Overview of Thermal Evaporation for Physical Vapor Deposition (PVD)

One common method of Physical Vapor Deposition (PVD) is Thermal Evaporation, a vacuum-based technology that applies coatings, or thin films, of pure materials onto various surfaces. These coatings range in thickness from angstroms to microns and may consist of single materials or multiple layers.

Thermal Evaporation Process

In Thermal Evaporation, materials to be deposited may be atomic elements (metals or nonmetals) or molecules (like oxides and nitrides). The object to be coated, known as the substrate, could be a semiconductor wafer, solar cell, optical component, or other surfaces.

How It Works:
The solid material is placed in a high-vacuum chamber and heated until it generates sufficient vapor pressure, forming a vapor cloud. This vapor travels across the chamber, condensing on the substrate to create a uniform coating. Often, the material source sits at the chamber’s bottom in an upright crucible, while substrates are inverted above it, facing downward toward the vapor source for optimal coverage.

Types of Thermal Evaporation Deposition

1. Resistive Thermal Evaporation Deposition
Resistive evaporation uses an electrical filament or heating element to vaporize the coating material. Known as “boats,” these filaments (often made from high-temperature metals like tungsten) hold the coating material. This method provides high deposition rates at a low cost and is effective for materials with low melting points, offering good directionality.

Advantages:

• High deposition rates at a low cost
• Useful with metals and dielectrics (e.g., chrome, aluminum, gold, calcium)
• Ideal for materials with low melting points

Disadvantages:

• Lower film density (though it can be enhanced with ion beam assistance)
• Limited scalability and potential for contamination compared to other PVD methods

2. E-Beam Thermal Evaporation
E-beam (electron beam) evaporation is a high-tech approach that uses an electron beam to heat and vaporize material in a crucible. The high-density electron beam offers excellent deposition rates and is ideal for high-melting-point materials like tungsten and yttrium oxide. E-beam systems often support multiple crucibles for layered processing.

Advantages:

• Higher deposition rates than resistive thermal evaporation
• Suitable for high-melting-point materials
• Produces films with high purity and directional control

Disadvantages:

• High-voltage equipment requires safety precautions
• More complex and costly setup and maintenance

3. Flash Thermal Evaporation
In Flash Evaporation, a fine wire or powder of the coating material is continuously fed into a hot crucible or element, vaporizing upon contact. While deposition rates with a wire are limited by diameter, flash systems with crucibles achieve faster coating. Accurate feeding of powders into the crucible ensures consistent film thickness.

Monitoring and Control: Quartz Crystal Control

Most Thermal Evaporation systems (resistive, e-beam, or flash) are equipped with quartz crystal monitors that track deposition rate in real-time, simplifying thickness control. Systems can also include various options like ion sources for substrate cleaning, multi-source co-deposition, or cycle load lock stations.

Additional Options and Accessories

Additional accessories, like residual gas analyzers (RGA) and cryogenic pumps, enhance system performance and control. These options make Thermal Evaporation a versatile choice, offering high deposition rates, real-time monitoring, and precise directional control for applications such as patterned coatings in lift-off processes.

Conclusion

Thermal Evaporation is a flexible and cost-effective PVD method suitable for a wide range of materials and applications, from semiconductor fabrication to high-quality optical coatings.