Capacitive MEMS Microphones With A Low-stress Ultra-thin Vibrating Membrane

With the development of Micro-Electro-Mechanical Systems(MEMS)technology,the core sensing components of traditional sensors are gradually becoming miniaturized,and Replaced by the MEMS chip which compatible with microelectronics processes and highly integrated with ASICs.For example,traditional electret microphones have been gradually replaced by MEMS microphones.In recent years,the rise of smart Internet of Things technology has not only provided more room for growth for MEMS microphones,but also raised more and higher technical requirements for MEMS microphones.Therefore,even if MEMS microphones have undergone more than 30 years of longterm research and development,their research still faces new challenges.At present,most domestic manufacturers can only do product packaging and module development of MEMS microphones,and do not master the core technology of their MEMS chips.They have long been subject to foreign MEMS chip research and development companies in core technologies.Therefore,the research on MEMS microphones and the exploration of the core structure and process of MEMS devices are of great significance.This paper will explore the implementation of MEMS microphone devices from three aspects: simulation design,process design and processing,and electrical acoustic testing of MEMS microphones.The simulation design mainly uses the finite element simulation software COMSOL to model the microphone device in three dimensions,simulate and calculate the main performances such as the pull-in voltage,diaphragm resonance frequency and frequency response of the microphone,and obtain the optimal device structural parameters,such as film stress and film thickness,etc.The process design and processing part designs a detailed process flow based on the simulated device parameters and is implemented by a semiconductor process.In the realization of process and device structure,this paper studies the problem of film stress and film bubble defects in detail,and proposes a method for trade-off stress and bubble defects in MEMS process.In addition,a lot of research experiments have been done in this paper.The etching selectivity ratio of reactive ion etching silicon nitride and silicon is explored,and detailed experimental data is presented.Through this experiment,a polysilicon diaphragm with a stress of 23.5 MPa was obtained,and the thickness of MEMS microphone diaphragm was thinned from 400 nm to 100 nm by reactive ion etching.The electrical acoustic test of MEMS microphone mainly performs C-V test,D-f test,sensitivity test,distortion test,etc.Then analyzes and compares the changes of microphone sensitivity,signal-to-noise ratio and distortion after the diaphragm is thinned.This paper successfully designed and implemented a microphone MEMS chip,which is characterized by a low stress,ultra-thin diaphragm.The size and stress of the MEMS microphone diaphragm generally determine the performance of the device.As the microphone chip becomes smaller and smaller,reducing the size of the diaphragm will result in a decrease in signal-to-noise ratio.The innovation of this paper lies in the proposed small-diameter diaphragm whose stress is reduced to a lower level by optimizing the deposition parameters and using rapid thermal annealing techniques.The diaphragm stress of this experimental sample is 23.5 MPa.Moreover,the etching selectivity ratio of silicon nitride to silicon in reactive ion etching is studied in detail,and the diaphragm is thinned to 100 nm,which is almost the ultimate thickness.First,the diaphragm thinning improves the signal-to-noise ratio of the MEMS microphone.The signal-to-noise ratio of this experimental sample is up to 64.18 d BA,with an average of 62.3 d BA.Second,the diaphragm thinning greatly reduces the bias voltage of the MEMS microphone.The bias voltage of this experimental sample is around 3.5 V,which greatly reduces the power consumption of the chip.The overall performance parameters of the MEMS microphone achieved in this paper have reached a high level,the sensitivity is-38 d BV,the total harmonic distortion is less than 0.5%,and the maximum sound pressure level is 121.6 d BSPL.Moreover,the realization of low-stress,ultra-thin diaphragms has wider application value for MEMS devices.