Abstract:
A process for manufacturing an integrated differential pressure sensor includes forming, in a monolithic body of semiconductor material having a first face and a second face, a cavity extending at a distance from the first face and delimiting therewith a flexible membrane, forming an access passage in fluid communication with the cavity, and forming, in the flexible membrane, at least one transduction element configured so as to convert a deformation of the flexible membrane into electrical signals. The cavity is formed in a position set at a distance from the second face and delimits, together with the second face, a portion of the monolithic body. In order to form the access passage, the monolithic body is etched so as to form an access trench extending through it.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an integrated differential pressure sensor and to a manufacturing process thereof. 
   2. Description of the Related Art 
   Differential pressure sensors are made using known semiconductor technology.  FIG. 1  illustrates by way of example a differential pressure sensor  1  of a piezoresistive type. 
   In detail, the differential pressure sensor  1  comprises a substrate  2  of semiconductor material (typically silicon), having a cavity  3  dug and accessible from the back of the substrate  2  and a flexible membrane  4  suspended above the cavity  3 . Piezoresistive elements  5 , connected in a Wheatstone-bridge configuration, are diffused in a surface portion of the flexible membrane  4  and are contacted by metallizations  6 , and a passivation layer  7 , made of thermal oxide, coats the top surface of the substrate  2 . The back of the substrate  2  is bonded to a base layer  8 , preferably made of Pyrex™ glass, or alternatively of silicon. The joining between the substrate  2  and the base layer  8  can be, for example, guaranteed by an intermediate layer  9 , of a lead-based paste (glass frit). An access opening  10  traverses the base layer  8  and the intermediate layer  9 , and reaches the cavity  3 . 
   In use, the top side of the flexible membrane  4  (i.e., the side opposite to the cavity  3 ) is placed in communication with a first chamber (not shown) containing a fluid at a first pressure, and the cavity  3  is placed in fluid communication with a second chamber (not shown), containing a fluid at a second pressure, through the access opening  10 . Consequently, the flexible membrane  4  is deformed as a function of the difference between the first pressure and the second pressure, and said deformation brings about an unbalancing of the Wheatstone bridge formed by the piezoresistive elements  5 . Said unbalancing may be detected by appropriate sensing electronics, which derives therefrom the desired differential pressure measurement. 
     FIG. 2  shows a package  11  of a known type housing the differential pressure sensor  1 . In detail, the package  11  is made of thermoplastic material, and has a chamber  12 , to a bottom internal surface of which the base layer  8  of the differential pressure sensor  1  is bonded via a layer of adhesive material  13 . The chamber  12  is filled with a silicone coating gel  14 , and is closed at the top by a metal cover  15 , which further delimits a main top surface of the package  11 . The silicone coating gel  14  surrounds and coats the differential pressure sensor  1 , and acts as a protection against the external environment. The metal cover  15  has a first opening  16 , which is placed, in use, in fluid communication with the first chamber. Furthermore, the base of the package  11 , in a position corresponding to the access opening  10 , has a second opening  17  connected to the access opening  10  and placed, in use, in fluid communication with the second chamber. The electrical connection between the differential pressure sensor  1  and the outside of the package  11  is provided via metal leads  19 , which come out of the package  11 . 
   Alternatively ( FIG. 3 ), packages  11  are known comprising a ceramic base  22 , bonded to which is the differential pressure sensor  1 , and a metal casing  23 , which encloses the differential pressure sensor  1  and rests on the ceramic base  22  in contact therewith. The metal casing  23  is open at the top to form a first port  24 , which is placed, in use, in fluid communication with the first chamber. Furthermore, the inside of the metal casing  23  is filled with a silicone coating gel  25 , which surrounds and coats the differential pressure sensor  1 . Through the ceramic base  22 , in a position corresponding to the access opening  10 , a passage  26  is provided, through which a second port  27  is placed in communication with the access opening  10 . Furthermore, in use, the second port  27  is placed in fluid communication with the second chamber. Electrical connection between the differential pressure sensor and the outside of the package  11  is provided via metal leads  28 , which come out of the ceramic base  22  through further passages  29  provided in the ceramic base  22 . 
   The pressure sensor described, though enabling a differential pressure measurement to be carried out, has, however, rather large dimensions, principally due to the need to perform a digging from the back of the substrate  2 . The manufacturing process, for similar reasons, is rather complex and costly, principally due to the need to perform the digging from the back (generally via a TMAH etching) and the bonding between the substrate  2  and the base layer  8 . Clearly, said disadvantages are particularly evident in applications wherein features such as economy and simplicity of production are constraining design characteristics. 
   BRIEF SUMMARY OF THE INVENTION 
   A differential pressure sensor is provided that will enable the disadvantages and problems referred to above to be overcome, and in particular that will be simple to manufacture at a low cost. 
   In one embodiment of the present invention, a process for manufacturing an integrated differential pressure sensor includes forming, in a monolithic body of semiconductor material having a first face and a second face, a cavity extending a distance from the first face and extending a distance from the second face. The cavity delimits in conjunction with the first face a flexible membrane and delimits in conjunction with the second face a portion of the monolithic body. Additionally, the process includes forming an access passage in fluid communication with the cavity, including etching the monolithic body to form an access trench extending through the monolithic body. The process further includes forming, in the flexible membrane, at least one transduction element configured to convert a deformation of the flexible membrane into electrical signals. 
   In another embodiment of the present invention, an integrated differential pressure sensor includes a monolithic body of semiconductor material having a first face and a second face, and a cavity buried within the monolithic body and extending in the monolithic body a distance from the first face and delimiting in conjunction with the first face a flexible membrane. Furthermore, the buried cavity extends a distance from the second face and delimits, in conjunction with the second face, a portion of the monolithic body. The sensor includes an access passage in fluid communication with the cavity, where the access passage includes an access trench extending through the monolithic body, and at least one transduction element formed in the flexible membrane and configured so as to convert a deformation of the flexible membrane into electrical signals. 
   In yet another embodiment of the invention, a pressure sensor system includes an integrated differential pressure sensor and a package configured to house the pressure sensor. The integrated differential pressure sensor includes a monolithic body of semiconductor material having a first face and a second face, and a cavity buried within the monolithic body and extending a distance from the first face and delimiting, in conjunction with the first face, a flexible membrane. Furthermore, the cavity extends a distance from the second face and delimits, in conjunction with the second face, a portion of the monolithic body. Additionally, the sensor includes an access passage in fluid communication with the cavity. The access passage has a buried connection channel in fluid communication with the cavity and an access trench extending through the monolithic body. In one embodiment, the access trench extends between the first face and the buried connection channel. The sensor includes at least one transduction element formed in the flexible membrane and configured to convert a deformation of the flexible membrane into electrical signals. 
   The package includes a first portion and a second portion mechanically coupled and defining an internal space facing the flexible membrane. The internal space is fluidally connected to a first opening of the package and insulated in a fluid-tight way from the access trench via a fluid-tight means. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the present invention, preferred embodiments thereof are described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
       FIG. 1  is a cross-sectional view of a differential pressure sensor, of a known type; 
       FIG. 2  is a cross-sectional view of a package of a known type housing the differential pressure sensor illustrated in  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of a different package of a known type housing the differential pressure sensor illustrated in  FIG. 1 ; 
       FIG. 4  shows a top plan view of a wafer of semiconductor material in an initial step of a process for manufacturing a differential pressure sensor; 
       FIG. 5  is a cross-sectional view at an enlarged scale of details of  FIG. 4 ; 
       FIGS. 6-9  show cross sections in subsequent steps of the manufacturing process, according to a first embodiment of the present invention; 
       FIG. 10  is a top plan view of a wafer of semiconductor material in a final step of a process for manufacturing a differential pressure sensor, according to a second embodiment of the present invention; 
       FIG. 11  is a cross-sectional view of the pressure sensor illustrated in  FIG. 10 , taken along the line XI-XI; 
       FIG. 12  is a cross-sectional view of a differential pressure sensor according to a third embodiment of the present invention; 
       FIG. 13  is a cross-sectional view of a package housing the pressure sensor illustrated in  FIG. 12 ; 
       FIG. 14  is a schematic perspective view of the package of  FIG. 13 ; 
       FIG. 15  is a cross-sectional view of a capacitive differential pressure sensor according to a fourth embodiment of the present invention; and 
       FIG. 16  is a cross-sectional view of a wafer of semiconductor material in an optional step of the manufacturing process according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of a process for manufacturing an integrated differential pressure sensor are now described. Such a manufacturing process is based upon the processes described in the patent application No. EP-A-1 324 382 and in the European patent application No. 04 425 197.3, filed in the name of the present applicant on Mar. 19, 2004. 
     FIG. 4  (which is not in scale, as neither are the following figures) shows a wafer  30  made of semiconductor material, for example monocrystalline silicon, comprising a substrate  31 , for example of an N type, designed to form the bulk of the differential pressure sensor, and having a front  30   a  and a back  30   b  (see also  FIG. 9 ). 
   In an initial step of the manufacturing process, a resist mask  32  is formed on the wafer  30  (see also to the cross section of  FIG. 5 ). In detail, the resist mask  32  has an approximately square area comprising a plurality of mask portions  32   a  having an approximately hexagonal shape, and defining a honeycomb lattice (as shown in the enlarged detail of  FIG. 4 ). In one embodiment, for example, the distance t between opposite sides of the mask portions  32   a  is 2 μm, whilst the distance d between facing sides of adjacent mask portions  32   a  is 1 μm. 
   Using the resist mask  32  ( FIG. 6 ), an anisotropic etching of the substrate  31  is performed, following upon which trenches  33  are formed, which delimit pillars  34  of silicon having a cross section corresponding to the mask portions  32   a . The trenches  33 , having for example a 10 μm depth, communicate with one another and together form a labyrinthine region  33   a  of a complex shape, with a cross section corresponding to the honeycomb lattice of the resist mask  32 . 
   Next ( FIG. 7 ), the resist mask  32  is removed and an epitaxial growth is performed in a deoxidizing environment (typically, in an atmosphere with high hydrogen concentration, preferably with trichlorosilane-SiHCl 3 ). Consequently, an epitaxial layer  35  (indicated only in  FIG. 7  and not distinguished from the substrate  31  in what follows), for example of an N type and of a thickness of 9 μm, grows on top of the pillars  34  and closes the labyrinthine region  33   a  at the top, entrapping the gas therein. A thermal annealing, for example for thirty minutes at 1190° C., is then performed preferably in a hydrogen atmosphere, or, alternatively, a nitrogen atmosphere. As discussed in the patent applications referred to above, the annealing step causes a migration of the silicon atoms, which tend to move into the position of lower energy. Consequently, and also thanks to the small distance between the pillars  34 , the silicon atoms migrate completely from the portions of the pillars  34  within the labyrinthine region  33   a , and a buried cavity  36  is formed, closed within the substrate  31 . For example, the buried cavity  36  has a side of 500 μm. On top of the buried cavity  36  there remains a thin silicon layer, made up in part by epitaxially grown silicon atoms and in part by migrated silicon atoms, which forms a membrane  37 , which is flexible, is suspended above the buried cavity  36 , and can be deflected in the presence of external stresses. 
   Next ( FIG. 8 ), piezoresistive elements  38  are formed in a surface portion of the membrane  37  opposite to the buried cavity  36  (the piezoresistive elements  38  are illustrated only in  FIG. 8  and no longer appear in the subsequent figures). In detail, the piezoresistive elements  38  are formed by means of P type diffusion or implantation, for example of boron atoms, and are connected to one another in a Wheatstone-bridge configuration. Alternatively to what is illustrated, the piezoresistive elements  38  can be made of polysilicon on top of the membrane  37 . 
   According to a first embodiment of the present invention (see  FIG. 9 ), a front/back alignment of the wafer  30  is then performed, followed by a digging from the back  30   b  via an anisotropic etching so as to provide an access trench  42 , which traverses a large part of the substrate  31  until it reaches the buried cavity  36 . The etching is performed during a fixed time interval, in such a way as not to reach the internal surface of the membrane  37  (in contact with the buried cavity  36 ). Next, the wafer  30  is cut so as to form dice, each of which comprises a differential pressure sensor. 
   In use, the external surface of the membrane  37  (i.e., the one opposite to the buried cavity  36 ) is placed in communication with a first chamber (not illustrated) containing a fluid at a first pressure, whilst the internal surface of the membrane  37  is placed in fluid communication with a second chamber (not illustrated) containing a fluid at a second pressure, through the access trench  42 . In this way, the external surface of the membrane  37  is subjected to the pressure of the fluid contained in the first chamber, whilst the internal surface of the membrane  37  is subjected to the pressure of the fluid contained in the second chamber, and the membrane  37  undergoes a deformation that is a function of the difference between the first pressure and the second pressure. Said deformation causes unbalancing of the Wheatstone bridge formed by the piezoresistive elements  38 , which, in a per se known and not illustrated manner, is detected by an appropriate electronic sensing circuit, generally comprising an instrumentation amplifier. From the detected unbalancing, the electronic sensing circuit derives the desired differential pressure measurement. 
   In order not to damage the membrane  37  during formation of the access trench  42 , thus changing the mechanical characteristics thereof, a second embodiment is proposed, which is illustrated in  FIGS. 10 and 11 . In detail, simultaneously with the formation of the buried cavity  36 , a connection channel  44  is formed, buried within the substrate  31 , in a lateral position with respect to the buried cavity  36  and in fluid communication therewith. For said purpose, process steps are performed that are substantially similar to the ones previously described (and for this reason are not described again), but starting from a resist mask  32  that laterally has a rectangular projection of a shape corresponding to the desired shape of the connection channel  44 . During the final steps of the manufacturing process, the access trench  42  is not provided in a position corresponding to the buried cavity  36 , but in a position corresponding to the connection channel  44  in such a way that a possible overetching will involve a portion of the wafer  30  overlying the connection channel  44 , instead of the membrane  37 . 
   A third embodiment, illustrated in  FIG. 12 , again envisages the formation of the connection channel  44  in a lateral position with respect to the buried cavity  36 , and in fluid communication therewith. However, unlike the second embodiment described, a digging from the front  30   a  of the wafer  30  is performed to provide the access trench  42 , which reaches the connection channel  44 . In this way, advantageously the digging step is considerably simplified, in so far as it is necessary to traverse a much smaller thickness of silicon (around 10 μm) as compared to the etching from the back  30   b  of the wafer  30 . Consequently, in this case, the access trench  42  is accessible from the front  30   a , instead of from the back  30   b  of the wafer  30 . 
   The first two embodiments do not impose any particular constraints on the package of the differential pressure sensor, which can be of a traditional type. On the contrary, the third embodiment imposes, to enable the differential pressure measurement, fluid-tight insulation between the area overlying the membrane  37  and the access trench  42  provided on the front  30   a.    
   According to an embodiment of the present invention, a package  50  suited for the purpose ( FIGS. 13 and 14 ) is consequently proposed. In detail, the package  50  is of a pre-molded plastic type, and comprises a base member  51  and a cover  52 . The base member  51  is open at the top and houses the differential pressure sensor inside it. In particular, the die of the differential pressure sensor is bonded to an internal bottom surface of the base member  51 , via a layer of adhesive material  53 . The base member  51  has, at the top, in an area corresponding to its open portion, a first threaded portion  54 , and the cover  52  has a second threaded portion  55  complementary to the first threaded portion  54 , designed to be screwed to the first threaded portion  54  so as to close the package  50 . The cover  52  has, on the top, a first opening  58 , which is connected to a first open duct  59 , placed in communication with the membrane  37 . The base member  51  has laterally a second opening  60 , which is connected to a second open duct  61 , placed in fluid communication with the access trench  42 . In addition, an internal surface of the cover  52 , facing the membrane  37 , is provided with slots  56 , which house a seal ring  57 , made of silicone resin. When the cover  52  is screwed to the base member  51 , the seal ring  57  bears upon the substrate  31 , outside the membrane  37  so as to insulate in a fluid-tight manner the access trench  42  from the first opening  58 . In particular, the seal ring  57  does not rest on the membrane  37  so as not to exert a pressure on the membrane  37  and hence so as not to alter mechanical characteristics thereof. Conveniently, an internal area  62  of the package  50  overlying the membrane  37  is filled with a protection gel, for example a silicone gel, to protect the membrane  37  from the external environment. 
   In use, the first and second open ducts  59 ,  61  are fluidally connected to the first and second chamber, respectively, in such a way that the membrane  37  is subjected to the difference of pressure of the fluids contained in the two chambers and is deformed accordingly. The seal ring  57  operates in such a way that the fluid contained in the second chamber will not come into contact with the top surface of the membrane  37 . 
   The assembly process of the package  50  envisages initially bonding of the substrate  31  to the internal bottom surface of the base member  51 ; then screwing of the cover  52  to the base member  51  so as to close the package  50  and to provide simultaneously fluid-tight insulation between the internal area  62  of the package  50  overlying the membrane  37  and the access trench  42 , via the seal ring  57 ; and finally, conveniently, introducing the protection gel through the first duct  59  so as to occupy the internal area  62 . 
   The various embodiments of the differential pressure sensor disclosed herein have reduced dimensions, in so far as they are integrated in a single monolithic body of semiconductor material and do not require the use of wafers of different materials and their consequent bonding. The manufacturing process is simple and inexpensive, and compatible with the integration of a corresponding electronic read circuit. 
   Furthermore, the sensitive part of the sensor (in particular, the membrane  37  and the piezoresistive elements  38 ) is automatically protected mechanically from the back  30   b  of the wafer by the fact that the buried cavity  36  is formed within the substrate  31 . In addition, given that the buried cavity  36  has a thickness of a few microns, the possibility of deflection of the membrane is limited in order to prevent any breakdown of the pressure sensor. 
   The mass of the membrane  37  is smaller than that of solutions of a known type, which means shorter response times of the differential pressure sensor. 
   Furthermore, the third embodiment described enables a further simplification of the manufacturing process, in so far as it eliminates the need for a front/back alignment of the wafer, given the absence of a digging from the back  30   b  of the wafer. Furthermore, the time for manufacturing is reduced, in so far as the etching from the front  30   a  is faster. 
   The differential pressure sensor described can advantageously be used in a plurality of applications, for example to measure the level of the water in washing machines and dish-washers, or else, in the automotive field, for monitoring pressure in airbags or inflation pressure of the tires, for monitoring the oil pressure or the fuel injection pressure, or for controlling the breakdown pressure of the ABS system. 
   Finally, it is clear that modifications and variations can be made to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the appended claims. 
   In particular, the described manufacturing process can be applied, with minor modifications, to the manufacturing of a differential pressure sensor of a capacitive type ( FIG. 15 ). In this case, the membrane  37  is doped, for example in a final step of the manufacturing process, via an implantation of dopant species of a P type conductivity, opposite to that of the substrate  31  (for example, using boron atoms). Then, implantations of a P +  type and of an N +  type and corresponding diffusion steps are performed to provide, respectively, first and second contact regions  70   a ,  70   b , in positions corresponding to the membrane  37  and to the bulk of the substrate  31 . Next, metal contacts  71   a ,  71   b  are formed on top of the first and second contact regions  70   a  and  70   b , respectively. In this way, the membrane  37  and the bulk of the substrate  31  form the electrodes of a capacitor  72  (represented schematically in  FIG. 15 ), the dielectric of which is constituted by the gas contained in the buried cavity  36 , and the capacitance of which varies following upon the deformations of the membrane  37 . Clearly, the manufacturing process does not envisage in this case the formation of the piezoresistive elements  38 , whilst the etching steps leading to the formation of the access trench  42  and of the connection channel  44  are substantially the same. 
   Furthermore (see  FIG. 16 ), the first embodiment described may envisage the formation of a stop oxide layer  73  on the internal walls of the buried cavity  36 , prior to etching from the back  30   b , which leads to the formation of the access trench  42 . In particular, for said purpose a digging from the front  30   a  is first performed via an anisotropic etching, to provide an oxidation trench  74  which reaches the buried cavity  36 . Then, a thermal oxidation is performed by supplying oxygen through the oxidation trench  74  so as to form the stop oxide layer  73 . Said embodiment is advantageous in so far as it eliminates the risk of overetching the membrane  37  during digging of the access trench  42 ; in fact, in this case, the etching can be performed with end-stop on the stop oxide layer  73 . 
   Furthermore, the geometrical shape of the membrane can be different, for example can be circular or generically polygonal. The structure of the resist mask  32  and the shape of the pillars  34  may vary with respect to what is illustrated. The pillars  34  can be replaced by walls of semiconductor material of a reduced thickness, or in general by other thin structures such as to enable migration of the silicon during the annealing step leading to the formation of the buried cavity  36 . For example, the walls can be rectilinear, parallel to one another, and separated by deep trenches. 
   Finally, in a final step of the manufacturing process, it is possible to integrate the electronic read circuit of the differential pressure sensor within the wafer  30 , i.e., together with the differential pressure sensor. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.