The patterns of integrated circuits are created on wafers by lithography. The steps of this critical manufacturing process are listed in Table 1.1. Each step will be discussed at length in later chapters of this book, but a brief description of each will be given here. Most of this book is devoted to photolithography, where optical methods are used to transfer the circuit patterns from master images-called masks or reticles-to the wafers. Photolithography is the method used for patterning nearly all integrated circuits fabricated today.
Adhesion promotion: The lithography process creates the patterns of integrated circuits in films of specialized materials called resists, which are coated onto the wafers on which the circuits are made. Resists typically do not adhere properly to untreated surfaces of silicon or silicon-containing materials, such as silicon dioxide and silicon nitride. To ensure proper adhesion, the wafer surfaces are treated prior to resist coating.
Resist coat: Resists are typically comprised of organic polymers applied from a solution. To coat the wafers with resist, a small volume of the liquid resist is first dispensed onto a wafer. The wafer is then spun about its axis at a high rate of spin, flinging off the excess resist and leaving behind, as the solvent evaporates, a thin (0.1-2 Î¼m, typically) film of solid resist.
Softbake: After the resist coating has been applied, the density is often insufficient to support later processing. A bake is used to densify the resist film and drive off residual solvent.
Alignment: Integrated circuits are fabricated by a series of patterning steps. These start with a lithography operation followed by an etch or ion implantation. Between patterning steps, there may be film depositions, planarizations, and other processes. Each new pattern must be placed on top of preceding layers, and proper overlay of the new layer to the circuit pattern already on the wafer is achieved during the alignment step by using specialized equipment.