Abstract:
A micro-electro-mechanical device formed in a monolithic body of semiconductor material accommodating a first buried cavity; a sensitive region above the first buried cavity; and a second buried cavity extending in the sensitive region. A decoupling trench extends from a first face of the monolithic body as far as the first buried cavity and laterally surrounds the second buried cavity. The decoupling trench separates the sensitive region from a peripheral portion of the monolithic body.

Description:
BACKGROUND 
       [0001]    Technical Field 
         [0002]    The present disclosure relates to a process for manufacturing MEMS (Micro-Electro-Mechanical System) devices having two buried cavities and to the micro-electro-mechanical device. 
         [0003]    Description of the Related Art 
         [0004]    As is known, sensors including micromechanical structures made, at least in part, of semiconductor materials employing MEMS technology are increasingly widely used, due to their advantageous characteristics of small dimensions, low manufacturing costs, and flexibility. 
         [0005]    A MEMS sensor generally comprises a micromechanical sensing structure, which transduces a physical or mechanical quantity to be detected into an electrical quantity (for example, correlated to a capacitive variation); and an electronic reading circuit, usually formed as an ASIC (Application-Specific Integrated Circuit), which carries out processing operations (i.e., amplification and filtering) of the electrical quantity and supplies an electrical output signal, either analog (for example, a voltage) or digital (for example a PDM—Pulse Density Modulation—signal). The electrical signal, possibly further processed by an electronic interface circuit, is then made available to an external electronic system, for example a microprocessor control circuit of an electronic apparatus incorporating the sensor. 
         [0006]    MEMS sensors comprise, for example, sensors for detecting physical quantities, such as inertial sensors, which detect acceleration or angular velocity data; sensors of derived signals, such as quaternions (data representing rotations and directions in three-dimensional space), gravity signals, etc.; motion sensors, such as step counters, running sensors, uphill sensors, etc.; and environmental signals, which detect quantities such as pressure, temperature, and humidity. 
         [0007]    To sense the physical/mechanical quantity, MEMS sensors of the considered type comprise a membrane or a mass formed in or on a semiconductor chip and suspended over a first cavity. The membrane may face the external environment or be in communication therewith via a fluidic path. 
         [0008]    U.S. Pat. No. 9,233,834 describes, for example, a MEMS device wherein a sensitive part of the device that forms the membrane is separated from the rest of the chip and supported by springs. The springs decouple the sensitive part from the rest of the chip and absorb the package stress, without transferring it to the sensitive part. In this device, the sensitive part is housed within or faces a second cavity that enables a limited movement of the sensitive part with respect to the rest of the chip. 
         [0009]    In practice, the device has two cavities, where a first cavity defines the membrane and a second cavity enables decoupling of the sensitive part of the device from the rest. In the known device, to obtain the two cavities, two semiconductor wafers are used, which are bonded together. If the device is provided with a cap, this is formed in a third wafer, which is also bonded, as discussed hereinafter with reference to  FIGS. 1 and 2 . 
         [0010]      FIG. 1  shows in a simplified way a MEMS sensor  1  formed in a chip  10  of semiconductor material, such as silicon. A cap  11  is fixed to a first face  10 A of the chip  10 , and a closing region  12  is fixed to a second face  10 B of the chip  10  via spacers  26 . 
         [0011]    The chip  10  comprises a suspended region  13  separated from a peripheral portion  18  of the chip  10  through a trench  14 . Elastic elements (also referred to as springs  15 ) support the sensitive region  13  and connect it mechanically to the peripheral portion  18 . The sensitive region  13  houses a buried cavity  16  delimiting a membrane  19 . The term “buried cavity” herein refers to an empty area (or an area filled with gas) within a semiconductor material body or chip, which extends at a distance from the two main faces of the body, being separated from these faces by portions of semiconductor and/or dielectric material. 
         [0012]    A second cavity  21  extends underneath the sensitive region  13 . The sensitive region  13  is provided with a stem  20  (also referred to as Z stopper) extending in the second cavity  21  and limiting oscillation of the sensitive region  13  in the event of impact or stresses that might damage the springs  15 . 
         [0013]    The cap  11  covers here at the top the entire first face  10 A of the chip  10  and protects the latter from the external environment. The cap  11  is fixed via bonding regions  22 , for example of metal such as gold, tin, or copper, or polymeric material or a glass material (glass-frit), fixed to the peripheral portion  18  and is thus spaced apart by a gap  23  from the first face  10 A due to the thickness of the bonding regions  22 . Further, the cap  11  has a through hole  24 , which fluidically connects the membrane  19  to the environment that surrounds the chip  10 . 
         [0014]    The closing region  12  has a protection function during handling of the MEMS sensor  1  (for example, during transport to an assembly system). In general, the closing region  12  is constituted by a second chip housing electronic components, such as an ASIC, but may be constituted by another support, such as a printed-circuit board, or the like. Generally, the closing region  12  has a containment trench  17 , to prevent material of the bonding regions  26  from reaching the mobile parts, limiting movement thereof in an undesired way. 
         [0015]    By virtue of the second cavity  21 , the sensitive region  13  bearing the sensitive part of the MEMS sensor (membrane  19 ) is free to move within certain limits in a vertical direction (perpendicular to the main extension plane of the chip  10  and thus to the faces  10 A,  10 B thereof) and is not affected by stress during manufacturing, in particular during packaging, in so far as the sensitive region  13  is mechanically decoupled from the peripheral portion. 
         [0016]    The device of  FIGS. 1 and 2  is formed by bonding three wafers together. In particular, initially ( FIG. 3A ) a first wafer  350  of monocrystalline silicon is processed for forming the buried cavities  16  delimiting the membranes  19  at the bottom. Formation of the buried cavities  16  may take place in various ways, for example as described in U.S. Pat. No. 8,173,513. Further, on a first face  350 A of the first wafer  350  a gold layer is deposited so as to form first bonding and electrical-connection structures  351 . In addition, the first wafer  350  is etched from the front using a silicon etching for laterally defining the trenches  14  and the springs  15 . 
         [0017]    In parallel, before or after ( FIG. 3B ), a second wafer  400  of monocrystalline silicon is provided with second bonding and electrical-connection structures  401  having a shape and dimensions that are congruent with those of the first bonding and electrical-connection structures  351 . Next, using a resist mask, a deep silicon etch is carried out to form holes  403  and trenches  404 . Etching is prolonged so that both the holes  403  and the trenches  404  have a greater depth than the thickness intended for the cap  11  ( FIG. 1 ). 
         [0018]    Then ( FIG. 3C ), the second wafer  400  is flipped over and fixed to the first wafer  350  via a wafer-to-wafer bonding process of a known type, to obtain a composite wafer  500 . 
         [0019]    Next ( FIG. 3D ), the first wafer  350  is thinned out from the back, to form the second cavities  21  and the stems  20 , and is etched once again from the back, to release the suspended regions  13  and the springs  15 . In addition, the second wafer  400  is thinned out until the bottom of the holes  403  and of the trenches  404  is reached. 
         [0020]    After bonding a third wafer  410  and dicing the composite wafer  500  of  FIG. 3D , the MEMS sensor  1  of  FIG. 1  is thus obtained. 
         [0021]    Consequently, in the process described, the MEMS device  1  is obtained by bonding three different wafers. 
         [0022]    Thus, its thickness is considerable. Further, the process is rather complex in so far as it specifies bonding of three wafers. 
       BRIEF SUMMARY 
       [0023]    One or more embodiments are directed to a MEMS device having two cavities and the manufacturing process thereof. 
         [0024]    According to one embodiment a micro-electro-mechanical device is provided. The micro-electro-mechanical device comprises a monolithic body of semiconductor material having a first face and a second face. A first buried cavity is in the monolithic body and a sensitive region is in the monolithic body facing the first buried cavity. The device comprises a movable element over a second cavity that faces the first buried cavity. The device comprises a decoupling trench extending from the first face of the monolithic body as far as the first buried cavity. The decoupling trench separates the sensitive region from a peripheral portion of the monolithic body. 
         [0025]    In at least one embodiment, the second cavity is buried in the sensitive region and the movable element is a membrane in the sensitive region and arranged between the second cavity and the first face. In another embodiment, the movable element and second cavity are spaced apart from the first face of the monolithic body and the movable element is supported by a structural element that is coupled to the first face of the monolithic body. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0026]    For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
           [0027]      FIGS. 1 and 2  are, respectively, a cross-section and a top view of a known MEMS sensor; 
           [0028]      FIGS. 3A-3D  are cross-sections of successive manufacturing steps of the MEMS sensor of  FIG. 1 ; 
           [0029]      FIGS. 4A-4F  show cross-sections of successive steps of an embodiment of the present manufacturing process; 
           [0030]      FIG. 5  is a top view of a detail of the wafer formed in step  4 E of the present process; 
           [0031]      FIGS. 6A-6D  are cross-sections of successive steps of another embodiment of the present manufacturing process; 
           [0032]      FIG. 7  is a top view of a detail of the wafer formed in step  6 B of the present process; 
           [0033]      FIG. 8  is a top view of a detail of the wafer formed in step  6 C of the present process; 
           [0034]      FIG. 9  is a cross-section of a different embodiment of the present MEMS device; and 
           [0035]      FIG. 10  shows an apparatus using the present MEMS device. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    The present manufacturing process will be described hereinafter with reference to manufacturing a single sensitive structure, it being understood that it is replicated a number of times in a wafer, prior to dicing the wafer, in a per se known manner to the person skilled of the art. 
         [0037]    Initially ( FIG. 4A ), a buried cavity is formed in an initial wafer  100  of semiconductor material. For example, to this end, the manufacturing process described in U.S. Pat. No. 8,173,513 and summarized briefly hereinafter may be used. 
         [0038]    In detail, on the initial wafer  100 , a resist mask  101  is formed having openings arranged according to a honeycomb configuration. Using the mask  101 , the initial wafer  100  is anisotropically etched for forming a plurality of trenches  102 , communicating with each other and delimiting a plurality of silicon columns  103 . 
         [0039]    With reference to  FIG. 4B , the mask  101  is removed and an epitaxial growth is carried out in reducing environment. Consequently, an epitaxial layer, for example of an N type with a thickness of 30 μm, grows above the columns  103 , closing the trenches  102  at the top and forming a first intermediate wafer  200 . 
         [0040]    A thermal annealing is carried out, for example for 30 minutes at 1190° C., preferably in hydrogen atmosphere, or, alternatively, in nitrogen atmosphere. 
         [0041]    As discussed in the patents referenced above, annealing causes migration of the silicon atoms, which tend to move into a lower-energy position. Consequently, and also by virtue of the short distance between the columns  103 , the silicon atoms of the latter migrate completely, and a first buried cavity  106  is formed. A thin silicon layer remains above the first buried cavity  106  and is formed in part by epitaxially grown silicon atoms and in part by migrated silicon atoms and forms a monosilicon closing layer  105 . 
         [0042]    In the embodiment shown ( FIG. 4C ), another epitaxial growth is carried out, of an N type or else a P type and of thickness of a few tens of micrometres, for example 50 μm, starting from the closing layer  105 . A second intermediate wafer  201  is thus formed, which includes a first thick monosilicon region  108  that overlies the first buried cavity  106 . 
         [0043]    With reference to  FIG. 4D , a second cavity  109  is formed in the first thick region  108 , for example repeating the manufacturing process described in U.S. Pat. No. 8,173,513 (see also  FIGS. 4A and 4B ). In this way, a sensor wafer  107  is formed having a first and a second face  107 A,  107 B and, above the first cavity  106 , a second thick region  114 . The second thick region  114  accommodates a second cavity  109  and a membrane  110 , which is delimited at the bottom of the second cavity  109  and faces the first face  107 A. The second thick region  114  has, for example, a thickness of approximately 50 μm, and the membrane  110  has, for example, a thickness of approximately 10 μm. 
         [0044]    If the application so specifies, electronic components  121  may be provided in the membrane  110 , for example piezoresistors, via diffusion or implantation of dopant ion species, here of a P type, in a known manner and not shown. Further, in a per se known manner, electrical interconnections (not shown) may be provided on the first face  107 A of the sensor wafer  107 . 
         [0045]    With reference to  FIG. 4E , using a masking layer (not shown), a deep silicon etch is carried out through the second thick region  114  until the first cavity  106  is reached. A trench  111  is thus formed, external to and surrounding the second cavity  109 . In particular, in the embodiment shown, the trench  111  has the shape of a square spiral. In this way, as may be seen in the top view of  FIG. 5 , the trench  111  is formed by five sides delimiting a sensitive portion  112 , and an arm or spring  113  connecting the sensitive portion  112  to the rest of the sensor wafer  107  (peripheral portion  104  and base  119 ). 
         [0046]    A cap wafer  115  is fixed to the first face  107 A of the sensor wafer  107 . To this end, for example, bonding regions  116 , for instance, of metal such as gold, tin, or copper, or of polymeric material or a glass based material (glass-frit) may be applied previously to the cap wafer  115  and/or to the sensor wafer  107 . In this way, it is possible to electrically connect the electronic components  121 , integrated in the second wafer  107 , with conductive structures (not illustrated) in or on the cap wafer  115 . The bonding regions  116  further form spacers between the first face  107 A of the sensor wafer  107  and the cap wafer  115 , thus delimiting a gap  117 . 
         [0047]    In the embodiment shown, the cap wafer  115  has a through hole  118  that enables fluidic connection between the gap  117  and the external environment and detection, by the membrane  110 , of the external pressure. 
         [0048]    The cap wafer  115  may further be provided with holes (not shown) for bonding wires (not shown). Alternatively, in a way not shown either, through-silicon vias (not shown) may be provided in the peripheral portion  104  of the sensor wafer  107  for electrical connection of the electrical components  121  with the second face  107 B of the sensor wafer  107 . 
         [0049]    After dicing the sensor wafer  107  into a plurality of MEMS devices  120 , each of them may be fixed to a support (not shown), for example an ASIC. Alternatively, the sensor wafer  107  may be fixed to a further wafer, prior to dicing, or to a printed-circuit board, in a way not shown. 
         [0050]    According to a different embodiment, the second cavity may be formed via removal of a sacrificial layer. 
         [0051]    In this case, the manufacturing process may comprise the same initial steps as those described above with reference to  FIGS. 4A-4C . 
         [0052]    Thus, starting from the structure of  FIG. 4C , where the first cavity  106  has already been formed in the second intermediate wafer  201 , a sacrificial region  130  is formed on the first thick region  108 . The sacrificial region  130  is formed, for example, by depositing a sacrificial layer (for instance, of silicon oxide) and its definition via known photolithographic techniques ( FIG. 6A ). A structural layer  131  is deposited over the sacrificial region  130 , for example a polycrystalline silicon layer grown by CVD, to form a sensor wafer  210  having a first, non-planar, face  210 A, comprising a projecting area, corresponding to the structural layer  131 , and a lowered area, corresponding to the exposed portion of the first thick region  108 . 
         [0053]    With reference to  FIG. 6B , the structural layer  131  is etched to define a micro-electro-mechanical structure of an inertial type, for example an accelerometer. In this case, as may be seen in the top view of  FIG. 7 , a suspended mass or platform  132 , springs  133 , connecting the platform  132  to the rest of the structural layer  131 , and mobile and fixed electrodes  134 , represented only schematically in  FIG. 7 , are defined. 
         [0054]    The sacrificial region  130  is removed by etching the sacrificial material, for example in hydrofluoric acid for releasing the platform  132  and the mobile electrodes, thereby obtaining the structure of  FIG. 6B , where a second cavity  125  extends underneath the platform  132 . 
         [0055]    Subsequently or previously, for example using a dry film ( FIG. 6C ) and analogously to what described with reference to  FIG. 4E , using a masking layer (not shown) a deep silicon etch is made through the first thick region  108 , outside the area of the structural layer  131 , and thus outside the platform  132 , until the first cavity  106  is reached. The trench  111  is thus formed, which, in top view (see  FIG. 8 ) surrounds the second cavity  125  and the platform  132 . Also here, the trench  111  has the shape of a square spiral and comprises five sides delimiting a sensitive portion  135 , and an arm or spring  136  connecting the sensitive portion  135  to the rest of the sensor wafer  210 , hereinafter also indicated as peripheral portion  137 . 
         [0056]    A cap wafer  140  is fixed to the first face  210 A of the sensor wafer  210  analogously to what described with reference to  FIG. 4F . In this case, since the surface  210 A of the sensor wafer  210  is not planar and the platform  132  projects above the thick region  108 , the cap wafer  140  has a recess  141  facing the sensitive region  135 . 
         [0057]    Also in this case, the cap wafer  140  may have holes (not shown) for passage of bonding wires, or, in a way not shown, through-silicon vias may be provided in the peripheral portion  137 . 
         [0058]    The sensor wafer  210  is diced into a plurality of MEMS devices  143 , and, analogously to what already described, each of them may be bonded to a support or the sensor wafer  210  may be fixed to a further wafer, prior to dicing. 
         [0059]    In a different embodiment ( FIG. 9 ), the cap is formed directly by an ASIC, and the hole for connection to the external environment is formed directly in the sensor wafer, instead of in the cap. 
         [0060]    In the embodiment shown, the sensor wafer  107  of  FIG. 4E  is used. The base portion  119  of the sensor wafer  107 , underneath the first cavity  106  in the view of  FIG. 4E , is here perforated by a deep silicon etch, analogous to the trench  111  etching. A connection hole  145  is thus obtained and connects the first cavity  106  to the external environment. 
         [0061]    First stoppers  146 , for example of dielectric material, such as silicon oxide, or metal material or polysilicon or a stack of different material layers, deposited and defined on the first face  107 A, in a per se known manner, are further formed on the first face  107 A of the sensor wafer  107 . 
         [0062]    Second stoppers  147  are formed on a face  150 A of an ASIC wafer  150 , in a position so as to face, at a distance, the first stoppers  146 . 
         [0063]    Spacer elements  151  as well as mechanical and electronic connection elements  152  are formed on the ASIC wafer  150  or on the sensor wafer  107 . 
         [0064]    The spacers  151  may be of materials including gold, copper, tin, glass-frit or polymers and may have a thickness of 5 μm. 
         [0065]    The mechanical and electronic connection elements  152  may, for example, be formed by so-called “solder balls”, arranged at contact pads  153 A,  153 B formed on the first face  107 A of the sensor wafer  107  and a face  150 A of the ASIC wafer  150 . 
         [0066]    The sensor wafer  107  and the ASIC wafer  150  are bonded together, with the first face  107 A of the sensor wafer and the face  150 A of the ASIC wafer  150  facing each other, thereby forming a composite wafer. Finally, the composite wafer is diced into a plurality of finished devices  160 . 
         [0067]    As an alternative to the above, the connection hole  145  may be formed at the end of the process, prior to dicing the composite wafer. 
         [0068]    In this way, between the two faces  107 A and  150 A a gap  154  is formed, the thickness thereof is defined by the spacer elements  151 , and the sensitive portion  112  may move in a limited way within the gap  154  or the first cavity  106 , and is thus decoupled from the peripheral portion  104 . 
         [0069]    In addition, the membrane  110  is connected to the external environment through the trench  111 , the first cavity  106 , and the hole  145 , thus forming a fluidic path. 
         [0070]    The mechanical and electronic connection elements  152  enable, in addition to bonding the sensor wafer  107  and the ASIC wafer  150 , their electrical connection. 
         [0071]    As an alternative to the above, the sensor wafer  107  and/or the ASIC wafer  150  may be diced prior to bonding, in a per se known manner. Further, it is possible to form the cap and ASIC also starting from the structure of  FIG. 6A , and thus with the second cavity  125  formed by removal of a sacrificial region. 
         [0072]      FIG. 10  is a schematic representation of an electronic apparatus  170  using the MEMS device  120 ,  143 ,  160 . 
         [0073]    The electronic apparatus  170  comprises, in addition to the MEMS device  120 ,  143 ,  160 , a microprocessor  174 , a memory block  175 , connected to the microprocessor  174  and an input/output interface  176 , also connected to the microprocessor  174 . Further, a speaker  178  may be present for generating a sound on an audio output (not shown) of the electronic apparatus  170 . 
         [0074]    In particular, the electronic apparatus  170  is fixed to a supporting body  180 , for example formed by a printed circuit. 
         [0075]    The electronic apparatus  170  is, for example, an apparatus for measuring blood pressure (sphygmomanometer), a household apparatus, a mobile communication device (a cellphone, a PDA—Personal Digital Assistant—, or a notebook) or an pressure measuring apparatus that may be used in the automotive sector or in the industrial field in general. 
         [0076]    In this way, the devices  120 ,  143 ,  160  may be formed with a lower number of wafers as compared to the devices currently produced, since both the cavities (i.e., the first cavity  106  and the second cavity  109  or  125 ) are formed in a same monolithic substrate, without bonding two wafers together. 
         [0077]    In this way, the manufacturing costs are considerably reduced. Further, it is possible to reduce the thickness of the finished device, for a same robustness. Finally, it is possible to reduce problems of contamination and/or delimitation of the gluing materials, without forming specific containment trenches. 
         [0078]    Finally, it is clear that modifications and variations may be made to the device and the manufacturing process described and illustrated herein, without thereby departing from the scope of the present disclosure. For example, the described embodiments may be combined for providing further solutions. In particular, the MEMS device  120  may be a sensor or an actuator of a different type, which may be obtained using MEMS technology and specify a mechanical decoupling from the rest of the chip. 
         [0079]    The various embodiments described above can be combined to provide further embodiments. 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.