Extreme ultraviolet
Extreme ultraviolet radiation (EUV or XUV) or high-
The main uses of extreme ultraviolet radiation are
EUV generation
Neutral atoms or
However, in 2011, Shambhu Ghimire et al. first observed high-harmonic generation in bulk crystals of zinc oxide. It draws interest to invest the possibility and mechanism of HHG in solid state. EUV radiation can be emitted in silicon dioxide or sapphire.
Direct tunable generation of EUV
EUV light can also be emitted by free electrons orbiting a synchrotron.
Continuously tunable narrowband EUV light can be generated by four wave mixing in gas cells of krypton and hydrogen to wavelengths as low as 110 nm.[2] In windowless gas chambers fixed four wave mixing has been seen as low as 75 nm.
EUV absorption in matter
When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter.[3]
The response of matter to EUV radiation can be captured in the following equations:
Point of absorption:
EUV photon energy = 92 eV, = Electron binding energy + photoelectron initial kinetic energy
Within 3 mean free paths of photoelectron (1–2 nm):
Reduction of photoelectron kinetic energy = ionization potential + secondary electron kinetic energy;
Within 3 mean free paths of secondary electron (~30 nm):
- Reduction of secondary electron kinetic energy = ionization potential + tertiary electron kinetic energy
- mNth generation electron slows down aside from ionization by heating (phonon generation)
- Final generation electron kinetic energy ~ 0 eV => dissociative electron attachment + heat, where the ionization potentialis typically 7–9 eV for organic materials and 4–5 eV for metals.
The photoelectron subsequently causes the emission of secondary electrons through the process of
Strictly speaking, photoelectrons, Auger electrons and secondary electrons are all accompanied by positively charged holes (ions which can be neutralized by pulling electrons from nearby molecules) in order to preserve charge neutrality. An electron-hole pair is often referred to as an exciton. For highly energetic electrons, the electron-hole separation can be quite large and the binding energy is correspondingly low, but at lower energy, the electron and hole can be closer to each other. The exciton itself diffuses quite a large distance (>10 nm).[4] As the name implies, an exciton is an excited state; only when it disappears as the electron and hole recombine, can stable chemical reaction products form.
Since the photon absorption depth exceeds the electron escape depth, as the released electrons eventually slow down, they dissipate their energy ultimately as heat. EUV wavelengths are absorbed much more strongly than longer wavelengths, since their corresponding photon energies exceed the bandgaps of all materials. Consequently, their heating efficiency is significantly higher, and has been marked by lower thermal ablation thresholds in dielectric materials.[5]
Solar minima/maxima
Certain wavelengths of EUV vary by as much as a factor of 50 between solar minima and maxima, [6] which may contribute to stratospheric warming and ozone production. These may in turn affect atmospheric circulation and climate patterns over short and long term solar cycles.[6]
EUV damage
Like other forms of ionizing radiation, EUV and electrons released directly or indirectly by the EUV radiation are a likely source of device damage. Damage may result from oxide desorption[7] or trapped charge following ionization.[8] Damage may also occur through indefinite positive charging by the Malter effect. If free electrons cannot return to neutralize the net positive charge, positive ion desorption[9] is the only way to restore neutrality. However, desorption essentially means the surface is degraded during exposure, and furthermore, the desorbed atoms contaminate any exposed optics. EUV damage has already been documented in the CCD radiation aging of the Extreme UV Imaging Telescope (EIT).[10]
Radiation damage is a well-known issue that has been studied in the process of plasma processing damage. A recent study at the University of Wisconsin Synchrotron indicated that wavelengths below 200 nm are capable of measurable surface charging.[11] EUV radiation showed positive charging centimeters beyond the borders of exposure while VUV (vacuum ultraviolet) radiation showed positive charging within the borders of exposure.
Studies using EUV femtosecond pulses at the Free Electron Laser in Hamburg (FLASH) indicated thermal melting-induced damage thresholds below 100 mJ/cm2.[12]
An earlier study[13] showed that electrons produced by the 'soft' ionizing radiation could still penetrate ~100 nm below the surface, resulting in heating.
See also
- Extreme Ultraviolet Explorer
- Extreme Ultraviolet Variability Experiment
- Extreme ultraviolet Imaging Telescope
- High harmonic generation
- CHIPSat
- Extreme ultraviolet lithography
References
- ^ "The periodic table of the elements by WebElements". www.webelements.com.
- PMID 19776917.
- ISSN 0021-8979.
- ISSN 0935-9648.
- ^ A. Ritucci et al., "Damage and ablation of large band gap dielectrics induced by a 46.9 nm laser beam", March 9, 2006 report UCRL-JRNL-219656 Archived January 25, 2017, at the Wayback Machine (Lawrence Livermore National Laboratory).
- ^ PMID 20833325.
- S2CID 136478856.
- ISSN 0021-8979.
- ISSN 0734-2101.
- .
- ^ J. L. Shohet, http://pptl.engr.wisc.edu/Nuggets%20v9a.ppt Archived 2006-08-29 at the Wayback Machine
- ^ R. Sobierajski et al., http://hasyweb.desy.de/science/annual_reports/2006_report/part1/contrib/40/17630.pdf
- ^ "FEL 2004 – VUV pulse interactions with solids" (PDF).
External links
- Media related to Extreme ultraviolet at Wikimedia Commons
- Mediawiki Extension:EUV