Piezo-z-position [nm]

Figure 3. Cantilever deflection versus sample-z-position curve on a stiff surface monitored via SFM.

Figure 3. Cantilever deflection versus sample-z-position curve on a stiff surface monitored via SFM.

techniques is given in [29] while in [30] results obtained by SFM in the field of nanotribology are reviewed.

Sensing of the z-position is usually done via the monitoring of the z-piezo voltage. This can cause ambiguities as piezo crystals exhibit hysteresis. This is overcome by monitoring the distance separately via inductive or fiber-optical sensors [31].

For dynamic modes [32] the application of an additional oscillation to the cantilever by a piezo crystal has to be performed. Magnetically driven cantilevers are used as well for actuation [33].

The quality of SFM data is essentially determined by the cantilever and the tip, influencing the resolution of topography and force measurements. Micromechanical properties of the cantilever and the shape and chemical composition of the tip, which comes into direct contact with the sample, are essential. High aspect ratios and small tip radii are desirable for imaging steep slopes and deep crevices. Depending on the mode of operation, different parameters have to be optimized, which can be realized according to the methods described in the following.

Silicon and silicon nitride cantilever fabrication based on photolithographic techniques is well established. Metalbased (Ni) cantilevers [34] and cantilevers made of piezoelectric material (lead zirconate titanate) are produced as well for independent actuation and sensing [35]. Cantilevers of various shapes (e.g., rectangular or V-shaped) and dimensions (usually 100 to 200 /m in length) are available. New approaches to a further miniaturization of the system have been made on microfabricated aluminum probes with length scales of 9 / m [36-39].

Coatings (Cr-Au, Pt, Al, TiN, W2C, TiO2, Co, Ni, Fe, Au with biological coating) can be applied to the cantilever to modify it against corrosion in the liquid phase or for different applications when conductive, magnetic, or biological properties are necessary.

The resonance frequency of the cantilever ranges between several kHz to several hundred kHz. Cantilevers with high resonance frequencies are used in dynamic modes as the tip oscillates with several hundred kHz above the surface.

The stiffness of the cantilever is defined by the force constant k and ranges between 0.01 and 100 N/m for the vertical deflection. The soft (k < 0.1 N/m) cantilevers are used in contact modes to minimize the disturbance of the sample. Rigid cantilevers with a force constant larger than 1 N/m are used in noncontact or dynamic modes since they exhibit high resonant frequencies and small oscillation amplitudes of about several nanometers. The force constant for the lateral twisting can be determined for friction measurements [40] and the movement of the cantilever has been modeled accordingly with finite element analysis [41]. The mechanical behavior and the determination of the spring constant are well described in literature [42-49].

The tip itself can consist of different materials or is coated according to the application. For some applications a hard surface is necessary and a diamond-like coating (DLC) is applied [34]. The production of DLC coatings is described in [50]. Different functionalities [51-53] can be applied. Cantilever tips can be modified by chemical [54] and biochemical [55] functionalization. Via silanization [56, 57] or via thiols, self-assembled monolayers (SAMs) are produced for further modification [53]. Proteins and bacteria [58] can be attached to the tip.

Additional materials which vary the shape of the tip can be deposited, such as polystyrene, borosilicate and silica spheres, C60 molecules [59], carbon-nanotubes in general [60-63], or single-wall nanotubes with diameters of <3 nm [64] or functionalized nanotubes [65]. The first are, for example, used to have a defined contact area. The latter are important because tips produced by lithographic methods are of pyramidal shape but do not present the necessary sharpness for high-resolution images. As the tip con-volutes with the surface features [66], its shape determines accordingly the lateral resolution, and the width the possibility to enter small features. Electrochemical etching can produce sharper tips with an open angle near the apex of 20°. For sharper tips an extra tip is grown on the apex of the conventional pyramidal tip using electron beam deposition (EBD) [67], which leads to a curvature radius of 2 to 20 nm, or the above-mentioned nanotube-functionalized tips are used. The real tip shape can be calculated and, by deconvolution, hidden information extracted [68-71]. The resolution of steep slopes can additionally be improved by slow scanning and a repeated z positioning at each pixel [72].

Usually the typical scan range is up to 150 /m but also larger scan areas can be reached with a scan area up to 25 mm2 [72].

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