Some Notes on Top Down vs. Bottom Up.

These notes provide some background and thoughts on the terms "Top Down" and "Bottom Up," as these terms pertain to molecular manufacturing and are frequently used in nanomanufacturing contexts.

Background

These phrases were coined by the Foresight Institute in 1989 in order to help people better understand the differences between molecular manufacturing (to mass-produce large atomically precise objects) and conventional manufacturing (which can mass-produce large objects that are not atomically precise). At that time, the terms nanotechnology and molecular manufacturing were essentially synonymous, and defined as, "a technology based on assemblers able to build systems to complex atomic specification under programmable control." The term "nanotechnology" fissioned into its broader, current usage after the launch of the NNI. It is now used to describe virtually any technology related to nanomaterials or nanostructures.

The distinction, then, is that conventional manufacturing processes are "top down," in which material is produced in bulk (e.g., polymers in tanks and steel in furnaces and single crystal silicon boules in crystal growers) and then shaped into a finished part through a variety of processes (e.g., casting, molding, rolling, forging, extruding, machining, and etching fine features such as in electronic circuits). In these processes, the positioning of each atom is not individually controlled during the operation and the location of each atom in the finished part is not precisely specified or achieved, resulting in defects and impurities. Even a single crystal silicon boule solidified from the melt is not defect free. So, while one can fashion many nanoscale features into an object made from a bulk process (like an integrated circuit), it is not possible to make large atomically precise objects with conventional manufacturing because you're starting with a piece of material that is not atomically precise.

By contrast, bottom up (aka "atom by atom") manufacturing processes are those in which atoms are positionally controlled during the manufacturing operation. One inexact example is the way ribosomes build proteins by "grabbing" transfer RNA and attaching amino acids to a growing polypeptide chain (which then folds into a protein). This is programmably controlled by messenger RNA. Although some relatively large atomically precise proteins can be made in this way, in biological systems these proteins are ultimately assembled together in ways that are not atomically precise (which is why the example is inexact). Recently, because we can manipulate individual atoms with scanning probe tips, we have been able to think about bottom up manufacturing of very small structures by directed assembly (DARPA BAA07-59). Some have referred to this as "top down" because we control the STM tip from the desktop but it's really "bottom up" by the original definition. In proposed molecular manufacturing systems, large atomically precise objects would be made via massive parallelization of molecular robotic systems that snap together small atomically precise modules into larger ones (Burch Animation).

The earliest source related to molecular manufacturing systems: Foresight Briefing #2, prepared in 1989 by Eric Drexler and Christine Peterson.

This usage of the terminology survived at least through the origination of the National Nanotechnology Initiative and can be found in:

    National Nanotechnology Initiative: The Initiative and its Implementation Plan,
    NSTC/NSET Report, July 2000 (see page 20)

Some difficulties with these terms

The terms "top down" and "bottom up" were never intended to describe all manufacturing processes, and certainly not the range of today's nanomanufacturing processes. For example, these terms were not intended to describe the production of industrial chemicals, performed under conditions where the random motion of molecules results in occasional favorable interactions to react and produce a high yield of the desired chemical compound. Atomically precise structures (molecules) thus can result from processes where the trajectories of molecules are not positionally controlled. The subsequent use of those chemicals in making products (e.g., molding plastic into a toothbrush) is where the usage of the terms would be appropriate (that would be top down, for the toothbrush). Similarly, neither of these terms would be applied to the methods of synthesis for carbon nanotubes, but would be applied to the subsequent manufacture of products from nanotubes and other chemicals (for example, a nanotube-reinforced polymer--again, top down).

Thin films can self-organize from solution (Langmuir-Blodgett films) or can be made from molecules in the gaseous or plasma states (atomic layer deposition). Under the original definition these would still be top down (bulk) processes because the trajectories of the molecules are not individually controlled and the products are not atomically precise. In the case of self-assembled films there are often plenty of defect structures, which allows us to maintain the distinction between the terms based on whether the product is atomically precise or not. The situation is getting murkier, however, as clever researchers such as Diana Huffaker have been able to tease atoms to be deposited onto precisely the right location on strained surfaces to allow the layering of electronic materials with highly dissimilar lattice constants (distance between the atoms in each crystal). As researchers get better and better at this, it blurs the distinction between top down and bottom up.

My recommendation

Maintain the usage of these terms in their original context, to explain how large objects can only be made to atomic precision from a "Bottom Up" molecular manufacturing process, not "Top Down" subtractive processes. Do not use these terms when referring to the production of nanoparticles, fullerenes, chemicals, thin films, or other nanomaterials.