Purposes of three-dimensional circuits

Three-dimensional (3-D) circuitry is a recognized concept. With the advent of silicon-on-insulator (SOI) technologies, this idea is being realized using techniques such as laser recrystallization of polysilicon, which allows fabrication of active devices stacked in two or more layers. Since silicon is a well-known microelectronic material, progress has been very fast and some laboratories have already succeeded in producing three-dimensional circuit cells [1]-[3]. In order to focus activity and rationally optimize products, technologists need to know the types of circuits required. This paper examines the problem from several points of view and tries to answer three fundamental questions: (1) What are the main advantages of 3-D circuitry? (2) What kind of circuits will yield best advantages? (3) Which technological problems have priority?