Abstract:
The MEMS Sensor Suite on a Chip provides the capability, monolithically integrated onto one MEMS chip, to sense temperature, humidity, and two axes of acceleration. The device incorporates a MEMS accelerometer, a MEMS humidity sensor, and a MEMS temperature sensor on one chip. These individual devices incorporate proof masses, suspensions, humidity sensitive capacitors, and temperature sensitive resistors (thermistors) all fabricated in a common fabrication process that allows them to be integrated onto one micromachined chip. The device can be fabricated in a simple micromachining process that allows its size to be miniaturized for embedded and portable applications. During operation, the sensor suite chip monitors temperature levels, humidity levels, and acceleration levels in two axes. External circuitry allows sensor readout, range selection, and signal processing.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Patent Application U.S. Ser. No. 60/619,421, entitled “MEMS Sensor Suite on a Chip” and filed on Oct. 15, 2004, which is fully incorporated herein by reference. 
     
    
     GOVERNMENT LICENSE RIGHTS 
       [0002]    This invention was made with Government support under contract number DAAE30-03-C-1076, awarded by the United States Army. The Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates generally to environmental sensors. More particularly, the present invention relates to micro-electromechanical (“MEMS”) humidity sensors, accelerometers, and resistors. 
         [0005]    2. Background of the Invention 
         [0006]    Embedding miniature sensors in structures, systems, storage and shipping containers, and other items allows the monitoring of those items to determine health, maintenance needs, lifetime, and other item characteristics. In addition, embedded sensors can be used to perform built in test and evaluation of products. Information from miniature accelerometers, temperature sensors, and humidity sensors can tell a user whether or not the item has been dropped sufficiently to cause damage, experienced temperature extremes beyond specifications, or seen humidity levels beyond those that can be handled. A multifunction sensor suite can be used to collect these environmental parameters, which can then be stored and analyzed by a monitoring system, test stand, or other external device. 
         [0007]    Current embedded sensor systems that perform this type of monitoring typically use sets of discrete sensor devices on a printed circuit board to form the sensor suite. The devices are typically individually packaged and interconnected to external electronics using printed circuit board traces. However, this approach limits the miniaturization of the sensor suite. The individual packages take up a lot of space, as do the relatively large circuit traces on a printed circuit board. Monolithic integration of sensors allows size reductions by removing the need for individual packaging for each sensor, as well as utilizing finer electrical interconnects. 
         [0008]    Most of the previous work on monolithically integrated sensors has focused on combining two sensor types onto the same chip. Typically, both an accelerometer and a temperature sensor, or both a temperature sensor and humidity sensor, may be monolithically integrated, but not all three. A number of designs and fabrication processes have been employed to perform the integration of two sensors. However, integrating all three sensing functions of accelerometers, temperature sensors, and humidity sensors (or integrating the two sensing functions of accelerometers and humidity sensors) is much more difficult. Integration challenges lie in the sensor design, sensor fabrication, and sensor packaging. 
         [0009]    The challenges are due to the sensor devices having substantially different functionality. In particular, the accelerometer requires a suspended inertial mass whereas a humidity sensor requires a material or structure that changes depending on humidity levels. These features often require widely different microfabrication approaches and techniques. Furthermore, a fundamental difference between accelerometers, temperature sensors, and humidity sensors is that the accelerometers and temperature sensors do not need to be exposed to the ambient air or gas. In contrast, humidity sensors need to be exposed to the external environment, as do chemical and biological sensors as well. Integrating exposed sensors and sensors that should not be exposed is difficult from a packaging perspective. 
         [0010]    As embedded sensor systems are miniaturized and incorporated into applications, the requirement to use integrated sensor suites becomes more important. It would therefore be desirable to combine accelerometers, temperature sensors, and humidity sensors in a small form factor. 
       SUMMARY OF THE INVENTION 
       [0011]    It is therefore an object of the present invention to provide multiple MEMS sensors on one chip for embedded sensing systems through a new sensor fabrication process, and associated sensor designs. The present invention achieves this goal by fabricating on a common substrate up to three different types of MEMS sensors. The three types of sensors are a three-axis accelerometer, a temperature sensor, and a humidity sensor. The accelerometer consists of a proof mass, suspension system, anchors, and capacitive position sensors. An external acceleration moves the proof mass against the suspension, thereby changing the capacitance of the position sensors. The temperature sensor consists of a doped silicon resistor with a resistance that is sensitive to temperature. Temperature changes will modify the resistance of the structure, which can then be measured by external circuitry. The humidity sensor consists of a humidity sensitive polymer layer sandwiched between two parallel-plate electrodes. Varying levels of humidity will alter the volume and dielectric constant of the polymer layer, which is then read by placing a modulated voltage across the electrodes. These three sensors are integrated on a common substrate and can be placed in a package that leaves the humidity sensor exposed to the environment through a filter structure. 
         [0012]    For purposes of summarizing the invention, certain aspects, advantages, and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any one particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
         [0013]    These and other embodiments of the present invention will also become readily apparent to those skilled in the art from the following detailed description of the embodiments having reference to the attached figures, the invention not being limited to any particular embodiment(s) disclosed. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a top view of one embodiment of the invention comprising the three sensor types on one chip. 
           [0015]      FIG. 2  is a top view of the accelerometer sensor. 
           [0016]      FIG. 3  shows the definition of parameters used in the design of the accelerometer. 
           [0017]      FIG. 4  is a top view of the temperature sensor. 
           [0018]      FIG. 5  shows the definition of parameters used in the design of the resistor. 
           [0019]      FIG. 6  is a top view of the humidity sensor. 
           [0020]      FIG. 7  shows the humidity sensor&#39;s capacitive response as a function of humidity level. 
           [0021]      FIGS. 8A-I  illustrate the steps in the fabrication process for a three-sensor chip. 
           [0022]      FIG. 9  illustrates an alternative embodiment of the MEMS sensor suite accelerometer. 
       
    
    
       [0023]    Repeat use of reference characters throughout the present specification and appended drawings is intended to represent the same or analogous features or elements of the invention 
       DETAILED DESCRIPTION 
       [0024]    The present invention and its advantages are best understood by referring to the drawings. The elements of the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
         [0025]    The illustrated embodiment of the invention is fabricated in a thick layer of silicon or other conductor material. Within this thick layer of material, the proof masses, flexures, capacitive position sensors, isolated resistors, humidity sensitive capacitors, and multiple anchors and pads are fabricated.  FIG. 1  shows one configuration of these structures that yields a sensor suite with a two-axis accelerometer  1 , a temperature sensor  2 , and a humidity sensor  3 . The three sensors occupy an area on the substrate surface  5  of about 0.5 square centimeters or less. Bonding pads  4  provide electrical connection to the three sensors. 
       The Accelerometer 
       [0026]      FIG. 2  shows one embodiment of the device&#39;s accelerometer, in which a moveable proof mass  10  responds to acceleration by deflecting the suspension flexures  11  suspending the proof mass  10  above the substrate surface  5 . When an acceleration is applied to the proof mass  10 , it will move in the +/−x, +/−y, or +/−z directions, in accordance with the direction of the acceleration, a distance that depends on the acceleration level, the amount of time the acceleration is applied, the size of the proof mass  10 , and the compliance of the suspension in that direction. This motion results in a deflection of the capacitive position sensors  14  in the x and/or y directions, and a change to the capacitance of those sensors. These deflections are proportional to the level of acceleration seen and can be measured by measuring the capacitance of the sensors. 
         [0027]    With reference to  FIG. 2 , the proof mass  10  is fabricated with holes  13  which are utilized during the fabrication process to allow chemicals to pass through to the substrate surface  5 . Four (4) suspension anchors  20  anchor eight (8) suspension flexures  11  located at the corners of the proof mass  10 . As can be seen in  FIG. 2 , four (4) of the suspension flexures allow displacement of the proof mass  10  in the x-direction and four (4) of the suspension flexures allow displacement of the proof mass  10  in the y-direction. Four (4) central flexures  15  decouple lateral motion of the proof mass  10  from the four (4) capacitive position sensors  14 . The capacitive position sensors  14  are comprised of a row of static comb-fingers  16  that are cantilevered to the comb-finger anchors  18  that are connected to the substrate surface  5 , and an interlocking (but not contacting) row of dynamic comb-fingers  17  that are attached to the flexure support  19 . As can be seen in  FIG. 2 , the components of the accelerometer  1  are “mirror-imaged” about the x- and y-axis in this embodiment. 
         [0028]      FIG. 3  shows the definition of the primary parameters used to design the accelerometer to detect specific levels of acceleration. The inertial force developed on the proof mass is then given by: 
         [0000]    
       
      
       F=m*a,  
      
     
         [0000]    where F is the inertial force, m is the mass of the proof mass, and a is the applied acceleration. 
         [0029]    The stiffness of the suspension provides a force against the inertial force. The stiffness in each axis of the device is given by: 
         [0000]    
       
         
           
             
               k 
               = 
               
                 
                   6 
                   * 
                   kb 
                 
                 = 
                 
                   
                     2 
                     * 
                     E 
                     * 
                     t 
                     * 
                     
                       wb 
                       3 
                     
                   
                   
                     lb 
                     3 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where k is the entire suspension stiffness, kb is the stiffness of one beam in the suspension, E is the Young&#39;s modulus of the material the device is made of, wb is the width  66  of a beam in the suspension, lb is the length  65  of a beam in the suspension, and t is the thickness of the material. 
         [0030]    The distance the proof mass will move under the applied acceleration is given by: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               y 
             
             = 
             
               F 
               k 
             
           
         
       
     
         [0031]    The capacitive position sensor capacitance will change with deflection of the proof mass. The capacitance change is given by: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               C 
             
             = 
             
               
                 
                   ɛ 
                   o 
                 
                 * 
                 N 
                 * 
                 t 
                 * 
                 Δ 
                  
                 
                     
                 
                  
                 y 
               
               
                 g 
                 o 
               
             
           
         
       
     
         [0000]    where C is the comb-drive capacitance, ε o  is the permittivity of free space, N is the number of fingers in the comb-drive, t is the thickness of the structure, l o  is the initial overlap  67  of the fingers, y is the amount of deflection, and g o  is the gap  68  between fingers in the comb-drive. Typical capacitive measurement circuits can measure capacitance changes on the order of 10 −18  Farads. 
       The Temperature Sensor 
       [0032]      FIG. 4  shows the temperature sensor that is comprised of a serpentine bridge  30  composed of doped silicon with a large temperature coefficient of resistance. The serpentine bridge  30  forms a thermistor that has a nominal resistance at ambient temperature that depends on the bridge geometry. As the external temperature changes, the resistance of the bridge  30  will change. Electrical contact with the resistor is made via bond pads  31 . A number of readout techniques can be utilized to measure that resistance change. 
         [0033]      FIG. 5  shows the definition of the primary parameters used to design the temperature sensors. The nominal resistance of the thermistor depends on the cross-sectional area of the device, the full device length, and the resistivity of the material, and is given by: 
         [0000]    
       
         
           
             
               R 
               o 
             
             = 
             
               
                 ρ 
                 * 
                 t 
                 * 
                 w 
               
               L 
             
           
         
       
     
         [0000]    where R o  is the resistance at ambient temperature, ρ is the material resistivity, t is the thickness of the material, w is the width  70  of the resistor, and L is the length of the resistor (i.e., the total length of all of the segments of length L o    71 ). 
         [0034]    For a temperature sensitive resistor, the resistance as a function of temperature is given by: 
         [0000]        R=R   o *(1+α( T−T   o )) 
         [0000]    , where R is the resistance at temperature, T, R o  is the resistance at ambient temperature, α is the temperature coefficient of the resistor material, and T o  is ambient temperature. Typical resistance measurement circuits can determine milliOhm changes in resistance. 
       The Humidity Sensor 
       [0035]      FIG. 6  illustrates the humidity sensor for the sensor suite, a parallel-plate capacitor with a humidity sensitive dielectric. The humidity-sensitive layer  40  is deposited above a bottom metal electrode  41 . A top metal electrode  42  is then deposited and patterned on the humidity sensitive layer  40 . That top metal layer is the top electrode, and is also patterned with a mesh  43  so as to allow the water vapor in the atmosphere to interact with the dielectric. Bond pads  44  and  45  provide electrical contact to the electrodes of the sensor. As humidity changes, the dielectric constant of the humidity sensitive polymer changes. As this parameters change, the capacitance of the sensor will also change. This can be measured using a capacitive sensing circuit well known in the art of humidity sensing devices. 
         [0036]    For the humidity sensor capacitor, the nominal capacitance depends on the area of the capacitor, A, the thickness of the dielectric layer, d, the permittivity of free space, ε o , and the nominal relative permittivity of the dielectric between the electrodes, ε r , and is given approximately by: 
         [0000]    
       
         
           
             
               C 
               o 
             
             = 
             
               
                 
                   ɛ 
                   o 
                 
                  
                 
                   ɛ 
                   r 
                 
                  
                 A 
               
               d 
             
           
         
       
     
         [0037]    In the presence of humidity, the polymer layer will absorb water vapor. The dielectric constant of water is approximately ε w =80, while that of the polymer layer is approximately ε p =3.5. This substantial difference in dielectric constant results in a change to the capacitance of the structure when in the presence of humidity. In the presence of humidity, the relative permittivity of the dielectric between the electrodes is given by a weighted average of the relative permittivities of the constituents: 
         [0000]      ε r   =f   w *ε w   +f   p *ε p    
         [0000]    , where f w  and f p  are the volume fraction of each component. Furthermore, in general: 
         [0000]      1 =f   w   +f   p    
         [0000]    , and therefore 
         [0000]      ε r =ε p   +f   w *(ε w −ε p ) 
         [0000]    , and 
         [0000]    
       
         
           
             C 
             = 
             
               
                 
                   
                     ɛ 
                     o 
                   
                    
                   A 
                 
                 d 
               
               * 
               
                 
                   ( 
                   
                     
                       ɛ 
                       p 
                     
                     + 
                     
                       
                         f 
                         w 
                       
                       * 
                       
                         ( 
                         
                           
                             ɛ 
                             w 
                           
                           - 
                           
                             ɛ 
                             p 
                           
                         
                         ) 
                       
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0038]    Therefore, the capacitance of the structure will be linear with the volume fraction absorption of water into the polymer. For the polymer used in one embodiment (PI2723 polyimide), the volume fraction absorption of water is 4% at 100% relative humidity, and 0% relative humidity, yielding an absorption of 0.04% per 1% RH. Other types of polymers could be used for the humidity sensitive layer of the capacitor depending upon the amount of water absorption that is desired. As can be seen from the above equations, the thickness of the layer of humidity sensitive polymer will vary depending upon the absorption rate of the polymer selected. 
         [0039]      FIG. 7  shows the theoretical sensor response as a function of humidity level. As can be seen in  FIG. 7 , nominal capacitance of 10 pF at 0% RH increases linearly to 20 pF at 100% RH. Typical capacitance measurement circuits can measure capacitance changes on the order of 10 −18  Farads, making this amount of capacitance change easy to detect. 
       The Fabrication Technique 
       [0040]      FIGS. 8A through 8I  illustrate the steps in the microfabrication process for one embodiment of the invention. This embodiment of the sensor suite is fabricated using a silicon-on-insulator (“SOI”) microfabrication process. 
         [0041]    Referring to  FIG. 8A , the starting material is a silicon-on-insulator wafer  50  with a handle layer  51  and a 15-micron thick active silicon layer  52  separated by a 2 micron thick silicon dioxide layer  53 . In addition, a thin coat of isolating oxide  54  is deposited on the top of the wafer  50  for the purpose of isolating the humidity sensor from the substrate. A 0.3 micron thickness of isolating oxide has been used in one embodiment, though other thicknesses could also be employed. 
         [0042]    The areas on the wafer  50  in which the accelerometer  1 , temperature sensor  2  and humidity sensor  3  will be fabricated are shown in  FIGS. 8B and 8C . As illustrated in  FIG. 8B , the wafer  50  is patterned to open areas in the isolation oxide  54 . These openings  55  will be areas to which electrical contact and structure definition for the accelerometer  1  and temperature sensor  2  can occur. Then, a metal layer  56  is deposited and patterned as shown in  FIG. 8C . This metal layer  56  makes contact to the silicon layer  52  through the openings in the oxide  54  where desired for the accelerometer and temperature sensors, and is also used to form the bottom electrodes  57  of the humidity sensor  3 . After the first metal layer is complete, the humidity sensitive polyimide  58  is deposited on the humidity sensor area  3  and patterned and cured, as shown in  FIG. 8D . After that step is complete, a second metal layer  59  is deposited, as shown in  FIG. 8E . This metal layer serves as the top electrode of the humidity sensitive capacitor  3  and can also be used to add another layer of metal to the bond pads for the accelerometer  1  and temperature sensor  2 , if desired. At this point, the humidity sensor  3  is complete. 
         [0043]    After completing the humidity sensor, a coating of silicon nitride  60  is deposited and patterned to completely encapsulate all exposed metal, as shown in  FIG. 8F . This encapsulating layer is required to protect the metal layers during the subsequent silicon and silicon dioxide etches. Plasma enhanced chemical vapor deposition (“PECVD”) silicon nitride was used in one embodiment to minimize the existence of pinholes that would allow damage to the isolated components during etching. PECVD silicon nitride also uses lower temperatures and therefore minimizes stress on the sensors during processing. Other processes could be used instead to manufacture the silicon nitride, however, provided that the resultant coating minimizes pinholes. 
         [0044]    As illustrated in  FIG. 8G , the SOI wafer is then patterned using standard lithography and etched via a deep silicon reactive ion etch (“DRIE”) to define the structures of the accelerometer  1  and resistor  2 . A Bosch DRIE process was used in one embodiment of the invention, though other DRIE processes could be used instead. 
         [0045]    Referring to  FIG. 8H , after defining the silicon structure, the silicon dioxide layer  53  in between the silicon layers is removed in a fashion that releases the moving parts of the accelerometer  1  structure but allows portions of the layer  53 , specifically those underneath the humidity sensor  3 , the resistor  2 , and anchors and bond pads (not illustrated) on the accelerometer  1 , to remain and hold the structure to the substrate. After the structure is released, the entire silicon nitride layer  60  is removed, as shown in  FIG. 8I , thereby completing processing of the device. 
         [0046]    Although the process discussed above incorporated one accelerometer, one resistor and one humidity sensor, any combinations of the three sensors on one chip that includes at least an accelerometer and a humidity sensor on one chip would be within the scope of the present invention. 
         [0047]    The sensor suite is connected to external electronics well known in the art of sensor systems that provide an oscillator that produces a modulation signal for monitoring the capacitors, a demodulator for detecting the capacitance-induced modulation of that signal, a current source to run through the thermistor, and amplifiers and filters to condition the signal. The electronics can be integrated in an application specific integrated circuit for further miniaturization. 
         [0048]    Using typical capacitance and resistance measurement circuits capable of monitoring changes in capacitance on the order of 10 −18  Farads and changes in resistance on the order of milliOhms, the ranges and resolutions for the sensors given the specific set of design parameters are shown in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical Resolutions and Ranges of the Sensors 
               
             
          
           
               
                   
                 Width of 
                 Length 
                   
                   
                   
                   
                   
               
               
                   
                 Proof 
                 of Proof 
               
               
                   
                 Mass or 
                 Mass or 
               
               
                   
                 Humidity 
                 Humidity 
                   
                 wb 
                 lb 
               
               
                   
                 Sensor 
                 Sensor 
                 Thickness t 
                 or w 
                 or L 
               
               
                 Sensor 
                 (μm) 
                 (μm) 
                 (μm) 
                 (μm) 
                 (μm) 
                 Resolution 
                 Range 
               
               
                   
               
             
          
           
               
                 Accelerometer 
                  500 
                  500 
                 15 
                 5 
                 500 
                 0.2 g 
                 ±1500 g 
               
               
                 Temperature 
                 N/A 
                 N/A 
                 15 
                 5 
                 500 
                 0.5° C. 
                 −40° C. to 
               
               
                   
                   
                   
                   
                   
                   
                   
                 250° C. 
               
               
                 Humidity 
                 1000 
                 1000 
                 3 
                 N/A 
                 N/A 
                 0.1% 
                 0-100% 
               
               
                   
                   
                   
                   
                   
                   
                   
                 RH 
               
               
                   
               
             
          
         
       
     
         [0049]    The initial intent of this invention was to miniaturize sensing devices in multifunctional embedded data acquisition systems, diagnostic devices, and test &amp; evaluation systems. However, the device could also be used in standalone applications where the sensor suite is connected to an RFID tag or other transmitter for remote determination of the environment seen by shipping containers and products. 
         [0050]    Although the current embodiment and some other potential embodiments and forms of this invention have been illustrated, it is apparent that other various modifications and embodiments of the invention can be made by those skilled in the art without departing from the scope and spirit of the present invention. For example, alternative designs of the accelerometer may be utilized including single axis versions such as the embodiment shown in  FIG. 9 , which incorporates a proof mass  90 , capacitive position sensors  91 , suspension flexures  92 , and anchors  93 . In other embodiments the humidity sensor could be configured as a chemical sensor depending on the specific formulation and characteristics of the polymer used in its creation.