Tracing the path of light through an EUV scanner

1. The source

Producing EUV light is a violent endeavor. In ASML’s sources, EUV light is generated using powerful lasers to knock off up to 13 electrons from tin atoms, creating a hot and dense plasma that emits a broad spectrum of radiation, including EUV. Specifically, tiny droplets of the metal – tens of thousands per second – are ejected from a nozzle and, while in flight, shot twice with a laser. The first, relatively weak shot expands and deforms the droplet into a pancake shape. The second laser shot carries much more energy and instantly turns the pancake into a plasma. A piece of optics shaped like the inside of a headlight (more on optics in section 5) collects and prefilters the broadband radiation and directs it toward the EUV scanner.

During the light-generation process, some tin debris scatters throughout the source environment. Unmitigated, this would damage the delicate and expensive EUV optics. That’s why a ‘blanket’ flow of oxygen and hydrogen gas protects the collector, capturing rogue tin fragments by turning them into gaseous compounds that can be removed with the gas stream. Because EUV radiation is absorbed by just about anything, and every EUV photon is precious, a vacuum environment is preferred, but for thermal stability, the presence of gases is necessary. Today’s ASML sources typically run well over a year before the collector mirror needs to be swapped for a clean one.

The sources that feed high-NA scanners are principally the same as those used for the first generation of EUV tools. A major difference is the angle at which the ‘raw’ EUV beam enters the scanner. Because Zeiss managed to reduce the number of mirrors in the illuminator from four to three (improving overall transmission), the light now needs to follow an almost horizontal path into the scanner. This required the source to be rotated by about 28 degrees.