The track is a necessary tool for lithographic processing, providing all of the material processes (coating, baking, rinsing, and developing) except exposure. Typically, in a mass production environment, the track is linked to an exposure tool to increase the process efficiency and reduce delays. The linked track and scanner are sometimes called the litho-cell or the litho-cluster. The track is made up of several modules; each module does one specific process with its respective chemicals. Photoresists and solvents are connected from a chemical storage cabinet to the module via pipelines, filters, and pumps. Before processing wafers on the track, a process flow must be defined.
Depending on the process requirements, various modules can be assembled. For example, a track used for 193-nm lithography consists of at least one coating module for BARC, one coating module for resist, one coating module for topcoat, several bake modules for post-apply bakes and post-exposure bake, and more than one development module. If more process steps are needed, more modules can be added. If the process needs to use the solvent-soluble topcoat for immersion, a topcoat coating module and a topcoat removal module must be added into the track design.
This chapter covers the main process modules: coating, baking, and developing. Each process module consists of various process steps. For each process step, we review its purpose, its possible failure mechanisms, and some solutions to these failure mechanisms. Resist line collapse, blob defects, and surfactant rinse are associated with the development module, and are, therefore, also included in this chapter. Immersion lithography requires some additional process modules. These immersion-specific requirements, as well as track performance monitoring, make up the last part of this chapter.
2.1 Coating Module
A coating module consists of a spin table and several dispense nozzles for different materials. Solvents are introduced into the module for pre-wetting the wafer surface, removing the edge bead, and rinsing the back side of the wafer. Figure 2.1(a) shows a diagram of a coating module, also called a coating bowl or a coating cup, due to its shape.