EUVL Light Source
Neutral atoms or condensed matter cannot emit EUV radiation. For matter to emit it, ionization must take place first. EUV light can only be emitted by electrons which are bound to multicharged positive ions; for example, to remove an electron from a +3 charged carbon ion (three electrons already removed) requires about 65 eV. Such electrons are more tightly bound than typical valence electrons. The thermal production of multicharged positive ions is only possible in a hot dense plasma, which itself strongly absorbs EUV. The Xe or Sn plasma sources for EUV lithography are either discharge-produced or laser-produced. Power output exceeding 100 W is a requirement for sufficient throughput. While state-of-the-art 193 nm ArF excimer lasers offer intensities of 200 W/cm2, lasers for producing EUV-generating plasmas need to be much more intense, on the order of 1011 W/cm2. This indicates the enormous energy burden imposed by switching from generating 193 nm light (laser output approaching 100 W) to generating EUV light (required laser or equivalent power source output exceeding 10 kW). An EUV source driven by a 20 kW CO2 laser with ~10% wall plug efficiency consumes an electrical power of ~200 kW, while a 100 W ArF immersion laser with ~1% wall plug efficiency consumes an electrical power of ~10 kW.
A further characteristic of the plasma-based EUV sources under development is that they are not even partially coherent, unlike the KrF and ArF excimer lasers used for current optical lithography. Further power reduction (energy loss) is expected in converting incoherent sources (emitting in all possible directions at many independent wavelengths) to partially coherent (emitting in a limited range of directions within a narrow band of wavelengths) sources by filtering (unwanted wavelengths and directions). On the other hand, coherent light poses a risk of monochromatic reflection interference and mismatch of multilayer reflectance bandwidth.
As of 2008, the development tools had a throughput of 4 wafers per hour with a 120 W source. For a 100 WPH requirement, therefore, a 3 kW source would be needed, which is not available in the foreseeable future. However, EUV photon count is determined by the number of electrons generated per photon which are collected by a photodiode; since this is essentially the highly variable secondary yield of the initial photoelectron, the dose measurement will be impacted by high variability. In fact, data by Gullikson et al. indicated ~10% natural variation of the photocurrent responsivity. More recent data for silicon photodiodes remain consistent with this assessment. Calibration of the EUV dosimeter is a nontrivial unsolved issue. The secondary electron number variability is the well-known root cause of noise in avalanche photodiodes.
If other problems are solved well enough to justify the investment, free electron lasers may provide the required light quality.
Read more about this topic: Extreme Ultraviolet Lithography
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