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Physical vapor deposition

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[[{{PAGENAME}}|Physical vapor deposition (PVD)]] is a type of deposition where source materials are transformed into a vapor or plasma using a physical process (typically heating or physical bombardment.) The vapor then moves towards a substrate where it condenses on the substrate surface.
More details: LNF Technology seminar: PVD February 27, 2015: [ User_Resources#LNF_Tech_Talks_.28technology_seminar_series.29 LNF Tech Talks], video recording] and [ complete slides]are available.
Many materials can be deposited using PVD. It is typically used for metals and harder insulators but, anything that can be heated to vaporization or bombarded, can be depositteddeposited. Typical limitations involve the quality of the deposited film (adhesion, other issues) or the suitability and safety of the material under vacuum. To see a complete list of PVD films currently supported in the LNF with maximum thicknesses listed, see [[LNF PVD Films]].
There are a variety of Physical Vapor deposition techniques including Pulsed Laser and Pulsed Electron Deposition, Cathode Arc depostion and many more techniques. The two forms of PVD used in the LNF are Evaporation and Sputter Deposition.
Evaporation is the method where source materials are heated to high temperatures where they melt and then evaporate or sublimate into a vapor. These atoms then precipitate into solid formonto surfaces, coating everything in the chamber, within line of sight, with a thin layer of the anode source material. Typically this deposition is done in a high vacuum chamber to allow for a collision-free path minimize gas collisions of the source material on its way to the substrate and to reduce unwanted reactions, contamination trapped gas layers and heat transfer.
The atoms in the vapor from evaporation have only thermal energy and strike the substrate with very little kinetic energy. When depositing thin films in a vacuum chamber that have or no kinetic energy, only the light and heat transfer from the source will cause any heat transfer hot sources to the waferssample is dominated by light radiation. All the The evaporators in the LNF are dome/liftoff tools with long throw distances with cold walled chambers and small/centered point sources. This means that, with the directionality of the evaporation, the material will strike the substrate as a normal angle and, with low heat transfer, the substrates do not get very hot as the films is deposited. This makes them ideal for liftoff applications, depositions where the substrates cannot handle any plasma heating and thicker films. They are poorly suited for any application requiring sidewall coverage or controlled stress or stoichiometry.
===Sputter deposition===
{{main|Sputter deposition}}
Sputter deposition (sputtering) involves exposing a target material to a plasma (typically Ar) which creates accelerated of ions and electrons that are used to "knock" off the target material into and make a cloud of source atoms. The source vapor then condenses onto the substrate forming a thin film.
Sputtering creates energetic atoms that move and collide as they travel thru the gas plasma towards the substrate. These atoms therefore come in at various angles and hit the substrate with some energy defined by the gas pressure and target voltage. Because of the non-normal nature of the plasma, sputtering does coat the sidewalls of the features on the substrate and the kinetic energy of the atoms also causes heating of the substrate during deposition. The heating and sidewall coverage make sputtering less desirable for liftoff applications but more useful when [[conformality|conformal]] coatings are needed. Film stress and chemistry can also be better tuned in sputtering using plasma power/pressure settings and by injecting reactive gasses during deposition.
==Figures of merit==
Uniformity measures the variation in thickness across a substrate and is usually expressed as a percentage. Typically: (Thickness Max - Thickness Min)/(2*Thickness Average).
Uniformity is typically set by the material being deposited and the geometry of the system: throw distance, substrate rotation and deposition angle.
Stress is a a measure of the force that the film exhibits on itself and the substrate. It is usually measure in megaMega-Pascals (MPa) with positive stress being called referred to as "tensile" and negative stress referred to as "compressive." Stress in thin films can affect devices and substrates as well as poorly affect adhesion and other properties. In terms of deposition parameters, stress is affected by the energy and angle of the material as it strikes the substrateas well as chamber pressure.
Resistivity is an electrical measurement of the characteristic of the film . It can be measured on electrical structures (lengths of wiring lines) or on blanket films using the four-point probe. It is expressed in many units, typically μ-ohm-cm.
Resistivity is typically used to measure the quality of the film in terms of source purity or vacuum purity , but it can be changed by altering the density of the film (pressure, power and bias during sputtering).
===Step coverage===
Bias can increase the energy of incoming atoms and change the stress of the film. Also, bias can cause collisions at the substrate level and redeposit atoms so it can be used to try and increase sidewall coverage.
==General Film Characteristics==
PVD films in generaly are dominated by island nucleation with low diffusion that then transforms into vertical fibers or columnar growth. Unless there is heating to temperatures that approach 50-70% of the melting point, the growth will not be crystalline with large grains. Evaporation typically grows with fibrous, domed structures while sputtering, which has a bit more energy and resputtering, approaches columnar growth.
If we combine the morphologicaly nature of the film with other factors (source size, gas collisions/mean free path and deposition angle) we can describe general trends in the film:
{| class="wikitable"
! style="font-weight: bold;" |Evaporation
! style="font-weight: bold;" |Sputtering
|'''Low Ion Energy'''
* Low heating/bombardment
* Pinholes at lower thicknesses
* Less energy for reactive films
|'''Higher Ion Energy'''
*Often denser films, better adhesion, smaller grain size
*Easier to make reactive films using injected gas.
|'''High Vacuum Process'''
*Higher directionality, poor step coverage
*Better for liftoff
*Lower impurity and less gas trapping
*Only optical/radiative heating
|'''Low Vacuum Process (process gas pressure)'''
*Less Directionality = better step coverage
*Chance of more gas entrapment in film
*Higher heat from plasma
|'''Point Source''' - poorer uniformity
|'''Larger source''' - better uniformity
|Rates dependent on melting point + vapor pressure - '''difficult to do alloys''' (co-dep recommended.) Some compounds dissociate with heating.
|Components typically sputter at similar rates when targets are alloyed to start with. Often knock off '''compounds as molecules'''.
==See also==
==Further reading==
* LNF Technology seminar: PVD February 27, 2015: [ video recording] and [https:/wiki/ complete slides29 LNF Tech Talk for PVD
; Basic Overviews of PVD and Thin Film technology:
:*Milton Ohring, ''Material Science of Thin Films'', 2nd Edition, Academic Press, 2002.:*J. L. Vossen and W. Kern, eds., ''Thin Film Processes''. Academic Press, 1978.:*Krishna Seshan, ed., ''Handbook of Thin Film Deposition'', 3rd Ed, Elsevier, 2012:*S. A Campbell, ''The Science and Engineering of Microelectronic Fabrication'', 2nd Ed, Oxford Press, 2001
[[Category:PVD| ]]
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