Nanolithography

Part of a series of articles on Nanoelectronics Single-molecule electronics Molecular scale electronics Molecular logic gate Molecular wires Solid state nanoelectronics Nanocircuitry Nanowires Nanolithography NEMS Nanosensor Related approaches Nanoionics Nanophotonics Nanomechanics See also Nanotechnology v · d · e

Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm. Nanolithography is used during the fabrication of leading-edge semiconductor integrated circuits (nanocircuitry) or nanoelectromechanical systems (NEMS).

As of 2007, nanolithography is a very active area of research in academia and in industry.

Optical lithography

Main article: Photolithography

Optical lithography, which has been the predominant patterning technique since the advent of the semiconductor age, is capable of producing sub-100-nm patterns with the use of very short wavelengths (currently 193 nm). Optical lithography will require the use of liquid immersion and a host of resolution enhancement technologies (phase-shift masks (PSM), optical proximity correction (OPC)) at the 32 nm node. Most experts feel that traditional optical lithography techniques will not be cost effective below 22 nm. At that point, it may be replaced by a next-generation lithography (NGL) technique.

Other nanolithography techniques

• X-ray lithography can be extended to an optical resolution of 15 nm by using the short wavelengths of 1 nm for the illumination. This is implemented by the proximity printing approach. The technique is developed to the extent of batch processing. The extension of the method relies on Near Field X-rays in Fresnel diffraction: a clear mask feature is "demagnified" by proximity to a wafer that is set near to a "Critical Condition". This Condition determines the mask-to-wafer Gap and depends on both the size of the clear mask feature and on the wavelength. The method is simple because it requires no lenses.
• A method of pitch resolution enhancement which is gaining acceptance is double patterning. This technique increases feature density by printing new features in between pre-printed features on the same layer. It is flexible because it can be adapted for any exposure or patterning technique. The feature size is reduced by non-lithographic techniques such as etching or sidewall spacers.
• Work is in progress on an optical maskless lithography tool. This uses a digital micro-mirror array to directly manipulate reflected light without the need for an intervening mask. Throughput is inherently low, but the elimination of mask-related production costs - which are rising exponentially with every technology generation - means that such a system might be more cost effective in the case of small production runs of state of the art circuits, such as in a research lab, where tool throughput is not a concern.
• The most common nanolithographic technique is Electron-Beam Direct-Write Lithography (EBDW), the use of a beam of electrons to produce a pattern — typically in a polymeric resist such as PMMA.
• Extreme ultraviolet lithography (EUV) is a form of optical lithography using ultrashort wavelengths (13.5 nm). It is the most popularly considered NGL technique.
• Charged-particle lithography, such as ion- or electron-projection lithographies (PREVAIL, SCALPEL, LEEPL), are also capable of very-high-resolution patterning. Ion beam lithography uses a focused or broad beam of energetic lightweight ions (like He⁺) for transferring pattern to a surface. Using Ion Beam Proximity Lithography (IBL) nano-scale features can be transferred on non-planar surfaces.^[1]
• Neutral Particle Lithography(NPL) uses a broad beam of energetic neutral particle for pattern transfer on a surface.^[2]
• Nanoimprint lithography (NIL), and its variants, such as Step-and-Flash Imprint Lithography, LISA and LADI are promising nanopattern replication technologies. This technique can be combined with contact printingand cold welding.
• Scanning probe lithography (SPL) is a promising tool for patterning at the deep nanometer-scale. For example, individual atoms may be manipulated using the tip of a scanning tunneling microscope (STM). Dip-Pen Nanolithography (DPN) is the first commercially available SPL technology based on atomic force microscopy.
• Atomic Force Microscopic Nanolithography (AFM) is a chemomechanical surface patterning technique that uses an atomic force microscope.^[3]
• Magnetolithography (ML) based on applying a magnetic field on the substrate using paramagnetic metal masks call "magnetic mask". Magnetic mask which is analog to photomask define the spatial distribution and shape of the applied magnetic field. The second component is ferromagnetic nanoparticles (analog to the photoresist) that are assembled onto the substrate according to the field induced by the magnetic mask.

Bottom-up methods

• Nanosphere lithography uses self-assembled monolayers of spheres (typically made of polystyrene) as evaporation masks. This method has been used to fabricate arrays of gold nanodots with precisely controlled spacings.^[4]

It is possible that molecular self-assembly methods will take over as the primary nanolithography approach, due to ever-increasing complexity of the top-down approaches listed above. Self-assembly of dense lines less than 20 nm wide in large pre-pattearned trenches has been demonstrated.^[5] The degree of dimension and orientation control as well as prevention of lamella merging still need to be addressed for this to be an effective patterning technique. The important issue of line edge roughness is also highlighted by this technique.

Self-assembled ripple patterns and dot arrays formed by low-energy ion-beam sputtering are another emerging form of bottom-up lithography. Aligned arrays of plasmonic ^[6] and magnetic wires and nanoparticles are deposited on these templates via oblique evaporation. The templates are easily produced over large areas with periods down to 25 nm.

See also

• Nanoimprint lithography
• Contact printing
• Nanopatterning
• Photolithography
• Soft lithography
• Liquid imaging
• LIGA
• PENs
• Magnetolithography
• Nanochannel glass materials

References

1. ^ Dhara Parikh, Barry Craver, Hatem N. Nounu, Fu-On Fong, and John C. Wolfe, "Nanoscale Pattern Definition on Nonplanar Surfaces Using Ion Beam Proximity Lithography and Conformal Plasma-Deposited Resist", Journal of Microelectromechanical Systems, VOL. 17, NO. 3, JUNE 2008
2. ^ J C Wolfe and B P Craver, "Neutral particle lithography: a simple solution to charge-related artefacts in ion beam proximity printing", J. Phys. D: Appl. Phys. 41 (2008) 024007 (12pp)
3. ^ R. C. Davis et al. (2003). "Chemomechanical surface patterning and functionalization of silicon surfaces using an atomic force microscope". Appl. Phys. Lett. 82 (5): 808–810. doi:10.1063/1.1535267.  Related article
4. ^ A. Hatzor-de Picciotto, A. D. Wissner-Gross, G. Lavallee, P. S. Weiss (2007). "Arrays of Cu(2+)-complexed organic clusters grown on gold nano dots" (^{[dead link]}). Journal of Experimental Nanoscience 2: 3–11. doi:10.1080/17458080600925807. http://www.alexwg.org/JExpNanosci2007.pdf. 
5. ^ Sundrani D, Darling SB, Sibener SJ (June 2004). "Hierarchical assembly and compliance of aligned nanoscale polymer cylinders in confinement". Langmuir 20 (12): 5091–9. doi:10.1021/la036123p. PMID 15984272. http://sibener-group.uchicago.edu/pubs/106.pdf. 
6. ^ T.W.H. Oates, A. Keller, S. Facsko, A. Muecklich (2007). "Aligned silver nanoparticles on rippled silicon templates exhibiting anisotropic plasmon absorption". Plasmonics 2 (2): 47–50. doi:10.1007/s11468-007-9025-z. http://www.springerlink.com/content/n17240g2lw7240x4/. 

External links

• George J. Kostas Nanoscale Technology and Manufacturing Research Center
• Commercialization of Nano Imprint Lithography (NIL)
• Nanolithography at Georgia Tech
• UCLA Nanoelectronics Research Facility
• Louisiana State University, Nanografted Rings
• Univ. of Cambridge and Imperial College of London, LithoEdit DNA
• Short Course on Nanostructure Fabrication at MIT