Kurt Niederer

Surface micromachined ultrasound transducers in CMOS technology

The paper presents a new technology for electrostatically driven ultrasound transducers, processed by only a few additional steps within a standard BiCMOS process. Measurements of the test structures showed good acoustic properties especially for application in fluids. The full integration with the electronic circuit allows the realization of a new generation of one- and two dimensional arrays for many different applications like low cost phased arrays or 2D arrays for specific medical applications like skin diagnosis.

Micromachined ultrasound transducers with improved coupling factors from a CMOS compatible process

For medical high frequency acoustic imaging purposes the reduction in size of a single transducer element for one-dimensional and even more for two-dimensional arrays is more and more limited by fabrication and cabling technology. In the fields of industrial distance measurement and simple object recognition low cost phased arrays are lacking. Both problems can be solved with micromachined ultrasound transducers (MUTs). A single transducer is made of a large number of microscopic elements. Because of the array structure of these transducers, groups of elements can be built up and used as a phased array. By integrating parts of the sensor electronics on chip, the cabling effort for arrays can be reduced markedly. In contrast to standard ultrasonic technology, which is based on massive thickness resonators, vibrating membranes are the radiating elements of the MUTs. New micromachining technologies have emerged, allowing a highly reproducible fabrication of electrostatically driven membranes with gap heights below 500 nm. A microelectronic BiCMOS process was extended for surface micromechanics (T. Scheiter et al., Proceedings 11th European Conference on Solid-State Transducers, Warsaw, Vol. 3, 1997, pp. 1595-1598). Additional process steps were included for the realization of the membranes which form sealed cavities with the underlying substrate. Membrane and substrate are the opposite electrodes of a capacitive transducer. The transducers can be integrated monolithically on one chip together with the driving, preamplifying and multiplexing circuitry, thus reducing parasitic capacities and noise level significantly. Owing to their low mass the transducers are very well matched to fluid loads, resulting in a very high bandwidth of 50-100% (C. Eccardt et al., Proceedings Ultrasonics Symposium, San Antonio, Vol. 2, 1996, pp. 959-962; P.C. Eccardt et al., Proceedings of the 1997 Ultrasonics Symposium, Toronto, Vol. 2, 1997, pp. 1609-1618). In the following it is shown how the BiCMOS process has been modified to meet the demands for ultrasound generation and reception. Bias and driving voltages have been reduced down to the 10 V range. The electromechanical coupling is now almost comparable with that for piezoelectric transducers. The measurements exhibit sound pressures and bandwidths that are at least comparable with those of conventional piezoelectric transducer arrays.

Micromachined transducers for ultrasound applications

Within the last decade many interesting solutions for micromachined electroacoustic transducers have been presented, most of them microphones for the audio range. So far they could not compete significantly on the market with miniaturized conventional transducer techniques such as electret microphones. One of the main reasons for this was a limited sensitivity due to a high noise level without offering significant cost advantages. Now two facts raised new attention to micromachined transducers: For high-frequency ultrasound imaging the reduction in size of a single transducer element for 1D and even more for 2D arrays is more and more limited by fabrication and cabling technology. On the other hand, new microfabrication technologies have emerged, allowing a highly reproducible fabrication of electrostatically driven membranes with gap heights below 400 nm. One of the most interesting facts is that with a recently developed process step micromechanical membranes can be fabricated within a modified BiCMOS process. This allows the combination of transducer elements with the driving, preamplifying and multiplexing electronics on a single chip, thus reducing parasitic capacities and noise level significantly. This paper first outlines the history of micromachined transducers. Then it describes the internal structure of a micromechanical transducer element and its acoustical properties. The main differences in comparison to piezoelectric bulk transducers are the significantly lower acoustic impedance of the membranes and the nonlinear electromechanical working principle, leading to consequences in array design, which are discussed. Mathematical models and experimental results for transducer bandwidth, membrane deflection and radiation patterns of transducer arrays are presented and compared with the properties of piezoelectric transducer arrays. The influence of the membranes relative deflection, the poling voltage, the cabling capacity and the preamplifier characteristics upon transmission level and signal-to-noise ratio are discussed. Finally an outlook for potential applications of micromachined transducers, especially in array configurations, is given.

Surface micromachined ultrasonic transducer

A spacer layer (7) with a cavity (8) etched out therein and a diaphragm (2) arranged thereabove on the spacer layer are located on a silicon substrate (1) with a doped region (5) formed therein, whereby the doped region and the diaphragm are electrically connected via terminal contacts (4, 6) to electronic components (13) that are likewise integrated in the substrate (1). The electronic component are a component part of the operating circuit that can also be used for the drive of the diaphragm and for evaluating the diaphragm oscillations. The integration makes it possible to arrange the micromachined transducer components as array that can be electronically driven as phased array.

Micromachined transducer design for minimized generation of surface waves

Due to their low membrane mass capacitive Micromachined Ultrasound Transducers (cMUTs) are very well matched to fluid loads. Besides the excellent transducer performance the good matching leads to an increased acoustic coupling between adjacent membranes and consequently to out of phase modes. These modes are lateral resonances of surface waves with a high quality factor. A method for suppressing the undesirable resonances has been developed and experimental results for the new transducer design are presented. The force of the electrostatic transducer increases with the square of the electric field in the gap. Due to their small gap spacing cMUTs can produce high sound pressures. Here we show how this spacing can be reduced beyond the limit of the CMOS fabrication process. On the other hand, the nonlinear force produces harmonic distortion. A procedure to find designs combining maximum sound pressure with minimum distortion has been developed. As a result, the sound pressure and according design parameters as a function of the distortion are discussed.

3D simulation of controlled micromachined capacitive ultrasound transducers

A finite element simulation scheme which allows the efficient calculation of electrostatic transducers immersed in an acoustic fluid is presented. This calculation scheme has been applied in investigations of the dynamical behavior of surface micromachined capacitive ultrasound arrays. In order to improve dynamics and efficiency of such transducers a controller has been designed and tested. Therewith, ring down time and acoustic crosstalk in the array are reduced.

Mikromechanische Ultraschallwandler für Flüssigkeiten und Gase

Mit Hilfe eines speziellen Mikromechanik-Prozesses lassen sich Ultraschallwandler in CMOS-Technologie realisieren. Diese Technologie ermöglicht die monolithische Integration von mikroelektronischen und mikromechanischen Bauteilen auf einem Silizium-Chip. Der eigentliche Wandler besteht dabei aus einem Array von elektrostatisch angetriebenen Membranen. Für die Anwendung als Luft-Ultraschallwandler wird ein mikromechanisches Herstellungsverfahren vorgestellt, das besonders gut für Frequenzen unter 1 MHz geeignet ist.