Optical beam bending technique

Figure 3. Mass diffusion coefficient D of the polyst3T ene-toluene solutions as a function of the polymer concentration. The open symbols indicate data obtained with an optical beam-bending technique [22] and the filled S3Tnbols data obtained from light scattering. The dashed line represents Eq. (5) and the solid curve represents Eq.(7).

The cantilever bending-technique requires a sensitive displacement detection such as a capacitance probe (Klokholm 1976, 1977), optical interferometry (Sontag and Tam 1986), a tunnelling tip (Wandass et al. 1988) or angular detection (e.g. laser beam deflection, Son-tag and Tam 1986 Trippel 1977 Tam and Schroeder 1988 Betz 1997 Sander et al. 1998). 

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. 

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... 

Schematic representation of an apparatus for the determination of mechanical film stresses using the bending beam technique with optical detection [150].

It is important to note that it is only possible to measure forces by optical detection of the cantilever bending, when the liquid crystal is in the isotropic phase. In the semi-ordered liquid crystal phases, the scattering of the laser beam decreases the sensitivity of the experiment. The limited temperature range is the most serious limitation of AFM force spectroscopy and can be overcome using a detection technique, based on piezo-resistive cantilevers, which is not considered here. 

The deflection of a thin, narrow beam (or disk - silicon technology) by the film stress is the most common technique for measuring film stress. The beam deflection can be measured using interferometry, an optical lever using a laser beam, or by capacitance measurements. The beam can be allowed to bend or be constrained from bending during the deposition. 

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