AM/FM Measurements Using Multiple Frequency of Atomic Force Microscopy

摘要

Apparatus and techniques presented combine 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 phase feedback from second resonant drive frequency operates in FM mode. In particular the first or second frequency may be used to measure the loss tangent, a dimensionless parameter which measures the ratio of energy dissipated to energy stored in a cycle of deformation.

主权项

1. A method of calibrating an atomic force microscope, which is equipped with a cantilever with a tip, a photodetector and a coarse and fine positioning system for controlling the position of the cantilever and the sample, comprising the following steps:
turn the drive mechanism off; position the detection laser on the side of the cantilever opposite the tip and in the center of the photodetector; calibrate the stiffness of the cantilever from a measured thermal spectrum taken far from the surface of the sample and record this as the natural frequency of the cantilever; approach the tip of the cantilever to close proximity with the sample, as determined by optical inspection of the tip and the sample using the coarse-positioning system; turn the drive mechanism on; set the drive frequency equal to the natural frequency of the cantilever as determined from the aforesaid thermal spectrum; set the drive amplitude to a desired free oscillation amplitude; approach the tip of the cantilever to the surface of the sample until contact is established by setting the feedback set point to a desired interaction amplitude that is slightly less than the free oscillation amplitude; fully separate the sample from the tip of the cantilever as determined by optical inspection using the fine-positioning system; acquire a cantilever tune by either setting the drive frequency to the natural frequency of the cantilever as determined from the aforesaid thermal spectrum or setting the drive frequency to a local amplitude maximum of the cantilever near the cantilever resonant frequency; turn off the drive mechanism and acquire an additional thermal spectrum of the cantilever that is closer to the imaging conditions; using a simple harmonic oscillator model and the data from the new thermal spectrum define the new natural frequency of the cantilever and determine the phase-to-drive-frequency relationship of the cantilever; turn on the drive mechanism and choose a drive frequency equal either to the new natural frequency of the cantilever or to a local amplitude maximum of the cantilever near the cantilever resonant frequency; choose a drive amplitude which achieves the desired cantilever free oscillation amplitude; set phase from the result of the aforesaid simple harmonic oscillator model.