Patent Publication Number: US-2010117485-A1

Title: Piezoelectric transducers with noise-cancelling electrodes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to commonly owned U.S. patent applications: MULTI-LAYER TRANSDUCERS WITH ANNULAR CONTACTS Ser. No. 11/11/604,478, to R. Shane Fazzio, et al. entitled TRANSDUCERS WITH ANNULAR CONTACTS and filed on Nov. 27, 2006; and Ser. No. 11/737,725 to R. Shane Fazzio, et al. entitled MULTI-LAYER TRANSDUCERS WITH ANNULAR CONTACTS and filed on Apr. 19, 2007. The entire disclosures of these related applications are specifically incorporated herein by reference. 
    
    
     BACKGROUND 
     Transducers are used in a wide variety of electronic applications. One type of transducer is known as a piezoelectric transducer. A piezoelectric transducer comprises a piezoelectric material disposed between electrodes. The application of a time-varying electrical signal will cause a mechanical vibration across the transducer; and the application of a time-varying mechanical signal will cause a time-varying electrical signal to be generated by the piezoelectric material of the transducer. One type of piezoelectric transducer may be based on film bulk acoustic resonators (FBARs) and bulk acoustic resonators (BAWs). As is known, disposed FBARs and certain BAW devices over a cavity in a substrate, or otherwise suspending at least a portion of the device will cause the device to flex in a time varying manner. Such resonators are often referred to as membranes. 
     As should be appreciated, among other applications, piezoelectric transducers may be used to transmit or receive mechanical and electrical signals. These signals may be the transduction of acoustic signals, for example, and the transducers may be functioning as microphones (mics) and speakers. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase as well. This has lead to comparatively small transducers, which may be micromachined according to technologies such as micro-electromechanical systems (MEMS) technology, such as described in the related applications. 
     While small feature size transducers do show promise, there are certain drawbacks to known devices that deleteriously impact their performance and thus their attractiveness for commercial implementation. One such drawback is their propensity to provide an unacceptably low signal-to-noise ration (SNR).  FIG. 1  shows an equivalent circuit of a transducer  101  (shown as an equivalent voltage source (V piezo ) and an equivalent capacitance C piezo ) connected to an amplifier  102 . As is known, small feature-size transducers comprise a comparatively small intrinsic capacitance (C piezo ) and provide a comparatively small piezoelectric effect. These factors tend to limit the signal amplitude due to the voltage divider circuit formed by Cpiezo and RL. Moreover, the comparatively large electrode area, makes the sensor susceptible to ambient noise (e.g., background electromagnetic signals). Finally, the transducer  101  has a comparatively large source impedance that when coupled with the required large load resistance (R L )  103 , can result in the ambient noise&#39;s dominating the signal. Notably, as shown in  FIG. 1 , at  104  the ambient electromagnetic noise from the transducer  101  ‘sees’ a comparatively high impedance load resistance  103  which can result in significant voltage noise at the amplifier&#39;s input terminal. Thus, the comparatively low signal amplitude of the desired signal from the transducer  101  is dominated by the ambient noise, a problem further exacerbated by electronic noise in the amplification circuit. 
     What is needed, therefore, is an apparatus that overcomes at least the drawbacks of known transducers discussed above. 
     SUMMARY 
     In accordance with a representative embodiment, an apparatus, comprises a transducer providing a first output; a capacitor providing a second output; a first load impedance connected to the first output; a second load impedance connected to the second output; and a differential amplifier having a first input connected to the first output and a second input connected to the second output. Illustratively, the first load impedance is connected electrically in parallel with the first input and the second load impedance is connected electrically in parallel with the second input. 
     In accordance with another representative embodiment, an apparatus configured to transmit acoustic signals or receive acoustic signals, or both, comprising: a membrane comprising a film bulk acoustic (FBA) transducer providing a first output; a capacitor device providing a second output; a first load impedance connected to the first output; a second load impedance connected to the second output; and a differential amplifier having a first input connected to the first output and a second input connected to the second output. Illustratively, the first load impedance is connected electrically in parallel with the first input and the second load impedance is connected electrically in parallel with the second input. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present teachings are best understood from the following detailed description when read with the accompanying drawing figures. The features are not necessarily drawn to scale. Wherever practical, like reference numerals refer to like features. 
         FIG. 1  shows a simplified schematic diagram of an equivalent circuit of a known transducer circuit. 
         FIG. 2A  shows a simplified schematic diagram of an equivalent circuit of a transducer circuit in accordance with a representative embodiment. 
         FIG. 2B  shows a simplified schematic diagram of an equivalent circuit of a transducer circuit in accordance with a representative embodiment. 
         FIG. 3A  shows a top view of a transducer and a capacitor on a common substrate in accordance with a representative embodiment. 
         FIG. 3B  shows a cross-sectional view of the transducer and capacitor shown in  FIG. 3A . 
         FIG. 3C  shows a top view of a transducer and a capacitor on a common substrate in accordance with a representative embodiment. 
         FIG. 3D  shows a cross-sectional view of the transducer and capacitor shown in  FIG. 3C . 
         FIG. 3E  shows a top view of a transducer and a capacitor on a common substrate in accordance with a representative embodiment. 
         FIG. 3F  shows a cross-sectional view of the transducer and capacitor shown in  FIG. 3A . 
         FIG. 4A  shows a top view of a transducer and a capacitor on a common substrate in accordance with a representative embodiment. 
         FIG. 4B  shows a cross-sectional view of the transducer and capacitor shown in  FIG. 4A . 
     
    
    
     DEFINED TERMINOLOGY 
     As used herein, the terms ‘a’ or ‘an’, as used herein are defined as one or more than one. 
     In addition to their ordinary meanings, the terms ‘substantial’ or ‘substantially’ mean to with acceptable limits or degree to one having ordinary skill in the art. For example, ‘substantially cancelled’ means that one skilled in the art would consider the cancellation to be acceptable. 
     In addition to their ordinary meanings, the terms ‘approximately’ mean 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. 
     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 representative 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. 
       FIG. 2A  shows a simplified schematic diagram of an equivalent circuit  200  of a transducer circuit in accordance with a representative embodiment. The circuit comprises a transducer  201 , which is illustratively a piezoelectric transducer based on film bulk acoustic (FBA) transducer technology or bulk acoustic wave (BAW) technology. Additional details of the transducer  201  are described in the referenced applications to Fazzio, et al. and below. Notably, the transducer  201  is a membrane device operative to oscillate by flexing over a substantial portion of the active area thereof. Moreover, the use of micromachined ultrasonic transducers (MUTs) and piezoelectric MUTs are also contemplated for use in the transducer of representative embodiments. These types of transducers are known to those of ordinary skill in the art. 
     The circuit  200  also comprises a capacitor device  202 , which in the present embodiment is not subject to the piezoelectric effect. As described below, the capacitor device is configured to provide an electromagnetic noise signal for cancellation of a noise signal garnered by the transducer  201 . 
     The circuit  200  includes a load resistance  203  connected to a first electrode  2   a  of the capacitor device  202  and a load resistance  204  connected to a first electrode  1   a  of the transducer  201 . As shown, in this configuration, the capacitor comprises a second electrode  2   b  connected to ground and the transducer  201  comprises a second electrode also connected to ground. First contacts  1   a  and  2   a  of the transducer  201  and the capacitor  202  provide a first output and a second output, respectively, which are also connected to a first (illustratively positive) input and a second (illustratively negative) input of a differential amplifier  205  of circuit  200 . Notably, second contacts  1   b,    2   b  of the transducer  201  and the capacitor  202 , respectively are connected to ground. 
     In operation, an incident signal on the transducer is converted from a mechanical wave to an electrical wave and emerges from the first output as a signal. This signal is provided to the positive input  205  and to the load resistance  204 . However, because of the parallel electrical connection shown, the signal ‘sees’ a comparatively high impedance value at the resistance  204 , and the voltage at the positive input of the differential amplifier  205  is reduced by the voltage divider circuit comprised of the transducer&#39;s output impedance and the resistance  204 . Unfortunately, noise can also be incident on the transducer  201  and the electrical wiring connecting the transducer to the resistance  204  and amplifier  205 . As described in connection with  FIG. 1 , the magnitude of the (desired) signal from the transducer can be small compared to the noise signal, and after amplification, can be lost in the noise. In accordance with a representative embodiment, beneficially the noise is substantially cancelled. In particular, the first contact  1   b  of the capacitor  202  provides an output that is connected to the second (in this example negative) input of the differential amplifier  205 . The noise signal is incident on the capacitor  202  and the electrical connections interconnecting the capacitor to the resistance  203  and amplifier  205  in a like manner as on the transducer and other electrical node, and thus is transmitted to the amplifier  205 . However, because the noise signal is provided to the negative input of the differential amplifier, its magnitude is substantially the same after amplification but its phase is opposite (i.e., everywhere π-radians out of phase) to the noise signal from the transducer  201 . Thus, the noise signal cancels and an output  206  from the amplifier is substantially the amplified (desired) transducer signal. 
       FIG. 2B  shows a simplified schematic diagram of an equivalent circuit of a transducer circuit in accordance with a representative embodiment. The equivalent circuit of  FIG. 2B  shares many common features with the circuit of  FIG. 2A , which are not repeated in order to avoid obscuring the details of the present representative embodiments. 
     As can be appreciated from a review of the embodiment of  FIG. 2B , instead of a capacitor  202 , the second differential input (in this case the negative input) of the presently described embodiment is connected to a second transducer  207 . The second transducer  207  is substantially identical to the first transducer  201 , however, is connected in an opposite manner to the second input of the differential amplifier  205 . The reversal of the connections to effect the desired phase may be effect as described in the referenced applications to Fazzio, et al. Thus, the phase of the (desired) signal at the output of the transducer (i.e., at contact  2   b ) is of substantially the same magnitude but opposite phase as the (desired) signal at the output (i.e., at contact  1  a) of the first transducer  201 . By contrast, because the noise signal is garnered by capacitive coupling at the transducers  201 ,  202 , the amplitude and phase of the noise signals provided at the respective outputs  1   a  and  2   b  are substantially the same. Thus, outputs  1   a  and  2   b  provide (desired) signals of substantially opposite phase and substantially in-phase noise signals to the first and second (differential) inputs of amplifier  205 . After amplification and combination, the output  206  of the amplifier  205  comprises an amplification of the sum of the (desired) signals from the transducers  201 ,  207 . In the illustrative embodiment, the amplitude of the output  206  is approximately twice that of the desired signals from the transducers  201 ,  207 . 
       FIG. 3A  shows a top view of transducer  201  and capacitor  202  on a common substrate  300  in accordance with a representative embodiment. The transducer  201  and capacitor may be fabricated using methods and materials in accordance with the teachings of the referenced applications to Fazzio, et al., or using other known methods and materials. Thus, fabrication sequences are omitted in order to avoid obscuring the descriptions of the representative embodiments. 
     The transducer comprises an upper electrode  301  and a piezoelectric layer  302  disposed over the substrate  300 . The capacitor  202  comprises an upper electrode  303  disposed over the substrate  300 . As shown, the electrodes  301 ,  303  are substantially circular and of approximately the same area. Contacts  1   b  and  2   b  are connected to the upper electrodes  301 ,  303  and contacts  1   a  and  2   a  are connected to lower electrodes (not shown in  FIG. 3A ). As should be appreciated, the arrangement of  FIG. 3A  provides the transducer  201  and capacitor  202  with connections as shown in  FIG. 2A . 
       FIG. 3B  shows a cross-sectional view of the transducer  201  and capacitor  202  shown in  FIG. 3A . The transducer  201  also comprises a lower electrode  304 , which spans a cavity  307  (commonly referred to as a ‘swimming pool’), that provides a membrane structure to the transducer  201 . Thus, the transducer  201  may flex over the cavity in response to electromagnetic or mechanical signals incident thereon. The capacitor also comprises a lower electrode  305 , which is illustratively of the same shape as the upper electrode  303 . However, this is not essential, and an electrode similar to that of lower electrode  304  can be provided. The area of the capacitor is of course dictated by the area of overlap of the upper and lower electrodes  303 ,  305 . Finally, the dielectric of the capacitor may be provided by piezoelectric layer  302  or by another suitable dielectric material. Usefully, the capacitance of the capacitor  202  and the transducer  201  are substantially the same so the noise signals delivered to the amplifier  205  are substantially the same. 
       FIG. 3C  shows a top view of transducer  201  and capacitor  202  on a common substrate  300  in accordance with a representative embodiment. The transducer  201  and capacitor may be fabricated using methods and materials in accordance with the teachings of the referenced applications to Fazzio, et al., or using other known methods and materials. Thus, fabrication sequences are omitted in order to avoid obscuring the descriptions of the representative embodiments. 
     The transducer comprises an upper electrode  308  and a piezoelectric layer  310  disposed over the substrate  300 . The capacitor  202  comprises an upper electrode  309  disposed over the substrate  300 . As shown, the electrodes  308 ,  309  are substantially circular and substantially concentric over a portion of an arc length. Beneficially, the areas of the electrodes  308 ,  309  are approximately the same. Contacts  1   b  and  2   b  are connected to the upper electrodes  308 ,  310  and contacts  1   a  and  2   a  are connected to lower electrodes (not shown in  FIG. 3A ). As should be appreciated, the arrangement of  FIG. 3C  provides the transducer  201  and capacitor  202  with connections as shown in  FIG. 2A . 
       FIG. 3D  shows a cross-sectional view of the transducer  201  and capacitor  202  shown in  FIG. 3C . The transducer  201  also comprises a lower electrode  311 , which spans cavity  307  (commonly referred to as a ‘swimming pool’), that provides a membrane structure to the transducer  201 . Thus, the transducer  201  may flex over the cavity  307  in response to electromagnetic or mechanical signals incident thereon. The capacitor  202  also comprises a lower electrode  312 , which is illustratively of the same shape as the upper electrode  309 . However, this is not essential, and an electrode similar to that of lower electrode  311  can be provided. The area of the capacitor  202  is of course dictated by the area of overlap of the upper and lower electrodes  309 ,  312 . Finally, the dielectric of the capacitor may be provided by piezoelectric layer  310  or by another suitable dielectric material. Usefully, the capacitance of the capacitor  202  and the transducer  201  are substantially the same so the noise signals delivered to the amplifier  205  are substantially the same. 
       FIG. 3E  shows a top view of transducer  201  and transducer  207  on a common substrate  300  in accordance with a representative embodiment. The transducers  201 ,  207  may be fabricated using methods and materials in accordance with the teachings of the referenced applications to Fazzio, et al., or using other known methods and materials. Thus, fabrication sequences are omitted in order to avoid obscuring the descriptions of the representative embodiments. 
     Transducer  201  comprises an upper electrode  315  and transducer  207  comprises an upper electrode  313 . A piezoelectric layer  314 , which is disposed between the upper electrodes  313 ,  315  and lower electrodes (not shown in  FIG. 3E ), is provided. As shown, the electrodes  313 ,  315  are substantially circular and substantially concentric over at least a portion of an arc length. Beneficially, the areas of the electrodes  313 ,  315  are approximately the same. Contacts  1   a  and  2   b  are connected to the upper electrodes  313 ,  315  and contacts  1   b  and  2   a  are connected to lower electrodes (not shown in  FIG. 3E ). As should be appreciated, the arrangement of  FIG. 3E  provides the transducers  201 ,  207  with connections as shown in  FIG. 2B . 
       FIG. 3F  shows a cross-sectional view of the transducers  201 ,  207  shown in  FIG. 3E . The transducer  201  also comprises a lower electrode  316 , which spans cavity  307  (commonly referred to as a ‘swimming pool’), that provides a membrane structure to the transducer  201 . Thus, the transducer  201  may flex over the cavity  307  in response to electromagnetic or mechanical signals incident thereon. The transducer  207  also comprises a lower electrode  317 , which is illustratively of the same shape as the upper electrode  315 . Usefully, the capacitance of the transducer  201  and the transducer  207  are substantially the same so the noise signals delivered to the amplifier  205  are substantially the same. 
       FIG. 4A  is a top view of a transducer structure  400  comprising ‘vertical’ electrodes in accordance with a representative embodiment.  FIG. 4A  shows the transducer structure comprising a substrate  401 , an upper electrode  405  and a second piezoelectric layer  405 .  FIG. 4B  shows a cross-sectional view of the transducer structure  400  comprising ‘vertical’ electrodes shown in  FIG. 4A . The transducer structure  400  may be fabricated using methods and materials in accordance with the teachings of the referenced applications to Fazzio, et al., or using other known methods and materials. Thus, fabrication sequences are omitted in order to avoid obscuring the descriptions of the representative embodiments. 
     The structure  400  comprises the substrate  401 , which comprises a cavity  402  provided therein. A lower electrode  403  is provided over the cavity  402  and substrate as shown. A first piezoelectric layer  406  is provided over the lower electrode  403 , and an inner electrode  404  is provided over the first piezoelectric layer  406 . The second piezoelectric layer  407  is provided over the inner electrode  404 , and the upper electrode  405  is provided over the second piezoelectric layer  407 . The lower, inner and upper electrodes  403 ,  405 ,  405  are provided in a substantially annular arrangement relative to one another. In a representative embodiment, the inner electrode  404  can be connected as the common electrode (e.g., with a single contact for contacts  1   b,    2   a  as shown) between one set of electrodes and the other set of electrodes. By appropriately connecting the outer electrodes to a readout circuit, the two sets of electrodes can be used in a differential configuration. For instance, if the neutral axis of the membrane stack is placed in the center electrode, the upper and common electrode would sense a piezoelectrically-developed voltage, and the common and bottom electrode would sense a piezoelectrically-developed voltage that is  180  degrees out of phase to the first voltage. 
     In view of this disclosure it is noted that the transducers and circuits useful for noise cancellation and amplification (gain) can be implemented in a variety of materials, variant structures, configurations and topologies. Moreover, applications other than small feature size transducers 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.