Abstract:
A diaphragm for a pressure sensor includes a central portion having a primary thickness and a surrounding secondary portion having a secondary thickness greater than the primary thickness. The pressure sensor includes the diaphragm, a fluid conduit capped by the diaphragm, and a piezoelectric bridge for each of the primary and secondary portions to generate a signal indicative of the displacement of the portions; and a method of producing the sensor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    To achieve high overpressure ratings (greater than 2×), the diaphragm thickness of present silicon pressure sensor designs have to be increased accordingly to that thickness required to reduce the stress in the diaphragm at the overpressure rating below the rupture point of the silicon. The need for a thicker diaphragm results in a reduced sensitivity over the normal operating pressure range, which is undesirable. One device providing increased sensitivity is disclosed in U.S. Pat. No. 6,877,380 to Lewis titled “Diaphragm for Bonded Element Sensor,” and herein incorporated by reference. While providing increased sensitivity, the device does not address the situation where higher overpressure ratings are needed along with sufficient sensitivity over the operating pressure range. What is needed is a diaphragm design that allows higher overpressure ratings while minimizing the reduction in sensitivity over the operating pressure range as well as providing improved linearity capability. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention provides silicon-based pressure sensors and methods of making the sensors. An example sensor includes a diaphragm formed by an etching process resulting in two concentric diaphragm portions each with a different diameter and thickness. 
         [0003]    By making the aspect ratio (diameter/thickness) of the primary (inner) diaphragm greater than the aspect ratio of the secondary (outer) diaphragm, the overpressure rating of the more sensitive inner diaphragm is increased due to a reduction in the stress at the edge of the inner diaphragm caused by the bending of the secondary diaphragm at the overpressure levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
           [0005]      FIG. 1-1  is a cross-section of a silicon pressure sensor according to the present invention; 
           [0006]      FIG. 1-2  is a top view of the silicon pressure sensor of  FIG. 1-1  showing piezoresistive elements; 
           [0007]      FIG. 2  is a top view of a rectangular diaphragm according to the present invention, shown without piezoresistive elements for clarity; 
           [0008]      FIG. 3  is a flow chart of an example method of manufacturing a device according to the present invention; 
           [0009]      FIG. 4  shows an intermediate step in the fabrication of a silicon pressure sensor according to the present invention; 
           [0010]      FIG. 5  shows an intermediate step in the fabrication of a silicon pressure sensor according to the present invention; 
           [0011]      FIG. 6  shows an electrical schematic of the dual bridge configured structure; 
           [0012]      FIG. 7  shows a theoretical (calculated) output of each bridge; and 
           [0013]      FIG. 8  shows the combined output of the two bridges of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]      FIGS. 1-1  and  1 - 2  show a cross-section and top views, respectively, of a pressure sensor assembly  10  constructed in accordance with an embodiment of the present invention. The assembly  10  includes a fluid conduit  11  defined by a tube  12  which may be made of glass, glass frit, or other materials depending on the intended use of the assembly  10 . The tube  12  is attached on a first end  14  by a diaphragm assembly  16 , and a second end  18  of the tube  12  is configured to allow attachment to a pressure vessel (not shown) or similar pressurized environment containing fluid to be monitored. 
         [0015]    The diaphragm assembly  16  includes a first layer  20  which is made of epitaxial silicon and a substrate layer  22  which is made of heavily doped silicon, though it will be appreciated that other materials having similar properties known to those having ordinary skill in the art may be used. The first layer  20  is an epitaxial layer grown on the substrate layer  22 . A diaphragm wall  24  including portions of the first layer  20  and the substrate layer  22  surrounds the diaphragm assembly  16 . In one embodiment, the diaphragm assembly  16  is attached to the tube  12  with a thermo-electric bond for the case where the conduit or tube is Pyrex®. A shoulder  38  has a thickness that increases gradually from a primary thickness  30  to a secondary thickness  36 . 
         [0016]    The primary aspect ratio is a primary diameter  28  divided by the primary thickness  30 , and the secondary aspect ratio is a secondary diameter  34  divided by the secondary thickness  36 . A secondary portion (diameter  28 ) is configured to exhibit less sensitivity than a primary portion (diameter  34 ), for example, one-fourth the sensitivity of the primary portion. By making the primary aspect ratio greater than the secondary aspect ratio, the overpressure rating of the more sensitive primary portion is increased due to a reduction in the stress experienced by the primary portion caused by deformation of the secondary portion at overpressure conditions. For applications where high overpressure ratings are not required, the assembly  10  can be used to significantly increase the dynamic range of the primary portion. 
         [0017]    In a particular embodiment, the primary aspect ratio, for example a primary diameter of 40 mils, a thickness of 2 mils and the secondary diameter of 80 mils and thickness of 8 mils in the calculation, is twice the secondary aspect ratio, while both portions have typical overpressure ratings of 1.5×. In this configuration, the operating pressure rating of the secondary portion will be four times the operating pressure rating of the primary portion (the operating pressure rating of a diaphragm is inversely proportional to the square of the diaphragm aspect ratio). For a sensor assembly  10  made according to this example, operating the assembly  10  at the pressure rating of the secondary portion will allow for an overpressure rating of the primary portion to be increased from 1.5× to 4×. 
         [0018]      FIG. 1-2  shows a top view of the assembly  10  of  FIG. 1-1 . The assembly  10  includes a primary piezoresistive bridge assembly  40  configured to generate a signal reflective of a displacement of a primary portion  26  and a secondary piezoresistive bridge assembly  42  configured to generate a signal reflective of a displacement of a secondary portion  32 . The piezoresistive bridge assemblies  40 ,  42  may be configured as shown in U.S. Pat. No. 6,718,830 to Johnson titled “Customized Span Compensation of SOI Pressure Sensor”, herein incorporated by reference though any configuration known to those having ordinary skill in the art may be used. In practice, the output of the bridge assemblies  40 ,  42  can be measured and added to increase the signal output. 
         [0019]      FIG. 2  is a top view of an alternate embodiment of the present invention including a high pressure rectangular plate diaphragms  35  surrounded by a rectangular diaphragm wall  41 , the output signal increases with increasing pressure. The incorporation of a diaphragm according to the present invention into the rectangular plate diaphragm design will cause a reduction in stress in the primary portion  37  as a function of bending of the secondary portion  39 , and thus improve the linearity of the primary portion  37 . 
         [0020]      FIG. 3  shows a block diagram of an example method  46  of making the sensor assembly  10 . While specific materials and steps are described herein, there are many materials and steps known to those having ordinary skill in the art which may be used. The diaphragm assembly  16 , as noted above, includes a first layer  20  of N-type epitaxial silicon which is grown on a substrate layer  22  of heavily doped P++ silicon layer at a block  48 . As shown in a block  50 , the substrate layer  22  is masked and standard electro-chemical etching is used to etch the substrate layer  22  to expose a bottom surface  44  ( FIG. 4 ) of the first layer  20 .  FIG. 4  is a cross-section of the diaphragm assembly  16  after the etching step of block  50 . 
         [0021]    After forming the structure shown in  FIG. 4 , the substrate layer  22  is masked, and plasma etching is used to etch the substrate layer  22  and the first layer  20 , as shown in a block  52 . Note that an initial thickness of the first layer  20  is equal to the final thickness of the primary portion plus the depth of the plasma etch.  FIG. 5  shows a cross-section of the diaphragm assembly  16  after the etching step of block  52 . After etching, the layers  20 ,  22  are lapped and polished as shown in a block  54 . At a block  56 , the primary and secondary bridge assemblies  40 ,  42  are formed to the primary and secondary portions, respectively, and configured. The diaphragm assembly  16  is attached to the tube  12  (as shown at a block  58 ) to produce the pressure sensor assembly  10  shown in  FIGS. 1-1  and  1 - 2 . 
         [0022]    The output signal that can be generated from such an arrangement is illustrated in  FIGS. 6 through 8 .  FIG. 6  shows an electrical schematic of the dual bridge configuration, where D 1  represents the primary bridge and D 2  represents the secondary bridge. RF 1 , RF 2 , RB 1  and RB 2  as shown can be used in conjunction with an operational amplifier to adjust the null and gain of the amplified bridge outputs, VP 1  and VP 2 . The governing equations are: 
         [0000]    
       
         
           
             
               1 
               ) 
             
              
             
                 
             
              
             Primary 
              
             
                 
             
              
             Pressure 
              
             
                 
             
              
             Channel 
              
             
                 
             
              
             Output 
              
             
                 
             
              
             in 
              
             
                 
             
              
             Voltage 
              
             
                 
             
              
             Ratio 
              
             
                 
             
              
             of 
              
             
                 
             
              
             VP 
              
             
                 
             
              
             1 
              
             
               / 
             
              
             Vref 
              
             
               : 
             
           
         
       
       
         
           
             
               VP 
                
               
                   
               
                
               
                 1 
                 / 
                 Vref 
               
             
             = 
             
               
                 
                   
                     
                       { 
                       
                         1 
                         + 
                         
                           RF 
                            
                           
                               
                           
                            
                           1 
                           * 
                           
                             [ 
                             
                               
                                 ( 
                                 
                                   
                                     1 
                                     / 
                                     Rr 
                                   
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 ) 
                               
                               * 
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       
                                         ( 
                                         
                                           Rr 
                                            
                                           
                                               
                                           
                                            
                                           1 
                                           * 
                                           Rr 
                                            
                                           
                                               
                                           
                                            
                                           2 
                                         
                                         ) 
                                       
                                       / 
                                       
                                         ( 
                                         
                                           Rt 
                                            
                                           
                                               
                                           
                                            
                                           1 
                                           * 
                                           Rt 
                                            
                                           
                                               
                                           
                                            
                                           2 
                                         
                                         ) 
                                       
                                     
                                   
                                   ) 
                                 
                                 / 
                               
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         ( 
                         
                           1 
                           + 
                           
                             Rt 
                              
                             
                                 
                             
                              
                             
                               2 
                               / 
                               Rr 
                             
                              
                             
                                 
                             
                              
                             2 
                           
                           - 
                           
                             
                               ( 
                               
                                 
                                   1 
                                   / 
                                   RB 
                                 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                               ) 
                             
                             * 
                             
                               ( 
                               
                                 Rr 
                                  
                                 
                                     
                                 
                                  
                                 
                                   1 
                                   / 
                                   Rt 
                                 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                       } 
                     
                   
                 
               
               
                 1 
                 + 
                 
                   Rr 
                    
                   
                       
                   
                    
                   
                     1 
                     / 
                     Rt 
                   
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
       
         
           
             2 
              
             
               ) 
             
              
             
                 
             
              
             Secondary 
              
             
                 
             
              
             Pressure 
              
             
                 
             
              
             Channel 
              
             
                 
             
              
             Output 
              
             
                 
             
              
             in 
              
             
                 
             
              
             Voltage 
              
             
                 
             
              
             Ratio 
              
             
                 
             
              
             of 
              
             
                 
             
              
             VP 
              
             
                 
             
              
             2 
              
             
               / 
             
              
             Vref 
              
             
               : 
             
           
         
       
       
         
           
             
                 
             
              
             
               
                 VP 
                  
                 
                     
                 
                  
                 
                   2 
                   / 
                   Vref 
                 
               
               = 
               
                 
                   
                     
                       
                         { 
                         
                           1 
                           + 
                           
                             RF 
                              
                             
                                 
                             
                              
                             2 
                             * 
                             
                               [ 
                               
                                 
                                   ( 
                                   
                                     
                                       1 
                                       / 
                                       Rr 
                                     
                                      
                                     
                                         
                                     
                                      
                                     4 
                                   
                                   ) 
                                 
                                 * 
                                 
                                   
                                     ( 
                                     
                                       1 
                                       - 
                                       
                                         
                                           ( 
                                           
                                             Rr 
                                              
                                             
                                                 
                                             
                                              
                                             3 
                                             * 
                                             Rr 
                                              
                                             
                                                 
                                             
                                              
                                             4 
                                           
                                           ) 
                                         
                                         / 
                                         
                                           ( 
                                           
                                             Rt 
                                              
                                             
                                                 
                                             
                                              
                                             3 
                                             * 
                                             Rt 
                                              
                                             
                                                 
                                             
                                              
                                             4 
                                           
                                           ) 
                                         
                                       
                                     
                                     ) 
                                   
                                   / 
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               Rt 
                                
                               
                                   
                               
                                
                               
                                 4 
                                 / 
                                 Rr 
                               
                                
                               
                                   
                               
                                
                               4 
                             
                             - 
                             
                               
                                 ( 
                                 
                                   
                                     1 
                                     / 
                                     RB 
                                   
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 ) 
                               
                               * 
                               
                                 ( 
                                 
                                   Rr 
                                    
                                   
                                       
                                   
                                    
                                   
                                     3 
                                     / 
                                     Rt 
                                   
                                    
                                   
                                       
                                   
                                    
                                   3 
                                 
                                 ) 
                               
                             
                           
                           ] 
                         
                         } 
                       
                     
                   
                 
                 
                   1 
                   + 
                   
                     Rr 
                      
                     
                         
                     
                      
                     
                       3 
                       / 
                       Rt 
                     
                      
                     
                         
                     
                      
                     3 
                   
                 
               
             
           
         
       
     
         [0023]      FIG. 7  shows the calculated output from each bridge for pressures as high as 4× the full scale pressure of the primary bridge.  FIG. 8  shows the calculated pressure from the combined bridges when configured as in  FIG. 6 . It is shown for a pressure range up to 4× the full scale pressure of the primary bridge, which would rupture the diaphragm of the primary bridge if it were not used in conjunction of the secondary diaphragm. The combined output is: 
         [0000]        V out/ V ref=( R 2/ R 1)* VP 1 +VP 2)/(1 +R 2/ R 1) 
         [0024]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.