Surface-micromachined MEMS microphones using membrane center holes for higher sensitivity and frequency response

Abstract

We achieve high sensitivity and frequency response surface-micromachined MEMS microphones composed of the membrane center holes and back plate supports. The surface-micromachined MEMS microphone having advantages of small size, low cost, high reliability, productivity and CMOS compatibility shows low sensitivity and poor frequency response due to the limitation of the surface micromachining process. The low sensitivity is mainly due to the small air gap height and the shallow back acoustic chamber depth. We propose the membrane center holes and the back plate supports. The air resistance of the air gap has a large effect on the frequency response of the microphone. The proposed membrane center holes reduce the air resistance at the center of the membrane so that the frequency response could be improved. The back plate supports having filled deep trenches allow the selectively deep etching of the back acoustic chamber and prevent the deformation of the back plate after the back acoustic chamber formation. Consequently, the sensitivity and the uniformity could be improved. The proposed surface-micromachined MEMS microphones are fabricated by using CMOS compatible MEMS processes. Deep trench of 100 μm depth is fabricated and filled to make the back plate supports and the etch-preventing wall. Polyimide sacrificial layer is used for the sensing air gap and the xenon difluoride etching of the silicon substrate is used for the deep back acoustic chamber. During the fabrication, The residual stress and deformation of the sensing membrane is minimized by using the PECVD (Plasma Enhanced Chemical Vapor Deposition) silicon nitride stress-control layer which is inserted in the aluminum sensing membrane. In the experimental study, we verify that the membrane center holes could successfully improve the sensitivity and the frequency response of the surface-micromachined MEMS microphones. After back acoustic chamber formation, the membrane height is reduced to 1.8 ± 0.1 μm f...