• AFM Probes and AFM Cantilevers

    An AFM cantilever with an AFM tip on its end is the main sensing component ultimately responsible for the quality of AFM imaging. The most common and at the same time very sensible scheme of data acquisition is an optical one based on registration of a laser beam reflected from the backside of the AFM cantilever with a sectioned (position sensitive) photodiode. For better reflection the backside of AFM cantilevers is often covered with aluminium or gold. There are a number of other deflection registration techniques and related peculiarities of AFM cantilever construction among which piezocantilevers are worth of particular consideration [160, 161, 210, 211, 390, 921, 922, 1382]. However, this report is limited to the description of AFM cantilevers for the optical registration scheme.

    AFM cantilever parameters

    • Material of the AFM cantilever
    • Geometry of the AFM cantilever
    • Stiffness of the AFM cantilever
    • Resonance frequency of the AFM cantilever
    • Q-factor

    AFM tip parameters

    •  Material of the tip

    • Geometrical parameters of the AFM tip

    There are a number of AFM tip defects, which cause artifacts. The most common of them are:

    a) Multiple peaks at the apex comprising atomic scale protrusions. Every peak during scanning contributes in the tip-sample interaction. In the simplest case of a double peak apex the features on the sample surface look double in the scans (Fig. 3, d). Absence of this defect is especially crucial when measuring single macromolecules.
    b) Blunt apex. This results in lowering of resolution power. The sharpness of the apex tends to decrease during consecutive contact mode scans of the sample surface (Fig. 3, a,b,c).
    c) Non-spherical apex. Results in geometry distortion of sample features.

    (a) 700 x 700 x 20 nm (b) 700 x 700 x 16 nm
    (c) 800 x 800 x 12 nm (d) 400 x 400 x 16 nm

    Fig. 3.  Imaging of sharp edges of CdF2 films grown in <111> orientation.
    a, b, c) Decreasing of AFM tip sharpness in the set. d) Double tip artifact.
    Image courtesy of Prof. Sergey Gastev, St. Petersburg

    As a matter of fact, the yield of AFM cantilevers with "good" AFM tips is occasionally not very close to 100% due to defects of one type or another including the most common mentioned above. Thus, development of a simple AFM tip shape characterization technique is quite desirable. Moreover, scanning probe techniques are utilized as the means of choice for critical dimension metrology (CDM) applications  [837, 1608, 1612] increasingly in recent years, and the AFM tip shape geometry becomes a crucial factor of success.

    Problem of tip-sample convolution. Methods of AFM tip characterization.

    Achieving the best possible resolution will always remain the outmost goal for many SPM studies. There are, though, some technical challenges to be overcome as well as fundamental limitations on the way to high resolution.

    One of the technical issues that has to be considered is the imperfect geometry and finite size of the AFM tip. The best approximation to the ideal AFM tip geometry (omitting various natural disturbing factors like thermal noise) is believed to be a carbon nanotube of several nanometers in diameter, several micrometers long and with a single atom at the sharp conical apex. Most of the commercial AFM probes are too far from this ideal case. In general, a conventional AFM tip is unable to penetrate high aspect ratio structures, to touch every point on the sample surface and to profile exactly surfaces with complex topography. In this way, the finite size of the AFM tip and its imperfectness contribute significantly to distortion of images.

    There are also some physical restrictions in achieving subnanometer resolution besides technical issues. Due to the long-range nature of van der Waals forces acting between AFM tip and sample, resulting force is determined by the mean interaction of a large number of atoms from both the AFM tip and the sample surface especially when the features are comparable in size with AFM tip apex. Therefore, the features of the sample surface become diluted by this interaction of collective nature.

    Thus, actually the image is a complex convolution of the AFM tip and the surface shapes. This convolution is unavoidable but there are ways to reconstruct a rather accurate image from the diluted one using special mathematical methods.

    The earliest attempts to formulate the task mathematically date from papers of Reiss et al. [1633] and Keller [1634]. Their works have become the basis for several similar methods of deconvolution where some simple particular geometries, e.g. spheres or parabolas are considered. These methods require intensive computation and evaluation of numerical derivatives and, therefore are comlicated enough for practical implementation.

    Another approach to solve the problem of deconvolution relies on mathematical morphology. This approach has been proposed and developed by many authors [1637] beginning from works of Gallarda and Jain [1635] and Pingali and Jain [1636]. It is applicable to general shapes (any AFM tip and sample which can be expressed as an array of heights in the usual fashion), and does not require numerical derivatives.

    These methods have one point in common - it is necessary beforehand to estimate the AFM tip geometry in order to perform proper deconvolution procedure thereafter. One of the most popular methods for AFM tip shape and size determination is based on the so-called AFM tip characterizers, which are the features of well-defined geometry at the nanoscale. Characterizers may be highly ordered edges of crystal facets (SrTiO3 [1603], MgO and NaCl [1604]), nanoparticles of well-defined spherical form [1605, 1607], sputtered cones [51] or nanoparticles [27] of InP, spike-like features in hydrothermally deposited ZnO films [268], macromolecules [788, 1455] and many others []. Additionally, specially designed calibration gratings can also serve as AFM tip characterizers. A computer analysis of the obtained scans can help restore the AFM tip shape and at the same time deduce what kind of defect the AFM tip contains.

    An alternative widely used approach for AFM tip shape determination has come to be known as "blind reconstruction". The term "blind" means that there is no need of a priory knowledge of the exact characterizer's actual geometry. Since the publication of the principles of general blind reconstruction by Villarrubia [1639] equivalent or similar methods independently are discovered later [1643, 1644]. The algorithms of blind reconstruction are published in [1645] and a speedier version is described and tested in [1646]. It is experimentally verified to work in a comparison between blind reconstruction of an AFM tip and an independent method [1647]. In other work Todd and Eppell report on the improvement of this method in respect to spatially anisotropic noise which generally takes place at the nanoscale and introduces an error in the AFM tip geometry determination [801]. The latest advances in evaluation of AFM tip performance in the blind reconstruction method can be found in the papers of Nie et al. [1654, 1656].

    Another modern technique developed by S. Xu et al. [23] is called nanografting. It is based on subsequent imaging of a thiol self-assembled monolayer. The authors state that this method features simplicity, high speed and the ability to characterize the very top portion of the AFM tip. Moreover, AFM tips with multiple asperities, which are difficult to investigate using other approaches, can be easily identified and characterized via nanografting.

    We would like to express special thanks to John Villarrubia from the National Institute of Standards and Technology (Gaithersburg, MD, USA) for the constructive discussion on the content of this paragraph.

    Please, send all comments and suggestions concerning these pages to info@mikromasch.com

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    1628 A Novel Self-Assembled Monolayer Coated Microcantilever for Low Level Cesium Detection
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    2357 Quality assessment of atomic force microscopy probes by scanning electron microscopy: correlation of tip structure with rendered images
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    2281 Microfabrication of a combined AFM-SNOM sensor
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    1831 Microprocess for fabricating carbon-nanotube probes of a scanning probe microscope
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    2291 Modeling of cylindrically tapered cantilevers for transverse dynamic force microscopy (TDFM)
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    1835 Quantitative Analysis of the Magnetic Properties of a Carbon Nanotube Probe in Magnetic Force Microscopy
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    1836 Reduction of Long-range Interactions using Carbon Nanotube Probes in Biological Systems
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    2506 Thermomechanical noise of a free v-shaped cantilever for atomic-force microscopy
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