Physical vapor deposition
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 bombardment.) The vapor then moves towards a substrate where it condenses on the surface.
|Physical vapor deposition|
|Equipment||List of PVD equipment|
More details: LNF Tech Talks, video recording and complete slides are available.
See the sub-categories for a general description of the PVD equipment at the LNF. For a complete list, please see list of PVD equipment.
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. 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 form onto surfaces, coating everything in the chamber, within line of sight, with a thin layer of the source material. Typically this deposition is done in a high vacuum chamber to minimize gas collisions of the source material on its way to the substrate and to reduce unwanted reactions, trapped gas layers and heat transfer.
The atoms in the vapor from 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 dominated by light radiation. 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 (sputtering) involves exposing a target material to a plasma (typically Ar) of ions and electrons that are used to "knock" off the target material 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 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
Deposition rate, usually expressed in Å/sec, is measured at the substrate using various methods. It is measured real-time in the evaporators and set using deposition time on the sputter tools.
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 measure of the force that the film exhibits on itself and the substrate. It is usually measure in Mega-Pascals (MPa) with positive stress 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 substrate as 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 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 to see more detailed descriptions on how deposition parameters can be used to alter these figures of merit:
Power in evaporation is typically heating boat (thermal) or electron emission current (e-beam) and while in the case of sputtering it is the plasma power which is dominated by current. Power is typically used to change deposition rate in the process. Power can be used to change film stress and substrate heating in sputtering but it usually does not change an evaporated film's characteristics.
Base or Starting pressure is a measure of the quality of the initial vacuum. Lower base pressure generally leads to films with less impurities, mostly unwanted O2, H2O and N2
Deposition pressure in the case of sputtering is the amount of Ar (and sometimes reactive gas) used during deposition. Higher pressure leads to more collisions on the target surface and more collision of the material as it makes it's way to the substrate. In magnetron sputtering there is typically a optimal pressure spot where the highest deposition rates occur but the process may be run higher or lower than this spot to tune the film for a stress value
Substrate Bias and Heating
Substrate heating can change the behavior of the deposited film by changing the stress, adhesion (typically driving off water) or chemical reactions in the film. The affect of heating varies with the material being deposited.
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:
|Low Ion Energy
||Higher Ion Energy
|High Vacuum Process
||Low Vacuum Process (process gas pressure)
|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.|
- 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