Extreme ultraviolet radiation (EUV[1] orXUV) or high-energyultraviolet radiation iselectromagnetic radiation in the part of theelectromagnetic spectrum spanningwavelengths shorter than the hydrogenLyman-alpha line from 121 nm down to the X-ray band of 10 nm. By thePlanck–Einstein equation the EUVphotons have energies from 10.26 eV up to 124.24 eV where we enter the X-ray energies. EUV is naturally generated by thesolar corona and artificially byplasma,high harmonic generation sources andsynchrotron light sources. Since theultraviolet C range extends to 100 nm, there is some overlap in the terms.
The main uses of extreme ultraviolet radiation arephotoelectron spectroscopy,solar imaging, andlithography. Inair, EUV is the most highlyabsorbed component of the electromagnetic spectrum, requiringhigh vacuum for transmission.
Neutral atoms orcondensed matter do not have large enoughenergy transitions to emit EUV radiation.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 typicalvalence electrons. The existence of multicharged positive ions is only possible in a hot denseplasma. Alternatively, the free electrons and ions may be generated temporarily and instantaneously by the intenseelectric field of avery-high-harmonic laser beam. The electrons accelerate as they return to the parent ion, releasing higher energy photons at diminished intensities, which may be in the EUV range. If the released photons constituteionizing radiation, they will also ionize the atoms of theharmonic-generating medium, depleting the sources of higher-harmonic generation. The freed electrons escape since the electric field of the EUV light is not intense enough to drive the electrons to higher harmonics, while the parent ions are no longer as easily ionized as the originally neutral atoms. Hence, the processes of EUV generation and absorption (ionization) strongly compete against each other.
In the beginning of the 21st century multiple EUV sources appeared based on thefree-electron laser and table top EUV sources based onhigh harmonic generation techniques.[2]: 579 First generation commercial systems for EUVlithography based onlaser-produced plasma (LPP) light generation have been shipped.[3]: 173
EUV light can also be emitted by free electrons orbiting asynchrotron.
Continuously tunablenarrowband EUV light can begenerated by four wave mixing in gas cells ofkrypton andhydrogen to wavelengths as low as 110 nm.[4] In windowless gas chambers fixed four wave mixing has been seen as low as 75 nm.
When an EUV photon is absorbed,photoelectrons andsecondary electrons are generated byionization, much like what happens whenX-rays or electron beams are absorbed by matter.[5]
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 3mean 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):
The photoelectron subsequently causes the emission of secondary electrons through the process ofimpact ionization. Sometimes, anAuger transition is also possible, resulting in the emission of two electrons with the absorption of a single photon.
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 anexciton. 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).[6]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.[7]
Certain wavelengths of EUV vary by as much as a factor of 50 betweensolar minima andmaxima,[8] which may contribute to stratospheric warming and regardingozone there is less energy needed to form ozone than to destroy it more energy from solar wind is rather damaging ozone layer. These in turn contribute to climate change related phenomena.[8]
Like other forms ofionizing radiation, EUV and electrons released directly or indirectly by the EUV radiation are a likely source ofdevice damage. Damage may result from oxide desorption[9] or trapped charge following ionization.[10] Damage may also occur through indefinite positive charging by theMalter effect. If free electrons cannot return to neutralize the net positive charge, positive ion desorption[11] 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).[12]
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.[13] EUV radiation showed positive charging centimeters beyond the borders of exposure whileVUV (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.[14]
An earlier study[15] showed that electrons produced by the 'soft' ionizing radiation could still penetrate ~100 nm below the surface, resulting in heating.
Media related toExtreme ultraviolet at Wikimedia Commons
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