Fabrication and testing of an integrated force sensor based on a MOS transistor for applications in scanning force microscopy

In this article an integrated force sensor based on a stress-sensing MOS transistor is introduced for applications in scanning force microscopy (SFM). The sensor configuration will be described, and theoretical and experimental investigations of the sensitivity will be presented. With the fabrication process, consisting of a standard CMOS-process and post-processing (conventional silicon bulk micromachining), cantilevers with MOS transistors integrated at the base for deflection detection were fabricated. Furthermore, Pt-tips were grown on some of the cantilevers by using focused ion beam (FIB) deposition techniques. The cantilevers have typically a spring constant of 1 N/m, and are 400 to 950 mu m in length, and have a mechanical resonance frequency between 6.2 and 35 kHz. It was found that the stress sensitivity of the MOS transistor changed depending on gate voltage, while being independent of the drain voltage. The cantilevers were successfully used for SFM-imaging in contact-mode. The resolution is noise limited to a few nanometer in the contact-mode, while atomic resolution is expected to be feasible in the dynamic-mode. These cantilevers together with integrated circuits are expected to be well suited for mass production because of their CMOS processing compatibility.