Low-Noise Cantilever Deflection Sensor

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. 

To date, various methods have been proposed for cantilever deflection measurement Among the most widely used methods is the OBD method, owing to its simple setup and high sensitivity. In the method, a focused laser beam is irradiated to the cantilever backside and the reflected beam is detected with a position-sensitive photodetector (PSPD). The PSPD is t3q)ically made of a two- or four-divided-photodiode array. 

The cantilever deflection induces the displacement of the laser spot on the PSPD. Thus, the difference between the photo-induced currents from the upper and lower elements varies in proportion to the cantilever deflection. The current signals are converted to the voltage signals using current-to-voltage (/-P) converters. These 

There are several noise sources in the OBD method, which includes the Johnson noise from the transimpedance resistor in the I-V converter, the shot noise from the photodiodes, and the noise arising from the laser beam. In most of the designs, the ultimate detection limit is determined by the photodiode shot noise. However, the laser beam noise is often predominant in actual AFM. This is particularly evident in the case of liquid-environment AFM. 

In liquid-environment AFM, the laser beam is reflected or scattered at the air-glass and glass-liquid interfaces, as shown in Fig. 18.4a. 

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To clarify these points, the author and coworkers carried out detailed experiments to compare the noise measured with the actual instruments and the noise expected from the operation principle. The results revealed that FM-AFM performance in air and liquid was limited not only by the Q factor but also by the noise from the cantilever deflection sensor. By developing a low-noise cantilever deflection sensor, true atomic-resolution imaging by FM-AFM became possible even in liquid in 2005. This dramatic improvement of FM-AFM performance in liquid triggered subsequent applications of FM-AFM to the studies in biology and chemistry. 

In summary, a stilf cantilever is necessary for high vertical resolution and high stability, while a small amplitude operation is desirable for achieving high lateral and vertical resolution. To achieve the force sensitivity required for detecting the short-range force with a stiff cantilever, a low-noise cantilever deflection sensor is necessary. With these operating conditions, it has become possible to obtain true atomic and molecular resolution even in liquid by FM-AFM. 

This result was soon confirmed by another research group. Hoogenboom et ah developed a low-noise cantilever deflection sensor using a Fabiy-Perot interferometer. With this deflection sensor, they obtained molecular-scale images of bR trimers and atomic-scale images of mica by FM-AFM in liquid. ... 

Until recently, the use of FM-AFM in liquid was very limited. However, several AFM research groups have already started to use it for their own applications. Kawakatsu et al developed a low-noise cantilever deflection sensor using a Doppler interferometer and obtained atomic-resolution images of mica in liquid. Umeda et al. reduced the deflection noise by modifying the cantilever... 

T. Fukuma, M. Kimura, K. Kobayashi, K. Matsushige, and H. Yamada, Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy. Rev. Sci. Instrum. 76,053704(2005). 

Equation 18.5 is valid only when the force sensitivity is limited by the thermal vibration of the cantilever. To satisfy this condition, the deflection noise density arising from the deflection sensor (i.e., zs) must be sufficiently smaller than that arising from the thermal Brownian motion (Wzb) around /o. Owing to the low Q factor in liquid, zB around /o is nearly constant. Thus, the above condition is described by... 

T. Fukuma and S. P. Jarvis, Development of liquid-environment frequency modulation atomic force microscope with low noise deflection sensor for cantilevers of various dimensions. Rev. Sci. Instrum. 77, 043701 C2006J. 

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