This article describes the Longitudinal Oscillation Contrast (LOC) module developed by CSM Instruments which is available as an optional extra for use with the standard contact mode scanning force microscope (SFM) system.
The SFM head houses a standard piezo tube which is feedback controlled in both horizontal axes by capacitive strain gauges. The deflection of the integrated cantilever beam is transformed into a measurement signal by a glass-fibre interferometer which serves as a position detector. This signal is sent to the control electronics where the tip position is continuously adjusted during scanning to compensate for height variations. This feedback control loop ensures that cantilever deflection remains constant during a measurement and is recorded as a direct representation of the surface topography of the sample.
High Resolution Frictional Contrast
The LOC module allows high resolution frictional contrast to be recorded simultaneously with the topography signal. The basic principle is shown in Fig. 1. During scanning in the conventional contact mode the scan head is oscillated parallel to the cantilever axis at a typical frequency of approximately 600 Hz. The type of friction encountered between the tip and the sample causes the cantilever to bend in one of two modes.
Figure 1. Schematic representation of the Longitudinal Oscillation Contrast (LOC) principle. The cantilever is oscillated along its axis and bends depending on whether static friction is encountered in the backward (a) or forward (b) direction. In the case of sliding friction, the cantilever bends to a much lesser extent. By positioning the optical fibre interferometer along the middle section of the cantilever, the resultant changes in intensity can be monitored depending on the type of interaction between tip and sample.
In the case of static friction (see Fig. 1 (a)), the cantilever bends towards the optical fibre (which is mounted above it) causing an increase in the intensity of the signal. In the case of sliding friction (see Fig. 1 (b)), the cantilever bends away from the fibre which results in a decrease in signal intensity. By mounting the optical fibre along the middle section of the cantilever allows the intensity difference to be optimised for either the ‘stick’ or the ‘slip’ condition.
At each measuring position, i.e., corresponding to each pixel of the image, the cantilever is oscillated in the LOC mode several times. By using a phase sensitive method (lock-in detection) allows the LOC signal to be separated from the topography signal. This means that, in practice, both a topography image and an LOC image can be acquired simultaneously.
By adjusting the amplitude of the LOC oscillation, the transition between static and sliding friction can be investigated in different materials as demonstrated in the results of Fig. 2. In this example, the friction signal has been plotted as a function of oscillation amplitude for chromium, gold and glass.
Figure 2. Friction (LOC) signal plotted as a function of oscillation amplitude for chromium, gold and glass samples scanned with a Si cantilever tip in contact mode. The low amplitude peak corresponds to the transition between static and sliding friction, whereas the constant signal at higher amplitudes gives a measure of the sliding friction encountered between tip and sample.
Starting at an amplitude of zero, the LOC signal is seen to increase linearly up to a certain maximum (depending on the characteristics of the measured sample material) before dropping sharply. The transition between static and sliding friction is characterised by this peak. The higher the peak, the greater the static friction between the tip and the sample. At higher amplitudes, the LOC signal remains almost constant at a certain level, this value being an indication of the sliding friction encountered between the tip and the sample measured.
Some typical results are presented in Figs 3 and 4 for a magnetic floppy disk surface and a glass slide respectively. In both cases, a large difference between the topography image and the LOC (friction) image is observed, with greatly enhanced contrast in the latter. This improved contrast is due to the variations in frictional characteristics between different phases of the surface structure. It can be particularly useful in cases where the surface topography is very flat and structureless, but where phases are present which have measurable variations in either static or sliding friction.
By adjusting the oscillation amplitude, the regime best suited to the measured sample can be used.
Figure 3. Topography (a) and LOC/friction (b) data obtained on a magnetic floppy disk surface. The topography image shows the general morphology of the surface, but the LOC image gives a large contrast enhancement between different phases.
Figure 4. Topography (a) and LOC/friction (b) data obtained on a standard glass substrate. Apart from a surface scratch, the topography image shows hardly any structure. In contrast, the LOC image shows the grain structure of the material.
The LOC technique is particularly well suited to frictional characterisation. Conventional Frictional Force Microscopy (FFM) relies on torsion of the cantilever beam to measure variations in friction. Such a method is very difficult to calibrate owing to the non-linear torsional deformation mode of the cantilever and so can only be considered as a qualitative technique. On the other hand, the LOC method can be calibrated quite accurately, provided that a sample is used with a known coefficient of friction between its surface and the material of the tip (usually Si).