Quantitative measurements using multiple frequency atomic force microscopy

摘要

The imaging mode presented here combines the features and benefits of amplitude modulated (AM) atomic force microscopy (AFM), sometimes called AC mode AFM, with frequency modulated (FM) AFM. In AM-FM imaging, the topographic feedback from the first resonant drive frequency operates in AM mode while the second resonant drive frequency operates in FM mode and is adjusted to keep the phase at 90 degrees, on resonance. With this approach, frequency feedback on the second resonant mode and topographic feedback on the first are decoupled, allowing much more stable, robust operation.

主权项

1. A method of operating an atomic force microscope, comprising:
using a feedback loop to control a distance between the base of a probe tip of a cantilever of the atomic force microscope, and a surface of a sample being measured by the atomic force microscope, to maintain the probe tip of the cantilever in a pre-established relationship with respect to the surface of the sample in a Z direction while scanning the sample by creating relative movement between the probe tip of the cantilever and the sample; exciting the a driving chip of the cantilever at a first and a second resonant frequencies of the cantilever which have been summed together by a circuit element; providing said first and said second resonant frequency as a reference signal to a lock-in amplifier circuit; controlling a distance between the base of the cantilever and the sample so that an amplitude of oscillation at the first resonant frequency of the probe tip is maintained essentially constant at an amplitude setpoint; detecting changes in phase and/or amplitude in the oscillation at the first resonant frequency while the amplitude of oscillation at the first resonant frequency is maintained essentially constant at the amplitude setpoint; detecting changes in phase and/or amplitude in the oscillation at the second resonant frequency while the amplitude of oscillation at the first resonant frequency is maintained essentially constant at the amplitude setpoint; and measuring the amplitude and/or phase of the cantilever at the first and second frequencies.