The Development of Atomic Force Microscopes

Typically, when we think of microscopes, we think of optical or electron microscopes. Such microscopes create a magnified image of an object by focusing electromagnetic radiation, such as photons or electrons, on its surface. Optical and electron microscopes can easily generate two-dimensional magnified images of an object's surface, with a magnification as great as 1000X for an optical microscope, and as large as 100,000X for an electron microscope. Although these are powerful tools, the images obtained are typically in the plane horizontal to the surface of the object. Such microscopes do not readily supply the vertical dimensions of an object's surface, the height and depth of the surface features.

The atomic force microscope (AFM), developed in the mid 1980's, uses a sharp probe to magnify surface features. With the AFM, it is possible to image an object's surface topography with extremely high magnifications, up to 1,000,000X. Further, the magnification of an AFM is made in three dimensions, the horizontal X-Y plane and the vertical Z dimension. As acknowledged by Binnig and Rohrer¹, the inventors of the tunneling microscope, such a powerful technique has its origins in the stylus profiler.

Magnification of the vertical surface features of an object, those features leaving the horizontal plane and extending in the vertical direction, have historically been measured by a stylus profiler. An example of an early profiler is shown in Figure 1. This profiler, invented by Schmalz² in 1929, utilized an optical lever arm to monitor the motion of a sharp probe mounted at the end of a cantilever. A magnified profile of the surface was generated by recording the motion of the stylus on photographic paper. This type of "microscope" generated profile "images" with a magnification of greater than 1000X.

A common problem with stylus profilers was the possible bending of the probe from collisions with surface features. Such "probe bending" was a result of horizontal forces on the probe caused when the probe encountered large features on the surface. This problem was first addressed by Becker³ in 1950 and later by Lee⁴. Both Becker and Lee suggested oscillating the probe from a null position above the surface to contact with the surface. Becker remarked that when using this vibrating profile method for measuring images, the detail of the images would depend on the sharpness of the probe.

In 1971 Russell Young⁵ demonstrated a non-contact type of stylus profiler. In his profiler, called the topographiner, Young used the fact that the electron field emission current between a sharp metal probe and a surface is very dependent on the probe sample distance for electrically conductive samples. In the topographiner, the probe was mounted directly on a piezoelectric ceramic used to move the probe in a vertical direction above the surface. An electronic feedback circuit monitoring the electron emission was then used to drive the piezoceramic and thus keep the probe sample spacing fixed. Then, with piezoelectric ceramics, the probe was used to scan the surface in the horizontal (X-Y) dimensions. By monitoring the X-Y and Z position of the probe, a 3-D image of the surface was constructed. The resolution of Young's instrument was controlled by the instrument's vibrations.

In 1981 researchers at IBM were able to utilize the methods first demonstrated by Young to create the scanning tunneling microscope⁶ (STM). Binnig and Rohrer demonstrated that by controlling the vibrations of an instrument very similar to Young's Topographiner, it was possible to monitor the electron tunneling current between a sharp probe and a sample. Since electron tunneling is much more sensitive than field emissions, the probe could be used to scan very close to the surface. The results were astounding; Binnig and Rohrer were able to see individual silicon atoms on a surface. Although the STM was considered a fundamental advancement for scientific research, it had limited applications, because it worked only on electrically conductive samples.

A major advancement in profilers occurred in 1986 when Binnig and Quate⁷ demonstrated the Atomic Force Microscope. Using an ultra-small probe tip at the end of a cantilever, the atomic force microscope could achieve extremely high resolutions. Initially, the motion of the cantilever was monitored with an STM tip. However, it was soon realized that a light-lever, similar to the technique first used by Schmalz, could be used for measuring the motion of the cantilever. In their paper, Binnig and Quate proposed that the AFM could be improved by vibrating the cantilever above the surface.

The first practical demonstration of the vibrating cantilever technique in an atomic force microscope was made by Wickramsinghe⁸ in 1987 with an optical interferometer to measure the amplitude of a cantilever's vibration.

Using this optical technique, oscillation amplitudes of between .3 nm and 100 nm were achieved. Because the probe comes into close contact with the surface upon each oscillation, Wickramsinghe was able to sense the materials on a surface. The differences between photo-resist and silicon were readily observed.

1. G. Binnig and H. Rohrer, Scanning Tunneling Microscopy—From Birth to Adolescence, Rev. of Mod. Phys, Vol 59, No. 3, Part 1 1987, P 615
2. Uber Glatte und Ebenheit als physikalisches und physiologishes Problem, Gustev Shmalz, Verein Deutscher Ingenieure, Oct 12, 1929, pp. 1461-1467
3. U.S. Patent 2,728,222
4. UK Patent 2,009,409
5. R. Young, J. Ward, F. Scire, The Topografiner: An Instrument for Measuring Surface Microtopography, Rev. Sci. Inst., Vol 43, No 7, p 999
6. G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel, Surface Studies by Scanning Tunneling Microscopy, Vol. 49, No 1, 1982, p 57
7. G. Binnig, C.F. Quate, Ch. Geber, Atomic Force Microscope, Phys. Rev. Letters, Vol. 56, No 9, 1986 p 930
8. Y. Martin, C.C. Williams, H.K. Wickramasinghe, Atomic Force Microscope-Force Mapping and Profiling on a sub 100-Å scale. J. Appl. Phys. Vol 61, No 10, 1987, p 4723