The present application relates to methods for manufacturing resonator structures and to corresponding resonator structures.
Filters are used in a variety of electronic circuits to filter out certain frequency components of a signal while letting other frequency components pass. For example, in communication circuits, filters may be used to block frequency components outside a frequency band or part of a frequency band used for communication and to be processed by further circuits.
To increase bandwidth, communication standards like wireless communication standards (for example LTE, Long Term Evolution) or also wire-based communication standards continually increase a used frequency range and a number of used frequency bands. In communication devices implementing such standards, often highly selective filters matching to the respective frequency bands are required. The frequency bands used may differ from country to country. Therefore, a plurality of filters having different filter characteristics (for example different passbands) are required. Furthermore, in what is referred to as carrier aggregation several frequency bands are operated at the same time. This requires specific filter designs for exactly those combinations. With a specific filter provided for each possible combination, the number of physical filters is actually much higher than the number of available bands. In order to reduce the number of different filters (2-port up to n-port filters) actually required in a communication device, tunable filters are highly desirable.
As highly selective bandpass filters in communication circuits and devices, surface acoustic wave (SAW) or bulk acoustic wave (BAW) technologies are frequently used. Conventional filters of such types are designed for fixed resonance or center frequencies. As a consequence, many filters are required to serve individual frequency bands or aggregated combinations of several frequency bands used in current communication standards like LTE or WiFi standards. Radio frequency (RF) switches are then used to select individual filters of the plurality of filters for example for desired signal paths between an antenna, a low noise amplifier or power amplifier. As such conventional approaches require a large number of mostly discrete components and as space is limited in mobile devices, tunable solutions are highly desirable.
One approach to provide tunability to BAW filters is to use coupled resonators with a first resonator and a second resonator, where the second resonator serves as the actual filter resonator and the first resonator, which is acoustically coupled to the second resonator, serves for tuning, for example by adjusting tunable capacitors coupled therewith. In some applications, different materials are desired for the first and second resonators. For example, for the second resonator, in many applications, a small resonator bandwidth is desired to provide small-band filters, while the second resonator should provide a large tuning range, which requirements may be implemented using different materials. However, such different materials may be difficult to integrate in a manufacturing process, for example due to different wafer sizes available and/or due to different material properties.
A method as defined in claim 1 or 6 and a resonator structure as defined in claim 19 are provided. The dependent claims define further embodiments.
According to an embodiment, a method for manufacturing a coupled resonator structure is provided, comprising:
processing a first wafer to form a processed first wafer, the processed first wafer comprising a first piezoelectric material,
processing a second wafer to form a processed second wafer, the processed second wafer comprising a second piezoelectric material,
singulating the first wafer to form at least one singulated wafer chip,
bonding the at least one singulated wafer chip to the second wafer to form a joint wafer, and
processing the joint wafer to form a resonator structure comprising a first resonator including the first piezoelectric material and a second resonator including the second piezoelectric material such that the first and second resonators are acoustically coupled with each other.
According to another embodiment, a method for manufacturing a resonator structure is provided, comprising:
processing a first wafer to form a processed first wafer, the processed first wafer comprising a first piezoelectric material,
processing a second wafer to form a processed second wafer, the processed second wafer comprising an acoustic termination at a first side thereof,
singulating the first wafer to form at least one singulated wafer chip,
bonding the at least one singulated wafer chip to the second wafer such that the first side of the second wafer faces the singulated wafer chip to form a joint wafer, and
processing the joint wafer to form a resonator structure comprising a first resonator including the first piezoelectric material.
According to another embodiment, a resonator structure is provided, comprising:
a diced first wafer piece comprising a first piezoelectric material,
a substrate comprising an acoustic termination at a first side thereof, wherein the diced wafer piece is bonded to the second wafer such that the first side faces the diced wafer piece.
The above summary is merely intended to give a brief overview over some implementations and is not to be construed as limiting.
In the following, various embodiments will be described in detail referring to the attached drawings. It should be noted that these embodiments serve illustrative purposes only and are not to be construed as limiting. For example, while embodiments may be described as comprising a plurality of features, elements or details, in other embodiments, some of these features, elements or details may be omitted and/or may be replaced by alternative features, elements or details. In addition to the features, elements or details explicitly described or shown in the drawings, other features, elements or details, for example components conventionally used in bulk acoustic wave (BAW) resonators or BAW-based filters, may be provided.
Features from different embodiments may be combined to form further embodiments unless noted to the contrary. Variations or modifications described with respect to one of the embodiments may also be applicable to other embodiments unless noted otherwise.
Embodiments discussed in the following relate to manufacturing of bulk acoustic wave (BAW) resonator structures, which may be used to build a BAW-based filter. For forming acoustic resonators like BAW resonators, generally a piezoelectric layer is provided between two electrodes (e.g. top and bottom electrodes), and acoustic waves propagate through the bulk of the piezoelectric material. The application of an electric field between the two electrodes generates a mechanical stress that is further propagated through the bulk of the structure as an acoustic wave. A resonance condition is established when the acoustical path in thickness direction of the structure corresponds to integer multiples of half the acoustic wave length.
In embodiments, at least two resonators are used which are acoustically coupled to each other to form a resonator structure. The at least two resonators comprise a first resonator using a first piezoelectric material and a second resonator using a second piezoelectric material. In some embodiments, the first piezoelectric material is provided as or on a first wafer, which is processed, diced and then provided to a second wafer. This way, by first dicing the first wafer and then providing the diced portions (also referred to as chips herein) to the second wafer enables the use of different materials which are provided in different wafer sizes.
For example, embodiments used herein may be used to integrate a thin lithium niobate (LiNbO3) crystal film into such a coupled resonator structures, for example based on silicon wafers. LiNbO3 crystals are usually available as wafers having a diameter of about 100 mm, while silicon (Si) wafers are available in larger sizes, for example as 200 mm or even 300 mm wafers. Using larger wafers for building resonator structures in many applications is preferable, as it enables parallel processing of more structures on a single wafer and may therefore serve to increase production yield. By using techniques disclosed herein, e.g. 100 mm LiNbO3 wafers may be used to form resonator structures on larger Si wafers like 200 mm wafers or 300 mm wafers. While LiNbO3 is used as an example material herein, other materials may also be used, for example lithium tantalate (LiTaO3).
Turning now to the figures,
At 10, the method of
At 11, the method comprises processing a second wafer. The second wafer in some embodiments may be a semiconductor wafer, and processing the second wafer may comprise forming a resonator comprising a second piezoelectric material on the second wafer. In some embodiments, the second wafer is a silicon wafer, and processing the second wafer may comprise depositing for example aluminum nitride as second piezoelectric material on the second wafer, and/or may comprise forming electrodes or other components of the second resonator.
At 12, the method comprises singulating the first wafer, also referred to as dicing, i.e. cutting individual pieces (also referred to as chips, chip dies or dies) from the first wafer, each piece being intended for one or more individual resonator structures. In other words, each piece may have a size to comprise one or more structures intended for one or more resonators structures in a final device.
It should be noted that the acts described with reference to numeral 10-12 may also be performed in a different order. For example, the second wafer may be processed prior to processing the first wafer, or may be processed in parallel to processing and/or singulating the first wafer.
At 13, the pieces singulated at 12 are transferred to the second wafer and bonded to the second wafer. It should be noted that pieces from more than one first wafer may be transferred to a single second wafer, for example to use a larger area of the second wafer for device manufacturing compared to an area of the first wafer. For example, in some implementations, the first wafer may be a LiNbO3 wafer, with sizes typically up to 100 mm diameter, while the second wafer may be a silicon wafer having a diameter of 200 mm or 300 mm.
At 14, the method then comprises processing of the joint wafers (second wafer with singulated pieces of first wafer bonded thereto) to finalize resonator structures.
Next, a more specific example for the general method and device described with reference to
At 30 of
At 31, the method comprises depositing a metal layer as a conductive material on the LiNbO3 wafer, which metal layer will later form a bottom electrode of a first resonator. In
At 32, optionally the method of
At 33, the method comprises deposition an oxide (or other isolation layer) on the LiNbO3 wafer, and subsequently planarizing the oxide. An example for a suitable oxide is silicon dioxide. The planarization may for example be performed by a standard CMP process (chemical mechanical polishing). An example result of the planarization is illustrated in
It should be noted that
At 34, the method of
At 35, the method then comprises providing a silicon substrate wafer with a resonator structure processed thereon. An example is shown in
The resonator structure of processed wafer 45 serves only as an example, and other conventional BAW resonator structures may also be used.
At 36, the chips of the LiNbO3 wafer generated by singulation are then transferred to the processed silicon wafer and bonded thereto. This is also illustrated in
It should be noted that a plurality of chips 44 may be placed on processed wafer 45 to be aligned with different resonator structured on processed wafer 45 for a production of a plurality of resonator structures or a plurality of devices with resonator structures on a single wafer. In this case,
At 37, after the chips 44 are bonded to processed wafer 45, the entire wafer 35 with the bonded chips 44 is then encapsulated for example by a compression molding process. This is illustrated in
Next, in the embodiment of
At 39, the method comprises structuring of the LiNbO3 layer, electrode deposition and structuring. This is illustrated in
In
At 310, the method comprises providing connections for the electrode to a desired location. For example, as shown in
Substrate wafer 46 may then be diced to form individual devices with parts of wafer 46 serving as substrate.
Just for better illustrating the purpose of resonator structures as discussed previously having two resonators stacked upon each other, operation and application of such resonator structures will now be explained using non-limiting examples referring to
Second resonator 50 has a first terminal 51 and a second terminal 52. Using first and second terminals 51, 52 which may for example correspond to or be coupled to electrodes of second resonator 50, the resonator structure of
Furthermore, tuning circuit 55 is coupled to first resonator 54. Tuning circuit 55 may comprise an impedance network, which may comprise variable elements like variable impedances, for example a variable capacitor, or switches like radio frequency (RF) switches. By changing a value of the variable element(s) of tuning circuit 55, resonances of the resonator structure of
In such an approach with first and second resonators, tuning circuit 55 is electrically decoupled from second resonator 50 and acts on second resonator 50 only via first resonator 54 and acoustic coupling 53. In some embodiments, this avoids adverse effects compared to tuning circuits directly coupled to second resonator 50. Second resonator 50 may also be referred to as a filter resonator, as it is to be incorporated into a filter structure using first and second terminals 51, 52. First resonator 54 may also be referred to as a frequency tuning resonator, as it is used for tuning resonance frequencies of the resonator structure of
As mentioned, second resonator 50, acoustic coupling 53 and first resonator 54 may be implemented in a process flow as discussed above.
For further illustration,
Each of resonators 73A to 73D, 74A to 74C may be a first resonator of a resonator structure as discussed previously with respect to
The resonator structure of
One electrode of the second resonator 82 is connected to ground (e.g. 88 in
In the example filter structure of
Furthermore, a tuning circuit is coupled between the electrodes of first resonator 85. In the example of
An inductance 86, e.g. an inductor, may increase a tuning range compared to a case where only a variable capacitor is used.
The electrodes of second resonator 92 are coupled with a first terminal 90 and a second terminal 91, respectively. Via first and second terminals 90, 91, the resonator element of
Furthermore, a tuning circuit is coupled to the electrodes of first resonator 95 comprising for example an inductance 96 and a variable capacitor 97. Inductance 96 and variable capacitance 97 may be implemented in a similar manner as explained for inductance 86 and variable capacitance 87 of
With the shunt resonator element of
It should be noted that the above filter structures and explanations only serve for further illustrating applications of the resonator structures discussed referring to
Above, methods for manufacturing coupled resonator structures have been discussed. In other embodiments, techniques as discussed above, in particular the singulating of a wafer made of a piezoelectric material followed by a bonding of one or more singulated pieces to another wafer, may be used to manufacture single resonator structures. In this way, for example a LiNbO3-based resonator may be integrated in a silicon environment, where for example an acoustic termination is provided in or on a silicon wafer as discussed above.
For such a manufacturing, the same process flow as discussed above may be used, where only the formation of a resonator in the second wafer is omitted. To illustrate this,
At least some embodiments are defined by the examples given below:
A method for manufacturing a coupled resonator structure, comprising:
processing a first wafer to form a processed first wafer, the processed first wafer comprising a first piezoelectric material,
processing a second wafer to form a processed second wafer, the processed second wafer comprising a second piezoelectric material,
singulating the first wafer to form at least one singulated wafer chip,
bonding the at least one singulated wafer chip to the second wafer to form a joint wafer, and
processing the joint wafer to form a resonator structure comprising a first resonator including the first piezoelectric material and a second resonator including the second piezoelectric material such that the first and second resonators are acoustically coupled with each other.
The method of example 1, wherein processing the second wafer comprises forming the second resonator on the second wafer.
The method of example 1, wherein the second piezoelectric material comprises at least one of aluminum nitride or scandium aluminum nitride.
The method of example 1, wherein the second piezoelectric material has a lower piezoelectric coupling constant than the first piezoelectric material.
The method of example 1, wherein the processed second wafer comprises an acoustic termination at a first side thereof, wherein said bonding is performed such that the first side of the second wafer faces the singulated wafer chip.
A method for manufacturing a resonator structure, comprising:
processing a first wafer to form a processed first wafer, the processed first wafer comprising a first piezoelectric material,
processing a second wafer to form a processed second wafer, the processed second wafer comprising an acoustic termination at a first side thereof,
singulating the first wafer to form at least one singulated wafer chip,
bonding the at least one singulated wafer chip to the second wafer such that the first side of the second wafer faces the singulated wafer chip to form a joint wafer, and
processing the joint wafer to form a resonator structure comprising a first resonator including the first piezoelectric material.
The method of example 6, wherein said acoustic termination comprises at least one of a cavity or an acoustic mirror.
The method of example 6, wherein the acoustic termination is encapsulated in the resonator structure.
The method of example 1, wherein the first wafer is made of the first piezoelectric material.
The method of example 1, wherein the first piezoelectric material is monocrystalline.
The method of example 1, wherein processing the first wafer comprises depositing a conductive material on the first wafer, at least part of the conductive material forming an electrode of the first resonator.
The method of example 11, further comprising structuring the conductive material.
The method of example 1, wherein the first piezoelectric material comprises at least one of lithium niobate or lithium tantalate.
The method of example 1, wherein processing the first wafer comprises providing a first dielectric layer on the first wafer, and wherein processing the second wafer comprises providing a second dielectric layer on the second wafer, wherein said bonding comprises bonding the first and second dielectric layers.
The method of example 14, wherein said first and second dielectric layers comprise an oxide.
The method of example 1, further comprising encapsulating the joint wafer.
The method of example 1, wherein said processing of the joint wafer comprises thinning the first piezoelectric layer.
The method of example 1, wherein said processing of the joint wafer comprises at least one of electrode deposition, electrode structuring or providing electrode connections.
A resonator structure, comprising:
a diced first wafer piece comprising a first piezoelectric material,
a substrate comprising an acoustic termination at a first side thereof, wherein the diced wafer piece is bonded to the second wafer such that the first side faces the diced wafer piece.
The resonator structure of example 19, wherein the resonator structure is manufactured by the method of example 1.
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