Thin Film Deposition by Thermal Evaporation Method
Thermal evaporation is one of the most popular among the deposition methods. Simplicity of operation and proper speed are the notable strengths of this deposition method. Thermal evaporation is one of the Physical Vapor Deposition (PVD) methods during which a thin film is deposited on the substrate during a physical process.
Thermal deposition mechanism
In the process of thermal evaporation, the target material is located inside an evaporation source (boat, coil, and basket) which is heated by the passage of electric current. The target material inside the evaporation source is heated to the evaporation point. Because heat generation is due to electrical resistance of the evaporation source, this method is also called resistive evaporation. After evaporation, the molecules of the target material move to the substrate and form a thin film on the surface of the substrate. Many materials can be deposited using this method, including Aluminum, Silver, Nickel, Chromium, Magnesium, etc.
One of the important features of vacuum evaporation systems is their ultimate pressure. In the process of thermal evaporation, if the evaporated molecules of the target material in their path to the substrate collide with the molecules of gases in the chamber, they will undergo undesired changes in their path and this will negatively affect the quality of the sample coverage. To prevent this, the thermal evaporative deposition process must be performed in a high vacuum environment. At a pressure of 10-5 Torr, the mean free path length of the molecules is 1 meter. This means that the average distance traveled by the molecules in the vacuum chamber, before colliding with another molecule, is 1 meter. As a result, it can be said that the molecules of the vaporized material can reach the substrate in a straight line without colliding with another molecules.
On the other hand, the presence of background gases in the deposition chamber may contaminate the deposited thin film. For example, the presence of a small amount of oxygen or moisture during thermal evaporation in the process of Organic Light-Emitting Diode (OLED) and organic photovoltaic devices fabrication, makes the active functional parts of these devices inefficient. By reducing the pressure of the deposition chamber to 10-6 Torr, the purity of the deposited thin film and the performance of the devices will increase significantly.
Applications of thermal evaporation deposition
Thermal evaporation deposition has variety of uses, including the following.
- Creating metal bonding layers in devices such as OLEDs, solar cells and thin film transistors.
- Create a thin layer of Indium on the back of the ceramic sputtering targets to connect the metal backing plate.
- Deposition of metal thin film (often aluminum) on polymers for use in food packaging and heat and sound insulation.
Metallized polymer films are polymer films that are coated with a thin layer of metal (usually aluminum) using thermal evaporation deposition method. These films have a shiny, metallic appearance like aluminum foil, but are lighter and cheaper than aluminum foil. These films are mostly used for decorating purpose and packaging food products.
Polypropylene and polyethylene terephthalate (PET) polymers are commonly used for metallization. Metallized PET films are used in NASA spacesuit to reflect heat to keep astronauts warm. PET in firefighter’s uniforms is also used to reflect the heat radiated from the side of the fire. Other applications of these metallized films, which are deposited by thermal evaporation coating, include emergency aluminum blankets, which are used to conserve the shock patients body heat. Metallized PET films are also used in the construction of anti-static electricity and anti-heat and soundproof enclosures in aircraft.
Thermal evaporation deposition system model DTT
Desktop, Turbo pumped, Thermal Evaporation deposition system (DTT) is a small vacuum deposition system that is suitable for research laboratories due to the small footprint. The DTT is equipped with a Turbo Molecular Pump (TMP) and the ultimate pressure of the deposition chamber reaches to 7*10-7 Torr. Thermal evaporation deposition at this pressure creates thin film with good quality and performance. The Turbo Molecular Pump used in the system are supplied by Leybold, a Germany company. TMP are much smaller than diffusion pumps with the same efficiencies as diffusion pumps. They occupy very little space in the laboratory.
In comparison with a diffusion pumps, the required time to reach the desired pressure in a thermal evaporation process is less than half when TMPs are used. TMPs do not need cooling water and oil. This eliminates chance of returning oil to the vacuum chamber when compared with diffusion pumps.
Relatively small vacuum chamber of this deposition system allows the pressure of the device to reach its minimum in a short time. Pressure of the deposition environment during the thermal evaporation process plays a significant role in the process of sample coverage and the quality of the thin film deposited by this method. For detailed information about the pressure inside the vacuum chamber, reliable and accurate pressure gauges need to be used. For this purpose, the pressure gauge used in the DTT model deposition system is full range pressure gauge made by Leybold Company which has the ability to display pressure in the range of atmosphere pressure up to 10-9 Torr.
The DTT is equipped with three separate evaporation sources that make it suitable for multilayers deposition. The sample holder in the DTT device has the ability to move and position accurately in front of any evaporator source (Boat selection option). The user can deposit multilayers without breaking the vacuum and just by moving the electrical connection to power each evaporation source. Another feature of this thermal evaporation deposition system is the possibility of creating alloys. Different materials can evaporate from two or three evaporation sources simultaneously in a controlled manner by connecting two or three sources of evaporation to the power supplies at the same time and with independent control of each other,
Separating walls embedded between evaporation sources prevents the effects of various evaporating materials on each other and contamination. DTT thermal evaporator is equipped with a Mass Flow Controller(MFC) for gas injection into the vacuum chamber for deposition of thin film in the presence of background gas. The high accuracy of the MFC used in the DTT thermal evaporator system allows the user to inject gas into the vacuum chamber with accuracy of one SCCM.
Another feature of DDT system is preheating to remove contaminants from evaporative materials before the coating process. This is to prevent deposition of the contaminants that rises from the evaporation sources or evaporative material due to the heat before evaporation of the main material. During the preheating process, electronic shutter embedded inside the vacuum chamber will be placed in front of the substrate and prevent deposition of the contamination on the samples.
For more information about the DTT thermal evaporation device, visit the website of Vac Coat Ltd.