Protection Layer Influence on Capacitive Micromachined Ultrasonic Transducers Performance
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1052-DD06-27
Protection Layer Influence on Capacitive Micromachined Ultrasonic Transducers Performance Edgard Jeanne1,2, Cyril Meynier3,4, Franck Teston3, Dominique Certon3, Nicolas Felix4, Mathieu Roy2, and Daniel Alquier1 1 Laboratoire de Microélectronique de Puissance, Université de Tours, Tours, 37071, France 2 R&D, STMicroelectronics, Tours, 37071, France 3 LUSSI CNRS FRE 2448, Université Francois Rabelais, Tours, 37000, France 4 Vermon SA, Tours, 37000, France ABSTRACT For MEMS technology, reliability is of major concern. The implementation of a protection and passivation layer, that may easily enhance reliability of capacitive Micromachined Ultrasonic Transducers (cMUTs) must be done without degrading device performance. In this work, realization, simulation and characterization of passivated cMUT are presented. Two materials, SiNx and Parylene C, were selected with regard to their mechanical and physical properties as well as their compatibility with device processing. Particular attention was paid on layer deposition temperature to avoid a structural modification of the top aluminium electrode and, hence, a membrane bulge. The characterization results are in good agreement with the simulations. The SiN passivation layer clearly impact device performance while Parylene C effectiveness is clearly pointed out even through ageing characterizations. If SiNx layer can be used for passivation with particular precautions, Parylene is definitely an interesting material for cMUT passivation and protection.
INTRODUCTION Capacitive Micromachined Ultrasonic Transducers (CMUTs) are a very attractive solution for high frequency ultrasound generation and, therefore, for medical ultrasound imaging. The first imaging assessment of the cMUT technology revealed a comparable image quality when faced to the standard piezoelectric ceramic technology [1]. A cMUT array element is composed of hundreds cells connected in parallel. A single cell can be assumed to be a parallel plate capacitor that consists of a highly doped polysilicon bottom electrode, a suspended membrane in silicon nitride over a vacuum gap, a top electrode in aluminum with eventually a final passivation layer [2]. To reach immersion requirements, a protection layer is mandatory. In order to efficiently protect the device, the chosen material must fill some essential mechanical and physical properties as well as compatibility with device manufacturing. In this work, after reviewing the protection layer requirements, two different types of materials will be investigated. First, silicon nitride (SiNx), deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) can be a material of choice due to its large use in the semiconductor industry, low deposition temperature and its good passivation properties [3]. The parylene C, which appears as a new material for MEMS and packaging issues [4-7], may be an alternative material. Indeed, it exhibits interesting properties such as low Young’s modulus, room temperature deposition and high hydrophobicity. Static and dynamic
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