Abstract:
On a semiconductor material body housing an electronic device a peripheral region of semiconductor material and at least one pad are initially formed. The peripheral region is connected to a first terminal of the electronic device and extends on at least one peripheral portion of the semiconductor material body. The pad is insulated from the semiconductor material body and is electrically connected to a second terminal of the electronic device. The semiconductor material body is fixed to a support body formed by a blank belonging to a reel. The pad is connected by a wire to an electrode formed by the blank. Next, a connection region is formed on the peripheral region and surrounds, at least partially, the semiconductor material body and the support body. The connection region is advantageously obtained by galvanic growth.

Description:
TECHNICAL FIELD 
     The present invention regards an electric connection structure for electronic power devices, and a method of connection. 
     BACKGROUND OF THE INVENTION 
     As is known, bipolar or discrete MOS power devices (operating at powers of over 0.5 W) are required, which are fast (with cut-off frequencies higher than 20 GHz) and have operating frequencies typical of the “S band” (2-4 GHz). These devices are extensively used in the radio-frequency range, in particular in analog and digital telephones, in cordless telephones, radars, satellite television, and high-frequency oscillators. 
     In particular, elementary cells are required comprising BJT NPN or MOS transistors with a high integration level so as to be inserted in ever smaller packages. Thereby, it is possible to reduce the parasitic capacitances and inductances introduced by the package, which would reduce the performance of the device. 
     For these devices, the self-aligned double-polysilicon technology is widely used, which enables cut-off frequencies higher than 50 GHz to be reached. In these devices, the individual transistors are insulated by trenches filled with insulating material, so as to reduce the parasitic capacitance towards the substrate and to reduce the size of the BJT device, or through diffused regions around the active area. The emitter, base and collector contacts are generally provided on the front of the wafer, at pads, and are connected to the pins of the device by wire bonding. 
     Since these are power devices, and therefore pass strong currents, the electrical connections call for pads having a considerable area (at least 60×60 μm 2 ), and require the use of wires of large diameter (for example, 25.5 μm), thus increasing the size of the die integrating the device, in contrast with what would be desirable. 
     In the surface portion of the die of certain devices there is, moreover, the need to connect one of the terminals (collector, base, emitter, drain, source, or gate) to the substrate, so as to obtain a bottom-collector, bottom-base, bottom-emitter, bottom-drain, bottom-source, or bottom-gate. This connection may be made inside the die through a diffused region that extends from the surface to the bottom part of the die, or else externally through a wire that provides the electrical connection between the particular pad and the portion of the lead frame (hereinafter also called support region) on which the die is bonded, alongside the die. In this case, in order to prevent excessive bending of the connection wire (or connection wires if a multiple connection is required), the support region must have a peripheral portion (around the die) of considerable dimensions. For example, at present it is required that the support region has a free wire-soldering area having a width (i.e., distance between the edge of the die and the edge of the support region) of at least 300 μm for each side on which a connection wire is to be bonded. 
     It follows that an electrical connection using the wire-bonding technique for power devices of the type specified above entails high encumbrance which is in conflict with the demand for ever smaller packages. In addition, the high number of required connection wires causes a high parasitic inductance and reduces the performance of the device. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide an improved electric connection structure. 
     According to the present invention, an electronic device and a relative method of connection are described as follows: 
     On a semiconductor material body housing an electronic device a peripheral region of semiconductor material and at least one pad are initially formed. The peripheral region is connected to a first terminal of the electronic device and extends on at least one peripheral portion of the semiconductor material body. The pad is insulated from the semiconductor material body and is electrically connected to a second terminal of the electronic device. The semiconductor material body is fixed to a support body formed on a blank belonging to a reel. The pad is connected by a wire to an electrode formed by the blank. Next, a connection region is formed, by galvanic growth, on the peripheral region, and surrounds, at least partially, the semiconductor material body and the support body, effecting an electrical connection between the peripheral region of the semiconductor material body and the support body. The connecting wire and electrode are surrounded by the same galvanic growth, resulting in an increase in the diameter of the wire. The device is then encapsulated according to known processes, and the blank is cut to obtain the finished product. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, a preferred embodiment thereof is now described, merely to provide a non-limiting example, with reference to the attached drawings, wherein: 
     FIG. 1 is a top view of a die bonded to a blank; and 
     FIGS. 2-4 are cross sections of the blank and of the die of FIG. 1, taken along cross-section line II—II, in subsequent manufacture steps. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show a portion of a sheet tape  1  (hereinafter referred to as “reel”), on which a die  2  is already bonded, the die being designed to house a power device of the bottom-collector, bottom-base, bottom-emitter, bottom-drain, bottom-source, or bottom-gate type. In particular, in the example shown the die  2  houses a bipolar gain transistor  3  (represented with its equivalent electric circuit in FIG.  2 ), of the high-frequency, high-power, and bottom-emitter type. 
     In a per se known manner, the reel  1  defines a plurality of blanks  5  identical to the one shown in FIG. 1, one for each die  2 , the geometry of which (after the electric connection, the formation of an encapsulation package, and cutting of the reel  1 ) is designed to form a support region for the die, as well as electrodes or pins. In detail, the blank  5  of FIG. 1 comprises a support region  10 , arranged at the center, and four electrodes  11 ,  12 ,  13 ,  14  laterally delimited by empty areas  9 . 
     In the example illustrated, two electrodes  11 ,  12 , arranged diagonally opposite to one another with respect to the support region  10 , are separate from the support region  10  and from one another, and are joined to the reel  1  only at external portions of the blank  5 . The electrodes  13 ,  14 , which also extend diagonally opposite to one another from the support region  10 , are connected to the support region  10 , and are hence at the same potential. In the example illustrated, the electrode  11  is intended to be connected to a base region, the electrode  12  is intended to be connected to a collector region, and the electrodes  13 ,  14  are intended to be connected to the emitter region, which is in turn appropriately biased, for example grounded. 
     The die  2  comprises a body  15  of P + -type semiconductor material, integrating the bipolar gain transistor  3 . The body  15  has a first face  15   a  fixed to the support region  10 , a second face  15   b  opposite to the first face, and a side surface  15   c . Typically, the body  15  has low resistivity, for instance 10-20 mΩ/cm. A passivation layer  16  extends over the second face  15   b  of the body  15 . The passivation layer  16  is open at two pads, or terminals,  20 ,  21 , which are electrically connected, in a known way that is not illustrated, respectively to the base region and collector region of the bipolar gain transistor  3 . In addition, the passivation layer  16  is removed at the front outer perimeter of the die  2  and leaves uncovered a metal strip  22 , which extends directly on the second face  15   b  of the body  15 , or is anyway electrically connected to the body  15 , forming, thereby, an additional terminal. The metal strip  22 , for example made of an aluminum and/or gold alloy, like the pads  20 ,  21 , and having, for example, a width of 20 μm and a depth of 3 μm, preferably extends along the entire perimeter of the second face  15   b  of the body  15 , as may be seen in particular in the top view of FIG.  1 . 
     The method of connection comprises initially forming the metal strip  22  and the pads  20 ,  21 , preferably simultaneously. Next, the die  2  is bonded on the support region  10 , according to known techniques, and the pads  20 ,  21  are electrically connected to the respective electrodes  11 ,  12 , through connection wires  24 , in a known way (wire-bonding step). The structure shown in FIG. 3 is thus obtained. 
     Subsequently, the reel  1  is immersed in a galvanic bath and connected to the anode or to the cathode of the latter. In this way, on all the electrically conductive parts in contact with the reel  1 , a conductive layer  25  of metallic material grows isotropically. The metallic material is preferably chosen from among the group comprising gold, copper, lead, tin, and nickel, or is a lead-tin alloy. The thickness of the conductive layer  25  depends upon the application requirements (typically upon the current flowing and the impedance desired at the envisaged operating frequency) and may, for instance, be 10 μm. The conductive layer  25  grows on the whole reel  1 , so as to coat the electrodes  11 - 14  (only the electrodes  11 ,  12  may be seen in FIG.  4 ), on the connection wires  24 , which become thicker, around the support region  10  (where the latter is uncovered), on the side surface  15   c  of the body  15 , and on the metal strip  22 . 
     In particular, a portion  25   a  of the conductive layer  25  electrically connects the metal strip  22 , and thus the emitter region of the bipolar gain transistor  3 , to the support region  10 , thus electrically connecting the emitter region to the potential of the surface  15   a  of the body  15 . This connection is possible thanks to the low conductivity of the body  15 , whose side surface  15   c  is also covered by the growing portion  25   a  of the conductive layer  25 . The structure shown in FIG. 4 is thus obtained. 
     Thereby, the base region and collector region of the bipolar gain transistor  3  are connected to their respective electrodes  11 ,  12  in a standard way, via the connection wires  24 , and the emitter region is connected to ground and to the electrodes  13 ,  14  via the portion  25   a  of the conductive layer  25 . 
     Next, the normal final steps of package molding and cutting the reel  1  are carried out to obtain the finished devices. 
     The advantages of the described connection system are the following. In the first place, the electronic device has a much smaller size than known devices with equal performance. In fact, the area necessary on the top side of the die  2  for forming the pads is reduced, considering that, for wires having a diameter of 25.6 μm a soldering area of 60×60 μm 2  is required, and that two adjacent connection wires must be positioned at least 150 μm apart for avoiding mutual inductance. 
     In addition, the support region  10  does not have to be sized so as to enable soldering of the wires, but can be chosen to be just slightly larger than the die  2 . 
     The electronic device thus obtained has high performance. In fact, the portion  25   a  of the conductive layer  25  represents a considerable metallic mass that is able to carry a high current (the metallic mass of the portion  25   a  is proportional to the size of the die  2 , as well as to the thickness of the conductive layer  25 ; consequently, with a 110 μm thick conductive layer  25 , a 50.6-μm section of side surface  15   c  of the body  15  is equivalent to a wire having a diameter of 25.6 μm). 
     In addition, the growth of the conductive layer  25  brings about a thickening of the connection wires  24 ; consequently, their parasitic resistance and inductance are reduced. At the end of the process, connection wires  24  are thus obtained having a diameter such as ones normally requiring much larger pads. 
     The replacement of some wires with the portion  25   a  of the conductive layer  25  allows a further increase in the performance of the electronic device, given the lower parasitic inductance of said device. 
     The growth of the conductive layer  25  is carried out simultaneously for all the blanks  5  of a reel  1 , thus obtaining simultaneous connection between all the metal strips  22  and their respective support regions  10  and dispensing with numerous soldering operations for separate connection by one or more wires. 
     Finally, it is clear that numerous modifications and variations may be made to the connection structure and to the electrical method of connection described and illustrated herein, all falling within the scope of the invention, as defined in the attached claims. In particular, the present method is applicable to all devices having a terminal connected to the bottom area of the body  15 , including integrated, as well as discrete, circuits. Furthermore, the metal strip  22  may also be discontinuous and not extend over the entire periphery of the die  2 , when it is not necessary to exploit the entire lateral surface of the die  2  for the connection. 
     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.