Page 19 - Chip Scale Review_March-April_2024-digital

Figure 3: Tolerances of the freeform couplers with respect to: a) linear translation, and b) rotation about each axis. X represents the direction of light propagation, Y the
vertical direction separating the two chips, and Z the in-plane direction perpendicular to light propagation.
rotation as seen in Figure 3. The tighter bonded together via U V- cu rable sample relative to the dummy Si chip
tolerances with respect to the rotations optically transparent epoxy Dymax OP- that is fixed in position. After alignment,
about X, the light propagation direction, 29. This epoxy is selected due to its fast epoxy is deposited onto the surface of
is due to the tilt about X redirecting the curing time (~2min), matching refractive the dummy PIC. The glass sample is
light beam produced by the reflectors index (n=1.5), and balanced viscosity then slowly brought towards the Si chip
causing it to miss the output waveguide (2500cP). The packaging process starts allowing capillary action to spread the
facet. The coupling efficiency (CE) of with six-axis alignment of the glass epoxy evenly across the surfaces of the
the free-form coupler was simulated to
be 84% at 1550nm, corresponding to an
insertion loss of 0.76dB.

Methodology
The packaging method used for the
3D-printed free-form coupler approach is
based on the variable solder bump height,
which can be controlled by varying the
bond pad sizes [9]. Additionally, these
solder bumps can also act as an electrical Save up to 50%
interconnection between the PIC and the
glass interposer. The vertical distance of your Gold Cost
between the PIC and glass interposer is
3
tested by using 5×5×0.5mm dummy Si Elevate Gold 7990 NBV HT
®
and glass chips. The Si chips have a 3µm A Superior Electrolytic
layer of oxide deposited on the surface Sulfite Gold Process
of the silicon to better mimic a PIC. In
the first case, where the solder bump
height is varied, arrays of 14×14 square • Up to 2.5 times longer bath life
gold bond pads are deposited on the • Up to 50% reduction in gold usage
dummy Si and glass chips with the bond
pad sizes varying from 50µm to 105µm. • Very stable electrolyte – provides zero risk of plate out
The variations are in steps of 5µm and • Slightly acidic pH eliminates defects caused by resist incompatibility
the pitch between the bond pads remains
constant at 100µm in all cases. SAC305 • Gold Metal Replenisher – 2 year shelf life
(8x longer than competitors)
solder bumps are jetted on the gold bond
pads of the silicon chip that is followed
by reflow in an LPKF Protoflow S oven
using the profile shown in Figure 4.
For the evanescent coupling approach,
the PIC and the glass interposer were www.technic.com

Chip Scale Review - Elevate Gold NBV HT 2023 - Gold savings.indd 1 3/6/2024 3:56:50 PM
3/6/2024 3:56:50 PM
Chip Scale Review - Elevate Gold NBV HT 2023 - Gold savings.indd 1
17
Chip Scale Review March • April • 2024 [ChipScaleReview.com] 17