(thin-film deposition and growth) and subtractive (etch) processes. This has evolved into an enormously sophisticated enterprise, which is proven for the low-cost, high-yield manufacture of extremely complex (~100 million transistors) and reliable circuits. However, limitations are on the horizon. One is the difficulty of extending today’s optically based lithographic techniques to the nanoscale, which is much smaller than ultraviolet optical wavelengths. Another is the limited number of materials used in ICs. The nanotechnology community is investigating many disparate technologies based on many different materials, but it is far from evident that the different processing requirements of these technologies can be reconciled.
Self-assembly is a radically different approach to fabrication at the nanoscale. It takes advantage of molecular and intermolecular forces to define atomic, nanoscale, and macroscale structures. Self-assembly depends on appropriate direction and control being exerted at all stages of the process by preprogramming of the subunits or building blocks such that the required recognition elements for self-assembly are contained in the subunits. Crystal growth is an example of self-assembly with exquisite long-range order. Living species are proof that complex three-dimensional structures with interacting functionality are possible. Integration of the top-down (lithography and pattern transfer) and the bottom-up (self-assembly) approaches offers an attractive approach to bridging the current gaps between these paradigms.
The incompatible materials issue may be addressed by individualized optimization of different devices and subsystems, followed by an assembly process akin to the automotive assembly line but at a vastly smaller scale. Here again, top-down (pick-and-place) and bottom-up, self-assembly inspired (DNA-assisted) approaches are among the many being investigated.