EUV Lithography
# EUV Tin Droplet Laser-Produced Plasma Source
EUV lithography's 13.5-nanometer light doesn't come from a lamp or a conventional laser — it comes from vaporizing microscopic droplets of molten tin into plasma, tens of thousands of times per second, inside the scanner itself. A droplet generator fires a stream of molten tin droplets, each only tens of micrometers across, into the path of a high-power CO2 laser. The laser doesn't hit each droplet just once: a low-power "pre-pulse" first flattens the droplet into a thin disk to maximize its surface area, and then a much higher-power main pulse slams into that flattened disk, instantly vaporizing it into a plasma hot enough to emit light at exactly the 13.5-nanometer wavelength EUV lithography needs. This two-pulse sequence, repeated at roughly 50,000 times per second, is the entire light source for every EUV scanner in production today.
The reason this elaborate droplet-and-laser scheme exists at all is that no material naturally emits a lot of usable light at 13.5 nanometers under ordinary conditions — tin plasma at exactly the right temperature and density is one of the few sources that does, and hitting that narrow window reliably is what makes EUV source engineering so difficult. Getting a plasma to emit strongly at 13.5 nm requires heating tin to a precise temperature and density window; too cold and the emission is weak, too hot and the plasma emits at different wavelengths that are wasted power and even damage the collector optics downstream. The two-pulse pre-pulse/main-pulse sequence exists specifically to control that temperature and density precisely, by first controlling the droplet's shape before the main pulse arrives, rather than just firing one pulse and hoping the resulting plasma happens to land in the right regime.
ASML's EUV scanners source this light from Cymer-designed droplet generators (ASML acquired Cymer in 2013), and source power output — measured in watts of usable 13.5nm light delivered to the wafer — has been the single biggest lever on EUV scanner throughput since the technology's introduction. Early EUV source power struggled to reach commercially viable levels for years, which is the main reason EUV lithography's industrial rollout lagged its initial research timelines by roughly a decade. Every subsequent generation of EUV source has pushed droplet-generator repetition rate and pulse precision higher specifically to raise watts delivered per hour, because scanner throughput — and therefore the economics of an entire EUV-based fab — scales directly with how much usable 13.5nm light the source can put out.
Secondary electron blur sets a resolution floor inside the resist after light arrives; the tin droplet plasma source determines how much of that light arrives at all, and how fast — between the two, EUV lithography's practical limits turn out to be set as much by a stream of molten metal droplets as by anything happening inside the optical column itself.