23. Top-Down Nanofabrication

Starting from a bulk material, the top-down fabrication process progresses to machine, modify, and shape it into the desired shape and size. In integrated circuit manufacturing, one takes a silicon wafer and carves patterns of specified dimensions by a series of lithographic steps through aligned masking levels, performs operations such as wet and dry chemical etching, ion implantation, diffusion, oxidation, metallisation and many others until the desired device/circuit has been obtained. The key to top-down nanofabrication has been the art of lithography which has been relentlessly improved to create patterns of smaller geometries with higher resolution. The illumination/irradiation source in lithography has been changed from an intense beam of deep UV photons to extreme UV photons, and focussed electrons. Due to its extremely short wavelength, the electron beam offers a very high diffraction-limited resolution but is a comparatively slow process. Another approach followed is to make patterns by mechanical pressure, e.g., by stamping and printing using designed templates. In block copolymer lithography, the directed self-assembly of block copolymers is synergistically integrated with common lithographic techniques for practical utilization by semiconductor industry. Scanning probe lithography can manipulate individual molecules but is a low throughput technique. The vast gamut of nanolithographic tools available to a semiconductor process engineer can be leveraged for fabrication of nanostructures of wide-ranging complexities.