BAW filter
Electrical filters which are formed from bulk acoustic wave resonators or stacked crystal filters are normally referred to as bulk acoustic wave (BAW) filters. Along with the popularization of mobile communication as represented by cellular phones, the demand for a high frequency filter operated in several hundred MHz-several GHz has been increased. At the moment, filters with ceramic electromagnetic resonators are used for this purpose in the high-frequency range. Surface acoustic wave (SAW) filters can be constructed so as to be substantially smaller. The reason for this is that the acoustic wavelength is smaller by 4 to 5 orders of magnitude than the electromagnetic wavelength. A disadvantage is, however, that surface acoustic wave filters often have a complicated construction and have to be protected by means of complicated housings. Among various filter technologies, the filter manufactured by using a bulk acoustic wave resonator is characterized by high frequency, compactness, excellent temperature characteristic, and excellent electric strength. The bulk acoustic wave resonator is a stacked resonator having a structure in which electrode films made of metal materials sandwiches an piezoelectric layer. The longitudinal resonant oscillation occurred by applying AC voltage to the stacked resonator is used. The structure of the bulk acoustic wave resonator is categorized by methods of confining the bulk acoustic wave in the piezoelectric film. BAW filters can be fabricated to include various known types of bulk acoustic wave (BAW) resonators. In principle, bulk acoustic wave resonators are built up from three components. The first component generates the acoustic wave and comprises a piezoelectric layer. Two electrodes, which are arranged above and below the piezoelectric layer, represent the second component. The third component has the task of acoustically insulating the carrier substrate from the oscillations generated by the piezoelectric layer.
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