Abstract:
A process for connecting two bodies forming parts of an electromechanical, fluid and optical microsystem, wherein a welding region is formed on a first body; an electrically conductive region and a spacing region are formed on a second body; the spacing region extends near the electrically conductive region and has a height smaller than the electrically conductive region. One of the first and second bodies is turned upside down on the other, and the two bodies are welded together by causing the electrically conductive region to melt so that it adheres to the welding region and collapses until its height becomes equal to that of the spacing region. Thereby it is possible to seal active parts or micromechanical structures with respect to the outside world, self-align the two bodies during bonding, obtain an electrical connection between the two bodies, and optically align two optical structures formed on the two bodies.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 10/060,068, filed Jan. 29, 2002, now pending, which application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for sealing and connecting parts of electromechanical, fluid and optical systems and to a device obtained thereby. 
     2. Description of the Related Art 
     Various solutions are known for connecting together devices formed in different chips. A known solution, referred to as “flip chip”, envisages connection of two or more chips, mounted on a same printed-circuit board, via connections formed by the same printed circuit. In another solution, referred to as “chip-to-chip wire bonding”, two or more chips are electrically connected though free wires that extend between pairs of chips. 
     In yet another solution, referred to as “chip-on-chip wire bonding”, a first chip is mounted on a second chip, generally of larger dimensions, and the two chips are connected together by means of free wires. 
     On the other hand, the need is increasingly felt of a process of welding and sealing parts of a same microsystem, given that the increase in the complexity of the systems imposes the need to form the individual parts of the same device in different wafers. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a process enabling connection and sealing of parts of a device formed on different wafers. 
     According to an embodiment of the present invention, a process is provided for connecting two bodies forming parts of an electromechanical, fluid and optical microsystem, including forming an electrically conductive region having a first height on a first body, forming a spacing region near said electrically conductive region on said first body, said spacing region having a second height, smaller than said first height forming a welding region on a second body, turning one between said first and second bodies upside down on top of the other, welding said electrically conductive region to said welding region by causing said electrically conductive region to reflow and collapse in such a way that said first height becomes equal to said second height. 
     Moreover, according to an embodiment of the invention, there is provided a device forming an electromechanical, fluid and optical microsystem including at least one first body and at least one second body welded together by a mechanical and electrical connection structure, wherein said mechanical and electrical connection structure comprises an electrically conductive region welded between said two bodies and a spacing region arranged near said electrically conductive region and surrounding an active region of said electromechanical microsystem. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       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: 
         FIG. 1  shows a cross-section of a device formed on two wafers before the latter are bonded together using the process according to the invention; 
         FIG. 2  shows the device of  FIG. 1  after bonding; 
         FIG. 3  is a perspective and sectional view of the device of  FIG. 3 ; 
         FIG. 3A  is a top plan view of the device of  FIG. 3 ; 
         FIG. 4  shows a cross-section of a connection structure according to the invention during self-alignment of two parts; 
         FIG. 5  shows the cross-section of  FIG. 4 , after bonding the two parts; 
         FIG. 6  shows a cross-section of an optical device formed in two wafers bonded using the process according to the invention; and 
         FIG. 7  is a top plan view of one of the two wafers of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a detail of a device  1  forming an integrated electromechanical microsystem having two parts, namely a first part formed in a first wafer  2  and a second part formed in a second wafer  3 . Of the two wafers  2 ,  3  only one portion is shown, wherein a mechanical and electrical connection structure  4  according to an embodiment of the invention is formed. 
     In detail, the first wafer  2  comprises a first insulating layer  5  having a surface  5   a  and housing a first connection line  6  connected to the surface  5   a  of the first insulating layer  5  through a first via  7  and a first contact pad  8 . A first metal region  9  extends on top of the surface  5   a  of the first insulating layer  5 , overlies and is in direct electrical contact with the first contact pad  8 . 
     The second wafer  3  comprises a second insulating layer  13  having a surface  13   a  and housing a second connection line  14  connected to the surface  13   a  of the second insulating layer  13  through a second via  15  and a second contact pad  16 . A second metal region  19  extends on top of the surface  13   a  of the second insulating layer  13 , overlies and is in direct electrical contact with the second contact pad  16 . 
     In addition, a plug region  20  and spacing regions  21  extend on top of the surface  13   a  of the second insulating layer  13 . In detail, the plug region  20  is formed on top of and in direct electrical contact with the second metal region  19  and has a greater height than that of the spacing regions  21 . The plug region  20  has the purpose of electrically connecting the first metal region  9  and the second metal region  19  and must reflow when bonding the two wafers  2 ,  3 . For this purpose, the material of the plug region  20  must be able to reflow at a sufficiently low temperature and in an inert atmosphere, whether a reducing atmosphere or a vacuum. For example, the material of the plug region  20  is a low-melting eutectic formed by alternating layers (typically of gold and tin) for a total height of, for instance, 10 μm. 
     The spacing regions  21  have the function of maintaining the wafers  2 ,  3  at a distance after bonding, sealing any active or micromechanical parts that may be present, and confining the plug region  20 . To this end, the spacing regions  21  are preferably made of a dielectric material with characteristics such as to be able to withstand the bonding temperature, be perfectly planar, and form an electrical insulator so as to be able to pass through possible metal regions without creating short circuits. For example, the spacing regions  21  may be of spun polymers, such as the material known as SU8 (Shell Upon 8), produced by SOTEC MICROSYSTEMS, or polyimide, of laminated polymer layers, such as photosensitive stick foils, for instance Riston, or else oxynitride layers deposited at low temperatures. 
     The spacing regions  21  may form part of a single region having an opening that forms a delimiting cavity  22  in an area corresponding to the plug region  20 , or else be two distinct adjacent regions that delimit, between them, the delimiting cavity  22 . In either case, the volume of the delimiting cavity  22  must be greater than that of the plug region  20  so as to enable collapse of the plug region  20  during bonding in such a way that the adhesion forces of the eutectic to the metal regions  9 ,  19  and the cohesion forces of the eutectic will guarantee stable sealing of the delimiting cavity  22 , as shown in  FIG. 2 . 
     The two wafers  2 ,  3  are manufactured in a known way, according to the devices that are to be made. In particular, on both of the surfaces  5   a  and  13   a  of the insulating layers  5  and  13 , metal regions  9 ,  19 , for example of titanium, nickel or gold, are formed. Next, by appropriate deposition and masking steps per se known, first the spacing regions  21  and then the plug region  20  are formed on one of the two wafers, whichever is the more convenient from the point of view of the process (in the example shown in  FIG. 1 , the second wafer  3 ). 
     Bonding then takes place by bringing the two wafers  2 ,  3  up to one another and applying a slight pressure at a low temperature (for example, 200° C.) so as to cause a weak bonding of the plug region  20 , which adheres to the first metal region  9  just enough to immobilize the two parts. By then increasing the temperature up to the reflow temperature of the material of the plug region  20  (for example, up to 300° C.), the latter region is made to collapse. Consequently, the surface  5   a  of the first insulating layer  5  belonging to the first wafer  2  is brought into contact with the spacing regions  21 , the height of which thus determines the spacing between the two wafers  2 ,  3 , as shown in  FIG. 2 . 
     At the end of the process, the first connection region  6  is electrically connected to the second connection region  14 , and the plug region  20  is confined within the delimiting cavity  22 . Consequently, the plug region  20  and the spacing regions  21  form the mechanical and electrical connection structure  4 . 
     Thanks to the mechanical and electrical connection structure  4  described above, it is possible to seal, with respect to the outside world, an active part of an electronic device and/or a micromechanical structure, as shown in  FIG. 3 . In  FIG. 3 , a first wafer  25  comprises a substrate  26  of semiconductor material, for example silicon, partially overlaid by a silicon-dioxide layer  27 , which is removed in a central portion. The silicon-dioxide layer  27  is in turn overlaid by an epitaxial layer  28 , the central portion of which defines a microstructure  29  which is suspended through arms (not shown). In the peripheral area, on top of an insulating layer (not shown), in which connection lines (not shown either) are formed, a spacing region  21  is present which completely surrounds the microstructure  29  (as is shown only for one half of the device, the other half being symmetrical to the half shown in  FIG. 3 ). The spacing region  21  moreover forms two delimiting cavities  22 , inside which two plug regions  20  are present. A second wafer  30  (shown in a ghost view) extends on top of the first wafer  25 , in contact with the spacing region  21  and the plug regions  20 . In particular, the second wafer  30  comprises metal regions  31  that extend on top of and in direct electrical contact with the plug regions  20 , and electrical connection regions (not shown either) connected to the metal regions. 
     By appropriately configuring the plug region  20  so that it completely surrounds active or micromechanical parts, it is possible to ensure perfect sealing (even vacuum sealing or sealing in a controlled environment) of these parts. 
     The mechanical and electrical connection structure  4  described above enables self-alignment between the two wafers during bonding, as is shown in  FIGS. 4 and 5 . In fact, when the eutectic is melted, it is liquid. In this condition, on the one hand adhesion forces are generated between the eutectic material of the plug regions  20  and the respective first metal regions  9 , and, on the other hand, the surface tension of the liquid tends to bring it to a condition of minimum volume. The combination of these two characteristics cause the eutetic material to behave like a spring, drawing the metal regions  9  and  19  as close together as possible and, in the process, aligning them vertically. If one of the two wafers  2 ,  3  is displaced laterally with respect to the other, as shown by the arrow in  FIG. 5 , the plug regions  20  automatically tend to assume a roughly paralellepipedal shape (or a cylindrical shape if the first metal regions  9  and the second metal regions  19  are circular) with a vertical axis, namely, with the metal regions  9 ,  19  aligned with respect to one another. 
     With the present mechanical and electrical connection structure  4  as described above it is possible to obtain optical alignment between the various parts in case of optical devices (the so-called MOEMS, i.e., MicroOpticalElectro-Mechanical Systems), as shown in  FIGS. 6 and 7 . 
     In detail,  FIGS. 6 and 7  show an optical module formed by a first body  35  of glass (quartz) carrying, on a bottom surface  35   a , a mirror  36  and a diffractive lens  48 , and, on a top surface  35   b , a plurality of mechanical and electrical connection structures  41  according to an embodiment of the invention. Each mechanical and electrical connection structure  41  comprises, analogously to the above, a plug region  38  that extends in a delimiting cavity  40  formed by spacing regions  39 . In the example illustrated, first metal regions  37  are formed on the top surface  35   b  of the first body  35  and extend laterally starting from respective plug regions  38 , passing underneath the spacing regions  39  which surround them as far as accessible external areas so as to connect electrically each plug region  38  to the outside. A metal region  49  is formed on the top surface  35   b  of the first body  35  and is of the same material as the metal regions  37  and functioning as an alignment mirror. 
     A second body  44 , of smaller dimensions than the first body  35  and of silicon/germanium, carries, on its bottom surface  44   a , second metal regions  45  that are to be bonded to as many plug regions  38  and are electrically connected to electrical connection regions  46 . In addition, a light-emitting diode  47 , made in a known way, is formed on the bottom surface  44   a  of the second body  44 . 
     A third body  50 , of smaller dimensions than the first body  35  and of semiconductor material, forms an optical component and carries, on its bottom surface  50   a , a third metal region  51 , which is U-shaped and is to be bonded to a plug region  38  having a corresponding shape (see  FIG. 7 ). 
     The second body  44  must be bonded in such a way as to be vertically aligned to the mirror  36 ; the third body  50  must be bonded in such a way as to be vertically aligned to the diffractive lens  48 . 
     Bonding of the second body  44  and the third body  50  is performed as described previously. 
     With the mechanical and electrical connection structure according to an embodiment of the invention it is therefore possible to connect together two wafers or a wafer and a chip, ensuring sealing of the active or micromechanical part with respect to the outside environment. In addition, the mechanical and electrical connection structure according to the present embodiment of the invention enables self-alignment between the two parts to be connected together, as explained previously; it also enables electrical connection between the two parts and, in the case of optical structures, it enables optical alignment to be achieved. 
     Finally, it is clear that modifications and variations may be made to the process and device described and illustrated herein, without thereby departing from the scope of the present invention. In particular, it is emphasized that the process and device enable even just mechanical connection between two parts, should it be necessary to connect the two parts also in points in which electrical connections are not required. In this case, the metal regions on which the corresponding plug regions are formed or to which they are bonded may be electrically floating. Alternatively, electrical connection to the metal regions may be obtained through interconnection regions formed inside or on top of the two parts, according to the requirements and materials of these parts. Insulation of the plug regions within closed delimiting cavities is not indispensable provided that there is no risk of contamination of the eutectic material of the plug regions. 
     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.