Solidly Mounted Bulk Acoustic Wave Filters
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Solidly Mounted Bulk Acoustic Wave Filters H.P. Loebl a, C. Metzmacher a, R.F. Milsom b, A. Tuinhout c, P.Lok c, F.van Straten c a
Philips Research Laboratories, Weisshausstr.2, D-52066 Aachen, Germany Philips Research Laboratories, Cross Oak Lane; Redhill; Surrey, RH1 5HA, UK c Philips Semiconductors, Mobile Communications; SI, Gerstweg2, NL-6534 AE Nijmegen, The Netherlands b
ABSTRACT Selective RF filters based on solidly mounted thin film bulk acoustic wave resonator filters (SMR filters) are of great interest for mobile and wireless applications in the GHz frequency range. They are small in size, robust and can be integrated on silicon. The SMR configuration comprises a textured piezoelectric layer of AlN or ZnO sandwiched between two electrodes, and an acoustic reflector, which confines the acoustic energy in the resonator region. The bulk acoustic wave (BAW) resonators are exited in the thickness extensional mode. In these paper examples of BAW resonators and filters for the frequency range between 1 and 8 GHz are shown. It will become clear that precise control of AlN film growth is very important. Modelling the resonator performance using a 1-dimensional electro acoustical model allows the extraction of important thin film parameters and thus simulation of the filter performance. Limitations of the 1-D model are discussed. INTRODUCTION Filtering of signals, which are closely separated in the frequency domain, is a key function in mobile radio front-ends. The trend to miniaturization promoted the use of electro-acoustic surface acoustic wave (SAW) filters, since the acoustic wavelength is 5 orders of magnitude smaller than the electromagnetic wavelength at a given frequency (at 1 GHz the acoustic wavelength is in the order of µm compared to cm for an electromagnetic wave). Therefore, SAW filters which exploit surface acoustic waves excited by inter-digital electrodes deposited on the surface of a piezoelectric single crystalline material (e.g.LiNbO3), have been used successfully for frequencies in the kHz up to 1-2 GHz range. However, at high frequencies and high power levels of 1-3 W, which are needed in transmit filters for mobile phones, SAW filters suffer from material problems related to acousto-migration due to strong acoustic fields. These high fields lead to deterioration and eventually shorts in the inter-digital electrodes used in SAW filters, because the electrodes spacing is much below 1µm at high frequencies. The concept of solidly mounted BAW filters (see figure 1), which exploits the thickness extensional mode of a thin piezoelectric film, seems to be more robust. All the layers are a few 100 nm up to a few µm thick. The lateral resonator dimensions determine the electrical impedance of the resonator or filter and are in the order of a few hundred µm. In contrast to SAW filters no sub-micron lithography is needed. However, the requirements on layer uniformity and layer quality are very high for BAW filters, since the thickness of the layers determines the resonance frequency and thus
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