Patent
1990-05-15
1991-10-22
Mintel, William
357 68, 357 45, 357 16, 357 71, H01L 3300
Patent
active
050600273
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns improvements in or relating to the manufacture of light emitting diode (LED) arrays, more particularly arrays comprised of many close-spaced diode elements.
There are many modern-day applications that require displays having a large number of display elements (pixels) on a fine geometry for use in optical systems. To provide good imagery requires as many as 500.times.500 pixels but it is possible to produce recognizable pictures with a much lower pixel count. The conventional cathode ray tube (CRT) can give extremely good definition but is bulky and requires high power and high voltages. Alternative display technologies can be more compact but also have their own particular problems. Passive liquid crystal device (LCD) displays use little power, but can be difficult to address and multiplex and also require back-lighting for use in the dark.
LED arrays made by hybrid techniques are relatively coarse in geometry, it being difficult to achieve a closer pitch than 0.5 mm. For finer geometry arrays in LED'S it is necessary to fabricate monolithic structures but this presents difficulties in addressing, since without specialized isolation techniques all the LED's in a monolithic structure have a common cathode and require individual connections. For fine geometries and for large numbers of pixels interconnection by conventional wirebonding is quite impracticable.
The present invention is intended to provide a remedy and thus enables the provision of an array of close-spaced elements complete with connections to LED driver circuitry.
The solution to the aforesaid problem is based upon:
a) the fabrication of the LED array in a material with a transparent substrate so that the array can be viewed from the reverse side; and
b) making the interconnections to the array by `flip-chipping` the array directly onto the silicon driving circuitry.
In accordance with a first aspect of this invention there is provided a light emitting diode array comprising an optically transparent substrate (1) of semiconductor material in which a multiplicity of light emitting diodes (10) are defined on one surface of the substrate, each of said diodes (10) having an electrical contact (4) on one surface of the substrate, said substrate (1) being provided with a common electrode of an optically transparent material for all the diodes (10), a further substrate (6) in which a multiplicity of driver circuits are defined corresponding to the number and positions of the light emitting diodes 10 on the substrate (1), each driver circuit having a corresponding contact and wherein the contacts of the light emitting diodes and the corresponding contacts of the driver circuits are in alignment and in electrical soldered contact.
In the foregoing array, light is transmitted in all directions from each addressed diode. That light which is transmitted in a reverse direction, that is, into the body of the optically transparent substrate, is used to provide the visual display.
In accordance with a second aspect of this invention there is provided a method of manufacturing a light emitting diode array, which comprises providing a semiconductor substrate (1) of optically transparent material having a multiplicity of light emitting diodes (10), and a common electrode of an optically transparent material for all of said diodes, each diode having an electrical contact (4) on one surface of the substrate, providing a further substrate (6) in which a multiplicity of contacts are defined corresponding to the number and positions of the light emitting diodes (10) on the substrate (1), depositing a gauged amounted of solder upon each contact of at least one of the aforesaid substrates, winging the substrates into surface contact which the electrical contact of each facing and in alignment, and reflowing the solder to form solder bump (8) connecting the light emitting diodes to the corresponding driver circuits.
The foregoing solution has additional advantages in LED performance. In conventional fine geometry LED structures driven at
REFERENCES:
patent: 4021838 (1977-05-01), Warwick
patent: 4074299 (1978-02-01), Kusano et al.
patent: 4122479 (1978-10-01), Sugawara et al.
patent: 4990971 (1991-02-01), Le Creff
Gooding John
Hart Peter B.
Mintel William
Plessey Overseas Limited
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