Abstract:
A process for manufacturing a suspended structure of semiconductor material envisages the steps of: providing a monolithic body of semiconductor material having a front face; forming a buried cavity within the monolithic body, extending at a distance from the front face and delimiting, with the front face, a surface region of the monolithic body, said surface region having a first thickness; carrying out a thickening thermal treatment such as to cause a migration of semiconductor material of the monolithic body towards the surface region and thus form a suspended structure above the buried cavity, the suspended structure having a second thickness greater than the first thickness. The thickening thermal treatment is an annealing treatment.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority from European patent application No. 05425676.3, filed Sep. 28, 2005, which is incorporated herein by reference.  
       TECHNICAL FIELD  
       [0002]     An embodiment of the present invention relates to a process for manufacturing thick suspended structures of semiconductor material, in particular that can be used as inertial (or seismic) masses in micro-electromechanical devices such as integrated accelerometers, to which the following description will make reference without this, however, implying any loss in generality.  
       BACKGROUND  
       [0003]     Processes for manufacturing thick suspended structures of semiconductor material are known to the art. Said processes initially envisage providing a layer of semiconductor material, and etching the layer of semiconductor material from the back, for example via an anisotropic wet chemical etch in TMAH (Tetra-Methyl Ammonium Hydroxide), so as to define a thick structure having a desired shape. Then, a covering layer is joined, for example via anodic bonding, to the layer of semiconductor material, underneath the structure previously defined. In particular, the covering layer has a recess in a position corresponding to said structure so that, following upon bonding between the two layers, the structure will be suspended above a cavity.  
         [0004]     By way of example,  FIG. 1  shows an accelerometer  1  of a piezoresistive type, comprising a thick suspended structure, in particular an inertial mass, made as described above.  
         [0005]     In detail, the accelerometer  1  comprises a first layer  2  and a second layer  3 , bonded to one another, for example, via anodic bonding. The first layer  2  is made of semiconductor material, whilst the second layer  3  may be made of semiconductor material, or, alternatively, of glass or plastic.  
         [0006]     The first layer  2  comprises a bulk region  4  and an inertial mass  5 , mechanically connected to the bulk region  4  via thin and deformable connection structures  6 . The inertial mass  5  is formed via a TMAH etching of the first layer  2 , made from the back; with the same etching the connection structures  6  are defined. The second layer  3  has a function of covering and mechanical support, and has a cavity  8 , in a position corresponding to the inertial mass  5 , so as to ensure freedom of movement for the inertial mass  5 . Piezoresistive detection elements  9 , for example constituted by regions doped by diffusion, are made in the connection structures  6  and connected in a bridge circuit.  
         [0007]     During operation, an acceleration sensed by the accelerometer  1  causes a displacement of the inertial mass  5 . Consequently, the connection structures  6 , fixed to the inertial mass  5 , undergo deformation, and the resistivity of the piezoresistive detection elements  9  varies accordingly, unbalancing the bridge circuit. Said unbalancing is then detected by a suitable electronic circuit, which derives therefrom the desired acceleration measurement.  
         [0008]     The described manufacturing process is rather complex, due to the presence of a wet etching to be carried out from the back of a layer of semiconductor material, and the need to provide a bonding with a covering layer. For this reason, micro-electromechanical devices comprising suspended structures formed through said process may be characterized by large overall dimensions and high costs.  
       SUMMARY  
       [0009]     An embodiment of the present invention is a process for manufacturing thick suspended structures of semiconductor material that will enable the aforementioned disadvantages and problems to be overcome, and in particular that will have a reduced complexity and lower production costs.  
         [0010]     Consequently, according to an embodiment of the present invention, a process for manufacturing a suspended structure of semiconductor material and a semiconductor structure comprising a suspended structure of semiconductor material are provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For a better understanding of embodiments of the present invention, an embodiment is now described, purely by way of non-limiting example and with reference to the attached drawings.  
         [0012]      FIG. 1  is a cross-sectional view of a micro-electromechanical structure of a known type.  
         [0013]      FIG. 2  is a top plan view of a wafer of semiconductor material, in an initial step of a process for manufacturing a suspended structure, according to an embodiment of the present invention.  
         [0014]      FIG. 3  is a cross-sectional view at an enlarged scale of details of the wafer of  FIG. 2 , taken along the line III-III, according to an embodiment of the invention.  
         [0015]      FIGS. 4-8  are cross-sectional views of the wafer of semiconductor material in subsequent steps of the manufacturing process according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     A process for manufacturing thick suspended structures of semiconductor material is now described. This process is based, in part, upon the process described in the European patent application 04 425 197.3, which is incorporated by reference.  
         [0017]      FIG. 2  (which, like the subsequent figures, is not drawn to scale) shows a wafer  10  of semiconductor material, in particular monocrystalline silicon of an N type with (100) orientation of the crystallographic plane, which comprises a bulk region  11 .  
         [0018]     In an initial step of the manufacturing process, a resist layer is deposited on a top surface  10   a  of the wafer  10 , and it is defined so as to form a mask  12  (see also the cross-sectional view of  FIG. 3 ). In detail, the mask  12  covers an approximately square area having sides I of, for example, 300 μm, with the sides parallel to the flat ( 110 ) of the wafer  10 . The mask  12  has a lattice structure  12   a  (as may be seen from the enlarged detail of  FIG. 2 ), defining a plurality of openings  13  of an approximately square shape. The openings  13  have sides t of approximately one micron, for example, 0.8 μm, and the distance d between opposite sides of adjacent openings  13  is also approximately one micron, for example, 0.8 μm.  
         [0019]     Using the mask  12  ( FIG. 4 ), an anisotropic dry chemical etching of the wafer  10  is then carried out, to form deep trenches  14  in a position corresponding to the openings  13 . The depth of the deep trenches  14  is of the order of microns or of tens of microns (for example, 10 μm), and the deep trenches  14  are separated from one another by walls  15  of semiconductor material, which form together a single separation structure, having a section corresponding to the lattice structure  12   a.    
         [0020]     Next, the mask  12  is removed, and an epitaxial growth is performed in a de-oxidizing atmosphere (typically, in an atmosphere with a high hydrogen concentration, preferably with trichlorosilane—SiHCl 3 ). Due to the epitaxial growth, a silicon closing layer  16  is formed (shown only in  FIG. 5 ), which has a thickness of the order of microns (for example, 5 μm) and closes the deep trenches  14  at the top, entrapping the gas present therein. In particular, before the deep trenches  14  are closed at the top, a growth of silicon occurs therein, causing a reduction in the dimensions of said trenches. At the end of the epitaxial growth, the deep trenches  14  consequently have an oval cross section elongated in a direction perpendicular to the top surface  10   a.    
         [0021]     A first thermal annealing treatment is then carried out in an atmosphere containing hydrogen or another inert gas (for example, nitrogen or argon) or else a combination of hydrogen and of another inert gas, at high temperature (around or higher than 1000° C.) for a first time interval, which lasts some minutes or some tens of minutes. Advantageously, the first thermal annealing treatment is carried out in a hydrogen atmosphere, at a temperature of 1200° C., and the first time interval is no longer than 30 minutes.  
         [0022]     The high temperature promotes a migration of the silicon atoms of the walls  15 , which tend to move into a position of lower energy. In particular, the silicon atoms migrate through adjacent lattice positions, preserving the lattice structure of perfect crystal of the silicon. On account of said migration, the individual deep trenches  14  evolve towards conformations with lower surface energy, for example, from oval shapes to shapes of a spherical type, and then merge together to form a single buried cavity  17 , which is uniform and entirely contained and insulated within the wafer  10  ( FIG. 6 ). For example, the buried cavity  17  has a thickness of 2 μm and a square cross section with sides of 300 μm. The main internal walls, i.e., the top and bottom walls, of the buried cavity  17  are substantially parallel to one another and to the top surface  10   a  of the wafer  10 . A surface region  18  of semiconductor material remains above the buried cavity  17 ; this surface region  18  is constituted in part by epitaxially grown silicon atoms and in part by migrated silicon atoms, and has a first thickness w 1  (in a direction orthogonal to the top surface  10   a ). For example, said surface region  18  can form a thin membrane, which is suspended in a flexible and deformable way above the buried cavity  17 .  
         [0023]     Next, according to an embodiment of the present invention, a second thermal annealing treatment is carried out at high temperature (around or above 1000° C.) for a second time interval, having a duration of tens of minutes or of some hours. The conditions and operative parameters of the second thermal annealing treatment may coincide with those of the first thermal treatment; i.e., the second treatment may also made in hydrogen atmosphere and at a temperature of 1200° C.; in addition, the duration of the second time interval may be longer than 30 minutes.  
         [0024]     Due to the second thermal annealing treatment, a further migration of the silicon atoms occurs: in particular, the silicon atoms of the bulk region  11  that “face” the inside of the buried cavity  17  migrate and are displaced, in the direction indicated by the arrows in  FIG. 7 , towards a central portion  18   a  of the surface region  18 . The resulting effect is that, whereas the ends of the buried cavity  17  remain substantially at the same depth with respect to the top surface  10   a  of the wafer  10 , the centre of the buried cavity  17  progressively shifts towards the bulk region, moving away from the top surface  10   a . The buried cavity  17  consequently assumes a profile having, in a section orthogonal to the top face  10   a , a central stretch substantially parallel to the top face  10   a , and lateral stretches, joined to the central stretch, inclined with respect to the top face  10   a  by an angle α of approximately 30°. The thickness of the central portion  18   a  of the surface region  18  progressively increases, and the surface region  18  is “strengthened” until it forms a suspended structure  20 , of large thickness (i.e., of tens of microns, for instance, more than 10 μm, or, more than 50 μm), above the buried cavity  17 . In particular, the suspended structure  20  has a central portion  20   a  and lateral portions  20   b , which surround the central portion  20   a . The central portion  20   a  has a second thickness w 2  greater than the thickness of the lateral portions  20   b  and than the first thickness w 1  of the surface region  18 . In addition, the suspended structure  20  has a bottom portion (adjacent to the buried cavity  17 ) having substantially the shape of a truncated pyramid turned upside down, and a top portion (adjacent to the top surface  10   a ) substantially corresponding to the surface region  18 . In particular, the side walls of the truncated pyramid are inclined by the angle α (of 30°) with respect to the top surface  10   a  of the wafer  10 , and the height of the pyramid is equal to the difference w 2 -w 1  between the second thickness and the first thickness.  
         [0025]     Proceeding further with the second thermal annealing treatment, the migration of the silicon atoms continues, and thus the dimensions of the inclined side walls and the second thickness w 2  of the suspended structure  20  increase, until the semiconductor structure of  FIG. 8  is obtained, with the suspended structure  20  that has a bottom portion having substantially the shape of a pyramid turned upside down, and with the buried cavity  17  that has a V-shaped profile in a section transverse to the top face  10   a.    
         [0026]     The second thickness w 2 , as likewise the shape (whether of a truncated pyramid or of a pyramid), of the suspended structure  20  is consequently a function of the duration of the second time interval, i.e., of the duration of the second thermal annealing treatment: for example, in one embodiment  FIG. 7  corresponds to a duration of 60 minutes, whilst  FIG. 8  corresponds to a duration of 6 hours. The value of the second time interval that leads to the formation of the suspended structure of  FIG. 8  (i.e., to the end of the process of migration of the silicon atoms) depends, as may be inferred, upon the starting dimensions of the surface region  18   a , or, in a similar way, upon the sides I of the mask  12 . In addition, also the value of the second thickness w 2  at the end of the process of migration is linked to the dimensions of the surface region  18   a  by simple trigonometric relations; for example, given a side I of 300 μm, said value is approximately equal to 90 μm.  
         [0027]     Advantageously, given the substantial uniformity of conditions and of operating parameters of the first and second thermal annealing treatments, just one thermal annealing treatment may be carried out, so that the second treatment is a continuation of the first treatment, with a total duration of the single thermal annealing treatment equal to the sum of the first and second time intervals. In general, said total duration is more than 30 minutes, for example between 60 and 600 minutes. The formation of the surface region  18  is in this case only an initial step of a single migration process of the silicon atoms, which then leads to the formation of the suspended structure  20 .  
         [0028]     The suspended structure  20  can advantageously be used within a micro-electromechanical structure, for example as inertial mass in an accelerometer. In this case, in a way not illustrated, the manufacturing process can proceed with the formation of thin and deformable connection structures between the suspended structure and the bulk region  11  of the wafer  10 , and with the formation of transduction elements, for example of a piezoresistive type, in said connection structures.  
         [0029]     The described manufacturing process has numerous advantages.  
         [0030]     In particular, it does not involve bonding steps, in so far as the suspended structure  20  and the underlying buried cavity are formed within a single monolithic body of semiconductor material, with advantages in terms of manufacturing costs and complexity.  
         [0031]     The suspended structure  20  can thus advantageously be used in semiconductor structures, for example as inertial mass in accelerometers of a resistive or capacitive type, or else in cantilever accelerometers (in this lafter case, the suspended structure  20  is carried by a beam, in a position corresponding to one end thereof, and is suspended above the buried cavity). The resulting semiconductor structures have small overall dimensions, given the absence of bonding between different layers and of wet etches carried out from the back.  
         [0032]     It is moreover possible to control the thickness (and the shape) of the resulting suspended structures in a precise way according to the duration of the thermal annealing treatment.  
         [0033]     The manufacturing process described enables integration of integrated circuits of a CMOS type within the suspended structure  20  (in a per se known manner which is not illustrated).  
         [0034]     Finally, modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present invention.  
         [0035]     For example, the step of epitaxial growth that leads to closing of the deep trenches  14  at the top ( FIG. 5 ) may not be envisaged. In fact, it is possible to obtain closing of the deep trenches  14  via the subsequent thermal annealing treatment and the consequent migration of the silicon atoms of the walls  15 .  
         [0036]     In the described manufacturing process wafers of semiconductor material of a P type, instead of an N type, may be used in an altogether equivalent way. The orientation of the crystallographic plane is advantageously (100), in so far as experimental tests have sometimes shown difficulty in obtaining the same structures starting from wafers with (111) orientation. In particular, in the case of (111) orientation, the deep trenches  14  may not merge into a single buried cavity  17  during the thermal annealing treatment.  
         [0037]     As an alternative to what has been described, via the mask  12  a hard mask can be obtained, for example made of oxide, which can then be used for the etching of the wafer  10  that leads to the formation of the deep trenches  14 .  
         [0038]     The structure of the mask  12  (or, in an equivalent way of the aforesaid hard mask) and the shape of the walls  15  and of the deep trenches  14  can vary with respect to what is illustrated. For example, the mask  12  can have a structure that is complementary to the one illustrated in  FIG. 2  and can comprise a plurality of portions of a polygonal shape (for example, square or hexagonal), arranged in a regular way to define an opening shaped like a (square or honeycomb) lattice. More in general, the walls  15  can be constituted by thin structures capable of enabling complete migration of the silicon atoms during the annealing step that leads to the formation of the buried cavity  17 . The masks  12  having a lattice structure are, however, the most advantageous for use in the manufacturing process described.  
         [0039]     Finally, the area over which the mask  12  extends may have different shapes; for example, it may have a rectangular or a generically polygonal shape.  
         [0040]     Moreover, the structure  10 , or a die or IC in which the structure is located, may compose part of an electronic system such as the air-bag-firing system of an automobile.