Target Utilization Improvement and Power Distribution of High Voltage Power Supply for Magnetron Sputtering
Magnetron sputtering is a widely used physical vapor deposition technique for producing thin films of metals, alloys, and compounds. The process employs a magnetron source consisting of a target material backed by permanent magnets that create a closed magnetic field trap above the target surface. Electrons become trapped in this magnetic field, enhancing the ionization efficiency and enabling high sputtering rates at moderate pressures. The high voltage power supply that drives the discharge affects both the target utilization efficiency and the power distribution across the target surface.
The sputtering process begins when a negative voltage applied to the target causes a glow discharge in the process gas, typically argon. Positive ions from the plasma are accelerated toward the negatively biased target, striking the surface with energy sufficient to eject target atoms through momentum transfer. The ejected atoms travel through the gas phase and condense on substrates positioned opposite the target, forming a thin film. The magnets behind the target create magnetic field lines that run parallel to the target surface, trapping electrons and concentrating the plasma in a closed loop above the target.
Target utilization refers to the fraction of the target material that is effectively used for film deposition before the target must be replaced. The magnetic field configuration creates a nonuniform plasma density above the target surface, with the highest density in the region where the magnetic field is parallel to the surface. This nonuniform plasma creates a nonuniform sputtering erosion pattern on the target, with deep erosion grooves forming under the high plasma density regions. When these grooves penetrate through the target, the remaining material cannot be effectively used, even though substantial material remains in less eroded regions.
The power distribution from the high voltage power supply affects the plasma distribution and thus the erosion pattern. The discharge voltage and current determine the total power delivered to the target. The spatial distribution of the current density across the target surface determines the local sputtering rates. The power supply output characteristics interact with the magnetron impedance to determine the operating point of the discharge. Power supplies with different output characteristics may produce different current distributions even with the same magnetron geometry.
Magnetic field design influences the target utilization by controlling the plasma confinement pattern. Conventional magnetrons with balanced magnetic fields produce a closed loop plasma with a single erosion track. Unbalanced magnetrons have magnetic field configurations that allow some field lines to extend toward the substrate, providing additional plasma in the substrate region. Rotating magnet designs move the magnetic field pattern during operation, spreading the erosion over a larger area of the target and improving utilization. The power supply must accommodate the varying impedance that may result from magnetic field rotation.
Power supply operating mode affects the target utilization and film properties. Direct current operation provides continuous sputtering with stable plasma conditions. Pulsed direct current operation, where the power is switched on and off at frequencies of tens to hundreds of kilohertz, can reduce arcing and improve process stability when sputtering reactive materials or insulating targets. Radio frequency operation enables sputtering of insulating targets by preventing charge accumulation. Each mode has implications for the power distribution and the resulting erosion pattern.
Arc handling capabilities of the power supply are important for maintaining stable operation and preventing target damage. Arcs can occur when local conditions cause a transition from glow discharge to arc discharge, with localized high current that can melt the target surface. The power supply must detect arcs quickly and reduce or reverse the voltage to extinguish the arc before significant damage occurs. Arc suppression algorithms that detect the early stages of arc development can prevent arcs from fully forming.
Power regulation modes include constant voltage, constant current, and constant power operation. Constant voltage operation maintains a fixed target voltage, allowing the current to vary with plasma conditions. Constant current operation maintains fixed current, with voltage varying. Constant power operation maintains the product of voltage and current constant, providing consistent energy delivery to the target. The choice of regulation mode affects the process stability and the response to changing conditions such as pressure variations or target surface changes.
Multi magnetron configurations for large area coating or co sputtering require coordinated power supply control. Multiple magnetrons may be operated from separate power supplies with synchronized control to ensure consistent coating across large substrates. Co sputtering from different target materials requires independent control of each magnetron power to achieve the desired film composition. The power supply design must enable the required coordination while maintaining stable operation of each individual discharge.
