After a long period of relatively low interest, science related to effects in the Extreme Ultraviolet (EUV) spectrum range experienced an explosive boom of publications in the last decades. A new application of EUV in lithography was the reason for such a growth. Naturally, an intensive development in such area produces a snowball effect of relatively uncharted phenomena. EUV-induced plasma is one of those. While being produced in the volume of a rarefied gas, it has a direct impact onto optical surfaces and construction materials of lithography machines, and thus has not only scientific peculiarity, but it is also of major interest for the technological application. The current article provides an overview of the existing knowledge regarding EUV-induced plasma characteristics. It describes common, as well as distinguishing, features of it in comparison with other plasmas and discusses its interaction with solid materials. This article will also identify the gaps in the existing knowledge and it will propose ways to bridge them.
Hydrogen radicals play an important role in, e.g., the cleaning of extreme ultraviolet reflective mirrors. Therefore, there is a need to quantify the surface radical flux in the various (plasma) setups where these effects are studied. In this paper, a catalytic radical sensor is presented, based on the measurement of the recombination heat of radicals on a surface, using dual probe thermopile heat flux sensors (HFSs). The first HFS1 has a high recombination (probability) coefficient coating, e.g., Pt. The second HFS2 has a low recombination coefficient coating, e.g., Al2O3. Signal subtraction largely eliminates common mode heat losses/gains such as conduction/convection and IR radiation, the net result representing the radical recombination heat. The signal can be improved by switching the radical source on/off at regular intervals. Radical recombination rates were measured in a remote microwave plasma chamber (38 Pa H2) over the range 1018−1021 atH/(m2 s), with nearly linear response as a function of plasma power setting. The sensor full scale limit is ∼1023 atH/(m2 s) and is dictated by the maximum allowable sensor surface temperature (<250 °C).
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