Abstract:
A tri-axis accelerometer includes a proof mass, at least four anchor points arranged in at least two opposite pairs, a first pair of anchor points being arranged opposite one another along a first axis, a second pair of anchor points being arranged opposite one another along a second axis, the first axis and the second axis being perpendicular to one another, and at least four spring units to connect the proof mass to the at least four anchor points, the spring units each including a pair of identical springs, each spring including a sensing unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Divisional of, and claims priority under 35 U.S.C. §120 to, U.S. patent application Ser. No. 11/513,669, filed on Aug. 30, 2006 now U.S. Pat. No. 7,757,555, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a tri-axis accelerometer having a single proof mass and fully differential output signals. 
     BACKGROUND INFORMATION 
       FIG. 1A  shows a cantilever beam accelerometer  100 , which includes a structure layer  101 , a sensing unit  102 , and an anchor  103 . The structure layer  101  is rectangular in form and attached on one end to the anchor  3 . The sensing unit  102 , which is attached to the structure layer  101 , senses a movement of the structure layer  101 , which is free to move at the end opposite the anchor  103 . Here, the sensing unit  102  may operate, for example, in a piezoelectric or piezoresistive manner. 
       FIG. 1B  shows the cantilever beam accelerometer  100  of  FIG. 1A  with a proof mass  104  attached to its tip to generate more strain along the structure layer  100 , which by its attachment to the anchor  103 , forms a suspended beam to support the proof mass  104 . As a result, the addition of the proof mass  104  may provide a more sensitive operation of the accelerometer of  FIG. 1B  as compared to the accelerometer of  FIG. 1A . 
     Although the design of the cantilever beam accelerometers of  FIGS. 1A  and B may be relatively simple and easy to fabricate, such designs may not be suitable for sensing acceleration in three axial directions. In particular, such designs may not provide a differential output signal in order to minimize noise. 
       FIG. 2  shows an accelerometer  200  based on a sensing membrane and a center proof mass, which is discussed, for example, by Li-Ping et al., J. MEMS, 2003, Volume 12, pages 433 to 439. The accelerometer  200  includes a sensing membrane, a center proof mass, a ring-shaped top electrode, a piezoelectric layer, and a mounting frame. Here, the balanced structure of accelerometer  200  minimizes cross sensitivity, and the extra mass significantly improves the overall sensitivity as compared to the designs shown in  FIGS. 1A and 1B . 
       FIGS. 3A-C  show top and side views of a tri-axis accelerometer  300  based on a piezoresistive sensing unit on top of a membrane, which is referred to, for example, by U.S. Patent Application Publication No. 2004/0027033. The tri-axis accelerometer  300  includes a disk-shaped suspension membrane  301 , a proof mass  302  attached underneath the disk-shaped membrane  301 , and piezoresistive sensing units  303  arranged on the surface of the disk-shaped membrane  301 . 
     The designs shown in FIGS.  2  and  3 A-C may be used for tri-axis sensing, but the suspension parts used in these designs are configured as whole membranes and therefore are relatively stiff. Here, the sensitivity of the piezoelectric sensing or piezoresistive sensing depends on the strain generated by the external acceleration. Therefore, a relatively stiff structure may not provide enough sensitivity. Moreover, having a strip or disc-like design may lead to cross talk since an acceleration in one direction, for example, may result in a deformation of the sensing films in other directions thereby requiring a more complicated signal processing to achieve a clean signal. Moreover still, devices having a suspension membrane design may occupy significant space on a microchip, which may increase the cost of such devices. 
     SUMMARY OF THE INVENTION 
     The exemplary embodiment of the present invention provides an exemplary tri-axis accelerometer, which may be provided on one monolithic microchip, and which detects output signals in three orthogonal axial directions, each direction being fully differential. 
     According to an exemplary embodiment of the present invention, a proof mass is arranged in the center of a sensor unit with center-symmetrical support/suspension springs, which are used to provide suspension to the proof mass, and to provide a connection between the proof mass and the anchor positions. Each spring unit may include two identical springs, which provide fully differential output signals. Sensing units may be arranged on top of the springs so that the sensor unit may be more easily fabricated via conventional batch processes. In this regard, the sensing units may be, for example, either piezoelectric or piezoresistive, and an output signal in each of three axial directions may be detected using a single common pick-up circuitry. 
     An exemplary embodiment of the present invention is directed to a tri-axis accelerometer, which includes a proof mass, at least four anchor points arranged in at least two opposite pairs, a first pair of anchor points being arranged opposite one another along a first axis, a second pair of anchor points being arranged opposite one another along a second axis, the first axis and the second axis being perpendicular to one another, and at least four spring units to connect the proof mass to the at least four anchor points, the spring units each including a pair of identical springs, each spring including a sensing unit. 
     Another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the sensing unit is arranged on the spring. 
     Yet another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the sensing unit includes a piezoelectric layer. 
     Still another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the sensing unit includes a piezoelectric resistor. 
     Yet another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the pair of identical springs are arranged to detect a fully differential output signal. 
     Still another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the pair of identical springs are arranged parallel to each other and perpendicular with respect to a longitudinal axis along the connection between the proof mass and a respective anchor point. 
     Yet another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, which includes only one pick-up circuit arrangement to detect the fully differential output signal of each pair of identical springs. 
     Still another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the tri-axis accelerometer is fabricated using one of a Surface MEMS process and a Bulk MEMS process. 
     Yet another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the proof mass, the at least four anchor points, and the at least four springs are arranged on one monolithic microchip. 
     Another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, which includes a proof mass, a plurality of anchor points arranged in at least two pairs opposite one another along at least two axial directions that are perpendicular to one another, and a plurality of spring units to connect the proof mass to the plurality of anchor points, the spring units each including a pair of identical springs, each spring including a sensing unit. 
     Yet another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the pair of identical springs are arranged to detect a fully differential output signal. 
     Still another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the pair of identical springs are arranged parallel to each other and perpendicular with respect to a longitudinal axis along the connection between the proof mass and a respective anchor point. 
     Yet another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, which further includes only one pick-up circuit arrangement to detect the fully differential output signal of each pair of identical springs. 
     Still another exemplary embodiment of the present invention is directed to a tri-axis accelerometer, in which the proof mass, the plurality of anchor points, and the plurality of springs are arranged on one monolithic microchip. 
     Another exemplary embodiment of the present invention is directed to a method of fabricating a tri-axis accelerometer, the method including providing a substrate layer, providing an intermediate layer on the substrate layer, providing a top layer on the intermediate layer, providing a sensing material on the top layer, patterning the top layer to form at least four pairs of sensing structure elements underneath the sensing material, and a proof mass arranged between the at least two pairs of sensing structure elements, the at least four pairs of sensing structure elements arranged along at least two axial directions that are perpendicular to one another, removing a portion of the intermediate layer underneath the sensing structure elements, and removing a portion of the intermediate layer underneath at least a part of the proof mass. 
     Yet another exemplary embodiment of the present invention is directed to a method of fabricating a tri-axis accelerometer, in which the sensing material includes at least one of a piezoelectric and piezoresistive material. 
     Still another exemplary embodiment of the present invention is directed to a method of fabricating a tri-axis accelerometer, in which the at least four pairs of sensing structure elements are arranged to detect a fully differential output signal. 
     Yet another exemplary embodiment of the present invention is directed to a method of fabricating a tri-axis accelerometer, in which each pair of sensing structure elements includes two parallel structures. 
     Still another exemplary embodiment of the present invention is directed to a method of fabricating a tri-axis accelerometer, the method including providing only one pick-up circuit arrangement to detect the fully differential output signal of each pair of identical springs. 
     Yet another exemplary embodiment of the present invention is directed to a method of fabricating a tri-axis accelerometer, the method including patterning the substrate layer to form an additional proof mass. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an available cantilever beam accelerometer. 
         FIG. 1B  shows the cantilever beam accelerometer of  FIG. 1A  with a proof mass attached to its tip to generate more strain along the suspension beam and thereby provide more sensitive output signals as compared to the accelerometer of  FIG. 1A . 
         FIG. 2  shows an accelerometer based on a sensing membrane and a center proof mass, whose balanced structure minimizes cross sensitivity, and extra mass provides an improved overall sensitivity as compared to the accelerometers of  FIGS. 1A and 1B . 
         FIG. 3A  is a top view of a tri-axis accelerometer, which includes piezoresistive sensing units arranged on the surface of a disk-shaped suspension membrane, and a proof mass attached underneath the membrane. 
         FIG. 3B  is a side view of the tri-axis accelerometer of  FIG. 3A , demonstrating a movement of its proof mass in a Z-axis direction. 
         FIG. 3C  is a side view of the tri-axis accelerometer of  FIG. 3A , demonstrating a movement of its proof mass in an X/Y direction. 
         FIG. 4A  shows an exemplary tri-axis accelerometer according to an exemplary embodiment of the present invention. 
         FIG. 4B  shows an exemplary tri-axis accelerometer, which is similar to the exemplary tri-axis accelerometer of  FIG. 4A  but with a differently arranged sensor element. 
         FIG. 4C  shows an exemplary tri-axis accelerometer, which is similar to the exemplary tri-axis accelerometer of  FIG. 4A  but with specially-configured springs. 
         FIG. 5  shows an exemplary Z-axis acceleration signal differential detection of the exemplary tri-axis accelerometer of  FIG. 4A . 
         FIG. 6A  is a schematic diagram illustrating the X and Y acceleration sensing pick-up circuitry of the exemplary tri-axis accelerometer of  FIG. 4A , where the device is fabricated using surface MEMS processes. 
         FIG. 6B  is a schematic diagram illustrating the X and Y acceleration sensing pick-up circuitry of the exemplary tri-axis accelerometer of  FIG. 4A , where the device is fabricated using Bulk MEMS processes. 
         FIG. 7  shows an exemplary tri-axis accelerometer, which is similar to the exemplary tri-axis accelerometer of  FIG. 4A  except that piezoresistive sensing elements are used instead of piezoelectric sensing elements. 
         FIG. 8A  is a schematic diagram illustrating exemplary electronic circuitry to detect a Z-axis differential acceleration output signal of the exemplary tri-axis accelerometer of  FIG. 7 . 
         FIG. 8B  is a schematic diagram illustrating exemplary electronic circuitry to detect X and Y differential acceleration output signals of the exemplary tri-axis accelerometer of  FIG. 7 , where the device is fabricated using surface MEMS processes. 
         FIG. 8C  shows an example of X and Y acceleration sensing pick-up of the exemplary tri-axis accelerometer of  FIG. 7 , where the device is fabricated using Bulk MEMS processes. 
         FIG. 9A  shows an exemplary first stage of an exemplary method for fabricating an exemplary surface MEMS tri-axis accelerometer. 
         FIG. 9B  shows an exemplary second stage of the exemplary method for fabricating an exemplary surface MEMS tri-axis accelerometer. 
         FIG. 9C  shows an exemplary third stage of the exemplary method for fabricating an exemplary surface MEMS tri-axis accelerometer. 
         FIG. 9D  shows an exemplary fourth stage of the exemplary method for fabricating an exemplary surface MEMS tri-axis accelerometer. 
         FIG. 9E  shows an exemplary fifth stage of the exemplary method for fabricating an exemplary surface MEMS tri-axis accelerometer. 
         FIG. 10A  shows an exemplary first stage of an exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer. 
         FIG. 10B  shows an exemplary second stage of the exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer. 
         FIG. 10C  shows an exemplary third stage of the exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer. 
         FIG. 10D  shows an exemplary fourth stage of the exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer. 
         FIG. 10E  shows an exemplary fifth stage of the exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer. 
         FIG. 10F  shows an exemplary sixth stage of the exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4A  shows an exemplary tri-axis accelerometer  400  according to an exemplary embodiment of the present invention. The exemplary tri-axis accelerometer  400  includes a center proof mass  401  surrounded by four spring units  402   a - d , which are each attached on one side to the center proof mass  401 , and on the other side to an anchor point  405 . The spring units  402   a - d  each include two identical suspension springs  403 . Each suspension spring  403  includes a sensing element  404  arranged on top of the suspension spring. In this regard, the sensing element  404  may be, for example, a piezoelectric layer or a piezoresistor. 
       FIG. 4B  shows an exemplary tri-axis accelerometer  450 , which is similar to the exemplary tri-axis accelerometer  400  of  FIG. 4A , but with a differently configured sensor element  403 , which is arranged on top of the suspension spring  403  so as to be facing away from the proof mass  401 , whereas in  FIG. 4A , only one of the sensor elements  403  of each pair of suspension springs  402   a - d  is arranged facing away from the proof mass  401 , the other sensor element  403  of the pair being arranged facing towards the proof mass  401 . 
       FIG. 4C  shows an exemplary tri-axis accelerometer  470 , which is similar to the exemplary tri-axis accelerometer  400  of  FIG. 4A , but with specially-configured springs  406  to connect each of the springs units  402   a - d  to the proof mass  401 . Here, the specially-configured springs  406  are flexible in one direction and rigid in another direction. That is, the specially-configured springs  406  are flexible in a direction that allows horizontal movement of the proof mass  401  with respect to the particular spring unit, and are rigid in a direction that prevents vertical movement of the proof mass  401  with respect to the particular spring unit. 
     Table 1 below shows a comparison of the exemplary accelerometers of the  FIGS. 4A-C  to the conventional accelerometers of  FIGS. 2 and 3 . As indicated in Table 1, the exemplary accelerometers of  FIGS. 4A-C  and the conventional accelerometers of  FIGS. 2 and 3  all provide tri-axis acceleration sensing and may operate according in a piezoelectric or piezoresistive manner. However, the device area occupied by the conventional accelerometers of  FIGS. 2 and 3  is greater that the device area occupied by the exemplary accelerometers of  FIGS. 4A-C , which have a greater output sensitivity and may be fabricated using MEMS and bulk MEMS processes. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Design in FIG. 2-3 
                 Design in FIGS. 4A-C 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Tri-axis acceleration 
                 Yes 
                 Yes 
               
               
                 sensing 
               
               
                 Output sensitivity which 
                 Small 
                 Large 
               
               
                 depends on the strain in 
               
               
                 the sensing elements 
               
               
                 Devices area 
                 Large 
                 Small 
               
               
                 It works for both piezo- 
                 Yes 
                 Yes 
               
               
                 electric sensing and 
               
               
                 piezoresistive sensing 
               
               
                 Design could be fabricated 
                 No 
                 Yes 
               
               
                 by using surface 
               
               
                 MEMS processes and 
               
               
                 bulk MEMS processes 
               
               
                   
               
             
          
         
       
     
       FIG. 5  shows an exemplary Z-axis acceleration signal differential detection of the exemplary tri-axis accelerometer of  FIG. 4A . Here, each pair of the four pairs of suspension springs has been designated according to their respective arrangement in the drawing. In particular, one of the four pairs of suspension springs is designated as the “upper” pair, one is designated as the “bottom” pair, one is designated as the “right” pair, and one is designated as the “right” pair. Additionally, one sensor element of each pair of suspension springs is designated as the “input”, and the other sensor element of each pair is designated as the “output”. The four output sensor signals “UpOut”, “BtOut”, “LtOut”, and “RtOut” from the output sensor elements of the upper, bottom, left and right pairs of suspension springs are combined together as one input into a comparator, and the four input sensor signals “UpIn”, “BtIn”, LtIn”, and “RtIn” from the input sensor elements on the upper, bottom, left and right pairs of suspension springs are combined together as another input to the comparator. 
       FIGS. 6A and 6B  each show in schematic form examples of X and Y acceleration sensing signal pick-up circuitry for the exemplary tri-axis accelerometer of  FIG. 4A . More specifically,  FIG. 6A  shows in schematic form an example of the sensing pick-up circuitry if the devices are fabricated using surface MEMS processes.  FIG. 6B  shows in schematic form an example of the sensing pick-up circuitry if the devices are fabricated using Bulk MEMS processes. 
     In  FIG. 6A , the left and right output sensor signals “LtOut” and “RtOut” from the output sensor elements on the left and right pairs of suspension springs are compared to left and right input sensor signals “LtIn” and “RtIn” from the input sensor elements on the left and right pairs of suspension springs to detect an external acceleration along the X axis, and the upper and bottom output sensor signals “UpOut” and “BtOut” from the two output sensor elements on the upper and bottom pairs of suspension springs are compared to the upper and bottom input sensor signals “UpIn” and “BtIn” from the input sensor elements on the upper and bottom pairs of suspension springs are compared to detect an external acceleration along the Y axis. In  FIG. 6B , by contrast, the output and input sensor signals “LtOut” and “LtIn” from the output and input sensor elements on the left pair of suspension springs are compared to output and input sensor signals “RtOut” and “RtIn” from output and input sensor elements on the right pair of suspension springs to detect an external acceleration along the X axis, and the output and input sensor signals “UpOut” and “UpOut” from the output and input sensor elements on the upper pair of suspension springs is compared to the output and input sensor signals “BtOut” and “BtIn” from the output and input sensor elements on the bottom pair of suspension springs are compared to detect an external acceleration along the Y axis. 
       FIG. 7  shows an exemplary tri-axis accelerometer  700 , which is similar to the exemplary tri-axis accelerometer  400  of  FIG. 4A , except that piezoresistive sensing elements are used instead of piezoelectric sensing elements. In this regard, the resistance of the piezoresistor sensing element will change when there is an external acceleration, as shown for example in Table 2 below. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Piezoresistor 
                 R uo   
                 R ui   
                 R bo   
                 R bi   
                 R ro   
                 R ri   
                 R lo   
                 R li   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Surface 
                 +X Acceleration 
                 0 
                 0 
                 0 
                 0 
                 − 
                 − 
                 + 
                 + 
               
               
                 MEMS 
                 +Y Acceleration 
                 − 
                 − 
                 + 
                 + 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 +Z Acceleration 
                 − 
                 + 
                 − 
                 + 
                 − 
                 + 
                 − 
                 + 
               
               
                 Bulk 
                 +X Acceleration 
                 0 
                 0 
                 0 
                 0 
                 − 
                 + 
                 + 
                 − 
               
               
                 MEMS 
                 +Y Acceleration 
                 − 
                 + 
                 + 
                 − 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 +Z Acceleration 
                 − 
                 + 
                 − 
                 + 
                 − 
                 + 
                 − 
                 + 
               
               
                   
               
             
          
         
       
     
       FIG. 8A  shows in schematic form exemplary electronic circuitry to detect a Z-axis differential acceleration output signals of the exemplary tri-axis accelerometer  700  of  FIG. 7 . Here, the voltage V 1  across the midpoint between the upper resistor pairs R ui  and R uo , and the right resistor pairs R ro  and R ri , is compared to the voltage V 2  across the midpoint between the bottom resistor pairs R bi  and R bo , and the left resistor pairs R lo  and R li . Note, for bulk MEMS and surface MEMS devices the signal pick-up circuitry may be the same. 
       FIGS. 8B and 8C  each show in schematic form examples of X and Y acceleration sensing signal pick-up circuitry of the exemplary tri-axis accelerometer of  FIG. 7 . More specifically,  FIG. 8B  shows in schematic form an example of the sensing pick-up circuitry that may be provided if the devices are fabricated using surface MEMS processes, and  FIG. 8C  shows in schematic form an example of the sensing pick-up circuitry that may be provided if the devices are fabricated using Bulk MEMS processes. 
       FIGS. 9A to 9E  show exemplary stages S 1  to S 5  of an exemplary method for fabricating an exemplary surface MEMS tri-axis accelerometer, which is formed from a three-layer structure consisting of a substrate layer  1101 , an intermediate layer  1102 , and a top layer  1103 . The substrate layer  1101  may include, for example, a single crystal silicon, the intermediate layer  1102  may include, for example, a thermally grown silicon dioxide layer or other type of deposited oxide layer, and the top layer  1103  may include, for example, a single crystal silicon. Alternatively, the substrate layer  1101  may include, for example, a grown polysilicon structure layer, the intermediate layer  1102  may include, for example, a thermal oxide or low temperature oxide layer, and the top layer  1103  may include, for example, an epitaxially grown polycrystalline silicon or silicon-germanium layer. 
     In stage S 1 , alignment marks  1104  are provided to the top layer  1103 , as shown in  FIG. 9A . In stage S 2 , piezo sensing units  1105  are provided, as shown in  FIG. 9B . Here, in this instance, a piezoresistive material may be implanted to form piezoresistive sensing units, or alternatively, a piezoelectric material may be deposited to form piezoelectric sensing units. 
     The piezoresistive material may implanted, for example, via ion implantation, and the piezoelectric material may be deposited, for example, via sputtering. 
     In stage S 3 , the top layer  1103  is patterned to form sensing structure elements  1106  and an upper proof mass  1107 , as shown in  FIG. 9C . Here, the sensing structure elements  1106  have been patterned so that the piezo sensing units remain as part of the sensing structure elements  1106 . In this regard, a deep reactive ion trench etching may be employed. 
     In stage S 4 , the portion of the intermediate layer  1103  underneath the sensing structure elements  1106  and proof mass  1107  has been removed, as shown in  FIG. 9D , so as to release sensor structure elements  1106  and the proof mass  1107 . In this regard, the portion of the intermediate layer  1102  underneath the sensing structure elements  1106  may be removed, for example, using a HF vapor etching process. In this instance, the proof mass  1107  may include perforations to facilitate a complete undercutting beneath the proof mass  1107 , which ensures that the proof mass  1107  is fully released. 
     In stage S 5 , contact pads  1108  are provided on the top layer  1103 , as shown in  FIG. 9E . Here, the contact pads  1108  may made of a metal material and be used, for example, to provide sensor output signals. In particular, the contact pads  1108  may be, for example, a sputtered deposited metal or shadow masked. 
       FIGS. 10A to 10F  show exemplary stages B 1  to B 6  of an exemplary bulk MEMS method for fabricating an exemplary tri-axis accelerometer, which is formed from a three-layer structure consisting of a substrate layer  1201 , an intermediate layer  1202 , and a top layer  1203 . The substrate layer  1201  may include, for example, a single crystal silicon, the intermediate layer  1202  may include, for example, a thermally grown silicon dioxide layer or other type of deposited oxide layer, and the top layer  1203  may include, for example, a single crystal silicon. Alternatively, the substrate layer  1201  may include, for example, a grown polysilicon structure layer, the intermediate layer  1202  may include, for example, a thermal oxide or low temperature oxide layer, and the top layer  1203  may include, for example, an epitaxially grown polycrystalline silicon or silicon-germanium layer. 
     In stage B 1 , alignment marks are provided to the top layer  1201 , as shown in  FIG. 10A . In stage B 2 , piezo sensing units  1205  are provided, as shown in  FIG. 10B . Here, in this instance, a piezoresistive material may be implanted to form piezoresistive sensing units  1205 , or alternatively, a piezoelectric material may be deposited to form piezoelectric sensing units  1205 . 
     In stage B 3 , the top layer  1201  is patterned to form sensing structure elements  1206  and an upper proof mass  1207 , as shown in  FIG. 10C . Here, the sensing structure elements  1206  have been patterned so that the piezo sensing units remain as part of the sensing structure elements  1206 . In this regard, a deep reactive ion trench etching may be employed. 
     In stage B 4 , the substrate layer  1201  is patterned, as shown in  FIG. 10D , to form the “backside” trenches  1208 , which define a lower proof mass  1209 . Hence, a part of the substrate layer  1201  is used to form an additional proof mass  1209 , which remains connected to the upper proof mass  1207  via intermediate layer  1202 . 
     In stage B 5 , a portion of the intermediate layer  1202  underneath the sensing structure elements  1206 , including a certain portion extending somewhat partially underneath the upper proof mass  1207 , is removed, as shown in  FIG. 10E , so as to release sensor structure elements  1206 , and the combined upper and lower proof mass structures  1207 / 1209 , which remain fixed together due to the unremoved portion of the intermediate layer that remains between them. In this regard, the upper proof mass  1207  does not include perforations so it will not be undercut completely and remain connected to the lower proof mass  1209 . Here, an HF vapor etching process may be used to removal the desired portion of intermediate layer. 
     In stage B 6 , contact pads  1210  are provided on the top layer  1201 , as shown in  FIG. 10F . Here, the contact pads  1210  may made of a metal material and be used, for example, to provide sensor output signals. In particular, the contact pads  1210  may be, for example, a sputtered deposited metal or shadow masked.