Silicon-Based Microcantilevers

The silicon resistor is defined in microcrystalline or single-crystal silicon and usually encapsulated in silicon nitride. The thickness of the deposited silicon nitride on either side of the cantilever is adjusted so that the neutral axis of the cantilever lies inside the silicon nitride layer rather than in the silicon layer. This asymmetry in material composition ensures that the resistor is placed close to one of the cantilever surfaces for optimal sensitivity. In addition, the silicon nitride serves as 

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Recently Micro Electro-Mechanical Systems (MEMS) have been emerging as sensor platform for the development of sensors with extreme high sensitivity [8-14]. Micromachined silicon cantilevers are the simplest MEMS sensors that can be micromachined and mass-produced. Microcantilever sensor technology is an upcoming sensing technique with broad applications in chemical, physical, and biological detection. With their compactness and potential low cost, detection techniques based on silicon-based cantilevers provide a path for the development of miniaturized sensors. 

Injection molding, an economical mass production technique, has also been used to fabricate microcantilevers out of thermoplastic polymers (McFarland et al. 2004 McFarland and Colton 2005a, b). In this process, a molten polymer such as polypropylene is forced under pressure into a steel cavity (mold) the shape of the cavity defines the dimensions of both the base and the cantilever(s). Injection-molded microcantilevers have been shown to be of equal caUber to commercial silicon miCTOcantile-vers. McFarland et al. (2004) and McFarland and Colton (2005a, b) specified in detail the fabrication of injection-molded microcantilevers. Despite their advantages over silicon-based cantilever arrays, polymeric cantilever arrays are not commercially available. 

Since the 1980s, silicon-based sensors and microelectromechanical devices (MEMS) have been used ex vivo for tasks such as pressure sensing, blood chemistry analysis, flow cytometry and electrophoresis [6]. The term BioMEMS was subsequently coined to describe the wide array of tools developed using silicon as a platform. Extremely sophisticated systems are now available, such as functionalized microcantilevers for molecule adsorption, recognition and quantification, as... 

Mass-produced cantilever sensors, however, have the potential to satisfy the conditions of selectivity, sensitivity, miniature size, low power consumption, and real-time operation [5, 6], Microcantilevers are micromachined from silicon or other materials and can easily be fabricated in multiple-element arrays. They resemble miniature diving boards measuring 100 to 200 pm long by about 20 to 40 pm wide by 0.3 to 1 pm thick and having a mass of a few nanograms. Their primary advantage originates from their sensitivity, which is based on the ability to detect their motion with subnanometer precision. 

Microcantilevers are associated with AFM, which basically are gold-coated surfaces based on a silicon core, which are associated with nanomechanics for biomolecular recognition (1,2,20,25). In AFM technology, a cantilever is in direct contact with the sample surface then the bending of the cantilever is determined by optical detection of the position of a laser beam (33). In fact, this method is a versatile tool for surface characterization and provides information concerning topological variations at the molecular level. 

The cantilevers can be fabricated of any shape and from substantially any material utilized in microelectronics industry, i.e. crystalline or poly-silicon, silicon nitride, silicon oxide, polymer materials (see Note 2). The rectangular shape beams are the most frequently used in biological research. In biological sensors based on the bending method, it is important to have the cantilevers flat and in plane with the base surface. Initial offset or curvature of the beams complicates adjustment of the experimental setup, especially, if working with arrays of cantilever. For this reason, the most common material for cantilevers fabrication nowadays is single crystalline silicon. A large variety of biomolecular interactions have been detected with silicon microcantilevers. 

In the best-case situation, both types of microcantilever sensors would be grouped in an array to provide a cross platform of sensitive and selective explosive detection system. Additional co-funded (TSA/ATF) work is eurrently on going in materials development for novel microeantilevers. This involves R D of silicon carbide (SiC) based cantilevers, for improvements in material properties (e.g., less fragile eompared to silicon) and to provide a platform for wide band gap type materials, like aluminum nitride (AIN). With an AlN/SiC based cantilever, the sensor can now work in the piezoeleetric resonator mode, providing enhance response and henee sensitivity to the analyte, along with a direct measurement by frequeney/resistance response, versus the more complex optieal deteetion... 

Pinnaduwage LA, Ji HE, Thundat T et al. (2005) Moore s law in homeland defense An integrated sensor platform based on silicon microcantilevers. IEEE Sens. J. 5 774-785. 

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Signal transduction from microcantilevers is based on optical, piezoelectric, and piezoresistive methods. Optical transduction is based on measurement of displacement of reflected light from the surface of microcantilever with the help of a position-sensitive photodetector [95]. However, this is not suitable for POC devices, because a larger path length might be required, which precludes portable instrumentation. Piezoelectric and piezoresistive detection involves integration of a piezoresistive (poly silicon) or piezoelectric material, respectively, on the microcantilever. In this case, the instrumentation can obviously be miniaturized. However, the requirement for a mechanically stable environment, difficulty in functionalization of a large number of cantilevers at the same time, fabrication losses, etc. are obstacles to commercialization of devices based on microcantilevers as POC products. 

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