Abstract:
A pressure sensor designed to detect a value of ambient pressure of the environment external to the pressure sensor includes: a first substrate having a buried cavity and a membrane suspended over the buried cavity; a second substrate having a recess, hermetically coupled to the first substrate so that the recess defines a sealed cavity the internal pressure value of which provides a pressure-reference value; and a channel formed at least in part in the first substrate and configured to arrange the buried cavity in communication with the environment external to the pressure sensor. The membrane undergoes deflection as a function of a difference of pressure between the pressure-reference value in the sealed cavity and the ambient-pressure value in the buried cavity.

Description:
BACKGROUND 
       [0001]    Technical Field 
         [0002]    The present disclosure relates to a pressure sensor in which the transduced pressure signal has a reduced ambient temperature dependence and to a manufacturing method thereof. 
         [0003]    Description of the Related Art 
         [0004]    As is known, a pressure transducer or sensor is a device that converts a variation of pressure into a variation of an electrical quantity (a resistance or a capacitance). In the case of a semiconductor sensor, the pressure variation is detected thanks to the presence of a membrane (or diaphragm) of semiconductor material that overlies a cavity and is able to undergo deflection in the presence of a force acting thereon. Deflection of the membrane is measured, for example, by piezoresistive elements constrained to the membrane itself (i.e., on the basis of the capacity of some materials to modify their resistivity as a function of the deflection to which they are subjected). 
         [0005]    Piezoresistors are normally provided on the edge of the suspended membrane and/or are connected together in Wheatstone-bridge configuration. Application of a pressure causes a deflection of the membrane, which in turn generates a variation of the offset voltage of the bridge. By detecting the variation of voltage with an appropriate electronic circuit it is thus possible to obtain the desired pressure information. 
         [0006]    Pressure sensors typically include a cavity in one side of the flexible membrane to enable deflection of the latter. This cavity forms a pressure reference and is provided so that said pressure reference is vacuum pressure. In this way, the membrane is under the influence of an absolute pressure. This type of pressure sensor finds wide application in the vacuum-technology industry, in space applications, and in all the sectors in which it is of interest to measure an atmospheric pressure with respect to an absolute reference that is minimally dependent upon external environmental conditions (e.g., the working temperature of the sensor itself). 
         [0007]    However, production of an absolute pressure sensor involves high-precision technological processes in order to couple the membrane perfectly on the cavity in vacuum conditions. 
         [0008]    Currently, various solutions have been proposed for manufacture of pressure sensors, amongst which: use of silicon-in-insulator (SOI) substrates; wet etching from the front (see for example U.S. Pat. No. 4,766,666); wet etching from the back; and other methods still (see, for example, U.S. Pat. No. 4,744,863). 
         [0009]    In all of the known aforementioned solutions, use of semiconductor technology for providing cavities underneath suspended structures and layers involves processes that are complex, costly, and in some cases scarcely compatible with the current steps used in the semiconductor industry for manufacture of integrated circuits. 
         [0010]      FIG. 1  is a lateral sectional view of a pressure sensor  10  of a known type, which provides a solution to the problems set forth above. According to the embodiment of  FIG. 1 , a membrane  1  extends in one upper side  5   a  of a wafer of semiconductor material  5 , for example silicon. The membrane  1  is suspended over a buried cavity  2 , which is formed according to the manufacturing method described in U.S. Pat. No. 8,173,513. 
         [0011]    The steps for obtaining the buried cavity  2 , described in U.S. Pat. No. 8,173,513, envisage a high-temperature processing of the wafer  5 , in an environment at controlled pressure (but not vacuum pressure). Following upon closing of the cavity  2  (completion of the formation of the overlying membrane  1 ), the wafer  5  is then cooled, thus generating a reduction of the pressure inside the buried cavity  2 . However, during use of the sensor  10  thus obtained, an increase in the temperature of the environment in which the sensor  10  operates causes a consequent increase in the pressure inside the buried cavity  2 . A variation of the absolute reference pressure provided by the pressure inside the buried cavity  2  leads to a variation of the transduced output signal of the sensor  10  also in the absence of a variation of the ambient pressure to be measured. These variations of the output signal may, at least in part, be compensated for during a step of processing of the output signal but are, however, undesirable. 
       BRIEF SUMMARY 
       [0012]    Some embodiments of the disclosure are a pressure sensor with reduced dependence upon the temperature of the transduced signal, and a manufacturing method thereof that will overcome the disadvantages of the known solutions. 
         [0013]    According to the present disclosure a pressure sensor and a manufacturing method thereof are provided. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    For an understanding of the present disclosure preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
           [0015]      FIG. 1  shows a cross-section of a pressure sensor of a known type; 
           [0016]      FIGS. 2 and 3  show in lateral sectional view and in top plan view, respectively, a pressure sensor according to one embodiment of the present disclosure; 
           [0017]      FIGS. 4 and 5  show in lateral cross-sectional view and in a top plan view, respectively, a pressure sensor according to a further embodiment of the present disclosure; 
           [0018]      FIG. 6  shows in lateral sectional view a pressure sensor according to a further embodiment of the present disclosure; 
           [0019]      FIG. 7  shows in lateral sectional view a pressure sensor according to a further embodiment of the present disclosure; 
           [0020]      FIG. 8A-14  show, in lateral sectional view, manufacturing steps of the pressure sensor of  FIGS. 2 and 3 ; and 
           [0021]      FIG. 15  shows an intermediate manufacturing step for obtaining the pressure sensor of  FIGS. 4 and 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 2  shows a lateral sectional view of a pressure transducer or sensor  11 , obtained in MEMS technology, according to one aspect of the present disclosure. The cross-section of  FIG. 2  is represented in a system of mutually orthogonal Cartesian axes X, Y, and Z, and is taken along a line of section shown in  FIG. 3 . The pressure sensor  11  comprises a body including a substrate  12 , of semiconductor material, such as silicon, coupled to a cap  14 , which is also of semiconductor material, such as silicon. The substrate  12  has a first face  12   a  and a second face  12   b , opposite to one another, along the axis Z. A cavity  16  extends within the substrate  12 , separated from the first face  12   a  by a thin portion of the substrate  12 , which forms a membrane  18  suspended over the cavity  16 . The membrane  18  has a thickness, along the axis Z, comprised between 4 and 40 for example 6 
         [0023]    The cavity  16  has a thickness, along the axis Z, smaller than the thickness, along Z, of the substrate  12 . In other words, the cavity  16  extends buried within the substrate  12 , between the first and second faces  12   a ,  12   b . According to one embodiment, the cavity  16  has, in a view in the plane XY, a circular or polygonal shape, for example square with side of 350 μm. Along Z, the cavity  16  has a depth comprised between 1 and 6 for example 4 μm. 
         [0024]    The cap  14  is coupled to the first face  12   a  of the substrate  12  in peripheral regions of the membrane  18 , by a coupling region  20 . Since the coupling region  20  is arranged in peripheral portions of the membrane  18 , during use the membrane  18  is free to undergo deflection and not undergo interference caused by the presence of the coupling region  20 . The coupling region  20  extends along the entire perimeter of the membrane  18  and is, for example, of a glass-frit type. Other types of bonding may be used, such as for example metal bonding (e.g., gold-gold), eutectic bonding (e.g., Al—Ge). 
         [0025]    The cap  14  has a recess  14 ′ directly facing the membrane  18 . The recess  14 ′ houses a getter layer  22 . The getter layer  22  has the function of generating, in use and when the recess  14 ′ is sealed (i.e., when the cap  14  is coupled to the substrate  12  by the coupling region  20 ), a reference pressure P REF  within the recess  14 ′ different from (in particular, lower than) the pressure P A  present in the environment outside the recess  14 ′. Materials used as getter layer  22  are known and comprise, for example, metals such as aluminum (Al), barium (Ba), zirconium (Zr), titanium (Ti), vanadium (V), iron (Fe), or corresponding mixtures or alloys, such as zirconium-aluminum, zirconium-vanadium-iron, zirconium-nickel, AND zirconium-cobalt (in particular, an alloy of Zr/Co/O). The getter layer  22  is, according to one embodiment, of a non-evaporable-getter (NEG) type, provided in the form of layer on the exposed surface of the recess  14 ′, in a manufacturing step prior to the step of coupling of the cap  14  to the substrate  12 . As is known, during the step of formation of the getter layer  22 , the material of which the getter layer  22  is made reacts with the surrounding air, causing formation of a passivating layer (typically of oxide or oxide/nitride) that coats the surface area of the getter layer  22  completely, rendering it inactive. Activation of the getter layer  22  occurs (following upon hermetic sealing of the recess  14 ′) by local activation in temperature in order to remove the passivating layer that has formed on the surface of the getter layer  22 . In this way, the getter layer  22  is activated and operates in a known way by reacting with residual gases within the recess  14 ′ and enabling a reduction of the reference pressure P REF  with respect to the ambient pressure P A . The reference pressure P REF  represents the vacuum pressure. 
         [0026]    It is evident that the getter layer  22  may be omitted in the case where the step of sealing of the recess  14 ′ takes place at controlled atmosphere and pressure. The extension of the recess  14 ′ along the axes X, Y, and Z is chosen so to generate, after coupling of the cap with the substrate  12 , a reference cavity  24  with a volume comprised between 1e-13 m 3  and 50e-13 m 3 , for example 10e-13 m 3 . The pressure P REF  inside the reference cavity  24  is the reference pressure for measurement of the absolute pressure by the pressure sensor  11 . It is consequently important for the coupling region  20  to seal the reference cavity  24  hermetically, preventing any exchange with the external ambient pressure P A . In use, i.e., following upon activation of the getter layer  22  or following upon the step of sealing of the recess  14 ′ should the sealing take place at controlled atmosphere and pressure, the reference pressure P REF  inside the reference cavity  24  has a value of approximately 0 mbar when measured at an ambient temperature of approximately 25° C. 
         [0027]    The pressure inside the cavity  16  is, in use, equal to the ambient pressure P A  to be measured. For this purpose, the cavity  16  is fluidically coupled to the environment external to the pressure sensor  11  so that its internal pressure stabilizes at the ambient pressure P A  (in this context, and in the following description, the fluid considered is air). For this purpose, according to one aspect of the present disclosure, the substrate  12  has one or more channels  26  (two channels  26  are illustrated in  FIG. 2 , with a dashed line in so far as they are not visible along the line of section of  FIG. 3 ), which connect the cavity  16  with the environment external to the pressure sensor  11 . The channels  26  are thus through holes provided in the substrate  12 . 
         [0028]    According to the embodiment of  FIG. 2 , the channels  26  have: a main extension along the axis X, for example of 100 μm; a thickness, along Z, equal to the thickness, along Z, of the cavity  16 , for example of 4 μm; and a dimension along Y for example of 10 μm. The dimension along Y of the channels  26  is smaller than the corresponding dimension of the cavity  16  in order not to generate undesirable structural weaknesses of the substrate  12 . 
         [0029]    According to one embodiment, the dimension of the channels  26  along Y is not constant, but is greater in the proximity of lateral faces  12   c ,  12   d  of the substrate  12  (which is shaped substantially like a squeezed funnel, or has a squeezed frustoconical shape, where the opening with smaller area directly faces the cavity  16 , and the opening with larger area faces the outside of the pressure sensor  11 ). In this way, any possible obstruction of the channels  26  by material deriving from outside the substrate  12 , for example during a dicing step for formation of the dice that integrate the pressure sensor  11 , is prevented. 
         [0030]    The channels  16  are formed, for example, during the same steps of creation of the cavity  16  and using the same manufacturing process (for example, the one described in U.S. Pat. No. 8,173,513). 
         [0031]    In this way, the cavity  16  is fluidically connected with the external environment, and the pressure inside the cavity  16  is the ambient pressure P A  to be measured. 
         [0032]      FIG. 3  is a top plan view of the pressure sensor  11  of  FIG. 2 , in the plane XY. As may be more fully appreciated from  FIG. 3 , according to this embodiment, four channels  26  are present that branch off symmetrically from respective regions of the cavity  16  and connect the latter with the outside of the sensor  11 . 
         [0033]    The membrane  18  further has one or more piezoresistive elements  28  arranged in peripheral regions of the membrane  18  and facing the inside of the reference cavity  24 . In other words, the piezoresistive elements  28  are formed in regions of the face  12   a  that, at the end of the manufacturing steps, are contained within the reference cavity  24 . The piezoresistive elements  28  may be protected by a thin layer of dielectric (e.g., silicon nitride with a thickness of approximately 0.5 or less) or else may face, or be exposed directly towards, the inside of the reference cavity  24 . In this case, since the reference cavity functions as protection for the piezoresistive elements  28 , they are not subject to deterioration caused by atmospheric agents present in the environment in which the pressure sensor  11  operates. Consequently, it is not necessary to provide a layer of protection of the piezoresistive elements  28 , with the advantage that these piezoresistive elements  28  are effectively housed in surface portions of the membrane  18 , which are more subject to stress during use. The sensitivity of the pressure sensor  11  is thus improved. 
         [0034]    According to one embodiment, the piezoresistive elements are provided as regions of a P type, formed by implantation of dopant atoms on the side  12   a  of the substrate  12 , whereas the portion of the substrate  12  that forms the membrane is of silicon with a doping of an N type. In  FIG. 3  four piezoresistors  28  are present, electrically connected together in Wheatstone-bridge configuration. The interconnections between the piezoresistors  28  (e.g., metal regions extending over an insulating layer), albeit present, are not represented in the figure exclusively for simplicity of representation. As an alternative to what has been said, the piezoresistors  28  may be made polysilicon, for example with a doping of a P type, deposited on the membrane  18 . A plurality of contact pads  30  extend in an area of the pressure sensor external to the membrane  18  and to the coupling region  20 . The contact pads  30  are of conductive material, such as metal, and form an interface for electrical connection with the outside of the sensor, for example for acquiring the transduced signal supplied at output by the Wheatstone-bridge circuit formed by the piezoresistors  28 . 
         [0035]    As is known, during the production processes, formed on a wafer of semiconductor material are a plurality of pressure sensors  11  of the type illustrated in  FIGS. 2 and 3 . A final processing step envisages dicing for forming dice, each of which houses a respective pressure sensor  11 . In the case where the channels  26  are formed during the same step of formation of the cavity  16 , it may happen that, during dicing, one or more of the channels  26  is occluded by waste material deriving from the dicing step itself. This effect is, obviously, undesirable. In order to overcome this drawback, one embodiment of the present disclosure envisages that the channels for connecting the cavity  16  with the outside are provided at the top, in the area of the cap  14 . This embodiment is illustrated in  FIGS. 4 and 5 .  FIG. 4  is, in particular, a cross-sectional view of the pressure sensor of  FIG. 5 , taken along the line of section V-V of  FIG. 5 . 
         [0036]    With reference to  FIG. 4 , a pressure sensor  31  is illustrated in lateral section. Elements of the pressure sensor  31  that are in common with those of the pressure sensor  11  of  FIG. 2  are designated by the same reference numbers and are not described herein any further. One or more channels  32  (two channels  32  are represented with a dashed line in  FIG. 4 , in so far as they are not visible along the line of section V-V), which have, in cross-sectional view in the plane XZ, the shape of an L or a T turned upside down, extend in the substrate  12  starting from the cavity  16 , in fluidic connection with the cavity  16 . A first portion  32 ′ of each channel  32  extends along the axis X as partial prolongation of the cavity  16  (the first portion  32 ′ is similar to the channels  26  of  FIGS. 2 and 3 ), whereas a second portion  32 ″ of each channel  32  extends along the axis Z starting from the first portion  32 ′ through the face  12   a  of the substrate  12 , and completely through the cap  14 . In this way, openings  36  are provided, fluidically connected with the cavity  16 , on an upper surface  14   a  of the cap  14 , exposed towards the external environment and thus at the pressure P A  to be measured. The second portions  32 ″ of the channels  32  are formed, for example, by anisotropic etching such as deep reactive ion etching (DRIE), whereas the first portions  32 ′ of the channels  32  are formed, for example, during the same step of formation of the cavity  16  (for instance, according to the process described in U.S. Pat. No. 8,173,513), or using some other technique. 
         [0037]      FIG. 5  shows, in a top plan view in the plane XY, the pressure sensor of  FIG. 4 . As may be noted from  FIG. 5 , four openings  36  are present at the front of the pressure sensor  31 , formed laterally with respect to the coupling region  20 . 
         [0038]    According to the embodiment of  FIGS. 4 and 5 , the second portions  32 ″ of the channels  32  extend outside the coupling region  20 . According to a further embodiment, illustrated in  FIG. 6 , a pressure sensor  39  comprises second portions  32 ″ of the channels  32  that extend through the coupling region  20 . In this case, during the step of formation of the coupling region  20 , through openings  40  may be provided in portions of the coupling region  20  in which a respective second portion  32 ″ of a respective channel  32  will be formed. 
         [0039]    According to a further embodiment, illustrated in  FIG. 7 , a pressure sensor  41  comprises channels  32  that extend exclusively in the substrate  12  (and not also through the cap  14 ). In this case, openings  44  for fluidic access towards the cavity  16  are provided on the face  12   a  of the substrate  12 , laterally with respect to the coupling region  20  and outside the reference cavity  24 . According to this embodiment, formation of the channels  32  is carried out prior to the steps of formation of the coupling regions  20  and of coupling between the substrate  12  and the cap  14 . In order to prevent the step of formation of the coupling region  20  from causing an undesirable obstruction of the channels  32  thus formed, it is expedient to provide the channels  32  at a sufficient distance from the portions of the face  12   a  in which the coupling region  20  is to be formed, for example at a distance, measured along the axis X, of 100 μm in the case of glass frit (in the case of use of some other bonding techniques, for example metal bonding, this distance may be different, either smaller or greater). In order to prevent the material used for the glass frit from expanding on the surface  12   a  of the substrate  12  in an undesirable way, there may be provided containment regions for example in the form of trenches (not illustrated) obtained by etching selective portions of the surface  12   a , alongside the surface area in which the coupling region  20  is to be formed. 
         [0040]    Described in what follows is a method for manufacturing the pressure sensor of  FIGS. 2 and 3 . 
         [0041]      FIG. 8A  is a cross-sectional view of a semiconductor wafer, made in particular of monocrystalline silicon, during an initial step of manufacture of the pressure sensor  11 . With reference to  FIG. 8A , the semiconductor wafer comprises a substrate  12  of an N type. Provided on the surface of the substrate  12  is a resist mask  103 . As may be seen in  FIG. 8B  (which shows a qualitative top plan view, not in scale, of the wafer of  FIG. 8A ), the mask  103  has a circular or generically polygonal sensor area  103 ′ (for example, square, as illustrated in  FIG. 8B ), and likewise has elongated regions  103 ″ that depart from the sensor area  103 ′ and define, as regards shape and dimensions, the channels  26  that are to be formed. The mask  103  defines a honeycomb lattice (as may be noted more clearly from the enlarged portion of  FIG. 8C ), which presents mask regions of a hexagonal shape arranged up close to one another. 
         [0042]    Using the mask  103  ( FIG. 9 ), etching of the substrate  12  is carried out to form a trench  106 , which has a depth of some microns, for example approximately 10 μm, and defines silicon columns  107  that are the same as one another and have a shape corresponding to the shape of the honeycomb regions defined by the mask  103 . Likewise illustrated in  FIG. 9  (with a dashed line) are trenches  108  that will lead, in subsequent processing steps, to formation of the channels  26 . 
         [0043]    Next ( FIG. 10 ), the mask  103  is removed and an epitaxial growth is carried out in deoxidizing environment (typically, in atmosphere that presents a high concentration of hydrogen, preferably using trichlorosilane —SiHCl 3 ). Consequently, an epitaxial layer  110  (hereinafter not distinguished from the substrate  12  and designated by the same reference number  12 ), of an N type, grows above the silicon columns  107  and closes the trench  106  at the top, trapping the gas present therein (here, molecules of hydrogen —H 2 ). The thickness of the epitaxial layer  110  is of some microns, for example between 1 and 40 μm. 
         [0044]    An annealing step is then carried out, for example for 30 minutes at 1190° C. The annealing step causes ( FIG. 11 ), in a per se known manner, a migration of the silicon atoms, which tend to move into the position of lower energy. Consequently, in the area of the trenches  106  and  108 , where the silicon columns are arranged close to one another, the silicon atoms migrate completely and form, respectively, the cavity  16 , closed at the top by the membrane  18 , and the channels  26 , which are also closed at the top by the silicon regions alongside the membrane  18 . Owing to the presence of the cavity  16 , the membrane  18  is flexible and may undergo deflection. 
         [0045]    Preferably, annealing is carried out in an H 2  atmosphere for preventing the hydrogen present in the trench  106  from escaping through the epitaxial layer  110  outwards and for increasing the concentration of hydrogen present in the cavity  16  and in the channels  26 , in the case where the hydrogen trapped during the step of epitaxial growth were not sufficient. Alternatively, annealing may be carried out in nitrogen environment. 
         [0046]    Next, in a way not illustrated in the figure, selective portions of the membrane  18  are doped via implantation of dopant species of a P type, for example boron, in order to provide the piezoresistive elements  28 . The step of formation of piezoresistors in selective portions of a membrane, as likewise their Wheatstone-bridge connection, is per se known and is thus not described herein any further. 
         [0047]    If so desired, it is possible to integrate electronic components, constituting the control circuitry of the pressure sensor  11 , and/or electrical contact pads (e.g., the pads  30  of  FIG. 3 ) within the substrate  12 , in regions external to the membrane  18 , in a per se known manner that does not form the subject of the present disclosure. 
         [0048]    Then ( FIG. 12 ), a wafer is laid, having a substrate  120 , in which the cap  14  is to be provided. For this purpose, the substrate  120  is etched on a face  14   b  thereof to form the recess  14 ′, which extends in depth into the substrate  120  for a thickness smaller than the thickness of the substrate  120  itself. The recess  14 ′ has dimensions, in the plane XY, such that, when the cap  14  is coupled to the substrate  12 , the recess  14 ′ is designed to surround the membrane  18  and the piezoresistors  28  completely. Formed within the recess  14 ′, as already described previously, is the getter layer  22 , in a per se known manner. 
         [0049]    Next, as illustrated in  FIG. 13 , the wafer comprising the substrate  12  (in the processing step of  FIG. 11 ) and the wafer comprising the substrate  120  (in the processing step of  FIG. 12 ) are coupled together (step known as “wafer-to-wafer bonding”) to form the coupling regions  20 , for example by the glass-frit technique, so that the recess  14 ′ faces, and completely surrounds, the membrane  18  (and the piezoresistors  28  integrated in the membrane  18 ). Preferably, the coupling regions  20  extend, on the face  12   a  of the substrate  12 , so to not project laterally (i.e., along X) with respect to the channels  26 . In this way, a subsequent dicing step is carried out exclusively through supports of semiconductor material (the cap  14  and the substrate  12 ), and not also through the coupling regions  20 , which could be of a material not compatible with the dicing tools used. 
         [0050]    Finally ( FIG. 14 ), the wafer is cut, along dicing lines  53 , into dice, each containing a respective pressure sensor  11  thus obtained. 
         [0051]    In order to manufacture the pressure sensor  31  of  FIGS. 4 and 5 , after the step of  FIG. 13 , step of  FIG. 15  is envisaged, in which a DRIE step is carried out (using an appropriate mask, not illustrated) on top regions of the cap  14  aligned, along Z, with respective portions of the channels formed during the step of  FIG. 9 . The second portions  32 ″ of the channels for access to the cavity  16  are thus formed (which, in  FIG. 15 , are represented with a dashed line in so far as they are not visible along the line of section VIII-VIII of  FIG. 8B ). Then the dicing step of  FIG. 14  is carried out. 
         [0052]    Appropriate alignment markers may be envisaged, in a per se known manner, in order to facilitate identification of the top regions of the cap  14  aligned, along Z, with respective portions of the channels formed during the step of  FIG. 9 . 
         [0053]    The advantages that may be achieved with the pressure sensor described emerge clearly from the foregoing description. 
         [0054]    In particular, the transduced pressure signal, generated at output from the pressure sensor according to any one of the embodiments described, does not depend upon the residual pressure that is present in a buried cavity. In fact, the reference pressure is now given by the pressure present in a cavity obtained by a process of coupling of substrates, which may be controlled with high precision (for example, using a getter layer). The reference pressure, according to the present disclosure, does not vary, or varies minimally, with the temperature of the environment in which the pressure sensor works. 
         [0055]    Furthermore, since the transducer elements (piezoresistors) face the inside of the reference cavity (which is hermetically closed), they are immune from any impurities and atmospheric agents (dust, humidity, etc.) that might damage them or generate variations of the signal transduced thereby that are unforeseeable and may not be compensated for. 
         [0056]    Finally, thanks to the manufacturing process described, the pressure sensor of silicon has a low cost and reduced dimensions, as well as an improved resistance to failure. In fact, since the cavity that receives the ambient pressure is of a buried type, to obtain it no further step of coupling between substrates is required. 
         [0057]    Finally, it is clear that numerous modifications and variations may be made to the pressure sensor described and illustrated herein, all of which fall within the scope of the inventive idea, as defined in the annexed claims. 
         [0058]    For instance, the transduced signal generated as a function of deflection of the membrane  18  may be generated by a capacitive coupling of conductive regions of the membrane  18  with a fixed reference electrode. The conductive regions of the membrane  18  comprise, for example, a thin metal layer, formed by deposition techniques of a known type. In this case, the piezoresistive elements are not necessary, and the cavity  24  further houses the fixed reference electrode; the latter faces the conductive regions of the membrane  18  so that the fixed reference electrode and the membrane  18  form respective plates of a capacitor. In use, deflection of the membrane causes a variation of the capacitance of the capacitor thus formed. The measurement of said variation of capacitance may be correlated to the deflection of the membrane  18  which in turn may be correlated to the ambient pressure P A  acting thereon. The ambient pressure P A  may thus be measured. 
         [0059]    Furthermore, the channel for connecting the cavity  16  with the external environment may be formed for connecting the cavity  16  with the face  12   b  of the substrate  12 , by providing fluidic access openings on said face  12   b . The process of formation of said openings is similar to the process already described with reference to  FIGS. 4 and 5  (e.g., DRIE of the substrate  12  starting from the face  12   b ). 
         [0060]    The various embodiments described above can be combined to provide further embodiments. All of the 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 are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0061]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.