Patent Publication Number: US-2018034440-A1

Title: Acoustic wave resonator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part under 37 C.F.R. §1.53(b) of, and claims priority under 35 U.S.C. §120 from, commonly-owned U.S. patent application Ser. No. 14/835,679 filed on Aug. 25, 2015, naming Stephen Roy Gilbert, et al. as inventors. The present application is also a continuation-in-part under 37 C.F.R. §1.53(b) of, and claims priority under 35 U.S.C. §120 from, commonly-owned U.S. patent application Ser. No. 14/866,273 filed on Sep. 25, 2015, naming Stephen Roy Gilbert, et al. as inventors. The present application is also a continuation-in-part under 37 C.F.R. §1.53(b) of, and claims priority under 35 U.S.C. §120 from, commonly-owned U.S. patent application Ser. No. 14/866,394, naming Stephen Roy Gilbert, et al. as inventors. The entire disclosures of U.S. patent application Ser. Nos. 14/835,679; 14/866,394; and 14/866,273 and are each specifically incorporated herein by reference. 
    
    
     BACKGROUND 
     Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a duplexer (diplexer, triplexer, quadplexer, quintplexer, etc.) for example, connected between an antenna (there could be several antennas like for MIMO) and a transceiver for filtering received and transmitted signals. 
     Various types of filters use acoustic (mechanical) resonators. The resonators convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. 
     Resonators may be used as band-pass filters with associated passbands providing ranges of frequencies permitted to pass through the filters. The passbands of the resonator filters tend to shift in response to environmental and operational factors, such as changes in temperature and/or incident power. For example, the passband of a resonator filter moves lower in frequency in response to rising temperature and higher incident power. 
     Cellular phones, in particular, are negatively affected by shifts in passband due to fluctuations in temperature and power. For example, a cellular phone includes power amplifiers (PAs) that must be able to deal with larger than expected insertion losses at the edges of the filter (duplexer). As the filter passband shifts down in frequency, e.g., due to rising temperature, the point of maximum absorption of power in the filter, which is designed to be above the passband, moves down into the frequency range of the FCC or government designated passband. At this point, the filter begins to absorb more power from the PA and heats up, causing the temperature to increase further. Thus, the filter passband shifts down in frequency more, bringing the maximum filter absorbing point even closer. This sets up a potential runaway situation, which is avoided only by the fact that the reflected power becomes large and the filter eventually settles at some high temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
         FIG. 1A  is a top view of an acoustic resonator structure according to a representative embodiment. 
         FIG. 1B  is the cross-sectional view of the acoustic resonator structure of  FIG. 1A  along line  1 B- 1 B. 
         FIG. 2  is a cross-sectional view of an acoustic resonator structure according to a representative embodiment. 
         FIG. 3  is a simplified schematic block diagram of a filter comprising an acoustic resonator structure according to a representative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings. 
     It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. Any defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings. 
     As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices. 
     As used in the specification and appended claims, and in addition to their ordinary meanings, the terms ‘substantial’ or ‘substantially’ mean to with acceptable limits or degree. For example, ‘substantially cancelled’ means that one skilled in the art would consider the cancellation to be acceptable. 
     As used in the specification and the appended claims and in addition to its ordinary meaning, the term ‘approximately’ means to within an acceptable limit or amount to one having ordinary skill in the art. For example, ‘approximately the same’ means that one of ordinary skill in the art would consider the items being compared to be the same. 
     Relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and “lower” may be used to describe the various elements&#39; relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be “below” that element. Similarly, if the device were rotated by 90° with respect to the view in the drawings, an element described “above” or “below” another element would now be “adjacent” to the other element; where “adjacent” means either abutting the other element, or having one or more layers, materials, structures, etc., between the elements. 
     The present teachings relate generally to temperature compensation of an acoustic stack in an acoustic resonator structure. As used herein, in certain representative embodiments, an acoustic stack comprises: a substrate having a first surface and a second surface; a piezoelectric layer disposed over the substrate, the piezoelectric layer having a first surface, and a second surface, wherein the first surface of the substrate, or the second surface of the piezoelectric layer, comprises a plurality of features; a plurality of electrodes disposed over the first surface of the piezoelectric layer, the plurality of electrodes configured to generate surface acoustic waves in the piezoelectric layer; and a temperature compensation layer disposed between the first surface of the substrate and the second surface of the piezoelectric layer. 
     Notably, the plurality of features described below in connection with representative embodiments are merely illustrative, and in other contemplated embodiments, these features are not provided in the acoustic stack. As such, the layers of the acoustic stack are comparatively smooth. 
     Furthermore, the acoustic stack may comprise more layers than those mentioned above, and described more fully below. By way of example, in certain embodiments, a silicon layer may be disposed between the temperature compensation layer and the piezoelectric layer. Further details of such an acoustic stack may be found in commonly owned U.S. patent application Ser. No. 15/009,801 filed on Jan. 28, 2016, naming Stephen Roy Gilbert, et al. as inventors. The entire disclosure of U.S. patent application Ser. No. 15/009,801 is specifically incorporated herein by reference. 
     In accordance with a representative embodiment, a temperature coefficient of frequency (TCF) of the acoustic stack is less negative compared to an acoustic stack that does not comprise the temperature compensation layer. Beneficially, the TCF of the acoustic stack is improved compared to known structures over a frequency range of Band 13. As will be appreciated by one of ordinary skill in the art, the downlink frequency range of Band 13 is 751 MHz to 787 MHz; and the uplink frequency range is 777 MHz to 787 MHz, with middle frequencies of the downlink and uplink being equally spaced from the respective upper and lower ends of the respective downlink and uplink frequencies. Due to the existence of a Public Safety Band 2 MHz below the uplink passband, the thermal drift of the filter must be approximately zero. 
       FIG. 1A  is a top view of a SAW resonator structure  100  according to a representative embodiment. Notably, the SAW resonator structure  100  is intended to be merely illustrative of the type of device that can benefit from the present teachings. Other types of SAW resonators, including, but not limited to dual mode SAW (DMS) resonators, and structures therefor, are contemplated by the present teachings. The SAW resonator structure  100  of the present teachings is contemplated for a variety of applications. By way of example, and as described in connection with  FIG. 3 , a plurality of SAW resonator structures  100  can be connected in a series/shunt arrangement to provide a ladder filter. 
     The SAW resonator structure  100  comprises a piezoelectric layer  103  disposed over a substrate (not shown in  FIG. 1A ). In accordance with representative embodiments, the piezoelectric layer  103  comprises one of lithium niobate (LiNbO 3 ), which is commonly abbreviated as LN; or lithium tantalate (LiTaO 3 ), which is commonly abbreviated as LT. 
     The SAW resonator structure  100  comprises an active region  101 , which comprises a plurality of interdigitated electrodes  102  disposed over a piezoelectric layer  103 , with acoustic reflectors  104  situated on either end of the active region  101 . In the presently described representative embodiment, electrical connections are made to the SAW resonator structure  100  using the busbar structures  105 . 
     As is known, the pitch of the resonator electrodes determines the resonance conditions, and therefore the operating frequency of the SAW resonator structure  100 . Specifically, the interdigitated electrodes are arranged with a certain pitch between them, and a surface wave is excited most strongly when its wavelength λ is the same as the pitch of the electrodes. The equation f 0 =v/λ. describes the relation between the resonance frequency (f 0 ), which is generally the operating frequency of the SAW resonator structure  100 , and the propagation velocity (v) of a surface wave. These SAW waves comprise Rayleigh or Leaky waves, as is known to one of ordinary skill in the art, and form the basis of function of the SAW resonator structure  100 . 
     Generally, there is a desired fundamental mode, which is typically a Leaky mode, for the SAW resonator structure  100 . By way of example, if the piezoelectric layer  103  is a 42° rotated LT, the shear horizontal mode will have a displacement in the plane of the interdigitated electrodes  102  (the x-y plane of the coordinate system of  FIG. 1A ). The displacement of this fundamental mode is substantially restricted to near the upper surface (first surface  110  as depicted in  FIG. 1C ) of the piezoelectric layer  103 . It is emphasized that the 42° rotated LT piezoelectric layer  103 , and the shear horizontal mode are merely illustrative of the piezoelectric layer  103  and desired fundamental mode, and other materials and desired fundamental modes are contemplated. 
       FIG. 1B  is a cross-sectional view of the SAW resonator structure  100  depicted in  FIG. 1A  along the lines  1 B- 1 B. The SAW resonator structure  100  comprises a substrate  108  disposed beneath the piezoelectric layer  103 , and a temperature compensation layer  109  disposed between the substrate  108  and the piezoelectric layer  103 . 
     As noted above, the piezoelectric layer  103  illustratively comprises one of LN or LT. Generally, in the representative embodiments described below, the piezoelectric layer  103  is a wafer that is previously fabricated, and that is adhered to the temperature compensation layer  109  by atomic bonding as described more fully below. 
     The materials selected for the piezoelectric layer  103  can be divided into two types: one which has been used for a long time and with a high degree of freedom in design is used for Rayleigh wave substrates; the other, with less freedom and limited in design, is for Leaky wave substrates with low loss characteristics, easily reaches the higher frequencies by high acoustic velocity, and is mainly used for mobile communications. LN and LT materials are often used for broadband filters, and according to the filter specifications the manufacturing materials and cutting angles differ. Filters for applications that require comparatively low loss mainly generally require Leaky wave materials, while Rayleigh wave materials are predominately used for communication equipment that requires low ripple and low group delay characteristics. Among Rayleigh wave materials, ST-cut crystal has the best temperature characteristics as a piezoelectric material. 
     As will become clearer as the present description continues, the temperature compensation provided to the SAW resonator structures of the present teachings results in two ways. 
     First, because of its thermal coefficient of expansion (TCE), the piezoelectric layer  103  (illustratively LT) tends to soften when heated rather rapidly compared to the substrate  108 . This softening impacts the acoustic velocity, and thus the frequency of acoustic waves in the piezoelectric layer  103 . However, as described more fully below, the piezoelectric layer  103  is comparatively thin. Moreover, because the piezoelectric layer  103  is atomically bonded to the substrate  108 , its propensity to soften when heated is reduced because of the comparatively low TCE of the substrate  108 , which is illustratively silicon. As such, the piezoelectric layer  103  is substantially mechanically “locked” to the substrate  108 . Stated somewhat differently, some temperature compensation results from the relative stiffness and inflexibility to change dimensions (due to the low TCE) of the substrate  108 . This forces the piezoelectric layer  103  to not expand or contract as rapidly with temperature compared to the same piezoelectric layer not disposed on the substrate  108 . Beneficially, the locking of the piezoelectric layer  103  provides an effective TCF of the piezoelectric layer  103  in the range of approximately −12 ppm/° C. to approximately −25 ppm/° C. 
     Second, the thickness of the piezoelectric layer is selected to be comparatively thin, allowing for greater thermal compensation by the temperature compensation layer  109 . To this end, the thicknesses (z-direction in the coordinate system of  FIG. 1B ) of the piezoelectric layer  103  and of the temperature compensation layer  109  are selected not only to realize electrical performance characteristics, but also to improve the overall temperature coefficient of frequency (TCF) of the acoustic stack. As will be appreciated by one of ordinary skill in the art, since the acoustic waves are surface acoustic waves, they generally do not extend too deep (i.e., z-direction) beneath the upper surface of the piezoelectric layer  103 . However, the thinner the piezoelectric layer is, the greater the likelihood of the interaction of the acoustic modes with the underlying layer(s). As such, the thicker the piezoelectric layer  103 , the less impact the underlying layer(s) have on its acoustic characteristics. However, the thicker the piezoelectric layer  103 , which has a negative TCF, the more deleterious is its impact on thermally-induced frequency. By way of example, LT has a TCF of −42 ppm/° C. Without compensation, over an allowed operating temperature range for a SAW resonator, this translates to approximately 4200 ppm, which is clearly unacceptable. 
     However, by the present teachings, the thickness of the piezoelectric layer  103  is selected to be comparatively thin, and the temperature compensation layer  109  is selected to be comparatively thick in order to provide suitable TCF characteristics. 
     In accordance with a representative embodiment, the piezoelectric layer  103  has a thickness (z-direction in the coordinate system of  FIG. 1B ) in a range of approximately 2λ to approximately 4λ. In some representative embodiments, the piezoelectric layer  103  has a thickness in the range of approximately 22 to approximately 5λ; and in yet other representative embodiments, the thickness of the piezoelectric layer  103  is in the range of approximately 2λ to approximately 10λ. By way of illustration, in absolute numbers, the piezoelectric layer  103  has a thickness in a range of approximately 0.5 μm to approximately 3.0 μm. 
     The temperature compensation layer  109  is deposited by a known method, such as chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD), or may be thermally grown. The temperature compensation layer  109  is polished to a thickness (z-direction in the coordinate system of  FIG. 1B ) in the range of approximately 0.25λ to approximately 4λ, and in other embodiments the temperature compensation layer  109  is polished to a thickness in the range of approximately 0.25λ to approximately 2λ. In absolute measure, in accordance with a representative embodiment, the temperature compensation layer has a thickness of approximately 0.5 μm to approximately 10.0 μm. 
     In accordance with a representative embodiment, the substrate 108 comprises crystalline silicon, which may be polycrystalline or monocrystalline, having a thickness of approximately 100.0 μm to approximately 800.0 μm. As will become clearer as the present description continues, the material selected for use as the substrate  108 , among other considerations, is selected for ease of micromachining, using one or more of a variety of known techniques. Accordingly, other polycrystalline or monocrystalline materials besides silicon are contemplated for use as the substrate  108  of the SAW resonator structure  100 . By way of example, these materials include, but are not limited to, glass, single crystal aluminum oxide (Al 2 O 3 ) (sometimes referred to as “sapphire”), and polycrystalline Al 2 O 3 , to name a few. In certain representative embodiments, in order to improve the performance of a filter comprising SAW resonator structure(s)  100 , the substrate  108  may comprise a comparatively high-resistivity material. Illustratively, the substrate  108  may comprise single crystal silicon that is doped to a comparatively high resistivity. 
     The temperature compensation layer  109  is illustratively an oxide material, such as SiO 2 , phosphosilicate glass (PSG), borosilicate glass (BSG), a thermally grown oxide, or other material amenable to polishing to a high degree of smoothness, as described more fully below. In accordance with a representative embodiment, the temperature compensation layer  109  comprises BSG having a doping level of approximately 2.0 atomic percent (atm %) to approximately 3.0 atm %. In another representative embodiment, the temperature compensation layer  109  has a doping level of less than approximately 5.0 atomic percent (atm %). 
     Beneficially, as noted above, providing the piezoelectric layer  103  with the noted thicknesses effectively locks the piezoelectric layer  103  to the substrate  108 . The inclusion of the temperature compensation layer  109  results in a TCF of the acoustic stack that is less negative compared to an acoustic stack that does not comprise the temperature compensation layer. Just by way of example, as noted above, the locking of the comparatively thin piezoelectric layer  103  of the representative embodiments results in it having a TCF that is approximately −12 ppm/° C. Providing the temperature compensation layer  109  of one of the noted materials, and having a thickness (z-direction in the coordinate system of  FIG. 1B ) in the noted range, results in a TCF of the acoustic stack that is approximately zero (0.0) over a frequency range of Band 13. It is emphasized that the TCF of the acoustic stack of the representative embodiment of  FIG. 1B  may be similarly nearly zero over other frequency ranges by selection of the piezoelectric layer  103  and the temperature compensation layer  109  of the materials noted above, and having thicknesses in the noted range without undue experimentation. 
     The piezoelectric layer  103  has a first surface  110 , and a second surface  111 , which opposes the first surface  110 . Similarly, the temperature compensation layer  109  has a first surface  112  and a second surface  113 . As depicted in  FIG. 1B , the first surface  112  of the temperature compensation layer  109  is atomically bonded to the second surface  111  of the piezoelectric layer  103 , as described more fully below. 
     The substrate  108  has a first surface  114  and a second surface  115  opposing the first surface  114 . The first surface  114  has a plurality of features  116  there-across. As noted above, undesired spurious modes are launched in the piezoelectric layer  103 , and propagate down to the first surface  114 . As described more fully in the above-incorporated applications, the plurality of features  116  reflect undesired spurious modes at various angles and over various distances to destructively interfere with the undesired spurious waves in the piezoelectric layer  103 , and possibly enable a portion of these waves to be beneficially converted into desired SAW waves. Again as described more fully below, the reflections provided by the plurality of features  116  foster a reduction in the degree of spurious modes (i.e., standing waves), which are created by the reflection of acoustic waves at the interface of the second surface  111  of the piezoelectric layer  103  and the first surface  112  of temperature compensation layer  109 . Ultimately, the reflections provided by the plurality of features  116  serve to improve the performance of devices (e.g., filters) that comprise a plurality of SAW resonator structures  100 . 
     As noted above, and as described more fully in the parent applications, the first surface  112  of temperature compensation layer  109  is polished, such as by chemical-mechanical polishing in order to obtain a “mirror” like finish with a comparatively low root-mean-square (RMS) variation of height. This low RMS variation of height significantly improves the contact area between the first surface  112  of the temperature compensation layer  109  and the second surface  111  of the piezoelectric layer  103  to improve the atomic bonding between the first surface  112  and the second surface  111 . As is known, the bond strength realized by atomic bonding is directly proportional to the contact area between two surfaces. As such, improving the flatness/smoothness of the first surface  112  fosters an increase in the contact area, thereby improving the bond of the temperature compensation layer  109  to the piezoelectric layer  103 . As used herein, the term “atomically smooth” means sufficiently smooth to provide sufficient contact area to provide a sufficiently strong bond strength between the temperature compensation layer  109  and the piezoelectric layer  103 , at the interface of their first and second surfaces  112 ,  111 , respectively. 
     As described in the parent applications, the shape, dimensions and spacing of the plurality of features  116  depends on their method of fabrication. For example, using a known etching technique, the plurality of features  116  are formed in the substrate  108 , and may have a generally pyramidal shape. Notably, some of the plurality of features  116  may have comparatively “flat” tops. The plurality of features  116  also have a height that may be substantially the same across the width of the interface between the substrate  108  and the temperature compensation layer  109 . Moreover, the width (x-dimension in the coordinate system of  FIG. 1C ) of the plurality of features  116  may be the same, or may be different. Generally, however, the width of the features is on the order of the desired fundamental mode of the SAW resonator structure  100 . 
     Alternatively, and again depending on the method of fabrication, the height of the plurality of features  116  may not be the same. Rather, by selecting the height of the plurality of features  116  to be different, a reduction in the incidence of more than one of the spurious modes can be realized. 
     The substrate  108  is illustratively single-crystal silicon, or other material having crystalline properties. The present teachings make use of the etching properties of the substrate  108  to realize the various characteristics of the plurality of features  116 . In one representative embodiment, the plurality of features  116  are formed by etching the substrate  108  along crystalline planes. In this case, the plurality of features  116  having pyramidal shapes and sides that are on a “slant” foster reflections at off-angles relative to the incident direction of the acoustic waves. 
     As noted above, there are multiple spurious modes, each having a different frequency and wavelength. In accordance with a representative embodiment, the height of the plurality of features  116  of the substrate  108  is approximately one-fourth (¼)λ, of one or more of the spurious modes. Selecting the height of the plurality of features  116  to be approximately one-fourth (¼)λ, of a particular spurious mode alters the phase of the reflected waves, results in destructive interference by the reflected waves, and substantially prevents the establishment of standing waves, and thus spurious modes. 
     In some embodiments, the height of the plurality of features  116  is substantially the same, and the height is selected to be approximately one-fourth (¼)λ, of one (e.g., a predominant) of the spurious modes. In other embodiments, the height of the plurality of features  116  is not the same, but rather each different height is selected to be approximately equal to one-fourth (¼)λ, of one of the multiple spurious modes (e.g., the spurious modes  107  depicted in  FIG. 1B ). By selecting this one height or multiple heights, the phase of the reflected waves is altered, and results in destructive interference by the reflected waves, thereby substantially preventing the establishment of standing waves of multiple frequencies, thus preventing the establishment of multiple spurious modes. 
     By way of example, if the spurious modes have a frequency of 700 MHz, the wavelength λ is approximately 6.0 μm. As such, the height would be approximately 1.5 μm. By contrast, if the spurious modes have a frequency of 4200 MHz, the λ is approximately 1.0 μm. In this example, the height would be approximately 0.25 μm. More generally, the height is in the range of less than approximately 0.25 μm (e.g., 0.1 μm) to greater than approximately 1.5 μm (e.g., 2.5 μm). As will be appreciated, the range for the height depends on the frequency of the fundamental mode. 
     The non-periodic orientation of the plurality of features  216 , the generally angled surfaces (e.g., sides of the features  216 ) provided by the plurality of features  216 , and providing the height of the plurality of features  216  to be in the noted range relative to the wavelength of the propagating spurious modes combine to alter the phase of the acoustic waves incident on the various features. Beneficially, these factors in combination result in comparatively diffuse reflection of the acoustic waves back through the temperature compensation layer  109  and into the piezoelectric layer  103 . This comparatively diffuse reflection of the acoustic waves from the plurality of features  216  will generally not foster constructive interference, and the establishment of resonance conditions. Accordingly, the plurality of features  216  generally prevent the above-noted parasitic acoustic standing waves (i.e., spurious modes) from being established from the acoustic waves generated in the piezoelectric layer  103 , which travel down and into the substrate  108 . 
     In other representative embodiments, the plurality of features  216  have random spacing, or random orientation, or random heights, or a combination thereof. As can be appreciated, such random spacing, orientations and heights, alone or in combination can foster comparatively diffuse reflection of the acoustic waves incident thereon. This diffuse reflection, in turn, alters the phase of the acoustic waves, and serves to reduce the propensity of standing waves (and thus spurious modes) from being established. 
     The random spacing, orientation, and heights of the plurality of features  216  can be effected by a number of methods. For example, the plurality of features  216  may be provided by simply using an unpolished wafer for the substrate  108 . Alternatively, the second surface  215  of the substrate  108  could be rough polished by CMP, for example. While the plurality of features  216  of such an embodiment would likely not have the height relative to the wavelength of the spurious modes, the random nature of such an unpolished surface would likely provide a useful degree of diffusive reflection to avoid the establishment of a resonant condition for the spurious modes. 
     Unfortunately, at the atomic level, the surfaces of such deposited films are very rough. However, the first surface  212  of temperature compensation layer  109  (e.g., PSG) can be polished by a known method to provide an atomically smooth surface. The surface of the temperature compensation layer  109  is first planarized by polishing with a slurry, using a known CMP method. The remaining PSG can then be polished using a more refined slurry. Alternatively, a single more refined slurry can be used for both polishing steps if the additional polishing time is not objectionable. As noted above, the goal is to create a “mirror” like finish that is atomically smooth in order to foster strong atomic bonding between the temperature compensation layer  109  and the piezoelectric layer  103 , at the interface of their first and second surfaces  212 ,  211  respectively. Further details of the polishing sequence can be found, for example, in U.S. Pat. No. 6,060,818 and U.S. Patent Application Publication No. 20050088257, to Ruby, et al. The entire disclosures of U.S. Pat. No. 6,060,818, and U.S. Patent Application Publication No. 20050088257 are specifically incorporated herein by reference. 
     Such an atomically smooth surface can be realized by providing the first surface  212  of temperature compensation layer  109  having an RMS variation in height of in the range of approximately 0.1 Å to approximately 10.0 Å; although beneficially, the RMS variation in height is less than approximately 5.0 Å. 
     As noted above, the forming of an atomically smooth first surface  212  provides an increased contact area at the interface of the first and second surfaces  212 ,  211 , respectively, of the temperature compensation layer  109  and the piezoelectric layer  103 . This increased contact area, in turn, fosters a comparatively strong atomic bond between the temperature compensation layer  109  and the piezoelectric layer  103 . Among other benefits, the strong atomic bond between the temperature compensation layer  109  and the piezoelectric layer  103  reduces separation or delamination of the temperature compensation layer  109  and the piezoelectric layer  103 , thereby increasing the reliability of devices comprising the SAW resonator structure  100  over time. 
       FIG. 2  is a cross-sectional view of the SAW resonator structure  200  in accordance with another representative embodiment. Many aspects and details of SAW resonator structure  200  are common to those of SAW resonator structure  200  described in connection with representative embodiments of  FIGS. 1A-1B . Many of these aspects and details are not repeated, but nonetheless are relevant to the presently described representative embodiments. 
     The SAW resonator structure  200  comprises substrate  208  disposed beneath the piezoelectric layer  203 , and temperature compensation layer  209  disposed between the substrate  208  and the piezoelectric layer  203 . 
     As noted above, the piezoelectric layer  203  illustratively comprises one of LN or LT. Generally, in the representative embodiments described below, the piezoelectric layer  203  is a wafer that is previously fabricated, and that is adhered to the temperature compensation layer  209  by atomic bonding. 
     The materials selected for the piezoelectric layer  203  can be divided into two types: one which has been used for a long time and with a high degree of freedom in design is used for Rayleigh wave substrates; the other, with less freedom and limited in design, is for Leaky wave substrates with low loss characteristics, easily reaches the higher frequencies by high acoustic velocity, and is mainly used for mobile communications. LN and LT materials are often used for broadband filters, and according to the filter specifications the manufacturing materials and cutting angles differ. Filters for applications that require comparatively low loss mainly generally require Leaky wave materials, while Rayleigh wave materials are predominately used for communication equipment that requires low ripple and low group delay characteristics. Among Rayleigh wave materials, ST-cut crystal has the best temperature characteristics as a piezoelectric material. 
     As described in connection with the representative embodiments of  FIG. 1B , the temperature compensation provided to the SAW resonator structures of the present teachings result in two ways. 
     First, because of its thermal coefficient of expansion (TCE), the piezoelectric layer  203  (illustratively LT) tends to soften when heated rather rapidly compared to the substrate  208 . This softening impacts the acoustic velocity, and thus the frequency of acoustic waves in the piezoelectric layer  203 . However, as described more fully below, the piezoelectric layer  203  is comparatively thin. Moreover, because the piezoelectric layer  203  is atomically bonded to the substrate  208 , its propensity to soften when heated is reduced because of the comparatively low TCE of the substrate  208 , which is illustratively silicon. As such, the piezoelectric layer  203  is substantially mechanically “locked” to the substrate  208 , and the effective TCF of the piezoelectric layer  203  is approximately −12 ppm/° C. 
     Second, the thickness of the piezoelectric layer  203  is selected to be comparatively thin, allowing for greater thermal compensation by the temperature compensation layer  209 . To this end, the thicknesses (z-direction in the coordinate system of  FIG. 1B ) of the piezoelectric layer  203  and of the temperature compensation layer  209  are selected not only to realize electrical performance characteristics, but also to improve the overall temperature coefficient of frequency (TCF) of the acoustic stack. As will be appreciated by one of ordinary skill in the art, since the acoustic waves are surface acoustic waves, they generally do not extend too deep (i.e., z-direction) beneath the upper surface of the piezoelectric layer  203 . However, the thinner the piezoelectric layer is, the greater the likelihood of the interaction of the acoustic modes with the underlying layer(s). As such, the thicker the piezoelectric layer  203 , the less impact the underlying layer(s) have on its acoustic characteristics. However, the thicker the piezoelectric layer  203 , which has a negative TCF, the more deleterious is its impact on thermally-induced frequency. By way of example, LT has a TCF of −42 ppm/° C. Without compensation, over an allowed operating temperature range for a SAW resonator, this translates to approximately 4200 ppm, which is clearly unacceptable. 
     However, by the present teachings, the thickness of the piezoelectric layer  203  is selected to be comparatively thin, and the temperature compensation layer  209  is selected to be comparatively thick in order to provide suitable TCF characteristics. 
     In accordance with a representative embodiment, the piezoelectric layer  203  has a thickness (z-direction in the coordinate system of  FIG. 1B ) in a range of approximately 2λ to approximately 4λ.In some representative embodiments, the piezoelectric layer  203  has a thickness in the range of approximately 2λ to approximately 5λ; and in yet other representative embodiments, the thickness of the piezoelectric layer  203  is in the range of approximately 2λ to approximately 20λ. By way of illustration, in absolute numbers, the piezoelectric layer  203  has a thickness in a range of approximately 0.5 μm to approximately 3.0 μm. 
     The temperature compensation layer  209  is deposited by a known method, such as chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD), or may be thermally grown. The temperature compensation layer  209  is polished to a thickness (z-direction in the coordinate system of  FIG. 1B ) in the range of approximately 0.25λ to approximately 4λ, and in other embodiments the temperature compensation layer  209  is polished to a thickness in the range of approximately 0.25λ to approximately 2λ. In absolute measure, in accordance with a representative embodiment, the temperature compensation layer  209  has a thickness of approximately 0.5 μm to approximately 10.0 μm. 
     In accordance with a representative embodiment, the substrate  208  comprises crystalline silicon, which may be polycrystalline or monocrystalline, having a thickness of approximately 100.0 μm to approximately 800.0 μm. As will become clearer as the present description continues, the material selected for use as the substrate  208 , among other considerations, is selected for ease of micromachining, using one or more of a variety of known techniques. Accordingly, other polycrystalline or monocrystalline materials besides silicon are contemplated for use as the substrate  208  of the SAW resonator structure  200 . By way of example, these materials include, but are not limited to, glass, single crystal aluminum oxide (Al 2 O 3 ) (sometimes referred to as “sapphire”), and polycrystalline Al 2 O 3 , to name a few. In certain representative embodiments, in order to improve the performance of a filter comprising SAW resonator structure(s)  200 , the substrate  208  may comprise a comparatively high-resistivity material. Illustratively, the substrate  208  may comprise single crystal silicon that is doped to a comparatively high resistivity. 
     The temperature compensation layer  209  is illustratively an oxide material, such as SiO 2 , phosphosilicate glass (PSG), borosilicate glass (BSG), a thermally grown oxide, or other material amenable to polishing to a high degree of smoothness, as described more fully below. In accordance with a representative embodiment, the temperature compensation layer  209  comprises BSG having a doping level of approximately 2.0 atomic percent (atm %) to approximately 3.0 atm %. In another representative embodiment, the temperature compensation layer  209  has a doping level less than approximately 5.0 atomic percent (atm %). 
     Beneficially, providing the piezoelectric layer  203  with the noted thicknesses effectively locks the piezoelectric layer  203  to the substrate  208 . The inclusion of the temperature compensation layer  209  results in a TCF of the acoustic stack that is less negative compared to an acoustic stack that does not comprise the temperature compensation layer. Just by way of example, as noted above, the locking of the comparatively thin piezoelectric layer  203  of the representative embodiments results in it having a TCF that is approximately −20 ppm/° C. Providing the temperature compensation layer  209  of one of the noted materials, and having a thickness (z-direction in the coordinate system of  FIG. 1B ) in the noted range, results in a TCF of the acoustic stack that is approximately zero (0.0) over a frequency range of Band 13. It is emphasized that the TCF of the acoustic stack of the representative embodiment of  FIG. 1B  may be similarly nearly zero over other frequency ranges by selection of the piezoelectric layer  203  and the temperature compensation layer  209  of the materials noted above, and having thicknesses in the noted range without undue experimentation. 
     As noted above, when connected in a selected topology, a plurality of SAW resonators can function as an electrical filter.  FIG. 3  shows a simplified schematic block diagram of an electrical filter  300  in accordance with a representative embodiment. The electrical filter  300  comprises series SAW resonators  301  and shunt SAW resonators  302 . The series SAW resonators  301  and shunt SAW resonators  302  may each comprise SAW resonator structures  100 ,  200  described in connection with the representative embodiments of  FIGS. 1A-2 . As can be appreciated, the SAW resonator structures (e.g., a plurality of SAW resonator structures  100 ,  200 ) that comprise the electrical filter  300  may be provided over a common substrate (e.g., substrate  108 ), or may be a number of individual SAW resonator structures (e.g., SAW resonator structures  100 ,  200 ) disposed over more than one substrate (e.g., more than one substrate  108 ,  208 ). The electrical filter  300  is commonly referred to as a ladder filter, and may be used for example in duplexer applications. It is emphasized that the topology of the electrical filter  300  is merely illustrative and other topologies are contemplated. Moreover, the acoustic resonators of the representative embodiments are contemplated in a variety of applications including, but not limited to duplexers. 
     The various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.