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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: [http://leccaplnf-wiki.engineecs.umich.edu/leccapwiki/viewer/r/EQfS99 Video User_Resources#LNF_Tech_Talks_.28technology_seminar_series.29 LNF Tech Talks], video recording] and complete slides are available. ==Equipment==See the sub-categories for a general description of the PVD equipment at the LNF. For a complete list, please see [[https:Category:PVD equipment|list of PVD equipment]].<categorytree mode=pages>PVD equipment<//umichcategorytree> ==Materials==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 deposited.box Typical limitations involve the quality of the deposited film (adhesion, other issues) or the suitability and safety of the material under vacuum.com/s/8cy227xts0yhcq0jnoll4kffh3k60rni  To see a complete slideslist of PVD films currently supported in the LNF with maximum thicknesses listed, see [[LNF PVD Films]].
==Technologies==
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===
{{main|Evaporation}}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 no kinetic energy, only the light from the source will cause any heat transfer to the wafers. All the evaporators in the LNF are dome/liftoff tools with long throw distances and small/centered point sources. 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. ====Thermal evaporation===={{main|Thermal evaporation}}In thermal evaporation, small amounts of source material is heated on a resistive "boat" which has current passed thru it.
====Electron-beam The atoms in the vapor from evaporation===={{main|Electron beam evaporation}}Electron Beam Evaporation have only thermal energy and strike the substrate with little or no kinetic energy and heat transfer from the hot sources to the sample is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off dominated by a tungsten filament under high vacuumlight radiation. The accelerated electrons strike evaporators in the target LNF are dome/liftoff tools with long throw distances with cold walled chambers and meltsmall/sublimate centered point sources. This means that, with the directionality of the evaporation, the material to transform into will strike the substrate as a normal angle and, with low heat transfer, the substrates do not get very hot as the gaseous phasefilms is deposited. Electron beam evaporation allows This makes them ideal for higher temperaturesliftoff applications, better purity depositions where the substrates cannot handle any plasma heating and more material versatility than thermalthicker films. However, there They are some stray low-level x-rays that may affect e-beam resistpoorly suited for any application requiring sidewall coverage or controlled stress or stoichiometry.
===Sputter deposition===
{{main|Sputter deposition}}
Sputter Deposition 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===
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===
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 negaitve 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===
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 Coveragecoverage===
Step coverage is the measure of how much coating is on the bottom/sidewall of a feature vs how much coating is on the top/field areas. It is highly dependent on the geometry of the features.
See the technology pages ({{main|Electron beam evaporation}}, {{main|Sputter deposition}}) to see more detailed descriptions on how deposition parameters can be used to alter these figures of merit. ==Method of operation==Samples are placed into a vacuum chamber (either set in and pumped or transferred into an already pumped chamber using a loadlock.) High purity source materials are also present in the chamber. Once the desired vacuum level is reached the tools begin heating or striking gas plasma to start the : {{main|Evaporation}}{{main|Sputter deposition process, usually under a closed shutter. The shutter opens and deposits the materials on the substrate and then the shutter closes once the desired amount is reached (time or measured deposition.) Typically PVD tools need cooling before the substrate can be vented back to atmosphere.}}
==Parameters==
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.
==MaterialsGeneral Film Characteristics==Many materials can be deposited using PVDfilms in generaly are dominated by island nucleation with low diffusion that then transforms into vertical fibers or columnar growth. It Unless there is typically used for metals and harder insulators but, anything heating to temperatures that can be heated to vaporization or bombardedapproach 50-70% of the melting point, can the growth will not be deposittedcrystalline with large grains. Typical limitations involve the quality of the deposited film (adhesion Evaporation typically grows with fibrous, domed structures while sputtering, other issues) or the suitability which has a bit more energy and safety of the material under vacuumresputtering, approaches columnar growth.
==Equipment==See If we combine the sub-categories for a general description morphologicaly nature of the PVD equipment at film with other factors (source size, gas collisions/mean free path and deposition angle) we can describe general trends in the LNF. For a complete list, please see [[film:Category:PVD equipment|list of PVD equipment]].
{| class="wikitable"! style="font-weight: bold;" |Evaporation! style=Complete tool list=="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'''.<categorytree mode=pages>PVD equipment</categorytree>|}
==See also==
* [[Thermal evaporation]]* [[Electron beam evaporationEvaporation]]
* [[Sputter deposition]]
* [[Deposition]]
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==References==
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==Further reading==
* LNF Technology seminar: PVD February 27, 2015: [http://leccaplnf-wiki.engineecs.umich.edu/leccap/viewer/r/EQfS99 Video recording] and [https:/wiki/umichUser_Resources#LNF_Tech_Talks_.box28technology_seminar_series.com/s/8cy227xts0yhcq0jnoll4kffh3k60rni 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.
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