University of Twente Student Theses


Low-cost silicon-based resistive load cell suitable for asymmetric loads

Leijen, Niels (2018) Low-cost silicon-based resistive load cell suitable for asymmetric loads.

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Abstract:Many different types of force sensors, also known as load cells, exist. Usually consisting of a steel or aluminum structure with resistive strain gauges attached to it to measure the deformation due to a load. These sensors cannot be easily miniaturized. For applications requiring limited thickness force sensors, e.g. in the order of a few millimeters, an alternative is needed. Existing solutions, like force sensitive resistors, suffer from large creep, hysteresis, and temperature dependence. Silicon force sensors can be a good alternative because they do not suffer from creep or hysteresis. In this thesis, a low-cost approach of realizing silicon force sensors is investigated. More specifically, it is investigated whether the boron doped silicon device layer in silicon-on-insulator (SOI) wafers can be used for piezoresistive force sensing. Fabrication requiring only a single photomask and etching step and therefore can be relatively cheap. Three sensor designs have been designed and realized, each with a different focus point in mind. Exploiting the buried oxide layer of a SOI wafer as sacrificial layer to realize freely suspended reference resistors that are used for temperature and stress compensation. The resistance of other resistors will change due to the piezoresistive character of p-type silicon. Measurements show a linear relation between the output voltage and the applied load with a sensitivity of 10 µV/N, corresponding to relative resistance changes in the order of 10-5N-1. A problem is that the sensors also show a relatively large temperature dependence, resulting in a change of output voltage in the order of 10 µV/K, indicating that better matching between force sensitive and reference resistors is needed. Furthermore, a significant creep of 8% in 10 minutes was observed due to assembly with epoxy. It can be concluded that fabrication of silicon load cells with resistive readout from a single SOI wafer is indeed possible. However, some changes in the proposed designs are necessary in order to make them less sensitive to temperature and prevent creep.
Item Type:Essay (Bachelor)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:52 mechanical engineering, 53 electrotechnology
Programme:Electrical Engineering BSc (56953)
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