If you’re unfamiliar with physical vapour deposition (PVD), here is a brief explanation. PVD is a process that vaporises a material within a vacuum chamber, which you may then deposit onto materials that can be both electrically conductive and insulating as a thin-film of alloyed material or as a pure metal coating. To ensure good results with this process, you must understand how substrate heating affects PVD.

Below, we explain everything you need to know about substrate heating in PVD for film uniformity. You will learn about the role of temperature control, various substrate heating methods, and factors influencing film uniformity.

Understanding Substrate Heating

Have you ever wondered why paying attention to PVD substrate heating is important? The main reason is that it can affect the outcome of the deposition process. Without proper heating, your substrate may be too thin and have inadequate PVD film uniformity. It may also end up with poor adhesive properties.

These are common problems for those who are unfamiliar with substrate heating. You may have even experienced such issues with your own substrates. For instance, perhaps you wished to apply a thin silicon oxide layer to your substrates but ended up with less-than-ideal results. Poor temperature control may be to blame.

To achieve uniform film deposition, you must understand the basics of substrate heating. We will discuss these basics next.

Principles of Substrate Heating in PVD

Substrate heating in PVD may seem complex, but it boils down to two simple factors. High temperatures favour the rapid formation of defect-free crystals, while low temperatures result in amorphous structures.

High temperatures are ideal for creating strong substrates with good adhesive properties, such as computer chips. Low temperatures are better for substances that must remain flexible, such as a contact lens or a sheet of plexiglass.

Heating the substrate is also effective during the cleaning process. Doing so removes water and dust from the surface of the substrate, allowing for better adhesion of the film.

Role of Temperature Control in Film Uniformity

Uniformity is key when producing thin-film coatings. It wouldn’t do to have a film that has a thickness that varies widely across a substrate. A lack of uniformity can cause films to perform poorly or in ways you didn’t intend.

Proper heating allows you to create the proper crystal structure for your films. Cooling is important for the deposition sources, too, as it protects your films from process radiation emanating from the vaporisation process.

Substrate Heating Methods

There are several substrate heating methods to choose from, although no method is necessarily better than the others. The best method depends on your budget, space constraints, and desired temperature range.

You can choose from resistive heating, inductive heating, or radiative heating methods. All of these methods are RHEED (reflection high-energy electron diffraction) compatible and capable of heating substrates to a minimum of 950 °C (depending on your equipment).

Resistive Heating Methods

Filament heating is the main method of resistive heating. This process requires a filament to have a current passed through it, in doing so, the filament will begin to heat up due to the electron flow. This can generate high temperatures but is limited by the need for it to be in contact with anything it needs to heat. 

This is a very common process due to its simplicity and is used in lots of vacuum systems.

Inductive Heating Methods

Inductive (or induction) heating uses an extrusion-based metal wire additive as an energy source. It is highly regarded for its ability to produce high deposition rates while reducing manufacturing times. It is also very efficient and boasts a longer life than resistance heating methods. It is not very applicable to substrate heating however due to larger radiative losses.

The main downside of using it as a deposition technique is that films can become detached from the substrate because of the formation of an oxide layer.

Radiative Heating Methods

Radiative heating is the only process that does not require direct contact between the heating source and the sample. This process is the transfer of heat through the radiation produced. Heat produced from a process such as resistive heating can radiate across the vacuum chamber and heat a sample. 

This process cannot produce as high temperatures as the others due to large losses of heat into the chamber. The radiation intensity will decrease by an inverse square law with relation to the distance between heat source and sample – therefore, as shorter distance is best. 

Factors Influencing Film Uniformity

When producing thin films, paying strict attention to uniformity is essential. Imagine what would happen if a chip in a patient’s medical device was too thick or too thin. The device might malfunction and lead to serious injury.

Several factors can affect film uniformity: substrate properties, deposition parameters, and process conditions.

Substrate Material and Properties

Substrates come in many different materials, including glass, metal, and concrete. Wood and paper products can also serve as substrates. These substrates are challenging to work with because they are anisotropic, which means that their surface properties differ depending on the direction of the substrate. 

The substrate’s surface condition can strongly impact the bond of subsequent layers and, in turn, their uniformity. For instance, it can be difficult to achieve uniformity with substrates that contain steps or trenches. If you wish to achieve the most uniform film possible, pay close attention to the molecular properties of your substrate and choose your heating method accordingly. 

Deposition parameters and process conditions can play a big role in film uniformity as well. Factors influencing film uniformity include:

• Distance between the substrate and the target
• Relative motion between the substrate and the target
• Target geometry
• The magnetron scattering source

Thermal Management Strategies

If you are having difficulties managing substrate heating in PVD for film uniformity, try these substrate heating techniques and thin film uniformity methods:

• Employ a system with independently heated filaments. Such a system counters the effects of the edges of substrate holders.
• Use tantalum shielding to cover electrical and mechanical parts to prevent overheating.
• Choose substrates with high thermal conductivity to keep temperatures low. This technique is necessary if producing boards for vehicles, industrial electronics, smartphones, and computers.
• Keep a close eye on your thermocouple attachments. For thermocouples to provide accurate readings, the wires must remain separate and only joined at the bead. If your profile graph shows an erratic reading, this indicates a loose attachment.

Optimising Substrate Heating for Uniform Film Deposition

During substrate heating, uneven temperatures are a recipe for poor deposition results. As the temperature has such a large impact on both the uniformity and the adhesion of the film – a varying temperature will create a film with varying properties.

For suitable results, you must maintain even temperatures throughout the heating process. Learn how to accomplish this below.

Temperature Profiling and Control

Temperature profiling, or thermal profiling, allows you to determine whether a thermal profile is outside of specifications or a set tolerance limit. In thermal profiling, the centre of a process window is zero, while the edges of the window are 99%. If the Process Window Index (PWI) is equal to or greater than 100%, it is outside of process limitations.

Two thermal profiling methods are ramp-soak-spike (RSS) and ramp to spike (RTS). RSS allows for a process limit of about 4 °C/second, while RTS permits approximately 1-2 °C/second.

Thermal Uniformity Across the Substrate Surface

As mentioned above, one way to maintain PVD film uniformity across a substrate surface is to use a system with independently heated filaments.  This allows you to control temperatures across the entire substrate surface so you don’t get any hot or cold zones.

Using the Rotating Wafer Thermal Imaging (RWTI) technique is another option. This technique employs multi-zone heating systems that produce temperature uniformity of less than 2 °C in the range of 600 °C to 1,100 °C.

Yet another method involves placing the substrate into a thermal processing chamber, rotating the substrate during heating, and measuring the temperature at a plurality of radial locations during rotation.

To Summarise Substrate Heating in PVD

If you wish to achieve good results, you must pay careful attention to substrate heating in PVD for film uniformity. Resistive, inductive, and radiative heating methods can help you maintain stable temperatures and reliable uniformity. You can also take advantage of thermal profiling to ensure that your temperature profiles are within a set tolerance limit for PVD or CVD.

Want to learn more about substrate heating or our HEX Series modular PVD systems? Reach out to Korvus Technology today.

References

[1] M. Ohring (2002). Film Uniformity, Materials Science of Thin Films 95-144, DOI: 10.1016/B978-012524975/50006-9

[2] R.K. Choubey, G.K Sharma & P.K. Jain (2023). On Identifying the Suitable Substrate Medium for Induction Heating-Based Metal Wire Additive Manufacturing, Recent Advances in Manufacturing and Thermal Engineering 249-259, DOI: 10.1007/978-981-19-8517-1_18

[3] B. Zhang (2016). Amorphous and Nano Alloys Ectroless Depositions, Technology, Composition, Structure, and Theory 323-381, DOI: 10.1016/B978-0-12-802685-4-00006-6

[4] C. Guo, M. Kong, C. Liu & B. Li (2013). Optimization of Thickness Uniformity of Optical Coatings on a Conical Substrate in a Planetary Rotation System, Applied Optics 52:4, B26-32, DOI: 10.1364/AO.52.000B26