MEMS Mechanical Sensors

A silicon-based transducer assembly coupled to a movable structure in a hearing instrument. The transducer assembly includes at least one microphone chip and an ASIC having multiple integrated components such as any combination of a DSP, an A/D converter, an amplifier, a filter, or a wireless interface. The movable structure may be a battery access door, a volume dial, a switch, or a touch pad. A protection strip can be disposed across the battery access door to prevent debris from clogging the silicon-based transducer assembly. The transducer assembly may also include an array of microphone chips to achieve adaptive beam steering or directionality. When equipped with a wireless interface, the hearing instrument wirelessly communicates with another hearing instrument or with a network.

Silicon-based transducer for use in hearing instruments and listening devices

MEMS technology is enabling the development of inexpensive, autonomous wireless sensor nodes with volumes ranging from cubic mm to several cubic cm. These tiny sensor nodes can form rapidly deployed, massive distributed networks to allow unobtrusive, spatially dense, sensing and communication. MEMS enable these devices by reducing both the volume and energy consumption of various components. This paper reviews some of the wireless sensor nodes under development and applicable MEMS devices for small and efficient optical communication, micropower generation, and sensing. In addition, CMOS post-process micromachining is discussed as a method of achieving low cost and high integration.

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MEMS for distributed wireless sensor networks

This review provides a comprehensive survey of noise research in MEMS. Some background on noise and on MEMS is provided. We review noise production mechanisms, and highlight work on the theory and modeling of noise in MEMS. Then noise measurements in the specific types of MEMS are reviewed. Inertial MEMS (accelerometers and angular rate sensors), pressure and acoustic sensors, optical MEMS, RF MEMS, surface acoustic wave devices, flow sensors, and chemical and biological MEMS, as well as data storage devices and magnetic MEMS, are reviewed. We indicate opportunities for additional experimental and computational research on noise in MEMS.

Noise in MEMS

An integrated optical interferometric detection method for micromachined capacitive acoustic transducers is reported. The back electrode of the capacitive transducer on a transparent substrate is shaped as an optical diffraction grating and the displacement of the transducer membrane is determined with interferometric resolution by measuring the intensity of the reflected diffraction orders. By applying voltage to deflect the membrane electrostatically, the detection sensitivity is kept at the optimum level and transmission signals are generated. Initial experiments on devices microfabricated on quartz substrates show a minimum detectable displacement of 2×10−4 A/√Hz around 250 kHz and low frequency detection capability for microphone applications. Ultrasonic pulse-echo experiments in air at 750 kHz are also demonstrated using both a HeNe laser and a 850 nm vertical cavity surface emitting laser as the light source.

Integrated optical interferometric detection method for micromachined capacitive acoustic transducers

The first differential silicon microphone is presented. This capacitive working device consists of two backplates with a membrane in between. Due to the balanced arrangement the air gap can be minimized. Thus, a higher electrical field and sensitivity can be achieved for low voltages. A dedicated process sequence has been developed in order to get the optimum mechanical and electrical properties for all structural layers. Furthermore, a sandwich structure has been developed to achieve a reproducible, very sensitive microphone membrane with a thickness of only 0.5 /spl mu/m and a stress of 45 MPa. The total sensitivity for a bias of 1.5 V was measured to be 13 mV/Pa and the A-weighted equivalent input noise was measured to be 22.5 dB SPLA. The upper limit of the dynamic range has been determined to be 118 dB SPL and the total harmonic distortion at 80 dB SPL is below 0.26%.

The first low voltage, low noise differential silicon microphone, technology development and measurement results

The first results of silicon microphones that are completely batch-packaged and integrated with signal conditioning circuitry in a chip stack are discussed. The chip stack is designed to be directly mounted into a system, such as a hearing instrument, without further single-chip handling or wire bonding. The devices are fully encapsulated and provided with a well-determined interface to the environment. The integrated microphones operate at a bias of 1.5 V and are expected to reach a sensitivity of 5 mV/Pa, an A-weighted equivalent input noise of 24 dB sound pressure level, and a power consumption of about 50 μW in the near future, thereby living up to the tight specifications of microphones for hearing instruments. Other potential applications include mobile phones, headsets, and wearable computers, in which space is constrained.

K. Rasmussen

Papers

The first low voltage, low noise differential silicon microphone, technology development and measurement results

Chip-size-packaged silicon microphones