Silicon nitride cantilevers

For imaging applications, minimisation of the damage to DNA molecules by the AFM tip is essential. One method of less-invasive imaging is based on phase (rather than height) imaging, which employs for instance 100 mm long silicon nitride cantilevers with narrow arms inserted into a fluid cell [65]. Alternatively, or better complementary, the operation in liquids [66], and... 

The minimum detectable signal for cantilever bending depends on the geometry and the material properties of the cantilever. For a silicon nitride cantilever of 200 pm long and 0.5 pm thick, with E = 8.5 x 1010 N/m2 and v = 0.27, a surface stress of 0.2 mJ/m2 will result in a deflection of 1 nm at the end. Because a cantilever s deflection strongly depends on geometry, the surface stress change, which is directly related to molecular adsorption on the cantilever surface, is a more convenient quantity of the reactions for comparison of various measurements. 

Cantilevers are usually microfabricated from silicon by using conventional pho-tomasking and etching techniques. Typical dimensions of a cantilever are 100 pm in length, 20 pm in width, and 1 pm in thickness. Silicon and silicon nitride cantilevers and cantilever arrays that utilize optical beam deflection for signal transduction are commercially available. Piezoresistive cantilever arrays are also commercially available. Piezoresistive cantilevers are 120 pm in length, 1 pm in thickness, and 40 pm in width. 

A homemade SMFS with a silicon nitride cantilever (Park, Sunnyvale, CA) was used. Each tip was calibrated by using a standard sample. The spring constants of these cantilevers were in the range 0.010-0.012 N/m. By moving the piezo tube, one could bring the sample into contact with the AFM tip so that some polymer chains were physically adsorbed onto the tip, resulting in a number of bridges . As the distance between the tip and the substrate increased, the chains were stretched and the elastic force deflected... 

Current instruments differ little, in principle, front the first AFM developed over I0 years ago. The biggest changes have come in the methods used to detect the deflection of the cantilever spring, as mentioned above, and the development of microfabricated silicon nitride cantilevers with integrated pyramidal tips. 

Figure 5.9 SEM micrographs of a triangular silicon nitride cantilever with integrated tip (a) 550 x and (b) higher magnification view.

The gold coating on the as-supplied silicon nitride cantilevers is stripped with aqua regia and a clean (111) oriented gold surface is then deposited by vacuum evaporation on the tip side of the lever. The gold film deposition is carried out at a pressure of less than 10 mbar and at a temperature of 270 C and the films are then annealed at the same temperature for 4 h before being allowed to cool down slowly (Balzers UTT400... 

Fig. 5 Scanning electron micrographs of a silica microsphere glued with polymer glue to the end of a silicon nitride cantilever (a). The polystyrene particle shown to the right was sintered at 120°C to the cantilever (b).

Silicon or silicon nitride cantilevers, such as those used in atomic force microscopy, are typically 100-200 pm long, 20-40 pm wide, and 0.6-pm thick (available from Digital Instruments, CA, and Park Scientific, CA). We have used commercially available silicon microcantilevers (Park Instrument, CA) in these experiments. The dimensions of the V-shaped microcantilevers were 200-pm length, 20-pm width, and 0.7-pm thickness. One side of the cantilever had a thin film of chromium (3 nm) followed by a 40-nm layer of gold deposited by e-beam evaporation. 

Prepare the cantilever Etched silicon Ccintilever substrates are generally used for NC-AFM or TM-AFM, and silicon nitride cantilevers are used for C-AFM. In both cases, the cantilever probe should be inspected under the microscope when being used for first time. Use the sharp-pointed tweezers to remove the cantilever substrate from the container. Grasp the sides of the substrate, away from the lever and probe tip. Be very careful about avoiding any contact with the probe lever, since it will immediately snap off. Silicon is very brittle. 

Photodiode Alignment After the laser beam is on the tip of the cantilever, adjust the photodiode positioner to maximize the sum signal on the elliptical bar graph (at the bottom of the scanner). This adjustment is much less sensitive than the laser position adjustment. The maximized value should be approximately 5 - 9 V for silicon nitride cantilevers. The value of this signal varies with many factors. It is important to note that it is possible to see a large response on the elliptical bar graph without having the laser beam on the cantilever. So it is important to visually verify that the... 

Fig. 7.2. Scanning electron micrograph of a microfabricated silicon nitride cantilever commonly used for force measurements. (A polystyrene microsphere glued at the end of the V-shaped cantilever can be seen pointing upward. The scale bar corresponds to 10 pm.) Reprinted from [10]. Copyright 1992, with kind permission from the American Chemical Society...

Methods. Physical adsorption. PEO was adsorbed either onto the silicon nitride cantilever tip or onto a piece of silicon nitride coated silicon wafer. The substrates were cleaned by placing them in an oxygen plasma (200 mm Hg, 25 W) for 5 minutes. Adsorption of the PEO to the clean surface was accomplished by placing the cantilever or the wafer piece in a 0.1% w/v solution of PEO in water. Adsorption was allowed to take place for at least 18 hours (sometimes as much as a week). The substrate was then placed in water for 22-24 hours to allow desorption of the weakly bound polymer chains. The substrate was vacuum dried prior to use. 

The silicon nitride cantilevers with tip were treated with an oxygen plasma to remove carbon contamination from the surface. Amine groups were incorporated onto the surface by placing the cantilever in a 5% v/v 3-aminopropyltriethoxysilane (APS) in water solution for 10 minutes and then rinsing thoroughly with water (20). 

Table 1. Summary of ESCA data for silicon nitride cantilevers as received and oxygen plasma modified and for a silicon nitride coated silicon wafer as received, oxygen plasma modified, APS modified, PEG bound and PEO adsorbed. The values are in atomic percent. The numbers in the parentheses in the carbon column indicate what percentage of carbon is in the ether form...

Eaton et al. used FV mapping for imaging of a phase-separated blend of PMMA and poly(dodecyl methacrylate) (PDDMA) [190,191). Unmodified silicon nitride cantilevers were used in ambient conditions in air or in water using a liquid cell. Differences in pull-off forces correlated with features in topography, making it possible to distinguish between the PDDMA-rich... 

Fig. 1.31. An electron micrograph showing the presence of a single E. coli bacterium on the surface of a silicon nitride cantilever oscillator which is coated with antibodies for this bacterium. Placeholder. (After Ilic et al. (2000).)...

Instrumentation. Commercial atomic force microscopes Explorer from Topometrix Inc., Nanoscope Illa Digital Instruments Co. Ltd.) which are based on the laser beam deflection detection scheme were used in conjunction with digital oscilloscopes of very stable low frequency (1-20 Hz) trigger system for lateral force (friction) measurements, and dual-phase lock-in amplifiers and function generators for force modulation measurements. Various triangular silicon nitride cantilevers were used. The lateral spring constants were determined with the "blind torsional calibration method discussed in more detail in the Appendix. 

Figure 2. Contact mode AFM ims es of surfaces after contact at 15 nN (top row) and 150 nN (bottom row). Images are at 1pm x 1pm and obtained with a silicon nitride cantilever, k = 0.58 N/m.

Figure 6. Lever deflection vs. sample displacement for all surfaces (IV deflection = 15 nN force, silicon nitride cantilever k=K).58 N/m). Measured surface penetration is less than 2 nm for all surfaces up to loads of 150 nN with the exception of elastomers.

Table IL Calculated surface stiffness and tip penetration depths for several materials using a silicon nitride cantilever (ks0.58N/m, tip radiuss40 nm) and...

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