12 years of resonator: From telephone to atomic force microscopy

Abstract

The history of MEMS resonators other than comb-drive is fairly short, 15 years at most. At the beginning, researchers together with engineers had the dream to replace quartz by silicon and they started working on resonators designed for filter application in cell phones. Today, MEMS resonators are surpassed by bulk acoustic wave technology and confined to time reference application. A couple of years ago, we decided the reorientation of our work on MEMS resonators towards atomic force microscopy (AFM) by fabricating integrated probes. In the World, besides US groups, like the Veeco group at Santa-Barbara, which does a lot of work for the AFM in liquid environment, there are still a few teams that aim to use dynamic AFM in liquid. The NanoScience Center from Munster (Resp. Fuchs) working with a private company (Atomic Force) has developed a signal processing that produces an artificial low damping coefficient in fluid. Also, it is worth to note that numerous teams from Japan are doing much effort to achieve an AFM oscillating system in liquid environment. For example, the Kawakatsu's group is focused on the development of an AFM in water with an oscillator vibrating at 20 MHz. Ando's Group has successfully imaged myosin motion onto a mica substrate with levers vibrating at 450 kHz (1 MHz in air) with 12 images/second within a 250 nm scanning window. High aspect ratio tip fabrication can be hardly achieved with such small lever dimensions. In order to clear this technological lock, deep changes are required. A paper reporting a joint study from Georgia Tech and Stanford [Onaran et al., Rev. Scientific Instruments 77 023501 2006]has shown an AFM tip mounted on a vibrating membrane. Using carefully designed micromachined mechanical structures with spring constants in the 1 N/m range and noise levels down to 10 fm/√Hz, they achieved piconewton force resolution with 10 kHz measurement bandwidth with this device. The dynamic modes of the AFM allow measurement of force variations under the picoNewton range. That is the reason why so many labs try to use the oscillating modes to probe soft matter or biological nanosystems dynamics in liquid environment. At present, these attempts face the difficulty of the liquid viscosity which dampens the oscillating cantilever. In order to minimize the hydrodynamic drag, we propose to change the overall oscillator and to choose an oscillation mode that reduces the liquid velocity gradient around the resonator. More precisely, the project aims to develop GHz MEMS/NEMS sensors for a new generation of high sensitivity Atomic Force Microscopes (AFM). This AFM will be a tool for in situ imaging of biological and chemical systems with a resolution better than the nanometer and the possibility of kinetic spectroscopy in liquids. In addition, it is intended to batch-fabricate the device in order to ease the possible industrial transfer thanks to the cost-effective approach.