Lithographic resists as amazing compact imaging systems – A review

Considering the goal of lithography under its most general aspect – that is, transferring and recording mask or template information in the form of contrast between the imaged and non-imaged areas of a resist film coated on a flat surface – three lithographic resist imaging mechanisms can be recognized. Depending on the nature of the resist film, this contrast may be based on intrinsic or photo- or radiation-induced differential solubility between the imaged and non-imaged part of the resist film in fine art lithography, photolithography, and radiation lithography, respectively, or pressure driven flow and confinement of resist in imprint lithography, or thermodynamically driven phase separation of resist constituents in directed self-assembly lithography. This contrast forms the basis of the printed image and ultimately derives from the forces that underlie the old chemist's rule: "Oil and water do not mix." Crucially, to create this contrast, the resist film must transform a two-dimensional image of the mask or template into a three-dimensional relief image on the substrate in a process that is highly non-linear. By creating the contrast in this manner, the resist film serves as a compact imaging system that senses, records, stores, and displays the mask image. Additionally, the resist film must maintain its structural and mechanical integrity to "resist" and withstand the harshness of other post-imaging processes such as etching, ion implantation, electroplating, etc. Following all necessary post-imaging processes, the resist film must be stripped or be left and incorporated into the final device. A versatile material that serves a multiplicity of functions and is operational in many dimensions is not only amazing but also forms the irreducible essence of lithography. By drawing on fundamental, theoretical, and experimental studies of molecular processes involved in lithographic resist imaging, this review paper explains how the resist film performs the above essential functions.