EUV is expected to be used as a light source in the next generation of lithography. In order to meet the joint requirements for commercialization, researchers and engineers working on EUV sources must satisfy many needs: for increased EUV power, high repetition rate, stability, improved source figure, and so forth. Compared with a source using an LPP, one based on a DPP has important advantages, such as its direct conversion from electrical energy to photon energy and, especially, its lower cost of ownership. The largest disadvantage of the DPP source is the need for tight debris mitigation because of erosion or sputtering of the electrodes and dielectric surface. Because debris easily damages EUV optics, the research and engineering effort has concentrated on the effect of debris generation on EUV source performance.
Although several schemes are available to create a high-energy-density plasma, such as the dense plasma focus (DPF), hollow-cathode-triggered Z pinch (HCT-Z), capillary discharge, and Z pinch, the source we are developing employs a combination of capillary discharge and Z pinch in view of their simplicity and stability. Capillary discharge can maintain a high-temperature plasma for a relatively long time because a small-diameter capillary (dielectric tube) is used, and a low-amplitude and long-duration current pulse drives the load. A conventional Z-pinch plasma, in contrast, is created in a large-diameter dielectric tube or vacuum. A large current pulse creates an azimuthal magnetic field around the cylindrical initial plasma or gas, and magnetic pressure compresses the plasma toward the central Z axis. The capillary discharge is good with respect to stability, but the Z pinch is better with respect to radiation brightness and the lifetime of the dielectric wall. Thus our source employs a medium-diameter capillary and a large-amplitude and short-duration current pulse, so that we are able to obtain high-power, high-brightness, and stable EUV radiation even at high repetition rates.
In this chapter the current development status of the capillary discharge, debris mitigation system, and collection optics and their evaluation will be described briefly.
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