Page 16 - Chip Scale Review_March-April_2024-digital

3D integration of photonic and electronic chips on a

glass substrate

By Saif Wakeel, Peter O’Brien [Tyndall National Institute, University College Cork, Ireland]
Luigi Ranno, Anuradha Agarwal, Lionel Kimerling [Massachusetts Institute of Technology]
3 D printed free-form couplers platform [1,4]. The 3D-printed free- approach. A UV-cured optical epoxy

and inverse tapers are two
different vertical integration form couplers allow for relatively can fill such a gap while maintaining
a relatively strong bond after curing.
large alignment tolerances and a high
strategies for coupling light between a bandwidth density between the PICs and Optically transparent epoxies are useful
photonic integrated circuit (PIC) and interposer waveguides by collimating and for packaging as they form a layer that
glass interposer. For the free-form redirecting the light from the waveguides is both a strong adhesive and refractive
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coupling scheme, 5×5×0.5mm dummy in the PIC to the waveguides in the index-matching. During curing, the layer
silicon dies are bonded to a glass glass interposer [4,5]. Similarly, the thickness is reduced due to shrinkage.
substrate using solder bumps via flip- overlapping inverse taper design allows When this effect is accounted for and
chip bonding. The gap distance between for high optical bandwidth (300nm/1dB) occurs in the same direction as the
the bonded chips is controlled by the and high-density of out-of-plane optical optical path, the coupling efficiency
solder height based on various bond pad interconnections between PIC waveguides can see improvement as the optical path
sizes. An array of 14×14 square gold and glass waveguides [1]. Both integration length is shortened after curing [6].
bond pads on the silicon chip is used with strategies require different packaging The free-from coupler Z-tolerance is
the bond pad length varying from 50µm approaches because of the varying not as strict as the tapered waveguides,
to 105µm in order to maintain a required vertical spacing requirements between so we demonstrated a different packaging
height. SAC305 solder is jetted onto the PIC and interposer. The 3D-printed method combining flip-chip bonding with
bond pads and an optimization of the free-form coupling scheme shown in solder and an epoxy underfill as bonding
reflow profile is performed to achieve Figure 1a requires ≤14µm vertical materials. This technique is widely used
defect-free, and high-wetting solder distance, whereas the inverse taper in electronic packaging industries as this
bumps. For the inverse taper coupling coupling method requires ≤3µm vertical method provides both strong mechanical
approach, the PIC is bonded to glass distance to facilitate evanescent coupling and electrical connections to a package
using an ultraviolet (UV)-curable optical as shown in Figure 1b. [7,8]. Recently, a group demonstrated
epoxy (n=1.5). The measured vertical The tapered waveguide approach using flip-chip attachment of a vertical cavity
distance falls within the 2.8µm required evanescent coupling requires a smaller surface emitting laser (VCSEL) onto a Si
Z-tolerance for evanescent coupling Z-tolerance than the free-form coupler PIC using solder bonds [9]. Solder was
according to simulation.

Introduction
The move towards all optical I/Os on a
switch package presents a viable solution
for enabling low energy consumption
(<2pJ/bit per transmitter (Tx)/receiver
(Rx)) in devices operating at >100Gbps
[1]. In these Tx/Rx packages, the PICs
are co-packaged with the electronic
integrated circuits (EICs) on a single
interposer to minimize radio frequency
(RF) track length [2]. This presents
a unique opportunity for using the
interposer material as a unified electrical-
optical interface. Glass can be used for
this purpose due to its low dielectric
constant, low-loss tangent and low-
loss optical wave guiding properties
[3]. New optical coupling strategies Figure 1: Packaging of PIC and EIC on a glass substrate: a) using solder bump flip-chip attachment between
such as free-form couplers and inverse the PIC and glass substrate to enable optical coupling by 3D printed coupler, and b) by epoxy bonding PIC and
tapers are promising interfaces for this glass substrate to enable evanescent coupling between PIC and glass waveguides.

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14 Chip Scale Review March • April • 2024 [ChipScaleReview.com]