PVD, or physical vapour deposition, is a technique for applying very thin coatings to materials such as glass, metals, plastics, computer chips, and even food packaging. During the PVD process, you may wish to apply a bias to a substrate. Substrate biasing in PVD is a technique that creates a localised plasma on the surface of the substrate.

Substrate biasing does require some extra preparation time; however, if you require a perfectly clean substrate, it is well worth the effort. Below, you will learn everything you need to know about substrate biasing in PVD processes.

Importance of Substrate Preparation

Substrate biasing has many benefits. The technique allows you to clean, oxidise, or etch a substrate in preparation for a film. Biasing provides gentle ion bombardment before or during deposition to improve adhesion, film density, and material adhesion. Next, we will discuss the importance of substrate preparation in more detail.

Criticality of Substrate Preparation

If you have ever experienced poor results during the deposition process, an inadequately prepared substrate may be to blame. Without proper preparation, materials may not adhere to the surface of the substrate as efficient as you might like. Should this happen, you must restart the thin-film deposition process, potentially wasting valuable time and materials. To help with this substrate biasing can improve the strength of the substrate and the overall properties of the film.

Bias is measured using voltage. The larger the bias voltage, the higher endurance and film strength are. A stronger substrate is capable of withstanding even the most demanding PVD processes, such as plasma-enhanced PVD.

Additionally, biasing can help to ensure that devices last for their expected lifespans. Poor films can increase the stress on a device, leading to reduced performance and degradation over time. This could have a negative effect on the manner in which consumers view your company’s products. Thus, substrate preparation is critical to avoid damage to your company’s reputation.

Effect of Surface Cleanliness

No matter how careful you are, substrates will naturally accumulate contaminants that can affect their performance. Contaminants may include:

• Grease from touching the substrate with non-gloved hands
• Wax from cutting oils
• Airborne bacteria
• Plasticisers that come from the container in which the substrate is shipped

Cleanliness affects the nucleation and growth of thin films. If your substrates are not perfectly clean, you may experience problems with adhesion and thin film growth. Proper cleaning ensures that materials adhere to the substrate properly.

Substrate Cleaning Techniques

There are several options for substrate cleaning: chemical cleaning, physical cleaning, and mechanical cleaning. Each option has advantages and disadvantages. We provide an overview of each technique for substrate biasing in PVD below.

Chemical Cleaning

Chemical cleaning, as the name implies, is a process that relies on various chemicals to clean the surface of a substrate. One advantage of chemical cleaning is gentleness. You do not risk damaging the substrate as you would with physical or mechanical cleaning. However, not all chemicals can remove all contaminants. You must take care to choose the proper chemical formulation.

Reactive ion etching (RIE) is a popular cleaning technique because it is the simplest process for providing directional etching. It uses a combination of physical and chemical processes to remove material from substrates.

Acid etching is another option. It involves the selective removal of material from a substrate using a chemical solution. 

Solvent cleaning is a preferred method because it is relatively simple. Commonly used solvents include acetone, xylene, and toluene. 

Physical Cleaning

Physical cleaning options include plasma cleaning and ion bombardment. These processes are highly regarded for their ability to remove surface oxides and activate the substrate surface. However, they require advanced knowledge of chemistry and specialised equipment that not everyone can access.

Mechanical Cleaning

Mechanical cleaning is ideal for removing particulate matter and roughening surfaces to improve adhesion. Mechanical cleaning may not be ideal for very delicate substrates.

Brushing and sandblasting are two mechanical cleaning techniques commonly used for substrates. Ultrasonic cleaning is another. It involves running substrates through an ultrasonic cleaning line and then rinsing them with deionised (DI) water.

Substrate Oxidation

Oxidation is a process that allows you to grow an oxide on certain substrates, an example of this would be the growth of a silicon oxide film on the surface of a silicon substrate. This technique allows precise control over the thickness of the oxide while producing highly adherent, dense, and uniform oxide with very good electrical properties.

There are two options for oxidation: thermal oxidation and plasma oxidation. Learn the differences between these techniques below.

Thermal Oxidation

Thermal oxidation forces an oxidising agent to diffuse into the wafer and react with it. Most commonly, thermal oxidation is used to produce a layer of silicon dioxide on silicon substrates, but you may use this technique on other types of materials as well. Thermal oxidation involves exposing silicon to extremely high temperatures (typically 800 to 1200 °C).

Plasma Oxidation

Plasma oxidation involves exposing substrate surfaces to reactive plasma species to induce surface oxidation and activation. You may also see this technique referred to as micro-arc oxidation (MAO).

This technique is ideal for producing ceramic layers on the surface of metals. These coatings have two to four times the hardness of steel or hard anodising.

The basic plasma oxidation process uses an electrolyte bath of a dilute aqueous solution, with reagents added based on desired surface coating properties. A high voltage is then passed through the electrolyte, generating plasma on the substrate’s surface.

Role of Biasing in PVD

During physical vapour deposition, you can apply a bias voltage to the surface of a substrate to adjust its ion energies, tailor the coating structure, and alter its mechanical properties.

In some cases, PVD substrate biasing can be self-generating. This occurs when insulating substrates are immersed in RF (radiofrequency) plasma. As the bias is increased, the bombardment of the substrate increases, improving the film’s dentification and adhesion.

Substrate biasing in PVD can help you to control the deposition rate and adjust film properties. Thus, it is a very useful technique for fine-tuning the surface of your substrate.

Effect on Deposition Rate

Deposition refers to the controlled growth, transfer, or synthesis of materials as thin films on substrates. These materials can be either crystalline, polycrystalline, or amorphous.

In PVD, biasing affects the deposition rate by influencing ion bombardment energy and flux onto the substrate surface. Substrate biasing during deposition is commonly used in both sputtering and thermal deposition.

Sputtering relies on vacuum deposition biasing to dislodge atoms from the target with a high-energy source. The atoms can then be deposited onto the target surface, typically using a plasma arc.

Learn more about sputtering systems from Korvus Technology.

Thermal deposition involves using vacuum pressure and very high temperatures to vaporise the target material.

You may also use substrate biasing in chemical vapour deposition (CVD). This process involves using volatile precursors to deposit gaseous source materials onto the surface of substrates.

Impact on Film Properties

Substrate bias control can influence film properties, such as crystallinity and thickness uniformity. Thus, substrate surface modification is useful when producing films that require specific uniformity or crystallinity, such as computer microchips and medical implant devices.

Enhancing PVD With Substrate Biasing Techniques

Proper substrate preparation is necessary to achieve uniform results with your substrates. Thorough cleaning, oxidation, and biasing will help to ensure that you achieve the desired outcome for your project.

If you would like to learn more about substrate biasing in PVD or our HEX Series PVD systems, contact Korvus Technology today.

References

[1] H.L. Brown, S.A. Thornley, S.J. Wakeham, M.J. Thwaites, R.J. Curry & M.A. Baker (2015). The Impact of Substrate Bias on a Remote Plasma Sputter Coating Process for Conformal Coverage of Trenches and 3D Structures, Journal of Physics D: Applied Physics 48:33, DOI 10.1088/0022-3727/48/33/335303

[2] L. Bardos, H. Barankova & L.E. Gustavsson (2006). Effect of Substrate Material and Bias of TiN Films Deposited in the Hybrid Plasma Reactor, Journal of Vacuum Science & Technology A 1655-1659, DOI: 10.1116/1.2167082

[3] H.B. Profijt, M.C.M. van de Sanden & W.M.M. Kessels (2013). Substrate-Biasing During Plasma-Assisted Atomic Layer Deposition To Tailor Metal-Oxide Thin Film Growth, Journal of Vacuum Science & Technology A 31, 01A106-1/10, DOI: 10.1116/1.4756906

[4] W. Zheng, L. Yi, K. Tao, Y. Ma, P. Chang & J. Wu (2006). The Influence of Substrate Bias in I-PVD Process on the Properties of Ti and Alloy Films, Journal of Materials Science: Materials in Electronics 931-935, DOI: 10.1007/s10854-006-0046-8