As the enabling factor/component for each lithographic printing technique, resists have oftentimes determined the successful implementation of each lithography. At times, new resists and related processes developed in anticipation of yet to be developed lithographic tools and methods. Other times, the invention of new lithographic tools and methods have preceded the invention of the corresponding resists. In Chapters 6 and 7, we trace the evolution of resist materials and processes throughout the entire history of lithography. In Chapter 8, we discuss general aspects of radiation chemistry of resist polymers. We pay particular attention to the factors that necessitated the changes and sometimes transitions in the evolution of lithographic resist materials and processes. The treatment on each representative class of resists is rendered here in a manner that employs a brief narrative of the historical background covering the invention of the resist to contextualize its usage, while also presenting detailed chemistry of the resist materials covering their synthesis, formulation, radiation chemistry, and image formation mechanisms, as well as the processing and applications of such materials in printing and fabrication. Because the resins of modern resists are almost exclusively composed of polymers, a generalized treatment of the fundamentals of radiation chemistry of resist polymers is provided in Chapter 8 as a way of highlighting the unique properties of these materials that make them ideal candidates for resist applications in semiconductor fabrication and general printing.
The evolution of resist materials and resist processes over time has closely paralleled the evolution of lithography. Generally, the evolution of resists can be divided into two main eras, namely, the dichromate era and the modern era (see Fig. 6.1). There have been many important discoveries in resist materials and processes, some of the most significant of which are covered in this chapter and the next.