Extreme ultraviolet
Encyclopedia
Extreme Ultraviolet radiation (EUV or XUV) is high-energy
ultraviolet
radiation, generally defined to be electromagnetic radiation
in the part of the electromagnetic spectrum
spanning wavelength
s from 120 nm down to 10 nm, and therefore (by the Planck–Einstein equation) having photon
s with energies from 10 eV up to 124 eV (corresponding to 124 nm to 10 nm respectively). EUV is naturally generated by the solar corona and artificially by plasma
and synchrotron light sources.
Its main uses are photoelectron spectroscopy, solar imaging
, and lithography.
In air, EUV is the most highly absorbed
component of the electromagnetic spectrum, requiring high vacuum
for transmission.
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 existence of multicharged positive ions is only possible in a hot dense plasma
. Alternatively, the free electrons and ions may be generated temporarily and instantaneously by the intense electric field of a very-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 constitute ionizing radiation
, they will also ionize the atoms of the harmonic-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.
EUV light can also be emitted by free electrons orbiting a synchrotron
.
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.
, EUV and electrons released directly or indirectly by the EUV radiation are a likely source of device damage
. Damage may result from oxide desorption or trapped charge following ionization. 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 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).
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. 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.
An earlier study showed that electrons produced by the 'soft' ionizing radiation could still penetrate ~100 nm below the surface, resulting in heating.
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation, generally defined to be electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
in the part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
spanning wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s from 120 nm down to 10 nm, and therefore (by the Planck–Einstein equation) having photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s with energies from 10 eV up to 124 eV (corresponding to 124 nm to 10 nm respectively). EUV is naturally generated by the solar corona and artificially by plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
and synchrotron light sources.
Its main uses are photoelectron spectroscopy, solar imaging
Extreme ultraviolet Imaging Telescope
The Extreme ultraviolet Imaging Telescope is an instrument on the SOHO spacecraft used to obtain high-resolution images of the solar corona in the ultraviolet range...
, and lithography.
In air, EUV is the most highly absorbed
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
component of the electromagnetic spectrum, requiring high vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
for transmission.
EUV generation
Neutral atoms or condensed matter cannot emit EUV radiation. IonizationIonization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
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
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
. Such electrons are more tightly bound than typical valence electrons. The existence of multicharged positive ions is only possible in a hot dense plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. Alternatively, the free electrons and ions may be generated temporarily and instantaneously by the intense electric field of a very-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 constitute ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
, they will also ionize the atoms of the harmonic-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.
EUV light can also be emitted by free electrons orbiting a synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
.
EUV absorption in matter
When an EUV photon is absorbed, photoelectronsPhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
and secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
are generated by ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
, 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
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
generation)
3. Final generation electron kinetic energy ~ 0 eV => dissociative electron attachment + heat
where the ionization potential
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
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
Impact ionization
Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers...
. Sometimes, an Auger transition
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...
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
Exciton
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...
. 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.
EUV damage
Like other forms of ionizing radiationIonizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
, EUV and electrons released directly or indirectly by the EUV radiation are a likely source of device damage
Radiation hardening
Radiation hardening is a method of designing and testing electronic components and systems to make them resistant to damage or malfunctions caused by ionizing radiation , such as would be encountered in outer space, high-altitude flight, around nuclear reactors, particle accelerators, or during...
. Damage may result from oxide desorption or trapped charge following ionization. Damage may also occur through indefinite positive charging by the Malter effect
Malter effect
The Malter effect is named after Zachary Malter, who first described the effect. Following exposure to ionizing radiation , secondary electron emission from the surface of a thin insulating layer results in the establishment of a positive charge on the surface...
. If free electrons cannot return to neutralize the net positive charge, positive ion desorption is the only way to restore neutrality. However, desorption
Desorption
Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption . This occurs in a system being in the state of sorption equilibrium between bulk phase and an adsorbing surface...
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).
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. EUV radiation showed positive charging centimeters beyond the borders of exposure while VUV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(Vacuum Ultraviolet) radiation showed positive charging within the borders of exposure.
Studies using EUV femtosecond pulses at the Free Electron Laser in Hamburg (FLASH
FLASH
FLASH, acronym of Free Electron LASer in Hamburg, a particle accelerator-based soft x-ray laser located at the DESY accelerator facilities in Hamburg, Germany. It can generate very powerful, ultrashort pulses of coherent radiation in the energy range 10 eV to 200 eV...
) indicated thermal melting-induced damage thresholds below 100 mJ/cm2.
An earlier study showed that electrons produced by the 'soft' ionizing radiation could still penetrate ~100 nm below the surface, resulting in heating.