WO2020025182A1

WO2020025182A1 - Radio frequency filter

Info

Publication number: WO2020025182A1
Application number: PCT/EP2019/061700
Authority: WO
Inventors: Christian Huck; Thomas Dengler
Original assignee: RF360 Europe GmbH
Priority date: 2018-07-30
Filing date: 2019-05-07
Publication date: 2020-02-06
Also published as: DE102018118384A1; DE102018118384B4

Abstract

A radio frequency filter comprises a series path coupled between an antenna port (111) and another port (112) which includes surface acoustic wave resonators (115a,..., 115e) that are coupled in series with each other. At least one parallel path (116a,..., 116d) is coupled between at least one resonator of the series path and a terminal for ground potential (117). One or more of the surface acoustic wave resonators (115a,..., 115e) of the series path are provided with a transversal mode suppression feature and the at least one surface acoustic wave resonator (116a,..., 116d) of the at least one parallel path is provided with another transversal mode suppression feature. Using the benefits of both transversal mode suppression features allows an enhanced performance of the filter.

Description

Radio frequency filter

Technical Field

The present disclosure relates to a radio frequency filter. Specifically, the present disclosure relates to a radio frequency filter coupled between an antenna port and another port including surface acoustic wave resonators coupled in series with each other. At least one parallel path including at least one surface acoustic wave resonator is coupled between one of the resonators of the series path and a terminal for ground potential.

Background

Radio frequency (RF) filters are widely used in electronic communication systems at the antenna frontend to select the wanted bandwidth from the antenna signal or provide the RF signal to the antenna. The RF filter may be composed of surface acoustic wave (SAW) resonators arranged in serial and parallel paths. A SAW resonator comprises a monocrystalline piezoelectric substrate and an interdigital transducer made of a metal material disposed on the piezoelectric substrate to input and output an electrical signal.

By application of the electrical RF signal to the transducer electrodes an acoustic resonating wave is generated in the piezoelectric substrate. A thin film SAW resonator comprises a relatively thin piezoelectric layer that is disposed on an insulating substrate. One or more intermediate functional layers may be provided such as a temperature compensating layer having a temperature coefficient of frequency (TCF) different from or opposite to the TCF of the piezoelectric material. Another temperature compensated SAW resonator comprises an insulating layer disposed on top of the

transducers and the relatively thick piezoelectric substrate.

Thin film and temperature compensated SAW resonators are provided with special structures to suppress the excitation of spurious transversal modes. Several transversal mode suppression techniques are available such as a piston mode design of the interdigital transducers, a slanted arrangement of the transducers and an apodized design of the transducers such as a cosine weighted apodized transducer design.

Each of the transversal mode suppression techniques has advantages and drawbacks with regard to the design of RF filters. For example, a piston mode design uses acoustic waveguide means to confine the wave energy transversally inside the acoustic track without substantially increasing space consumption. On the other hand, the piston mode

transversal mode suppression technique may be insufficient in the stop band frequency region of the resonator resulting in unwanted attenuation dips in the filter passband. A slanted filter design has an almost flat passband with reduced insertion attenuation, however, slanted resonators have higher losses in the lower passband area. Additionally, the slanted orientation of the transducers of the resonators generates increased complexity in the filter topology and may result in a higher space consumption. A filter using cosine aperture weighted resonators exhibits a relatively flat passband, however, with increased insertion attenuation over the entire passband. Furthermore, in order to maintain the same static capacitance and the same effective aperture as unweighted resonators the maximum aperture of cosine apodized resonators must be increased resulting in a higher space consumption. Consequently, the use of a specific transversal mode suppression technique generates also certain drawbacks in the RF filter, especially when all resonators use the same suppression technique.

It is an object of the present disclosure to provide a radio frequency filter using surface acoustic wave resonators that avoids one or more of the above-mentioned drawbacks.

It is another object of the present disclosure to provide a radio frequency filter using surface acoustic wave resonators that has reduced insertion loss and a relatively flat shape of the passband.

Summary

According to the present disclosure, one or more of the above-mentioned objects are achieved by a radio frequency filter that comprises an antenna port and another port; a series path coupled between the antenna port and the other port including surface acoustic wave resonators coupled in series with each other; at least one parallel path coupled between at least one of the surface acoustic wave resonators of the series path and a terminal for ground potential including at least one surface acoustic wave resonator;

wherein one or more surface acoustic wave resonators of the series path are provided with a transversal mode suppression feature and the at least one surface acoustic wave resonator of the at least one parallel path is provided with another transversal mode suppression feature. According to an embodiment, a radio frequency (RF) filter comprises an antenna port which is to be connected to an antenna and another port which may be a send (Tx) or receive (Rx) port to be connected to the circuitry in an electronic device. A series path of resonators is connected between the antenna port and the other port and includes a multitude of surface acoustic wave (SAW) resonators serially connected with each other. From at least one of the resonators a parallel port is connected to a terminal for ground

potential. In a typical ladder type filter structure each coupling node between two serially connected resonators of the serial path is coupled to a resonator of a parallel path connected to ground potential. According to the present disclosure, one or more of the resonators of the series path are manufactured with one common transversal mode suppression technique, and the at least one resonator of the one or more parallel paths are manufactured according to another common transversal mode suppression technique. According to a preferred embodiment, all resonators of the series path are manufactured with one common transversal mode suppression technique. The transversal mode suppression technique of the series path is different from the transversal mode

suppression technique of the one or more parallel paths. The selection of different transversal mode suppression

techniques for the series and the parallel paths,

respectively, allows to eliminate or reduce disadvantages of one suppression technique and benefit from the advantages of the different employed suppression techniques combined in a filter design. The surface acoustic wave resonators of the series path are provided with a transversal mode suppression feature and at least one surface acoustic wave resonator of the at least one parallel path is provided with another transversal mode suppression feature. Using the benefits of both transversal mode suppression features allows an enhanced performance of the filter.

While one or more of the resonators of the series path are manufactured with one common transversal mode suppression technique, at least one resonator of the series path may be manufactured with another one transversal mode suppression technique. The latter resonator may be manufactured according to the transversal mode suppresion technique of the resonator of the at least one parallel path or even yet another

transversal mode suppression technique. The decision which one(s) of the series resonators is to be manufactured with the other transversal mode suppression technique may depend on the frequency, the dissipative losses and/or the thermal connection of the respective series resonator (s) and/or the noise mode position of the respective series resonator (s) relative to Tx/Rx. In dependence on the impact of the

respective resonator (s) on the above mentioned criteria, it may use the other transversal mode suppression technique, e.g. to save space. The resonator employing the other one transversal mode suppression technique may in most cases be that resonator connected to the antenna port.

It is useful to use the piston mode resonator design for the resonators in the series path. The resonators of the at least one parallel path in a ladder type filter have another transversal mode suppression technique to avoid drawbacks of the piston mode design which is an attenuation dip at the upper edge of the filter passband. The SAW resonators of the parallel paths may exhibit a slanted track design of the resonator that avoids an attenuation dip at the upper edge of the filter passband. As an alternative, resonators with an apodized transversal mode suppression feature may be used in the parallel paths. The apodized design of the parallelly connected SAW resonators maintains a relatively flat shape of the filter passband.

The use of different transversal mode suppression techniques in series and parallel paths of a RF filter are particularly useful for SAW technologies that are susceptible to

transversal mode excitations. These SAW technologies may exhibit non-satisfying piston mode realizations or other means that induce transversal mode excitations. Such

technologies may include a thin film SAW structure or a temperature compensated SAW structure. A thin film SAW resonator comprises a relatively thin monocrystalline

piezoelectric substrate or layer disposed on an insulating carrier substrate such as silicon providing a high velocity layer to vertically confine the acoustic wave within the thin film piezoelectric layer. The piezoelectric substrate may be crystalline lithium tantalate or crystalline lithium niobate having a suitable cut angle. The electrodes of the

interdigital transducer having interdigitized fingers are made from a metal material and are disposed on the

piezoelectric substrate. Furthermore, one or more additional intermediate layers may be disposed between the silicon carrier substrate and the thin film piezoelectric layer to provide additional functions. A functional layer of an insulating material may provide a temperature compensation by a temperature coefficient of frequency (TCF) different from or opposite to the TCF of the piezoelectric material. The insulating material may be, e.g., silicon dioxide. Other materials are also useful. Due to the parasitic surface charges, which are generated at the interface of the silicon substrate and the silicon dioxide layer, another functional layer may be disposed at this interface to reduce the unwanted electrical conduction, e.g., a poly-silicon layer that has the function of a trap rich layer.

Another temperature compensated SAW resonator type comprises a self-supporting crystalline piezoelectric layer, e.g. a bulk substrate, on which the metal interdigitated fingers of the transducers are disposed. The piezoelectric layer may be made of crystalline lithium tantalate or crystalline lithium niobate. The upper surface of the resonator is covered with an insulating layer of defined thickness to achieve

temperature compensation. The insulating layer may be silicon dioxide that covers the upper surface of the metal transducer and the upper surface of the piezoelectric substrate. Another material having a TCF different from or opposite to the piezoelectric material is also useful.

The use of different transversal mode suppression features in series and parallel paths of a RF filter such as piston mode design, slanted design, apodized design requires no change in the deposition sequence of layers. The different transversal mode suppression features can be easily generated during the manufacturing process without changing the conventional production procedure. It just requires a suitably adapted reticle to generate the required structure in the metal layer intended for the transducers which is then used in the standard photolithography process followed by an evaporating step to generate the transducers and achieve the

corresponding transversal mode suppression feature at the transducers of the SAW resonator. This requires only an adaption of the design of the transducers and no additional steps in the production sequence and no deposition of an additional layer. Accordingly, the present disclosure

achieves an optimized filter design whereby the advantages of different transversal mode suppression techniques are

maintained while reducing the drawbacks of each transversal mode suppression technique.

A piston mode transversal mode suppression technique useful for the resonators of the series path of the RF filter, in more detail, includes a specific design of the end portions of the interdigitated fingers of the transducer electrodes. A suitable pattern of different acoustic velocities in

longitudinal direction causes the acoustic energy to be maintained in the inner area of the interdigitated transducer fingers. The main portions of the electrodes extend in a transversal direction, wherein the end portions are widened and have an additional extension in the longitudinal

direction perpendicular to the transversal direction.

Furthermore, the end portions having the longitudinal

extension of the fingers may have an increased thickness of the metal electrode layer or an additional metal layer on top of the metal electrode for increased mass loading.

Transversally adjacent on the outer side to the

longitudinally extended end portions of the fingers there is a transversal gap without stub fingers resulting in only one finger per wavelength. The acoustic velocity in the

longitudinal direction is higher in the transversal gap than in the main portions of the electrodes and the area of the longitudinally extended end portions of the fingers so that the acoustic wave is substantially confined within the inner, main finger portions of the transducer electrodes. This generates a convexe slowness. Other possible piston mode designs empoly concave slowness. Embodiments of resonators according to the piston mode design are described in US patent 9,257,960 B2 (US 2013/0051588 Al), WO 2016/095967 A1 and WO 2015/007319 Al herein incorporated by reference. All piston mode design approaches described in said publications may be used in connection with the principles of the present disclosure .

A slanted resonator design useful for the resonators of the parallel paths of the RF filter includes interdigitated fingers that are each shifted relative to one another so that a slanted orientation of the transducer electrodes is

achieved. Each neighboring finger is shifted a fraction of a finger length in the transversal direction to render the slanted orientation of the acoustic track. The direction of extension of the transducer encloses an angle with the longitudinal direction which is a direction of the

propagation of the acoustic wave which is perpendicular to the transversal direction of the extension of the finger. The angle is more than 0 degrees and may range between 0 degrees and 30 degrees at most. In practice, the angle between the direction of the extension of the transducer and the

longitudinal direction of the propagating acoustic wave may be between 5 degrees and 15 degrees. In a preferred

embodiment, that angle may be between 8 degrees and

12 degrees.

An additional parameter to optimize the design of the

transducers of a slanted resonator may be the length of the stub fingers that are disposed in the space opposite the end portions of the fingers. The length of the stub fingers may be in the range of 0.5 l to 4.0 l, wherein l is the length of the resonating wave of the main mode. Practically, the length of the stub fingers may be in the range of 1.5 l to 2.5 l. In a preferred embodiment, the stub finger length may be at or about 2 l. Yet another parameter to optimize the design of the

transducers of a slanted resonator may be the transversal gaps between the electrode fingers and corresponding stub fingers. The transversal gaps may be in the range of 250 nm to 500 nm. In general, the transversal gaps should be as small as possible, but practical limitations such as

manufacturing accuracy and electrostatic discharge as well as power durability must be considered.

SAW resonators according to the slanted transversal mode suppression feature are described in co-pending DE patent applications 102018118003.9 and 102018118005.5 both filed on July 25, 2018. All slanted transversal mode suppression features described in said patent application may be used in connection with the principles of the present disclosure.

An apodized transversal mode suppression feature useful for the resonators of the parallel paths of the RF filter

comprises interdigitated fingers of the transducer electrodes of which the length of the fingers is determined such that the finger end portions define an envelope curve that has a specific form. According to the principles of the present disclosure the envelope curve may have a cosine form or cosine weighting.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or

framework to understand the nature and character of the claims. The accompanying drawings are included to provide a further understanding and are incorporated in, and constitute a part of, this description. The drawings illustrate one or more embodiments, and together with the description serve to explain principles and operation of the various embodiments. The same elements in different figures of the drawings are denoted by the same reference signs.

Brief Description of the Drawings

In the drawings:

Figure 1 shows a schematic diagram of a ladder type filter;

Figure 2 shows a comparative diagram of the real part of the admittance of resonators having piston mode or slanted or cosine weighted transversal mode suppression features;

Figure 3 shows a comparative diagram of the upper portion of the passband of RF filters with resonators having piston mode or slanted or cosine weighted transversal mode suppression features only;

Figures 4A and 4B show comparative diagrams of the upper portion of the passband and the unitary violation of RF filters with resonators having piston mode or slanted

transversal mode suppression features only in comparison with a RF filter with a combination of resonators having piston mode and slanted transversal mode suppression features;

Figures 5A and 5B show comparative diagrams of the upper portion of the passband and the unitary violation of RF filters with resonators having piston mode or cosine weighted transversal mode suppression features only in comparison with a RF filter with a combination of resonators having piston mode and cosine weighted transversal mode suppression

features ; Figures 6A and 6B show comparative diagrams of the upper portion of the passband and the unitary violation of a RF filter with a combination of resonators having piston mode and cosine weighted transversal mode suppression features in comparison with a RF filter with a combination of resonators having piston mode and slanted transversal mode suppression features ;

Figures 7A and 7B show top and cross-sectional views of a transducer having a piston mode transversal mode suppression feature ;

Figure 8 shows a top view of a resonator having a slanted transversal mode suppression feature;

Figure 9 shows a top view of a resonator having a cosine weighted aperture transversal mode suppression feature; and

Figures 10A and 10B show cross-sectional views of a thin film SAW resonator and a temperature compensated SAW resonator.

Detailed Description of the Embodiments

The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings showing embodiments of the disclosure. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will fully convey the scope of the disclosure to those skilled in the art. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the

disclosure . Figure 1 illustrates a ladder type RF filter. The filter comprises an antenna port 111 to which the antenna of an electronic device is to be connected. Another port 112 may be the send (Tx) or receive (Rx) port of the filter that is to be connected to the internal circuitry of the electronic device for further processing the filtered signal or for receiving the RF signal to be transmitted over the antenna. A serial path of resonators 115a, 115b, 115c, 115d, 115e is connected between ports 111, 112. The resonators 115a, ...,

115e are connected in series with each other. Parallel paths are connected between the serial path and a terminal 117 for ground potential. Each parallel path includes a resonator such as resonators 116a, 116b, 116c, 116d, wherein each resonator is connected to the coupling node of two serial resonators such as coupling node 118 between resonators 115a, 115b and to the ground terminal 117. The filter depicted in Figure 1 includes nine resonators in an SPSPSPSPS

configuration (S: serial; P: parallel) . Another number of more or less resonators in the RF filter may also be useful.

According to an embodiment of the present disclosure, the resonators of the series path are fabricated according to a transversal mode suppression feature that is common to all the resonators 115a, ..., 115e disposed in the serial path. The resonators 116a, ..., 116d of the parallel paths are manufactured with another transversal mode suppression feature common to all the resonators 116a, ... 116d of the four parallel paths depicted.

Figure 2 illustrates the real part of the complex admittance of one resonator that exhibits one of a cosine weighted aperture (curve 201), a piston mode design (curve 202) and a slanted design (curve 203) . The piston mode approach may not work properly over the entire stop band of the resonator, as can be especially recognized by the peaks in the area between 1950 and 2050 MHz of curve 202. On the other hand, the piston mode approach uses acoustic waveguides that confine the wave energy transversally inside the acoustic track so that it does not increase realization area and space compared to other solutions. The cosine weighted aperture approach depicted with curve 201 exhibits a relatively smooth curve, however, has increased insertion loss and increased

attenuation. A resonator according to the cosine aperture weighting approach may require more space than a resonator according to the piston mode design. A transversal mode suppression feature using a slanting of the acoustic track by a specific angle depicted at curve 203 shows a relatively smooth curve with lower losses inside the stop band and higher losses beneath both stop band edges. The slanted design, however, increases complexity for filter designs due to the slanting of the acoustic tracks.

Figure 3 shows comparative curves of the upper portion of the passband of a ladder type filter with nine resonators in SPSPSPSPS architecture wherein, within one filter, all the serial and parallel resonators have the same transversal mode suppression feature, i.e. a cosine weighted (curve 301), a piston mode (curve 302) or a slanted (curve 303) transversal mode suppression feature. The cosine weighted aperture approach according to curve 301 shows a relatively flat passband, however, an increased insertion attenuation over the entire passband in that curve 301 is the lowermost curve having considerable attenuation. The characteristic of a filter having piston mode resonators according to curve 302 shows dips at the right-sided edge of the passband of increased attenuation resulting from the stop band peaks shown in Figure 2. A filter using slanted resonators

according to curve 303 shows an almost flat passband with reduced insertion attenuation when compared to the cosine weighted filter design and without dips at the right filter skirt. However, the left filter skirt and the left half of the passband have increased attenuation in that the left portion of curve 303 has a lower level compared to the right portion of curve 303 and the left portion of curve 302.

Figures 4A and 4B relate to a hybrid filter design according to the principles of the present disclosure wherein the series connected resonators 115a, ..., 115e exhibit a piston mode transversal mode suppression feature and the parallel connected resonators 116a, ..., 116d exhibit a different, slanted transversal mode suppression feature. The hybrid slanted/piston mode filter is depicted with curve 401. By way of comparison, the conventional filters using only piston mode or only slanted design for all serial and parallel resonators are depicted with curves 302, 303 (also shown in Figure 3) . As can be gathered from Figure 4A, the hybrid filter using piston mode resonators in the serial path and slanted resonators in the parallel paths exhibits less attenuation in the left half of the passband as curve 401 is above curves 302 and 303 at most portions of the upper region of the passband. Due to their reduced losses below resonance frequency, the piston mode resonators in the serial path yield a reduced attenuation in the left half of the passband when compared to curve 303. The parallel resonators are replaced by resonators of the slanted design in the hybride piston mode/slanted filter so that drawbacks from

conventional piston mode resonators of the parallel paths as shown in Figures 2 and 3 are avoided. Thereby, a relatively flat passband over the entire width of the passband is obtained with almost no attenuation dips. The unitarity violation for all above mentioned filter scenarios depicted in Figure 4B is representative of the globally arising losses in the filter. Curve 405 shows the unitarity violation for the hybrid slanted/piston mode approach, curve 406 for the slanted only filter approach, and curve 407 for the piston mode only filter approach. Curve 405 shows the overall best loss situation combining the advantages of both underlying transversal mode suppression approaches, whereas curves 406 and 407 depict the drawbacks mentioned above.

Turning now to Figures 5A and 5B, the filter characteristics of a hybrid filter having piston mode resonators 115a, ...,

115e in the series path and cosine weighted aperture

resonators 116a, ..., 116d in the parallel paths is shown with curve 501 in Figure 5A. By way of comparison, curves 301 and 302 from Figure 3 relating to the cosine weighted only and piston mode only filter designs are also depicted in Figure 5A. As can be gathered from Figure 5A, curve 501 has a considerably flat passband at a relatively low attenuation level, that means that curve 501 has a higher level than cosine weighted only curve 301. Furthermore, curve 501 avoids the attenuation dip at the right edge of the passband of a piston mode only filter of curve 302. Figure 5B shows a comparative diagram of the unitarity violation representative of the the globally arising losses in the filter. Curve 506 depicts the unitarity violation for the hybrid cosine/piston mode filter design wherein curves 407 and 505 depict the corresponding unitarity violation for conventional piston mode only and cosine weighting only filter designs. It is to be noted that curve 506 is in most portions below curves 407 and 505 at the right half of the passband and the

attenuation dip at the right edge of the passband is avoided. Figures 6A and 6B show comparative curves for passband attenuation and unitarity violation for a hybrid

slanted/piston mode filter design (curves 601, 602) and a hybrid cosine weighted/piston mode design (curves 603, 604). As can be gathered from Figures 6A and 6B, a hybrid filter design comprising piston mode resonators in the serial path and slanted resonators in the parallel path exhibit less insertion attenuation in the passband and lower global losses over the full passband range. It is to be noted that all curves depicted in Figures 2 are based on measurements from actual resonator implementations and all curves depicted in Figures 3 through 6B are obtained by filter synthesis based on resonator measurements shown in Figure 2.

Figures 7A and 7B show a top and a cross-sectional view of a resonator employing a piston mode design. Figure 7 depicts two electrodes 710, 720 of an interdigital transducer that may be made of a metal material disposed on a piezoelectric substrate. The longitudinal direction X depicts the

propagation direction of the acoustic main mode. Direction Y depicts the transversal direction perpendicular to the longitudinal direction X. A multitude of fingers from

electrodes 710, 720 are interdigitized with each other so that one finger of one of the electrodes is situated between two fingers of the other electrode. The electrodes have different sections such as a central section 731 and end portions 732, 733 adjacent to the central section. A

transversal gap 745 is disposed between the end portion 744 of finger 742 and the bus bar portion of electrode 710. The fingers may have outer portions 734, 735 which are disposed in the non-overlapping regions of the fingers. According to the piston mode design of the transducer of the SAW

resonator, the end portions 732, 733 are extended into the longitudinal direction X. The width of the end portions 732, 733 in the longitudinal direction X is larger than in the central portion 731. The velocity of an acoustic wave is higher in the transversal gap so that the acoustic energy is maintained in the inner portion of the transducer fingers reducing transversal acoustic leakage. The velocity of an acoustic wave is lower in the end portion area and adopted such that transversal modes are suppressed. The widening of the end portions 732, 733 does not require more overall space of the transducer. Figure 7B shows a cross-sectional view along the length of the finger 741 exemplified for all fingers so that all fingers of electrodes 710, 720 exhibit a corresponding cross-sectional shape. The end portions 732,

733 have an increased thickness of the metal layer compared to the central portion 731. This may be realized by the same metal as in the central portion 731 or by using a different metal or generally another material. The thickness of the end portions 732, 733 is higher than the thickness of the central portion 731. The piston mode resonator design depicted in Figures 7A and 7B may be used for the series resonators

115a, ..., 115e of a ladder type filter architecture.

Figure 8 depicts a top view on a resonator according to the slanted transversal mode suppression design. The metal transducer shown in Figure 8 shows first and second

transducer electrodes 810, 820 having interdigitated fingers. For example, finger 811 is connected to electrode 810 and adjacent finger 821 is connected to the other electrode 820. The acoustic track of the depicted transducer is slanted by angle with respect to the longitudinal propagation

direction X of the acoustic main mode. The angle may be in the range from 0 degrees to 30 degrees. Preferably, the angle is between 5 degrees to 15 degrees and most preferably between 8 degrees to 12 degrees. The direction XY represents the direction of extension of the transducer which is

parallel to the busbars 810a, 820a of the electrodes 810,

820. The space between one of the fingers and the opposite busbar may be empty or, preferably, may be filled with a stub finger. For example, finger 811 connected to electrode 810 is associated with a stub finger 831 connected to electrode 820. The length of the stub finger may be from 0.5 to 4 times the wavelength l of the acoustic main mode. Preferably, the stub finger length is in the range of between 1.5 l to 2.5 l, most preferably of 2 l. A transversal gap 841 between electrode finger 811 and corresponding stub finger 831 is to be

minimized in consideration of electrostatic discharge and power durability as well as manufacturing accuracy and may be in the range between 250 nm to 500 nm (nanometers) . The slanted resonator design depicted in Figure 8 may be used for the parallel resonators 116a, ..., 116d of a ladder type filter architecture.

Turning now to Figure 9, a top view on a SAW resonator employing an apodized transducer design having cosine

weighting is depicted. The transducer comprises electrodes 910, 920 having interdigitated fingers. Finger 911 is

connected to the electrode 910 and a stub finger 931 is associated with finger 911 connected to the other electrode 920. The transversal gap between finger and corresponding stub finger is located at different positions from finger to finger. An envelope curve 940 that connects these transversal gaps forms a cosine waveform so that the apodized function of the transducer is cosine weighted. Stated otherwise, also the end portions of the fingers may be connected by an envelope curve that exhibits a cosine waveform. Other functions may also be useful, wherein a cosine weighting may be most common .

Figures 10A and 10B show possible embodiments of a SAW resonator in cross-sectional view. Figure 10A depicts a thin film SAW resonator comprising a carrier substrate 1110 on which a thin film of a piezoelectric substrate 1111 is disposed. The piezoelectric substrate may be lithium

tantalate or lithium niobate. An additional intermediate functional layer 1113 may be optionally disposed between carrier substrate 1110 and piezoelectric thin film layer 1111. Carrier substrate layer 1110 has a higher acoustic velocity so that the acoustic energy propagating in the piezoelectric layer 1111 is vertically confined to the piezoelectric layer. Layer 1113 may have the function of a temperature compensating layer to reduce temperature induced frequency drifts or the function of a trap rich layer reducing electrical conduction induced by parasitic surface charges. Several layers exhibiting temperature compensating and trap rich functions may be provided. A functional layer for temperature compensation may be made of an insulating material such as silicon dioxide. Other insulating materials are also useful. A trap rich layer may be made of

polysilicon. A transducer electrode 1112 made of a metal material such as a composition of copper and aluminum is disposed on thin film piezoelectric layer 1111. A seed layer (not shown) may be disposed between piezoelectric layer 1111 and metal transducer electrode layer 1112.

Figure 10B depicts a cross-sectional view of a temperature compensated SAW resonator. Provided is a piezoelectric layer 1120 which may comprise a bulk substrate. A transducer 1121 is disposed on the piezoelectric substrate 1120. The surface of the resonator is conformly covered with a layer 1122 of an insulating dielectric material such as silicon dioxide.

During operation of the resonator, substantial heat is generated by the electroacoustic operation of the resonator. The electrical parameters are very susceptible to temperature variations and may shift with increasing temperature which is to be avoided. The silicon dioxide layer 1122 substantially compensates the effects arising from the heating of the layers and the increased temperature due to its opposite TCF compared to the inherent TCF of the piezoelectric material 1120.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosure as laid down in the appended claims. Since modifications, combinations, sub combinations and variations of the disclosed embodiments incorporating the spirt and substance of the disclosure may occur to the persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims.

Claims

Claims :

1. A radio frequency filter, comprising:

- an antenna port (111) and another port (112);

- a series path coupled between the antenna port and the other port including surface acoustic wave resonators (115a, 115b, 115c, 115d, 115e) coupled in series with each other;

- at least one parallel path coupled between at least one of the surface acoustic wave resonators of the series path and a terminal (117) for ground potential including at least one surface acoustic wave resonator (116a, 116b, 116c, 116d);

- one or more surface acoustic wave resonators of the series path provided with a transversal mode suppression feature and the at least one surface acoustic wave resonator of the at least one parallel path provided with another transversal mode suppression feature.

2. The radio frequency filter according to claim 1, wherein the one or more surface acoustic wave resonators (115a, 115b, 115c, 115d, 115e) of the series path comprise a piston mode transversal mode suppression feature.

3. The radio frequency filter according to claim 1 or 2, wherein the at least one surface acoustic wave resonator (116a, 116b, 116c, 116d) of the at least one parallel path comprise a slanted track.

4. The radio frequency filter according to claim 1 or 2, wherein the at least one surface acoustic wave resonator (116a, 116b, 116c, 116d) of the at least one parallel path comprises an apodized transversal mode suppression feature having cosine weighting.

5. The radio frequency filter according to any of claims 1 to 4, wherein the surface acoustic wave resonators of the series path (115a, 115b, 115c, 115d, 115e) and the at least one parallel path (116a, 116b, 116c, 116d) are thin film

resonators that comprise a piezoelectric layer (1111)

disposed on a substrate (1110) and at least one metal

transducer (1112) disposed on the piezoelectric substrate.

6. The radio frequency filter according to claim 5, wherein one or more intermediate functional layers (1113) are

disposed between the substrate (1110) and the piezoelectric layer (1111) to perform at least one of temperature

compensation, electrical conduction compensation and another function .

7. The radio frequency filter according to any of claims 1 to 4, wherein the surface acoustic wave resonators of the series path (115a, 115b, 115c, 115d, 115e) and the at least one parallel path (116a, 116b, 116c, 116d) are temperature compensated resonators that comprise a piezoelectric layer (1120) and at least one metal transducer (1122) disposed on the piezoelectric substrate and an insulating layer (1122) disposed on the metal transducer.

8. The radio frequency filter according to claim 2, wherein the surface acoustic wave resonators (115a, 115b, 115c, 115d, 115e) of the series path comprise transducers that comprise at least two electrodes (710, 720) having interdigitated fingers (741) extending in a transversal direction (Y) , wherein an end section (732, 733) of the fingers has an extension in a longitudinal direction (X) perpendicular to the transversal direction larger than another section of the fingers .

9. The radio frequency filter according to claim 8, wherein the transducers are formed of a metal layer that has

increased layer thickness at the end sections (732, 733) .

10. The radio frequency filter according to claim 3, wherein the at least one surface acoustic wave resonator (116a, 116b, 116c, 116d) of the at least one parallel path comprises transducers that comprise at least two electrodes (810, 820) having interdigitated fingers (811, 821) extending in a transversal direction (Y) , wherein each finger of one of the electrodes is shifted in the transversal direction (Y) with regard to another one of the fingers of said electrode.

11. The radio frequency filter according to claim 10, wherein a propagation direction of acoustic waves is along a

longitudinal direction (X) perpendicular to the transversal direction (Y) and an angle (a) between a direction of

extension of the transducer and the longitudinal direction encloses a non-perpendicular angle.

12. The radio frequency filter according to claim 10 or 11, wherein a propagation direction of acoustic waves is along a longitudinal direction (X) perpendicular to the transversal direction (Y) and an angle (a) between a direction of

extension of the transducer and the longitudinal direction encloses an angle of between 0 degrees and 30 degrees, preferably between 5 degrees and 15 degrees and most

preferably between 8 degrees to 12 degrees.

13. The radio frequency filter according to any of claims 10 to 12, wherein a stub finger (831) is disposed opposite at least one of the fingers (811) of the interdigital

transducers and the length of the stub finger is in the range of 0.5 l to 4.0 l, preferably, in the range of 1.5 l to 2.5 l and most preferably in the range of 2 l, wherein l is the wavelength of the acoustic main mode.

14. The radio frequency filter according to claim 13, wherein a transversal gap (841) between a finger (811) and a stub finger (831) is in the range of 250 nm to 500 nm.

15. The radio frequency filter according to claim 4, wherein the at least one surface acoustic wave resonator (116a, 116b, 116c, 116d) of the at least one parallel path comprises transducers that comprise at least two electrodes (910, 920) having interdigitated fingers (911), transversal gaps between overlapping and stub fingers forming an envelope curve (940) having a cosine form.