The need for using miniature and high performance filters in wireless communication devices has led to the widespread usage of Surface Acoustic Wave (SAW) filters. In addition to Surface Acoustic Wave (SAW) filters Bulk Acoustic Wave (BAW) filters can also be used. Bulk Acoustic Wave (BAW) filters typically include several Bulk Acoustic Wave (BAW) resonators. In a Bulk Acoustic Wave (BAW) filter, acoustic waves propagate in a direction that is perpendicular to the filter's layer surfaces. In contrast, acoustic waves which propagate within a Surface Acoustic Wave (SAW) filter do so in a direction that is parallel to the layer surfaces of the filter.

It is known to fabricate monolithic filters that include at least a Bulk Acoustic Wave (BAW) resonator device (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 Bulk Acoustic Wave (BAW) resonators and Stacked Crystal Filters (SCFs) is the number of layers that are included in the structures of the respective devices. By example, Bulk Acoustic Wave (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. Stacked Crystal Filter (SCF) devices, in contrast, typically include two piezoelectric layers and three electrodes. In the Stacked Crystal Filter (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.

Bulk Acoustic Wave (BAW) filters can be fabricated to include various known types of Bulk Acoustic Wave (BAW) resonators. These known types of Bulk Acoustic Wave (BAW) resonators comprise three basic portions. A first one of the portions, which is used to generate acoustic waves, includes an acoustically-active piezoelectric layer. This layer may comprise, by example, zinc-oxide (ZnO), aluminum nitride (AlN), zinc-sulfur (ZnS), or any other suitable piezoelectric material that can be fabricated as a thin film. A second one of the portions includes electrodes that are formed on opposite sides of the piezoelectric layer. A third portion of the Bulk Acoustic Wave (BAW) resonator includes a mechanism for acoustically isolating the substrate from vibrations produced by the piezoelectric layer. Bulk Acoustic Wave (BAW) resonators are typically fabricated on silicon, gallium arsenide, or glass substrates using thin film technology (e. g., sputtering, chemical vapor deposition, etc.). Bulk Acoustic Wave (BAW) resonators exhibit series and parallel resonances that are similar to those of, by example, crystal resonators. Resonant frequencies of Bulk Acoustic Wave (BAW) resonators can typically range from about 0.5 GH to 5 GHz, depending on the layer thicknesses of the devices.

If a contaminating or otherwise harmful external material comes into contact with any of these layers, the performance of the Bulk Acoustic Wave (BAW) filters can

become degraded. In order to avoid this problem, these layers are typically protected using a semi-hermetic packaging.

Packaging of SAW or BAW filters is special because of the need to have a sealed cavity above the active filter structures. The reason is that any package or passivation material that would get in contact with the surface of acoustically active structures will start to vibrate itself and thus propagate acoustic waves and dissipate energy outside the active structures. The effects would include at least reduction of quality factors Q, shift of resonance or passband frequencies, increase of insertion loss, or complete non-functionality at all. Therefore, acoustic wave filter devices (e. g. SAW and/or BAW) can not be packaged into standard plastic mold packages, for example, and it is hard to construct chip-scale packages for them.

One known method of protecting these layer surfaces during assembly includes assembling the filters using, for example, flip-chip technology in a hermetic environment. As can be appreciated, this technique can be tedious to perform.

Another known method of protecting layer surfaces of SAW filters includes packaging the SAW filters in hermetically sealed ceramic packages. After being packaged in this manner, the SAW filters can then be surface mounted to a circuit board. Unfortunately, the costs of using semi-hermetic packaging can be high. Thus, it would be desirable to provide a novel, inexpensive technique for protecting these surfaces.

The foregoing and other problems are overcome by a method of fabricating filter devices as specified in independent claim 1. According to a further aspect of the present invention there is provided a filter device as specified in independent claim 20. Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be

understood as a first non-limiting approach of defining the invention in general terms.

According to a first aspect of the present invention there is provided a method of fabricating filter devices, comprising the steps of: providing a carrier wafer carrying a plurality of filters, providing a capping wafer, bonding the capping wafer to the carrier wafer, so that said filters are arranged in cavities located between the carrier wafer and the capping wafer, separating the bonded wafers into single filter devices, so that each filter device comprises a carrier substrate carrying at least one filter and a capping substrate, whereby said filter is arranged in at least one cavity located between the carrier substrate and the capping substrate.

Furthermore, the present invention provides a filter device comprising: a carrier substrate carrying at least one filter; and a capping substrate, so that said filter is arranged in at least one cavity located between the carrier substrate and the capping substrate.

Due to the use of a capping wafer/substrate no conventional package needed to protect the sensible filters, which reduces both product size and product costs significantly. Even though additional packaging is possible (i. e. in plastic molded packages, or in glob-top packages), it is not required for the reliability of the filters. A lot

of different wafer-bonding techniques are available, so that for many substrate and/or capping wafer materials an optimal package can be provided. According to the present invention, the capping of the sensible filters, especially the sensible acoustic wave filters, is be performed on wafer-level, i. e. in one process step for thousands of filters on a single wafer (batch processing).

The wafer capping can be performed within the wafer fab where the clean room facilities are best and allow to achieve and to maintain optimal surface conditions for the filters (i. e. minimal particulate contamination). Wafer-level packaging according to the present invention allows to dice the wafers after the packaging process, i. e. when the filter structures are already sealed inside a cavity. Thus, no additional protection of the filter surface is needed. In contrast, all conventional packaging solutions require to singulate the individual chips before the packaging process, which requires protection of the filter surfaces during the usually quite dirty sawing process. Furthermore, the formation of the interconnects (e. g. bumping) of the combined wafer can be performed using available standard techniques because of the complete protection/sealing of the filters.

The present invention basically avoids all the problems that conventional packaging methods provide to acoustic wave filters due to their need of an acoustic decoupling between device surface and package. The resulting filter device can then be used and assembled like standard chips, using for example wire bonding or a flipchip technology.

According to the present invention, packaging of the filters, especially the acoustic wave filters, is performed by bonding the carrier (substrate) wafer carrying the manufactured filters with a second wafer, called capping wafer. In principle, several wafer-bonding techniques are known for substrate materials that are used in semiconductor

industry. For example, silicon substrate wafers can be bonded with silicon capping wafers or with glass capping wafers (like PYREX, which is well adapted to silicon with respect to the thermal expansion coefficient). Wafer bonding techniques include silicon direct bonding, anodic bonding, eutectic bonding, solder bonding, and gluing.

According to a preferred embodiment either the capping wafer, or the carrier wafer, or both wafers are structured/patterned with a certain topography that guarantees that the filters are positioned well in a protecting sealed cavity after the bonding process. This patterning can preferably be done by micromaching techniques, for example, which are also already established for semiconductor or glass materials.

According to a further preferred embodiment, the combined wafers can be grinded and/or etched from either the top side, or the bottom side, or even both sides, after the wafer-bonding process, in order to reduce the height of the wafer-level package to a minimum. Preferably basic contact pads/metallizations or plating bases are protected during such thinning. Once the combined wafer is thinned, is preferred that the contact pads/plating bases are enhanced by metal deposition or by electroplating processes. According to a further preferred embodiment, so called"bumps"can be produced that are in contact with the pad metallization and that are large enough in diameter to allow for a stable assembly process on the customer's circuitry board.

Solder bumps and/or metal bumps, respectively, can be created using different methods like for example : Electroplating of alloys or of individual metals followed by a melting process; Vapour-deposition under vacuum; Chemical deposition (using auto-catalytic Ni solutions, for example); Solder transfer: electroplating of solder material onto a structured temporary support target/wafer, followed by a

transfer of the material deposits onto the substrate wafer (for example by heating of the solder material above the melt temperature); Use of nailhead bonds placed by a wire-bonder (wires made of Au, or PbSn, or SnAg, for example); Solder- ball bumpers: Placing of (preformed) solder balls (like PbSn or AuSn) on top of the pads with UBM (like NiAu). In the second step, the placed solder ball will be melt using a laser pulse (from a ND-YAG laser, for example).

According to a further preferred embodiment, there can be additional filters, especially acoustic wave filters, and/or active/passive ICs placed as flip chip on top of the carrier wafer within the cavitiy/cavities protected by the capping wafer. According to a still further preferred embodiment, additional passive components, e. g. capacities and/or inductivities, can be provided on the capping wafer.

Some of the above indicated and other more detailed aspects of the invention will be described in the following description and partially illustrated with reference to the figures. Therein: Fig. 1 shows schematically a cross-section of an exemplary Bulk Acoustic Wave (BAW) resonator that includes an air gap; Fig. 2 shows a top view of the Bulk Acoustic Wave (BAW) resonator of Fig. 1; Fig. 3 shows schematically a cross-section of an exemplary Bulk Acoustic Wave (BAW) resonator that includes an acoustic mirror; Fig. 4 shows schematically a cross-section of an exemplary Stacked Crystal Filter (SCF) that includes an air gap;

Fig. 5 shows schematically a top view of a portion of the Stacked Crystal Filter (SCF) of Fig. 4; Fig. 6 shows schematically a cross-section of an exemplary solidly-mounted the Stacked Crystal Filter (SCF) that includes an acoustic mirror; Fig. 7 shows a top view of a portion of the Stacked Crystal Filter (SCF) of Fig. 6; Figs. 8 to 10 show a method of fabricating filter devices according to a first embodiment of the present invention; Fig. 11 show a filter device according to a further embodiment of the present invention; Fig. 12 show a filter device according to a further embodiment of the present invention; Fig. 13 show a filter device according to a further embodiment of the present invention; and Fig. 14 show a filter device according to a further embodiment of the present invention.

Figs. 1 and 2 show a cross-section (side view) and a top view, respectively, of a Bulk Acoustic Wave (BAW) resonator 10 having a membrane or bridge structure 11. The Bulk Acoustic Wave (BAW) resonator 10 comprises a piezoelectric layer 12, a first protective layer 13a, a second protective layer 13b, a first electrode 14, a second electrode 15, the membrane 11, etch windows 16a and 16b, an air gap 17, and a substrate 18. The piezoelectric layer 12 comprises, by example, a piezoelectric material that can be fabricated as a

thin film such as, by example, zinc-oxide (ZnO), or aluminum- nitride (A1N).

The membrane 11 comprises two layers, namely, a top layer 19 and a bottom layer 20. The top layer 19 is made of, by example, poly-silicon or aluminum-nitride (A1N), and the bottom layer 20 is made of, by example, silicon-dioxide (Si02) or gallium arsenide (GaAs). The substrate 18 is comprised of a material such as, by example, silicon (Si), Si02, GaAs, or glass. Through the etch windows 16a and 16b, a portion of the substrate 18 is etched to form the air gap 17 after the membrane layers have been deposited over the substrate 18.

In Fig. 3, another Bulk Acoustic Wave (BAW) resonator 30 is shown. This resonator 30 has a similar structure as that of the Bulk Acoustic Wave (BAW) resonator 10 of Fig. 1, except that only a single protective layer 13 is provided, and the membrane 11 and the air gap 17 are replaced with an acoustic mirror 31 which acoustically isolates vibrations produced by the piezoelectric layer 12 from the substrate 18.

The acoustic mirror 31 preferably comprises an odd number of layers (e. g., from three to nine layers). The acoustic mirror 31 shown in Fig. 3 comprises three layers, namely a top layer 31a, a middle layer 31b, and a bottom layer 31c. Each layer 31a, 31b and 31c has a thickness that is, by example, approximately equal to one quarter wavelength. The top layer 31a and bottom layer 31c are made of materials having low acoustic impedances such as, by example, silicon (Si), poly-silicon, aluminum (Al), or a polymer. Also, the middle layer 31b is made of a material having a high acoustic impedance such as, by example, gold (Au), molybdenum (Mo), or tungsten (W). The ratio of the acoustic impedances of consecutive layers is large enough to permit the impedance of the substrate to be transformed to a lower value. As a result, the substrate 18 may be comprised

of various high acoustic impedance materials or low acoustic impedance materials (e. g., Si, Si02, GaAs, glass, or a ceramic material).

Reference will now be made to Figs. 4 to 7, which show various embodiments of another type of BAW device, namely a Stacked Crystal Filter (SCF). Figs. 4 and 5 show a Stacked Crystal Filter (SCF) 40. The Stacked Crystal Filter (SCF) 40 comprises a first piezoelectric layer 12a, a first protective layer 13a, a second protective layer 13b, a first electrode 14, a second electrode 15, the membrane 11, etch windows 16a and 16b, an air gap 17, and a substrate 18. The piezoelectric layer 12a comprises, by example, a piezoelectric material that can be fabricated as a thin film such as, by example, zinc-oxide (ZnO), or aluminum-nitride (A1N). Thereby, the second, middle electrode 15 is usually employed as a ground electrode.

In addition to these layers, the Stacked Crystal Filter 40 also includes an additional piezoelectric layer 12b that is disposed over the second electrode 15 and over portions of the first piezoelectric layer 12a. Furthermore, the Stacked Crystal Filter (SCF) 40 includes a third, upper electrode 41 that is disposed over a top portion of the piezoelectric layer 12b. The electrode 41 may comprise similar materials as the electrodes 14 and 15, and the piezoelectric layers 12b may comprise similar materials as the piezoelectric layer 12a.

FIG. 6 shows a solidly-mounted Stacked Crystal Filter 50 that is similar to the Stacked Crystal Filter 40 shown in Fig. 4. However, instead of an air gap 17 the solidly-mounted Stacked Crystal Filter 50 comprises an acoustic mirror 31 which acoustically isolates vibrations produced by the piezoelectric layers 12a and 12b from the substrate 18. As descrobed with respect to Fig. 3, the acoustic mirror 31 preferably comprises an odd number of layers (e. g., from

three to nine layers). The acoustic mirror 31 shown in Fig. 6 also comprises three layers, namely a top layer 31a, a middle layer 31b, and a bottom layer 31c. Each layer 31a, 31b and 31c has a thickness that is, by example, approximately equal to one quarter wavelength. The top layer 31a and bottom layer 31c are made of materials having low acoustic impedances such as, by example, silicon (Si), poly-silicon, aluminum (Al), or a polymer. Also, the middle layer 31b is made of a material having a high acoustic impedance such as, by example, gold (Au), molybdenum (Mo), or tungsten (W). It should be noted that a membrane or tuning layer (not shown) may also be provided between the acoustic mirror 30 and the electrode 14 of the device 50, if needed for tuning the device 50 to enable it to provide desired frequency response characteristics.

Figs. 8 to 10 show a method of fabricating filter devices according to a first emebodiment of the present invention.

As shown in Fig. 8 a silicon carrier wafer 50 is provided, which already contains a finalized acoustic wave filter 51. The acoustic wave filter 51 can be selected from a wide range of different acoustic wave filter types as Surface Acoustic Wave (SAW) filters, Bulk Acoustic Wave (BAW) filters and/or Stacked Crystal Filters (SCF). Preferably, acoustic wave filter 51 is includes at least one Bulk Acoustic Wave (BAW) resonator and/or Stacked Crystal Filters (SCF) as described with respect to the Figs. 1 to 7. In addition to the acoustic wave filter 51, the carrier wafer 50 comprises an integrated circuit (IC) (not shown), preferably a radio- frequency integrated circuit (RF-IC). Furthermore, the carrier wafer 50 comprises pads 52 which are later used to connect the acoustic wave filter 51 to the outside world.

In order to protect the acoustic wave filter 51 from contaminating or otherwise harmful external material, a

silicon capping wafer 53 is provided, which will be bonded to the carrier wafer 50. In the present embodiment the capping wafer 53 is structured to provide pad openings 54 and a recess 55, so that a cavity for the acoustic wave filter 51 is provided once the wafer bonding process is finished. On the surface of the capping wafer 53 which confronts the carrier wafer 51, a layer 56 of solder material is provided.

In the present embodiment an AuSi layer is provided as solder material. Preferably, the wafer bonding is compatible to the temperature budget in the later processing, i. e. as seen during the reflow process of bump formation and the reflow soldering during assembly of the product later on. The process according the present embodiment ensures this by using AuSi eutectic bonding for the wafer bonding process, because the AuSi eutectic temperature T = 363°C is well abvove the melting point of alloys like Sn/Pb (T = 183°C for composition 63/37) and typical reflow temperatures around 230°C as used in later process stages.

After the AuSi eutectic wafer bonding process is finished, the acoustic wave filter 51 is arranged in a cavity 56 located between the carrier wafer 51 and the capping wafer 53. Due to the form of the recess 55 in the capping wafer 53 and the nature of the AuSi eutectic wafer bonding process, the acoustic wave filter 51 is hermetically sealed within cavity 57. Accordingly, a high reliability of the acoustic wave filter 51 can be guaranteed. The AuSi eutectic wafer bonding process is preferably performed within the wafer fab where the clean room facilities are best and allow to achieve and to maintain optimal surface conditions for the acoustic wave filter 51 (i. e. minimal particulate contamination).

Following the AuSi eutectic wafer bonding process, the combined wafer 51 and 53 are grinded both sides, in order to reduce the height of the wafer-level package to a minimum.

Preferably basic contact pads/metallizations or plating bases

are protected during such thinning (not shown). The resulting structure is shown in Fig. 9.

Following the thinning process the interconnections are produced. According to the present embodiment a so called "bumping process"is used to fabricate the interconnections.

Bumping processes usually require some under-bump metallization (UBM, not shown), which has already been deposited on the pads 52 before the wafer bonding.

Preferably, a structured deposition of bump materials (dump deposits) using selective deposition methods, like microform electroplating or lift-off techniques, is performed.

Thereafter the remaining under-bump metallization (UBM) is etched utilizing the bump deposists as an etch mask, and a bump formation is performed by a reflow process that melts the alloy and forms the bump balls 58. The resulting structure is shown in Fig. 10.

Thereafter, a wafer dicing process is performed which separates the bonded wafers into single filter devices, so that each filter device comprises a carrier substrate carrying at least one filter and a capping substrate, whereby said filter is arranged in at least one cavity located between the carrier substrate and the capping substrate. The resulting filter device may then be connected to a wiring substrate using a standard flipchip technology.

In case no"bumping process"is used, the wafer dicing process can be performed directly after the thinning of the wafer package. The interconnections are then produced after wafer dicing process, for example with the help of a conventional wire bonding process whereby wires 59 are used to contact the pads 52. The resulting filter devices is shown in Fig. 11.

The filter devices shown in Fig. 10 comprise a capping wafer 53 that was structured to provide pad openings 54. Fig.

12 show a filter device according to a further embodiment of the present invention wherein the carrier substrate 60 is structured to provide openings. Accordingly, the bumping process that is used to fabricate the interconnections 68 is applied to back surface of the carrier wafer.

Fig. 13 show a filter device according to a still further embodiment of the present invention. The filter device shown in Fig. 13 is similar to the filter device shown in Fig. 10 except for the fact, that the acoustic wave filter now comprises two Bulk Acoustic Wave (BAW) resonators 51 located in the sealed cavity. This allows for a multi-band operation of the final filter device.

Fig. 14 show a filter device according to a still further embodiment of the present invention. The filter device shown in Fig. 14 comprises a carrier substrate 70 and an additional substrate (chip) which are electrically and mechanically connected to the substrate 70 by a flip-chip technique. Both the carrier subtrate 70 and the flip-chip- mounted substrate 71 have active or passive IC components 72 and Bulk Acoustic Wave (BAW) resonators 73 on it. The capping substrate 74 also contains additional passive components 75, like a coil which acts as an inductivity.

The carrier subtrate 70 and the flip-chip-mounted substrate (die) 71 are covered by the capping wafer 74 and sealed within the cavity 76. Using the connections 78 the filter device shown in Fig. 14 may then be connected to a wiring substrate using a standard flipchip technology.