Electron beam evaporation
|Electron beam evaporation|
|Other Names||e-beam evaporation|
|Materials||Ag, Al, Al2O3, Au, Cr, Cu, Fe, Ge, In, MgF, Ni, NiCr, Pd, Pt, Sn, SiO2, SiO, Ti, TiO2, Zn, ZnSe|
Electron beam evaporation (e-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. These atoms then precipitate into solid form, coating everything in the vacuum chamber (within line of sight) with a thin layer of the anode material.
Method of operation
This section requires expansion.
This section requires expansion.
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 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.
- 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 e-beam 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 4 e-beam evaporators
- 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.