Abstract:
A bistable sensor with a tunable threshold for use in microelectromechanical systems. The sensor uses electrostatic force to modify the threshold and to disable the sensor in a deflected position once a sustained extreme in vibration is detected. Potential applications include mechanical implementations of signature analysis to automatically eliminate large amplitude noise at a specific frequency, shock detection without requiring quiescent DC power consumption, and determination of the magnitude of a shock.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of acceleration and shock sensors, and more particularly to bistable threshold sensors. 
     BACKGROUND OF THE INVENTION 
     Micro-Electro-Mechanical Systems (MEMS) integrate mechanical elements, such as microsensors and microactuators, and electronics on a common substrate through the utilization of microfabrication technology. MEMS are typically micromachined using integrated circuit (IC) compatible batch-processing techniques that selectively etch away parts of a silicon wafer or add new structural layers. MEMS range in size from several micrometers to many millimeters. These systems sense, control, and actuate on a micro scale and function individually or in arrays to generate effects on a macro scale. 
     Microsensors, such as acceleration and shock sensors, are known in the prior art. While shock sensors come in many shapes and sizes, they typically involve the use of a suspended structure to detect vibration with peak excursions of that structure closing an electrical contact to indicate that a shock has occurred or a threshold has been exceeded. Acceleration sensors typically use a resonant structure to detect motion. One type of detector includes a silicon mass suspended by silicon beams with ion implanted piezoresistors on the beams to sense motion. Another type of detector uses capacitance changes to detect movement of the beam. Another type employs a shift in a physical load to produce a shift in the structure&#39;s resonant frequency. 
     A conventional shock sensor is shown in FIG.  1 . Shock sensor  10  includes substrate  11 , insulating layer  12 , conductive cantilever  13  having a free end and fixed end, and contact conductor  14 . Voltage is applied to the conductive cantilever  13  that serves as the top electrode. Contact conductor  14  serves as the bottom electrode. A shock with sufficient magnitude causes the free end of conductive cantilever  13  to touch contact conductor  14  completing the circuit. Current is detected by an ammeter ( 17 ). Once the circuit is completed, however, it immediately opens again. Thus, the detector must be continuously monitored to detect a shock. This can be a problem for one time use applications that require a sensor to disable itself when an extreme vibration is detected. Another problem is that the amplitude of a shock that causes the cantilever to close the electrical contact is fixed by the material properties and geometry of the sensor. Thus, variation of the detection threshold can only be made by physical modification of the distance between the electrodes. 
     An example of a conventional acceleration sensor is provided by U.S. Pat. No. 4,855,544. It discloses acceleration sensor  20  including a cantilevered beam  23  having an integral end mass at the free end of the beam as shown in FIG.  2 . Prior art acceleration sensor  20  further includes substrate  21 , insulating layer  22 , and bottom conductor  24 . Acceleration causes deflection of the free end of beam  23  from a relaxed condition. As acceleration increases, beam  23  will move to an increasingly strained condition with the free end moving towards bottom conductor  24 . Movement of beam  23  is typically detected by a capacitance measurement. One problem with this prior art accelerometer is that there is no simple mechanism for threshold detection. Thus, it cannot automatically disable itself to eliminate large amplitude noise at a specific frequency. 
     In light of the foregoing, there is a need for bistable threshold sensors that can detect extremes in acceleration and that allow the detection threshold to be electrically modified. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to bistable micromechanical sensors that can detect extremes in acceleration and that allow the detection threshold to be electrically modified that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     In accordance with the purposes of the present invention, as embodied and broadly described, the invention provides a bistable threshold sensor including a substrate, a resonant structure over the substrate with a fixed portion and a free portion as a first electrode, a ground conductor layer on the substrate as a second electrode, and an insulating layer over the ground conductor. The sensor further includes a contact conductor on a portion of the insulating as a third electrode, wherein the free portion of the resonant structure contacts the contact conductor when the resonant structure is in a deflected position, and a voltage source for providing a bias voltage between the first electrode and the second electrode. 
     In another embodiment, the invention provides a method for threshold detection including providing a bistable threshold sensor having a detection threshold. The sensor includes a resonant structure as a first electrode, wherein the resonant structure has an elastic restoring force, a ground conductor as a second electrode, a contact conductor as a third electrode, wherein a gap exists between the first and second electrode, a bias voltage between the first and a second electrode creating a nonlinear electrostatic force, and wherein the threshold is determined by the elastic restoring force, the gap, and the electrostatic force. The method further includes the step of electrostatically locking the resonant structure in a deflected position once the detection threshold is reached. 
     The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the objects, advantages, and principles of the invention. 
     FIG. 1 is a schematic side view of a prior art shock sensor. 
     FIG. 2 is a schematic side view of a prior art acceleration sensor. 
     FIG. 3 shows a bistable threshold sensor consistent with one embodiment of the present invention. 
     FIG. 4 shows a bistable threshold sensor in a post-threshold mode consistent with one embodiment of the present invention. 
     FIG. 5 shows a bistable threshold sensor consistent with another embodiment of the present invention. 
     FIG. 6 shows a bistable threshold sensor consistent with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 3 depicts a bistable threshold sensor consistent with one embodiment of the present invention. Sensor  30  includes substrate  31  that can be a silicon wafer or any other semiconductor base material. Ground conductor  35  is formed over substrate  31  and insulating layer  32  is formed over ground conductor  35 . Resonant structure  33  is fixed to insulating layer  32  and has an equilibrium or undeftected position. Contact conductor  34  is formed over a portion of insulating layer  32 . Contact conductor  34  is positioned with respect to cantilevered beam  33  so that contact can only occur when cantilevered beam  33  is in a deflected state. Voltage source  38  provides a bias voltage between resonant structure  33  and ground conductor  35 . No voltage is applied between resonant structure  33  and contact conductor  34 . Sensor  30  is fabricated using conventional micromachining techniques from materials known in the art. 
     Resonant structure  33  is shown in FIG. 3 as a cantilevered beam having a fixed end and a free end. To avoid stiction problems due to charging or welding effects, dimples can be provided on the surface of beam  33  that touches the contact conductor or on the contact conductor itself. Although it is shown as a cantilevered beam in FIG. 3, resonant structure  33  can be any structure typically used in accelerometers. Examples include proof mass structures, doubly fixed beams, as well as more complicated geometries. 
     When used to detect a certain threshold of high amplitude sustained vibration at a frequency close to the resonant frequency of resonant structure  30 , the bistable sensor is preferably used in a vacuum. When used as a shock sensor, sensor  30  is preferably used in an ambient atmosphere. 
     The free end of cantilevered beam  33  serves as a top electrode and ground conductor  35  serves as the bottom electrode. Contact conductor  34  acts as another electrode. The voltage provided by voltage source  38  between cantilever  33  and ground conductor  35  can be scaled to a wide range of voltages and depends on a number of factors including the stiffness of the cantilever, the mass of the cantilever, and the gap distance between the free end of the cantilever and the contact conductor. The voltage, however, is preferably on the order of several volts. 
     Detection threshold of sensor  30  can be modified in the conventional manner by changing the gap distance between cantilevered beam  33  and the contact conductor  34 , as well as by modifying the materials of construction or the geometry of cantilevered beam  33 . The detection threshold of sensor  30 , however, can also be electrically modified by changing the bias voltage. As the bias voltage increases, the detection threshold decreases due to the increase in direct current (DC) force pushing cantilevered beam  33  towards ground conductor  35 . Thus, by adjusting the detection threshold, a sensor can be configured to detect a shock occurring at a particular moment or during a momentary system configuration of interest. The maximum threshold is set by the gap distance, as well as the material properties and the geometry of resonant structure  33 . The minimum threshold can be set close to zero, if a sufficient DC voltage is applied to allow resonant structure  33  to deflect by itself. 
     When the amplitude vibration due to shock or acceleration is below the detection threshold, sensor  30  functions in the same manner as conventional shock and acceleration sensors. In other words, cantilevered beam  33  responds by deflecting and eventually returning to its equilibrium position. When the amplitude of the shock or vibration increases beyond the threshold of detection, however, the large displacement of cantilevered beam  33  causes it to enter a region in which the nonlinear electrostatic attractive forces created by the bias voltage overcome the elastic restoring force of the cantilevered beam. Once this occurs, cantilevered beam  33  deflects and is electrostatically locked in a position against contact conductor  34 . In the post-threshold mode of operation shown in FIG. 4, cantilevered beam  33  remains electrostatically locked in the deflected position regardless of any subsequent shock or vibration. Since no voltage is applied between beam  33  and contact conductor  34 , no quiescent power is drawn. Occurrence of the post-threshold mode can then be sensed at a later time by applying voltage between beam  33  and contact conductor  34 . 
     In another embodiment, a constant voltage is applied between resonant structure  33  and contact conductor  34  to detect the timing of the occurrence of the post-threshold mode. 
     In still another embodiment, bistable threshold sensor  30  includes additional sensors to measure the motion of resonant structure  33 . Suitable sensors include capacitive, piezoresistive or any other sensor used to measure motion of a resonant structure. 
     One application of bistable threshold sensors consistent with the present invention is the measurement of the magnitude of a shock. This can be accomplished by providing a plurality of sensors each having a different electrically set threshold. All sensors having a threshold below the magnitude of the shock will be locked in the post-threshold or deflected position. Sensors having a threshold above the magnitude of the shock will be in the equilibrium or undeflected position. 
     Another application of bistable threshold sensors consistent with the present invention is a signal processing fuse. Specifically, an array of sensors can be used to detect a specific frequency spectrum, including specific frequency characteristics of interest, within complex vibration stimuli. One example is environment noise centered at a specific frequency that dominates the response of a conventional detector. Bistable threshold sensors consistent with the present invention, however, can disable themselves when the high-amplitude signals are detected. Once the amplitude of the noise rises above the threshold, cantilevered beam  33  is electrostatically locked in the deflected position regardless of any subsequent shock or vibration. Once the amplitude of the noise falls below the threshold, cantilevered beam  33  is released by decreasing the bias voltage. The sensor is then ready to detect the desired frequency spectrum or characteristic. 
     Another embodiment of a bistable threshold sensor consistent with the present invention is shown in FIG.  5 . In this configuration, current-limiting resistor  59  is used to achieve bistablity with only two electrodes. Sensor  50  includes substrate  51 , ground conductor  55  formed over substrate  51 , contact conductor  54 , and insulating layer  52  formed over ground conductor  55 . Resonant structure  53  is fixed at one end to insulating layer  52 . Resonant structure  53  is again shown as a cantilevered beam in FIG. 5, but can be any structure used in accelerometers. The post-threshold mode is determined by detection of a voltage at ammeter  57 . 
     Another embodiment of a bistable threshold sensor consistent with the present invention is shown in FIG.  6 . Sensor  60  includes substrate  61  with insulating layer  62  formed over substrate  61 . Ground conductor  65  and contact conductor  64  are formed on different portions of insulating layer  62 . Resonant structure  63  is formed on another portion of insulating layer  62  so as to have a fixed end and a free end. Voltage source  69  provides a bias voltage between ground conductor  65  and resonant structure  63 . To avoid stiction problems due to charging or welding effects, dimples can be provided on the surface of beam  63  that touches the contact conductor  64  or on the contact conductor itself. These dimple also prevent beam  63  from contacting insulator  62  or ground conductor  65 . The manner of operation of this sensor, however, is similar to previously discussed embodiments. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the bistable threshold sensor. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.