Low Noise AFMs
The enormous potential of the low noise AFM is the ability to obtain atomic resolution in aqueous environment. In order to achieve that, the cantilever needs to be driven at an amplitude comparable to the range of tip-sample forces which are in the order of a few Angstroms. A home built bespoke AFM was used since the detection of very small cantilever displacement is not possible with any current commercially available AFM.
In the development of this microscope two main improvements were made to parts of the optical beam deflection (OBD) system: the low noise cantilever detection sensor and the stability of the laser. The diameter of the laser beam in commercially available AFMs is usually broader than the width of the cantilever, causing light reflection back into the laser and instabilities of the laser light intensity. These fluctuations introduce deflection noise in the detection of the cantilever position rendering it impossible to monitor the sub-nanometre oscillation amplitude of the cantilever. Therefore, by focusing the laser beam onto the cantilever using the objective lens, noise is reduced and the amount of light reflected in the position sensitive photodetector (PSPD) is increased, which enhances sensitivity. A schematic illustration of the system is show in Figure 1.
Figure 1: A schematic illustration of the Low noise AFM
The implement ion of a focused laser beam significantly reduces backscattering but other sources can remain (e.g. measured in ambient conditions a glass slide is normally put in the laser beam which reflects light back in the laser. Even if there is no glass the liquid-air interface would be a inevitable source of backscattering. These reflections cause fluctuations in the laser beam power thus introducing noise. Optical noise can be suppressed by modulating the laser power with a radio frequency (RF) signal (typically 300-500 MHz). The RF laser power modulation generates a more stable multimode resonance in the laser which is less affected by backscattering. Although in theory higher laser power reduces the deflection noise density, mode hopping noise occurs above certain laser intensity and therefore the laser power is set below this threshold.
Figure 2: (a) The influence of RF modulation of laser noise. (b) example of small spot size. The low noise AFM allows for variable spot sizes to suit the dimension of the cantilever and improve performace. (c) Thermally limited deflection noise density in air (d) thermally limited deflection noise density in liquid.
References
- Development of Liquid-Environment Frequency Modulation Atomic Force Microscope with Low Noise Deflection Sensor for Cantilevers of Various Dimensions , Fukuma, T., and Jarvis, S. P., Review of Scientific Instruments, 77, 043701, (2006).