Investigation of Different Piezoresistive Materials to be Integrated into Micromechanical Force Sensors Based on SU 8 Photoresist

The aim of this scientific work is to present different piezoresistive materials suitable to be integrated into micromechanical force sensors. As material for the mechanical structure of the sensors SU-8 has been chosen because it features favorable characteristics, such as flexible and simple fabrication of micro components through the use of standard UV lithography for forming three dimensional geometries such as cantilevers and membranes. In addition, on the basis of a significantly lower Young’s modulus compared to silicon, great opportunities to improve the force sensitivity of such sensors are offered by SU-8.However, SU-8 photoresist does not have piezoresistive properties, and therefore it has to be combined with an additional, beneficial piezoresistive material. A well-controlled and frequently used material for piezoresistive elements is doped silicon. This paper provides an overview of characteristics such as gauge factor and temperature coefficient of resistance (TCR) for a variety of commonly used piezoresistive materials, namely metals, silicon, conductive composite materials and diamond-like carbon. As a characteristic factor for the estimated sensitivity of the force sensor, the ratio of the gauge factor k to the Young´s modulus E of the structural material is presented for the different material combinations. A classification of conventional silicon based tactile force sensors is made to build a basis for comparison. Furthermore the suitability of different piezoresistive materials for the integration into an SU 8-based sensor is investigated.