Patent Publication Number: US-2023146234-A1

Title: Fabrication Method of MEMS Transducer Element

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date under 35 U.S.C. § 119(a)-(d) of European Patent Application No. 21206693.0, filed on Nov. 5, 2021. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of fabricating a plurality of microelectromechanical (MEMS) transducer element and a microelectromechanical (MEMS) sensor arrangement as well as to a micromechanical (MEMs) transducer element for monitoring at least one measurand and for generating an electrical output signal correlated with the at least one measurand. 
     BACKGROUND 
     MEMs sensor arrangements are known in the art and comprise a transducer element for monitoring at least one measurand and generating an electrical output signal correlated with the at least one measurand. The medium, which is to be monitored, must gain access to defined sensitive elements of the sensor arrangement while it must be ensured that a potentially aggressive and/or humid environment does not damage and/or impair the remaining parts. This is in particular true for electronic components of the sensor arrangement. 
     Furthermore, component suppliers provide sensor components in a not yet fully assembled state to the original equipment manufacturers (OEM). Thus, providing a sealing that protects the electronic components must be facilitated. This sealing should allow for an automated assembly procedure performed outside the premises of the component supplier. 
     Such a MEMS sensor arrangement is known from EP 3 456 682 A1. A side cut view of the sensor arrangement  100  is shown in  FIG.  1   . The sensor arrangement  100  comprises a ceramic sensor  102  with a transducer element  104  mounted onto a substrate  106 . A channel  108  is provided in the substrate  106 . The medium to be monitored enters through the channel  108  to impinge onto the transducer element  104 . The ceramic sensor  102  further comprises a transducer substrate  110  and side wall electrodes  112 . Using contact pads  114 , the electrodes  112  are connected to electrical leads  116  that are provided on the surface of the substrate  106  to electrically connect the transducer element  104  to further electrical components  118  of the sensor arrangement  100 . 
     A solder seal  120  is provided between the transducer element  104  and the substrate  106  around the media channel  108  to seal and protect the electrical components  118 , the electrical leads  116 , the contact pads  114  and the electrodes  112  from the media channel  108 . In addition, a protective cover  122  made from plastics, ceramic, glass or from an electrically conductive material, is provided to protect the transducer element  102 . 
     Typically, the ceramic sensor  102  is fabricated by the sensor manufacturer which also provides a first level packaging and sealing of the sensor and it is then delivered to an OEM who takes care of the electrical connections and pressure port sealing. The sensor sealing and its connectivity to the electrical components  118  is still challenging, as it is time consuming and quality control still tedious. 
     SUMMARY 
     A method of fabricating a plurality of individual microelectromechanical transducer elements includes forming a plurality of microelectromechanical transducer elements on a wafer. Each microelectromechanical transducer element has a sensitive region with a membrane and a sensing element monitoring at least one measurand and generating an electrical signal correlated with the at least one measurand, and an electrical contact outputting the electrical signal. The method includes providing, for each microelectromechanical transducer element, a sealing structure around a sensitive region and an electrical connection connected to the electrical contact. The sealing structure and the electrical connection are made out of a reflow solder material. The method includes dicing the wafer to form individual microelectromechanical transducer elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood by reference to the following description taken in conjunction with the accompanying figures, in which reference numerals identify features of the invention. 
         FIG.  1    illustrates a schematic side cut view of a microelectromechanical (MEMS) sensor system according to the state of the art; 
         FIG.  2   a    illustrates a schematic view of a fabrication method of a microelectromechanical transducer element according to a first embodiment of the invention; 
         FIG.  2   b    illustrates a side view of a microelectromechanical transducer element according to a variant of the first embodiment of the invention; 
         FIG.  2   c    illustrates a top view of the microelectromechanical transducer element obtained with the fabrication method according to the first embodiment of the invention; 
         FIG.  2   d    illustrates a schematic view of a fabrication method of a microelectromechanical sensor arrangement according to a variant of the first embodiment of the invention; 
         FIG.  2   e    illustrates a schematic view of a fabrication method of a microelectromechanical sensor arrangement according to a second variant of the first embodiment of the invention; 
         FIG.  2   f    illustrates a schematic view of a fabrication method of a microelectromechanical sensor arrangement according to a third variant of the first embodiment of the invention; 
         FIG.  3   a    illustrates a schematic view of a fabrication method of a microelectromechanical transducer element according to a second embodiment of the invention; 
         FIG.  3   b    illustrates a schematic view of a microelectromechanical sensor arrangement according to a variant of the second embodiment of the invention; 
         FIG.  3   c    illustrates a schematic view of a microelectromechanical sensor arrangement according to a second variant of the second embodiment of the invention; 
         FIG.  4   a    illustrates a schematic view of a fabrication method of a microelectromechanical transducer element according to a third embodiment of the invention; 
         FIG.  4   b    illustrates a schematic view of a fabrication method of a microelectromechanical sensor arrangement according to a variant of the third embodiment of the invention; 
         FIG.  5   a    illustrates a schematic view of a fabrication method of microelectromechanical transducer element fabricated according to a fourth embodiment of the invention; 
         FIG.  5   b    illustrates a schematic view of a microelectromechanical sensor arrangement according to a variant of the fourth embodiment of the invention; 
         FIG.  6   a    illustrates a schematic view of a fabrication method of microelectromechanical transducer element fabricated according to a fifth embodiment of the invention; 
         FIG.  6   b    illustrates a schematic view of a microelectromechanical sensor arrangement according to a variant of the fifth embodiment of the invention; 
         FIG.  7   a    illustrates a schematic view of a transducer element fabricated according to a sixth embodiment of the invention; 
         FIG.  7   b    illustrates a schematic view of a microelectromechanical sensor arrangement according to a variant of the sixth embodiment of the invention; 
         FIG.  8   a    illustrates a schematic view of a transducer element fabricated according to a seventh embodiment of the invention; 
         FIG.  8   b    illustrates a schematic view of a microelectromechanical sensor arrangement fabricated according to a variant of the seventh embodiment of the invention; 
         FIG.  8   c    illustrates a schematic view of a microelectromechanical sensor arrangement fabricated according to a second variant of the seventh embodiment of the invention; 
         FIG.  8   d    illustrates a schematic view of a microelectromechanical sensor arrangement fabricated according to a third variant of the seventh embodiment of the invention; and 
         FIG.  9    illustrates a schematic view of a microelectromechanical sensor system according to an eighth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT(S) 
       FIG.  2   a    shows a schematic diagram of a method of fabrication of a plurality of individual microelectromechanical transducer elements according to a first embodiment of the invention. The method comprises a step a) of realizing a plurality of individual transducer elements  202   a ,  202   b ,  202   c  on a wafer  200 , e.g. a silicon wafer, using a microelectromechanical device fabrication process as known in the art. The individual microelectromechanical transducer elements  202   a ,  202   b ,  202   c  of the plurality of microelectromechanical transducer elements all have the same structural features. Here, only transducer element  202   a  will be described in detail, transducer elements  202   b ,  202   c  like all other transducer elements on the wafer  200  are realized in the same way. 
     The transducer element  202   a  comprises a sensitive region  204  for monitoring at least one measurand and generating an electrical signal correlated with the at least one measurand, and one or more electrical contacts  206   a ,  206   b  for outputting the electrical signal. The sensitive region  204  comprises a membrane  208 , also called diaphragm, carrying a plurality of sensing elements  210   a ,  210   b  electrically connected to the one or more electrical contacts  206   a ,  206   b . The membrane  208  of the sensitive region  204  is provided above a cavity  212  in the wafer  200 . In the embodiment illustrated, the sensing elements  210   a ,  210   b  are facing the inner cavity  212 . In an alternative, they could be provided on the other side of the membrane  208  thus facing away from the cavity  212 . The sensing elements  210   a ,  210   b  can be strain gauges or capacitive structures. 
     In the embodiment shown in  FIG.  2   a   , the sensing elements  210   a ,  210   b  are provided between the membrane  208  and the inner cavity  212 . In an alternative, the sensing elements  210   a ,  210   b  can also be provided above the membrane  208 , on the surface  226  of the wafer. 
     The electrical contacts  206   a ,  206   b  are electrically connected with the sensing elements  210   a ,  210   b  respectively. An insulating or non-conductive layer  218  is deposited at least partially over the electrical contacts  206   a ,  206   b  respectively. A portion  220   a ,  220   b  of the electrical contacts  206   a ,  206   b  remains uncovered. 
     According to the invention, the method then comprises a step b) shown in  FIG.  2   a    of providing, for each microelectromechanical transducer element  202   a ,  202   b ,  202   c , a sealing structure  222  around its sensitive region  204  and electrical connections  224   a ,  224   b  electrically connected respectively to the electrical contacts  206   a ,  206   b.    
     According to the invention, the sealing structure  222  and the electrical connections  224   a  and  224   b  are made out of the same material, in particular a solder material, more in particular a reflow solder material. Thus, the sealing structure  222  and the electrical connections  224   a ,  224   b  can be realized during the same process step. To realize the sealing structure  222  and the electrical connections  224   a ,  224   b , a layer of a solder material is deposited on the wafer  200  and then patterned e.g. using screen printing or photolithography techniques known in the art. Alternatively, bumping or electrolytic metal deposition techniques can be used as well. As a result, the process is less complex and can be realized faster since only one step is required for providing both the sealing structure  222  and the electrical connections  224   a ,  224   b  for the plurality of transducer elements  202   a ,  202   b ,  202   c  on the wafer  200 . 
     In this embodiment, the sealing structure  222  and the electrical connections  224   a ,  224   b  are provided on the same surface side  226  of the wafer  200 , namely the side with the membrane  208  of the transducer elements  202   a ,  202   b ,  202   c . The sealing structure  222  and the electrical connection structure  224   a ,  224   b  are separated by a gap  228  to be electrically isolated from each other. According to a variant of the invention, the step b) can comprise providing the sealing structure  222  and the electrical connection structure  224   a ,  224   b  over opposite sides of the wafer  200 . In this variant, the electrical contacts  206   a ,  206   b  can be arranged further away from the potentially aggressive and/or humid environment provided by the measurand entering the device. 
     The sealing structure  222  is provided at least partially over the insulating layer  218  to be electrically isolated from the electrical contacts  206   a ,  206   b  of the transducer element  202   a . In contrast thereto, the electrical connections  224   a ,  224   b  are arranged at least partially on the free portion  220   a ,  220   b  of the electrical contacts  206   a ,  206   b  to realize an electrical contact with the electrical contacts  206   a ,  206   b . The sealing structure  222  is realized such that it surrounds the active region  204  of the membrane  208 . 
     Then, according to step c) of the inventive method shown in  FIG.  2   a   , the wafer  200  is diced to form individual microelectromechanical transducer elements. Here only the individual transducer element  230   a  is illustrated. In the following a wafer  200  that has been diced, will be called a transducer substrate  214 . 
       FIG.  2   b    illustrates a transducer element  230   b  according to a variant. The only difference with respect to the transducer element  230   a  of  FIG.  2   a    is a different shape of the cavity  212   a . In this variant, a protrusion  201  remains after realizing the cavity  212   a  in the wafer  200 . The protrusion extends into the cavity  212   a  towards the membrane  208 . The protrusion  201  can have varying heights hp and widths wp. Typically wp is less than the distance  208   a  between the sensing elements  210   a ,  210   b . The protrusion  201  allows limiting the extent of the flexure of the membrane  208 . 
       FIG.  2   c    is a schematic view onto an active surface side  226  of the transducer element  230   a . As illustrated, the sealing structure  222  surrounds the sensitive region  204  of the transducer element  230   a . The sealing structure  222  in this embodiment has a square-shape. However, other forms, like a ring shape, could be used as long as the sealing structure  222  surrounds the sensitive region  204 . The electrical connections  224   a ,  224   b ,  224   c    224   d  are positioned on the electrical contacts  206   a ,  206   b ,  206   c ,  206   d  spaced apart by a gap  228  from the sealing structure  222  via the insulating layer  218 . 
     In  FIG.  2   c   , four electrical connections  224   a ,  224   b ,  224   c  and  224   d  on four electrical contacts  206   a ,  206   b ,  206   c ,  206   d  are illustrated. However, more or less connection structures and electrical contacts of different shape and size may be used depending on the requirements and the number of sensing elements used. 
     According to the invention, the individual microelectromechanical transducer element  230   a  comprises a sealing structure  222  and electrical connections  224   a ,  224   b ,  224   c  and  224   d  on the active surface side  226  made of the same material and realized already during the MEMS level process steps at the OEM prior to dicing and not on packaging level at the customer site. This simplifies the integration of the transducer element  230   a  at the customer site. 
     According to a first variant of the embodiment according the invention illustrated in  FIG.  2   d   , the manufacturing process continues after step c) with a step d) of providing a transducer element  230   a  and a substrate  232 , e.g. at a customer site. The substrate  232  can be chip carrier, in particular a ceramic chip carrier, a PCB, a flexible circuit board, a leadframe or the like. 
     The substrate  232  comprises at least one media channel  234  that extends through the substrate  232 . The substrate  232  comprises further contact pads  236   a ,  236   b ,  236   c  provided on a surface  238  of the substrate  232 . The contact pads  236   a ,  236   b ,  236   c  are made of a conductive material, in particular metal. 
     In the subsequent step e), the transducer element  230   a  is positioned on the substrate  232 . Here, the electrical connection  224   a  is aligned with the contact pad  236   a  and the sealing structure  222  is aligned with the contact pad  236   b  on the one side of the channel  234  of the substrate  232 . On the other side of the channel  234 , the sealing structure  222  and the electrical connection  224   b  are aligned with the contact pad  236   c.    
     Subsequently, a soldering step is realized, illustrated by step f) in  FIG.  2   d   , to thereby seal the media channel  234  from the electrical connections  224   a  and  224   b  with the sealing structure  222 . At the same time, the transducer element  230   a  is electrically connected to the substrate  232  via the electrical connections  224   a  and  224   b.    
     The soldering in step f) may be performed by a reflow soldering technique during which the substrate  232  and the transducer element  230   a  are heated beyond the fusion point of the solder material used for the sealing structure  222  and the electrical connection structures  224   a ,  224   b . After cooling down, reliable solder connections  240 ,  242 ,  244  are established between the substrate  232  and the transducer element  230   a , in particular between the contact pad  236   a ,  236   b ,  236   c  of the substrate  232  and the sealing structure  222  and the electrical connection structures  224   a ,  224   b  of the transducer elements  230   a.    
     After step f) a MEMs sensor arrangement  250   a  is obtained that realizes reliable electrical contacts and a reliable protection of the parts of the sensor that are outside the media channel  234 . 
     As shown in step f) of  FIG.  2   d   , the solder connection  240  for sealing between the contact pads  236   b  and the sealing structure  222  is separated from the solder electrical connection  242  between the contact pad  236   a  and the electrical connection structure  224   a . To the contrary, the solder connection  244  electrically connects the sealing structure  222  and the electrical connection structure  224   a  via the contact pad  236   c . According to an alternative, the sealing structure  222  may be electrically isolated from all sensing elements  210   a ,  210   b , but connected to ground using an additional contact pad on the substrate  232 . The formed solder connections  240 ,  242 ,  244  form the electrical connections as well as provide a secure sealing against the ingress of humidity and/or aggressive chemicals coming from the media channel  234  into the interface between the transducer element  230   a  and the substrate  232 . 
     The method provides a plurality of transducer elements  302   a  on wafer level with a channel  234  for a measurand. With such a transducer further integrated pressure sensors can be realized and/or differential pressure sensors having media channels on both sides of the measuring membrane  208  can be realized. 
     A second variant of the invention is shown in  FIG.  2   e    illustrating a MEMs sensor arrangement  250   b  comprising a transducer element  230   b  connected to a substrate  232 . The only difference between this variant and the transducer element  230   a  and MEMs sensor arrangement  250   a , illustrated in  FIG.  2   d    is a different electrical connection between the sealing structure  222  and the electrical connection  224   b . All other features remain the same and reference made is to the description above. 
     Instead of realizing the electrical connection between the sealing structure  222  and the electrical connection  224   b  on the substrate  232  side using contact pad  236   c  as shown in  FIG.  2   d   , the electrical connection is realized on the transducer element  202   b  side using an electrically conductive layer  248 . 
     The electrically conductive layer  248  also electrically connects the electrical contact  206   b  with the electrical connection  224   b . The insulating layer  218  remains present between the electrical contact  206   a  and the sealing structure  222 . 
     The solder connection  240  with the substrate  232  is then realized between the sealing structure  222  and contact pad  236   b  and the solder electrical connection  242  is realized between the electrical connection structure  224   a  and the contact pad  236   a  and the electrical connection structure  224   b  and an additional contact pad  236   d.    
     The electrically conductive layer  248  allowing the electrical connection between the sealing structure  222  and the electrical connection  224   b  can be provided at least partially around the media channel  234  or even extend entirely around it. In this case, the isolating layer  218  is arranged such that an electrical isolation between the sealing structure  222  and the other electrical connections  224   a ,  224   c  and  224   d  are guaranteed. 
     A third variant of the invention is shown in  FIG.  2   f    illustrating a MEMs sensor arrangement  250   c  comprising a transducer element  230   c  connected to a substrate  232 . The only difference between this variant and the transducer element  230   b  and MEMs sensor arrangement  250   b  illustrated in  FIG.  2   e    is a different electrical connection between the electrical connection  224   a  and the electrical contact pad  206   a  of the sensing element  210   a . All other features remain the same and reference is made is to the description above. 
     In this variant, the electrical connection structure  224   a  is not directly connected to the electrical contact  206   a  like in the other embodiments but via an electrically conductive layer  250 , which can be realized at the same time as the electric conductive layer  248 . Like in the other embodiments, the electrical connection structure  224   a  is electrically isolated from the sealing structure  222  using the insulating layer  218 . 
     In use, a measurand from a measurement volume enters the MEMs sensor arrangement  250   a  or  250   b  via the media channel  234 . The membrane  208  of the transducer element  230   a  deforms under the pressure difference between the measurand and the pressure in the cavity  212 . The deformation or stress is sensed by the sensing elements  210   a  and  210  and electrical signals proportional to the pressure are output via the contact pads  236   a  and  236   b  to be treated in the further electrical components, like components  118  shown in  FIG.  1   . 
     Using the same solderable material as sealing structure and as electrical connection structure allows realizing the sealing and the electrical connection step of the transducer element with the substrate during the same manufacturing step at the transducer element level. Thus, the assembly process can be shortened and facilitated. 
     In addition, by providing the electrical connections  224   a ,  224   b  below the membrane  208 , it is no longer necessary to provide electrical contacts  112  on the side of the like in the prior art as shown in the art. This allows reducing the size of the final transducer element  230   a.    
     In a third variant of the embodiment, not shown, the steps d), e) and f) are performed before step c) of dicing the wafer  200 . In this variant, a substrate is provided that comprises a plurality of channels corresponding to the number of transducer elements present on the wafer. 
     For the following embodiments of the invention and their variants and alternatives, the features in common with the first embodiment and its variants and alternatives will not be described in detail again, but reference is made to their description above and the same reference numbers will be used. 
       FIG.  3   a    illustrates a fabrication method of a MEMS transducer element according to a second embodiment of the invention. 
     In this embodiment, the step a) of realizing a plurality of microelectromechanical transducer elements  302   a ,  302   b ,  302   c  on a wafer  200  comprises the additional patterning process step a 1 ) of realizing a groove  304 , in an embodiment, for each transducer element  302   a  of the plurality of transducer elements realized on the wafer  200 . All the other features of the transducer element  302   a  are the same as the features of the transducer element  202   a  described in the first embodiment, and reference is made to their description in the first embodiment. Where applicable, the same reference numbers will be used. 
     Step a 1 ) is realized before step b). The groove  304  is realized around the sensitive region  204  of the transducer element  302   a , on the active surface side  226  of the transducer element  302   a . The minimum width w g  of the groove  304  is set by the limits of the manufacturing process, and can typically range from 10 μm to 400 μm. The minimum depth t g  of the groove  304  is deeper than the thickness t s  of the sensitive region  204 . A groove  304  having a depth larger than the thickness t s  of the sensitive region  204  will provide better stress isolation. The groove  304  separates the electrical contacts  206   a ,  206   b  in two parts  206   a _ 1  and  206   a _ 2 ,  206   b _ 1  and  206   b _ 2 , one part on each side of the groove  304 . The groove  304  also separates the insulating layer  218  in two parts  218   a _ 1  and  218   a _ 2 ,  218   b _ 1  and  218   b - 2 , one on each side of the grove  304 . 
     To maintain the electrical connection between the two parts of the electrical contacts  206   a _ 1  and  206   a _ 2 ,  206   b _ 1  and  206   b _ 2 , respective electrically conductive layers  306   a  and  306   b  are deposited on the side walls  308  of the groove  304  during step a 2 ) as illustrated in  FIG.  3   a   . The electrically conductive layers  306   a ,  306   b  can be metallic layers, e.g. an aluminum or copper layer. The electrically conductive layers  304 ,  306   b  are partially deposited within the groove  304  in order to avoid creating an electrical short circuit between the electrical contact pads  206   a ,  206   b ,  206   c ,  206   d  of the transducer element  302   a . The electrically conductive layer  304   a ,  306   b  is deposited within the groove  304  so as to provide an electrical connection with the electrical contact pads  206   a _ 1 ,  206   b _ 1  locally. 
     For this embodiment, the steps b) and c) are realized in the same way as in the first embodiment and its variants and alternatives. However, as illustrated in step b 1 ) of  FIG.  3   b   , the sealing structure  222  is arranged around the sensitive region  204  on the outer side of the groove  304 . Thus, both the sealing structure  222  and the electrical connections  224   a ,  224   b  on the electrical contacts  206   a _ 2  and  206   b _ 2  are arranged on the outer side of the groove  304 . Indeed, since both the sealing structure  222  and the electrical connections  204  can negatively affect the transducer element  302   a , the groove  304  is positioned so as to decouple both areas. The groove  304  is positioned between the sensitive area  204  of the transducer element  302   a  and both the sealing structure  222  and the electrical connections  224   a ,  224   b , in order for the groove  304  to serve as an outside stress decoupling feature to reduce the influence of external stress onto the sensing elements  210   a ,  210   b . The groove  304  therefore leads to a more reliable transducer element with a reduced sensibility to outside vibrations at the transducer element level. 
     Furthermore, the sealing structure  222  is deposited on the insulating layer  218   a _ 2 ,  218   b _ 2  and spaced apart from the electrically conductive layer  306   a ,  30   b , such that a portion  218   c  of the insulating layer  218   a _ 2 ,  218   b _ 2  is not covered by the sealing structure  222 . Thus, an electrical contact between the sealing structure  222  and the electrically conductive layer  306   a ,  306   b  of the groove  304  can be prevented. In this embodiment, the electrically conductive layer  306   a ,  306   b , the sealing structure  222  and the electrical contact  224   a ,  224   b  can be made of the same material. In addition, since the groove  304  is integrated into the wafer  200 , the transducer element  302   a  offers a compact design. 
     Thus, after step c), a transducer element  330   a  is obtained having all the features of the transducer element  230   a  of the first embodiment but in addition, the stress decoupling feature in the form of the groove  304 . 
       FIG.  3   b    illustrates the transducer element  330   a  soldered to substrate  232  to obtain a MEMs sensor arrangement  350   a  according to a first variant of the second embodiment. The process steps to obtain the MEMS sensor arrangement  350   a  correspond to the steps d) to f) of the first variant of embodiment 1. 
     A second variant is shown in  FIG.  3   c    illustrating a MEMs sensor arrangement  350   b  comprising a transducer element  330   b  connected to a substrate  232 . The only difference between this variant and the transducer element  330   a  and MEMs sensor arrangement  350   a , illustrated in  FIG.  3   b    is a different electrical connection between the sealing structure  222  and the electrical connection  224   b  and between the electrical connection  224   a  and the electrical contact  206   a _ 2 . All other features remain the same and reference made is to the description above. 
     Instead of realizing the electrical connection between the sealing structure  222  and the electrical connection  224   b  on the substrate  232  side using the contact pad  236   c  as shown in  FIG.  2   d   , the electrical connection is realized on the transducer element  302   b  side using the electrically conductive layer  306   b  present within the groove  304 . 
     In this variant, the sealing structure  222  is deposited on top of the insulating layer  218   b _ 2  but in contact with the electrically conductive layer  306   b , and thus is also electrically connected with the electrical contact  206   b _ 2  and with the electrical connection  224   b . The insulating layer  218   b _ 2  remains, however, present between the electrical contact  206   b _ 2  and the sealing structure  222 . According to a variant, a layer  218  could be present like in  FIG.  2     f.    
     The solder connection  240  with the substrate  232  is then realized between the sealing structure  222  and contact pad  236   b  and the solder electrical connection  242  between the electrical connection structure  224   a  and the contact pad  236   a  and the electrical connection structure  224   b  and an additional contact pad  236   d . The solder connection  240  extends around the media channel  234 . 
       FIG.  4   a    illustrates a fabrication method of a MEMS transducer element according to a third embodiment of the invention. 
     In this embodiment, the step a) of realizing a plurality of microelectromechanical transducer elements on a wafer  200  comprises additional process steps of providing vias  404   a ,  404   b , e.g. so called through silicon vias (TSV), through the wafer  200  of the transducer element  402   a ,  402   b ,  402   c.    
     First corresponding through holes are realized through the wafer  200  which are then filled with an electrically conductive material, in particular metal. All the other features of the transducer element  402   a  are the same as the features of the transducer element  202   a  described in the first embodiment, and reference is made to their description in the first embodiment. In addition, the same reference numbers are used where appropriate. 
     The vias  404   a ,  404   b  are positioned such as to allow an electrical connection with the electrical contacts  206   a ,  206   b  on the opposite surface side  246 , opposite to the active surface side  226 . 
     During step b) of this embodiment, the electrical connections  224   a ,  224   b  and the sealing structure  222  are then realized on opposite surface sides of the wafer  200 , i.e. of the transducer element  402   a.    
     As shown in step b) of  FIG.  4   a   , the sealing structure  222  is provided on the surface side  226  of the transducer element  202   a  where the sensitive region  204 , i.e. the membrane  208 , is provided. 
     The electrical connection structures  224   a ,  224   b  are provided on the opposite side  246  of the active surface side  226  of the transducer element  202   a , in direct contact with the vias  404   a ,  404   b  respectively. 
     After dicing, as shown in step c), an individual transducer element  430   a  is obtained. 
     This transducer element  430   a  can then be mounted to two different substrates  432  and  432 ′ as illustrated in  FIG.  4   b   , to realize a MEMs sensor arrangement  450  according to a variant of the third embodiment. The transducer element  430   a  is mounted on its active surface side  226  to a substrate  432  using the sealing structure  222 . Furthermore, the transducer element  430  is mount to the substrate  432 ′ using the electrical connections  224   a ,  224   b  on its other surface side  246 . The transducer element  430   a  is thus sandwiched between two substrates  432 ,  432 ′. 
     The substrate  432  comprises a media channel  434 , like substrate  232  and electrical conductive pad  436  so that the sealing structure  222  can be attached using soldering like in the first and second embodiment. 
     The second substrate  432 ′ comprises electrical conductive pads  436   a ,  436   b  to realize the electrical connections with the electrical contacts  206   a  and  206   b.    
     By arranging, the sealing on the one side and the electrical connections on the other side a more compact design can be realized and, in addition, the electrical components can be arranged further away from the media channel  434 . 
       FIG.  5   a    illustrates a schematic view of a fabrication method of microelectromechanical (MEMs) transducer elements  502   a ,  502   b ,  502   c  fabricated according to a fourth embodiment of the invention. 
     In this embodiment, an additional process step is realized during step a) to provide a media channel  534  for each transducer element  502   a ,  502   b ,  502   c  in the wafer  200 . The media channel  534  is realized such that it extends from the opposite surface side  246  with respect to the active surface side  226  up until the cavity  212  and the membrane  208 . All the other features of the transducer element  502   a  are the same as the features of the transducer element  202   a  described in the first embodiment, and reference is made to their description in the first embodiment. In addition, the same reference numbers are used where appropriate. 
     Otherwise, the transducer element  502   a  is realized using the same process steps as described above concerning the first embodiment. Thus, during step b) of this embodiment, the electrical connections  224   a ,  224   b  and the sealing structure  222  are realized on the transducer elements  502   a ,  502   b ,  502   c  and after dicing of step c) an individual transducer element  530   a  with a media channel  534 , the electrical connections  224   a ,  224   b  and the sealing structure  222  is obtained. 
       FIG.  5   b    illustrates a schematic view of a microelectromechanical sensor arrangement  550  according to a variant of the fourth embodiment of the invention using the transducer element  530   a  with the media channel  534  and the substrate  232  with the media channel  234  to realize a differential pressure sensor. Again, the transducer element  530   a  is attached to the substrate  232  by heating the electrical connections  224   a  and  224   b  and the sealing structure  222  above their fusion point. 
     In this configuration, a first media channel, media channel  234  is provided through which a first media under pressure P 1  can impinge on the membrane  208  and a second media channel, media channel  534 , is provided through which a second media under pressure P 2  can impinge on the membrane  208  from the other side. The sensing elements  210   a ,  210   b  detect the displacement or stress of the membrane  208  induced by the pressure difference P 1 -P 2  between media acting on the two sides of the membrane  208 , indicated by the double arrow. Thus, a differential pressure measurement can be realized. 
     Again, according to the invention, the sensor  550  can be integrated using the sealing structure  222  and the electrical connections  224   a ,  224   b  already provided at the OEM, thus at wafer level. 
       FIG.  6   a    illustrates a schematic view of a fabrication method of microelectromechanical transducer element fabricated according to a fifth embodiment of the invention. This embodiment combines the features of the third and fourth embodiment. 
     In this embodiment, step a) consists in providing a transducer elements  602   a ,  602   b ,  602   c  comprising vias,  404   a ,  404   b  connecting the opposite surface side  246  with the electrical contacts  206   a  and  206  on the membrane  208  on the active surface side  226 , like in the third embodiment as shown in  FIG.  4   a   , and a media channel  534 , as shown in the  FIG.  5   a    in the fourth embodiment. The description of the method will therefore not be repeated again but it is referred to the detailed description of the third and fourth embodiment. Furthermore, all the other features of the transducer element  602   a  are the same as the features of the transducer element  202   a  described in the first embodiment, and reference is made to their description in the first embodiment. In addition, the same reference numbers are used where appropriate. 
     The method according to the fifth embodiment then comprises a step b) of providing a sealing structure  222  and electrical connections  224   a ,  224   b  on the opposite surface side  246 . The electrical connection structures  224   a ,  224   b  are provided in direct contact with the vias  404   a ,  404   b  respectively like in the third embodiment. The sealing structure  222  is realized to surround the media channel  534 . 
     After dicing, like illustrated by step c) in  FIG.  6   a   , an isolated transducer element  630   a  is obtained. 
       FIG.  6   b    illustrates a schematic view of a microelectromechanical sensor arrangement  650  according to a variant of the fifth embodiment. In this embodiment, the transducer element  630   a  is attached with its opposite surface side  246  to a substrate  232  with media channel  234  using process steps d) to f) as illustrated in  FIG.  2   d   . The attachment is realized by heating the solder material above its fusion point and cooling down like in the other embodiments. 
     The soldering step takes place as in the other embodiments between the sealing structure  222 , the electrical connection structures  224   a ,  224   b  and the electrical contact pads  236   a ,  236   b ,  236   c  of the substrate  232  to form a seal and an electrical connection. 
     In this embodiment, the seal realized by the sealing structure  222  and the substrate  232  protects the electrical connection structures  224   a ,  224   b  from any media in the media channel  234 . 
     The electrical connection structures  224   a ,  224   b  provide an electrical connection between the sensing elements  210   a ,  210   b  of the membrane  208 , in particular the piezoresistive gauge  210   a ,  210   b  of the membrane  208 , via the electrical contacts  206   a ,  206   b  and the vias  404   a ,  404   b  with the substrate  232  and other electrical component present in a sensor arrangement. 
     A cap  652 , shown in  FIG.  6   b   , is provided to realize a reference volume  654  on the active surface side  226  of the membrane  208  of the transducer element  630   a . In this configuration, a pressure sensor is realized in which the media enters via the media channel  234  and the media channel  534  to deform the membrane  208  against the pressure in the reference volume  654 . 
       FIG.  7   a    illustrates a schematic view of a transducer element  730   a  fabricated according to a sixth embodiment of the invention. The fabrication process to obtain the transducer element  730   a  according to the sixth embodiment is similar to the one of the fifth embodiment, except that in step b) a second sealing structure  722  is provided on the active surface side  226 . Besides that, all features of the transducer element  730   a  are the same as for the transducer element  630  illustrated in  FIG.  6   a   , reference is therefore made to its description above. The second sealing structure  722  is made of the same material as the sealing structure  222  and is deposited in the same way either before or after the process step of realizing structure  222 . 
       FIG.  7   b    illustrates a schematic view of a microelectromechanical sensor arrangement  750  according to a variant of the sixth embodiment. In this embodiment, like fourth embodiment illustrate in  FIG.  5   b   , a differential pressure sensor is realized. To do so a substrate  232  is attached to the opposite surface side  246  of the transducer element  730   a . Attachment is realized by heating the solder material above its fusion point and cooling down like in the other embodiments. 
     The soldering step takes place as in the other embodiments between the sealing structure  222 , the electrical connection structures  224   a ,  224   b  and the electrical contact pads  236   a ,  236   b ,  236   c  of the substrate  232  to form a seal and an electrical connection. 
     In this embodiment, the seal realized by the sealing structure  222  and the substrate  232 , protects the electrical connection structures  224   a ,  224   b  from any media in the media channel  234  and  534 . 
     The electrical connection structures  224   a ,  224   b  provide an electrical connection between the piezoresistive gauge  210   a ,  210   b  of the membrane  208  via the electrical contacts  206   a ,  206   b  and the vias  404   a ,  404   b  with the substrate  232  and other electrical component present in a sensor arrangement. 
     A second substrate  432 , like already used in the third embodiment as illustrated in  FIG.  4   b    is attached on the active surface side  226 , as shown in  FIG.  7     b.    
     The soldering step takes place between the second sealing structure  722  and the conductive pad  436 . Thus, a second seal is realized by the sealing structure  722  and the substrate  432  to protect the electrical connection structures  224   a ,  224   b  from any media in the second media channel  434 . 
     In this configuration, a first media channel, media channel  234  and  534  is provided through which a first media under pressure P 1  can impinge on the membrane  208  and a second media channel, media channel  434 , is provided through which a second media under pressure P 2  can impinge on the membrane  208  from the other side. The sensing elements  210   a ,  210   b  detect the displacement of the membrane  208  induced by the pressure difference P 1 -P 2  between the media acting on the two sides of the membrane  208 , indicated by the double arrow. Thus, a differential pressure measurement can be realized like in the variant of the fourth embodiment illustrated in  FIG.  5     b.    
       FIG.  8   a    illustrates a schematic view of a fabrication method of microelectromechanical (MEMs) transducer element  830   a  fabricated according to a seventh embodiment of the invention. This embodiment is similar to the fourth embodiment illustrated in  FIGS.  5   a  and  5   b   . The difference between the two embodiments is the use of a snubber structure  860  as media channel instead of the media channel  534  illustrated in  FIGS.  5   a  and  5   b   . In pressure sensors, snubber structures are used to mitigate transient events of high pressure, e.g. pressure spikes, which can cause damage of the membrane when the pressure peak leads to a membrane deformation beyond its predetermined yield point, as already known from EP3748325A1, the description of which is incorporated herewith by reference. 
     Besides the use of a snubber structure  860 , all other features are the same as in the fourth embodiment and the transducer element  830   a  can be realized using the same process steps and is not described in detail again. Instead, reference is made to the detailed description of the fourth embodiment. 
     Instead of realizing the media channel  534 , microelectromechanical production steps as known in the art, for example a succession of dry or wet etching and wafer bonding steps or other alternatives like 3D glass laser structuring, are realized to provide the wafer  200  with an integrated snubber structure  860 . 
     The integrated snubber structure  860  in this embodiment comprises a through channel  862  reaching from the opposite surface side  246  of the transducer element  830   a  to the cavity  212 . The channel  862  comprises two or more portions, in this example four portions  864   a ,  864   b ,  864   c ,  864   d , with changing directions to mitigate transient pressure events. Providing integrating snubber structures  860  inside the wafer  200  allows reducing the size of the transducer element  830   a  and improves the integration into a complete pressure sensor. 
     According to the invention, the transducer element  830   a  of the sixth embodiment furthermore comprises a sealing structure  222  and electrical connections  224   a ,  224   b  on the active surface side  226 . 
       FIG.  8   b    illustrates a variant of the sixth embodiment. The transducer element  830   a  is attached to substrate  232 , similar to the variant of the fourth embodiment illustrated in  FIG.  5   b   , to form a microelectromechanical sensor arrangement  850  according to a variant of the seventh embodiment of the invention using the transducer element  830   a  with the snubber structure  860  and the substrate  232  with the media channel  234  to realize a differential pressure sensor. Also in this variant, the transducer element  830   a  is attached to the substrate  232  by heating the electrical connections  224   a  and  224   b  and the sealing structure  222  above their fusion point. 
     In this configuration, a first media channel, media channel  234  is provided through which a first media under pressure P 1  can impinge on the membrane  208  and a second media channel, snubber structure  860 , is provided through which a second media under pressure P 2  can impinge on the membrane  208  from the other side. The sensing elements  210   a ,  210   b  detect the displacement of the membrane  208  induced by the pressure difference P 1 -P 2  between media acting on the two sides of the membrane  208 , indicated by the double arrow. Thus, a differential pressure measurement can be realized. 
     According to the invention, the sensor  850  can be integrated at the site of an OEM, thus already at wafer level, by using the sealing structure  222  and the electrical connections  224   a ,  224   b.    
     According to further variants, the transducer element  830   a  and the Mems sensor arrangement  850  could be combined with features of the other embodiment. E.g. vias  404   a ,  404   b  could be used to provide the electrical contact on the opposite side surface  246 . Furthermore, instead of realizing a differential pressure sensor, a pressure sensor having only one media channel, the snubber structure  860 , and using a cap  652  as illustrated in the variant of the fifth embodiment of  FIG.  6   b   , could be realized. 
     A second variant of a Mems sensor arrangement  870  according to the seventh embodiment comprises a transducer element  872  with an integrated snubber structure  880  attached to a substrate  232  as illustrated in  FIG.  8   c   . Also in this variant, the transducer element  872  is attached to the substrate  232  by heating the electrical connections  224   a  and  224   b  and the sealing structure  222  above their fusion point. The same process steps can realize this sensor arrangement  870  as the one illustrated in  FIG.  8     b.    
     The integrated snubber structure  880  comprises a first channel  882   a  perpendicular to the cavity  212  behind the membrane  208 , followed by a second cavity  884  parallel to the first cavity  212  and a second channel  882   b  again perpendicular which extends through to the opposite surface side  246 . 
     The sensor arrangement  870  as illustrated in  FIG.  8   c    is a differential pressure sensor, as the one illustrated in  FIG.  8   b    but could also be realized as a pressure sensor having only one medial channel and a cap like illustrated in the variant of the fifth embodiment of  FIG.  6   b   . Furthermore, also in this sensor arrangement  870  vias could be used to move the electrical connections to the opposite surface side  246 . 
     A third variant of a Mems sensor arrangement  890  according to the seventh embodiment comprises a transducer element  892  with an integrated snubber structure  900  attached to a substrate  232  as illustrated in  FIG.  8   d   . Also in this variant, the transducer element  892  is attached to the substrate  232  by heating the electrical connections  224   a  and  224   b  and the sealing structure  222  above their fusion point. The same process steps as the one illustrated in  FIG.  8   b    or  8   c  can realize this sensor arrangement  890 . 
     The integrated snubber structure  900  in  FIG.  8   d    comprises a first channel  902  in connection with the cavity  212  and an internal cavity  904 . The internal cavity  904  in turn is connected to a second channel  906  that extends through to the opposite surface side  246 . A pressure mitigation element  908  is furthermore provided inside the internal cavity  904 . This pressure mitigation member  908  is a movable element, like a piston, that is configured and formed from a material that enables it to move within the separate cavity to block the first channel  902  under a pressure spike. 
     The sensor arrangement  890  as illustrated in  FIG.  8   d    is a differential pressure sensor, like the one illustrated in  FIG.  8   b    or  8   c , but could also be realized as a pressure sensor having only one medial channel and a cap like illustrated in the variant of the fifth embodiment of  FIG.  6   b   . Furthermore, also in this sensor arrangement  890  vias could be used to move the electrical connections to the opposite surface side  246 . 
       FIG.  9    illustrates a schematic view of a microelectromechanical sensor system  950  according to an eight embodiment of the invention. 
     In this embodiment, a Mems sensor arrangement  250  is mount on a circuit carrier  960 . The circuit carrier  960  can be part of a printed circuit board or a flexible board, having further electronic components mounted thereon. According to variants, any one of the Mems sensor arrangements  350   a ,  350   b    450 ,  550 ,  650 ,  750 ,  870 ,  850  or  890  according to one of the embodiments two to seven and their variants could be mount instead. 
     In  FIG.  9   , the circuit carrier  960  comprises a media channel  962  aligned with the media channel  234  of the sensor arrangement  250   a  so that a measurand can impinge on the membrane  208 . 
     A solder seal  964  seals the media channel  962  at the interface between the circuit carrier  960  and the substrate  232  of the MEMs sensor arrangement  250   a . The circuit carrier  960  further comprises electrical contact pads  966   a  and  966   c  electrically connected with the electrical contact pads  236   a  and  236   c  of the substrate  232 , e.g. using vias  968   a ,  968   c  in the substrate  232  and solder connections  970   a  and  970   c.    
     Also in this embodiment, the solder seal  964  and the solder connections  970   a  and  970   c  can be of the same material, so that the sealing and electrical connections can be realized in one step. 
     A number of embodiments of the invention have been described. Nevertheless, it is understood that various modifications and enhancements may be made without departing the following claims.