A new technique measures forces, down to 5 x 10E-16 N on a sub-micron scale. A small tip, having a diameter D less than or equal to 1OOA, at the end of a spring is vibrated close to a solid. Linear and non linear AC contact forces between the tip and the solid are Measured through the resulting tip displacement with a highly sensitive laser probe. Profiling and "contact" inspection can be done nondestructively on an Angstrom scale.
In operation, a metallic spring tip is mounted on a small PZT transducer and Vibrated close to the surface of the solid to be examined. A laser heterodyne probe accurately measures the AC displacement of the spring. When the tip is brought close to the surface, interactive forces between the tip and the surface (van der Waals forces. or electronic repulsion -contact force-) modify the AC displacement at the fundimental frequency giving some information concerning the absorbed mechanical power, and generate harmonics of the AC vibration frequency. Both fundamental and harmonic vibrations can be measured in order to: (a) acquire surface information, and (b) control the tip-surface distance.
The AC driving voltage on the FZT and the DC position at the tip over the surface are input parameters that can be controlled in order to monitor the AC and DC force of the tip on the solid. Spectroscopy at non-linear tip-surface interactions could be achieved. Interaction with particles could also be examined.
Positioning and stabilizing the tip, vertically and laterally could be achieved with controlled and focussed laser beams. The absolute distance of the tip over the surface and/or of the flexion of the spring could also be obtained with a phase-sensitive microscope, which allows one to measure displacements with a precision an the order of an Angstrom.
Copyright IBM Corp 1987