Abstract:
A process for bonding two distinct substrates that integrate microsystems, including the steps of forming micro-integrated devices in at least one of two substrates using micro-electronic processing techniques and bonding the substrates. Bonding is performed by forming on a first substrate bonding regions of deformable material and pressing the substrates one against another so as to deform the bonding regions and to cause them to react chemically with the second substrate. The bonding regions are preferably formed by a thick layer of a material chosen from among aluminum, copper and nickel, covered by a thin layer of a material chosen from between palladium and platinum. Spacing regions ensure exact spacing between the two wafers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a process for bonding and electrically connecting Microsystems integrated in several distinct substrates.  
           [0003]    2. Description of the Related Art  
           [0004]    As is known, numerous technological approaches allow manufacturing integrated circuits wherein the electronic circuitry coexists with a sensor element or an actuator (micro-electromechanical device). The traditional approaches envisage the production of the sensors/actuators and circuitry in a same silicon substrate (surface and epitaxial sensors). The most recent approaches envisage, instead, several substrates and the electronic circuit, the micro-electromechanical device or parts thereof are formed in distinct wafers that are subsequently bonded together and finally diced.  
           [0005]    Bonding of the wafers is obtained by causing one or more metals to react with one another, with the silicon of one of the substrates or with metal alloys. To this aim, one or more metals are deposited in sequence on the surface of one or both of the wafers. Then the surfaces to be bonded are brought into intimate contact through a piston that applies a pre-determined pressure, as shown in FIG. 1, which illustrates a substrate  1 , a first wafer  2 , a layer of bonding material  3 , a second wafer  4 , and a piston  5  which presses the second wafer  4  against the first wafer  2 .  
           [0006]    Under the pressure of the piston  4 , the bonding material reacts only where the surfaces are in a mechanical contact, and the areas that are not in contact are not bonded.  
           [0007]    With this solution, bonding between the wafers depends to a large extent upon the mechanical force of the piston; in particular, criticalities are linked, on the one hand, to the uniformity of pressure applied by the piston and, on the other, to the possible presence of foreign bodies.  
           [0008]    In particular, for example in the presence of non-planar areas, the pressure applied by the piston may be non-uniform over the entire surface or over the entire area where bonding is to be obtained. In this case, the presence of areas of the two wafers that are not in contact prevents bonding of these areas.  
           [0009]    In addition, the presence of particles, acting as spacers, also entails absence of contact, which prevents bonding, as shown, by way of example, in FIG. 1, wherein a particle  7  prevents bonding in an area of the surfaces of the wafers  2 ,  4 .  
           [0010]    On the other hand, application of excessive pressure in an attempt to achieve uniform contact in the areas to be bonded may be counterproductive. In fact the deformation of the substrate thus induced causes stresses in the material that persist over time, weakening the bonding joints and/or subsequently causing undesired deformations, in particular in case of suspended structures. For example, a mobile part (such as a rotor of a micro-actuator), once it is released, tends to relieve the accumulated stresses. In this case, the mobile part may get deformed and undergo an undesired spatial displacement, such as might impair proper operation of the structure or, in any case, reduce efficiency thereof.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    An embodiment of the present invention provides a manufacturing process allowing a good bonding quality to be achieved between wafers of semiconductor material.  
           [0012]    According to the present invention there are provided a process for bonding distinct substrates, as well as a device obtained thereby.  
           [0013]    According to an embodiment of the invention, a process for bonding two distinct substrates is provided, comprising the steps of forming micro-integrated devices in at least one of two substrates, using micro-electronic processing techniques and bonding the substrates by forming, on a first of the substrates, bonding structures of deformable material and by pressing the substrates against each other so as to deform the bonding structures and cause the bonding structures to react chemically with a second substrate.  
           [0014]    The bonding structures may comprise a stack of layers including a soft layer and a bonding layer. They may also include a diffusion barrier layer between the soft layer and the bonding layer.  
           [0015]    The method may also include forming spacing regions on the first substrate, the spacing regions having a thickness less than the thickness of the bonding structures.  
           [0016]    Another embodiment of the invention provides an integrated device comprising first and second substrates, distinct from each other, and bonding structures arranged between said first and second substrates, including portions of deformed material and portions of material derived from the reaction between said structures and said second substrate.  
           [0017]    The structures may comprise a stack of layers including a soft layer and a bonding layer. A diffusion barrier layer may be formed between the soft layer and the bonding layer. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0018]    For a better understanding of the present invention, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:  
         [0019]    [0019]FIG. 1 shows the bonding of two wafers according to the prior art;  
         [0020]    [0020]FIG. 2 illustrates a cross-section of two bonded wafers, according to one aspect of the invention;  
         [0021]    FIGS.  3 - 10  show, at an enlarged scale, successive steps of the process for bonding two wafers of semiconductor material, according to the invention;  
         [0022]    [0022]FIGS. 11 and 12 show two steps for bonding two wafers of semiconductor material, in a non-planar surface area; and  
         [0023]    [0023]FIG. 13 shows a cross-section of a device formed in two bonded substrates, in a different embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    The invention is based upon the use of a material having characteristics allowing good-quality bonding of two substrates (namely, two wafers of semiconductor material in which electronic devices and/or micro-electromechanical structures are integrated), even in presence of non-planar areas and/or undesired particles acting as spacers, as generally happens in the case of substrates that have undergone previous fabrication processes.  
         [0025]    According to one aspect of the invention, on one of two substrates a layer (possibly a composite layer) is formed having characteristics of high deformability (soft material) and capacity for reaction with the other substrate (bonding material).  
         [0026]    In what follows, the term “soft material” or “deformable material” refers to a material which at standard bonding pressures and at a low temperature (of less than 450-500° C., usable in the final fabrication steps) undergoes deformation without causing stresses on the substrates (for example, a material that has a modulus of elasticity of less than one tenth that of silicon).  
         [0027]    According to another aspect of the invention, as shown in FIG. 2, on the first substrate (first wafer  10 ) there is deposited and possibly defined a stack of layers comprising at least one soft layer  11 , of a material having good plastic characteristics and low cost (such as aluminum, copper or nickel), and at least one bonding layer  12 , which reacts with the material present on the surface of the second substrate (second wafer  13 ), forming a eutectic or a silicide. A suitable material is, for example, palladium or platinum.  
         [0028]    According to yet another aspect of the invention, between the soft layer  11  and the bonding layer  12  a diffusion barrier layer  14  may extend, with the dual function of enabling good adhesion between the soft layer  11  and the bonding layer  12  and of constituting a barrier against the diffusion of the various materials of the soft layer  11  and bonding layer  12 . A suitable material is, for instance, chromium or titanium.  
         [0029]    Typically, the material of the bonding layer  12  has a high cost, such as to require minimization of its use, adopting low thicknesses.  
         [0030]    In order to carry out bonding, the first wafer  10  and second wafer  13  are brought into mechanical contact with one another. A temperature cycle (for example, at 400° C.) and mechanical pressure (through a piston similar to the piston  5  of FIG. 1) is carried out so as to cause the bonding layer  12  and the second wafer  13  to react and bond. In this step, the soft layer  11  undergoes deformation and adapts to the existing geometry, compensating for any non-planar regions and/or for the presence of foreign bodies. In practice, the soft layer  11 , which has a low cost and hence can be deposited with a large thickness, behaves like a cushion and enables a more even distribution of the pressure exerted by the piston, in such a way as to obtain uniform mechanical contact over the entire area to be bonded and in such a way that any foreign bodies are completely surrounded and embedded in said layer.  
         [0031]    Hereinafter there will be described an example of a sequence of steps of a process for bonding and electrically connecting two wafers, one of which houses electrical circuits and the other houses a micro-electromechanical device.  
         [0032]    With reference to FIG. 3, first electrical components  18  are formed, in a known way, in a body  17  of semiconductor material. On top of the body  17  various insulating layers—illustrated, for simplicity, as a single insulating layer  19 —and various conductive layers, including polycrystalline-silicon regions and various metal levels (not shown in detail) are formed and defined. On top of the insulating layer  19  a top metal layer, for example of aluminum, is formed and defined, thereby forming a contact region  20 , connected to the electrical components  18 , as schematically shown in FIG. 3. Then a protection layer  21 , preferably of silicon dioxide, is deposited and opened, so as to form an opening above the contact region  20 .  
         [0033]    Next (FIG. 4), a first and a second conductive layers  23 ,  24  are deposited in succession. For example, the first conductive layer  23  may be of tantalum/aluminum, and the second conductive layer  24  may be of aluminum. Then (FIG. 5), a spacing layer  25 , for example of silicon dioxide, is deposited.  
         [0034]    Next (FIG. 6), the spacing layer  25  is defined so as to form spacing regions  25 ′, and (FIG. 7) first the second conductive layer  24  and then the first conductive layer  23  are defined. Thus, connection lines  26  are formed by the overlaid first and second conductive layers  23 ,  24 , and stator electrodes  27  formed by the first conductive layer  23  alone.  
         [0035]    Next (FIG. 8), a sacrificial layer  28 , for example of polyimide, is deposited and opened where the bonding regions are to be formed. As shown in FIG. 9, a soft layer  30  (for example of aluminum) and a bonding layer  31  (for example of palladium) are deposited and defined, thus forming bonding regions  32  that extend in part on top of the sacrificial layer  28 . In particular, the thickness of the soft layer  30  (which determines, to a first approximation, the thickness of the bonding regions  32 ) is greater than the thickness of the spacing layer  25 . The bonding regions  32  are thus deeper than the spacing regions  25 ′. Subsequently, the sacrificial layer  28  is removed, and finally (FIG. 10) the wafer  33  thus obtained is bonded to a second wafer  34  in which micromechanical structures (not shown) have been formed.  
         [0036]    In this step, the wafers  33 ,  34  are pressed against one another at a low temperature (for instance, at about 400° C.). Consequently, the aluminum of the soft layer  30 , which melts at 600° C., softens and spreads out, thus enabling the second wafer  34  to abut against the spacing regions  25 ′, which thus ensure proper spacing between the wafers  33 ,  34 , while the bonding layer  31  reacts with the second wafer  34  to form a suicide or a eutectic, ensuring bonding of the wafers. Then bonding joints  35  are formed, that buckle with respect to the bonding regions  32 . The spacing regions  25 ′ may moreover be shaped in such a way as to surround the bonding joints  35  and isolate them from the outside environment.  
         [0037]    By making the bonding regions  32  of an appropriate depth, equal to at least the sum of the depth of the spacing regions  25 ′ and the possible depressions in the second wafer  34 , it is possible to ensure bonding even in the non-planar areas of the wafers  33 ,  34 , as shown in FIGS. 11 and 12, wherein the second wafer  34  has a central depression which would prevent bonding thereof to the first wafer  33 . As shown in FIG. 12, the central bonding region  32  is deformed less than the lateral regions, but ensures bonding even so.  
         [0038]    Finally, the final fabrication steps are performed, which include, if so envisaged, thinning-out of the first wafer  33  and/or second wafer  34 , freeing of the suspended structures, dicing, packaging, etc.  
         [0039]    In certain applications, it may be necessary to have two or more bonding regions  32  arranged in parallel, so as to obtain a section with adequate contact. In fact, to ensure a sufficient deformability of the bonding regions  32 , the portion of soft material cannot have an excessive width, i.e., a width greater than a certain value, which can be determined experimentally. In this case, it is possible to arrange a plurality of bonding joints  35  in parallel. For example, as shown in FIG. 13, two bonding joints  35  are connected to a same connection line  26  on the first wafer  33  and to a same conductive region  36  in the second wafer  34 . The conductive region  36  is electrically insulated from the remainder of the second wafer  34  by insulating regions  37 .  
         [0040]    [0040]FIG. 13 also shows two spacing regions  25 ″ which do not surround bonding joints  35  and are formed at regions removed from the second wafer  34 . In this case, the spacing regions  25 ″ have a function of mechanical support to the second wafer, wherein a linear electrostatic motor is formed, in order to prevent collapse of the suspended regions. Here, the spacing regions  25 ″ are arranged in the proximity of a “spring”  40  which connects a fixed region  41  of the second wafer  34  (which houses the conductive region  36 ) to a mobile region (“rotor”)  42  provided with mobile electrodes  43 . The spacing regions  25 ″ face a removed portion that surrounded the spring  40 . Possibly further spacing regions  25 ″ having the function of temporary mechanical suspension may be provided also at the suspended regions and must be removed after the bonding step.  
         [0041]    Finally, it is clear that numerous modifications and variations may be made to the process and device described herein, without thereby departing from the scope of the present invention.  
         [0042]    In particular, the invention may be applied to integrated devices of any type formed in at least two substrates.  
         [0043]    The material of the bonding regions may vary. The diffusion barrier material may be present or not, according to the materials used and to the requirements. The soft layer may be modified in terms of hardness, for example by adding copper to the aluminum. Alternatively, the soft layer may be made entirely of copper, possibly coated with a thin layer of platinum, which forms the bonding layer. The soft material may be nickel protected by a very thin layer of palladium, which is exhausted during bonding and enables the formation of a nickel silicide; in this case, then, the nickel layer works both as a soft material, which undergoes deformation and enables adaptation of the bonding joints to the existing geometry, and as a bonding material, which ensures mechanical connection between the two wafers.  
         [0044]    Obviously, the same process can be used to bond three or more wafers together.  
         [0045]    All of the above 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.  
         [0046]    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. Accordingly, the invention is not limited except as by the appended claims.