Patent Publication Number: US-2021184653-A1

Title: Acoustically coupled radio frequency (rf) filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to Provisional Application Ser. No. 62/947,011, filed Dec. 12, 2019, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to the field of RF filters, and more specifically, to an acoustically coupled RF filter. 
     BACKGROUND 
     The rapid evolution of wireless technology has improved the exchange of information globally. Due to extensive worldwide research and development, wireless technology has become more widely available and affordable. Consequently, the number of users of wireless technology scaled tremendously within two decades. The wireless communication market revolutionized with the first release of the smart phone. An exponential growth of services is occurring in a limited range of frequencies and therefore crowding the limited range significantly. However, high bandwidth is a very important requirement for the quality of specific operation. Communication channels need to be displaced by some unused frequencies to ensure zero interference. In order to efficiently use a particular spectrum, high selective filters must be employed. Acoustically coupled filters are amongst commercial candidates which have been under extensive research for several decades. Furthermore, low volume, low power, low cost, and good coupling are the main key drivers for acoustically coupled RF filters. 
     SUMMARY 
     Disclosed is an acoustically coupled RF filter with large bandwidth reconfigurability. The acoustically coupled RF filter is realized in ferroelectric aluminum-scandium-nitride films. In some embodiments, the acoustically coupled RF filter system comprises a first conductive layer, a ferroelectric layer, and a second conductive layer comprising a plurality of interdigital transducers (IDTs). 
     In some embodiments, the ferroelectric layer is positioned above the first conductive layer. The ferroelectric layer may be sputtered over the first conductive layer. In some embodiments, a trench (or a semi-trench) is formed in the ferroelectric layer. The trench forms a trapezoid-shape region in the ferroelectric layer. In some embodiments, the first conductive layer is positioned above an insulation layer. In some embodiments, the trench extends fully through the ferroelectric layer. In some embodiments, the trench extends fully through the ferroelectric layer and partly into the first conductive layer. In some embodiments, the trench extends fully through the ferroelectric layer and the first conductive layer. In some embodiments, the trench extends fully through the ferroelectric layer and the first conductive layer, and partly into the insulation layer. In some embodiments, the trench extends fully through the stack of the ferroelectric layer, the first conductive layer, and the insulation layer. In some embodiments, an access to the first conductive layer is formed in the ferroelectric layer. Both the trench and the access to the first conductive layer may be formed in the ferroelectric layer by an etching technique. In some embodiments, the plurality of IDTs is formed by patterning the second conductive layer. The IDTs may comprise at least two sets of IDTs. Each set of IDTs may comprise about 10 IDT fingers. The IDTs may have a pitch of about 5 micrometers. 
     In some embodiments, the plurality of IDTs forms an input and an output for the acoustically coupled RF filter system. In some embodiments, the first conductive layer comprises a same material as the second conductive layer. In some embodiments, the first conductive layer and the second conductive layer comprise Molybdenum (Mo). 
     In some embodiments, the ferroelectric layer comprises aluminum-scandium-nitride (Al 1-x Sc x N) films. The Al 1-x Sc x N provides significantly higher electromechanical coupling compared to its aluminum-nitride (AlN) counterparts. Furthermore, Al 1-x Sc x N becomes ferroelectric when Sc-content (e.g., x) exceeds 27%. In some embodiments, the ferroelectric layer comprises Al 1-x Sc x N where x is about 0.27. 
     The large electromechanical coupling of the Al 1-x Sc x N film (x &gt;0.27), leads to large bandwidths. In some embodiments, the Al 1-x Sc x N films are about 1 micrometer in thickness. In some embodiments, the aluminum scandium nitride films are engineered to implement 2.3 GHz filters with −3 dB bandwidths (e.g., BW−3 dB). In some embodiments, the −3 dB bandwidth is demonstrated over 70-117 MHz. In some embodiments, an insertion loss of −6 dB is observed. The observed insertion loss is dominated by a routing line resistance, in accordance with some embodiments. In some embodiments, bandwidth tuning of about 15 MHz (i.e., about 15% of the bandwidth) is achieved through application of a DC voltage of about 60 volts, using a bias-tee. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the present disclosure can be understood by those of ordinary skill in the art, a more detailed description can be had by reference to aspects of some illustrative embodiments, some of which are shown in the accompanying drawings. 
         FIGS. 1A-1I  are different views of an exemplary acoustically coupled RF filter system, in accordance with some embodiments. 
         FIG. 1J  is an expanded view of interdigital transducer (IDT) of an acoustically coupled RF filter, in accordance with another exemplary embodiments of the present invention. 
         FIG. 2A  compares simulated results of different responses of an exemplary acoustically coupled RF filter with aluminum scandium nitride, in accordance with some embodiments, and an exemplary acoustically coupled RF filter with aluminum nitride. 
         FIG. 2B  demonstrates measured filter response of an exemplary acoustically coupled RF filter system with aluminum scandium nitride, in accordance with some embodiments. 
         FIG. 3  illustrates a bandwidth tunability of an exemplary acoustically coupled RF filter system, through application of different DC voltages to an input IDT set, in accordance with some embodiments. 
         FIG. 4  illustrates a measured temperature coefficient of frequency (TCF) of an exemplary acoustically coupled filter system, in accordance with some embodiments. 
         FIG. 5  illustrates an exemplary method of fabricating an exemplary acoustically coupled RF filter, in accordance with some embodiments. 
     
    
    
     In accordance with common practice some features illustrated in the drawings cannot be drawn to scale. Accordingly, the dimensions of some features can be arbitrarily expanded or reduced for clarity. In addition, some of the drawings cannot depict all the components of a given system, method or device. Finally, like reference numerals can be used to denote like features throughout the specification and figures. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various described embodiments. However, it will be apparent to one of ordinary skill in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail so as not to unnecessarily obscure aspects of the embodiments. 
     Disclosed herein is an acoustically coupled filter system with large bandwidth reconfigurability, realized in ferroelectric aluminum-scandium-nitride films. 
     It will also be understood that, although the terms first, second, and/or the like are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first contact could be termed a second contact, and, similarly, a second contact could be termed a first contact, without departing from the scope of the various described embodiments. The first contact and the second contact are both contacts, but they are not the same contact, unless the context clearly indicates otherwise. 
     The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described embodiments and the appended claims, the singular forms “a,” “an,” and “the” are intended to comprise the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     As used herein, the term “if” is, optionally, construed to mean “when” or “upon” or “in response to determining” or “in response to detecting,” depending on the context. Similarly, the phrase “if it is determined” or “if [a stated condition or event] is detected” is, optionally, construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event],” depending on the context. 
     It should be appreciated that in the development of any actual implementation (as in any development project), numerous decisions must be made to achieve the developers&#39; specific goals (e.g., compliance with system and business-related constraints), and that these goals will vary from one implementation to another. It will also be appreciated that such development efforts might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art of image capture having the benefit of this disclosure. 
     Since the majority of acoustically coupled RF filters cannot be tuned, a hardware manufacturer is restricted to integrate big bank of filters into a single functional device. This is a vital functional requirement in order to ensure there is no interference among different channels. The existing solution requires high power and big space in the device in order to ensure a reliable functionality. 
     Biasing operation frequency via a DC voltage will replace banks into a single highly tunable RF filter. Ferroelectric materials are ideal candidates to be used in the RF filters. Good tunability, significantly low dielectric loss and room temperature functionality, as well as fast response, are among the reasons to choose ferroelectric materials. 
     Coupling multiple RF filters is mostly performed through electric coupling. However, in the case of electric coupling RF filters, a lot of energy leakage occurs when the power is conveyed from one resonator to another. This energy leakage is translated into high insertion in-band losses. Bandwidth in electric coupling is also smaller due to pole-zero characteristic of the transfer function. Acoustically coupled RF filters, however, can cure this drawback. 
     Realization of high-performance RF filters within a monolithic structure has been intriguing in acoustic device community as it reduces over foot-print of the filter, eliminates various fabrication, packaging and reliability challenges, and reduces routing parasitic inherent to electrically coupled acoustic filters (such as Bulk Acoustic Wave (BAW), ladder, lattice, and/or the like). While the idea of acoustic coupling of two resonance modes within a single structure promises realization of a monolithic filter, achieving comparable performance with electrically coupled counterparts have been challenging. Specifically, acoustically coupled filters suffer from lower bandwidth and larger insertion loss; both of which resulted from partial electrode-coverage inherent to the interdigital transducer (IDT) design of the two-port acoustically coupled filter. In order to overcome these disadvantages, the present application uses a ferroelectric material in an acoustically coupled RF filter. 
       FIGS. 1A-1I  are different views of an exemplary acoustically coupled RF filter system  100 , in accordance with some embodiments.  FIG. 1A  and  FIG. 1G  illustrates different layers of the acoustically coupled RF filter system  100 , according to some embodiments.  FIG. 1B  illustrates a top view of the acoustically coupled RF filter system  100 , according to some embodiments.  FIG. 1C  is a cross-sectional view of acoustically coupled RF filter system  100  shown in  FIG. 1B  when cut along lines AA′ near the center, according to some embodiments.  FIG. 1D-1F  and  FIG. 1H-1I  are possible cross-sectional views of acoustically coupled RF filter system  100  shown in  FIG. 1B  when cut along lines AA′ near the center, according to some other embodiments. In some embodiments, the acoustically coupled RF filter system  100  comprises a first conductive layer  110 , the ferroelectric layer  120 , a second conductive layer  140  and a plurality of Interdigital Transducers (IDTs)  150 . In some embodiments, the acoustically coupled RF filter system  100  also comprises an insulation layer  105 , as shown in  FIG. 1G . 
     In some embodiments, the ferroelectric layer  120  is positioned above the first conductive layer  110 . The ferroelectric layer  120  may be sputtered over the first conductive layer  110 . In some embodiments, the first conductive layer  110  is positioned above the insulation layer  105 . In some embodiments, the ferroelectric layer  120  comprises a semi-trench (or a trench), e.g., the semi-trench  130 . In some embodiments, the semi-trench  130  is formed in the ferroelectric layer  120 . In some embodiments, the semi-trench  130  extends fully through the ferroelectric layer  120 . In some embodiments, the semi-trench  130  extends fully through the ferroelectric layer  120  and partly into the first conductive layer  110 . In some embodiments, the semi-trench  130  extends fully through the ferroelectric layer  120  and the first conductive layer  110 . In some embodiments, the semi-trench  130  extends fully through the ferroelectric layer  120  and the first conductive layer  110 , and partly into the insulation layer  105 . In some embodiments, the semi-trench  130  extends fully through the stack of the ferroelectric layer  120 , the first conductive layer  110 , and the insulation layer  105 . In some embodiments, the semi-trench  130  forms a trapezoid-shape region in the ferroelectric layer  120 . In some embodiments, the second conductive layer  140  is positioned above the ferroelectric layer  120 . In some embodiments, the plurality of IDTs  150  is patterned on the second conductive layer  140 . The plurality of IDTs  150  may comprise at least two sets of IDTs. Each set of IDTs may comprise about  10  IDT fingers. In some embodiments, the plurality of IDTs  150  has a pitch of about 5 micrometers. 
     In some embodiments, the plurality of IDTs  150  forms an input, e.g., RF In  160   a , and an output, e.g., RF Out  160   b , for the acoustically coupled RF filter system  100 . In some embodiments, the insulation layer  105  comprises an insulation material. In some embodiments, the insulation layer  105  comprises silicon (Si). In some embodiments, the first conductive layer  110  comprises a same material as the second conductive layer  140 . In some embodiments, one or more of the first conductive layer  110  or the second conductive layer  140  comprises a metal. In some embodiments, one or more of the first conductive layer  110  or the second conductive layer  140  comprises Molybdenum (Mo). In some embodiments, the first conductive layer  110  and the second conductive layer  140  comprise Molybdenum (Mo). In some embodiments, one or more of the first conductive layer  110  or the second conductive layer  140  each has a thickness of about 100 nanometers. 
       FIG. 1J  is an expanded view of IDT  300  of an acoustically coupled RF filter, in accordance with another exemplary embodiments of the present invention. It is understood that  FIG. 1J  does not show the remaining elements (e.g., trench) of the acoustically coupled RF filter. IDT  300  is shown as including  8  exemplary interdigitated fingers (also referred to herein as fingers)  200   1 ,  200   2 . . .  200   7 ,  200   8 . As is seen, finger  200   2  is disposed between fingers  200   1  and  200   3 ; finger  200   3  is disposed between fingers  200   2  and  200   4 , and the liken. 
     Fingers  200   1  and  200   3  are shown as being coupled to one another to form a first electrode of the RF filter. Fingers  200   2  and  200   4  are shown as being coupled to one another to form a second electrode of the RF filter. Fingers  200   5  and  200   7  are shown as being coupled to one another to form a third electrode of the RF filter. Fingers  200   6  and  200   8  are shown as being coupled to one another to form a fourth electrode of the RF filter. To operate the filter, a first voltage is applied across the first and second electrodes, and a second voltage is applied across the third and fourth electrodes. The first and second voltage may be different from one another. 
     Although not shown, it is understood that in other embodiments, a different arrangement of the fingers may be used to form the electrodes. For example, in some embodiments, fingers  200   1  and  200   7  may be a part of the same conductive trace to form a first electrode; fingers  200   2  and  200   8  may be a part of another conductive trace to form a second electrode, and the like. It is also understood that embodiments of the present invention are not limited by the number of fingers, number of electrodes formed using the fingers, and the number/level of voltages applied to the IDT of an acoustically coupled RF filter. 
     In some embodiments, to overcome the above-mentioned problems with the acoustically coupled RF filters, the ferroelectric layer  120  comprises aluminum-scandium-nitride films (Al 1-x Sc x N). The Al 1-x Sc x N provides significantly higher electromechanical coupling compared to its aluminum-nitride (AlN) counterparts. Furthermore, Al 1-x Sc x N becomes ferroelectric when Sc-content (e.g., x) exceeds 27%. This ferroelectric property can be utilized for agile reconfiguration of the acoustically coupled RF filter system  100  using nonlinear electro-strictive effect. In some embodiments, the ferroelectric layer  120  comprises aluminum-scandium-nitride films Al 1-x Sc x N where x is at least 0.27. In some embodiments, the ferroelectric layer  120  has a thickness of about 1 micrometer. 
     The large electromechanical coupling of the Al 1-x Sc x N film when x &gt;0.27, leads to large bandwidths. As a non-limiting example, in some embodiments, the Al 1-x Sc x N film possesses a bandwidth about 3-5 times larger than that of its acoustically coupled AlN counterparts. Furthermore, disclosed acoustically coupled RF filter system  100  uses ferroelectricity in Al 1-x Sc x N films (x &gt;27%) for large bandwidth tuning of the filter. The architecture of the acoustically coupled filter provides superior characteristics compared to conventional electrically coupled filters (e.g., lattice, ladder, and/or the like) comprising lithographical bandwidth definition through proper IDT patterning, and realization of the filter within a single and/or monolithic structure. The thickness of each of the multiple layers in the acoustically coupled RF filter system  100 , the depth and the shape of the semi-trench (or trench), and the number of fingers and the pitch of the plurality of IDTs can be adjusted to achieve a required large bandwidth. Besides, the ferroelectric properties of the Al 1-x Sc x N film enable the use of nonlinear electro-strictive effect for local tuning of the electromechanical coupling and dielectric constant of the film using a DC bias. This capability directly translates to the reconfiguration of filter bandwidth and center frequency. 
     In some embodiments, the aluminum scandium nitride films are acoustically engineered to implement  2 . 3  GHz filters with about −3 dB bandwidths (e.g., BW−3 dB). In some embodiments, the −3 dB bandwidths (or the bandwidths) is demonstrated over a range of about 70 MHz to about 117 MHz. In some embodiments, an insertion loss of about −6 dB is observed. The observed insertion loss is dominated by a routing line resistance, in accordance with some embodiments. In some embodiments, bandwidth tuning of about 15 MHz (about 15% of the bandwidth) is achieved through application of a DC voltage of about 60 volts, using a bias-tee. In some embodiments, the acoustically coupled RF filters system  100  enables realization of reconfigurable RF front-end for multi-band wireless systems with extensive compatibility with a wide variety of communication standards used in the 5G communications. 
     In some embodiments, the semi-trench  130  forms a trapezoid region in the ferroelectric layer  120 . The semi-trench  130  may be formed by an etching process. In some embodiments, a chlorine etchant may be used to form the semi-trench  130  in the ferroelectric layer  120 . In some embodiments, a depth of the semi-trench  130  is smaller than the thickness of the ferroelectric layer  120 , thus, confining the semi-trench  130  in the ferroelectric layer  120 . In some embodiments, the first conductive layer  110  and the second conductive layer each has a thickness of about 100 nanometers. In some embodiments, the thickness of the ferroelectric layer  120  is about 1 micrometer and the depth of the semi-trench is about 350 nanometers. 
     In some embodiments, the Al 1-x Sc x N film (x &gt;27%), e.g., the ferroelectric layer  120 , is sandwiched between two Mo layers, e.g., between the first conductive layer  110  and the second conductive layer  140 . In some embodiments, the plurality of IDTs  150  is patterned on the top Mo layer, e.g., the second conductive layer  140 , to define the input and output transduction ports, e.g., RF In  160   a  and RF Out  160   b , of the acoustically coupled RF filter system  100 . In some embodiments, a trapezoid geometry with non-parallel edges are patterned through etching semi-trench  130  in the Al 1-x Sc x N films. 
     In some embodiments, the acoustically coupled RF filter system  100  is implemented in reactively sputtered Al 0.7 Sc 0.3 N films, e.g., the ferroelectric layer  120 . The filters are created by etching semi-trenches, e.g., the semi-trench  130 , in Al 0.7 Sc 0.3 N, e.g., the ferroelectric layer  120 , to define a trapezoid acoustic cavity for efficient energy trapping of two thickness-extensional Lamb modes and create a bandpass filter. In some embodiments, the Al 0.7 Sc 0.3 N film, the ferroelectric layer  120 , is etched by using a high-power chlorine-based recipe to provide an access to bottom Mo electrode, e.g., the first conductive layer  110 . 
       FIG. 2A  compares simulated results of different responses of the disclosed acoustically coupled RF filter system, e.g., one with Al 0.7 Sc 0.3 N as ferroelectric layer, in accordance with some embodiments, and the other with aluminum nitride (AlN) replacing Al 0.7 Sc 0.3 N. Referring to  FIG. 2A , the simulation is performed by considering a hypothetical case of similar elastic constants for both films, while reflecting the different piezoelectric coupling coefficient. No material loss is considered to accurately explore the effect of the electromechanical coupling on bandwidth. The inset compares the passband, highlighting significant enhancement of the bandwidth in the disclosed acoustically coupled RF filter system. While the bandwidth of the acoustically coupled filter system should be defined by a pitch size of the plurality of IDTs, the low electromechanical coupling of AlN results in large pass-band ripples which exceed about −3 dB. The simulation confirms that the disclosed acoustically coupled RF filter system with Al 0.7 Sc 0.3 N as ferroelectric layer achieves a BW−3 dB of about 3-times compared to the one with AlN. 
       FIG. 2B  demonstrates measured filter response of the disclosed acoustically coupled RF filter system with Al 0.7 Sc 0.3 N as ferroelectric layer, showing a BW−3 dB of about 70 MHz. The acoustically coupled RF filter system shows a sharp roll-off at the lower end, which makes it suitable for Rx applications. The inset shows a lumped-element model of the filter, with its response layered on top of the measured response, confirming its accuracy. The measured response is compared with the lumped model that is equivalent to 1.5-stage ladder architecture, e.g., three resonances. This response highlights a promise of the acoustically coupled RF filter system to reduce the footprint. 
       FIG. 3  shows a bandwidth tunability of the acoustically coupled RF filter system, through application of different DC voltages to input IDT set, in accordance with some embodiments. As shown on  FIG. 3 , a large BW−3 dB tunability of about 15 MHz (e.g., about 15%) is demonstrated using a DC voltage of about 60 V. A reconfigurability of the bandwidth is provided for the ferroelectric effect in Al 1-x Sc x N that facilitates local tuning of the dielectric constant and electromechanical coupling by DC fields through the nonlinear electro-strictive effect. 
       FIG. 4  shows a measured temperature coefficient of frequency (TCF) of the acoustically coupled RF filter system., in accordance to some embodiments. The TCF measurements was performed across a temperature range of 40-100° C. The TCF highlights a value of about 28 ppm/° C. 
       FIG. 5  illustrates a method of fabricating the acoustically coupled RF filter  500 , in accordance with some embodiments. In some embodiments, the method  500  comprises forming a first conductive layer above an insultation layer, as represented by block  501 . In some embodiments, the method  500  comprises forming a ferroelectric layer above a first conductive layer, as represented by block  502 . In some embodiments, a semi-trench (or trench) is formed in the ferroelectric layer. As represented by block  504 , in some embodiments, the method  500  comprises etching the semi-trench (or trench) onto the ferroelectric layer. As represented by block  506 , the method  500  further comprises forming a trapezoid-shape region in the ferroelectric layer with the semi-trench (or trench). As represented by block  510 , in some embodiments, the method  500  comprises forming a second conductive layer above the ferroelectric layer. In some embodiments, the method  500  further comprises forming a plurality of IDTs by patterning the second conductive layer, as represented by block  512 . The plurality of IDTs may form an RF filter input and an RF filter output. 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best use the invention and various described embodiments with various modifications as are suited to the particular use contemplated.