Vacuum Coating Multi-Target Reversible-Polarity High-Voltage Switching

Multi-target reactive sputtering of optical stacks and barrier layers requires instantaneous polarity reversal between cathode pairs every 8–40 µs at voltages up to 1200 V to suppress arcing on poisoned targets while maintaining film stoichiometry. Reversible-polarity switching systems must therefore transition from +1100 V / –1100 V to the opposite state in <1.8 µs with zero dead-time and arc energy below 0.42 mJ per event.

The topology uses two independent 1.8 kV full H-bridges of 1.7 kV silicon carbide MOSFETs driving a common 1:1 air-core transformer with center-tapped secondary. Polarity reversal is executed by simultaneously changing the conduction sequence of both bridges in a single 1.4 µs switching cycle, eliminating the voltage zero-crossing dwell that previously allowed arc re-ignition. Rise/fall times are <380 ns through minimized output inductance (<42 nH) and active gate clamping.

Arc management employs a three-tier response: micro-arcs (<0.3 mJ) are quenched by instantaneous bridge disable for 1.1 µs followed by soft restart at 68 % voltage; macro-arcs trigger full reverse-voltage override at +220 V for 6 µs to actively extract electrons; persistent arcs initiate a 42 µs blanking period with programmed reverse cleaning pulse. Detection sensitivity reaches 38 A/µs via di/dt monitoring on each target return line.

Target-to-target isolation exceeds 28 kV through fiber-optic gate drive and floating bridge supplies powered by individual 1:1 isolation transformers. Independent current monitoring per target enables asymmetric reverse amplitude (typically 12–28 % of negative peak) optimized for Al₂O₃ versus SiO₂ processes without hardware change.

Process synchronization supports dual-magnetron co-sputtering: the switching supply accepts an external clock with <68 ns jitter and phase-locks reversal timing to the magnetron pulse trailing edge, ensuring ion bombardment occurs during the optimal metal-rich phase for maximum refractive index control.

Thermal design uses immersion cooling in fluorinated fluid with redundant magnetically coupled pumps, maintaining junction temperatures below 68 °C at 8 kW average output per target pair. These reversible-polarity systems routinely achieve defect densities below 0.06 cm⁻² on 200 mm AR coatings and refractive index uniformity <0.0008 across 1.2 m architectural glass panels at deposition rates >18 nm·m/min.