Force/Displacement Detection Using Electron Interference

Hiroshi Yamaguchi and Yoshiro Hirayama
Physical Science Laboratory

@Ultra-small force and displacement have been detected using a small bar, clamped at one end like a diving board, known as a cantilever. It is already used in practical systems, such as in the pickups for analog disks, atomic force microscopes, and the accelerometers of automobile airbag systems. Especially, the micron-scale cantilevers play essential roles in the technology using microelectromechanical systems (MEMS), which has made rapid advances in the last few years. For detecting the cantilever deflection, both optical and electrical methods are widely used. Optical methods, such as optical levers and laser interferometers, offer higher detection sensitivity than electrical ones. They have been recently used even for the detection of single spins [1]. In contrast, electrical methods are advantageous for downscaling and integration. In addition, we could strongly enhance the detection sensitivity in piezoresistive cantilevers, which is one of the most important electrical methods, using the quantum effects in semiconductor low-dimensional structures.
@At low temperatures, electrons exhibit wave-like behavior, so they can be used for highly accurate sensing just like the photons in a laser interferometer. We have succeeded in detecting the displacement of a micromechanical cantilever by using electron interference [2]. The device used was an InAs/AlGaSb pieozoresistive cantilever with the thicknesses of conductive InAs and insulating AlGaSb films of 15 nm and 285 nm, respectively. A scanning electron microscope image of a typical fabricated device is shown in Fig. 1. The resistance change induced by the cantilever deflection was measured under a magnetic field (Fig.2). The magnetic field was used to adjust the phase differences among different electron paths in the InAs films. The resistance change, i.e. the cantilever deflection sensitivity, has a strong and aperiodic B dependence, which was reproducible in repeated measurements. At 6.9 T, the resistence change had a maximum value, which was nearly one order of magnitude larger than that at zero magnetic field. This clearly demonstrates the possibility of highly sensitive quantum mechanical displacement sensing, which is promising for future micromechanical cantielver applications.

[1] D. Ruger et al., Nature 430 (2004) 329.
[2] H. Yamaguchi et al, Phys. Rev. Lett. 93 (2004) 036603.


Fig. 1 SEM image of a fabricated piezoresistive cantilever.
Fig. 2 Measured resistance change as a function of magnetic field.