High-Voltage Phase-Locked Power Systems for Excimer Laser Repetition Rate Control
Excimer lasers, operating on transitions of rare gas-halide molecules, are pulsed light sources critical for applications such as photolithography, micromachining, and medical procedures. Their operation involves the rapid discharge of a high-voltage pulse across a gas mixture to create a population inversion. The stability and precision of the laser's output, particularly at high repetition rates, are intimately tied to the performance and synchronization of its high-voltage charging and switching system. Achieving consistent pulse energy and timing requires more than just a robust high-voltage power supply; it necessitates a phase-locked loop architecture where the high-voltage system acts as a precisely controlled actuator within a broader feedback system.
The fundamental process begins with the charging of a large storage capacitor, known as the main storage capacitor, to a high voltage, typically in the range of 15 to 40 kilovolts. This capacitor will later discharge through thyratron or solid-state switches into the laser gas. The charging power supply, often a resonant switching converter, must replenish this capacitor to an exact voltage level within the brief interval between laser pulses. At repetition rates of several hundred hertz to kilohertz, this interval is extremely short. The challenge is to ensure the capacitor voltage at the moment of triggering is identical for every pulse, as this voltage directly determines the energy deposited into the gas discharge and, consequently, the output pulse energy.
A simple open-loop charging system is insufficient for high-precision applications. Variations in line voltage, load impedance, or gas mixture characteristics cause the final charging voltage to drift, leading to pulse energy instability. The solution is a closed-loop, phase-locked system. Here, a master timing oscillator sets the repetition rate. The high-voltage power supply receives this timing signal as a phase reference. Its control loop does not merely aim for a voltage setpoint; it is designed to achieve that setpoint at a specific, predetermined phase point relative to the master clock. This is the essence of the phase-lock for the high-voltage subsystem.
The power supply employs a fast control algorithm that monitors the capacitor voltage via a high-resolution, high-bandwidth divider and feedback circuit. As the capacitor charges, the controller compares the actual voltage ramp to an ideal, time-referenced ramp profile. It dynamically adjusts the charging current to ensure the voltage trajectory intersects the desired final value precisely at the moment the laser is scheduled to fire, which is defined by the phase of the master clock. This compensates for any disturbances within the charging cycle itself. Advanced designs use predictive correction based on the voltage level from the previous pulse and the known time remaining, effectively creating a learning system for charging behavior.
Furthermore, the phase-locking extends beyond just the charging supply. The trigger generator for the main high-voltage switch must also be precisely synchronized. Jitter in the trigger pulse directly translates to timing jitter in the optical output. Therefore, the entire high-voltage pulse generation chain—from the charging supply's completion signal to the trigger circuit's delay—is disciplined by the same master clock, often using digital delay generators with picosecond-level adjustment granularity. This ensures the electrical discharge is initiated with minimal temporal uncertainty.
The high-voltage components themselves must be designed for this pulsed, repetitive duty. The charging supply must have a high peak power capability to deliver energy quickly but also exhibit excellent efficiency to manage heat in a compact laser head. Resonant topologies are favored as they reduce switching losses and electromagnetic interference. The switches, whether thyratrons or solid-state stacks, must have fast recovery times to handle the high repetition rate without latch-up or performance degradation. Thermal management of these components is critical, as even a small increase in temperature can alter gas density and switch characteristics, breaking the phase-lock stability.
Integration with the laser's optical feedback is the next level of sophistication. Many systems incorporate a pulse energy monitor that samples a fraction of the output light. This measurement is fed back to the high-voltage control system, which adjusts the target charging voltage setpoint to maintain constant pulse energy, despite gradual changes in gas aging or optics contamination. This creates a nested control loop: a fast inner loop for phase-locked voltage timing, and a slower outer loop for energy stabilization. The high-voltage system must respond smoothly to these setpoint adjustments without introducing instability into its primary phase-locking function.
In practice, such a phase-locked high-voltage system is what enables excimer lasers to perform reliably in the most demanding industrial and scientific roles. It allows the laser to operate as a true tool, with predictable output that can be seamlessly integrated into larger automated processes, such as stepping a wafer in a lithography scanner or synchronizing with a part handler in a micromachining station. The high-voltage system, therefore, transitions from being a simple power source to being the core timing and energy regulation engine of the entire laser apparatus.
