Abstract
Atomic force microscopy (AFM) is used in the semiconductor industry for inspection and quality control. Frequency modulated AFM (FM-AFM) extracts surface topography by measuring the frequency shift created by the Van der Waals (VdW) interaction forces between the tip and the sample. To improve the measurement speed and address complex geometries emerging in industrial microchip constructions, several enhancements are introduced. While most FM-AFM devices operate in a single vibrating mode, this article proposes a method for multidimensional sensing using frequency modulation of 2 orthogonal vibration modes, simultaneously. The concept was tested on a large-scale experimental system, where VdW forces were replaced by magnetic forces, using a magnetic tip and ferromagnetic samples. To emulate the VdW forces accurately, the designed ratio between the base frequency and frequency shift was kept to mimic a Nano-scale AFM. By utilizing an Autoresonance (AR) control scheme for faster locking onto resonance and a curve fitting frequency estimation algorithm it is possible to sense the minute changes in frequency experienced by several modes, simultaneously. Experimental results employ 3D relevant topographies such as inclined surfaces, steep walls and trenches that were reconstructed experimentally with 4 (μm) resolution or better. Downscaling to typical AFM dimensions would theoretically yield sub-nanometer resolution.
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