Abstract:
Described herein is a transparent device for display of information superimposed on a background, said device comprising a plurality of LED sources, addressable individually or in groups through a series of conductive paths deposited on a transparent underlayer and connected to a control electronics, in which: i) said LED sources are integrated in the form of dice, i.e., of elements obtained by dividing up a semiconductor wafer and without package; and ii) at least one of said conductive paths is with interrupted stretches and replaced by stretches of metal wire, bonded to said paths through a wire-bonding operation.

Description:
This application claims priority from European Patent Application No. 05425255.6, filed on Apr. 21, 2006, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device for the presentation of information, characterized in that it is at the same time transparent, i.e., it enables vision of the scene at the rear of the device, and emissive, i.e., it presents the user with information in the form of light emitted by the device itself. 
     A device of this type is of particular interest for the vehicle sector in so far as it enables presentation of the information on two different planes, namely, the background, i.e., the external scene visible to the user through the windows of the vehicle, such as the windscreen, rear window and side windows, and the light image generated by the device. 
     The interest of this type of devices is not limited, however, to the vehicle sector; the advertising sector or that of information to the public are just two examples of the possible fields of application of the device. 
     Transparent devices based upon light-emitting diodes (LEDs) are already present in the known art; for example, the patent application No. EP1460609 describes a solution for transparent and programmable LED displays comprising: an array of LEDs in a row-column configuration, each row of said LEDs being connected by an electric wire, each column being connected to a transparent-conductive-oxide (TCO) layer deposited on the underlayer, said transparent layer being electrically insulated from said row of wires; and means for electrical turning-on associated to said array. 
     The use of metal wires for interconnecting the LEDs of each row enables a reduction in the voltage drops due to the electrical resistance of the row and hence use of decidedly lower supply voltages. Said metal wires are bonded on the underlayer of TCO only in areas corresponding to purposely provided pads obtained by cutting the layer of TCO along a closed circular path, so as to insulate said pads electrically from the corresponding column of TCO. 
     The solution described in the patent application No. EP1460609 presents a series of limitations:
         the transparency is limited on account of the dimensions of the LED components in SMD format, which becomes particularly critical if the aim is to increase the density of the LEDs or reduce the dimensions of the device;   the process steps regarding the positioning of the wires constituting the rows and the corresponding operations of bonding on the pads are not obtainable using standard technologies and machinery for SMD technology, thus rendering fabrication of the device more costly;   if the dimensions of the device are reduced, the technological steps described above become more complex and the production yields decrease; and   the formation of insulated pads along the columns of TCO produces a local narrowing of the paths and hence an increase in the in-series resistance and thus limits the number of addressable rows.       

     SUMMARY OF THE INVENTION 
     In order to overcome the drawbacks described above, the subject of the invention is a display according to claim  1 . Further advantageous characteristics are indicated in claims  2 - 19  and  25 - 36 . Also forming the subject of the invention is a method for making the display, having the characteristics indicated in claim  20 . 
     The present invention is consequently able to overcome the drawbacks of the known art through:
         the use of LED sources in the form of dice, i.e., of semiconductors not provided with package; this enables a reduction to the minimum of the dimension of the source and an increase in the transparency of the display; and   the use of the wire-bonding process for electrical connection of groups of dice between one another.       

     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 typical applications are light signalling devices, headlights or other lights for motor vehicles, devices for information to the public, etc. 
     The technique for fabrication of said devices goes under the name of chip-on-board (COB) technology and consists in the mounting of arrays of LED chips directly on an appropriate underlayer. Said technology comprises first the process known by the term “die-bonding” (i.e., thermal 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 the die-bonding techniques, flip-chip methodology envisages the turning of the chip upside down and electro-thermal connection to the circuit of its pads without using wires for the electrical connection, thus excluding a further 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 packaging or protection of the source from the external stresses using appropriate resins. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the annexed plate of drawings, in which: 
         FIGS. 1-4  illustrate a preferred embodiment of the invention; 
         FIGS. 5-7  illustrate an alternative embodiment; 
         FIGS. 8 and 9  illustrate two further variants; and 
         FIGS. 10 and 11  illustrate a further variant. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the preferred embodiment represented in  FIGS. 1 ,  2 ,  3  and  4 , the LED sources are arranged in a matrix configuration, in which each LED is positioned at the point of crossing-over of a row  20  and a column  10 , in such a way as to be addressable individually through the application of an appropriate potential difference between said row and said column. 
     In the solution represented in  FIGS. 1 and 2 , the process of fabrication comprises the following steps: 
     1) preparation of the (glass or plastic) underlayer coated with transparent conductive oxide (for example, indium-tin oxide—ITO); 
     2) separation of the columns of TCO  10  (through a photolithographic process or a laser-ablation process); 
     3) formation of metal pads, two for each die, of which one,  11 , at least in part set on top of the respective column of TCO and the other,  22 , set between said column of TCO and the adjacent column of TCO, in areas corresponding to the row; the formation of the pads can be obtained, for example, through a technique of shadow masking, or else through a uniform evaporation of the metal and a subsequent removal via photolithography; this step is necessary for guaranteeing a good adhesion of the metal bond on the underlayer; 
     4) pick-and-place of the die  2  on the respective metal pad  22 ; 
     5) die-attach of the die  2  with electrically and thermally conductive resin; 
     6) wire bonding between the top electrode of each individual die and the respective metal pad  11 ; 
     7) wire bonding between the metal pad  22  of each individual die and the metal pad  22  of the subsequent die along the same address row; and 
     8) protective coating with transparent resin (for example, silicone or epoxy resin) and overlayer (of glass or plastic). 
     The use of an overlayer  1 ′ (of glass or plastic) has the function of guaranteeing 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 view of the background and/or will not introduce optical power. 
     In the embodiment described above (represented in  FIGS. 1 and 2 ), the dice are of the vertical-electrode type, i.e., the anode and cathode are arranged, respectively, on the two opposite faces of the die, in such a way that the electrical contact with the rows  20  is guaranteed by the conductive resin used for the die-attach, where the electrical contact with the columns  10  of TCO is guaranteed by the connection via wire bonding between the electrode on the top face of the die and the respective pad  11 . 
     In a further embodiment, represented in  FIG. 3 , it is possible to use dice with horizontal electrodes, i.e., ones on the same face; in this case, the dice are positioned with both of the electrodes on the top face and the die-attach operation only has the function of bonding the die to the underlayer and guaranteeing thermal dissipation, but not guaranteeing electrical conduction. 
     The steps of the process described above are replaced by the following steps: 
     1) preparation of the (glass or plastic) underlayer coated with transparent conductive oxide (TCO; for example, indium-tin oxide—ITO); 
     2) separation of the columns of TCO  10  (through a photolithographic process or a laser-ablation process); 
     3) formation of metal pads, 3 for each LED chip, of which the first ( 11 ) at least in part overlying the respective column of TCO, the second ( 12 ) and the third ( 22 ) set between said column of TCO and the adjacent column of TCO, in areas corresponding to the row; the formation of the pads can be obtained, for example, through a technique of shadow masking, or else through a uniform evaporation of the metal and a subsequent removal via photolithography; this step is necessary for guaranteeing a good adhesion of the metal bond on the underlayer. 
     4) pick-and-place of the die on the respective metal pad  12 ; 
     5) die-attach of the die with thermally conductive resin; 
     6) wire bonding between the electrodes of one and the same type (for example, the cathodes) of each individual die and the respective pads  11 ; 
     7) wire bonding between the electrodes of the other type (for example, the anodes) and the respective pads  22 ; 
     8) wire bonding between the metal pad  22  of each individual LED chip and the metal pad  22  of the subsequent LED chip along the same address row; 
     9) protective coating with transparent resin (for example, silicone or epoxy resin) and overlayer (glass or plastic). 
       FIG. 4  illustrates a variant of  FIG. 2 , in which an additional insulating layer  14  is provided in the central part of the columns, in such a way as to prevent the wire from coming into contact with the columns  10  of TCO, on account of the pressure exerted on the wire used for wire bonding during the process of packaging, and from thus impairing operation of the corresponding row and column. 
     In a further embodiment, represented in  FIGS. 5 ,  6  and  7 , 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 or in series. 
       FIG. 5  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 or in series. Each segment, which is addressable in an independent way, can be constituted by a pair of parallel paths ( FIGS. 6 and 7 ), 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 interrupting the paths  20  of one of said segments so as to enable the passage of the paths  10  of another of said segments and by connecting the stretches  22  of said paths  20  with the stretches  21  of conductive wire deposited through a wire-bonding process ( FIGS. 6   a ,  6   b ,  7 ). 
       FIG. 6   b  presents a variant, in which an insulating layer is deposited on top of the paths  10  in areas corresponding to said points of intersection  35  so as to prevent said stretches of wire  21  from coming into electrical contact with said paths  10  on account of a deformation of the wire. 
     A further variant of the present invention is represented in  FIG. 8 . In this variant one of the two segments is constituted by a set of LEDs in series, whilst the other is constituted by a set of LEDs in parallel. In the point of intersection  35  between the two segments, the electrical insulation between the path  20  of the first segment and the paths  10  of the second segment is obtained in a way similar to what is indicated with reference to  FIGS. 6 and 7 . 
     A further variant of the present invention is represented in  FIG. 9 . In this variant, 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  10  of the second segment is obtained in a way similar to what is described with reference to  FIGS. 6 and 8 . 
     Again with reference to  FIG. 5 , 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. 
     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. 10 and 11 ) the aim is to provide on the external surface of the underlayer  1  and/or of the overlayer  1 ′, in areas corresponding to the dice, appropriate micro-indentations  36  along the line of connection of the dice, said micro-indentations  36  having the function of extracting light from the underlayer  1  and/or from the overlayer  1 ′ so as to connect the points of light and generate light images in the form of continuous lines. 
     Said effect can be further strengthened 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  10 ,  20  that connect the sources, in the case where said paths are made of metal instead of TCO). In fact, said pads tend to reflect part of the light emitted by the lateral surfaces of the die; the reflected light impinges upon the micro-indentations  36 , which produces an increase in the actual dimensions of the source. 
     A further solution that can be adopted is to deposit the protective resin  1 ″ in the form of paths that connect the different dice. The light emitted by the dice 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  1 ′ is used, on the surface of said overlayer. 
     Said micro-indentations  36  may be in the form of cylindrical microlenses with axis perpendicular to the line of connection of the dice ( FIG. 10   a ), generic grooves made along an axis perpendicular to the line of connection of the dice ( FIG. 10   b ), or cylindrical lenses (one for each die) with axis perpendicular to the line of connection of the dice ( FIG. 10   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  1  and passing through the centre of one of said dice. 
     The micro-indentations  36  can also be simply areas with high roughness, such as to diffuse the light emitted by the LEDs. 
     According to a further variant of the present invention, said effect of dashed appearance of the image can be reduced or eliminated using a density of LED sources, i.e., a number of dice 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. 
     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 LEDs is approximately 1 minute of arc, equal to the resolution of the eye in the fovea. 
     It is known, however, how the eye will tend to merge points angularly separated by up to 3 minutes of arc, which would enable the spacing between the pixels to be increased up to 1 mm, thus reducing the number of sources necessary by a factor of 3.