Modern EUV scanners rely on LPP technology: powerful CO₂ laser pulses strike tiny tin droplets (roughly 30 microns in diameter), turning them into plasma that emits 13.5-nm EUV light. The light is collected by a 0.5-meter elliptical multilayer mirror and directed through a vacuum optical path. ASML has gradually increased source power from around 250W to around 500W and plans to reach 1000W in the coming years, while nearly doubling droplet generation to 100,000 per second.
Free-electron lasers offer a different approach: accelerating electrons to near light speed and passing them through an undulator — a series of alternating magnets — to emit coherent EUV radiation. Proponents argue FEL eliminates tin droplets and the associated debris that require protective pellicles on photomasks, and could provide substantially higher power. A single FEL might even feed multiple scanners via a beam-distribution system.
However, the tradeoffs are severe: a particle accelerator adds enormous complexity, requiring radiation shielding, electron-beam control, and delicate optics capable of handling extreme power levels. Achieving the availability, efficiency, and cost required for semiconductor fabs would take years — exactly the kind of effort ASML has already invested in LPP.
Elon Musk has publicly endorsed FEL in a past tweet (the link appears to date from 2012). Yet JPMorgan notes that given recent laser advances, ASML sees no reason to explore the alternative. The company's incremental strategy has already yielded consistent power improvements, and it appears committed to evolving its LPP platform rather than starting from scratch with an unproven technology.