface in the form of SiO2 under an oxidizing heat‐treatment atmosphere.

Kinetics of TiSi2 formation by thin Ti films on Si

Silicide formation with Ti deposited on single crystal Si and Ti deposited on amorphous Si layers sequentially without breaking the vacuum was investigated using backscattering spectrometry and glancing-angle x-ray diffraction. For Ti deposited on amorphous Si, TiSi2 was formed with a rate proportional to (time)^1/2 and an activation energy of 1.8±0.1 eV. For Ti deposited on single crystal Si, the reaction rate was slower and the silicide layer was nonuniform in thickness. We attribute the difference in behavior to the presences of interfacial impurities in the case where Ti was deposited on single crystal Si.

Compensating impurity effect on epitaxial regrowth rate of amorphized Si

The epitaxial regrowth of ion‐implanted amorphous layers on 〈100〉 Si with partly compensated doping profiles of 11B, 75As, and 31P was studied. Single implants of these impurities are found to increase the regrowth rate at 475 and 500 °C. The compensated layers with equal concentrations of 11B and 31P or 11B and 75As show a strong decrease of the regrowth whereas for the layers with overlapping 75As and 31P profiles no compensation has been found.

Ion implantation and low-temperature epitaxial regrowth of GaAs

Channeling and transmission electron microscopy have been used to investigate the parameters that govern the extent of damage in ion‐implanted GaAs and the crystal quality following capless furnace annealing at low temperature (∼400 °C). The implantation‐induced disorder showed a strong dependence on the implanted ion mass and on the substrate temperature during implantation. When the implantation produced a fully amorphous surface layer the main parameter governing the regrowth was the amorphous thickness. Formation of microtwins after annealing was observed when the initial amorphous layer was thicker than 400 A. Also, the number of extended residual defects after annealing increased linearly with the initial amorphous thickness and extrapolation of that curve predicts good regrowth of very thin (<400 A) GaAs amorphous layers produced by ion implantation. A model is presented to explain the observed features of the low‐temperature annealing of GaAs.

Interaction of metal layers with polycrystalline Si

Auger electron spectroscopy, MeV /sup 4/He/sup +/ backscattering spectrometry, and scanning electron microscopy were used to investigate interactions between Al films and polycrystalline layers of chemical vapor deposited Si on SiO/sub 2/. Depth profiling techniques showed that intermixing of the Al and Si occurred in 400--560/sup 0/ t. Dissolution of the polycrystalline Si into the Al film occurs followed by nucleation and growth of Si crystallites in the Al film. The morphology of the final structure depends on the relative thicknesses of the as-deposited Al and Si layers. For Al layers thinner than those of the Si, a nearly continuous film is formed on the outer surface. The thickness of this final Si film is approximately that of the original Al layer. The remaining Si and the Al form a two-phase layer between the outer Si film and the SiO/sub 2/ substrate. (WDM)