"More Than a Feeling," Feature Article, April 2006

FEATURE FOCUS: Microsystems

more than a feeling

The atomic force microscope is enabling engineers to understand mechanical systems at the most basic level.

by F. Michael Serry

THE 19th-century German chemist Friedrich August Kekulé had spent years studying carbon compounds, trying to deduce their molecular shape. Then one night, he later claimed, the shape of the molecule benzene came to him in a dream of atoms engaged in a giddy dance. The six carbons in benzene, Kekulé declared, were arranged in a ring.

Ever since scientists first adopted the atomic model of matter, many have been dreaming of finding a way to see—in all three dimensions—the atoms and molecules that make up the stuff around us. Optical microscopes are incapable of resolving details at the nanoscale and scanning electron microscope images are two-dimensional. X-ray crystallography can gather some information on molecular structures, but only indirectly.

For nearly 20 years, however, researchers have been able to make the dreams of previous generations come true through the use of atomic force microscopes. The AFM and its predecessor, the scanning tunneling microscope, or STM, started out as powerful imaging tools that revealed the three-dimensional structure of surfaces with unprecedented lateral and vertical resolution, down to the atomic level. But the AFM has evolved swiftly since its invention in 1987; it is now a highly modular instrument with numerous new techniques for probing the properties of materials on the nanometer scale and for modifying matter on that scale.

Combined with secondary modes such as phase imaging AFM, and with environmental options such as imaging with the sample fully submerged in a liquid, AFM and STM pushed research and material development into new directions and extended the applications in areas as diverse as contact lenses and magnetic data storage.

The heart of the AFM is a probe comprising a microfabricated cantilever with an extraordinarily sharp tip. The tip's radius of curvature at the apex may be as small as 2 nanometers, and this is what makes contact with matter. The spring constant of the cantilever may be smaller than the force constant holding atoms together in most solids, so that the cantilever flexes as it crosses the surface of a sample.

The AFM tip can be thought of as a nanometer-scale finger that we have at our disposal to interface with matter on the scale of individual molecules, and even atoms. This interface is mechanical, which brings us very close to having the kind of tactile interaction that we are used to, and on which we rely heavily to understand the world around us, but which, absent an AFM, is largely inaccessible to us on the scale of individual molecules.

INTUITIVE INTERFACE

The traditional use of an AFM as an imaging tool is still providing many insights. For instance, the makers of a hydrogel contact lens had developed a process to make a lens with two regions that have different affinities for water to study the effect on biocompatibility. Although the manufacturer's process had established a difference between the two regions, several analytical techniques failed to confirm it. Only by using an AFM on a sample immersed in liquid could researchers demonstrate the difference between the two regions on this experimental contact lens material.

But as engineers and scientists become more immersed in studying nanoscale material and creating complex microsystems and nanosystems, the newer uses of more advanced AFM will become increasingly important. The atomic force microscope is our most direct, intuitive interface with the nanometer-scale physical world. It is the only instrument that allows us to "touch" the surface of a sample with nanometer-scale resolution and atomic-level force sensitivity.

This is the reason the AFM has become much more than a high-resolution imaging tool. Two related areas where the AFM is now indispensable are nanomanipulation and nanotribology.

In nanomanipulation, the structure and arrangement of nanometer-scale features on a surface can be rearranged in the plane of the surface. The level of sophistication in hardware and software, and the interaction between the two that is required to make this type of work possible, repeatable, and relatively easy, has taken well over a decade to attain.

A sample of an experimental magnetic recording as imaged in TappingMode AFM topography (top) and LiftMode Magnetic Force Microscopy (below). LiftMode uses mechanical resonance to detect long-range interactions between the AFM tip and sample, and is widely used in disk drive R&D.

Nanomanipulation out of the plane of the sample uses the same AFM probe, but it involves controlling and monitoring the probe's motion perpendicular to the plane of the sample surface. This often involves pulling or pushing on the sample with the probe tip positioned accurately and precisely at a given location—for example, the location of an individual protein molecule. When a chemical or physical bond breaks, the molecule snaps and so does the AFM tip attached to it. This event is recorded with very high time resolution in what is usually called "force spectroscopy" or "force-distance" plot. These plots make it possible to learn the details—with molecular-scale resolution—of how matter is held together, and how the bonds that hold it together can break, and how different environments (air, liquid, pH variation, etc.) alter the picture.

Where is this used? Take the case of improving the mechanical performance in materials under stress, for example. From parts used in cars and airplanes, to biomimetic implants, materials with high fracture toughness are in demand, as are materials that can repair themselves mechanically.

Like most other mechanical properties, fracture toughness used to be studied, tested, and modeled mainly as a bulk property of matter. But the fracture toughness of materials such as composites can be enhanced if the material somehow provides energy-dissipation mechanisms on the molecular scale. This type of mechanism exists in naturally occurring materials, such as bone and abalone shell. When this mechanism is reversible, or at least in part reversible, then it also serves as a self-healing process post-deformation.

Investigating such processes requires highly controlled, precision nanomechanical interface with materials. The AFM provides precisely this. Researchers using AFM have now established that after relatively weak bonds break, untying segments of a relatively large structural molecule (such as a naturally occurring polymer), the energy needed to stretch the untied segment can be orders of magnitude larger than the broken bond's energy.

This type of work is now the subject of intense research, and the AFM's out-of-plane nanomanipulation capability is front and center in the quest for understanding the nature and dynamics of such mechanisms. Once more is known about structure and function of naturally occurring "sacrificial bonds" and "hidden lengths" as these are called, then synthetic versions may be designed and manufactured, which may also be tested the same way—using an AFM, or other instrumentation based on AFM.

Friction is another area of study in which the description of bulk matter differs from that of individual atoms and molecules. Nanomechanical testing and evaluation, including nanotribology, is one area where AFM technology is opening new windows that provide insight into how matter behaves. You need AFM to visualize, and to sense reaction to forces on the nanometer scale.

Researchers are using an AFM to study friction between the sliding parts of microelectromechanical systems and other microsystems. One of the most accessible model systems to study solid lubricants experimentally is graphite in general, and especially highly oriented pyrolytic graphite, known as HOPG.

AFM and scanning tunneling microscope imaging of HOPG goes back to the early days of the instruments. HOPG provided one of the best test samples to image for atomic-scale resolution—in-plane and out of plane of the sample—in ambient air or in liquid.

An AFM tip can move rather large samples, depending on how strongly they are adsorbed to the substrate. Here, plates of hydrocarbons atop one region of this HOPG layer are shown to be aligned differently when the AFM tip scans upwardly (top) than when the tip scans down (bottom). The tip combs the plates as it scans over them.

Work using AFM is making significant progress in validating—or challenging—the understanding of and models for mechanisms such as stick-slip and super-lubricity in solid lubricants. The AFM is indispensable to this work, because it is the only instrument that offers precision nanopositioning with sub-nanonewton force control, and highly accurate angstrom-level movement of a nanometer-scale tactile interface with solid matter.

The same is true of liquid lubricants and stiction-reducing chemicals. It is now a matter of routine to use anti-stiction thin films to treat those surfaces that make intermittent contact in MEMS, or that execute motion in close proximity to each other. The AFM is probably the most widely used and cited instrument today for investigating the nanometer-scale adhesive and viscoelastic properties of lubricants and stiction-reducing thin films, including monolayers.

Much of this work with atomic force microscopy is done for MEMS or for other microsystems in which stiction must be dealt with—including between the read-write head and the recording media in magnetic storage systems.

MINUTE FORCE CHANGES

The AFM has evolved into a highly modular instrument. Avanced AFMs such as the BioScope II from Veeco Instruments operate in liquid to image and probe biologically important matter, both organic and synthetic.

Also, there are AFMs for operating in vacuum, useful in investigating properties of matter without a water layer adsorbed on it, or for probing tip-sample interactions with highly sensitive probes in long range or in contact. Compared to operation in air, an AFM probe working in a vacuum can have 100 times more sensitivity.

In air, in liquid, or in vacuum, the AFM probe delivers the means to investigate minute force changes between the tip and the sample. Increasingly, tip-sample interactions are studied by recording and analyzing the changes in the dynamics of the cantilevers, such as the frequency, amplitude, and phase shifts of the driven cantilever at or near its resonance frequency.

Nanotechnology-related research has also pushed the development of an astonishingly wide array of AFM probes, from the general-purpose type at about $10 each to application-specific ones that can cost up to several hundred dollars each. Instrument manufacturers and probe manufacturers are driving each other's capabilities, and the ultimate driver of all that is the growing sphere of AFM work across science and engineering disciplines, including mechanical engineering.

Just as Kekulé dreamed of seeing the carbon atoms in a benzene ring, today's researchers dream of manipulating the very building blocks of matter to construct nanoscale machines and materials. The atomic force microscope, which made Kekulé's dream come true, is laying the groundwork for bringing these newer, more ambitious visions to reality.

F. Michael Serry is a senior applications scientist at Veeco Instruments in Santa Barbara, Calif.