Interference lithography
Encyclopedia
Interference lithography (or holographic lithography) is a technique for patterning regular arrays of fine features, without the use of complex optical
systems or photomask
s.
or holography
. An interference pattern between two or more coherent
light waves
is set up and recorded in a recording layer (photoresist
). This interference pattern consists of a periodic series of fringes representing intensity minima and maxima. Upon post-exposure photolithographic
processing, a photoresist pattern corresponding to the periodic intensity pattern emerges.
For 2-beam interference, the fringe-to-fringe spacing or period is given by (λ/2)/sin(θ/2), where λ is the wavelength and θ is the angle between the two interfering waves. The minimum period achievable is then half the wavelength.
By using 3-beam interference, arrays with hexagonal symmetry can be generated, while with 4 beams, arrays with rectangular symmetry are generated. Hence, by superimposing different beam combinations, different patterns are made possible.
. Second, it is preferred to use a monochromatic or temporally coherent light source. This is readily achieved with a laser but broadband sources would require a filter. The monochromatic requirement can be lifted if a diffraction grating is used as a beam splitter, since different wavelengths would diffract into different angles but eventually recombine anyway. Even in this case, spatial coherence and normal incidence would still be required.
are Lloyd´s mirror
s, prism
s and diffraction grating
s.
[1,2]. Spacings of a few nanometers[1] or even less than a nanometer[2] have been reported using electron holograms. This is because the wavelength of an electron is always shorter than for a photon of the same energy. The wavelength of an electron is given by the de Broglie relation h/p, where h is the Planck constant
and p is the electron momentum. For example, a 1 keV
electron has a wavelength of slightly less than 0.04 nm. A 5 eV
electron has a wavelength of 0.55 nm. This yields X-ray-like resolution without depositing significant energy. In order to ensure against charging, it must be ensured that electrons can penetrate sufficiently to reach the conducting substrate.
A fundamental concern for using low-energy electrons (<<100 eV) with this technique is their natural tendency to repel one another due to Coulomb forces
as well as Fermi-Dirac statistics
, though electron anti-bunching has been verified only in a single case.
or 193 nm immersion
). In addition, interfering laser beams of high-power pulsed lasers provides the opportunity of applying a direct treatment of the material's surface (including metals, ceramics and polymers) based on photothermal and/or photochemical mechanisms. Due to the above mentioned characteristics, this method has been called in this case “Direct Laser Interference Pattering” (DLIP) [3-5]. Using DLIP, the substrates can be structured directly in one-step obtaining a periodic array on large areas in a few seconds. Such patterned surfaces can be use for different applications including tribology (wear and friction reduction) or biotechnology.
Electron interference lithography[6,7] may be used for patterns which normally take too long for conventional electron beam lithography
to generate.
The drawback of interference lithography is that it is limited to patterning arrayed features only. Hence, for drawing arbitrarily shaped patterns, other photolithography techniques are required. In addition, non-optical effects, such as secondary electrons from ionizing radiation
or photoacid generation and diffusion, cannot be avoided with interference lithography. For instance, the secondary electron range is roughly indicated by the width of carbon contamination (~20 nm) at the surface induced by a focused (2 nm) electron beam[7]. This indicates that the lithographic patterning of 20 nm half-pitch features or smaller will be significantly affected by factors other than the interference pattern, such as the cleanliness of the vacuum.
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
systems or photomask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
s.
Basic principle
The basic principle is the same as in interferometryInterferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
or holography
Holography
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...
. An interference pattern between two or more coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
light waves
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
is set up and recorded in a recording layer (photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
). This interference pattern consists of a periodic series of fringes representing intensity minima and maxima. Upon post-exposure photolithographic
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
processing, a photoresist pattern corresponding to the periodic intensity pattern emerges.
For 2-beam interference, the fringe-to-fringe spacing or period is given by (λ/2)/sin(θ/2), where λ is the wavelength and θ is the angle between the two interfering waves. The minimum period achievable is then half the wavelength.
By using 3-beam interference, arrays with hexagonal symmetry can be generated, while with 4 beams, arrays with rectangular symmetry are generated. Hence, by superimposing different beam combinations, different patterns are made possible.
Coherence requirements
For interference lithography to be successful, coherence requirements must be met. First, a spatially coherent light source must be used. This is effectively a point light source in combination with a collimating lens. A laser or synchrotron beam are also often used directly without additional collimation. The spatial coherence guarantees a uniform wavefront prior to beam splittingBeam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...
. Second, it is preferred to use a monochromatic or temporally coherent light source. This is readily achieved with a laser but broadband sources would require a filter. The monochromatic requirement can be lifted if a diffraction grating is used as a beam splitter, since different wavelengths would diffract into different angles but eventually recombine anyway. Even in this case, spatial coherence and normal incidence would still be required.
Beam splitter
Coherent light must be split into two or more beams prior to being recombined in order to achieve interference. Typical methods for beam splittingBeam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...
are Lloyd´s mirror
Lloyd's mirror
Lloyd's mirror is a classic optics experiment and was first described in 1834 and again in 1837 by Humphry Lloyd in the proceedings of the Royal Irish Academy of Science. In the experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come...
s, prism
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
s and diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
s.
Electron holographic lithography
The technique is readily extendible to electron waves as well, as demonstrated by the practice of electron holographyElectron holography
Electron holography is holography with electron waves. Dennis Gabor invented holography in 1948 when he tried to improve resolution in electron microscope. The first attempts to perform holography with electron waves were made by Haine and Muley in 1952; they demonstrated recorded with 60keV...
[1,2]. Spacings of a few nanometers[1] or even less than a nanometer[2] have been reported using electron holograms. This is because the wavelength of an electron is always shorter than for a photon of the same energy. The wavelength of an electron is given by the de Broglie relation h/p, where h is the Planck constant
Planck constant
The Planck constant , also called Planck's constant, is a physical constant reflecting the sizes of energy quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory, who discovered it in 1899...
and p is the electron momentum. For example, a 1 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
electron has a wavelength of slightly less than 0.04 nm. A 5 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...
electron has a wavelength of 0.55 nm. This yields X-ray-like resolution without depositing significant energy. In order to ensure against charging, it must be ensured that electrons can penetrate sufficiently to reach the conducting substrate.
A fundamental concern for using low-energy electrons (<<100 eV) with this technique is their natural tendency to repel one another due to Coulomb forces
Coulomb's law
Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...
as well as Fermi-Dirac statistics
Fermi-Dirac statistics
Fermi–Dirac statistics is a part of the science of physics that describes the energies of single particles in a system comprising many identical particles that obey the Pauli Exclusion Principle...
, though electron anti-bunching has been verified only in a single case.
Atom holographic lithography
The interference of atomic deBroglie waves is also possible provided one can obtain coherent beams of cooled atoms. The momentum of an atom is even larger than for electrons or photons, allowing even smaller wavelengths, per the deBroglie relation. Generally the wavelength will be smaller than the diameter of the atom itself.Uses of interference lithography
The benefit of using interference lithography is the quick generation of dense features over a wide area without loss of focus. Hence, it is commonly used for testing photoresist processes for lithography techniques based on new wavelengths (e.g., EUVExtreme ultraviolet lithography
Extreme ultraviolet lithography is a next-generation lithography technology using an extreme ultraviolet wavelength, currently expected to be 13.5 nm.-EUVL light source:...
or 193 nm immersion
Immersion lithography
Immersion lithography is a photolithography resolution enhancement technique for manufacturing integrated circuits that replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one. The resolution is increased by a factor...
). In addition, interfering laser beams of high-power pulsed lasers provides the opportunity of applying a direct treatment of the material's surface (including metals, ceramics and polymers) based on photothermal and/or photochemical mechanisms. Due to the above mentioned characteristics, this method has been called in this case “Direct Laser Interference Pattering” (DLIP) [3-5]. Using DLIP, the substrates can be structured directly in one-step obtaining a periodic array on large areas in a few seconds. Such patterned surfaces can be use for different applications including tribology (wear and friction reduction) or biotechnology.
Electron interference lithography[6,7] may be used for patterns which normally take too long for conventional electron beam lithography
Electron beam lithography
Electron beam lithography is the practice of emitting a beam of electrons in a patterned fashion across a surface covered with a film , and of selectively removing either exposed or non-exposed regions of the resist...
to generate.
The drawback of interference lithography is that it is limited to patterning arrayed features only. Hence, for drawing arbitrarily shaped patterns, other photolithography techniques are required. In addition, non-optical effects, such as secondary electrons from 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...
or photoacid generation and diffusion, cannot be avoided with interference lithography. For instance, the secondary electron range is roughly indicated by the width of carbon contamination (~20 nm) at the surface induced by a focused (2 nm) electron beam[7]. This indicates that the lithographic patterning of 20 nm half-pitch features or smaller will be significantly affected by factors other than the interference pattern, such as the cleanliness of the vacuum.