We review the application of scanning probe microscopes to dimensional measurement of topographic features. Probe microscopes show great promise as metrology tools because they produce three-dimensional data over almost all solids in a wide range of ambients. Even though these microscopes readily achieve atomic resolution, there are several aspects of their behavior that can cause them to exhibit large measurement errors. The actuators that drive the probe exhibit hysteresis and creep, so they must be independently monitored. In addition, the geometry of an extended probe moving across the surface makes probe-sample interaction intrinsically nonlinear. Forces on the probe that cause it to flex are another source of inaccuracy. Probe fabrication and characterization are, consequently, important issues. We describe present understanding of these problems and the techniques being developed to solve them.

We report on the use of a force-balance atomic force microscope to obtain images with nanometer-scale resolution. In addition, force curve measurements were obtained on a mica surface in air with a sensitivity of 10-8 N.

Forces between a probe and sample have been measured in air and vacuum using a rocking beam force balance sensor capable of simultaneously obtaining both force and tunneling topographs. Force versus distance curve measurements demonstrate the effects of capillary forces in air but not in vacuum. Simultaneous scanning tunneling microscope (STM) imaging and force measurement in air show spatial force variations consistent with repulsive force tunneling. Our results support observations of large repulsive forces in STM occurring from surface contaminants. These results show how forces can affect image formation in both STM and scanning force microscopy.

Precision probe metrology requires that the probe be carefully formed and measured. We demonstrate a method to accurately measure the shape of a probe in situ by scanning special measurement structures.

Scanning probe microscopes have unusual advantages as measurement tools. They achieve high resolution simultaneously in all three dimensions, over almost any solid, in ambients ranging from high vacuum to fluid electrolytes. They offer the prospect of performing dimensional metrology at the atomic level with the calibration linked directly to crystal lattice constants. Application of these microscopes to measurement is not completely straightforward, however. As with optical and electron microscopes, accurate measurement is not possible without a thorough understanding of the instrument's properties. We discuss here two aspects of probe microscope behavior that affect position measurement because they exhibit strong nonlinearities.The piezo ceramic actuators commonly used to generate the probe motion are ferroelectric so they suffer from hysteresis and creep. Consequently, the probe motion must be independently monitored. We have adopted a capacitive scheme for monitoring the probe position in all three dimensions. This scheme allows the tube position to be measured to within 10 nm, though there are distortions caused by the tube bending that must be corrected.

There exists interest in the measurement of small forces for applications such as microtopography of semiconductor devices and atomic force microscopy. A new method is introduced here in which a small silicon beam, that is acted on by the external force of interest, has its position sensed by an rf phase shift technique. The position information in turn is fed back via electrostatic forces to continuously rebalance the beam about its central support. This force-feedback approach provides high sensitivity, submillisecond response, inherent force calibration, and electronically controlled stiffness.

For accurate linewidth measurement in scanning probe metrology the shape and size of the probe tip must be known. Since the probe can be degraded during a scan, quick in situ characterization is desirable. A technique is described employing an array of known structures that allows tip characterization with the probe microscope itself. This technique can be used to measure either the shape of a probe tip or the flexing caused by attractive forces near a sidewall. The results suggest that the sharpest probes may experience significant bending in the vicinity of a wall.

Probe tips for scanning tunneling microscopy have been sharpened using focused ion beam milling. Reproducible tips were formed on polycrystalline W and Pt-Ir shanks, but this technique is not limited to these materials. The tips were found to have cone angles of 12±3° and radii of curvature as sharp as 4 nm. Focused ion beam machining allows precise control of the final shape of the tips which is important in metrology measurements of various nanostructure devices.

A serious impediment to precision metrology with scanning probe microscopes is the unreproducible, nonlinear behavior of the piezo ceramic actuators. We have developed a simple solution to the problem by measuring the position of the scanning head with capacitors. The circuit monitoring the gap between the plates has a linear response with slope 12 μm/V and noise of 0.1 nm/(Hz)1/2. The linearity of the system is verified by comparing the capacitor output with a scan of a grating that is periodic in two dimensions. We analyze the errors associated with the technique and show how to reduce them to acceptable levels.