Silicon microcantilevers

Silicon microcantilever sensors that can be mass-produced using currently available microfabrication techniques, however, have the potential to satisfy the conditions of sensitivity, miniature size, low power consumption, and real-time operation [2], Microcantilevers are generally micromachined from silicon wafers using conventional techniques. Typical dimensions of a micromachined cantilever are 100 p,m in length, 40 p,m in width, and 1 xm in thickness. The primary advantage of a cantilever beam originates from its ability to sensitively measure displacements with sub-nanometer precision. Sensitive detection of displacement leads to sensitive detection of forces and stresses. 

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. 

The key to achieving chemical selectivity using microcantilevers is the ability to functionalize one surface of the silicon microcantilever so that explosive molecules will be preferentially bound to that surface upon... 

Pinnadnwage L. A., Thnndat T., Gehl A., Wilson S. D., Hedden D. L., and Lareau R. T., Desorption characteristics of nncoated silicon microcantilever surfaces for explosive and common nonexplosive vapors. Ultramicroscopy, 100(3), 211-216, 2004. 

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. 

Photomechanical Responses of Azobenzene-Containing Materials. Ji et al. (2004) reported that a self-assembled monolayer (SAM) of azobenzene molecules on a silicon microcantilever created a photon-driven switch. Commercial silicon microcantilevers were electron-beam coated on one side with 3nm of primer layer (chromium), followed by 20 nm of gold. An azobenzene derivative with a decanthiol substituent on one ring was used to functionalize the gold surface. The thiol (-SH) end attached to the gold surface and the other azobenzene end disposed outward. 

Figure 13.22. Bending responses as a function of time for silicon microcantilevers coated with (a) a monolayer of the trans-azobenzene compound on the gold surface, (b) a bare gold surface, (c) a monolayer of 1-dodecanethiol on the gold surface after exposure to a 365 nm UV light. Source From Ji et al., 2004. Reprinted with permission.

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. 

Novel mechanical tests are being developed and shown to be useful tools to investigate thermal and structural relaxation. These include the modulated-temperature-thermomechanometry technique (mT-TM) which has been recently employed to investigate blends of core cross-linked (CCS) PS and PMMA (Spoljaric et al. 2011) and nanomechanical thermal analysis. By using silicon microcantilever deflection measurements, the latter technique can provide a measure of temperature-dependent thermal stresses and therefore investigate the influence of physical aging (Yun et al. 2011). 

Amfrola J, Rodriguez A, Castaner L, Santos JP, Gutienez J, Horrillo MC (2005) Micromachincd silicon microcantilevers for gas sensing applications with capacitive read-out. Sens Actuators B 111-112 247-253 Balkus KJ, Ball LJ, Gnade BE, Anthony JM (1997) A capacitance type chemical sensor based on AlPO -5 molecular sieves. Chem Mater 9 380-386... 

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. 

Fig. 15.2. Fabrication of silicon nitride microcantilevers with integrated tips, (a) A...

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. 

One of the most important issues for simulation of microsystems is to know the properties of the materials. In the case of microcantilevers, young modulus, Poisson s ratio, and density are the main parameters to be considered. If monocrystalline silicon is used, as reported in this work, the mechanical properties are well-known (see Table 1). For an anisotropic material such as silicon, the Young s modulus, Poisson s ratio, and shear modulus depend on which crystal direction the material is being stretched. The appropriate values for each direction have to be introduced in the model. 

The structural materials that conform the cantilevers can have intrinsic stresses and stress gradients because of the fabrication processes. These can change the mechanical performance of the microcantilevers. If stresses and stress gradients are high, they must be previously measured by using specific test structures and their values should be included in the simulations. As monocrystalline silicon will be used in the fabrication of our devices, these stresses can be neglected. 

Fig. 3. (A) Static method for determining the cantilever spring constant. Force-distance curves obtained over a hard surface and over a fabricated microcantilever. The inset shows the method configuration scheme. (B) A profile of silicon cantilevers fabricated by the technology described above. The cantilever is flat, but the stresses at the anchoring area produce initial cantilever displacement. The profile has been measured by an optical confocal profilometer.

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. 

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... 

Figure 2. Bending response for a silicon cantilever coated with self assembled monolayer of triethyl-12-mercaptododecyl-ammonium bromide on the gold surface of a microcantilever as a function of the change in concentration of...

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. 

Lang W (1996) Silicon microstructuring technology. Mater Sci Eng R 17 1-55 Lang HP, Gerber C (2008) Microcantilever sensors. Top Curr Chem 285 1-27... 

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