Microelectromechanical systems (MEMS) microphones are a critical component in modern audio technology, enabling high-performance sound capture in compact form factors. These devices leverage microfabrication techniques to achieve precise acoustic sensing, with capacitive and piezoelectric transduction being the two dominant mechanisms. Their applications span consumer electronics, medical devices, and IoT systems, driven by advancements in fabrication, noise reduction, and directional sensing capabilities.

Capacitive MEMS microphones operate based on variable capacitance between a movable diaphragm and a fixed backplate. Sound pressure waves deflect the diaphragm, altering the capacitance, which is converted into an electrical signal. The diaphragm is typically made of silicon-based materials, while the backplate is perforated to allow air damping control. A key advantage of capacitive transduction is high sensitivity and low noise, making it suitable for applications requiring high-fidelity audio. However, these microphones require a bias voltage to maintain the capacitive field, adding complexity to the system.

Piezoelectric MEMS microphones utilize materials such as aluminum nitride or lead zirconate titanate that generate an electric charge in response to mechanical strain. The diaphragm incorporates a piezoelectric layer that directly converts acoustic vibrations into electrical signals without needing a bias voltage. This simplifies the design and reduces power consumption, but piezoelectric microphones often exhibit lower sensitivity compared to capacitive variants. Recent improvements in piezoelectric material deposition have narrowed this gap, enabling their use in applications where power efficiency is critical.

Fabrication of MEMS microphones involves sophisticated processes such as sacrificial layer deposition and backplate etching. In capacitive microphones, a sacrificial layer, often made of silicon dioxide or photoresist, is deposited between the diaphragm and backplate during manufacturing. After structuring the diaphragm and backplate, the sacrificial layer is removed via etching, creating an air gap. This step is crucial for achieving the desired mechanical compliance and sensitivity. Backplate etching involves patterning holes to control air damping and reduce squeeze-film damping effects, which can degrade frequency response.

For piezoelectric microphones, the fabrication process includes depositing and polarizing the piezoelectric layer atop the diaphragm. Challenges include ensuring uniform stress distribution and minimizing residual stresses that could affect performance. Advanced techniques like atomic layer deposition enable precise thickness control, improving piezoelectric coupling efficiency.

Key performance parameters for MEMS microphones include signal-to-noise ratio (SNR) and sensitivity. SNR, measured in decibels, quantifies the ratio of desired audio signal to background noise, with higher values indicating better performance. High-end capacitive MEMS microphones achieve SNRs exceeding 65 dB, while piezoelectric versions typically range between 55 dB and 60 dB. Sensitivity, expressed in dBV/Pa, reflects the microphone’s output voltage for a given sound pressure level. Capacitive microphones often exhibit sensitivities around -38 dBV/Pa, whereas piezoelectric versions may range from -42 dBV/Pa to -45 dBV/Pa.

Directional microphones represent a growing trend in MEMS technology, leveraging multiple diaphragms or port configurations to achieve directional sound capture. By combining signals from different diaphragms with controlled phase delays, these microphones can emphasize sound from specific directions while suppressing ambient noise. This is particularly useful in smartphones and hearing aids, where isolating a speaker’s voice in noisy environments is essential. Advanced algorithms further enhance directionality through beamforming techniques.

Noise cancellation is another critical advancement, often implemented using multiple microphones to capture and subtract ambient noise. Active noise cancellation systems integrate MEMS microphones with digital signal processing to generate anti-phase signals that destructively interfere with unwanted noise. This technology is widely adopted in headphones, smartphones, and hearing aids, significantly improving audio clarity.

In smartphones, MEMS microphones are integral to voice calls, voice assistants, and audio recording. Their small size allows integration into bezels or beneath displays without compromising device aesthetics. Modern smartphones employ multiple microphones for beamforming and noise suppression, enhancing voice recognition accuracy.

Hearing aids benefit from MEMS microphones due to their low power consumption and high SNR. Directional microphones help users focus on conversations in crowded settings, while feedback cancellation algorithms prevent whistling artifacts. The miniaturization of MEMS technology has enabled nearly invisible hearing aid designs, improving user comfort and adoption.

IoT devices utilize MEMS microphones for voice control and environmental monitoring. Smart home assistants rely on far-field voice pickup, requiring high SNR and directionality. Low-power piezoelectric microphones are favored in battery-operated IoT nodes, where energy efficiency is paramount.

Emerging trends include the integration of MEMS microphones with machine learning for real-time audio processing. Edge computing enables on-device noise suppression and keyword detection, reducing latency and privacy concerns. Additionally, advancements in packaging technologies, such as wafer-level packaging, are reducing costs while improving robustness against environmental factors like humidity and dust.

Future developments may focus on improving piezoelectric materials to match the performance of capacitive microphones while maintaining power efficiency. Research into novel diaphragm materials, such as graphene, could further enhance sensitivity and frequency response. The demand for MEMS microphones will continue growing as voice interfaces and smart devices proliferate, driving innovation in fabrication techniques and signal processing algorithms.

In summary, MEMS microphones are a cornerstone of modern audio technology, with capacitive and piezoelectric transduction offering distinct advantages for different applications. Fabrication techniques like sacrificial layer etching and backplate patterning enable precise control over performance metrics such as SNR and sensitivity. Directional sensing and noise cancellation are increasingly critical, particularly in consumer electronics and medical devices. As the industry evolves, MEMS microphones will play an even greater role in enabling seamless, high-quality audio capture across diverse use cases.