Abstract:
A method of fabrication of transparent LED devices, of the type comprising the operations of: i) providing a series of conductive paths on a transparent underlayer; ii) connecting said conductive paths to electronic control means; iii) associating to said underlayer an array of LED sources addressable individually or in groups through said conductive paths, in which i) said LED sources are integrated in the form of chips, i.e., of elements obtained by dividing up a semiconductor wafer and without package, via technologies of the chip-on-board type; ii) said method envisages the use of the flip-chip technique for die bonding, i.e., the electrical connection of the chip to the underlayer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method for the fabrication of transparent displays using light-emitting diodes (LEDs), in which the following operations are envisaged:  
         [0002]     providing a completely transparent underlayer with conductive paths;  
         [0003]     associating to said underlayer an array of LEDs in the form of chips by means of an operation of die bonding, in particular using the flip-chip technique;  
         [0004]     using an anisotropic conductive paste for connection of the LED chips to the underlayer; and  
         [0005]     protecting the light-emitting devices from mechanical and external environmental stresses via the deposition on the chip of a packaging material.  
         [0006]     As is known, LED sources can be integrated directly in the form of chips or dice (multilayer semiconductor elements that emit light radiation if electrically supplied on a printed circuit. Some possible applications are light-signalling devices, headlights or other lights for motor vehicles, devices for providing information to the public, etc.  
         [0007]     The technique for the fabrication of said devices goes under the name of chip-on-board (COB) technology and consists in mounting an array of LED chips directly on an appropriate underlayer. Said technology comprises first the process known by the term “die bonding” (thermal connection or electro-thermal connection of the die to the underlayer), associated to which are possible operations of wire bonding (electrical connection of the chip to the circuit). Amongst die-bonding techniques, the flip-chip methodology envisages turning-over of the chip and electro-thermal connection to the circuit of its pads without using wires for the electrical connection, thus excluding an additional wire-bonding process. In the flip-chip process, the connections of the pads are typically obtained by means of metal bumps (balls). As final step, the COB process envisages the packaging or protection of the source from the external stresses by means of appropriate resins.  
         [0008]     Represented in  FIG. 1  is a LED in the form of chip  10  with both of the metal connections (pads)  11  on the top surface of the die connected to a generic printed circuit  12 , which carries conductive paths  13 , by means of the wire-bonding technique (a) and flip-chip technique (b). In the first case (a), the electrical connections between the die and the circuit are made via metal wires  14 ; in the second case (b), the die  10  is turned upside down, and the metal pads  11  are directly connected to the paths  13  of the circuit.  
         [0009]     There is known the technique of fabrication of devices using LED chips by means of the consolidated technology of wire bonding for the electrical connection of the chip to the transparent underlayer. In the case where the aim is to provide a transparent device, it is necessary to use a transparent underlayer (for example, plastic or glass) and conductive paths that are also transparent (made, for example, of transparent conductive oxide or TCO). In this case, however, in addition to the transparent conductive paths made of TCO, it is necessary to provide isles made of metal (usually gold, Au), to enable bonding of the wire to the underlayer, with the consequent following disadvantages:  
         [0010]     lower transparency of the device on account of the presence of the metal isles;  
         [0011]     longer time and higher costs of fabrication in so far as it is necessary to introduce a step of selective deposition of the metal pads and a wire-bonding step;  
         [0012]     need for protecting also the metal wires used for wire bonding with the packaging resin to prevent breaking thereof; and  
         [0013]     light emission of the LED chip in part masked by the wires.  
         [0014]     The wire-bonding step can moreover lead to a reduction of the production yields, following upon detachment or breaking of the wire during the process of packaging.  
       SUMMARY OF THE INVENTION  
       [0015]     The purpose of the present invention is to overcome the aforesaid problems using the flip-chip technique for connection of the LED chips to the underlayer.  
         [0016]     According to the invention, said purpose is achieved through a method according to claim  1 . The subject of the invention is also a device obtained with said method and preferably having the characteristics of one or more of claims  11 - 30 .  
         [0017]     The realization of said invention envisages the use of LED chips with both of the metal pads on the same face (usually the bottom face, if mounted in a flip-chip configuration) and with transparent underlayer (usually SiC or Al 2 O 3 ). These types of chips are designed in such a way that light emission is favoured in the flip-chip configuration, see  FIG. 1 , detail (b).  
         [0018]     According to a preferred embodiment, described in claim  2 , the present invention envisages the use of anisotropic electrically conductive paste for the electrical connection of the chip directly to the paths. The anisotropy of the paste means that electrical conduction will occur only in a direction substantially perpendicular to the underlayer  15 , thus preventing the two electrodes of the LED chip, albeit both physically in contact with the same drop of paste, from being mutually short-circuited.  
         [0019]     Unlike the known art of die bonding, which uses metal bumps, the technique of bonding using anisotropic conductive resin does not require the use of metal pads. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The invention will now be described with reference to the annexed plate of drawings, in which:  
         [0021]      FIG. 1  illustrates the known solution described above; and  
         [0022]      FIGS. 2-10  illustrate different variants of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIGS. 2   a,    2   b,    2   c  are schematic illustrations of the process of fabrication of the device according to a preferred embodiment. The process involves the following steps:  
         [0024]     1. fabrication of a transparent underlayer  15  with conductive paths  16  made of TCO ( FIG. 2   a );  
         [0025]     2. dispensing of the anisotropic paste in areas corresponding to the positions envisaged for the LED chips  10 ;  
         [0026]     3. positioning of the LED chips  10  with metal pads facing the underlayer  15  (flip-chip) in areas corresponding to the path of TCO  16  ( FIG. 2   b );  
         [0027]     4. thermal curing of the anisotropic paste; and  
         [0028]     5. packaging of the individual chip (or set of chips) with appropriate resin for increasing the light emission thereof and for protecting it from external stresses; as may be seen from  FIG. 2   c,  each of the chips is coated with a so-called glob-top  17 , i.e., a resinous package, substantially in the form of a dome.  
         [0029]     According to a variant of the present invention, the use of a (glass or plastic) overlayer  15 ′ is envisaged. Said overlayer  15 ′ has the function of guaranteeing the planarity of the protective layer of transparent resin, not only in order to ensure transparency of the device but also to ensure that the panel will not distort the vision of the background and/or will not introduce optical power.  
         [0030]     In this case, the glob-top  17  is replaced by a continuous layer  17 ′ of resin that remains encapsulated between the underlayer  15  and the overlayer  15 ′ ( FIG. 9 ).  
         [0031]     From the foregoing description, the advantages of the method proposed emerge clearly. The biggest advantage lies in the excellent transparency of the device: only in areas corresponding to the LED chips (typically having a surface area of 0.1 mm 2 ) will dark areas be formed. Furthermore, with this method it is possible to obtain displays with higher densities of points of light, eliminating the space necessary for the electrical connections via wires.  
         [0032]     The conductive paths made of TCO can be replaced by metal paths. This enables reduction of the costs of the device, even though the transparency of the device proves evidently lower. The lower resistivity of metal as compared to TCO enables, given the same thickness and width of the paths, reduction of the supply voltages, or else, given the same supply voltage, reduction of the dimensions of the paths.  
         [0033]     In a further preferred embodiment, represented in  FIG. 3 , the LED chips are arranged in a matrix configuration, in which each LED is positioned at the point of crossing-over between a row  20  and a column  30 , said row  20  and column  30  being constituted by paths of conductive material, in such a way that each single LED chip is addressable individually through the application of an appropriate potential difference between said row and said column.  
         [0034]     The rows are electrically insulated from the columns through a layer  40  of electrically insulating material, for example, silicon oxide, deposited on said rows  20  (for example, through an operation of thermal evaporation, e-beam evaporation, sputtering, CVD, spinning, dipping, etc.).  
         [0035]     Subsequently deposited on said layer  40  are the columns  30 , constituted by electrically conductive material, for example, a metal or a transparent conductive oxide TCO.  
         [0036]     In a preferred embodiment represented in  FIGS. 3   b  and  3   c,  the layer  40  of electrically insulating material is removed only in areas corresponding to purposely provided pads  21 , one for each LED, so as to enable access to the underlying row  20  of conductive material. Finally, according to the present invention, the LED chips are positioned on the underlayer by means of the flip-chip technique, in such a way that one of the two electrodes is in an area corresponding to a pad  21  and the other electrode in an area corresponding to a column  30  of conductive material. The electrical contact between the electrodes and the respective conductive paths is obtained with the anisotropic conductive resin  18 .  
         [0037]     In a variant of this embodiment, represented in  FIGS. 4   a,    4   b  and  4   c,  the layer  40 , as well as in the areas corresponding to said pads  21 , is also removed in areas corresponding to the spaces located between adjacent rows  20 .  
         [0038]     In a further embodiment, represented in  FIGS. 5   a  and  5   b , the layer  40  is removed from the entire underlayer except for purposely provided pads positioned at the point of crossing-over between said rows  20  and said columns  30 .  
         [0039]     In another embodiment, represented in  FIG. 6 , the layer  40  is removed in areas corresponding to the spaces set between adjacent columns  30 .  
         [0040]     In yet a further embodiment, represented in  FIGS. 7 and 8 , the LEDs are not arranged in a matrix configuration (i.e., individually addressable), but rather are addressable in groups, with the LEDs of each group electrically connected together in parallel ( FIG. 8   a ) or in series ( FIG. 8   b ).  
         [0041]      FIG. 7  illustrates an example of an image that can be presented on a display according to the present invention. Each segment ( 31 ,  32 ,  33  and  34 ) represents a set of LEDs electrically connected in parallel ( FIG. 8   a ) or in series ( FIG. 8   b ).  
         [0042]     In the embodiment represented in  FIG. 8   a,  each segment ( 20  and  30 ), which is addressable in an independent way, is constituted by a pair of parallel paths, one of which is electrically connected to the electrodes of the same type (for example, the cathodes) of a parallel of LEDs, whilst the other is electrically connected to the electrodes of the other type (for example, the anodes). In the points of intersection  35  between two or more segments it is necessary to insulate electrically the paths belonging to different segments. This is obtained, according to the present invention, by depositing on the paths  20  of one of said segments, in areas corresponding to the point of intersection  35 , a pad  40  of electrically insulating material, on which the paths  30  of the second of said segments are subsequently deposited.  
         [0043]     Presented in  FIG. 8   b  is a variant in which both of the segments are constituted by a set of LEDs connected together in series. In the point of intersection  35  between the two segments, the electrical insulation between the path  20  of the first segment and the path  30  of the second segment is obtained in a way similar to what has been described with reference to  FIG. 8   a.    
         [0044]     Once again with reference to  FIGS. 7 and 8 , it is evident that the smaller the number of LEDs used to obtain the segments  31 ,  32 ,  33  and  34 , the more the appearance of said segments of the image will be dashed.ccc  
         [0045]     In order to limit the number of LED sources, at the same time reducing this effect of dashed appearance of the image, according to a variant of said invention ( FIGS. 9 and 10 ) the aim is to provide on the outer surface of the underlayer  15  and/or of the overlayer  15 ′, in areas corresponding to the LED chip, appropriate micro-indentations  36  along the connection line of the LED chip, said micro-indentations  36  having the function of extracting light from the underlayer  15  and/or from the overlayer  15 ′, so as to connect the points of light and generate light images in the form of continuous lines.  
         [0046]     The above effect can be further reinforced by the metal pads that are deposited on the paths made of TCO for the purpose of improving adhesion of the bonding operations (or else directly by the conductive paths  20 ,  30  that connect the sources, in the case where said paths are made of metal instead made of TCO). In fact, said pads tend to reflect part of the light emitted by the lateral surfaces of the LED chip; the light reflected impinges upon the micro-indentations  36 , which produce an increase in the effective dimensions of the source.  
         [0047]     A further solution that can be adopted is to deposit the protective resin  17 ′ in the form of paths that connect the different LED chips. The light emitted by the LED chips is thus in part entrapped by said paths of resin (light-guide effect) and subsequently extracted by purposely provided micro-indentations  36  made on the surface of said path of resin, or else, in the case where an overlayer  15 ′ is used, on the surface of said overlayer.  
         [0048]     Said micro-indentations  36  can be in the form of cylindrical microlenses with axis perpendicular to the connection line of the LED chip ( FIG. 9   a ), generic grooves made along an axis perpendicular to the connection line of the LED chip ( FIG. 9   b ), cylindrical lenses (one for each chip) with axis perpendicular to the line of connection of the chips ( FIG. 9   c ). Alternatively, said micro-indentations can be in the form of microlenses with rotational symmetry, each microlens having its axis of symmetry perpendicular to said underlayer  15  and passing through the centre of one of said LED chips.  
         [0049]     The micro-indentations  36  may also be simply areas with high roughness, such as to diffuse the light emitted by the LEDs.  
         [0050]     According to a further variant of the present invention, the aforesaid effect of dashed appearance of the image can be reduced or eliminated using a density of LED sources, i.e., a number of LED chips per unit length, such that the angular separation between two sources with respect to the eye of the user is comparable with the angular resolution of the eye.  
         [0051]     By way of example, if the display is installed at a distance of 1 m from the driver and the distance between two adjacent dice is 0.3 mm, i.e., comparable with the dimensions of the die, the angular separation between the LED appears as approximately 1 minute of arc, equal to the resolution of the eye in the fovea.  
         [0052]     It is known, however, how the eye tends to merge points that are angularly separated by up to 3 minutes of arc, which would enable the spacing between the pixel up to 1 mm to be increased, thus reducing the number of sources necessary by a factor of 3.