Characterizing Materials on the Nanoscale

By Dan Day


Since the 1970s the materials and biological sciences have undergone a major change in scale. In the days before the recent semiconductor revolution, most materials were characterized on the micron (millionth of a meter) level. Today, the realm of interest has shrunk one thousand times to the nanometer scale (one-billionth of a meter)! Scientists are now assembling microdevices in layers only tens to hundreds of atoms thick. A major obstacle to this work is being able to see what one is doing to enable control of the fabrication processes. An important tool developed to fill this need is an amazing device that fits on a desktop—the atomic force microscope, or AFM.

The AFM was invented by researchers Binnig, Quate, and Gerber of IBM Research, Zurich, Switzerland in 1986. Since its inception, the AFM has been used extensively in surface science, nanotechnology, polymer science, semiconductor fabrication, and microbiology. At the Lawrence Livermore National Laboratory, the AFM is being used to characterize biologically synthesized inorganic materials like seashells. With the data produced, scientists can fabricate similar structures using other materials, and describe the surface coatings on metals exposed to caustic solutions so that metallurgists can perform chemical attacks on metal alloys.

A Lilliputian Scale

In its simplest form, the AFM consists of a minute cantilever or thin ceramic "plank" from which a very tiny pointed stylus protrudes. In many ways it resembles a phonograph needle on a Lilliputian scale.

The material to be analyzed is placed on a small cylindrical ceramic pedestal beneath the cantilever tip. This ceramic is a unique material (piezoelectric) that shrinks or expands when an electric voltage is applied across it. The pedestal can be manipulated in the horizontal and vertical dimensions by strategically placed electrodes used to control its movement. The delicacy of this control is accurate enough to allow the stylus to traverse the surface of the material in much same the way as a television electron gun scans the screen to produce an image.

Movement of the stylus is tracked by a laser beam reflected off the cantilever top surface above the stylus tip. A photodiode detector translates the laser beam deflection into vertical and horizontal components. Computerized images on the display screen are color-coded to depict vertical and horizontal tip displacements as a "topographic map" of the scanned surface. The AFM can be used in the contact mode just described to measure surface topography, or it can assess frictional drag between the sample and the cantilever tip. By varying the force applied to the cantilever, the AFM can measure sample hardness, and by floating the tip over the sample it can monitor electromagnetic attractive forces.

The sensitivity of the AFM depends on the sharpness of the cantilever tip. Cantilevers measure about 100 microns (one-millionth of a meter) long and are made of gold-coated silicon oxynitride with a 3-micron-long pyramid-shaped silicon nitride tip protruding from one end. Both the cantilever and the tip are made by the same photolithographic techniques (derived from the negative/print process in photography) used to manufacture semiconductor chips. The more sensitive cantilevers have an ultra-thin spine deposited on the end of the tip that has a radius of only 10 nanometers. The smaller the tip radius, the more sensitive it is to the atomic structure of the surface beneath it.

Many Uses for a Unique Instrument

The AFM is only one of about two dozen scanning probe-type microscopes (SPMs) that are used to monitor various physical properties of materials. The AFM has a wide range of applications in biology, polymer science, nanotechnolgy, semiconductor fabrication, and surface science. These remarkable devices are used to characterize biological specimens, to probe the hardness of materials, to determine surface roughness, to explore the electrical and magnetic properties of surfaces, to monitor crystal growth patterns, and to test the strength of tiny nano-filaments of materials now being synthesized to make the next generation of electronic microcircuits.

AFM Resources

More detailed discussion of the AFM can be found at these web sites:

Diagram: Reprinted with permission from Arunan Nadarajah, University of Toledo
Photo: Digital Instruments MultiMode

The Devil Mountain Views -- Mar/Apr 2002
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