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Chapter 18:
Plasma Capillary Source
Capillary discharges provide a simple means to produce hot and dense plasmas that are brilliant radiators in the EUV and soft x-ray spectral regions. Early investigations by Bogen et al. revealed strong line emission down to 3 nm, and McCorkle was the first to point out a possible application to lithography. Capillary discharges are usually compact, allow efficient energy coupling, and are characterized by a small diameter-to-length ratio. The radiating hot plasma is either produced from a gas filling the discharge tube or, when operated at near-vacuum, from material ablated from the wall by an initial sliding spark, radiation from the hot plasma and heat flow to the wall providing further wall ablation. The smaller the capillary diameter, the stronger the wall ablation is. Also, a two-step operation employing a prepulse to evaporate wall material for the main discharge has been demonstrated. Gas-filled capillaries have diameters typically larger than 1 mm, the diameters of ablative setups being 1 mm and smaller. The high current through the capillary results in pinching of the plasma to form the hot plasma column along the axis. In fast discharges this pinch effect is preceded by a cylindrical shock wave with consequent heating. Although in quasi-steady state, standard Z pinches are inherently unstable against the growth of the m = 0 sausage and the m = 1 kink instability over an Alfvén transit time, in capillary discharges weakly ionized plasma and∕or the influx of ablated wall material more or less bridges the region between the wall and the hot plasma column and stabilizes the latter. Exceptions are possible, and the plasma becomes unstable if conditions can be found under which the plasma indeed is fully detached from the wall. Capillary discharges can be characterized also by the period of the discharge and consequently by the current driver. Extremely fast discharges with a first half-cycle duration of the current well below 100 ns are driven by transmission lines, which are pulse-charged by appropriate Marx generators. Operation of such fast discharges is usually at high voltages. The fast rise time is mandatory for driving collisionally excited soft-x-ray lasers, but the short duration of EUV emission discourages application to lithography, where the delivered radiant energy is important. An extraordinarily short duration of emission, 5 ns, has been achieved in a device that combines the features of a transient hollow-cathode discharge with the inherent characteristics of a capillary discharge. Half-cycle durations of 100 ns and above are achieved by conventionally discharging capacitors but in carefully designed low-inductance arrangements. Rather straightforward designs of the discharge circuit are used for pulse durations of 0.5 μs and longer.
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