Patent Publication Number: US-2011073969-A1

Title: Sensor system and method for manufacturing same

Description:
RELATED APPLICATION INFORMATION 
     The present application claims priority to and the benefit of German patent application no. 10 2009 045 164.1, which was filed in Germany on Sep. 30, 2009, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a sensor system, which is intended for use in an aggressive measuring environment. It includes a sensor element having circuit elements integrated into the top side and a carrier for the sensor element. A sensor system of this type may thus include a sensor element, into whose surface entire circuit parts of an analysis circuit are integrated, or also a sensor element without an analysis circuit, in whose surface, for example, only piezoresistors for measured value detection having supply lines and terminal pads are implemented. Furthermore, the present invention relates to a method for manufacturing such a sensor system. 
     BACKGROUND INFORMATION 
       FIG. 1  shows a sensor system  10  for absolute pressure detection, as is known from practice. This sensor system  10  includes a micromechanical sensor element  1 , in whose top side a diaphragm  11  is implemented over a cavern  12 . Cavern  12  is closed in a pressure-tight manner on the rear. For this purpose, sensor element  1  was bonded to a glass carrier  2 . A defined reference pressure for the measurement pressure, which acts on the front side of diaphragm  11 , prevails within cavern  12 . The resulting diaphragm deformations are detected with the aid of piezoresistive transducer elements  13 , which are integrated into the diaphragm front side. In the exemplary embodiment shown here, the composite made of glass carrier  2  and sensor element  1  was installed on a circuit board  3  using an adhesive layer  4 . The electrical connection between sensor element  1  and circuit board  3  is formed by bonding wires  5 , which are led from the top side of sensor element  1  to circuit board  3 . In order to protect circuit elements  13  in the surface of sensor element  1  and bonding wires  5  against aggressive measuring media, sensor element  1  is embedded together with bonding wires  5  in a gel layer  7 , which is implemented inside a gel ring  6  on circuit board  3 . 
     Both the assembly and the installation of the known sensor system have proven to be problematic in practice. 
     The configuration of the circuit elements and the electrical terminals of the sensor element on the top side of the sensor element, which faces toward the measurement medium, requires that the electrical connection lines also come into contact with the measurement medium, if additional protective measures are not taken, such as embedding in a suitable protective gel. This form of “packaging” may only be implemented individually after the isolation of the sensor systems and after their installation. 
     The known sensor system may only be used in a limited pressure range because of the above-described assembly and connection technology. In addition, the known sensor system has a comparatively high lateral acceleration sensitivity because of the additional mass of the protective gel on the sensor diaphragm. Moreover, the media resistance and the temperature resistance of the sensor system are substantially a function of the type, composition, and resistance of the gel passivation. 
     SUMMARY OF THE INVENTION 
     The exemplary embodiments and/or exemplary methods of the present invention provide an assembly and connection technology for a sensor system of the type cited at the beginning; which is simple and robust and, after the isolation of the sensor elements, does not require any further packaging measures for protecting the circuit elements and electrical terminals of the sensor elements. 
     For this purpose, the carrier of the sensor system according to the present invention is provided with through contacts and the sensor element is installed in flip-chip technology on the carrier, so that the top side of the sensor element is at least regionally capped by the carrier and the circuit elements of the sensor element may be electrically contacted from the rear side of the carrier via the through contacts. 
     Accordingly, a carrier substrate having embedded through contacts and flip-chip connection technology is essential according to the present invention, and is used to create both a fixed composite made of sensor element and carrier and also a reliable electrical contact of the sensor element. According to the invention, the carrier thus has at least three functions. As in the related art, the carrier is used as an installation base of the sensor system. Furthermore, the carrier forms a cap for the sensor element surface and thus protects the circuit elements and any micromechanical structures in the surface of the sensor element. In addition, the sensor element is electrically contacted via the through contacts implemented in the carrier. 
     Fundamentally, there are various possibilities for the implementation of a sensor system according to the present invention with respect to the function of the sensor element. A sensor system according to the present invention may thus be designed, for example, as an acceleration sensor. The implementation of pressure sensors represents a particularly advantageous application of the assembly and connection technology according to the present invention. 
     Various materials and also various connection technologies may fundamentally be used for the sensor element and carrier in the scope of the present invention. However, these are to be adapted to one another. Thus, it proven to be advantageous with respect to the use of the sensor system, according to the present invention at varying ambient temperatures if the coefficient of thermal expansion of the carrier material is adapted to the coefficient of thermal expansion of the sensor element. For example, glass, or ceramic carriers, which are both cost-effective and simple to process, fulfill this condition for sensor elements based on silicon. 
     In addition, through contacts in the form of metal pins or silicon through contacts may be embedded easily and reliably in these carrier materials. Silicon through contacts are advantageously metal plated on both sides for better electrical connection. Furthermore, it is advantageous that standard methods are available to produce a reliable bond between a silicon sensor element and a glass/ceramic carrier, such as anodic bonding or seal glass gluing with the aid of a suitable glass solder. The electrical connection between the sensor element and the through contacts of the carrier is advantageously created by thermal compression bonding. The glass/ceramic carrier may be made of a multilayer ceramic system, so that printed conductors may be guided parallel to the surface in the interior of the carrier, enclosed on both sides by carrier material. A special corrosion protection is possible as a result of these embedded printed conductors. 
     As already noted, sensor systems according to the present invention are particularly suitable for pressure detection. Exemplary embodiments for absolute pressure detection and relative pressure detection are explained in greater detail hereafter in connection with the figures. 
     The claimed method for manufacturing sensor systems of this type for pressure detection provides that firstly the surface of a semiconductor wafer is processed in order to produce circuit elements for signal detection, such as piezoresistors having electrical supply lines, in the sensor surface of a plurality of sensor elements. Micromechanical structures may also be generated in and/or below the surface of the semiconductor wafer in the scope of the surface processing of the semiconductor wafer, in particular caverns as reference volumes. 
     The sensor diaphragms are produced in that the semiconductor wafer is thinned down to diaphragm thickness starting from the rear side at least in an area of the sensor elements. This may be performed, for example, by appropriate structuring of the wafer rear side. 
     Independently of the processing of the semiconductor wafer, a glass/ceramic carrier substrate, which is provided with through contacts, is structured in order to produce reference volumes or pressure connection openings for a plurality of sensor elements. The processed surface of the semiconductor wafer is situated aligned on the structured glass/ceramic carrier and installed, the mechanical connection being created by anodic bonding or seal glass gluing and the electrical contact of the sensor elements to the through contacts of the carrier substrate being created simultaneously, i.e., in one installation step, by thermal compression bonding. 
     In a particularly advantageous variant of the method according to the present invention, the sensor diaphragms are only manufactured after the manufacturing of the composite made of semiconductor wafer and carrier substrate, in that the semiconductor wafer is thinned over its entire area down to diaphragm thickness in the composite with the glass/ceramic carrier. For this purpose, the semiconductor wafer may simply be ground down, for example, to an SOI layer functioning as a grind stop layer, which is particularly cost-effective. 
     The sensor systems are only isolated thereafter by cutting apart the composite made of semiconductor wafer and glass/ceramic carrier, for example, by sawing. 
     At least a part of the circuit elements in the surface of the individual sensor elements is capped by the carrier. The requirement for further protective measures after the isolation process is dispensed with for these circuit elements. In the event of corresponding assembly of the sensor system, further packaging of the sensor elements after the isolation may even be completely dispensed with. The sensor systems may be extremely miniaturized, in particular if an integrated compensation or analysis circuit is dispensed with. 
     As already explained above, there are various possibilities for advantageously implementing and refining the teaching of the exemplary embodiments and/or exemplary methods of the present invention. For this purpose, reference is made to the Patent Claims subordinate to the independent Patent Claims, on the one hand, and to the following description of multiple exemplary embodiments of the present invention on the basis of the figures, on the other hand. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic sectional view of a sensor system  10  according to the related art. 
         FIG. 2   a  shows a schematic sectional view of a sensor system  20  according to the present invention for absolute pressure detection. 
         FIG. 2   b  shows a schematic sectional view of the composite made of a processed semiconductor wafer and a carrier substrate before the isolation of sensor systems  20  shown in  FIG. 2   a.    
         FIG. 3  shows a schematic sectional view of a sensor system  30  according to the present invention for relative pressure detection. 
         FIG. 4  shows a schematic sectional view of a second sensor system  40  according to the present invention for absolute pressure detection. 
         FIG. 5   a  shows a schematic sectional view of a third sensor system  50  according to the present invention for absolute pressure detection. 
         FIG. 5   b  shows a schematic sectional view of the composite made of a processed semiconductor wafer and a carrier substrate before the isolation of sensor systems  50  shown in  FIG. 5   a.    
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2   a  shows a sensor system  20 , made of a sensor element  21  and a carrier  22 . Sensor system  20  is used for absolute pressure detection. 
     In the present exemplary embodiment, carrier  22  is a glass carrier having silicon through contacts  23 , which are provided on both sides with metal plating for better contacting. A cavernous opening  24  is implemented in the surface of glass carrier  22  between through contacts  23 . 
     Sensor element  21  is implemented here in the form of a silicon substrate which is thinned down to diaphragm thickness over its entire area, and which is installed upside down in flip-chip technology on glass carrier  22 . Prior to the installation of sensor element  21 , piezoresistive transducer elements  25  having highly doped electrical supply lines  26 , which may be anodically bonded, were integrated into the surface of sensor element  21 . Piezoresistive transducer elements  25  are situated in the active diaphragm area above cavernous opening  24  and electrical supply lines  26  are guided outward to through contacts  23 . 
     The mechanical connection between sensor element  21  and glass carrier  22  was created by anodic bonding. Cavernous opening  24  was closed in a pressure-tight manner, so that a defined reference pressure prevails here. In the same installation step, the electrical connection between supply lines  26  and through contacts  23  was also created, in that the contact surfaces of sensor element  21  were connected using thermal compression bonding to the metal-plated contact pads of through contacts  23  of glass carrier  22 , which were situated in a matching manner. For this purpose, high-melting-point solders were used, which, to avoid cavities, may be flux-free and compatible with electrical supply lines  26 , which may be made of temperature-resistant alloys made of gold, platinum, palladium with tin or niobium for high-temperature applications. The supply lines are led via corresponding diffusion barriers at least in the silicon contact area. Depressions are implemented around the contacting pads of through contacts  23  in the surface of glass carrier  22 , in order to receive solder bumps  27 , which are bonded to the corresponding contact areas of sensor element  21  before the actual bonding procedure. 
     Transducer elements  25  of sensor element  21  may thus be electrically contacted via through contacts  23  from the rear side of glass carrier  22 . 
     Sensor element  21  was advantageously only thinned after the production of the composite with glass carrier  22 , and even before the isolation of sensor system  20  manufactured in the wafer composite, which is illustrated by  FIG. 2   b.    
     A glass wafer  220  having embedded highly doped silicon through contacts  23  is shown here, whose surface is structured so that a cavernous opening  24  is implemented between each two through contacts  23 , which are spaced apart corresponding to the diaphragm dimensions. 
     Independently of glass wafer  220 , circuit elements, piezoresistive transducer elements having electrical supply lines here, and a plurality of sensor elements  21  are integrated into the front side of a silicon wafer  210 , the positions of these circuit elements being adapted to the configuration of through contacts  23  in glass wafer  220 . 
     The processed front side of silicon wafer  210  was then anodically bonded against the structured surface of glass wafer  220 . The requirement for the anodic bond is that the surface of glass wafer  220  is smooth, for example, polished, and the front side of silicon wafer  210  is smooth and unpassivated in the bond areas. In this manner, the circuit elements in the front side of silicon wafer  210  are capped by glass wafer  220 . The electrical connections between the supply lines and through contacts  23  were also created during the bonding process, as previously explained. 
     In the exemplary embodiment described here, silicon wafer  210  was thinned over its entire area down to diaphragm thickness in the composite with glass wafer  220 , an SOI layer being able to be used as the grind stop boundary. The isolation of the sensor systems is only performed thereafter by cost-effective sawing of the glass-silicon sandwich, which is indicated here by sawing trenches  28 . Sensor system  20  shown in  FIG. 2   a  is the result of this isolation process. 
     In the case of sensor system  20 , the pressure is applied to the unprocessed rear side of sensor element  21 , so that the measurement medium does not come into contact with circuit elements  25  and  26  on the front side of sensor element  21 , which proves to be advantageous in particular in the case of aggressive measuring media. The thus implemented corrosion, protection of circuit elements  25  and  26  is also supplemented by a non-stick layer  29  on the open silicon and glass surfaces, by which deposits on sensor system  20  are prevented and the effects of icing may also be reduced. For high-usage temperatures, for example, non-stick layers made of silicon carbide or silicon nitride are suitable. Silanization of the surfaces is suitable for low-usage temperatures. 
     Sensor system  30  shown in  FIG. 3  essentially corresponds to sensor system  20 , but is designed differently thereto for relative pressure measurements. For this purpose, sensor system  30  includes a sensor element  31  which—like sensor element  21 —is implemented in the form of a silicon substrate thinned down to diaphragm thickness over its entire area. The diaphragm deformations are also detected here with the aid of piezoresistive transducer elements  35  having highly doped supply lines  36  which may be anodically bonded, and which are integrated into the top side of sensor element  31 . Sensor element  31  is installed upside down in flip-chip technology on a glass carrier  32  according to the exemplary embodiments and/or exemplary methods of, the present invention. In contrast to glass carrier  22 , this glass carrier is provided with a through opening  34  as a pressure connection opening, so that the active diaphragm area of sensor element  31  spanning through opening  34  may have pressure applied to it on both sides. Piezoresistive transducer elements  35  are situated in the active diaphragm area over through opening  34  and electrical supply lines  36  are led outward, where they are connected via solder bumps  37  to through contacts  33 , which are embedded in glass carrier  32 . As in the case of sensor system  20 , the mechanical connection between sensor element  31  and glass carrier  32  was created by anodic bonding. Two concentric connection areas result. One connection area is located between through opening  34  and through contacts  33 , and the second connection area extends outward from through contacts  33 . These two connection areas enclose the metal-plated contact pads of through contacts  33  and protect them against corrosion. A non-stick layer  39  on the open surfaces of sensor element  31  and carrier  32  forms an additional layer. 
     A second exemplary embodiment of a sensor system  40  according to the present invention, which is intended for the absolute pressure measurement, is shown in  FIG. 4 . This sensor system includes a sensor element  41 , in whose surface a diaphragm  411  was exposed by superficial micromechanical processing. Diaphragm  411  spans a closed cavern  412  in the substrate of sensor element  41 , in which a defined reference pressure for the absolute pressure measurement prevails. Piezoresistive transducer elements  45  having electrical supply lines  46 , which are led outward, are implemented in diaphragm  411 . Sensor element  41  is installed in flip-chip technology on a glass carrier  42  having through contacts  43  and a through opening  44  as a pressure connection opening for diaphragm  411  and contacted via solder bumps  47 . As in the case of sensor element  30 , two connection areas, which are implemented concentrically to one another, also protect the contacting pads of through contacts  43  and parts of an integrated analysis circuit, which is optionally implemented on the surface of sensor element  41 , against the attack of the measurement medium. 
     Sensor system  50  shown in  FIG. 5   a  is also conceived for the absolute pressure measurements. It includes a sensor element  51 , which is installed on a carrier  52  in flip-chip technology. As in the case of sensor element  20 , carrier  52  is a glass or ceramic carrier having embedded through contacts  53 , which are implemented here in the form of metal pins, however. Metal pins  53  open into depressions in the carrier top side, where they terminate flatly, and function on the carrier bottom side as contact pins for sensor system  50 . A cavernous opening  54  is implemented in the carrier surface between metal pins  53  and is closed in a pressure-tight manner by sensor element  51 , so that a defined reference pressure for the absolute pressure measurement prevails within cavernous opening  54 . 
     Sensor element  51  is a silicon chip, in whose top side a diaphragm  511  is implemented. This diaphragm  511  was exposed by structuring the chip rear side and accordingly has pressure applied to it via the chip rear side. The diaphragm deformations are also detected here with the aid of piezoresistive transducer elements  55 , which were integrated together with highly doped electrical supply lines  56  in the diaphragm surface. Piezoresistors  55  are equipped with a diffusion barrier and are thus high-temperature resistant. 
     Supply lines  56  are implemented, for example, in the form of polysilicon, gold, or platinum printed conductors. 
     The mechanical connection between the top side of sensor element  51  and the carrier top side was implemented here in the form of a glass-glued joint. The glass solder used for this purpose is just as high-temperature-capable as sensor element  51 , carrier  52 , and solder bumps  57 , which are situated in the depressions in the carrier top side and via which the electrical connection is created between supply lines  56  of sensor element  51  and metal pins  53  of carrier  52 . 
     Because of the high-temperature resistance of above-described sensor system  50 , additional heating resistors via which the sensor system may be heated in a targeted manner may be embedded in the glass carrier, for example. Organic deposits on the sensor system, which may occur after a long operating time in spite of non-stick layer  59 , may thus be burned away. 
     The composite shown in  FIG. 5   b  made of a processed silicon wafer  510  and a glass wafer  520  illustrates the manufacturing of sensor system  50  shown in  FIG. 5   a.    
     Metal pins  53  are embedded as through contacts in glass wafer  520 . In addition, the surface of glass wafer  520  is structured so that a cavernous opening  54  is implemented between each two metal pins  53 , which are spaced apart corresponding to the diaphragm dimensions. 
     In the exemplary embodiment shown here, the front side of silicon wafer  510  was processed in order to integrate circuit elements, piezoresistive transducer elements having electrical supply lines, and a plurality of sensor elements here, the positions of these circuit elements being adapted to the configuration of metal pins  53  and cavernous openings  54 . In addition, the rear side of silicon wafer  510  was structured by potassium hydroxide etching, diaphragms  511  of individual sensor elements  51  being exposed. 
     The front side of silicon wafer  510  was then glued on the structured surface of glass wafer  520 . This glued joint was created with the aid of a glass solder layer  60 . The electrical connections between the supply lines and metal pins  53  were also created in the same installation step. 
     Silicon wafer  510  may then also be thinned further over its entire area, for example, by grinding, in the composite with glass wafer  520 . 
     The isolation of the sensor systems by cost-effective sawing of the glass silicon sandwich is only performed thereafter, which is indicated here by sawing trenches  58 . Sensor system  50  shown in  FIG. 5   a  is the result of this isolation process. 
     The assembly and connection technology, which is also explained above on the basis of the examples, allows the implementation of very cost-effective, highly miniaturized sensor systems, which are insensitive to installation stress and are suitable for high temperatures, and which may be processed further in manifold ways, for example, by conductive gluing or soldering directly onto a circuit board. The sensor systems according to the present invention are therefore particularly suitable for use in rough measuring environments in the event of very restricted space conditions. The assembly and connection technology according to the present invention may advantageously be used, for example, for implementing absolute pressure sensors in the exhaust train of vehicles, differential pressure sensors in diesel particulate filters or catalytic converters, combustion chamber pressure sensors, pressure sensors in particulate filters of solid fuel heating systems, pressure sensors for turbines and media-resistant pressure sensors in liquids for measuring fill levels, flow rates, or pressure, for example, in oil pressure sensors.