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Chapter 37:
Erosion of Condenser Optics Exposed to EUV Sources
EUVL is a leading candidate for integrated circuit (IC) chip manufacturing below the 45-nm node. The technical basis for the approach is the use of a much smaller wavelength, 13.5 nm (92.5 eV), than is used in current manufacturing (193–248 nm). The very small radiation wavelength allows for much higher imaging resolution at relatively small numerical aperture (NA), all with an acceptable depth of field. Since the initial development of EUVL in the early 1990s, followed by the much more intensive development effort conducted by the Virtual National Laboratory (VNL) in collaboration with the EUV Limited Liability Company (LLC), an important challenge to the technology has been the development of a commercially viable EUV light source. The source of 13.5-nm radiation must provide sufficient EUV flux to enable high-volume manufacturing (HVM): the production of one hundred 300-mm-diameter wafers per hour while being highly reliable. Other contributors have discussed the types of EUV sources (all based on localized plasmas) that have been developed, including their spectral output, stability, size, and other operating characteristics. In this chapter, the focus will be on the degradative effects on optics directly exposed to plasma sources of EUV radiation. In particular, the tendency of the plasma to physically remove optical coatings, a process we call erosion, will be discussed. There are two distinct types of commercially viable EUV sources: laser-produced plasmas (LPPs) and discharge sources. In an LPP source, an intense laser beam (typically Nd-YAG, 1064 nm) is directed onto a target. The target can be solid, liquid, or gas. Under intense laser radiation, the target is dissociated and vaporized into a highly ionized plasma. Electron acceleration within the plasma (producing continuum radiation) as well as fluorescence from ionized atomic species leads to radiative emission over a broad range of wavelengths from the IR to the soft-x-ray. Of particular interest for EUVL is that the emission can also take place near 13.5 nm. Much work in the 1980s and early 1990s was performed on solid targets such as gold, tungsten, and tin, although it was very quickly realized that in addition to creating large numbers of EUV photons, the sources produced a large amount of debris consisting of vapor and particles from the solid target. This debris produced undesirable deposition on collecting optics and plasma diagnostic equipment. This debris-induced deposition is a serious limitation to the use of solid targets as a reliable source of EUV radiation. In an effort to circumvent the problem of debris-induced deposition, a great deal of work has focused on laser plasma sources using targets composed of liquid or frozen Xe, the idea being that once the Xe was volatilized by the laser beam, the gaseous Xe could not deposit on the collecting optics. This expectation was in fact realized, and Xe LPP sources have since been the focus of a great deal of work. However, experience has shown that it is still possible to generate condensable debris from a Xe LPP source if the nozzle producing the condensed Xe flow is degraded by exposure to the plasma. A second type of source is generally termed a discharge-produced plasma (DPP). In a DPP source, an electrical discharge is struck in the target medium, producing a plasma that emits EUV radiation. DPP sources include those based on a capillary electric discharge, as well as those based on a Z pinch. If a condensable target material is used in these discharge sources, large amounts of target material can be deposited on collector optics, degrading reflectivity. Even if a non-condensable target material is used (such as Xe), degradation of the electrodes by plasma operation can also lead to deposition at the collector optics.
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