Abstract:
Periodic signal generators include an oscillator circuit, which is configured to generate a first periodic signal at an output thereof, and a piezoelectric-based microelectromechanical resonator. The resonator is configured to generate a second periodic signal at a first electrode thereof, which is electrically coupled to the oscillator circuit. A variable impedance circuit is provided, which is electrically coupled to a second electrode of the piezoelectric-based microelectromechanical resonator. The variable impedance circuit is configured to passively modify a frequency of the second periodic signal by changing an induced electromechanical stiffness in at least a portion of the piezoelectric-based microelectromechanical resonator.

Description:
REFERENCE TO PRIORITY APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 61/393,760, filed Oct. 15, 2010, the disclosure of which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to integrated circuit devices and, more particularly, to microelectromechanical resonator devices. 
     BACKGROUND OF THE INVENTION 
     Frequency references provided by oscillators are required in every clocked electronic system, including communication circuits, microprocessors, and signal processing circuits. Oscillators frequently consist of high performance piezoelectric crystals, such as quartz oscillators. The advantages of quartz oscillators are their stable operating frequency and high quality factor. However, the disadvantages of quartz oscillators are their relatively large size and unsuitability for high integration with electronic circuitry (e.g., CMOS circuits). 
     Based on these limitations of conventional oscillators, there is a strong interest in the development of fully integrated silicon oscillators. Integration is important not only for reduced size but also reduced power consumption. It is possible to realize an integrated silicon oscillator using the mechanical properties of silicon devices. For example, silicon microelectromechanical (MEMS) resonators can provide small form factor, ease of integration with conventional semiconductor fabrication techniques and high f•Q products. Accordingly, MEMS resonators are considered a desirable alternative to quartz resonators in real-time and other clock applications. 
     One example of a MEMs resonator includes lateral-mode piezoelectric resonators, such as thin-film piezoelectric-on-silicon (TPoS) resonators, which have been successfully incorporated in low-power and low-noise oscillators. Some examples of these types of resonators are disclosed in U.S. Pat. No. 7,939,990 to Wang et al., entitled “Thin-Film Bulk Acoustic Resonators Having Perforated Bodies That Provide Reduced Susceptibility to Process-Induced Lateral Dimension Variations,” and in U.S. Pat. No. 7,888,843 to Ayazi et al., entitled “Thin-Film Piezoelectric-on-Insulator Resonators Having Perforated Resonator Bodies Therein,” the disclosures of which are hereby incorporated herein by reference. Unfortunately, frequency tuning has not been studied extensively in these types of resonators. 
     Active frequency tuning techniques that include application of a DC voltage on the piezoelectric layer have been demonstrated, but such tuning typically requires relatively large voltages, which may be incompatible with the low operating voltages of conventional oscillator circuits. Some examples of active frequency tuning in micromechanical resonators are disclosed in U.S. Pat. Nos. 7,639,105 and 7,843,284 to Ayazi et al., entitled “Lithographically-Defined Multi-Standard Multi-Frequency High-Q Tunable Micromechanical Resonators,” and in U.S. Pat. No. 7,924,119 to Ayazi et al., entitled Micromechanical Bulk Acoustic Mode Resonators Having Interdigitated Electrodes and Multiple Pairs of Anchor Supports,” and in U.S. Pat. No. 7,800,282 to Ayazi et al., entitled Single-Resonator Dual-Frequency Lateral-Extension Mode Piezoelectric Oscillators, and Operating Methods Thereof,” the disclosures of which are hereby incorporated herein by reference. Based on limitations of active frequency tuning, cost effective passive tuning techniques have been considered. 
     SUMMARY OF THE INVENTION 
     Periodic signal generators according to embodiments of the invention utilize passively-tuned microelectromechanical resonators. In some of these embodiments, a periodic signal generator is provided with an oscillator circuit, which is configured to generate a first periodic signal at an output thereof. A piezoelectric-based microelectromechanical resonator is also provided. This resonator is configured to generate a second periodic signal at a first electrode thereof, which is electrically coupled to the oscillator circuit. A variable impedance circuit is provided, which is electrically coupled to a second electrode of the piezoelectric-based microelectromechanical resonator. The variable impedance circuit is configured to passively modify a frequency of the second periodic signal by changing an induced electromechanical stiffness in at least a portion of the piezoelectric-based microelectromechanical resonator. 
     According to some embodiments of the invention, the piezoelectric-based microelectromechanical resonator may be a bulk-lateral-mode microelectromechanical resonator, such as a thin-film piezoelectric-on-semiconductor resonator. The resonator may also include a pair of interdigitated electrodes. A first one of the pair of interdigitated electrodes may include a center finger that is electrically connected to the variable impedance circuit. A second one of the pair of interdigitated electrodes may include a pair of fingers that are spaced on opposing sides of the center finger. The center finger is preferably located over a position of maximum stress within the resonator. The width of the center finger relative to the pair of fingers influences the tuning range of the resonator. According to some preferred embodiments of the invention, a width of the center finger should be at least 1.5 times greater than a width of each of the pair of fingers. 
     The variable impedance circuit may include a variable capacitance device, such as a varactor diode, and a fixed capacitor connected in series between a first terminal of the varactor diode and the first one of the pair of interdigitated electrodes. A relatively high resistance resistor may also be provided, which has a first terminal electrically connected to the first terminal of the varactor diode and a second terminal configured to receive a reference voltage. The variable impedance circuit is configured to change a voltage across the varactor diode in response to changes in magnitude of the reference voltage. The resonator may further include a piezoelectric layer, which is sandwiched between the pair of interdigitated electrodes and an underlying reference electrode. This reference electrode may be electrically coupled to the oscillator circuit. An inductor may also be provided, which has a first terminal electrically coupled to the first one of the pair of interdigitated electrodes and a second terminal electrically coupled to the reference electrode. Preferably, the inductor has an inductance of sufficient magnitude to cancel out at least a majority of a parasitic capacitance of the thin-film piezoelectric-on-semiconductor resonator, and thereby improve its tuning range. 
     According to still further embodiments of the invention, a periodic signal generator is provided with a microelectromechanical resonator, an oscillator circuit and a variable impedance circuit. The resonator includes a pair of interdigitated electrodes, a reference electrode and a piezoelectric layer extending between the reference electrode and the pair of interdigitated electrodes. The oscillator circuit is configured to generate a periodic signal at an output thereof. The oscillator circuit has a first terminal/node electrically coupled to a first of the pair of interdigitated electrodes and a second terminal/node electrically coupled to the reference electrode. The variable impedance circuit is electrically coupled to a second of the pair of interdigitated electrodes. The variable impedance circuit is configured to modify an electromechanical stiffness in the piezoelectric layer when the resonator is excited (e.g., in a fundamental lateral-extensional mode of operation). The variable impedance circuit may include a varactor diode and a fixed capacitor electrically connected in series between a first terminal of the varactor diode and the second of the pair of interdigitated electrodes. A performance enhancing inductor may also be provided. This inductor, which has a first terminal electrically coupled to the second of the pair of interdigitated electrodes and a second terminal electrically coupled to the reference electrode, is configured to cancel out parasitic capacitance within the resonator, which would otherwise limit a maximum tuning range of the resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic of a periodic signal generator according to an embodiment of the present invention; 
         FIG. 2  is an electrical schematic of a piezoelectric-based microelectromechanical resonator that utilizes an inductor to compensate for parasitic capacitance associated with the resonator; 
         FIG. 3A  is a cross-sectional view of an alternative microelectromechanical resonator that may be utilized in the periodic signal generator of  FIG. 1 ; 
         FIG. 3B  is a perspective view of an embodiment of a resonator that is similar to the embodiment of  FIG. 3A ; 
         FIG. 4A  is a cross-sectional view of an alternative microelectromechanical resonator that may be utilized in the periodic signal generator of  FIG. 1 ; 
         FIG. 4B  is a perspective view of an embodiment of a resonator that is similar to the embodiment of  FIG. 4A ; and 
         FIG. 5  is a cross-sectional view of an alternative microelectromechanical resonator that may be utilized in the periodic signal generator of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The present invention now will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. 
     It will be understood that when an element or layer is referred to as being “on,” “connected to” or “coupled to” another element or layer (and variants thereof), it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer (and variants thereof), there are no intervening elements or layers present. Like reference numerals refer to like elements throughout. 
     It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprising”, “including”, having” and variants thereof, when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. In contrast, the term “consisting of” when used in this specification, specifies the stated features, steps, operations, elements, and/or components, and precludes additional features, steps, operations, elements and/or components. 
     Embodiments of the present invention are described herein with reference to cross-section and perspective illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a sharp angle may be somewhat rounded due to manufacturing techniques/tolerances. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is an electrical schematic of a periodic signal generator  100  according to an embodiment of present invention. This generator  100  is illustrated as including a tunable oscillator circuit  30 , a piezoelectric-based microelectromechanical resonator  20 , which is shown as a lateral mode thin-film piezoelectric-on-semiconductor (TPoS) resonator, and a variable impedance circuit  10 . The oscillator circuit  30  includes two amplification stages within an oscillation loop, which are provided by first operational amplifier A 1  and a phase-correcting second operational amplifier A 2 , and a Schmitt trigger comparator ST 1 . The comparator ST 1  converts an output signal from the second operational amplifier A 2  into a square wave output signal that can support jitter measurements. The oscillator circuit  30  further includes resistors R 2 -R 7 , connected as illustrated. The variable impedance circuit  10  includes a large capacitor C, a varactor diode VD 1 , which provides a voltage-controlled variable capacitance, and an input resistor R 1 , connected as illustrated. The input resistor R 1  includes a terminal responsive to a tuning voltage (V DC ), which controls the magnitude of the capacitance provided by the varactor diode VD 1 . The large capacitor C, which operates as an AC short circuit for high frequency signals, is connected in series between a cathode of the varactor diode VD 1  and a second electrode of the piezoelectric-based microelectromechanical resonator  20 . This second electrode is embodied as a center electrode  28   b  of a pair of interdigitated electrodes  28   a ,  28   b , which are provided on an upper surface of a piezoelectric layer  26 . This piezoelectric layer  26  is sandwiched between the interdigitated electrodes  28   a ,  28   b  and a bottom reference electrode  24 , which may, in some embodiments of the invention, be connected to the output of second operational amplifier A 2 . The electrode  28   a  with the pair of fingers on opposing sides of the center electrode  28   b  is electrically connected to the first operational amplifier A 1 . The resonator  20  further includes a semiconductor (e.g., silicon) resonator body  22 , which can be suspended opposite an opening in a surrounding substrate (not shown). In some embodiments of the invention, the electrodes  24 ,  28   a - 28   b  and the piezoelectric layer  26  may be formed of molybdenum (Mo) and aluminum nitride (AlN), respectively. 
     Although not wishing to be bound by any theory, the application of an adjustable tuning voltage V DC  to the varactor diode VD 1  operates to vary a shunt impedance at a port of the resonator  20 . This variation in shunt impedance causes the resonant frequency of the resonator  20  and consequently the frequency of the periodic output signal generated by the second operational amplifier A 2  to change in response to changes in the tuning voltage V DC . In particular, by separating a tuning port from a signal port, the termination impedance provided by the variable impedance circuit  10  can be altered between extremes (open and short) in order to expand the tuning range of the resonator  20  without significantly adversely affecting the performance of the oscillation loop. This expansion in the tuning range is achieved because the shunt impedance provided by the variable impedance circuit  10  changes the effective modulus of the piezoelectric layer  26 , which means the minimum and maximum impedances provided by the variable impedance circuit  10  set the resonance frequency limits of the resonator  20 . These frequency limits may be further increased by using the center electrode finger  28   b  as the tuning port because this electrode finger overlaps a portion of the piezoelectric layer  26  that undergoes the maximum stress in a fundamental mode. In this regard, it is advantageous that a ratio of a width of the center electrode finger  28   b  to a width of the fingers in the electrode  28   a  be greater than about 1.5 because the frequency tuning range of the resonator  20  typically increases as this ratio increases. Furthermore, as illustrated at  FIG. 2 , in order to maintain the frequency tuning range of a microelectromechanical resonator  20 ′ at near its theoretical limits, a parallel inductor L may be added to cancel out at least a majority of a parasitic capacitance C P  at the tuning port, which may be generated by wire bonds and printed circuit board traces. These and other aspects of the periodic signal generator  100  of  FIG. 1  are described in an article by M. Shahmohammadi, entitled “Passive Tuning in Lateral-Mode Thin-Film Piezoelectric Oscillators,” Proceedings of The International Frequency Control Symposium (IFCS 2011), San Francisco, May 2011, the disclosure of which is hereby incorporated herein by reference. 
       FIG. 3A  illustrates a cross-sectional view of a pure-piezoelectric resonator  20   a  that may be utilized in the periodic signal generator of  FIG. 1 . It can be expected that the tuning range of the resonator  20   a  of  FIG. 3A  may be greater than an otherwise equivalent TPoS resonator because the contribution of the piezoelectric modulus of the piezoelectric layer  26  on the overall effective modulus is greater by virtue of the elimination of the underlying resonator body  22 .  FIG. 3B  illustrates a perspective view of a resonator  20   a  (with resonator body  22 ) that may be substituted for the resonator  20   a  of  FIG. 3A . As shown by  FIGS. 3A-3B , a pair of single-finger electrodes  28   a  may be connected to a first node of an oscillator circuit  30  and a center-finger electrode  28   c  may be connected to a second node of the oscillator circuit  30 . With reference to the signal generator  100  of  FIG. 1 , the first node of the oscillator circuit  30  may be embodied as the positive input terminal of the first operational amplifier A 1  and the second node of the oscillator circuit  30  may be embodied as the output terminal of the second operational amplifier A 2 .  FIG. 3B  further illustrates that a variable impedance tuning circuit  10  may be connected to the two-finger electrode  28   b  and the bottom reference electrode  24 . 
       FIG. 4A  illustrates a cross-sectional view of a pure-piezoelectric resonator  20   b  that may be utilized in the periodic signal generator of  FIG. 1 . This resonator  20   b  includes a dual-stack of piezoelectric layers  26   a - 26   b  with an intervening reference electrode  24 .  FIG. 4B  illustrates a perspective view of a resonator  20   b  (with resonator body  22 ) that may be substituted for the resonator  20   b  of  FIG. 4A . As shown by  FIGS. 4A-4B , a dual-finger electrode  28   a  may be connected to a first node of an oscillator circuit  30  and a center-finger electrode  28   c  may be connected to a second node of the oscillator circuit  30 .  FIG. 4B  further illustrates that a variable impedance tuning circuit  10  may be connected to the planar electrode  28   b  and the reference electrode  24 , which is sandwiched between the piezoelectric layers  26   a ,  26   b.    
       FIG. 5  illustrates a cross-sectional view of a pure-piezoelectric resonator  20   c  that may be utilized in the periodic signal generator of  FIG. 1 . This resonator  20   c  includes a dual-stack of piezoelectric layers  26   a - 26   b  with an intervening middle electrode  28   a . This middle electrode  28   a  and the bottom surface electrode  28   c  are connected to first and second nodes of the oscillator circuit  30 , respectively. In addition, the middle electrode  28   a  and the upper surface electrode  28   b  are connected to the tuning circuit. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.