The Development of Atomic Force Microscopes

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.