Piezoresistive sensors on AFM cantilevers with atomic resolution

Abstract The use of AFM is simplified, its application field is extended, and its acceptance is increased by substituting the optical detection of cantilever deflections by integrated electrical sensors. Accordingly, four piezoresistors are fabricated on the existing Si cantilevers with sharp tips. The resistors with positive and negative piezoresistivity coefficients form a bridge and are located at a position of maximum strain. With a CMOS like processing technique the electrical sensors are fabricated. The major process steps are ion implantation for doping p-type resistors, e-beam lithography to protect the tip, metal evaporation for the Ohmic contacts, striplines, and contact pads. All process parameters where optimised to obtain a maximum sensitivity and to preserve the high quality of the 10 μm long and 20 nm sharp tip at the free end of the cantilever. Theoretically, the sensitivity of the piezoresistive sensors is limited by the thermal and shot noise, and by the dissipation power. Experimentally, the sensitivity of the sensors was determined by inserting the cantilever in an AFM and by measuring e. g. the atomic steps on the cleaved surface of pyrolytic graphite. By many other AFM results the usefulness of these sensitive cantilevers was demonstrated. In dynamic and static measurement modes various problems were investigated: the oxidation of allotactically grown CoSi 2 , the dislocation network of stressed SiGe layers on Si wafers, the information storage on hard disk, the topography of written compact disks, the structure of standards for the scanning microscopy.