Top-Down vs. Bottom-Up

 

There are two major subclasses of nanotechniques in solid phase nanoengineering: “top down” and “bottom up”. The former is what has been practiced with great success by the electronics industry to manufacture integrated circuits; the latter however is thought by many nano-practitioners to be the logical process for future self-assembling nano products.

The top-down approach involves molding or etching materials into smaller components. This approach has traditionally been used in making parts for computers and electronics. According to Scientific American, (Sept 2001), p. 40, the US then produced 3 × 109 transistors per second using top down manufacturing. This is an astounding feat of manufacturing prowess, especially in face of the need to actively engineer each step. Thus starting with a thin sheet of a single crystal silicon wafer, the surface is cleaned, coated, preferentially etched using highly focused optics in as many as 100 separate operations before the final integrated circuit is complete.

“What could we do with layered structures with just the right layers? What would the properties of materials be if we could really arrange the atoms the way we want them? They would be very interesting to investigate theoretically. I can't see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do.”(Feynman, "There's Plenty of Room at the Bottom",)

This module has as its purpose a review of methods of self-assembly from the gaseous phase. The sketch shows the essence of bottom up manufacturing. While the technology of vapor-phase deposition (another name for what we are doing) has been around for many years, the level of extrapolation needed to get true self-assembly is far off. And, in meaningful ways, self-assembly from condensed phases (such as in biological systems) has outstripped vapor-phase self-assembly; however, there are two principle vapor-phase technologies that are useful and widely practiced: molecular beam Epitaxy (MBE) and vapor-deposition. The latter has two subdisciplines, physical vapor-deposition (PVD) and chemical vapor deposition (CVD). The difference between the last two is that in PVD the actual active species are directly evaporated or injected into the gas phase; in CVD, a precursor is used that, on transporting into the vapor space, is chemically decomposed into the required species. We will use the term “CVD” to represent both of these forms and simply bear in mind that in some cases, the reactive gaseous species is simply added to the reactor and in the other it is created by chemical decomposition of an injected species.