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.
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.
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.