Angstrom Advanced AFM / SPM Modes and Scanner Technology

SPM Tapping Mode:

In Tapping Mode, a cantilever is oscillating in free air at its resonant frequency. A piezo stack excites the cantilever’s substrate vertically, causing the tip to bounce up and down. As the cantilever bounces vertically, the reflected laser beam is deflected in a regular pattern over a photodiode array, generating a sinusoidal electronic signal. And this signal is converted to a root mean square (RMS) amplitude value. 

When the same cantilever is oscillating at the sample surface, Although the piezo stack continues to excite the cantilever’s substrate with the same energy, the tip is deflected in its encounter with the surface. The reflected laser beam reveals information about the vertical height of the sample surface.

The feedback system controls the scanner’s Z voltage to maintain the tip-sample force constant, which leads the Up-Down signal equals to the Setpoint. The Z voltage is recorded for calculating the sample topography. Figure 1: No force between tip and sample, no cantilever deflection. Figure 2: Repulsion between tip and sample, cantilever deflects upside. Figure 3: Repulsion between tip and sample, cantilever deflects upside: X: Deflection of cantilever k: Force constant of cantilever F=kx

The sample is placed on the Piezo Electric Scanner and is scanned under a stationary cantalever tip (there are AFM models in which the tip is scanned over a stationary sample). The PES is a very precise component and is able to accuratly move the sample through a scan (a back and forth raster pattern) of only a few hundred nanometers. Scanners are made of piezo tubes and are steady held in the SPM base.Voltage applied to piezoelectric scanner tube housed inside moves sample precise increments back and forth. Size at bottom indicates maximum size scan possible,each scanner has its own specified parameters. Samples are held in place on the scanner with a removable cap.Piezo scanner can extend and retract 3-dimentionally based on the applied X, Y and Z voltage placed across the individual elements.