Abstract:
A method for the electrical and/or mechanical interconnection of components of a microelectronic system includes at least one first component and at least one second component to be connected, and at least one local Joule-effect micro-heater is incorporated in one of the first and second components at a respective soldering point therebetween. The method includes supplying electrical energy to the micro-heater to utilize the heat produced therefrom by the Joule effect to solder the first and second components at the respective soldering point.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of microelectronics, and, more particularly, to a multi-component microelectronic system, such as a micro-electromechanical system (MEMS) or a multichip system in which two or more component parts of the system are electrically and/or mechanically interconnected. 
     BACKGROUND OF THE INVENTION 
     In the production of complex micro-electromechanical systems (MEMS) that include several mechanically and/or electrically interconnected subsystems, the greatest difficulties are encountered during the interfacing and connection of the various subsystems. For example, in hard-disk reading/writing units having micrometric actuation for the fine positioning of the reading/writing transducers (heads or sliders), it is necessary to interconnect the suspension, the micro-actuator, and the slider. 
     The slider is fixed to the rotor of the micro-actuator, and is usually glued to a support plate which in turn is anchored to the rotor. The slider is also soldered at several points to flexible mechanical connection elements or springs suspended above the rotor and connected in a cantilevered manner to the static part of the micro-actuator. The electrical terminals of the slider are soldered to further flexible electrical connection elements or springs also suspended above the rotor, and connected in a cantilevered manner to the static portion of the micro-actuator. There are at least four electrical terminals, two for reading and two for writing. 
     The soldering operations are generally performed by a ball bonding technique and are extremely delicate operations. Moreover, if the micro-actuator is found to be faulty during the subsequent functional testing stage, the entire micro-electromechanical system has to be rejected since the functioning components of the system cannot easily be disconnected for reuse. 
     Similar problems arise in multichip systems in which two or more chips, each incorporating a respective integrated circuit, have to be mechanically and electrically interconnected in predetermined regions to form a single microelectronic system. In this case also, in addition to the difficulty of the soldering operation, production output is greatly penalized by the fact that it is very difficult to disconnect faulty chips from functioning ones to be able to reuse the latter. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is an object of the present invention to provide a method for the electrical and/or mechanical connection of component parts of a microelectronic system which overcomes the problems mentioned above. 
     According to the present invention, this object is achieved by a method for the electrical and/or mechanical interconnection of components of a microelectronic system, characterized in that it provides for the formation of at least one local Joule-effect micro-heater. The micro-heater is incorporated at a respective soldering point between a first component and a second component of the micro-electromechanical system. The method further includes providing electrical energy to the micro-heater to utilize the heat produced by the micro-heater by the Joule effect for the soldering of the first and second components at the soldering point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the present invention will become clear from the following detailed description of two practical embodiments, illustrated by way of non-limiting examples in the appended drawings, in which: 
     FIG. 1 is a cross-sectional perspective view of a hard-disk reading/writing transducer and a respective micro-actuator according to a first embodiment of the present invention; 
     FIG. 2 is a perspective view showing on an enlarged scale an element for electrical connection to the reading/writing transducer shown in FIG. 1; 
     FIG. 3 is a cross-sectional view taken along the line III—III of FIG. 2; 
     FIG. 4 is a top plan view of the reading/writing transducer and the respective micro-actuator showing electrical and mechanical connection elements according to a variation of the first embodiment of the present invention; 
     FIG. 5 is a perspective view of two chips of a multichip system before their interconnection according to a second embodiment of the present invention; and 
     FIG. 6 is a detailed cross-sectional view of the two chips shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a cross-sectional perspective view of a micro-actuator  1  of the type used in a hard-disk reading/writing unit for the fine positioning of a reading/writing transducer (head or slider)  2  is shown. 
     The micro-actuator  1  in the embodiment shown is an electrostatic micro-actuator comprising an inner rotor  3  and an outer stator  4 , both made of polysilicon. Movable electrodes  30  projecting radially from the periphery of the rotor  3  are interdigitated with fixed electrodes  40  which project radially from the stator  4  towards the rotor  3 . A micro-actuator of this type is described, for example, in the European Patent Application No. 913921, which is incorporated herein by reference in its entirety, and is assigned to the assignee of the present invention. 
     The slider  2  is fixed to the rotor  3 , and is typically glued to a support plate, which in turn is anchored to the rotor  3 . The support plate is not shown for greater clarity of the drawings. Two suspended and flexible electrical connection elements  5   a ,  5   b  are visible in FIG.  1  and extend from the outer periphery of the micro-actuator  1  in a circular-sector-shaped region, in which the stator and the rotor are interrupted, to the respective electrical terminals  6   a ,  6   b  of the slider  2 . 
     Referring now to the enlarged view of FIG. 2, each electrical connection element comprises a pair of substantially parallel bars  7   a ,  7   b  anchored in a cantilevered manner to a layer of insulating material  16  and suspended above an underlying semiconductor substrate  160 . The bars  7   a ,  7   b  are made, for example, from the same polysilicon layer from which the rotor and the stator are made. 
     Each bar  7   a ,  7   b  has an enlarged head  8   a ,  8   b , anchored to the layer  16 , and the end of the bar remote from the head is joined to the other bar of the pair by a crosspiece  9 . A pad  11   a ,  11   b  of conductive material, such as aluminium or copper, is formed on each bar  7   a ,  7   b  on the head  8   a ,  8   b . A respective track  10   a ,  10   b  is also made of conductive material, such as aluminium or copper, that extends from the pad. 
     The track  10   b  terminates substantially in the region of the crosspiece  9 , whereas the track  10   a  extends to merge with a pad  12  of conductive material, such as aluminium or copper. The soldering area  120  is gold-plated and is formed on the crosspiece  9  substantially beneath the electrical terminal  6   a ,  6   b  of the slider  2 . 
     The pads  11   a ,  11   b  and  12 , the tracks  10   a  and  10   b , and the portion  14  of the crosspiece  9  between the end of the pad  12  and the end of the track  10   b  form a conductive path for the overall resistance of which is concentrated substantially in the polysilicon portion  14 . This resistance has greater resistivity than aluminum or copper, as shown schematically by the resistor R. 
     As shown in greater detail in FIG. 3, the bar  7   b  is anchored in a cantilevered manner to the insulating layer  16  which, for example, is made of silicon oxide. The bar  7   a  contacts through an opening  17  in the oxide layer  16  an underlying polysilicon layer  18  which extends out of the micro-actuator. This layer is contacted through another opening  19  in the oxide layer  16  by another polysilicon bar  20 . 
     Polysilicon bar  20  extends as far as the periphery of the chip containing the micro-actuator. This is for the soldering of electrical wires which will be connected to the electrical terminals  6   a ,  6   b  of the slider. The tracks  10   a ,  10   b  are covered by a layer of passivating material  21  which has openings on the pad  12  to permit soldering to the electrical terminals  6   a ,  6   b  of the slider, and on the pads  11   a ,  11   b  to permit contact by electrical-energy supply probes  13   a ,  13   b.    
     The electrical terminals  6   a ,  6   b  of the slider are formed on a dielectric layer  22  which thermally insulates the body of the slider. To solder the terminals of the slider, the two probes  13   a ,  13   b  are placed on the pads  11   a ,  11   b  and an electric pulse is applied to the probes to close the electrical circuit. The electric pulse may include a voltage V as shown schematically in FIG.  2 . 
     The current which flows in the circuit brings about localized heating by the Joule effect in the region of the resistor R, that is, the portion of the circuit having greater resistance. Resulting heat melts a spot  15  of soldering material, such as a lead/tin alloy previously applied to the electrical terminal  6   a ,  6   b  of the slider, for example. As the spot  15  melts, it drops onto the pad  12  as indicated schematically  15 ′ in chain line. When the voltage supply to the electric circuit is interrupted, the molten soldering material sets. This causes soldering of the slider terminal to the pad  12 , and hence the electrical connection to the pad  11   a.    
     In other words, the circuit portion with greater resistance R acts as an integrated local Joule-effect micro-heater. The heat generated by the micro-heater by the Joule effect is utilized to melt the solder  15 . The micro-heater is activated by the application of a suitable voltage upstream or downstream thereof by supplying electrical energy thereto. 
     Clearly, variations and/or additions may be applied to the embodiment described and illustrated above. For example, instead of providing pairs of polysilicon bars, the electric soldering circuit may be formed on a single bar of sufficient width. The two pads  11   a  and  11   b  for the probes  13   a  and  13   b , the two tracks  10   a  and  10   b , and the pad  12  may be on the single bar of sufficient width. 
     Moreover, the bars  7   a  and  7   b  and the crosspiece  14  may be made of materials other than polysilicon, possibly even of insulating material. The micro-heaters may be formed on the bars by the deposition of a refractory and sufficiently resistive material instead of being formed by respective portions of the bars. 
     The plan view of FIG. 4 shows a variation of the embodiment described above in which the method according to the invention is used not only for soldering electrical terminals  60   a-   60   d  of the slider  2  to respective electrical connection elements  50   a-   50   d  similar to the elements  5   a  and  5   b  of FIG. 1, but also for soldering points  52   a-   52   d  for the mechanical anchoring of the slider  2  to respective flexible elements (springs)  51   a-   51   d . This anchoring is for the mechanical connection of the slider to the chip containing the micro-actuator. 
     Both the electrical connection springs  50   a-   50   d  and the mechanical connection springs  51   a-   51   d  are formed as shown in FIGS. 2 and 3, and as described with reference thereto. The points  52   a-   52   d  for the mechanical anchoring of the slider are soldered to the respective mechanical connection springs  51   a-   51   d  in the manner described above. 
     By virtue of the present invention, the soldering operation for the electrical and/or mechanical interconnection of the slider to the micro-actuator poses no problems, and there is no risk of damaging the delicate structure of the micro-actuator. The method according to the present invention may also advantageously be used for unsoldering the slider from the micro-actuator. This is done, for example, if faults in the micro-actuator, in the circuitry integrated in the micro-actuator chip, or in the slider are detected during testing. 
     It is possible, by proceeding as described above for the soldering, to unsolder the slider and to replace the faulty component with a functioning one. The need to discard the entire microelectronic system because of a fault which affects a single component is thus avoided. 
     FIGS. 5 and 6 show schematically a second possible embodiment of the invention for the electrical and/or mechanical interconnection of two chips  70  and  71 , each incorporating respective integrated circuits to form a multichip system. Local Joule-effect micro-heaters  72  is incorporated in the chip  70  in positions corresponding to those of soldering pads  73  of the chip  71 . The micro-heater  72  comprises resistive elements connected in an electrical circuit comprising a pair of tracks  74  and  75  of conductive material. The conductive material may be aluminum or copper, for example. The pair of tracks  74  and  75  terminate in respective pads  76 ,  77  on which probes  78 ,  79  can be placed. 
     As shown in FIG. 6, the micro-heater  72  comprises a resistive element  80  of refractory material of sufficient resistivity, such as polysilicon, separated from a semiconductor substrate  81  of the chip  70  by a dielectric layer  82  of low thermal conductivity, such as silicon oxide. The two tracks  74 ,  75  contact the resistive element  80  at two respective points through respective openings in an insulating layer  83 . A dielectric layer  84  covers the resistive element  80  and the tracks  74  and  75 . A soldering pad  85  formed above the dielectric layer  84  comprises an enlarged head of a track  86 , such as aluminium or copper, for electrical interconnection between the two chips  70  and  71 . A layer of gold  87  is preferably formed on the pad  85 . 
     Spots  88  of solder, typically a lead/tin alloy, are applied to the pads  73  of the chip  71  or, may equally be applied to the corresponding pads  85  of the chip  70 . The two chips are then brought together to bring the pads  73  adjacent the respective pads  85 . The probes  78  and  79  are placed on the pads  76  and  77  and a voltage V is applied thereto. The current which conducts in the electrical circuit formed by the tracks  74  and  75 , and by the resistive element  80  brings about heating of the resistive element  80  by the Joule effect. The heat thus produced causes the spot of solder  88  to melt, soldering the pads  73  and  85  together, thus interconnecting the two chips  70 ,  71  electrically and mechanically. 
     In this embodiment also, the soldering is achieved easily and does not pose problems of damage to the structures incorporated in the two chips. Moreover, if at a testing stage subsequent to the interconnection it is found that one of the two chips is defective, the two chips can be unsoldered and the faulty chip can be replaced by a new, functioning chip. The unsoldering is performed in exactly the same way as the soldering. 
     Clearly, variations of the embodiment described and illustrated may also be provided in this second embodiment of the invention. For example, the pad  85  could be connected in the conductive path  74 - 77  for the supply of electrical energy to the resistive element  80 .