Abstract:
A method of manufacturing a microelectronic device including a first component hybridized with a second component via electric interconnects, involves the steps of: (i) forming the first and second components, the second component being transparent to ultraviolet radiation at least in line with locations provided for the interconnects; (ii) forming interconnection elements including copper oxide on the second component at the locations provided for the interconnects; (iii) placing the first and second components on each other; and (iv) applying the ultraviolet radiation through the second component on the elements including copper oxide to implement an ultraviolet anneal converting copper oxide into copper.

Description:
FIELD OF THE INVENTION 
     The invention relates to the “flip-chip” hybridization of two microelectronic components. 
     BACKGROUND OF THE INVENTION 
     Flip-chip hybridization is a technique where two microelectronic components, for example, an array of photosensitive elements and a circuit for reading it, are independently formed, and where these components are placed on each other while electric interconnects, necessary to their operation, are formed therebetween. 
     According to the state of the art, the interconnects are made in the form of solder bumps, arranged between metal areas wettable by the material forming the bumps, formed on opposite surfaces of the components. The hybridization method then comprises forming said areas on the surfaces of the components to be hybridized, depositing a solder material on the areas of a first component, placing the second component on the first component while aligning the areas of the two components, and then heating the assembly to a temperature melting the solder material to form the electric interconnects. 
     This type of hybridization however has the disadvantage of submitting the components to a significant thermal shock, the melting temperature of the materials of the bumps being usually greater than 152° C., this temperature being the melting temperature of the solder material, that is, tin. Such a heating may thus embrittle the components and/or forbid the use of materials which cannot withstand such temperatures, such as plastic materials, particularly PEN (polyethylene naphthalate) and PET (polyethylene terephthalate), which have a vitreous transition temperature usually much lower than the melting temperature of the solder materials. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing a method of manufacturing a device comprising a first and a second components hybridized by means of electric interconnects, which does not require taking the entire device to a high temperature to form said interconnects. 
     For this purpose, the invention aims at a method of manufacturing a microelectronic device comprising a first component hybridized with a second component by means of electric interconnects, comprising:
         forming first and second components, the second component being transparent to ultraviolet radiation at least in line with locations provided for the interconnects;   forming interconnection elements comprising copper oxide on the second component at the locations provided for the interconnects;   placing the first and second component on each other; and   applying the ultraviolet radiation through the second component on the elements comprising copper oxide to implement an ultraviolet anneal converting copper oxide into copper.       

     “Transparent” here means a material which lets through enough ultraviolet radiation to allow an anneal of the copper oxide. 
     In other words, the reduction of the copper oxide into copper by a UV anneal requires no general heating of the device. Further, the application of the UV anneal does not induce, at the electric interconnects, a significant temperature rise, and thus enables to use materials such as plastics for the component manufacturing. 
     According to an embodiment, the forming of interconnection elements comprises, for each of these:
         forming an area on the second component, the area comprising a first region at least partially transparent to ultraviolet radiation surrounded with a second region less transparent than the first region;   and depositing copper oxide on said area at least on the first region thereof.       

     Particularly, the second region is made of a material absorbing ultraviolet radiation or of a material reflecting ultraviolet radiation, advantageously a second region made of gold, titanium, or silver, having a thickness greater than 30 nanometers. 
     In other words, it is possible to control the copper oxide portion which is reduced into copper by delimiting it with a region which absorbs or reflects ultraviolet radiation. It is thus possible to form interconnects accurately limited in space, even in the case where a copper oxide ink, which spreads once deposited, is used. 
     According to an embodiment, the second component is made of PEN, PET, or glass, which materials are transparent to UVs and inexpensive. Further, PEN and PET have the advantage of being flexible materials. 
     According to an embodiment, the forming of the interconnects comprises forming a transparent metal oxide layer, particularly made of ITO (Indium tin oxide), of ATO (Antimony tin oxide), or of another electrically-conductive metal oxide transparent to UVs, on the second component. Such electrically-conductive metal oxides let through UVs and are used as electric connection pads after the conversion of CuO. 
     According to an embodiment, the ultraviolet anneal is achieved by a photonic pulse having a duration in the range from 0.5 millisecond to 2 milliseconds and having a fluence in the range from 200 Joules to 1,500 Joules, particularly a 1.5-millisecond duration and a 1,400 Joule fluence. 
     Such values thus enable to convert CuO into Cu from the rear surface of the flexible substrate, for example having a 125-μm thickness. In this case, it is spoken of applied energy. An order of magnitude for the fluence, which is 5.8 J/cm 2 , 2 inches (5 cm) away from the Xenon lamp, may be mentioned. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a simplified cross-section view of a device according to the invention; 
         FIG. 2  is a view of a detail of  FIG. 1 , illustrating an interconnect according to the invention; 
         FIGS. 3 and 4  respectively are simplified cross-section views along plane III-III and IV-IV of  FIG. 2 ; 
         FIGS. 5 to 14  are simplified top and cross-section views illustrating a method of manufacturing the device of  FIG. 1 ; 
         FIGS. 15 and 16  are simplified top and cross-section views of an alternative interconnect according to the invention; 
         FIGS. 17 and 18  respectively are simplified top and cross-section views of an alternative interconnect according to the invention; 
         FIG. 19  is a simplified cross-section view of a device according to the invention with electrically non-conductive ink dispensed between the two hybridized components; 
         FIG. 20  is a top view of one of the components illustrating the shape of the ink. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A device  10  according to the invention will be described in relation with  FIGS. 1 to 4 . Device  10  comprises a first and a second components  12 ,  14  hybridized to each other by mean of electric interconnects  16 . First component  12  for example comprises an array of photosensitive elements, and the second component for example comprises a circuit for reading said array, interconnects  16  connecting each photosensitive element of the array to the read circuit. The second component is further transparent to ultraviolet radiation at least in line with interconnects  16 . Particularly, the second component may be essentially made of flexible, low-cost and transparent plastic materials, such as PEN or PET, or also be made of glass, in which case the second component is transparent over its entire surface. 
     Interconnects  16  each comprise a solid conductive area  18  formed on a surface  20  of first component  12 , for example, a metal area made of gold, silver, copper, platinum, titanium, or others, and an interconnection element  22  formed on a surface  24  of second component  14 . 
     More particularly, interconnection element  22  comprises:
         a solid conductive area  26  transparent to ultraviolet radiation, formed on surface  24  of component  14 , advantageously an area made of a conductive metal oxide, such as ITO (indium tin oxide) or ATO (antimony tin oxide), or of another conductive metal oxide transparent to UVs, such an oxide further having the advantage of bonding well to plastics and of having a good electric conductivity;   a ring  28  of material absorbing ultraviolet radiation, formed on transparent conductive area  26 , for example, a gold, titanium, or silver layer;   a copper pad  30  (Cu) formed in ring  28 , and in contact with conductive area  18  of first component  12  and with conductive area  26  of second component  14 ;   a copper pad  30  (Cu) formed in ring  28 , and in contact with conductive area  18  of first component  12  and with conductive area  26  of second component  14 .       

     Each interconnect  16  thus comprises an electrically-conductive path between first and second components  12 ,  14 , formed of conductive areas  18 ,  26  and of copper pad  30 . 
     A method of manufacturing above-described device  10  will now be described in relation with  FIGS. 5 to 14 , and more particularly a method which does not require using a high temperature to form electric connections  16 . Except for possible optional thermal anneal steps described hereafter, the method is advantageously carried out at ambient temperature, no heating being implemented to form interconnects  16  between the two components  14 ,  16 . 
     The method starts with the forming of component  14 , transparent at least in line with the locations provided for interconnects  16 , for example, a flexible low-cost component such as a PEN or PET plastic, or a glass component  14 . 
     The method then carries on with the forming of interconnection elements  22 . More particularly, for each of these, a square solid conductive area  26  transparent to ultraviolet radiation is formed on surface  24  of component  14 , for example, by means of a full plate vapor deposition of a layer of conductive transparent oxide such as ITO, followed by a wet or plasma etching to define area  26  ( FIG. 5  in top view and  FIG. 6  in cross-section view along plane VI-VI of  FIG. 5 ). 
     At a next step, a ring  28 , having a square or circular cross-section and made of a material absorbing ultraviolet radiation, is deposited on area  26  while leaving the latter exposed inside of ring  28 . For example, ring  28  is made of gold, platinum, or silver, and is formed by full plate vapor deposition followed by a wet or plasma etching ( FIG. 7  in top view and  FIG. 8  in cross-section view along plane VIII-VIII of  FIG. 7 ). 
     The thickness of ring  28  is selected to sufficiently block the incident ultraviolet radiation on ring  28  through component  14 , to avoid an ultraviolet anneal of copper oxide subsequently deposited on ring  28 . Advantageously, when ring  28  is made of gold, silver, or titanium, the thickness of ring  28  is greater than or equal to 30 nanometers. Optionally, molecules having a high ultraviolet absorption power are mixed with the metal of ring  28  or directly deposited on the flexible substrate in the form of a ring if the deposition techniques are well mastered, for example, pyrene molecules, which for example enables to decrease the thickness of ring  28 . 
     The method carries on with the deposition of a copper oxide  50  in ring  28 , particularly the silk-screening deposition of a copper oxide ink, for example, ink “Metalon® ICI-020” or “Metalon® ICI-021” of Novacentrix, Tex., USA, to have the copper oxide extend higher than ring  28 . 
     The ink, due to its nature, then spreads on the edge of ring  28 , thickness W abs  of ring  28  being advantageously selected to provide enough surface area for the ink to spread, and to prevent the latter from overflowing from ring  28  ( FIG. 9  in top view and  FIG. 10  in cross-section view along plane X-X of  FIG. 11 ). 
     Copper oxide ink  50  is then dried by thermal anneal to form copper oxide pads. 
     Independently from the manufacturing of component  14  and of elements  26 ,  28 ,  50  just described, the method comprises forming first component  14  and solid conductive areas  18  of square cross-section, for example, by means of a full plate vapor deposition of a gold, titanium, platinum, copper, or silver layer on surface  20  of component  12 , followed by a wet or plasma etching to define each area  18  ( FIG. 11  in top view and  FIG. 12  in cross-section view along plane XII-XII of  FIG. 11 ). 
     Once components  12  and  14  and their interconnection elements have been formed as previously described, first component  12  is placed on second component  14  by aligning areas  18  with copper oxide pads  50  ( FIG. 13  in cross-section). 
     The method then carries on with the application of a photonic pulse in the ultraviolet wavelength range  52 , particularly having a wavelength in the range from 200 nanometers to 700 nanometers, through second component  14  at least in line with each interconnect  16 , the pulse being for example applied to the entire surface of second component  14  ( FIG. 14 ). Ultraviolet pulse  52  has a duration in the range from 0.5 millisecond to 2 milliseconds, and an energy in the range from 200 Joules to 1,400 Joules, advantageously a 1.5-millisecond duration and a 1,400-Joule energy. Such energy values enable to convert CuO into Cu from the rear surface of the plastic PEN or PET substrate having a 125-μm thickness. Pulse  52  is for example produced by a flash UV lamp, particularly a “XENON® PulseForge” flash lamp produced by Xenon Corporation. Advantageously, the distance separating the lamp from component  14  is in the range from 3 centimeters to 7 centimeters, and more particularly 5 centimeters. This distance being the distance at which the optical system of the Xenon lamp focuses the sent light pulse, that is, the distance at which the energy is maximum. 
     UV pulse  52  then crosses second transparent component  14  and transparent conductive areas  26  and is incident on copper oxide portion  50  comprised in ring  28  of absorbing material. 
     At the same time, ring  28  of absorbing material at least partly blocks part of the ultraviolet pulse and thus at least partially prevents this pulse from reaching the copper oxide portion formed on ring  28 . Ultraviolet pulse  52  then induces an anneal of the copper oxide in ring  28 , which undergoes a reduction, thus forming copper pads  30  between conductive areas  18  and  26 . Thus copper oxide, which is a poor electric conductor with a resistance per square in the order of 10 6 Ω/□ and a poor heat conductor with a low heat conductivity equal to 33 W/m·K, is reduced into copper, which is a good electric conductor having a resistance per square equal to 60 mΩ/□, and a good heat conductor having a heat conductivity equal to 403 W/m·K. 
     Further, the anneal is carried out locally, that is, at the level of interconnects  16 , and not on the assembly formed of components  12 ,  14  and of the interconnection elements, but the anneal further induces a temperature lower than 100° C., and thus a temperature lower than the vitreous transition temperature of PEN. 
     It should further be noted that rings  28  of absorbing material enable to accurately define the geometry of copper pads  30 , and this, even if the copper oxide has been deposited by a technique which does not enable to accurately control this deposition. 
     Previously-described interconnects  16  exhibit a conductive area  26  on component  14 , area  26  being used for the current flow between components  12  and  14 . For example, there exist within the thickness of component  14  electric connections in contact with areas  26 . 
     As a variation, as illustrated in  FIG. 15  in top view and in  FIG. 16  in cross-section view along plane XII-XII of  FIG. 15 , a metal track  54  forming one piece with conductive ring  28  is formed. Track  54  is in particular made of the same material as ring  28  and is formed jointly therewith. This track can thus be used for an electric connection with interconnect  16  on surface  24  of component  14 . 
     Interconnects  16  having a square cross-section have been described. Of course, the interconnects may take any geometric shape, for example, a circular shape as illustrated in  FIG. 17  in top view and in  FIG. 18  in cross-section view along plane XVIII-XVIII of  FIG. 17 . 
     Particularly, the shape of interconnects  16  may thus be dictated by the shape of areas  26  formed on second component  14 . Minimum distance d between two interconnects  16  may in particular be in the order of 30 micrometers with a minimum width W pad  equal to 40 in the case of the square shape and a minimum diameter D PAD  of 40 micrometers in the case of a circular shape. Minimum thickness W abs  of rings  28  is for example 5 micrometers. 
     Similarly, rings of absorbing material for blocking ultraviolet radiation and thus preventing the annealing of the copper oxide laid on the rings have been described. As a variation, the ring is made of a reflective material which also blocks ultraviolet radiation. Also as a variation, the rings are omitted and component  14  is coated with a reflective or absorbing layer having openings in line with the locations provided for copper pads  32 . Still as a variation, rings  28  are omitted, for example, if the application does not require an accurate definition of the copper pad geometry. 
     After the final hybridization of the two components and the conversion of CuO into Cu at the connection level, a non electrically-conductive ink NCP (“Non Conductive Paste”) is dispensed, this step being followed by an anneal between 60 and 80° C. for a few minutes to mechanically strengthen the two hybridized components. For example, the ink is a non electrically-conductive epoxy resin. The dispensing is performed manually or automatically, and the ink may also be deposited by silk-screening. 
     For example, the NCP ink occupies the entire volume between the two components. As a variation, as illustrated in the cross-section view of  FIG. 19 , NCP ink  60  is deposited around interconnects  16 , for example, in the form of a cord, as illustrated in  FIG. 20  which is a top view of component  14 , or of pads. 
     As a variation, the NCP ink is deposited on component  14  before component  12  is placed thereon, for example, by silk screening, after which component  12  is installed. An anneal such as described hereabove is then applied to solidify the ink. 
     The invention thus has the following advantages:
         the possibility of hybridizing a transparent component on another or a transparent substrate on an opaque substrate such as silicon, for example;   the geometry of the rings blocking the ultraviolet radiation enables to obtain a small hybridization pitch;   good electric and heat conductivities of the interconnects;   a possibility of alignment from the transparent rear surface;   a low cost and a mechanical flexibility, particularly due to the use of plastic;   a direct passivation by the copper oxide of the vertical walls of the copper pads;   an adaptation of the thermal expansion coefficients of the different assembled layers by the use of CuO ink; and   a fast manufacturing.