One of the common methods of Physical Vapor Deposition (PVD) is Thermal Evaporation. This is a form of Thin Film Deposition, which is a vacuum technology for applying coatings of pure materials to the surface of various objects. The coatings, also called films, are usually in the thickness range of angstroms to microns and can be a single material, or can be multiple materials in a layered structure.
Thermal Evaporation Process
The materials to be applied with vacuum Thermal Evaporation techniques can be pure atomic elements including both metals and non metals, or can be molecules such as oxides and nitrides. The object to be coated is referred to as the substrate, and can be any of a wide variety of things such as: semiconductor wafers, solar cells, optical components, or many other possibilities.
Thermal Evaporation Deposition involves heating a solid material inside a high vacuum chamber, taking it to a temperature which produces some vapor pressure. Inside the vacuum, even a relatively low vapor pressure is sufficient to raise a vapor cloud inside the chamber. This evaporated material now constitutes a vapor stream, which traverses the chamber and hits the substrate, sticking to it as a coating or film.
Since, in most instances of Thermal Evaporation processes the material is heated to its melting point and is liquid, it is usually located in the bottom of the chamber, often in some sort of upright crucible. The vapor then rises above this bottom source, and the substrates are held inverted in appropriate fixtures at the top of the chamber. The surfaces intended to be coated are thus facing down toward the heated source material to receive their coating.
Steps may have to be taken to assure film adhesion, as well as control various film properties as desired. Fortunately, Thermal Evaporation system design and methods can allow adjustability of a number of parameters in order to give process engineers the ability to achieve desired results for such variables as thickness, uniformity, adhesion strength, stress, grain structure, optical or electrical properties, etc.
What is Resistive Thermal Evaporation Deposition?
E-Beam Evaporation Process
There are three primary means of heating the source material.
One method, often referred to as Resistive Evaporation Deposition, uses a simple electrical resistive heating element, or filament to evaporate the coating material. There are numerous different physical configurations of these resistive evaporation filaments, including many known as “boats” – essentially thin sheet metal pieces of suitable high temperature metals (such as tungsten) with formed indentations or troughs into which the material is placed. The resistive filament heating source offers the safety of low voltage, although very high current is required, usually several hundred amps.
Some advantages of Resistive Thermal Deposition are that it can provide high deposition rates at low cost compared to other PVD processes. It is a relatively simple PVD coating process that can be used with metals or non-metals/dielectrics including chrome, aluminum, indium, gold, silver, calcium, lithium and more. It can be used with materials with low melting points and provides good directionality.
Some disadvantages with Resistive Thermal Vapor Deposition are that film densities are relatively low but can be improved with Ion beam assisted deposition. While it is a relatively lower cost to set up, it has limited scalability. Compared to more complicated PVD processes it has higher levels of possible contamination.
What is E-Beam Thermal Evaporation?
The other common heat source is an Electron Beam or E-Beam, and this is generally known as E-Beam Evaporation. This is certainly a more “high tech” approach to heating a material up, and involves some dangerous high voltage (usually 10,000 volts), so E-Beam systems always include extra safety features. The source itself is an E-Beam “gun”, where a small and very hot filament boils off electrons which are then accelerated by the high voltage, forming an electron beam with considerable energy.
This beam is magnetically directed into the crucible where the material awaits. At the standard 10 kV, even 0.1 amp of this beam current will deliver 1 kilowatt of concentrated power, and this heats the material, which is contained in a hearth that is water cooled to prevent its own destruction. It is quite common for these commercially available E-Beam guns to have multiple crucibles and thus be capable of holding several different materials at one time and easily switch between them for multi layer processing.
Some advantages of E-Beam Evaporation are that it has superior deposition rates than Resistive Thermal Evaporation or Sputtering. It’s better for higher melting point materials as compared to resistive thermal evaporation. It produces films with high levels of purity, high coating utilization efficiency and good directionality.
E-Beam Evaporation is good for coating materials that have high melting points including yttrium oxide, hafnium oxide, molybdenum, silicon, tantalum, tungsten and others.
Good for high-volume batch production, it is widely used for the making of solar panels, eyeglasses and architectural glass, and laser optics.
There are some disadvantages included with E-Beam Evaporation Systems as they involve very high voltages that can be hazardous and require extensive safety precautions. It requires moderately complex set-up and maintenance costs.
What is Flash Thermal Evaporation Deposition?
Flash Thermal Evaporation is where a fine wire or powder of coating material is used and sometimes continuously fed into a hot ceramic crucible or heating element that evaporates nearly instantly when the power is applied, or when continuously fed, on contact to the hot element. While deposition rates and thicknesses using a wire are limited by the wire’s diameter, Flash Thermal Evaporation using crucibles is faster. How coating powders are fed into the crucible is a crucial success factor for achieving consistent thickness in films.
Quartz Crystal Control
Most Thermal (Resistive, E-Beam or Flash) Evaporation PVD systems include quartz crystal deposition control, whereby real time deposition rate monitoring and control takes most of the work out of achieving the right thickness. Thin Film Evaporation systems can also be configured with various hardware or software options. These can include ion source capability either for in situ cleaning of substrate surfaces or for ion assisted deposition, or substrate pre heat stations.
Other options can include multiple quartz crystals, co-deposition with multiple sources (either type), or fast cycle load lock stations.
Accessories such as residual gas analyzers (RGA’s), and other custom features and specialized automation are also available to help you perfect your Thermal Evaporation system and process methods. Cryogenic pumps are the most popular type of high vacuum pump for Evaporation, but other options are available if desired. Regardless of which options are selected, Thin Film Evaporation systems can offer the advantages of relatively high deposition rates, real time rate and thickness control, and (with suitable physical configuration) good evaporant stream directional control for such purposes as Lift Off processing to achieve direct patterned coatings.
Norm Hardy is a Process Engineer at Semicore Equipment Inc., a worldwide supplier of high performance thermal evaporation deposition systems. Please allow our helpful support staff answer any questions you have regarding “What is Thermal Evaporation Deposition?” and the best methods and techniques for your specific Vacuum Thermal Evaporation Deposition Equipment needs by contacting us at sales@semicore.com or by calling 925-373-8201.
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