The light source has been identified as being the most critical single component on the way to realizing EUVL. The requirements on the source are derived from a throughput model for the processing of up to 120 wafers per hour in a commercial EUV scanner. In the current status of design of the scanner, this means a requirement of 115-W collected inband EUV. The 115 W refers to the power emitted in a 2% band around the center wavelength of 13.5 nm. For etendue limits imposed by the optical system, the radiation must be emitted out of a small volume of a few cubic millimeters in order to be collectable by the optical system. Typical collector openings are limited to a geometrical opening of 2–3 sr; collector efficiencies are on the order of 50%–80%. The source power is defined to include all of these above effects.
Philips Extreme UV, as a joint venture of Philips and Fraunhofer ILT, is running an extensive program to realize high-power sources well suited for production. The source concept is the hollow-cathode-triggered (HCT) gas discharge. Many aspects of it have been published in a series of preceding papers. The program includes fundamental research on ignition and pinch formation, but mainly focuses on the many engineering problems, such as power scaling, electrode lifetime, collector protection, and closed-loop dose control. So far, Xe has been used as a radiator, which, however, is considered as having insufficient conversion efficiency (CE) to meet the long-term power requirements. To address this issue, a research program using Sn as a radiator has been started. Sn is well known for its favorable spectral distribution; however, it has the disadvantage of being more difficult to handle. Handling problems occur both in its delivery to and in its removal from the system. Nevertheless, it seems to be the only possibility for ever achieving the required high power levels.
The chapter addresses some of the problems associated with both Xe and Sn sources. In Sec. 13.2, the physics of ignition and pinching of an HCT source will be described. Section 13.3 summarizes the results achieved so far in developing commercial EUV sources. Emphasis is put on the physical principles rather than the engineering aspects.
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