Atomic force microscopy

High-resolution surface profiling can be performed by several techniques that are collectively known as scanning probe microscopy (SPM). This includes scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). In the former,

Figure 2.6: A schematic of the NanoScope III multimode scanning probe microscope that was used for all AFM measurements. The inset shows the design of the Si cantilevers used for most of the TappingMode measurements, and was adapted from [31].

a very small tip is brought to within 1 nm of the sample, and an electric current is tunnelled across the gap to deduce topography. Although this technique produces very high resolution images, STM is fundamentally limited to conductive samples [30]. In AFM, the tip is located at the end of a cantilever and by monitoring its deflection, the surface of any material can be profiled. These cantilevers and tips are usually fabricated from Si or Si3N4.

The spring constant of a surface atom can be approximated to be about 10 N/m. To minimise surface damage, AFM cantilevers are long, thin and have spring constants between 0.1 and 50 N/m [30], although specific cantilevers vary with different AFM modalities. Contact AFM involves simply scanning the tip across a sample, and they are in physical contact with each other. Generally, a piezoelectric scanner is used to move the sample while the cantilever deflection is optically monitored. A control system varies the scanner height in order to maintain a constant cantilever deflection, and hence deduces the surface profile. The forces generated between the tip and the sample range from 10−8 _{to 10}−6 _{N, which can easily damage soft}

materials. Non-contact AFM, on the other hand, uses a stiff cantilever to probe the surface while only imparting forces of about 10−_{12 N [30]. This is achieved}

by vibrating the cantilever at 100–400 kHz, and measuring amplitude changes that result from the electrostatic and van der Waals forces at the surface. This approach minimises damage to the sample and tip degradation. The small vibration amplitude conventionally used in non-contact AFM means that the tip-to-sample

distances are usually less than 10 nm, so it is possible for the tip to stick on the surface and hence degrade the image quality [32].

The AFM measurements presented in this thesis were performed on a Digital Instruments NanoScope III multimode scanning probe microscope in the Depart- ment of Applied Mathematics. Samples were profiled using the TappingModeTM

technique, which uses a larger vibration amplitude than standard non-contact AFM and hence the tip has sufficient energy to avoid sticking. There is negligible sample damage as tip-sample forces with TappingMode are only about 10−_{10 to 10}−_{9 N,}

which is much less than in contact mode [32]. The cantilever is driven at a high enough frequency to ensure that the lateral resolution is limited by the radius of the tip, as in stylus profiling.

The NanoScope III includes a cylindrical piezoelectric scanner, on which the sample is mounted. This provides controlled lateral scanning and height adjustment. When configured for TappingMode, the cantilever substrate is mounted on a separate piezoelectric oscillator and this is used to vibrate the tip. The tip holder sits inside the optical head, where a 670 nm laser is aligned onto the back of the cantilever. As shown in figure2.6, the reflected light is incident on a mirror, which directs the signal onto a segmented photodiode. The differential signal between the upper and lower detectors is used to measure the deflection of the cantilever, whereas the left-right differential indicates the lateral forces. In TappingMode, the root-mean-square (RMS) amplitude of the cantilever vibration is measured and compared with a setpoint value. The control electronics adjust the scanner height to keep the vibration amplitude constant, and the voltage applied to the scanner is converted to a value for the surface height.

For the roughness measurements discussed in chapter4, Tap300Al Si AFM probes were used, as depicted in the inset of figure 2.6. These contain 125 µm long and 4 µm thick Si cantilevers, with force constants of about 40 N/m and resonant frequencies of about 300 kHz [31]. NSG20 probes were also used to characterise some samples with large-scale surface roughness. These cantilevers are stiffer than the Tap300Al model, with typical resonant frequencies of 420 Hz. Before each set of measurements, the cantilever is tuned and the driving frequency is chosen to be slightly less than the resonance peak, so the vibrational amplitude is about 70% of the maximum [32]. The back of the cantilevers are coated in Al or Au to improve the reflection of the laser and tip radii are less than 10 nm for high lateral resolution. Image analysis was performed with Gwyddion—free software for visualisation and analysis of SPM data.

Figure 2.7: A Wyko NT9100 surface profiler was used to examine surfaces with high resolution. A schematic of the entire system is shown in (a) and the design of the through- transmissive-media module is shown in (b). Both figures were modified from [33] with permission from Bruker Nano Surfaces Business, formerly Veeco Metrology Group.