EUV induced plasma
EPG investigates intensively the type of plasma that is induced when highly energetic photons ionize a low pressure gas.
The practical application here is the future generation lithography tools. In order to keep up with Moore’s law (double the amount of transistors on a chip every two years) lithography processes have been dictated to reduce the wavelength of light with which lithography is performed. Currently, the used light – which is generated by irradiating a tin droplet by an intense laser beam - has a wavelength as small as 13.5 nm (92 eV photons) and is called Extreme Ultraviolet (EUV) radiation. The absorbance of this radiation in air at atmospheric pressure would be so high that EUV lithography is forced to be performed in vacuum. However, due to technological issues, there still remains the necessity of a low pressure background gas to be used.
Although at a much smaller rate than would have been the case in atmospheric gas pressures, also the low pressure background gas will be ionized by the EUV radiation passing through, creating a low pressure background plasma (see Fig.1). When touching delicate surfaces – such as the very expensive multilayer mirrors – local electric fields might accelerate ions reducing life-time of these optical components. Therefore understanding how the background plasma is being built up and the properties of it is of major importance for lithography industry to develop future systems with sufficient life-time and acceptable cost of ownership levels.
EPG researches EUV induced plasma by several state-of-the-art diagnostics. One of the most important plasma parameters, the density of free electrons is explored and monitored with the microwave cavity resonance technique [link]. Other diagnostics applied are the usage of Langmuir probes, Faraday cups and fast plasma-imaging techniques. More recently, EPG has started to investigate the properties of fast ions in the induced plasma. Of interest here are the density of ions and their fluxes and energies towards the surfaces. Diagnostics applied to this end are retarding field grid analysis and mass spectrometry. Experiments are done in close collaboration with ASML research one Xe-based discharge produced plasma EUV sources [see Fig.2].