Articles techniques et présentations

MEMS microphones consist of a conductive backplate and a conductive flexible membrane. The backplate and the membrane are at a certain distance from each other to be interpreted as parallel plates of a capacitor. When a bias voltage is applied to the electrodes, membrane deflection caused by received acoustic signals are converted into electrical signals which are then processed by the ASIC signal chain. The microphone consists of a central region and an outer perimeter region. The central region is the active region of the microphone which is acoustically active. This central region is surrounded by the outer perimeter region that is acoustically inactive. In the outer perimeter region, there is a formation of inactive capacitance between the membrane and substrate as well as between back plate and substrate. Further breaking down each capacitance, the total membrane and substrate capacitances consist of capacitance between membrane and bond pad for electrical connection and capacitance due to membrane and substrate overlap. Similarly, the total back plate and substrate capacitances consist of capacitance between the backplate and metal trace and back plate and bond pad as shown in Figure 1. The parasitic capacitance between the substrate and membrane here is zero as both are sorted together. The capacitor's acoustically inactive section deteriorates the electro-acoustics performance of the microphone due to parasitic capacitance. This parasitic capacitance affects the signal quality of the microphone. Therefore a good estimation of parasitic, in general, is very important to help in improving the acoustic characteristics of the microphone. The microphone consists of a system of stacked layers whose thicknesses plays an important role in estimating the parasitic capacitances. The fringing field effect is also taken into the account.

2D and 3D models have been implemented to estimate the parasitic capacitance in the AC/DC Module of the COMSOL Multiphysics^® simulation software. Electrostatics physics setup is used to calculate the capacitance between the diaphragm and substrate and backplate and substrate. Electric potential and electric field distribution can be easily plotted with the help of COMSOL Multiphysics^® simulation software. All the inbuilt material properties have been used in the model. The effective parasitic capacitance is calculated by integrating over the desired surface.

The estimated parasitic capacitance between the membrane and substrate from the simulation is 0.031pF. And the estimated parasitic capacitance between the back plate and substrate is 0.079pF. The total parasitic capacitances added together is 0.11pF which is slightly lower than the measured data acquired empirically that gives 0.12pF. There is an approximate ~8% difference between the two results which is good enough to give a rough estimate about the degradation in the acoustic performance of the microphone.