Integrated Circuits for High-Voltage Servo-Loop in a MEMS Microphone

Micro Electromechanical Systems (MEMS) microphones are a type of microphone that is widely used in smart-phones, smart-watches, laptops, tablets, and smart assistants. The popularity of MEMS microphones can in part be attributed to their small size and low power consumption, which makes them ideal for mobile devices.

However, MEMS microphones do leave something to be desired in terms of the ratio between the acoustic sensitivity and the background noise of the microphone, also known as the Signal-to-Noise Ratio (SNR). Features like smart assistants, e.g. Amazons Alexa and Apples Siri, both in dedicated smart assistant devices and in smart-phones, have created a push from the smart-device industry for MEMS microphone manufacturers to increase the SNR of MEMS microphones, as a higher SNR makes it easier for smart assistants to distinguish voice commands.

One of the major limitations to increasing the SNR in MEMS microphones right now is a physical phenomenon called the squeeze film damping effect, which behaves as a thermal noise source and thereby limits the achievable SNR. A next generation MEMS microphone seeks to reduce the noise from squeeze film damping by encapsulating certain components of the MEMS microphone in a vacuum, however, this approach creates a new challenge.

If the backchamber of a MEMS microphone is kept at a vacuum pressure the noise from squeeze film damping is reduced, but the ambient pressure will create a pressure difference across the microphone diaphragm, which will exert a force on the diaphragm that can reduce the acoustic sensitivity and thereby result in a reduced the SNR.

To achieve a high SNR the new microphone seeks to maintain a high sensitivity by using an actively generated electrostatic force to compensate the pressure induced force. An additional challenge to this approach is that the ambient pressure is not static and the electrostatic force must be adjusted continuously to accommodate for changes in ambient pressure.

Furthermore, to generate the necessary electrostatic force the MEMS microphone requires an adjustable bias voltage of 80.0 to 200.0 V, which is far beyond the bias voltages observed in MEMS microphones today. This work presents a servo-loop configuration which can continuously adjust the generated electrostatic force in the next generation MEMS microphone based on a feedback signal of the microphone diaphragm displacement.

The servo-loop configuration makes it possible to compensate the force exerted by a non-static ambient pressure and thereby makes it possible to achieve a high SNR. To realise the servo-loop two fully integrated circuits are proposed, namely a High-Voltage (HV) bias generator and a Capacitance-to-Digital Converter (CDC).

To implement the HV bias voltage generator circuit an analysis of Charge Pumps (CPs) is carried out in two parts, where the first part focuses on the challenge of boosting a low supply voltage to 200.0 V and the second part focuses on the challenges of regulating the output voltage of a capacitively loaded CP.

Based on the analysis of CPs a cascaded CP implementation is proposed to implement a HV bias voltage generator capable of boosting 1.4 V to the voltage range 80.0 V to 200.0 V. Design of a CDC for the servo-loop is based on a review of capacitance sensing implementations found in the literature, where a Capacitance-to-Digital Conversion approach appear favourable given the area restrictions found in MEMS microphones.

A CDC implementation based on an Incremental ADC (IADC) is proposed, but because no physical version of the MEMS module is available, only the IADC component of the CDC is implemented. Two physical ICs have been designed and fabricated during the project, using a 180 nm Silicon-On-Insulator (SOI) process, for evaluation of the proposed circuits and a Hardware-In-Loop platform has been implemented to emulate the MEMS and evaluate the servo-loop performance.

The fabricated circuits have been measured by themselves to evaluate their individual performance and have also been measured in a servo-loop configuration with the HIL platform to evaluate the servo-loop performance. The HV bias generator prototype is capable of boosting 1.4 V to the voltage range 41.0 V to 188.8 V at a power consumption of 554.1 µW and to the voltage range 41.0 V to 161.2 V at a power consumption of 53.1 µW.

The measured voltage-step resolution of the HV bias generator is 20 mV. The IADC prototype has a 10-bit resolution and a sample-rate of 971 samples/second, with a differential non-linearity and integral non-linearity of less than 0.5 least significant bits. The prototype circuits for the servo-loop occupy an area of 0.710 mm2 and consumes as little as 53.1 µW when the servo-loop is configured to operate in a low-power mode.

In the low-power mode the servo-loop has a bandwidth of <1.0 Hz and when configured to operate in a high-performance mode the large-signal bandwidth of the servo-loop is 25.1 Hz. The implemented circuits are not able to meet the targeted specifications of the servo-loop, but future work is suggested on how to improve the performance of the implemented circuits.