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Intermittent Contact Mode AFM


In Intermittent Contact Mode AFM, the cantilever’s oscillation amplitude (and phase relative to the drive signal) are the main quantities that are measured. Usually, the quantity that the feedback system strives to control is the cantilever’s oscillation amplitude.

The measured amplitude is compared with a setpoint value. The difference between the two is called the error signal, and is the input into the feedback system, the output of which (after amplification) drives the Z-actuator. This output signal is the quantity that is most often plotted for each X,Y coordinate to synthesize the 3-dimensional image that is usually called the Topography Image or the Height Image in Intermittent Contact Mode AFM.

The measured amplitude itself is rarely used to compose an image, but the error signal, the difference between it and the set point value, frequently is. This image is usually displayed and captured side-by-side the topography image, and is called “the Amplitude Image,” clearly a confusing choice of name. In many ways the Amplitude Image in Intermittent Contact Mode AFM is similar to the Deflection Image in Contact Mode AFM. On harder samples, the Amplitude Image highlights the edges of features, and on softer samples, it often depicts subsurface features better than the topography image does.

Different methods are used to drive the cantilever in all Dynamic Mode AFMs, including in Dynamic Vertical Mode AFM, and specifically, in Intermittent Contact Mode AFM. These methods are described in MAC Mode, and AC Mode, and Acoustic Mode.

Regardless of the method of drive, the driving force F is usually sinusoidal:

Where the drive frequency, ƒ, is typically at or near one of the cantilever’s eigenfrequencies. Most often, this is the fundamental eigenfrequency ƒ ο and the free end of the cantilever oscillates with an amplitude, A, that is proportional to the quality factor Q, of the resonance around the eigenfrequency, and inversely proportional to the cantilever’s spring constant k.

Absent any tip-sample interactions, the cantilever oscillations, z(t), are also sinusoidal if the drive amplitude, Fo, is small enough to keep the cantilever motion small compared with the cantilever thickness.

When the cantilever and the sample are close enough, during each oscillation cycle, the tip moves through an interaction potential that includes longrange attractive and short term repulsive components. There are also non-conservative forces that are not accounted for in this potential. The tip-sample forces are highly nonlinear and complex, especially around the lower excursion point of the cantilever, when the tip is closest to the sample or temporarily in contact with it.

One of the main advantages of Intermittent Contact Mode AFM over Contact Mode AFM is that since the tip only intermittently contacts the sample surface, lateral shear forces that may alter (dull) the tip or rearrange (damage) the sample surface during raster scanning are significantly reduced or nearly eliminated. This was the main impetus behind the development of Dynamic Vertical Mode Force Microscopy. But it was soon realized that Dynamic Force Microscopy provides additional cantilever signals to probe the tip-sample interaction, chief among them are the relative phase φ and frequency shift Δf of the cantilever.

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