"A Diode Array"
The invention relates to a diode array and more particularly to an array of light emitting diodes (LED) and a method for producing and interfacing such an array.
For the purposes of this specification the term array is taken to include a single light emitting diode. A potential difference across the terminals of an LED causes an electrical current to flow from one terminal through the device to the other terminal, emitting light. Arrays of such LEDs are used for a wide variety of applications including displays, communication, measurement and illumination. They are also widely used in alignment and test systems.
Heretofore, these arrays have been produced by mounting each LED of the array directly onto an electrically conductive surface of an IC package. While this produces a operable array it presents a particular problem for manufacturers in that the correct alignment of multiple LEDs or LED arrays is extremely difficult and critical to the operation of the final product. This is particularly problematic in applications where it is necessary to address individual LED components without influencing the operation of adjacent devices. Control at this level requires the provision of and individual control contact for each and every LED which must be correctly aligned. A further problem arises in that electronic circuitry cannot be integrated with the LED or LED array within a conventional integrated circuit (IC) package.
There is therefore a need for an improved diode array which will overcome the aforementioned problems.
Accordingly there is provided a diode array of the type having an electronic control circuit, at least one diode and contact means for connecting the diode to the control circuit characterised in that the contact means is provided by a carrier material having a high thermal conductivity.
In one embodiment the carrier material is an insulator.
In another embodiment the carrier material is a semiconductor.
In a preferred arrangement the carrier material incorporates an electronic circuit.
Ideally the carrier material incorporates an insulating layer having at least one opening for connecting the diode to the electronic circuit.
Preferably at least one opening is a registration opening.
According to one aspect of the invention there is provided a diode array of the type having an electronic control circuit, a plurality of diodes and contact means for connecting the diodes to the control circuit characterised in that the contact means is provided by a carrier incorporating a plurality of registration slots, each registration slot formed for receiving a diode and locating the received diode in a desired orientation with respect to adjacent diodes.
Preferably the carrier material is mounted on an integrated circuit.
According to another aspect of the invention there is provided a method for producing a diode array comprising the steps of:-
selecting a carrier material;
defining an electronic circuit on the carrier material to produce a patterned material;
covering the patterned material with an electrically insulating film to form a sub-carrier;
etching the insulating film of the sub carrier to define an electrical contact point and a registration slot; and
placing and bonding a diode in the registration slot of the etched sub carrier to form a sub assembly.
Preferably the method further comprises the step of mounting and bonding the sub assembly on an integrated circuit package.
In one arrangement at least one terminal of the diode is connected to a pin on the IC package.
Preferably the method further comprises the step of coating the sub assembly with a protective transparent material .
Ideally the step of defining an electronic circuit on the carrier material includes metallisation of at least one contact .
In one arrangement a plurality of etched sub carriers are produced on a single carrier wafer.
A diode array formed in accordance with the invention ensures the accuracy of placement of individual LEDs or LED arrays in a simple manner. It further facilitates the automatic placement of such components. As accuracy is defined by the microelectronic processing used to fabricate the alignment slots sub-micrometer tolerances may be achieved.
Beneficially, by ensuring correct design of the circuitry on the sub carrier, an electronic logic capability can be added to the package which can significantly reduce the size and pin-out of the IC package and eliminate external circuitry, thereby reducing complexity and cost. Furthermore, electronic circuitry can be fabricated on the Sub-Carrier and thus integrated with the LED or LED array within one and the same IC package.
The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only with reference to the accompanying drawings, in which: -
Fig. 1 is a top view of a diode array in accordance with the invention; and
Fig. 2 is a diagrammatic side view of a the diode array of Fig. 1 with the diodes removed.
Referring to the drawings there is illustrated a diode array indicated generally by the reference numeral 1. In order not to unnecessarily obscure the present invention specific details relating to coating, etching and circuit fabrication have been omitted to aid clarity as they do not form part of the present invention. Similarly, while reference is made throughout the specification to light
emitting diodes it will be readily apparent to those skilled in the art that the invention may be more widely applied.
The diode array 1 has an electronic control circuit (not shown) connected to an integrated circuit (IC) package 2, sixty light emitting diodes 3 and a carrier material 4 for connecting the light emitting diodes (LED) 3 to the IC package 2 and thus to the control circuit. The carrier material 4 may be either electrically insulating or semiconducting but has a high thermal conductivity and is compatible with microelectronic processing techniques and the fabrication of ICs . Possible examples of such material are Silicon, diamond, diamond-like-carbon, Gallium Arsenide, Indium Phosphide, sapphire and glass.
The material 4 incorporates an electronic circuit 10 defined and fabricated onto the material 4 by conventional microelectronic fabrication techniques such as photolithography, metallisation, ion implantation, etching and deposition. The circuit 10 may optionally include one or more metal interconnect tracks corresponding to each LED element or to more than one LED which track or tracks may include one or more metal contact or bonding pads to enable electrical contact to be made by conventional microelectronic bonding techniques between the LED or LEDs and the pins of conventional IC packages. The circuit 10 may similarity include logic, driving, control, active or passive electronic functionality.
The material 4 is covered by an electrically insulating film 11 having holes 12 to define pads where electrical contact can be made and to define registration slots or grooves to allow accurate positional alignment of the LEDs or LED arrays.
Referring now in more detail to the drawings the array 1 has sixty individual LED elements 3 configured in three linear arrays each comprising twenty LEDs 3. The material 4 is silicon either intrinsically insulating or with a top surface layer of insulating oxide. The material 4 is patterned with deposited metal conductor contact pads/areas and interconnect tracks as set out in Fig 1 to provide the circuit 10 on a patterned material. The patterned material 4 is covered by a Silicon Dioxide film 11 to form a sub carrier. Contact pad areas and alignment/registration slots 12 are opened in the film 11 by wet or plasma etching (see Fig 2). Each of the twenty LED 3 arrays is dropped into one of the empty alignment/registration slots 12 preceded by an Indium solder pre-form or conducting paste and heat bonded to the material 4 to form a sub assembly. The sub assembly with LEDs is placed into a 24 pin Dual-In-Line (DIL) window package 2, and epoxy bonded. Wire bonding from each LED 3 to the corresponding metal contact pad and from the sub assembly to the DIL package pad is then carried out. Clear epoxy protection (not shown) can be applied. In this embodiment of the invention the three LED arrays are accurately aligned along their major axes which requirement is critical for measurement, testing and alignment applications. Furthermore, each individual LED can be separately addressed and accessed electrically as required from the use of only 24 pins. For example, LED number 41 from the left can be switched on and off by passing an electrical current from pin number 2 to pin number 13 ( using the standard DIL package pin numbering convention that pin 1 is at bottom left hand corner of the package ( usually marked with a dot on the package) and the pin numbers increase anti-clockwise around the package) without affecting any other LED.
Many variations on the specific embodiments of the invention described will be readily apparent and accordingly the invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail within the scope of the appended claims.