Research on Pre-ionization Control Technology in Excimer Laser High-Voltage Power Supplies

Excimer lasers (e.g., ArF, KrF) serve as high-power pulsed light sources in the deep ultraviolet spectrum and are indispensable in lithography, micro-machining, and medical applications. Their operation relies on high-energy pulsed discharges generated by high-voltage power supplies, where discharge uniformity and stability directly determine laser output energy stability, beam quality, and device longevity. Gas pre-ionization technology is the core element for achieving high-uniformity discharges, and its control precision significantly impacts the laser’s overall performance. 
1. Technical Significance of Pre-ionization
Gas discharge in excimer lasers occurs only when the free electron density in the discharge region meets a threshold condition. The pre-ionization mechanism involves ionizing trace impurities via photons generated by spark or corona discharge before the main discharge, creating an initial electron density that satisfies the threshold. Insufficient pre-ionization causes the main discharge to degenerate into localized arcs, leading to uneven energy deposition, gas degradation, electrode erosion, and laser energy fluctuations. Studies show that the time interval between pre-ionization and the main discharge must be controlled within tens of nanoseconds (ns): deviations reduce discharge uniformity and beam quality. 
2. Key Pre-ionization Methods
1. Corona Pre-ionization: 
   Ultraviolet photons are generated by corona discharge at electrode edges, suitable for small-to-medium lasers. An improved front-face corona structure positions pre-ionization electrodes at the front of the main electrodes, using quartz tubes to form a uniform plasma layer. This increases discharge spacing from 4.5 mm to 7.3 mm, boosting output energy and efficiency. Advantages include simple structure, low energy consumption (pre-ionization energy is minimal), and reduced electrode sputtering contamination, extending gas life (single-charge lifetime >6 hours). 
2. UV Pre-ionization: 
   Ideal for high-pressure (>2 atm) scenarios. In XeCl lasers, spark gap arrays generate UV photons for ionization, achieving 43 mJ output at 2 atm with an energy density of 3 J/L and efficiency of 1.4%. Efficiency depends on gas composition: polyatomic molecules (e.g., CCl₄) absorb UV photons, reducing ionization efficiency. Using HCl as a chlorine donor, dissociation products replenish chlorine via photochemical cycles, extending laser life to 5,000 pulses. 
3. Surface Sliding Flash Pre-ionization: 
   Utilizes multi-channel discharge along insulating dielectric plates (e.g., ceramic or epoxy) to produce UV light. It triples electron yield compared to free-spark methods and operates at lower voltages. Narrow metal strips (e.g., nickel) form parallel channels with the main cathode via a dielectric plate, each connected to an independent high-voltage ceramic capacitor for discharge synchronization. The plate angle must be <30° to optimize airflow and reduce turbulence loss during high-repetition operation. 
3. Core Timing Control Technologies
Timing drift between pre-ionization and main discharge is a key stability challenge. Traditional open-loop control cannot dynamically compensate for circuit noise or temperature drift. Modern solutions adopt closed-loop feedback control: 
• Time Measurement Unit: Photodiodes monitor main discharge start time (T₂), while pickup coils determine pre-ionization start time (T₁). 
• Magnetic Switch Adjustment: Independent pulse compression circuits for pre-ionization and main discharges regulate saturation cutoff time by adjusting reset current (I_b) in saturable inductors (magnetic switches). This alters magnetic flux swing (ΔB), with Δt ∝ N·A_c·ΔB / V (N: coil turns, A_c: core area). 
• Dynamic Compensation Algorithm: If the measured time difference T_D=T₂-T₁ deviates from the target T_DT, the control unit recalculates current based on ΔI_b ∝ ΔT_D, enabling real-time correction. 
4. Impact on Laser Performance
1. Enhanced Discharge Uniformity: Pulse fronts optimized to 50–100 ns suppress discharge channel contraction and localized arcs. 
2. Extended Gas Lifetime: Uniform discharges reduce halogen gas consumption; e.g., XeCl lasers with HCl gas sustain >200 hours of continuous operation. 
3. Reduced Operational Energy: Resonant trigger control (e.g., LC circuit oscillation) superimposes high-frequency pulses on DC voltage, avoiding wasteful ionization from sustained high voltage and cutting energy consumption by >20%. 
5. Conclusion and Future Perspectives
Gas pre-ionization control is a pivotal technology for high-voltage power supplies in excimer lasers, evolving from open-loop to intelligent closed-loop systems. Future advancements require exploring novel insulating materials (e.g., BaTiO₃ dielectric plates) for erosion resistance and multi-physics coupling models (discharge-flow-temperature) for real-time regulation, enabling higher repetition rates (>1 kHz) and longer-life industrial lasers.