Peak Current Control Technology of High Voltage Power Supply for High Power Pulsed Magnetron Sputtering

 

The fundamental operating principle of high power pulsed magnetron sputtering involves applying high power density pulses to a conventional magnetron cathode. During each pulse, the power density at the target surface reaches levels orders of magnitude higher than in conventional direct current magnetron sputtering, typically achieving peak power densities of several hundred watts per square centimeter. This intense power input generates a dense plasma with electron densities approaching or exceeding those of the sheath, leading to substantial ionization of the sputtered atoms before they reach the substrate. The resulting high flux of ionized deposition material enables excellent film density, adhesion, and conformal coverage even at low substrate temperatures.

 

The pulse characteristics including peak current, pulse duration, repetition frequency, and duty cycle collectively determine the time averaged power input and the instantaneous plasma conditions during each pulse. The peak current directly influences the discharge voltage through the current voltage characteristic of the magnetron discharge, with higher currents generally corresponding to higher voltages and increased sputtering rates. The peak current also affects the plasma density and the degree of ionization of sputtered material, with higher currents producing more intense plasmas and greater ionization fractions. Controlling the peak current to a consistent value pulse after pulse ensures reproducible plasma conditions and deposition characteristics.

 

Peak current control in pulsed power supplies typically employs feedback regulation based on current sensing during each pulse. The current measurement may utilize Hall effect sensors, current transformers, or resistive shunts depending on the current magnitude, pulse duration, and accuracy requirements. Hall effect sensors provide galvanic isolation and good accuracy across a wide current range but may have limited bandwidth for very short pulses. Current transformers offer excellent isolation and high bandwidth but cannot measure direct current components and may exhibit saturation effects at high pulse currents. Resistive shunts provide accurate measurement with excellent bandwidth but introduce power dissipation and require careful signal isolation.

 

The feedback control system compares the measured peak current with a desired setpoint and adjusts the power supply output to minimize the error. The control action may modulate the pulse width, the charging voltage of an energy storage capacitor, or the conduction angle of switching elements depending on the power supply topology. Proportional integral control algorithms provide the basic regulation function, with the integral term eliminating steady state error and the proportional term providing rapid response to deviations. The control bandwidth must be sufficient to correct for pulse to pulse variations while avoiding instability that could cause oscillations or overshoot in the current waveform.

 

Pulse to pulse current variations arise from several sources including fluctuations in the plasma ignition conditions, variations in the target surface condition, and instabilities in the power supply charging and switching circuits. The plasma ignition at the beginning of each pulse depends on the presence of seed electrons to initiate the discharge, which may vary with the time since the previous pulse and the background plasma conditions. Target surface conditions including oxidation, contamination, and surface roughness affect the secondary electron emission and sputtering yield, influencing the discharge current voltage characteristics. Power supply variations in the charging voltage or switching timing cause corresponding variations in the available energy and current delivery during each pulse.

 

Advanced peak current control strategies may incorporate predictive or feedforward elements to anticipate and compensate for known sources of variation. The relationship between charging voltage and peak current can be characterized and used to predict the charging voltage required for a desired peak current, enabling rapid adjustment without waiting for feedback from the actual pulse. Similarly, the influence of pulse repetition frequency on plasma conditions and discharge characteristics can be incorporated into feedforward compensation schemes. These predictive elements reduce the reliance on feedback correction and improve the pulse to pulse consistency of the peak current.

 

The current waveform shape during the pulse also influences the plasma behavior and deposition characteristics beyond the peak current value alone. An ideal rectangular current pulse maintains constant current throughout the pulse duration, providing steady plasma conditions from pulse initiation to termination. However, practical current waveforms may exhibit overshoot at pulse initiation, droop during the pulse due to energy depletion from storage capacitors, or ringing from interactions between the plasma impedance and power supply output impedance. Controlling these waveform characteristics in addition to the peak current value ensures that the plasma experiences consistent conditions throughout each pulse.

 

Overshoot at pulse initiation can cause transient plasma conditions with higher density and ionization than the steady state pulse conditions. While this transient may contribute beneficially to plasma ignition and stabilization, excessive overshoot can cause target arcing or non reproducible plasma conditions. The power supply design must balance the need for rapid current rise to achieve the desired peak current with the avoidance of excessive overshoot that could degrade process performance. Snubber circuits, gate drive timing, and output network design all influence the current rise characteristics and overshoot magnitude.

 

Current droop during the pulse arises when the energy storage capacitor discharges significantly during the pulse, reducing the available voltage and consequently the discharge current. The droop magnitude depends on the ratio of pulse energy to stored energy, with larger capacitance values reducing droop at the expense of increased charging time and power supply size. Some power supply architectures employ active pulse shaping circuits that maintain constant current despite capacitor voltage droop, improving pulse flatness at the cost of increased circuit complexity.

 

The thermal management of the magnetron target under high power pulsed operation imposes constraints on the pulse parameters and average power delivery. While the peak power density during pulses is very high, the average power must be limited to prevent excessive target heating that could cause melting, cracking, or degraded sputtering performance. The duty cycle, defined as the ratio of pulse duration to pulse period, determines the relationship between peak and average power. Peak current control must operate within the constraints imposed by target thermal management, potentially limiting the achievable peak current for given pulse durations and repetition frequencies.

 

Process integration considerations for high power pulsed magnetron sputtering include coordination with substrate bias, gas flow control, and other process parameters. The substrate bias during deposition influences the energy of ions arriving at the growing film surface, affecting film density, stress, and microstructure. Synchronizing the substrate bias with the magnetron pulses enables time resolved control of ion energy during different phases of the deposition process. Gas flow dynamics affect the pressure and composition in the deposition region, with pulsed operation potentially causing pressure transients that influence plasma characteristics and deposition rates.