AC Mode on an AFM
by Grant Drenkow
Those unfamiliar with the flexibility of an AFM (atomic force microscope) don’t realize that one can oscillate the tip up and down. Contact mode AFM often has a disadvantage for samples that are either weakly bound or soft because the tip can simply move or damage the surface feature and the resulting images are generally not high resolution. The advent of AC mode AFM, which operates in the intermittent contact regime or in the non-contact regime, provides a solution to this problem. The Agilent AFM’s have two oscillating modes - magnetic and acoustic.
Magnetic AC Mode (MAC)
To achieve MAC Mode imaging, a cantilever coated with a magnetic material is driven into oscillation by an AC magnetic field generated by a solenoid positioned close to the cantilever housing. The result of MAC Mode™ is a gentle, clean cantilever response that has no spurious background signals (“forest of peaks”) like other AC modes can have. Because the cantilever (and only the cantilever) is driven directly by the magnetic field, the need to shake the cantilever holder at large amplitudes is eliminated. Background resonance is absent, signal to noise is improved and setup becomes straightforward. Better signal-to-noise means that much smaller amplitudes can be used, This decreases damage to the sample and preserves asperities on the probe, contributing to greatly improved resolution. MAC Mode has even greater advantages when the cantilever is vibrated in liquid.
Acoustic AC Mode (AAC)
In Acoustic AC Mode (AAC) the cantilever is excited by high frequency acoustic vibration from a piezoelectric transducer attached to the cantilever holder. AAC mode can be classified into two categories, intermittent contact mode and non-contact mode, depending on the force regime and the tip-sample separation distance. The interaction between the tip and the sample is predominately vertical, thus negligible lateral forces are encountered. Consequently, AC mode AFM does not suffer from the tip or sample degradation effects that are sometimes observed after many scans in contact mode AFM, and it is a technique for imaging soft samples. In AC mode, tip-sample force interactions cause changes in amplitude, phase and the resonance frequency of the oscillating cantilever. The spatial variation of the change can be presented in height (topography) or interaction (amplitude or phase) images that can be collected simultaneously. The system monitors the resonant frequency or amplitude of the cantilever and keeps it constant by a feedback circuit that moves the scanner up and down. The motion of the scanner at each probe location is used to generate a topographic data set. The amplitude change at each probe location forms the amplitude image. The phase data is the result of the phase lag between the AC drive input and the cantilever oscillation output at each probe location. Consequently, contrast in phase images, which are due to differences in material properties, can provide very useful information. In addition, fine morphological features are easily observed in amplitude and phase images.
If you would like to learn more about the Agilent AFM’s - go to www.agilent.com/find/afm.
