The atomic force microscope (AFM) is one of the primary instruments used in research and industry to analyze materials and objects, being able to provide measurements of topography and material properties with nanometer-scale resolution. At the heart of the AFM is a microcantilever with a sharp probe tip, which is scanned over the surface of a sample while mapping the varying intermolecular forces that exist between the probe tip and the sample.
The AFM is conventionally implemented as a macroscale system, with the instrument itself being many orders of magnitude larger than the size of the scan window. The in-plane scanning of the microcantilever is typically performed using piezoelectric tube scanners or flexure-guided nanopositioners, while the deflection of the cantilever is measured using a laser and an optical sensor. In general, the relatively large size, complexity, and cost of conventional AFMs restricts their use to specialized laboratory-based applications.
As an extension of our ongoing research in the field of MEMS nanopositioning, LDCN has developed a novel silicon-on-insulator MEMS AFM that is designed to operating in tapping mode. The device features a central stage that is positioned along the in-plane axes using electrostatic actuators, while electrothermal sensors are implemented to enable closed-loop position control of the stage. A silicon microcantilever is integrated at the end of the stage, and is actuated in the out-of-plane direction using an aluminum nitride piezoelectric transducer. This transducer is simultaneously used to measure the deflection of the cantilever using a novel high-side charge sensing implementation, therefore avoiding the requirement to use a conventional laser and optical detector. The device has been successfully used to obtain tapping-mode AFM images of a sample, with an imaging range of up to 8 μm × 8 μm in closed loop.
The miniaturization of the AFM using MEMS technology has to the potential to lead to significant cost and portability improvements that may allow the AFM to move beyond its current role as a specialized scientific instrument. LDCN's research in this area is continuing with the goal of achieving video-rate AFM using a single microfabricated device.
Further details about the on-chip AFM are reported in IEEE Journal of Microelectromechanical Systems.
The fabricated on-chip AFM, with close-up scanning electron microscope images highlighting the major components of the device.
Tapping-mode images of a calibration grating obtained using the on-chip AFM.