Cantilever-Based Tactile Sensor with Improved Sensitivity for Dimensional Metrology of Microcomponents

Abstract:
The paper reports on a highly sensitive tactile force sensor for dimensional metrology of microstructures comprising large aspect ratios, steep sidewalls, and deep narrow drillings. The sensor consists of an up to 7-mm-long cantilever (width 150 µm, height 380 µm) serving as a tactile element suspended by a thin silicon membrane hinge (thickness 100 µm). The membrane hinge comprises an implanted quadratic p-well as piezoresistive stress sensor element with four peripheral p+-contacts. The sensor exploits the pseudo-Hall effect in silicon. Upon application of a mechanical stress sigmaxx and biasing two opposite contacts, an offset voltage VpH is measured between the orthogonal contact pair. To investigate and improve the stress sensitivity of these stress sensing elements, 90×90 micrometer2 p-wells comprising non-conducting islands of different sizes were implemented. The device is microstructured using deep reactive ion etching of single-crystal silicon. In contrast to existing processes, the thicknesses of membrane and beam can be chosen independently. In this way it is possible to optimize the sensor with respect to mechanical stiffness, i.e. force sensitivity, and beam size, i.e. minimal drilling diameter to be characterized. In comparison to sensor elements without non-conducting island, the introduction of a non-conducting region of 60×60 micrometer2 improves the sensitivity of the sensor by 41%. This improvement is consistent with finite element simulations. Depending on the size of the non-conducting island, stress sensitivities between 7.74 mV/V/mN and 10.9 mV/V/mN are achieved.