Abstract:
A method of forming a hybridized device comprising forming a first microelectronic component provided, on a surface, with metal balls, and a second microelectronic component provided, on a surface, with connection elements corresponding to said metal balls, and hybridizing the first and second components to attach the metal balls of the first component to the connection elements of the second component. The manufacturing of the second microelectronic component comprises forming a substrate provided with cavities at the locations provided for the connection elements, and forming resistive elements made of fusible metal respectively suspended above the cavities. The hybridizing of the first and second components comprises transferring the first component onto the second component to have the metal balls rest on the suspended resistive elements, and circulating an electric current through the resistive elements to melt said elements.

Description:
FIELD OF THE INVENTION 
     The invention relates to the assembly of microelectronic components, and more particularly to “flip-chip” hybridization. 
     BACKGROUND 
     An assembly by the so-called “flip-chip” technique usually comprises forming electrically-conductive balls on a surface of a first electronic component and forming electrically-conductive connection elements, particularly balls or connection areas, on a surface of a second component according to a predetermined connection pattern. The first component is then transferred on the second component with the balls in front of corresponding connection elements, after which the assembly is pressed and heated. The balls then deform and melt to form electric connections perpendicular to the main plane of the electronic component, generally in the form of a wafer. 
     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. 
     However, the melting of a metal ball requires high temperatures, higher than 170° C. Now, such temperatures are not compatible with organic components, particularly components made of plastic such as PEN (polyethylene naphthalate) and PET (polyethylene terephthalate), which have lower glass transition temperatures, of 120° C. for PEN and 70° C. for PET, for example. Thus, when a plastic component is submitted to the melting temperature of the solder balls, the plastic component takes a rubbery state and strongly deforms, or even destroys. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The aim of the present invention is to provide a method providing a hybridization which implements a local heating of the metal balls while thermally insulating in an efficient way the component provided with the connection elements. For this purpose, the invention aims at a method of forming a hybrid device comprising forming a first microelectronic component provided, on a surface, with metal balls, and a second microelectronic component provided, on a surface, with connection elements corresponding to said metal balls, and hybridizing the first and second components to attach the metal balls of the first component to the connection elements of the second component. 
     According to the invention:
         the manufacturing of the second microelectronic component comprises:
           forming a substrate provided with cavities at the locations provided for the connection elements; and   forming resistive elements made of fusible metal respectively suspended above the cavities;   
           and the hybridization of the first and second components comprises:
           transferring the first component onto the second component to have the metal balls rest on the suspended resistive elements; and   applying an electric current through resistive elements to melt said elements.   
               

     In other words, the substrate is isolated from the molten element by air until the molten metal flows into the cavity. Further, the melting of the resistive elements causes the settling of the first component under its own weight. The metal balls are thus introduced into the cavities in contact with the molten metal and bond to the substrate once the metal has cooled down. 
     According to an embodiment, the resistive elements are made in the form of coils or rods. Particularly, the resistive elements are made of a metal of low melting point, for example, indium or a gold and tin alloy. 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. 
     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. 
     According to an embodiment, the suspended resistive elements are obtained as follows:
         deposition of two metal areas around each cavity;   filling of the cavities with resin;   forming of the resistive elements on the resin and on the metal areas, while leaving at least one passage of access to the resin; and   removal of the resin from the cavities through the at least one access passage.       

     Particularly, the metal connection areas have a surface area greater than or equal to twice the surface area of the cavity. 
     According to an embodiment, the method comprises, before the forming of the suspended resistive elements, depositing a metal layer at the bottom of each cavity and at least one electric connection in contact with said layer. A mechanical and electrical interconnection of the first and second components is thus obtained. 
     According to an embodiment, the height of the cavities is greater than or equal to ⅛ of the ball height. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS. 1 to 3  are simplified cross-section views illustrating a method of hybridizing a first and a second microelectronic components according to the invention; 
         FIGS. 4-6 ,  7 A,  7 B,  8 A,  8 B,  9 A,  9 B,  10 A and  10 B are simplified views illustrating a method of manufacturing a second component according to the invention provided with fusible resistive elements suspended above cavities; and 
         FIG. 11  is a simplified top view of a second component illustrating a diagram of electric connection of the fusible resistive elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method of hybridizing a first microelectronic component  10  with a second microelectronic component  12  according to the invention is now described in relation with the simplified cross-section views of  FIGS. 1 to 3 . 
     First microcomponent  10 , for example, a silicon-based microelectronic chip comprises, on one of its surfaces  14 , metal balls  16 , for example, soldered to metal areas  18 , as known per se in the state of the art. 
     Second microcomponent  12  comprises a substrate  20 , for example, a flexible plastic substrate, particularly made of PET or of PEN, in a surface  22  of which are formed cavities  24  intended to respectively receive balls  14  of first component  10 . 
     A resistive element  26  made of a fusible material, for example, indium, or of an alloy of gold and tin, is further suspended above each cavity  24  and is connected, for example, via metal tracks  28  formed on surface  22  of second component  12 , to electric power supply terminals  30 , to enable the flowing of an electric current therethrough. 
     Optionally, the bottom of each cavity  24  further comprises a metal area  32  for the electric connection of ball  14  subsequently introduced into it with, for example, electronic circuits present in substrate  12 . 
     The hybridization method starts by the transfer of first component  10  onto second component  12  by aligning balls  14  with resistive elements  26  ( FIG. 1 ). First component  10  then rests on second component  12  and applies its weight P thereto ( FIG. 2 ). 
     An electric current I is then applied to each of fusible resistive elements  26 , for example, by means of a voltage or current source  34 , to said elements them by Joule effect. The molten material then flows into cavities  24 . Since it is no longer supported by elements  26  which have been destroyed by melting to form the mechanical and electric contact, first component  10  then settles under the effect of its own weight, and balls  16  are introduced into the cavities along with the molten material  36  ( FIG. 3 ). The method then ends with the cooling of the molten material  36 , which solidifies by bonding to the bottom of cavities  24  and to balls  16 , so that mechanical and electrical connections are thus formed between components  10 ,  12 . 
     Thus, during the entire heating period of fusible resistive elements  26 , substrate  20  is protected from an excessive heating by the air filling cavities  24  and only comes into contact with the molten material at the last moment, that is, only when fusible elements  26  are destroyed. Further, the cooling of the fusible elements starts immediately after their destruction since this destruction interrupts the current flow, and accordingly the Joule effect heating. 
     Preferably, height e of cavities  24  is, in the present case, in the range from 2 to 8 micrometers. Generally, height e of the cavities is greater than or equal to  1 / 8  of the ball height. 
     A method of manufacturing second component  12  will now be described in relation with  FIGS. 4 to 11 . 
     The method starts with the manufacturing of substrate  20 , for example, a flexible PET or PEN substrate (cross-section view of  FIG. 4 ) and carries on with the forming of cavities  24  in a surface  22  of substrate  20 , for example, by application of an oxygen plasma through a mask (cross-section view of  FIG. 6 ). 
     Advantageously, the walls of cavity  24  have a slightly inward slope to mechanically block balls  16  after the cooling. 
     A metal layer  40 , for example a gold layer, is then deposited full plate on substrate  20 , for example, by evaporation (cross-section view of  FIG. 6 ). Particularly, a metal layer is deposited inside and around of each cavity  24 . 
     Metal layer  40  is then etched, for example, by means of a photolithography and of a wet or plasma etch step, to define, for each cavity  24 , two connection areas  42 ,  44  on surface  22 , electrically insulated from the metal deposited in cavity  24 , as well as two connection areas  46 ,  48  on surface  22 , electrically insulated from areas  42 ,  44  and in contact with metal  32  deposited in cavity  24  (cross-section view of  FIG. 7A  and top view of  FIG. 7B ). 
     The method then carries on with the deposition of a resin  50  in each cavity  22  to protect metal  32  deposited in cavities  24  and define a solid surface on which fusible resistive elements  26  will be subsequently deposited (cross-section view of  FIG. 8A  and top view of  FIG. 8B ). Advantageously, the resin does not totally fill cavities  24  and is slightly recessed with respect to surface  22  of substrate  20  to enable the resistive fusible element to bend downwards as an effect of its weight. Connection areas  42 ,  44 ,  46 ,  48  are then cleaned, for example, by means of an oxygen plasma, of a power in the range from 30 W to 50 W, applied for some ten seconds. 
     Fusible resistive elements  26  are then formed on resin  50 , above each cavity  24 , for example, by means of a photolithography depositing fusible metal through a mask of desired shape for elements  26  (cross-section view of  FIG. 9A  and top view of  FIG. 9B ). Each resistive element  26  further extends in two metal areas  52 ,  54  respectively deposited on connection areas  42  and  44 . Each element  26  can thus be supplied with current via areas  42 ,  44 . 
     Advantageously, connection metal areas  42  and  44  have a surface area greater than or equal to twice the surface area of cavities  24  to promote a temperature rise on both sides of the cavity when current is flowing. Such a local temperature rise will induce an expansion of element  12  which will further open cavity  24 , which eases the insertion of fusible heating element  26  and of ball  16  into cavity  24  once the current has been applied. 
     Fusible resistive elements  26  advantageously take the shape of a coil. A coil indeed has a high electric resistance due to its great length and to its small width, and thus a high Joule effect heating, and does not totally cover resin  50  which is thus partially exposed, which enables the subsequent removal of the resin. Other shapes are of course possible for fusible elements  26 , such as, for example, parallel rods or strips connected between areas  42 ,  44 . Preferably, the material and the geometrical characteristics of the heating elements are selected so that they melt when they conduct an electric current of a few milliamperes according to the nature of the fusible heating element. 
     It can be shown that heating ΔT per second of an element  26  conducting a current of intensity I can be modeled at the first degree according to relation: 
               Δ   ⁢           ⁢   T     ⁢           =         ρ   ×     l   2         2   ×     k   p     ×     w   2     ×     h   2         ×     I   2             
where:
         ρ is the resistivity per meter of the material forming element  26 , in Ω/m;   l is the length in meter of element  26 ;   w is the width in meter of element  26 ;   h is the height in meter of element  26 ;   k p  is the heat conductivity of the material forming element  26 , in W/K·m; and   I is the intensity in ampere of the current flowing through element  26 .       

     Several parameters are thus available, that is, the dimensions of element  26 , the nature of the material(s) forming it, as well as the value of the current that it conducts, to control the Joule effect heating of elements  26 . It is however possible to select elements  26  which very rapidly melt under the effect of a current in the range from a few milliamperes to a few tens of milliamperes. 
     The thickness of fusible resistive elements  26  is also selected between 5 micrometers and 15 micrometers to obtain a quantity of molten material in cavities  24  sufficient to attach balls  16 . Elements  26  are advantageously made of a metal having a good wettability and bonding, once melted and cooled down, to metal  32  deposited at the bottom of cavities  24 , for example, indium or a gold and tin alloy. Advantageously, elements  26  are made of indium, this metal bonding well to metal and having a low melting point equal to 152° C. 
     The method carries on with the removal of resin  50  filling cavities  26 , for example, by wet etching or by means of a plasma, which avoids deteriorating substrate  20  (cross-section view of  FIG. 10A  and top view of  FIG. 10B ). 
       FIG. 11  is a top view of a second component  12  illustrating an example of electric connection for the power supply of an array of N rows of resistive elements  26 , for example, three rows, and of M columns of resistive elements, for example, three columns. For clarity, connection areas  46 ,  48  are not shown. 
     The electric connection advantageously is a parallel connection of fusible resistive elements  26 . For example, the connection comprises, for each column in the array, a first metal track  60 , formed on surface  22  of component  12 , and connecting each of connection areas  42  of elements  26  of the column to a first common metal track  62  formed on surface  22 . Similarly, the connection comprises, for each column in the array, a second metal track  64 , formed on surface  22  of component  12 , and connecting each of connection areas  44  of elements  26  of the column to a second common metal track  66  formed on surface  22 . Common tracks  64 ,  66  are each connected to an electric power supply terminal  30 , so that the application between these terminals of a voltage V enables to circulate a substantially identical electric current in each of elements  26 . Of course, other connection diagrams are possible. 
     A series connection may also be achieved in the case where balls  16  are aligned and laid above fusible heating element  26 . Then, a current is applied to the series assembly and all balls are heated at the same time. 
     It should also be noted that the volume of the fusible heating element can be adaptable to the volume of balls  16 . For example, if balls  16  do not all have the same volume, it is possible to take into account the variable volume by increasing or decreasing the volume of the material of fusible heating element  26  to provide a good uniformity of the connections after the cooling of the material.