Abstract:
A method for producing optoelectronic devices, including the following successive steps: providing a substrate having a first face; on the first face, forming sets of light-emitting diodes including wire-like, conical or frustoconical semiconductor elements; covering all of the first face with a layer encapsulating the light-emitting diodes; forming a conductive element that is insulated from the substrate and extends through the substrate from the second face to at least the first face; reducing the thickness of the substrate; and cutting the resulting structure in order to separate each set of light-emitting diodes.

Description:
[0001]    The present application claims the priority of French application FR13/59413 that is incorporated by reference. 
       BACKGROUND 
       [0002]    The present invention generally relates to methods of manufacturing optoelectronic devices based on semiconductor materials. The present invention more specifically relates to methods of manufacturing optoelectronic devices comprising light-emitting diodes formed by three-dimensional elements, particularly semiconductor microwires or nanowires. 
       DISCUSSION OF THE RELATED ART 
       [0003]    The term “optoelectronic devices with light-emitting diodes” designates devices capable of converting an electric signal into an electromagnetic radiation, and particularly devices dedicated to emitting an electromagnetic radiation, particularly light. Examples of three-dimensional elements capable of forming light-emitting diodes are microwires or nanowires comprising a semiconductor material based on a compound mainly comprising at least one group-III element and one group-V element (for example, gallium nitride GaN), called III-V compound hereafter, or mainly comprising at least one group-II element and one group-VI element (for example, zinc oxide ZnO), called II-VI compound hereafter. 
         [0004]    The three-dimensional elements, particularly semiconductor microwires or nanowires, are generally formed on a substrate which is then sawn to delimit individual optoelectronic devices. Each optoelectronic device is then arranged in a package, particularly to protect the three-dimensional elements. The package may be attached to a support, for example, a printed circuit. 
         [0005]    A disadvantage of such an optoelectronic device manufacturing method is that the steps of protecting the three-dimensional semiconductor elements have to be performed separately for each optoelectronic device. Further, the bulk of the package may be significant as compared with the active area of the optoelectronic device comprising the light-emitting diodes. 
       SUMMARY 
       [0006]    Thus, an object of an embodiment is to at least partly overcome the disadvantages of previously-described optoelectronic devices comprising light-emitting diodes, particularly with microwires or nanowires. 
         [0007]    Another object of an embodiment is to suppress the individual protection packages of optoelectronic devices comprising light-emitting diodes. 
         [0008]    Another object of an embodiment is for optoelectronic devices comprising light-emitting diodes made of semiconductor material to be capable of being manufactured at an industrial scale and at a low cost. 
         [0009]    Thus, an embodiment provides a method of manufacturing optoelectronic devices comprising the successive steps of:
       (a) providing a substrate comprising a first surface;   (b) forming, on the first surface, assemblies of light-emitting diodes comprising, conical or tapered wire-shaped semiconductor elements;   (c) forming, for each assembly of light-emitting diodes, an electrode layer covering each light-emitting diode of said assembly and a conductive layer covering the electrode layer around the light-emitting diodes of said assembly;   (d) covering the entire first surface of a layer encapsulating the light-emitting diodes;   (e) decreasing the substrate thickness, the substrate comprising, after step (e), a second surface opposite to the first surface;   (f) forming a conductive element insulated from the substrate and crossing the substrate from the second surface all the way to at least the first surface, the conductive element being in contact with the conductive layer;   (g) forming, on the second surface, at least one first conductive pad in contact with the substrate; and   (h) cutting the obtained structure to separate each assembly of light-emitting diodes.       
 
         [0018]    According to an embodiment, the method comprises, at step (f), forming, on the second surface, at least one second conductive pad in contact with the conductive element. 
         [0019]    According to an embodiment, the method comprises forming at least one additional conductive element, insulated from the substrate and crossing the substrate from the second surface all the way to at least the first surface, and in contact with the base of at least one of the light-emitting diodes. 
         [0020]    According to an embodiment, the forming of the conductive element successively comprises, after step (e), etching an opening in the substrate from the second surface, forming an insulating layer at least on the lateral walls of the opening, and forming a conductive layer covering the insulating layer, or filling the opening with a conductive material. 
         [0021]    According to an embodiment, the forming of the conductive element comprises, before step (b), etching an opening in the substrate from the first surface across a portion of the substrate thickness, the opening being opened on the second surface after the substrate thinning step. 
         [0022]    According to an embodiment, the electrode layer and the conductive layer are further formed in the opening. 
         [0023]    According to an embodiment, the method comprises, before step (b), forming an insulating portion at least on the lateral walls of the opening and filling the opening with a conductive material. 
         [0024]    According to an embodiment, at step (e), the substrate is totally removed. 
         [0025]    According to an embodiment, the method further comprises, for each assembly of light-emitting diodes, depositing at least one conductive layer in contact with the bases of the diodes of said assembly. 
         [0026]    According to an embodiment, the method comprises, before step (e), a step of attaching a support to the layer encapsulating the light-emitting diodes. 
         [0027]    According to an embodiment, the layer encapsulating the light-emitting diodes comprises phosphors between the light-emitting diodes. 
         [0028]    According to an embodiment, the method comprises a step of forming a layer of phosphors covering the layer encapsulating the light-emitting diodes or covering the support. 
         [0029]    According to an embodiment, the method comprises a step of forming a layer, between the layer encapsulating the light-emitting diodes and the phosphor layer, capable of transmitting the light rays emitted by the light-emitting diodes and of reflecting the light rays emitted by the phosphors. 
         [0030]    According to an embodiment, the method comprises a step of forming reflectors around the light-emitting diodes between the substrate and the layer encapsulating the light-emitting diodes and having a height greater by 50% than the height of the light-emitting diodes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0032]      FIG. 1  is a partial simplified top view of an example of a semiconductor substrate wafer having a plurality of optoelectronic devices comprising microwires or nanowires formed thereon; 
           [0033]      FIGS. 2A to 2F  are partial simplified cross-section views of structures obtained at successive steps of an embodiment of a method of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0034]      FIGS. 3A and 3B  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0035]      FIGS. 4 and 5  are partial simplified cross-section views of structures obtained by other embodiments of methods of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0036]      FIGS. 6A to 6C  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0037]      FIGS. 7A and 7B  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0038]      FIGS. 8 to 10  are partial simplified cross-section views of structures obtained by other embodiments of methods of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0039]      FIGS. 11A to 11D  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0040]      FIGS. 12A to 12E  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices comprising microwires or nanowires; 
           [0041]      FIG. 13  is a partial simplified cross-section view of an embodiment of an optoelectronic device comprising microwires or nanowires formed on a substrate wafer before sawing of the substrate; 
           [0042]      FIG. 14  is a partial simplified top view of the optoelectronic device of  FIG. 13 ; and 
           [0043]      FIGS. 15 to 27  are partial simplified cross-section views of embodiments of optoelectronic devices comprising microwires or nanowires.\ 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, as usual in the representation of electronic circuits, the various drawings are not to scale. Further, only those elements which are useful to the understanding of the present description have been shown and will be described. In particular, the optoelectronic device control means described hereafter are within the abilities of those skilled in the art and are not described. 
         [0045]    In the following description, unless otherwise indicated, the terms “substantially”, “approximately”, and “in the order of” mean “to within 10%”. Further, “compound mainly formed of a material” or “compound based on a material” means that a compound comprises a proportion greater than or equal to 95% of said material, this proportion being preferentially greater than 99%. 
         [0046]    The present description relates to optoelectronic devices comprising three-dimensional elements, for example, microwires, nanowires, conical elements, or tapered elements. In the following description, embodiments are described for optoelectronic devices comprising microwires or nanowires. However, these embodiments may be implemented for three-dimensional elements other than microwires or nanowires, for example, pyramid-shaped three-dimensional elements. 
         [0047]    The term “microwire” or “nanowire” designates a three-dimensional structure having an elongated shape along a preferential direction, having at least two dimensions, called minor dimensions, in the range from 5 nm to 2.5 μm, preferably from 50 nm to 2.5 μm, the third dimension, called major dimension, being at least equal to 1 time, preferably at least 5 times, and more preferably still at least 10 times, the largest of the minor dimensions. In certain embodiments, the minor dimensions may be smaller than or equal to approximately 1 μm, preferably in the range from 100 nm to 1 μm, more preferably from 100 nm to 300 nm. In certain embodiments, the height of each microwire or nanowire may be greater than or equal to 500 nm, preferably in the range from 1 μm to 50 μm. 
         [0048]    In the following description, the term “wire” is used to mean “microwire or nanowire”. Preferably, the average line of the wire which runs through the centers of gravity of the cross-sections, in planes perpendicular to the preferential direction of the wire, is substantially rectilinear and is called “axis” of the wire hereafter. 
         [0049]      FIG. 1  is a partial simplified top view of a wafer  10  of a semiconductor substrate having wires formed thereon. As an example, it is a single-crystal silicon wafer having an initial thickness in the range from 500 μm to 1,500 μm, for example, approximately 725 μm, and having a diameter in the range from 100 mm to 300 mm, for example, approximately 200 mm. Advantageously, it is a silicon wafer currently used in methods of circuit manufacturing in microelectronics, particularly based on metal-oxide field-effect transistors or MOS transistors. As a variation, any other single-crystal semiconductor compatible with microelectronics manufacturing methods such as germanium may be used. Preferably, the semiconductor substrate is doped to decrease the electric resistivity of the substrate to an acceptable level for the series resistance of the light-emitting diode and to a resistivity close to that of metals, preferably smaller than a few mohm·cm. 
         [0050]    A plurality of optoelectronic devices  14  comprising light-emitting diodes are simultaneously formed on wafer  10 . Dotted lines  12  show an example of separation limits between optoelectronic devices  14 . The number of light-emitting diodes may be different according to optoelectronic devices  14 . Optoelectronic devices  14  may take up portions of wafer  10  having different surface areas. Optoelectronic devices  14  are separated by steps of sawing wafer  10  along sawing paths shown by lines  12 . 
         [0051]    According to an embodiment, the method of manufacturing optoelectronic devices  14  comprising light-emitting diodes formed of three-dimensional elements, particularly semiconductor wires, comprises the steps of: 
         [0052]    forming the light-emitting diodes of the optoelectronic devices on a first surface of wafer  10 ; 
         [0053]    protecting the assembly of light-emitting diodes with an encapsulation layer; 
         [0054]    forming contact pads for the biasing of the light-emitting diodes for each optoelectronic device on the side opposite to the encapsulation layer; and 
         [0055]    sawing wafer  10  to separate the optoelectronic devices. 
         [0056]    The encapsulation layer protects the light-emitting diodes during the contact pad forming steps and is kept after the optoelectronic devices have been separated. The encapsulation layer keeps on protecting the light-emitting diodes after the substrate has been sawn. It is then not necessary to provide, for each optoelectronic device, a protection package for the light-emitting diodes, attached to the device after the optoelectronic devices have been separated. The bulk of the optoelectronic device may be decreased. 
         [0057]    Further, the step of protecting the light-emitting diodes of optoelectronic devices  14  is carried out by encapsulation of the wires in an encapsulation layer which is deposited all over wafer  10  before the step of sawing wafer  10 . This step is thus carried out only once, for the all the optoelectronic devices  14  formed on wafer  10 . The manufacturing cost of each optoelectronic device is thus decreased. 
         [0058]    Thus, the encapsulation is entirely performed at the wafer scale after the microwire or nanowire manufacturing steps. Such a collective encapsulation at the wafer scale enables to decrease the number of steps dedicated to the encapsulation, and thus the encapsulation cost. Further, the surface area of the final encapsulated optoelectronic component is almost identical to that of the active area of the chip taking part in the light emission, which enables to decrease the dimensions of the optoelectronic component. 
         [0059]      FIGS. 2A to 2F  are partial simplified cross-section views of obtained structures corresponding to an optoelectronic device at successive steps of an embodiment of a method of manufacturing optoelectronic devices formed with wires such as previously described and capable of emitting an electromagnetic radiation.  FIGS. 2A to 2F  correspond to one of the optoelectronic devices formed on substrate  10 . 
         [0060]      FIG. 2A  shows a structure comprising, from bottom to top in  FIG. 2A :
       semiconductor substrate  10  comprising an upper surface  22 ;   seed pads  24  promoting the growth of wires and arranged on surface  22 ;   wires  26  (two wires being shown) of height H 1 , each wire  26  being in contact with one of seed pads  24 , each wire  26  comprising a lower portion  28 , of height H 2 , in contact with seed pad  24  and an upper portion  30 , of height H 3 , continuing lower portion  28 ;   an insulating layer  32  extending on surface  22  of substrate  10  and on the lateral sides of lower portion  28  of each wire  26 ;   a shell  34  comprising a stack of semiconductor layers covering each upper portion  30 ;   a layer  36  forming a first electrode covering each shell  30  and further extending on insulating layer  32 ; and   a conductive layer  38  covering electrode layer  36  between wires  26  without extending on wires  26 .       
 
         [0068]    The assembly formed by each wire  26 , the associated seed pad  24 , and shell  34  forms a light-emitting diode DEL. The base of diode DEL corresponds to seed pad  24 . Shell  34  particularly comprises an active layer which is the layer from which most of the electromagnetic radiation delivered by light-emitting diode DEL is emitted. 
         [0069]    Substrate  10  may correspond to a one-piece structure or correspond to a layer covering a support made of another material. Substrate  10  for example is a semiconductor substrate, preferably a semiconductor substrate compatible with manufacturing methods implemented in microelectronics, for example, a substrate made of silicon, germanium, or an alloy of these compounds. The substrate is doped so that the substrate resistivity is lower than a few mohm·cm. 
         [0070]    Preferably, substrate  10  is a semiconductor substrate, such as a silicon substrate. Substrate  10  may be doped with a first conductivity type, for example, N-type doped. Surface  22  of substrate  20  may be a&lt;100&gt; surface. 
         [0071]    Seed pads  24 , also called seed islands, are made of a material promoting the growth of wires  26 . As a variation, seed pads  24  may be replaced with a seed layer covering surface  22  of substrate  10 . In the case of seed pads, a treatment may further be provided to protect the lateral edges of the seed pads and the surface of the substrate portions which are not covered with the seed pads to prevent wires from growing on the lateral sides of the seed pads and on the surface of the substrate portions which are not covered with the seed pads. The treatment may comprise forming a dielectric region on the lateral sides of the seed pads and extending on top of and/or inside of the substrate and connecting, for each pair of pads, one of the pads of the pair to the other pad in the pair, with no wire growth on the dielectric region. 
         [0072]    As an example, the material forming seed pads  24  may be a nitride, a carbide, or a boride of a transition metal from column IV, V, or VI of the periodic table of elements or a combination of these compounds. As an example, seed pads  24  may be made of aluminum nitride (AlN), boron (B), boron nitride (BN), titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), hafnium (Hf), hafnium nitride (HfN), niobium (Nb), niobium nitride (NbN), zirconium (Zr), zirconium borate (ZrB2), zirconium nitride (ZrN), silicon carbide (SiC), tantalum carbo-nitride (TaCN), magnesium nitride in MgxNy form, where x is approximately equal to 3 and y is approximately equal to 2, for example, magnesium nitride in Mg3N2 form or magnesium gallium nitride (MgGaN), tungsten (W), tungsten nitride (WN), or a combination thereof. 
         [0073]    Seed pads  24  may be doped with the same conductivity type as substrate  10  or with the opposite conductivity type. 
         [0074]    Insulating layer  32  may be made of a dielectric material, for example, silicon oxide (SiO2), silicon nitride (SixNy, where x is approximately equal to 3 and y is approximately equal to 4, for example, Si3N4), aluminum oxide (Al2O3), hafnium oxide (HfO2), or diamond. As an example, the thickness of insulating layer  32  is in the range from 5 nm to 800 nm, for example, equal to approximately 30 nm. 
         [0075]    Wires  26  may be at least partly formed based on at least one semiconductor material. The semiconductor material may be silicon, germanium, silicon carbide, a III-V compound, a II-VI compound, or a combination of these compounds. 
         [0076]    Wires  26  may be at least partly formed of semiconductor materials mainly comprising a III-V compound, for example, III-N compounds. Examples of group-III elements comprise gallium (Ga), indium (In), or aluminum (Al). Examples of III-N compounds are GaN, AN, InN, InGaN, AlGaN, or AlInGaN. Other group-V elements may also be used, for example, phosphorus or arsenic. Generally, the elements in the III-V compound may be combined with different molar fractions. 
         [0077]    Wires  26  may be at least partly formed based on semiconductor materials mainly comprising a II-VI compound. Examples of group-II elements comprise group-IIA elements, particularly beryllium (Be) and magnesium (Mg), and group-IIB elements, particularly zinc (Zn) and cadmium (Cd). Examples of group—VI elements comprise group-VIA elements, particularly oxygen (O) and tellurium (Te). Examples of II-VI compounds are ZnO, ZnMgO, CdZnO, or CdZnMgO. Generally, the elements in the II-VI compound may be combined with different molar fractions. 
         [0078]    Wires  26  may comprise a dopant. As an example, for III-V compounds, the dopant may be selected from the group comprising a group-II P-type dopant, for example, magnesium (Mg), zinc (Zn), cadmium (Cd), or mercury (Hg), a group-IV P-type dopant, for example, carbon (C), or a group-IV N-type dopant, for example, silicon (Si), germanium (Ge), selenium (Se), sulfur (S), terbium (Tb), or tin (Sn). 
         [0079]    The cross-section of wires  26  may have different shapes, such as, for example, oval, circular, or polygonal, particularly triangular, rectangular, square, or hexagonal. It should thus be understood that the term “diameter” mentioned in relation with a cross-section of a wire or of a layer deposited on this wire designates a quantity associated with the surface area of the targeted structure in this cross-section, corresponding, for example, to the diameter of the disk having the same surface area as the wire cross-section. The average diameter of each wire  26  may be in the range from 50 nm to 2.5 μm. Height H 1  of each wire  26  may be in the range from 250 nm to 50 μm. 
         [0080]    Each wire  26  may have an elongated semiconductor structure along an axis D substantially perpendicular to surface  22 . Each wire  26  may have a general cylindrical shape. 
         [0081]    The axes of two wires  26  may be distant by from 0.5 μm to 10 μm, and preferably from 1.5 μm to 4 μm. As an example, wires  26  may be regularly distributed. As an example, wires  26  may be distributed in a hexagonal network. 
         [0082]    As an example, lower portion  28  of each wire  26  is mainly formed of the III-N compound, for example, gallium nitride, having a doping of the first conductivity type, for example, silicon. Lower portion  28  extends up to a height H 2  which may be in the range from 100 nm to 25 μm. 
         [0083]    As an example, upper portion  30  of each wire  26  is at least partially made of a III-N compound, for example, GaN. Upper portion  30  may be doped with the first conductivity type, or may not be intentionally doped. Upper portion  30  extends up to a height H 3  which may be in the range from 100 nm to 25 μm. 
         [0084]    In the case of a wire  26  mainly made of GaN, the crystal structure of wire  26  may be of wurtzite type, the wire extending along axis C. The crystal structure of wire  26  may also be of cubic type. 
         [0085]    Shell  34  may comprise a stack of an active layer covering upper portion  30  of the associated wire  26  and of a bonding layer between the active layer and electrode  36 . 
         [0086]    The active layer is the layer from which most of the radiation delivered by light-emitting diode DEL is emitted. According to an example, the active layer may comprise confinement means, such as multiple quantum wells. It is for example formed of an alternation of GaN and of InGaN layers having respective thicknesses from 5 to 20 nm (for example, 8 nm) and from 1 to 10 nm (for example, 2.5 nm). The GaN layers may be doped, for example of type N or P. According to another example, the active layer may comprise a single InGaN layer, for example, having a thickness greater than 10 nm. 
         [0087]    The bonding layer may correspond to a semiconductor layer or to the stack of semiconductor layers and enables to form a P-N or P-I-N junction with the active layer and/or upper portion  30 . It enables to inject holes into the active layer via electrode  36 . The stack of semiconductor layers may comprise an electron barrier layer made of a ternary alloy, for example, aluminum gallium nitride (AlGaN) or aluminum indium nitride (AlInN) in contact with the active layer and an additional layer, to provide a good electric contact between electrode  36  and the active layer, for example, made of gallium nitride (GaN) in contact with the electron barrier layer and with electrode  36 . The bonding layer may be doped with the conductivity type opposite to that of portion  30 , for example, P-type doped. 
         [0088]    Electrode  36  is capable of biasing the active layer of each wire  26  and of letting through the electromagnetic radiation emitted by light-emitting diodes DEL. The material forming electrode  36  may be a transparent and conductive material such as indium tin oxide (ITO), aluminum zinc oxide, or graphene. As an example, electrode  36  has a thickness in the range from 10 nm to 150 nm according to the desired emission wavelength. 
         [0089]    Conductive layer  38  may be a single layer or correspond to a stack of two layers or of more than two layers. Conductive layer  38  may further be capable of at least partly reflecting the radiation emitted by light-emitting diodes DEL. As an example, conductive layer  38  corresponds to a metal monolayer. According to another example, conductive layer  38  corresponds to a stack of layers for example comprising a metal layer covered with a dielectric layer or with a plurality of dielectric layers. The metal layer of conductive layer  38  may be formed on a bonding layer, for example, made of titanium. As an example, the material forming the metal layer of conductive layer  38  (monolayer or multilayer) may be aluminum, an alloy based on aluminum, particularly AlSiz, AlxCuy (for example, with x equal to 1 and y equal to 0.8%), silver, gold, nickel, chromium, rhodium, ruthenium, palladium, or an alloy of two of these compounds or of more than two of these compounds. As an example, conductive layer  38  (monolayer or multilayer) has a thickness in the range from 100 nm to 2,000 nm. 
         [0090]    An embodiment of a manufacturing method providing the structure shown in  FIG. 2A  comprises the steps of: 
         [0091]    (1) Forming, on surface  22  of substrate  10 , seed pads  24 . 
         [0092]    Seed pads  24  may be obtained by a method such as chemical vapor deposition (CVD) or metal-organic chemical vapor deposition (MOCVD), also known as metal-organic vapor phase epitaxy (MOVPE). However, methods such as molecular-beam epitaxy (MBE), gas-source MBE (GSMBE), metal-organic MBE (MOMBE), plasma-assisted MBE (PAMBE), atomic layer epitaxy (ALE), hydride vapor phase epitaxy (HVPE) may be used, as well as an atomic layer deposition (ALD). Further, methods such as evaporation or reactive cathode sputtering may be used. 
         [0093]    When seed pads  24  are made of aluminum nitride, they may be substantially textured and have a preferred polarity. The texturing of pads  24  may be obtained by an additional treatment carried out after the deposition of seed pads  24 . It for example is an anneal under an ammonia flow (NH3). 
         [0094]    (2) Protecting the portions of surface  22  of substrate  10  which are not covered with seed pads  24  to avoid the subsequent growth of wires on these portions. This may be obtained by a nitriding step which causes the forming, at the surface of substrate  10 , between seed pads  24 , of silicon nitride regions (for example, Si3N4). 
         [0095]    (3) Growing lower portion  28  of each wire  26  up to height H 2 . Each wire  26  grows from the top of the underlying seed pad  24 . 
         [0096]    Wires  26  may be grown by a process of CVD, MOCVD, MBE, GSMBE, PAMBE, ALE, HVPE type. Further, electrochemical processes may be used, for example, chemical bath deposition (CBD), hydrothermal processes, liquid aerosol pyrolysis, or electro-deposition. 
         [0097]    As an example, the wire growth method may comprise injecting into a reactor a precursor of a group-III element and a precursor of a group-V element. Examples of precursors of group-III elements are trimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn), or trimethylaluminum (TMA 1 ). Examples of precursors of group-V elements are ammonia (NH3), tertiarybutylphosphine (TBP), arsine (AsH 3 ), or unsymmetrical dimethylhydrazine (UDMH). 
         [0098]    According to an embodiment of the invention, in a first phase of growth of the wires of the III-V compound, a precursor of an additional element is added in excess, in addition to the precursors of the III-V compound. The additional element may be silicon (Si). An example of a precursor of silicon is silane (SiH4). 
         [0099]    As an example, in the case where upper portion  28  is made of heavily-doped N-type GaN, a MOCVD-type method may be implemented by injection, into a showerhead-type MOCVD reactor, of a gallium precursor gas, for example, trimethylgallium (TMGa) and a nitrogen precursor gas, for example, ammonia (NH3). As an example, a showerhead-type 3×2″ MOCVD reactor commercialized by AIXTRON, may be used. A molecular flow ratio between trimethylgallium and ammonia within the 5-200 range, preferably within the 10-100 range, enables to promote the growth of wires. As an example, a carrier gas which ensures the diffusion of organometallic elements all the way to the reactor charges with organometallic elements in a TMGa bubbler. The latter is set according to the standard operating conditions. A flow of 60 sccm (standard cubic centimeters per minute) is for example selected for TMGa, while a 300-sccm flow is used for NH3 (standard NH3 bottle). A pressure of approximately 800 mbar (800 hPa) is used. The gaseous mixture further comprises silane injected into the MOCVD reactor, which material is a precursor of silicon. The silane may be diluted in hydrogen at 1,000 ppm and a 20-sccm flow is provided. The temperature in the reactor is for example in the range from 950° C. and 1,100° C., preferably from 990° C. to 1,060° C. To transport species from the outlet of the bubblers to the two reactor plenums, a 2,000-sccm flow of carrier gas, for example, N2, distributed between the two plenums, is used. The previously-indicated gas flows are given as an indication and should be adapted according to the size and to the specificities of the reactor. 
         [0100]    The presence of silane among the precursor gases results silicon being incorporated within the GaN compound. A lower N-type doped portion  28  is thus obtained. This further translates as the forming of a silicon nitride layer, not shown, which covers the periphery of portion  28  of height H 2 , except for the top, as portion  28  grows. 
         [0101]    (4) Growing upper portion  30  of height H 3  of each wire  26  on the top of lower portion  28 . For the growth of upper portion  30 , the previously-described operating conditions of the MOCVD reactor are, as an example, maintained but for the fact that the silane flow in the reactor is decreased, for example, by a factor greater than or equal to 10, or stopped. Even when the silane flow is stopped, upper portion  30  may be N-type doped due to the diffusion in this active portion of dopants originating from the adjacent passivated portions or due to the residual doping of GaN. 
         [0102]    (5) Forming by epitaxy, for each wire  26 , the layers forming shell  34 . Given the presence of the silicon nitride layer covering the periphery of lower portion  28 , the deposition of the layers forming shell  34  only occurs on upper portion  30  of wire  26 . 
         [0103]    (6) Forming insulating layer  32 , for example, by conformally depositing an insulating layer over the entire structure obtained at step (5) and etching this layer to expose shell  34  of each wire  26 . In the previously-described embodiment, insulating layer  32  does not cover shell  34 . As a variation, insulating layer  32  may cover a portion of shell  34 . Further, insulating layer  32  may be formed before shell  34 . 
         [0104]    (7) Forming electrode  36 , for example, by conformal deposition. 
         [0105]    (8) Forming conductive layer  38 , for example, by physical vapor deposition (PVD) over the entire structure obtained at step (7) and etching this layer to expose each wire  26 .  FIG. 2B  shows the structure obtained after having deposited an encapsulation layer  40  over the entire wafer  10 . The maximum thickness of encapsulation layer  40  is in the range from 12 μm to 1,000 μm, for example, approximately 50 μm, so that encapsulation layer  40  totally covers electrode  36  at the top of light-emitting diodes DEL. Encapsulation layer  40  is made of an at least partially transparent insulating material. 
         [0106]    Encapsulation layer  40  may be made of an at least partially transparent inorganic material. 
         [0107]    As an example, the inorganic material is selected from the group comprising silicon oxides, of type SiOx where x is a real number between 1 and 2, or SiOyNz where y and z are real numbers between 0 and 1, and aluminum oxides, for example, Al2O3. The inorganic material may then be deposited by low-temperature CVD, particularly at a temperature lower than 300° C.-400° C., for example by PECVD (plasma enhanced chemical vapor deposition). 
         [0108]    Encapsulation layer  40  may be made of an at least partially transparent organic material. As an example, encapsulation layer  40  is a silicone polymer, an epoxide polymer, an acrylic polymer, or a polycarbonate. Encapsulation layer  40  may then be deposited by a spin coating method, by an inkjet printing method, or by a silk-screening method. A method of dispensing by time/pressure dispenser or by volumetric dispenser is also possible in automated mode on programmable equipment. 
         [0109]      FIG. 2C  shows the structure obtained after attaching an additional support  42 , called handle, on encapsulation layer  40 . As an example, the handle has a thickness in the range from 200 μm to 1,000 μm. 
         [0110]    According to an embodiment, handle  42  is intended to be kept on the optoelectronic devices once sawn. Handle  42  then is made of an at least partly transparent material. It may be glass, particularly a borosilicate glass, for example, Pyrex, or sapphire. An observer perceives the light rays emitted by light-emitting diodes DEL which cross surface  43  of handle  42  opposite to encapsulation layer  40 . 
         [0111]    According to another embodiment, handle  42  is intended to be removed at a subsequent step of the manufacturing method. In this case, handle  42  may be made of any type material compatible with the subsequent steps of the manufacturing method. It may be silicon or any planar substrate compatible with microelectronics flatness criteria. 
         [0112]    Handle  42  may be attached to encapsulation layer  40  by any means, for example, by bonding, for example by using a layer of organic temperature-crosslinkable glue, not shown, or also by molecular bonding (direct bonding) or optical bonding with UV-cured glue. When encapsulation layer  40  is made of an organic material, this material may be used as glue for handle  42 . When a glue layer is used, it should be at least partially transparent. 
         [0113]      FIG. 2D  shows the structure obtained after a step of thinning substrate  10 . After thinning, the thickness of substrate  10  may be in the range from 20 μm to 200 μm, for example, approximately 30 μm. The thinning step may be carried out by one or more than one milling or etching step, and/or by chemical mechanical polishing methods (CMP). Thinned substrate  10  comprises a surface  44  opposite to surface  22 . Surfaces  22  and  44  are preferably parallel. 
         [0114]      FIG. 2E  shows the structure obtained after the steps of:
       forming an insulating layer  45 , for example, made of silicon oxide (SiO2) or of silicon oxynitride (SiON), on the rear surface of substrate  10 . Insulating layer  45  is for example carried out by conformal deposition by PECVD;   etching, for each optoelectronic device, at least one opening  46  crossing insulating layer  45 , substrate  10 , insulating layer  32 , and electrode  36  to expose a portion of metal layer  38 . The etching of substrate  10  may be a deep reactive ion etching (DRIE). The etching of the portion of insulating layer  32  is also performed by plasma etching with the chemistry adapted to insulating layer  32 . At the same time, electrode layer  36  may be etched. As a variation, layer  36  may be removed from the areas where vias  46  are formed before the step of forming metal layer  38 . Opening  46  may have a circular cross-section. The diameter of opening  46  may then be in the range from 5 μm to 200 μm according to the size of unit optoelectronic component  14  such as shown in  FIG. 1 , for example, approximately 15 μm. A plurality of circular openings  46  may then be simultaneously formed to create connections in parallel. This enables to decrease the resistance of connections. Such connections may be arranged at the periphery of the area where light-emitting diodes DEL are formed. As a variation, opening  46  may correspond to a trench, for example extending along at least one side of the optoelectronic device. Preferably, the trench width is in the range from 15 μm to 200 μm according to the size of unit optoelectronic component  14  such as shown in  FIG. 1 , for example, approximately 15 μm;   forming an insulating layer  48 , for example, made of SiO2 or SiON, on the internal walls of opening  46  and, possibly on layer  45 , the portion of layer  48  covering layer  45  not being shown in the drawings. Insulating layer  48  is for example formed by conformal PECVD. Insulating layer  48  has a thickness in the range from 200 nm to 5,000 nm, for example, approximately 3 μm;   etching insulating layer  48  to expose conductive layer  38  at the bottom of opening  46 . This etching is anisotropic; and   etching at least one opening  50  in insulating layer  45  to expose a portion of surface  44  of substrate  10 . To perform this etching, opening  46  may be temporarily obstructed, for example, with a resin.       
 
         [0120]      FIG. 2F  shows the structure obtained after the forming of a second electrode  52  in opening  50  and of a conductive layer  54  on insulating layer  48 , conductive layer  54  covering the internal walls of opening  46  to come into contact with metal portion  36 , and extending on surface  44  around opening  46 . Electrode  52  and conductive layer  54  may comprise a stack of two layers, as shown in the drawings, or more than two layers. It for example is TiCu or TiAl. This layer may be covered with another metal layer, for example, gold, copper, or eutectic alloys (Ni/Au or Sn/Ag/Cu) to implement a soldering method. Second electrode  52  and conductive layer  54  may be formed, particularly in the case of copper, by electrochemical deposition (ECD). The thickness of electrode layer  52  and conductive layer  54  may be in the range from 1 μm to 10 μm, for example, approximately 5 μm. 
         [0121]    The assembly comprising opening  46 , insulating layer  48 , and conductive layer  54  forms a vertical connection  56  or TSV (Through Silicon Via). Vertical connection  56  enables to bias first electrode  36  from the rear surface of substrate  10  while the biasing of wires  26  is obtained by second electrode  52  through substrate  10 . 
         [0122]      FIGS. 3A and 3B  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices with wires, which comprises all the steps described in relation with  FIGS. 2A to 2E . 
         [0123]      FIG. 3A  shows the structure obtained after the steps of:
       forming in opening  50  of insulating layer  44  a conductive pad  60 ;   depositing an insulating layer  62  particularly covering metal pad  60 . Insulating layer  62  may be made of silicon oxide or of silicon nitride or may correspond to a stack of two stacked layers or more and have a thickness in the range from 200 nm to 1,000 nm; and   etching openings  64  in insulating layer  62  to expose portions of conductive pad  60 .       
 
         [0127]      FIG. 3B  shows the structure obtained after steps similar to what has been previously described in relation with  FIG. 2F  to form second electrodes  66  in openings  64  and conductive layer  54  in opening  46 . 
         [0128]    The embodiment described in relation with  FIGS. 3A and 3B  advantageously enables to adjust the positions and the dimensions of second electrodes  66 . 
         [0129]      FIG. 4  illustrates another embodiment of a manufacturing method comprising, after the steps previously described in relation with  FIG. 2F , the steps of:
       depositing an insulating layer  68  particularly covering pad  52  and filling opening  46 . It may be an insulating polymer, for example, a BCB (benzocyclobutene) resist having a thickness in the range from 2 μm to 20 μm, or a silicon oxide, or silicon nitride, or both, and have a thickness in the range from 200 nm to 1,000 nm;   forming openings  70  in insulating layer  68  to expose portions of second electrode  52  and conductive layer  54 . It may be a plasma-type etching when insulating layer  68  is made of an inorganic material or steps of illumination and development when insulating layer  68  is made of a resist; and   forming conductive bumps  72  in openings  70 . Bumps  72  are made of materials compatible with soldering operations in electronics, for example tin- or gold-based alloys. Bumps  72  may be used to attach the optoelectronic device to a support, not shown.       
 
         [0133]    In the previously-described embodiments, the current flows between first electrode  36  and second electrode  52 ,  66  through substrate  10 . 
         [0134]      FIG. 5  illustrates another embodiment where the light-emitting diodes are directly biased at the base of wires  26 . Wires  26  are formed on a seed layer  74 , which is then common to an assembly of light-emitting diodes DEL of the optoelectronic device. 
         [0135]    A vertical connection  76  is formed in substrate  10 , for example, similarly to vertical connection  56 , with the difference that vertical connection  76  is connected to seed layer  74 . 
         [0136]      FIGS. 6A to 6C  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices with wires which comprises all the steps described in relation with  FIGS. 2A to 2E . 
         [0137]      FIG. 6A  shows the structure obtained after the deposition of a thick metal layer  80 , for example, copper. It may be an ECD. The thickness of insulating layer  80  is for example in the order of 10 μm. Metal layer  80  is sufficiently thick to fill opening  46 . 
         [0138]      FIG. 6B  shows the structure obtained after a step of polishing metal layer  80  to delimit a metal portion  82  in opening  50  and a metal portion  84  in opening  46 . The step of planarizing layer  80  may be carried out by CMP. 
         [0139]      FIG. 6C  shows the structure after steps similar to those previously described in relation with  FIGS. 3A and 3B , comprising depositing an insulating layer  86  over the entire rear surface of substrate  10  and forming a second electrode  88  crossing layer  86  in contact with metal portion  82  and a conductive pad  90  crossing layer  86  in contact with metal portion  84 . A passivation layer, particularly made of polymer, may be deposited on the structure on the rear surface side, openings being formed in the passivation layer to expose electrode  88  and conductive pad  90 . 
         [0140]    The assembly comprising opening  46 , insulating layer  48 , metal portion  84 , and metal pad  90  forms a TSV  91  which plays the same role as previously-described TSV  56 . Metal pads  88  and  90  are used to assemble the optoelectronic component encapsulated on its final support, for example, a printed circuit. The assembly methods may be carried out by soldering. The metal stack is selected to be compatible with solder operations used in electronics, and particularly with the soldering used, for example, in Cu with an organic solderability preservative finish (OSP) or Ni—Au finish (by a process which may be chemical (ENIG, Electroless nickel immersion gold) or electrochemical), Sn, Sn—Ag, Ni—Pd—Au, Sn—Ag—Cu, Ti—Wn—Au, or ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold). 
         [0141]      FIGS. 7A and 7B  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices with wires. 
         [0142]    The initial steps may comprise the steps previously described in relation with  FIG. 2A , with the difference that, before steps (5) to (7), an opening  92  is formed in substrate  10 . Opening  92  may be formed by a DRIE-type etching. The depth of opening  92  is strictly greater than the thickness of substrate  10  after the thinning step. As an example, the depth of opening  92  is in the range from 10 μm to 200 μm, for example, in the order of 35 μm. 
         [0143]    During the implementation of steps (5) to (7), insulating layer  32 , electrode  36 , and conductive layer  38  are also formed in opening  92 . 
         [0144]      FIG. 7B  shows the structure obtained after the carrying out of the steps of:
       depositing encapsulation layer  40  similarly to what has been previously described in relation with  FIG. 2B . Encapsulation layer  40  partially or totally penetrates into opening  92 ;   installing handle  42  similarly to what has been previously described in relation with  FIG. 2C ;   thinning substrate  10  similarly to what has been previously described in relation with  FIG. 2D  all the way to opening  92 ;   forming an insulating layer  94  on rear surface  44  of substrate  10  while protecting opening  92 ; and   forming an opening  96  in insulating layer  94  to expose a portion of substrate  10 .       
 
         [0150]    The assembly comprising opening  92  and the portions of insulating layer  32 , of electrode layer  36 , and of conductive layer  38  extending in opening  92  forms a TSV  98  which plays the same role as previously-described TSV  56 . 
         [0151]    The subsequent steps of the method may be similar to what has been previously described in relation with  FIG. 2F . 
         [0152]      FIG. 8  shows an embodiment where substrate  10  is at least sawn once at the level of a TSV which may correspond to one of previously-described TSVs  56 ,  91 , or  98 . The sawing exposes a portion of the conductive layer which extends on the internal walls of the TSV. The biasing of first electrode  36  of light-emitting diodes DEL may then be performed from the side of the optoelectronic device. As an example, the optoelectronic device may be attached to a support  100  by a connection pad  102  in contact with the rear surface of substrate  14  and by a connection pad  104  in contact with the lateral exposed portion of the TSV. 
         [0153]      FIG. 9  shows an embodiment where a TSV  106  is provided at the level of each wire  26  of the optoelectronic device. Each TSV  106  comes into contact with seed pad  24  of the associated wire  26 . TSVs  106  may be unconnected to one another. Wires  26  can then be separately biased. As a variation, an electrode, not shown, provided on the side of rear surface  44  of substrate  10 , may be connected to all the vertical connections  106  associated with a same optoelectronic device. 
         [0154]      FIG. 10  shows an embodiment where a TSV  110  simultaneously comes into contact with seed pads  24  of a plurality of wires  26 . Vertical connections  106 ,  110  may be formed according to any of the manufacturing methods previously described for the forming of TSVs  56 ,  91 , and  98 . 
         [0155]      FIGS. 11A to 11D  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices with wires. 
         [0156]      FIGS. 11A and 11B  show structures obtained after the carrying out of steps before step (1) previously described in relation with  FIG. 2A . 
         [0157]      FIG. 11A  shows the structure obtained after the steps of:
       etching an opening  120  in substrate  10 . Opening  120  may be formed by an etching of reactive ion etching type, for example, a DRIE etching. The depth of opening  120  is strictly greater than the targeted thickness of substrate  10  after the thinning step. As an example, the depth of opening  120  is in the range from 10 μm to 200 μm, for example, approximately 35 μm. The distance between the lateral walls of opening  120  is in the range from 1 to 10 μm and for example 2 μm; and   forming an insulating portion  122 , for example, made of silicon oxide, on the lateral walls of opening  120 , for example, by a thermal oxidation method. At this step, an insulating portion may also form at the bottom of opening  120  and on the rest of substrate  10 . The thickness of insulating portion may be in the range from 100 nm to 3,000 nm, for example, approximately 200 nm.       
 
         [0160]      FIG. 11B  shows the structure obtained after carrying out the steps of:
       anisotropically etching the insulating portion at the bottom of opening  120  and the insulating portion covering surface  22  of substrate  10 , to keep insulating portions  122  on the lateral sides of opening  120 . As an example, the etching of the insulating portion covering surface  22  of substrate  10  may be omitted. In this case, a mask formed by photolithography may be provided to protect said unetched insulating portions;   filling opening  120  with a filling material, for example, polysilicon, tungsten, or a refractory metallic material which supports the thermal budget during the previously-described steps carried out a high temperatures, particularly in relation with steps 2A to 2D, for example, deposited by LPCVD. Polysilicon advantageously has a thermal expansion coefficient close to that of silicon and thus enables to decrease the mechanical stress during the previously-described steps carried out at high temperatures, particularly in relation with steps 2A to 2D;   removing the layer of filling material, for example, by a CMP-type method. In the case where the etching of the insulating portion covering surface  22  of substrate  10  has been omitted during the anisotropic etching of the insulating portion at the bottom of opening  122 , said unetched layer may advantageously be used as a stop layer during the removal of the layer of filling material. In this case, the removal of the layer of filling material is followed by a step of etching the insulating portion covering surface  22  of substrate  10 . A portion  124  of the filling material is thus obtained.       
 
         [0164]      FIG. 11C  shows the structure obtained after the implementation of steps similar to what has been previously described in relation with  FIGS. 2A to 2D , with the difference that it comprises, before the forming of conductive layer  38 , a step of etching an opening  125  in electrode layer  36  and of insulating layer  32  so that conductive layer  38  comes into contact with portion  124 . 
         [0165]      FIG. 11D  shows the structure obtained after implementation of the following steps, similarly to what has been previously described in relation with  FIGS. 7B, 3A , and  3 B:
       thinning substrate  10  to reach conductive portion  124 ;   forming an insulating layer  126  on rear surface  44  of substrate  10 ;   forming, in insulating layer  126 , an opening  128  to expose a portion of rear surface  44  of substrate  10  and an opening  130  to expose conductive portion  124 ;   forming a conductive pad  132  in opening  128  in contact with substrate  10  and with a conductive pad  134  in opening  130  in contact with conductive portion  124 ;   forming an insulating layer  136  covering insulating layer  126  and conductive pads  132 ,  134 ;   forming, in insulating layer  136 , an opening  138  to expose a portion of conductive pad  132  and an opening  140  to expose conductive pad  134 ; and   forming a second electrode  142  in opening  138  in contact with conductive pad  132  and a conductive pad  144  in opening  130  in contact with conductive pad  134 .       
 
         [0173]    The assembly comprising portion  124  of the filling material delimited by insulating portions  122  forms a TSV  145  which plays the same role as previously-described TSV  56 . Conductive portion  124  which connects pad  144  to metal layer  38  is formed by portion  124  of the filling material. 
         [0174]    As a variation, insulating layer  126  may be absent and conductive pads  132 ,  144  may be directly formed on substrate  10 . 
         [0175]    According to another variation, instead of forming a portion  124  of a filling material insulated from substrate  10  by insulating portions, the method may comprise steps of forming insulating trenches delimiting a portion of the substrate which then plays the role of portion  124 . Preferably, heavily-doped silicon, for example having a dopant concentration greater than or equal to 1019 atoms/cm3, is used to decrease the resistance of this connection. This conductive portion may be formed by one or a plurality of silicon trenches around the active area or by one or a plurality of insulated silicon vias. 
         [0176]    The embodiment previously described in relation with  FIGS. 11A to 11D  may be implemented to form vertical connections  106  and  110  previously described in relation with  FIGS. 9 and 10 . 
         [0177]      FIGS. 12A to 12E  are partial simplified cross-section views of structures obtained at successive steps of another embodiment of a method of manufacturing optoelectronic devices with wires. The initial steps may comprise the steps previously described in relation with  FIGS. 2A to 2C , with the difference that conductive layer  38  is not present. 
         [0178]      FIG. 12A  shows the structure obtained after a step of removing substrate  10 . The removal of substrate  10  may be performed by one or more than one etch step. The rear surface of the structure thus exposed after removal of the substrate is designated with reference numeral  150 . In  FIG. 12A , the etching is stopped on insulating layer  32  and on seed pads  24 . As a variation, the method may further comprise removing seed pads  24 . 
         [0179]      FIG. 12B  shows the structure obtained after carrying out the steps of:
       etching an opening  152  in insulating layer  32 ;   depositing a mirror layer  154  on rear surface  150  and in opening  152 ; and   depositing a conductive layer  156  covering mirror layer  154 .       
 
         [0183]    Mirror layer  154  may be a single layer or correspond to a stack of two layers or of more than two layers. As an example, mirror layer  154  corresponds to a metal monolayer. According to another example, mirror layer  154  corresponds to a stack of layers comprising a metal layer covered with a dielectric layer or with a plurality of dielectric layers. The metal layer of mirror layer  154  may be formed on a bonding layer, for example, made of titanium. The thickness of mirror layer  154  (monolayer or multilayer) is greater than 15 nm, for example, in the range from 30 nm to 2 μm. Mirror layer  154  may be deposited by ECD. 
         [0184]    According to an embodiment, mirror layer  154  is capable of at least partly reflecting the radiation emitted by light-emitting diodes DEL. 
         [0185]    According to an embodiment, the complex optical indexes of the materials forming seed pads  24  and mirror layer  154  (monolayer or multilayer) and the thicknesses of seed pads  24  and of mirror layer  154  are selected to increase the mean reflectivity of seed pads  24  and mirror layer  154 . The mean reflectivity of a layer or of a stack of layers is the mean of the ratio of the electromagnetic energy reflected by the layer or the stack of layers to the incident energy for all possible angles of incidence at a given wavelength. It is desirable for the mean reflectivity to be as high as possible, preferably greater than 80%. 
         [0186]    The complex optical index, also called complex refraction index, is a dimensionless number which characterizes the optical properties of a medium, particularly the absorption and the diffusion. The refraction index is equal to the real part of the complex optical index. The extinction coefficient, also called attenuation coefficient, measures the energy loss of an electromagnetic radiation crossing this material. The extinction coefficient is equal to the opposite of the imaginary part of the complex refraction index. The refraction index and the extinction coefficient of a material may be determined, for example, by ellipsometry. A method of analyzing ellipsometric data is described in the work entitled “Spectroscopic ellipsometry, Principles and Applications” by Hiroyuki Fujiwara, published by John Wiley &amp; Sons, Ltd (2007). 
         [0187]    As an example, the material forming the metal layer of mirror layer  154  (monolayer or multilayer) may be aluminum, silver, chromium, rhodium, ruthenium, palladium, or an alloy of two of these compounds or of more than two of these compounds. 
         [0188]    According to an embodiment, the thickness of each seed pad  24  is smaller than or equal to 20 nm. 
         [0189]    According to an embodiment, the refractive index of each seed pad  24  is in the range from 1 to 3 for a wavelength in the range from 380 nm to 650 nm. 
         [0190]    According to an embodiment, the extinction coefficient of each seed pad  24  is smaller than or equal to 3 for a wavelength in the range from 380 nm to 650 nm. 
         [0191]    As an example, the material forming each seed pad  24  may correspond to the previously-indicated examples. 
         [0192]    Conductive layer  156  may be made of aluminum, of silver, or of any other conductive material. As an example, it has a thickness in the range from 30 nm to 2,000 nm. Conductive layer  156  may be deposited by ECD. Mirror layer  154  and conductive layer  156  may be confounded. 
         [0193]      FIG. 12C  shows the structure obtained after a step of etching conductive layer  156  and mirror layer  154  to delimit a pad  158 , comprising a portion  160  of mirror layer  154  and a portion  162  of conductive layer  156 , connected to electrode layer  36  and a pad  164 , comprising a portion  166  of mirror layer  154  and a portion  168  of conductive layer  156 , connected to seed pads  24 . 
         [0194]      FIG. 12D  shows the structure obtained after the steps of:
       depositing an insulating layer  170  extending on pads  158 ,  164  and between pads  158 ,  164 ;   etching, in insulating layer  170 , an opening  172  exposing conductive pad  158  and an opening  174  exposing conductive pad  164 ; and   depositing a conductive layer  176  covering insulating layer  170  and penetrating into openings  172 ,  174 .       
 
         [0198]    Insulating layer  170  may be made of silicon dioxide deposited by low-temperature PECVD or an organic material of BCB, Epoxy type having a thickness of a few microns, typically 3-5 μm. 
         [0199]    Conductive layer  176  may be made of TiCu or TiAl. As an example, it has a thickness in the range from 500 nm to 2 μm. 
         [0200]      FIG. 12E  shows the structure obtained after a step of etching conductive layer  176  to delimit a conductive pad  178  connected to conductive pad  158 , a second electrode  180  connected to conductive pad  164 , and a conductive portion  182  in contact with insulating layer  170 . Conductive portion  182  may play the role of a radiator. Insulating layer  170  may particularly enable to electrically insulate heat sink  182  from electric contact pad  158  and/or from conductive layer  156 . 
         [0201]    The embodiment described in relation with  FIGS. 12A to 12E  has the advantage of suppressing the series resistance due to substrate  10 . 
         [0202]      FIGS. 13 and 14  respectively are a cross-section view and a top view, partial and simplified, of an embodiment of an optoelectronic device  190  with wires formed on a wafer  10  of a substrate after the step of thinning substrate  10  and before the sawing of substrate  10 . In  FIG. 13 , optoelectronic devices  192  adjacent to optoelectronic device  190  have further been partially shown. 
         [0203]    Each optoelectronic device  190 ,  192  is surrounded with one or a plurality of trenches  194  (two in the present example) filled with an insulating material, which extend across the entire thickness of thinned substrate  10 . As an example, each trench has a width greater than 1 μm, for example, approximately 2 μm. The distance between the two trenches  194  is greater than 5 μm, for example, approximately 6 μm. The sawing lines of substrate  10 , shown by short-dashed lines  196 , are formed between trenches  194  of optoelectronic device  190  and trenches  194  of the adjacent optoelectronic devices  192 . Trenches  194  provide a lateral electric insulation of the silicon substrate and thus of optoelectronic device  190  after sawing. 
         [0204]    As shown in  FIG. 14 , additional trenches  198  connect the external trenches  194  of two adjacent optoelectronic devices  190 ,  192 . After the sawing, a portion  200  of substrate  10  remains at the periphery of each optoelectronic device  190 ,  192 . Trenches  198  enable to divide peripheral portion  200  into a plurality of insulated segments  202 . This enables to decrease short-circuit risks in the case where conductive pads would come into contact with these segments. 
         [0205]    According to an embodiment, the optoelectronic device further comprises phosphors capable, when they are excited by the light emitted by the light-emitting diodes, of emitting light at a wavelength different from the wavelength of the light emitted by the light-emitting diodes. As an example, the light-emitting diodes are capable of emitting blue light and the phosphors are capable of emitting yellow light when they are excited by blue light. Thereby, an observer perceives a light corresponding to a composition of the blue and yellow lights which, according to the proportion of each light, may be substantially white. The final color perceived by the observer is characterized by its chromatic coordinates such as defined by the standards of the International Committee on Illumination. 
         [0206]    According to an embodiment, a layer of phosphors is provided within encapsulation layer  40 . Preferably, the mean diameter of the phosphors is selected so that at least part of the phosphors distribute between wires  26  during the step of forming encapsulation layer  40 . Preferably, the phosphors have a diameter in the range from 45 nm to 500 nm. The phosphor concentration and the thickness of the phosphor layer are then adjusted according to the targeted chromatic coordinates. 
         [0207]    The extraction ratio of an optoelectronic device is generally defined by the ratio of the number of photons escaping from the optoelectronic device to the quantity of photons emitted by the light-emitting diodes. Each light-emitting diode emits light in all directions, and particularly towards the neighboring light-emitting diodes. The active layer of a light-emitting diode tends to capture photons having a wavelength smaller than or equal to the transmission wavelength. Thereby, part of the light emitted by a light-emitting diode is generally captured by the active layers of the neighboring light-emitting diodes. An advantage of arranging phosphors between wires  26  is that the phosphors convert part of the light, for example, blue, emitted by a light-emitting diode into a light at a higher wavelength, for example, yellow, before the blue light reaches the neighboring light-emitting diodes. Since yellow light is not absorbed by the active layers of the neighboring light-emitting diodes, the extraction ratio of the optoelectronic device is increased. 
         [0208]    Another advantage is that since the phosphors are located close to substrate  10 , the discharge by the substrate of the heat generated during the heating of the phosphors in operation is improved. 
         [0209]    Another advantage is that since the phosphors are not arranged in a separate layer, the total thickness of the optoelectronic device is decreased. 
         [0210]    Another advantage is that the homogeneity of the light emitted by the optoelectronic device is improved. Indeed, the light which escapes from encapsulation layer  40  in all directions corresponds to a composition of the light emitted by the light-emitting diodes and of the light emitted by the phosphors. 
         [0211]      FIG. 15  shows an embodiment of an optoelectronic device  205  comprising all the elements shown in  FIG. 2F  and further comprising, between encapsulation layer  40  and handle  42 , a layer of phosphors  206  extending on encapsulation layer  40  and possibly a layer of glue  208  extending of phosphor layer  206 , handle  42  extending on glue layer  208 . The thickness of phosphor layer  206  may be in the range from 50 μm to 100 μm. Phosphor layer  206  may correspond to a layer of silicone or of an epoxide polymer having the phosphors embedded therein. Phosphor layer  206  may be deposited by a spin coating method, by an inkjet printing method, or by a silk-screening method or by a sheet deposition method. The phosphor concentration and the thickness of phosphor layer  206  are adjusted according to the targeted chromatic coordinates. As compared with the embodiment where the phosphors are present in encapsulation layer  40 , phosphors of larger diameter may be used. Further, the phosphor distribution in phosphor layer  206  and the thickness of phosphor layer  206  may be more easily controlled. 
         [0212]      FIG. 16  shows an embodiment of an optoelectronic device  210  comprising all the elements of optoelectronic device  205  shown in  FIG. 15 , with the difference that phosphor layer  206  covers handle  42 . A protection layer, not shown, may cover phosphor layer  206 . In the present embodiment, phosphor layer  206  is advantageously formed in the last steps of the optoelectronic device manufacturing method. The colorimetric properties of the optoelectronic device may thus further be modified during the most part of the optoelectronic device manufacturing method. Further, the colorimetric properties of the optoelectronic device may easily be corrected at the end of the process if necessary by modifying the phosphor layer, for example, by adding an additional phosphor layer. 
         [0213]      FIG. 17  shows an embodiment of an optoelectronic device  215  comprising all the components of optoelectronic device  210  shown in  FIG. 16  and further comprising trenches  216  extending in handle  42  and filled with phosphor layer  206 . Preferably, trenches  216  extend across the entire thickness of handle  42 . The distance between the lateral walls of each trench  216  is preferably substantially equal to the thickness of phosphor layer  206  covering handle  42 . 
         [0214]    For optoelectronic device  210 , shown in  FIG. 16 , part of the light emitted by light-emitting diodes DEL may escape from the lateral edges of handle  42  without having crossed phosphor layer  206 . The color of the laterally-escaping light is thus different from the color of the light having crossed phosphor layer  206 , which may not be desirable if a light of homogeneous color is desired. For optoelectronic device  215 , the light laterally escaping from handle  42  crosses trenches  216  filled with phosphor layer  206 . The light escaping from handle  42 , through surface  43  or laterally, thus advantageously has a homogeneous color. 
         [0215]      FIG. 18  shows an embodiment of an optoelectronic device  220  comprising all the elements of optoelectronic device  205  shown in  FIG. 15 , with the difference that glue layer  208  is not shown and that an intermediate layer  222  is interposed between encapsulation layer  40  and phosphor layer  206 . 
         [0216]    Intermediate layer  222  is capable of letting through the light rays emitted by light-emitting diodes DEL at a first wavelength or in a first wavelength range and of reflecting the light rays emitted by the phosphors at a second wavelength or in a second wavelength range. The extraction ratio of optoelectronic device  220  is then advantageously increased. As an example, intermediate layer  222  may correspond to a dichroic mirror, which is a mirror reflecting light rays having a wavelength within a certain range and letting through light rays having a wavelength which does not belong to this range. A dichroic mirror may be formed of a stack of dielectric layers having different optical indexes. 
         [0217]    According to another example, intermediate layer  222  may be a monolayer made of a material having a refractive index smaller than the refractive index of encapsulation layer  40  and smaller than the refractive index of the phosphor layer. Intermediate layer  222  may correspond to a silicone or epoxide polymer layer. Further, a surface treatment, called texturing, is applied to surface  224  of encapsulation layer  40  before the forming of intermediate layer  222  to form raised areas on surface  224 . Interface  226  between intermediate layer  222  and phosphor layer  206  is substantially planar. The light rays emitted by light-emitting diodes DEL cross interface  224  which is irregular even if the refractive index of intermediate layer  222  is smaller than the refractive index of encapsulation layer  40  while the light rays emitted by the phosphors mainly reflect on interface  226 , given that interface  226  is planar and that the refractive index of intermediate layer  222  is smaller than the refractive index of phosphor layer  206 . 
         [0218]    A texturing method causing the forming of raised areas at the surface may be applied to free surface  43  of handle  42  and/or to surface  228  of phosphor layer  206  in contact with handle  42 . 
         [0219]    For a layer made of an inorganic material, the method of texturing a surface of the layer may comprise a chemical etching step or a mechanical abrasion step, possibly in the presence of a mask protecting portions of the treated surface in order to promote the forming of raised areas at the surface. For a layer made of an organic material, the method of texturing a surface of the layer may comprise a step of embossing, moulding, etc. 
         [0220]    For the previously-described optoelectronic devices, part of the light emitted by light-emitting diodes DEL may escape through the lateral edges of encapsulation layer  40 . This is generally not desirable since this light is not perceived by an observer in normal operating conditions of the optoelectronic device. According to an embodiment, the optoelectronic device further comprises means capable of reflecting the light rays laterally escaping from the optoelectronic device to increase the quantity of light escaping from surface  43  of handle  42 . 
         [0221]      FIG. 19  shows an embodiment of an optoelectronic device  230  comprising all the components of the optoelectronic device shown in  FIG. 2F  and further comprising blocks  232  arranged on insulating layer  32  and at least partially surrounding the assembly of light-emitting diodes DEL. Each block  232  is covered with a metal layer  234 , for example corresponding to an extension of conductive layer  38 . As an example, blocks  232  may correspond to resist blocks formed on insulating layer  32  before the deposition of encapsulation layer  40 . Preferably, the height of blocks  232  is smaller than the maximum height of encapsulation layer  40 . In  FIG. 19 , lateral edges  236  of blocks  232  are substantially perpendicular to surface  22  of substrate  10 . As a variation, lateral sides  236  may be inclined with respect to surface  22  to promote the reflection of light rays towards surface  43  of handle  42 . 
         [0222]      FIG. 20  shows an embodiment of an optoelectronic device  240  comprising all the components of the optoelectronic device shown in  FIG. 2F  and further comprising blocks  242  arranged on insulating layer  32  and at least partially surrounding the assembly of light-emitting diodes DEL. Blocks  242  are made of a reflective material. It may be silicone filled with reflective particles, for example, titanium oxide particles (TiO2). As an example, blocks  242  may be formed on insulating layer  32  by a silk-screening method before the deposition of encapsulation layer  40 . Preferably, the height of blocks  242  is smaller than the maximum height of encapsulation layer  40 . In  FIG. 20 , lateral edges  244  of blocks  242  are substantially perpendicular to surface  22  of substrate  10 . As a variation, lateral edges  244  may be inclined with respect to surface  22  to promote the reflection of light rays towards surface  43  of handle  42 . 
         [0223]      FIG. 21  shows an embodiment of an optoelectronic device  245  comprising all the components of the optoelectronic device shown in  FIG. 2F , with the difference that light-emitting diodes DEL are formed in a cavity  246  formed in substrate  10 . Lateral sides  248  of cavity  246  are covered with an insulating layer  250 , for example corresponding to an extension of insulating layer  32 , and with a metal layer  252 , for example corresponding to an extension of conductive layer  38 . Preferably, the depth of cavity  246  is smaller than the maximum height of encapsulation layer  40 . In  FIG. 21 , lateral sides  248  of the cavity are substantially perpendicular to surface  43  of handle  42 . As a variation, lateral sides  248  may be inclined with respect to surface  43  to promote the reflection of light rays towards surface  43  of handle  42 . 
         [0224]      FIG. 22  shows an embodiment of an optoelectronic device  255  comprising all the components of the optoelectronic device shown in  FIG. 2F  and further comprising trenches  256  surrounding light-emitting diodes DEL, a single trench being shown in  FIG. 22 . Trenches  256  cross substrate  10  and encapsulation layer  40 . The internal walls of each trench  256  are covered with a reflective layer  258 , for example, a metal layer, for example, made of silver or aluminum or a varnish layer, having a thickness in the range from 30 nm to 2,000 nm. An insulating layer, not shown, may be provided to insulate reflective layer  258  from substrate  10 . Trenches  256  may be formed after the step of thinning substrate  10  previously described in relation with  FIG. 2D . An advantage over optoelectronic devices  230 ,  240 , and  245  is that encapsulation layer  40  may be formed on a planar surface, which makes its deposition easier. 
         [0225]      FIG. 23  shows an embodiment of an optoelectronic device  260  comprising all the components of the optoelectronic device shown in  FIG. 2F  and further comprising trenches  262  formed in encapsulation layer  40  and surrounding light-emitting diodes DEL, a single trench being shown in  FIG. 23 . Trenches  262  may be filled with air. Trenches  262  may be formed by etching after the step of forming encapsulation layer  40  in the case where encapsulation layer  40  is made of an inorganic material. Trenches  262  delimit, in encapsulation layer  40 , a central block  264  having the light-emitting diodes embedded therein and peripheral blocks  266  at least partially surrounding central block  264 . Each peripheral block  266  is covered with a metal layer  268 , for example, made of silver or aluminum and having a thickness in the range from 30 nm to 2,000 nm. A glue layer  269  may be provided between handle  42  and blocks  264 ,  266 . In  FIG. 23 , lateral sides  270  of peripheral blocks  266  are substantially perpendicular to surface  22  of substrate  10 . As a variation, lateral sides  270  may be inclined with respect to surface  22  to promote the reflection of light rays towards surface  43  of handle  42 . An advantage over optoelectronic devices  230 ,  240 , and  245  is that encapsulation layer  40  may be formed on a planar surface, which makes its deposition easier. 
         [0226]      FIG. 24  shows an embodiment of an optoelectronic device  275  comprising all the components of the optoelectronic device shown in  FIG. 2F  and further comprising an insulating layer  276  extending on electrode layer  32  between light-emitting diodes DEL, without covering light-emitting diodes DEL. Insulating layer  276  is covered with a reflective layer  278 . Reflective layer  278  preferably corresponds to a metal layer, for example, made of aluminum, of an aluminum-based alloy, particularly AlSiz, AlxCuy (for example, with x equal to 1 and y equal to 0.8%), of silver, gold, nickel, or palladium. As an example, reflective layer  278  has a thickness in the range between 30 nm and 2,000 nm. Reflective layer  278  may comprise a stack of a plurality of layers, particularly comprising a bonding layer, for example, made of titanium. The thicknesses of insulating layer  276  and of reflective layer  278  are selected to that surface  280  of the reflective layer in contact with encapsulation layer  40  is close to the end of shell  34 , for example, less than 1 μm away from the end of shell  34 . As compared with the previously-described embodiments, reflective surface  280  advantageously enables to avoid for light rays emitted by shell  34  of a light-emitting diode DEL to the outside of the light-emitting diode to penetrate into lower portion  28  of the light-emitting diode or lower portions  28  of the neighboring light-emitting diodes. The extraction ratio is thus increased. 
         [0227]      FIG. 25  shows an embodiment of an optoelectronic device  285  comprising all the components of optoelectronic device  275  shown in  FIG. 24 , with the difference that insulating layer  276  and reflective layer  280  are replaced with a reflective layer  286  extending on electrode layer  32  between light-emitting diodes DEL, without covering light-emitting diodes DEL. It may be a silicone layer filled with reflective particles, for example, TiO2 particles, or a TiO2 layer. The thickness of reflective layer  286  is selected so that surface  288  of reflective layer  286  in contact with encapsulation layer  40  is close to the end of shell  34 , for example, less than 1 μm away from the end of shell  34 . The extraction ratio is thus increased. 
         [0228]    According to an embodiment, one or a plurality of lenses are provided on surface  43  of handle  42 . The lenses enable to increase the focusing of light rays escaping from surface  43  along the direction perpendicular to surface  43  and thus to increase the quantity of light rays perceived by a user watching surface  43 . 
         [0229]      FIG. 26  shows an embodiment of an optoelectronic device  290  comprising all the components of optoelectronic device  230  shown in  FIG. 19 , with the difference that handle  42  is not present. Further, optoelectronic device  290  comprises, for each light-emitting diode DEL, a converging lens  292  arranged on encapsulation layer  40 . 
         [0230]      FIG. 27  is a view similar to  FIG. 26  of an embodiment  295  where a lens  296  is associated with a plurality of light-emitting diodes DEL. 
         [0231]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. Further, although, in the previously-described embodiments, each wire  26  comprises a passivated portion  28  at the base of the wire in contact with one of seed pads  24 , passivated portion  28  may be absent. 
         [0232]    Further, although embodiments have been described for an optoelectronic device for which shell  34  covers the top of the associated wire  26  and a portion of the lateral sides of wire  26 , it is possible to only provide the shell at the top of wire  26 .