Pulse Energy Feedback Control in High-Voltage Power Supplies for Excimer Lasers

Excimer lasers (e.g., ArF, KrF, XeCl) are indispensable high-power pulsed light sources in the deep ultraviolet spectrum, critical for semiconductor lithography, medical aesthetics, and micro-nano processing. Their core operation relies on nanosecond-scale high-energy pulsed discharges generated by high-voltage power supplies. The stability of discharge quality directly determines the energy uniformity, efficiency, and lifespan of laser output. Here, closed-loop pulse energy feedback control technology is pivotal for enhancing system performance, particularly in mitigating challenges such as gas degradation and load fluctuations. 
1. Technical Challenges: Roots of Pulse Energy Instability
Excimer laser excitation requires gas ionization and population inversion within tens of nanoseconds. Traditional open-loop power systems face multiple issues: 
• Dynamic Gas Degradation in Discharge Cavity 
  During high-voltage discharges, halogen gases (e.g., F₂ in Ar/F₂ mixtures) react with electrode materials, causing continuous depletion of fluorine concentration. This alters discharge impedance and leads to pulse energy decay. For example, in lithography applications, a 1% fluctuation in fluorine concentration can result in >5% laser energy drift. 
• Transient Power Fluctuations 
  Thyratron switches typically exhibit pulse rise times exceeding 100 ns, prone to localized arcing and non-uniform energy deposition. This causes electrode erosion and energy drift. Additionally, aging and temperature drift of storage capacitors contribute to charging voltage deviations. 
• Time-Varying Load Characteristics 
  In medical applications (e.g., 308 nm XeCl lasers for vitiligo treatment or vascular ablation), laser transmission through UV optical fibers demands precise power management. Peak power >10⁷ W accelerates fiber end-face damage, while pulse widths <30 ns exacerbate power density stress. 
2. Implementation Strategies for Pulse Energy Feedback
To address these challenges, modern excimer laser power supplies adopt all-solid-state topologies combined with closed-loop control: 
• All-Solid-State Pulse Power Modulation 
  Replacing thyratrons with IGBT/MOSFET switches and multi-stage magnetic pulse compression (MPC) technology. For instance, a three-stage LC peaking circuit compresses pulse rise times to 50–100 ns, improving discharge uniformity while supporting long-term stable operation at kHz-level repetition rates. 
• Dual Closed-Loop Feedback Architecture 
  • Direct Energy Feedback Loop: Real-time monitoring of each pulse's output energy, with dynamic adjustment of capacitor charging voltage via PID algorithms. For example, upon detecting energy drop, the control system elevates the DC reference voltage within 1 ms to compensate for losses. 
  • Indirect Gas-State Feedback Loop: Gas composition tracking via spectroscopy or electrical parameter inversion. 3D array photosensors capture UV radiation intensity to estimate fluorine consumption, triggering automatic gas replenishment. This extends gas lifespan from 3 days to 15 days. 
• Pulse Width-Energy Co-Optimization 
  For fiber transmission scenarios, pulse width broadening is achieved by adjusting LC parameters in pulse-forming networks (PFN). Experiments show that a 4-stage LC peaking circuit extends 308 nm laser pulses from 30 ns to 60 ns, reducing peak power from 18.8 MW to 8.4 MW. Fiber transmission efficiency increases by >40%, while energy transfer efficiency rises from 1.53% to 1.73%. 
3. Application Value and Emerging Trends
• Precision Assurance in Semiconductor Lithography 
  In 193 nm immersion lithography, energy feedback systems limit pulse energy instability to ±0.5% and spectral bandwidth (E95) to ≤0.35 pm, ensuring critical dimension accuracy for sub-7 nm processes. 
• Enhanced Reliability in Medical Devices 
  For 308 nm vitiligo treatment systems, feedback control achieves output energy instability <4% at 1–200 Hz, with spot uniformity error ≤10%, effectively preventing skin burns. 
• Energy Efficiency Leap in Industrial Processing 
  In display panel annealing, power combining from multiple lasers requires real-time energy synchronization. Closed-loop control boosts single-module energy conversion efficiency to >2.5%—a 50% improvement over conventional systems—while reducing thermal erosion of electrodes. 
Conclusion
Pulse energy feedback technology in excimer laser power supplies fundamentally transforms transient discharges into controllable energy outputs through a sense-decide-act closed-loop architecture. With advancements in wide-bandgap semiconductor switches, multi-physics simulation, and adaptive PID algorithms, next-generation systems will integrate pulse rise-time modulation, gas lifespan prediction, and fault self-diagnostics. This evolution promises megawatt-level peak power with million-pulse longevity and sub-millijoule energy precision, driving progress in high-end manufacturing and medical equipment while setting new paradigms for pulsed power supply design.