John J. Neumann

CMOS-MEMS membrane for audio-frequency acoustic actuation

Using CMOS-MEMS micromachining techniques we have constructed a prototype earphone that is audible from 1 to 15 kHz. The fabrication of the acoustic membrane consists of only two steps in addition to the prior post-CMOS micromachining steps developed at CMU. The ability to build a membrane directly on a standard CMOS chip, integrating mechanical structures with signal processing electronics will enable a variety of applications including economical earphones, microphones, hearing aids, high-fidelity earphones, cellular phones and noise cancellation. The large compliance of the CMOS-MEMS membrane also promises application as a sensitive microphone and pressure sensor.

A fully-integrated CMOS-MEMS audio microphone

We report on the construction of a microphone and associated electronics fabricated entirely within a standard CMOS (complementary metal oxide semiconductor) die. An A-weighted noise level of 46 dB SPL was achieved with a total diaphragm area of 0.61 mm/sup 2/. Because the microphone uses the same processing sequence as CMOS-MEMS (microelectromechanical systems) microspeakers it is now possible to create acoustic systems-on-chip for applications in such areas as hearing aids, in-ear translators, and active noise cancellation. Because electret materials are not used, the microphone can withstand temperatures up to 250/spl deg/C with no degradation in performance. The frequency-modulated output provides a convenient, low-noise way to transmit the signal off chip, and is directly compatible with digital circuitry and FM radios.

Use of Lamb waves to monitor plates: experiments and simulations

Lamb waves at ultrasonic frequencies travel with little attenuation in thin elastic plates, and we demonstrate their use in pulse-echo behavior to monitor plate integrity. We envision using a single PZT wafer-type transducer to generate waves and to receive reflections from distant flaw or boundary locations. However, Lamb waves generally have multiple modes, each of them highly dispersive, and in consequence pulse dispersion can become pronounced and can make difficult or impossible the interpretation of pulse-echo responses. We show that selective generation of the S0 wave will overcome those difficulties; therefore, selection of transducer dimensions and pulse characteristics to achieve selective generation should be considered mandatory for most intended applications. We first review the work of others identifying a basic relationship between transducer dimension and excitation frequency for selective generation of the S0 wave. We then summarize our extensive experimental studies of wafer-type transducers with particular attention to S0 and A0 mode behavior, both in transmission and reception. We next report our two-dimensional finite element simulation of the same problem performed in FEMLAB, requiring transient simulation of the coupled electromechanical problem. We simulate the piezoelectric response of the wafer-type transducer coupled to the elastic plate, both as transmitter and receiver, as well as the development of Lamb waves within the source region and their subsequent propagation along the plate. Simulations illustrate the development and separation of the S0 and A0 modes and reproduce the expected group velocities and dispersion behavior. We show good agreement between our experiments and our simulations regarding S0 mode behavior, and we summarize the results to guide a designer in choosing transducer dimensions. In particular, good selectivity between the S0 and A0 mode generation can be obtained with appropriate choice of transducer size and center frequency. We show the results of experiments on an aluminum plate in which excitation of a single PZT wafer-type transducer at 6.5 V (peak-to-peak) produces reflected signals of ample strength (tens of mV) from distant boundaries and from partial thickness flaws.

Comparison of piezoresistive and capacitive ultrasonic transducers

MEMS ultrasonic transducers for flaw detection have heretofore been built as capacitive diaphragm-type devices. A diaphragm forms a moveable electrode, placed at a short gap from a stationary electrode, and diaphragm movement has been detected by capacitance change. Although several research teams have successfully demonstrated that technology, the detection of capacitance change is adversely affected by stray and parasitic capacitances, limiting the sensitivity of such transducers and typically requiring relatively large diaphragm areas. We describe the design and fabrication of what to our knowledge is the first CMOS-MEMS ultrasonic phased array transducer using piezoresistive strain sensing. Piezoresistors have been patterned within the diaphragms, and diaphragm movement creates bending strain which is detected by a bridge circuit, for which conductor losses will be less significant. The prospective advantage of such piezoresistive transducers is that sufficient sensitivity may be achieved with very small diaphragms. We compare transducer response under fluid-coupled ultrasonic excitation and report the experimental gauge factor for the piezoresistors. We also discuss the phased array performance of the transducer in sensing the direction of an incoming wave.

Robust capacitive MEMS ultrasonics transducers for liquid immersion

Capacitive diaphragm MEMS ultrasonic transducers are of great interest because they offer wide bandwidth and ready integration into arrays. However, fragility of these transducers is a significant barrier to their application. In this talk, we report on robust transducers which have been fabricated using the MUMPS process. The transducer design has been optimized to minimize stray capacitance between the output node and the substrate. We report the use of a protective silicone layer which protects the transducers from liquid exposure and, to a degree, from mechanical damage. The silicone layer has been applied with high transducer yield without the need for prior closure of the etch holes, and coated transducers survive extended immersion in water. The thickness of the silicone layer must be carefully controlled, however, in order to prevent pulse distortion.

Digital sound reconstruction using arrays of CMOS-MEMS microspeakers

We report on the development of a direct digital method for producing sound using an array of CMOS-MEMS microspeakers. In a previous article we described the theory behind digital sound reconstruction, characterized the acoustic response of a single CMOS-MEMS membrane, and showed experimental results involving a 3-bit (7 speaker) array composed of individual microspeaker chips. In this work, we report on the design, fabrication, and operation of an 8-bit (255 speaker) array integrated on a single chip. With this new device, we have been able to further study sound reconstruction and understand the sources of harmonic distortion and waveform quality that are essential to creating a complete digital sound system.