Evaporator

Evaporation is another common form of PVD (Physical Vapor Deposition) used for coating samples for observation in the electron microsocpe as well as the preperation of thin films of carbon used as sample support films in the transmission microscope.

The basic opperating mechanism of the evaporator is described by its very name, evaporation. The substance which is to be coated upon the sample is heated to the point where neutral atoms can escape the bulk of the substance. These atoms then travel in a streight line from the surface they where liberated from, then recondence on a surface of lower temperature then the bulk substance. The atoms may further be scattered by colision with other atoms of the same substance or residual gas atoms within the vacuum chamber. If coating is performed at high pressure >1e-4 mBar, the atoms will scatter in directions not within line of sight from source to sample, thus depositing in areas not directly visible from the source. This comes at the cost of corser surfaces, inferrior adhesion and inpurities within the thin film.

Evaporator types

Thermal Evaporator

The thermal evaportator is among the simplest of the evaporation type setups. It consists of a filamanet or metal strip with indentation (boat) which is loaded or made up of the material to be deposited. In the case of filaments, the source material is usually in the form of a wire, which is either hooked or wrapped around the filament. Where as in the boat it is usually granuals placed within the troth.

Filament Type

The filament type thermal evaporator is the most common used in laboratory enviroments, especially in the past. This is mostly due to the ease with which the filments can be made and replaced, though this is dependent on the shape and or coating applied to the filament in use.

The filament shapes of the evaporation source effect the source size, heat distribution and amount of material which can be evaporated. The most common shape of the filament is a V shape. The source material is hooked or wound onto the apex of the inverted V. This type of filament has a very small source size, which results in sharp shadow contrast due to line of sight effects.

Other common filament shapes are a simple bar, which was used to coat samples for the scanning electron microscope, due to the large area from which material evaporates from, this filament type is more likely to fully coat the surface and features of complex structures viewed in the SEM.

Another common shape is the spiral, either flat or conical. This filament type allows the evaporator to be used with granuals instead of wires, but caries with it a larger source size and higher complexity of making then the V shaped filament.

Typically the filament is made of Tungsten wire, though Molybdenum and Tantalum wires may also be used.

Furthermore the filaments may be coated in aluminium oxide or other refractory ceramics to prevent alloying of the source material with the filament, such as in the case of aluminium and tungsten.

Boats

These are a strip of metal, typically molybdenum, which has either an indent pressed into it, or otherwise has a receptical formed to allow the source material to be placed within. The boats also typically have a thinner section at the middle when viewed from the top, this is used to increase resistance in that area, thus focusing the heat.

Carbon Rod

The Carbon Rod evaporator source consists of 2 carbon rods, both narrowing to a point, where one is a truncated cone and the other a fully cone. These rods are then mechanically pressed against eachother, making an electrical connection. When current is passed through these rods, the very tip sublemates, thus releasing carbon vapor into the vacuum chamber. Typically this is done in short high current impulses.

In addition to use of pure carbon (Spectral Carbon), the tips may be further furnished with a thin wire of metals like Gold or Platnium. These will evaporate together with the carbon, and are used in some high resolution coating schemes such as that described by Hitachi in the S-800 User manual.

Electron Beam Evaporator

The electron beam evaporator is a more refined methode of heating the source material to a temperature at which it evaporates. There are two basic setups by which this is acomplished. Heat for this proscess is supplied by means of a high current beam of electrons impacting the surface of the source material, thereby heating it. The electron beam evaporator has a few advantages over the pure thermal evaporator, mostly in that the maximum temperature atainable is not limmited to the melting point of the support material, as is the case with thermal evaporators. Thus refractory metals like tungsten can be evaporated as well. Additionally the resulting layer on the substrate is usually of much higher purity then with a thermal evaporator.

The electron source can have several geometries, these include but are not limmited to the fallowing:

Radial

This type of Electron beam evaporator source, also known as an electron beam hearth, is among the simpler and more compact setups, as it dose not rely on the use of magnets to deflect the electron beam on to the target material. This source consists of shielding block surounding a anular filament at which center is placed the target material, typically in the form of a rod. A small aperture above this rod allows the gasseos atoms to escape the hearth and make their way to the substrate.

An example of such a hearth design may be found in the BAF-300 from Balzers.

Beam deflection

Unlike the Radial source, the Beam deflection source has the advantage of being able to change the source material under vacuum. This is due to the electron gun not surounding the target, but instead being 270° rotated away from the target. The electron beam is then deflected in a 270° arc onto the target material, which typically is in the form of pellets placed within a copper crucible, which intern is placed within a water coole carosell atop the evaporator source.

Due to the need to form the electron beam away from the target, and having to bend it by 270°, this source is typically much larger in size, when compared to the radial type.