Accelerometers and gyroscopes are useful in a variety of applications including motion detection and motion compensation. Additionally, certain applications require accelerometers and gyroscopes of comparatively small dimensions. For example, video and still cameras beneficially include gyroscopes to detect angular motion (pitch, yaw and rotation) caused by user movement.
In accordance with an illustrative embodiment an accelerometer includes a substrate having a cavity, a cantilevered transducer disposed over the cavity and having an upper electrode, a lower electrode and a piezoelectric element therebetween. An acceleration causes a movement of the cantilevered transducer that is proportional to a magnitude of the acceleration.
In accordance with another illustrative embodiment, an accelerometer includes a substrate having a cavity with a lower surface, and a side surface. The accelerometer also includes a cantilevered transducer comprising: a piezoelectric element having an upper surface and a lower surface; a first edge electrode and an upper electrode each disposed over the upper surface; and a lower electrode disposed over the lower surface of the piezoelectric element. In addition, the accelerometer includes a second edge electrode disposed over the side surface of the cavity; and an electrode disposed over the lower surface of the cavity.
In accordance with another representative embodiment, a gyroscope includes a substrate having a cavity. A cantilevered transducer is disposed over the cavity and includes an upper electrode, a lower electrode and a piezoelectric element therebetween.
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.
The terms ‘a’ or ‘an’, as used herein are defined as one or more than one.
The term ‘plurality’ as used herein is defined as two or more than two.
The term ‘cantilevered transducer’ as used herein includes a membrane disposed over a cavity and attached at least partially about a perimeter of the cavity. The membrane comprises a piezoelectric layer disposed between electrodes.
In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of example embodiments according to 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 hardware, software, firmware, materials and methods may be omitted so as to avoid obscuring the description of the illustrative embodiments. Nonetheless, such hardware, software, firmware, materials and methods that are within the purview of one of ordinary skill in the art may be used in accordance with the illustrative embodiments. Such hardware, software, firmware, materials and methods are clearly within the scope of the present teachings.
The accelerometers and gyroscopes described in connection with representative embodiments are contemplated for use in a wide variety of sensing, control and correction applications in motor vehicles, consumer electronics, industrial equipment and manufacturing, to mention only a few. For example, the accelerometers and gyroscopes may be used for vehicle stability sensing, video equipment motion compensation, robotic vehicle motion, and avionic gyroscope applications. It is emphasized that the noted applications are merely illustrative, and that other applications within the purview of one of ordinary skill in the art having had the benefit of the present disclosure are contemplated.
Illustratively, the accelerometers and gyroscopes of the representative embodiments may be micromachined using methods referenced herein as well as other methods known to those of ordinary skill in the micro-electromechanical systems (MEMS) arts. Beneficially, the accelerometers and gyroscopes can be fabricated in comparatively small dimensions, thereby fostering their use in many electronics applications where component size is a factor. Moreover, the accelerometers and gyroscopes may be fabricated in large scale (e.g., wafer scale) fabrication.
Furthermore, a variety of materials may be used in fabricating the accelerometers and gyroscopes of the representative embodiments. Notably, the substrates of the representative embodiments may be semiconductor materials such as silicon; the piezoelectric materials may be AlN, ZnO, lead zirconium titanate (PZT) or combinations thereof; the electrodes may be metal such as Al, Mo, Pt, Au or metal alloys; and the mass loading layers may be dielectrics, ceramics, piezoelectric materials and metals. It is emphasized that the noted materials are merely illustrative.
Still alternatively, the areal shape of the cantilevered transducers may be square or may be of an irregular shape. The noted areal shapes are intended only to be illustrative and in no way limiting of the possible cantilevered transducer shapes. Furthermore, and as will be appreciated upon review of the present description, attachment to the edge(s) of the cavity 105 can depend on the areal shape of the cantilevered transducer. For example, a rectangular areal shaped cantilevered transducer may be attached on one or more sides thereof to one or more corresponding edges of the cavity 105. By contrast, an elliptical areal shaped cantilevered transducer may be connected at least partially about the perimeter of the cavity 105.
In certain representative embodiments, the cantilevered transducer 101 may comprise a cantilevered piezoelectric structure such as described in U.S. Pat. No. 6,384,697 entitled “Cavity Spanning Bottom Electrode of Substrate Mounted Bulk Wave Acoustic Resonator” to Ruby, et al. and assigned to the present assignee. The disclosure of this patent is specifically incorporated herein by reference.
Illustratively, a known deep reactive ion etching (DRIE) method, such as the Bosch Method, may be used to form the cavity 105. Sacrificial material may then be provided in the cavity 105 for fabrication of the cantilevered transducer 101 in a similar manner as described in the referenced patent to Ruby, et al.; or as described in co-pending and commonly assigned U.S. patent application entitled “Piezoelectric Microphones” to R. Shane Fazzio, et al., having Ser. No. 11/588,752. This application, filed Oct. 27, 2006, is specifically incorporated herein by reference.
In certain embodiments, it may be useful for the electrodes 102,104 to be of dissimilar materials. Alternatively, or additionally, the thickness of the electrodes may be different. Moreover, a mass loading layer 111 is optionally provided and may be used to modify the location of the neutral axis of the cantilevered transducer 101 with respect to the piezoelectric element 103. The mass loading layer 111 may be disposed substantially coincident with or near the geometric center of the upper electrode 102 (as shown); or over substantially the entire surface of the upper electrode 102; or in other locations over the upper electrode electrodes. As will become clearer as the present description continues, among other effects, electrodes of dissimilar materials, electrodes of differing thicknesses, and mass loading may function to provide proof masses and to provide an asymmetry in the transducer 101.
Displacement of the piezoelectric element 103 and the charge displacement in the piezoelectric element 103 are augmented through the use of mass loading layer 111 or dissimilar electrodes, or both, allowing for the generation of a signal of sufficient magnitude during deflection to provide a proper measure of the acceleration. In addition, the resonance frequency of the cantilevered transducer 101 may be modified by the mass loading layer 111. Additional details of mass loading layer 111 may be found in U.S. Pat. No. 6,469,597, entitled “Method of Mass Loading of Thin Film Bulk Acoustic Resonators (FBAR) for Creating Resonators of Different Frequencies and Apparatus Embodying the Method” to Ruby, et al. The disclosure of this patent is specifically incorporated herein by reference.
In operation, a force along the +y-direction of the coordinate system shown in
As will be appreciated by one of ordinary skill in the art, the optional mass loading layer 111 disposed substantially coincident with or near the geometric center of the upper electrode 102 serves to increase the mass and thus the reactionary force. The augmented reactionary force increases the charge displacement in the piezoelectric element 103 and thereby the induced voltage. This beneficially improves the sensitivity of the accelerometer 101.
The accelerometer 101 of the presently described representative embodiment is also adapted to detect an acceleration along a second axis. In particular, in the present embodiment the upper electrode 102 is connected to the substrate 106 by a contact 107 and the lower electrode 104 is connected to the substrate 106 by a contact 108. If an acceleration is in the +z-direction (i.e., into and out of the plane of the page), the reactionary force creates a shearing action between the upper and lower electrodes 102, 103 that results in a shear force on the piezoelectric element 103 indicative of the acceleration along the z-axis. Moreover, an acceleration in the y-direction will create a shear stress in 103 due to pinning of electrodes 102,104 on opposite sides of the cavity 105. Beneficially, the optional mass loading layer 111 augments or magnifies the shearing action between the upper and lower electrodes 102, 103 and thus the induced voltage.
In a representative embodiment, the upper electrode contact 107 and the lower electrode contact 108 connect respective electrodes 102, 104 to circuitry (not shown) adapted to provide an output based on the acceleration. The circuitry adapted to process a signal indicative of the acceleration (e.g., direction and magnitude) may be one of a variety of circuits/components known to one of ordinary skill in the art. Details of this circuitry are generally omitted to avoid obscuring the description of the representative embodiments.
It is emphasized that the placement of the upper electrode contact 107 from the substrate 106 to the upper electrode 102 of the accelerometer 101 may be other than shown in
In the present embodiment, the upper electrode contact 107 is disposed along a different side of the accelerometer 109. Like the accelerometer 101, the accelerometer 109 is adapted to measure acceleration in two directions, illustratively along the y-axis and along the z-axis in substantially the same manner as described in connection with the representative embodiments of
In a representative embodiment, the cantilevered transducer 101 and capacitor connected in parallel form a resonant circuit useful in providing an indication of a linear acceleration of the accelerometer 109 and the magnitude thereof. In particular, in one embodiment, a time-varying electrical (carrier) signal is applied to the transducer 101. This signal causes the transducer 101 to oscillate. Upon movement due to acceleration along the y-axis, the lower electrode 104 is moved closer to or farther away from the electrode 110, depending on the direction of the acceleration along the y-axis. The change in the distance between the electrodes (plates of the capacitor) 104,110 and change in the charge displacement in the piezoelectric element result in a variation in the capacitance of the resonant circuit and modulation of the output signal of the resonant circuit. The modulation of the output may be provided to circuitry (not shown) indicative of an acceleration (e.g., direction, or magnitude, or both) as desired.
The previously described accelerometers include cantilevered transducers disposed over a cavity in a substrate and connected at least partially about the perimeter (e.g., to one or two sides) of the substrate, for example by contacts 107, 108. However, as shown in
In representative embodiments, the accelerometers 101, 109, 110 or 113 may be provided in an electronic device and are adapted to provide a simple security feature. For example, the accelerometers 101,109 may be provided in a cell phone or personal digital assistant (PDA) having a global positioning function. The device may then be disposed in an item of value (e.g., luggage). If the item is moved by a would-be thief, an acceleration results in an alarm signal and ready tracking due to the GPS capability. It is emphasized that this is merely an illustrative implementation of the accelerometers 101,109 and, as noted previously, that many other applications are contemplated.
In the representative embodiment described in connection with
The accelerometer 200 includes a first cantilevered transducer 201 and a second cantilevered transducer 202 provided over a substrate 203. The first cantilevered transducer 201 includes a first upper electrode 204 and the second cantilevered transducer 202 includes a second upper electrode 205. The first cantilevered transducer 201 is disposed over a first cavity 206 and the second cantilevered transducer 202 is disposed over a second cavity 207. Optionally, a single cavity may be provided, rather than two cavities as shown. The first and second cantilevered transducers 201,202 also include respective lower electrodes (not shown) and piezoelectric elements (not shown) between the respective upper and lower electrodes.
The accelerometer 200 includes a first connection 208 that connects the lower electrode (not shown) of the first cantilevered transducer 201 to the second upper electrode 205 of the second cantilevered transducer 202; and a second connection 209 connects the first upper electrode 204 to the lower electrode (not shown) of the second cantilevered transducer 202. As will be readily appreciated, the connections to the electrodes of the transducers 201, 202 are ‘crossed.’
In a representative embodiment, the cantilevered transducers 201, 202 are substantially the same and have piezoelectric elements comprised of film stacks with the neutral axis in the same plane. Illustratively, the neutral axis may be at the interface of one of the electrodes and the piezoelectric element of the cantilevered transducer. In addition, the c-axis of the piezoelectric elements for both cantilevered transducers 201, 202 are aligned in the same direction.
Application of a time-dependent electrical signal will induce motion of the transducers 201, 202 opposite to one another. In the present embodiment, an additional electrode (not shown) may be provided on a lower surface of one of the transducers 201, 202. This lower electrode is illustratively electrically isolated from the electrode used to drive the cantilevered transducer, and is capacitively coupled to the electrode in a lower surface of the cavity 206. Then a differential capacitance, of roughly the same magnitude may be established. As will be appreciated, the first cantilevered transducer 201 and electrode in the first cavity 206 provide substantially the same structure as the accelerometer 110 described in connection with
Known circuitry (not shown) may be implemented to garner a differential signal from the differential capacitance. Upon application of a pressure, or acceleration (e.g., along the z-axis of the reference coordinate system), deflection of the cantilevered transducers 201, 202 will be in the same direction, and will increase or decrease both capacitances simultaneously. This change in the capacitance will modulate the signals in the differential signal enabling detection of acceleration or pressure to occur.
In another illustrative embodiment, the electrode in the lower surface of the cavity is foregone. As in the previously described embodiment, the neutral axes are along one of the piezoelectric/electrode interfaces and that the c-axes of the piezoelectric elements are aligned. Furthermore, only one set of connections to the electrodes are crossed. In this embodiment, application of a bias voltage deflects the cantilevered transducers 201, 202 in opposite directions, putting the piezoelectric layer in one of the cantilevered transducers in compression and the other of the cantilevered transducers in tension. Application of a force, pressure, or acceleration, being in the same direction (e.g., the z-axis in the coordinate system shown) for each cantilevered transducer will increase compression in one and decrease tension in the other. This will increase the potential difference across one of the cantilevered transducers 201, 202 and decrease the potential difference across the other cantilevered transducer. This difference may then be extracted differentially. Usefully, the bias has a comparatively high impedance, and two of the electrodes on the cantilevered transducers 202, 202 need to have high impedance between them. Moreover, the differential readout will have a comparatively lower impedance.
Certain embodiments contemplate at least two cantilevered transducers each operating as a resonator at parallel resonance.
In the present embodiment, a first cantilevered transducer 301 (the first resonator) operates at a slightly different resonant frequency than a second cantilevered transducer 302 (the second resonator). This difference in resonant frequency may be achieved, for example, by providing a mass loading layer to the first cantilevered transducer 301 that differs slightly relative to the mass loading layer (if any) provided to the second transducer 302.
In accordance with representative embodiments, a variable capacitance in parallel to the plate capacitance C0 is provided to at least one of the cantilevered transducers 301, 302.
The electrode 304 selectively connects with a first upper electrode 306, thereby forming a capacitance Cv in parallel with the plate capacitance Co. An equivalent circuit representation for a resonator including this additional capacitance is shown in
When deflected by an acceleration or some other force in the y-direction of the coordinate system shown, the first cantilevered transducer 301 deflects in the −y-direction, changing the distance between the first lower electrode 303 and the electrode 304, thereby changing the capacitance Cv. This variance in CV results in a ‘pulling’ of the resonant frequency fp, as will be appreciated from Eqn. 2. The first cantilevered transducer 301 and the second cantilevered transducer 302 may be operated to produce a beat frequency determined by the relative mass loading of the two resonators. When first cantilevered transducer 301 is deflected by an acceleration (or other force or pressure), pulling of the resonant frequency fp induces a modulation of this beat frequency. This modulation may then be measured in order to measure the level of deflection and subsequently the applied force, pressure, or acceleration.
The accelerometer 400 includes a substrate 401 having a cavity 402 therein. A first outer electrode 403, a second outer electrode 405, and a center electrode 404 are disposed over a piezoelectric element 406. A first edge electrode 407 and a second edge electrode 408 are provided on side walls of the cavity 402. Finally, a first lower electrode (not shown) and an electrode (not shown) disposed over a bottom surface (not shown) of the cavity 402 are also provided. These electrodes are, respectively, substantially the same as electrodes 104, 112 described in conjunction with
The accelerometer 400 is adapted to sense acceleration in the ±x-direction in substantially the same manner as described in connection with previously described embodiments. Additionally, the accelerometer 400 is adapted to sense acceleration in the ±y-direction. Notably, an acceleration in the +y-direction will cause a reactionary force that both causes charge displacement in the piezoelectric element 406 and results in the distance between the first outer electrode 403 and the first edge electrode 407 to become greater; and the distance between the second outer electrode 405 and the second edge electrode 408 to become smaller. As will be readily appreciated, this provides a differential capacitive measurement that is indicative of the acceleration in the +y-direction.
Mass loading layers (not shown) may be disposed over the piezoelectric element 406, or over the electrodes 404, 405, 407, or a combination thereof. As described previously, these mass loading layers usefully augment the charge displacement and movement of the cantilevered transducer due to acceleration, and thereby usefully improve the sensitivity of the cantilevered transducers to acceleration.
In representative embodiments, contacts 409 and 410 provide signals representative of the capacitance between the upper outer electrode 403 and the first edge electrode 407; and contacts 411 and 412 provide signals representative of the capacitance between the lower outer electrode 405 and the second edge electrode 408. These signals may be provided to circuitry (not shown) to provide an indication of the differential in the capacitance and thus the magnitude and direction (sign) of y-axis acceleration. Illustratively, this circuitry may be a difference amplifier known to one of ordinary skill in the art.
Contact 413 is connected to the lower electrode (not shown) and contact 414 is connected to the center electrode 404. As described in various embodiments previously, signals from these contacts are provided to circuitry to determine the magnitude and direction of x-axis acceleration.
To this point, the representative embodiments have related to accelerometers. However, gyroscopes, which are adapted to sense angular acceleration, are contemplated. Gyroscopes often require actuation of a rotor or rotational mechanism and a sense element for external perturbations imposed upon the rotor axis orientation. A change in the orientation or tilt of the rotor results in a reactive force that is measurable either in the non-inertial reference frame of the rotor or in the inertial reference frame of the device. Either actuation or sensing, or both, can be effected by a piezoelectric element such as described in connection with the accelerometers previously.
Piezoelectric cantilevers such as cantilevered transducer 101 shown in
Forces and displacements resulting from the perturbation of the rotational axis described can be sensed by capacitative elements or piezoelectric elements as described in connection with certain accelerometers of the representative embodiments.
The gyroscope rotor thus described may be actuated piezoelectrically and reaction to an imposed rotational perturbation may be sensed piezoelectrically or capacitatively. Alternatively, the gyroscope rotor may be actuated electromagnetically and the reaction to an externally applied perturbation measured piezoelectrically.
In connection with illustrative embodiments, piezoelectric accelerometers and gyroscopes are described. One of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. These and other variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.