Multiple distinct plasmas in extreme ultraviolet lithography sources

To create light at 13.5±0.2 nm for extreme ultraviolet (EUV) lithography, a plasma must be created that can excite ionized species, which will radiate in that band. This can be done by a laser hitting an appropriate solid or liquid material, creating a laser-produced plasma, or by heating up such a material in a discharge-produced plasma. The desired light is produced in the first tens of nanoseconds, and then the plasma expands. We address the effects of that expansion and the plasmas produced by it. If the source chamber is filled with hydrogen gas (which is typical in EUV sources), first, the photons can produce a plasma both in the gas and by photoelectron emission on the chamber surfaces. Then, fast electrons leave the expanding plasma, which could do the same things. Finally, fast ions lead the way and again can interact with the gas or surfaces. Finally, ambipolar diffusion takes place for the remaining bulk plasma. These plasmas were measured by a triple Langmuir probe as a function of distance from the initial plasma, gas pressure, and initial plasma source gas. The experimental results are consistent with diffusion theory, taking into account the higher initial temperature of the source plasma, and with estimates of the density given knowledge about the initial plasma.