Asymmetric Bipolar Pulsed High-Voltage Supply for Magnetron Sputtering
Magnetron sputtering is a dominant physical vapor deposition technique. While DC sputtering is common, pulsed operation, particularly in the mid-frequency range (10-350 kHz), is employed to prevent target poisoning when depositing insulating materials and to improve film properties. Asymmetric bipolar pulsed sputtering takes this a step further by applying a high-voltage waveform where the positive and negative pulse phases have deliberately different amplitudes, durations, or shapes. This asymmetry provides independent control over the ion bombardment energy during the different phases, enabling unprecedented tuning of film microstructure, stress, and adhesion. The high-voltage power supply that generates this complex waveform is the enabling core of the process.
In a standard bipolar pulsed DC supply, the output alternates between a negative voltage (for sputtering, e.g., -500V) and a positive voltage (for discharging the target, e.g., +100V), with similar pulse widths. The asymmetric design intentionally breaks this symmetry. A common configuration features a long, moderately negative sputtering phase followed by a short, highly positive reversal phase. During the long negative phase, argon ions bombard the target, ejecting material. The voltage during this phase primarily controls the sputtering rate. During the short positive phase, electrons are attracted to the target to neutralize positive charge buildup. The key insight is that by making this positive pulse much higher in voltage (e.g., +200V to +400V) but very short, it can briefly attract a flux of high-energy positive ions from the plasma to the substrate or the growing film, without significantly interrupting the net sputtering flux from the target.
This controlled ion bombardment during the film growth is transformative. It can be used to densify the film, reduce intrinsic stress from compressive to tensile or vice versa, improve adhesion by creating an intermixed interface, and alter crystallographic orientation. The independent control of the negative sputtering voltage (magnitude and duration) and the positive ion bombardment voltage (magnitude and duration) through the asymmetric waveform provides two powerful, decoupled knobs for process engineers.
Designing the power supply to deliver such waveforms is non-trivial. It must be a true four-quadrant amplifier, capable of sourcing and sinking current at both positive and high negative voltages. The switching topology often involves a full-bridge inverter feeding a high-voltage transformer, with sophisticated control of the switching pattern to shape the output. The transitions between negative and positive high voltages must be extremely fast to maintain the defined pulse shapes, requiring minimal stray inductance in the output circuit and switches with high dv/dt capability. The supply must also handle the highly dynamic and non-linear load presented by the magnetron plasma, whose impedance changes drastically between the metallic and insulating modes of the target surface.
Feedback and stability are critical challenges. The plasma impedance is not constant; it depends on gas pressure, target condition, and the history of the pulse itself. The power supply typically operates in a constant power, constant current, or more advanced modes like pre-programmed voltage-time profiles. It must include fast feedback loops to regulate the output according to the set mode despite the changing load. This often requires digital signal processors to implement adaptive algorithms that can adjust pulse parameters in real-time to maintain the desired process conditions. Arc handling is another paramount concern. Any arc at the target must be detected within microseconds, and the output voltage must be suppressed or reversed to quench the arc before it damages the target or the power supply. In an asymmetric system, the arc detection and suppression logic must account for the different phases of the waveform.
Integration with the deposition system extends beyond just the output terminals. The power supply must be synchronized with other pulsed systems in the chamber, such as a substrate bias supply, to create synchronized or phase-shifted bombardment schemes. It must also communicate with the facility's water cooling system, as the asymmetric waveform can change the average power dissipation in the target and the supply itself compared to DC operation. By providing this level of precise, asymmetric control over the plasma potentials, this specialized high-voltage supply moves magnetron sputtering from a simple coating technique into the realm of precision surface engineering, enabling the deposition of films with tailored properties for demanding applications in optics, tribology, and microelectronics.
