Magnetic resonance force microscope (MRFM), which is one kind of SPM technology using microcantilever-based force sensors for molecular imaging, has succeeded in detecting a single electron spin. The next challenge is to detect a single proton spin, which has a magnetic moment 500 times smaller than that of an electron spin. Therefore, cantilever force sensors with much higher sensitivity are required. Nanocantilevers exhibit unprecedented force sensitivity and lower air damping effect than that of microcantilevers. In addition, the fundamental resonance frequency of nanocantilevers can achieve to several hundred megahertz which is matched to the spin procession frequency.
In this paper, we propose a piezoresistive nanocantilever for MRFM application. The sensitivity of the cantilever is inversely proportional to its width and the square of its thickness. Furthermore, it is reported that p-type silicon with width less than 300 nm and thickness less than 100 nm exhibits a giant piezoresistive effect. With above considerations, the designed nanocantilvers have a thickness of 100 nm, widths varying from 100 nm to 500 nm, and lengths from 2.5 um to 10 um.
The fabrication process starts from boron diffusion using a spin-on dopping method on a high resistivity SOI wafer. Electron beam (EB) lithography and fast atom beam (FAB) etching is utilized to fabricate the thin beam of the cantilevers. After FAB, electrodes are formed by plasma sputtering of aluminium and lift-off process. Finally, the nanocantilevers are released from the SOI substrate by deep reactive ion etching (DRIE), buffered hydrofluoric acid (BHF) and critical point drying. Though some cantilevers are bended due to the tensile stress after boron diffusion, the piezoresistive nanocantilvers are successfully fabricated after optimal mask design.
The high resonance frequency of the piezoresistive nanocantilever leads to a high bandwidth of the force sensor. However, it also presents a barrier in measurement due to the impedance match problem since the output impedance of the cantilevers is several hundred kilohms. The nanocantilvers are optically driven and a piezoresistive frequency modulation method is utilized for measuring the piezoresistive nanocantilevers.
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