High-Speed Atomic Force Microscopy


value, but this process takes finite time and thus is a bottleneck toward attainment of higher scan speeds. Te speed at which the cantilever returns to equilibrium


Figure 4: Two high-resolution 20 × 20 µm AFM scan frames show PHBV crystal growth from amorphous phase after quenching. The image on the left shows one AFM image frame of spherulite growth. The image on the right shows a single AFM image frame of the dominant lamellae slowing their growth to a stop. Each of these 13-megapixel images was captured in minutes, enabling researchers to further explore the sample’s micro- and nano-scale features offline.


constant tip-sample interaction (commonly call the setpoint). When attempting to scan faster, the AFM Z-scanner feedback loop must respond correspondingly faster or the forces between the tip and the sample will deviate from the setpoint, resulting in image distortion that can degrade the AFM tip’s resolution and possibly damage soſt or delicate samples. Because feedback loops are only as fast as their slowest


component, producing a high-speed AFM that is free of image distortion, resolution degradation, or sample damage requires that the speed (or bandwidth) of every component in the AFM feedback loop be improved significantly. Te most challenging components to develop in the feedback loop are the Z scanner bandwidth and the AFM cantilever dynamics. High-bandwidth Z scanner design. Te Z-axis scanner


represents a significant design challenge for high-speed AFMs (Figure 5). In recent years academic researchers have demonstrated high-bandwidth Z scanners that traded off speed increases for greatly reduced Z range and accuracy. To maximize the value of high-speed AFM to its users, the AFM Z scanner must demonstrate at least one order of magnitude increase in bandwidth yet have enough range to allow a broad range of samples, while maintaining ultra-low-noise Z position-sensor capabilities


to enable accurate surface


metrology. In addition, the design should facilitate easy AFM probe exchange and waterproofing of the scanner surfaces for liquid operation and cleaning. Te latest generation of high-speed AFMs addresses these needs. Attaining fast cantilever dynamics. Te most common


mode of AFM operation is TappingModeTM (also called “dynamic mode” or “intermittent contact mode”), in which the cantilever is oscillated at or near its resonant frequency. Tis resonant frequency is defined by the cantilever effective mass and spring constant (expressed in N/m), which are in turn defined largely by the cantilever material and geometry. As the tip is brought into proximity of the surface, the oscillation amplitude is reduced by tip-sample forces. Te AFM feedback loop maintains a constant oscillation amplitude by actuating the Z scanner as the tip is scanned over the sample surface. Maintaining sample measurement accuracy requires that the cantilever amplitude be allowed to return to its equilibrium


14


depends primarily on three factors: the cantilever’s oscillation setpoint, resonant frequency, and quality factor (Q). Cantilever speed increases with decreasing oscillation setpoint, and thus decreasing the oscillation setpoint is a common tactic to improve scan speed. However, doing so increases imaging force, which can damage delicate samples and degrade tip sharpness unless the cantilever spring constant is also decreased. Cantilever speed also increases with increasing resonant frequency and with decreasing Q. Te obvious way to increase cantilever resonant frequency


is to increase its thickness, but doing so greatly increases the spring constant and therefore the imaging force. Tus, to image faster while maintaining low force, it is necessary to decrease all cantilever dimensions. By properly balancing all the dimensional and materials constraints, it is possible to design a cantilever that enables over one order of magnitude increase in AFM scan speed while maintaining low tip-sample force. Balancing these design factors with scalable, economic manufacturing techniques is critical to the development of commercial AFMs that meet the challenges of emerging high-bandwidth applications.


Improved Laser Optics Cantilever deflection is measured as the angle change of


the reflected laser beam induces movement of the laser spot on a four-quadrant photo detector. Te signal generated is proportional to the movement of the projected spot divided by the size of the spot. Te cantilever acts as a mirror, so the incoming beam must be more convergent to produce a smaller spot, and this beam, when reflected, is equally divergent. Tus, a smaller laser spot on the back of the cantilever


results in a larger laser spot on the photo detector, resulting in reduced optical deflection sensitivity. Tis detrimental characteristic is offset by the increase in deflection sensitivity with decreasing cantilever length. Tus, high-speed AFMs that use small cantilevers can still attain atomic resolution. However, to maximize optical deflection sensitivity for a broad range of cantilever lengths, the high-speed AFM needs to allow fast and easy adjustment of the laser spot size to enable the user to select an appropriate spot size, namely, the largest spot size that will not spill off the edge of the cantilever and generate interference and noise. Te new generation of high-bandwidth AFMs provide a selection of spot sizes that accommodates different cantilevers, including a small spot size required for the smaller cantilevers used for high-bandwidth scanning.


Tip Scanning Versus Sample Scanning AFM scanners can be configured as “tip scanning,”


where the sample is stationary and the tip is moved in three dimensions as it scans over the surface, or as “sample scanning,” where the tip is stationary and the sample is moved in three dimensions as it is scanned under the tip. High-speed sample scanning systems are inherently simpler to design and manufacture but are highly restrictive for the user. Sample size is limited to what can fit on the sample scanning stage (usually ~10 mm in diameter by a few mm thick), and the sample’s mass negatively impacts the AFM scanner dynamics. In contrast, a high-speed tip scanning AFM can accept very large samples,


www.microscopy-today.com • 2011 November


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