Patent Publication Number: US-2011071775-A1

Title: Pressure measurement using a mems device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/141,326, filed Jun. 18, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field of the Invention 
     Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs, transmits, and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed. 
     The device of the current invention is intended to measure the pressure about the device, and although it has a similar structure as an interferometric modulator, it may or may not have the optical properties of a typical interference modulator. 
     SUMMARY 
     One aspect of the invention is a device for measuring pressure comprising at least one element comprising two layers separated by a space, wherein a dimension of the space changes over a variable time period in response to a voltage applied across the two layers and a measuring module configured to measure the time period, wherein the time period is indicative of the ambient pressure about the device. 
     Another aspect of the invention is a method of measuring ambient pressure comprising applying a voltage across two layers of a MEMS device, measuring a value characteristic of the response time of the device, and determining a pressure about the device based on the measured value. 
     Yet another aspect of the invention is a device for measuring pressure comprising at least one element comprising two conductive layers separated by a space, wherein a dimension of the space changes over a variable time period in response to a change in voltage applied across the two layers, a measuring module configured to measure current flowing between the two conductive layers as a function of time when there is a change in voltage applied between the two conductive plates, and a processor configured to determine the time difference between when the voltage pulse is applied and when the local maximum of the motion current occurs, wherein the processor is further configured to associate the time difference with an ambient pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an embodiment of the invention in the released state. 
         FIG. 1B  shows an embodiment of the invention in the actuated state. 
         FIG. 1C  shows an applied voltage and the current response of one embodiment of the invention due to the applied voltage. 
         FIG. 2  is a flowchart which shows a method of measuring pressure according to one embodiment of the invention. 
         FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of a MEMS device. 
         FIG. 4  is an isometric view depicting a portion of one embodiment of the invention as an interferometric modulator display in which the first layer, a movable reflective layer, of a first interferometric modulator is in a relaxed position and the first layer of a second interferometric modulator is in an actuated position. 
         FIG. 5  is an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     Typical usage of an interferometric light modulator involves taking advantage of the optical properties of the device. In some embodiments, the interferometric light modulator is a bistable device having two states, each with different optical properties. The state a particular modulator is in is controllable by the application of an appropriate electrical signal. Thus, the interferometric light modulator is well-suited for display applications. However, other properties of a MEMS device having similar structure to an interferometric light modulator, can be used for other purposes, e.g. the measurement of the ambient pressure about the device. 
       FIG. 1A  shows an embodiment of the invention in the relaxed state at a first time, and  FIG. 1B  shows the same MEMS device in the actuated state at a second time.  FIG. 1C  shows an applied voltage, capable of causing actuation, and the current response of one embodiment of the invention due to the applied voltage. The MEMS device shown in  FIGS. 1A and 1B  comprises a first layer  24  and a second layer  26 . The two layers may be separated by supports  28 . When a voltage is immediately applied across the two layers of the device, the device takes some time to change states, from the relaxed state shown in  FIG. 1A  to the actuated state shown in  FIG. 1B . It is appreciated that the device may be designed without supports  28 , or that both layers may move towards each other upon the application of a voltage. 
     The time it takes for such a MEMS device to actuate or release depends on the design of the device, the voltage signal, and the ambient pressure. In one embodiment, the actuation time of the modulator is a linear function of the ambient pressure. As such, the MEMS device can be used, with appropriate control and measurement circuitry, as a pressure sensor. 
     In one embodiment, an applied voltage  30  changing from a first value  32  to a second value  34  causes the first layer  24  to begin moving towards the second layer  26 . The resulting current response  36  may exhibit multiple peaks, as measured by an ammeter  22  connected between the first layer  24  and the second layer  26 . In general, the current response  36  can be described by the following equation: 
     
       
         
           
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     The first peak  38  is caused by the change in the applied voltage  30  from a first value  32  to a second value  34 , as described by the first term in the above equation. The second peak  40  is caused by the change in capacitance associated with the movement of the first layer  24  relative to the second layer  26 , as described by the second term in the above equation. The motion of the first layer  24  relative to the second  26 , and thus, the second peak  40  in the current response  36  is affected by the ambient pressure. 
     As the first layer  24  picks up speed as it moves towards the second layer  26  there is an increase in the measured current corresponding to the left half of the second peak  40 . At a point of maximum velocity, the first layer  24  begins to slow down as it pushes air out from between it and the second layer  26 . This point is indicated by a maximum in the second peak  40  of the current response  36 . Finally, the first layer  24  comes to rest, with the device in an actuated state. The actuation time of the device can thus be measured in a number of ways. For instance, the actuation time can be considered the amount of time required for the first layer  24  to reach maximum velocity. The actuation time can also be considered the amount of time required for the first layer to move fully from a relaxed state to an actuated state. The actuation time may be characterized by measuring the sharpness of the second peak  40 , e.g. measuring the time between when the second peak  40  reaches 50% of the maximum for the first time while increasing and when the second peak  40  reaches 50% of the maximum for the second time while decreasing. The release time, i.e. the time it takes the change from the actuate state to the release state, is also a function of the pressure and can also be used to measure the ambient pressure to which the device is exposed. 
       FIG. 2  is a flowchart which shows a method  50  of measuring pressure according to one embodiment of the invention. In a first stage, a voltage is applied across two layers of a MEMS device  72 . In the next stage, the current response resulting from this applied voltage is measured  74 . In the next stage, a value characteristic of the response time of the device is obtained based on the measured current response  76 . In the final stage, the pressure about the device is obtained based on the value characteristic of the response time  78 . 
     The bistable nature of such a MEMS device is enabled by a linear mechanical force competing with a nonlinear electrostatic force. This creates hysteresis in the device. In one embodiment of the invention, as shown in  FIG. 3 , when no voltage is applied across the first layer  24  and the second layer  26 , the position of the first layer  24  is apart from the second layer, and the device is in the relaxed state, indicated as point A. As the voltage across the two layers is increased, the MEMS device remains in the relaxed state until a threshold is reached, indicated as point B. After the voltage across the two layers exceeds this threshold, the first layer  24  changes position to be closer to the second layer  26  and the MEMS device is in the actuated state, indicated by point C. As the voltage is decreased, the MEMS device stays in the actuated state even as the voltage is decreased below the voltage which first caused the actuation of the element, indicated by point D. After the voltage has been decreased below a second threshold, the MEMS device enters the relaxed state, indicated again by point A. The hysteresis effect occurs regardless of the polarity of the voltage, i.e. the same is true if a negative voltage rather than a positive voltage is used. 
     As just described, one embodiment of invention exhibits hysteresis. Thus, the applied voltage to actuate or release the device can take many forms. As shown in  FIG. 1C , the applied voltage may be a step function from a release voltage to an actuation voltage. However, a change from a hold voltage (e.g. 5 volts) to an actuation voltage (e.g. 10 volts) can also cause actuation in a device in the release state. Similarly, although a step function is shown in  FIG. 1C , a periodic function, such as a square wave, may be advantageous to repeatedly measure the pressure about the device as it repeatedly changes states. 
     The use of a bilayer MEMS device or an array of such MEMS devices, as a pressure sensor, has many advantages over typical pressure sensors. In some embodiments, the construction of such a MEMS device lends itself to the creation of an array of such devices. The use of an array adds redundancy to the measurement. If an element or even a fraction of the elements fail to operate, the device as a whole can still be used to measure pressure. 
     As a pressure sensor, the device may be used to measure ambient pressure as a barometer. The device may be used as an altimeter. The device may be used to measure blood pressure as part of a sphygmomanometer. With appropriate design, the device may be used to record pressure applied by a user. In one embodiment of the invention, the device is further configured as an interferometric modulator has particularly configurable optical properties, as described below. As such, it may be possible to use the device as part of a touch-screen display. Also, although the device is described to measure air pressure, it is noted that other forms of pressure may also be measured such a configured MEMS device. 
       FIG. 4  is an isometric view depicting a portion of one embodiment of an interferometric modulator array in which a movable reflective layer of a first interferometric modulator is in a relaxed position and a movable reflective layer of a second interferometric modulator is in an actuated position. As described, a MEMS device designed to measure pressure can be further configured with specific optical properties. Similarly, standard interference modulators can be used as a general MEMS device to measure ambient pressure. The depicted portion of the modulator array in  FIG. 4  includes two adjacent interferometric modulators  12   a  and  12   b . In the interferometric modulator  12   a  on the left, the first layer, a movable reflective layer  14   a , is illustrated in a relaxed position at a predetermined distance from the second layer, an optical stack  16   a , which includes a partially reflective layer. In the interferometric modulator  12   b  on the right, the movable reflective layer  14   b  is illustrated in an actuated position adjacent to the optical stack  16   b.    
     The optical stacks  16   a  and  16   b  (collectively referred to as optical stack  16 ) typically comprise several fused layers, which can include an electrode layer, such as indium tin oxide (ITO), a partially reflective layer, such as chromium, and a transparent dielectric. The optical stack  16  is thus electrically conductive, partially transparent, and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate  20 . The partially reflective layer can be formed from a variety of materials that are partially reflective such as various metals, semiconductors, and dielectrics. The partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials. 
     In some embodiments, the layers of the optical stack  16  are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers  14   a ,  14   b  may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of  16   a ,  16   b ) deposited on top of posts  18  and an intervening sacrificial material deposited between the posts  18 . When the sacrificial material is etched away, the movable reflective layers  14   a ,  14   b  are separated from the optical stacks  16   a ,  16   b  by a defined gap  19 . A highly conductive and reflective material such as aluminum may be used for the reflective layers  14 , and these strips may form column electrodes in a display device. 
     With no applied voltage, the gap  19  remains between the movable reflective layer  14   a  and optical stack  16   a , with the movable reflective layer  14   a  in a mechanically relaxed state, as illustrated by the pixel  12   a  in  FIG. 4 . However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable reflective layer  14  is deformed and is forced against the optical stack  16 . A dielectric layer (not illustrated in this Figure) within the optical stack  16  may prevent shorting and control the separation distance between layers  14  and  16 , as illustrated by interferometric light modulator  12   b  on the right in  FIG. 4 . The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation that can control the reflective vs. non-reflective pixel states is analogous in many ways to that used in conventional LCD and other display technologies. 
     Some embodiments of the invention may include a display element with which to output the measured ambient pressure. The display element may be an LCD display, such as those used in wristwatches, or the display element may be an interferometric array. In the case an interferometric array is used to display the ambient pressure, it may be possible to configure the array to both measure and display the ambient pressure about the device. 
       FIG. 5  is an exemplary embodiment of the invention. In this embodiment, the device  50  comprises a processor  52 , a memory  54 , an input  56 , an image source module  58 , a transceiver  60 , a measuring module  62 , a controller  64 , a driver  66 , and a display  68 . In an exemplary operation, a user wishing to measure the pressure about the device indicates this desire using the input  56 . The processor  52  passes this instruction the controller  64 , which activates a driver  66  which drives the display  68 . A measuring module  62  connected to display  68  measures the current response of at least one of the MEMS device embodied in the display  68  and passes this information to the processor  52 . The processor may access memory  54  storing code to calculate the pressure about the device from the measured current response. The device may be embodied in a general display unit which receives images from an image source module  58 . Further, the image source module  58  is connected to a transceiver  60 , which may act as a transmitter and a receiver, in order to receiver new images. 
     The foregoing description sets forth various preferred embodiments and other exemplary but non-limiting embodiments of the inventions disclosed herein. The description gives some details regarding combinations and modes of the disclosed inventions. Other variations, combinations, modifications, modes, and/or applications of the disclosed features and aspects of the embodiments are also within the scope of this disclosure, including those that become apparent to those of skill in the art upon reading this specification. Thus, the scope of the inventions claimed herein should be determined only by a fair reading of the claims that follow.