Yoshinobu Yasuno

High-Performance Condenser Microphone With Single-Crystalline Silicon Diaphragm and Backplate

This paper presents a high-performance silicon condenser microphone fabricated with a new process using single-crystalline silicon. This simple fabrication process, which requires only two photolithography steps and two wet-etching steps, is suitable for low-cost mass production. We designed the structure of a high-performance microphone and simulated it with an equivalent acoustic-circuit model. We then fabricated a prototype microphone based on this design, and experimental measurements on the prototype confirmed its excellent acoustic characteristics, such as a high sensitivity of -43.5 dB, a wide frequency range of 30 Hz to 20 kHz, and a high maximum sound pressure level (1 kHz at 1% THD) of 122 dBSPL. The measured equivalent noise (A-weighted) is 30.5 dBASPL. The measured frequency responses showed good agreement with those estimated from the simulation, indicating that the high controllability of the process enabled us to fabricate the prototype as we designed it. These results show that it is feasible to economically mass produce such high-performance microphones for purposes ranging from broadcasting to consumer use

Soft X-ray Charging Method for a Silicon Electret Condenser Microphone

A novel charging method using soft X-ray irradiation for forming an electret of a silicon condenser microphone was developed. Soft X-rays can penetrate through the microphone diaphragm and are suitable for ionizing gas inside the microphone element. The surface potential of a positively biased SiO2/Si3N4 dielectric film negatively increased with irradiation time, since induced anions and electrons are dragged toward the film by an applied electric field. The charging rate is inversely proportional to the bias voltage and electrode-gap distance. Charged electrets have excellent retention properties; therefore, the method is promising for the realization of silicon-electret condenser microphones.

Electret charging method based on soft X-ray photoionization for MEMS transducers

An electret charging method employing soft X-ray irradiation for microelectromechanical systems (MEMS) electret transducers is investigated. Soft X-rays can penetrate obstructions/narrow gaps and ionize gas molecules inside air gaps. This allows embedded electrets even after fabrication processes and vertical electrets on the sidewalls of high-aspect-ratio structures to be charged. The charging properties of polymer electrets subjected to soft X-ray irradiation were systematically examined. The surface potential of embedded electrets increased linearly with increasing irradiation time. The potential could be precisely controlled by the applied bias voltage. The surface potential of vertical electrets after charging was uniform to a depth of 20-30 times the gap opening. Since the photoionization rate depends on the collision diameter of the gas molecules with X-rays, a high charging rate was realized by using Xe gas. In the present charging method, photoionization was found to play a dominant role in generating electrons from gas molecules. Experiments performed at different gas pressures revealed that the effects of photoelectric phenomena and carrier displacement could be neglected. The present charging method was found to have a charge stability as high as that of corona charging. These results indicate that the present charging method has the potential for designing various new MEMS electret transducers.

Water-repellent coating for mobile phone microphones

SummaryA water-repellent coating material has been developed which is composed of low molecular-weight poly(tetrafluoroethylene) (PTFE) particles and anatase-type photocatalytic TiO2 nano particles, fluoro-oil (perfluorinated polyether) and a polyvinylidene fluoride binder. The presence of these PTFE particles in the coating material makes it possible for the water-repellency properties of this material to be improved by achieving a water contact angle of more than 150°. The photocatalytic activity of the TiO2 particles incorporated in the water-repellent coating inhibits the surface contamination which previously had caused the deterioration of the water-repellent properties. This water-repellent coating can be applied to mobile phone microphones. The water repellency of the electret condenser microphone improves the durability of the mobile phone.

A basic concept of direct converting digital microphone

An electroacoustic system which directly converts analog acoustic signals to digital electric signals is described. The system consists of a subtractor, a sampling and holding circuit, a sigma–delta modulator as a comparator, an accumulator, and a local direct digital-to-analog converting transducer similar to a typical electronic analog-to-digital converter. The subtractor is an electrostatic device which has a diaphragm, driving electrodes, and a detecting electrode. The surface area of the driving electrodes corresponds to the significant bits in the digital signal, as an electroacoustic digital-to-analog converter. The detecting electrode produces an electrical signal proportional to the displacement of the diaphragm driven by subtracting the received acoustic signal from the electrostatically driven force. This is regarded as a subtractor. The detected signal is amplified and sampled-held and modulated by the sigma–delta procedure and generates a signal of ±1 bit, which is added to the accumulator me...

Charging efficiencies and heat resistance in three types of SiO 2 /Si 3 N 4 electrets

Charging efficiencies and heat resistance of three types of silicon dioxide/silicon nitride double-layer electrets were investigated. These electrets were fabricated with SiO₂ films deposited from SiH₄ via plasma-enhanced chemical vapor deposition (PECVD SiO₂) or with tetraethylorthosilicate (TEOS) using PECVD (TEOS SiO₂) or were thermally grown SiO₂. Electrets with thicker SiO₂ films were found to have higher charging efficiencies and heat resistance than those with thinner SiO₂ layers. Moreover, the expected lifetimes of the SiO₂ electrets were significantly longer than that of a fluorinated ethylene propylene copolymer electret.

Sensitivity change with practical use of electret condenser microphone

Dr. Sessler and Dr. West invented the electret condenser microphone (ECM) in 1966. It has since been applied in various ways as a sound input device. The ECM has become an important component as a microphone for communications because of its stable sensitivity frequency characteristic. Materials and production methods have been improved continually up to the present. In particular, the ECM reliability is based on the electret’s stability. For that reason, the electret surface charge decay is the main factor in ECM sensitivity degradation. This study analyzed the changes of an ECM preserved for 28 years in the laboratory and actually used for an outdoor interphone unit for 29 years. The change of diaphragm stiffness and electret surface voltage were compared with the evaluation result of a heat‐acceleration test and verified. A degradation estimate of sensitivity change of ECM was performed. Regarding the life of the electret predicted in the report of former study [K. Miura and Y. Yasuno, J. Acoust. Soc. Jpn. (E) 18(1), 29–35 (1997)], the validity was verified using actual data from this long‐term observation.

Environmentally Robust Electret Condenser Microphone

A new structure of front filter of ECMs has been introduced to protect in practical use from such common hazards as water or alcohol. This protection structure is composed of acoustically transparent mesh with a water-repellent coating on an electrically-conductive layer. This water-repellent coating has a contact angle of more than 150 degree for water on the filter having less than 50 /spl mu/m aperture which chokes water invasion but is enough for acoustical transmission. The new protection structure also has an electrically conductive layer inside which copes with the induction noise problem by the effect of electrostatic shielding. The reliability of the coating was confirmed by several accelerated tests such as long term immersion in water, heat cycling in the presence of both high and low moisture, heat shocks and surface scratch. These tests showed no damage to the coating. The fabricated electret condenser microphone has met the requirements for the IEC 60526 class 7 which is 30 minutes under water at a depth of 1 meter. The induction noise was reduced by approximately 10 dB at 2 GHz.

Development of high-performance single-crystalline silicon condenser microphone

We have developed a high-performance silicon condenser microphone fabricated with a new process using single-crystalline silicon. Our fabrication process, requiring only two photolithography steps and two wet-etching steps, is simple and highly controllable, making it suitable for low-cost mass production. We designed, fabricated, and tested a prototype microphone, and the experimental results confirmed its excellent acoustical characteristics such as high sensitivity of -43.5 dB, wide frequency range of 30 Hz to 20 kHz, and wide dynamic range of 134 dBSPL. The simulated and measured frequency responses showed excellent agreement, indicating the prototype was successfully fabricated as designed by our highly controllable process. These results show that it is feasible to economically mass-produce such high-performance microphones for purposes ranging from broadcasting to mobile communications

A conceptual experiment of direct converting digital microphone

A transducer which directly converts signals from analog acoustic to digital electric is described. This consists of a subtracter, a sampling and holding circuit, a sigma‐delta modulation circuit as a comparator, an accumulator, and a local digital‐to‐analog converter similar to a typical electric analog‐to‐digital converter. The subtracter is an electrostatic device which has a diaphragm, driving electrode, and detecting electrode. The driving electrode is divided into corresponding digital signal bits by the ratio of each area which enables one to drive the diaphragm digitally and is regarded as an electroacoustic digital‐to‐analog converter. The detecting electrode makes electric signals proportional to the displacement of the diaphragm driven by subtracting the incoming acoustic signal and electrically driven force. This is regarded as the subtracter. This detected signal is amplified and sampled‐held and modulated by the sigma‐delta procedure and generates subtracted signal to + or −1 bit which is ad...