Condensation and Nucleation

Atoms that impinge on a surface in a vacuum environment either are reflected immediately, reevaporate after a residence time, or condense on the surface. The ratio of the condensing atoms to the impinging atoms is called the sticking coefficient. If the atoms do not immediately react with the surface, they will have some degree of surface mobility over the surface before they condense. Re-evaporation is a function of the bonding energy between the adatom and the surface, the surface temperature, and the flux of mobile adatoms. For example, the deposition of cadmium on a steel surface having a temperature greater than about 200 °C (390 °F) will result in total re-evaporation of the cadmium.

Surface Mobility. The mobility of an atom on a surface will depend on the energy of the atom, atom-surface interactions (chemical bonding), and the temperature of the surface. The mobility on a surface can vary due to changes in chemistry or crystallography. The different crystallographic planes of a surface have different surface free energies that affect the surface diffusion. For example, for face-centered cubic metals the surface free energy of the (111) surface is less than that of the (100) surface, and the surface mobility of an adatom is generally higher on the (111) surface than on the (100) surface. This means that different crystallographic planes will grow at different rates during adatom condensation. Various techniques have been developed to study surface mobility and the surface diffusion rate of adatoms on a surface (Ref 6, 7).

The surface mobility of adatoms can be an important factor in surface coverage. For example, the surface coverage of a silicon device is improved by depositing an Al/Cu metallization on a TiN barrier layer at 500 °C (930 °F). The Al/Cu has a higher surface mobility on the TiN surface than on the silicon surface and is able to completely fill a 0.5 /'m diameter by 0.8 pm deep "via" holes. Also, by direct current magnetron sputter-depositing the Al-5Ga eutectic alloy at 300 °C (570 °F), "via" holes with aspect ratios of up to 4:1 have been successfully filled.

Nucleation. Atoms condense on a surface by losing energy. They lose energy by:

• Making and breaking chemical bonds with the substrate surface atoms

• Finding preferential nucleation sites (e.g., lattice defects, atomic steps, impurities)

• Colliding with other diffusing surface atoms (same species)

• Colliding or reacting with adsorbed surfacespecies

Nucleation by Surface Reaction. The condensing atoms react with the surface to form atom-to-atom chemical bonds. The chemical bonding may be by metallic (homopolar) bonding where the atoms share orbital electrons, by electrostatic (coulombic, heteropolar) bonding where ions are formed due to electron loss/gain, or by electrostatic attraction (van der Waals forces) due to polarization of molecules. If the atom-atom interaction is strong, surface mobility is low and each surface atom can act as a nucleation site. If the reaction is strong, the atom is said to be chemisorbed. In some cases the chemisorbed adatom displaces the surface atoms, giving rise to a "pseudomorphic" surface structure.

The bonding energy of atoms to surfaces can be studied by thermal desorption techniques. The chemisorption energy for some materials on clean surfaces are:

Material

Energy, eV

Rb on W

2.6

Cs on W

2.8

B on W

6.1

N2 on Fe

3.0

Ni on Mo

2.1

Ag on Mo

1.5

Au on W

3.0

O2 on Mo

7.5

The bonding between a metal atom and an oxide surface is proportional to the metal-oxygen free energy of formation, with the best adhesion produced by the formation of an intermediate oxide interfacial layer. In many instances the surface composition can differ significantly from that of the bulk of the material, and/or the surface can have an nonhomogeneous composition. Examples are the glass-bonded (Si-O) sintered, alumina ceramics shown in Fig. 2. Film atoms prefer to nucleate and react with the glassy Si-O phase, and if this material is leached from the surface during surface preparation, the film adhesion suffers (Ref 9). Preferential sputtering of a compound or alloy substrate surface can change the surface chemistry. For instance, preferential sputtering of an Al2O3 surface removes oxygen, leaving an aluminum-rich surface (Ref 10). Surface contamination can greatly influence the nucleation density, interfacial reactions, and nuclei orientation. When a two-phase binary alloy is deposited, the two materials may react differently with the surface, resulting in segregation on the surface.

Fig. 2 Surface morphology of an as-sintered 96% alumina ceramic such as is used in hybrid circuitry. 1000x

Nucleation at Preferential Nucleation Sites. If the adatom-surface interaction is weak, the adatom will have a high surface mobility and will condense at preferential nucleation sites where there is stronger bonding due either to a change in chemistry (element or electronic) or an increase in coordination number (e.g., at a step). Preferential nucleation sites can be:

• Morphological surface discontinuities such as steps or scratches

• Lattice defects in the surface such as point defects or grain boundaries

• Foreign atoms in the surface

• Charge sites in insulator surfaces

• Surface areas that have a different chemistry or crystallographic orientation

Steps on a surface can act as preferential nucleation sites. For example, gold deposited on cleaved single-crystal NaCl or KCl shows preferential nucleation on cleavage steps. Steps on Si (100) and GaAs (100) surfaces can be produced by polishing at an angle of several degrees to a crystal plane. This procedure produces an "off-cut" or "vicinal" surface (Ref 11) comprised of a series of closely spaced steps. These steps aid in dense nucleation for epitaxial growth of GaAs on Si and AlxGai_x As on GaAs. Scratches on the substrate surface provide nucleation sites in the deposition of diamond films.

Lattice defects can act as preferential nucleation sites. For example, amorphous carbon films have a high density of defects that act as nucleation sites for gold deposition. When depositing adatoms on electrically insulating substrates, charge sites on the surface can act as preferential nucleation sites. Electron irradiation, ultraviolet radiation (UV), and ion bombardment can be used to create charge sites.

Nucleation by Collision. Mobile surface adatoms can nucleate by collision with other mobile surface species to form stable nuclei. Thus the nucleation density can depend on the deposition (arrival) rate. For example, in the deposition of silver on lead it has been shown that at a deposition rate of 0.1 nm/min, the silver is completely re-evaporated, while at 10 nm/min, the atoms are completely condensed. When depositing silver on glass, improved adhesion can be obtained by a rapid initial deposition rate, followed by a lower rate to build up the thickness.

Nucleation by Reaction with Adsorbed Atoms. Mobile surface species can react with adsorbed surface species such as oxygen. For example, chromium deposition after oxygen plasma cleaning of glass generally results in improved

adhesion compared to a glass surface that has not been oxygen-plasma cleaned. This is probably due to the adsorption of oxygen on glass, which increases the nucleation density of deposited gold (Ref 12). The adsorption of reactive species can also have an important effect in reactive deposition processes (Ref 13).

Nucleation of Unstable Surfaces. Some surfaces are unstable and change their nature when atoms are added to the surface. For example, the adatom may interact with the surface lattice and cause atomic rearrangement such that a pseudomorphic surface is formed that presents a different surface to atoms subsequently deposited (Ref 8). Some polymers, particularly glassy polymers (i.e., those above their glass transition temperatures), have surfaces that are unstable and into which the depositing adatom will sink and possibly even nucleate below the polymer surface (Ref 14). Polyethylene and polypropylene are examples of polymers that are glassy at room temperature.

Nucleation Density. In general, the number of nuclei per unit area, or "nucleation density," should be high in order to form a dense film and obtain complete surface coverage at low film thickness. The nucleation density and growth behavior can vary with different substrate locations due to phase distribution, surface morphology, or crystallographic orientation of the substrate surface. The variation of nucleation density and subsequent film growth can result in film property variations over the surface.

Characterization of Nucleation Density. The relative and absolute nucleation density can be determined by a number of techniques (Ref 15), including:

• Optical density of the deposited film as a function of mass deposited

• Behavior of the thermal coefficient of resistivity (TCR) as a function of mass deposited

• Transmission electron microscopy (TEM) and ultrahigh vacuum TEM

• Auger electron spectroscopy (AES)

• Low-energy electron diffraction (LEED)

• Reflection high-energy electron diffraction (RHEED)

• Work function change as a function of mass deposited

• Scanning electron microscopy (SEM)

• Scanning tunneling microscopy (STM) (Ref 16)

• Atomic force microscopy (AFM)

• Photon tunneling microscopy (PTM) (Ref 17)

Optical Adsorption. On transparent substrate materials, the optical density of a film formed by depositing a given amount of material can be used to measure the comparative nucleation density. The optical density (OD) is defined as: OD = log [(% visual light transmitted through the substrate)/(% visual light transmitted through a metallized substrate)]. A good electrical conductor having a high nucleation density is optically opaque to the human eye when the film thickness is about 1000 A (100 nm). A comparison of optical densities is often a good "quick check" on process reproducibility.

The temperature coefficient of resistance (TCR) of very thin metal films on electrically insulating substrates depends on the growth of the nuclei. Isolated nuclei result in a negative TCR (increasing temperature/decreasing resistance) due to thermally activated tunneling conduction between nuclei. Connected nuclei, which form a continuous film, have a positive TCR as would be expected in a metal. Thus TCR measurements can be used to provide an indication of nucleation density and growth mode by determining the nature of the TCR as a function of mass deposited.

Surface Analytical Techniques. Using low-energy electron diffraction (LEED) it has been shown that very low coverages of contamination can inhibit interfacial reaction and epitaxial growth. The field ion microscope has been used to field-evaporate deposited material and observe the "recovered" substrate surface. Using this technique to study the deposition of copper on tungsten, it was shown that electroplating results in interfacial mixing similar to that produced by high-temperature vacuum deposition processing.

Modification of Nucleation Density. There are a number of ways to modify the nucleation of depositing atoms on substrate surfaces, including:

• Change deposition temperature (increasing temperature can increase reaction with surface and increases surface mobility; decreasing temperature can decrease surface mobility)

• Increase deposition rate to increase collision probability of diffusing species

• Change surface chemistry to make the surface more reactive

• "Sensitizing" the surface by the addition of "nucleating agents"

• Generate nucleation sites on the surface (lattice defects, charge sites on insulators) by mechanically disrupting the surface to produce defects and disturb contaminant layers ("mechanical activation"), using ion bombardment to produce lattice defects (Ref 18), incorporating species by ion implantation or chemical substitution, or using electron bombardment or photon bombardment (Ref 19) to charge centers on insulator surfaces

• Produce codeposition or absorption of reactive species

• Change surface roughness

• Create a new surface--"basecoat" or "glue layer"

Adsorption and Codeposition or Reactive Species. Adsorbed or codeposited reactive species can affect the surface chemistry and thus the nucleation of the deposited species. The presence of adsorbed oxygen or oxygen in a plasma or bombarding oxygen ion beam during deposition has been shown to aid in the adhesion of gold (Ref 20) and oxygen-active film materials (Ref 21) to oxide substrates. The increased adhesion is attributed to the increased nucleation density. In the case of a plasma system such as plasma-enhanced chemical vapor deposition (PECVD), the radicals, unique species, and excited species formed in the plasma may play an important role in adsorption and deposition from a gaseous precursor. For example, in the deposition of silicon from silane by PECVD, it has been proposed that the formation of disilane and trisilane in the plasma and its adsorption on the surface, along with low-energy particle bombardment, is important to the low-temperature, high-rate deposition of amorphous silicon (Ref 22).

Surface roughness can also play an important role in nucleation. The typical 94% alumina used in microelectronics has a surface roughness that looks like a field of boulders several microns in diameter (see Fig. 2). Deposition on such a surface results in a high nucleation density on the tops of the "boulders" and a lower nucleation density on the sides and in the pores. Flowed glass surfaces, on the other hand, are smooth and the nucleation density can be uniform over the surface.

Establishing a New Surface. In the extreme case, a new surface layer ("glue layer," basecoat) can be used to provide a better surface for the deposition of the desired material. This is often done in the metallization systems used in microelectronics and for interconnects in integrated circuit technology. In these cases a material is deposited on the oxide/semiconductor surface that forms a desirable oxide interface (e.g., titanium or chromium). Then a surface layer is deposited that alloys with the first layer and provides the desired property (e.g., gold, copper, silver). The new surface can also be used to smooth or "planarize" the initial surface (e.g., a "flowed" basecoat layer).

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