Extreme Ultraviolet (EUV) optical components used in EUV lithography tools are continuously impacted by an exotic and highly transient type of plasma: EUV-induced plasma. Such an EUV-induced plasma is generated in a repetitive fashion upon sending a pulsed beam of high energy (92 eV) photons through a low-pressure background gas. Although its formation occurs on a time scale of ∼100 ns, it is the plasma's decay dynamics on longer time scales that dictates the fluxes and energy distribution of the produced ions. Therefore, the plasma decay also determines the overall impact on plasma-facing EUV optical components. Enabled by electron density measurements using Microwave Cavity Resonance Spectroscopy at a much higher sensitivity, we clearly show the breakdown of the ambipolar field in an EUV photon-induced plasma below electron densities of ∼2 × 1012 m−3 and the—until now—unidentified transition from ambipolar diffusion-driven decay into a decay regime driven by free diffusion. These results not only further improve the understanding of elementary processes in this type of plasma but also have a significant value for modeling and predicting the stability and lifetime of optical components in EUV lithography.

For the microwave cavity resonance spectroscopy based non-destructive beam monitor for ionizing radiation, an addition—which adapts the approach to conditions where only little ionization takes place due to, e.g., small ionization cross sections, low gas pressures, and low photon fluxes—is presented and demonstrated. In this experiment, a magnetic field with a strength of 57 ± 1 mT was used to extend the lifetime of the afterglow of an extreme ultraviolet-induced plasma by a factor of ∼5. Magnetic trapping is expected to be most successful in preventing the decay of ephemeral free electrons created by low-energy photons. Good agreement has been found between the experimental results and the decay rates calculated based on the ambipolar and classical collision diffusion models.

Understanding extreme ultraviolet (EUV) photon-induced plasma dynamics is key to increasing the lifetime of the new generation of lithography machines. The plasma decay times were determined by means of a non-destructive microwave method, microwave cavity resonance spectroscopy, for unmagnetized and magnetized EUV photon-induced plasma afterglows with the argon pressure ranging from 0.002 to 10 Pa. As a result of an external magnet with a magnetic field strength of (57 ± 1) mT, the plasma decay times were extended by two orders of magnitude. Good agreement was found between these measured plasma decay times and four diffusion models, i.e. the ion acoustic, ambipolar, classical-collision, and Bohm’s diffusion model.