Patent Publication Number: US-7585743-B2

Title: Manufacturing method for a semiconductor substrate comprising at least a buried cavity and devices formed with this method

Description:
PRIORITY CLAIM 
     This application is a Divisional of U.S. patent application Ser. No. 10/726,005, filed on Dec. 1, 2003 now U.S. Pat. No. 7,193,256 which claims priority from European patent application No. 02425742.0, filed Nov. 29, 2002, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention relate generally to a method for manufacturing a semiconductor substrate comprising at least a buried cavity. 
     More specifically, embodiments of the invention relate to a method for manufacturing a semiconductor substrate of a first concentration type comprising a buried insulating cavity. 
     Embodiments of the invention also relate to devices manufactured with a method according to the embodiments of the invention. 
     Embodiments of the invention relate particularly, but not exclusively, to a method for manufacturing a semiconductor substrate comprising buried microchannels and the following description is made with reference to this field of application for convenience of illustration only. 
     BACKGROUND 
     As it is well known, demands for circuits with higher operating speeds, affecting at present most electronic applications, particularly at radio frequency, and the need to produce chip integrated mechanical structures (MEMS-Micro Electro-Mechanical System), make the use of SOI (Semiconductor On Insulator) substrates more and more frequent. In fact, most of the limitations of electronic-circuit performance are due to the dispersion and parasitic effects occurring between the integrated devices and the substrate. 
     A first prior-art solution to insulate the semiconductor integrated electronic devices from the substrate provides for the creation under said devices of highly doped “buried” wells, which form charge-depletion areas in order to block the charge flow towards the substrate. 
     Although advantageous in many aspects, this first solution has several drawbacks. In fact, with the increase of circuit operating frequencies, this solution cannot meet the insulation requirements needed for the correct operation of the single devices. 
     As already mentioned, a second solution provides, on the contrary, the use of SOI substrates, whose structure is shown in  FIG. 1 . 
     In particular, a SOI structure comprises a first substrate  1  and a second substrate  2  insulated from each other by an oxide layer  3 . Such a structure allows the first substrate  1 , whereon all devices are integrated, to be electrically insulated from the second substrate  2 . 
     It is evident that the dispersion currents between the first and second substrates are almost null, and the parasitic capacitance effects are reduced, allowing, therefore, the integration of devices operating at higher cut-off frequencies than typically possible in a non-SOI structure. 
     As already said, SOI structures are used considerably also in MEMS and MOEMS (Micro Optical-Electro-Mechanical System) applications to form suspended structures, because it is possible to selectively remove the buried oxide layer since it is used as sacrificial layer. 
     Nevertheless, the formation of SOI structures is rather expensive, and it heavily affects the calculation of the final cost of the finished product. Moreover, these manufacturing processes are rather critical and often involve a significant decrease in yield, both because of structural defects and because of the effective reduction in useable area. 
     The main process steps of a traditional SOI structure manufacturing process are now described. 
     A first and second substrate  1  and  2  are superficially oxidized, so that the thickness of the oxide layer  3  thus obtained on the surface of the two substrates  1  and  2  is about half the thickness of the final layer  3  as shown in  FIG. 2 . 
     After performing a polishing step of the surfaces which will come into contact, the first and second substrates  1  and  2  are initially aligned with micrometric precision, and are then put in surface contact. By means of a pressure exerted by a body B, in order to eliminate the air between the oxide layers  3  belonging to two substrates  1  and  2 , a virtually perfect adhesion is obtained as shown in  FIG. 3 . 
     The first and second substrates  1  and  2  are then subjected to a thermal process at high temperature (e.g., 1100÷1200° C.) which allows the oxide layer  3  to reflux and then bond of the two substrates ( FIG. 4 ). 
     The SOI structure thus obtained is superficially refaced and polished in order to obtain the surface ready for the integration of the electronic devices. The thickness of the first substrate is set at a chosen value, which varies according to the applications used as shown in  FIG. 5 . 
     Although the process can seem simple, the high cost of the single SOI structure is, however, due to the low yield. In fact, in order to make the SOI structure usable, the alignment of the two substrates  1  and  2  must be practically perfect, with the ideal being to keep the crystallographic orientation of the substrates  1  and  2  the same in the SOI structure. 
     Moreover the thermal processes create a series of stress phenomena which produce the so-called “warp” effect. This effect causes a bending of the SOI structure because of a bending-radius decrease by more than one order of magnitude. This “warp” effect is even higher if the oxide layer  3  on the back side of the substrate  2  facing away from the substrate  1 , of the SOI structure is removed, because this layer balances at least partially the “warp” effect induced on the SOI structure by the oxide layer  3  on the first substrate  1 . The removal of this back oxide layer  3  is quite common in most devices having a back contact. In some SOI structures, the bending is so high as to create problems in the photolithography both in the alignment step and in the focusing step. Moreover, in some cases, the “warp” effect of the SOI structure does not allow the vacuum system that holds the SOI structure in place to seal. 
     It is therefore expensive from the economic point of view to manufacture SOI structures characterized by close tolerances from the point of view of thickness, uniformity, and “warp” effect. Moreover the external circular oxide crown called the “terrace” must be taken into consideration for each SOI structure, because it reduces, by more than 1 cm, the diameter of the area effectively usable for the integration. Also, the terrace negatively affects the wafer yield, greatly on small and middle-sized substrates (4, 5 and 6 inches), and in a more limited way on large-sized substrates (8 and 12 inches). 
     A second method to form higher-quality substrates, but also more expensive, exploits the so-called SIMOX method, whose main steps are indicated herebelow. 
     With particular reference to  FIGS. 6 to 9 , a semiconductor substrate  4  is shown, which is implanted with high-energy and high-dose oxygen ions. The implanted layer  5  which is formed is positioned at about 1 μm from the surface. 
     An annealing is then performed at extremely high temperature in an inert atmosphere for the diffusion of the implanted ions and the restoration of the crystalline characteristics of the semiconductor substrate above the layer  5  damaged by ion implantation, with subsequent extension of the area which will become the SOI structure oxide layer. 
     A further annealing step at extremely high temperature in an oxidizing atmosphere is performed to realize the oxidation of the buried layer  5  and complete the SOI substrate structure oxide layer  5 ′. 
     The surface oxide layer  6  is then removed to complete the definite SOI structure. 
     Although the SIMOX method allows improved substrates to be obtained, from the physical-mechanical characteristic point of view, this process is, as already mentioned, rather expensive. Moreover, limitations exist on the achievable thickness; for example, the thickness of the buried oxide layer typically does not exceed 500 nm, and the thickness of the overall SOI structure typically does not exceed one micron. 
     If compared to the first method for manufacturing a SOI structure, this last method allows an improved thickness tolerance, a greater uniformity, the absence of the terrace and, thus, a larger exposable surface for the same wafer size, and finally the so-formed SOI structures are less affected by the “warp” effect. 
     Therefore, a need has arisen for a structure comprising a low-cost buried insulation layer capable of providing good physical-mechanical characteristics and a wide thickness choice of the buried oxide layer and of the final structure, overcoming the limitations and drawbacks which still limit the SOI structures formed according to the prior art. 
     SUMMARY 
     An embodiment of the present invention provides a cavity in a semiconductor substrate starting from trench structures formed therein by exploiting the properties of the silicon self-organizing surface migration process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the device according to the invention will be apparent from the following description of an embodiment thereof given by way of non-limiting example with reference to the attached drawings. 
       In the drawings: 
         FIGS. 1 to 5  are respective schematic-section views of a semiconductor substrate portion during the steps of a first method for manufacturing SOI structures according to the prior art; 
         FIGS. 6 to 9  are respective schematic-section views of a semiconductor substrate portion during the steps of a second method for manufacturing SOI structures according to the prior art; 
         FIGS. 10 to 16  are respective schematic views of a semiconductor substrate portion during the steps of a method for manufacturing SOI structures according to an embodiment of the invention; 
         FIGS. 17   a  to  17   f  are respective schematic views of a trench formed in a semiconductor substrate during the steps of a manufacturing method according to an embodiment of the invention; 
         FIGS. 18   a  to  18   d  are respective schematic views of a plurality of trenches formed in a semiconductor substrate during the steps of a manufacturing method according to an embodiment of the invention; 
         FIGS. 19 and 20  are respective schematic views of a trench formed in a semiconductor substrate during the steps of a manufacturing method according to an embodiment of the invention; 
         FIG. 21  is a top view of a semiconductor substrate according to an embodiment of the invention; 
         FIG. 22  shows, according to the depth L and the radius R of trenches formed in the substrate, the number of void spheres formed in the semiconductor layer in consequence of the silicon self-organizing surface migration process according to an embodiment of the invention; 
         FIGS. 23 and 24  show possible geometries of cavities which can be obtained from differently-located trenches according to an embodiment of the invention; 
         FIG. 25  shows a curve of the feature voltage v. current according to the semiconductor substrate doping concentration and type during the transformation step of the crystalline silicon into porous silicon according to an embodiment of the invention; 
         FIGS. 26   a  to  26   f  are respective schematic views of a first embodiment of the manufacturing method according to embodiments the invention; 
         FIGS. 27   a  to  27   g  are respective schematic views of a second embodiment of the manufacturing method according embodiments of the invention; 
         FIGS. 28   a  to  28   c  are respective schematic views of a third embodiment of the manufacturing method according to embodiments of the invention; 
         FIGS. 29   a  to  29   e  are respective schematic views of a fourth embodiment of the manufacturing method according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, a method for manufacturing SOI structures according to an embodiment of the invention is now described. 
     In particular, as shown in  FIG. 10 , through a traditional photolithographic technique, several trenches  8  are formed in a semiconductor substrate  7 . 
     The semiconductor substrate  7  is formed by a crystalline silicon wafer with any P-type crystallographic orientation, for example doped with acceptor ions with a concentration between 1E13 and 5E20 [cm −3 ] (P − , P, P + , P ++  type etc.). In alternative, the semiconductor substrate  7  is doped with donor ions (N-type) with a concentration between, for example, 1E13 and 5E20. 
     If the substrate  7  is of the N −  type, an N + -type epitaxial layer is formed on the substrate  7 . The so-formed epitaxial layer thickness can vary by several microns, according to the chosen application type. 
     In a first embodiment, the trenches  8  are equally-spaced grooves formed in the substrate  7 , for example, having a width between 0.3 and 5 μm, a depth between 1 and 40 μm, and having a distance between adjacent trenches of between 10 and 40 μm. 
     As shown in  FIG. 12 , after the trench  8  formation a semiconductor material layer  9  is formed on the substrate  7  surface. This semiconductor material layer  9  is formed in order not to completely close trenches  8 , so as to create a planar surface layer. 
     This semiconductor material layer  9  is formed for example with epitaxy. 
     The semiconductor material layer  9  is of the same dopant type and with the same impurity concentration as the substrate  7  (relative to the P-type substrate  7  case) or as the first epitaxial layer being formed (in the case of the N − -type substrate). The thickness of this semiconductor material layer  9  is, for example, between 1 and 10 μm. 
     In order to partially close trenches  8  the silicon self-organizing surface migration process can be also exploited. 
     Particularly, cylindrically developing trenches  10  are formed in the substrate  7 , which have a diameter comprised, for example, between 0.2 and 2 μm and a depth comprised between 2 and 30 μm, as shown in  FIG. 11 . 
     It is however possible to form trenches  8  developing in parallel lines, as shown for example in  FIG. 10 . 
     After forming trenches  10  in the substrate, an annealing process is performed at high temperature, for example between 1000° C.÷1300° C., in a non-oxidizing environment, for example H2, for about some tens of minutes. 
     A structural rearrangement of atoms towards the lowest energy layers occurs in the substrate  7 , so that the cylindrically-developing trench  10  turns into a spherical cavity  10   a  buried in correspondence with the surface-distal trench and forming a surface layer  7   a  on the cavities. 
     Particularly, the annealing thermal process changes the morphology of trenches  10  as shown in  FIGS. 17   a  to  17   f.    
     This silicon self-organizing surface migration process has also some advantages with respect to the traditional methods for manufacturing SOI structures, in fact it does not produce defects in the upper crystalline structure. 
     If very close trenches  10  are formed in the substrate  7 , it is possible to form the structure shown in  FIGS. 18   a  to  18   d  wherein the cylindrically developing trenches  10  turn at first into essentially skittle-shaped cavities to become afterwards spherical cavities  10   a  joining the adjacent cavities, forming thus a sole area or microchannel  10   b.    
     Experimental tests have shown that, with R s  indicating the radius of the depleted sphere  10   a  produced at the end of the annealing process by the trench  10 , and R R  indicating the radius of the starting trench  10 , having cylindrical symmetry, it results:
 
R s =1.88 R R  
 
     As shown in  FIG. 19 , if the trench  10  is very deep with respect to its width, the self-aggregation process forms different vertically-aligned spheres  10   a,  and λ being the distance between two spheres  10   a  formed inside the same trench  10 , the result is that λ is equal to 8.89 R R , as shown in  FIG. 20 . 
     The number of depleted spheres  10   a  formed inside the semiconductor layer  7  is highlighted, depending on the depth L and the radius R of the trench  10 , in the diagram of  FIG. 22 . 
     By changing the depth L of the trench  10 , it is possible to determine the number and the radius of the depleted areas formed after the silicon self-organizing surface migration process inside the single trench  10 . 
     Advantageously, by forming the trenches  10  in such a way that the distance D NT  between the centers of two consecutive trenches is equal to 2R S =3.76 R R , each depleted sphere  10   a  being formed can thus join the first sphere close thereto. In this way an infinite number of depleted spheres  10   a  can be connected obtaining several geometries as shown in  FIGS. 23 and 24 . The quantity D NT  is typical of the process as far as the microchannel  10  size and the thickness of the buried depleted layer are concerned, and it is linked to these parameters by the equation:
 
 R   P =(8.86 R   R   3   /D   NT ) 1/2  
 
     Once the trench  8  or  10  closing process is completed, through the deposition of the surface layer  9   a  or the annealing process, the SOI structure manufacturing process continues, however, with the formation on the surface layer  7   a,    9   a  of the semiconductor material layer  9  which forms the final SOI structure final substrate layer. 
     The type of doping and the concentration of this semiconductor material layer  9  are conveniently selected on the basis of the application. 
     If the starting substrate  7  is of the N-type, the N-type epitaxial layer exists between this semiconductor material layer  9  and the microchannels. 
     As shown in  FIG. 13 , a series of trenches  11  being orthogonal to the first series of trenches  8  or microchannels  10   b  is formed in the semiconductor material layer  9 . These trenches  11  are so deep as to intersect microchannels  10   b.    
     The distance between these orthogonal trenches  11  depends on the size of the device, or of the circuit, which is to be integrated on the SOI structure (even several mms). 
     The structure thus obtained is subjected to a step of transforming of a layer  10   c  comprising microchannels  10   b  into a porous silicon layer. Particularly, the structure is subjected to a wet electroerosive process of the crystalline silicon in a hydrofluoric acid water solution. Therefore the layer  10   c  comprising microchannels  10   b  turns into a porous silicon layer  12 . 
     The electrochemical process occurs in a cell wherein the crystalline silicon layer  10   c  is the positive electrode, while the cathode is a metallic electrode. 
     Particularly, once an operating voltage is set, porous silicon is not formed in the N −  type layers, because it is as if a reverse-biased junction preventing the current flow was formed. Vice versa, in the P + , N +  and P −  type substrate portion, with the same voltage, porous silicon is formed, since it is as if a directly biased junction allowing the current to flow is formed, closing the electric circuit being created. Nothing prevents operating selectively between areas P +  and N + , it is simply enough to correctly define the process parameters. 
     The diagram of  FIG. 25  shows a curve of the characteristic voltage v. current depending on the doping concentration and type, which explains the selectivity phenomenon of the porous silicon formation process. 
     After the electrochemical etching, the structure obtained therefrom is the same as in  FIG. 14  wherein the N +  type silicon layer  10   c  in which microchannels  10   b  are formed turns into a porous silicon layer  12 . 
     The structure obtained comprises therefore, on a crystalline silicon substrate  7 , a porous silicon layer  12  crossed by a series of equally-spaced microchannels  10   b,  whereon a semiconductor material layer  9  is formed on which it is possible to perform the integration of devices and circuits. The semiconductor material layer  9  is crossed by a series of trenches  11  being orthogonal to the microchannels  10   b  which, through their intersection, create a network of channels allowing the electrochemical solution to permeate during the electroerosion process, and the output of reaction products of the process for forming porous silicon. Afterwards the same channels  11  will serve to carry the gas required for the oxidation, or the thermal treatment of the porous silicon layer  12 . 
     A scheme of the structure obtained seen from above is the one shown hereafter in  FIG. 21 . 
     As shown in  FIGS. 15 and 16 , the structure thus obtained further is subjected to an oxidizing process, for example in oven, and given the large surface provided by the porous silicon layer  12  (˜500 m 2 /cm 3 ), the reactivity thereof and the small size of the crystalline structures therein, the porous silicon layer  12  is totally oxidized in significantly less time than the crystalline silicon typical oxidation rate, forming a dielectric layer  12   a.  Therefore, in the same time required to totally oxidize the porous silicon layer  12 , on the surface only a very thin oxide layer is formed. 
     After the oxidation step the filling of the surface trenches  11  is performed through a dielectric layer formation step, for example by depositing a TEOS (Tetra Ethyl Orthosilane) layer or a PVAPOX layer. The surface dielectric layer is then removed, respectively for the case of the N-type substrate as shown in  FIG. 15  and of the P-type substrate as shown in  FIG. 16 . The so-filled trenches  11  are the device or circuit integration edges, i.e. the “scribe lines”. 
     Several additional embodiments of the invention are now described. 
     In a first embodiment of the process according to the invention, as shown in  FIGS. 26   a  to  26   f,  on a substrate  7  through a traditional photolithographic technique are defined the geometries of trenches  10  to be formed. 
     Through an anisotropic removal some trenches  8  and  10  of predetermined size are opened. 
     An annealing step is performed at high temperature in a hydrogen environment so as to let the self-organizing surface migration process occur, which, by planarizing the substrate  7  surface, leaves the round-section channel  10   b  buried. 
     Nothing prevents these trenches  8  from being closed by a semiconductor material layer  9   a  formed on the substrate  7 . 
     Once the “trench closing” is performed, a semiconductor material layer  9  is formed, as shown in  FIG. 26   c.  This semiconductor material layer  9  is formed, for example, by epitaxial growth and with a dopant concentration being different from the substrate  7 . 
     The semiconductor material layer  9  is formed in order to be selective during the porous layer formation. In alternative, a doped polysilicon layer is deposited, also in this case what has been said for the formation process selectivity due to the dopant type and quality is still valid. 
     Through a masking step and an etching process some trenches  11  are Opened on the surface layer, which intersect the underlying microchannels  10   b , which will serve as channel input, output as shown in  FIG. 26   f.    
     As shown in  FIG. 26   e  the so-formed structure is then put in an hydrofluoric acid water solution and by electrochemical etching the porous silicon layer  12  is formed locally around the microchannel  10   b  and isotropically. 
     Through a basic etching, at room temperature, the porous silicon layer  12  is selectively removed, leaving therefore a far wider microchannel  10   d  than the microchannel formed by the thermal process. It is however possible to form surface trenches  11  after forming the microchannel  10   d.    
     This embodiment is particularly advantageous in the microfluid field. In fact, by exploiting the self-organizing surface migration principle and the following porous silicon selective formation, buried microchannels for “lab on chip”-type applications are formed. 
     A second embodiment of the process according to the invention is shown in  FIGS. 27   a  to  27   g.  The geometries of trenches  8  and  10  to be formed are defined on a substrate  7  through a traditional photolithographic technique. 
     Through an anisotropic removing process some trenches  10  of predetermined size are opened. 
     An annealing step is performed at high temperature in an hydrogen environment so as to let the self-organizing surface migration process occur, which, by planarizing the substrate  7  surface, leaves a depleted portion of microchannel  10   b  buried, whose form will depend directly on the geometry defined by trenches  10 , covered by a layer  7   a  or membrane, totally formed by defectless crystalline silicon. 
     Once the “trench thermal closing” is completed, if it is necessary to have a membrane being more than some microns thick, an epitaxial growth of a semiconductor material layer  9  can be performed without any problem in order to reach the desired thickness. 
     If a higher thickness, and thus volume, of the microchannel  10   b  cavity is required, it may be necessary to grow epitaxially a further layer having a different doping with respect to the substrate  7  and being selective in the porous layer  12  formation. In the alternative, a doped polysilicon deposition can be performed. At this point, after opening on the surface small trenches  11  connecting the cavity  10   b  with the surface, the porous silicon layer  12  is formed, through an anodizing electrochemical process in an hydrofluoric acid solution. This porous silicon layer  12  is dissolved afterwards by a basic etching (KOH, NaOH, etc.). 
     The final structure is shown in  FIG. 27   e,  wherein an extension of the microchannel  10   d  cavity, being controllable as one likes, can just be seen. Obviously, trenches  11  previously opened on the semiconductor material layer  9  surface can be closed through a localized deposition process or be an integral part of the membrane geometry as in the case of a microphone. 
     If a condenser microphone is to be formed as shown in  FIG. 27   f,  after manufacturing the silicon membrane  7   a,  a metallization layer  13  is formed. This layer  12  is conveniently configured by means of traditional techniques. The metallization layer  13  is deposited on the membrane  7   a,  for example, by sputtering, and configured through a traditional photolithographic and etching technique. The metallization layer  13  is a first electric contact, while the second contact is the substrate  7  lower portion. In fact, since the substrate  7  is highly doped, it has excellent conductive features. 
     A second embodiment to obtain an integrated microphone is shown in  FIG. 27   g.  This second embodiment provides a non-frontal communication port  7   b  of the microchannel  10   b  with the environment, i.e. from the membrane  7   a  itself, in the back side substrate  7 , being obtained by forming a window  7   c  in the substrate  7  back side. 
       FIGS. 28   a  to  28   c  show a third embodiment of the method according to the invention. Particularly, a method for manufacturing a piezoresistive-type pressure sensor is described. 
     After defining the features to be possessed by a membrane, trenches  10  are defined on the substrate  7  by photolithography and anisotropic etching. 
     Afterwards, the annealing at high temperature is performed to form the buried microchannel  10   b  which will have the desired geometry previously defined. 
     After manufacturing the membrane  7   a  the thickening thereof or the microchannel  10   b  extension can be performed by forming a further layer. Finally a layer  14  of polysilicon or other material having piezoresistive features is deposited on the membrane and the geometries of the piezoresistors which will transduce the pressure variation signal into an easily-detectable electric signal are defined through photomasking and etching. A scheme of a pressure sensor structure is shown in  FIG. 28   c.    
     This third embodiment of the method according to the invention is thus particularly advantageous when manufacturing suspended structures, as in the case of microphones and pressure sensors, and generally in each device requiring a stiff membrane  7   a.    
     Advantageously, the structure and geometry of the membrane  7   a  are formed with different features according to the pressure field to be undergone in the application in terms of rigidity, elasticity, thickness. 
     A fourth embodiment of the process according to the invention to manufacture a MOSFET transistor with buried channel (ISFET Ion sensitivity FET) is now described. 
     Trenches  10  with predetermined size are formed on the silicon substrate  7 .  FIG. 29   a  shows a series of cylindrically-developing trenches  10  being formed along two parallel lines. 
     Through a thermal process at high temperature in an hydrogen environment trenches are closed, in order to planarize the surface and leave a microchannel  10   b  buried. 
     Advantageously, through a selective formation and removal process of transformation of the silicon layer into a porous silicon layer  12 , the extension step of the buried microchannel  10   d  is performed, as shown in  FIGS. 29   c  and  29   d.    
     Advantageously, on the previously obtained structure, a semiconductor material layer  9  of the same type of material which will form the microchannel  10   b  is made to grow epitaxially. 
     Afterwards, drain and source regions  15  are formed in the substrate  7  or in the semiconductor material layer  9  if it is present. These drain and source regions are formed at the edges of the buried microchannel  10   c  being formed in the substrate  7 . 
     A gate oxide layer and the MOSFET gate region  16  are then formed, as shown in  FIG. 29   d.    
     At this point the contacts of the MOSFET gate  16 , drain and source  15  regions are formed conventionally. 
     The so-formed device will have, far from the electrode area, two large ports according to the application and the package type, which intercept the buried microchannel  10   b    10   d  and serve as inlet/outlet, through which gases can go in and out, and the solutions which will be required to etch chemically the surface or which are to be monitored by the system. 
     Obviously the MOSFET manufactured with the method according to embodiments of the invention will be biased in a static way by means of surface electric contacts in the transistor active area. The interaction with the solutions and molecules anchored to the inner surface of the buried microchannel  10   b  will produce a modulation of the MOSFET channel which, if detected, allows the chemical composition of the fluids which are caused to flow in the buried microchannel  10   b  to be studied in real-time. 
     An application of an ISMOSFET transistor manufactured with a method according to an embodiment of the invention is for example the detection of the AND anchorage for determining the genetic expression. 
     In fact ISFETs (Ion sensitivity FET) manufactured with a method according to an embodiment of the invention are sensitive to the changes induced by the chemical bonds between the molecules and the surface of the buried microchannel  10   b.  This device allows, after a convenient calibration, PH, charge, molecular bond changes to be studied and detected. 
     In conclusion, the above-described methods allow SOI structures to be manufactured, which are to be used in the manufacturing of fast electronic circuits having reduced costs which affect negligibly the product final price. 
     In fact these substrates are formed only by combining different techniques, such as wet etching of the silicon surfaces for trench definition, electrochemical etching of the crystalline silicon for porous silicon formation, exploitation of the selectivity with respect to the dopant type and quantity of the crystalline substrate of this etching, exploitation of the crystalline silicon surface self-organization properties and, finally, epitaxial growths. 
     Therefore, devices are manufactured, which combine the crystalline silicon surface self-organization technique with the following formation of a conductive layer, the selective formation of a porous silicon layer  12  around the cavity  10   b  and the following oxidation thereof, the hole through the already-formed buried cavities  10   b  (microchannels) allowing a porous silicon buried layer to be uniformly formed around the cavities  10   b.  Advantageously, layer  12  is oxidized. This layer  12  can be then removed with a very short etching time. The so-formed SOI substrates easily allow therefore traditional electronic devices to be integrated on the so-formed surface portion. Also, the cavity  10   b  can remain void (or be made void by removal of the layer  12 ) to allow formation of other devices such as microphones, transducers, and ISFETs. An integrated circuit having such a substrate structure can be used in electronic systems such as computer system. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.