Piezoresistive cantilever

The piezoresistive readout technique has several advantages over commonly used optical beam deflection methods. For example, optical beam deflection probes the bending of the free end of the cantilever. It is assumed that the bending is uniform along the length of the cantilever. The piezoresistive method, however, measures the integrated bending of the cantilever. Piezoresistive cantilevers can be encapsulated in silicon nitride... 

FIGURE 3.12 Temporal response of the cantilever piezoresistive sensor to water vapor diluted in (room temperature, 1 atm) [94]. (Reproduced with permission Allendorf, M.D. et al., J.4 i Chem Soc 130,14404 (2008), with permission). 

Chui, B.W., Stowe, T.D., Kenny, T.W., Mamin, H.J., Terris, B.D. and Rugar, D., Lowstiffness silicon cantilevers for thermal writing and piezoresistive readback with the atomic force microscope. Appl. Phys. Leu., 69(18), 2767-2769 (1996). 

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. 

Figure 12.5 Relationship between the area under the curve obtained from the peaks shown in Figure 12.2 and the mass of adsorbed TNT calculated from the observed changes in resonance frequency of the piezoresistive cantilever.

Special cantilevers have been designed for in-vacuum as well as in-liquid operation. Piezoresistive cantilevers [205] and micromachined cantilevers with an integrated electronic sensor [206] make the deflection measurements more simple, however demonstrate lower force sensitivity. 

There exist a number of readout techniques based on optical beam deflection, variation in capacitance, piezoresistance, and piezoelectricity. Piezoelectricity is more suited for a detection method based on resonance frequency than the method based on cantilever bending. The capacitive method is not suitable for liquid-based applications. The piezoresistive readout has many advantages, and it is ideally suited for handheld devices. 

Fig. 2. Scanning electron micrograph of a piezoresistive cantilever array.

For the pattern recognition (PARC) approach, we have coated the piezoresistive cantilevers with different selective layers. Each piezoresistive cantilever had four cantilever elements. Two of these cantilevers were coated with gold, whereas the other two served as reference cantilevers. We have used four separated chips arranged into an array in a single vapor chamber. Each cantilever chip was coated with a different selective agent. The four coatings used in our study include 4-MBA, Au (evaporated), CH3(CH2)n-SH, and a complex of (i-cyclodextrin and alkane. 

Fig. 7. Response of a 4-mercaptobenzoic acid (4-MBA)-coated piezoresistive cantilever as a function to the exposure to PETN vapor. The readout electronics were designed to produce a positive signal in response to...

There are several techniques suitable for the cantilever response read-out as the optical beam deflection, piezoresistivity, piezoelectricity, interferometry and capacitance, among the most important. However, the majority of the biochemical applications carried out with the cantilever-based sensor are based in the optical beam deflection method, because of its high sensitivity and its simplicity. 

C. elegans has demonstrated free mobility behavior in liquid and on agar, and the mobility properties can be sensed by the micropiUar-based mechanical sensor in channel microfluidics. Park et al. [39] fabricated micrometer-scale piezoresistive cantilevers as force-displacement sensors, and measured the mechanical properties of C. elegans. For the same purpose, Doll et al. [40] used SU-8 force sensing pillars to characterize the scale of interaction forces that worms generated during their locomotion. 

In AFM lithography, the tip rests on the end of a cantilever, with the force (1-100 nN) between the tip and sample being determined by measurement of the deflection of the cantilever. This measurement can be optical, capacitive, or piezoresistive. Here, the tip-to-sample separation is adjusted to maintain constant force between the tip and the sample. ... 

Kim, S., Rahman, T., Senesac, L. R., Davison, B. H., and Thundat, T. 2009. Piezoresistive cantilever array sensor for consolidated bioprocess monitoring. Scanning 31,204-210. 

The deflections of a microcantilever can be measured with sub-Angstrom resolution using current techniques perfected for die AFM, such as optical reflection, piezoresistive, capacitance, and piezoelectric detection methods. One great advantage of flie cantilever technique is diat five resonance response parameters (resonance fiequency, phase, anqilitude, Q-flictor, and deflection) can be simultaneously detected. 

R. Mukhopadhyay, M. Lorentzen, J. Kjems, and F. Besenbacher, Nanomechanical sensing of DNA sequences using piezoresistive cantilevers, Langmuir 21, 8400-8408 [2005). 

A number of sensitive magnetic field detection devices have been developed as biosensors giant magnetoresistive (GMR) sensors [4], piezoresistive cantilevers [5], inductive sensors [6], superconducting quantum interference devices (SQUIDs) [7, 8], anisotropic magnetoresistive (AMR) rings [9], and miniature Hall crosses [10]. 

Integration of the detector into the cantilever has also been accomplished using the piezoresistive properties of silica. This is beneficial for samples that are light sensitive and following the movements of cantilevers in an array. It can be used in conjunction with the integrated piezoelectric actuator howevei it can be difficult to decouple the signals. 

Piezoresistive readout can be utilized by using a piezoresistive cantilever, in which a sensing element (piezoresistor) is embedded at the clamped end thereby it is sometimes called a self-sensing cantilever (Fig. 4.3.3). In contrast to the optical readout method, no bulky and complex peripherals are required because a piezoresistive readout electrically measures the change in resistance of a piezoresistor with a simple circuit (Fig. 4.3.4). By means of the piezoresistive readout, it is also possible to perform measurements even in... 

To realize the appropriate scheme for the enhanced sensitivity, it is important to note the basic properties of a piezoresistive cantilever for surface stress sensing [3]. One notable parameter is piezocoefBcient. P-type piezoresistors created by boron diffusion onto a single-crystal silicon (100) surface can be effectively utilized because of its high piezocoefficient [30-32]. Plain stress (i.e., = 0) is assumed because of the intrinsically... 

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