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Aluminum Nitride is the most favoured material for industrial fabrication of thin film BAW microwave filters. There are certainly good reasons for it, such as the high acoustic quality factor, the high sound velocity, and the excellent chemical compatibility with semiconductor front end materials. The most limiting property of AlN is probably the coupling factor, the determining factor for bandwidth. There are materials exhibiting much larger coupling factors, especially among ferroelectrics. However, no high enough quality factors have been achieved to date with ferroelectric films. In this article, the state of the art of AlN thin film properties is first reviewed. Results on ferroelectric TFBAR's are summarized. The role of domain configuration in ferroelastics is discussed for the case of KNbO 3 . The problematics of losses due to relaxation of domain wall motions is addressed. It is tried to make a conclusion on potentially good candidates. Aluminum nitride, ferroelectrics, bulk acoustic waves I.0-7803-9383-X/05/$20.00 (c) 2005 IEEE

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