Mechanical specifications, construction details

AN4426

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DocID025704 Rev 1

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Mechanical specifications, construction details

A microphone is a dual-die device consisting of two components, the integrated circuit and

the sensor, which are housed in a package using techniques that are proprietary to ST.

Figure 1. MEMS microphone inside package

The sensor uses MEMS technology (Micro-Electrical-Mechanical Systems) and it is

basically a silicon capacitor. The capacitor consists of two silicon plates/surfaces. One plate

is fixed while the other one is movable (respectively, the green plate and the grey one

shown in the following figure). The fixed surface is covered by an electrode to make it

conductive and is full of acoustic holes which allow sound to pass through. The movable

plate is able to move since it is bonded at only one side of its structure. A ventilation hole,

allows the air compressed in the back chamber to flow out and consequently allows the

membrane to move back. The chamber allows the membrane to move inside but also, in

combination with the chamber created by the package will affect the acoustic performance

of the microphones in terms of frequency response and SNR.

Figure 2. MEMS transducer mechanical specifications

So basically the microphone MEMS sensor is a variable capacitor where the transduction

principle is the coupled capacitance change between a fixed plate (back plate) and a

movable plate (membrane) caused by the incoming wave of the sound.

AM17596v1

MEMS sensor

ASIC (Application specific

integrated circuit)