Mubashir Hussain

Vacuum in Thin Films Deposition

Vacuum

An enclosed space from which matter, especially air, has been partially removed

so that the matter or gas remaining in the space exerts less pressure than the

atmosphere

Historical facts

• Early references to the science of thin film deposition include the research

conducted by Michael Faraday in 1857.

• In this series of experiments, Faraday created thin metallic films by exploding

metal wires in a vacuum vessel.

Pressure Regimes

• Viscous flow regime

Describes the case where gas flows as a fluid, where the

mean free path of the gas molecules is much smaller than the dimensions of

the apparatus.

• Molecular flow regime

Describes the high-vacuum case, where the mean free path is much longer than

the characteristic dimensions of the apparatus.

Physical Vacuum Deposition (PVD)

Process

1. Creation of an evaporant from the source material

2. Transport of the evaporant from the source to the substrate (item to be

coated)

3. Condensation of the evaporant onto the substrate to form the thin film

deposit.

Reasons For Using A Vacuum When

Depositing Thin Films

• Keeping reactive gases out of the growing film.

• Keeping a high arrival energy of the depositing species.

To Quantify Requirements

• Monolayer formation time

• Mean free path

Monolayer formation time

• To get the monolayer formation time we assume all of the arriving gas sticks

to a surface and that it forms a neat monolayer of stuck atoms. The number

we get will depend on the gas species, the pressure and the gas temperature.

Example

• If we are growing our thin film at 1 nm/s, a typical atom is say 0.2nm

diameter, so our film is growing at say 5 monolayers per second. The time

taken to form a monolayer of thin film is then 0.2 seconds. To get a pure

film we need to make sure that a monolayer of reactive gas takes much

longer than 0.2 seconds to form.

Mean free path

• The thin film microstructure depends on the arrival energy of the species

forming the film. The energetic species have to make it from the sputter

source or ion gun to the substrate without losing energy. Collisions with gas

atoms will cause these energetic species to lose energy. The distance between

collisions will again depend on the species involved, the pressure and the gas

temperature.

This illustrates that a basic knowledge of vacuum

technology and gas kinetics is required when dealing

with thin film deposition processes.

Why this process is best conducted under

vacuum

• The process of evaporation involves significant amounts of heat, if oxygen

were present, any reactive metal would form oxides

• Collisions with gas molecules during the transport of evaporant from source

to substrate would reduce the net deposition rate significantly, and would

also prevent growth of dense films.

Nucleation

In nucleation, the atoms and molecules which are arriving (called ad atoms) at

the surface lose thermal energy to the surface, and the surface absorbs that

energy. Depending on the amount of thermal energy the atoms and the surface

have, the atoms move about on the surface until they lose the thermal energy

required to move about the surface.

Desirable Properties

1) High chemical purity.

2) Good adhesion between the thin film and substrate.

3) Control over mechanical stress in the film.

4) Deposition of very thin layers, and multiple layers of different materials.

5) Low gas entrapment.

Strategies for making better vacuum system

• There are adsorbed gases from contaminated surfaces, they may be there due

to weak van der Waals interaction or chemically bonded to surface by strong

ionic or covalent bonds.

• Elevated temperature facilitate degassing.

• Adsorbed water from surface removal

• Traces quantities of water vapor may form particles or oxide defects in

semiconductor processing steps.

Strategies for making better vacuum system

• Beyond thermal methods, electron, ion and photon beams have also been

employed in detaching adsorbants from vacuum hardware. Collision of

energetic charged particles with adsorbed gases enhance chemical reaction

that lead towards their removal. In this way lower pressures attained and in

shorter time.

Comparison between PVD and CVD

• Some physical vapor deposition processes have

to be carried out in ultra-high vacuum where almost no collisions between

residual gas molecules and the coating material will take place.

• In contrast to this, there are chemical vapor deposition processes, where

frequent gas collisions must take place to obtain uniform coatings of

irregular substrates.

EvaporationMean free path

Reactive gases

Monolayer formation time

NucleationDecrease in K.E and effect the deposition

Thank you