How Do We Make Small Things? - Introduction to BioMEMS - page 38

Scanning
electron beam

P = 10 -6 Torr

Scan

Coat with resist Dissolve/Etch

Scanning
tip

FIGURE 1.36 Nanoscale.lithography.

growth, charge deposition, etc.—by temporarily disabling the proximity signal feedback. If the
tip has been used to modify a resist layer, the resist must be developed with a developer solution
to transfer the pattern to the substrate.

1.9 Fabrication Based on Self-Assembly: A “Bottom-Up” Approach
In his oten-cited visionary lecture “ here is plenty of room at the bottom ” in 1959 at the
American Physical Society meeting at Caltech, the physicist Richard Feynman explained very
graphically the vast amount of information storage density available on surfaces down to nano-
meter scales. In it, he explained how one could envision manipulating atoms if it were possible to
build nanoscale machines. his talk historically marked the birth of the ield of nanotechnology.
he present view, however, is radically diferent. Although it is true that there exist nanoscale
machines that manipulate molecules atom by atom, it is also true that they existed well before
humans: Nature has been inventing them for millions of years in the shape of proteins and RNA
enzymes. Nevertheless, Feynman's talk inspired a whole generation of scientists. Yet (undoubt-
edly for lack of time) the talk did not address a crucial aspect of nanoscale machines and sur-
faces: at short distances, surfaces exert enormous attractive forces that will keep most molecular
entities stuck to them like glue.
Did someone say glue? Maybe that can be put to some good use. Perhaps at very small
scales, we can make use of those natural attractive forces to cause objects to stick to each other
spontaneously in ordered arrays. Actually, this “bottom-up” approach already has a technical
name: it has been termed self-assembly . Self-assembly is actually based on an old observation:
that if you take thousands of spheres, such as balls or oranges, and put them in a container,
they always pack to minimize the spacing let between them—because that minimizes their
gravitational energy. his can be generalized to beads on a sticky surface or in thin liquid
ilms, as was known since the 1980s: place a drop of water containing latex or silica beads on a
surface, let it dry, and the surface tension as the water recedes creates a monolayer of beads in
a near-perfect hexagonal array arrangement—which minimizes the capillary forces between
the spheres.
All systems have a tendency to settle into a minimal energy state. It is straightforward to
realize that, as long as the beads form a monolayer (i.e., as long as they do not stack up), the