• Photolithography:

Photolithographic process consists of producing a mask carrying the requisite pattern information and subsequently transferring that pattern using some optical technique into a photoactive polymer or photoresist (or simply resist). Figure 40.2 depicts the basic steps of the photolithographic process: (a) At first, the resist material is applied as a thin coating over some base. (b) They are exposed in an image-wise fashion through a mask, such that light strikes selected areas of the resist material, (c) The exposed resist is then subjected to a development step. The exposed areas may be rendered more soluble in some developing solvent than the unexposed areas, thereby producing a positive tone image of the mask. (d) Conversely, the exposed areas may be rendered less soluble, producing a negative tone image of the mask. (e) The effect of this process is to produce a three-dimensional relief image in the resist material that is a replication of the opaque and transparent areas of the mask. (f) The areas of resist that remain following the imaging and developing processes are used to mask the underlying substrate for subsequent etching or other image transfer steps. (g) The resist material resists the etchant and prevents it from attacking the underlying substrate in those areas where it remains in place after development. (h) Following the etching process, the resist is removed by stripping to produce a positive or negative tone relief image in the underlying substrate. In principle, the diffraction sets the limit of the maximum resolution or the minimum size of the individual elements by photolithography.

• Electron beam lithography (EBL):

A fine focused beam of electrons with a size less than few nanometers can be deflected accurately and precisely over a surface either by electromagnetically or by electrostatically. When the surface is coated with a radiation sensitive polymeric material, the electron beam can be used to write patterns of very high resolution [8]. As we know well that electrons possess both particle and wave properties and their wavelength is on the order of a few tenths of angstrom, their resolution may not be limited by diffraction conditions. But, the resolution of EBL is limited by forward scattering of the electrons in the resist layer and back scattering from the underlying substrate. Nevertheless, EBL is the most powerful tool for the fabrication of materials as small as 3-5 nm [9].