Abstract:
A Micro Electro Mechanical Systems (MEMS) G-switch includes one or more actuators formed between fixed driving stages and moveable driving stages. A proof mass is attached to the moveable driving stages and flexibly attached to a substrate through one or more spring members. A voltage control circuit applies working voltages to the driving stages. With a first working voltage applied between the moveable and the fixed driving stages, moving of the driving stages&#39; sensing direction towards gravity at a first critical angle will cause moveable driving stages to collapse and touch the fixed driving stage on the substrate and thus turn on the MEMS G-switch. After turning on the G-switch, a second working voltage is applied and moving of the driving stages&#39; sensing direction away from gravity at a second critical angle will cause moveable electrodes to deviate from the fixed electrodes and thus turn off the MEMS G-switch.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to MEMS G-switches, and more particularly, to a MEMS structure having a larger capacitance when the MEMS G-switch is turned on and a smaller capacitance when the MEMS G-switch is turned off. 
     (2) Description of the Related Art 
     Micro-electro-mechanical systems (MEMS) are devices which may be fabricated using semiconductor thin film technology in order to reduce the characteristic dimensions and, thus, the cost of the devices. MEMS have been spotlighted recently because of their increasing application to a wider range of fields. Many micro-mechanical sensing devices are now well known. Such devices include sensors of all types. These devices are termed “micro-mechanical” because of their small dimensions on the order of a few centimeters square or smaller. The small size is generally achieved by employing photolithographic technology similar to that employed in the fabrication of integrated circuit (IC) dies. With this technology, the devices are as small as microelectronic circuits, and many such devices are often fabricated in a batch on a single substrate, thereby dividing the cost of processing among many individual devices. The resulting low unit cost increases the application for such devices. 
     The MEMS G-switch is one of the physiological sensing or intelligent monitoring devices providing miniature, lightweight and ultra-low power as required for health monitoring applications or consumer electronics, for example. 
     U.S. Pat. No. 6,765,160 (Robinson) discloses a G-switch that is closed when a proof mass makes contact with a bottom ring electrode. U.S. Pat. No. 7,316,186 (Robinson et al) describes a submunition having a MEMS G-switch that closes upon impact. U.S. Pat. No. 6,035,694 (Dupuie et al) describes calculating stray capacitance by sensing the position of a MEMS proof mass. U.S. Pat. No. 6,550,330 (Waters et al) and U.S. Patent Applications 2006/0161211 (Thompson et al) and 2003/0140699 (Pike et al) disclose various MEMS accelerometers. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide a cost-effective and very manufacturable method of fabricating a highly reliable MEMS G-switch. 
     Another object of the invention is to provide a highly reliable MEMS G-switch. 
     In accordance with the objects of this invention an improved MEMS G-switch device is achieved. The MEMS G-switch device has a proof mass flexibly attached to a substrate. The device includes one or more actuator driving stages comprising fixed driving stages on the substrate and moveable driving stages attached to the proof mass. The proof mass is connected to the substrate through one or more spring elements which can be specially designed and optimized. A voltage control circuit applies voltage between the moveable driving stages and the fixed driving stages wherein two different working voltages are generated by the voltage control circuit when the G-switch turns on and off. 
     Also in accordance with the objects of the invention, a MEMS G-switch is achieved comprising a substrate, a proof mass suspended by a spring member, a fixed driving stage attached to the substrate, a moveable driving stage attached to the proof mass, and a bump attached between the fixed and moveable driving stages. When the MEMS G-switch is turned off, a first working voltage is applied to the moveable driving stage, thereby moving the sensing direction of the moveable driving stage toward gravity at a first critical angle, and turning on the MEMS G-switch. When the MEMS G-switch is turned on, a second working voltage, which is lower than the first working voltage, is applied to the moveable driving stage, thereby moving the sensing direction of the moveable driving stage away from gravity at a second critical angle, turning off the MEMS G-switch. 
     Also in accordance with the objects of the invention, a MEMS G-switch is achieved comprising a substrate, a proof mass suspended onto said substrate, one or more fixed driving stages attached to the substrate, one or more moveable driving stages attached to the proof mass, and one or more bumps attached either to the fixed driving stage or to the moveable driving stage. When the MEMS G-switch is turned off, a first working voltage is applied to the one or more moveable driving stages, thereby moving the moveable driving stages&#39; sensing direction towards gravity at a first critical angle will turn on the MEMS G-switch. When the MEMS G-switch is turned on, a second working voltage is applied to the one or more moveable driving stages, thereby moving of the sensing direction of the moveable driving stages&#39; sensing direction away from gravity at a second critical angle will turn off the MEMS G-switch. 
     Also in accordance with the objects of the invention, a method for manufacturing a MEMS G-switch is achieved. A substrate is provided. A proof mass is formed in or above the substrate. One or more fixed driving stages are attached to the substrate. One or more moveable driving stages are attached to the proof mass. A plurality of mechanical springs is provided to suspend the one or more moveable driving stages above the substrate. One or more bumps are attached between the fixed and moveable driving stages. A voltage control circuit is provided to control two working voltages applied to the one or more driving stages alternatively when the G-Switch turns on and off. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. In the accompanying drawings forming a material part of this description, there is shown: 
         FIGS. 1A and 1B  are schematic diagrams of a G-switch embodiment configured as a G-switch turned off in the horizontal direction and turned on in the vertical direction according to a first preferred embodiment of the present invention. 
         FIG. 2  is a graphical representation of the working status of the G-switch according to the first preferred embodiment of the present invention. 
         FIGS. 3A and 3B  are schematic diagrams and cross-sectional views, respectively, for showing a simple model of a MEMS G-switch according to a second preferred embodiment of the present invention. 
         FIG. 4A  and  FIG. 4B  are schematic diagram and cross-sectional views, respectively, for showing a simple model of a MEMS G-switch according to a third preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention can be viewed as providing principles for designing MEMS G-switches. Other systems, methods, features, and advantages of the present invention will be or will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by accompanying claims. 
     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Common constituent elements will be explained with like reference numerals throughout the disclosure. The described exemplary embodiments are intended to assist in understanding the invention and are not intended to limit the invention in any way. 
       FIGS. 1A and 1B  are the schematic diagrams of a G-switch embodiment configured as a G-switch turned off in the horizontal direction and turned on in the vertical direction according to an exemplary embodiment of the present invention. The horizontal direction is any direction which is perpendicular to the direction of gravity. The G-switch is designed to detect the gravity and its components. In  FIG. 1A , the sensing direction of the switch is perpendicular to gravity, so it is open. And in  FIG. 1B , the sensing direction of the switch is in the same direction as gravity, so it is closed. 
     In  FIG. 1A , a proof mass  2  is connected to a substrate  1  with a spring  5 . Actuator driving stages are servomechanism driving stages that supply and transmit a measured amount of energy for the operation of another mechanism or system. The actuator driving stages of the invention consist of fixed driving stages on the substrate and moveable driving stages. Usually, moveable driving stages are connected to the substrate with the springs or beams. A moveable driving stage  4  is attached to the proof mass through beams  3 . The driving stage  6  is a fixed stage that is fixed on the substrate  1 . 
     There are some bumps  7  on the fixed or moveable driving stage. The bumps may be on either the fixed driving stage  4  or the moveable driving stage  6  or on both the fixed and moveable driving stages. 
     A working voltage V is applied by a voltage control circuit between the fixed driving stage  4  and moveable driving stage  6 . The G-switch  100  according to an exemplary embodiment of the present invention has a working voltage applied between the moveable driving stage  4  attached to the mass  2  and the fixed driving stage  6  attached to the substrate  1 . The dynamic behavior of the proof and the driving stages depends on the applied working voltage, the spring constant of the spring  5 , and the rest position of the proof mass  2 . 
     Here, the proof mass  2  and the driving stage  4  move in the orientation sensitive, or sensing direction, of the G-switch, which is perpendicular to the substrate  1 , by electrostatic force between the moveable driving stage  4  and the fixed driving stage  6 . When the applied voltage increases, the moveable driving stage  4  comes closer to the fixed driving stage  6 . Once a critical voltage is achieved, the electric force becomes dominant with respect to the restoring force due to the gravity and the restoring force of spring  5 , and the moveable driving stage  4  will collapse and stick to the fixed driving stage  6 , as shown in  FIG. 1B . This critical voltage is called the static snap-in voltage. For a different rest position of the proof mass  2 , the value of the critical voltage is different. On the other hand, for a certain critical voltage, there is a corresponding rest position of the proof mass  2 , which is determined by the restoring force of spring  5  and the angle between the orientation sensitive and the gravity of the proof mass  2 . So, for an applied voltage and a certain spring constant of spring  5 , modifying the angle between the gravity and the orientation sensitive can result in the snap-in of the moveable driving stage  4  onto the fixed driving stage  6 . 
     As there are bumps  7  between the moveable driving stage and the fixed driving stage, after snap-in, the moveable driving stage rests on the bumps  7 . The G-switch  100  is on and the electrostatic force is determined by the height of bumps  7 . The electrostatic force can be reduced by decreasing the applied voltage while keeping the moveable driving stage  4  still resting on the fixed driving stage  6 . 
     To keep the moveable driving stage  4  on the fixed driving stage  6 , the electrostatic force between the driving stages should be larger than the restoring force of spring  5 . Once the critical angle between the gravity and the orientation sensitive is larger than a certain value, the restoring force will become dominant and, as a result, the moveable driving stage  4  deviates from the fixed driving stage  6  and the G-switch  100  is turned off, as shown in  FIG. 1A . After the switch is turned off and returned to the initial status, the electrostatic force can be increased by changing the applied voltage to the critical voltage for snap-in while keeping the switch open. 
     The electrostatic force between the moveable driving stage and the fixed driving stage can be expressed in Equation 1. 
                     F   E     =         1   ⁢     ɛ   0       2     ⁢       AV   2       h   2                 [     Equation   ⁢           ⁢   1     ]               
In Equation 1, F E  denotes the electrostatic force,
         ε 0  denotes permittivity of free space,   A denotes the area of the driving stages,   V denotes the voltage applied on driving stages, and   h denotes the distance between the moveable driving stages and fixed driving stage.       

     Meanwhile, the relationship between the restoring force of the spring and the gravity is expressed in Equation 2 as below.
 
 F   m   =kδ−mg  cos θ  [Equation 2]
 
In Equation 2, F m  denotes restoring force,
         k denotes a spring constant,   δ denotes displacement of the moveable driving stage,   m denotes the value of the proof mass,   g denotes acceleration due to the gravity, and   θ denotes the angle between the gravity and the orientation sensitive.       

       FIG. 2  is a graphical representation of the status of the G-switch  100  according to the preferred embodiments of the present invention according to Equation 1 and Equation 2. In  FIG. 2 , the horizontal axis stands for the normalized displacement of the moveable driving stage  4 . The vertical axis stands for the normalized electrostatic force between the moveable driving stage  4  and the fixed driving stage  6  and the restoring force of spring  5 . Referring to Equation 1, the curves  201  and  202  show the electrostatic forces with two different working voltages. Referring to Equation 2, the lines  203  and  204  show the relationship between the restoring force of the spring and the gravity with two different values of θ (which denotes the angle between gravity and the orientation sensitive). 
     When a voltage is applied by an external voltage supply, an electrical field will form in the air gap between the driving stages and result in an electrostatic force between the moveable driving stage and the fixed driving stage. For a given bias voltage, there are two values of normalized displacement in  FIG. 2 . The lower value is the stable equilibrium point, because the derivative of the net force is negative. The higher value is not stable and the net force has a positive derivative at this value. 
     In  FIG. 2 , assuming the G-switch is turned off initially, the moveable driving stage (and the proof mass) rests near the stable equilibrium point  205  with a voltage V 1  and an inclined angle of θ to gravity. As the inclined angle decreases, the working point of the proof mass moves toward snap-in point  206  along the curve  201 . Once the angle between the gravity and the orientation sensitive arrives at a critical angle θ 1 , the electric force between the driving stages becomes dominant and the snap-in occurs. The working point of the proof mass moves to point  207  rapidly along the curve  201 . The moveable driving stage rests onto the fixed driving stage. The G-switch is turned on. Then, the applied voltage is reduced and the working point moves to point  208 . Now, once the angle between the gravity and the orientation sensitive increases and arrives at another critical angle θ 2 , the restoring force becomes dominant, the working point of the proof mass moves back near the point  205 , and returns to its initial open status and the G-switch is turned off. 
     The G-switch shows a much bigger capacitance value when the switch is turned on, compared with a smaller capacitance when the switch is turned off. A simple voltage control circuit can be used to detect the current pulses caused by the variation of the capacitance between the moveable driving stage and the fixed driving stage. 
     A second preferred embodiment of the present invention will be described with reference to  FIGS. 3A and 3B .  FIG. 3A  is an orthogonal view and  FIG. 3B  is a cross-sectional view showing a simple model of a MEMS G-switch according to the second preferred embodiment of the invention. 
     In  FIG. 3A , a proof mass  302  is located in the center of a substrate  301 . The material of the proof mass can be silicon or polysilicon. It can be formed all the way through the substrate or partly through the substrate. The proof mass  302  is supported by wide beams  303  connected to a driving stage  304 . The driving stage  304  is located over the substrate  301  supported by a beam  305  extended from four anchors  306 , the anchors  306  supporting the beams  305 . The driving stage  304  is provided with driving electrodes  304   a ,  304   b ,  304   c  and  304   d  in four symmetrical orientations and the proof mass  302  is connected between the driving electrodes  304   a ,  304   b ,  304   c  and  304   d . The beams  303  and beams  305  also act as signal lines for switching and are connected to the contact sections  307  on the anchors  306 . Here, the substrate  301  is also used as a common fixed electrode under driving electrodes  304   a ,  304   b ,  304   c  and  304   d.    
       FIG. 3B  is a schematic diagram that provides a close-up view of the interface comprising the beams, the electrodes, the gaps, and the bumps shown along the cross-section A-A′ of  FIG. 3A . In  FIG. 3B , anchors  306  are shown on the substrate  301 ; electrodes  304  which are suspended by beam members  303  are connected to proof mass  302 . The electrodes  304  are also connected to the anchors  306  by beams  305 . The gaps between the moveable electrodes  304  and the fixed electrode  301  are denoted by  356 . Bumps  352 , which are made of a nonconductive material such as polysilicon, are attached to either the moveable electrodes  304 , as shown in the figure, or to the fixed electrode, or to both fixed and moveable electrodes. 
     The G-switch  300  according to the second preferred embodiment of the present invention has a voltage applied by a voltage control circuit to the driving electrodes. The mass  302  and the driving electrodes move in the Z direction which is perpendicular to the substrate  301  by electrostatic force between the driving electrodes  304   a ,  304   b ,  304   c  and  304   d  and common fixed electrode  301 . When the applied voltage increases, the driving electrodes  304   a ,  304   b ,  304   c  and  304   d  come closer to the common fixed electrode  301 . Once a critical voltage is achieved, the electrostatic force becomes dominant with respect to the mechanical force and the driving electrodes  304   a ,  304   b ,  304   c  and  304   d  will press onto the fixed common electrode, here substrate  301 . 
     When the switch is open, the restoring force of the beams  305  is equal to the electrostatic force of electrodes  304 . The stage in closing the switch is to reduce the angle between the gravity and the orientation sensitive, or sensing direction. As the angle θ becomes smaller, due to the component of the gravity of the proof mass in the Z-direction, the electrodes  304  come closer to the substrate  301 . Once a critical rest position of the proof mass  301 , and thus the rest position of the electrodes  304 , is achieved (referring to Equation 2), the rest position of the electrodes  304  and the proof mass  302  corresponds to an angle θ 1 , the electrostatic force becomes dominant, snap-in occurs, and the electrodes  304  collapse and stick onto the substrate  301 . Referring to Equation 1 and Equation 2, the snap-in voltage of the electrodes is V 1 . The critical rest position corresponding to the angle θ 1  shows a restoring force F m1  and the electrodes with applied voltage of V 1  show an electrostatic force F E1 . As there are bumps  352  under electrodes  304 , the electrodes  304  rest on the bumps. The G-switch  300  is on and the electrostatic force is determined by the height of bumps  352 . After the G-switch is turned on, the electrostatic force is decreased by changing the applied voltage from V 1  to V 2 , while keeping the electrodes  304  still at rest on the substrate  301 . 
     To keep the electrode  304  onto the substrate  301 , electrostatic force F E2  should be larger than the restoring force F m2 . Referring to Equation 2, there is a critical angle θ 2 . Once the angle between the gravity and the orientation sensitive is larger than θ 2 , the restoring force F m2  will become dominant, and as a result, the electrodes  304  deviate from substrate  301  and the G-switch  300  is open. This turns the switch off and returns it to its initial status. Referring to Equation 1, the electrostatic force can be increased by changing the applied voltage from V 2  to V 1  while keeping the switch open. 
     A third preferred embodiment of the present invention will be described with reference to  FIGS. 4A and 4B .  FIG. 4A  is an orthogonal view and  FIG. 4B  is a cross-sectional view across A-A′ of  FIG. 4A  showing a simple model of a MEMS G-switch according to the third preferred embodiment of the invention. The G-switch  400  includes the substrate  401 , the proof mass  402 , electrodes  404   a - 404   d , and support members  405 . Preferably, here, the proof mass  402  and electrodes  404  are shown supported by a U-type beam  405   a - 405   d  suspended above the surface of the substrate, as shown by the gap  456  in  FIG. 4B . The U-type beam  405   a - 405   d  formed by cambered beams is connected to anchors  406 . The bumps  452  are under the electrodes  404 . When the G-switch  400  is inclined, there is an in-plane component of gravity. Compared with other kinds of beams, U-type beams of the present invention show smaller cross-talk caused by the in-plane component of gravity. Furthermore, with a U-type beam, the stability and reliability of the G-switch and the operating factor of the wafer are both improved. 
     The operation of the G-switch of this embodiment is the same as described above in the second embodiment. 
     It should be emphasized that in the above-described embodiments of the present invention, preferred embodiments are merely possible examples of implementations based on the principles of the MEMS G-switch of the present invention set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.