Design and Fabrication of a Low-Stress Membrane for Microphones via a low Temperature Process
Paul C.- P. Chao
|關鍵字:||CMOS麥克風;乾蝕刻;低溫過程;低電壓;CMOS Microphone;Dry Etching;Low Temperature Processes;Low Voltage|
This paper aims to develop a novel process to fabricate a low-stressed vibrating membrane for MEMS microphones. This membrane is made of aluminum via the dry etching technique. In this way, the low residual stress in the membrane and low pull-in voltage can both be achieved by the low-temperature process to form a multi-layer structure. The simple five layers for the device are metal (aluminum), silicon, metal (aluminum) and oxide. The merit of low-temperature process offered by this proposed process is made possible by the sputtering of aluminum for membrane and the dry-etching of sacrificial layers. The experimental results show that the aluminum sputtering leads to a low-stressed membrane; furthermore, the dry-etching technique is better than wet-etching, since the etching rate of dry etching can be controlled in an easier way, and with no over-etching to other structures or materials in the device, such as by anisotropic etching effect. Note that dimples remain on the aluminum layer when the sacrificial layer is etched. These dimples can prevent the top and bottom electrodes from contacting each other, when the high voltage is applied; thus, the lifetime of electrodes through a number of operations is significantly increased. Measurements show that the residual stress of the membrane can be as low as 16 MPa, while the resonance (cutoff) frequency is up to 20 kHz, the upper limit of human audible range. It appears that the performance of the fabricated membrane reaches a level of satisfaction for a number of applications, including the MEMS microphone.