|Other Names||Electron beam evaporation, e-beam evaporation, thermal evaporation|
|Materials||Ag, Al, Al2O3, Au, Cr, Cu, Fe, Ge, In, MgF, Ni, NiCr, Pd, Pt, Sn, SiO2, SiO, Ti, TiO2, Zn, ZnSe|
Evaporation Evaporation is a form of physical vapor deposition in which a target material is heated under high vacuum. The heated target material will then melt and evaporate or sublime to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber (within line of sight) with a thin layer of the target material.
- 1 Method of operation
- 2 Applications
- 3 Figures of merit
- 4 Materials
- 5 Equipment
- 6 See also
- 7 Further reading
Method of operation
Evaporation can be done on blanket substrates (for later etching or just blanket deposition), thru shadow masks or samples that are prepared for liftoff by photo-patterning. Samples should be clean and vacuum-safe and it is recommended that if there is any organic processing done to them (particularly photolithography) that they are cleaned in an oxygen plasma.
Samples are placed in a vacuum chamber with the proper source materials and then pumped down to high vacuum (typically 10-6 Torr.) The source material is then slowly heated while being shielded from the samples by a shutter. Once the preheat recipe is finished, the shutter opens and the deposition is measured real-time by a quartz crystal monitoring system which controls the heating power to maintain a set deposition rate and then closes the shutter when final deposition thickness is reached. The shutter is closed, power is ramped down and the tool is allowed to cool for a few minutes. Finally the tool is vented and the samples are removed.
Method of Heating
The LNF uses two types methods to apply heat to the source material. In thermal evaporation, small amounts of source material are heated on a resistive "boat" which has high current passed thru it while in electron beam evaporation the source material is heated by electrons that are created by a tungsten filament and then accelerated toward the target material. The two methods have their advantages and disadvantages. E-beam evaporation is directed at heating only the source material and therefore is better from a purity standpoint and for evaporating higher temperature metals or depositing at higher rates. Thermal evaporation is typically used for lower melting point materials or in the case of liftoff on e-beam resist where there is potential for damage from stray x-rays created in e-beam evaporation. Both methods have similar operation, parameters and figures of merit.
Evaporation is typically used to deposit metals and certain other materials in thin films. Because it involves a small point-source and does not give kinetic energy to the source material, evaporation is very directional and the heat transfer to the substrate is typically minimal.
This makes evaporation best for:
- Lift-off processing of metals and some dielectrics
- Any process where substrate heating has to be minimized
- Processes where directional coatings are desirable
Evaporation is a poor choice for:
- Conformal coatings
- Thin, pinhole-free coatings
- Coatings of alloys or compounds where melting points vary for each component
- Films where stress tuning and/or certain crystal structures are needed.
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 using quartz crystal monitors.
- In the Evaporators deposition rate is measured real-time using quartz crystal monitors. The number measured on these monitors is then converted (via the Tooling Factor) to actual deposition on the substrates which is displayed on the controller. Deposition rate can programmed by the user and the controller uses the crystal feedback to vary the electron-beam power (filament current) or current on the source material to match the programmed rate.
- The Tooling Factor is determined by the geometry of the chamber, the size and shape of the beam sweep and the nature of the cone of material evaporated. It is tuned periodically using monitor runs to get accurate film thickness on the substrate.
- Deposition rate does NOT affect stress, uniformity or most other film properties in evaporation.
- Heat transfer to the substrate is not usually decreased with lower deposition rate as most of the heating is optical and exposing the substrate to the optical heating for longer time often creates more heating.
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.
- In evaporation we have a point-like source that puts out a cone of material. Deposition on a substrate is mostly determined by 1/(throw distance)2 so films are typically uniform over a spherical surface.
- On a flat surface, the thickness variation center to edge is dominated by the difference in throw distance from the center to the edge of the substrate. : Uniformity = (Substrate Width)2/2*(Throw Distance)2
- In general, longer throw distances, closer to the center axis of the source will yield the best uniformity as the throw distance of the center and edge of the wafer are minimized. Longer throw distance does lower the deposition rate and efficiency of material, so a balance must be struck.
- There is some angular affect so "uniformity plates" are installed in the larger-domed evaporators in the LNF to minimize the lower deposition on the outer edges. These plates work in conjunction with substrate rotation.
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 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.
- In evaporation, without ability to heat or bias the substrate, the low energy of the atoms being evaporated does not allow for stress tuning
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.
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.
- In evaporation, the LNF tools are all designed for liftoff so step coverage is minimized. You can deposit at an angle but it will only coat one sidewall at a time.
The following materials can be deposited at the LNF using evaporation:
- Aluminum oxide(Al2O3)
- Gold (Au)
- Magnesium flouride (MgF)
- Nickel (Ni)
- Nichrome (NiCr)
- Silicon dioxide(SiO2)
- Silicon monoxide(SiO)
- Titanium dioxide(TiO2)
- Zinc selenide(ZnSe)
The LNF has 5 evaporators
- Angstrom Engineering Evovac Evaporator
- Enerjet Evaporator
- Cooke Evaporator
- SJ-20 Evaporator
- SJ-26 Evaporator
Angstrom Engineering Evovac Evaporator
- Materials Deposited: Ag, Al, Au, Cr, Cu, Ge, Ni, Pt, Pd, Ti, Al2O3, SiO2, TiO2
- The Angstrom Engineering Evovac is a combination electron beam evaporator and thermal evaporator used to deposit Ag, Al, Au, Cr, Cu, Ge, Ni, Pt, Pd, Ti, Al2O3, SiO2 and TiO2 with the ability to heat the substrates and do in-situ substrate cleaning using an ion mill. It is an open loop box coater with a dome that can run multiple wafers and substrates in each run.
- Materials Deposited: Al,Cr,Au,Pt,Ti
- The Enerjet is an e-beam evaporator used to deposit materials on a 9-wafer dome in a batch process. It is typically used for depositing common metal stacks for blanket coating or liftoff processing.
- Materials Deposited: Ni or NiCr, Au or Ag, In or Sn or Cu or Pd, Zn
- The Cooke is an e-beam evaporator used to deposit materials on a single-wafer fixture. It has a 4-pocket gun which supports multiple materials and is typically used for depositing less common metal stacks.
- Materials Deposited: Au,Ge,Ni,Ti
- The SJ-20 is an e-beam evaporator originally used to deposit contacts on III-V substrates (it can be used for any substrate now.) It has a 4-pocket gun and a 9-wafer dome.
- Materials Deposited: Ag, Al2O3, Au, Fe, Ge, MgF, MgO, Pd, Px, SiO2, SiO, Ti, TiO2, ZnSe
- The SJ-26 is an e-beam evaporator used to deposit optical compounds and dielectric materials. It has a multi-wafer dome for thinner depositions and a single-wafer jig for thicker stacks. There are metals supported on this tool but, due to the dirty nature of some optical compounds, the base pressure of the tool is much higher and the films may be of lower quality.