Exploration of Energy Recovery Mechanism for High Voltage Pulse Power Supply of Excimer Laser in Lithography Machine Light Source
Excimer lasers have served as the primary light source for deep ultraviolet lithography, enabling the production of integrated circuits with feature sizes down to the nanometer scale. The high voltage pulse power supply that drives the excimer laser operates at high repetition rates with substantial peak power. The energy recovery mechanism in these power supplies significantly affects the overall system efficiency, thermal load, and operational costs of the lithography tool.
Excimer lasers operate on the principle of exciting rare gas halide molecules to create population inversion and stimulated emission. Common excimer species include argon fluoride at one hundred ninety-three nanometers and krypton fluoride at two hundred forty-eight nanometers. The laser requires high voltage electrical discharges to excite the excimer molecules, with discharge voltages typically in the range of tens of kilovolts.
The pulse power supply for an excimer laser must deliver short, high current pulses at high repetition rates. Typical pulse durations are in the range of tens to hundreds of nanoseconds. Repetition rates for lithography lasers can reach several kilohertz. The peak power during each pulse can be megawatts, while the average power is tens of kilowatts. This pulsed operation presents unique challenges for power supply design and efficiency.
In a conventional pulse power supply, energy is transferred from the power source to the laser discharge during each pulse. After the discharge, any remaining energy in the pulse forming network is typically dissipated as heat. At high repetition rates, this dissipated energy represents a significant power loss. Energy recovery mechanisms capture this residual energy and return it to the power supply for use in subsequent pulses.
The pulse forming network stores energy before each pulse and shapes the current waveform delivered to the laser. The network typically consists of capacitors and inductors arranged to produce the desired pulse characteristics. After the laser discharge terminates, the pulse forming network may contain residual charge and magnetic energy. Conventional designs dissipate this energy through resistors or allow it to decay through circuit losses.
Energy recovery circuits redirect the residual energy back to the storage capacitors instead of dissipating it. Several approaches have been developed for energy recovery in pulsed power systems. Resonant energy recovery uses resonant circuits to transfer energy between the pulse forming network and the storage capacitors. Active energy recovery uses power semiconductors to control the energy transfer with higher efficiency.
The resonant energy recovery approach exploits the natural oscillation of inductor-capacitor circuits. When the laser discharge terminates, the residual energy in the pulse forming network causes the circuit to oscillate. By properly timing a switch, the oscillation can be directed to charge the storage capacitor. This approach is relatively simple but has limited efficiency due to circuit losses during the resonant transfer.
Active energy recovery uses controlled switches to manage the energy transfer. After the discharge, switches connect the pulse forming network to the storage capacitor through an inductor. The switch timing controls the energy transfer, and the inductor limits the current and enables soft switching. Active recovery can achieve higher efficiency than resonant recovery but requires more complex control.
The efficiency improvement from energy recovery depends on the operating conditions and the recovery circuit design. At high repetition rates, the recovered energy represents a larger fraction of the total energy per pulse. The recovery efficiency depends on the circuit losses during the transfer, including conduction losses in switches and inductors, and switching losses if hard switching is used.
Thermal benefits accompany the efficiency improvement. The energy that would have been dissipated as heat now remains in the system as stored electrical energy. This reduces the cooling requirements for the pulse power supply and improves the overall system reliability. Lower temperatures also help maintain component performance and extend operational life.
The integration of energy recovery with the pulse generation requires careful timing coordination. The recovery operation must complete before the next pulse is initiated. At high repetition rates, the available time for recovery is limited. The recovery circuit must operate quickly enough to complete the energy transfer within the inter-pulse period.
Control of the energy recovery circuit must adapt to varying operating conditions. The laser discharge characteristics vary with the gas composition, pressure, and electrode condition. These variations affect the residual energy available for recovery. The control system must adjust the recovery timing and parameters to maintain efficient operation across the range of conditions.
The impact of energy recovery extends beyond the pulse power supply itself. The reduced input power requirement eases the demand on the facility power infrastructure. The reduced thermal load simplifies the cooling system design. These system-level benefits contribute to lower operating costs and improved tool availability for lithography manufacturing.
