Cantilever stress sensor

Summary. Scanned probe methods for imaging electrochemical deposition on surfaces are now well established. For such methods the smface structure at the atomic scale can be measured so that surface strains may be inferred. Here we demonstrate how extremely sensitive and fast stress sensors can be constructed from atomic force microscope (AFM) cantilevers for studies of interfacial processes such as adsorption and reconstruction. The surface stress sensor has submonolayer sensitivity for use in electrochemistry, whereby simultaneous cyclic voltammograms and stress changes can be recorded. This is demonstrated with measurements of the electrocapillary curve of gold, and stress changes associated with the underpotential deposition of silver on gold (111). 

Semiconductor fabrication processes permit construction of small, sensitive, stress sensors. In fact the levers used in atomic force microscopes are almost ideal for this purpose. The combination of the mechanical properties of silicon nitride and the geometry of the cantilever mean that the lever has a high resonant firequency and a low spring constant [32]. The low spring constant is beneficial for sensor applications because it means that a small applied force can be transduced to a measurable deflection, which lies at the heart of any sensor [33]. When combined with the highly sensitive optical lever AFM detection system, both of these factors mean that this arrangement is a fast and highly sensitive stress sensor. 

Capacitive membranes sensor typically is based upon the deformation of the membrane due to the interaction between bio-probe molecules at the functionalized surface and the corresponding bio-targets. The working mechanism is similar as how cantilever surface stress sensors operate. Surface stress-based sensor is comprised of stretchable membrane over which surfaces is functionalized by bio-probes. The interaction with the corresponding bio-targets results in surface stress and finally makes the membrane deform. Figure 3 shows the schematic structure... 

Mechanical nanosensors possess comparative advantages over optical nanosensors and electromagnetic nanosensors for the measurement of nanoscale mechanical properties [2]. Examples of mechanical nanosensors include CNT-based fluidic shear-stress sensors [3] and the nanomechanical cantilever sensors [4]. 

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 use of polymer-coated cantilevers such as microfabricated beams of silicon is becoming more popular as the basis of nanomechanical sensors [11]. These devices detect physical and chemical interactions between the reactive layer on the surface and the environment [8]. When the polymer interacts with a gaseous species, it swells and causes the cantilever to bend as a result of surface stresses when used in the static mode. In the dynamic mode, the cantilever acts as a microbalance, which responds to changes in resonance frequency. Savran s group at Purdue University has been researching the micromechanical detection of proteins by use of aptamer-based receptor molecules [12]. 

FIGURE 4.3.6 Sensor output distributions of 100 simulated cantilevers (a) and a MSS (b) while a randomly distributed surface stress is applied on their surfaces. Reprinted from Ref. [34], with permission from Elsevier B.V. 

FIGURE 4.3.11 (a) Visualization of the two systems of two-dimensional stress induced on a cantilever-type nanomechanical sensor, (a) Type A system in which the stress is induced on the top surface of the coating layer, (b) Type B system in which the stress is induced at the interface between the silicon cantilever and the coating layer. Note that the dimensions and deflections of the cantilever and the coating layer are exaggerated. Thickness dependence on the deflection of the cantilever in the case of (c) Type A and (d) Type B. Poisson s ratio of coating layers is 0.30 for all plots. Reprinted from Ref [12], with permission from the American Scientific Publishers. 

Analytes adsorbed/absorbed on a cantilever sensor can induce two-dimensional stress on the surface of the receptor layer (type A) or on the interface between the receptor layer and the cantilever (type B) as depicted in Fig. 4.3.11. The type A stress is induced when the surface of a cantilever is modified by functional groups including self-assembled monolayers. Analytes that induce charge distribution at the interface by dipole interactions can cause the type B stress. Here, we focus on these two cases [12]. 

The deflection of a cantilever sensor is investigated by FEA simulation. The length, width, and thickness of the cantilever are set at 500, 100, and 1 pm, respectively. The results are depicted in Fig. 4.3.11c and d. The deflection is plotted as a function of thickness. As we have seen in the previous section, the deflection increases by decreasing the Young s modulus for the type A stress. There is an optimal thickness f,.op written as Eq. (4.3.7). The... 

Surface stress can be measured by a number of experiments, including piezoelectric elements [145], ribbon electrodes [146], and more recently developed micromechanical sensors [147-149]. It can be also measured with AFM and STM, as demonstrated by several groups [150,151]. The AFM and STM-based surface stress measurements usually detect the bending of a thin plate (cantilever) as a result of a surface stress change at one of the two surfaces. If the plate is isotropic and freely standing, the change in the surface stress can be calculated from the amount of bending by [152]... 

Kg. 1.9 Schematic diagrams of mass-sensitive gas sensors (a, b) quartz crystal microbalance (QCM) device (c) surface acoustic wave (SAW) device (d, e) microcantilever - (d) dynamic mode absorption of analyte molecules in a sensor layer leads to shift in resonance frequency, and (e) static mode the cantilever bends owing to adsorption of analyte molecules and change of surface stress at the cantilever surface (Reprinted with permission from Battison et al. (2001). Copyright 2001 Elsevier)... 

Here 1 show how the use of atomic force microscope (AFM) cantilevers as sensitive stress, mass, and temperature sensors can be employed to monitor the evaporation of microdrops of water. Starting drop diameters are always below 100 p,m. The foremost interest lies in exploring the last stages of the evaporation process. 

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