High-Voltage Power Supplies for Magnetron Sputtering of High-Entropy Alloy Coatings

 

Magnetron sputtering operates by creating a plasma in a low-pressure inert gas environment, typically argon. A high negative voltage is applied to the target material, causing positive gas ions to accelerate and bombard the target, ejecting atoms which then travel to coat the substrate. For HEA targets, the sputtering yields—the number of atoms ejected per incident ion—can vary significantly between the different constituent elements. This inherent disparity can lead to a deposited film composition that deviates from the target composition if the process is not carefully managed. The high-voltage power supply directly addresses this through its control over the discharge power, voltage, and current density.

 

Modern systems predominantly utilize pulsed DC or advanced mid-frequency AC power supplies, which have largely superseded pure DC for reactive or insulating target applications. While HEA metallic targets are conductive, pulsed power offers distinct advantages. By operating in a pulsed mode, with precise control over frequency, duty cycle, and pulse shape, the power supply can manage the average power delivered to the target, thereby controlling its temperature and mitigating issues like thermal runaway or uneven erosion of the target face. More critically, the high peak currents achievable during the pulse on-time can enhance the ionization density of the sputtered material. This creates a denser plasma near the substrate, often referred to as a high-power impulse magnetron sputtering (HiPIMS) regime when using very short, high-power pulses. This increased ionization fraction allows for additional control over the film's microstructure, density, and adhesion through substrate biasing, which is crucial for achieving the desired mechanical and chemical properties in HEA coatings.

 

Furthermore, the stability and ripple characteristics of the high-voltage output are non-negotiable. Any instability or excessive noise in the output voltage can translate into fluctuations in the sputtering rate and plasma density. For a multi-element system like an HEA, such fluctuations can cause momentary variations in the elemental flux arriving at the substrate, potentially leading to layered inhomogeneities or columnar growth defects in the coating. Therefore, the power supply must exhibit exceptional stability, with tightly regulated output and minimal ripple, even under dynamic load conditions as the target erosion profile changes over time.

 

Another key consideration is arc handling. Micro-arcs can occur on the target surface due to dielectric inclusions or local melting. In a DC system, an arc can sustain and cause catastrophic damage to the target and power supply. Advanced high-voltage power supplies for this application incorporate extremely fast arc detection and suppression circuits. Upon detecting the characteristic voltage drop and current spike of an arc, the supply can cut off output within microseconds, often reversing polarity briefly to quench the arc, and then seamlessly restore power. This capability is essential for protecting expensive HEA targets and maintaining a consistent, arc-free deposition process. In summary, the high-voltage power supply for magnetron sputtering of HEAs is not just a source of energy; it is a precision instrument for plasma control. Its ability to deliver stable, clean, and precisely modulated power—coupled with intelligent arc management—is fundamental to unlocking the full potential of high-entropy alloy coatings, enabling reproducible deposition of films with tailored compositions and superior performance characteristics.