Abstract:
A FBAR ladder filter which may yield less degradation in the stopband near the passband edges than conventionally grounded FBAR ladder filters. For that purpose a thin film resonator (FBAR) ladder filter having a plurality of serially-coupled FBAR elements, each serially-coupled FBAR element including an upper metal electrode and a lower metal electrode, and a plurality of shunt-coupled FBAR elements, each shunt-coupled FBAR element including an upper metal ground electrode providing a ground node and a lower metal electrode, is provided with at least one capacitor element including an upper metal electrode and a lower metal electrode, wherein each capacitor element is serially coupled between two ground nodes so that the inductive coupling of the shunt-coupled FBAR elements is compensated.

Description:
PRIORITY 
   This application claims priority to European application no. EP03029610.7 filed Dec. 22, 2003. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to grounding of thin film bulk acoustic wave resonator ladder filters. Some principles of grounding of such ladder filters are known for example from the U.S. Pat. No. 6,323,744. 
   DESCRIPTION OF THE RELATED ART 
   Thin film bulk acoustic wave resonators (hereinafter “FBAR”) are typically used in high-frequency environments ranging from several hundred megahertz (MHz) to several gigahertz (GHz).  FIG. 1  illustrates the general cross-section of a conventional FBAR component  100 . In  FIG. 1 , FBAR component  100  includes a piezoelectric material  110  interposed between two conductive electrode layers  105  and  115 , with electrode layer  115  which may be formed on a membrane or sets of reflecting layers deposited on a solidly mounted semiconductor substrate  120  which may be made of silicon or quartz, for example. The piezoelectric material is typically a dielectric, preferably one selected from the group comprising at least ZnO, CdS and AlN. Electrode layers  115  and  105  are formed from a conductive material, preferably of Al, Mo or W, but may be formed from other conductors as well or from composite layers of such conductors as disclosed in the U.S. Pat. No. 6,291,931 the content of which is incorporated hereby by reference. 
   These FBAR components are often used in electrical signal filters, more particularly in FBAR filters applicable to a lot of telecommunication technologies. For example, FBAR filters may be employed in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems. 
   The desire to render these increasingly complicated information and communication systems portable and even hand-held place significant demands on filtering technology, particularly in the context of increasingly crowded radio frequency resources. Therefore, FBAR filters must meet strict performance requirements which include:
         (a) being extremely robust,   (b) being readily mass-produced and   (c) being able to sharply increase performance to size ratio achievable in a frequency range extending into the gigahertz region.       

   However, in addition to meeting these requirements, there is a need for low passband insertion loss simultaneously coupled with demand for a relatively large stopband attenuation. Moreover, some of the typical applications noted above for these FBAR filters require passband widths up to 4% of the center frequency, which is not easily achieved using common piezoelectrics such as AIN. 
   A standard approach to designing FBAR filters out of resonators is to arrange them in a ladder configuration alternately in a series-shunt relationship (i.e., a “shunt” resonator connected in shunt at a terminal of a “series” resonator), as disclosed for example in the US 6,262,637 the content of which is incorporated hereby by reference. 
   Currently, the conventional way of designing FBAR ladder filters is to design simple building blocks of FBAR components which are then concatenated together (connected or linked up in a series or chain).  FIG. 2  illustrates a simple building block in circuit form, commonly known as a T-Cell. Referring specifically to  FIG. 2 , a T-Cell  125  includes three FBAR components  130 A,  130 B and  135 . FBARs  130 A and  130 B each are “series arm” portions of the T-Cell block. They are connected in series between an input port  132  and node  136 , and node  136  to an output port  134  of T-Cell building block  125 . Further, FBAR components  130 A or  130 B may be part of a series arm for an adjacently connected T-Cell, as will be discussed later. The resonator element  135  comprises the “shunt leg” portion of T-Cell building block  125 , being connected in shunt between terminal  136  and ground. A plurality of these T-Cells  125  chained together form a FBAR ladder filter. 
     FIGS. 3A and 3B  illustrate ideal and conventional grounding patterns for FBAR ladder filters. Ideally, FBAR ladder filters would like to see perfect isolated ground paths from each of its shunt legs to the final external ground of a package or carrier that the die rests on, so that there are no avenues of feedback or coupling between the shunt resonators. The die is an integral base on which the individual serially and shunt-coupled FBAR components of the ladder filter are fabricated on. 
   The die typically rests upon or is situated within a carrier or package. Such an ideal grounding arrangement is illustrated by the FBAR ladder filter circuit  150  shown between input port  149  and output port  151  of  FIG. 3A . As shown in  FIG. 3A , shunt FBAR elements  152  and  153  are directly grounded to the final external ground  155  of a carrier or package. Since all of the ground nodes of the ladder filter are top electrodes and are usually grouped next to each other, it is common practice to tie all the grounds together into one large ground pad, or “bus”, and then wirebond this pad to the final package ground with one or more wirebonds. Such a grounding arrangement is illustrated in  FIG. 3B . 
   In  FIG. 3B , the die grounds of shunt elements  162  and  163  of FBAR ladder filter  160  are “tied” together to form a single metal strip  164  (i.e., a common die ground from the top metal electrodes) which is connected to the final external ground  166  on the carrier by wirebond  165 . Although this provides somewhat adequate grounding, there is a significant degradation of the ladder filter performance in the stopband near the passband edges, due to the aforementioned coupling and/or feedback between the shunt resonators caused by this common die bus. 
   These stopband performance “glitches” near the passband of a FBAR ladder filter can be somewhat minimized by adding multiple wirebonds.  FIG. 4A  illustrates a simplified view of a FBAR ladder filter circuit using multiple wirebonding, and  FIG. 4B  depicts a representation of the multiple wirebond arrangement of  FIG. 4A . In  FIG. 4A , the FBAR ladder filter  200  consists of two T-cells  205  and  215  concatenated together, T-cell  205  having serially-coupled FBAR elements  207  and  209  and shunt FBAR element  210 , T-cell  215  having serially coupled FBAR elements  217  and  219  and shunt FBAR element  220 . Similar to  FIG. 3B , the grounds of the shunt FBAR elements  210  and  220  are tied together to form a single metal strip  230 ; however, instead of a single wirebond, two wirebonds  225  and  235  connect the common ground to the final external ground of the carrier or package (not shown). 
     FIG. 4B  is a schematic view from the top of the FBAR ladder filter  200  of  FIG. 4A . Specifically, in die  250  there is shown the arrangement of the top metal electrodes corresponding to the FBAR elements in T-cells  205  and  215  of  FIG. 4A , as well as the wirebond connections to the final external ground. 
   Specifically, top electrodes  207 A and  209 A and  217 A and  219 B correspond to series FBAR elements  207 ,  209  and  217 ,  219  and metallization  270  is a common ground strip for connecting shunt FBAR elements  210  and  220  (analogous to the singular metal strip  230  connecting the shunt FBAR elements of the ladder filter  200 ) to an external ground (not shown). Connectors  281  and  282  connect the FBAR ladder filter to other components adjacent thereto within a particular system (not shown). Bottom electrodes (not shown) are common to respective FBAR elements in T-cells  205  and  215  respectively. As can be seen from  FIG. 4B , two wirebonds  295 A and  295 B (corresponding to wirebonds  225  and  235  in  FIG. 4A ) are for connecting the common ground strip of the adjacent FBAR shunt elements to the final external ground on a carrier or package on that the die rests on (not shown). 
   The use of multiple wirebonds somewhat improves the stopband glitches near the passband edges of the FBAR ladder filter, as compared to using the single wirebond shown in  FIG. 3B . This is because by increasing the number of wirebonds to a final external ground, the overall inductance and resistance is lowered, which in turn helps to isolate the common die bus from the final package ground. This somewhat limits the deteriorating effects due to the feedback/coupling phenomena. However, the improvement is still unacceptable when compared to the response achievable by employing an ideal grounding arrangement as illustrated in  FIG. 3A . Accordingly, there is a need for a FBAR ladder filter having an amended grounding arrangement which yields to superior filter performance than the aforementioned standard grounding techniques. 
   SUMMARY OF THE INVENTION 
   The present invention provides a FBAR ladder filter which may yield less degradation in the stopband near the passband edges than conventionally grounded FBAR ladder filters. By forming a capacitor between at least two of the plurality of shunt-coupled FBAR elements of the ladder filter is achieved by forming a capacitor on the die of the ladder filter. Thus, all shunt-coupled FBAR elements are capacitively decoupled from one another, reducing the feedback/coupling effects prevalent in filters with common die grounds or split die grounds. 
   Preferably, the capacitor element is formed using the same materials that are deposited in steps for fabricating the array of the acoustic resonators. For example, the electrodes and the piezoelectric layer that are deposited to fabricate the FBAR elements may be utilized in the formation of the capacitor element. 
   This is preferably achieved without any additional process steps to the FBAR element fabrication. The tuning of the capacity of the capacitor element can be provided merely by properly adjusting the area of its upper and lower electrodes. Of course, the tuning of the capacitor element could be done also by adjusting the thickness of the piezoelectric layer. However, it is preferred to tune the capacity of the capacitor element by adjusting the area of its electrodes and thus providing an capacitor element having the same thickness as the FBAR elements on the die. It only has to be ensured that this additional “series resonator” element functions as a capacitor, especially as a decoupling capacitor, and not as a resonator. That means that the resonance frequency of that additional “series resonator” element has to be in a frequency range far away of the relevant ladder filter frequencies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become fully understood from the detailed description given herein below and the accompanying drawings, wherein like elements represent like reference numerals, which are given by way of illustration only and thus are not limitative of the invention and wherein: 
       FIG. 1  is a side view of a conventional thin film resonator; 
       FIG. 2  illustrates a T-Cell block used in a conventional FBAR ladder filter; 
       FIG. 3A  illustrates a FBAR ladder filter circuit with an ideal grounding arrangement; 
       FIG. 3B  illustrates a FBAR ladder filter circuit with a single wirebond grounding arrangement; 
       FIG. 4A  illustrates a FBAR ladder filter circuit with a multiple wirebond grounding arrangement; 
       FIG. 4B  is a physical representation of the FBAR ladder filter circuit of  FIG. 4A ; 
       FIG. 5A  illustrates a FBAR ladder filter circuit with a capacitor element between the shunt FBAR elements in accordance with the present invention; 
       FIG. 5B  is a schematic view from the top of the FBAR ladder filter circuit of  FIG. 5A ; and 
       FIG. 6  is a block diagram showing the front-end circuit  60  of a conventional cellular telephone, personal communication system (PCS) device or other transmit/receive apparatus. 
       FIG. 7  illustrates the passband response characteristics for the FBAR filter circuit of the present invention of  FIG. 5A  and the grounding arrangements of  FIG. 4A . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention is directed to a thin film resonator ladder filter which provides improved performance in the stopband near the passband edges by providing a capacitor element between the shunt-coupled FBAR element in the ladder filter, so that the inductive coupling of the shunt-coupled FBAR elements among each other is compensated. Stopband performance “glitches” near the passband are significantly reduced, as compared to the conventional single and multiple wirebond grounding arrangements currently used in grounding a FBAR ladder filter to the carrier or package on which it rests. 
     FIG. 5A  illustrates a FBAR ladder filter circuit with a decoupling capacitor grounding arrangement in accordance with the present invention; and  FIG. 5B  is a schematic top view of the FBAR ladder filter circuit of  FIG. 5A . The FBAR elements of  FIGS. 5A and 5B  are in respect to the FBAR elements almost identical to those previously identified in  FIGS. 4A and 4B , so only those differences from the capacitor arrangement discussed in  FIGS. 4A and 4B  are emphasized. 
   In  FIG. 5A , the FBAR ladder filter  300  consists of two T-cells  305  and  315  concatenated together, T-cell  305  having serially-coupled FBAR elements  307  and  309  and shunt FBAR element  310 , T-cell  315  having serially coupled FBAR elements  317  and  319  and shunt FBAR element  320 . Two wirebonds  325  and  335  connect the shunt FBAR elements  310  and  320  to the final external ground of the carrier or package  340 . Additionally, a capacitor element  330  is serially connected between the two shunt FBAR elements  317  and  319 . 
   As will be evident below, providing this capacitor element  330  between the two shunt-coupled FBAR element  317  and  319  in the FBAR ladder filter  300  enables the FBAR ladder filter  300  to better control parasitic stopband glitches near the passband, as compared to conventional single or multiple wirebonding from a single common die ground or from split die grounds to an external ground of a carrier or package. 
     FIG. 5B  is a schematic view from the top of the FBAR ladder filter  300  of  FIG. 5A . Specifically, in die  350  there is shown the arrangement of the top metal electrodes corresponding to the FBAR elements in T-cells  305  and  315  of  FIG. 5A , as well as the wirebond connections to the final external ground. 
   Specifically, top electrodes  307 A and  309 A and  317 A and  319 B correspond to series FBAR elements  307 ,  309  and  317 ,  319  and the metallization  370  is a common ground strip for connecting the shunt FBAR elements  310  and  320  to an external ground (not shown). 
   Connectors  381  and  382  connect the FBAR ladder filter to other components adjacent thereto within a particular system (not shown). Bottom electrodes (not shown) are common to respective FBAR elements in T-cells  305  and  315  respectively. As can be seen from  FIG. 5B , two wirebonds  395 A and  395 B (corresponding to wirebonds  325  and  335  in  FIG. 5A ) are for connecting the common ground strip of the adjacent FBAR shunt elements to the final external ground on a carrier or package on that the die rests on (not shown). 
   Between the two top ground electrodes  310 A and  320 A of the shunt FBAR elements  310  and  320  a capacitor element  330  is connected in series. The capacitor element  330  has an upper metal electrode  330 A and a lower metal electrode (not shown). The lower metal electrode is connected with the upper ground metal electrode  310 A of the shunt FBAR element  310  and the upper metal electrode  330 A is connected with the upper ground metal electrode  320 A of the shunt FBAR element  320 . The capacitor element  330  is designed to act as a pure capacitor and not as a resonator. The capacitor element  330  is manufactured in the same way as the FBAR elements, i.e. the materials deposited are the same materials used for the FBAR elements. 
     FIG. 7  illustrates a comparison of the passband response for the FBAR filter circuit of the embodiment with the grounding arrangements of  FIGS. 4A and 5A . Specifically,  FIG. 7  depicts the passband performance in dB (y-axis) vs. unit frequency (x-axis, 0.02 GHz/div.) for a FBAR filter circuit having: (a) a multiple-wirebond grounding from a common die ground (illustrated as “w/o capacitor” in the key of  FIG. 7 ); and (b) a shunt resonator divided into separate elements, each having an individual shunt top ground electrode and a capacitor element connected in series between these two top ground electrodes, as illustrated in  FIG. 5A  (denoted as “with capacitor” in the key of  FIG. 7 ). 
   The “shoulders” of each of the respective responses in  FIG. 7  are labeled as A and B to denote the edges of the passband and to illustrate the distinction between the out-of-band rejection characteristics obtainable by the two different grounding arrangements. In the case where the FBAR filter is connected to the external ground in the conventional way, the out-of-band rejection characteristics is poor (see point “A” to the left of the passband). 
   However, in accordance with the present invention, the out-of-band response at the passband edges may be improved drastically over that attained with the conventional wirebonding arrangement of  FIGS. 4A and 4B . This response is a response corresponding to the circuit structure of  FIG. 5A , where a de-coupling capacitor element is connected in series between the two electrodes of the shunt resonators. 
   Therefore, the use of the FBAR ladder filter of the present invention allows especially for improved stopband response near the passband edge, as compared to conventional filters. The FBAR filter of the present invention reduces the disadvantageous effects of coupling/feedback by providing a capacitor element between the shunt legs of the ladder filter. Moreover, the grounding arrangement of the present invention more closely approximates the response attainable by perfect grounding than any conventional grounding arrangement. Further, the FBAR ladder filter grounding arrangement of the present invention is a perfect way for adjusting the conventional single and multiple-wirebond grounding arrangements from a common die ground, which are currently used to ground FBAR ladder filter circuits to a carrier or package. 
   In a preferred embodiment of the present invention these ladder filters a incorporated in a duplexer, typically in a full duplexer, for telephony applications. To enable a full duplexer to be used, the transmit signal must be at a different frequency from the receive signal. The full duplexer lacks a switch and incorporates bandpass filters that isolate the transmit signal from the receive signal according to the frequencies of the signals.  FIG. 6  shows a conventional front-end circuit  610  such as that used in a cellular telephone, personal communication system (PCS) device or other transmit/receive apparatus. In this, the output of the power amplifier  612  of the transmitter  614  and the input of the low-noise amplifier  616  of the receiver  618  are connected to the duplexer  620 , which is a full duplexer. Also connected to the duplexer is the antenna  622 . 
   The duplexer  620  is a three-port device having a transmit port  624 , a receive port  626  and an antenna port  628 . The antenna port is connected to the transmit port through the Tx bandpass filter  630  and to the receive port through the series arrangement of the 900 phase shifter  634  and Rx bandpass filter  632 . The pass bands of the bandpass filters  630  and  632  are respectively centered on the frequency range of the transmit signal generated by the transmitter  614  and that of the receive signals to which the receiver  618  can be tuned. In the example shown, bandpass filters are configured such that the high-frequency stop band of the Tx bandpass filter  630  overlaps the pass-band of the Rx bandpass filter  632  and the low-frequency stop band of the Rx bandpass filter  632  overlaps the pass-band of the Tx bandpass filter  630 . 
   The requirements for the bandpass filters  630  and  632  constituting the duplexer  620  are quite stringent, especially the requirements for the Rx bandpass filter  632  is very stringent. The bandpass filters isolate the very weak receive signal generated by the antenna  622  and fed to the input of the low-noise amplifier  616  from the strong transmit signal generated by the power amplifier  612 . In a typical embodiment, the sensitivity of the low noise amplifier  616  is of the order of −100 dBm, and the power amplifier  612  can feed power levels of about 28 dBm into the duplexer. In such an example, the duplexer must attenuate the transmit signal by about 50 dB between the antenna port  628  and the receive port  626  to prevent the residual transmit signal mixed with the receive signal at the receive port from overloading the low-noise amplifier. Any feedback/coupling of the FBAR elements in the bandpass filters  630  and  632  has to be avoided since feedback/coupling would cause detrimental effects on the filter characteristics of the bandpass filters  630  and  632  and therefore on the performance of the whole duplexer. 
   The FBAR filters of the present invention also provide an ideal additional circuit element for connection to elements such as auxiliary inductors, auxiliary capacitors or variable capacitors. It is known that these components can be advantageously used to shape filter performance when added to input, output and/or shunt paths of the filter. If the conventional method of common die grounding is used, additional types of feedback/coupling between all shunt FBAR elements and further added additional circuit elements can be detrimental to the filter performance. However, in accordance with the filter arrangement of the present invention, desired circuit elements can be placed between the shunt top ground electrodes and its final ground, each desired circuit element capacitive decoupled from the rest of the shunt elements, thereby reducing the detrimental effects of feedback/coupling. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Although the T-cell structure is illustrated in designing a ladder filter, the grounding method may be used in filters designed by other methods that do not use the T-Cell as a building block. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and also to modifications as would be obvious to one skilled in the art or intended to be included within the scope of the following claims.