Development and Experimental Evaluation of a Novel Piezoresistive MEMS Strain Sensor

This paper presents the experimental evaluation of a new piezoresistive microelectromechanical systems strain sensor. The sensing chip is highly capable of measuring biaxial state of strain/stress. The sensing elements are p-type piezoresistors on (100) single crystal silicon aligned along [110] and its in-plane transverse. The concept of introducing geometric features to enhance the sensor sensitivity is investigated. The results of experimental evaluation and finite-element analysis (FEA) proved the viability of this concept to improve the sensor sensitivity. The microfabrication process utilizes five doping concentrations to explore the effect of doping level on the sensor performance. The sensor is developed considering applications under varying temperature conditions. Therefore, high doping concentration (more than 1 ×10¹⁹ atoms/cm³) is favorable to reduce the sensor thermal drift. As a result, the sensor sensitivity is significantly reduced. Hence, geometric features are introduced in the sensor silicon carrier to compensate for the signal loss through stress concentration effect, which magnified the strain field in the proximity of the sensing elements. In addition, the use of full-bridge configuration reduced the overall temperature coefficient of resistance (TCR). At doping concentration of ~5 ×10¹⁹ atoms/cm³, the measured strain sensitivity is 0.035 mV/με for input voltage of 5 volts, which corresponds to an effective gauge factor of ~7 and piezoresistive gauge factor of ~44. The effective gauge factor includes all the signal losses and the effect of bonding adhesive. Design and analysis, prototyping, and experimental evaluation are presented. Finally, guidelines to select the bonding adhesive and packaging scheme are provided.