Compound semiconductor LED fabrication (fab) is the process of creating an electronic circuit (in our case LEDs) on an insulating non-conductive or conductive substrate. This process begins with the epi-wafer (see Tech Brief: Epitaxial Wafer Foundry) growth process,and then flows to the “fab ”processes.These processes encompass all those associated with actual individual device fabrication including: Photolithography, Etching, Deposition, and associated mechanical Thinning and Dicing processes.

Insulating Substrate Epiwafer (InGaN/Sapphire): Electrical contacts to the p- and n-layers are on the top surface of the LED. In highy doped conducting p-type GaN, electrical current does not spread laterally within the thin p-type layers. The area of ohmic contact to the p-layer is maximized to promote current spreading, which in turn maximizes light propagation and minimizes turn-on voltage and series resistance. In InGaN/Sapphire LEDs, much of the light generated at the p-n junction escapes the LED through the top surface, therefore the large-area p-contact is made with transparent metalization in all areas outside those dedicated for electrical bond wire attachment pads.

Conducting Substrate Epiwafer (AlInGaP/GaAs): In UNIROYAL Optoelectronics AlInGaP/GaAs device structure the conducting substrate serves to transition the n-contact to a metalized back-die contact, while the p-layer contact is patterned on the top-die surface.

Photolithography: This area transfers the photolithographic mask of the LED die geometry, usually a glass master, to the wafer using light to effect the transfer via a light-sensitive photo-emulsive film (photoresist) applied to the wafer surface. The process is analogous to photoengraving.

Etching: Here the challenge is to maintain relatively fine line and layer detail within III-V materials, such as GaN which has a hardness and chemical inertness resembling diamond, to expose the n-layer. Halogen-based etch chemistries are used, as no production wet methods exhibiting reliable and repeatable practical implementation that are practical have been found to date. Thus the die mask is transferred from the photoresist to the wafer.

Metals Deposition: A vapor deposition process that deposits combinations of metal onto the wafer at elevated temperature and reduced pressure to promote chemical reaction. For high reliability purposes, UNIROYAL Optoelectronics utilizes gold top layer metalization for its wire bond pad contacts.

Passivation Deposition: A proprietary deposition process through which UNIROYAL Optoelectronics deposits a specialized Silicon Dioxide layer to the outer LED die surface for durability purposes.

By repeating these steps through the process flow, the various structures and top layer topology of the LED die are established.

Thinning/Dicing: Thinning Sapphire (to 3.5 to 5 mils) and GaAs (to 6 to 8 mils) with highly stressed and compositionally diverse active regions, without breaking,requires rigorous process details and regimen along with full understanding of device structures, crystallography, equipment and fabrication methodologies. UNIROYAL Optoelectronics has developed complementary proprietary processes for this area of die processing that are unique in the industry. The results include maintaining die thickness and x/y die geometry more consistent than previously obtained.