Abstract:
The method inlcudes the steps of forming a sacrificial buried region of insulating material on a substrate of monocrystalline semiconductor material, epitaxially growing a first semiconductor material layer on the substrate, the first semiconductor material layer including a polycrystalline region over the sacrificial buried region and a monocrystalline region elsewhere, the substrate and the semiconductor material layer surrounding the sacrificial buried region on all sides, and removing the sacrificial buried region. The portion of the polycrystalline region surrounded by the trench thus forms a suspended structure separated and isolated thermally from the rest of the semiconductor material layer. Using microelectronics processes, electronic components are formed in the monocrystalline region, and dedicated regions are formed at the suspended structure, so that the electronic components are integrated in the same chip with static, kinematic or dynamic microstructures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 09/110,692, filed Jul. 7, 1998, entitled MANUFACTURING METHOD AND INTEGRATED MICROSTRUCTURES OF SEMICONDUCTOR MATERIAL AND INTEGRATED PIEZORESISTIVE PRESSURE SENSOR HAVING A DIAPHRAGM OF POLYCRYSTALLINE SEMICONDUCTOR MATERIAL, now abandoned, which in turn is a continuation-in-part of application Ser. No. 08/903,164, filed Jul. 30, 1997, entitled METHOD FOR FABRICATING INTEGRATED MICROSTRUCTURES OF SEMICONDUCTOR MATERIAL, now abandoned, which prior applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of fabricating integrated microstructures of semiconductor material and, in particular, to a method of fabricating an integrated piezoresistive pressure sensor having a diaphragm of polycrystalline semiconductor material. The present invention also relates to an integrated piezoresistive pressure sensor having a diaphragm of polycrystalline semiconductor material. 
     2. Discussion of the Related Art 
     Micromachining, which is based on microelectronic fabrication techniques, provides fabrication methods for microsystems such as microsensors, microactuators and special micromechanisms. In recent years, micromachining techniques have been developed for producing integrated pressure microsensors of semiconductor material. These sensors provide numerous advantages in comparison to traditional sensors: reduced cost, high degree of performance and reliability, better signal-to-noise ratio, possibility of integration with memory circuits for producing intelligent sensors, possibility of on-line self-testing, and greater reproducibility. As such, integrated pressure microsensors are increasingly used in the automotive industry, contributing to greater safety and environmental protection with absolutely no increase in vehicle cost. 
     Currently marketed semiconductor pressure microsensors are typically based on three physical effects: 1) a piezoresistive effect whereby the pressure-induced inflection of a silicon diaphragm unbalances a Wheatstone bridge comprising resistors diffused in the diaphragm; 2) a capacitive effect whereby pressure induces a displacement of a diaphragm forming the movable electrode of a capacitor (thus resulting in a variation in capacitance); and 3) a resonant effect whereby the pressure inflects a diaphragm to which a resonant structure is fixed and this mechanical deformation varies the oscillation frequency of the structure. For the sensor to operate effectively, the diaphragms must be of uniform, accurately controlled thickness with no intrinsic mechanical stress. These characteristics are typically achieved by forming the microstructures by plasma or wet etching, isotropically (for profiles coincident with the crystal faces) or anisotropically (for generating profiles which are independent from the crystal orientation). 
     Many specialized techniques have been developed to fabricate such microstructures. One such fabrication technique employs etching solutions such as ethyldiaminapyrocatechol (EDP), whereby a structure is formed and separated from the substrate by etching the bulk silicon from the front of the wafer using what is known as the “front bulk micromachining” technique. 
     In the mid 80&#39;s, “surface micromachining” was proposed, whereby the sensitive element or micromechanism was formed of polycrystalline silicon, and which provides for forming suspended structures by depositing and subsequently removing sacrificial layers of different types, e.g. silicon oxide or nitride, porous silicon, aluminum, photoresist, polyimide, etc. Surface micromachined suspended structures, however, are characterized by poor flexural rigidity, and have a tendency to collapse onto the underlying layer, thus impairing thermal or mechanical isolation. A general review of surface micromachining technology (as well as bulk micromachining and the characteristics of each) is to be found, for example, in the article entitled “Micromachining and ASIC Technology” by Axel M. Stoffel, in Microelectronics Journal, 25 (1994), p. 145-156. 
     Several industrial laboratories and research centers have produced prototype integrated microstructures using the surface micromachining technique. Details of these are to be found, for example, in the articles: “Novel fully CMOS-compatible vacuum sensor”, by O. Paul, H. Baltes, in Sensors and Actuators A 46-47 (1995) p. 143-146, in which a diaphragm of dielectric material is formed on a sacrificial metal layer; “Surface-Micromachined Piezoresistive Pressure Sensor” by T. Lisec, H. Stauch, B. Wagner, in Sensor 95 Kongressband, AO1.2, p. 21-25, in which both the sacrificial layer and the diaphragm are of polysilicon and separated by a small layer of silicon oxide; and “Surface-Micromachined Microdiaphragm Pressure Sensors” by S. Sugiyama, K. Shimaoka, O. Tabata, in Sensors and Materials 4, 5 (1993), p. 265-275, in which use is made of a sacrificial polysilicon layer and a silicon nitride layer as the diaphragm. 
     Though they do in fact provide for better integrating the devices, the above surface micromachining techniques pose serious problems with regard to the quality of the deposited films (amorphous or polycrystalline) to form the diaphragms, stiction of the suspended structures on the silicon substrate, and packaging difficulties. 
     In the early 90&#39;s, a further microstructure fabrication technique, known as “silicon fusion bonding”, was devised, whereby a cavity is formed in a monocrystalline silicon wafer onto which a further monocrystalline silicon wafer, in which the sensor is formed, is bonded. 
     A similar microstructure fabrication technique employs dedicated or nondedicated SOI (Silicon-on-Insulator) substrates. 
     Other highly specialized techniques, such as “wafer dissolving,” provide for forming silicon microstructures by means of dedicated processes which are totally incompatible with standard planar microelectronics technology. In a sense, these “ad hoc” processes simply consist of transferring onto silicon what is currently done using other materials, and only provide for fabricating the sensitive portion, so that the processing and control circuit must be formed on a separate chip. 
     Yet another highly specialized technique is the LIGA method—a German acronym for Lithographie Galvanoformung Abformung—which comprises three processing steps: synchrotron x-ray lithography; galvanic deposition of metal films; and formation of plastic molds (see, for example, S.M.Sze&#39;s “Semiconductor Sensors”, John Wiley &amp; Sons, Inc., Chapter 2, p. 75-78). 
     At present, diaphragms of semiconductor material (silicon) are produced using the bulk micromachining technique, which is described in detail, for example, in the articles “CMOS Integrated Silicon Pressure Sensor” by T. Ishihara, K. Suzuki, S. Suwazono, M. Hirata and H. Tanigawa, IEEE Journal Sol. St. Circuits, vol. Sc-22, April 1987, 151-156, and “Micromachining and ASIC Technology” by A. M. Stoffel, Microelectronics Journal 25 (1994) 145-156. Bulk micromachining typically involves processing a silicon wafer on both faces to exploit the excellent mechanical properties of monocrystalline silicon. However, front-to-rear processing, and the need for particular handling of the wafers make bulk micromachining incompatible with current integrated circuit fabrication technology. 
     Another method, known as the electrochemical stop method using a PN junction, provides for more accurately controlling the thickness of the diaphragm and eliminating any process-induced tensile or compressive stress. The electrochemcial stop method involves forming a diaphragm in an N-type monocrystalline semiconductor layer (e.g. the epitaxial layer) on a P-type substrate. The N-type layer is masked except for a previously implanted anode contact region. The rear of the substrate is masked with a mask presenting a window aligned with the region in which the diaphragm is to be formed; a positive potential difference is applied between the N-type layer and the substrate, via the anode contact region, after which the P-type substrate is chemically etched for a few hours at a relatively low temperature (e.g. 90° C.). The chemical etching terminates automatically at the PN junction, forming the diaphragm from the N-type layer at the removed substrate region. 
     FIGS. 1A,  1 B and  1 C illustrate typical fabrication steps for forming an absolute piezoresistive pressure microsensor using the electrochemical stop method. The initial steps are those commonly adopted in the fabrication of integrated circuits. In particular, starting from a wafer  1  of monocrystalline silicon comprising a P-type substrate  2  and an N-type epitaxial layer  3 , P-type junction isolation regions  4  extending from the upper surface of wafer  1  to substrate  2  are formed in epitaxial layer  3 . An integrated circuit is then formed, for example, an NPN transistor with an N+-type collector contact region  6 , a P-type base region  7 , and an N+-type emitter region  8 , as shown in FIG.  1 A. Simultaneously with the integrated circuit, the diffused resistors (only one of which, comprising a P-type resistive layer  10 , is shown) and one anode region for each wafer and each diaphragm (N+-type region  11  in FIG. 1A) are formed. The resistors are preferably formed in the same implant step of base region  7  of the NPN transistor. Anode region  11  is preferably formed in the same step as one of the N-type regions of the integrated circuit (e.g. when implanting collector contact region  6  or emitter region  8 ). A dielectric layer  12  is then deposited, and metal contacts  13  are formed. 
     Thereafter, wafer  1  is masked with a front mask  15  and a rear mask  16 . The front mask  15  (of silicon oxide) covers the whole of the upper surface of wafer  1  except for a window at anode region  11 , and the rear mask  16  (of silicon nitride or oxide) covers the whole of the lower surface of the wafer except for the region in which the diaphragm is to be formed, as shown in FIG.  1 B. The rear of the wafer is then subjected to anisotropic etching while epitaxial layer  3  is biased, via anode region  11 , at a positive voltage (e.g. 5V) with respect to substrate  2 . Anisotropic etching terminates automatically at epitaxial layer  3 , and the portion of epitaxial layer  3  at the removed portion of substrate  2  forms the diaphragm  18 . 
     Following removal of masks  15  and  16 , wafer  1  is bonded to a sheet of glass  17  (FIG. 1C) using an anodic bonding method whereby a medium-high voltage (e.g. 500 V) is applied between wafer  1  and sheet  17  for a few hours at a temperature of 300 to 400° C. Alternatively, wafer  1  may be bonded to the sheet of glass  17  using the glass frit bonding method, which typically requires lower temperatures. Finally, sheet  17  is fixed to container  19 . 
     The above method presents the following drawbacks: it is incompatible with batch processing techniques, due to the electric contacts on each wafer; the rear etching of wafer  1  poses problems in terms of front-rear alignment; the thickness of wafer  1  demands prolonged etching; the scaling problems involved are such as to preclude the integration of structures smaller than a few hundred micrometers; and, once the diaphragm is formed, wafer  1  must invariably be bonded to a glass support, both for absolute and differential sensors (which require holes aligned with the diaphragm, thus posing further alignment problems). 
     Furthermore, the anisotropy of etching causes a reduction of the otherwise available area of wafer  1 ; the chemical agents used, (including, for example, potassium or sodium), may contaminate the chip, causing problems in the circuit integration; and it is very difficult to control the thickness of the active diaphragm, which causes problems with sensor reproducibility. 
     The above drawbacks make it difficult to integrate the method in currently used integrated circuit technology. Therefore, several pressure microsensor manufacturers have opted to form a double integrated chip: one chip contains the diaphragm microstructure, while the other provides for processing the signal. Single-chip integrated sensors also exist, but are not batch processed. 
     None of the above methods provides a method for fabricating low-cost sensors comparable with those of microelectronics technology, using known, highly controllable, noncritical fabrication steps, and which may be integrated with the control electronics in one chip. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide microstructures of semiconductor material designed to overcome the drawbacks typically associated with current technology, as well as methods of fabricating such microstructures. 
     The method of the present invention provides, in one embodiment, a sacrificial buried region of insulating material formed on a monocrystalline semiconductor material substrate. A polycrystalline region for forming the sensitive element or micromechanism is formed over the sacrificial buried region, a monocrystalline region is grown elsewhere, and after the electronic components are formed, portions of the polycrystalline region are selectively removed via trenches in the polysilicon to form a suspending region in which static, kinematic or dynamic microstructures are formed. 
     According to one embodiment of the method of the present invention, the sacrificial buried oxide region can be formed by sequentially depositing an oxide layer and a nitride layer over the substrate, photolithographically defining a window in the nitride layer to expose a portion of the underlying oxide layer, followed by growth of a thermal oxide layer. 
     According to another embodiment, the sacrificial buried region can be formed by photolithographically defining a window in both the nitride and oxide layers, using the nitride as an etch mask to define a recess in the substrate, followed by growth of a thermal oxide layer in the recess. 
     According to another embodiment, the sacrificial buried oxide region can be formed by depositing or growing an oxide region and photolithographicaly defining a region of oxide on the substrate. 
     After the formation of the sacrificial buried oxide region by any of the methods previously described, a semiconductor material layer is deposited over the substrate and then selectively removed to provide a germ region for epitaxial growth over the sacrificial buried oxide region. The semiconductor material layer can be amorphous or polycrystalline silicon. 
     At this point in the method of the present invention, standard fabrication techniques can be used to form integrated electronic components in the first semiconductor material layer. 
     In order to isolate the electronic components, regions surrounding the sensitive regions are photolithographically defined, followed by etching the regions and the sacrificial buried oxide region to form a suspended area. 
     The present invention also provides an integrated microstructure of semiconductor material, formed in a semiconductor material layer over a substrate where the semiconductor material layer is polycrystalline and monocrystalline elsewhere. The microstructure is formed over a sacrificial buried region and also includes an opening located between the substrate and the polycrystalline region. The opening is a trench that extends between an outer surface of the semiconductor material layer and the opening. The polycrystalline region thus forms, over the opening, a suspended structure supporting integrated microsensors and micromechanisms. The suspended structure is supported by connecting and supporting portions that extend between an inner portion of the polycrystalline region in which the microstructures are formed and outer regions of the semiconductor material layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1A is a cross-sectional illustration of a wafer of semiconductor material showing an NPN transistor formed during the fabrication of a known sensor according to the electrochemical stop method; 
     FIG. 1B is a cross-sectional illustration of the wafer of semiconductor material of FIG. 1A after ansotropic etching through the rear surface of the wafer to form a diaphragm; 
     FIG. 1C is a cross-sectional illustration of the wafer of semiconductor material of FIG. 1A after bonding to a sheet of glass; 
     FIG. 2A is a cross-sectional illustration of a wafer of semiconductor material having successive depositions of silicon oxide and nitride, followed by the photolithographic and etch steps to define an opening in the nitride during the fabrication of a sacrificial buried region according to one aspect of the present invention; 
     FIG. 2B shows an intermediate structure formed after the steps illustrated in FIG. 2A, in which a recess is formed in the semiconductor wafer; 
     FIG. 2C illustrates the window of FIG. 2B after filling the recess with oxide; 
     FIG. 2D illustrates the wafer of FIG. 2C after deposition of a polysilicon crystalline layer; 
     FIG. 2E illustrates the wafer of FIG. 2D showing an epitaxial layer formed after the polycrystalline layer deposited on the wafer shown in FIG. 2D was etched to form a germ for the growth of the epitaxial layer; 
     FIG. 3 is a cross-sectional illustration of an alternate method of forming the sacrificial buried oxide region, according to aspect of the present invention; 
     FIG. 4A is a cross-sectional illustration of a wafer of semiconductor material on which a layer of silicon oxide has been formed during the fabrication of a sacrifical buried region according to another aspect of the present invention; 
     FIG. 4B shows an intermediate structure formed after defining the oxide of FIG. 4B; 
     FIG. 4C illustrates the wafer of FIG. 4B after deposition of a polysilicon crystalline layer; 
     FIG. 4D illustrates the wafer of FIG. 4C the polysilicon crystalline layer has been defined to form a germ for growth of an epitaxial layer; 
     FIG. 4E illustrates the wafer of FIG. 4D showing the epitaxial layer formed after the polycrystalline layer deposited on the wafer shown in FIG. 4D was etched to form a germ for the growth of the epitaxial layer; 
     FIG. 5 is a cross-sectional illustration of the semiconductor wafer of FIGS. 2E,  3 , or  4 E, after the formation of integrated structures on and openings in the front surface; 
     FIG. 6 is a cross-sectional illustration of the semiconductor wafer of FIG. 5 after the sacrificial buried region has been etched isotropically via openings in the front surface; 
     FIG. 7 shows a top plan view of a portion of the FIG. 8 wafer; 
     FIG. 8A is a cross-sectional illustration of FIGS. 2E,  3 , or  4 E after the formation of piezoresistive sensors on the front surface of the semiconductor wafer, followed by anisotropic etching of the rear surface to form opening therein; 
     FIG. 8B is a cross-sectional illustration of the wafer of FIG. 8A after isotropically etching the sacrificial buried oxide region to form the final piezoresistive sensor, including an air gap formed in the wafer; 
     FIG. 9 is a top-view illustration of a detail of the embodiment illustrated in FIGS. 8A and 8B; and 
     FIG. 10 is an illustration of another embodiment of the present invention, in which the piezoresistive sensors are integrated into the diaphragm. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a method of fabricating integrated microstructures of semiconductor material that include suspended structures. In preferred embodiments, the suspended microstructures are diaphragms formed from polycrystalline semiconductor material. The suspended structures of the present invention are formed by etching a sacrificial buried layer of silicon dioxide formed on the surface of a semiconductor wafer. The present invention provides various methods for forming and etching the sacrificial buried layer. In a preferred embodiment, the method of the invention provides a piezoresistive pressure sensor including a suspended diaphragm of polycrystalline semiconductor material. 
     FIGS. 2A through 2E illustrate one embodiment of the present fabrication method for forming the sacrifical buried oxide region. Referring initially to FIG. 2A, a silicon oxide layer  22  is formed, preferably thermally grown, on a P-type single crystal silicon wafer substrate  21 . A silicon nitride layer  23  is then deposited on oxide layer  22 , followed by photolithographic and etching steps, which are performed to remove that portion of nitride corresponding to the region in which a suspended structure is to be formed. The resist masking layer is then removed to obtain the intermediate structure shown in FIG. 2A, in which nitride layer  23  has a window  24 . Thereafter, silicon oxide layer  22  and substrate  21 , masked by nitride layer  23 , are successively etched to obtain the intermediate structure shown in FIG. 2B, in which oxide layer  22  and part of substrate  21  form a recess  25 . As shown in FIG. 2C, recess  25  is subsequently filled with an oxide, which is preferably thermally grown in the present embodiment, to form a sacrificial buried oxide region  26 , preferably having a thickness of about 2μm. 
     After filling recess  25 , silicon nitride layer  23  is removed, followed by the deposition of an amorphous or polycrystalline silicon layer  27 , as shown in FIG.  2 D. Polysilicon layer  27 , with the exception of the portion over sacrificial buried oxide region  26 , and oxide layer  22  over substrate  21 , are then removed by means of photolithographic and etching steps. Thus, a silicon region  27 ′ is obtained over sacrificial buried oxide region  26 , as shown in FIG. 2E, which represents the germ for the next step of growing an epitaxial layer. If desired, doping species may also be implanted to form buried structures prior to growing the epitaxial layer, such as, for example, but not limited to, buried layer  9  in FIGS.  5  and  6 ). An epitaxial layer  28  is then grown, thereby forming a polycrystalline structure (polysilicon region  29 ) over sacrificial buried oxide region  26 , and a monocrystalline structure (epitaxial region  30 ) elsewhere, as shown in FIG.  2 E. Thus, a wafer  32  is obtained, as shown in FIG. 2E, preferably including an epitaxial layer  28  thickness of between about 5 μm and about 10 μm. 
     An alternative method for forming the sacrificial buried region is illustrated in FIG.  3 . According to this embodiment, the intermediate structure shown in FIG. 2A, as masked by nitride layer  23 , is oxidized locally to form a sacrificial buried oxide region  26 ′ at window  24 . The intermediate structure typically includes “bird&#39;s beaks”, as shown, that extend beneath nitride layer  23 . The successive steps shown in FIGS. 2C-2E are then performed. Since, in the present embodiment, the sacrificial buried oxide region is grown directly by local oxidation, without forming recess  25  beforehand, the etching of oxide layer  22  and substrate  21  are eliminated. 
     A further embodiment for forming the sacrificial buried oxide region is shown in FIGS. 4A-4E. According to this embodiment, a sacrificial buried oxide region  26 ″ is formed by depositing or thermally growing an oxide layer  22  on substrate  21 , as shown in FIG.  4 A. The silicon oxide layer  22  is defined to form sacrificial buried oxide region  26 ″, as shown in FIG. 4B, by photolithography and etch steps. An amorphous or polycrystalline silicon layer  27  is then deposited, as shown in FIG.  4 C. The polycrystalline layer  27  is then etched, and removed, except for the portion over sacrificial buried oxide region  26 ″, to form region  27 ′, representing a germ for epitaxial growth, as shown in FIG.  4 D. An epitaxial layer  28  is then grown to obtain the structure shown in FIG. 4E, which corresponds to FIG. 2E, showing polysilicon region  29  and epitaxial region  30 . 
     Formation of the sacrificial buried oxide region, by any of the described methods, is typically followed by standard integrated circuit fabrication steps. For example, as shown in FIG. 5, P-type junction isolation regions  34 , extending from surface  31  of epitaxial layer  28  to substrate  21 , and an NPN transistor, presenting an N+ type collector contact region  42 , a P-type base region  43 , and an N-type emitter region  44 , are formed in N-type epitaxial layer  28 . If necessary, diffused regions for implementing the microsensors, microactuators or micromechanisms provided for in the process may be formed simultaneously with the integrated circuit. For example, when implanting the base region  43  of the NPN transistor, a heating element (not shown) may be formed in the case of a chemoresistive gas sensor, or piezoresistive elements (not shown) may be formed in the case of a pressure sensor. A dielectric layer  35  is then deposited and metal contacts  36  are formed, thereby obtaining the structure shown in FIG. 5 (which also shows an N+type buried layer  9 ). 
     At this point, the suspended structures are formed by isotropically etching through the top surface of the wafer. According to the present embodiment, a trench  20 , extending from and including dielectric layer  35  to sacrificial buried oxide region  26 ,  26 ′,  26 ″ is formed. As shown in FIG. 5, photolithographic masking and chemical etching are used to remove portions of dielectric layer  35  and polysilicon region  29  over sacrificial buried oxide region  26  to form trench  20 . In the present embodiment, trench  20  preferably extends in a closed line, e.g. along the sides of a rectangle (FIG. 7) or along the circumference of a circle, so as to laterally define inner portions  29 ′,  35 ′ of polysilicon region  29 ′ and dielectric layer  35 , and to separate them from respective outer portions  29 ″,  35 ″, with the exception of connecting and supporting arms  52  extending between inner portions  29 ′,  35 ′ and respective outer portions  29 ″,  35 ″. Trench  20  advantageously extends close to the periphery of sacrificial buried oxide region  26 ,  26 ′,  26 ″. 
     At this point, sacrificial buried oxide region  26 ,  26 ′,  26 ″ is removed using hydrofluoric acid to form an opening or air gap  41  beneath inner polysilicon portion  29 ′, as shown in FIG.  6 . Removal of sacrificial buried oxide region  26 ,  26 ′,  26 ″ creates a suspended region, wherein structures isolated thermally or mechanically from the substrate (static structures such as chemoresistive gas or piezoresistive pressure sensors), movable structures for detecting external signals (kinematic structures such as flow or motion sensors), or mechanical structures (dynamic structures such as micromotors or microactuators) may be formed. 
     Before or after the sacrificial buried oxide region  26 ,  26 ′,  26 ″ is removed, regions or layers may be formed according to the type of structure to be fabricated. For example, before forming trench  20 , a tin oxide film  14  may be deposited and defined over inner polysilicon portion  29 ′ and respective inner dielectric portion  35 ′ to form a chemoresistive gas sensor shown schematically by  50  in the FIG. 7 plan view, in which the dotted line  45  indicates the edge of air gap  41 , and which shows trench  20  separating inner and outer dielectric portions  35 ′ and  35 ″. 
     Preferred Devices: Piezoresistor Formed Near Edges of Poly 
     In a preferred embodiment, shown in FIGS. 8A and 8B, N-type well regions  55 , extending from surface  56  of epitaxial layer  28  to substrate  21 , and an NPN transistor having an N+-type collector contact region  51 , a P-type base region  53 , and an N+-type emitter region  54 , are formed in P-type epitaxial layer  28 . Thereafter, a dielectric layer  49  is deposited and a P-type polycrystalline silicon layer (doped with boron) having a thickness of 0.2-0.4 μm is deposited; the polysilicon layer is then shaped to form piezoresistive resistors  46 . In the present embodiment, which is intended to increase sensor sensitivity, piezoresistors  46  are formed near the edges of the polycrystalline region  29  intended to form the diaphragm, i.e. in the maximum sensitivity region, as shown in FIG.  9 . In particular, the resistors are oriented such that when the diaphragm is pressure stressed, the resistance of resistors  46 ′ increases, while the resistance of resistors  46 ″ decreases. Metal contacts  47  and  48  and interconnection lines are then formed to connect the piezoresistors to define a Wheatstone bridge, as shown in FIG.  9 . As a consequence, when a pressure is exerted, voltage V A  at node A decreases, while voltage V B  at node B increases. Thus, bridge unbalancing allows the exerted pressure to be measured. 
     In another embodiment of the invention, the suspended structures may be formed by anisotropically etching, preferably by Reactive Ion Etching (RIE), through the rear surface of the wafer. According to this embodiment, one or more dielectric and passivation layers are deposited (illustrated as a single layer  70  in FIG.  8 A). The rear surface  62  of the wafer is completely covered by mask  64  with the exception of windows corresponding to the sacrificial buried oxide region  26 ,  26 ′,  26 ″, and substrate  21  is then anisotropically RIE etched to form openings  63  having substantially vertical sidewalls, as shown in FIG.  8 A. Alternatively, instead of a RIE etching step, an etch with a TMAH (TetraMethylAmmoniumHydroxide) solution may be carried out, which gives rise to openings with inclined sidewalls having an angle of about 54°, and thus a substantially frusto-conical shape, by which it is possible to precisely control the diaphragm thickness by virtue of the high etching selectivity with respect to oxide. 
     Thereafter, sacrificial buried oxide region  26 ,  26 ′,  26 ″ is etched and completely removed through openings  63  using hydrofluoric acid, thus forming an air gap  65  extending beneath inner polycrystalline region  29 , as shown in FIG.  8 B. Polycrystalline region  29  thus forms the diaphragm of the pressure sensor. After removing mask  64 , the final structure shown in FIG. 8 b  forms a sensor for detecting the absolute and/or differential pressure. 
     According to a different embodiment of the just described process, instead of being formed by a polysilicon layer deposited above surface  56  of wafer  32 , the piezoresistors are integrated in the diaphragm  29 , as shown in FIG. 10, wherein piezoresistors  60  are formed using the usual steps of masking and implanting a suitable doping agent of N-type, in this case simultaneously with other components of the signal processing circuit. Electrical contacts  48  having portions extending through openings  61  of the dielectric layer  66  allow the piezoresistors  60  to be connected according to the diagram of FIG. 10, as above described. 
     ADVANTAGES 
     The advantages of the fabrication methods described are as follows. In particular, the suspended region is thicker, therefore providing better mechanical characteristics and fewer signal processing problems as compared with deposited films. In addition, if the suspended structure is used to form a pressure sensor, it is more sensitive compared to similar types formed using deposited films. 
     Because the suspended structure is formed from an epitaxial layer, it has a high degree of flexural rigidity, and is therefore highly unlikely to collapse and adhere to the substrate. Thus, the suspended structures of the present invention possess better mechanical characteristics compared to surface micromachined structures. 
     The suspended structure may be integrated with electronic control components on one chip, and may be formed using only three additional masks as compared with standard analog/digital fabrication processes, i.e. for defining sacrificial buried oxide region  26 ,  26 ′,  26 ″, defining the germ for epitaxial growth  27 ′, and etching trench  20  or  63 . 
     The method described is extremely low-cost as compared with those involving the use of SOI or silicon-fusion-bonded substrates. 
     Finally, the buried oxide region may be formed with a desired degree of flatness. That is, the embodiment in FIGS. 2A-2E may be used in cases requiring a highly flat chip, and the FIG.  3  and FIGS. 4A-4C embodiments when flatness is not particularly essential. 
     The described process allows considerable simplification of the working process with respect to known solutions and avoids many problems during packaging, by eliminating the anodic bonding step. The sensor may be integrated together with the relative circuitry, since the fabrication does not require the use of alkaline ion-based pollutant chemical agents. A spatial integration may be obtained which is comparable with that of surface micromachined structures and which is one to two order of magnitude greater than bulk-micromachined sensors. The polycrystalline structure of region  29  is not problematic to the finished sensor, since this region does not accommodate signal processing components—only, in the onstances shown, piezoresistors. On the other hand, polycrystalline region  29  as grown at high temperatures, has very good mechanical features, and in particular a very low internal mechanical stress and a Young&#39;s modulus which is comparable with that of monocrystalline silicon. Therefore, a diaphragm formed from polycrystalline region  29  has mechanical characteristics which are much better that surface micromachined structures. Finally, with the described techniques, precise control of the diaphragm  29  thickness is obtained, and thus, very good reproducibility. 
     Clearly, changes may be made to the method as described and illustrated herein without, however, departing from the scope of the present invention. In particular, isolation regions  34  in epitaxial layer  28  may be of a type other than that shown, e.g. dielectric as opposed to junction; the electronic components integrated in the same chip may be both bipolar and MOS types; the conductivity of the various regions may be other than as shown; and the structure may be used for any sensor or mechanism employing suspended regions of the type described. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.