Introduction to Thermal Evaporation for Thin Film Deposition

By AZoNano

Table of Contents

Introduction
Thermal Evaporation
Heat Source
Other Features
Conclusion
About Semicore Equipment

Introduction

Thermal evaporation is one of the most widely used techniques of physical vapor deposition (PVD). It is a type of thin film deposition, which is a vacuum process wherein coatings of pure materials are applied over the surface of many different objects. The deposited coatings or films usually have a thickness in the range of angstroms to microns and are composed of a single material or layers of multiple materials. The coating materials can be either molecules, including nitrides and oxides, or pure atomic elements such as both metals and non-metals. The object to be coated is called the substrate, which can be many different things, including semiconductor wafers, optical components, and solar cells.

Thermal Evaporation

Thermal evaporation is a process wherein a solid material is heated inside a high vacuum chamber to a temperature which generates some vapor pressure. Inside the vacuum, even a very low vapor pressure is adequate to create a vapor cloud within the chamber. This evaporated material now consists of a vapor stream, which passes through the chamber, and strikes and sticks onto the substrate as a film or coating. Since, in a majority of cases, the material becomes liquid by heating it to its melting temperature, it is normally placed in the bottom of the chamber, often in some form of upright crucible. The vapor then rises above from this bottom source and reaches the substrates that are held inverted in suitable fixtures at the top of the chamber, with surfaces to be coated facing down toward the rising vapor to acquire their coating.

Various measures may have to be taken in order to ensure film adhesion and control different film properties as desired. The evaporation system design allows process engineers to adjust a number of parameters to obtain desired results for variables such as grain structure, uniformity, thickness, stress, adhesion strength, optical or electrical properties, and much more.

Heat Source

Two heat sources are predominantly used for heating the source material, namely the filament source and the electron beam (e-beam) source. The method that uses the filament source is called as filament evaporation, wherein the heat source is a simple electrical resistive heat element or filament. There many different physical configurations available for these filaments, including the ‘boats,’ which are basically thin sheet metal pieces of appropriate high temperature metals like tungsten with formed indentations or troughs to place the material. The filament source provides the safety of low voltage, but requires very high current, typically in the range of several hundred amps.

The method that uses the e-beam as heat source is referred as e beam evaporation. It is a sophisticated technique to heat up a material and involves some risky high voltage (normally 10,000 volts). Hence, extra safety features are always incorporated into the e-beam systems. The source itself is an e-beam ‘gun,’ where electrons are boiled off by a tiny and very hot filament, followed by subsequent acceleration by the high voltage to create an electron beam with substantial energy. This beam is magnetically directed into the chamber and heats the material contained in a hearth that is cooled down by water to avoid its own destruction. These commercially available e-beam guns have multiple crucibles and can hold many different materials simultaneously, thus allowing easy switching between materials for multi layer processing.

Other Features

Most thermal evaporation PVD systems feature quartz crystal deposition control to provide real time deposition rate monitoring and control in order to achieve the desired thickness. It is also possible to configure thin film evaporation systems with many different hardware or software options. These options include ion source capability for substrate pre heat stations, ion assisted deposition, or for in situ cleaning of substrate surfaces. Fast cycle load lock stations, co-deposition with multiple sources, and multiple quartz crystals are the other options available. Moreover, accessories like residual gas analyzers and other custom automation and features are also available. Although cryogenic pumps are the most widely used high vacuum pump type for evaporation, other options are also available if required.

Conclusion

Irrespective of whatever may be the options selected, thin film evaporation systems can offer the benefits of relatively high deposition rates, rate and thickness control in real time, and better evaporant stream directional control for applications such as lift off processing to obtain direct patterned coatings. Semicore supplies a suite of high performance thermal evaporation systems that provide coatings on many different materials.

About Semicore Equipment

Semicore is a manufacturer and worldwide supplier for the electronics, optical, solar energy, medical, automotive, military and related high technology industries.

Our high-performance production or R&D vacuum sputtering and thin film evaporation systems provide coatings on a variety of materials including plastic films, glass, ceramics, metals and hybrid substrates.

Whether you want to take advantage of our proven industrial solutions for vacuum system automation, process control and supervisory monitoring applications or need to develop some unique new application of your own design you will find Semicore’s staff and facilities to be competent, open-minded and eager to help.

This information has been sourced, reviewed and adapted from materials provided by Semicore Equipment.

For more information on this source, please visit Semicore Equipment.

Date Added: May 14, 2013 | Updated: Jun 11, 2013
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