Frequency Modulation mode
In
Non-Contact and
Semicontact modes of operation the cantilever is driven at a constant frequency near resonance, and force gradients are detected as variations in the amplitude or phase of the cantilever vibrations. In this scheme the signal-to-noise ratio for a given bandwidth can be increased by increasing the Q of the cantilever resonance. However, increasing the Q also decreases the maximum available bandwidth of the system. This circumstance became very important when operating in vacuum. In vacuum cantilever Q can be > 50 000. So high Q offers high sensitivity, but the very small bandwidth (< 1 Hz) causes too slow AFM operation speeds for most applications.
For increasing AFM sensitivity through higher Q without placing any restriction on bandwidth or dynamic range an alternative detection method (so called Frequency Modulation mode) was proposed
[1].
In the Frequency Modulation (FM) detection system, a high-Q cantilever vibrating on resonance serves as the frequency-determining component of an constant amplitude oscillator. Changes in the force gradient cause instantaneous changes in the oscillator frequency, which are detected by a FM demodulator. The cantilever is kept oscillating at its resonant frequency (with small shift ~ 50 Hz) by applying positive feedback through the oscillator control amplifier.
References
1. J. Appl. Phys. 69, 668 (1991).
