Actual and prospective communication systems make high demands on components used in its transceiver/receiver circuits. The reason for the high requirements is that, on the one hand, new systems are developed more effectively and, on the other hand, more and more frequency ranges for new bands of mobile communications are allocated in a frequency range of 500 MHZ to 6 GHZ, which is applicable for mobile communication systems. Since mobile communication systems, such as mobile phones, have to be operated around the world in different frequency bands high demands are made in relation to the edges of band-pass filters provided in the mobile phones due to the reduction of distances between neighboring bands. Furthermore, a high suppression of a stop band of a band-pass filter is necessary concurrently with a small insertion loss of the filter. In terms of duplexers and generic multiplexers high suppression of the stop band of a filter in the frequency range of a neighboring filter is required.
A compromise has to be found between the suppression in the stop band and the insertion loss in the pass band of a filter of a destined technology. In a BAW based filter technology, serial arm BAW resonators and parallel arm BAW resonators are disposed on a substrate so as to define a ladder-type filter circuit which has a serial arm having input and output terminals and a plurality of parallel arms located between the serial arm and a reference potential. A BAW resonator comprises a piezoelectric layer which is disposed between a top electrode and a bottom electrode, the stack of layers is disposed on a supporting substrate such as a wafer of silicium.
In the FBAR (film bulk acoustic resonator) technology an air gap is provided on top and bottom of the stack so that an acoustic bulk wave is reflected at the boundary between the top and bottom electrode of the stack and the air gap. In another technology, the so-called BAW-SMR (bulk acoustic wave-solidly mounted resonator) technology, the stack of electrode-piezoelectric layer-electrode is supported by a sequence of further thin films with alternating low and high acoustic impedance. These alternating layers are effective as an acoustic reflector and uncouple the resonator acoustically from the supporting substrate.
In general, a high suppression in the stop band of a ladder-type filter result in a low, i.e., bad, insertion loss of the filter. Thus, the suppression in the stop band of a filter is increased only in a destined local frequency range, rather than reducing the level of the stop band over a wide frequency range.
A possibility to generate a local pole in the transmission curve S21 of a filter is to couple the resonators in the parallel arms of the ladder-type structure of a filter to a reference potential by inductivities that are connected in series with the parallel arm resonators. Another possibility to generate local poles in the filter transmission curve S21 which is often used in antenna duplexers is to provide phase inversion circuits in the duplexer so that a mutual influence of the receiver and transmitter filter of the antenna duplexer can be avoided. Another approach uses inductivity/capacity-networks comprising inductivities and capacities which provide a short circuit or an open circuit effective for destined local frequency ranges.
If multiple pairs of poles are required to realize a destined filter curve in the stop band, more than one parallel arm resonator has to be connected to the reference potential via a comparatively high inductivity. In consequence, the steepness of the left edge of the pass band diminishes and the suppression directly below the pass band is increased. A possibility to overcome this effect is to introduce additional ladder-type stages so that more parallel arm resonators are provided which have to be connected in series with inductivities to the reference potential. However, this kind of embodiment is applicable in a practical design for up to only four ladder-type stages since the required area for additional components increases and also because the insertion loss is raised, which has a negative effect in relation to the pass band of the filter.
Accordingly, there has been a demand for a bulk acoustic wave filter capable of increasing the suppression in a local frequency region near the pass band without significantly increasing the insertion loss.