Resistive Thermal Evaporation
Resistive evaporation is a popular physical vapor deposition technique because of its simplicity. During this process, a material in a high-vacuum environment is heated to its evaporation point using electrical energy. The vaporized molecules then travel from the source to the substrate where they nucleate together, forming a thin-film coating. Materials that can be deposited using this technique include aluminum, silver, nickel, chrome, magnesium, among many others.
High levels of vacuum are needed for resistive evaporation for two reasons. The first is that when gas is evacuated from a chamber, vapour molecules inside it can travel longer distances before they collide with a gas molecule.
Collisions with gas molecules are undesirable during evaporation because they change the direction of travel of material vapour and thus can adversely affect the coverage of the substrate. When the gas pressure is below 10-5 Torr, the average distance of travel of a vapor molecule before colliding with a gas molecule (also called the mean free path) is greater than 5 meters, which is typically larger than the chamber dimensions. This means that the molecules would travel in a straight line from the source to the substrate, making resistive evaporation highly directional. This is an important characteristic when doing a deposition for a lift-off process in micro and nano-fabrication.
The second reason why high vacuum is important is film purity. Gases that are present in air can be deleterious to film properties if they somehow get incorporated into the deposition. This is especially true of oxygen, which can oxidize metal films. By pumping away most of the gases and going into the 10-6 Torr range or below, the purity of evaporated films is greatly improved.
There are many applications that use resistive thermal evaporation. Many of our customers use it to deposit metallic contact layers for their thin film devices, such as OLEDs, solar cells, and thin-film transistors. Some of our customers also use this technique to deposit thick indium layers for wafer bonding. The deposition rate can be controlled using a quartz crystal rate sensor, temperature, or optical monitoring systems to ensure consistent high-quality results.
We routinely create custom deposition systems that feature resistive thermal evaporation sources, either by themselves or in conjunction with other deposition techniques.
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