Nanolithography

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 1 and 100 nm. Different approaches can be categorized in serial or parallel, mask or maskless/direct-write, top-down or bottom-up, beam or tip-based, resist-based or resist-less methods. As of 2015, nanolithography is a very active area of research in academia and in industry. Applications of nanolithography include among others: Multigate devices such as Field effect transistors (FET), Quantum dots, Nanowires, Gratings, Zone plates and Photomasks, nanoelectromechanical systems (NEMS), or semiconductor integrated circuits (nanocircuitry).

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 optical wavelengths. Several optical lithography techniques require the use of liquid immersion and a host of resolution enhancement technologies like phase-shift masks (PSM) and optical proximity correction (OPC). Multiple patterning is a method of increasing the resolution by printing features in between pre-printed features on the same layer by etching or creating sidewall spacers, and has been used in commercial production of microprocessors since the 32 nm process node e.g. by directed self-assembly (DSA). Extreme ultraviolet lithography (EUVL) uses ultrashort wavelengths (13.5 nm) and as of 2015, is the most popularly considered Next-generation lithography (NGL) technique for mass-fabrication.

Electron-beam lithography

Main article: Electron beam lithography

Electron beam lithography or Electron-Beam Direct-Write Lithography (EBDW) scans a focused beam of electrons on a surface covered with an electron-sensitive film or resist (e.g. PMMA or HSQ) to draw custom shapes. By changing the solubility of the resist and subsequent selective removal of material by immersion in a solvent, sub-10 nm resolutions have been achieved. This form of direct-write, maskless lithography has high resolution and low throughput, limiting single-column e-beams to photomask fabrication, low-volume production of semiconductor devices, and research&development. Multiple-electron beam approaches have as a goal an increase of throughput for semiconductor mass-production.

Nanoimprint lithography

Main article: Nanoimprint lithography

Nanoimprint lithography (NIL), and its variants, such as Step-and-Flash Imprint Lithography, LISA and LADI are promising nanopattern replication technologies where patterns are created by mechanical deformation of imprint resist, typically a monomer or polymer formulation that is cured by heat or UV light during imprinting. This technique can be combined with contact printing and cold welding.

Multiphoton lithography

Main article: Multiphoton lithography

Multiphoton lithography (also known as direct laser lithography or direct laser writing) patterns surfaces without the use of a photomask, whereby two-photon absorption is utilized to induce a change in the solubility of the resist.

Scanning probe lithography

Main article: Scanning probe lithography

Scanning probe lithography (SPL) is a tool for patterning at the nanometer-scale down to individual atoms using scanning probes. Dip-pen nanolithography is an additive, diffusive method, thermochemical nanolithography triggers chemical reactions, thermal scanning probe lithography creates 3D surfaces from polymers, and local oxidation nanolithography employs a local oxidation reaction for patterning purposes.

Other techniques

• Molecular self-assembly (as bottom-up approach) of dense lines less than 20 nm wide in large pre-patterned trenches has been demonstrated. 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 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.
• Stencil lithography is a resist-less and parallel method of fabricating nanometer scale patterns using nanometer-size apertures as shadow-masks.
• X-ray lithography can be extended to a resolution of 15 nm by using the X-ray wavelengths of 1 nm as 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.
• Laser printing of single nanoparticles In this method, the optical forces induced via scattering and absorption of photons on nanoparticles are used to direct single nanoparticles to specific locations on substrates and attach them via van-der Waals forces. This technique has been demonstrated on metallic nanoparticles, which are easier to print due to their large plasmonically-induced scattering and absorption cross sections, in both serial and parallel printing methods.
• 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.
• 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.
• Proton beam writing uses a focused beam of high energy (MeV) protons to pattern resist material at nanodimensions.
• Charged-particle lithography, such as ion- or electron-projection lithographies (PREVAIL, SCALPEL, LEEPL), are also capable of 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.
• Neutral particle lithography (NPL) uses a broad beam of energetic neutral particle for pattern transfer on a surface.

External links

Nanotechnology at DMOZ