Surface Approach¶
see also Piezo Approach panel
Overview¶
During a surface approach, the NanoFrazor’s cantilever is moved towards the sample and the distance sensor’s signal is monitored. The results provide three pieces of information:
- The piezo extension required to bring the tip into contact with the surface.
- The calibration data for the height sensor.
- The adhesion between the tip and sample.
Description of the procedure¶
The procedure begins with the tip of the NanoFrazor’s thermal cantilever separated from the surface by several micrometers. The NanoFrazor moves the thermal cantilever towards the surface of the substrate by extending the Z-piezo scanner. The Z-piezo scanner incorporates a deflection sensor, which is calibrated before being installed in the NanoFrazor. This is used in conjunction with a control loop to achieve accurate, calibrated positioning of the piezo scanner. The extension of the piezo scanner reduces the air gap separating the substrate and the cantilever’s thermal distance sensor. Consequently, the sensor cools down and the reader signal increases. A full description of the thermal distance sensor’s operation can be found in [Duerig2005].
Once the tip touches the surface, the reader to surface distance is determined by the length of the tip, and the reader signal stops increasing. This change in slope signals to the NanoFrazor that the surface has been found and it begins to withdraw the cantilever from the surface while continuing to record the reader signal. The withdrawal continues until the cantilever returns to its original height above the surface.
Measured data¶
Results¶
Figure 8(a) shows a plot of the data collected during such a surface approach curve. The graphs show a close-up of the data collected when the tip was within a few hundred nanometers of the surface. The arrows indicate the time flow. The light blue curve is measured before the contact with the surface (approach), the red curve afterwards (retract).
At point 1 in the approach curve in Figure 8 (a), the cantilever moves towards the surface and so the reader signal increases. Point 2 shows a jump in the reader signal, this is the so-called “snap-in” event where short range, attractive adhesion forces pull the tip into contact with the surface. Thereafter, as outlined above, the reader signal becomes relatively insensitive to further piezo extension yielding the portion of the data to the right of point 2. The points to the left of point 2 were measured as the cantilever was withdrawn from the surface. Initially, during retraction, the tip is kept in contact with the surface by adhesion forces, the cantilever bends and the reader to surface distance changes by less than the movement of the Z-piezo scanner. That is why the slope between point 2 and 3 is much lower than the slope at point 1. At point 3, the elastic restoring force of the cantilever is bigger than the adhesion force between the tip and the surface, and the tip “snaps-off”. The difference in piezo extension between point 2 and point 3 is called the “adhesion length”. At point 4, the reader signal during retraction overlaps with the reader signal during approach, since the cantilever’s separation from the surface is now controlled only by the Z-piezo’s position.
Analysis¶
Point 2 gives the extension of the piezo required to bring the cantilever into contact with the surface. The curve before the contact with the surface provides the height sensor calibration. An extrapolation of this curve provides the calibration for negative heights (depths). The adhesion length multiplied by the cantilever’s stiffness gives the adhesion force between the tip and sample.
Because the adhesion force between the surface and the tip depends on the contact area, the diameter of the tip can be estimated from the adhesion length measured during the approach and retract. In particular, for adhesion lengths between 10 nm and 50 nm and when performing approach onto a PPA surface, the adhesion length is roughly equal to the diameter of the tip [1]. The approach curve in Figure 8 (a) is from a blunt tip which has been used extensively. It has an adhesion length of 170 nm. The surface approach curve in Figure 8 (b) is from a new cantilever, the tip diameter is around 10 nm.
[1] | This is only a rough estimate. The exact ratio between adhesion length and diameter of the tip depends on the microscopic properties of the substrate surface, on the relative humidity, on the exact shape of the tip and the stiffness of the cantilever. |
[Duerig2005] | Urs Duerig, Fundamentals of micromechanical thermoelectric sensors, Journal of Applied Physics, 98, 044906 (2005). |