Abstract:
A micromechanical device may include one or more piezoresistive elements whose electrical resistance changes in response to externally or internally induced strain. The present invention leverages the piezoresistive properties of such devices to sense the positional state of the device. A sensing circuit may be integrated into the device that senses an electrical resistance of at least a portion of the micromechanical device and provides information regarding the positional state of the micromechanical device. The micromechanical device may be a compliant device that includes relatively flexible members such as mechanical beams or ribbons. The positional states may be continuous positional states (such as the position of an actuator) or discreet positional states (such as the positional state of a bistable memory device). In certain embodiments, the micromechanical device is a threshold detector that latches to a particular stable configuration when an applied force exceeds a selected value.

Description:
RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Patent Application No. 60/627,736 entitled “Piezoresistive Micro Displacement MEMS Sensing” and filed on 11 Nov. 2004 for Robert K. Messenger, Timothy W. McLain, and Larry L. Howell, and to U.S. Provisional Patent Application No. 60/735,408 entitled “Piezoresistive Sensing of Bistable Micro Mechanism State” filed 9 Nov. 2005 for Jeffrey Anderson, Larry L. Howell, Timothy W. McLain, and Robert Messenger. Each of the aforementioned applications is incorporated herein by reference. 

   GOVERNMENT LICENSE 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. CMS-0428532 awarded by the National Science Foundation. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to micro-electro-mechanical systems (MEMS) and more particularly relates to apparatus, systems, and methods for positional sensing of micromechanical elements. 
   2. Description of the Related Art 
   Micro-electro-mechanical systems (MEMS) are typically fabricated using semiconductor processes that etch away and dope selected areas to form electrical and mechanical devices on a common substrate. The integration of electrical and mechanical devices facilitates providing low-cost high performance components. Typical applications include sensors, transducers, accelerometers, optical switching, and multi-colored projection. 
   One issue related to controlling the mechanical devices on MEMS chips is sensing the position of various mechanical elements and adjusting their position to achieve a desired position. In particular, the small geometries involved with MEMS systems impose significant challenges to sensing the position of mechanical elements in a cost effective manner. For example, optical techniques used in large scale applications are typically impractical for the small scales involved with MEMS devices. As a result, a need exists for an apparatus, system, and method to sense the positional state of a micromechanical device in a cost effective manner. 
   SUMMARY OF THE INVENTION 
   The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available MEMS devices and methods. Accordingly, the present invention has been developed to provide an apparatus, method, and system to sense a positional state of a micromechanical device that overcome many or all of the above-discussed shortcomings in the art. 
   In one aspect of the present invention, an apparatus to sense a positional state of a micromechanical device includes a micromechanical device comprising a piezoresistive material, the micromechanical device configured to have a plurality of positional states, and a sensing circuit configured to sense an electrical resistance of at least a portion of the micromechanical device. In some embodiments, the micromechanical device may be formed entirely of a piezoresistive material such as polysilicon and possess substantially homogeneous piezoresistive properties that are leveraged to sense the positional state of the device. 
   The micromechanical device may be a compliant device comprised of one or more relatively flexible members such as mechanical beams, strips, or ribbons. In certain embodiments, the micromechanical device is a threshold detector that latches to a particular stable configuration when an applied force exceeds a selected value. The electrical resistance of the micromechanical device (or a selected portion of the micromechanical device) may correspond to the amount of strain within the micromechanical device and therefore the positional state. The positional states may be continuous positional states (such as the position of an actuator) or discreet positional states (such as the positional state of a bistable memory device). 
   To increase sensitivity, the sensing circuit may be electrically connected across the longest dimension of the micromechanical device. In one embodiment, the sensing circuit comprises a Wheatstone bridge wherein one branch of the Wheatstone bridge comprises a portion of the micromechanical device. 
   In another aspect of the present invention, a method to sense a positional state of a micromechanical device includes providing a micromechanical device comprising a piezoresistive material, the micro-mechanical device configured to have a plurality of positional states, and sensing the electrical resistance of at least a portion of the micromechanical device. The method may also include detecting a positional state of the micromechanical device from the sensed electrical resistance. 
   In another aspect of the present invention, a system to sense a positional state of a micromechanical device includes a micromechanical device having piezoresistive properties, a sensing circuit configured to sense an electrical resistance of at least a portion of the micromechanical device, and a processing module configured to receive a signal from the sensing circuit and detect a positional state of the micromechanical device. The positional state of the micromechanical device may be sensitive to various actuation means such as pressure, temperature, force, acceleration, voltage, current, light, magnetic fields, thermal radiation, and the like. 
   The present invention facilitates sensing a positional state of a micromechanical device in a non-obtrusive cost effective manner. It should be noted that references to features, advantages, or similar language within this specification does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
   Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
   The aforementioned features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To enable the advantages of the invention to be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
       FIG. 1  is a block diagram depicting one embodiment of a prior art MEMS control system; 
       FIG. 2  is a layout diagram depicting one example of a MEMS positional sensor; 
       FIG. 3  is a block diagram depicting one embodiment of a MEMS control system of the present invention; 
       FIG. 4  is a layout diagram depicting one embodiment of a MEMS positional sensing device of the present invention; 
       FIG. 5  is a layout diagram depicting another embodiment of a MEMS positional sensing device of the present invention; 
       FIG. 6  is a flow chart diagram depicting one embodiment of a MEMS control method of the present invention; 
       FIG. 7  is a layout diagram depicting one embodiment of a MEMS positional sensing device of the present invention integrated with a Wheatstone bridge sensing circuit; and 
       FIG. 8  is a layout diagram depicting one embodiment of a peak acceleration sensing array of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Some of the functional units described in this specification have been explicitly labeled as modules, (while others are assumed to be modules) in order to emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits, MEMS devices, or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete electrical or mechanical components. A module may also be implemented in programmable hardware devices or systems such as field programmable gate arrays, programmable array logic, and programmable logic devices. 
   Modules may also be implemented in software for execution by various types of processors such as embedded processing units, microcontrollers, or the like. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module. 
   Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. 
   Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     FIG. 1  is a block diagram depicting one embodiment of a MEMS control system  100 . As depicted, the MEMS control system  100  includes a processing module  110 , and a MEMS substrate  120  with one or more control circuits  130 , micromechanical devices  140 , and in some embodiments one or more sensing circuits  150 . The MEMS control system  100  illustrates a typical approach to controlling the micromechanical devices  140 . 
   The processing module  110  may receive sensor information and provide control information in the form of digital or analog signals, or other means known to those of skill in the art. The control circuits  130  control the micromechanical devices  140  as directed by the processing module  110 . For example, the control circuits  130  may activate mechanical actuators or switches that control the position and orientation of optical or mechanical elements. In some embodiments, the sensing circuits  150  sense the position of the micromechanical devices  140  and provide feedback information  152  to the control circuits  130 . 
     FIG. 2  is a layout diagram depicting one example of a MEMS positional sensing device  200 . As depicted, the positional sensing device  200  includes a thermal actuator  210  and a capacitive sensor  220 . The positional sensing device  200  facilitates providing positional feedback corresponding to the positional state of the thermal actuator  210 . 
   The thermal actuator  210  includes a pair of actuation signal pads  212   a  and  212   b  that receive a differential control signal (not shown) that provides current to a set of beams  214 . The beams  214  may be thermally heated and elongated proportional to the current provided to the signal pads  212 . The elongation of the beams  214  may actuate a movable shuttle  216  to a new position  218 . 
   The depicted capacitive sensor  220  includes a lower plate  222  and an upper plate  224  integrally formed with the movable shuttle  216 . As the overlap between the upper plate  224  and the lower plate  222  changes, a change in capacitance may be sensed by an appropriate sensing circuit  150  (see  FIG. 1 ) connected to the lower plate  222 . In turn, the control circuits  130  or the processing module  110  (see  FIG. 1 ) may receive the capacitance information and estimate the position of the movable shuttle  216 . 
   Although the positional sensing device  200  provides valuable feedback, significant circuitry may be required to support the positional sensing device  200 . For example, sensing the capacitance between the upper plate and the lower plate may require generation of an AC signal comprising one or more high frequency components. Furthermore, stray capacitance and other non-linear effects may necessitate significant calibration and/or signal processing to provide an accurate estimate of the position of the movable shuttle  216 . In addition, significant substrate real estate may be consumed by the capacitive sensor  220  and its associated support circuitry. Furthermore, additional layers, processing steps, or materials may be required to properly fabricate the capacitive sensor  220 . 
   As subsequently described and claimed herein, various embodiments of the present invention address many of the aforementioned issues.  FIG. 3  is a block diagram depicting one embodiment of a MEMS control system  300  of the present invention. In addition to the elements described in conjunction with the MEMS control system  100  or similar elements, the MEMS control system  300  includes one or more piezoresistive elements  345  that may be integrated into the micromechanical devices  140  and/or the sensing circuits  150 . Piezoresistive elements experience changes in electrical resistance in response to changes in strain. 
   The piezoresistive elements  345  may be constructed of the same material as the micromechanical devices  140 . In one embodiment, the micromechanical devices  140  are fabricated from one or more layers of polysilicon. By sensing the electrical resistance of the piezoresistive elements  345  (and therefore the mechanical strains within the micromechanical devices  140 ), information and/or feedback corresponding to the positional states of the micromechanical devices  140  may be provided to the controls circuits  130  and/or the processing module  110 . In certain embodiments, the correspondence between the feedback information and the micromechanical states may be substantially linear. In some embodiments, the piezoresistive elements that are measured include primarily those portions of the micromechanical devices (such as beams) that experience significant changes in strain in response to changes in positional state. 
     FIG. 4  is a layout diagram depicting one embodiment of a positional sensing device  400  of the present invention. As depicted, the positional sensing device  400  includes a pair of stationary pads  410   a  and  410   b , a movable shuttle  420 , and one or more piezoresistive elements  345 . The positional sensing device  400  is one example of a MEMS device that leverages the piezoresistive elements  345  for both mechanical structure and electronic feedback purposes—a concept the applicants consider unique to the present invention. 
   The stationary pads  410   a  and  410   b  may receive a sensing signal from a sensing circuit used to estimate the electrical resistance of the device  400  in general and of the piezoresistive elements  345  in particular. To increase sensitivity, the sensing circuit may be electrically connected across the longest dimension of the device  400 . 
   The movable shuttle  420  may be moved by an external force such as acceleration, or by forces imposed by other elements integrated onto the MEMS substrate  120 . Such forces may induce movement on the shuttle  420  and a corresponding strain on the piezoresitive elements  345  and cause the device  400  to assume a particular positional state. 
   In certain embodiments, the positional sensing device  400  is a bistable device. For example, the device  400  may be a compliant bistable device that is induced into the stable positional state  440   b  when the device is subjected to acceleration that exceeds a selected threshold. In the depicted embodiment, the piezoresistive elements  345  are elongated beams  425  having a length to width ratio of greater than 5 and the device  400  has bistable positional states  440   a  and  440   b . The first stationary pad  410   a  may be separated from the moveable shuttle  420  by a first separation distance  430   a  and the second stationary pad  410   b  may be separated from the moveable shuttle  420  by a second separation distance  430   b . The length of the beams  425   a  that connect the first stationary pad  410   a  to the movable shuttle may be longer than the first separation distance  430   a . Similarly, the length of the beams  425   b  that connect the second stationary pad  410   b  to the movable shuttle may be longer than the second separation distance  430   b . The beams  425  and the moveable shuttle  420  interact mechanically and essentially form a plurality of interacting elements within the device  400 . Having beams  425  that are longer than the separation distance that they span facilitates providing two mechanically stable positions for the movable shuttle corresponding to the positional states  440   a  and  440   b  for the device  400 . As is depicted in  FIG. 4 , in the first stable position the elongated beams have an unbent shape while the elongated beams have a slight ‘S’ shape for the second stable position. The stable positions or states  440   a  and  440   b  may be maintained without consuming power and without an electrostatic force acting on the device  400  or the elements thereof. The bulk piezoresistivity across the device  400  and/or the plurality of interacting elements can be measured via the stationary pads  410  (i.e. across the stationary pads  410   a  and  410   b ) to sense whether the device  400  is in the first stable state  440   a  or the second stable state  440   b . For more information on compliant devices and discrete positional states, the reader is referred to the textbook  Compliant Mechanisms  authored by one of the applicants (Larry L. Howell) and published by John Wiley and Sons, Inc. 
   In certain embodiments, the positional sensing device  400  is a bistable device. For example, the device  400  may be a compliant bistable device that is induced into the stable positional state  440   b  when the device is subjected to acceleration that exceeds a selected threshold. In the depicted embodiment, the device  400  has bistable positional states  440   a  and  440   b . In another embodiment, the device  400  is a tristable device. For more information on compliant devices and discrete positional states, the reader is referred to the textbook  Compliant Mechanisms  authored by one of the applicants (Larry L. Howell) and published by John Wiley and Sons, Inc. 
   The sensing signal (not shown) provided to the stationary pads may be used to measure the electrical resistance of one or more of the piezoresistive elements  345 . For example, in one embodiment, a pair of DC reference voltages (one of which may be a ground voltage) are applied to the pads  440  and the current flowing through the device  400  is measured to estimate the positional state of the device  400 . The use of DC reference voltages may simplify circuit design and circuit layout. 
     FIG. 5  is a layout diagram depicting another embodiment of a positional sensing device of the present invention, namely, the positional sensing device  500 . In addition to the elements of the sensing device  400  or similar elements, the sensing device  500  includes an active device  510  and a reference device  520 . The active device  510  may assume a number of positional states that change the electrical resistance of the device. Use of the reference device  520  may normalize measurement of the electrical resistance of the piezoresistive elements  345 . 
   The depicted reference device  520  includes a stationary element  530  that anchors the reference device  520  into a certain positional state. The reference device  520  may also include a set of piezoresistive reference elements  545  that are substantially identical to the piezoresistive elements  345  yet held in a constant positional state. As a result, the depicted sensing device  500  may function as a voltage divider wherein a voltage measured at a measurement pad  410   c  may be proportional to the ratio of the resistance of one of the devices to the total resistance of both devices. The use of such a voltage divider may factor out process variations and environmental factors such as humidity, and facilitate more accurate measurement of the positional state of the active device  510 . 
     FIG. 6  is a flow chart diagram depicting one embodiment of a MEMS control method  600  of the present invention. As depicted, the MEMS control method  600  includes providing  610  a piezoresistive micromechanical device, sensing  620  an electrical resistance, and deriving  630  a positional state. The MEMS control method  600  may be conducted in conjunction with the MEMS control system  300  depicted in  FIG. 3 . 
   Providing  610  a piezoresistive micromechanical device may include providing a micromechanical device  140  that includes one or more piezoresistive elements  345 . Sensing  620  an electrical resistance may include measuring a response to a particular measurement signal. Deriving  630  a positional state may include processing the signal response to detect relative changes in resistance and mapping the response values to particular positional states. In certain embodiments, a mapping function is determined by executing a calibration sequence that places a micromechanical device in known positional states. 
     FIG. 7  is a layout diagram depicting one embodiment of a MEMS positional sensing device  700  of the present invention integrated with a Wheatstone bridge sensing circuit. As depicted, the sensing device  700  includes a thermal actuator  710  with a moveable shuttle  715 , an active device  720 , and three reference devices  730 . The depicted active device  720  and the reference devices  730  form a Wheatstone bridge circuit that facilitate providing a differential sensing voltage to the differential measurement pads  740 . The use of a differential sensing voltage may increase the sensitivity of the positional sensing device  700 . 
   The active device  720  may have an electrical resistance corresponding to the positional state of the movable shuttle  715 . The depicted active device  720  and the reference device  730   a  form a voltage divider that is parallel to the voltage divider formed by the reference devices  730   b  and  730   c . Each voltage divider should experience substantially similar noise conditions. As a result, the voltages provided to the measurement pads  740  may provide a differential signal that enables detection of small changes in electrical resistance associated with the active device  720 . In certain embodiments, the differential signal is amplified by a differential amplifier (not shown) to provide a larger amplitude feedback signal to the control circuits  130 , or the like. Using a Wheatstone bridge sensing circuit similar to the depicted circuit may be particularly useful for extremely small bistable devices such as MEMS memory devices. 
     FIG. 8  is a layout diagram depicting one embodiment of a sensing array  800  of the present invention. As depicted, the sensing array  800  includes a two dimensional array comprising rows  802  and columns  804  of bistable positional sensing devices  810 . The depicted bistable positional sensing devices  810  are similar to the positional sensing device  500  depicted in  FIG. 5 . In one embodiment, each bistable sensing device  810  is a MEMS memory device. In another embodiment, each bistable sensing device  810  is a thresholded acceleration detector. 
   In addition to the elements described in conjunction with  FIG. 5 , the bistable positional sensing devices  810  may include one or more of electrostatic combs  820  attached to the movable shuttle  420  and corresponding combs  830  proximate to the combs  820 . The electrostatic combs  820  and  830  facilitate moving the active device  510  to a selected positional state. 
   For example, by applying a sufficiently large voltage to pad  410   d  the electrostatic combs  820   a  and  830   a  may attract each other and induce a primary stable state illustrated with solid lines. Similarly, by applying a sufficiently large voltage to pad  410   e  electrostatic combs  820   a  and  830   a  may attract each other and induce a secondary stable state illustrated with dashed lines. Consequently, the positional state of the active device  510  may be selectively programmed. Furthermore, the positional state of each active device  510  may be detected by applying a voltage difference to pads  410   a  and  410   b  and measuring the voltage response at pad  410   c.    
   In some embodiments, each positional sensing device  810  has a different sized movable shuttle  420  (i.e. shuttles of various masses) that requires a corresponding level of acceleration to move the shuttle from the primary stable state to the secondary stable state  440   b  (see  FIG. 4 ). In those embodiments, the sensing array  800  may capture and retain a peak acceleration level experienced by the array. Furthermore, since power is only needed to read or reset the sensing devices  810 , the information may be captured and retained without consuming power. In such embodiments, the combs  820   b  and  830   b  may not be necessary for valid operation and may be omitted from the positional sensing devices  810 . 
   The present invention provides improved positional sensing of micromechanical devices. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.