Fujitsu has developed high-performance, miniature aluminum nitride (AlN) thin Film Bulk Acoustic Resonators (FBAR) filters for 5-GHz Wireless Local Area Network (WLAN) applications. Filters with higher frequencies of around 5 GHz and wider bandwidths will be needed in future mobile communication systems. Therefore, we developed filters using thin-film Bulk Acoustic Wave (BAW) technology, which shows promise as a possible solution to these higher frequency applications. There are two issues related to the development of FBAR filters. The first has been the need for miniaturization. To achieve this, we developed a new resonator configuration that employs bulk micro-machining techniques. The second issue has been to increase the bandwidth. Five-GHz WLAN applications require a 200-MHz bandwidth at 5250 MHz, which corresponds to a fractional bandwidth of 3.8%. However, the piezoelectric coupling coefficient (k(2)) of AlN cannot satisfy this requirement. To achieve wider filter bandwidth, we developed two approaches. They utilize epitaxial AlN film-growth.,and reactance-controlled flip-chip package design technologies. The packaging technology also enabled us to miniaturize the filters.
