Good of you to repeat my post FJ, especially for those that have me on ignore!

 

The text of paper (without graphics/images):

 

A Wafer Scale Hybrid Integration Platform for Co-packaged Photonics using a

CMOS based Optical InterposerTM

 

Abstract: In this paper, we present a unique hybrid integration

platform for wafer scale passive assembly of electronics and

photonics devices using a CMOS based Optical Interposer.

Our optical interposer enables seamless communications

between electronics and photonics chips that are assembled on

it using visually assisted passive flip chip bonding techniques.

This unique integration platform is the first such platform in

the industry adapted to directly modulated lasers and enables

the world’s smallest single chip Transmit/Receive Optical

engine for 100G-400G optical engines.

 

Introduction: The explosive and exponential growth in data

rates has necessitated a co-packaged multi-chip module

integrating both electronics and optical components (or

photonic chiplets)[1, 2]. This work explores the use of a silicon

interposer modified with multiple optical waveguide layers to

communicate both electrically and optically and enable a low

cost wafer scale integration platform for such co-packaging

applications. The platform has been applied to 100G DML-

based Optical engines, to demonstrate the essential features of

the hybrid integration platform and is readily extensible to

400G with compatible photonics components. Due to its

modular construction, with pre-validated building blocks, the

platform is easily adaptable to other applications for either high

speed data transport or for applications that benefit from

proximity placements of electronics and photonics. The

electrical interfaces are accomplished with conventional

electrical interposer functionality (metal traces and through

silicon vias (TSVs), whereas the optical interfaces are

accomplished with multiple layers of non-interacting optical

waveguides layered above the electrical traces (Fig.1).

 

The Optical Interposer’s Features: The Optical Interposer

is constructed using CMOS compatible wafer fabrication

methods using a high resistivity silicon substrate to enable high

speed RF communications. Electrical traces required for

component interconnectivity are first formed on the silicon

substrate. Integrated heat sinks are incorporated under the

metal and in regions of the interposer that house the lasers,

enabling a low thermal resistance for the lasers, which is

critical to ensure laser functionality in uncooled applications.

Thereafter, multiple waveguide layers are monolithically

integrated on the wafer. These waveguides are configured to

perform the various optical functions required such as

multiplexers, de-multiplexers, vertical and in-plane couplers,

interferometers and directional coupler-based power taps. The

Optical Interposer is then finished with low loss vertical

coupling mirrors for out-of-plane optical connections and with

mirror like etched facets for in-plane coupling of lasers and

fiber connections. The features of such an Optical interposer

are shown in Fig 2. Self-referencing pedestals, fiducial marks

and mechanical guides enable visually-assisted passive

placement of the optical devices integrated on the platform.

Finally, eutectic solder is deposited on the interposer to

promote flip-chip bonding of the optical and electrical

components that are subsequently assembled on the platform.

Fig.2B/C shows the optical micrograph of a completed optical

interposer chip prior to and post assembly, highlighting the

various components of the interposer.

 

Results and Discussion: Waveguides: Our waveguides are

designed to both be CMOS compatible and provide low loss

characteristics. The material loss through the waveguides

characterized by prism spectroscopy is <0.3dB/cm and is about

one order of magnitude better than typically observed in small

core silicon waveguides used in most other silicon photonics

technologies. Moreover, our waveguides are largely athermal

(dn/dT=12pm/°C) and are non-birefringent. A proprietary Spot

Size Converter has been designed for chip facet fiber coupling

achieving facet coupling losses of 0.25dB. Optical Passive

device performance: Waveguides must be designed and

configured into high performance optical passive devices for

use in any optical application. Multiplexers and

Demultiplexers form the backbone of any direct detect data-

communications WDM (wavelength division multiplexing)

system. These devices have to be precisely engineered for the

required bandwidth spectrum used (for example, FR4, LR4).

Fig.3 shows the measured optical spectrum for the passive

demux/mux devices built on the Optical Interposer for FR4 and

LR4 systems respectively. Excellent insertion loss, crosstalk,

channel uniformity are achieved exceeding requirements.

Vertical Mirrors: In addition to in plane coupling of light,

POET’s optical interposers utilize vertical mirrors to enable

out of plane coupling. The vertical mirrors are used with top-

entry photodetectors and for wafer level test. Fig. 4 shows the

construction and performance (0.5dB coupling loss) achieved

by the vertical mirrors when coupled to a PD. Thermal

Performance: Achieving low thermal resistance is a critical

requirement for hybrid integration. Fig.5 shows a comparison

of the thermal resistance of a conventional P-up lasers mounted

on a standard AlN submount to that achieved with a P-down

laser attached to the Optical Interposer. Equivalent thermal

resistances are obtained suggesting a good thermal path from

the heat source (laser) to the back of the interposer through the

integrated heat sink. Laser Coupling: One of the key benefits to

using the Optical Interposer is its visually assisted wafer scale

passive placement of optical devices such as lasers while

simultaneously achieving good coupling efficiency to the

waveguides. Fig. 6 shows the coupling performance for CW

lasers with integrated spot size converters. 90% (1dB)

coupling efficiency has been achieved which is best in class for

such passive alignment techniques.

 

Product Demonstration: The features of the Optical

Interposer have been used along with compatible optical

components to create the world’s smallest single chip

100/200G Optical engine. At 6mmx9mm, this optical engine

incorporates 4 lasers, 4 high speed photodetectors, 4 monitor

photo diodes and a multiplexer/de-multiplexer pair. Fig. 7

shows an image of a completed and assembled optical engine.

The optical engine is so small, that one can fit four such

engines inside a standard QSFP-DD module, thus quadrupling

data rates for a given faceplate density. Fig. 8A shows the

achieved eye diagrams for 100G transmission with excellent

eye margins. Finally, Fig. 8B shows the BER/Sensitivity

performance of 100G/200G receivers built using this

technology again showing excellent characteristics and

meeting 10km LR4 system requirements.

 

Conclusions: We have demonstrated a unique silicon-based

wafer scale hybrid integration photonics packaging platform

for applications in current and future datacenter applications.

The versatile and low loss platform called the Optical

Interposer includes all the features necessary for high speed

datacenter applications: Excellent RF performance, low loss,

athermal and non-birefringent waveguides and low loss chip-

fiber coupling, high coupling efficiency passive placement of

optical devices, excellent thermal properties with low thermal

resistance and low cost through wafer scale assembly and test.

Products assembled with the Optical Interposer show excellent

performance for 100/200/400G applications.

 

Fig. 1: A) Electrical interfaces and B) Optical interfaces with multiple

 

layers of non-interacting optical waveguides

 

Fig. 2: A) Optical Interposer features, Optical Interposer chip A)

 

before and C) after assembly

 

Fig. 3: (a) FR4 Demux (b) LR4 Demux (c) FR4 Mux & (d) LR4 Mux

 

spectral characteristics with excellent insertion loss and Xtalk

 

Fig. 4: Construction and performance of a low loss vertical mirror 

 

Fig. 5: Comparison of thermal resistance of a P-up laser on-AlN

submount to that of a P-down laser attached to the Interposer

 

Fig. 6: Comparison of thermal resistance of a P-up laser on AlN sub-

mount to that of a P-down laser attached to the Interposer

 

Fig. 7: World’s smallest fully assembled Optical Engine for 100G-

400G FR4 applications

 

Fig. 8: (A) Eye Diagrams and (B) Receiver sensitivity for 100G

optical engines meeting CWDM4 MSA specifications