Changes in Microstructure and Morphology during Deposition

Film microstructure, morphology, and properties can be influenced by processes that occur after adatom condensation but during film growth. The processes that change the film properties include:

• Mass transport, such as growth of the interfacial region and crystal defect formation and void coalescence

• Recrystallization and grain growth

• Phase precipitation and growth

• Chemical reaction of codeposited species

Stress annealing

Many of these changes are time- and temperature-dependent and therefore depend on the thermal history of the film during deposition. This thermal history depends on the deposition temperature, condensation energy release, deposition rate, deposition time, thermal conductivity of the film and substrate materials, heat removal mechanisms, and so on.

Modification of Film Morphology. The film morphology and microstructure can be modified during growth. Methods of modifying the columnar growth include continuous or periodic:

• Bulk or surface heating by laser or radiant heating

• Codeposition of alloying species

• Reaction with ambient gaseous species

• Changing angle of incidence of adatom flux

• Mechanical disruption of the growth

• Changing deposition rate

• Changing gas pressure

• Bombardment by massive energetic particles

Heating. Increasing the deposition (substrate) temperature changes the film morphology, as shown in Fig. 4. Surface heating by a laser or by a radiant heat source can change the surface mobility of the adatom, thereby changing the growth mode of the deposited material.

Codeposition of Alloying, Impurity, or Dopant Species. Codeposition of alloying species can change the surface mobility and thus the film morphology. Alloying species that lower the melting point of the deposited material are particularly effective.

Impurities are small amounts of material incorporated into the structure unintentionally. The addition of small amounts of an impurity can have a significant influence on the microstructure and properties of the growing film. Small amounts of nitrogen in aluminum films change the electromigration properties. Residual gases present during deposition can affect film stress.

Dopants are small amounts of materials that are deliberately incorporated into the structure to affect microstructure or film properties. For example, the inclusion of gold, nickel, or cobalt (5 wt%) in sputter-deposited MoS2 films for solid film lubricants produces "compact" films that are physically stronger to greater thicknesses than MoS2 films sputtered without the additions.

Periodic Injection of Reactive Gas. Periodic reaction of the surface of the growing film with ambient species generates a new surface chemistry. This can force the adatoms to renucleate on the "foreign" surface, thereby changing the film morphology. For example, the periodic injection of oxygen during aluminum deposition suppresses the columnar growth morphology (Ref 38). The same effect is seen for nitrogen on beryllium films. The effect is similar to the "brightening agents" used in electrodeposition.

The angle of incidence of the adatom flux is important to the growth of the columnar morphology. The angle of incidence can be modified by proper fixturing and position control. Generally it is not possible to optimize the angle of incidence for all surface features.

Mechanical Disruption during Deposition. Mechanical disruption of the growing surface can be used to change the columnar morphology. Periodic mechanical brushing of a surface during deposition has been used to densify and eliminate porosity in sputter-deposited aluminum films and in CVD-tungsten deposits.

Changing Gas Pressure during Deposition. For gaseous environments, where vapor phase collision processes are important, changes in the ambient pressure change the scattering and thermalization of the adatom flux, and, in the case of sputter deposition, the flux and energy of reflected high-energy neutrals. These factors are important in determining the film morphology, stress (Ref 39), and other properties.

Concurrent Massive Energetic Particle Bombardment. In ion plating, the growing film is subjected to continuous or periodic energetic particle bombardment during deposition. When massive energetic particles bombard a surface, they release their energy rapidly by physical collisions with the near-surface atoms. These physical collisions cause a number of effects, some of which modify the columnar morphology of the growing film. These effects include:

• Sputtering and redeposition

• Input of "heat" into the surface and the near-surface region

• Increased or decreased surface mobility of the depositing atoms

• Generation of point defects that act as nucleation sites

Typically, a film sputtering rate of 20 to 40% of the deposition rate or an energy input of about 20 eV per depositing atom is necessary to completely disrupt the columnar growth process.

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