Extreme ultraviolet

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Extreme Ultra-Violet radiation (EUV) is generally considered to be the part of the electromagnetic spectrum spanning from 120 nm down to 10 nm. Its main uses are photoelectron spectroscopy, solar imaging, and lithography. EUV is naturally generated by the solar corona and artifically by plasma sources. EUV is the most highly absorbed component of the electromagnetic spectrum, requiring high vacuum for transmission.

[edit] 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[1].

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 + kinetic energy of the emitted photoelectron

Within 3 mean free paths of photoelectron (1-2 nm):
Kinetic energy of photoelectron = Electron binding energy + kinetic energy of secondary electron + remaining kinetic energy of photoelectron

Within 3 mean free paths of secondary electron (~30 nm):
Kinetic energy of Nth (final generation) secondary electron ~ 0-5 eV = Chemical dissociation energy + heating

where the electron binding energy is 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 impact ionization. Sometimes, an Auger 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 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. 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.

[edit] EUV Damage

Like other forms of ionizing radiation, EUV and EUV-generated electrons are a likely source of device damage. Damage may result from oxide desorption[2] or trapped charge following ionization[3]. 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[4] 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)[5].

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.[6] EUV radiation showed positive charging centimeters beyond the borders of exposure while VUV radiation showed positive charging within the borders of exposure.

Studies using EUV femtosecond pulses at the FLASH synchrotron beam facility indicated thermal melting-induced damage thresholds below 100 mJ/cm2.[7]

[edit] References

  1. ^ B. L . Henke et al., J. Appl. Phys. 48, pp. 1852-1866 (1977).
  2. ^ D. Ercolani et al., Adv. Funct. Mater. 15, pp. 587-592 (2005).
  3. ^ D. J. DiMaria et al., J. Appl. Phys. 73, pp. 3367-3384 (1993).
  4. ^ H. Akazawa, J. Vac. Sci. & Tech. A 16, pp. 3455-3459 (1998).
  5. ^ J-M. Defise et al., Proc. SPIE 3114, pp. 598-607 (1997).
  6. ^ J. L. Shohet, http://pptl.engr.wisc.edu/Nuggets%20v9a.ppt
  7. ^ R. Sobierajski et al., http://hasyweb.desy.de/science/annual_reports/2006_report/part1/contrib/40/17630.pdf