Page 135 - High Power Laser Handbook

104 Diode Lasers Semiconductor Laser Diodes 105

n-type GaAs substrate
Epitaxial growth

Epitaxial characterization 3-inch wafer
(2~4 inch typ.)
Waveguide formation
p-side metallization

Wafer thinning
~300 low-power bars
(1 mm × 10 mm)
n-side metallization -or-
~100 high-power bars
Metallization alloy (3–4 mm × 10 mm)
Bar cleave
25 laser die
Facet passivation (3–4 mm × 0.4 mm)

Facet mirror coating
Dice into individual chips
Figure 5.3 Typical wafer process flow for a semiconductor laser diode.

grown epitaxially by MOCVD or MBE and form the optical cladding
and waveguide layers, as well as carrier confinement. This growth
step is critical for both the laser’s proper initial performance and its
reliability. Sophisticated analytical tools are used to confirm material
composition, layer thickness, doping levels, and defect density.
Structures for electrical and optical confinement in the lateral
direction are then defined. Photolithography is used to pattern the
desired laser geometry onto the wafer’s surface. Various methods
of dielectric deposition, etching, or ion implantation are used. On the
p side of the wafer, a metallization stack is deposited to create an ohmic
contact to the semiconductor, while also providing a stable surface for
subsequent solder reflow or wire bonding. The wafer is then polished
to a thickness of 100 to 150 mm for ease of subsequent cleaving and low
electrical resistance through the substrate. Metallization is deposited
on the n side of the thinned wafer and then briefly heated to alloy the
contact to the semiconductor for low resistance. The wafer is cleaved
into bars, which are then passivated and coated with a dielectric mate-
rial to form the front (output) and rear mirror facets.
For high-power lasers, the foremost differentiating process is facet
passivation and mirror coating. Because the facet power densities are
2
so high—on the order of 100 MW/cm —care must be taken in the design
and control of these processes. The details of these processes are tightly
controlled trade secrets, and there are several competing methods.
The purity and control of the epitaxial layers is the second key process
required to ensure high reliability, high performance, and high yield.