Method for the electrical and/or mechanical interconnection of components of a microelectronic system

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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:

[0010] 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;

[0011] 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;

[0012] FIG. 3 is a cross-sectional view taken along the line III—III of FIG. 2;

[0013] 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;

[0014] 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

[0015] FIG. 6 is a detailed cross-sectional view of the two chips shown in FIG. 5.

[0016] 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.

[0017] 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.

[0018] 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 5a, 5bare 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 6a, 6bof the slider 2.

[0019] Referring now to the enlarged view of FIG. 2, each electrical connection element comprises a pair of substantially parallel bars 7a, 7banchored in a cantilevered manner to a layer of insulating material 16 and suspended above an underlying semiconductor substrate 160. The bars 7a, 7bare made, for example, from the same polysilicon layer from which the rotor and the stator are made.

[0020] Each bar 7a, 7bhas an enlarged head 8a, 8b, 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 11a, 11bof conductive material, such as aluminium or copper, is formed on each bar 7a, 7bon the head 8a, 8b. A respective track 10a, 10bis also made of conductive material, such as aluminium or copper, that extends from the pad.

[0021] The track 10bterminates substantially in the region of the crosspiece 9, whereas the track 10aextends 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 6a, 6bof the slider 2.

[0022] The pads 11a, 11band 12, the tracks 10aand 10b, and the portion 14 of the crosspiece 9 between the end of the pad 12 and the end of the track 10bform 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.

[0023] As shown in greater detail in FIG. 3, the bar 7bis anchored in a cantilevered manner to the insulating layer 16 which, for example, is made of silicon oxide. The bar 7acontacts 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.

[0024] 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 6a, 6bof the slider. The tracks 10a, 10bare covered by a layer of passivating material 21 which has openings on the pad 12 to permit soldering to the electrical terminals 6a, 6bof the slider, and on the pads 11a, 11bto permit contact by electrical-energy supply probes 13a, 13b.

[0025] The electrical terminals 6a, 6bof 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 13a, 13bare placed on the pads 11a, 11band 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.

[0026] 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 6a, 6bof 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 11a.

[0027] 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.

[0028] 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 11aand 11bfor the probes 13aand 13b, the two tracks 10aand 10b, and the pad 12 may be on the single bar of sufficient width.

[0029] Moreover, the bars 7aand 7band 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.

[0030] 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 60a-60dof the slider 2 to respective electrical connection elements 50a-50dsimilar to the elements 5aand 5bof FIG. 1, but also for soldering points 52a-52dfor the mechanical anchoring of the slider 2 to respective flexible elements (springs) 51a-51d. This anchoring is for the mechanical connection of the slider to the chip containing the micro-actuator.

[0031] Both the electrical connection springs 50a-50dand the mechanical connection springs 51a-51dare formed as shown in FIGS. 2 and 3, and as described with reference thereto. The points 52a-52dfor the mechanical anchoring of the slider are soldered to the respective mechanical connection springs 51a-51din the manner described above.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.