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-based 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 Equipment
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 equipment’s vacuum chamber, even a relatively low vapor pressure is sufficient to raise a vapor cloud. 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. Variables include thickness, uniformity, adhesion strength, stress, grain structure, optical properties, electrical properties, and more.
Resistive Thermal Evaporation
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.” These are 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 in those cases, a very high current, usually several hundred amps, is required.
Some advantages of Resistive Thermal Deposition are that it can provide high deposition rates at a low cost compared to other PVD coating machines and processes. It is a relatively simple PVD coating system 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.
A disadvantage of Resistive Thermal Vapor Deposition is that film densities are relatively low, but this can be improved with Ion beam assisted deposition. While the equipment is typically lower cost to set up, it has limited scalability compared to more complicated PVD processes. It also has higher levels of possible contamination.
E-Beam Thermal Evaporation
E-Beam Evaporation Process
The other common heat source is an Electron Beam or E-Beam which is used in a process known as E-Beam Evaporation. This is certainly a more “high-tech” approach to heating a material, and involves potentially dangerous high voltage, usually 10,000 volts. As a result, 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. 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.
An advantage of E-Beam Evaporation equipment is that it has superior deposition rates to Resistive Thermal Evaporation or Sputtering, which is better for higher melting point materials. 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. As it is also good for high-volume batch production, it is widely used in the production of solar panels, eyeglasses, 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 also requires moderately complex equipment set-up and maintenance costs.
Flash Thermal Evaporation
In Flash Thermal Evaporation 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 equipment that uses crucibles is faster. The manner in which coating powders are fed into the crucible is a crucial determining factor for achieving consistent thickness in films.
Quartz Crystal Control
It is common amongst Thermal Evaporation systems (Resistive, E-Beam, or Flash) to include quartz crystal deposition control, whereby real-time deposition rate monitoring and control helps to be more precise in achieving the right thickness. Thin Film Evaporation systems can also be configured with various hardware or software options. These can include ion source capability for in situ cleaning of substrate surfaces, ion beam 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.
ccessories 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. Regardless of which options are selected, Thin Film Evaporation systems can offer multiple advantages including 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.
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.
Semicore’s CAPOS all-in-one PVD Thermal Evaporation & Sputtering System
Semicore’s breakthrough CAPOS thin film deposition system is the PVD Industry’s first cost effective “Open-platform” design, that is ideal for both precision R&D and batch production.
Flexible enough that it can be used for either sputtering or evaporation coatings, this platform is also used for Semicore’s CAPOS-CT series, as a highly versatile “Cluster Tool” platform that can be configured with multiple Process Modules (PM) and cassette-to-cassette operation that speeds production cycles.
For more detailed information on these units’ specifications please download the PDF.
Matt Hughes is President of 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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