Metrology with an atomic force microscope

Abstract

We describe applications of the atomic, magnetic, and electrostatic force microscopes (AFM, MFM and EFM), to metrology problems, especially profiling, magnetic field measurements and potentiometry. The force microscope has many advantages. It is capable of much higher lateral resolution than optics or low-voltage SEM, is nondestructive, operates in air, and does not require samples with special properties, such as electrical conductivity or extreme smoothness. By changing the tip-sample spacing, the speed/resolution tradeoff can be optimized for each application. The basic force probe consists of an L-shaped wire cantilever mounted on a piezoelectric bimorph which serves for terrain following and excitation. The wire is electro-etched to reduce its diameter and form a sharp tip. When the tip and sample are close together, the gradient of the force between them changes the effective spring constant (and thus the resonant frequency of the cantilever; the shift is detected as a changed response to a slightly off-resonance excitation. Force gradients of 10-4 N/m and force increments of 10-13N have been measured in this way, and the current lateral re-solution is 50Å; there seems to be no fundamental impediment to achieving atomic resolution. The electrostatic force microscope is an AFM with a voltage applied to the tip. It makes possible potentiometry through nonconducting passivation layers. The magnetic force microscope (MFM) is very similar. It uses a steel wire tip to make magnetic images with a lateral resolution of better than 1000 A. which we hope to improve to 100 A. We present data illustrating usefulness of all these techniques in metrology problems. © 1988, SPIE.