Abstract:
A method of forming a hybridized device including forming a first component provided with metal bumps, and a second component provided with connection elements, attaching the bumps to the connection elements. The manufacturing of the second component includes forming, on a surface of a substrate, resistive elements at the locations provided for the connection elements; depositing an electric insulator layer at least on the resistive elements; and forming the connection elements, each comprising a metal well having an opening capable of receiving the corresponding metal bump of the first microelectronic component and at least partially filled with a fusible element, particularly indium or an alloy of tin and gold, or with a conductive ink, particularly based on silver or copper. Further, the attachment of the balls to the connection elements comprises applying an electric current through the resistive elements to heat the bumps.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the assembly of microelectronic components, and more particularly to “flip-chip” hybridization. 
       BACKGROUND 
       [0002]    An assembly by the so-called “flip-chip” technique usually comprises forming electrically-conductive bumps on a surface of a first electronic component and forming electrically-conductive connection elements, particularly bumps or connection areas, on a surface of a second component according to a predetermined connection pattern. The first component is then transferred onto the second component to place the bumps in correspondence with the connection elements, after which the assembly is pressed and heated. The bumps then deform and melt to form electric connections perpendicular to the main plane of the electronic component, generally in the form of a wafer. 
         [0003]    The most conventional technique to form the assembly is to perform a general heating of the assembly, for example, by placing the two components under a temperature-controlled atmosphere. 
         [0004]    However, the melting of a metal bump requires high temperatures, typically higher than 156° C. Now, such temperatures are not compatible with organic components, particularly plastic components such as PEN (polyethylene naphthalate) and PET (polyethylene terephthalate), which have glass transition temperatures lower than these temperatures, respectively of 120° C. for PEN and 70° C. for PET, for example. Thus, when submitting a plastic component to the solder bump melting temperature, the plastic component takes a rubbery state and strongly deforms, or even destroys. 
         [0005]    Methods implementing a local heating of the metal bumps are also known. For example, U.S. Pat. No. 7,810,701 describes a component comprising contacts in the form of resilient spirals covered with a resistive layer having the bumps deposited thereon. By circulating a high-frequency electric current in the spirals, an electromagnetic field which causes the melting of the resistive layer and of the bumps is created. This method is thus irreversible and does not enable to separate the two hybridized components without causing their destruction. 
       DISCUSSION OF THE INVENTION 
       [0006]    The present invention aims at providing a method and a device enabling to achieve a hybridization which is reversible while being mechanically resistant, and which implement a local heating of metal bumps. 
         [0007]    For this purpose, the invention aims at a method of manufacturing a hybridized device, comprising:
       forming a first microelectronic component provided on a surface with metal bumps, and a second microelectronic component provided on a surface with connection elements corresponding to said metal bumps;   placing the bumps of the first microelectronic component into contact with the connection elements of the second microelectronic component; and   attaching the bumps to the connection elements.       
 
         [0011]    According to the invention:
       the forming of the second microelectronic component comprises:
           forming a substrate;   forming, on a surface of the substrate, resistive elements at the locations provided for the connection elements;   depositing an electric insulator layer at least on the resistive elements; and   forming the connection elements, each comprising a metal well having an opening capable of receiving the corresponding metal bump of the first micro-electronic component and at least partially filled with a fusible element, particularly indium or an alloy of tin and gold, or with a conductive ink, particularly based on silver or copper,   
           and the attachment of the bumps to the connection elements comprises applying an electric current through the resistive elements to heat the bumps.       
 
         [0018]    In other words, the connection elements and the bumps are locally heated by convection through an electrically-insulating layer, without for the heating elements to be themselves destroyed. Not only is the heating performed locally, which limits as much as possible the heating of the second microelectronic component, but it is further possible to dehybridize the two microelectronic components by having current circulate again in the heating elements, thus allowing their separation. 
         [0019]    Further, the wells particularly enable to mechanically strengthen the hybridized assembly. The ink, annealed on heating, or the fusible material partially filling the wells enables to define a wettable surface for the bumps and to form an electric connection therewith, while also mechanically strengthening the interconnect between the two hybridized circuits. 
         [0020]    More particularly, the manufacturing of the metal well comprises forming a metal area and cutting with a laser an opening in said metal area. The laser etching, for example, by means of an excimer laser, has the advantage of providing a very local heating, and thus of limiting unwanted thermal effects on the second microelectronic component, while allowing a high photonic energy input. The size of the well opening is for example adjusted by means of the difference in thermal expansion coefficients between the material of the metal area and the material of the substrate. 
         [0021]    According to an embodiment, the electric insulator is a fluorinated polymer of low dielectric constant, particularly a dielectric constant equal or close to 2. Fluorinated polymers have a very high chemical stability, a very high melting temperature, as well as a very good thermal conductivity. Further, the insulator is advantageously obtained by applying an anneal of steep slope to an emulsion of this polymer in one or several solvents, which enables to very rapidly evaporate the solvent(s) contained in said emulsion, which generates micropores filled with air in the finally-obtained insulator. 
         [0022]    According to an embodiment, the resistive elements are made in the form of coils or rods. Particularly, the resistive elements are series-connected. The resistive elements thus have a strong electric resistance. It is thus possible to obtain a strong heating with a low current and/or within a very short time. 
         [0023]    According to an embodiment, on application of the current, the surface of the second microelectronic component, opposite to the surface provided with the connection elements, is cooled down, which enables to decrease the impact of the heating, even local, on the second microelectronic component. 
         [0024]    According to an embodiment, the substrate of the second microelectronic component comprises plastic, particularly PEN or PET. For example, the substrate is a flexible substrate. 
         [0025]    The invention also aims at a hybridized device comprising:
       a first microelectronic component provided on one surface with metal bumps; and   a second microelectronic component provided on one surface with connection elements corresponding to said metal bumps, the metal bumps and the connection elements being mechanically attached.       
 
         [0028]    According to the invention, the second microelectronic component comprises:
       a substrate;   resistive elements at the longitudinal station of the connection elements;   an electric insulator layer interposed between the resistive elements and the connection elements; and   electric terminals for the application of an electric current in the resistive elements.       
 
         [0033]    Further, the connection elements of the device each comprise a metal well receiving the corresponding metal bump of the first microelectronic component, at least partially filled with a fusible element, particularly indium or an alloy of tin and gold, or of a conductive ink, particularly based on silver or copper. 
         [0034]    The invention also aims at a device intended to be hybridized to another device provided with metal bumps, comprising:
       a substrate;   connection elements, arranged on a surface of the substrate and capable of being rigidly attached to said metal bumps, said elements each comprising a metal well having an opening capable of receiving the corresponding metal bump of the first microelectronic component and at least partially filled with a fusible element, particularly indium or an alloy of tin and gold, or with a conductive ink, particularly based on silver or copper;   resistive elements at the longitudinal station of the connection elements;   an electric insulator layer interposed between the resistive elements and the connection elements; and   electric terminals for the application of an electric current in the resistive elements.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    The 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: 
           [0041]      FIG. 1  is a simplified cross-section view of an device according to the invention comprising two hybridized microelectronic components; 
           [0042]      FIGS. 2 to 12  are simplified views illustrating a manufacturing method according to a first embodiment of the invention; 
           [0043]      FIGS. 13 and 14  are simplified views illustrating a manufacturing method according to a second embodiment of the invention; 
           [0044]      FIG. 15  is a simplified top view of a microelectronic component provided with resistive elements according to the invention; and 
           [0045]      FIG. 16  is a perspective view of a resistive element. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    In  FIG. 1 , a device according to the invention comprises a first microelectronic component  10 , for example, a microelectronic chip based on silicon hybridized to a second microelectronic component  12 . 
         [0047]    The two components  10  and  12  are hybridized by means of metal bumps  14  arranged on wettable areas of a surface  16  of first component  10 , and mechanically attached to corresponding connection elements  18  arranged on a surface  20  of second component  12 , as will be described in further detail hereafter. 
         [0048]    Each connection element  18  is further formed on a layer made of an electrically-insulating material  22 , particularly a dielectric layer, deposited on a resistive element  24  formed on top and/or inside of second component  12 . Finally, metal terminals  26  are provided on top and/or inside of second component  12  for the application of an electric current in resistive elements  24 . 
         [0049]    A method for manufacturing the device just described according to a first embodiment of the invention will now be described in further detail in relation with  FIGS. 2 to 12 . 
         [0050]    The method starts with the manufacturing of a substrate  30  of second component  12  (cross-section view of  FIG. 2 ). Substrate  30  is for example a flexible plastic substrate particularly made of PET or of PEN, a silicon substrate comprising microelectronic circuits, or the like. 
         [0051]    A metal layer  32  is then deposited full plate on a surface of substrate  30 , for example, by evaporation or sputtering (cross-section view of  FIG. 3 ). Layer  32  is for example a gold, titanium, nickel, copper, or tungsten layer and has a thickness in the range from 30 nanometers to 300 nanometers, to keep the flexibility of substrate  30  by limiting the effects of the difference between the thermal expansion coefficient of substrate  30  and that of metal layer  32 . 
         [0052]    Layer  32  is then etched to form resistive elements  24 . The shape of elements  24  is selected according to the local heating desired on hybridization of components  10 ,  12 . 
         [0053]    Preferably, elements  24  are threadlike and have a great curvilinear length for a small width and/or a small height, which enables to define a high electric resistance, and accordingly a strong Joule effect heating. Resistive elements  24  are for example foamed of coils each arranged between two areas  26 ,  28  (top view of  FIG. 5 ) or a set of strips or rods, for example, two strips, connected in parallel between two areas  30 ,  32  (top view of  FIG. 6 ). The material and the thickness of layer  32 , and thus of resistive elements  24 , are also selected according to the local heating desired on hybridization of components  10 ,  12 , as will be explained in further detail hereafter. 
         [0054]    Elements  24  may also be formed by locally depositing layer  32  and by etching it, or directly by means of a photolithography and an etching. 
         [0055]    The method carries on with the deposition of an electric insulator layer  34 , for example, a dielectric layer, at least on resistive elements  24  (cross-section view of  FIG. 7 ) to avoid any short-circuit between them and subsequently-manufactured connection elements  18 , and thus also to avoid any short circuit between resistive elements  24  and metal bumps  14  of first component  10 . Layer  34  is for example deposited full plate over the entire substrate  30 , or only on resistive elements  24 . 
         [0056]    Preferably, layer  34  is made of a material which is also a good heat conductor and which has a higher melting temperature than metal bumps  14 , to avoid the melting of layer  34  on hybridization. Advantageously, layer  34  is thus made of a fluorinated polymer of low dielectric constant which has a high chemical stability as well as a good heat conductivity if annealed very rapidly. Layer  34  is for example deposited by silk-screening or by inkjet. 
         [0057]    A metal area  36 , also called “UBM” area (Under Bump Metallization&gt;&gt;), is then deposited on insulating layer  34  at the longitudinal station of each resistive element  24 , this layer being typically made of gold, titanium, nickel, copper, or tungsten, and for example made of the same metal as resistive elements  24  (cross-section view of  FIG. 8 ). 
         [0058]    A central area  38  of each area  36  is then submitted to a local heating, advantageously by means of a laser, for example, of excimer type, which enables to limit thermal effects on substrate  30  while providing a significant photon energy. 
         [0059]    Metal area  36  and insulating layer  34  expand under the effect of the local heat. Since the materials forming them are different, area  36  and layer  34  thus have different thermal expansion coefficients, with the metal of area  36  having, in particular, a coefficient greater than that of layer  34 . 
         [0060]    The heating thus results in an expansion difference which causes the separation of central area  36  and the forming of one of several beads defining a well  40  (cross-section view of  FIG. 9 ). Width x of well  40  is especially determined by the difference in thermal expansion coefficient between area  36  and layer  34  and is selected to be greater than the diameter of bump  14  which will be subsequently introduced into well  40 . This especially enables to adjust the dimension of well  40  according to the temperature of heating element  24  during a hybridization, but also a dehybridization. Particularly, the local heating increases expansion differences between area  36  and layer  34 , which enables to further open wells  40  to receive bumps  14 . 
         [0061]    The method carries on with the deposition, for example, by silk-screening or inkjet, of a conductive ink  42  into each well  40 , particularly a metal ink containing silver or copper (cross-section view of  FIG. 10 ). Ink  42  especially enables to wet a metal bump  14  introduced into well  40  during the hybridization and forms an electric connection with this bump. The presence of ink  42  thus enables to use bumps  14  which do not or only very slightly wet the metal of area  36 , such as for example gold bumps  14 . 
         [0062]    The method then carries on with the actual hybridization of microelectronic components  10  and  12 . 
         [0063]    More particularly, first component  10  is transferred onto second component  12  by alignment thereof, and then by application of a force to first component  10  ( FIG. 11 ), to introduce bumps  14  into wells  40 , and thus place bumps  14  into contact with ink  42  ( FIG. 12 ). 
         [0064]    The surface of second component  12 , opposite to that provided with connection elements  22  formed of wells  40  and of ink  42 , is further placed into contact with a cooling system, for example, a refrigerated plate  44 . 
         [0065]    Finally, an electric current I is applied to each of resistive elements  24 , which then heat up by Joule effect. The generated heat is transferred by convection to bumps  14  which melt, ink  42  being further submitted to an anneal which will further increase the conductivity thereof by the total evaporation of the solvents present in this metal ink. Area  36  does not melt under the effect of the heating since its melting temperature is selected to be higher than the temperature obtained by the heating. Particularly, the material forming area  36  has a melting temperature greater than that of bumps  14 . Once the bumps have melted, the injection of current I is stopped, and bumps  14  cool down and solidify. Wells  40  especially enable to reinforce the mechanical resistance of hybridized components  10 ,  12 . 
         [0066]    According to an alternative embodiment, bumps  14  and the metal of area  36  are selected so that the bumps wet metal  36 . For example, bumps  14  are made of a so-called “solder” metal, such as for example indium, tin, and gold and silver alloys. Ink  42  is then omitted. 
         [0067]    A manufacturing method according to a second embodiment of the invention will now be described in relation with  FIGS. 13 and 14 . 
         [0068]    This method for example starts identically to the steps of the first embodiment described in relation with  FIGS. 2 to 8 . Unlike the first embodiment, the method carries on with the direct deposition of ink  42  on metal areas  36  with no forming of well  40  (cross-section view of  FIG. 13 ). In the same way as for the first embodiment, the ink enables to use metal bumps  40  which do not or very slightly wet metal area  36 , while providing an electric connection between bumps  40  and areas  36 . 
         [0069]    First component  10  is then transferred onto second component  12  by alignment of bumps  14  with areas  36  ( FIG. 14 ). The surface of second component  12 , opposite to that provided with connection elements  22  formed of areas  36  and of ink  42 , is then cooled as previously described, and an electric current I is applied to each of resistive elements  24  which heat up by Joule effect. The generated heat is then transferred by convection to bumps  14  which melt, ink  42  further undergoing an anneal. 
         [0070]    According to an alternative embodiment, bumps  14  and the metal of area  36  are selected so that the bumps wet metal  36 . For example, bumps  14  are made of a so-called “solder” metal, such as for example indium and gold and tin alloys. Ink  42  is then omitted. 
         [0071]    The hybridized devices thus obtained can be simply dehybridized. It is indeed sufficient to have a current circulate again through resistive elements  24 , which have not been destroyed during the hybridization, to induce the melting of bumps  14  again, which enables to separate components  10 ,  12 . 
         [0072]      FIG. 15 , which is a top view of an embodiment of the second component  30 , illustrates a connection diagram of resistive elements  24 . Advantageously, resistive elements  24  are series-connected, so that they conduct a same current, for example in the range from 1 mA to 300 mA, and heat up substantially identically as soon as they are themselves identical. For example, elements  24  are formed from a single conductive metal strip having its ends forming terminals  26  for the application of current I. The current is for example injected by a current source via a current mirror. 
         [0073]      FIG. 15  further illustrates an arrangement of a connection element  22  corresponding to a peripheral sealing of first circuit  10  on second circuit  12 . Of course, the hybridization pattern depends on the targeted application. 
         [0074]      FIG. 16  is a perspective view of an element  24  in the form of a rectilinear element, it being understood that the following remains valid for non-rectilinear elements, length l to be considered being the curvilinear length of the element. 
         [0075]    It can be shown that heating ΔT per second of an element  24  conducting a current of intensity I can be modeled at the first degree according to relation: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               T 
             
             = 
             
               
                 
                   ρ 
                   × 
                   
                     l 
                     2 
                   
                 
                 
                   2 
                   × 
                   
                     k 
                     p 
                   
                   × 
                   
                     w 
                     2 
                   
                   × 
                   
                     h 
                     2 
                   
                 
               
               × 
               
                 I 
                 2 
               
             
           
         
       
     
         [0000]    where:
       ρ is the resistivity per meter of the material forming element  24 , in Ω/m;   l is the length in meter of element  24 ;   w is the width in meter of element  24 ;   h is the height in meter of element  24 ;   k p  is the heat conductivity of the material forming element  24 , in W/K·m; and   I is the intensity in ampere of the current flowing through element  24 .       
 
         [0082]    Several parameters are thus available, that is, the dimensions of element  24 , the nature of the material(s) forming it, as well as the value of the current that it conducts, to control the heating that it generates. Particularly, it is possible to select elements  24  which allow a very intense and/or very fast heating of metal bumps  14 . 
         [0083]    Hybridization devices and a hybridization method where a melting of the metal bumps is obtained have been described. 
         [0084]    The invention also applies to hybridizations where the bumps are not melted. In particular, so-called “cold” insertions where protruding connection elements, for example, hollow cylinders, are force-fit into bumps made of a ductile material, are known. Such a hybridization is for example described in document US-A-2011/0035925. The invention may also advantageously be used to locally heat the bumps in order to soften them without melting them, and thus ease the insertion of the connection element into the bump.