Physical vapor deposition
|Physical vapor deposition|
|Equipment||List of PVD equipment|
Physical vapor deposition (PVD) describes a variety of vacuum deposition methods used to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces (e.g., onto semiconductor wafers). The coating method involves purely physical processes such as high-temperature vacuum evaporation with subsequent condensation, or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in chemical vapor deposition.
- 1 Technologies
- 2 Figures of merit
- 3 Method of operation
- 4 Parameters
- 5 Materials
- 6 Equipment
- 7 See also
- 8 Further reading
Evaporation is the method where source materials are heated to high temperatures where they melt or sublimate into a vapor. These atoms then precipitate into solid form, coating everything in the chamber, within line of sight, with a thin layer of the anode material. Typically this deposition is done in a vacuum to allow for a collision-free path of source material to the substrate and to reduce unwanted reactions, contamination 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.
In thermal evaporation, small amounts of source material is heated on a resistive "boat" which has current passed thru it.
Electron Beam Evaporation is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a tungsten filament under high vacuum. The accelerated electrons strike the target and melt/sublimate the material to transform into the gaseous phase. Electron beam evaporation allows for higher temperatures, better purity and more material versatility than thermal. However, there are some stray low-level x-rays that may affect e-beam resist.
Sputter Deposition involves exposing a target material to a plasma (typically Ar) which creates accelerated ions and electrons to "knock" off the target material into 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 energy of the atoms 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)/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 mega-Pascals (MPa) with positive stress being called "tensile" and negaitve 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.
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.
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 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.
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.
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 depositted. Typical limitations involve the quality of the deposited film (adhesion, other issues) or the suitability and safety of the material under vacuum.
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.
Complete tool list
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