Measuring the electron density in an Extreme Ultra-Violet generated plasma

In order to meet the demand of increasing computer speed and memory capacity, industries are striving to reduce the size of computer chips. This miniaturization of computer chips can be achieved by reducing the wavelength in lithography machines to Extreme Ultra-Violet (EUV, 92 eV). The low-pressure (around 1 Pa) transparent background gas (e.g. H¬2 and He) in the lithography machine is partially ionized by the absorption of EUV photons. The study of these low-density (1015 m-3) pulsed plasmas is interesting and experimentally challenging. Two different diagnostics are investigated to determine the electron density in these EUV-generated plasmas: coherent microwave scattering (CMS) and microwave cavity resonance spectroscopy (MCRS). CMS is based on the scattering of microwaves off the plasma, the scattered power has a resonance at the plasma frequency. In MCRS measurements the resonance frequency in a cavity is determined, this frequency depends on the electron density in the cavity. These two diagnostics are tested on a simulation plasma with similar dimensions and density as the EUV plasma. The results of MCRS on the simulation plasma show a detection limit of 1014 m-3 and a time-resolution of 16 ns. Currently a new setup is built to measure the electron density in the EUV plasma.