In modern vacuum coating technologies, bias voltage control is a critical parameter that directly influences thin film microstructure, density, internal stress, and adhesion strength. Whether in hard coatings, decorative films, or optical coatings, proper control of the substrate bias voltage not only modulates plasma dynamics, but also enhances the functionality and reliability of the resulting films.
No.1 What Is Bias Voltage Control?
Bias voltage control refers to the technique of applying a negative potential to the substrate during deposition, making it electrically lower than the surrounding plasma. This technique is widely used in PVD (Physical Vapor Deposition) processes, especially in magnetron sputtering, ion plating, and cathodic arc deposition systems.
The substrate bias can be applied via DC (Direct Current), MF (Mid-Frequency), or RF (Radio Frequency) power supplies. Its primary role is to accelerate positive ions in the plasma toward the substrate surface, enabling ion bombardment that promotes desirable film growth characteristics.
No.2 How Bias Voltage Affects Film Properties
The fundamental mechanism of bias voltage control lies in modifying film growth kinetics through the energy of incoming ions. Its impact is reflected in several key aspects:
Densification:
A suitable negative bias increases the kinetic energy of ions arriving at the substrate, promoting surface mobility and rearrangement of adatoms. This leads to denser films with improved corrosion resistance, hardness, and wear resistance.
Stress Regulation:
Ion bombardment also introduces residual stress within the film. Excessive bias can induce compressive stress, potentially causing cracking or delamination. Therefore, optimal bias levels must be carefully selected based on film material, substrate type, and coating thickness.
Adhesion Enhancement:
Bias voltage enhances interfacial interactions by promoting interlayer mixing or forming graded interfaces, thereby improving film-to-substrate adhesion—especially critical for hard coatings or multilayer structures.
Particle Suppression and Surface Smoothing:
Appropriate bias can suppress macro-particle incorporation and reduce surface roughness, thereby decreasing scattering loss in optical films and improving surface quality.
No.3 Types of Bias Control Methods
DC Bias: Commonly used for conductive substrates, offering simple control and fast response. Typical in decorative coatings and hard coatings.
RF Bias: Ideal for non-conductive substrates such as glass, ceramics, and polymers. Offers wide material compatibility but requires more sophisticated system integration and process tuning.
Pulsed Bias: Involves applying periodic bias pulses, balancing deposition rate and ion energy. Well-suited for low-temperature coatings or complex geometries.
Additionally, some advanced systems employ closed-loop bias control, which monitors plasma characteristics and bias current in real time to maintain a stable process window and ensure coating uniformity across batches.
—This article was published by vacuum coating equipment manufacturer Zhenhua Vacuum
Post time: Jul-17-2025