Growth of Nuclei

When atoms condense they form nuclei. If the surface is of the same material as the depositing atoms, the process is called homogeneous nucleation; if they are of different materials, the process is call heterogeneous nucleation. In semiconductor terminology, heterogeneous nucleation forms heterojunctions. Three types of nucleation mechanisms have been identified; they differ according to nature of interaction between the deposited atoms and the substrate material (Ref 23): (a) the van der Merwe mechanism leading to a monolayer-;by-monolayer growth; (b) the Volmer-;Weber (V-;W) mechanism, characterized by a three-dimensional nucleation and growth; and (c) the Stranski-Krastanov (S-K) mechanism, where an altered surface layer is formed by reaction with the deposited material to generate a strained or pseudomorphic structure, followed by nucleation on this altered layer. The S-K nucleation is common with metal-on-metal deposition and at low temperatures where the surface mobility is low. The conditions for these types of growth is generally given in terms of thermodynamics and surface energy considerations.

Often the adsorption is accompanied by surface reconstruction, surface lattice strain, or surface lattice relaxation, which change the lattice atom spacing or the surface crystallography to give a pseudomorphic structure. The interaction of the depositing material with the surface can form a structure on which subsequent depositing atoms nucleate and grow in a manner different from that of the initially depositing material. This may alter the subsequent film structure. For example, the unique beta-tantalum structured films are stabilized by deposition on an as-grown tantalum silicide interfacial material.

Nuclei Coalescence and Agglomeration. The nuclei grow by collecting atoms that diffuse over the surface. Isolated nuclei grow laterally and vertically on the surface to form a continuous film (Ref 24). The higher the nucleation density, the less the amount of material needed to form a continuous film. The principal growth mode of the nuclei may be laterally over the substrate surface ("wetting growth"), or the nuclei may prefer to grow in a vertical mode ("dewetting growth"). Examples of wetting growth are: gold on copper and chromium, iron on W-O surfaces, and titanium on SiO2. Examples of dewetting growth are nickel and copper on W-O surfaces, and gold on carbon, Al2O3, and SiO2. Growth and coalescence of the nuclei can leave interfacial voids or structural discontinuities at the interface, particularly if there is no chemical interaction between the nuclei and the substrate material and dewetting growth occurs.

In cases where there is little chemical interaction between the nucleating atoms and the substrate, the isolated nuclei grow together, giving the so-called island-channel-continuous film growth stages. Before coalescence the nuclei can have a "liquid-like" behavior that allows them to rotate and align themselves crystallographically with each other, giving an oriented (epitaxial) overgrowth.

Agglomeration of nuclei occurs when the temperature of the nuclei is high enough to allow atomic diffusion and rearrangement such that the nuclei "ball-up" to minimize the surface area. Agglomeration of evaporated gold films is increased at high deposition rates, at high substrate temperatures, and in high-rate electron beam evaporation. Gold is often used for replication in electron microscopy, and agglomeration of pure gold can be a problem; therefore, gold alloys such as 60Au:40Pd are used to reduce the agglomeration tendencies and provide better replication. Agglomeration can occur after deposition if there is appreciable columnar growth (high surface area) in the film and the film is heated.

Where there is strong interaction between the adatoms and the substrate but little diffusion or compound formation with the substrate, the crystal orientation of the deposited material can be influenced by the substrate crystallographic orientation, producing a preferential crystallographic orientation in the nuclei. This type of oriented overgrowth is called epitaxial growth. Lattice mismatch between the nuclei and the substrate at the interface can be accommodated by lattice strain or by the formation of "misfit" dislocation networks, and under proper conditions a single crystal epitaxial film can be grown. This is often the goal in molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) of semiconductor thin films. In the growth of semiconductor materials it is desirable to form an interface that is defect-free so that electronically active sites are not generated. Such an interface can be formed if there is lattice parameter matching between the deposited material and the substrate, or if the deposited material is thin enough to allow lattice strains to accommodate the lattice mismatch without producing dislocation networks. This latter condition produces a "strained layer superlattice" structure (Ref 25).

At the other extreme of growth are amorphous materials where rapid quenching, bond saturation, limited surface diffusion, and the lack of substrate influence results in a highly disordered material. Comparison between amorphous materials formed by coevaporation and those formed by rapid quenching show some indication of a lower degree of short range ordering in the codeposited material, as indicated by the lower crystallization temperature and lower activation energy for crystallization than in the low-temperature deposited films. Amorphous conductive materials, such as W75Si25, have been proposed as a diffusion barrier film in semiconductor metallization. Because amorphous films have no grain boundaries, they are expected to show lower diffusion rates than films that have grain boundaries, in that grain boundary diffusion rates are generally higher than bulk diffusion rates.

Heating by Condensation. At high deposition rates, the condensation energy can produce appreciable substrate heating. When a thermally vaporized atom condenses on a surface it releases energy from several sources, including:

• Heat of vaporization or sublimation (enthalpy of vaporization)--a few electron volts per atom

• Energy to cool to ambient--depends on heat capacity and temperature change

• Energy associated with reaction, which may be exothermic, where heat is released, or endothermic, where heat is adsorbed--heat of reaction (Table 1)

• Energy released on solution--heat of solution

Table 1 Heats of reaction





























Au in Si






(a) Heat of solution

(a) Heat of solution

The thermal vaporization energy for gold from tungsten is about 3 eV per atom, and the kinetic energy of the vaporized atom is about 0.3 eV per atom. Thus the kinetic energy is only a small part of the energy being released during deposition. However, it has been shown, using mechanical velocity filters, that the kinetic energy of the depositing gold particles is important to the film structure, properties, and annealing behavior.

If the depositing atom has greater than thermal energy, due either to being vaporized by sputtering (and not "thermalized" during transport from source to substrate), or to being accelerated as an ion (film ion), the kinetic energy that it releases will be greater than thermal. If the depositing species is excited or ionized, it also releases the excitation energy or the ionization energy. In these situations the energy released also includes excess kinetic energy, excitation energy (if an excited species), or ionization energy (if an ionized species).

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