Laser deflection

According to the distance from probe to the sample, three operation modes can be classified for the AFM. The first and foremost mode of operation is referred to as contact mode or repulsive mode. The instrument lightly touches the sample with the tip at the end of the cantilever and the detected laser deflection measures the weak repulsion forces between the tip and the surface. Because the tip is in hard contact with the surface, the stiffness of the lever needs to be less than the effective spring constant holding atoms together, which is on the order of 1 — 10 nN/nm. Most contact mode levers have a spring constant of <1 N/m. The defection of the lever can be measured to within 0.02 nm, so for a typical lever force constant at 1 N/m, a force as low as 0.02 nN could be detected [50]. 

Recently, the research laboratories of the microchip producer AMD began to use TERS for characterizing patterned silicon surfaces. Metallized AFM tips that have been prepared by sputter deposition of thin Ag films onto quartz tips and sharpened by focused ion beam (FIB) miUmg were used. With a top-illumination set-up, line profiles of patterned samples were recorded and the influence of laser deflection at the tip and laser heating on silicon stress measurements were studied [44-46]. 

There have been some initial studies of thermal-wave detection using the techniques described above. Ash and his colleagues have performed some imaging experiments with the laser interferometric technique, (8-11) while Amer and his colleagues have used both the laser interferometric and a laser deflection (surface deformation) technique for spectroscopic studies on amorphous silicon. (12-13) These various investigations were all performed at low to moderate modulation frequencies (<100 kHz) only. 

The SFM tip is placed in the near-field of the source and in contact with the sample surface acting as a detector of the SAW. Due to laser deflection detection with a 4-quadrant photodiode vertical and lateral oscillation of the cantilever can be detected with lock-in amplifiers leading to amplitude as well as phase signals... 

Micromechanical flow meters measure shear forces produced as a result of fluid flow. The deflection of a micromechanical plate due to the shear force can be measured optically using a system coupled to a laser deflection system Figure 7 shows a schematic of such a plate, encapsulated in a channel. The major interaction between the fluid and the plate is through the stress field, which contains both normal (pressure) and shear stresses. 

Scanning ion conductance microscopy has also been paired with atomic force microscopy (AFM)." For an SICM-AFM configuration, a bent nanopipette was used as the cantilever probe. Probe-surface distance was recorded by laser deflection off the back of the pipette, and ion current was collected at the nanopipette opening." With this modification, conductive pathways of synthetic polycarbonate membranes were recorded." Other hybridizations of AFM and SICM have been used to identify the mechanosensitive properties of living cells - and guide neuronal growth cone development. ... 

Figure 11.4 Laser deflections in the module (a) position of the laser sheet, (b) deflection of the laser inside the channel (top figure) and outside of the channel (bottom figure).

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

Infrared laser lines involving. .. 2p 5s —. .. 2p 4p transitions in the 3.39 pm region are not particularly usefiil. However, they do cause some problems in a 632.8 nm laser because they deplete the populations of the. ., 2p 5s states and decrease the 632.8 nm intensity. The 3.39 pm transitions are suppressed by using multilayer cavity mirrors designed specifically for the 632.8 nm wavelength or by placing a prism in the cavity orientated so as to deflect the infrared radiation out of the cavity. 

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. 

Several practical issues of the scatterometer must be considered in the case of characterizing nominally smooth surfaces. The incident laser beam may be collimated, but more commonly it is brought to a focus at a distance defined by the arc in which the detector rotates. In addition, a deflection mirror or an optical fiber might be used to direct light to the detector element. These features permit measurements close to the specular and transmitted beams, and this is critical to folly characterize the scattered light. This is especially significant since the scattered light intensity... 

Fig. 5.5. Schematic view of the deflection sensing system as used in the NanoScope III AFM (Digital Instruments, Santa Barbara, CA, USA). The deflection ofthe cantilever is amplified by a laser beam focused on the rear ofthe cantilever and reflected towards a split photodiode detector.

Yet another modification is atomic force microscopy (ATM), in which a fine tip attached to a tiny flexible beam is scanned across the surface. The atom at the end of the tip experiences a force that pulls it toward or pushes it away from the atoms on the surface. The deflection of the beam, which shows the shape of the surface, can he monitored by using light from a laser. 

Let us first consider the case where the laser beam is emitted through the telescope. The outgoing beam undergoes a tilt deflection through the turbulent atmosphere as does the backscattered light. The round trip time to the mesosphere is 0.6 ms at zenith. It is much shorter than the tilt coherence time ... 

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

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