Patent Publication Number: US-2023160767-A1

Title: Pressure sensor with trim resistors

Description:
BACKGROUND 
     The present disclosure relates to pressure sensors, and in particular, to a pressure sensor with a trim die. 
     A pressure sensor is configured to measure the pressure of a fluid. Pressure sensors can be absolute pressure sensors that measure a pressure of a first fluid compared to a reference pressure (typically a vacuum). Pressure sensors can also be differential pressure sensors that measure a difference in pressure between a first fluid and a second fluid. Pressure sensors can measure pressure in a variety of ways. For example, a pressure sensor can have one or more diaphragms that deform based on the pressure of a first fluid and/or a second fluid and one or more piezoresistive strain gauge sensors on the diaphragms can measure the strain in the diaphragms caused by the deformation of the diaphragms. 
     A pressure sensor can include piezoresistors arranged in a Wheatstone bridge configuration. The piezoresistors are configured such that each individual piezoresistor changes in different directions in response to an applied pressure. The change in resistance can be measured to determine the change in pressure. 
     SUMMARY 
     A pressure sensor includes a Wheatstone bridge circuit including a first resistor, a second resistor, a third resistor, and a fourth resistor having matching output characteristics. The pressure sensor further includes a first trim resistor in series with the Wheatstone bridge circuit, wherein the first trim resistor has output characteristics matching the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. The pressure sensor additionally includes a second trim resistor in parallel or a parallel loop with the Wheatstone bridge circuit, wherein the second trim resistor has output characteristics matching the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     A pressure sensor includes a Wheatstone bridge circuit including a first resistor, a second resistor, a third resistor, and a fourth resistor having matching output characteristics. The pressure sensor further includes a first trim resistor electrically coupled to the Wheatstone bridge circuit, wherein the first trim resistor has output characteristics matching the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. The pressure sensor additionally includes a second trim resistor electrically coupled to the Wheatstone bridge circuit, wherein the second trim resistor has output characteristics that are mismatched to the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     A pressure sensor includes a first sensor portion having a pressure die with piezoresistors located in a Wheatstone bridge configuration, and a second sensor portion having a trim die with a first trim resistor positioned on the trim die. The pressure die in the first sensor portion is electrically coupled to the trim die in the second sensor portion. The first sensor portion and the second portion are positioned apart from one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a circuit diagram of a pressure sensor including a Wheatstone bridge and trim resistors. 
         FIG.  2    is a side view of a first embodiment of a diaphragm wafer of the pressure sensor. 
         FIG.  3    is a cross-sectional view of the pressure sensor. 
         FIG.  4    is a circuit diagram of a trim die of the pressure sensor. 
         FIG.  5 A  is a side view of a second embodiment of a diaphragm wafer. 
         FIG.  5 B  is a side view of a first embodiment of a trim wafer. 
         FIG.  6 A  is a side view of a third embodiment of a diaphragm wafer. 
         FIG.  6 B  is a side view of a second embodiment of a trim wafer. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a circuit diagram of pressure sensor  10  including Wheatstone bridge  12  and trim resistors  14 .  FIG.  2    is a side view of diaphragm wafer  30  of pressure sensor  10 .  FIGS.  1 - 2    will be discussed together.  FIG.  1    shows pressure sensor  10 , including Wheatstone bridge  12 , trim resistors  14 , input voltage terminal  16 , positive voltage terminal  18 , negative voltage terminal  20 , and ground  22 . Wheatstone bridge  12  includes resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4  (referred to collectively as resistors R 1 -R 4 ). Trim resistors  14  include resistor RG, resistor RS, resistor RZ, and resistor RV.  FIG.  2    shows diaphragm wafer  30  having outer portion  32  and diaphragm  34 , and Wheatstone bridge  12 . 
     A circuit diagram of pressure sensor  10  is shown in  FIG.  1   . Pressure sensor  10  can be a high temperature pressure sensor. Pressure sensor  10  includes Wheatstone bridge  12  that forms a sensing portion of pressure sensor  10 . Wheatstone bridge  12  includes resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4 . Pressure sensor  10  also includes trim resistors  14  positioned in series and in parallel with Wheatstone bridge  12 . Trim resistors  14  include resistor RG, resistor RS, resistor RZ, and resistor RV. Resistor RG and resistor RS are positioned in series with Wheatstone bridge  12 , and resistor RZ and resistor RV are positioned in parallel with Wheatstone bridge  12 . 
     Pressure sensor  10  include input voltage terminal  16  that is electrically coupled to a voltage source (not shown in  FIG.  1   ). Input voltage terminal  16  receives a voltage from the voltage source. Pressure sensor  10  also includes positive output terminal  18  and negative output terminal  20 . A sensing device is electrically coupled to positive output terminal  18  and negative output terminal  20  to receive a signal from Wheatstone bridge  12 . Pressure sensor  10  also includes ground  22 . 
     Wheatstone bridge  12  includes resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4  arranged in a bridge circuit with resistor R 1  and resistor R 3  on a first leg of the bridge and resistor R 2  and resistor R 4  on a second leg of the bridge. Resistor RG and resistor RS of trim resistors  14  are arranged in series with resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4  of Wheatstone bridge  12 . Resistor RZ of trim resistors  14  is arranged in parallel with resistor R 1  of Wheatstone bridge  12 . Resistor RV of trim resistors  14  is arranged in parallel with resistor R 4  of Wheatstone bridge  12 . 
     A voltage is input into the circuit at input voltage terminal  16 . Input voltage terminal  16  is arranged in series with resistor RG and resistor RS. Current flows from input voltage terminal  16  through resistor RG and resistor RS. The current is then split between resistor R 1  and resistor R 3  of Wheatstone bridge  12 , resistor RZ, and resistor RV. The currents flowing through resistor R 1  and resistor RZ will then flow through resistor R 3  to ground  22 . The currents flowing through resistor R 4  and resistor RV will then flow through resistor R 2  to ground  22 . The output of pressure sensor  10  can be measured as an output voltage between positive output terminal  18  and negative output terminal  20 . 
     Wheatstone bridge  12  is an electrical circuit known in the art used in resistive based sensors. Resistors R 1 -R 4  of Wheatstone bridge  12  are positioned on diaphragm wafer  30 , shown in  FIG.  2   . Diaphragm wafer  30  includes outer portion  32  and diaphragm  34 . Outer portion  32  supports diaphragm  34 , which is configured to deflect under pressure. Resistors R 1 -R 4  are piezoresistors formed on diaphragm  34  and a voltage is applied across Wheatstone bridge  12 . Upon an input of pressure, diaphragm  34  will deform and cause the surface of diaphragm  34  to undergo a mechanical strain. The resistance of each of resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4  will experience a change in resistance in response to the induced mechanical strain in diaphragm  34 . The change in resistance will cause the output voltage being sensed at positive output terminal  18  and negative output terminal  20  to change in accordance with the change in resistance. This change in output voltage can be used to determine the pressure being applied to diaphragm  34 . 
     Wheatstone bridge  12  is configured such that the individual resistors R 1 -R 4  change in different directions in response to the applied pressure. Resistors R 1 -R 4  are made out of the same material and have the same thickness so they have the same output characteristics, however two of resistors R 1 -R 4  are oriented such that the resistance increases with applied pressure and the other two of resistors R 1 -R 4  are oriented such that the resistance decreases with applied pressure. The nominal voltage output of Wheatstone bridge  12  when no pressure is applied can be referred to as Vnull. When no pressure is applied, the nominal voltage output (Vnull) should be a known set value, which is typically zero. However, variations in the processes used to manufacture resistors R 1 -R 4  and Wheatstone bridge  12  can cause variation in the nominal voltage output (Vnull) of Wheatstone bridge  12 . The amount of voltage that changes with applied full scale pressure is referred to as span. Variations in the processes used to manufacture diaphragm wafer  30 , and specifically variations in the thickness of diaphragm  34 , can cause variations in span. When diaphragm wafer  34  is assembled in a sensor package, the assembly process can also cause variations in the nominal voltage output (Vnull) and span. 
     The piezoresistive effect in Wheatstone bridge  12  is responsible for the large change in resistance proportional to diaphragm  34  strain that allows Wheatstone bridge  12  to accurately sense slight changes in pressure. The piezoresistive effect is temperature sensitive and causes the span to change with temperature, which can be referred to as the span temperature coefficient (STC). Wheatstone bridge  12  is useful in that common mode changes in resistance, such as resistance temperature sensitivity, is rejected. However, in practice there is some temperature sensitivity that is not common mode and changes the output of Wheatstone bridge  12  with temperature, which can be referred as the zero temperature coefficient (ZTC). 
     The materials that are used for diaphragm  34  and Wheatstone bridge  12  of pressure sensor  10  are selected to be sensitive to strain. Resistors R 1 -R 4  will experience a large change in resistance with a small pressure change due to the sensitivity of the materials. This allows pressure sensor  10  to accurately sense slight changes in pressure, but it is also makes pressure sensor  10  susceptible to changes to the nominal voltage output (Vnull), span, span temperature coefficient (STC), and zero temperature coefficient (ZTC) based on variations caused by manufacturing and assembly. The nominal voltage output (Vnull), span, span temperature coefficient (STC), and zero temperature coefficient (ZTC) can be considered the sensor characteristics of pressure sensor  10 . 
     Applications of Wheatstone bridge  12  require fixed voltage outputs at zero pressure (Vzero) and at full scale pressure (Vfullscale) with a linear correlation between pressure in and voltage out between the voltage output at zero pressure (Vzero) and at full scale pressure (Vfullscale). Trim resistors  14  are used to transform the sensor characteristics (nominal voltage output (Vnull), span, span temperature coefficient (STC), and zero temperature coefficient (ZTC)) of pressure sensor  10 , which vary with process variation, into the fixed outputs that applications of Wheatstone bridge  10  require. Trim resistors  14  can be trimmed, for example by laser ablation, fuse blowing, diode blowing, by selecting precise discrete component values from an array of possible choices, or another suitable trimming process, to attenuate, or change, the sensor characteristics to correct for the variations caused by the process variations. 
     To correct for the four sensor characteristics, a minimum of four trim resistors  14  are needed. Resistor RG and resistor RS are trimmed to change the voltage being delivered to Wheatstone bridge  12  and thus affect the span of Wheatstone bridge  12 . Resistor RG changes the span, and resistor RS compensates for temperature variations of span, thus affecting the span temperature coefficient (STC). Resistor RV and resistor RZ are trimmed to change the nominal voltage output (Vnull) of Wheatstone bridge  12 . When no pressure is being applied to pressure sensor  10 , the nominal voltage output (Vnull) should be zero. Resistor RV changes the nominal voltage output (Vnull), and resistor RZ compensates for temperature variations of the nominal voltage output (Vnull), thus affecting the zero temperature coefficient (ZTC). 
     Resistor RG is matched to and has the same output characteristics as resistors R 1 -R 4  of Wheatstone bridge  12 . Specifically, resistor RG has the same temperature coefficient of resistance as resistors R 1 -R 4  of Wheatstone bridge  12 . Resistor RG can be trimmed to change the span of Wheatstone bridge  12 . Because resistor RG has the same temperature coefficient of resistance as resistors R 1 -R 4  of Wheatstone bridge  12 , resistor RG changes the voltage going into Wheatstone bridge  12  at all temperatures. This allows resistor RG to be trimmed to change the span of Wheatstone bridge  12  without introducing temperature sensitivity into Wheatstone bridge  12 . Resistor RG can be matched to resistors R 1 -R 4  in any suitable manner. For example, resistor RG can be manufactured out of the same materials and using the same manufacturing processes as are used to manufacture resistors R 1 -R 4  to match resistor RG to resistors R 1 -R 4 . 
     Resistor RS is a trim resistor that is purposely not matched to and has different output characteristics than resistors R 1 -R 4  of Wheatstone bridge  12 . Specifically, resistor RS has a lower temperature coefficient of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 . Resistor RS can be trimmed to temperature compensate the span of Wheatstone bridge  12 . Because resistor RS has a different temperature coefficient of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 , resistor RS temperature compensates the voltage going into Wheatstone bridge  12 . If the resistance of Wheatstone bridge  12  increases in resistance due to an increase in temperature, resistor RS can be designed to decrease in resistance due to an increase in temperature to temperature compensate the span of Wheatstone bridge  12 . Alternatively, resistor RS can be designed to have low to no temperature sensitivity to temperature compensate the span of Wheatstone bridge  12 . 
     Resistor RV is matched to and has the same output characteristics as resistors R 1 -R 4  of Wheatstone bridge  12 . Specifically, resistor RV has the same temperature coefficient of resistance as resistors R 1 -R 4  of Wheatstone bridge  12 . Resistor RV can be trimmed to change the nominal voltage output (Vnull) of Wheatstone bridge  12 . Because resistor RV has the same temperature coefficient of resistance as resistors R 1 -R 4  of Wheatstone bridge  12 , resistor RV will change the voltage going into Wheatstone bridge  12  at the same level at all temperatures. This allows resistor RV to be trimmed to change the nominal voltage output (Vnull) of Wheatstone bridge  12  without introducing temperature sensitivity into Wheatstone bridge  12 . In the embodiment shown in  FIG.  1   , resistor RV is parallel with resistor R 4 , and will thus effectively change resistor R 4  to change the nominal voltage output (Vnull). In alternate embodiments, resistor RV can be parallel to resistor R 1 , resistor R 2 , resistor R 3 , or resistor R 4  in Wheatstone bridge  12 . Resistor RV can be matched to resistors R 1 -R 4  in any suitable manner. For example, resistor RV can be manufactured out of the same materials and using the same manufacturing processes as are used to manufacture resistors R 1 -R 4  to match resistor RV to resistors R 1 -R 4 . 
     Resistor RZ is a trim resistor that is purposely not matched to and has different output characteristics than resistors R 1 -R 4  of Wheatstone bridge  12 . Specifically, resistor RZ has a lower temperature coefficient of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 . Resistor RZ can be trimmed to temperature compensate the nominal voltage output (Vnull) of Wheatstone bridge  12 . Because resistor RZ has a different temperature coefficient of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 , resistor RZ temperature compensates the nominal voltage output (Vnull) of Wheatstone bridge  12 . If the resistance of Wheatstone bridge  12  increases in resistance due to an increase in temperature, resistor RZ can be designed to decrease in resistance due to an increase in temperature to temperature compensate the nominal voltage output (Vnull) of Wheatstone bridge  12 . Alternatively, resistor RZ can be designed to have low to no temperature sensitivity to temperature compensate the nominal voltage output (Vnull) of Wheatstone bridge  12 . In the embodiment shown in  FIG.  1   , resistor RZ is parallel with resistor R 1 , and will thus effectively introduce temperature sensitivity into resistor R 1  to temperature compensate the nominal voltage output (Vnull). In alternate embodiments, resistor RZ can be parallel to any of resistor R 1 , resistor R 2 , resistor R 3 , or resistor R 4  in Wheatstone bridge  12 . 
     The embodiment of pressure sensor  10  shown in  FIG.  1    is one example of a circuit diagram for pressure sensor  10 . In alternate embodiments, trim resistors  14  can be positioned in different locations in the circuit and each trim resistor  14  shown in  FIG.  1    can include one or more resistors in alternate embodiments. For example, resistor RV and resistor RZ are typically much bigger than resistors R 1 -R 4  and a t-network can be used with the lower resistance components. When used in a t-network, resistor RV and resistor RZ are each in a parallel loop with one of resistors R 1 -R 4  of Wheatstone bridge  12 . 
     Traditionally, trim resistors are discrete components that are selected from an inventory and soldered to a board. When discrete components like that are used, thousands of discrete components are needed to be stored in inventory to properly trim Wheatstone bridge  10 . Further, the use of discrete components provides less control, as it is difficult to match the passive components to Wheatstone bridge  12 . Manufacturing trim resistors  14  according to the present disclosure eliminates the need for an inventory of thousands of discrete components. Further, trim resistors  14  can be matched to Wheatstone bridge  12 , which allows for greater control when attenuating and temperature compensating Wheatstone bridge  12 . 
     Trim resistors  14  can be included on the same die as Wheatstone bridge  12  or on a separate trim die, discussed below with respect to  FIGS.  3 - 4   . 
       FIG.  3    is a cross-sectional view of pressure sensor  10 .  FIG.  4    is a circuit diagram of trim die  82  of pressure sensor  10 .  FIGS.  3 - 4    will be discussed together.  FIG.  3    shows pressure sensor  10  that includes first sensor portion  50 , second sensor portion  52 , and feedthroughs  54 . First sensor portion  50  includes housing  60 , cavity  62  (which includes first cavity portion  62 A and second cavity portion  62 B), isolator  64 , filler material  66 , pressure transfer fluid  68 , pressure die  70  (which includes diaphragm wafer  30 ), and wire bonds  74 . Second sensor portion  52  includes trim board  80 , trim die  82 , and wire bonds  84 .  FIG.  4    shows trim die  82  that includes trim resistors  14 , input voltage terminal  16 , positive voltage terminal  18 , negative voltage terminal  20 , and bridge terminal  90 . Trim resistors  14  include resistor RG, resistor RS, resistor RZ, and resistor RV. 
     Pressure sensor  10  includes first sensor portion  50  that forms a sensing portion of pressure sensor  10  and second sensor portion  52  that forms a trim portion of pressure sensor  10 . Second sensor portion  52  is positioned away from first sensor portion  50 . Feedthroughs  54  extend between first sensor portion  50  and second sensor portion  52 . 
     First sensor portion  50  includes housing  60  that forms a body portion of first sensor portion  50 . Cavity  62  is formed in housing  60  and extends from a first end to a second end of housing  60 . Cavity  62  includes first cavity portion  62 A extending from a first end of housing  60 , and second cavity portion  62 B extending from first cavity portion  62 A to a second end of housing  60 . Isolator  64  is positioned on the first end of housing  60  over a first end of cavity  62 . Filler material  66  is positioned in second cavity portion  62 B of cavity  62 . Filler material  66  can be a glass seal in one embodiment. Pressure transfer fluid  68  is positioned in first cavity portion  62 A of cavity  62  between isolator  64  and filler material  66 . Pressure transfer fluid  68  can be oil in one embodiment. 
     Pressure die  70  is positioned in first cavity portion  62 A of cavity  62 . Pressure die  70  is positioned on filler material  66 . Pressure die  70  includes diaphragm wafer  30 . Feedthroughs  54  extend through filler material  66  in second cavity portion  62 B and into pressure transfer fluid  68  in first cavity portion  62 A. Wire bonds  74  extend between and electrically couple pressure die  70  to feedthroughs  54 . Pressure die  70  is hermetically sealed in first sensor portion  50  of pressure sensor  10  between isolator  64  and filler material  66 . 
     Second sensor portion  52  includes trim board  80  that forms a base of second sensor portion  52 . Trim die  82  is positioned on trim board  80 . Wire bonds  84  extend between and electrically couple trim die  82  to trim board  80 . Trim die  82  includes trim resistors  14 , input voltage terminal  16 , positive voltage terminal  18 , and negative voltage terminal  20 , as shown in  FIG.  4   . Trim resistors  14  include resistor RG, resistor RS, resistor RZ, and resistor RV. Resistor RG, resistor RS, resistor RZ, and resistor RV have the same characteristics as discussed above with respect to  FIG.  1   . Trim die  82  further includes bridge terminal  90 , which can be electrically coupled to pressure die  70  by feedthroughs  54 . 
     Resistors R 1 -R 4  of Wheatstone bridge  12  (shown in  FIGS.  1 - 2   ) are positioned on diaphragm wafer  30  of pressure die  70 . Pressure die  70  is positioned in first cavity portion  62 A of cavity  62  in housing  60  and surrounded with pressure transfer fluid  68 . As discussed above with respect to  FIGS.  1 - 2   , variations in the sensor characteristics of resistors R 1 -R 4  of Wheatstone bridge  12  can be caused due to the manufacturing and assembly of pressure sensor  10 . When first sensor portion  50  of pressure sensor  10  is fully assembled, resistors R 1 -R 4  are inaccessible, as they are enclosed in and hermetically sealed in first cavity portion  62 A of cavity  62  of housing  60 . This prevents resistors R 1 -R 4  from being able to be directly trimmed to compensate for variations caused by the assembly of first sensor portion  50  of pressure sensor  50 . Further, if trim resistors were positioned directly on pressure die  70 , they would also be inaccessible when first sensor portion  50  of pressure sensor  10  is fully assembled and any variation caused by the assembly of first sensor portion  50  could not be compensated for. The design of pressure sensor  10  shown in  FIGS.  3 - 4    resolves this issue, as trim resistors  14  will be accessible after first sensor portion  50  has been fully assembled. 
     Placing trim resistors  14  on trim die  82  and positioning it away from first sensor portion  50  allows for attenuation and temperature compensation of Wheatstone bridge  14  after first portion  50  has been fully assembled. Trim die  82  is made out the same materials as pressure die  70 , for example silicon, oxide, nitride, poly silicon, or nichrome, so that it has the same characteristics as pressure die  70 . Trim die  82  can be manufactured according to the same manufacturing process as pressure die  70 . This allows resistor RG and resistor RV on trim die  82  to be matched to and have the same output characteristics as resistors R 1 -R 4  of Wheatstone bridge  12 . For example, resistor RG and resistor RV can have the same temperature coefficient of resistance as resistors R 1 -R 4 . Further, resistor RG and resistor RV can be made out of the same material as, be doped to the same concentration as, and have the same thickness as resistors R 1 -R 4  of Wheatstone bridge  12 . Resistor RS and resistor RZ can be purposefully mismatched to and have different output characteristics than resistors R 1 -R 4  of Wheatstone bridge  12 . For example, resistor RS and resistor RZ can have different temperature coefficients of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 . Trim die  82  can be manufactured using any process that allows the output characteristics of resistor RG and resistor RV to be matched to resistors R 1 -R 4  of Wheatstone bridge  12 . Trim resistors  14  on trim die  82  can be trimmed, for example by laser ablation, fuse blowing, diode blowing, or another suitable trimming process. 
     Manufacturing trim die  82  out of the same materials and according to the same process as pressure die  70  ensures that trim die  82  and pressure die  70  will respond to temperature changes in the same way. Because trim die  82  is manufactured out of the same materials as pressure die  70 , it will be sensitive to strain. Trim die  82  is also advantageous, as it can be used to compensate for all four sensor characteristics of Wheatstone bridge  12 . Trim die  82  makes it easier and most cost effective to change and temperature compensate Wheatstone bridge  12 . 
     Further, trim die  82  will age in the same way and at the same rate as pressure die  70 . As pressure die  70  ages, some variation in the sensor characteristics of Wheatstone bridge  12  can occur. However, if trim die  82  ages in the same way and at the same rate as pressure die  70 , it will undergo the same variations and will not impart differences into the sensor characteristics of Wheatstone bridge  12 . 
       FIGS.  5 A- 6 B  discussed below show examples of diaphragm wafers and trim wafers that can be positioned on pressure die  70  and trim die  82 . The diaphragm wafers and trim wafers are designed to have similar components being made out of similar materials and having similar thicknesses to match the diaphragm wafer and the trim wafer. 
       FIG.  5 A  is a side view of diaphragm wafer  100 .  FIG.  5 B  is a side view of trim wafer  120 .  FIGS.  5 A- 5 B  will be discussed together below.  FIG.  5 A  shows Wheatstone bridge  12  (including resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4 ), diaphragm wafer  100  having outer portion  102  and diaphragm  104 , first insulating layer  110 , second insulating layer  112 , and protection layer  114 .  FIG.  5 B  shows trim resistors  14  (including resistor RG, resistor RS, resistor RZ, and resistor RV), trim wafer  120  including wafer portion  122 , first insulating layer  130 , second insulating layer  132 , and protection layer  134 . 
     Diaphragm wafer  100 , shown in  FIG.  5 A , is one embodiment of diaphragm wafer  30  that can form part of pressure die  70  of pressure sensor  10  shown in  FIG.  3   . Diaphragm wafer  100  includes outer portion  102  and diaphragm  104 . Outer portion  102  supports diaphragm  104 , which is configured to deflect under pressure. First insulating layer  110  is positioned on a first surface of outer portion  102  and diaphragm  104 . Resistor R 1 -R 4  of Wheatstone bridge  12  are positioned on a first surface of first insulating layer  110  over diaphragm  104 . Second insulating layer  112  is positioned on a first surface of first insulating layer  110  and first surfaces of resistors R 1 -R 4  of Wheatstone bridge  12 . Protection layer  114  is positioned on a first surface of second insulating layer  112 . 
     Trim wafer  120 , shown in  FIG.  5 B , is one embodiment of a trim wafer that can form part of trim die  82  of pressure sensor  10  shown in  FIG.  3   . Trim wafer  120  includes wafer portion  122 . First insulating layer  130  is positioned on a first surface of wafer portion  122 . Resistor RG, resistor RS, resistor RZ, and resistor RV of trim resistors  14  are positioned on a first surface of first insulating layer  130 . Second insulating layer  132  is positioned on a first surface of first insulating layer  130  and first surfaces of resistor RG, resistor RS, resistor RZ, and resistor RV of trim resistors  14 . Protection layer  134  is positioned on a first surface of second insulating layer  132 . Resistor RG and resistor RV are matched to resistors R 1 -R 4  of Wheatstone bridge  12  (the matched trim resistors), and resistor RS and resistor RZ are mismatched to resistors R 1 -R 4  of Wheatstone bridge  12  (the unmatched trim resistors). 
     Similar components of diaphragm wafer  100  and trim wafer  120  are made out of the same materials. Outer portion  102  and diaphragm  104  of diaphragm wafer  100  are made out of the same materials as wafer portion  122  of trim wafer  120 . Resistors R 1 -R 4  of Wheatstone bridge  12  of diaphragm wafer  100  are made out of the same materials and doped to the same concentration as resistor RG and resistor RV (the matched trim resistors) of trim resistors  14  of trim wafer  120 . First insulating layer  110  of diaphragm wafer  100  is made out of the same material as first insulating layer  130  of trim wafer  120 . Second insulating layer  112  of diaphragm wafer  100  is made out of the same material as second insulating layer  132  of trim wafer  120 . Protection layer  114  of diaphragm wafer  100  is made out of the same material as protection layer  134  of trim wafer  120 . 
     Further, similar components of diaphragm wafer  100  and trim wafer  120  have the same thickness. Outer portion  102  of diaphragm wafer  100  has the same thickness as wafer portion  122  of trim wafer  120 . Resistors R 1 -R 4  of Wheatstone bridge  12  of diaphragm wafer  100  have the same thicknesses as resistor RG and resistor RV (the matched trim resistors) of trim resistors  14  of trim wafer  120 . First insulating layer  110  of diaphragm wafer  100  has the same thickness as first insulating layer  130  of trim wafer  120 . Second insulating layer  112  of diaphragm wafer  100  has the same thickness as second insulating layer  132  of trim wafer  120 . Protection layer  114  of diaphragm wafer  100  has the same thickness as protection layer  134  of trim wafer  120 . 
     Resistor RS and resistor RZ (the unmatched trim resistors) can be made out of a different material than or doped to a different concentration than resistors R 1 -R 4  of Wheatstone bridge  12 . More specifically, resistor RS and resistor RZ can be made out of a different material than or doped to a different concentration than resistors R 1 -R 4  of Wheatstone bridge  12  so that resistor RS and resistor RZ have a different temperature coefficient of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 . 
     Diaphragm wafer  100  and trim wafer  120  can be manufactured according to the same process to match the materials and thicknesses of similar components on diaphragm wafer  100  and trim wafer  120 . Alternatively, diaphragm wafer  100  and trim wafer  120  can be manufactured using different processes that allow the materials and thicknesses of similar components of diaphragm wafer  100  and trim wafer  120  to be matched. 
     Having similar components of diaphragm wafer  100  and trim wafer  120  be matched allows diaphragm wafer  100  and trim wafer  120  to have the same drift characteristics. As the components of diaphragm wafer  100  and trim wafer  120  age, the output characteristics of diaphragm wafer  100  and trim wafer  120  will drift. Using the same materials and thicknesses for similar components of diaphragm wafer  100  and trim wafer  120  allows diaphragm wafer  100  and trim wafer  120  to have the same drift characteristics. This allows Wheatstone bridge  12  to stay in balance, which results in low to no output error drift. 
     In an alternate embodiment, trim wafer  120  can have extra layers and trim resistors  14  can be positioned on different layers. For example, resistor RG and resistor RV can be positioned on a matched doped silicon layer, and resistor RS and resistor RZ can be positioned on an unmatched Nichrome layer. 
       FIG.  6 A  is a side view of diaphragm wafer  140 .  FIG.  6 B  is a side view of trim wafer  160 .  FIGS.  6 A- 6 B  will be discussed together below.  FIG.  6 A  shows Wheatstone bridge  12  (including resistor R 1 , resistor R 2 , resistor R 3 , and resistor R 4 ), diaphragm wafer  140  having outer portion  142  and diaphragm  144 , insulating layer  150 , and protection layer  152 .  FIG.  6 B  shows trim resistors  14  (including resistor RG, resistor RS, resistor RZ, and resistor RV), trim wafer  160  including wafer portion  162 , insulating layer  170 , and protection layer  172 . 
     Diaphragm wafer  140  is another embodiment of diaphragm wafer  30  that can form part of pressure die  70  of pressure sensor  10  shown in  FIG.  3   . Diaphragm wafer  140  includes outer portion  142  and diaphragm  144 . Outer portion  142  supports diaphragm  144 , which is configured to deflect under pressure. Resistors R 1 -R 4  of Wheatstone bridge  12  are positioned in a top side of diaphragm  144 . Insulating layer  150  is positioned on first surfaces of outer portion  142  and diaphragm  144  and on first surfaces of resistors R 1 -R 4  of Wheatstone bridge  12 . Protection layer  152  is positioned on a first surface of insulating layer  150 . 
     Trim wafer  160  is another embodiment of a trim wafer that can form part of trim die  82  of pressure sensor  10  shown in  FIG.  3   . Trim wafer  160  includes wafer portion  162 . Resistor RG, resistor RS, resistor RZ, and resistor RV of trim resistors  14  are positioned in a top side of wafer portion  162 . Insulating layer  170  is positioned on a first surface of wafer portion  162  and first surfaces of resistor RG, resistor RS, resistor RZ, and resistor RV of trim resistors  14 . Protection layer  172  is positioned on a first surface of insulating layer  170 . Resistor RG and resistor RV are matched to resistors R 1 -R 4  of Wheatstone bridge  12  (the matched trim resistors), and resistor RS and resistor RZ are mismatched to resistors R 1 -R 4  of Wheatstone bridge  12  (the unmatched trim resistors). 
     Similar components of diaphragm wafer  140  and trim wafer  160  are made out of the same materials. Outer portion  142  and diaphragm  144  of diaphragm wafer  140  are made out of the same materials as wafer portion  162  of trim wafer  160 . Resistors R 1 -R 4  of Wheatstone bridge  12  of diaphragm wafer  140  are made out of the same materials and doped to the same concentration as resistor RG and resistor RV (the matched trim resistors) of trim resistors  14  of trim wafer  160 . Insulating layer  150  of diaphragm wafer  140  is made out of the same material as insulating layer  170  of trim wafer  160 . Protection layer  152  of diaphragm wafer  140  is made out of the same material as protection layer  172  of trim wafer  160 . 
     Further, similar components of diaphragm wafer  140  and trim wafer  160  have the same thickness. Outer portion  142  of diaphragm wafer  140  has the same thickness as wafer portion  162  of trim wafer  160 . Resistors R 1 -R 4  of Wheatstone bridge  12  of diaphragm wafer  140  have the same thicknesses as resistor RG and resistor RV (the matched trim resistors) of trim resistors  14  of trim wafer  160 . Insulating layer  150  of diaphragm wafer  140  has the same thickness as insulating layer  170  of trim wafer  160 . Protection layer  152  of diaphragm wafer  140  has the same thickness as protection layer  172  of trim wafer  160 . 
     Resistor RS and resistor RZ (the unmatched trim resistors) can be made out of a different material than or doped to a different concentration than resistors R 1 -R 4  of Wheatstone bridge  12 . More specifically, resistor RS and resistor RZ can be made out of a different material or doped to a different concentration than resistors R 1 -R 4  of Wheatstone bridge  12  so that resistor RS and resistor RZ have a different temperature coefficient of resistance than resistors R 1 -R 4  of Wheatstone bridge  12 . 
     Diaphragm wafer  140  and trim wafer  160  can be manufactured according to the same process to match the materials and thicknesses of similar components on diaphragm wafer  140  and trim wafer  160 . Alternatively, diaphragm wafer  140  and trim wafer  160  can be manufactured using different processes that allow the materials and thicknesses of similar components on diaphragm wafer  140  and trim wafer  160  to be matched. 
     Having similar components of diaphragm wafer  140  and trim wafer  160  be matched allows diaphragm wafer  140  and trim wafer  160  to have the same drift characteristics. As the components of diaphragm wafer  140  and trim wafer  160  age, the output characteristics of diaphragm wafer  140  and trim wafer  160  will drift. Using the same materials and thicknesses for similar components of diaphragm wafer  140  and trim wafer  160  allows diaphragm wafer  140  and trim wafer  160  to have the same drift characteristics. This allows Wheatstone bridge  12  to stay in balance, which results in low to no output error drift. 
     In an alternate embodiment, trim wafer  160  can have extra layers and trim resistors  14  can be positioned on different layers. For example, resistor RG and resistor RV can be positioned on a matched doped silicon layer, and resistor RS and resistor RZ can be positioned on an unmatched Nichrome layer. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A pressure sensor includes a Wheatstone bridge circuit including a first resistor, a second resistor, a third resistor, and a fourth resistor having matching output characteristics. The pressure sensor further includes a first trim resistor in series with the Wheatstone bridge circuit, wherein the first trim resistor has output characteristics matching the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. The pressure sensor additionally includes a second trim resistor in parallel or a parallel loop with the Wheatstone bridge circuit, wherein the second trim resistor has output characteristics matching the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     The pressure sensor of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Two of the first resistor, the second resistor, the third resistor, and the fourth resistor are oriented to increase in resistance due to an applied pressure, and two of the first resistor, the second resistor, the third resistor, and the fourth resistor are oriented to decrease in resistance due to the applied pressure. 
     The first trim resistor and the second trim resistor have a temperature coefficient of resistance that is the same as a temperature coefficient of resistance of the first resistor, the second resistor, the third resistor, and the fourth resistor. 
     The first trim resistor and the second trim resistor are made out of the same material and/or have the same thickness as the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     The first trim resistor can be trimmed to change a span of the Wheatstone bridge circuit, and the second trim resistor can be trimmed to change a nominal voltage output of the Wheatstone bridge circuit. 
     The pressure sensor further includes a third trim resistor in series with the Wheatstone bridge circuit, wherein the third trim resistor has output characteristics that are mismatched to the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge, and a fourth trim resistor in parallel with the Wheatstone bridge circuit, wherein the fourth trim resistor has output characteristics that are mismatched to the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     The third trim resistor and the fourth trim resistor have a temperature coefficient of resistance that is different than a temperature coefficient of resistance of the first resistor, the second resistor, the third resistor, and the fourth resistor. 
     The first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge are positioned on a pressure die, and the first trim resistor, the second trim resistor, the third trim resistor, and the fourth trim resistor are positioned on a trim die separate from the pressure die. 
     A pressure sensor includes a Wheatstone bridge circuit including a first resistor, a second resistor, a third resistor, and a fourth resistor having matching output characteristics. The pressure sensor further includes a first trim resistor electrically coupled to the Wheatstone bridge circuit, wherein the first trim resistor has output characteristics matching the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. The pressure sensor additionally includes a second trim resistor electrically coupled to the Wheatstone bridge circuit, wherein the second trim resistor has output characteristics that are mismatched to the output characteristics of the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     The pressure sensor of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The first trim resistor has a temperature coefficient of resistance that is the same as a temperature coefficient of resistance as the first resistor, the second resistor, the third resistor, and the fourth resistor, and the second trim resistor has a temperature coefficient of resistance that is different than the temperature coefficient of resistance as the first resistor, the second resistor, the third resistor, and the fourth resistor. 
     The first trim resistor is made out of the same material and/or has the same thickness as the first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge. 
     The first trim resistor can be trimmed to change a span or a nominal voltage output of the Wheatstone bridge circuit, and the second trim resistor can be trimmed to temperature compensate a span or a nominal voltage output of the Wheatstone bridge circuit. 
     The first resistor, the second resistor, the third resistor, and the fourth resistor of the Wheatstone bridge are positioned on a pressure die, and the first trim resistor and the second trim resistor are positioned on a trim die separate from the pressure die. 
     A pressure sensor includes a first sensor portion having a pressure die with piezoresistors located in a Wheatstone bridge configuration, and a second sensor portion having a trim die with a first trim resistor positioned on the trim die. The pressure die in the first sensor portion is electrically coupled to the trim die in the second sensor portion. The first sensor portion and the second portion are positioned apart from one another. 
     The pressure sensor of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The first sensor portion further includes a housing, and a cavity extending from a first end to a second end of the housing, wherein the pressure die is positioned in a first cavity portion of the cavity and surrounded by a pressure transfer fluid to hermetically seal the pressure die in the first sensor portion. 
     The pressure die further includes a diaphragm wafer having an outer portion and a diaphragm, a first insulating layer on first surfaces of the outer portion and the diaphragm, wherein the piezoresistors of the Wheatstone bridge are positioned on a first surface of the first insulating layer over the diaphragm, a second insulating layer on a first surface of the first insulating layer and first surfaces of the piezoresistors of the Wheatstone bridge, and a protection layer on a first surface of the second insulating layer. The trim die further includes a trim wafer having a wafer portion, a first insulating layer on a first surface of the wafer portion, wherein the first trim resistor is positioned on a first surface of the first insulating layer, a second insulating layer on a first surface of the first insulating layer and a first surface of the first trim resistor, and a protection layer on a first surface of the second insulating layer. The outer portion and the diaphragm of the diaphragm wafer are made out of the same material as the wafer portion of the trim wafer. The first insulating layer of the diaphragm wafer is made out of the same material and/or has the same thickness as the first insulating layer of the wafer portion. The piezoresistors of the Wheatstone bridge on the diaphragm wafer are made out of the same material and/or have the same thickness as the first trim resistor on the wafer portion. The second insulating layer of the diaphragm wafer is made out of the same material and/or has the same thickness as the second insulating layer of the wafer portion. The protection layer of the diaphragm wafer is made out of the same material and/or has the same thickness as the protection layer of the wafer portion. 
     The pressure die further includes a diaphragm wafer having an outer portion and a diaphragm, wherein the piezoresistors of the Wheatstone bridge are positioned in the diaphragm, an insulating layer on first surfaces of the outer portion and the diaphragm and first surfaces of the piezoresistors of the Wheatstone bridge, and a protection layer on a first surface of the insulating layer. The trim die further includes a trim wafer having a wafer portion, wherein the first trim resistor is positioned in the wafer portion, an insulating layer on a first surface of the wafer portion and a first surface of the first trim resistor, and a protection layer on a first surface of the insulating layer. The outer portion and the diaphragm of the diaphragm wafer are made out of the same material as the wafer portion of the trim wafer. The piezoresistors of the Wheatstone bridge on the diaphragm wafer are made out of the same material and/or have the same thickness as the first trim resistor on the wafer portion. The insulating layer of the diaphragm wafer is made out of the same material and/or has the same thickness as the insulating layer of the wafer portion. The protection layer of the diaphragm wafer is made out of the same material and/or has the same thickness as the protection layer of the wafer portion. 
     The trim die is manufactured using the same process as the pressure die. 
     The first trim resistor has a temperature coefficient of resistance that is the same as a temperature coefficient of resistance of the Wheatstone bridge. 
     The first trim resistor has a temperature coefficient of resistance that is different than a temperature coefficient of resistance of the Wheatstone bridge. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.