Patent Publication Number: US-2022221363-A1

Title: Pressure Sensor Device and Method for Forming a Pressure Sensor Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. patent application Ser. No. 16/333,671, entitled “Pressure Sensor Device and Method for Forming a Pressure Sensor Device”, which was filed on Mar. 15, 2019, which is a national phase fling under section 371 of PCT/EP2017/074953, filed Oct. 2, 2017, which claims the benefit of European patent application 16191894.1, filed on Sep. 30, 2016, all of which are incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present application relates to a pressure sensor device and a method for forming a pressure sensor device. 
     BACKGROUND 
     Pressure sensors are sensitive to stress and therefore need to be protected from undesired stress which can be caused, for example, by mechanical deformation. If a pressure sensor is arranged on a chip stress can be induced due to different coefficients of thermal expansion of different parts of the chip. Also, if the processing of the pressure sensor involves heating and solidification steps, additional stress can be exerted on the pressure sensor. As another example, if a pressure sensor is arranged within a device, mechanical forces of the device can occur and induce stress on the pressure sensor. It is desirable to maintain the conditions of the calibration of the pressure sensor, this means additional stress should be avoided in order to guarantee a correct pressure reading of the pressure sensor. 
     Additional stress induced on a pressure sensor can be avoided, for example, by the use of compliant layers which can be silicone-based. Such layers can be employed in land grid array packages. However, the use of compliant layers increases the total thickness of the device and the packages of the device are usually larger than the footprint of the pressure sensor. 
     SUMMARY 
     Embodiments provide a pressure sensor device with an increased accuracy in pressure sensing. Further embodiments provide a method for forming such a pressure sensor device with an increased accuracy. 
     In one embodiment of the pressure sensor device, the pressure sensor device comprises a substrate body. The substrate body can be a wafer, a substrate or bulk material and it can comprise silicon or glass. The substrate body can comprise a complementary metal oxide semiconductor device and a substrate. The pressure sensor device further comprises a pressure sensor comprising a membrane. The pressure sensor is arranged on top of the substrate body, for example on top of the complementary metal oxide semiconductor device. The pressure sensor can also comprise a cavity below the membrane. 
     In one embodiment, the pressure sensor device comprises a cap body comprising at least one opening. The cap body can comprise silicon or glass and can be of the same material as the substrate body. It is also possible that the cap body and the substrate body comprise different materials. The cap body and the substrate body can comprise materials with similar coefficients of thermal expansion. The cap body can be a wafer, a substrate or a bulk material which is arranged on top of the pressure sensor. This means the cap body is arranged on the side of the pressure sensor which faces away from the substrate body. The opening in the cap body can be formed by deep reactive ion etching in combination with grinding. The diameter or a lateral extension of the opening can, for example, amount to approximately 800 μm. Advantageously, the diameter or the lateral extension of the opening is small in comparison to the lateral extension of the cap body. 
     In one embodiment of the pressure sensor device, the pressure sensor is arranged between the substrate body and the cap body in a vertical direction which is perpendicular to the main plane of extension of the substrate body and the mass of the substrate body equals approximately the mass of the cap body. The main plane of extension of the substrate body extends in a lateral direction of the substrate body. The thickness of the substrate body is given in the vertical direction and the thickness of the substrate body is small in comparison to the lateral extension of the substrate body. This means, the vertical direction corresponds to a stacking direction of the pressure sensor device. This means, the pressure sensor is arranged on top of the substrate body in stacking direction and the cap body is arranged on top of the pressure sensor in stacking direction. The mass of the substrate body can, for example, amount to at least 80% of the mass of the cap body and at most 120% of the mass of the cap body. Optionally, the mass of the substrate body can amount to at least 90% of the mass of the cap body and at most no % of the mass of the cap body. Optionally, the mass of the substrate body can amount to at least 95% of the mass of the cap body and at most 105% of the mass of the cap body. This means, the masses of the substrate body and the cap body are approximately equal within a tolerance of manufacturing. 
     If the masses of the substrate body and the cap body are approximately equal, the pressure sensor or the membrane of the pressure sensor are located in a plane of minimal mechanical stress change when subjected to externally applied, mechanical or thermal induced deformations. This means, there is a plane of minimal stress within the device. Advantageously, the membrane of the pressure sensor is located in the plane of minimal stress within the pressure sensor device. 
     The cap body can be patterned before it is attached on the pressure sensor and the substrate body. For example, a recess can be formed in the cap body so that the cap body is not in direct contact with the membrane of the pressure sensor. In order to keep the masses of the substrate body and the cap body approximately equal, the cap body needs to be thicker than the substrate body because of the opening in the cap body. The opening of the cap body can have a lateral extension of approximately one third of the lateral extension of the cap body. In this case, the cap body needs to be thicker than the substrate body in order to position the pressure sensor in the plane of minimal stress of the pressure sensor device. It is also possible that the lateral extension of the opening of the cap body is larger than the lateral extension of the membrane of the pressure sensor. Therefore, the thickness of the cap body depends on the lateral extension of the opening in the cap body. 
     If the substrate body is transparent for light or electromagnetic radiation, an optical sensor can advantageously be integrated in the device next to the pressure sensor. 
     By positioning the pressure sensor in the plane of minimal stress, it is protected against strain which can be caused, for example, by different coefficients of thermal expansion of the pressure sensor device and an underlying printed circuit board on which the pressure sensor is mounted. The pressure sensor can also be protected against additional stress induced from other parts of the pressure sensor device or the surroundings of the pressure sensor device. If the additional stress on the pressure sensor is minimized by placing the pressure sensor in the plane of minimal stress, the pressure sensor can operate under the same conditions as during the calibration of the pressure sensor, thus guaranteeing a correct and more accurate pressure reading. 
     Furthermore, by positioning the cap body on top of the pressure sensor, the thickness of the pressure sensor device is increased such that the substrate body can be thin enough to allow a processing from a bottom side of the substrate body, where the bottom side of the substrate body faces away from the pressure sensor. This means that through silicon vias can be etched in the substrate body to electrically contact the pressure sensor from the backside of the pressure sensor facing away from the cap body. Moreover, it is possible to release the membrane of the pressure sensor after the processing steps which can comprise the etching of the through silicon vias which avoids a potential membrane fracture during the processing steps. Additionally, the employment of a cap body and a substrate body allows an efficient packaging of the pressure sensor since no compliant or glue layers are required for the decoupling of the membrane. Therefore, the thickness of the packaging can be reduced and also the footprint of the packaging. The thickness of the packaging can for example amount to 300 to 600 μm and the footprint of the packaging can amount to 1 to 2 mm 2 . Furthermore, the assembly costs of the packaging can be reduced. 
     In one embodiment the pressure sensor device comprises a substrate body, a pressure sensor comprising a membrane, and a cap body comprising at least one opening. The pressure sensor is arranged between the substrate body and the cap body in a vertical direction which is perpendicular to the main plane of extension of the substrate body, and the mass of the substrate body equals approximately the mass of the cap body. 
     In one embodiment of the pressure sensor device, the mass of the substrate body amounts to at least 95% of the mass of the cap body and at most 105% of the mass of the cap body. This means the masses of the cap body and the substrate body are approximately equal so that the pressure sensor is located in the plane of minimal stress of the pressure sensor device. It is also possible that the mass of the substrate body amounts to at least 80% of the mass of the cap body and at most 120% of the mass of the cap body. Optionally, the mass of the substrate body amounts to at least 90% of the mass of the cap body and at most no % of the mass of the cap body. 
     In one embodiment of the pressure sensor device, the pressure sensor comprises a capacitive pressure sensor comprising a cavity below the membrane. The pressure sensor can be formed, for example, by providing a sacrificial layer above a bottom electrode and electrically conductive vias which are arranged around the area which is supposed to be the cavity of the pressure sensor. A top electrode can be deposited on top of the sacrificial layer and the electrically conductive vias, and the top electrode can be patterned to form the membrane. The sacrificial layer is etched away through etch holes in the membrane such that the cavity is formed below the membrane. The membrane can be sealed by plasma-enhanced chemical vapor deposition of silicon nitride, which can be silicon-rich, or silicon nitride and silicon oxide such that a compressive film is formed on top of the membrane. Advantageously, a compressive film is less permeable for gases from the surroundings into the cavity. 
     In one embodiment of the pressure sensor device, the substrate body comprises at least one vertical electrically conductive via and/or the pressure sensor device is surface mountable. Vertical in this case means along the vertical direction which means that the electrically conductive via extends from the bottom side of the substrate body which faces away from the pressure sensor in the direction of the pressure sensor. The electrically conductive via electrically contacts the pressure sensor or another electrical contact, for example a contact of a complementary metal oxide semiconductor device. The electrically conductive via can be a through silicon via which is formed by patterning of the substrate body, for example by etching. A trench patterned in the substrate body can be coated with an isolation material and with an electrically conductive material to contact the pressure sensor. 
     The electrically conductive material of the electrically conductive via can comprise titanium and/or titanium nitride and tungsten or tantalum and/or tantalum nitride and copper. In order to form the electrically conductive via in the substrate body, the substrate body can be thinned to, for example, 100 to 200 μm. Since the cap body is arranged on top of the pressure sensor, it is possible to thin the substrate body so that the electrically conductive via can be formed. With this, the pressure sensor device can be surface mountable. Surface mountable means that the pressure sensor device can be electrically contacted from the bottom side of the substrate body. The electrically conductive via can be electrically contacted on the bottom side of the substrate body by a solder ball. 
     In one embodiment of the pressure sensor device, the pressure sensor is positioned on top of an integrated circuit. The integrated circuit can be a complementary metal oxide semiconductor device. With this, the total size of the device can be reduced since the pressure sensor is arranged on top of the integrated circuit. 
     In one embodiment of the pressure sensor device, a top layer covers the pressure sensor on the side of the pressure sensor facing the cap body, and the top layer and the cap body are connected via direct bonding. The top layer can be deposited onto the pressure sensor before the membrane is released. The top layer can, for example, comprise a bond oxide and it can be, for example, 2 to 3 μm thick. The top layer can comprise silicon dioxide and/or silicon nitride. The advantages of direct bonding are that a very stiff connection can be formed between the cap body and the top layer and that the bond is compatible with high temperatures which can employed during the processing of the pressure sensor device. 
     For a stiff connection, it is important that the surface of the top layer is topographically flat. Therefore, the surface of the top layer can be flattened by chemical mechanical polishing before connecting with the cap body. A good mechanical coupling and stiffness of the connection between the top layer and the cap body is important for the stress compensation in the plane of the membrane of the pressure sensor. If the connection between the top layer and the cap body is not stiff enough, strain cannot be transferred to the cap body and the plane of minimal stress is less well defined. Therefore, the stiffness of the connection can also influence the required thicknesses of the cap body and the substrate body. It is also possible that the top layer and the cap body are connected via a glue. 
     If the top layer comprises a metal it can be connected with the cap body by eutectic bonding. A eutectic bond is a very stable bond and it is also stable at high temperatures. Therefore, in the case of eutectic bonding other processing steps, such as for example the formation of an electrically conductive via with tungsten in the substrate body, are possible. 
     In one embodiment of the pressure sensor device, the top layer comprises at least one electrically conductive wall which is arranged on top of the pressure sensor surrounding the opening and which is in direct contact with the pressure sensor and the cap body. The top layer can comprise one or more electrically conductive walls. The electrically conductive wall is arranged to protect the top layer against the etching of the cavity such that the top layer is not etched away where it is arranged between the cap body and the substrate body. This means the electrically conductive wall is arranged around the membrane of the pressure sensor and around the area where the cavity is etched. With this, it is avoided that the top layer is etched away below the cap body. Therefore, the electrically conductive wall can be formed before the membrane is released. The electrically conductive wall can also serve to stabilize the pressure sensor device. 
     In one embodiment of the pressure sensor device, the opening in the cap body is positioned above the pressure sensor in vertical direction and extends over the total lateral extension of the pressure sensor. The opening is introduced into the cap body so that the pressure sensor can measure the pressure of the surroundings of the pressure sensor device. Therefore, the membrane with a sealing needs to be in direct contact with the air or the gas from the surroundings of the pressure sensor device. 
     The lateral extension of the opening can be the same as the lateral extension of the pressure sensor or the lateral extension of the opening can be larger than the lateral extension of the pressure sensor. The lateral extension refers to an extension in two dimensions which are given by two directions which are perpendicular to the vertical direction. The opening can be shaped circular. It is also possible to introduce more than one opening in the cap body. 
     The cap body can be thinned before the opening is introduced. The thickness of the cap body depends on the size of the opening. This means if the lateral extension of the opening is small, the cap body can be thinner in order to keep the plane of minimal stress in the plane of the membrane. 
     In one embodiment of the pressure sensor device, the opening in the cap body is positioned above the pressure sensor in vertical direction and the lateral extension of the opening is smaller than the lateral extension of the pressure sensor. It is advantageous to keep the lateral extension of the opening small and, therefore, also the thickness of the cap body in order to reduce the total height of the pressure sensor device. It is also possible that the opening is not positioned above the membrane in vertical direction but besides the membrane in vertical direction. 
     Furthermore, a method for forming a pressure sensor device is provided. The pressure sensor device may be produced by means of one of the methods described here. This means all features disclosed for the pressure sensor device are also disclosed for the method for forming a pressure sensor device and vice-versa. 
     According to at least one embodiment of the method for forming a pressure sensor device, the method comprises providing a pressure sensor on a substrate body, the pressure sensor comprising a membrane, and depositing a top layer on top of the substrate body and the pressure sensor. The method further comprises connecting a cap body with the top layer, the mass of the cap body being approximately equal to the mass of the substrate body, and introducing at least one opening in the cap body. 
     The mass of the substrate body can, for example, amount to at least 95% of the mass of the cap body and at most 105% of the mass of the cap body. It is also possible that the mass of the substrate body amounts to at least 80% of the mass of the cap body and at most 120% of the mass of the cap body. Optionally, the mass of the substrate body amounts to at least 90% of the mass of the cap body and at most no % of the mass of the cap body. This means, the masses of the substrate body and the cap body are approximately equal within a tolerance of manufacturing, such that the pressure sensor is positioned in the plane of minimal stress. 
     The pressure sensor can be a capacitive pressure sensor with a membrane and a cavity. The substrate body can comprise silicon or glass, and it can also comprise a complementary metal oxide semiconductor device. The top layer can be a bond oxide, for example silicon dioxide or silicon nitride, which needs to be topographically flat. The cap body and the top layer can be connected by direct bonding, via gluing or via eutectic bonding. The opening can be introduced in the cap body by deep reactive ion etching or by grinding. It is also possible that the cap wafer is patterned before connecting with the top layer, for example with a recess for the membrane so that the cap body is not in direct contact with the membrane. 
     By positioning the pressure sensor in the plane of minimal stress, it is protected against strain which can be caused, for example, by different coefficients of thermal expansion of the cap body and the substrate body. The pressure sensor can also be protected against additional stress induced from other parts of the pressure sensor device or the surroundings of the pressure sensor device. If the additional stress on the pressure sensor is minimized by placing the pressure sensor in the plane of minimal stress, the pressure sensor can operate under the same conditions as during the calibration of the pressure sensor, thus guaranteeing a correct and more accurate pressure reading. 
     According to at least one embodiment of the method for forming a pressure sensor device, the mass of the substrate body amounts to at least 95% of the mass of the cap body and at most 105% of the mass of the cap body. It is also possible that the mass of the substrate body amounts to at least 80% of the mass of the cap body and at most 120% of the mass of the cap body. Optionally, the mass of the substrate body amounts to at least 90% of the mass of the cap body and at most no % of the mass of the cap body. This means the masses of the cap body and the substrate body are approximately equal so that the pressure sensor is located in the plane of minimal stress of the pressure sensor device. 
     According to at least one embodiment of the method for forming a pressure sensor device, a handling wafer is connected to the substrate body at the bottom side of the substrate body by an adhesive material which can be removed. Furthermore, the handling wafer and the adhesive material are removed. The handling wafer can be connected to the substrate body in order to protect electrically conductive vias in the substrate body from mechanical damage or damage from an etching step. For example, during the thinning of the cap body or the etching of the opening, electrically conductive vias in the substrate body can be protected by the handling wafer. The adhesive material can be high-temperature compatible and it can be removed. These features are advantageous for the processing of the pressure sensor device since in some processing steps, high temperatures can be required. If only lower temperatures are required during processing, it is also possible to employ a temporary bonding or tape as the adhesive which is compatible only to low temperatures and which can be removed. 
     According to at least one embodiment of the method for forming a pressure sensor device, a handling wafer is connected to the substrate body at the bottom side of the substrate body by an adhesive material which can be patterned. Furthermore, the handling wafer is removed and the adhesive material is not removed. Advantageously, the adhesive material is compatible with high temperatures. If the adhesive material cannot be removed, it is required that it can be patterned such that the pressure sensor and also, for example, a complementary metal oxide semiconductor device can be electrically contacted from the bottom side of the substrate body. 
     According to at least one embodiment of the method for forming a pressure sensor device, a vertical electrically conductive via is etched in the substrate body before the membrane is released. With the vertical electrically conductive via, the pressure sensor can be electrically contacted and also, for example, other devices in the pressure sensor device. If the vertical electrically conductive via is etched in the substrate body before the membrane is released, the membrane is protected by the sacrificial layer during the etching of the electrically conductive via and other processing steps. Therefore, potential membrane fracture during processing is avoided. 
     According to at least one embodiment of the method for forming a pressure sensor device, a vertical electrically conductive via is etched in the substrate body after the membrane is released. This means the membrane is released before further processing which can be the bonding of the cap wafer to the top layer. With this, it is possible to keep the opening in the cap body small since it is not required to etch the cavity through the opening of the cap body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description of figures may further illustrate and explain exemplary embodiments. Components that are functionally identical or have an identical effect are denoted by identical references. Identical or effectively identical components might be described only with respect to the figures where they occur first. Their description is not necessarily repeated in successive figures. 
         FIG. 1  shows a cutaway view of an exemplary embodiment of the pressure sensor device; 
         FIG. 2  shows a schematic cutaway view of a pressure sensor device on a printed circuit board; 
         FIG. 3  shows the stress level as a function of cap body thickness; 
       With  FIGS. 4A to 4O  an exemplary embodiment of the method for forming a pressure sensor device is described; 
       With  FIGS. 5A to 5L  another exemplary embodiment of the method for forming a pressure sensor device is described; 
         FIG. 6  shows an exemplary embodiment of the pressure sensor device with electrically conductive walls; 
         FIG. 7  shows an exemplary embodiment of the pressure sensor device with a reduced opening in the cap body; 
         FIGS. 8 and 9  show exemplary embodiments of the pressure sensor device with a reduced opening in the cap body and a sealing on the membrane; and 
     
    
    
     With  FIGS. 10A to 10K  another exemplary embodiment of a method for forming a pressure sensor device is described. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  shows a schematic cutaway view of an exemplary embodiment of the pressure sensor device  10  where the plane of minimal stress is indicated by the dashed line. On top of a substrate body  11  a pressure sensor  12  with a cavity  16  and a membrane  13  is arranged. The pressure sensor  12  is electrically contacted by two electrically conductive vias  17 . The electrically conductive vias  17  extend through the substrate body  11  from a bottom side  18  of the substrate body  11  towards the pressure sensor  12 . An isolation material  19  electrically isolates the electrically conductive vias  17  from each other and from the substrate body  11 . On top of the pressure sensor  12  a top layer  20  is positioned which connects the pressure sensor  12  with a cap body  14 . Thus, the pressure sensor  12  is arranged between the substrate body  11  and the cap body  14  in a vertical direction z. The cap body  14  and the top layer  20  can be connected via direct bonding. The cap body  14  comprises an opening  15  which is arranged above the pressure sensor  12 . The plane of minimal stress is approximately located in the plane of the pressure sensor  12  and advantageously in the plane of the membrane  13  of the pressure sensor  12 . In order to place the plane of minimal stress in the plane of the membrane  13  the mass of the substrate body  11  equals approximately the mass of the cap body  14 . In other embodiments, this means for example that the mass of the substrate body  11  amounts to at least 95% of the mass of the cap body  14  and to at most 105% of the mass of the cap body  14 . 
       FIG. 2  shows a schematic cutaway view of a pressure sensor device  10  mounted onto a printed circuit board. The pressure sensor device  10  with the printed circuit board is schematically depicted as one device body  44 . Due to for example different coefficients of thermal expansion of the pressure sensor device and the printed circuit board, the device body  44  is bent in such a way that the top part is compressed and the bottom part experiences a tensile stress. The tensile and the compressive stresses are indicated by arrows. In one plane through the device body  44  the stress is minimal which is indicated by the dashed line. 
       FIG. 3  shows the stress level y as a function of cap body thickness x for a constant substrate body thickness. The additional stress is introduced on the pressure sensor device  10  by soldering the pressure sensor device  10  onto a printed circuit board. The optimal thickness x of the cap body  14  is at zero stress. The stress change is plotted for two different diameters of the opening  15  in the cap body  14 . In the case of the straight line the diameter of the opening  15  amounts to 500 μm and in the case of the dashed line the diameter of the opening  15  amounts to 250 μm. The inset in  FIG. 3  shows a schematic of a pressure sensor device  10  with the opening  15  in the cap body  14 . 
     With  FIGS. 4A to 4O  an exemplary embodiment of the method for forming a pressure sensor device  10  is described. In the process flow described with  FIGS. 4A to 4O  the membrane  13  of the pressure sensor  12  is released before the electrically conductive via  17  is formed in the substrate body  11 . 
       FIG. 4A  shows the substrate body  11  which comprises an integrated circuit  21  which can be for example a complementary metal oxide semiconductor device. The integrated circuit  21  is arranged on top of a substrate  43  which is comprised by the substrate body  11  and which can comprise silicon. On top of the integrated circuit  21  the pressure sensor  12  is arranged. By arranging the pressure sensor  12  on top of the integrated circuit  21  instead of next to it, the footprint of the device is reduced. In this exemplary embodiment the pressure sensor  12  comprises a bottom electrode  22  on which a dielectric etch stop layer  23  is arranged. A sacrificial layer  24  is deposited on top of the dielectric etch stop layer  23 . In order to form the cavity  16  of the pressure sensor  12 , the sacrificial layer  24  is patterned in such a way that trenches are formed in the sacrificial layer  24 . The trenches are filled with an electrically conductive material  34  as for example tungsten. In order to form the membrane  13 , an electrically conductive layer  25  is deposited on top of the sacrificial layer  24 . Below the electrically conductive layer  25  there is an adhesion layer  26  and there is another adhesion layer  27  on top of the electrically conductive layer  25 . The electrically conductive layer  25  and the adhesion layers  26  and  27  comprise several etch openings  28 . The bottom electrode  22  of the pressure sensor  12  is electrically connected with the integrated circuit  21  by electrically conductive walls  29 . The integrated circuit  21  comprises a back contact  30 . 
     As shown in  FIG. 4B , in the next step of the method for forming a pressure sensor device  10 , a top layer  20  is deposited on top of the pressure sensor  12 . The top layer  20  can comprise a bond oxide and can be for example 2 to 3 μm thick. Furthermore, additional electrically conductive walls  29  are formed around the membrane  13 . The further electrically conductive walls  29  are also connected by an electrically conductive layer  25  around which adhesion layers  26  and  27  are arranged. A mask  31  is positioned on top of the top layer  20  in order to pattern the top layer  20  above the additional electrically conductive walls  29 . 
     In the next step of the method, as shown in  FIG. 4C , the top layer  20  is patterned in such a way that electrically conductive walls  29  are formed around the membrane  13  in the top layer  20 . The electrically conductive walls  29  can be filled with tungsten. After the deposition of the material of the electrically conductive walls  29  the surface of the top layer  20  is planarized for example by chemical mechanical polishing. In order to achieve a stiff connection between the top layer  20  and the cap body  14  it is necessary that the surface of the top layer  20  is topographically flat. 
       FIG. 4D  shows that a mask  31  is positioned on the top layer  20 . The mask  31  should be able to withstand an etching step with for example hydrogen fluoride vapor. In the next step, the mask  31  is patterned in such a way that the area above the membrane  13  on the pressure sensor  12  is free of the mask  31 . 
       FIG. 4E  shows that in the next step of the method the sacrificial layer  24  below and around the membrane  13  is etched away. With this, the membrane  13  of the pressure sensor  12  is released. Also, parts of the top layer  20  around the membrane  13  are etched away. The electrically conductive walls  29  and the additional electrically conductive walls  29  act as an etch stop so that not the whole top layer  20  is etched away. 
     As shown in  FIG. 4F  in the next step a sealing layer  32  is deposited on top of the membrane  13  and the mask  31 . The sealing layer  32  can comprise silicon nitride. The sealing layer  32  is deposited at elevated temperatures as for example temperatures above 400° C. by plasma enhanced chemical vapor deposition, and therefore after cooling down the sealing layer  32  is compressive. With this, the sealing layer  32  is less permeable for gases from the surroundings into the cavity  16 . 
     In  FIG. 4G  it is shown that another top layer  20  is deposited on top of the sealing layer  32 . The other top layer  20  can also comprise a bond oxide. The other top layer  20  is deposited in order to improve the bonding strength between the top layer  20  and the cap body  14 . The surface of the top layer  20  needs to be topographically flat in order to achieve a stiff connection to the cap body  14 . 
     As shown in  FIG. 4H  in the next step of the method the cap body  14  is connected with the sealing layer  32 . It is also possible that another top layer  20  is arranged between the sealing layer  32  and the cap body  14  in order to achieve a stiff connection. The cap body  14  are connected with the sealing layer  32  or the top layer  20  at elevated temperatures which could be for example around 450° C. As shown in  FIG. 4H  the sealing layer  32  on top of the membrane  13  is not in direct contact with the cap body  14 . Therefore, the membrane  13  is still released and no additional stress is imposed on the membrane  13  from the cap body  14 . With the cap body  14  being connected with the pressure sensor  12  and the substrate body  11 , the pressure sensor device  10  is turned upside down and the substrate  43  is thinned to a required thickness by for example grinding. A required thickness of the substrate  43  can be 100 to 200 μm. 
       FIG. 4I  shows that the substrate body  11  is patterned. This means a trench is formed in the substrate  43 , for example by deep reactive ion etching. The trench extends through the whole substrate  43  in vertical direction z from the bottom side  18  to the integrated circuit  21 . In a next step the oxide on the bottom of the integrated circuit  21  is also etched away such that the back contact  30  of the integrated circuit  21  can be electrically contacted. In the next step an isolation layer  33  is deposited on the bottom side  18  of the substrate body  11  and within the trench in the substrate  43 . 
     As shown in  FIG. 4J , in the next step of the method the isolation layer  33  is etched within the trench such that the back contact  30  of the integrated circuit  21  is free of the isolation layer  33 . For etching the isolation layer  33  a mask  31  is employed. In the next step, an electrically conductive material  34  is deposited within the trench and around the trench on the bottom side  18  of the substrate body  11 . The electrically conductive material  34  can be deposited by physical vapor deposition or by chemical vapor deposition. For example, the electrically conductive material  34  can comprise titanium, tantalum, tantalum nitride or copper. With a mask  31  applied on the bottom side  18  of the substrate body  11  the electrically conductive material  34  is patterned and etched away in such a way that a part of the bottom side  18  of the substrate body  11  is free of the electrically conductive material  34 . 
     In  FIG. 4K  it is shown that in the next step of the method another isolation layer  33  is deposited on the bottom side  18  of the substrate body  11 . With this, an electrically conductive via  17  is formed in the substrate  43 . The cap body  14  is thinned in such a way that the masses of the substrate body  11  and the cap body  14  are approximately equal which means that the membrane  13  of the pressure sensor  12  is positioned in the plane of minimal stress of the pressure sensor device  10 . 
     In  FIG. 4L  it is shown that a mask  31  is positioned on top of the cap body  14 . With the mask  31  the cap body  14  is patterned in such a way that an opening  15  is formed in the cap body  14 . The opening  15  is a trench through the whole cap body  14 . In this case the opening  15  is positioned besides the membrane  13  on the pressure sensor  12  in a lateral direction. This means, the opening  15  extends in a vertical direction z, but it is not positioned directly above the membrane  13 . Since the membrane  13  is already released, it is not required anymore to etch the sacrificial layer  24  and thus the opening  15  can be small and it is not necessary that the opening  15  is positioned directly above the membrane  13 . If the opening  15  is small, the stress compensation within the pressure sensor device  10  is more well-defined than for a larger opening  15 . 
     At next as shown with  FIG. 4M  a mask  31  is attached to the bottom side  18  of the substrate body  11  in order to etch a part of the isolation layer  33 . With this, a part of the isolation layer  33  is etched in such a way that a part of the electrically conductive material  34  is free of the isolation layer  33 . With this, a bottom contact  35  is formed at the bottom side  18  of the substrate body  11  where a solder ball  38  is soldered. Therefore, the pressure sensor device  10  is surface-mountable. As a last step, the pressure sensor device  10  is singulated by dicing. 
       FIG. 4N  shows an exemplary embodiment of the pressure sensor device  10  processed by the method described with the  FIGS. 4A to 4M  with an additional top layer  20  on top of the sealing layer  32 . In this case the lateral extent of the membrane  13  has to be large enough to enable pressure sensing with the thicker and stiffer membrane  13  because of the additional top layer  20  on top of the membrane  13 . 
       FIG. 4O  shows an exemplary embodiment of a pressure sensor device  10  processed by the method shown with the  FIGS. 4A to 4M  without additional electrically conductive walls  29 . In this case a larger amount of the top layer  20  is etched away during the etching of the cavity  16 . This reduces the stiffness of the connection between the cap body  14  and the substrate body  11  and it reduces the stability of the whole pressure sensor device  10 . 
     With the  FIGS. 5A to 5L  another exemplary embodiment of the method for forming a pressure sensor device  10  is described. In this process flow the electrically conductive via  17  is formed in the substrate  43  before the membrane  13  is released. 
       FIG. 5A  shows the pressure sensor  12  on top of the substrate body  11  as described for  FIG. 4A . 
       FIG. 5B  shows the deposition of the top layer  20  on top of the pressure sensor  12  as described for  FIG. 4B . 
     In  FIG. 5C  it is shown that in the next step of the method the cap body  14  is connected with the top layer  20 . The cap body  14  and the top layer  20  are for example connected via direct bonding. Advantageously, the connection is stable at elevated temperatures, so that it is possible to deposit a final passivation or a sealing layer  32  on the membrane  13  in order to hermetically seal the membrane  13 . 
     As shown in  FIG. 5D  the substrate  43  is thinned to for example 100 to 200 μm via grinding. The substrate  43  is thinned in order to introduce an electrically conductive via  17  in the substrate  43 . 
     In the next step of the method as shown in  FIG. 5E  an electrically conductive via  17  is formed in the substrate  43 . The electrically conductive via  17  comprises two isolation layers  33  and an electrically conductive material  34 . 
     In  FIG. 5F  it is shown that in the next step of the method, a temporary handling wafer  36  is attached to the substrate  43  with an adhesive  37 . By attaching a temporary handling wafer  36  to the bottom side  18  of the substrate body  11 , the electrically conductive via  17  is protected from etching processes and mechanical damages. Therefore, advantageously the handling wafer  36  is only removed after the membrane  13  is released. The electrically conductive via  17  is also protected during grinding or etching of the cap body  14 . If the pressure sensor device  10  is heated to elevated temperatures during the further processing, the adhesive  37  can be a high temperature compatible material which is removed after the processing. It is also possible that the adhesive  37  is a high temperature compatible material which remains on the substrate body  11 . However, in this case the adhesive  37  needs to be patterned in such a way that the electrically conductive via  17  can be electrically contacted from the bottom side  18  of the substrate body  11 . If only lower temperatures are employed during the further processing the adhesive  37  can be a bonding or a tape which is compatible only with lower temperatures and which can be removed. It is also possible to employ a permanent bonding. 
       FIG. 5G  shows that in the next step of the method the opening  15  is introduced in the cap body  14 . Before introducing the opening  15 , the cap body  14  is thinned. The opening  15  can be introduced by deep reactive ion etching of the cap body  14 . 
     In  FIG. 5H  it is shown that the membrane  13  of the pressure sensor  12  is released. As described for  FIG. 4E , the sacrificial layer  24  and the top layer  20  are etched away through the opening  15  and the etch openings  28 . Since no additional electrically conductive walls  29  are introduced in the top layer  20  a part of the top layer  20  is also etched away below the cap body  14 . Preferably, the lateral distance that the top layer  20  is etched below the cap body  14  is less than 10 μm. After etching the pressure sensor device  10  is annealed at an elevated temperature in order to remove all residues and water. 
     As shown in  FIG. 5I  in a next step the temporary handling wafer  36  is removed. The isolation layer  33  on the back side  18  of the substrate body  11  is patterned in such a way that a part of the electrically conductive material  34  is free of the isolation layer  33  and a bottom contact  35  of the pressure sensor device  10  is formed. 
       FIG. 5J  shows that a sealing layer  32  is deposited onto the membrane  13 . As described for  FIG. 4F , the sealing layer  32  is a compressive film. Advantageously, the membrane  13  is only released after the formation of the electrically conductive via  17  in order to avoid membrane fracture during the processing and cleaning. 
     In  FIG. 5K  it is shown that the bottom contact  35  is contacted by a solder ball  38 . At this processing stage the pressure sensor device  10  can be tested. As a next step, the pressure sensor device  10  is singulated by for example dicing. 
       FIG. 5L  shows the dicing step. A dicing foil  42  is arranged on top of the pressure sensor device  10  which means that it is attached to the cap body  14  or the sealing layer  32  on top of the cap body  14 . This arrangement has the advantage that the membrane  13  is protected during the dicing and it does not have to be cleaned after the dicing step. Furthermore, because of the thickness of the cap body  14  the dicing foil  42  will not stick to the membrane  13 . Therefore, the dicing foil  42  can be removed without damaging the membrane  13 . 
       FIG. 6  shows an exemplary embodiment of the pressure sensor device  10  with electrically conductive walls  29  arranged around the membrane  13 . As described for  FIG. 4C  the electrically conductive walls  29  protect the top layer  20  from being etched during the release of the membrane  13 . Here, it is shown that the electrically conductive walls  29  are arranged on top of the electrically conductive layer  25  with the adhesion layers  26  and  27 . The handling wafer  36  is attached to the substrate body  11  by the adhesive  37  and the handling wafer  36  can be removed during processing. Advantageously, the adhesive  37  is compatible with high temperatures. If the adhesive  37  cannot be removed, it is required that it can be patterned such that the pressure sensor  12  and the integrated circuit  21  can be electrically contacted from the bottom side  18  of the substrate body  11 . 
       FIG. 7  shows an exemplary embodiment of the pressure sensor device  10  where the opening  15  in the cap body  14  is positioned above the pressure sensor  12  in vertical direction z, and the lateral extension of the opening  15  is smaller than the lateral extension of the pressure sensor  12 . A smaller opening  15  in the cap body  14  increases the mechanical stiffness of the pressure sensor device  10 . 
       FIG. 8  shows an exemplary embodiment of the pressure sensor device  10  where the lateral extension of the opening  15  in the cap body  14  is reduced, and the sealing layer  32  covers the membrane  13  and the cap body  14 . For a reduced opening  15 , it is still necessary to cover the whole membrane  13  with the sealing layer  32 . 
       FIG. 9  shows an exemplary embodiment of the pressure sensor device  10  with a reduced lateral extension of the opening  15  in the cap body  14  and the sealing layer  32  coating the membrane  13  and the cap body  14 . In this case the opening  15  is very small and the membrane  13  comprises fewer etch openings  28  in order to guarantee that all etch openings  28  of the membrane  13  are covered with the sealing layer  32 . 
     With  FIGS. 10A to 10K  an exemplary embodiment of the method for forming a pressure sensor device  10  is described. In this process flow the membrane  13  is released before introducing the electrically conductive via  17  in the substrate  43 . 
       FIG. 10A  shows the pressure sensor  12  on top of an integrated circuit  21  which is on top of the substrate  43 . The membrane  13  of the pressure sensor  12  is released and it is covered with the sealing layer  32 . 
     As shown in  FIG. 10B  in the next step of the method a photosensitive glue layer  39  is deposited on top of the sealing layer  32 . The glue layer  39  can be for example approximately 25 μm thick. The glue layer  39  can be polyimide-based and it can withstand elevated temperatures. At next, the glue layer  39  is patterned by employing a mask  31  in such a way that the area of the membrane  13  is not covered with the glue layer  39 . The glue layer  39  can be patterned by lithography. 
     As shown in  FIG. 10C  the cap body  14  is patterned or etched in such a way that one or more recesses are formed in the cap body  14 . The recesses can be formed by deep reactive ion etching and they can for example be approximately 300 μm deep. The recesses furthermore have a similar lateral extension as the membrane  13 . The patterned cap body  14  is positioned on top of the glue layer  39  in such a way that a recess is positioned above the membrane  13  in vertical direction z. The cap body  14  is connected with the glue layer  39  for example at an elevated temperature of 250° C. 
       FIG. 10D  shows that the substrate  43  is thinned to a thickness of approximately 100 to 200 μm. 
     As shown in  FIG. 10E  a trench which is formed in the substrate  43  is coated with an isolation layer  33 . The isolation layer  33  can comprise an oxide, a nitride or a polymer. As described for  FIG. 4I  the isolation layer  33  is in direct contact with the back contact  30  of the integrated circuit  21 . 
     In  FIG. 10F  it is shown that the isolation layer  33  is removed on the back contact  30 . At first, a resist layer  40  is deposited on the bottom side  18  of the substrate body  11 . The resist layer  40  is patterned in such a way that the isolation layer  33  is free of the resist layer  40  around the back contact  30 . The isolation layer  33  is removed from the back contact  30  by etching while the rest of the isolation layer  33  is protected by the resist layer  40 . At next, the resist layer  40  is removed. 
     As shown in  FIG. 10G  in the next step of the method an electrically conductive material  34  is deposited on the bottom side  18 . For example at first, tantalum or tantalum nitride can be deposited by physical vapor deposition, and in the next step copper is deposited also by physical vapor deposition. At next, another resist layer  40  is deposited and patterned. Afterwards, another electrically conductive material  34  which can be copper is deposited by electroplating through the resist layer. 
     In  FIG. 10H  it is shown that the remaining resist layer  40  and a part of the electrically conductive material  34  which is not within the trench are removed. The electrically conductive material  34  is removed by wet etching. Moreover, a further isolation layer  33  is deposited on the bottom side  18  of the substrate body  11 . 
       FIG. 10I  shows that the further isolation layer  33  is patterned in such a way that a part of the electrically conductive material  34  is free of the isolation layer  33  and a bottom contact  35  of the pressure sensor device  10  is formed. The bottom contact  35  is electrically contacted with a solder ball  38 . 
     In  FIG. 10J  it is shown that a grinding tape  41  is applied to the bottom side  18  of the substrate body  11 . At next, the cap body  14  is thinned to a thickness of for example approximately 230 μm. Then, the opening  15  is introduced in the cap body  14 , such that the mass of the substrate body  11  equals approximately the mass of the cap body  14 . 
     In  FIG. 10K  is shown that after removing the grinding tape  41  the pressure sensor device  10  can be tested and calibrated. The pressure sensor device  10  can be electrically contacted at the solder ball  38  and the membrane  13  can be exposed to air or other gas. Afterwards, the pressure sensor device  10  can be applied to a dicing foil  42  and it can be singulated via dicing. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.