Langmuir 2000, 16, 6267-6277
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Evaluation of Intermittent Contact Mode AFM Probes by HREM and Using Atomically Sharp CeO2 Ridges as Tip Characterizer Bjo¨rn Skårman* and L. Reine Wallenberg National Center for High Resolution Electron Microscopy, Department of Inorganic Chemistry 2, Lund University, SE-221 00 Lund, Sweden
Sissel N. Jacobsen and Ulf Helmersson Department of Physics, Linko¨ ping University, SE-581 83 Linko¨ ping, Sweden
Claes Thelander Department of Solid State Physics, Lund University, SE-221 00 Lund, Sweden Received January 19, 2000. In Final Form: April 17, 2000 The imaging process of the atomic force microscope (AFM) in contact, noncontact, and intermittent contact mode is still debated after more than a decade of widespread use, in particular when imaged features are approaching atomic dimensions. Several models for the interaction between the tip and the surface have been suggested, but generally they all need an exact description of the geometry of either the tip, the surface, or both. We present here a tip characterizer with close to reproducible geometry, exactly known angles of all surfaces, and sharp features with close to atomic dimension. It has been tested on three commercial AFM probes and a laboratory-made electron-beam-deposited tip, sharpened by oxygen plasma etching. High-resolution transmission electron microscopy has been used to unambiguously verify the tip shapes down to atomic dimensions, both before and after imaging in intermittent contact mode. The effect on the recorded AFM images is shown of tip shape, tip wear, spallation, and accumulation on the tip of amorphous and crystalline debris. The imaging is shown to be a dynamic event, with a continuously changing tip and occasional catastrophic events that give abrupt changes in imaging conditions. The tips are severely worn down already after scanning a few centimeters, but accumulated amorphous material may still give it imaging capabilities in the nanometer range, even with having a tip radius exceeding 130 nm. Accumulated amorphous material seems to be more important than previously believed. Procedures for tip in situ characterization and reliable imaging are suggested.
Introduction The information that can be obtained by scanning probe microscopy (SPM) depends strongly on the interaction between the probe tip and the surface. The shape of the probe tip must be carefully controlled, and its dimensions must be accurately known if the true profile of the surface is to be determined. Since real tips used on corrugated sample surfaces cannot be regarded as an infinitively sharp point probe over a flat specimen, tip effects must be taken into account when interpreting the data acquired. The tip effects occur due to that different parts on the probe tip will interact with the sample depending on the geometry of the surface, leading to a (nonlinear) convolution of the sample features with the tip shape. The fact that every atomic force microscopic (AFM) image contains information about the tip makes it possible to use the AFM image itself to determine the tip geometry. If the object is steeper or sharper than the tip, the approach is known as inverse AFM, or self-imaging. This is analogous to (direct) AFM imaging and gives a reconstruction of the tip, or rather of each point of the tip that was in contact with the sample during the image acquisition.1,2 It is commonly assumed that the AFM imaging performance * Corresponding author. E-mail:
[email protected]. (1) Montelius, L.; Tegenfeldt, J. O. Appl. Phys. Lett. 1993, 62, 2628. (2) Montelius, L.; Tegenfeldt, J. O.; van Heeren, P. J. Vac. Sci. Technol., B 1994, 12, 2222.
can be substantially improved by the use of very sharp tips. For a tip to be considered “sharp”, that is, for it to be capable of creating images without significant convolution, it must have an end radius less than the smallest radius of curvature of interest in the sample, and it must have an opening angle less than the angle of the steepest feature of the sample. However, it has been shown that even very blunt tips can be used to achieve high-resolution images over flat surfaces,3,4 which suggests that a blunt tip may in fact have one, or several, small protrusions, which are acting as the actual sensing tip. Another tip effect may arise if the shape of the tip changes during scanning,5-7 e.g., tip wear, tip disruption, or adsorption of surface contaminants on the tip. When scanning relatively soft materials, such as biological or polymer materials, one can assume that the tip shape is maintained (unless surface material has been adsorbed onto it). On the other hand, if the sample consists of hard solid material and exposes sharp features, the tip shape is very likely to be deformed, either abruptly, or gradually (3) Gru¨tter, P.; Zimmermann-Edling, W.; Brodbeck, D. Appl. Phys. Lett. 1992, 60, 2741. (4) Sheiko, S. S.; Mo¨ller, M.; Reuvekamp, E. M. C. M.; Zandbergen, H. W. Ultramicroscopy 1994, 53, 371. (5) DeRose, J. A.; Revel, J.-P. Microsc. Microanal. 1997, 3, 203. (6) Heuberger, M.; Dietler, G.; Schlapbach, L. J. Vac. Sci. Technol., B 1996, 14, 1250. (7) Neves, B. R. A.; Vilela, J. M. C.; Russell, P. E.; Reis, A. C. C.; Andrade, M. S. Ultramicroscopy 1999, 76, 61.
10.1021/la000078t CCC: $19.00 © 2000 American Chemical Society Published on Web 06/22/2000
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during the scanning. Since such effects are difficult to anticipate, they are not easily corrected for, and consequently, the resulting images are difficult to interpret. To minimize the risk of erroneous image interpretation, the tip should be regularly checked in situ, by means of inverse AFM using a well-defined sample surface, a “tip characterizer”. The ideal tip characterizer would be an infinitely narrow spike, yielding a perfect image of the tip. Something close to this has been achieved, where unique structural features of the sample surface, i.e., needles or columns, were inversely imaged.1,8,9 The region around the tip apex is however not imaged correctly, since the columns have a finite thickness. Using narrower columns would be a simple solution, but these columns would bend or break during AFM scanning. Characterizers consisting of lithographically patterned arrays of square pillars have been used.10,11 The corners of these pillars could generate an image of the tip, but the limited sharpness of these corners and the size uncertainty of the pillars makes them a poor choice. Spheres of latex12,13 or gold14-16 are good candidates, since knowledge of the exact shape of the tip characterizer is crucial for the success of inverse AFM. They are available commercially with diameters ranging from 5 to 500 nm with a narrow size distribution. The verification of the actual tip shape is however not straightforward.5,15,17 Latex spheres offer a wide size range, but are not readily available in the interesting 1-10 nm range, where also charging could cause problems.18 It is likely that the smallest gold particles (150b 120b 4-10 120° b
