To use all functions of this page, please activate cookies in your browser.
my.chemeurope.com
With an accout for my.chemeurope.com you can always see everything at a glance – and you can configure your own website and individual newsletter.
 My watch list
 My saved searches
 My saved topics
 My newsletter
Interference lithographyInterference lithography (or holographic lithography) is a technique for patterning regular arrays of fine features, without the use of complex optical systems or photomasks. Additional recommended knowledge
Basic principleThe 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 postexposure photolithographic processing, a photoresist pattern corresponding to the periodic intensity pattern emerges. For 2beam interference, the fringetofringe 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 3beam 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 requirementsFor 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 splitterCoherent 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 mirrors, prisms and diffraction gratings. Electron holographic lithographyThe 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 deBroglie 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 Xraylike 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 lowenergy electrons (<<100 eV) with this technique is their natural tendency to repel one another due to Coulomb forces as well as FermiDirac statistics, though electron antibunching has been verified only in a single case. Atom holographic lithographyThe 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 lithographyThe 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., EUV or 193 nm immersion). Electron interference lithography[3,4] 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, nonoptical 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[4]. This indicates that the lithographic patterning of 20 nm halfpitch features or smaller will be significantly affected by factors other than the interference pattern, such as the cleanliness of the vacuum. References


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Interference_lithography". A list of authors is available in Wikipedia. 