Wideband micromachined capacitive microphones with radio frequency detection

Abstract

Silicon microphones based on capacitive micromachined ultrasonic transducer membranes and radio frequency detection overcome many of the limitations in bandwidth, uniformity of response, and durability associated with micromachined condenser microphones. These membranes are vacuum-sealed to withstand submersion in water and have a flat mechanical response from dc up to ultrasonic frequencies. However, a sensitive radio frequency detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing small membranes. In this paper we develop a mathematical model for calculating the expected output signal and noise level and verifies the model with measurements on a fabricated microphone. Measurements on a sensor with 1.3 mm 2 area demonstrate less than 0.5 dB variation in the output response between 0.1 Hz to 100 kHz under electrostatic actuation and an A-weighted equivalent noise level of 63.6 dB͑A͒ SPL in the audio band. Because the vacuum-sealed membrane structure has a low mechanical noise floor, there is the potential for improved sensitivity using higher carrier frequencies and more sophisticated detection circuitry.

MEMS/NEMS Techniques and Applications

“…Air-coupled non-destructive evaluation [31], microphones with RF [32] and optical [33,34] detection schemes, surface and bulk acoustic wave devices [35,36], and smart microfluidic channels [25,37] are among these applications. Since the first demonstration of CMUTs in the early 1990s, extensive research has been conducted on fabrication and modeling of this new transducer technology.…”

“…This topology may be problematic, especially when the gap height is comparable or thinner than the back electrode metal thickness. Because LPCVD poly-silicon is highly resistive (>10 6 -cm) as deposited, it can be patterned by selective doping, which does not generate a significant surface topology [32]. Although it is possible to achieve reasonable conductivity on selectively doped regions, the resistance of the ground plane remains an issue.…”

“…Then, a phase detection method can be used to detect the pressure wave that causes membrane vibrations [115,116]. Hansen et al later developed the technology for vacuum sealed cMUTs and demonstrated wideband sound detection using cMUTs and RF detection [32].…”

Wide-band piezoresistive microphone for aero-acoustic applications

“…Therefore microphones with a bandwidth of several hundreds of kHz and a dynamic range covering 40Pa to 4kPa are still needed for aero-acoustic measurements. Some research works have been done on micro-fabricated aero-acoustic microphones using different sensing mechanisms, such as piezoresistive [3,4], capacitive [5,6], piezoelectric [7] and optical [8]. Most of them have a predicted bandwidth of ~100kHz and none of them were successfully calibrated in a real acoustic field with frequency higher than 20kHz.…”

Design Approaches of MEMS Microphones for Enhanced Performance

“…For this purpose, a number of microphones have been proposed in the literature. For example, a microphone with a bandwidth from 0.1 to 100 kHz was proposed by Hansen et al [177,178]. This microphone [178] has a higher noise floor of 63.6 dBA.…”