WO2012049374A1

WO2012049374A1 - Balanced-unbalanced filter based on laterally coupled baw thin films

Info

Publication number: WO2012049374A1
Application number: PCT/FI2011/050895
Authority: WO
Inventors: Johanna Meltaus; Tuomas Pensala; Markku Ylilammi
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2010-10-14
Filing date: 2011-10-14
Publication date: 2012-04-19
Anticipated expiration: 2013-04-14
Also published as: FI20106060A0; FI20106060A7; FI20106060L

Abstract

The invention describes an acoustic balanced-unbalanced (balun) or balanced-balanced thin-film BAW filter based on lateral acoustic coupling. In laterally acoustically coupled thin- film BAW filters (LBAW) one can realize transformation from unbalanced to balanced electric signal if the electrodes of the balanced port are placed on the opposite sides of the piezoelectric film. The manufacturing process is simpler than in the corresponding component based on vertical acoustical coupling. The device can also realize impedance transformation.

Description

BALANCED-UNBALANCED FILTER BASED ON LATERALLY COUPLED BAW

THIN FILMS

FIELD OF INVENTION The present invention relates generally to the balancing and unbalancing of electronic signals in laterally coupled bulk acoustic wave (LBAW) thin film filters.

BACKGROUND OF THE INVENTION

In certain applications, such as radio applications, a conversion from a balanced signal to an unbalanced signal, or vice versa, is often required. Traditionally, the primary way to accomplish a balanced-unbalanced conversion is with a dedicated component. Having a separate component increases the requisite size and cost of such a device. Therefore it is advantageous if a conversion can be realized in another component.

An example of a component that is typically present in devices requiring balanced- unbalanced signal conversion is a filter. Several types of filter arrangements are known which can provide required signal conversion in addition to their standard required filter capabilities.

Bulk acoustic wave (BAW) filters, when arranged using vertical acoustic coupling (CRF), can potentially be used for balanced-unbalanced signal conversion. Balanced BAW filters are realized by vertically coupling two piezoelectric layers. However, due to the presence of two layers, these filters are extremely sensitive to the thickness variations between the piezoelectric layers. An additional disadvantage to using CRF BAW filters is that they are both difficult and expensive to manufacture.

Another type of filter that can be utilized is a surface acoustic wave (SAW) filter. Balun transformation, as well as impedance transformation, can be realized relatively easily in SAW components. However, compared to BAW, SAW components have much worse power handling capabilities. SAW filters have additional distinct disadvantages. SAW filters are often too large to be used in certain devices, specifically in small compact devices. Another disadvantage is that there are difficulties associated with patterning at high frequencies, e.g. above 2 GHz.

SUMMARY OF THE INVENTION The object of the present invention is realized with laterally acoustically coupled thin-film BAW (LBAW) filters.

It is an object of some embodiments of the present invention to provide a filter and methods for manufacturing said filter which are capable of balanced to unbalanced signal transformation. An LBAW filter according to an embodiment of the present invention is characterized by having a single piezoelectric layer having electrodes on both a top and bottom surface.

It is also an aspect of some embodiments of the present invention to provide a filter and methods for manufacturing said filter which are capable of unbalanced to balanced signal transformation which overcome at least some of the disadvantages of the prior art. It is a further aspect of some embodiments of the present invention to provide a filter and methods for manufacturing said filter which are capable of balanced to balanced signal transformation which overcome at least some of the disadvantages of the prior art.

Furthermore, it is an aspect of some embodiments of the present invention that impedance transformation can be realized. In LBAW filters, balun conversion is possible when the bottom electrode is patterned to correspond to the top electrode. Then, one signal of the balanced port can be taken from the top of the piezoelectric layer and the other signal from the bottom of the single piezoelectric layer.

Typically, the coupled electrodes in LBAW components should be narrow, e.g. interdigital fingers. Due to this structure it makes possible the impedance transformation between input and output ports. However, patterning of a bottom electrode into narrow fingers or other narrow patterns creates multiple steps in the bottom electrode area. Piezoelectric material, such as A1N, can have difficulties growing well on such steps because discontinuities in the underlying material may cause crystal defects. Therefore, there is herein described a method of manufacturing LBAW components according to embodiments of the present invention which is independent of piezoelectric material selection.

A method of the present invention is characterized in the characterizing portion of independent claim 1.

Due to the design of LBAW devices according to embodiments of the present invention there are realized several distinct advantages over the prior art. Since only one piezoelectric layer is necessary in certain embodiments, the coupling is not particularly sensitive to its thickness. This makes the fabrication process simpler and less expensive. Additionally, as LBAW devices can be realized on thin films, the devices are easily capable of operation at GHz frequencies.

The present invention will now be discussed in more detail with respect to the figures and exemplary embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 : Principle of LBAW balun filter. Acoustic vibration is excited in a piezoelectric thin film layer. Port 1 is unbalanced and port 2 is balanced. Balanced signals have opposite phase and are taken from the top and bottom of the piezoelectric layer.

Fig 2: SEM image of a LBAW filter (top electrode). Electrode length is 300 μιη and width and gaps are less than 5 μιη. Balun function requires the patterning of bottom electrode into a similar structure. Other electrode configurations and shapes are possible.

Fig. 3a-f: Fabrication steps for fabricating a LBAW filter using a layer transfer technique (LTT).

Fig 4: LBAW balun filter on an acoustic mirror. Fig 5: LBAW balun filter on an air gap.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS The general operation principle of a balun LBAW filter 10 is depicted in Figure 1. In the example of Figure 1, unbalanced signal 14 is input to the balun LBAW filter 10 on a first side of a piezoelectric layer 12. Opposite-phased signals 15 and 17 in the balanced port are then taken from opposite sides of the piezoelectric layer 12. Additionally, there is a ground 16 located opposite the input 14. The input and output ports can be interchanged to obtain balanced input and unbalanced output.

Figure 2 shows an example of an LBAW filter electrode structure. One side of the finger electrodes is the input port while the other side is the output port. In general, however, the electrodes need not be alternatingly connected to input and output ports. Some electrodes may be grounded while other may be left floating. Also, the number of ports is not restricted to 2. Electrode and gap widths are also not restricted but can vary within the electrode structure. In a balun filter, the bottom electrodes should be patterned in a corresponding manner as the top electrodes. More specifically, the bottom electrode should be similar and match the top electrode. One of the additional advantages to a balun LBAW filter is that the structure can also realize an impedance transformation.

In the present invention, a thin film balun filter is realized using LBAW technology.

Acoustic vibration is generated in a piezoelectric thin film. Example materials that are usable for such a thin film are as A1N, ZnO and PZT. One of ordinary skill in the art will recognize other suitable piezoelectric materials. The frequency response, central operating frequency and bandwidth, of the device is determined primarily by the piezoelectric material used, the thickness of the piezoelectric layer, the composition of nearby layers and to some extent the dimensions of the electrodes. A signal conversion is created by patterning the bottom electrode to correspond to the top electrode. A number of different structures and patterns can be used for the top electrode, for example a finger structure as shown in Figure 2, as long as the bottom electrode corresponds.

Several problems arise in the fabrication process when attempting to create a device having a piezoelectric layer with electrodes on the bottom side of the piezoelectric layer.

Patterning of a bottom electrode into narrow fingers or other narrow patterns creates multiple steps in the bottom electrode area. Piezoelectric material, such as A1N, can have difficulties growing well on such steps because discontinuities in the underlying material may cause crystal defects. Therefore, although it is disadvantageous to merely fabricate a bottom electrode, then grow a piezoelectric layer and finally fabricate a top electrode on the new top surface of the piezoelectric layer, it is a possible method for creating a device according to the present invention. The smooth surface required by the piezoelectric layer can be obtained by filling the gaps between a patterned bottom electrode, for example using a Si02 layer, and then polishing the surface (CMP). This process, planarizing, requires that the electrode material is compatible with the CMP process.

The CMP process described above is material-dependent and limits the electrode materials. To achieve maximal lateral coupling it is better to be able to choose the materials more freely. A materials-independent way to combine a high-quality piezoelectric film to any electrode material is a layer transfer technique (LTT). A more detailed description of LTT is described in patent publications FI20060832, US 2010/0067167 and WO 2008/034940, the subject matter of which is herein incorporated by reference. Therefore, there is herein described an exemplary method for fabricating a device according to the present invention. Figures 3a-f show a method of fabrication for a balun LBAW 30 using a layer transfer technique (LTT). Here the piezoelectric film can be deposited first and the bottom electrode processed on top of it. Using wafer bonding the piezoelectric layer and the electrode are transferred to another wafer, after which the process can be continued on top of the piezoelectric layer.

Figure 3 a shows a piezoelectric layer 32 which has been grown on a substrate 31. An example of a suitable substrate is Si. The important characteristic of substrate 31 is that the desired piezoelectric material which makes up the piezoelectric layer 32 should grow well on it. Since the substrate 31 will not normally make up a part of the final device, the selection of the substrate material is not limited to any particular material.

Once a piezoelectric layer is formed on the substrate 31 then the bottom electrode 33 can be fabricated on the free surface of the piezoelectric layer 32. Electrode 33 can be patterned by any suitable method known in the art. On top of the patterned bottom electrode 33 a dielectric layer 34 is deposited and polished as shown in Figure 3b. The dielectric layer 34 will eventually form the first layer below the piezoelectric layer in the final thin-film. The advantage to the present approach is that dielectric materials are not as affected by the multiple steps of the bottom electrode as growing a piezoelectric layer is when grown on the bottom electrode. Once the dielectric material is deposited and polished then there is a fresh, polished surface on which another layer of the thin- film can be formed. The acoustic properties of the filter, i.e. the thin film composition and the layer stack, must be designed such that the acoustic coupling between electrodes is possible, and the acoustic energy does not escape from the device. Coupling between electrodes can be realized either with an evanescent of with a propagating wave.

Figure 3c shows layer 35 which is formed on top of the dielectric layer 34. Layer 35 can be one or more prefabricated layers which will form part or all of the final thin-film. For example, layer 35 can be a carrier wafer which is bonded to the polished surface by any well known process. At this point, additional layers can be added if desired or the process can continue to creating the top electrode.

Figure 3d shows the device 30 of Figure 3c inverted and with the substrate 31 removed. Substrate 31 can be removed with any known method such as thinning or etching so as to expose at least a portion of the piezoelectric layer 32. Although it is not necessary, it is often ideal to remove all of the substrate material 31. Additionally, it is possible at this stage to modify the now open surface of the piezoelectric layer 32 by adding or removing material if necessary. At one or more locations on the piezoelectric layer it can be required to form an electrical via to connect one or more portions of the bottom electrode to the top electrode. Figure 3e, as an example, shows an electrical via 36 formed at an output end of device 30. Any suitable method for creating such an electrical via, known in the art, can be used where desired. On top of the prepared open surface of the piezoelectric layer 32, now the top surface either with or without one or more electrical vias, the top electrode 37 is manufactured. As discussed above, it is important to pattern the top electrode 37 so that it corresponds to the previously manufactured bottom electrode 33, as shown in Figure 3f. The device of Figure 3f can be a complete device or it can proceed to further necessary manufacturing steps as required. A balun LBAW filter according to the present invention can be realized in several embodiments. Examples of such embodiments which will be discussed herein are on an acoustic mirror and on an air-gap. The present invention is not limited to the example embodiments herein described. One of ordinary skill in the art may recognize other embodiments of a balun LBAW filter according to the present invention which do not depart from the scope of the present application.

An acoustic mirror, or acoustic Bragg reflector, is composed of thin film layers 47 and 48 with alternating high and low acoustic impedances, for example W and Si02. An example of a balun LBAW filter on an acoustic mirror 40 is shown in Figure 4. Using an acoustic mirror gives flexibility to the design of the filter's acoustic properties. In a balun filter it may be necessary to use a wholly dielectric mirror to avoid electrical parasitic

capacitances.

Figure 4 shows the balun LBAW filter 40 with a piezoelectric layer 41 having a top electrode (42, 45a, 44a, 43) on a first, top, surface and a bottom electrode (45b, 44b) on a second, bottom, surface. While only a few electrodes are shown, the number of electrodes may be chosen appropriately so that the desired frequency response is obtained. Below the piezoelectric layer is the Bragg reflector consisting of low acoustic impedance layers 47, which can be Si02, alternated with high acoustic impedance layers 48, which can be W. While only three layers are shown, an appropriate number of additional alternating layers with the same, similar or different materials can be added. The Bragg reflector is then supported by the substrate 49.

It can be seen from Figure 4 that the top and bottom electrodes, though not identical, correspond to each other. The top electrode includes an input electrode 42 and a first output 44a. Connected to the top electrode by 43 and an electrical via 46 is the output of the bottom electrode, 44b. An LBAW balun filter on a mirror can be processed using any of the above described techniques.

A balun LBAW filter with an air gap, as shown in Figure 5, requires fewer layers than a mirror stack. However, its acoustic properties cannot be modified as freely as those of a mirror structure. Patterning of an air gap structure can be realized either with planarization or with LTT. The structure can be released, for example, by etching through the substrate 58 from the back side. Alternatively, it can be achieved by doing a release etch from the top with the help of release holes. The film can also be transferred onto a cavity-SOI wafer. To design the acoustic properties, the materials can be selected appropriately and/or an extra layer or layers 59 can be deposited on top of the structure.

Figure 5 shows an LB AW balun filter 50 on an air gap with a piezoelectric layer 51 having a top electrode (52, 55a, 54a, 53) on a first, top, surface and a bottom electrode (55b, 54b) on a second, bottom, surface. Below the piezoelectric layer is air gap fabricated into substrate 58 with structures supported by the dielectric material layer 57, which can be Si02.

It can be seen from Figure 5 that the top and bottom electrodes, though not identical, correspond to each other. The top electrode includes an input 52 and a first output 53. Connected to the top output 53 by an electrical via 56 is the output of the bottom electrode 54b. The second output of the balanced signal is taken from electrode 54a.

Typically, the coupled electrodes in LBAW components should be narrow, e.g. interdigital fingers. For example, depending on the operation frequency, the electrodes should normally be of the order of around 1 to 10 μιη. For example, the electrodes, top and/or bottom, should be manufactured having widths between 0.01 to 100 μιη, preferably between 0.01 to 50 μιη, more preferably between, 0.05 to 15 μιη.

In some embodiments, the electrodes should be even narrower, such as on the order of the lateral wavelength at the desired operation frequency. For example, such electrodes should be ± 50% of the lateral wavelength of a desired operation frequency, preferably ± 20% of the lateral wavelength of a desired operation frequency, more preferably ± 10% of the lateral wavelength of a desired operation frequency.

Impedance transformation can be realized as an aspect of the present invention. The impedance levels at input and output can be modified by designing the electrode structures appropriately. Compared to a corresponding single-single filter, a balun filter has double impedance in its output port. Therefore, if an unbalanced component is designed for 50

Ohms, a similar balun filter with balanced output transforms the impedance from 50 to 100 Ohms.

Port impedance can also be modified by modifying a port's static capacitance CO. Static capacitance is affected by the area of the electrode(s), which can be changed by changing the width, shape, or number of electrodes. Both input and output port impedances can be modified this way. Devices according to the present invention generally have the advantages over the prior art of simpler and therefore less expensive fabrication process especially when compared to vertically coupled BAW filters. The use of LTT is makes manufacture in material independent allowing for better device design. Additionally, the better power handling, especially when compared to SAW filters allows for smaller size and higher operation frequencies.

Devices according to embodiments of the present invention can be used in at least the following applications; small size RF balun filters, for example in cell phones, WLAN and other wireless devices, especially at GHz frequencies. While the present invention has been described with the aid of the drawings and exemplary embodiments, one of ordinary skill in the art will recognize variations not disclosed herein but which do not part from the scope of the present invention. Further examples are described in U.S. Provisional application 61/392,955 which is incorporated herein by reference in its entirety.

Claims

A method of manufacturing an LBAW element (30) comprising the steps of:

- fabricating one or more first electrodes (33) on a first surface of a

piezoelectric layer (32),

CHARACTERIZED BY,

- forming a dielectric layer (34) on at least a portion of the exposed first surface of the piezoelectric layer (32) and at least a portion of the one or more first electrodes (33), and

- fabricating one or more second electrodes (37) on a second surface of a piezoelectric layer (32) opposite of the first surface, wherein the second electrodes (37) correspond to the first electrodes (33).

A method in accordance with claim 1 characterized by,

- fabricating the one or more first electrodes (33) on a first surface of a piezoelectric layer (32) which has a substrate (31) affixed to the second surface of the piezoelectric layer, and

- removing the substrate (31) prior to fabricating the one or more second electrodes (37).

A method in accordance with claim 1 or 2 characterized by

- planarizing the exposed surface of the dielectric layer (34), and

- forming one or more additional layers (35) on the planarized surface of the dielectric layer (34).

A method in accordance with claim 3 characterized by,

- forming one or more additional layers (35) includes bonding one or more prefabricated layers.

A method in accordance with claim 4 characterized by,

- the one or more prefabricated layers is a carrier wafer.

6. A method in accordance with claims 3-5 characterized by,

- the one or more additional layers is part or all of an acoustic mirror or an air- gap structure.

7. A method in accordance with any of the preceding claims, characterized by the area of the electrodes being chosen to provide a desired impedance level transformation.

8. A method in accordance with any of the preceding claims, wherein said first and/or second electrodes are inderdigital fingers.

9. A method in accordance with any of the preceding claims, wherein said first and/or second electrodes have widths on the order of a desired operation frequency of the LBAW element.

10. A method in accordance with claim 9, wherein said first and/or second electrodes are fabricated having electrode widths between ± 50% of the lateral wavelength of a desired operation frequency, preferably between ± 20% of the lateral wavelength of a desired operation frequency, more preferably between ± 10% of the lateral wavelength of a desired operation frequency.

11. A method in accordance with any of the preceding claims, wherein said first and/or second electrodes are fabricated having widths between 0.01 to 100 μιη, preferably between 0.01 to 50 μιη, more preferably between 0.05 to 15 μιη, still more preferably between 1 to 10 μιη.

12. A method in accordance with any of the preceding claims, wherein said first

electrodes are not identical to said second electrodes.

13. A method in accordance with any of the preceding claims, further comprising the steps of fabricating at least one port and/or input on one or more sides of said piezoelectric layer.

14. A method according to claim 13, wherein a plurality of ports are arranged with respect to each other such that balanced-unbalanced signal conversion is achievable by said LBAW element.

15. A method in accordance with any of the preceding claims, wherein said steps are carried out in the order recited.

16. An LBAW element (30) having a predefined desired operating frequency manufactured by the ordered steps comprising:

- fabricating one or more first electrodes (33) on a first surface of a

piezoelectric layer (32),

- forming a dielectric layer (34) on at least a portion of the exposed first

surface of the piezoelectric layer (32) and at least a portion of the one or more first electrodes (33), and

- fabricating one or more second electrodes (37) on a second surface of a

piezoelectric layer (32) opposite of the first surface, wherein the second electrodes (37) correspond to the first electrodes (33) wherein said first and second electrodes as well as said dielectric layer combine to effect lateral acoustical coupling.

17. An LBAW element according to claim 16, wherein said first and second electrodes have widths on the order of the desired operation frequency of the LBAW element.

18. An LBAW element according to either claim 16 or 17, wherein said first and/or second electrodes are fabricated having electrode widths between ± 50% of the lateral wavelength of a desired operation frequency, preferably between ± 20% of the lateral wavelength of a desired operation frequency, more preferably between ± 10% of the lateral wavelength of a desired operation frequency.

19. An LBAW element according to any of claims 16-18, wherein said first and/or second electrodes are fabricated having widths between 0.01 to 100 μιη, preferably between 0.01 to 50 μιη, more preferably between 0.05 to 15 μιη, still more preferably between 1 to 10 μιη.

20. An LBAW element manufactured by any of claims 1-13.

21. An LBAW balun filter manufactured by any of claims 1-13.

22. An LBAW balanced - balanced filter manufactured by any of claims 1-13.