Nanolithography

Nanolithography — or lithography at the nanometer scale — refers to the fabrication of 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 or nanoelectromechanical systems (NEMS).

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

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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 photomask 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 30 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.

• 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 printing.

• 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.

• The furthest developed NGL remains X-ray lithography which is extensible to 15 nm resolution by use of "demagnification" in the Near Field.

• Chemomechanical Surface Patterning using an atomic force microscope is another type of nanolithography. ^[1]

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.^[2]

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-patterned trenches has been demonstrated (see e.g., D. Sundrani et al., Langmuir, vol. 20, 5091-5099 (2004)). 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.

See also

• Nanoimprint lithography
• Contact printing
• Nanopatterning
• Photolithography
• Soft lithography
• Liquid imaging
• LIGA

References

1. ^ R. C. Davis et al., Chemomechanical Surface Patterning and Functionalization of Silicon Surfaces Using an Atomic Force Microscope, Appl. Phys. Lett. 82 (5): 808-810 (2003). Related article
2. ^ 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". Journal of Experimental Nanoscience 2: 3-11.