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Chapter 31:
Electron-Based EUV Sources for At-Wavelength Metrology
Editor(s): Vivek Bakshi
Author(s): Egbert, André; Chichkov, Boris
During the past few years, significant progress has been achieved in the development of multilayer-coated optical components for the EUV spectral range. At present, the characterization of EUV optics is carried out at synchrotron radiation facilities. Synchrotrons are high-quality sources that are ideal for scientific research, but they are very complex and expensive for applications. Therefore, compact, cheap, and easy-to-operate tools and systems are badly required for at-wavelength control and inspection, directly at the mirror fabrication labs and companies. Several attempts to develop laboratory EUV sources for metrology, based on laser- or discharge-produced plasmas, are currently being made. Unfortunately, these sources have rather complex setups and high running costs. They operate at a relatively low repetition rate, exhibit considerable temporal and spatial fluctuations, and always require special efforts for debris mitigation. Usually it is overlooked that one can produce EUV radiation directly from solid targets, avoiding all these plasma-related drawbacks. This can be done by using electron-induced characteristic emission from solids. The electron-based EUV source can be considered as an analog to a conventional x-ray tube. Electrons are generated by a tungsten filament, accelerated in a high-voltage electric field toward an anode, and focused onto a solid target, which allows the generation of characteristic emission in the EUV spectral range. This radiation is produced by electron-impact ionization (excitation) of atomic inner shells, which is followed by radiative decay. In Fig. 31.1 (a), dependences of the wavelengths of the characteristic K- and L-shell emission on the atomic number Z are shown. The Kα line of beryllium (Z = 4) and the L-shell radiation of silicon (Z = 14) are located at 11.4 and 13.5 nm, respectively. Therefore, electron-induced emission from these materials (especially from silicon) is potentially of high practical importance for EUV-related applications. An EUV source operating via this mechanism (EUV tube) fulfills practically all requirements for a metrology source: stable, long-term, and debris-free operation; compact, low-cost, and user-friendly setup; and well-defined, easy-to-calibrate, and controllable output. For several decades soft-x-ray sources based on the electron bombardment of solid targets have been studied and applied to the calibration of soft-x-ray spectrographs and detectors. However, the possibility of using this technique for the generation of EUV radiation has fallen into oblivion. Now, due to the rapid technological developments in EUVL, this technique has been rediscovered as a promising tool for EUV metrology. An interesting but more complicated version of an EUV source based on narrowband Si L-edge Cherenkov emission induced by rel-ativistic electrons also has been suggested.
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