Extreme Ultraviolet - EUV Absorption in Matter

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.

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):

1. Reduction of secondary electron kinetic energy = ionization potential + tertiary electron kinetic energy

2. Nth generation electron slows down aside from ionization by heating (phonon generation)

3. Final generation electron kinetic energy ~ 0 eV => dissociative electron attachment + heat

where the ionization potential 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. The exciton itself diffuses quite a large distance (>10 nm). 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.