Characterizing dusty argon-acetylene plasmas as a first step to understand dusty EUV environments

In extreme ultraviolet (EUV) lithography, ionic and particulate debris coming from the plasma source plays an important role. We started up a project looking at the principles of particle formation in plasmas and the interaction with EUV radiation. To this end, we study a low-pressure (10 Pa) capacitively-coupled radio-frequency (13.56 MHz) argon-acetylene plasma. In low-pressure hydrocarbon plasmas, dust particles spontaneously form under certain conditions. The whole process occurs in a matter of seconds to minutes after igniting the plasma and results in a cloud of particulates up to micrometer sizes levitating in the plasma. Several techniques are used to gain information about the plasma dynamics. Microwave cavity resonance spectroscopy is used to determine the global electron density. Mie laser scattering is used to locally determine the dust particle density. Suspended dust particles will scatter the laser light, resulting in a visible trace through the cavity, which is also imaged from below with a video camera. The electrical characteristics of the plasma (current, voltage, impedance, dissipated power) are determined using a commercially available plasma impedance monitor. All of these techniques are time resolved and are complemented with scanning and transmission electron microscopy of the grown dust particles to infer their sizes. We conclude by presenting an overview of approaches for future research predominantly aimed at the interactions in a more EUV-like environment.