Abstract:
A process for manufacturing an integrated membrane made of semiconductor material includes the step of forming, in a monolithic body of semiconductor material having a front face, a buried cavity, extending at a distance from the front face and delimiting with the front face a surface region of the monolithic body, the surface region forming a membrane that is suspended above the buried cavity. The process further envisages the step of forming an insulation structure in a surface portion of the monolithic body to electrically insulate the membrane from the monolithic body; and the further and distinct step of setting the insulation structure at a distance from the membrane so that it will be positioned outside the membrane at a non-zero distance of separation.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a process for manufacturing a membrane made of semiconductor material that is integrated in, and electrically insulated from, a substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    A large number of integrated semiconductor sensors are known, made with micromachining techniques, which base their operation on detection of a deformation of a thin membrane (also defined as diaphragm), suspended above a cavity. Among these sensors it is possible, for example, to mention absolute or differential, capacitive or piezoresistive, pressure sensors, inertial sensors, chemical sensors, magnetic-field sensors, and microphones. It is also known that it is often advantageous, or even necessary, to provide an electrical, and possibly thermal, insulation of the membrane. 
         [0005]    Main known techniques for manufacturing electrically insulated membranes envisage the use of a SOI (Silicon On Insulator) wafer, or of surface micromachining techniques, with selective removal of sacrificial layers (e.g., a field-oxide or buried-oxide layer). According to these techniques, a substrate of semiconductor material is used only as a mechanical support. 
         [0006]    The aforesaid techniques are in general complex to implement and involve considerable manufacturing costs. 
         [0007]    European patent application No. EP-A-1 577 656, filed in the name of the present applicant, discloses a process for the manufacturing of a membrane of monocrystalline silicon suspended above a cavity, buried and entirely contained within a substrate, which is also made of silicon. This process is extremely simple and economically advantageous, and compatible with the integrated manufacturing of corresponding electronic circuits; also, the resulting structure has small dimensions. 
         [0008]    However, the problem of electrical or thermal insulation of the membrane thus obtained has not yet been adequately solved, so that this manufacturing process is not currently completely exploitable in applications in which the insulation of the membrane from the substrate from which it is obtained is required. In particular, it is advisable to prevent structures for electrical insulation of said membrane from altering the mechanical behavior thereof, in particular the capacity of deformation due to external stresses, and from causing in general reliability problems. 
       BRIEF SUMMARY 
       [0009]    The present disclosure is directed to a process for manufacturing an electrically insulated membrane of semiconductor material that will enable the aforesaid disadvantages and problems to be overcome. 
         [0010]    In accordance with one embodiment of the present disclosure, a process for manufacturing an insulating membrane made of semiconductor material is provided, the process including forming, in a monolithic body of semiconductor material having a front face, a buried cavity extending at a distance from said front face and delimiting with said front face a surface region of said monolithic body, said surface region forming a membrane that is suspended above said buried cavity; forming an insulation structure in a surface portion of said monolithic body such as to electrically insulate said membrane from said monolithic body; and setting said insulation structure at a distance from said membrane so that said insulation structure is positioned outside, and at a non-zero distance of separation from said membrane. 
         [0011]    In accordance with another embodiment of the present disclosure, a microelectromechanical structure is provided that includes a monolithic body of semiconductor material having a front face; and a buried cavity, extending at a distance from said front face and delimiting with said front face a membrane, which is suspended above said buried cavity; and an insulation structure in a surface portion of said monolithic body, surrounding said membrane at a non-zero distance of separation and configured to electrically insulate said membrane from said monolithic body. 
         [0012]    In accordance with another embodiment of the present disclosure, a process is provided that includes forming at least two voids in a substrate and a membrane formed from the substrate material to completely cover at least one of the at least two voids; forming a trench completely around the membrane at a non-zero distance from the membrane and extending into the substrate to a depth that is at least a depth of at least one of the at least two voids in the substrate and in communication with at least one of the at least two voids; and forming a buried insulation region completely around the trench and all of the at least two voids via the trench to electrically insulate the membrane from a remainder of the substrate. 
         [0013]    In accordance with another aspect of the foregoing embodiment, the process includes filling the trench and all but one of the at least two voids with a dielectric material to form a single uniform electrical and thermal insulation structure around the membrane while leaving a remaining void below the membrane to provide functionality to the membrane. 
         [0014]    In accordance with another aspect of the foregoing embodiment, the process of forming at least two voids and the membrane includes forming at least one buried cavity and at least one buried channel and filling all but one of the at least one buried cavity and all of the buried channels with the dielectric material via the trench. 
         [0015]    In accordance with another aspect of the foregoing embodiment, the trench is filled with the dielectric material. 
         [0016]    In accordance with another aspect of the foregoing embodiment, the process includes etching the substrate to form a plurality of columns and performing epitaxial growth on the columns to merge a top portion of the columns to form the membrane of monocrystalline silicon that is flexible and adapted to flex above one of the at least two voids in the presence of external stress. 
         [0017]    In accordance with another aspect of the foregoing embodiment, the trench is formed away from the membrane and anchorages of the membrane to the substrate a distance of separation to prevent mechanical stresses on the membrane due to discontinuity represented by a silicon-oxide junction created by the insulation structure. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]    For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached plate of drawings, wherein: 
           [0019]      FIG. 1  shows a top plan view of a wafer of semiconductor material in an initial step of a process for manufacturing a membrane according to a first embodiment of the present disclosure; 
           [0020]      FIGS. 2-4  show cross sections taken along the line II-II of the wafer of  FIG. 1  in subsequent steps of the manufacturing process; 
           [0021]      FIG. 5  shows a top plan view of the wafer of  FIG. 4  in a subsequent step of the manufacturing process; 
           [0022]      FIGS. 6 and 7  show cross sections taken along the line VI-VI of the wafer of  FIG. 5  in conclusive steps of the manufacturing process; 
           [0023]      FIG. 8  shows a cross section of a wafer of semiconductor material in a manufacturing process in accordance with a second embodiment of the disclosure; 
           [0024]      FIGS. 9   a - 9   c  show top plan views of the wafer of  FIG. 8  in a subsequent step of the manufacturing process, according to different variants; 
           [0025]      FIGS. 10 and 11  show cross sections taken along the line X-X of the wafer of  FIG. 9   a  in conclusive steps of the manufacturing process; and 
           [0026]      FIG. 12  shows a top plan view of the wafer of semiconductor material at the end of the manufacturing process according to a further embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Embodiments of a process for manufacturing a suspended membrane of semiconductor material are now described. This process is based, in part, on the processes disclosed in the aforesaid European patent application No. EP-A-1 577 656, and in the European patent application No. EP-A-1 324 382, which is also filed in the name of the present applicant. 
         [0028]      FIG. 1  (not in scale, as neither are the subsequent figures) shows a wafer  1  of semiconductor material, for example monocrystalline silicon, that includes a substrate  2 , for example of an N +  type (such as to provide an electrical path with low resistance). In an initial step of the manufacturing process, a resist layer is deposited on a top surface  1   a  of the wafer  1 ; the resist layer is then defined so as to form a mask  4  (see also the cross-sectional view of  FIG. 2 ). In detail, the mask  4  includes a first mask region  4   a , extending, for example, over an approximately square inner area, and a second mask region  4   b , adjacent to, and extending as a frame around, the first mask region  4   a , surrounding it entirely. The first and second mask regions  4   a ,  4   b  are made of a plurality of respective mask portions  5   a ,  5   b  having a generic polygonal shape (for example a square shape), arranged in a regular manner in rows and columns and defining together a grid-shaped mask opening  6 . In particular, the mask portions  5   a  of the first mask region  4   a  are set at a first separation distance d 1 , whilst the mask portions  5   b  of the second mask region  4   b  are set at a second separation distance d 2 , greater than the first separation distance d 1 . In addition, the mask portions  5   b  of the second mask region  4   b  have dimensions greater than the mask portions  5   a  of the first mask region  4   a . For example, the first separation distance d 1  (equal to the side of the first mask portions  5   a ) is in the range of 0.5 and 1 μm, whilst the second separation distance d 2  (equal to the side of the second mask portions  5   b ) is in the range of 1 and 2 μm. 
         [0029]    Using the mask  4  ( FIG. 2 ), an anisotropic chemical etching of the front of the wafer  1  is carried out, forming first and second deep trenches  8   a ,  8   b  in positions corresponding to the mask opening  6 , respectively of the first mask region  4   a  and the second mask region  4   b . The deep trenches  8   a ,  8   b , having for example a depth of 10 μm, delimit silicon columns, with a section corresponding to the mask portions  5   a ,  5   b . In particular, first columns (or pillars)  9   a  are formed in a first region  2   a  of the substrate  2  underneath the first mask region  4   a , and second columns  9   b  in a second region  2   b  of the substrate  2 , surrounding like a frame the first region  2   a , underneath the second mask region  4   b . The first columns  9   a  are also consequently set at the first separation distance d 1 , and the second columns  9   b  at the second separation distance d 2 , and moreover the second columns  9   b  have in cross-section larger dimensions than the first columns  9   a . The deep trenches  8   a ,  8   b  communicate with one another and form, together, a labyrinthine region of complex shape, with a section corresponding to the grid of the mask opening  6 . 
         [0030]    Next, the mask  4  is removed, and an epitaxial growth is performed in a deoxidizing environment (typically, in an atmosphere with a high concentration of hydrogen, preferably with trichlorosilane-SiHCl 3 ). Consequently ( FIG. 3 ), an epitaxial layer  10  of monocrystalline silicon grows in a controlled way on top of the columns  9   a ,  9   b  and closes the aforesaid labyrinthine region at the top, trapping the gas present therein. In particular, before the deep trenches  8   a ,  8   b  are closed at the top, a growth of silicon occurs also inside them, which causes a reduction of their dimensions. At the end of the epitaxial growth step, the first and second deep trenches  8   a ,  8   b  form respective first and second buried channels  11   a ,  11   b  having an oval and elongated cross section in a direction perpendicular to the top surface  1   a  of the wafer  1 . 
         [0031]    A step of thermal annealing is then performed, preferably in a hydrogen atmosphere for 30 minutes at 1190° C., or, alternatively, in a nitrogen atmosphere or an atmosphere of another inert gas (for example, argon), or else of a combination of hydrogen and of another inert gas. As discussed in the aforementioned patent applications (and as described in detail, for example, in “A New Substrate Engineering for the Formation of Empty Space in Silicon (ESS) Induced by Silicon Surface Migration” T. Sato, N. Aoki, I. Mizushima, and Y. Tsunashima, IEDM 1999, pp. 517-520), the annealing step causes a migration of the silicon atoms, which tend to move into a position of lower surface energy, through adjacent lattice positions and preserving intact the perfect crystal-lattice structure of silicon. 
         [0032]    In particular ( FIG. 4 ), on account of the small separation distance between the first columns  9   a  and their small width, the first buried channels  11   a  within the first region  2   a  of the substrate  2  merge completely (or in a similar way, the silicon atoms migrate completely from the first columns  9   a ), and consequently a single buried cavity  12  is formed, having, for example, a square cross section of a side of 500 μm and a thickness of 0.5 μm, closed and totally insulated within the substrate  2 . Above the buried cavity  12  there remains a thin layer of silicon, having for example a thickness of approximately 5-10 μm, constituted in part by silicon atoms grown epitaxially and in part by silicon atoms that have migrated from the first columns  9   a . There is thus formed a membrane  14  of monocrystalline silicon, which is flexible, is suspended above the buried cavity  12 , and can deflect in the presence of external stresses. Instead, given the greater separation distance between the second columns  9   b  in the second region  2   b  of the substrate  2 , the silicon atoms do not migrate completely from the second columns  9   b , which thin out during the annealing process, but do not disappear. The second buried channels  11   b , which tend to move into a lower energy condition, assume an approximately circular cross section, without merging with one another. 
         [0033]    Next ( FIG. 5 ), the wafer  1  is etched from the front to form a connection trench  15 , extending vertically (in a direction transverse to the top surface  1   a ) and having a depth and a dimension such as to reach the second buried channels  11   b . The connection trench  15  extends along a closed line within the second region  2   b  of the substrate  2 , externally with respect to the buried cavity  12  and to the membrane  14 , in particular at a given distance of separation I from the membrane  14 , comprised between 1 and 100 μm, preferably between 1 and 50 μm. In particular, portions of the connection trench  15  extend at the distance of separation I on corresponding sides of the membrane  14 . 
         [0034]    Oxygen is then supplied through the connection trench  15  during a thermal oxidation step ( FIG. 6 ) so as to oxidize completely the second columns  9   b , the portions of silicon surrounding the second buried channels  11   b  and the buried cavity  12 , and partially the internal walls of the connection trench  15 . A buried insulation region  16  is thus formed, which separates the membrane  14  electrically from the substrate  2  in a direction transverse to the top surface  1   a , and also a vertical insulation region  17 , which separates the membrane  14  electrically from the substrate  2  in a direction parallel to the top surface  1   a.    
         [0035]    Next, a layer of dielectric material, for example TEOS (tetraethyl orthosilicate) oxide can be deposited on the wafer  1  so as to fill the opening to the connection trench  15  completely ( FIG. 7 ) with an insulating filling region  18 . Possibly, the dielectric material can fill also the second buried channels  11   b  (either completely or partially), and form with the buried insulating region  16  a single uniform and compact electrical insulation structure. Within this insulation structure voids may remain, without, however, this altering its properties of electrical insulation. Furthermore, the substantial continuity of the buried insulation region  16  and of the vertical insulation region  17  ensures electrical and thermal insulation of the membrane  14  from the substrate  2 . 
         [0036]    It is to be noted in particular that the vertical insulation region  17  is advantageously made at a distance from, and outside of, the membrane  14  so as to prevent reliability problems. In fact, at the discontinuity represented by the silicon-oxide junction due to the aforesaid insulation region, the mechanical stresses, which could cause damage to, or even failure of, the microelectromechanical structure, are maximum. The location of the insulation region far from the membrane  14  and from its anchorages to the substrate  2  enables prevention of the aforesaid reliability problems. 
         [0037]    The layer of dielectric material can then be removed from the surface of the wafer, and further surface insulation structures (for example, of a LOCOS type) can be formed. In any case, a thin passivation layer  19  is deposited on the top surface  1   a  of the membrane  14 . 
         [0038]    The manufacturing process then envisages formation of a first metal contact  20  and of a second metal contact  21 , for electrically contacting the membrane  14  and the substrate  2 , respectively. In detail, and as illustrated in  FIG. 7 , the first metal contact  20  is set on the membrane  14  at a central portion thereof, and the second metal contact  21  is set on the back of the wafer  1 . Alternatively (in a way not illustrated), the first metal contact  20  can be arranged as a circular ring in a position external to the membrane  14  but inside the vertical insulation region  17 , and the second metal contact  21  can be set also on the front of the wafer, outside the vertical insulation region  17 . The area external to the insulation structure can house a reading/control electronics, for example made with BCD processes with junction insulation (“top-bottom”), or with oxide insulation. 
         [0039]    It is thus possible to obtain a capacitive pressure sensor in which the first and second metal contacts  20 ,  21  contact the plates of a detection capacitor, the value of capacitance of which varies with deformation of the membrane  14  (in turn caused by a pressure acting on its external surface). In a per-se known manner (which is not described in detail), further electrical components can be integrated within the membrane  14  with appropriate integration masks; for example, piezoresistive elements can be integrated to obtain a pressure piezoresistive sensor. 
         [0040]    A second embodiment of the present disclosure envisages a different process for formation of the vertical insulation region; also in this case, process steps are carried out such that this region is set outside the membrane  14 , at a certain distance of separation I, so as to prevent any mechanical stresses therein. 
         [0041]    In detail, the process envisages ( FIG. 8 ) formation of the buried cavity  12  within the substrate  2  and the membrane  14 , as described previously. It is to be noted that, in this case, the buried insulation region is not formed at the side of the cavity  12  (basically, just the first mask region  4   a  is used for formation of the first columns  9   a  at the first separation distance d 1 ). On the top surface  1   a  of the wafer a resist layer is then formed, which is appropriately defined so as to form an etching mask  24 . 
         [0042]    Next ( FIGS. 9   a - 9   c ), through the etching mask  24 , an external peripheral portion of the membrane  14  is etched, and a plurality of trenches  25  are opened, each of which extends through the membrane  14  and has a depth such as to reach the buried cavity  12 . In detail, in the variant illustrated in  FIG. 9   a , the trenches  25  are aligned at regular intervals parallel to external sides of the membrane  14 , throughout the perimeter of the membrane. Each trench  25  extends parallel to a respective external side of the membrane  14 . Suspension bridges  26  are present between adjacent and consecutive trenches  25 ; suspension bridges  26  are made of silicon, connect the membrane  14  to the substrate  2 , and have the function of supporting the membrane above the buried cavity  12 , during the etching step. In the variant of  FIG. 9   b , the trenches  25  are again set at a regular distance along the external sides of the membrane  14 , but each trench  25  extends in a direction transverse to a respective one of the external sides. The variant illustrated in  FIG. 9   c  envisages, instead, the presence of two series of trenches  25 , set inside one another, each series being formed by aligned trenches extending parallel to the external sides of the membrane  14  (in a similar way to what is illustrated in  FIG. 9   a ). 
         [0043]    Next ( FIG. 10 ), a thermal oxidation of the trenches  25  is performed, which leads to formation of an oxide layer  27  on the internal walls of the trenches  25  and of the buried cavity  12 , and to complete oxidation of the suspension bridges  26 . A vertical insulation region is thus formed, now designated by  17 ′, which surrounds the membrane  14  completely, insulating it electrically and thermally from the substrate  2 . 
         [0044]    Next, a filling region  28 , for example of polysilicon, is introduced within the trenches  25 , for example using the CVD technique; the filling region  28  fills the trenches totally, and it also fills an external peripheral portion of the buried cavity  12 , consequently reducing the dimensions thereof. Accordingly, the dimensions of the membrane  14 , suspended above the buried cavity  12 , are also reduced. Advantageously, the filling region  28  penetrates within the buried cavity  12  for a distance of approximately 1-50 μm. Introduction of the filling region  28  thus again enables mechanical decoupling of the membrane  14  from the vertical insulation region  17 ′, which in fact occupies a position at a certain distance of separation I from the membrane. In this way, stressing of the silicon-oxide discontinuity caused by deformation of the membrane is prevented. In this second embodiment, the resulting dimensions of the membrane  14  depend on the amount of polysilicon introduced, and hence on the extent of introduction of the filling region  28  within the buried cavity  12 . 
         [0045]    According to a further embodiment of the present disclosure, the process described can be modified to obtain a differential pressure sensor  30 , illustrated schematically in  FIG. 12 . 
         [0046]    In detail, simultaneously with formation of the buried cavity  12 , a connection channel  31  is formed, buried within the substrate  2 , in a lateral position with respect to the buried cavity  12  and in fluid communication therewith. For this purpose, process steps are performed that are substantially similar to the ones previously described (and for this reason are not described again), but starting from a first mask region  4   a  that laterally has a rectangular projection of a shape corresponding to the one desired for the connection channel  31 . 
         [0047]    During the final steps of the manufacturing process, after formation of the vertical insulation region  17  (or  17 ′), which in this case extends also around the connection channel  31 , an access trench  32  is dug by digging from the front of the wafer  1 ; this trench traverses a surface portion of the substrate and reaches the connection channel  31 . Digging to form the access trench  32  can also be performed from the back of the substrate  2 , with the disadvantage, however, of traversing a greater thickness of silicon. Through the access trench  32  and the connection channel  31 , it is possible to gain access to an internal wall of the buried cavity  12 . This enables detection of differential pressure in the case where a pressure is exerted both on the internal face and on the external face of the membrane  14 . 
         [0048]    The foregoing may be applied both to the first and to the second embodiment described previously. In the case where the buried insulation region  16  is present, the second mask region  9   b  has a conformation such as to surround, in contact, the first mask region  9   a  (which also has a lateral projection in a position corresponding to the connection channel  31 ). 
         [0049]    The process described affords numerous advantages. In particular, it enables in a simple and inexpensive way a membrane of monocrystalline silicon to be obtained, integrated in a silicon substrate, suspended above a buried cavity, electrically and thermally insulated from the substrate from which it has been obtained. In particular, vertical insulation regions are made far from the membrane and from its anchorages, separated by a distance sufficient to prevent mechanical stresses and the consequent risk of damage or failure. 
         [0050]    The process does not envisage the use of SOI wafers or of complex techniques of surface micromachining, is extremely versatile and robust, easy to control, and of low cost. 
         [0051]    The resulting structures (for example, the pressure sensor described) have small dimensions and are consequently suited for very small packages, for example LGA packages, which are compatible with “wafer-level” package techniques. Piezoresistive structures and/or structures sensitive to particular chemical compounds can moreover be easily integrated in the membrane. 
         [0052]    Finally, it is clear that modifications and variations can be made to what is described and illustrated herein, without thereby departing from the scope of the present disclosure, as defined in the annexed claims. 
         [0053]    In particular, in the first embodiment, the buried insulation region  16  can be confined to just the sensor (hence surrounding only the membrane  14 ), or else can extend throughout a silicon die, as far as the scribing lines in which cutting of the wafer  1  is performed. 
         [0054]    Furthermore, the structure of the mask  4  and the shape of the deep trenches  8   a ,  8   b  and of the columns  9   a ,  9   b  can vary with respect to what is illustrated herein. For example, the mask  4  can have a structure complimentary to the one described previously, and consequently including a grid-shaped structure defining a plurality of openings of a polygonal shape, or else the same mask can be shaped like a grating to define openings having a striplike shape. The same columns  9   a ,  9   b  can also have a different shape (for example, with hexagonal cross section). 
         [0055]    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 and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0056]    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.