Deflection detection systems

Fig. 7. (a) Laser beam deflection detection systems as often used in ambient AFMs. 

The sensitivity of the optical deflection detection system is easily calibrated by recording an f-d curve on a stiff substrate, e.g., a glass slide or a piece of silicon. The slope of the hard wall contact region in this photodetector - piezo displacement plot must be unity, as for the movement of 1 nm in z direction the tip and cantilever move upwards 1 nm as well. This function is typically implemented in the AFM software (Fig. 2.29). 

Nakabeppu et al. [58] describe the use of composite cantilevers made from tin or gold deposited on conventional silicon nitride AFM probes to detect spatial variations in temperature across an indium/tin oxide heater. Differential thermal expansion of the bimetallic elements causes the beam to bend. This movement is monitored using the AFM optical lever deflection detection system. In order to separate thermal deflection of the beam from displacement of the cantilever caused by the sample topography, an intermittent contact mode of operation is employed. Measurements were made under vacuum so as to minimize heat loss. A more practical use of this technology is in the form of miniature chemical and thermal sensors [59]. This approach has been used to perform thermal analysis on picolitre volumes of materisd deposited on the end of a bimetallic cantilever [60]. Arrays of such devices have applications as highly sensitive electronic noses . 

For SFM, maintaining a constant separation between the tip and the sample means that the deflection of the cantilever must be measured accurately. The first SFM used an STM tip to tunnel to the back of the cantilever to measure its vertical deflection. However, this technique was sensitive to contaminants on the cantilever." Optical methods proved more reliable. The most common method for monitoring the defection is with an optical-lever or beam-bounce detection system. In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode. A given cantilever deflection will then correspond to a specific position of the laser beam on the position-sensitive photodiode. Because the position-sensitive photodiode is very sensitive (about 0.1 A), the vertical resolution of SFM is sub-A. 

FIGURE 7.4 The acousto-optical detection system. Deflection of the incident laser beam by the acousto-optic crystal. Deflection angle is determined by the frequency of the acoustic wave [683]. Reprinted with permission from John Wiley Sons. 

Fig. 1.1 (a) Schematic diagram of contact mode AFM and SEM images of (b) V-shaped AFM cantilevers and (c) tip. The basic elements of transducer (piezo positioner), cantilever and tip assembly, and ultrasensitive force detection system (optical beam deflection/photodiode detector) are common for most AFM techniques, while details may differ (reproduced with permission from [4])... 

Another kind of impulse generator-based accelerator based on a pulse transformer was developed in Novosibirsk, USSR, in the 1970s. One such unit (ELIT-1,1 MeV) has been operating at the Time-Resolved Spectroscopy Laboratory of the University of Leipzig. It is equipped with a beam deflection unit that can reduce the electron pulse width within the sample to less than a nanosecond and it has been coupled to a Fourier transform EPR detection system. 

The cantilever position is held fixed in the AFM laser head while the sample is scanned imder the tip. See Fig. 6.7. The feedback system relies on an optical detection system that monitors the cantilever vertical deflection, dz, as the tip follows the contours of the sample surface. In the constant force mode of operation, as in the STM constant current case, dz is translated into a cor-... 

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. 

The sample (or tip) is moved using a piezoelectric tube similar to that employed in STM instrumentation. Deflections of the cantilever are monitored using a suitable detection system. The most common is the optical lever detection system in which a laser beam is reflected off the rear of the cantilever and into a multisegment... 

Instrumentation. Commercial atomic force microscopes Explorer from Topometrix Inc., Nanoscope Illa Digital Instruments Co. Ltd.) which are based on the laser beam deflection detection scheme were used in conjunction with digital oscilloscopes of very stable low frequency (1-20 Hz) trigger system for lateral force (friction) measurements, and dual-phase lock-in amplifiers and function generators for force modulation measurements. Various triangular silicon nitride cantilevers were used. The lateral spring constants were determined with the "blind torsional calibration method discussed in more detail in the Appendix. 

The cantilever deflection measurement system is one of the most important elements in AFM. In FM-AFM, the cantilever deflection signal is used for producing the cantilever excitation signal as well as for detecting Af. Thus, having a clean deflection signal is important for stable cantilever oscillation as well as for sensitive force detection. 

Figure 2.8. Schematic of the contact mode AFM. The normal and lateral forces, and F, result in deflection and torsion of the cantilever, respectively. They can be monitored using an optical detection system and used for the purpose of feedback control and/or image contrast formation.

The lateral forces result in a torsion of the cantilever, and this can be measured using the same optical detection system that measures vertical deflection (see Fig. 2.8). This signal from the tip-specimen interaction leads to another operational mode, the lateral or frictional force microscope (LFM or FFM) [142, 143]. In LFM the cantilever is designed to be... 

Deflection detection schemes have been developed that use self-sensing piezoresistive cantilevers [104] or optical interferometry [105, 106]. But in nearly all practical systems cantilever deflection is measured with an optical lever system [107], as shown in Fig. 3.22. A small spot of laser light is reflected from the back of the cantilever and the reflected light is directed to an adjustable mirror. This in turn reflects the light onto a position-sensitive detector, a four-quadrant photo diode, shown schematically in Fig. 3.23. 

As mentioned above, the thermal deactivation-induced acoustic waves were detected by sensitive microphones in a majority of PA studies. In order to improve the sensitivity of the teehnique, a new detection system with optical microphone was used to detect the acoustic wave generated from the sample. In this system, the acoustic wave-induced change in the position of a laser beam on a pellicle is used to detect the signal. Instead of a microphone to detect the signal, a laser beam positioned on a Mylar pellicle (10 /u-M thickness) monitors the acoustic waves. The vibrations of the pellicle caused by acoustic waves deflect the laser beam, which is in turn is detected by a silicon photodiode (Fig. 5). 

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