Surface micromachined ultrasound transducers in CMOS technology

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Abstract
Abstracr - The paper presents a new technology for electrostatically driven ultrasound transducers, processed by only a few additional steps within a standard CMOS process. Measurements of the test structures showed good acoustic properties espe- cially 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. TECHNOLOGY FOR INTEGRATED TRANSDUCERS Unlike previously reported solutions, the trans- ducer cells reported in this paper have been inte- grated completely into a standard BiCMOS - process. Only the standard layers of the BiCMOS process are used to build up the transducer. In contrast to all integrated micromechanical systems presented so far (e.g. (2), (3)) neither pre- nor postprocessing of the system is necessary. Com- pared to conventional surface micromachining (4), (S), the additional processing effort for the sensor realization was reduced dramatically. Due to the fact, that only the very thin standard layers of a BiCMOS process are used to build up the sensor, the distance between the transducer elec- trodes becomes as small as 600 nm. Thus electro- static excitation of the transducers by means of volt- ages in the range between 5 V and 100 V becomes possible. PROCESS The integrated system process is based on a 0.8 micron analog BiCMOS process (16 masks). The transducer membrane is formed by the 400 nm thick capacitor polysilicon layer of the process. The field oxide (600 nm) serves as a sacrificial layer for the membrane. The effective gap is reduced by a BPSG layer of high permittivity to 450 nm.

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