Micromachined silicon cantilevers and tips for bidirectional force microscopy

Scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs), known under the expression scanning probe microscope (SPM), are well established methods to analyze surfaces, and the principle of their operation is published widely [1-3]. In AFM a point probe reacts mechanically on interatomic forces of the sample and generates a topographic view of the surface with an atomic resolution. The mechanical part typically consists of a cantilever as a spring element with a sharp tip at its end. In the field of solid-state sensors and actuators, micromachining techniques were developed to become the appropriate tool for cantilever and tip fabrication [4]. Silicon dioxide (SiO 2) as well as silicon nitride (Si3N 4) cantilevers have been reported [5]. Recently also a polysilicon cantilever was presented [6] using a sacrificial-layer technique. In microelectronics itself such S T M / A F M are employed as profilers. Thereby the chip surface is measured in three dimensions, which allows in the comparison with an ideal chip a control of quality and even the localization of defects. For this application it would be very important to have arrays in order to reduce the scanning time, which is only possible with batch-fabricated heads. Similar requirements hold for writing with SPM and maybe also for the observation of dynamic processes. The advantage of using thin films to form cantilevers for S T M / A F M is the possible high aspect ratio to obtain a high stiffness in x and y direction (in the plane) and an adjustable stiffness in z direction (out of the plane), without affecting the resonance frequency. We have fabricated such SiO 2 cantilevers [4] which have been successfully used in AFM applications [3]. Recently, some interest has arisen to measure not only the force in the z direction but also parallel to the scanning direction (say y direction) especially to study tribological behaviour of surfaces. The effect of this force at the tip is either pushing the cantilever sideways or, when this one is too still in this direction, twisting it. Both possibilities have been applied for friction measurements. For the torsional type, micromachined cantilevers have been used [7,8], whereas in the case of the lateral spring, only conventional techniques have been reported, e.g. by Neubauer et al., a round wire with iridium, tungsten and diamond tips [9]. For a micromachined cantilever, one with a square cross section would be ideal to realize equal spring constants in both directions. This is no longer achievable with thin film cantilevers since the width of the beam should