Interference lithography

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Interference lithography (or holographic lithography) is a technique for patterning regular arrays of fine features, without the use of complex optical systems or photomasks.

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Basic principle

The basic principle is the same as in interferometry 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 , 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 or 3D photonic crystals are generated. With multi wave interference (by inserting a diffuser into the optical path) aperiodic patterns with defined spatial frequency spectrum can be originated. 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 splitting. 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 splitting are Lloyd´s mirrors, prisms and diffraction gratings.

Electron holographic lithography

The technique is readily extendible to electron waves as well, as demonstrated by the practice of electron holography. [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 , where is the Planck constant and 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.

Atom holographic lithography

The interference of atomic de Broglie 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 de Broglie relation. Generally the wavelength will be smaller than the diameter of the atom itself.

Uses

The benefit of using interference lithography is the quick generation of dense features over a wide area without loss of focus. Seamless diffraction gratings on areas of more than one square meter have been originated by interference lithography. [3] Hence, it is commonly used for the origination of master structures for subsequent micro or nano replication processes [4] (e.g. nanoimprint lithography) or for testing photoresist processes for lithography techniques based on new wavelengths (e.g., EUV 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 Patterning" (DLIP). [5] [6] [7] 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 used for different applications including tribology (wear and friction reduction), photovoltaics (increased photocurrent), [8] or biotechnology. Electron interference lithography [9] [10] 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 or uniformly distributed aperiodic patterns only. Hence, for drawing arbitrarily shaped patterns, other photolithography techniques are required. In addition, for electron interference lithography 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. [10] 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.

Related Research Articles

<span class="mw-page-title-main">Diffraction</span> Phenomenon of the motion of waves

Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1660.

<span class="mw-page-title-main">Wave interference</span> Phenomenon resulting from the superposition of two waves

In physics, interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference. The resultant wave may have greater intensity or lower amplitude if the two waves are in phase or out of phase, respectively. Interference effects can be observed with all types of waves, for example, light, radio, acoustic, surface water waves, gravity waves, or matter waves as well as in loudspeakers as electrical waves.

In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect selected areas of it during subsequent etching, deposition, or implantation operations. Typically, ultraviolet light is used to transfer a geometric design from an optical mask to a light-sensitive chemical (photoresist) coated on the substrate. The photoresist either breaks down or hardens where it is exposed to light. The patterned film is then created by removing the softer parts of the coating with appropriate solvents, also known in this case as developers.

<span class="mw-page-title-main">Diffraction grating</span> Optical component which splits light into several beams

In optics, a diffraction grating is an optical grating with a periodic structure that diffracts light into several beams traveling in different directions. The emerging coloration is a form of structural coloration. The directions or diffraction angles of these beams depend on the wave (light) incident angle to the diffraction grating, the spacing or distance between adjacent diffracting elements on the grating, and the wavelength of the incident light. The grating acts as a dispersive element. Because of this, diffraction gratings are commonly used in monochromators and spectrometers, but other applications are also possible such as optical encoders for high-precision motion control and wavefront measurement.

<span class="mw-page-title-main">Holography</span> Recording to reproduce a three-dimensional light field

Holography is a technique that enables a wavefront to be recorded and later reconstructed. It is best known as a method of generating real three-dimensional images, but also has a wide range of other applications. In principle, it is possible to make a hologram for any type of wave.

<span class="mw-page-title-main">Interferometry</span> Measurement method using interference of waves

Interferometry is a technique which uses the interference of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy, quantum mechanics, nuclear and particle physics, plasma physics, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms.

In physics, coherence expresses the potential for two waves to interfere. Two monochromatic beams from a single source always interfere. Physical sources are not strictly monochromatic: they may be partly coherent. Beams from different sources are mutually incoherent.

In physics and chemistry, Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a large crystal lattice. It encompasses the superposition of wave fronts scattered by lattice planes, leading to a strict relation between wavelength and scattering angle, or else to the wavevector transfer with respect to the crystal lattice. Such law had initially been formulated for X-rays upon crystals. However, it applies to all sorts of quantum beams, including neutron and electron waves at atomic distances if there are a large number of atoms, as well as visible light with artificial periodic microscale lattices.

The photorefractive effect is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index. The effect can be used to store temporary, erasable holograms and is useful for holographic data storage. It can also be used to create a phase-conjugate mirror or an optical spatial soliton.

Masklesslithography (MPL) is a photomask-less photolithography-like technology used to project or focal-spot write the image pattern onto a chemical resist-coated substrate by means of UV radiation or electron beam.

Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures on various materials.

<span class="mw-page-title-main">Nanoimprint lithography</span> Method of fabricating nanometer scale patterns using a special stamp

Nanoimprint lithography (NIL) is a method of fabricating nanometer-scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting. Adhesion between the resist and the template is controlled to allow proper release.

Contact lithography, also known as contact printing, is a form of photolithography whereby the image to be printed is obtained by illumination of a photomask in direct contact with a substrate coated with an imaging photoresist layer.

Electron holography is holography with electron matter waves. Dennis Gabor invented holography in 1948 when he tried to improve image resolution in electron microscope. The first attempts to perform holography with electron waves were made by Haine and Mulvey in 1952; they recorded holograms of zinc oxide crystals with 60 keV electrons, demonstrating reconstructions with approximately 1 nm resolution. In 1955, G. Möllenstedt and H. Düker invented an electron biprism, thus enabling the recording of electron holograms in off-axis scheme. There are many different possible configurations for electron holography, with more than 20 documented in 1992 by Cowley. Usually, high spatial and temporal coherence of the electron beam are required to perform holographic measurements.

Lloyd's mirror is an optics experiment that was first described in 1834 by Humphrey Lloyd in the Transactions of the Royal Irish Academy. Its original goal was to provide further evidence for the wave nature of light, beyond those provided by Thomas Young and Augustin-Jean Fresnel. In the experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come from a virtual source as a result. The reflected light interferes with the direct light from the source, forming interference fringes. It is the optical wave analogue to a sea interferometer.

A distributed-feedback laser (DFB) is a type of laser diode, quantum-cascade laser or optical-fiber laser where the active region of the device contains a periodically structured element or diffraction grating. The structure builds a one-dimensional interference grating, and the grating provides optical feedback for the laser. This longitudinal diffraction grating has periodic changes in refractive index that cause reflection back into the cavity. The periodic change can be either in the real part of the refractive index or in the imaginary part. The strongest grating operates in the first order, where the periodicity is one-half wave, and the light is reflected backwards. DFB lasers tend to be much more stable than Fabry–Perot or DBR lasers and are used frequently when clean single-mode operation is needed, especially in high-speed fiber-optic telecommunications. Semiconductor DFB lasers in the lowest loss window of optical fibers at about 1.55 μm wavelength, amplified by erbium-doped fiber amplifiers (EDFAs), dominate the long-distance communication market, while DFB lasers in the lowest dispersion window at 1.3 μm are used at shorter distances.

Microlithography is a general name for any manufacturing process that can create a minutely patterned thin film of protective materials over a substrate, such as a silicon wafer, in order to protect selected areas of it during subsequent etching, deposition, or implantation operations. The term is normally used for processes that can reliably produce features of microscopic size, such as 10 micrometres or less. The term nanolithography may be used to designate processes that can produce nanoscale features, such as less than 100 nanometres.

Plasmonic nanolithography is a nanolithographic process that utilizes surface plasmon excitations such as surface plasmon polaritons (SPPs) to fabricate nanoscale structures. SPPs, which are surface waves that propagate in between planar dielectric-metal layers in the optical regime, can bypass the diffraction limit on the optical resolution that acts as a bottleneck for conventional photolithography.

Optical holography is a technique which enables an optical wavefront to be recorded and later re-constructed. Holography is best known as a method of generating three-dimensional images but it also has a wide range of other applications.

In materials science, direct laser interference patterning (DLIP) is a laser-based technology that uses the physical principle of interference of high-intensity coherent laser beams to produce functional periodic microstructures. In order to obtain interference, the beam is divided by a beam splitter, special prisms, or other elements. The beams are then folded together to form an interference pattern. Sufficiently high power of the laser beam can thus result in the removal of material at the interference maximums thanks to ablation phenomenon, leaving the material intact at the minimums. In this way, a repeatable pattern can be permanently fixed on the surface of a given material. DLIP can be applied to almost any material and can change the properties of surfaces in many technological areas with regard to electrical and optical properties, tribology (friction and wear), light absorption and wettability (e.g., which can be related to hygienic properties).

References

  1. 1 2 Dunin-Borkowski, RE; Kasama, T; Wei, A; Tripp, SL; Hÿtch, MJ; Snoeck, E; Harrison, RJ; Putnis, A (2004). "Off-axis electron holography of magnetic nanowires and chains, rings, and planar arrays of magnetic nanoparticles". Microsc. Res. Tech. 64 (5–6): 390–402. CiteSeerX   10.1.1.506.6251 . doi:10.1002/jemt.20098. PMID   15549694. S2CID   432466.
  2. 1 2 Hasselbach, F. (1997). "Selected topics in charged particle interferometry". Scanning Microscopy. 11: 345–366.
  3. ↑ Wolf, Andreas J.; Hauser, Hubert; Kübler, Volker; Walk, Christian; Höhn, Oliver; Bläsi, Benedikt (2012-10-01). "Origination of nano- and microstructures on large areas by interference lithography". Microelectronic Engineering. Special issue MNE 2011 – Part II. 98: 293–296. doi:10.1016/j.mee.2012.05.018.
  4. ↑ Bläsi, B.; Tucher, N.; Höhn, O.; Kübler, V.; Kroyer, T.; Wellens, Ch.; Hauser, H. (2016-01-01). "Large area patterning using interference and nanoimprint lithography". In Thienpont, Hugo; Mohr, Jürgen; Zappe, Hans; Nakajima, Hirochika (eds.). Micro-Optics 2016. Vol. 9888. pp. 98880H–98880H–9. doi:10.1117/12.2228458. S2CID   32333348.
  5. ↑ Lasagni, A.; Holzapfel, C.; Mücklich, F. (2005). "Periodic Pattern Formation of Intermetallic Phases with Long Range Order by Laser Interference Metallurgy". Adv. Eng. Mater. 7 (6): 487–492. doi:10.1002/adem.200400206. S2CID   137980942.
  6. ↑ Lasagni, A.; Mücklich, F.; Nejati, M. R.; Clasen, R. (2006). "Periodical Surface Structuring of Metals by Laser Interference Metallurgy as a New Fabrication Method of Textured Solar Selective Absorbers". Adv. Eng. Mater. 8 (6): 580–584. doi:10.1002/adem.200500261. S2CID   135516098.
  7. ↑ Lasagni, A.; Holzapfel, C.; Weirich, T.; Mücklich, F. (2007). "Laser interference metallurgy: A new method for periodic surface microstructure design on multilayered metallic thin films". Appl. Surf. Sci. 253 (19): 8070–8074. Bibcode:2007ApSS..253.8070L. doi:10.1016/j.apsusc.2007.02.092.
  8. ↑ Ring, Sven; Neubert, Sebastian; Schultz, Christof; Schmidt, Sebastian S.; Ruske, Florian; Stannowski, Bernd; Fink, Frank; Schlatmann, Rutger (2015-01-01). "Light trapping for a-Si:H/µc-Si:H tandem solar cells using direct pulsed laser interference texturing". Physica Status Solidi RRL. 9 (1): 36–40. Bibcode:2015PSSRR...9...36R. doi:10.1002/pssr.201409404. ISSN   1862-6270. S2CID   93490614.
  9. ↑ Ogai, Keiko; Kimura, Yoshihide; Shimizu, Ryuichi; Fujita, Junichi; Matsui, Shinji (1995). "Nanofabrication of grating and dot patterns by electron holographic lithography". Appl. Phys. Lett. 66 (12): 1560–1562. Bibcode:1995ApPhL..66.1560O. doi:10.1063/1.113646.
  10. 1 2 Fujita, S.; Maruno, S.; Watanabe, H.; Kusumi, Y.; Ichikawa, M. (1995). "Periodical nanostructure fabrication using electron interference fringes produced by scanning interference electron microscope". Appl. Phys. Lett. 66 (20): 2754–2756. Bibcode:1995ApPhL..66.2754F. doi:10.1063/1.113698.
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