1. Mubashir Hussain
Vacuum in Thin Films Deposition

2. 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

3. 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.

4. 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.

5. 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.

6. 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.

7. To Quantify Requirements
• Monolayer formation time
• Mean free path

8. 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.

9. 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.

10. 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.

11. This illustrates that a basic knowledge of vacuum
technology and gas kinetics is required when dealing
with thin film deposition processes.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. Evaporation Mean free path
Reactive gases
Monolayer
formation time
NucleationDecrease in K.E and effect the deposition

19. Thank you