ABSTRACT
Friction versus load curves can be obtained by the following procedure. First, the AFM control electronics must be able to vary the applied load on the cantilever while simultaneously monitoring and recording the normal and lateral signals while scanning across the surface. This can be accomplished in a number of different ways. Some software and controller packages, such as RHK's SPM 100 controller and SPM 32 software (RHK Technology), allow the applied load to be automatically ramped in a stepwise fashion so that an image can be acquired with every scan line obtained at a different load. Some other AFM controller/software packages do not allow the user sufficient flexibility to complete such an experiment. In this case, the user may apply an external signal to the AFM itself. One way is to supply a voltage signal directly to the z-piezo of the AFM, thus altering the load on the cantilever. This may be accomplished by either modifying the electronics of the AFM controller, or by using an external connection, such as a breakout box. The downside to applying a signal directly to the z-piezo is that the feedback system needs to be turned off, so no line-by-line feedback can be maintained. A better method is to send a signal to the AFM that changes the feedback setpoint value, i.e. the applied normal load in feedback control. An external signal is sent directly into the AFM electronics to vary the setpoint, again either by modification of the AFM electronics or using a breakout box. This method allows for feedback control while the load is varied during scanning, which is beneficial for measurements on rough or sloped surfaces. Load and friction are continuously measured by monitoring the position of the laser spot on the photodetector in the normal and lateral directions, respectively. Usually it is easier to apply a continuously varying voltage, as opposed to the stepwise voltage ramp as in the RHK system for example. However, instead of each line being a separate load value, the load is continuously varied even during a single scan line. To account for this, the load and friction variations on a single line are averaged (excluding the edges of the image where the tip is sticking, and not sliding, with respect to the surface) to obtain a single data point of average load and friction for every scan line. This is reasonable to do if the variation in voltage (and, thus, force) during a single scan line is small compared to the range of voltage (force) applied during the experiment.
