Long-Term Effects of Relative Humidity on the Performance of ZnO-Based MEMS Acoustic Sensors

This paper investigates the long-term repercussions of relative humidity on capacitance and dissipation factor tan δ of ZnO-based MEMS acoustic sensors. During the fabrication process, a ZnO layer covered with a 0.3-μm-thick PECVD layer was sandwiched between two aluminum (Al) electrodes on a 25-μm-thick silicon diaphragm made by a bulk micromachining technique. The fabrication of an acoustic sensor chip was then completed by etching a ZnO layer in the presence of strong acid (HCl) and weak acid (NH₄Cl with electrolytically added Cu ions), separately. Post fabrication, under the humid environment conditions prevailing over a long period of time, viz., 150 days, with relative humidity between 60% and 80%, the capacitance values were found to be 1.5 times higher than the original values in the case of strong acid. The corresponding losses tan δ increased from 0.03 to 0.06. However, under the same conditions, the capacitance values did not change for the acoustic chips fabricated using weak acid. The deterioration in frequency and sensitivity responses of the packaged device has been also observed in the case of etching using strong acid. The investigations showed that a 0.3-μm-thick PECVD silicon dioxide as a passivating layer could protect the sensors from ambient humidity over a long period of time, because of a positive slope of a ZnO edge. However, the response of the devices for a negative slope of a ZnO edge was affected due to nonuniform step coverage of a ZnO layer.