...

Korvus Technology

Optimising Film Uniformity With Substrate Heating in PVD

If you’re unfamiliar with physical vapour deposition (PVD), here is a brief explanation. PVD is a process that produces metal vapour, which you may then deposit onto electrically conductive materials as a thin alloy or 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.

Role of Temperature Control in Film Uniformity

Uniformity is key when producing thin-film coatings. It wouldn’t do to have one film that is 0.1 micrometres, while another measures 0.01 micrometres. 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, too, as it protects your films from process radiation.

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.

Resistive Heating Methods

Filament evaporation is the main method of resistive heating. During this process, metal pellets are placed on filaments. Then, under vacuum deposition heating, the pellets are heated to their melting point using a resistive filament. The liquid metal produces a vapour that travels in a straight line from source to sample. Deposition rates average 1 nm/second.

Resistive heating is ideal for materials such as quartz, graphite, molybdenum, and tungsten. This heating method is fairly popular because it’s flexible and budget friendly. The main drawback is that contamination can occur if the filament and source are heated to the same temperature.

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. The main downside is that samples can become detached from the substrate because of the formation of an oxide layer.

Radiative Heating Methods

E-beam evaporation is a type of radiative heating you might want to consider. During this process, thermal emissions from electrons from a filament source heat samples to high temperatures. This method is preferred if desired temperatures are too high for thermal evaporation.

Radiative heating is ideal for large substrate sizes. It can produce temperatures of up to 1,100 °C.

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 [2]. 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 [1]. 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

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 [3]. Picture a cake baked at 250 °C for 20 minutes, followed by 400 °C for 30 minutes more. Such a cake would probably be inedible, or at least it wouldn’t taste very good.

      Although substrate heating in PVD for film uniformity is more complicated than baking a cake, the idea remains the same. 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 [4]. 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

      Facebook
      Twitter
      LinkedIn
      Seraphinite AcceleratorOptimized by Seraphinite Accelerator
      Turns on site high speed to be attractive for people and search engines.