It is known to fabricate monolithic filters that include Bulk Acoustic Wave (BAW) resonator devices (also known in the art as "Thin Film Bulk Acoustic Wave Resonators (FBARs)"). Presently, there are primarily two known types of Bulk Acoustic Wave devices, namely, BAW resonators and Stacked Crystal Filters (SCFs). One difference between BAW resonators and SCFs is the number of layers that are included in the structures of the respective devices. By example, BAW resonators typically include two electrodes and a single piezoelectric layer that is disposed between the two electrodes. One or more membrane layers may also be employed between the piezoelectric layer and a substrate of the respective devices. SCF devices, in contrast, typically include two piezoelectric layers and three electrodes. In the SCF devices, a first one of the two piezoelectric layers is disposed between a first, lower one of the three electrodes and a second, middle one of the three electrodes, and a second one of the piezoelectric layers is disposed between the middle electrode and a third, upper one of the three electrodes. The middle electrode is generally used as a grounding electrode.
BAW resonators are often employed in bandpass filters having various topologies. M. M. Driscoll et al. (Driscoll), entitled "Recent Advances in Monolithic Film Resonator Technology", Ultrasonic Symposium, 1986, pp. 365-369. The Driscoll publication discloses a multiple-pole filter that includes BAW resonators connected in a series configuration and a number of tuning elements, namely, inductors, that are each connected between ground and a respective node located between a respective pair of the BAW resonators. An equivalent circuit of an individual BAW resonator is shown in FIG. 4b. The equivalent circuit includes an equivalent inductance (Lm), an equivalent capacitance (Cm), and an equivalent resistance (R), that are connected in series, and a parallel parasitic capacitance (Co).
One concern relating to the design of filters is the elimination of the parasitic capacitance (Co). The parasitic capacitance (Co) associated with each BAW resonator of the filter can be canceled at the filter's center frequency by connecting an additional tuning element (e.g., an inductor) in parallel with each BAW resonator in the manner disclosed in the Driscoll publication. Unfortunately, however, this technique does not cause the parasitic capacitance (Co) to be canceled at out-of-band frequencies. Also, the use of the tuning elements adds undesired complexity and size to the overall structure of the filter.
Filters that include BAW resonators are often constructed to have ladder topologies. For the purposes of this description, ladder filters that are constructed primarily of BAW resonators are also referred to as "BAW ladder filters". The design of ladder filters is described in a publication entitled "Thin Film Bulk Acoustic Wave Filters for GPS", by K. M. Lakin et al. (Lakin), IEEE Ultrasonic Symposium, 1992, pp. 471-476. As is described in this publication, BAW ladder filters are typically constructed so that one or more BAW resonators are series-connected within the filters and one or more BAW resonators are shunt-connected within the filters. An exemplary BAW ladder filter 41 that includes two BAW resonators 42 and 43 is shown in FIG. 8d. Another exemplary (single) BAW ladder filter 44 that includes two series-connected BAW resonators 43 and 45 and two shunt-connected BAW resonators 42 and 46 is shown in FIG. 8f. An equivalent circuit of the BAW ladder filter 44 is shown in FIG. 8h. Still another exemplary BAW ladder filter 47 is shown in FIG. 8i. This filter 47 has a "balanced" topology, and is similar to the filter 44 of FIG. 8f, but also includes a BAW resonator 48 and a BAW resonator 49. An equivalent circuit of this filter 47 is shown in FIG. 8j.
BAW ladder filters are typically designed so that the series-connected resonators (also referred to as "series resonators") yield a series resonance at a frequency that is approximately equal to, or near, the desired (i.e., "design") center frequency of the respective filters. Similarly, the BAW ladder filters are designed so that the shunt-connected resonators (also referred to as "shunt resonators" or "parallel resonators") yield a parallel resonance at a frequency that is approximately equal to, or near, the desired center frequency of the respective filters.
BAW ladder filters yield passbands having bandwidths that are a function of, for example, the types of materials used to form the piezoelectric layers of the BAW resonators, and the respective thicknesses of the layer stacks of the BAW resonators. Typically, the series-connected BAW resonators of BAW ladder filters are fabricated to have thinner layer stacks than the shunt-connected resonators of the filters. As a result, the series and parallel resonances yielded by the series-connected BAW resonators occur at somewhat higher frequencies than the series and parallel resonant frequencies yielded by the shunt-connected BAW resonators (although the series resonance of each series-connected BAW resonator still occurs at a frequency that is near the desired filter center frequency on the frequency spectrum). In a BAW ladder filter, the parallel resonances yielded by the series-connected BAW resonators cause the filter to exhibit, a notch above the upper edge or skirt of the filter's passband, and the series resonances yielded by the shunt-connected BAW resonators cause the filter to exhibit a notch below the lower edge of the filter's passband. These notches have "depths" that are defined by the acoustic and electric losses of the series-connected and shunt-connected BAW resonators (i.e., the notches are defined by quality factors of the shunt and series BAW resonators).
The difference in the thicknesses of the layer stacks of the series-connected and shunt-connected BAW resonators can be achieved during the fabrication of the devices. By example, in a case in which the BAW resonators include one or more membrane layers, an additional layer of suitable material and thickness may be added to the membrane layers of the shunt-connected devices during fabrication so that, after the devices are completely fabricated, the shunt-connected devices will have thicker layer stacks than the series-connected resonators. As another example, the series resonators can be fabricated to have thinner piezoelectric layers than the shunt resonators, and/or the thicknesses of the upper electrodes of the series resonators can be reduced by a selected amount using a suitable technique, after the deposition of the upper electrode layers. These steps require the use of masking layers.
The performance of BAW ladder filters may be further understood in view of the element equivalent circuit of the BAW resonator shown in FIG. 4b. The series resonance of the individual BAW resonator is caused by the equivalent inductance (Lm) and the equivalent capacitance (Cm). At the series resonant frequency of the BAW resonator, the impedance of the BAW resonator is low (i.e., in an ideal case, where there are no losses in the device, the BAW resonator functions like a short circuit). At frequencies that are lower than this series resonant frequency, the impedance of the BAW resonator is capacitive. At frequencies that are higher than the series resonant frequency of the BAW resonator, but which are lower than the parallel resonant frequency of the device (the parallel resonance results from equivalent capacitance (Co)), the impedance of the BAW resonator is inductive. Also, at higher frequencies than the parallel resonant frequency of the BAW resonator, the impedance of the device is again capacitive, and, at the parallel resonant frequency of the device, the impedance of the BAW resonator is high (i.e., in an ideal case the impedance is infinite and the device resembles an open circuit at the parallel resonant frequency).
For an exemplary case in which two BAW resonators (e.g., a shunt BAW resonator and a series BAW resonator) having equivalent circuits similar to the one shown in FIG. 4b are employed in a BAW ladder filter, a lowest resonant frequency of the filter is one at which the series resonance of the shunt BAW resonator occurs. At this frequency, an input of the BAW ladder filter is effectively shorted to ground, and thus a frequency response of the BAW ladder filter exhibits a deep notch below the passband of the filter. The next highest resonant frequencies of the BAW ladder filter are the series resonant frequency of the series BAW resonator and the parallel resonant frequency of the shunt BAW resonator. These resonant frequencies are within the passband frequencies of the BAW ladder filter, and are located at or near the desired center frequency of the BAW ladder filter on the frequency spectrum. At the parallel resonant frequency of the shunt BAW resonator, the shunt BAW resonator behaves like an open circuit, and at the series resonant frequency of the series BAW resonator, the series BAW resonator behaves like a short circuit (and thus provides a low-loss connection between input and output ports of the BAW ladder filter). As a result, for a case in which a signal having a frequency that is approximately equal to the center frequency of the BAW ladder filter is applied to the input of the BAW ladder filter, the signal experiences minimum insertion loss (i.e., it encounters low losses) as it traverses the filter circuit between the filter's input and output.
A highest resonant frequency of the BAW ladder filter is one at which the series-connected BAW resonator yields a parallel resonance. At this frequency, the series BAW resonator behaves like an open circuit and the shunt BAW resonator behaves like a capacitor. As a result, the filter's input and output are effectively de-coupled from one another, and the frequency response of the filter includes a deep notch above the filter's passband.
The frequency response of a BAW ladder filter that includes no tuning elements typically has deep notches and steeply-sloped upper and lower passband edges (i.e., skirts). Unfortunately, however, these types of ladder filters tend to provide poor stopband attenuation (i.e., out-of-band rejection) characteristics. An example of a measured frequency response of a BAW ladder filter that exhibits deep notches, steeply-sloped passband edges, and poor stopband attenuation, and which includes four BAW resonators and no tuning elements, is shown in FIG. 9a.
Another exemplary frequency response is shown in FIG. 8e, for the BAW ladder filter 41 of FIG. 8d. The BAW ladder filter 41 yields the frequency response of FIG. 8e assuming that 1) the resonators 43 and 42 include the layers listed in respective Tables 1 and 2 below, 2) the layers of resonators 43 and 42 have thicknesses and include the materials listed in respective Tables 1 and 2, 3) the filter 41 is connected between 50 Ohm terminals, and 4) the filter 41 includes no tuning elements.
TABLE 1 ______________________________________ SERIES BAW RESONATOR (43, 45) Layer ______________________________________ Upper electrode: Molybdenum 308 nm (Mo) Piezoelectric 2147 nm layer: Zinc-oxide (ZnO) Lower electrode: Molybdenum 308 nm (Mo) first membrane layer: 90 nm silicon-dioxide (SiO.sub.2) area of upper electrode 225 um* 225 um ______________________________________
TABLE 2 ______________________________________ SHUNT BAW RESONATOR (42, 46) Layer ______________________________________ Upper electrode: 308 nm Molybdenum (Mo) Piezoelectric layer: 2147 nm Zinc-oxide (ZnO) Lower electrode: 308 nm Molybdenum (Mo) first membrane layer: 90 nm (SiO.sub.2) second membrane 270 nm layer: (SiO.sub.2) area of upper electrode 352 um* 352 um ______________________________________
As can be appreciated in view of Tables 1 and 2, the BAW resonator 42 includes two membrane layers, and the BAW resonator 43 includes only a single membrane layer. The employment of two membrane layers in the resonator 42 causes the resonant frequencies yielded by the resonator 42 to be lower than those yielded by the series-connected resonator 43, as was described above.
The level of stopband attenuation provided by a BAW ladder filter can be increased by including additional BAW resonators in the filter and/or by constructing the filter so that the ratio of the areas of the filter's parallel-connected BAW resonators to the areas of the filter's series-connected BAW resonators is increased. FIG. 8g shows an exemplary "simulated" frequency response of the filter 44 (which includes a greater number of resonators than the filter 41), assuming that 1) the resonators 43 and 45 include the layers having the thicknesses and materials listed in Table 1, 2) the resonators 42 and 46 include the layers having the thicknesses and materials listed in Table 2, and 3) the filter 44 includes no tuning elements.
As can be appreciated in view of FIGS. 8e and 8g, the degree of attenuation provided by the filter 44 at out-of-band frequencies is improved somewhat over the attenuation level provided by the filter 41 that includes only two BAW resonators. Unfortunately, however, the employment of additional BAW resonators in a filter increases the filter's overall size and can cause an undesirable increase in the level of insertion loss of the filter. This is also true in cases in which the filter's parallel-connected BAW resonators are fabricated to have larger areas than the series-resonators. Moreover, even if such measures are taken in an attempt to improve the filter's passband response, the level of stopband attenuation provided by the filter may be insufficient for certain applications.
As shown in FIGS. 8e and 8g, the center frequencies of the passbands of respective filters 41 and 44 are located at about 947.5 MHz on the frequency spectrum, and the minimum passband bandwidth yielded by each of the filters 41 and 44 is approximately 25 MHz. As can be appreciated by those having skill in the art, these frequency response characteristics are required of filters that are employed in GSM receivers.
It is known to employ one or more SCF devices in a passband filter. An advantage of employing SCF devices in passband filters is the better stopband attenuation characteristics provided by these filters in general, as compared to the stopband attenuation characteristics of typical BAW ladder filters. An exemplary lumped element equivalent circuit of a SCF is shown in FIG. 8b. The equivalent circuit includes an equivalent inductance (2Lm), an equivalent capacitance (Cm/2), an equivalent resistance (2R), and parasitic capacitances (Co). As can be appreciated in view of FIG. 8b, the SCF can be considered to be an LC resonator having parallel capacitances (Co) connected to ground.
Like the BAW ladder filters described above, filters that are comprised primarily of SCF devices can also suffer from a number of drawbacks. One drawback is that SCFs generally yield frequency responses that do not exhibit such desired characteristics as deep notches and steeply-sloped passband edges. This can be seen in view FIG. 8c, which shows an exemplary frequency response of a SCF. The frequency response of a filter that is comprised primarily of one or more SCF components can be improved to some extent by connecting an inductor between each SCF structure, as is described in U.S. Pat. No. 5,382,930. Unfortunately, however, the addition of these inductors adds to the overall size and complexity of the filter, and can also increase the filter's level of insertion loss, owing to losses in the inductors. Another drawback associated with these types of filters is that it can be difficult to control the passband bandwidths of the filters.
In view of the above description, it can be appreciated that it would be desirable to provide a filter which can yield the desired frequency response characteristics provided by both BAW ladder filters and Stacked Crystal Filters. That is, it would be desirable to provide a filter which exhibits a frequency response having deep notches and steeply-sloped upper and lower passband edges, and which also yields stopband attenuation levels that are similar to those generally yielded by Stacked Crystal Filters. It is also desirable that the filter be small in size, and be able to exhibit the desired frequency response characteristics without the use of tuning elements.
Another concern of this invention relates to duplex filters. Duplex filters (also referred to as "duplexers") are conventionally employed as three port devices in transceivers to isolate the receiver (RX) and transmitter (TX) portions of the transceiver from one another and to provide frequency selectivity for each of the RX and TX portions of the transceiver. Duplex filters typically include a bandstop filter for filtering signals that are output by the TX portion of the transceiver, before the signals are transmitted from the transceiver via an antenna. The bandstop filter attenuates signals having frequencies within the filter's stopband, which normally includes the same frequencies of the transceiver's receive band. Duplex filters also typically include a passband filter for filtering signals that are received by the antenna, before the signals are provided to the RX portion of the transceiver.
Most conventional duplexers suffer from a number of drawbacks. For example, one conventional type of duplexer, namely, ceramic duplexers, which are often employed in mobile telephone transceivers, generally are undesirably large in size. Also, some conventional duplexers employed in mobile telephones sometimes include Surface Acoustic Wave (SAW) devices, which unfortunately cannot operate at certain high RF power levels such as those often employed in GSM transmitters. It can thus be appreciated that it would be desirable to provide a duplexer which overcomes these problems.