Bulk acoustic resonator electrical impedance transformers

An electrical impedance transformer comprises a first film bulk acoustic resonator (FBAR), having a first electrical impedance and a first resonance frequency. The electrical impedance transformer also comprises: a second FBAR, having a second electrical impedance and a second resonance frequency, and being disposed over the first FBAR. The electrical impedance transformer also includes a decoupling layer disposed between the first and the second FBARs. The first electrical impedance differs from the second electrical impedance and the first and second resonance frequencies are substantially the same.

BACKGROUND

In many applications, it is useful to provide an electrical impedance transformation from an input having one impedance to an output having another electrical impedance. For example, in many communication devices, an antenna is used to receive signal and to transmit signals. The received signals are provided to a receiving amplifier of a receiver of the communication device. Moreover, the antenna may receive signals from a transmitter amplifier of a transmitter. Regardless of whether the transmission/reception of signals is half or full duplex, or even simplex, often times the antenna has an impedance that varies from the impedance of the amplifier (receiver or transmitter). As should be appreciated, mismatched impedances result in reflections and losses that are beneficially avoided.

Among other technologies, electrical impedance transformers can be based on bulk acoustic waves (BAW) devices. One type of electrical impedance transformer is based on a film bulk acoustic resonator (FBAR) structure. The transformer includes two acoustic stacks, each comprising a layer of piezoelectric material disposed between two electrodes. A decoupling material is disposed between the acoustic stacks. Acoustic waves achieve resonance across the acoustic stacks, with the resonant frequency of the waves being determined by the materials in the acoustic stack.

FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns, and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to known resonators. However, certain known BAW-based electrical impedance transformers suffer from, among other drawbacks, insertion loss and reduced bandwidth.

There is a need, therefore, for an electrical impedance transformer that overcomes at least the shortcoming of known electrical impedance transformers discussed above.

SUMMARY

In a representative embodiment, an electrical impedance transformer, comprises a first film bulk acoustic resonator (FBAR), having a first electrical impedance and a first resonance frequency. The first FBAR comprises: a first set of electrodes, having a first acoustic impedance; and a first piezoelectric layer having a first thickness. The first piezoelectric layer is disposed between the first set of electrodes. The electrical impedance transformer also comprises: a second FBAR, having a second electrical impedance and a second resonance frequency, and being disposed over the first FBAR. The second FBAR comprises: a second set of electrodes, having a second acoustic impedance, which differs from the first acoustic impedance; and a second piezoelectric layer having a second thickness. The second piezoelectric layer is disposed between the second set of electrodes. The electrical impedance transformer also includes a decoupling layer disposed between the first and the second FBARs. The first electrical impedance differs from the second electrical impedance and the first and second resonance frequencies are substantially the same.

In another representative embodiment, a communication device, comprising: a first port; a second port; and an electrical impedance transformer. The electrical impedance transformer comprises a first film bulk acoustic resonator (FBAR), having a first electrical impedance and a first resonance frequency. The first FBAR comprises: a first set of electrodes, having a first acoustic impedance; and a first piezoelectric layer having a first thickness. The first piezoelectric layer is disposed between the first set of electrodes. The electrical impedance transformer also comprises: a second FBAR, having a second electrical impedance and a second resonance frequency, and being disposed over the first FBAR. The second FBAR comprises: a second set of electrodes, having a second acoustic impedance, which differs from the first acoustic impedance; and a second piezoelectric layer having a second thickness. The second piezoelectric layer is disposed between the second set of electrodes. The electrical impedance transformer also includes a decoupling layer disposed between the first and the second FBARs. The first electrical impedance differs from the second electrical impedance and the first and second resonance frequencies are substantially the same.

DEFINED TERMINOLOGY

As used herein, the terms ‘a’ or ‘an’, as used herein are defined as one or more than one.

As used herein, the term “electric impedance” refers to a measure of the impediment to the flow of alternating current caused by a combination of resistance and reactance and typically is measured in ohms at a given frequency.

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. Descriptions of known devices, materials and manufacturing methods may be omitted so as to avoid obscuring the description of the example embodiments. Nonetheless, such devices, materials and methods that are within the purview of one of ordinary skill in the art may be used in accordance with the representative embodiments.

Representative embodiments are described in the context of FBAR-based electrical impedance transformers. It is noted that the term ‘electrical impedance’ may be referred to simply as ‘impedance.’ The term ‘acoustic impedance’ will always be referred to as such to avoid its being confused with the electrical impedance. As will become clearer as the present description continues, the electrical impedance transformers of the representative embodiments may comprise stacked FBARs of the representative having an acoustically decoupling layer between FBARs have certain features common to those described in commonly owned U.S. Pat. No. 7,019,605 to Larson, III and entitled “Stacked Bulk Acoustic Resonator Band-Pass Filter with Controllable Pass Bandwidth;” and certain features common to those described in commonly owned US Patent Publication 20070176710, to Jamneala, et al. The disclosures of this patent and patent publication are specifically incorporated herein by reference.

FIG. 1Ais a cross-sectional view of an electrical impedance transformer100in accordance with a representative embodiment. The impedance transformer100comprises a substrate101having a cavity102(or acoustic mirror) therein. A first FBAR stack resonator103is disposed over the cavity102of the substrate101resulting in an FBAR membrane. The impedance transformer100comprises a second FBAR resonator104disposed over the first FBAR resonator103. An acoustic decoupler105is disposed between the first and second FBARs as shown. In representative embodiments, decoupler105comprises dielectric polymer SiLK® commercially provided by E.I. Dupont, Inc., USA. In other embodiments, the acoustic decoupler105is formed of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the balun. It is emphasized that other materials and arrangements for layer105are contemplated. Some illustrative materials may be found in the incorporated references to Larson III, et al., and Jamneala, et al.

The cavity or reflector (e.g., a mismatched acoustic Bragg reflector)102and its fabrication may be as described in commonly owned U.S. Pat. No. 6,107,721, to Lakin, the disclosure of which is specifically incorporated into this disclosure by reference in its entirety. Moreover, the cavity102may be fabricated according to known semiconductor processing methods and using known materials. Illustratively, the cavity102may be fabricated according to the teachings of U.S. Pat. Nos. 5,587,620, 5,873,153 and 6,507,983 to Ruby, et al. The disclosures of these patents are specifically incorporated herein by reference. It is emphasized that the methods described in these patents are representative and other methods of fabrication and materials within the purview of one of ordinary skill in the art are contemplated.

The first FBAR resonator103comprises a first set of electrodes106,108, and a first piezoelectric layer107therebetween. In a representative embodiment, the first piezoelectric layer107comprises aluminum nitride (AlN) although other materials within the purview of one of ordinary skill in the art may be used in place of or in addition to AlN. As described more fully herein, the electrodes106,108are selected based on the desired impedance of the first FBAR resonator.

The second FBAR resonator104comprises a second set of electrodes109,111, and a second piezoelectric layer110therebetween. In a representative embodiment, the second piezoelectric layer110comprises aluminum nitride (AlN) although other materials within the purview of one of ordinary skill in the art may be used in place of or in addition to AlN. As described more fully herein, the electrodes109,111are selected based on the desired impedance of the second FBAR resonator104.

In a representative embodiment, the first piezoelectric layer107has a first thickness (t) and the second piezoelectric layer110has a second thickness (t+∂t), which is greater that ‘t.’ The thicknesses of the layers107,110are selected to provide a different electrical impedance for the first and second FBAR resonators. In particular, the capacitance of the FBAR is inversely proportional to the thickness of the piezoelectric layer between the electrodes if the areas are equal. In turn, the electrical impedance is inversely proportional to the capacitance; and as such the electrical impedance is proportional to the thickness of the piezoelectric layer of the FBAR resonator. In representative embodiments, the ratio of the impedances (relative value) of the first and second FBAR resonators103,104is set by selecting the ratio of the thicknesses t and t+∂t. For example, in the exemplary embodiment shown inFIG. 1A, the thickness t of the first FBAR resonator103is smaller than the thickness (t+∂t) of the second FBAR resonator104. In this way, the electrical impedance of the second FBAR resonator104is larger than the electrical impedance of the first FBAR103. As will be described more fully herein, the absolute value of the electrical impedances of the first and second FBAR resonators103,104are set by selecting the areal dimensions of the FBAR resonators. In representative embodiments, the areas of the two FBARs are substantially equal and are substantially overlapping.

As will be appreciated, a change in the thickness of the piezoelectric material will result in a change in the resonant frequency (f0) of the FBAR resonator. However, the resonant frequency of the two FBAR resonators103,104of the electrical impedance transformer100must be substantially the same. (By substantially the same, the resonant frequencies are the same within some acceptable tolerance). One way to maintain the same the same resonant frequencies in the first and second FBARs103,104, the overall acoustic thicknesses of the first and second FBARs103,104are made substantially the same by increasing the thickness of the first set of electrodes106,108. However, while providing the desired resonant frequency, the electromechanical coupling coefficient, kt2, which is proportional to the ratio of the electric energy density in a particular volume to the acoustic energy in a particular volume, can be comparatively degraded by providing relatively thick electrodes. As should be appreciated, degradation of kt2results in an undesirable reduction in insertion loss and decrease in bandwidth.

In accordance with representative embodiment ofFIG. 1A, the resonant frequency of the first and second FBARs103,104is set to substantially the same value, while the coupling coefficient, kt2, is substantially unchanged in the FBAR having decreased electrical impedance by selecting a material for the first set of electrodes106,108that has a greater acoustic impedance (and is more mechanically rigid (i.e., ‘stiffer’)) than the material selected for the second set of electrodes109,111. Stated differently, the piezoelectric layer having a comparatively reduced thickness has a set of electrodes of a material that has a greater acoustic impedance. As should be appreciated, because the material having a greater acoustic impedance is used for the first set of electrodes106,108, their thickness can be comparatively reduced, and thus the desired resonant frequency can be attained. Moreover, because the overall thickness of the electrodes106,108is also comparatively reduced, the deleterious impact of thicker electrodes on the coupling coefficient, kt2, can also be substantially avoided.

In a representative embodiment, the first set of electrodes106,108are made of or comprise tungsten (W) or an alloy thereof, whereas the second set of electrodes109,111are made of or comprise molybdenum (Mo). With these selected materials the lower impedance first FBAR103is provided with substantially the same resonant frequency and coupling coefficient, kt2, as the second FBAR104. By way of comparison, for transformers working at a frequency of about 2 GHz and achieving a 2:1 impedance transformation ratio, the thickness of the piezoelectric layers107is approximately 1.0 μm, the thicknesses of electrodes (illustratively Tungsten)106,108are about 2500 Angstroms, the thickness of piezoelectric layer110is approximately 2 μm and the thickness of the electrodes (illustratively molybdenum)109,111are approximately 2000 Å.

The selection of the thicknesses, t and t+∂t, of the piezoelectric layers107and110determine the ratio of the impedances of the first FBAR103and the second FBAR104for a desired resonant frequency and coupling coefficient. For instance, the thicknesses of the various materials of the FBARs may be selected to provide a 2:1 impedance ratio, where the second FBAR104, having a comparatively thick piezoelectric material, has twice the electrical impedance of the first FBAR103. Selection of the absolute impedance (e.g., 50Ω to 100Ω transformation) is determined by the suitable selection of the areal dimension of the overlapping stacked FBARs.

FIG. 1Bis a top view of the electrical impedance transformer100. The electrodes111,109,108and106may be selectively apodized and may include mass loading layers and other performance enhancing features. The use of apodization and mass loading are known to those of ordinary skill in the art and details thereof are generally omitted in order to avoid obscuring the description of the representative embodiments. For example, details of apodization may be found in U.S. Pat. No. 7,629,865, entitled “Piezoelectric Resonator Structures and Electrical Filters” to Richard C. Ruby, et al. In addition, details of mass loading may be found in U.S. Pat. No. 7,280,007, entitled “Thin Film Bulk Acoustic Resonator with Mass Loaded Perimeter” to Hongjun Feng, et al.; and U.S. patent application Ser. No. 11/713,726, entitled “Piezoelectric Resonator Structures and Electrical Filters having Frame Elements” to Jamneala, et al. Furthermore, the FBARs103,104may include frame structures such as described in U.S. Pat. No. 7,714,684, entitled “Acoustic Resonator Performance Enhancement Using Alternating Frame Structure” to Richard C. Ruby, et al.

As should be appreciated, the first and second FBARs103,104comprise a coupled resonator structure. As such, only the area of overlap function as coupled acoustic resonators. Therefore, the shapes and dimensions of the electrodes111,109,108and106, and intervening piezoelectric layers107,110, are selected to substantially the same (i.e., to within manufacturing tolerances). Moreover, the electrodes and piezoelectric layers are aligned to substantially overlap (again to within manufacturing tolerances), thereby substantially optimizing the overlap of the FBARS103,104.

The area of the first and second FBARs103,104is determined by the area of the electrodes111,109,108and106, and intervening piezoelectric layers107,110. Moreover, the absolute impedances of the first and second FBARs103,104are determined by the areal dimensions of the first and second FBARs103,104. Thus, the selection of the areal dimensions of the electrodes111,109,108and106, and intervening piezoelectric layers107,110is used to select the absolute impedances of the first and second FBARs103,104.

In practice, once the thicknesses of the piezoelectric layers107,110are determined for the desired ratio of the impedance of the first FBAR103to the impedance of the second FBAR104, the layers of the stacked FBAR structure are formed by known processing methods, such as described in one or more of the incorporated references above. To provide the absolute impedance of the first FBAR103and of the second FBAR104, the electrodes111,109,108and106and piezoelectric layers107,110are fabricated with the required areal dimensions to effect the desired impedances.

Continuing with the previous illustrative ratio, the thickness of the piezoelectric layers107,110are selected to provide a 2:1 impedance ratio of the second FBAR104to the first FBAR103; and the electrodes111,109,108and106, and piezoelectric layers107,110are sized so that the areal dimensions provide a second FBAR104of 100Ω and a first FBAR103of 50Ω. Of course, the areal dimensions could provide other impedances; for instance the second FBAR104could be 200Ω and the first FBAR103could 100Ω by selection of the areal dimension. Alternatively, the ratio could be changed to another value and the areal dimension selected to suit a particular need.

FIG. 2is a simplified schematic view of a portion of a communication device200including an electrical impedance transformer in accordance with a representative embodiment. By ‘portion’ is meant only the elements germane to the present discussion are shown. Naturally, a communication device (e.g., a mobile phone) includes a large number of components, which while necessary to its function, need not be described in order to properly describe the function of the transformer100within the device200. Thus, these components are not described to avoid obscuring the description of the illustrative embodiments.

The device200includes the electrical impedance transformer100including the first and second FBARs103,104. The substrate101is not illustrated for simplicity. Notably, many details of the transformer100described in conjunction withFIGS. 1A and 1Bare common to the device200and are not repeated. An input201is connected to an antenna202. The input201is connected to the first FBAR103having a first electrical impedance. An electrical signal from the input201is converted to an acoustic signal and is coupled to the second FBAR104having a second electrical impedance. The second FBAR104converts the acoustic signal to an electrical signal and provides the electrical signal to an output203. In the representative embodiment, the output is connected to a receiver amplifier204, which amplifies the signal for further processing at a receiver (not shown inFIG. 2.) Continuing with the previous example, with the thicknesses and areal dimensions selected, a signal from a 50Ω input201is provided to a 100Ω output203by the transformer100of an illustrative embodiment.

FIG. 3is a simplified schematic view of a portion of a communication device300including an electrical impedance transformer in accordance with a representative embodiment. Again, by ‘portion’ is meant only the elements germane to the present discussion are shown. Naturally, a communication device (e.g., a mobile phone) includes a large number of components, which while necessary to its function, need not be described in order to properly describe the function of the transformer100within the device300. Thus, these components are not described to avoid obscuring the description of the illustrative embodiments.

The device300includes the electrical impedance transformer100including the first and second FBARs103,104. The substrate101is not illustrated for simplicity. Notably, many details of the transformer100described in conjunction withFIGS. 1A and 1Bare common to the device300and are not repeated. An input301is connected to a transmit amplifier302. The input301is connected to the first FBAR103having a first electrical impedance. An electrical signal from the input301is converted to an acoustic signal and is coupled to the second FBAR104having a second electrical impedance. The second FBAR104converts the acoustic signal to an electrical signal and provides the electrical signal to an output303. In the representative embodiment, the output303is connected to an antenna304, which transmits the signal to a receiver (not shown inFIG. 3.) Continuing with the previous example, with the thicknesses and areal dimensions selected, a signal from a 50Ω input301is provided to a 100Ω output303by the transformer100of an illustrative embodiment.

Applicants note that the device200,300are merely illustrative and not intended to limit the scope of the present teachings. Notably, variations of the transformer100and connections thereto are contemplated. For instance, the transformation of input to output can be different than the step-up described. As such, the inputs201,301could be at a higher impedance (e.g., 100Ω) and the outputs203,303could be at a lower impedance (e.g., 50Ω). Furthermore, the electrical transformer100could be used in a wide variety of applications, and thus is not limited to the communications applications described. Generally, the electrical transformer100may be used in many types of electronic device to perform such functions as transforming impedances, linking single-ended circuitry with balanced circuitry or vice versa and providing electrical isolation.

FIG. 4is a graphical representation of a Q-circle (S11and S22parameters) of two FBARs forming the acoustic transformer in accordance with a representative embodiment. The two FBARs have substantially the same resonant frequency, but the electrodes of one of the FBARs comprise a material with a comparatively higher acoustic impedance (e.g., W), while the electrodes of the other FBAR have a comparatively lower acoustic impedance (e.g., Mo). Due to the enhanced coupling coefficient provided by the electrodes having a higher acoustic impedance, the thickness of the piezoelectric layer (in this case AlN) is reduced with no significant variation in coupling coefficient, kt2, or resonant frequency, f0.

FIG. 5Ais a graphical representation of a Q-circle (S11and S22) of an electrical impedance transformer of a representative embodiment. The FBARs are arranged in a stacked (DSBAR) configuration with approximately 1300 Angstroms of SiLK used as the acoustic decoupler. The resultant return loss plots for input, S11, and the output, S22, where the source impedances are 32 Ohms and 50 Ohm respectively.

FIG. 5Bis a graphical representation of the transmission coefficient (S12) versus frequency of the electrical impedance transformer described in connection withFIG. 5A. In the representative embodiment, the 2 dB bandwidth of 108 MHz BW is achieved.

FIG. 6is a simplified schematic view of a communications device600in accordance with a representative embodiment. The communications device600includes an electrical impedance transformer601, which is substantially identical to one of the impedance transformers described previously. The communications device includes an input/output602to an antenna; a transmitter603and a receiver604. The device600may function in half or full duplex mode; and the impedance transformer601may provide a step-up or a step-down electrical impedance transformation. Moreover, the device may include more that one electrical impedance transformer601; and may be configured in a different manner than shown.

In view of this disclosure it is noted that the various acoustic resonator filters described herein can be implemented in a variety of materials and variant structures. Moreover, applications other than resonator filters may benefit from the present teachings. Further, the various materials, structures and parameters are included by way of 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 materials and equipment to implement these applications, while remaining within the scope of the appended claims.