Fabrication of a Novel MEMS Capacitive Microphone using Lateral Slotted Diaphragm

In this paper, we have fabricated a new microphone with lateral slotted diaphragm positioned over the back plate electrode using 8 anchors. It consists of a 3µm thick aluminum (Al) as a diaphragm, a 0.5µm as a back plate electrode and 1.3 µm thick resist (AZ1500) as a sacrificial layer on 4 inches silicon wafer. The novelty of this method relies on diaphragm which includes some lateral slots to reduce the effect of residual stress and stiffness of diaphragm for increasing the microphone sensitivity and also air damping in the gap has been reduced laterally. By this way, it is not needed to make acoustic holes in back plate, thus the effective surface of diaphragm has been increased and it causes to increase the microphone capacitance. Compared with previous works, the chip size of this microphone is reduced, the complex and expensive fabrication process can be avoided and the sensitivity can be increased. The measured zero bias capacitance of microphone is 17.5 pF, and its pull-in voltage is 25 V. The capacitance of the microphone was measured as a function of the applied bias voltage using C-V analyzer. The capacitance increased with increasing bias voltage due to the decrease in the air gap thickness as the Al membrane is electrostatically pulled towards the back plate. The microphone has been tested with external amplifier and speaker, the external amplifier was able to detect the sound waves from microphone on speaker and oscilloscope. The maximum amplitude of output speech signal of amplifier is 45 mV, and the maximum output of MEMS microphone is 1.125 µV.