Abstract:
Electro-mechanical connection component provided on one connection surface with conductive inserts intended to be inserted at ambient temperature into respective ductile conductive pads formed on a surface of another connection component for a face-to-face type hybridization. Each insert of the component includes: a hollow metal core formed of a bottom arranged on the connection surface and of a lateral wall protruding from said bottom, defining an internal surface of the insert, at least a portion of said internal surface being non-oxidized; and a metal layer substantially only covering the internal surface of the metal core.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the connection of two components according to the flip-chip technique, and more specifically to the connection of two electronic components by insertion at ambient temperature of metal-type inserts in a metal forming pads. 
         [0002]    The invention thus specifically applies in so-called chip-on-chip, chip-on-wafer, and wafer-on-wafer assemblies. 
       STATE OF THE ART 
       [0003]    To replace flip-chip hybridizations with solder balls, it is known to provide on a surface of a first electronic component inserts made of a hard metal, for example, titanium nitride, and on a surface of a second electronic components, pads made of a ductile metal, for example, silver, and then to hybridize the two components by inserting at low temperature the inserts into the pads, which thus creates mechanical and electrical interconnects between components. 
         [0004]      FIGS. 1 and 2  schematically illustrate the flip-chip hybridization of a first and of a second microelectronic components  10 ,  12 . First component  10  comprises on one or its surface  14  a set of electrically-conductive inserts  16 , intended to penetrate into respective electrically-conductive pads  18 , pads  18  being arranged on surface  20  of second component  12 . 
         [0005]    The bottom of each insert  16  is further in contact with a connection area  22  formed in the thickness of first component  10 , area  22  forming the interface with, for example, an electronic circuit  24 . Similarly, each pad  18  is in contact with a connection area  26  formed in the thickness of second component  12 , area  26  forming the interface, for example, with an electronic circuit  28 . 
         [0006]    To perform the hybridizing, preferably at cold temperature, electronic components  10  and  12  are aligned to present each insert  16  in front of a pad  18 , and an appropriate pressure, illustrated by the arrows, is for example exerted on the first component, which is mobile ( FIG. 1 ). Inserts  16 , which have a greater hardness than pads  18 , then penetrate into them. Interconnects  30  between first and second microelectronic components  10 ,  12  are thus formed ( FIG. 2 ). Interconnects  30  mechanically fasten components  10 ,  12  together, while creating electric connections therebetween. 
         [0007]    As an example, first component  10  is a detection matrix formed of a plurality of sensitive detection elements, especially for detecting electromagnetic radiation, and second component  12  is a circuit for reading said sensitive elements. Interconnects  30  thus form the electric connection of the read circuit with each of the sensitive elements of first component  10 . 
         [0008]    However, a recurrent problem in this type of hybridizing by metal-in-metal insertion lies in the fact that, with no specific operation, the insert surface oxidizes, which creates electric connections of poor quality between inserts and the pads into which they are inserted. Indeed, metals adapted for a cold insertion, more commonly “hard” metals, such as for example titanium nitride, are oxidizable. 
         [0009]    A solution used to avoid the insertion of oxidized inserts is to manufacture inserts  16 , each formed of a central metal core having a greater hardness than pads  18 , and to cover this core, before its oxidation, with a layer of a noble metal, which is thus non-oxidizable, such as gold or platinum. The core and the layer of noble metal are thus inserted together into a pad  18  without for oxide capable of affecting the quality of the electric connection to appear. 
         [0010]    An example of a method of low cost manufacturing of hollow cylindrical inserts  16  covered with a layer of noble metal is described hereafter in relation with the simplified cross-section views of  FIGS. 3 to 10 . 
         [0011]    The method starts with the deposition of a sacrificial layer  40  having a thickness e on surface  14  of component  10 , for example, a polyimide-type resin layer, followed by a photolithography to form circular holes  42  in sacrificial layer  40  all the way to surface  14  of component  10  ( FIG. 3 ). Thickness e corresponds to the height desired for the insert core and the diameter of circular holes  42  corresponds to the outer diameter of the core. 
         [0012]    The method carries on with the full-plate deposition of a layer or a multilayer of hard metal  44 , for example, titanium nitride or an alloy containing titanium nitride, of a thickness corresponding to the thickness of the core of inserts  16 . The deposition is for example a chemical vapor deposition, or CVD, performed at a temperature compatible with the microelectronic elements of component  10 , especially a temperature lower than 425° C. for a component  10  implementing a CMOS technology ( FIG. 4 ). 
         [0013]    A removal of the portion of hard metal  44  deposited between holes  42  is then performed, for example, by means of a damascene or gap fill etching, well known per se. 
         [0014]    For example, according to the gap-fill etching, a full-plate deposition of fluid resin layer  46  is performed, said layer thus filling holes  42  and planarizing the assembly obtained at the previous step ( FIG. 5 ). Once solidified, resin layer  46  is then uniformly etched, for example, by mechanical or chem.-mech. polishing, until metal layer surface  46  is reached. Holes  42  however remain filled with resin  46  to protect the metal covering them during subsequent steps ( FIG. 6 ). An etching of metal  44  arranged between holes  42  is then implemented in a manner known per se ( FIG. 7 ). 
         [0015]    The method then carries on with the removal of resin  46  contained in holes  42 , for example, by means of a decoating based on an O 2  plasma, followed by the removal of sacrificial layer  40 , for example, by means of a decoating based on an O 2  plasma ( FIG. 8 ). Cores  50  of inserts  16  are thus formed. 
         [0016]    A layer  52  of noble metal, for example a gold or platinum layer, is then deposited full plate, for example, by means of a CVD ( FIG. 9 ), after which the portion of layer  52  placed between cores  50  is removed, for example, by means of the conventional photolithography technique ( FIG. 10 ). 
         [0017]    However, this method has a number of disadvantages. First, even though the method has a low cost and provides a high manufacturing efficiency, it implements a large number of complex steps. 
         [0018]    Then, this type of method makes it difficult or even impossible to decrease interconnect pitches, that is, the minimum space between two insert/pad interconnects, if low-cost techniques of manufacturing the noble metal layer covering the metal cores of the inserts are used. Indeed, low-cost manufacturing techniques comprise performing a full-plate deposition of a noble metal layer on the surface of the component comprising the inserts, and then etching the noble metal layer present between inserts. Now, the only low-cost etching technique of the state of the art applicable to noble metals such as gold and platinum is a liquid chemical etching which does not enable, to date, to etch surfaces having dimensions smaller than 10 micrometers, or even 15 micrometers. Only an ion machining etching currently enables to etch interconnect pitches smaller than 10 micrometers. However, such a technique has a very low efficiency, especially due to the cleanings required between each deposition, and it thus expensive. 
         [0019]    This difficulty to decrease the interconnect pitch is illustrated in the simplified cross-section views of  FIGS. 11 and 12 . 
         [0020]    Particularly, as can be seen in  FIG. 11 , noble metal layer  52  is not totally removed between inserts  16  due to the accuracy of the liquid etching used. A ring of noble metal  54  thus remains around each insert  16 . 
         [0021]    With the simplified assumption of cylindrical connection areas  22  aligned with their respective inserts  16 , minimum interconnect pitch P is equal to the sum of width L 1  of connection area  22 , of twice width G 1  between the external diameter of core  50  of an insert  16  and the edge of connection area  22 , of twice width G 2  between the external diameter of core  50  and the edge of ring  54  surrounding an insert  16 , and of width L 4  separating two rings  54  of adjacent inserts  16 , that is, P=L 1 +2G 1 +2G 2 +L 4 . 
         [0022]    Width L 1  is mainly determined by the maximum current intended to cross interconnect  30  formed of an insert  16  inserted in a pad  18  and is thus substantially independent from the method of manufacturing inserts  16 . A minimum width L 1 , for example, in imagers, is 3 micrometers. 
         [0023]    Width G 1  depends on the accuracy of the photolithography used to form openings  42  in sacrificial layer  40 . By means of current etching techniques, the minimum possible width G 1  is approximately 1 micrometer. 
         [0024]    Width G 2  depends on the accuracy of the photolithography used to remove noble metal layer  52  between inserts  16 . By means of current etchings, the minimum possible width G 2  is approximately 1 micrometer. 
         [0025]    Finally, width L 4  also represents the distance separating two adjacent conductive interconnect elements  30 . It can be estimated that a minimum width L 4  of approximately 3 micrometers is appropriate to avoid any risk of short-circuit between interconnects. 
         [0026]    Further, noble metals, for example, gold or platinum, are highly reflective, which hinders the photolithographic etching. Further, etch chemistries are very aggressive for circuits, particularly image sensors. 
         [0027]    Thus, the minimum interconnect pitch P which can currently be achieved is approximately 10 micrometers, that is, an interconnect surface density equal to approximately 10 4  interconnects/mm 2 . 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0028]    The present invention aims at providing inserts enabling to decrease the interconnect pitch and which can be achieved with current manufacturing techniques, as well as at a low-cost method of manufacturing such inserts, and particularly a method also enabling to perform a hybridization of microcomponents at ambient temperature. 
         [0029]    For this purpose, an object of the invention is a method for manufacturing an electro-mechanical connection component provided on one connection surface with conductive inserts intended to be inserted at ambient temperature into respective ductile conductive pads formed on a surface of another connection component for a face-to-face type hybridization. 
         [0030]    According to the invention:
       the method comprises, for each insert:
           forming a hollow metal core formed of a bottom arranged on the connection surface and of a lateral wall protruding from said bottom, defining an internal surface of the insert, at least a portion of said internal surface being non-oxidized; and   forming a metal layer substantially only covering the internal surface of the metal core and intended to be in contact with the pad associated with the insert;   
           and the metal layer is made of an inoxidizable metal, especially a noble metal, or the forming of the metal layer comprises:
           forming a first metal sub-layer non-oxidized over at least a portion of its surface, covering at least said non-oxidized portion of the internal surface of the core, the first sub-layer having a greater plasticity than the core; and   forming a second sub-layer covering at least the first sub-layer over its non-oxidized portion and having a lower plasticity than the first sub-layer.   
               
 
         [0037]    “Ambient temperature” here means a temperature remote from the melting temperature of the material forming the first layer and the pads, for example, temperatures on the order of 300° K for which no significant softening of this first layer and of the pads where the inserts are intended to be inserted can be observed. The “cold” or “ambient temperature” hybridization of the invention thus differs from “thermo-compressive”-type hybridizations during which both a pressure and a heating are applied, the heating aiming at softening or melting the pads in order to ease the insertion of inserts. 
         [0038]    In other words, it is chosen not to totally cover the central metal core of inserts, but only its hollow portion, and thus to take no specific measure to avoid the oxidation of the external surface of the central core. Avoiding covering the external surface of the core with a layer of noble metal enables to do away with the presence of a ring of noble metal around the inserts. 
         [0039]    Indeed, attempting to protect the entire oxidizable surface of the central core by implementing low-cost manufacturing methods, and particularly a full-plate deposition, results in depositing an unwanted metal layer between inserts, which unwanted layer then has to be removed by means of a low-cost etching of limited accuracy. Thus, metal rings are inevitably formed around the inserts, which limits the minimizing of the interconnect pitch. 
         [0040]    It should be noted that as large a conductive surface area as possible has been desired up to now for an insert, in order to maximize the quality of the electric interconnect formed of an insert and of a pad. This, in particular, used to be the reason to totally cover the central conductive core with a layer of noble metal to avoid any oxidizing of the core surface. This thus implies, in the context of a low-cost manufacturing, disengaging the central core of the inserts from the sacrificial layer in which it is formed, to perform a full-plate deposition, to totally coat this core with a layer of noble metal. 
         [0041]    However, the inventors have observed that this approach is actually based on a technically erroneous assumption, that is, that the external surface and the internal surface of the core define electric paths of similar importance. 
         [0042]    Actually, noble metals, and more generally non-oxidizable metals, have a lower electric resistivity than the hard metals used to form the central core. Thus, the current preferably runs through the noble metal layer rather than through the central core. 
         [0043]    Referring to the simplified cross-section view of  FIG. 13 , when a total current I tot  flows through an interconnect  30 , for example, a current injected via area  26  of pad  18 , total current I tot  breaks down into a first current I int  flowing in internal portion  60  of noble metal layer  52  and a second current i ext  flowing in external portion  62  of layer  52 . Now, as can be seen, external current i ext  has to cross twice central core  50 , which has a greater resistivity than layer  52 , to reach connection area  22  of insert  16 . However, internal current I int  only crosses the core once, at bottom  64  thereof. Measurements thus show that internal current I int  amounts to 90% of total current I tot  and thus that external current i ext  only amounts to 10% of this current. 
         [0044]    Thus, external portion  62  of noble metal layer  52  can be considered as a dead arm with a negligible electric conduction and its removal causes no substantial alteration of the electric conduction of an interconnect  30 . 
         [0045]    Further, according to a first variation, the metal layer being formed of an inoxidizable metal, particularly a noble metal such as gold or platinum, this enables to define a non-oxidized surface at the interface with the pad when the insert is inserted into the pad. 
         [0046]    However, this variation has a number of disadvantages, including:
       a high cost due, on the one hand, to the cost of noble metals and, on the other hand, to the great number of complex steps of be implemented to only cover the inserts with a layer of such a metal;   the impossibility to decrease the interconnect pitch, as previously described, due to the limited accuracy of liquid etchings of noble metals;   an interdiffusion and an electromigration of the noble metal covering the inserts. Thus, the interconnects end up being formed of a complex multilayer made of the insert material, of the noble metal, and of the pad material, which make interconnects very sensitive to solid/solid-type diffusion, to the creation of Kirkendall-type holes, and of holes at the interfaces of the areas made of different materials; and   a crossed contamination of gold. Indeed, gold is a highly-doping material for the silicon usually present in electronic components. All the manufacturing steps using gold should thus be carried out in manufacturing areas different from those where exposed silicon is present.       
 
         [0051]    The second variation provides a flip-chip hybridization by insertion of metal inserts into metal pads ensuring an electric connection without using noble metals. 
         [0052]    Under the effect of the insertion into a pad, the different regions of an insert are deformed. Since it has a greater plasticity than the core, the first sub-layer will thus be more strongly deformed than said core during the penetration into a pad. Since, further, the second sub-layer has a lower plasticity than the first sub-layer, this second sub-layer cannot deform as much as the first sub-layer without breaking. Since it cannot follow the deformation undergone by the first sub-layer, the second sub-layer “cracks”. 
         [0053]    If the adherence of the second sub-layer to the first sub-layer is low, the second sub-layer “peels off” by sliding on the first sub-layer on insertion and remains outside of the pad, thus totally exposing the first sub-layer, which is non-oxidized, and thus is a good electric conductor. 
         [0054]    If the adherence of the second sub-layer to the first sub-layer is strong, the second sub-layer also penetrates into the pad, with cracks due to the plasticity difference with the first sub-layer. The cracks thus define as many non-oxidized electric “paths” towards the first non-oxidized sub-layer, which are good electric conductors, thus providing a good electric conduction of the interconnect formed by the insert and the pad. 
         [0055]    This result is obtained independently from the oxidizable nature of the first sub-layer which is thus advantageously selected from among non-noble materials. It is also not necessary to use a deoxidizing flow on insertion, since the electric paths have formed, and this, even if the second sub-layer is oxidized. 
         [0056]    According to an embodiment, the adherence of the second sub-layer to the first sub-layer is low so that the second sub-layer slides on the first sub-layer under the effect of a shearing applied to the stack of the first and second sub-layers. Thereby, on insertion of the insert into a pad, the second sub-layer peels off and remains outside of the pad. 
         [0057]    According to an embodiment, the first sub-layer is made of an oxidizable metal and the second sub-layer is made by oxidizing the first sub-layer to create a layer of native oxide of the metal of the first sub-layer having a plasticity lower than that of the first sub-layer. 
         [0058]    “Native oxide layer” designates an oxide layer obtained by natural oxidation of the metal when in contact with oxygen. 
         [0059]    The native oxide layer has the double property of being very brittle and of having a very low adherence to the metal from which it originates. Under the effect of the penetration of the insert into the pad, an “ice-on-mud” phenomenon can thus be observed, that is, the native oxide layer cracks in plates which slide on the first sub-layer during the insertion. The second sub-layer is thus “peeled off” and remains outside of the pad. 
         [0060]    Further, the native oxide layer has the advantage of having a very small thickness, in the order of a few nanometers, totally defined by the nature of the metal. Thus, whatever the time of exposure of the first sub-layer to oxygen, the thickness of the second sub-layer remains constant. Moreover, the electronic component provided with inserts can thus be stored in oxidizing conditions, such as air, for example, with no specific precaution. 
         [0061]    Thus, the first sub-layer is not a noble metal and, additionally and preferably, it is made of oxidizable metals, in order to form a native oxide layer. 
         [0062]    Advantageously, the first sub-layer is made of a metal selected from the group comprising aluminum, tin, indium, lead, silver, copper, zinc, and alloys based on these metals. These materials are advantageously very plastic, and can be used in low-cost processes exploitable for low interconnect pitches lower than 10 micrometers, or even than 5 micrometers. More specifically, the first sub-layer is made of aluminum, which also has the advantage of being a ductile material which, at the same time, keeps a constant hardness for a wide range of temperatures due to its high melting temperature, greater than 500° C. 
         [0063]    The advantage of using a hollow insert is that this decreases the insert bearing surface on the pad, and thus eases the insertion, or even allows a cold insertion at ambient temperature. Due to the decreased bearing surface area, the surface pressure exerted on the surface of the first and second layers bearing on the pad is also increased, which facilitates the deformation of the first layer, and thereby the cracking of the second layer. This also increases the shearing effect and helps peeling off the second layer in case of a low adherence thereof to the first layer. It should be noted that a cylindrical shape optimizes such effects, the hollow inserts thus advantageously having this shape. 
         [0064]    According to a variation, the first sub-layer has a ductility substantially equal to that of the pads, which facilitates the deformation undergone by the first sub-layer on insertion, and thus also eases the cracking of the second sub-layer. 
         [0065]    According to an embodiment, the method comprises:
       depositing a sacrificial layer on the connection surface of the first component;   forming openings in the sacrificial layer vertically in line with the locations desired for the inserts;   depositing a first metal layer at least in the opening to form the metal core for each insert;   depositing a second metal layer at least in the openings to form a metal layer covering the core of each insert; and   removing the sacrificial layer.       
 
         [0071]    Not only does this method enable to manufacture inserts which are not surrounded with metal rings, but also does it enable to decrease the manufacturing cost. Indeed, the cost is mainly inherent to the number of manufacturing sequences requiring a change of material. In particular, in the manufacturing method described in relation with  FIGS. 3 to 10 , a first manufacturing sequence relative to the forming of openings  42  ( FIG. 3 ), a second sequence relative to the deposition of the metal forming core  50  of inserts  16  ( FIG. 4 ), a third sequence relative to the removal of sacrificial layer  40  ( FIGS. 5 to 8 ), a fourth sequence relative to the deposition of the noble metal layer  52  ( FIG. 9 ), and a fifth sequence relative to the removal of layer  52  of noble metal between inserts  16  ( FIG. 10 ) can be observed. As will more easily be understood on reading of the following, the method according to the present invention only comprises three manufacturing sequences, which significantly decreases the manufacturing cost. 
         [0072]    Another object of the invention is a method of face-to-face type hybridization of a micro-electronic component obtained according to a method of the previously-mentioned type with a microelectronic component having, on one of its surfaces, respective conductive pads having a lower hardness than the metal core of the hollow inserts, comprising the insertion at ambient temperature of the inserts, provided with their second metal sublayer, into the pads. 
         [0073]    Advantageously, the interconnect pitch between microcomponents is smaller than 10 micrometers. 
         [0074]    According to an embodiment, the pressure exerted on a bearing surface of each insert during their insertion into the pads is greater than 1,800 megaPascals, which allows an efficient peeling-off of the oxide layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0075]    The present invention will be better understood on reading of the following description provided as an example only in relation with the accompanying drawings, where the same reference numerals designate the same or similar elements, among which: 
           [0076]      FIGS. 1 and 2  are simplified cross-section views of the hybridizing of a first and of a second microelectronic components by insertion of inserts into pads; 
           [0077]      FIGS. 3 to 10  are simplified cross-section views illustrating a method of manufacturing inserts comprising an external layer made of noble metal; 
           [0078]      FIG. 11  is a cross-section view of two adjacent inserts manufactured according to the method of  FIGS. 3 to 10  and inserted into respective pads; 
           [0079]      FIG. 12  is a simplified cross-section of an insert of  FIG. 11  according to plane A-A; 
           [0080]      FIG. 13  is a simplified view of an insert manufactured according to the method of  FIGS. 3 to 10  and inserted into a pad, illustrating the electric currents in the insert; 
           [0081]      FIGS. 14 to 18  are simplified cross-section views illustrating a first method of manufacturing inserts according to the invention; 
           [0082]      FIG. 19  is a cross-section view according to a first alternative embodiment of the invention of two adjacent inserts manufactured according to the method of  FIGS. 14 to 18  and inserted into respective pads. 
           [0083]      FIG. 20  is a simplified cross-section of an insert of  FIG. 19  along plane B-B; 
           [0084]      FIG. 21  is a cross-section view of an insert according to a second alternative embodiment of the invention; 
           [0085]      FIG. 22  is a simplified cross-section of an insert of  FIG. 3  along plane C-C; 
           [0086]      FIG. 23  is a simplified cross-section view illustrating the penetration of the insert of  FIG. 21  into a ductile pad. 
       
    
    
     DETAILED DESCRIPTION 
       [0087]    A method according to the invention of manufacturing inserts for a flip-chip hybridizing of a first and of a second microelectronic components similar to that described in relation with  FIGS. 1 and 2  will now be described in relation with  FIGS. 14 to 18 . 
         [0088]    It should be noted that the inserts may take any shape, although inserts having a decreased bearing surface area, such as hollow cylinders, for example, are preferred to decrease the pressure necessary for their insertion into the pads. In the following, U-shaped cylindrical and hollow inserts will however be described, this shape being a preferred embodiment. However, it should be understood that considerations bearing on the materials forming the inserts and the pads are independent from the shape selected for them. For example, the inserts may be solid and/or have a triangular, square, and more generally polygonal, star, or other shape. 
         [0089]    The method starts similarly to the manufacturing steps described in relation with  FIGS. 3 and 4 . Metal  44  forming core  50  of the inserts has a greater hardness than pads  18  to be insertable into them. For this purpose, central core  50  preferably has a Young&#39;s modulus greater than 1.5 time the Young&#39;s modulus of the material of pads  18 . 
         [0090]    Advantageously, metal  44  forming central core  50  is made of a hard metal, such as titanium nitride (TiN), tungsten nitride (NiW), copper (Cu), vanadium (V), molybdenum (Mo), nickel (Ni), titanium tungstenate (TiW), WSi, or tungsten (W), for example, and pads  18  are made of a ductile metal, for example, aluminum, tin, indium, lead, silver, copper, zinc, or an alloy of these metals. 
         [0091]    The method carries on with the full-plate deposition of a metal layer or multilayer  70  having the function of protecting the internal surface of central core  50  of the inserts from oxidation and optionally having a smaller electric resistivity than the metal  44 , metal  44  forming the core remaining non-oxidized at this step of the method. The deposition for example is a chemical vapor deposition or CVD carried out at a temperature compatible with the microscopic elements of component  10 , especially a temperature smaller than 425° C. for a component  10  implementing CMOS technology ( FIG. 14 ). 
         [0092]    Layer  70  is preferably made of aluminum, this metal having the advantage of having a very high melting temperature greater than 500° C. 
         [0093]    A removal of the portion of hard metal  44  deposited between holes  42  is then performed, for example, by means of a damascene or gap-fill etching well known per se. 
         [0094]    For example, a gap-fill etching similar to that described in relation with  FIGS. 5 to 8  is implemented, comprising:
       the full-plate deposition of a fluid resin layer  46  ( FIG. 15 ),   the uniform etching of solidified layer  46  to reach the surface of metal layer  70  ( FIG. 16 ),   the etching of the stack of metal layers  44 ,  70  arranged between holes  42  ( FIG. 17 ), and   the removal of resin  46  contained in holes  42 , followed by the removal of sacrificial layer  40  ( FIG. 18 ).       
 
         [0099]    The method thus comprises three manufacturing sequences, that is, a first manufacturing sequence relative to the forming of openings  42  ( FIG. 3 ), a second sequence relative to the deposition of metals  44  and  70  ( FIG. 14 ), a third sequence relative to the removal of sacrificial layer  40  ( FIGS. 15 to 18 ). 
         [0100]      FIGS. 19 and 20  are simplified cross-section views illustrating inserts  72  manufactured according to the method just described. As illustrated, inserts  72  are formed of a central core  50 , having only its internal surface covered with a metal layer of protection against oxidation  70 . 
         [0101]    Still with the simplified assumption of cylindrical connection areas  22  aligned with their respective inserts  16 , the minimum interconnect pitch P obtained due to the invention is equal to the sum of width L 1  of connection area  22 , of twice width G 1  between the external diameter of core  50  of an insert  16  and of width L 5  separating the external diameters of adjacent inserts  72 , that is, P=L 1 +2G 1 ±L 5 . 
         [0102]    Taking the previously-described numerical examples, that is, a minimum value of L 1  equal to 3 micrometers, a minimum value of G 1  equal to 1 micrometer, and a value of L 5  equal to 3 micrometers, a minimum interconnect pitch P equal to 8 micrometers is obtained, that is, an interconnect surface density equal to approximately 1.625. 10 4  interconnects/mm 2 . 
         [0103]    According to a first variation, metal layer  70  is made of a noble metal, such as gold or platinum, for example. 
         [0104]    According to a second variation, illustrated in simplified cross-section view in  FIGS. 21 and 22 , metal layer  70  is formed of a first metal sub-layer  80  formed on core  50  of insert  72 , sub-layer  80  being itself covered with a protection sub-layer  82 . 
         [0105]    First metal sub-layer  80 , apart from being electrically conductive and from strongly adhering to central core  50  of insert  72  due to the metal-metal interface that it forms with core  50 , has the function of deforming, while remaining attached to core  50 , during the penetration of the insert into a pad  18 . It has, for this purpose, a greater plasticity than core  50 . Sub-layer  80  may thus be formed of a ductile metal. Particularly, a ductile metal having a Young&#39;s modulus greater than 1.5 time that of the material of core  50  has an appropriate plasticity. 
         [0106]    Preferably, sub-layer  80  has a ductility substantially equal to that of pads  18  to enable the penetration of hard core  50  without breaking and obtain relative deformations of sublayer  80  and of pad  18  in substantially equal fashion. 
         [0107]    Sub-layer  80  is thus advantageously made of aluminum, tin, indium, lead, silver, copper, zinc, or an alloy of these metals. Further, metal sub-layer  80  is not oxidized. 
         [0108]    Protection sub-layer  82  has as a first function to protect metal sub-layer  80  from oxidation and as a second function to expose at least a portion of metal sub-layer  80  on insertion of insert  72  into a pad  18  to create an electric connection between the material of pad  18  and central core  50 . To achieve this, protection sub-layer  82  is selected to crack under the effect of the deformation of metal sub-layer  80 . Protection sub-layer  82  thus has a lower plasticity than metal sub-layer  80 . 
         [0109]    Preferably, protection sub-layer  82  is selected to have a very low breakage threshold under deformation stress, that is, is very “brittle”. Protection sub-layer  82  may be a protection film placed on metal sub-layer  80 , such as for example an epoxy resist or a polymer layer such as parylene, for example, or a hard metal layer or a layer of hard and brittle insulator, such as for example SiO 2  or SiN. 
         [0110]    Preferably, protection sub-layer  82  is made of the native oxide of the metal forming metal sub-layer  80 , which has the triple advantage of:
       providing a very thin protection layer  82 , in the order of a few nanometers,   being hard and brittle, and especially having a plasticity and a ductility much lower than those of the actual metal  80 , and   having a very low adherence to metal sub-layer  80 .       
 
         [0114]    Further, this embodiment has the advantage that no specific measures are necessary to avoid the oxidation of inserts during their storage since inserts  72  are left to oxidize on purpose. 
         [0115]    As illustrated in  FIG. 22 , during the penetration of insert  72  into pad  18 , a deformation, even light, of metal sub-layer  80 , breaks oxide layer  82  into plates, and, under the effect of shearing, the plates of native oxide slide on metal sub-layer  80  while remaining outside of pad  18 . Oxide layer  82  is thus peeled off during the insertion, thus exposing metal sublayer  80 , thereby creating a high-quality, and especially oxide-less, electric connection. 
         [0116]    A central core  50  non oxidized over its entire internal surface has been described. As a variation, only a portion of the internal surface of central core  50  is non-oxidized. Central core  50  is then covered with layer  70  at least on this non-oxidized portion. In the second variation, the sub-layer covers at least this non-oxidized portion and protection sub-layer  82  covers at least the portion of sub-layer  80  covering the non-oxidized portion of core  50 , this portion of sub-layer  80  being non-oxidized. 
         [0117]    As mentioned hereabove, inserts  72  are preferably hollow cylinders having very small bearing surfaces S ( FIGS. 3 and 4 ), that is, to be able to perform a cold insertion, under ambient atmosphere, that is, under an ambient temperature much lower than the melting temperature of pads  18 , for example, a temperature of approximately 300° K and under atmospheric pressure. Further, very small bearing surfaces result in increasing the stress exerted on the different regions of the inserts, and thus particularly the deformation and shear forces, which facilitates the cracking of protection sub-layer  82  as well as its peeling-off in the case of a protection layer of low adherence to metal sub-layer  80 . Reference should advantageously be made to document FR 2 928 033 for the calculation of a bearing surface area allowing a cold insertion under ambient atmosphere. 
         [0118]    Advantageously, the pressure exerted on bearing surface S on insertion of inserts comprising a first aluminum sub-layer  80  covered with a native oxide layer  82  (alumina Al 2 O 3 ) in aluminum pads  18  is greater than 1,800 megaPascals. The inventors have indeed observed that for lower pressure values, the interconnects formed of inserts  16  in pads  18  have a high electric resistance, which means that the peeling-off of oxide layer  82  is not complete. The inventors have however observed that for the previous configuration of inserts and pads, pressures greater than 1,800 megaPascals (MPa) provide high-quality interconnects, that is, having an electric resistance close to that of aluminum, which means that the oxide layer has been almost fully peeled off 
         [0119]    Advantageously, the general insertion force or, equivalently, the general insertion pressure, exerted on circuits  10  and  12  to hybridize them, for example that exerted on circuit  10  such as illustrated by arrows in  FIG. 1 , and bearing surface S of the hollow cylinder, are thus selected to obtain said minimum pressure. 
         [0120]    For example, a hollow cylinder having a diameter equal to 4 μm, with a wall thickness equal to 0.2 μm, has a bearing surface area S equal to 2.512 μm 2 . When such an insert is submitted to a 5-mN insertion force, the pressure exerted on its bearing surface S is equal to 1,990 MPa. 
         [0121]    Knowing the general insertion force and the number of interconnects between circuits  10  and  12 , the unit insertion force applied to each insert  16  can be deduced. Knowing the unit insertion force, a maximum bearing area to obtain at least the minimum 1,800-MPa pressure can thus be deduced. Finally, bearing surface S of a hollow cylinder being provided by relation S=2×π×(R 2 −R 1 )×R 2 , where R 2 −R 1  is the thickness of the walls of inserts  16  and 2×R 2  is the external diameter of inserts  16  ( FIG. 4 ), thickness and diameter couples can easily be deduced. The selection of specific values for the thickness and the diameter can thus be performed according to other considerations, especially considerations relative to thicknesses that can be achieved according to the manufacturing method used or considerations relative to the mechanical robustness of inserts.