Design and fabrication of a novel integrated floating-electrode-“electret”-microphone (FFEM)

In this study, a novel principle “electret” microphone is proposed and implemented by a single-chip fabrication technique. The microphone uses a charged floating electrode surrounded by highly insulated materials as “electret” to excite electric field. Net free-electric-charges (not “bonded” charges as in traditional electret) in the floating electrode can be easily charged by use of the “hot” electron technique, available using the so-called avalanche-breakdown technique of p⁺-n junction. Thus the electret microphone is rechargeable, which can greatly increase the lifetime of the microphone. Preamplifier has been on-chip integrated in a JFET type by use of ion implantation. “Electret” charges are bonded in a deep potential trap, different from that of traditional electret of which the charges are in a surface shallow potential trap, thus this microphone can operate at high temperature (high as 300°C) and has high stability and reliability. Experiments show that the prototype microphone has a 3 mV/Pa sensitivity and 2l kHz frequency bandwidth in a 1 mm ×1 mm diaphragm area. Microphone performance can be further improved by use of the optimized processes and designs. The fabrication is completely IC-compatible, hence the microphone is promising in the integrated acoustic systems