The principle of CMOS silicon microphone and the principle of capacitive micro microphone

Principle of CMOS silicon microphone head

With the rise of smart phones, the need for sound quality and lightness and shortness has attracted more and more attention. In recent years, noise suppression and echo cancellation technologies have been widely used to improve sound quality.

Compared with traditional electret microphones (ECM), capacitive microelectromechanical microphones are made of silicon semiconductor materials, which facilitates the integration of analog amplifier circuits and ADC (Σ-? ADC) circuits to realize analog or digital microelectromechanics Microphone components, as well as the manufacture of miniaturized components, are very suitable for light, thin and short portable devices.

Principle of Capacitive Microphone

MEMS Microphone is a miniature sensor. The principle is to use the force gradient generated by the sound change to cause the acoustic diaphragm of the capacitive micro microphone to be deformed by the sound pressure interference, thereby changing the capacitance value between the acoustic diaphragm and the silicon back plate.

The change of the capacitance value is converted by the capacitance voltage conversion circuit into the output change of the voltage value, and the MEMS sensor generates a voltage amplified output, thereby converting the sound pressure signal into a voltage signal. Here, a high-impedance resistor must be used to provide a bias voltage VPP for the MEMS sensor, so as to generate a fixed charge on the MEMS sensor. The final output voltage will be proportional to the deformation of VPP and the diaphragm? D.

The deformation of the diaphragm is related to its rigidity. The lower the rigidity, the greater the deformation; on the other hand, the output voltage is inversely proportional to d (air gap), so the lower the air gap, the better the output voltage and sensitivity, but this Will be limited by the pull-in voltage of the MEMS sensor, that is, limited by the maximum limit value of the electrostatic field of the MEMS sensor

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CMOS MEMS microphone circuit design

In the design of CMOS micro microphones, the circuit is a very important link, which will affect the operation, sensing, and system sensitivity of the micro microphones.

The induced charge of the electret capacitor micro microphone is generated by the electret charge provided by the electret material itself, while the condensed capacitor micro microphone uses a bias voltage extracted from the CMOS operating voltage. A high-impedance resistor is provided to the acoustic diaphragm of the micro microphone to provide a fixed source of charge.

At this time, if the acoustic diaphragm is driven by sound pressure to produce a displacement change, the voltage of the electrode plate will change. Finally, by amplifying the signal through the circuit amplifier, the circuit design of the analog microphone can be realized;

If you add a ∑-? ADC analog-to-digital conversion circuit, you can complete the circuit design of the digital microphone (generally, the output signal of the digital microphone is a 1-bit PDM output). From the perspective of the manufacture of MEMS microphones, as far as the current technology is concerned, MEMS components with integrated CMOS circuits can be divided into three types.

Technology: first make MEMS structure, then make CMOS components; Intra-CMOS MEMS

Process: CMOS and M EMS component process mixed manufacturing; Post-CMOS MEMS

Process: First realize the CMOS components, and then proceed to the MEMS structure manufacturing.

Generally speaking, the first two methods cannot be carried out in traditional fabs, while Post-CMOS MEMS can be produced in semiconductor foundries.

The manufacturing method of Post-CMOS MEMS. Special attention should be paid to the Post-CMOS MEMS process, and additional heat treatment or high temperature process should not be allowed to affect the physical characteristics of the CMOS components and the stress state of the MEMS, so as not to affect the initial stress of the diaphragm.