Nanolithography is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures. From Greek, the word can be broken up into three parts: "nano" meaning dwarf, "lith" meaning stone, and "graphy" meaning to write, or "tiny writing onto stone." Today, the word has evolved to cover the design of structures in the range of 10−9 to 10−6 meters, or structures in the nanometer range. Essentially, field is a derivative of lithography, only covering significantly smaller structures. All nanolithographic techniques can be separated into two categories: those that etch away molecules leaving behind the desired structure, and those that directly write the desired structure to a surface. The field of nanolithography was born out of the need to increase the number of transistors in an integrated circuit in order to maintain Moore's Law. While lithographic techniques have been around since the late 18th century, none were applied to nanoscale structures until the mid-1950s. With evolution of the semiconductor industry, demand for techniques capable of producing micro- and nano-scale structures skyrocketed. Photolithography was applied to these structures for the first time in 1958 beginning the age of nanolithography. Since then, photolithography has become the most commercially successful technique, capable of producing sub-100 nm patterns. There are several techniques associated with the field, each designed to serve its many uses in the medical and semiconductor industries. Breakthroughs in this field contribute significantly to the advancement of nanotechnology, and are increasingly important today as demand for smaller and smaller computer chips increases. Further areas of research deal with physical limitations of the field, energy harvesting, and photonics.
Important techniques
Optical lithography
Optical Lithography is one of the most important and prevalent sets of techniques in the nanolithography field. Optical lithography contains several important derivative techniques, all that use very short light wavelengths in order to change the solubility of certain molecules, causing them to wash away in solution, leaving behind a desired structure. Several optical lithography techniques require the use of liquid immersion and a host of resolution enhancement technologies like phase-shift masks and optical proximity correction. Some of the included techniques in this set include multiphoton lithography, X-Ray lithography, light coupling nanolithography, and extreme ultraviolet lithography. This last technique is considered to be the most important next generation lithography technique due to its ability to produce structures accurately down below 30 nanometers.
Electron-beam lithography
Electron beam lithography or electron-beam direct-write lithography scans a focused beam of electrons on a surface covered with an electron-sensitive film or resist 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. EBL can be utilized for selective protein nanopatterning on a solid substrate, aimed for ultrasensitive sensing.
, and its variants, such as Step-and-Flash Imprint Lithography and laser assisted directed imprint are promising nanopattern replication technologies where patterns are created by mechanical deformation of imprint resists, typically monomer or polymer formations that are cured by heat or UV light during imprinting. This technique can be combined with contact printing and cold welding. Nanoimprint lithography is capable of producing patterns at sub-10 nm levels.
Miscellaneous techniques
Charged-particle lithography
This set of techniques include ion- and electron-projection lithographies. Ion beam lithography uses a focused or broad beam of energetic lightweight ions for transferring pattern to a surface. Using Ion Beam Proximity Lithography nano-scale features can be transferred on non-planar surfaces.
Magnetolithography
Magnetolithography is 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 that are assembled onto the substrate according to the field induced by the magnetic mask.
Nanosphere lithography
Nanosphere lithography uses self-assembled monolayers of spheres as evaporation masks. This method has been used to fabricate arrays of gold nanodots with precisely controlled spacings.
Plasmonic lithography uses surface plasmon excitations to generate beyond-diffraction limit patterns, benefiting from subwavelength field confinement properties of surface plasmon polaritons.
Proton beam writing
This technique uses a focused beam of high energy protons to pattern resist material at nanodimensions and has been shown to be capable of producing high-resolution patterning well below the 100 nm mark.
Stencil lithography
Stencil lithography is a resist-less and parallel method of fabricating nanometer scale patterns using nanometer-size apertures as shadow-masks.
Quantum optical lithography, is a diffraction-unlimited method able to write at 1 nm resolution by optical means, using a red laser diode.Complex patterns like geometrical figures and letters were obtained at 3 nm resolution on resist substrate. The method was applied to nanopattern graphene at 20 nm resolution.
