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Large Scan Area High-Speed Atomic Force Microscopy Using a Resonant Scanner B

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Large Scan Area High-Speed Atomic Force Microscopy Using a Resonant Scanner B REVIEW OF SCIENTIFIC INSTRUMENTS 80, 093707 ͑2009͒ Large scan area high-speed atomic force microscopy using a resonant scanner B. Zhao,1 J. P. Howard-Knight,1 A. D. L. Humphris,2 L. Kailas,1 E. C. Ratcliffe,3 S. J. Foster,3 and J. K. Hobbs1 1Department of Physics and Astronomy and Department of Chemistry, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, United Kingdom 2Infinitesima Ltd., Oxford Center for Innovation, Mill Street, Oxford OX2 0JX, United Kingdom 3Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, United Kingdom ͑Received 22 July 2009; accepted 24 August 2009; published online 22 September 2009͒ A large scan area high-speed scan stage for atomic force microscopy using the resonant oscillation of a quartz bar has been constructed. The sample scanner can be used for high-speed imaging in both air and liquid environments. The well-defined time-position response of the scan stage due to the use of resonance allows highly linearized images to be obtained with a scan size up to 37.5 ␮min 0.7 s. The scanner is demonstrated for imaging highly topographic silicon test samples and a semicrystalline polymer undergoing crystallization in air, while images of a polymer and a living bacteria, S. aureus, are obtained in liquid. © 2009 American Institute of Physics. ͓doi:10.1063/1.3227238͔ I. INTRODUCTION levers, enhancing the scanner system rigidity, and/or increas- ing the feedback bandwidth by using fast electronics and The atomic force microscope ͑AFM͒ is one of the most control methods. Each of these approaches is particularly widely used tools for imaging and manipulating nanoscale suitable for a particular application. structures in many areas of science and technology including Taking a different approach, Humphris et al.5 con- biology, nanotechnology, and material science. Its versatility structed a high-speed scanning probe microscope making use and wide applicability come from the combination of high of the resonant oscillation of the scan stage, and achieved a spatial resolution, relatively noninvasive nature, ease of tip velocity of more than 20 cm/s. A high-speed scanning sample preparation, and the large number of different mea- near-field optical microscope and an AFM have been suc- surements of surface properties that can be made with a cessfully implemented by this approach.5,6 The high-speed single instrument.1 However, a major disadvantage of com- AFM, termed VideoAFM,7 significantly enhanced the scan- mercial AFMs is the low rate at which images can be cap- ning speed of AFM by using a micromachined tuning fork tured, which limits their application for both the study of oscillator as a fast scanner. Through the use of very soft dynamic processes that occur at high rate, and for rapid sur- cantilevers and a pinning or “direct” force that maintains face inspection at nanometer resolution for industrial quality tip-sample contact, the probe tip is forced to track the sample control purposes. surface at the high tip velocities used for imaging. This high The limitation in imaging speed arises mainly from the tip velocity limits the scan size in VideoAFM systems to requirement for high mechanical stability of the AFM scan 4–5 ␮m, larger areas being acquired by tiling multiple system, from the bandwidth of the feedback loop that main- smaller images. tains tip-sample contact, and from the response time of the Although high-speed operation of AFMs has been inves- cantilever. Several approaches have been adopted in the at- tigated and realized by several groups, most of the previous tempt to improve imaging speed. By using a flexure structure instruments sacrifice scan size to ensure a stable and linear to increase the rigidity of the scanner system and small can- image. For many applications, such as following processes tilevers to respond fast, Hansma et al.2 have attained fast occurring at a cellular scale, for surface patterning, or for scan images up to 13 ␮m at 4 frames/s. The use of complex more general surface inspection, there remains a need for control algorithms for the scanner motion that learn from and rapid scanning over the scale of tens of micrometers, i.e., then counteract the scanner’s instability have pushed this over scan areas comparable to those found in most commer- even further, with line frequencies up to nearly 8 kHz. High- cial AFMs. Here we present a new sample scanner capable of speed imaging by using a cantilever with an integrated pi- large area high-speed scanning, and its integration into an ezoelectric actuator has been reported by Sulchek et al.,3 atomic force microscope. minimizing the mass that must be moved by the z-feedback loop. Ando et al.4 have developed a high-speed AFM system II. INSTRUMENTATION using a rigid scanner and fast electronics with small cantile- vers, and obtained a scan area of 240ϫ240 nm2 with 25 A. Analysis of VideoAFM tuning fork scanners frames/s in dynamic mode. Most research has focused on In previous works,5,6 the resonance of a quartz crystal increasing the scan speed by using smaller/integrated canti- tuning fork was utilized to provide the fast scan axis of 0034-6748/2009/80͑9͒/093707/5/$25.0080, 093707-1 © 2009 American Institute of Physics Downloaded 03 Oct 2009 to 133.28.47.30. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp 093707-2 Zhao et al. Rev. Sci. Instrum. 80, 093707 ͑2009͒ rather the lateral vibration is coupled to a rotational oscilla- tion resulting in the surface of the fork experiencing a varia- tion in height of approximately 200 nm. As this variation occurs at a frequency that is considerably greater than the bandwidth of the conventional AFM feedback, it will result in a variation in force over the sample surface of 5–10 nN depending on the exact stiffness of the VideoAFM cantilever used, as well as the formation of a background signal due to variation in cantilever deflection across the image. Such a background is indeed observed in unflattened experimental VideoAFM data, and could not be explained by a FEM that did not include the tuning fork in the model.9 Such out of plane oscillations are not desirable. For a robust large scan FIG. 1. ͑Color online͒͑a͒ and ͑b͒ show a FEM of a tuning fork used for area scanning system that uses resonance, tuning forks are VideoAFM scanning. In ͑a͒, the color scale shows the total displacement not suitable unless the added mass can be very significantly from rest, while in ͑b͒ it shows only the displacement perpendicular to the plane of the fork. ͑c͒ shows the total displacement as a function of time from reduced or somehow counterbalanced. the point at which the displacement is 3 ␮m, while ͑d͒ shows the corre- sponding vertical displacement. B. Scanner design For resonant scanning, any object with a well-defined a scanning probe microscope. The Infinitesima Ltd. resonant frequency can be used. Ideally this should have a VideoAFM uses AFM cantilevers attached to a quartz tuning high quality factor ͑Q-factor͒, the resonance to be driven fork, scanning the tip rather than the sample and hence al- should be well separated in frequency from other resonances lowing larger samples to be imaged. More recently, an alter- of the body, and motion should be largely constrained to a native approach using a brass bar8 has been used for similar single plane. To avoid the problems with tuning forks out- ͑tens of kilohertz͒ high-speed scanning. In the current work lined above, we have chosen to use a macroscopic quartz bar the aim is not ultimate scan speed, but rather to obtain high with rectangular cross-section as illustrated schematically in scan speed and large area scanning. Previously we have Fig. 2͑a͒. Quartz has been widely used in mechanics and found that tip velocities greater than ϳ20 cm s−1 are diffi- electronics due to its excellent mechanical stability and ge- cult to maintain with any except the flattest samples, as the neric high Q factor. As variations in resonant frequency will force required to maintain tip-sample contact becomes so lead to changes in scan area when scanning at resonance, the large that the surface and/or tip are damaged, so simply driv- high thermal stability of quartz is also advantageous. ing the existing microtuning forks to very large amplitudes is In the new sample scanner the macroscopic quartz bar is not appropriate. In liquid, this problem is even more in evi- used as a passive resonator, the scanner consisting of a clamp dence as it is much more difficult to apply a force directly with an integrated mini piezoactuator to excite oscillation of between the tip and the sample to maintain tip-sample con- the bar, and a nanopositioning stage with its travel direction tact, and the tuning forks are inevitably heavily damped from oriented parallel to the long axis of the bar, as illustrated immersion in the liquid. schematically in Fig. 2͑b͒. Fast axis scanning is realized by A secondary issue with the use of tuning fork based the resonant oscillation of the quartz bar at its natural fre- scanners comes from the effect of adding mass to the tuning quency. The natural resonant frequency of a rigid body is fork legs. The tuning fork legs are finely tuned to act as a determined by its material property and geometric size. By pair of coupled oscillators, giving a very high quality factor controlling the length of the effective resonator, we can de- resonance. However, once masses are added to the legs, even fine resonant frequencies which correspond to different fast if this is done equally on both legs, the mechanics of the scan line rates.
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